chapter xvi. of the _Riddle_). It seems that the hereditary bias

towards mysticism and superstition is not yet eliminated even from the educated mind of our time. It is to be explained phylogenetically by inheritance from prehistoric barbarians and savages, in whom the earliest religious ideas were wholly dominated by animism and fetichism.

IV

THE SCIENCE OF LIFE

Object of biology--Relation to the other sciences--General and special biology--Natural philosophy--Monism: hylozoism, materialism, dynamism--Naturalism--Nature and spirit--Physics--Metaphysics--Dualism--Freedom and natural law--God in biology--Realism--Idealism--Branches of biology--Morphology and physiology--Anatomy and biogeny--Ergology and perilogy.

The broad realm of science has been vastly extended in the course of the nineteenth century. Many new branches have established themselves independently; many new and most fruitful methods of research have been discovered, and have been applied with the greatest practical success in furthering the advance of modern thought. But this enormous expansion of the field of knowledge has its disadvantages. The extensive division of labor it has involved has led to the growth of a narrow specialism in many small sections; and in this way the natural connection of the various provinces of knowledge, and their relation to the comprehensive whole, have been partly or wholly lost sight of. The importation of new terms which are used in different senses by one-sided workers in the various fields of science has caused a good deal of misunderstanding and confusion. The vast structure of science tends more and more to become a tower of Babel, in the labyrinthic passages of which few are at their ease and few any longer understand the language of other workers. In these circumstances, it seems advisable, at the commencement of our philosophic study of "the wonders of life," to form a clear idea of our task. We must carefully define the place of biology among the sciences, and the relation of its various branches to each other and to the different systems of philosophy.

In the broadest sense in which we can take it, biology is the whole study of organisms or living beings. Hence not only botany (the science of plants) and zoology (the science of animals), but also anthropology (the science of man), fall within its domain. We then contrast with it all the sciences which deal with inorganic or lifeless bodies, which we may collectively call abiology (or anorganology); to this belong astronomy, geology, mineralogy, hydrology, etc. This division of the two great branches of science does not seem difficult in view of the fact that the idea of life is sharply defined physiologically by its metabolism and chemically by its plasm; but when we come to study the question of abiogenesis (chapter xv.) we shall find that this division is not absolute, and that organic life has been evolved from inorganic nature. Moreover, biology and abiology are connected branches of cosmology, or the science of the world.

While the idea of biology is now usually taken in this broad sense in most scientific works and made to embrace the whole of living nature, we often find (especially in Germany) a narrower application of the term. Many authors (mostly physiologists) understand by it a section of physiology--namely, the science of the relations of living organisms to the external world, their habitat, customs, enemies, parasites, etc. I proposed long ago to call this special part of biology œcology (the science of home-relations), or bionomy. Twenty years later others suggested the name of ethology. To call this special study any longer biology in the narrower sense is very undesirable, because it is the only name we have for the totality of the organic sciences.

Like every other science, biology has a general and a special part. General biology contains general information about living nature; this is the subject of the present study of the wonders of life. We might also describe it as biological philosophy, since the aim of true philosophy must be the comprehensive survey and rational interpretation of all the general results of scientific research. The innumerable discoveries of detailed facts which observation and experiment give us, and which are combined into a general view of life in philosophy, form the subject of empirical science. As the latter, on the side of the organic world, or as empirical biology, forms the first object of the science of life, and seeks to effect in the system of nature a logical arrangement and summary grouping of the countless special forms of life, this special biology is often wrongly called the science of classification.

The first comprehensive attempt to reduce to order and unity the ample biological material which systematic research had accumulated in the eighteenth century was made by what we call "the older natural philosophy" at the beginning of the nineteenth century. Reinhold Treviranus (of Bremen) had made a suggestive effort to accomplish this difficult task on monistic principles in his _Biology, or Philosophy of Living Nature_ (1802). Special importance attaches to the year 1809, in which Jean Lamarck (of Paris) published his _Philosophie Zoologique_, and Lorentz Oken (of Jena) his _Manual of Natural Philosophy_. I have fully appreciated the service of Lamarck, the founder of the theory of descent, in my earlier writings. I have also recognized the great merit of Lorentz Oken, who not only aroused a very wide interest in this science by his _General Natural History_, but also put forward some general observations of great value. His "infamous" theory of a primitive slime, and the development of infusoria out of it, is merely the fundamental idea of the theory of protoplasm and the cell which was long afterwards fully recognized. These and other services of the older natural philosophy were partly ignored and partly overlooked, because they went far beyond the scientific horizon of the time, and their authors to an extent lost themselves in airy and fantastic speculations. The more scientists confined themselves in the following half-century to empirical work and the observation and description of separate facts, the more it became the fashion to look down on all "natural philosophy." The most paradoxical feature of the situation was that purely speculative philosophy and idealist metaphysics had a great run at the same time, and their castles in the air, utterly destitute of biological foundation, were much admired.

The magnificent reform of biology which Darwin initiated in 1859 by his epoch-making _Origin of Species_ gave a fresh impulse to natural philosophy. As this work not only used the rich collection of facts already made in proof of the theory of descent, but gave it a new foundation in the theory of selection (Darwinism properly so called), everything seemed to call for the embodiment of the new conception of nature in a monistic system. I made the first effort to do this in my _General Morphology_ (1866). As this found few supporters among my colleagues, I undertook in my _History of Creation_ (1868) to make the chief points of the system accessible to the general reader. The remarkable success of this book (a tenth edition of it appearing in 1902) emboldened me at the end of the nineteenth century to state the general principles of my monistic philosophy in my _Riddle of the Universe_. About the same time (1899) there appeared the work of the Kiel botanist, Johannes Reinke, _The World as Reality_; and two years afterwards he followed it up with a supplementary volume, _Introduction to Theoretic Biology_. As Reinke treats the general problems of natural philosophy from a purely mystic and dualistic point of view, his ideas are diametrically opposed to my monistic and naturalistic principles.

The history of philosophy describes for us the infinite variety of ideas that men have formulated during the last three thousand years on the nature of the world and its phenomena. Überweg has given us, in his excellent _History of Philosophy_, a thorough and impartial account of these various systems. Fritz Schultze has published a clear and compendious "tabulated outline" of them in thirty tables in his genealogical tree of philosophy, and at the same time shown the phylogeny of ideas. When we survey this enormous mass of philosophic systems from the point of view of general biology, we find that we can divide them into two main groups. The first and smaller group contains the monistic philosophy, which traces all the phenomena of existence to one single common principle. The second and larger group, to which most philosophic systems belong, constitutes the dualistic philosophy, according to which there are two totally distinct principles in the universe. These are sometimes expressed as God and the world, sometimes as the spiritual world and material world, sometimes as mind and matter, and so on. In my opinion, this antithesis of monism and dualism is the most important in the whole history of philosophy. All other systems are only variations of one or the other of these, or a more or less obscure combination of the two.

The form of monism which I take to be the most complete expression of the general truth, and which I have advocated in my writings for thirty-eight years, is now generally called hylozoism. This expresses the fact that all substance has two fundamental attributes; as matter (_hyle_) it occupies space, and as force or energy it is endowed with sensation (_cf._ chapter xix.). Spinoza, who gave the most perfect expression to this idea in his "philosophy of identity," and most clearly treated the notion of substance (as the all-embracing essence of the world), clothes it with two general attributes--extension and thought. Extension is identical with real space, and thought with (unconscious) sensation. The latter must not be confused with conscious human thought; intelligence is not found in substance, but is a special property of the higher animals and man. Spinoza identifies his substance with nature and God, and his system is accordingly called pantheism; but it must be understood that he rejects the anthropomorphic, personal idea of deity.

A good deal of the infinite confusion that characterizes the conflicts of philosophers over their systems is due to the obscurity and ambiguity of many of their fundamental ideas. The words "substance" and "God," "soul" and "spirit," "sensation" and "matter," are used in the most different and changing senses. This is especially true of the word "materialism," which is often wrongly taken to be synonymous with monism. The moral bias of idealism against _practical_ materialism (or pure selfishness and sensualism) is forthwith transferred to theoretical materialism, which has nothing to do with it; and the strictures which are justly urged against the one are most unjustifiably applied to the other. Hence it is important to distinguish very carefully between these two meanings of materialism.

Theoretical materialism (or hylonism), as a realistic and monistic philosophy, is right in so far as it conceives matter and force to be inseparably connected, and denies the existence of immaterial forces. But it is wrong when it denies all sensation to matter, and regards actual energy as a function of dead matter. Thus, in ancient times Democritus and Lucretius traced all phenomena to the movements of dead atoms, as did also Holbach and Lamettrie in the eighteenth century. This view is held to-day by most chemists and physicists. They regard gravitation and chemical affinity as a mere mechanical movement of atoms, and this, in turn, as the general source of all phenomena; but they will not allow that these movements necessarily presuppose a kind of (unconscious) sensation. In conversation with distinguished physicists and chemists I have often found that they will not hear a word about a "soul" in the atom. In my opinion, however, this must necessarily be assumed to explain the simplest physical and chemical processes. Naturally I am not thinking of anything like the elaborate psychic action of man and the higher animals, which is often bound up with consciousness; we must rather descend the long scale of the development of consciousness until we reach the simplest protists, the monera (chapter ix.). The psychic activity of these homogeneous particles of plasm (for instance, the chromacea) rises very little above that of crystals; as in the chemical synthesis in the moneron, so in crystallization we are bound to assume that there is a low degree of sensation (not of consciousness), in order to explain the orderly arrangement of the moving molecules in a definite structure.

The prejudice against theoretical materialism (or materialistic monism) which still prevails so much is partly due to its rejection of the three central dogmas of dualist metaphysics, and partly to a confusion of it with hedonism. This practical materialism in its extreme forms (as Aristippus of Cyrene and the Cyrenaic school, and afterwards Epicurus, taught it) finds the chief end of life in pleasure--at one time crude, sensual pleasure, and at others spiritual pleasure. Up to a certain point, this thirst for happiness and a pleasant and enjoyable life is innate in every man and higher animal, and so far just; it only began to be censured as sinful when Christianity directed the thoughts of men to eternal life, and taught them that their life on earth was only a preparation for the future. We shall see afterwards, when we come to weigh the value of life (chapter xvii.), that this asceticism is unjustifiable and unnatural. But as every legitimate enjoyment can become wrong by excess, and every virtue be turned into vice, so a narrow hedonism is to be condemned, especially when it allies itself with egoism. However, we must point out that this excessive thirst for pleasure is in no way connected with materialism, but is often found among idealists. Many convinced supporters of theoretical materialism (many scientists and physicians, for instance) lead very simple, blameless lives, and are little disposed to material pleasures. On the other hand, many priests, theologians, and idealist philosophers, who preach theoretical idealism, are pronounced hedonists in practice. In olden times many temples served at one and the same time for the theoretic worship of the gods and for practical excesses in the way of wine and love; and even in our day the luxurious and often vicious lives of the higher clergy (at Rome, for instance) do not fall far short of the ancient models. This paradoxical situation is due to the special attractiveness of everything that is forbidden. But it is utterly unjust to extend the natural feeling against excessive and egoistic hedonism to theoretical materialism and to monism. Equally unjust is the habit, still widely spread, of depreciating matter, as such, in favor of spirit. Impartial biology has taught us of late years that what we call "spirit" is--as Goethe said long ago--inseparably bound up with matter. Experience has never yet discovered any spirit apart from matter.

On the other hand, pure dynamism, now often called energism (and often spiritualism), is just as one-sided as pure materialism. Just as the latter takes one attribute of substance, matter, as the one chief cause of phenomena, dynamism takes its second attribute, force (_dynamis_). Leibnitz most consistently developed this system among the older German philosophers; and Fechner and Zöllner have recently adopted it in part. The latest development of it is found in Wilhelm Ostwald's _Natural Philosophy_ (1902). This work is purely monistic, and very ingeniously endeavors to show that the same forces are at work in the whole of nature, organic and inorganic, and that these may all be comprised under the general head of energy. It is especially satisfactory that Ostwald has traced the highest functions of the human mind (consciousness, thought, feeling, and will), as well as the simplest physical and chemical processes (heat, electricity, chemical affinity, etc.), to special forms of energy, or natural force. However, he is wrong when he supposes that his energism is an entirely new system. The chief points of it are found in Leibnitz; and other Leipzig scientists, especially Fechner and Zöllner, had come very close to similar spiritualistic views--the latter going into outright spiritism. Ostwald's chief mistake is to take the terms "energy" and "substance" to be synonymous. Certainly his universal, all-creating energy is, in the main, the same as the substance of Spinoza, which we have also adopted in our "law of substance." But Ostwald would deprive substance of the attribute of matter altogether, and boasts of his _Refutation of Materialism_ (1895). He would leave it only the one attribute, energy, and reduce all matter to immaterial points of force. Nevertheless, as chemist and physicist, he never gets rid of space-filling substance--which is all we mean by "matter"--and has to treat it and its parts, the physical molecules and chemical atoms (even if only conceived as symbols), daily as "vehicles of energy." Ostwald would reject even these in his pursuit of the illusion of a "science without hypotheses." As a fact, he is forced every day, like every other exact scientist, to assume and apply in practice the indispensable idea of matter, and its separate particles, the molecules and atoms. Knowledge is impossible without hypotheses.

Monism is best expressed as hylozoism, in so far as this removes the antithesis of materialism and spiritualism (or mechanicism and dynamism), and unites them in a natural and harmonious system. Our monistic system has been charged with leading to pure naturalism; one of its most vehement critics, Frederick Paulsen, attaches so much importance to this stricture that he thinks it as dangerous as dogmatic clericalism. We may, therefore, usefully consider the idea of naturalism, and point out in what sense we accept it and identify it with monism. The key to the position is in our monistic anthropogeny, our unprejudiced conviction, supported by every branch of anthropological research, of "man's place in nature," as we have established it in the first section of the _Riddle_ (chapters ii.-v.). Man is a purely natural being, a placental mammal of the order of primates. He was phylogenetically evolved in the course of the Tertiary Period from a series of the lower primates (directly from the anthropoid apes, but earlier from the cynocephali and lemures). Savage man, as we have him to-day in the Veddah or Australian negro, is physiologically nearer to the apes than to highly civilized men.

Anthropology (in the widest sense) is only a particular branch of zoology, to which we must assign a special position on account of its extreme importance. Hence all the sciences which relate to man and his psychic activity--especially what are called the moral sciences--must be regarded from our monistic point of view as special branches of zoology and as natural sciences. Human psychology is inseparably connected with comparative animal psychology, and this again with that of the plants and protists. Philology studies in human speech a complicated natural phenomenon, which depends on the combined action of the brain-cells of the phronema, the muscles of the tongue, and the vocal cords of the larynx, as much as the cry of mammals and the song of birds do. The history of mankind (which we, in our curious anthropocentric mood, call the history of the world), and its highest branch, the history of civilization, is connected by modern prehistoric science directly with the stem-history of the primates and the other mammals, and indirectly with the phylogeny of the lower vertebrates. Hence, when we consider the subject without prejudice, we do not find a single branch of human science that passes the limits of natural science (in the broadest sense), any more than we find nature herself to be supernatural.

Just as monism, or naturalism, embraces the totality of science, so on our principles the idea of nature comprises the whole scientifically knowable world. In the strict monistic sense of Spinoza the ideas of God and Nature are synonymous for us. Whether there is a realm of the supernatural and spiritual beyond nature we do not know. All that is said of it in religious myths and legends, or metaphysical speculations and dogmas, is mere poetry and an outcome of imagination. The imagination of civilized man is ever seeking to produce unified images in art and science, and when it meets with gaps in these in the association of ideas it endeavors to fill them with its own creations. These creations of the phronema with which we fill the gaps in our knowledge are called _hypotheses_ when they are in harmony with the empirically established facts, and _myths_ when they contradict the facts: this is the case with religious myths, miracles, etc. Even when people contrast mind with nature, this is only a result, as a rule, of similar superstitions (animism, spiritism, etc.). But when we speak of man's mind as a higher psychic function, we mean a special physiological function of the brain, or that particular part of the cortex of the brain which we call the phronema, or organ of thought. This higher psychic function is a natural phenomenon, subject, like all other natural phenomena, to the law of substance. The old Latin word _natura_ (from _nasci_, to be born) stands, like the corresponding Greek term _physis_ (from _phyo_--to grow), for the essence of the world as an eternal "being and becoming"--a profound thought! Hence physics, the science of the _physis_, is, in the broadest sense of the word, "natural science."

The extensive division of labor which has taken place in science, on account of the enormous growth of our knowledge in the nineteenth century and the rise of many new disciplines, has very much altered their relations to each other and to the whole, and has even given a fresh meaning and connotation to the term. Hence by physics, as it is now taught at the universities, is usually understood only that part of inorganic science which deals with the molecular relations of substance and the mechanism of mass and ether, without regard to the qualitative differences of the elements, which are expressed in the atomic weight of their smallest particles, the atoms. The study of the atoms and their affinities and combinations belongs to chemistry. As this province is very extensive and has its special methods of research, it is usually put side by side with physics as of equal importance; in reality, however, it is only a branch of physics--chemistry is the physics of the atoms. Hence, when we speak of a physico-chemical inquiry or phenomenon, we might justly describe it briefly as _physical_ (in the wider sense). Physiology, again, a particularly important branch of it, is in this sense the physics of living things, or the physico-chemical study of the living body.

Since Aristotle dealt with the eternal phenomena of nature in the first part of his works, and called this _physics_, and with their inner nature in the second part, to which he gave the name of _metaphysics_, the two terms have undergone many and considerable modifications. If we restrict the term "physics" to the empirical study of phenomena (by observation and experiment), we may give the name of metaphysics to every hypothesis and theory that is introduced to fill up the gaps in it. In this sense the indispensable theories of physics (such as the assumption that matter is made up of molecules and atoms and electrons) may be described as metaphysical; such also is our assumption that all substance is endowed with sensation as well as extension (matter). This monistic metaphysics, which recognizes the absolute dominion of the law of substance in all phenomena, but confines itself to the study of nature and abandons inquiry into the supernatural, is, with all its theories and hypotheses, an indispensable part of any rational philosophy of life. To claim, as Ostwald does, that science must be free from hypotheses is to deprive it of its foundations. But it is very different with the current dualistic metaphysics, which holds that there are two distinct worlds, and which we find in a hundred forms as philosophic dualism.

If we understand by metaphysics the science of the ultimate ground of things, springing from the rational demand for causes, it can only be regarded, from the physiological point of view, as a higher and late-developed function of the phronema. It could only arise with the complete development of the brain in civilized man. It is completely lacking among savages, whose organ of thought rises very little above that of the most intelligent animals. The laws of the psychic life of the savage have been closely studied by modern ethnology. It teaches us that the higher reason is not found in savages, and that their power of abstract thought and of forming concepts is at a very low level. Thus, for instance, the Veddahs, who live in the forests of Ceylon, have not the general idea of trees, though they know and give names to individual trees. Many savages cannot count up to five; they never reflect on the ground of their existence or think of the past or future. Hence it is a great error for Schopenhauer and other philosophers to define man as a "metaphysical animal," and to seek a profound distinction between man and the animal in the need for a metaphysic. This craving has only been awakened and developed by the progress of civilization. But even in civilized communities it (like consciousness) is not found in early youth, and only gradually emerges. The child has to learn to speak and think. In harmony with our biogenetic law, the child reproduces in the various stages of its mental development the whole of the gradations which lead from the savage to the barbarian, and from the barbarian to the half-civilized, and on to the fully educated man. If this historical development of the higher human faculties had always been properly appreciated, and psychology had been faithful to the comparative and genetic methods, many of the errors of the current metaphysical systems would have been avoided. Kant would not then have produced his theory of _a priori_ knowledge, but would have seen that all that now seems to be _a priori_ in civilized man was originally acquired by _a posteriori_ experiences in the long evolution of civilization and science. Here we have the root of the errors which are distinctive of dualism and the prevailing metaphysical transcendentalism.

Like all science, biology is _realistic_--that is to say, it regards its object, the organisms, as really existing things, the features of which are to an extent knowable through our senses (_sensorium_) and organ of thought (_phronema_). At the same time, we know that these cognitive organs, and the knowledge they bring us, are imperfect, and that there may be other features of organisms that lie beyond our means of perception altogether. But it by no means follows from this that, as our idealist opponents say, the organisms (and all other things) exist only in our mind (in the images in our cortex). Our pure monism (or hylozoism) agrees with realism in recognizing the unity of being of each organism, and denying that there is any essential distinction between its knowable phenomenon and its internal hidden essence (or noumenon), whether the latter be called, with Plato, the eternal "idea," or, with Kant, the "thing in itself." Realism is not identical with materialism, and may even be definitely connected with the very opposite, dynamism or energism.

As realism generally coincides with monism, so idealism is usually identical with dualism. The two most influential representatives of dualism, Plato and Kant, said that there were two totally distinct worlds. Nature, or the empirical world, is alone accessible to our experience, while the spiritual or transcendental world is not. The existence of the latter is known to us only by the emotions or by practical reason; but we can have no idea of its nature. The chief error of this theoretical idealism is the assumption that the soul is a peculiar, immaterial being, immortal and endowed with _a priori_ knowledge. The physiology and ontogeny of the brain (together with the comparative anatomy and histology of the phronema) prove that the soul of man is, like that of all other vertebrates, a function of the brain, and inseparably bound up with this organ. Hence this idealist theory of knowledge is just as inconsistent with realistic biology as is the psycho-physical parallelism of Wundt or the psychomonism of more recent physiologists, which in the end issues in a complete dualism of body and mind. It is otherwise with _practical_ idealism. When this presents the symbols or ideals of a personal God, an immortal soul, and the free-will as ethical stimuli, and uses them for their pedagogical worth in the education of the young, it may have a good influence for a time, which is independent of their theoretical untenability.

The many branches of biology which have been developed independently in the course of the nineteenth century ought to remain in touch with one another, and co-operate with a clear apprehension of their task, if they are to attain their high purpose of framing a unified science embracing the whole field of organic life. Unfortunately, this common aim is often lost sight of in the specialization of study; the philosophical task is neglected in favor of the empirical. The confusion that has ensued makes it desirable to determine the mutual positions of the various biological disciplines. I went into this somewhat fully in my academic speech on the development and aim of zoology in 1869. But as this essay is little known, I will briefly resume the chief points of it.

In correspondence with the long-established distinction between the plant and the animal, the two chief branches of biology, zoology and botany, have developed side by side, and are represented by two different chairs in the universities. Independently of these, there arose at the very beginning of scientific activity that field of inquiry which deals with human life in all its aspects--the anthropological disciplines and the so-called "mental sciences" (history, philology, psychology, etc.). Since the theory of descent has proved man's origin from vertebrate ancestors, and thus anthropology has been recognized as a part of zoology, we have begun to understand the inner historic connection between these various branches of anthropology, and to combine them in a comprehensive science of man. The immense extent and the great importance of this science have justified the creation of late years of special chairs of anthropology. It seems desirable to do the same for the science of the protists, or unicellular organisms. The cell theory, or cytology, as an elementary part of anatomy, has to be dealt with in both botany and zoology; but the lowest unicellular representatives of both kingdoms, the primitive plants (protophyta) and the primitive animals (protozoa), are so intimately connected, and throw so great a light, as independent rudimentary organisms, on the tissue cells in the _histon_, or multicellular organism, that we must regard as a sign of progress the recent proposal of Schaudinn to found a special institute and journal for the science of protists. One very important section of it is bacteriology.

The practical division of biology, according to the extent of the organic kingdom, leads us to mark out four chief provinces of research: protistology (the science of the unicellulars), botany (the science of plants), zoology (the science of animals), and anthropology (the science of man). In each of these four fields we may then distinguish morphology (the science of forms) and physiology (the science of functions) as the two chief divisions of scientific work. The special methods and means of observation differ entirely in the two sections. In morphology the work of description and comparison is the most important as regards both outer form and inner structure. In physiology the exact methods of physics and chemistry are especially demanded--the observation of vital activities and the attempt to discover the physical laws that govern them. As a correct knowledge of human anatomy and physiology is indispensable for scientific medicine, and the work requires a particularly large apparatus, these two sciences have long been studied separately, and have been handed over to the medical facility in the division of the academic curriculum.

The broad field of morphology may be divided into anatomy and biogeny; the one deals with the fully developed, and the other with the developing, organism. Anatomy, the study of the formed organism, studies both the external form and the inner structure. We may distinguish as its two branches the science of structures (tectology) and the science of fundamental forms (promorphology). Tectology investigates the features of the structure in the organic _individual_, and the composition of the body out of various parts (cells, tissues, and organs). Promorphology describes the real form of these individual parts and of the whole body, and endeavors to reduce them mathematically to certain fundamental forms (chapter viii.). Biogeny, or the science of the evolution of organisms, is also divided into two parts--the science of the individual (ontogeny) and of the stem or species (phylogeny); each follows its own peculiar methods and aims, but they are most intimately connected by the biogenetic law. Ontogeny deals with the development of the individual organism from the beginning of its existence to death; as embryology it observes the growth of the individual within the fœtal membranes; and as metamorphology (or the science of metamorphoses) it follows the subsequent changes in post-fœtal life (chapter xvi.). The task of phylogeny is to trace the evolution of the organic stem or species--that is to say, of the chief divisions in the animal and plant worlds, which we describe as classes, orders, etc.; in other words, it traces the genealogy of species. It relies on the facts of paleontology, and fills up the gaps in this by comparative anatomy and ontogeny.

The science of the vital phenomena, which we call physiology, is for the most part the physiology of work, or ergology; it investigates the functions of the living organism, and has to reduce them as closely as possible to physical and chemical laws. Vegetable ergology deals with what are called the vegetative functions, nutrition and reproduction; animal ergology studies the animal activities of movement and sensation. Psychology is directly connected with the latter. But the study of the relations of the organism to its environment, organic and inorganic, also belongs to physiology in the wider sense; we call this part of it perilogy, or the physiology of relations. To this belong chorology, or the science of distribution (also called biological geography, as it deals with geographical and topographical distribution), and œcology or bionomy (also recently called ethology), the science of the domestic side of organic life, of the life-needs of organisms and their relations to other organisms with which they live (biocenosis, symbiosis, parasitism).

THIRD TABLE

SYNOPSIS OF THE CHIEF BRANCHES OF BIOLOGY (1869)

BIOLOGY = THE SCIENCE OF LIFE

The four chief branches of systematic biology.

I. Protistology = the science of single cells--unicellular organisms. II. Botany = the science of plants--tissue plants (metaphyta). III. Zoology = the science of animals--tissue animals (metazoa). IV. Anthropology = the science of man--speaking primates.

┌───────────────────────────────────────────────────────────────────────┐ │ A. MORPHOLOGY = THE SCIENCE OF FORMS. │ │ Anatomy and biogeny of organisms. │ ├────────────────────────────────────┬──────────────────────────────────┤ │A I. ANATOMY. │A II. BIOGENY. │ │The science of structure. │The science of development. │ │1. TECTOLOGY. │3. PHYLOGENY. │ │The science of structure. │Stem history. │ │Cytology, science of cells. │Paleontology and genealogy. │ │Histology, science of tissues. │Transformism or theory of descent.│ │Organology, science of organs. │Natural classification. │ │Blastology, science of persons. │ ──── │ │Kormology, science of trunks. │ │ │ ──── │4. ONTOGENY. │ │2. PROMORPHOLOGY. │4_a_. Embryology. │ │The science of fundamental │(Development within the │ │forms. Knowledge of the geometrical │fœtal membranes.) │ │ideal forms (mathematically │4_b_. Metamorphology. │ │definable) in relation │(Modification of the organism │ │to the concrete real form of │after fœtal life.) │ │the individual. │ │ └────────────────────────────────────┴──────────────────────────────────┘ ┌──────────────────────────────────────────────────────────────────┐ │ B. PHYSIOLOGY = THE SCIENCE OF FUNCTIONS. │ │ Physics and chemistry of the organism. │ ├─────────────────────────────┬────────────────────────────────────┤ │B I. ERGOLOGY. │B II. PERILOGY. │ │ Physiology of work. │Physiology of relations. │ │5. Vegetal ergology. │7. Chorology. │ │Physiology of the vegetative │The science of distribution. │ │functions. │Biological geography and topography.│ │5_a_. Trophonomy. │The science of migrations. │ │The science of metabolism. │ │ │5_b_. Gonimatology. │ ──── │ │The science of reproduction. │ │ │ ──── │ │ │6. Animal ergology. │8. ŒCOLOGY. │ │6_a_. Phoronomy. │(or bionomy or ethology). │ │The science of movement. │The science of domestic life. │ │6_b_. Sensonomy. │Biological economy. │ │The science of sensation. │Relations of the organism to │ │6_c_. Psychology. │the environment, and to other │ │ │organisms with which it lives. │ └─────────────────────────────┴────────────────────────────────────┘

V

DEATH

Life and death--Individual death--Immortality of the unicellulars--Death of the protists and tissue-organisms--Causes of physiological death--Using up of the plasma--Regeneration--Biotonus--Perigenesis of the plastidules: memory of the biogens--Regeneration of protists and tissue-organisms--Senile debility--Disease--Necrobiosis--The lot of death--Providence--Chance and fate--Eternal life--Optimism and pessimism--Suicide and self-redemption--Redemption from evil--Medicine and philosophy--Maintenance of life--Spartan selection.

Nothing is constant but change! All existence is a perpetual flux of "being and becoming"! That is the broad lesson of the evolution of the world, taken as a whole or in its various parts. Substance alone is eternal and unchangeable, whether we call this all-embracing world-being Nature, or Cosmos, or God, or World-spirit. The law of substance teaches us that it reveals itself to us in an infinite variety of forms, but that its essential attributes, matter and energy, are constant. All individual forms of substance are doomed to destruction. That will be the fate of the sun and its encircling planets, and of the organisms that now people the earth--the fate of the bacterium and of man. Just as the existence of every organic individual had a beginning, it will also undeniably have an end. Life and death are irrevocably united. However, philosophers and biologists hold very different views as to the real causes of this destiny. Most of their opinions are at once out of court, because they have not a clear idea of the nature of life, and so can have no adequate idea of its termination--death.

The inquiry into the nature of organic life which we instituted in the second chapter has shown us that it is, in the ultimate analysis, a chemical process. The "miracle of life" is in essence nothing but the metabolism of the living matter, or of the plasm. Recent physiologists, especially Max Verworn and Max Kassowitz, have pointed out, in opposition to modern vitalism, that "life consists in a continuous alternation between the upbuild and the decay of the highly complicated chemical unities of the protoplasm. And if this conception is admitted, we may rightly say that we know what we mean by death. If death is the cessation of life, we must mean by that the cessation of the alternation between the upbuild and the dissolution of the molecules of protoplasm; and as each of the molecules of protoplasm must break up again shortly after its formation, we have in death to deal only with the definite cessation of reconstruction in the destroyed plasma-molecules. Hence a living thing is not finally dead--that is to say, absolutely incompetent to discharge any further vital function--until the whole of its plasma-molecules are destroyed." In the exhaustive justification with which Kassowitz follows up this definition in the fifteenth chapter of his _General Biology_, the natural causes of physiological death are fully described.

Among the numerous and contradictory views of recent biologists on the nature of death we find many errors and misunderstandings, due to a lack of clear distinction between the duration of the living matter in general and that of the individual life-form. This is particularly noticeable in the contradictory views which have been elicited by August Weismann's theory (1882) of the immortality of the unicellulars. I have shown in the eleventh chapter of the _Riddle_ that it is untenable. But as the distinguished zoologist has again taken up his theory with energy in his instructive _Lectures on the Theory of the Descent_ (1902), and has added to it erroneous observations on the nature of death, I am obliged to return to the point. Precisely because this interesting work gives most valuable support to the theory of evolution, and maintains Darwin's theory of selection and its consequences with great effect, I feel it is necessary to point out considerable weaknesses and dangerous errors in it. The chief of these is the important theory of the germ-plasm and the consequent opposition to the inheritance of acquired characteristics. Weismann deduces from this a radical distinction between the unicellular and the multicellular organisms. The latter alone are mortal, the former immortal; "between the unicellular and the multicellular lies the introduction of physiological--that is to say, normal--death." We must say, in opposition to this, that the physiological individuals (_bionta_) among the protista are just as limited in their duration as among the histona. But if the chief stress in the question is laid, not on the individuality of the living matter, but on the continuity of the metabolic life-movement through a series of generations, it is just as correct to affirm a partial immortality of the plasm for the multicellulars as for the unicellulars.

The immortality of the unicellulars, on which Weismann has laid so much stress, can only be sustained for a small part of the protists even in his own sense--namely, for those which simply propagate by cleavage, the chromacea and bacteria among the monera (chapter ix.), the diatomes and paulotomes among the protophyta, and a part of the infusoria and rhizopods among the protozoa. Strictly speaking, the individual life is destroyed when a cell splits into two daughter-cells. One might reply with Weismann that in this case the dividing unicellular organism lives on as a whole in its offspring, and that we have no corpse, no dead remains of the living matter, left behind. But that is not true of the majority of the protozoa. In the highly developed ciliata the chief nucleus is lost, and there must be from time to time a conjugation of two cells and a mutual fertilization of their secondary nuclei, before there can be any further multiplication by simple cleavage. However, in most of the sporozoa and rhizopoda, which generally propagate by spore formation, only one portion of the unicellular organism is used for this; the other portion dies, and forms a "corpse." In the large rhizopods (thalamophora and radiolaria) the spore-forming inner part, which lives on in the offspring, is smaller than the decaying outer portion, which becomes the corpse.

Weismann's view of the secondary "introduction of physiological death in the multicellulars" is just as untenable as his theory of the immortality of the unicellulars. According to this opinion, the death of the histona--both the metaphyta and metazoa--is a purposive outcome of adaptation, only introduced by selection when the multicellular organism has reached a certain stage of complexity of structure, which is incompatible with its original immortality. Natural selection would thus kill the immortal and preserve only the mortal; it would interfere with the multiplication of the immortals in the bloom of their years, and only use the mortal for rearing posterity. The curious conclusions which Weismann reached in developing this theory of death, and the striking contradictions to his own theory of the germ-plasm which he fell into, have been pointed out by Kassowitz in the forty-ninth chapter of his _General Biology_. In my opinion, this paradoxical theory of death has no more basis than the germ-plasm theory he has ingeniously connected with it. We may admire the subtlety and depth of the speculations with which Weismann has worked out his elaborate molecular theory. But the nearer we get to its foundations the less solid we find them. Moreover, not one of the many supporters of the theory of germ-plasm has been able to make profitable use of it in the twenty years since it was first published. On the other hand, it has had an evil influence in so far as it denied the inheriting of acquired characters, which I hold, with Lamarck and Darwin, to be one of the soundest and most indispensable supports of the theory of descent.

In discussing the question of the real causes of death, we confine our attention to normal or physiological death without considering the innumerable causes of accidental or pathological death, by illness, parasites, mishaps, etc. Normal death takes place in all organisms when the limit of the hereditary term of life is reached. This limit varies enormously in different classes of organisms. Many of the unicellular protophyta and protozoa live only a few hours, others several months or years; many one-year plants and lower animals live only a summer in our temperate climate, and only a few weeks or months in the arctic circle or on the snow-covered Alps. On the other hand, the larger vertebrates are not uncommonly a hundred years old, and many trees live for a thousand years. The normal span of life has been determined in all species in the course of their evolution by adaptation to special conditions, and has then been transmitted to offspring by heredity. In the latter, however, it is often subject to considerable modifications.

The organism has been compared, on the modern "machine theory" of life, to an artificially constructed mechanism, or an apparatus in which the human intelligence has put together various parts for the attainment of a certain end. This comparison is inapplicable to the lowest organisms, the monera, which are devoid of such a mechanical structure. In these primitive "organisms without organs" (chromacea and bacteria) the sole cause of life is the invisible chemical structure of the plasm and the metabolism effected by this. As soon as this ceases death takes place (_cf._ chapter ix.). In the case of all other organisms the comparison is useful in so far as the orderly co-operation of the various organs or parts accomplishes a certain task by the conversion of virtual into active force. But the great difference between the two is that in the case of the machine the regularity is due to the purposive and consciously acting will of man, whereas in the case of the organism it is produced by unconscious natural selection without any design. On the other hand, the two have another important feature in common in the limited span of life which is involved in their being used up. A locomotive, ship, telegraph, or piano, will last only a certain number of years. All their parts are worn out by long use, and, in spite of all repairing, become at last useless. So in the case of all organisms, the various parts are sooner or later worn out and rendered useless; this is equally true of the organella of the protist and the organs of the histon. It is true that the parts may be repaired or regenerated; but sooner or later they cease to be of service, and become the cause of death.

When we take the idea of regeneration, or the recuperation of parts that have been rendered useless, in the widest sense, we find it to be a universal vital function of the greatest importance. The whole metabolism of the living organism consists in the assimilation of plasm, or the replacing of the plasma-particles which are constantly used up by dissimilation (_cf._ chapter x.). Verworn has given the name of _biogens_ to the hypothetical molecules of living matter--which I regard with Hering as endowed with memory, and (1875) have called plastidules. He says: "The biogens are the real vehicles of life. In their constant decay and reconstruction consists the process of life, which expresses itself in the great variety of vital phenomena." The relation of assimilation (the building-up of the biogens) to dissimilation (the decay of the biogens) may be expressed by a fraction to which the name _biotonus_ is given A/D. It is of radical importance in the various phenomena of life. The variations in the size of this fraction are the cause of all change in the life-expression of every organism. When the biotone increases, and the metabolism quotient becomes more than one, we have growth; when, on the other hand, it falls below one, and the biotone decreases, we have atrophy, and finally death. New biogens are constructed in _regeneration_. In _generation_ or reproduction groups of biogens (as germ-plasm) are released from the parent in consequence of redundant growth, and form the foundation of new individuals.

The phenomena of regeneration are extremely varied, and have of late years been made the subject of a good deal of comprehensive experiment, especially on the side of what is called "mechanical embryology." Many of these experimental embryologists have drawn far-reaching conclusions from their somewhat narrow experiments, and have partly urged them as objections to Darwinism. They imagine that they have disproved the theory of selection. Most of these efforts betray a notable lack of general physiological and morphological knowledge. As they also generally ignore the biogenetic law, and take no account of the fundamental correlation of embryology and stem history, we can hardly wonder that they reach the most absurd and contradictory conclusions. Many examples of this will be found in the _Archiv für Entwickelungsmechanik_. When, however, we make a comprehensive survey of the interesting field of regeneration processes, we discover a continuous series of development from the simplest repair of plasm in the unicellular protists to the sexual generation of the higher histona. The sperm-cells and ova of the latter are redundant growth-products, which have the power of regenerating the whole multicellular organism. But many of the higher histona have also the capacity to produce new individuals by regeneration from detached pieces of tissue, or even single cells. In the peculiar mode of metabolism and growth which accompanies these processes of regeneration, the memory of the plastidule, or the unconscious retentive power of the biogens, plays the chief part (_cf._ my _Perigenesis of the Plastidule_, 1875). In the most primitive kinds of the unicellular protists we find the phenomena of death and regeneration in the simplest form. When an unnucleated moneron (a chromaceum or bacterium) divides into two equal halves, the existence of the dividing individual comes to an end. Each half regenerates itself in the simplest conceivable way by assimilation and growth, until it, in turn, reaches the size of the parent organism. In the nucleated cells of most of the protophyta and protozoa it is more complicated, as the nucleus becomes active as the central organ and regulator of the metabolism. If an infusorium is cut into two pieces, only one of which contains the nucleus, this one alone grows into a complete nucleated cell; the unnucleated portion dies, being unable to regenerate itself.

In the multicellular body of the tissue-forming organisms we must distinguish between the partial death of the various cells and the total death of the whole organism, or cell-state, which they make up. In many of the lower tissue-plants and tissue-animals the communal link is very loose and the centralization slight. Odd cells or groups of cells may be set loose, without any danger to the life of the whole histon, and grow into new individuals. In many of the algæ and liverworts (even in the _bryophyllum_, closely related to the stone-crop, or _sedum_)--as well as in the common fresh-water polyp, hydra, and other polyps--every bit that is cut off is capable of growing into a complete individual. But the higher the organization is developed and the closer the correlation of the parts and their co-operation in the life of the centralized stock or person, the slighter we find the regenerative faculty of the several organs. Even then, however, many used-up cells may be removed and replaced by regenerated new cells. In our own human organism, as in that of the higher animals, thousands of cells die every day, and are replaced by new cells of the same kind, as, for instance, epidermic cells at the surface of the skin, the cells of the salivary glands or the mucous lining of the stomach, the blood-cells, and so on. On the other hand, there are tissues that have little or nothing of this repairing power, such as many of the nerve-cells, sense-cells, muscle-cells, etc. In these cases a number of constant cell-individuals remain with their nucleus throughout life, although a used-up portion of their cell-body may be replaced by regeneration from the cytoplasm. Thus our human body, like that of all the higher animals and plants, is a "cell-state" in another sense. Every day, nay, every hour, thousands of its citizens, the tissue-cells, pass away, and are replaced by others that have arisen by cleavage of similar cells. Nevertheless, this uninterrupted change of our personality is never complete or general. There is always a solid groundwork of conservative cells, the descendants of which secure the further regeneration.

Most organisms meet their death through external or accidental causes--lack of sufficient food, isolation from their necessary environment, parasites and other enemies, accidents and disease. The few individuals who escape these accidental causes of death find the end of life in old age or senility, by the gradual decay of the organs and dwindling of their functions. The cause of this senility and the ensuing natural death is determined for each species of organisms by the specific nature of their plasm. As Kassowitz has lately pointed out, the senility of individuals consists in the inevitable increase in the decay of protoplasm and the metaplastic parts of the body which this produces. Each metaplasm in the body favors the inactive break-up of protoplasm, and so also the formation of new metaplasms. The death of the cells follows, because the chemical energy of the plasm gradually falls off from a certain height, the acme, of life. The plasm loses more and more the power to replace by regeneration the losses it sustains by the vital functions. As, in the mental life, the receptivity of the brain and the acuteness of the senses gradually decay, so the muscles lose their energy, the bones become fragile, the skin dry and withered, the elasticity and endurance of the movements decrease. All these normal processes of senile decay are caused by chemical changes in the plasm, in which dissimilation gains constantly on assimilation. In the end they inevitably lead to normal death.

While the gradual decay of the bodily forces and the senile degeneration of the organs must necessarily cause the death of the soundest organism in the end, the great majority of men pass away through illness long before this normal term of life is reached. The external causes of this are the attacks of enemies and parasites, accidents, and unfavorable conditions of life. These cause changes in the tissues and their component cells, which first occasion the partial death of particular sections, and then the total death of the whole individual. The modifications of the living matter which produce disease and premature death are called _necrobioses_. They consist partly of _histolyses_--that is to say, degeneration of the cells by atrophy, dissolution, withering (mortification), or colliquation; and partly of _metaplasmosisms_, or metamorphoses of the plasm--fatty, mucous, chalky, or amyloid metamorphoses of the cells. It was the great merit of Rudolph Virchow that he proved, in his epoch-making _Cellular Pathology_ (1858), that all diseases in man and other organisms may be reduced to such modifications of the cells which make up the tissues. Hence disease, with its pain, is a physiological process, a life under injurious and dangerous conditions. As in all normal vital phenomena, so in abnormal or pathological, the ultimate ground must be sought in the physical and chemical processes in the plasm. Pathology is a part of physiology. This discovery has cut the ground from under the older notion of disease as a special entity, a devil, or a divine punishment.

The natural physical explanation of death, which has been made possible by modern physiology and pathology, has shattered, not only all the old superstitious ideas about disease and death, but also a number of important metaphysical dogmas which built upon them. Such was, for instance, the naïve belief in a conscious Providence, controlling the fate of individuals and determining their death. I do not fail to appreciate the great subjective value which such a trust in a protecting Providence has for men amid their countless dangers. We may envy the childish temper for the confidence and hope which it derives from this belief. But as we do not seek to have our emotions gratified by poetic fictions, we are bound to point out that reason cannot detect the shadow of a proof of the existence and action of this conscious Providence, or "loving Father in heaven." We read daily in our journals of accidents and crimes of all kinds that cause the unexpected death of happy human beings. Every year we read with horror the statistics of the thousands of deaths from shipwreck and railway accidents, earthquakes and landslips, wars and epidemics. And then we are asked to believe in a loving Providence that has decreed the death of each of these poor mortals! We are asked to console ourselves in face of the tragedy with the hollow phrases: "God's will be done," or "God's ways are wonderful." Simple children and dull believers may soothe themselves with such phrases. They no longer impose on educated people in the twentieth century, who prefer a full and fearless knowledge of the truth.

When our monistic and rational conception of death is described as dreary and hopeless, we may answer that the prevalent dualistic view is merely an outcome of hereditary habits of thought and mystic training in early youth. When these are displaced by progressive culture and science, it will be clear that man has lost nothing, but gained much, as regards his life on earth. Convinced that there is no eternal life awaiting him, he will strive all the more to brighten his life on earth and rationally improve his condition in harmony with that of his fellows. If it is objected that then everything will depend on mere "chance," instead of being controlled by a conscious Providence or a moral order of the world, I must refer the reader for my reply to the close of the fourteenth chapter of the _Riddle_, where I have dealt with fate, providence, end, aim, and chance. And if it is further claimed that our realistic view of life leads to pessimism, there is no better ground for such an accusation.

I have given, in the eleventh chapter of the _Riddle_, the scientific reasons which forbid us to accept the personal immortality of the soul. But as the most vehement attacks have been made on this chapter by metaphysicians of the prevailing school and by Christian theologians, I must return to the question here. I am convinced, from numbers of letters I have received and conversation with educated people of all classes, that no other dogma is so firmly established and highly valued as athanatism, or the belief in personal immortality. Most men will not give up at any price the hope that a better life awaits them beyond the grave, which will compensate them for all the pain and suffering they endure here. In the picturing of this future life the mediæval geocentric idea still forms the chief feature. Troelslund has shown, in his _Idea of Heaven and of the World_, how this theory still dominates the metaphysics of the majority of men; in spite of Copernicus and Laplace, heaven is still for most people the semicircular blue glass bell that overarches the earth. We still hear the praises of our life in this heaven sung daily in sermons and speeches and festive orations. The orator extends his right hand "upward" to the infinite starry space of heaven, forgetting that the radius of the direction he is pointing towards changes every second, and in twelve hours reaches the precisely opposite direction, and becomes "downward." Other believers endeavor to be still more concrete, and point out definite celestial bodies as the homes of immortal souls. Modern cosmology, astronomy, and geology entirely exclude these pretty fictions from science; and modern psychology, physiology, ontogeny, and phylogeny rigorously refuse an inch of ground for athanatism.

Optimism regards the world on its good and bright and admirable side: pessimism looks to the shades and tragedies of life. In some philosophic and religious systems one or other of these tendencies is consistently and exclusively worked out; but in most systems the two are mingled. Pure and consistent realism is generally neither optimistic nor pessimistic. It takes the world as it is, a unified whole, the nature of which is neither good nor bad. Dualistic idealism, however, generally combines the two, and distributes them between its two worlds; it describes this world as a "vale of tears," and the next as a glorious city of joy and happiness. This view is a conspicuous feature in most of the dualistic religions, and has still a considerable influence, both practically and theoretically, on the minds of educated people.

The founder of systematic optimism was Gottfried Leibnitz, whose philosophy sought to achieve an ingenious harmony between divergent systems, but is really a form of dynamism, or a monism somewhat akin to the energism of Ostwald. Leibnitz gave a compendious statement of his system in his _Monadology_ (1714). He taught that the world consists of an infinite number of monads (which almost correspond to our psychic atoms), but this pluralism was converted into a monism by making God, as the central monad, bind all together in a substantial unity. In his _Theodicy_ (1710) he taught that God (the "all-wise, all-good, and almighty creator of the world") had with perfect consciousness created "the best of all possible worlds"; that his infinite goodness, wisdom, and power are seen everywhere in the pre-established harmony of things; but that the individual human being, and humanity taken as a whole, have only a limited capacity for development. The man who knows the real features of the world, who has honestly confronted the tragic struggle for life that rules throughout living nature, who has sympathy for the infinite sum of misery and want of every kind in the life of men, can scarcely understand how an acute and informed thinker like Leibnitz could entertain such optimism as this. It would be more intelligible in the case of a one-sided and nebulous metaphysician like Hegel, who held that "all that is real is rational and all that is rational is real."

Pessimism is the direct opposite of systematic optimism. While the one holds the universe to be the best, the other regards it as the worst, of all possible worlds. This pessimistic conception has found expression in the oldest and most popular religions of Asia, Brahmanism and Buddhism. Both these Hindoo religions were originally pessimistic, and at the same time atheistic and idealistic. Schopenhauer especially pointed out this, declaring that they were the most perfect of all religions, and importing their leading ideas into his own system. He considers it "a glaring absurdity to attempt to prove this miserable world the best of all possible ones--this cock-pit of tortured and suffering beings, who can only survive by destroying one another, in which the capacity for pain grows with knowledge, and so reaches its height in man. Truly optimism cuts so sorry a figure in this theatre of sin, suffering, and death that we should have to regard it as a piece of sarcasm if Hume had not given us an explanation of its origin (the wish to flatter God and hope for some result from it). To the palpable sophistry of Leibnitz, who would prove this world the best of all possible, we can oppose a strict and honest proof that it is the worst of all possible." However, neither Schopenhauer nor the most important of modern pessimists, Edward Hartmann, has drawn the strict practical conclusion from pessimism. That would be to deny the will to live, and put an end to suffering by suicide.

The mention of suicide as the logical consequence of pessimism may serve as an occasion to glance at the curious and contradictory views that are expressed about it. There are few problems of life (apart from immortality and the freedom of the will) on which such absurd and contradictory things have been said even down to our own time. The theist who regards life as a gift of God may hesitate to reject or return it--although the offering of one's self as a victim for other men is considered a high virtue. Most educated people still look upon suicide as a great sin, and in some countries (such as England) the attempt is punished by law. In the Middle Ages, when a hundred thousand men were burned alive for heresy or witchcraft, suicides were punished by a disgraceful burial. As Schopenhauer says: "Clearly there is nothing in the world to which a man has a plainer right than his own life and person. It is simply ridiculous for criminal justice to deal with suicide." The advance of embryology in the last thirty years has made it clear that the individual life of a man (and all other vertebrates) begins at the moment when the male sperm-cell and the maternal ovum coalesce. In this blind chance plays an important part, as in so many other important aspects of life--taking "chance" in the scientific sense, which I have explained in chapter xiv. of the _Riddle_. Hence, the real cause of personal existence is not the favor of the Almighty, but the sexual love of one's earthly parents; very often this consequence of the act of love has been anything but desired. If, then, the circumstances of life come to press too hard on the poor being who has thus developed, without any fault of his, from the fertilized ovum--if, instead of the hoped-for good, there come only care and need, sickness and misery of every kind--he has the unquestionable right to put an end to his sufferings by death. Every religion assents to this under certain conditions, even Christianity when it says: "If thine eye scandalize thee, cast it from thee." It is true that the conventional morality condemns suicide under any circumstances; but the reasons it alleges are ridiculously slight, and are not improved by having the mantle of religion wrapped about them.

The voluntary death by which a man puts an end to intolerable suffering is really an act of redemption. We should, therefore, describe it as self-redemption, and look on it with Christian sympathy, not brand it pharisaically as "self-murder." As a fact, this contemptuous phrase has no meaning, since murder is the taking away of a man's life against his will, while the suicide dies voluntarily. Hence, he usually deserves our sympathy, not contempt, and certainly not punishment. Our conventional morality is, as so often happens, full of senseless contradictions. Modern states have introduced conscription; they demand that every citizen shall give up his life for his country on command, and kill as many other men as he can (an admirable commentary on the Scriptural "Love your enemies") for some political reason or other. But they never secure to each citizen the means of honorable existence and free development of his personality--not even the right to work by which he may maintain himself and his family.

I fully recognize the advance that social politics has made in improving the conditions of the poorer classes, the promotion of hygiene and education and the bodily and mental welfare of citizens; but we are still very far from the attainable ideal of general prosperity and happiness which reason dictates to every civilized nation. Misery and want are increasing among the poor, as the division of labor and over-population increase. Thousands of strong and active men come to grief every year without any fault of theirs, often precisely because they were quiet and honest; thousands are hungry because, with the best will in the world, they cannot find work; thousands are sacrificed to the heartless demands of our iron age of machinery with its exacting technical and industrial requirements. On the other hand, we see thousands of contemptible characters prospering because they have been able to deceive their fellows by unscrupulous speculations, or because they have flattered and served the higher authorities. It is no wonder that the statistics of suicide increase so much in the more civilized communities. No feeling man who has any real "Christian love of his neighbor" will grudge his suffering brother the eternal rest and the freedom from pain which he has obtained by his self-redemption.

The seventh petition of the Lord's Prayer, which is repeated daily by millions of Christians, is: "Deliver us from evil." Luther explains this as a prayer to be saved "from all evil of body and soul" in this life and the next. When we consider this in the light of our monistic principles, we have naturally to set aside the superstitious ideas of the Middle Ages regarding the future life, and deal only with the petition as regards this life. The number and variety and gravity of these evils have grown in civilized communities in the nineteenth century, notwithstanding all the progress we have made in art and science and the rational reform of our personal and social life. Civilization has gained infinitely in value by the change we have made in our conceptions of time and space in this age of steam and electricity. We can make our domestic and public life much pleasanter, and avail ourselves of a far greater number of luxuries, than was possible to our grandfathers a hundred years ago. But all this has caused a much greater expenditure of nerve-energy. The brain has to bear a much greater strain, and is worn out earlier, the body is more stimulated and overworked than it was a hundred years ago. Many diseases of modern civilization are making appalling progress; neurasthenia, especially, and other diseases of the nerves, carry off more victims every year. Our asylums grow bigger and more numerous every year, and we have sanatoria on every side in which the baited victim of modern civilization seeks refuge from his evils. Some of these evils are quite incurable, and the sufferers have to meet a certain death in terrible pain. Many of these poor creatures look forward to their redemption from evil and the end of their miserable lives. The important question arises whether, as compassionate men, we should be justified in carrying out their wish and ending their sufferings by a painless death.

This question is of great importance, both in practical philosophy and in juridical and medical practice, and, as opinions differ very much on the subject, it seems advisable to deal with it here. I start from my own personal opinion, that sympathy is not only one of the noblest and finest functions of the human brain, but also one of the first conditions of the social life of the higher animals. The precepts of Christian charity which the gospels rightly place in the very foreground of morality, were not first discovered by Christ, but they were successfully urged by him and his followers at a time when refined selfishness threatened the Roman civilization with decay. These natural principles of sympathy and altruism had arisen thousands of years before in human society, and are even found among all the higher animals that live a social life. They have their first roots in the sexual reproduction of the lower animals, the sexual love and the care of the young on which the maintenance of the species depends. Hence the modern prophets of pure egoism, Friedrich Nietzsche, Max Stirner, etc., commit a biological error when they would substitute their morality of the strong for universal charity, and when they ridicule sympathy as a weakness of character or an ethical blunder of Christianity. It is just in its insistence on sympathy that the Christian teaching is most valuable, and this part of its system will survive long after its dogmas have sunk into oblivion. However, this lofty duty must not be confined to men, but extended to "our relations," the higher vertebrates, and, in fact, to all animals whose brain-organization seems to point to the possession of sensation and a consciousness of pleasure and pain. Thus, for instance, in the case of the domestic animals which we use daily in our service, and which have an undoubted psychic affinity to ourselves, we must take care to increase their pleasures and mitigate their sufferings. Faithful dogs and noble horses, with which we have lived for years and which we love, are rightly put to death and relieved from pain when they fall hopelessly ill in old age. In the same way we have the right, if not the duty, to put an end to the sufferings of our fellow-men. Some severe and incurable disease makes life unbearable for them, and they ask for redemption from evil. However, medical men hold very different opinions on the matter, as I have found in conversation with them. Many experienced physicians, who practise their profession in a spirit of sympathy and without dogmatic prejudice, have no scruple about cutting short the sufferings of the incurable by a dose of morphia or cyanide of potassium when they desire it; very often this painless end is a blessing both to the invalids and their families. However, other physicians and most jurists are of opinion that this act of sympathy is not right, or is even a crime; that it is the duty of the physician to maintain the life of his patients as long as he can in all circumstances. I should like to know why.

While I am dealing with this important and--for the medical conscience--difficult question of social ethics, I may take the opportunity to consider the general attitude of physicians to the monistic philosophy. It is now half a century since I visited the wards in the Julius hospital at Würtzburg as a medical student. It is true that--happily for me and my patients!--I practised the profession only for a short time after I had passed my examinations in 1857; but the thorough acquaintance with the human organism, its anatomic structure and physiological functions, which I then obtained has been of incalculable service to me. I owe to it not only the solid empirical foundation of the special study of my life, zoology, but also the monistic tendency of my whole system. As the medical training in its widest sense includes anthropology--and so should include psychology also--its value for speculative philosophy cannot be exaggerated. The scholastic metaphysicians who still regard the chairs of philosophy at our universities as their monopoly would have avoided most of their dualistic errors if they had had a thorough training in human anatomy, physiology, ontogeny, and phylogeny. Even pathology, the science of the diseased organism, is very instructive for the philosopher. The psychologist especially acquires, by the study of mental disease and the visiting of the asylum wards, a profound insight into the mental life which no speculative philosophy could give him. There are few experienced and thoughtful physicians who retain the conventional belief in the immortality of the soul and God. What would the immortal soul do on the other side of eternity when it is already utterly ruined in this life, or was even born as an idiot? How can a just God condemn the criminal to the fires of hell when he himself has tainted the man with an hereditary bias, or has placed him in an environment in which, seeing the absence of free-will, crime was a necessity for him? And how can this all-loving God answer for the immeasurable sum of want and misery, and pain and unhappiness, which he sees accumulated before him every year in the lives of families and states, cities and hospitals? It is no wonder that the old saying ran: _Ubi tres medici, duo sunt athei_ (Of three doctors two are sure to be atheists). One of my medical colleagues was an old, experienced, and sympathetic physician who had travelled all over the world, and had then, as director of a large hospital, been a close witness of the sufferings of humanity. Religiously educated by pious parents, and endowed with keen sensitiveness, he was, after long struggles, forced by his medical studies to part with the faith of his boyhood--like myself, in his twenty-first year. We were talking about the great mysteries of life shortly before his death, and he said to me: "I have been unable to reconcile belief in the immortality of the soul and the freedom of the will with my psychological experiences, and I have been just as unable to discover throughout the whole world a single trace of a moral order or a beneficent providence. If it is true that an intelligent Deity rules the world, he cannot be a God of love, but an all-powerful demon, whose constant entertainment is an eternal and merciless play of being and becoming, building up and destroying." However, we do still find here and there informed and intelligent physicians who adhere to the three central dogmas of metaphysics--a proof of the immense power of dogmatic tradition and religious prejudice.

We must class as a traditional dogma the wide-spread belief that man is bound under all circumstances to maintain and prolong life, even when it has become utterly useless--a source of pain to the incurable and of endless trouble to his friends. Hundreds of thousands of incurables--lunatics, lepers, people with cancer, etc.--are artificially kept alive in our modern communities, and their sufferings are carefully prolonged, without the slightest profit to themselves or the general body. We have a strong proof of this in the statistics of lunacy and the growth of asylums and nerve-sanatoria. In Prussia alone there were 51,048 lunatics cared for in the asylums (six thousand in Berlin) in 1890; more than one-tenth of them were quite incurable (four thousand of them suffering from paralysis). In France, in 1871, there were 49,589 in the asylums (or 13.8 per thousand of the population), and in 1888 there were 70,443 (or 18.2 per thousand); thus, in the course of seventeen years, the absolute number of the unsound rose nearly 30 per cent. (29.6), while the total population only increased 5.6 per cent. In our day the number of lunatics in civilized countries is, on the average, five-sixths per thousand. If the total population of Europe is put at three hundred and ninety to four hundred millions, we have at least two million lunatics among them, and of these more than two hundred thousand are incurable. What an enormous mass of suffering these figures indicate for the invalids themselves, and what a vast amount of trouble and sorrow for their families, what a huge private and public expenditure! How much of this pain and expense could be spared if people could make up their minds to free the incurable from their indescribable torments by a dose of morphia! Naturally this act of kindness should not be left to the discretion of an individual physician, but be determined by a commission of competent and conscientious medical men. So, in the case of other incurables and great sufferers (from cancer, for instance), the "redemption from evil" should only be accomplished by a dose of some painless and rapid poison when they have expressed a deliberate wish (to be afterwards juridically proved) for this, and under the control of an authoritative commission.

The ancient Spartans owed a good deal of their famous bravery, their bodily strength and beauty, as well as their mental energy and capacity, to the old custom of doing away with new-born children who were born weakly or crippled. We find the same custom to-day among many savage races. When I pointed out the advantages of this Spartan selection for the improvement of the race in 1868 (chapter vii. of the _History of Creation_) there was a storm of pious indignation in the religious journals, as always happens when pure reason ventures to oppose the current prejudices and traditional beliefs. But I ask: What good does it do to humanity to maintain artificially and rear the thousands of cripples, deaf-mutes, idiots, etc., who are born every year with an hereditary burden of incurable disease? Is it not better and more rational to cut off from the first this unavoidable misery which their poor lives will bring to themselves and their families? It is no use to reply that religion forbids it. Christianity also bids us give up our life for our brethren, and to cast it from us when it hurts us--that is to say, when it only causes useless pain to us and our friends. The truth is, the opposition is only due to sentiment and the power of conventional morality--that is to say, to the hereditary bias which is clothed in early youth with the mantle of religion, however irrational and superstitious be its foundation. Pious morality of this sort is often really the deepest immorality. "Laws and rights creep on like an eternal sickness;" this is equally true of the social customs and morals on which laws and rights are founded. Sentiment should never be allowed to usurp the place of reason in these weighty ethical questions. As I pointed out in the first chapter of the _Riddle_, sentiment is a very amiable, but a very dangerous, function of the brain. It has no more to do with the attainment of the truth than what is called revelation. That is well seen in Kant's dualism, for his _mundus intelligibilis_ is essentially an outcome of his religious sentimentality

VI

PLASM

Plasm is the universal living substance--Definition of protoplasm, chemically and morphologically--Physical character--Viscous condition--Chemical analysis--Colloid character of albumin--Albuminoid molecules--Elementary structure of plasm--Work of plasm--Protoplasm and metaplasm--Structures of metaplasm--Frothy structure--Skeletal structure--Fibrous structure--Granular structure--Molecular structure--Plasma molecules--Plastidules and biogens--Micella and biophora--Caryoplasm and cytoplasm--Nuclear matter--Chromatin and achromin--Nucleolus and centrosoma--Caryotheka and caryolymph--Cellular matter--Plasma products--Internal plasma products--External plasma products--Cell membranes--Intercellular matter--Cuticular matter.

By plasm, in the widest sense of the word, we mean the living matter, or all bodies that are found to constitute the material foundations of the phenomena of life. It is usual to give this matter the name of protoplasm; but this older and historically important designation has suffered so many changes of meaning through the variety of its applications that it is better now to use it only in the narrower sense. Moreover, recent research on protoplasm has been greatly developed, and several new names have been invented, which are formed from the word "plasm" with a qualifying prefix. These are special varieties of the general idea of plasm, or special modifications of the general matter, such as metaplasm, archiplasm, and so on.

The botanist, Hugo Mohl, who first introduced the name "protoplasm" in 1846, used it to designate a part of the contents of the ordinary plant-cell--namely, the viscous matter that Schleiden called "cell-mucus," which is found on the inner surface of the cell-wall, and often forms a varying net-work or skeleton in the watery fluid in the cell, and exhibits characteristic movements. Mohl gave the name of "primordial skin" to this important wall-layer (the chief element of the plant-cell), and called the material of it, as being chemically different from the other parts of the cell, _protoplasm_--that is to say, the first (_proton_) or earliest formation of the organism. It is important to notice that Mohl, the author of the name, conceived it in a purely chemical, not a morphological, sense, like Oscar Hertwig and other recent cytologists. I intend to retain this early chemical idea of protoplasm--or, briefly, plasm. It was also taken in this sense by Max Schultze, who pointed out (in 1860) its extreme significance and wide distribution in all living cells, and introduced an important reform of the cell-theory which we will see later.

The mixing of the chemical and the morphological ideas of protoplasm has been very mischievous in recent biology, and has led to great confusion. It generally comes from a failure to formulate clearly the difference between the two essential elements of the modern notion of the cell--the anatomic distinction between the nucleus and the body of the cell. The internal nucleus (or _caryon_) had the appearance of a solid, definite, morphologically distinct constituent of the cell; the outer and softer mass which we now call the cell-body (_celleus_ or _cytosoma_) seemed to be a formless and only chemically definable protoplasm. It was only discovered at a later date that the chemical composition of the nucleus is closely akin to that of the cell-body, and that we may properly associate the _caryoplasm_ of the one with the _cytoplasm_ of the other under the general heading of _plasm_. All the other materials that we find in the living organism are products or derivatives of the active plasm.

In view of the extraordinary significance which we must assign to the plasm--as the universal vehicle of all the vital phenomena (or "the physical basis of life," as Huxley said)--it is very important to understand clearly all its properties, especially the chemical ones. This is rendered somewhat difficult from the circumstance that the plasm is, in most of the organic cells, closely bound up with other substances--the various plasma products; it can rarely be isolated in its purity, and can never be had pure in any quantity. Hence we are for the most part dependent on the imperfect, and often ambiguous, results of microscopic and microchemical research.

In every case where we have with great difficulty succeeded in examining the plasm as far as possible and separating it from the plasma-products, it has the appearance of a colorless, viscous substance, the chief physical property of which is its peculiar thickness and consistency. The physicist distinguishes three conditions of inorganic matter--solid, fluid, and gaseous. Active living protoplasm cannot strictly be described as either fluid or solid in the physical sense. It presents an intermediate stage between the two which is best described as viscous; it is best compared to a cold jelly or solution of glue. Just as we find the latter substance in all stages between the solid and the fluid, so we find in the case of protoplasm. The cause of this softness is the quantity of water contained in the living matter, which generally amounts to a half of its volume and weight. The water is distributed between the plasma molecules, or the ultimate particles of living matter, in much the same way as it is in the crystals of salts, but with the important difference that it is very variable in quantity in the plasm. On this depends the capacity for absorption or imbibition in the plasm, and the mobility of its molecules, which is very important for the performance of the vital actions. However, this capacity of absorption has definite limits in each variety of plasm; living plasm is not soluble in water, but absolutely resists the penetration of any water beyond this limit.

The chemistry of living matter is the most important and interesting, but at the same time the most difficult and obscure, part of the whole of biological chemistry. In spite of the innumerable and careful investigations which have been made of it by the ablest physiologists and chemists in the second half of the nineteenth century, we are still far from a satisfactory solution of this fundamental problem of biology. This is due partly to the extraordinary difficulty of isolating pure living plasm and subjecting it to chemical analysis, and partly to the many errors and misunderstandings that have arisen through one-sided treatment of the subject, and especially through confusion of the chemical and morphological features of plasm. We can thus understand the contradictory views that are still put forward by distinguished chemists and physiologists, zoologists and botanists. As I cannot deal here with the very extensive, elaborate, and contradictory literature of the subject, I must be content to give a brief summary of the conclusions I have reached by my reading and my own studies of plasm (begun in 1859).

To begin with, we must clearly understand that protoplasm--in the most general sense in which we here take it--is a _chemical_ substance, not a "mixture of different substances," or a "mixture of a small quantity of solid matter with a good deal of fluid." As Richard Neumeister very well observes: "We seek the nature of protoplasm in the peculiar processes which take place in its constituent matter. Protoplasm is for us a chemical matter, so pronounced, in fact, that the highest chemical actions that we know of are embodied in it." I must, from my point of view, entirely reject Oscar Hertwig's conception of living matter as a "mixture" of a number of chemical elements; because chemistry applies this phrase to various gases and powdery substances which are completely indifferent to each other--a property which we certainly do not find in the constituents of protoplasm. When we speak of the living matter or protoplasm, the general phrase does not imply that the substance may not have a distinctive composition in each particular case. And when we find many biologists still conceiving protoplasm as a mixture of various substances, the error is generally due to a confusion of the chemical idea with the morphological, and to a belief that certain structural features of the plasm are primary, whereas they are only secondary, products of the vital process itself in the cell-body.

The older biologists who first introduced the name protoplasm and studied it carefully recognized that this living matter belonged to the albuminous (or proteid) group. The many characteristics which distinguish these nitrogenous carbon-compounds from all other chemical compounds--their behavior towards acids and bases, their peculiar color-reaction towards certain salts, their decomposition-products, etc.--are found in all the plasma-substances, and in all the other albuminoids. This is quite in agreement with the results of quantitative analysis. However differently the various plasma-substances behave in detail, they always exhibit the same general composition as the other albuminoids out of the five "organogenetic elements"--namely, in point of weight, fifty-one to fifty-four per cent. carbon, twenty-one to twenty-three per cent. oxygen, fifteen to seventeen per cent. nitrogen, six to seven per cent. hydrogen, and one to two per cent. sulphur. However, there is a good deal of variety and complication in the way in which the atoms of these five elements are combined in albumin and their molecules are grouped. Hence the question of the chemical nature of the plasma-substances compels us now to look for a moment at the larger group of albuminoids to which they belong.

The carbon-compounds which we comprise under the chemical title of the albumins or proteids are the most remarkable, but also, unfortunately, the least known, of all bodies. The attempt to examine them closely encounters extraordinary difficulties, greater than in any other group of chemical compounds. Everybody is familiar with the appearance of ordinary albumin, from the transparent viscous albumin that surrounds the yolk in the hen's egg, and which becomes a white, opaque, and solid mass when it is cooked. However, this special form of albumin, which we can get so easily in any quantity from the eggs of birds and reptiles, is only one of the innumerable kinds of albumin, or species of protein, that are to be found in the bodies of the various animals and plants. Chemists have hitherto tried in vain to master the chemical structure of these obscure protein-compounds. They are only rarely to be found in chemically pure form as crystals. As a rule, they are in the colloid form, or uncrystallized jelly-like masses, which offer a much greater resistance than crystals to the passage through a porous medium by diosmosis (see p. 39). However, although we have not yet succeeded in penetrating the molecular constitution of the albumins, the laborious research of chemists has yielded some general results which are of great importance for our purpose. We have, in the first place, a general idea of their molecular constitution.

Molecules are the smallest homogeneous parts into which a body can be divided without altering its chemical character. Hence the molecules of every chemical compound are made up of two or more atoms of different kinds. The greater the number of atoms in each compound, the higher is its molecular weight. The space between the molecules and their component atoms is filled with imponderable and highly elastic ether. As even the largest molecules occupy only a very tiny space, and remain far below the range of the most powerful microscope, all our ideas of their composition depend on general physical theories and special chemical hypotheses. Nevertheless, stereochemistry, the modern science of the molecular structure of chemical compounds, is not only a perfectly legitimate section of natural philosophy, but it yields the most important conclusions as to the mutual attractions of the elements and the invisible movements of the atoms in combining. It further enables us to calculate approximately the relative size of the molecules and the number of atoms that are grouped together in them. However, the albuminoids present the greatest difficulty of all in this calculation, and their structural features are still very obscure. Nevertheless, science has reached certain general conclusions, which we may formulate in the following propositions:

1. The molecule of albumin is unusually large, and therefore its molecular weight is very high (higher than in most or all other compounds).

2. The number of atoms composing it is very large (probably much more than a thousand).

3. The disposition of the atoms and groups of atoms in the albuminous molecule is very complicated, and at the same time very unstable--that is to say, very changeable and easily altered.

These characters, which are ascribed to all albuminous bodies by modern chemistry, hold good of all plasma-substances; and, in fact, are true in a higher degree of these, as the metabolism of the living matter causes a constant displacement of the atoms. This is caused, according to the view of Franz Hofmeister and others, by the formation of ferments or enzyma--in other words, by catalysators of a colloidal structure. Verworn has, on physiological grounds, given the name of biogens to these plasma-molecules.

The profound insight which comparative anatomy has given us into the significance and nature of organs, and comparative histology into those of the cells, has naturally excited a desire to penetrate in the same way the mystery of the elementary structure of the plasm, the chief active constituent of the cell. The improved methods of modern cytology, and the great progress which this science of the cell owes to the microtome and to microchemistry with its delicate coloring processes, etc., have prompted many observers of the last three decades to study the finest structural features of the elementary organism, and on this foundation build hypotheses as to the elementary structure of protoplasm. In my opinion, all these theoretical ideas, in so far as they would explain the finer structure of pure plasm, have a very serious defect; they relate to microscopic structures which do not belong to the plasm as such (as a chemical body), but to the cell-body (or cytosoma), the chief active constituent of which is certainly the plasm. These microscopic structures are not the efficient causes of the life-process, but products of it. They are phylogenetic outcomes of the manifold differentiations which the originally homogeneous and structureless plasm has undergone in the course of many millions of years. Hence I regard all these "plasma-structures" (the comb, threads, granules, etc.), not as original and primary, but as acquired and secondary. In so far as these structures affect the plasm as such, it must take the name of metaplasm, or a differentiated plasm, modified by the life-process itself. The true protoplasm, or viscous and at first chemically homogeneous substance, cannot, in my opinion, have any anatomic structure. We shall see, when we come to consider the monera, that very simple specimens of such organisms without organs still actually exist.

By far the greater part of the plasm that comes under investigation as active living matter in organisms is metaplasm, or secondary plasm, the originally homogeneous substance of which has acquired definite structures by phyletic differentiations in the course of millions of years. To this modified plasm we must oppose the original simple primary plasm, from the modification of which it has arisen. The name "protoplasm," in the narrower sense, could very properly be retained for this originally homogeneous form of structureless plasm; but, as the term has now almost lost definite meaning and is used in many different senses, it is, perhaps, better to call this pure homogeneous primary plasm _archiplasm_. It is still found--firstly, in the body of many (but not all) of the monera, part of the chromacea and bacteria, and the protamœba and protogenes; and, secondly, in the body of many very young protists and tissue-cells. In the latter case, however, there is already a chemical differentiation of the inner caryoplasm and outer cytoplasm. When we examine these young cells under a high power of the microscope, with the aid of the modern coloring methods, their protoplasm seems to be perfectly homogeneous and structureless, or, at the most, there are merely very fine granules regularly distributed in it which are believed to be products of metabolism. This is best seen in many of the rhizopods, especially the amœbæ, thalamophora, and mycetozoa. There are large amœbæ, which thrust out strongly mobile feet from their unicellular body, broad, flaplike processes of the naked cell body which constantly change their form, size, and place. If they are killed and examined with the aid of the best methods of coloring, it is quite impossible to detect any structure in them; and this is also true of the pseudopodia of the mycetozoa and many other rhizopods. Moreover, the slow flowing movement of the fluid protoplasm shows clearly that there cannot be any composition out of fine fixed elements in the body. This is particularly clear in those amœbæ and mycetozoa in which a hyaline, firm, and non-granulated skin-layer (hyaloplasm) is more or less separated from a dark, softer, and granulated marrow-layer (polioplasm); as both of them are viscous and pass into each other without sharp limits, there cannot be any constant and fixed structural features in them.

Organic life--in its lowest and simplest form--is nothing but a form of metabolism, and therefore a purely chemical process. The whole vital activity of the chromacea, the simplest and oldest organisms that we know, is confined to that process of metabolism which we call plasmodomism or carbon-assimilation. The homogeneous and structureless globules of protoplasm, which represent the whole frame of these primitive protophyta (chroococcus, aphanocapsa, etc.) in the simplest conceivable way, expend their whole vital power in the process of self-maintenance. They maintain their individuality by a simple metabolism; they grow by the addition of fresh plasm obtained by it, and they split up into two equal globules of plasm when the growth passes a certain limit--reproduction by clevage, maintenance of the species. Thus these chromacea have neither special organs, or organella, that we can distinguish in their simple plasma-bodies, nor different functions in their life-process; it is wholly taken up with the primitive work of their vegetal metabolism. We shall see later on that this is a purely chemical process, something like catalysis in inorganic combinations; and for this neither special organs nor fine elementary structures in the plasm are needed. The "end" of their existence, self-maintenance, is attained just as simply as in the catalysis of any inorganic compound, or the formation of a crystal in its mother-water.

If we compare this very rudimentary life-process of the monera with that of the highly differentiated protists (diatomes, desmidiacea, radiolaria, and infusoria), the biological distance between them seems to be immense; and it is, naturally, far greater when we extend the comparison to the histona, the highly organized metaphyta and metazoa, in the bodies of which millions of cells co-operate in the work of the various tissues and organs.

In the great majority of cells--either the autonomous cells of the protists or the tissue-cells of the histona--we can detect more or less definite and constant fine structures in the plasm. We must regard these always as phyletic, secondary products of the life-process, and so call the differentiated plasm by the name of metaplasm. The very different interpretations of the microscopic pictures which this metaplasm affords have led to a good deal of controversy. In this the desire to discover in these secondary plasma-structures the first causes of vital action, or the real elementary organella of the cell, has played a great part. The most important of the theories that have been formulated are those of the frothy structure, the skeletal structure, the fibrous structure, and the granulated structure of the plasm. All these theories of structure apply to plasm in general, but particularly to its two chief forms, the caryoplasm of the nucleus and the cytoplasm of the cell-body.

Among the many different attempts to discover a definite structure in living matter, the theory of the frothy structure (also called the honeycomb structure) has lately found the most favor. Otto Bütschli, of Heidelberg, especially, has endeavored, on the basis of many years of careful study and experiment, to make it the foundation of his view of the plasm. It is undeniable that the living matter of many cells shows a delicate structure which may best be compared with fine soap-suds; innumerable globules are crowded close together in a fluid, and flatten each other by their pressure into polyhedrical shapes. In 1892 Bütschli artificially produced fine oil-suds by beating up cane sugar or potash in olive oil, and then put a small drop of the stuff in a drop of water under the microscope. The small particles of sugar then exercised an attractive action by diffusion on the particles of water; the latter penetrated into the oily matter, released the sugar, and formed tiny vesicles with it. As the vesicles of sugar do not mix with oil, they look like cavities isolated on all sides, and polyhedrically flattened by mutual pressure. The striking resemblance of this artificially produced "oil soap-suds" to the natural and microscopically visible structures of many kinds of plasm is strengthened from the fact that Bütschli, Georg Quincke, and others, have also observed similar flowing movements in both; and as these apparently spontaneous movements can be explained physically and reduced to adhesion, imbibition, and other mechanical causes, there seemed a prospect of reducing the "vital" movements of the living and flowing plasm to purely physical forces. Quite recently Ludwig Rhumbler, of Göttingen, an authority on the rhizopods, has endeavored to give in this sense a _Physical analysis of the vital phenomena in the cell_. To-day the froth theory is much the most popular of the many attempts to detect a fine plasm-structure as the essential anatomic foundation of an explanation of the physiological functions. It must be noted, however, that frequently very different phenomena are confused under this name, especially the coarser froth-formation by taking up water in the living matter and the invisible hypothetical molecular structure. Both these must be distinguished from the finer plasma-structure which is visible under a powerful microscope; but the limit between them is difficult to determine.

A second view of the finer structure of the plasm, which had been greatly esteemed before the acceptance of the froth theory, was formulated in 1875 by Carl Frommann and Carl Heitzmann, and supported by Leydig, Schwitz, and others. It puts another interpretation on the net-like appearance of the microscopic plasma-structure. It assumes that the plasma consists of a skeleton of fine threads or fibrils combined in the form of a net, and that these spread and cross in the body of the cell which is filled with fluid. It is also compared to a sponge, and is said to have a spongy structure. We can artificially produce such a skeletal structure by, for instance, causing coagulation in a thick solution of glue or albumin by adding alcohol or chromic acid. It is unquestionable that there are these "plasma-skeletons" both in the nucleus and the body of the cell; but they are generally (if not always) secondary products of organization in the elementary organism (or cell-organs), not primitive structures of its plasm. Moreover, an optical transverse action of a froth-structure or honeycomb, examined as a flat surface in the microscope, shows the same configuration as a fine skeleton. We can hardly see any difference between the two. We cannot accept the skeletal formation as a fundamental structure of the plasm.

As we notice very fine threads in the plasm of many cells, both in the caryoplasm of the nucleus and the cytoplasm of the cell body, the cytologist Flemming, of Kiel (1882), believed it was possible to discover them in the plasm of all cells, and based on this his filar theory of plasm. He says that we must distinguish two chemically different kinds of plasm in living matter--the filar (threadlike) and the inter-filar matter. The fine threads of the former are of different lengths, and sometimes run separately, at other times are bound in a sort of net-work (_mitoma_ and _paramitoma_). In certain conditions of cell-life, especially in indirect cell-division, these filar formations play a great part; and also in the functions of highly differentiated cells, such as the ganglionic cells. But in many cases these plasma threads may be merely parts of a skeletal or frothy structure (honeycomb walls in section). In any case, we cannot regard the thread formation as a general elementary structure of plasm; in my opinion, it is always a secondary phyletic product of living matter, and never a primary feature of it.

Totally different from the three preceding theories of the finer structure of the plasm is the granular theory of Altmann (1890). He supposes that all living matter is originally made up of tiny round granules, and that these independently living _bioblasts_ are the real "elementary organisms," the microscopic ultimate individuals; hence the cells which are formed by the combination of these granules must be looked on as individuals of the second order. Between the granules of the granulated substance (the real active living matter) there is always an inter-granular substance; the granules are regularly distributed and arranged in these. The granules themselves, or the bioblasts, are homogeneous, sometimes globular, and sometimes oval, or of other shapes. However, the distinction between these substances is quite arbitrary, and neither chemically nor morphologically well defined. Under the head of granules Altmann throws together the most different contents of the cell--fat granules, pigment granules, secretory granules, and other products of metabolism. Hence his granular theory is now generally rejected. However, there was a sound idea at the bottom of it--namely, the idea of explaining the vital properties and functions of living matter by small separate constituents which make up the plasm, and move in a viscous medium. But these real elementary parts are not microscopically visible; they belong to the molecular world, which lies far below the limit of microscopic power. In my opinion, Altmann's visible granules, like Flemming's threads and Frommann's skeleton and Bütschli's honeycomb, are not primary structures, but secondary products of plasma differentiation.

As the special properties and activities of any natural body depend on its chemical constitution, and this is, in the long-run, determined by the composition of its molecules, it is a matter of the greatest interest in biology to form as clear and distinct an idea as possible of the nature and properties of the molecules of plasm. Unfortunately, it is only possible to do this approximately, and to a slight extent. As the hypotheses of modern structural chemistry on the molecular formation of complicated organic compounds are often very unsafe, this is bound to be the case in the highest degree as regards the albuminoids and, the most important of all, the living matter or plasm. We have as yet no knowledge of the fundamental features of its very variable chemical structure. The one thing that bio-chemists have told us about it is that the molecule of plasm is very large, and made up of a great number of atoms (over a thousand); and that these are combined in smaller or larger groups, and are in a state of very unstable equilibrium, so that the life process itself causes constant changes in them.

Since the great problem of heredity was forced by Darwin in 1859 into the foreground of general biology, many different hypotheses and theories of it have been framed. All these have in the end to trace it to molecular features in the plasm of the germ-cells; because it is this germ-plasm of the maternal ovum and the paternal sperm-cell that conveys the characteristics of the parents to the child. Hence the great progress that has been made recently in the study of conception and heredity, by means of a number of remarkable observations and experiments, has been of service to our ideas on the molecular structure of the plasm. I have dealt with the chief of these theories in the ninth chapter of my _History of Creation_, and must refer the reader thereto. In chronological order we have: (1) the pangenesis theory of Darwin (1868), (2) the perigenesis theory of Haeckel (1875), (3) the idioplasm theory of Nägeli (1884), (4) the germ-plasm theory of Weismann (1885), and (5) the mutation-theory of De Bries (1889). None of these attempts, and none of the later theories of heredity, has given us a satisfactory and generally admitted idea of the plasma-structure. We are not even clear as to whether in the last resort life is to be traced to the several molecules, or to groups of molecules, in the plasm. With an eye to this latter difference, we may distinguish the plastidule and micellar theories as two different groups of relevant hypotheses.

In my essay on "The Perigenesis of the Plastidules" (1875) I formulated the hypothesis that in the last instance the plastidules are the vehicles of heredity--that is to say, plasma-molecules which have the property of _memory_. In this I found support in the ingenious theory of the distinguished physiologist, Ewald Hering, who had declared in 1870 that "memory is a general property of organic matter." I do not see still how heredity can be explained without this assumption! The very word "reproduction," which is common to both processes, expresses the common character of psychic memory (as a function of the brain). By plastidules I understand simple molecules; the homogeneous nature of the plasm in the monera (both chromacea and bacteria and rhizomonera) and the primitive simplicity of their life-functions do not dispose us to think that special groups of molecules are to be distinguished in these cases. Max Verworn has recently (1903) formulated his biogen-hypothesis in the same sense, as a "critical-experimental study of the processes in the living matter." He also takes the active plasma-molecules, which he calls biogens, as the ultimate individual factors of the life-process, and is convinced that in the simplest cases the plasm consists of homogeneous biogen-molecules.

The hypothesis of Nägeli (1884) and Weismann (1885) is totally different from the hypothesis of the plastidules and biogens as simple molecules of the plasm. According to this, the ultimate "vital unities" or individual vehicles of the life-process are not homogeneous plasma-molecules, but groups of molecules, made up of a number of different molecules. Nägeli calls them _micella_, and assigns them a crystalline structure. He supposes that these micella are combined chainwise into micellar ropes, and that the variety of the many forms and functions of plasm is due to the different configuration and arrangement of these. Weismann says: "Life can only arise by a definite combination of different kinds of molecules, and all living matter must be made up of these groups of molecules. A single molecule cannot live, can neither assimilate nor grow nor reproduce." I do not see the justice of this observation. All the chemical and physiological properties which Weismann afterwards attributes to his hypothetical _biophora_ may be ascribed to a single molecule just as well as to a group of molecules. In the simplest forms of the monera (both the chromacea and the bacteria) the nature of their rudimentary life can be explained on the one supposition just as well as the other. Naturally, this does not exclude a very complicated chemical structure in the large plastidule or biogen as a single molecule. Verworn's biogen-hypothesis seems to me quite satisfactory when it represents the primitive molecule of living matter as really the ultimate factor of life.

The chief process in the evolutionary history of the plasm is its separation into the inner nuclear matter (caryoplasm) and the outer cellular matter (cytoplasm). When both kinds of plasm arose by differentiation from the originally simple plasm of the monera, there also took place the morphological separation of the nucleus (caryon) and cell-body (cytosoma or celleus). As these two chief forms of living matter are chemically different but nearly related, and as they may in certain circumstances (for instance, during indirect cell-division and the partial caryolysis connected therewith) enter into the closest mutual relations, we must suppose that the original severance of the two substances took place gradually and during a long period of time. It was not by a sudden bound or transformation, but by a gradual and progressive formation of the chemical antithesis of caryoplasm and cytoplasm, that the real nucleated cell (cytos) arose from the unnucleated cytode (or primitive cell). Both may correctly be comprised under the general head of _plastids_ (or formative principles), as "ultimate individualities."

I regard as the chief cause of this important differentiation of the plasm the accumulation of hereditary matter--that is to say, of the internal characteristics of the plastids acquired by ancestors and transmitted to their descendants--within the plastids while their outer portion continued to maintain the intercourse with the outer world. In this way the inner nucleus became the organ of heredity and reproduction, and the outer cell-body the organ of adaptation and nutrition. I put forward this hypothesis in 1866 in my _General Morphology:_ "The two functions of heredity and adaptation seem to be not yet distributed between differentiated substances in the unnucleated cytodes, but to inhere in the whole of the homogeneous mass of the plasm; while in the nucleated cell they are divided between the two active constituents of the cell, the inner nucleus taking over the transmission of hereditary characters and the outer plasm undertaking adaptation, or the accommodation to the features of the environment." This hypothesis was afterwards (1873) confirmed by the discoveries of Strasburger, the brothers Hertwig, and others, with regard to cell-cleavage and fertilization; it is particularly supported by the phenomena of _caryokinesis_(the movement of the nucleus) in sexual generation. Hence we can understand how it is that in the monera (chromacea and bacteria), which propagate by simple cleavage, there is no sexual generation and no nucleus.

The great significance of the nucleus in the life of the cell, as central organ of heredity, and also probably as "the soul of the cell," depends chiefly on the chemical properties of its albuminous matter, the caryoplasm. This one indispensable nuclear element is chemically akin to the cytoplasm of the cell-body, but differs from it in certain respects. The caryoplasm has a greater affinity for many coloring matters (carmine, hæmatoxylin, etc.) than the cytoplasm; and the former coagulates more quickly and firmly than the latter through acids (such as acetic and chromic acid). Hence we need only add a drop of diluted (two per cent.) acetic acid to cells that seem homogeneous to make perfectly clear the separation between the inner nucleus and outer body. As a rule, the firmer nucleus then stands out sharply as a globular or oval particle of plasm; occasionally it has other forms (cylindrical, conical, spiral, or branched). The caryoplasm seems to be originally quite homogeneous and structureless, as we find in many of the protists and many young cells of histona (especially young embryos). But in the great majority of cells the caryoplasm is divided into two or more different substances, the chief of them being chromatin and achromin.

The most common division of the caryoplasm in the cells of the animal and plant body, and the one of chief significance for their vital activity, is that into two chemically different substances, which are usually called chromatin (or nuclein) and achromin (or linin). Chromatin has a greater affinity for coloring (_chromos_) matter (carmine, hæmatoxylin, etc.), and so this "colorable nuclear matter" is particularly regarded as the vehicle of heredity. The achromin (or achromatin, or linin) is either not at all or less easily colorable, and is akin to the cytoplasm; in direct cell-division it enters into close relations with the latter. Achromin is usually found in the form of slender threads, and hence called "nuclear thread-matter" (linin). Chromatin is generally found in roundish or rod-shaped granules (chromosomata), which exhibit very characteristic changes of form (loop formation, etc.) in indirect cell-division. The chemical, physiological, and morphological difference between chromatin and achromin must not be regarded as an original property of cell nuclei (as is wrongly stated sometimes), but is the outcome of a very early phylogenetic differentiation in the originally homogeneous caryoplasm; and this holds also of two other parts of the nucleus--the nucleolus and centrosoma.

In a good many cells, but by no means universally, we find two other constituents of the nucleus, which owe their rise to a further differentiation of the caryoplasm. The nucleolus is a small globular or oval particle, which may be found singly or in numbers in the nucleus, and behaves somewhat differently towards coloring matter than the closely related chromatin. It has a special affinity for acid aniline colors, gosin, etc. Its substance has, therefore, been distinguished as _plastin_ or _paranuclein_. The nucleolus is especially found in the tissue-cells of the higher animals and plants as an independent constituent; it is wanting in many of the unicellular protists. The same may be said of the centrosoma, or "central body" of the cell. This is an extremely small granule, on the very limit of visibility, the chemical composition of which is not known very well. We should have paid no attention to this constituent of the cell (distinguished in 1876) if it did not play an important, and perhaps leading, part in indirect cell-division. As the "polar body in the division of the nucleus," the centrosoma exercises a peculiar attraction on the granules distributed in the cytoplasm, which arrange themselves radially about this centre. The centrosomata grow independently and increase by cleavage, like the chromoplasts (chlorophyll particles, etc.). When they have split up, each of the daughter-microsomata acts in turn as a centre of attraction on its half of the cell. However, the great importance which modern cytologists have ascribed to it on this account is discounted by two circumstances. In the first place, we have not succeeded, in spite of all efforts, in discovering a centrosoma in the cells of the higher plants and many of the protists; and, in the second place, a number of recent chemical experiments have succeeded in producing centrosomata artificially (for instance, by the addition of magnesium chloride) in the cytoplasm. Hence many cytologists regard the centrosoma as a secondary product of differentiation in the cell-body, not the nucleus.

Two other parts of the nucleus that we find very often, but by no means universally, in the cells of the animal and plant body are the nuclear membrane (caryotheca) and the nuclear sap (caryolymph). A large number of cells--but not all--have the appearance of vesicles, having a thin skin enclosing a liquid content, the nuclear sap. The achromin then usually forms a frame-work of threads, with chromatin granules in its meshes or knots, within this round vesicle. This very thin nuclear membrane (often only visible as its contour) or caryotheca may be regarded as the result of surface-strain (at the planes of contact of caryoplasm and cytoplasm). The watery and usually clear and transparent nuclear sap (caryolymph) is formed by imbibition of watery fluid (like the frothy structure of the plasm in general). The separation of the nuclear membrane and nuclear sap is not a primary property of the nucleus, but is due to a secondary differentiation in the originally homogeneous caryoplasm.

Like the caryoplasm of the nucleus, the cytoplasm of the cell-body is originally a chemical modification of the simple and once homogeneous plasm (the archiplasm). This is clearly shown by the comparative biology of the protists, their unicellular organism presenting a much greater variety of stages of cell-organization than the subordinate tissue-cells in the bodies of the multicellular histona. However, in the great majority of cells the cytoplasm is separated into several, and frequently very numerous, parts, which have received diverse forms and functions in the division of labor. We then see very conspicuously the regularity of cell-organization, which is altogether wanting in the simple homogeneous plasma granules of the monera. As this great differentiation of the advanced elementary organism is incorrectly generalized by some recent cytologists and described as a universal feature of cells, it is necessary to insist explicitly that it is a secondary phylogenetic development, and is altogether wanting in the primitive organisms. The complexity of the physiological division of labor and the accompanying morphological separation of parts is extremely great in the cytoplasm. When we wish to arrange them in a few large groups from a general point of view, we may distinguish the active plasma-formations from the passive plasma-products; the former are due to a chemical metamorphosis of the living plasm, the latter lifeless excretions from it.

Under the head of plasm-formations, or products of differentiation in the cytoplasm, we comprise all formations that are due to partial metamorphosis of the living cell-body--not lifeless excretions from it, but living parts of its substance, undertaking special functions, and therefore chemically and morphologically differentiated from the primary cytoplasm. One of the commonest differentiations of this kind is the separation of the firm hyaline skin-layer (hyaloplasm) from the softer granular marrow-layer (polioplasm); though the two often pass into each other without clear limits. In most plant-cells special granules of plasm, mostly globular or roundish, are developed, called _trophoplasts_, and these undertake the work of metabolism. To this class belong the amyloplasts, which produce starch (amylum), the chloroplasts or chlorophyll-granules which form the green matter (chlorophyll) in the leaf, and the chromoplasts which form color-crystals of various sorts. In the cells of the higher animals the myoplasts form the special contractile tissue of the muscles, and the neuroplasts the psychic tissue of the nerve-matter. On the other hand, the distinction between the body-plasm (somoplasma) and the germ-plasm (germoplasma), which serves as the base of Weismann's untenable theory of the germ-plasm (_cf._ chapter xvi.), is purely hypothetical and without direct observation to support it.

The infinite variety of parts of the cell which arise as excretions of the living active cytoplasm, and so must be regarded as lifeless plasma-products, may be divided into two chief groups--internal and external. The former are stored within the living cytoplasm, the latter thrust out from it.

Internal plasma-products of common occurrence are the microsomata, very small and opaque particles which are generally regarded as products of metabolism. They consist sometimes of fat, sometimes of derivatives of albumin, sometimes of other substances of which we do not know the chemical composition. The same may be said of the large and variously-colored pigment-granules, which are very common and determine the color of tissues. Also very common in the cytoplasm are large accumulations of fat in the shape of oil-globules, fat-crystals, etc., besides other crystals of a very different sort, partly organic crystals (for instance, albuminous crystals in the aleuron-granules of plants), partly inorganic crystals (for instance, of oxalic-acid salts in many plant-cells, of calcareous salts in many animal-cells). The watery cell-sap (cytolymph) plays an important part in many of the larger cells. It is formed by the accumulation of fluid in the cytoplasm, and is found in its frothy structure. The large empty spaces which it forms are called vacuoles, with very regularly disposed alveoles. When the cell-sap gathers in great abundance within the cell, we get the large vesicular cells which are found in the tissues of the higher plants, the cartilages, etc.

As external excretions of the living cytoplasm that have acquired some importance, especially as protective organs, in the majority of cells, we have first of all the cell-membranes, the firm capsules or protective skins which enclose the soft cell-body, like a snail in its house. In the first period of the cell-theory (1838-1859) such an integument was ascribed to all cells, and often regarded as their chief constituent; but it was discovered afterwards that this protective skin is altogether wanting in many (especially animal) cells, and that it is not found in many when they are young, but grows subsequently. We now distinguish between naked cells (gymnocytes) and covered cells (thecocytes). As examples of naked cells we have the amœbæ, and many of the infusoria, the spores of algæ, the spermatozoa, and many animal tissue-cells.

The cell-covering (cytotheca) varies very much in size, shape, composition, and chemical character, especially in the rhizopods among the unicellular protists. The flint shells of the radiolaria and diatomes, the chalky cells of the thalamophora and calcocytea, the cellulose shells of the desmidiacea and syphonea, show the extraordinary plasticity of the constructive cytoplasm (_cf._ chapter viii.). Among the histona the tissue-plants are remarkable for the infinite variety of shape and differentiation of their cellulose capsules. The familiar properties of wood, cork, bast, the hard shells of fruit, etc., are due to the manifold chemical modification and morphological differentiation which the cellulose membrane undergoes in the tissues of plants. This is less frequently seen in the tissues of animals; but, on the other hand, the intercellular and the cuticular matter play a greater part in these.

The intercellular matter, an important external plasma-product, is formed by the social cells in the tissues of the histona thrusting out in common firm protective membranes. These protective structures are very common among communities of protists, in the form of masses of jelly, in which a number of cells of the same kind are united; such are the zooglœa of many of the bacteria and chromacea, the common jelly-like envelope of the volvocina and many diatomes, and the globular cell-communities of the polycyttaria (or social radiolaria). The chief part is played by intercellular matter in the body of the higher animals, in the form of mesenchyma-tissue; the connecting tissue, cartilages, and bones owe their peculiar property to the amount and quality of the intercellular matter that is deposited between the social cells.

When the socially joined epidermic cells at the surface of the tissue-body thrust forth in common a protective covering, we get the cuticles, which are often thick and solid armor-plates. In many of the metaphyta wax and flinty matter are deposited in the cellulose cuticles. The strongest formation is found in the invertebrate animals, where the cuticle often determines the whole shape and articulation, as in the calcareous shells of mollusks (mussel-shells, snail-shells, cockle-shells, etc.); and especially the coats of the articulata (the crab's coat of mail, and the skins of spiders and insects).

VII

UNITIES OF LIFE

Units of life--Simple and complex organisms--Morphological and physiological individuals--Morphonta and bionta--Stages of individuality: cell, person, stem--Actual and virtual bionta--Partial and genealogical bionta--Metaphysical individuals--Cells (elementary organisms)--Cell membranes--Unnucleated cells--Plastids (cytodes and cells)--Primitive cells and nucleated cells--Organella (cell organs)--Cell communities (cœnobia)--Tissues of histona--Systems of organs--Organic apparatus--Histonal individuals (sprouts and persons)--Articulation of the histona (metamerism)--Stems of the histona--Animal states.

The dissection of the body of the higher animal and plant into its various organs soon prompted comparative anatomists to draw a distinction between simple and complex organisms. Then, when the cell-theory developed in the course of the last half-century, the common anatomic groundwork of all living forms was recognized in the cell; and the conception of the cell as the elementary organism led to the further belief that our own frame, like that of all the higher animals and plants, is a cell-state, composed of millions of microscopic citizens, the individual cells, which work more or less independently therein, and co-operate for the common purposes of the entire community. This fundamental principle of the modern cell-theory was applied with great success by Rudolph Virchow to the diseased organism, and led to most important reforms in medicine. The cells are, in his view, independent "life-unities or individual life-centres," and the unified life of the whole man is the combined result of the work of his component cells. In this way the cells are the real life-unities of the organism. Their individual independence is at once seen in the permanently unicellular protists, of which several thousand species are already known to us.

On the other hand, we find among the lower animals and the higher plants a composition of homogeneous parts, which represents a higher stage of life-unity. The tree is an individual, but it is made up of a number of branches or individual sprouts, each of which consists in like manner of an axial stem with leaves attached. If we detach such a branch and plant it in the ground, it takes root and grows into an independent plant. So the coral-stem is made up of a number of individual animals or persons, each of which has its own stomach and mouth with a crown of tentacles. Each several coral-individual is equivalent to a single living polyp (actinia). Thus the stem (_cormus_) is a higher unity, both in the animal and the plant world. Even the herds of gregarious animals, the swarms of bees and ants, and the communities of human beings, are similar unities; with the difference that the individual persons or citizens are not physically connected, but held together by common interests. We can, therefore, distinguish three stages of organic individuality, one building upon the other--the cell, the person (or sprout), and the stem or state (cormus). Each higher unity represents an intimate union of lower individuals. Morphologically, in relation to their anatomic structure, the latter are independent; but physiologically, in respect of the life-unity of the whole, they are subordinated to the former.

This relation is quite clear in the familiar examples I have quoted. But there are other organisms in which this is not so, and where the question of the real individuality is very difficult to answer. Thus, fifty years ago, we came to recognize floating animal-stems in the remarkable siphonophora, or social medusæ, which had hitherto been regarded as individual animals, or medusæ with a multiplicity of organs; further study proved that each of these apparent organs is really a modified medusa, and the whole united structure a stem. This example throws a good deal of light on the important question of association and division of labor. The whole floating siphonophoron is, physiologically considered (in respect of its vital activity), a harmoniously organized animal with a number of different organs; but from the morphological point of view (in respect of form and structure) each dependent organ is really an independent medusa.

It is clear, from these few illustrations, that the question of organic individuality is by no means so simple as it seems at first sight, and that it receives different answers according as we look at the form and structure (morphologically) or the vital and psychic activity (physiologically). We must, therefore, distinguish at once between morphological (_morphonta_) and physiological (_bionta_) individuals. The tree and the siphonophoron are bionta, or individuals of the highest order, made up of a number of similar branches or persons, the social morphonta. But, when we further dissect the latter anatomically into their various organs, and these again into their microscopic elements, the cells, each branch or person seems to be a bion, and their cells to be morphonta. Each multicellular organism is, however, developed in the beginning from a single cell, the stem-cell (cytula) or fertilized ovum; this is at once a morphon and a bion, a simple individual both morphologically and physiologically. The whole process of its development into a multicellular organism consists in a repeated cleavage of the stem-cell, the resultant cells being joined in a higher unity, and assuming different forms in consequence of the division of work.

The complicated modern state, with its remarkable achievements, may be regarded as the highest stage of individual perfection which is known to us in organic nature. But we can only understand the structure of this extremely complex "organism of the highest order," and its social forms and functions, when we have a sociological knowledge of the various classes that compose it, and the laws of their association and division of labor; and when we have made an anthropological study of the nature of the persons who have united, under the same laws, for the formation of a community and are distributed in its various classes. The familiar arrangement of these classes, and the settling of the rank in the mass and the governing body, show us how this complex social organism is built up step by step.

But we have to look in the same way on the cell-state, which is made up from the separate individualities in human society or in the kingdom of the tissue-animals, or the branches in the kingdom of the tissue-plants. Their complex organism, composed of various organs and tissues, can only be understood when we are acquainted with their constituent elements, the cells, and the laws according to which these elementary organisms unite to form cell-communities and tissues, and are in turn modified in the divers organs in the division of labor. We must, therefore, first establish the scale of the morphonta, and the laws of their association and ergonomy, according to which the several stages or conditions of morphological individuality build on each other. Three such stages may be at once distinguished: (1) the cell (or, more correctly, the plastid), (2) the person (animal) or branch (vegetal), and (3) the stem or cormus. But we shall find that there are further subordinate stages under each of these three. It is only in the case of the protists that the morphological unity is bound up with the physiological. In the case of the histona, the multicellular, tissue-forming organisms, this is only so at the beginning of individual existence (at the stage of the stem-cell). As soon as the multicellular body arises from this cytula by repeated segmentation, it is raised to the stage of a higher individuality, the cell-state.

Our own human frame is, in its mature condition, like that of all the higher animals, a very complete cell-state, but a single cell at the beginning of its existence. We speak of the life-unity of the former as an actual bion, and that of the latter as a virtual bion; in other words, the physiological individual or the life-unity has in the first case reached the highest stage of individual development that pertains to its species, while in the second case it remains at the lowest stage of virtual formation, and has only the capacity of rising to the higher stage. In the higher plants and animals only one cell of the organism, or the two combined sexual cells (ovum and spermium), are the potential bion which may develop into an actual one. There are, however, exceptions. In the fresh-water polyp (hydra) and cognate cnidaria each piece of the body-wall, in the bath-sponge (euspongia) and similar sponges each piece of tissue, and in many plants (for instance, marchantia among the crytogams and bryophyllum among the phanerogams) each portion of a branch or leaf, has the power to develop into a mature organism, and is, therefore, a virtual bion.

From these virtual bionta (parts of the body that may grow into whole organisms) we must distinguish the partial bionta which have not this property. These are separated parts of the body that live for a time after being cut off from the whole organism, but then die off. Thus, for instance, the heart of a tortoise beats for a long time after being cut out. A flower that has been plucked may, if put in water, keep fresh and alive for many days. In some highly organized cephalopods one of the eight arms of the male develops into an independent body, swims about, and accomplishes the fertilization of the female (_hectocotylus_ among the _argonauta_, _philonexis_, etc.). It was at first thought to be an independent animal parasite. The same thing happens with the remarkable foldlike dorsal appendages of a large naked snail (_thetys_), which get detached and creep about. The body of many of the lower animals may be cut in pieces and yet may live for weeks. The life-properties of these partial bionta are important in view of the general question of the nature of life and its apparent unity in most of the higher organisms. As a fact, even here the cells and organs lead their separate individual life, though they are subordinate to and dependent on the whole.

It has been attempted to answer this question of organic individuality in the sense of counting all organisms individuals which develop from a single fertilized ovum. Thus, the Italian botanist Gallesio, in 1816, regarded all plants that arise by asexual generation (budding or segmentation)--sprouts, branches, slips, bulbs, etc.--as merely portions of a single individual that came from an egg (the seed). So also Huxley, in 1855, considered the sum of all the animals that have been produced by asexual propagation, but from a single sexually generated animal, to be parts of one individual. In practice, however, this principle is useless. We should have to say that the millions of plant-lice which arise parthenogenetically from unfertilized germ-cells, but are originally descended from one impregnated ovum, are one single individual; so also all the weeping-willows in Europe, because they all came from shoots of one single sexually-produced tree.

Many attempts have been made in the course of the nineteenth century to give a generally satisfactory answer to this difficult question of the content and connotation of the idea of the organic individual. None of these has found general favor. I have compared and criticised them in the third book of my _General Morphology_. I there paid special attention to the views of Goethe, Alexander Braun, and Nägeli among the botanists, and Johannes Müller, Leuckart, and Victor Carus among the zoologists. When we consider the striking divergence of the views of such distinguished scientists and thinkers on so important a biological question, we can understand that opinions are still very divided to-day. Hence we must not be too hard on the metaphysical philosophers when--in complete ignorance of the real facts--they rear the most extraordinary theories in their airy speculations on "the principle of individuation". Compare, for instance, the opinions of the school-men and those of recent thinkers such as Arthur Schopenhauer and Edward Hartmann. As a rule, the psychological side of the problem--the question of the individual soul--is very prominent, without much attention being paid to its material substratum--the anatomic basis of the organism. Many metaphysicians, who, in their one-sided anthropism, make man here also the measure of all things, would assign personal consciousness as the basis of the idea of individuality. It is obvious that this is not a practicable test even for the higher animals, to say nothing of the lower animals and plants. In these we have a far greater variety of individuality on the one hand, and a far greater simplicity of construction on the other. I have tried to show, in my essay on "The Individuality of the Animal Body" (1878), the easiest way to answer these complicated tectological questions, and to support it by the science of structure. It suffices to distinguish the three chief stages I have mentioned, and to explain clearly their physiological significance on the one hand and morphological on the other. We will therefore consider the cell first, then the person (or sprout), and, finally, the stock (or cormus).

Ever since the middle of the nineteenth century the cell theory has been generally and rightly considered one of the most important theories in biology. Every anatomical, histological, physiological, and ontogenetic work must build on the idea of the cell as the elementary organism. Nevertheless, we are still very far from having a general and clear agreement as to this universal and fundamental idea. On the contrary, the ablest biologists still differ considerably as to the nature of the cell or the elementary individual, its relation to the whole of the multicellular organism, and so on. This divergence of views is partly due to the intricacy of the phenomena we find in the life of the cell, and partly to the many and extensive changes that have been made in the meaning of the term in the course of its employment. Let us first cast a glance at the various stages of its history.

When in the last third of the seventeenth century a number of scientists, especially Malpighi in Italy and Crew in England, used the microscope for the first time in the anatomic study of plant structure, they noticed a certain build of the tissue that closely resembled the honeycomb. The closely packed wax cells, filled with honey, of the hive, which show a hexagonal appearance in section, are like the wood cells that contain the sap in the plant. It was the great merit of Schleiden, the real founder of the cell theory, to prove that _all_ the different tissues of plants are originally composed of such cells (1838). Theodor Schwann soon afterwards proved the same for the animal tissues; in 1839 he extended the theory to the whole organic world. Both these scientists regarded the cell as essentially a vesicle, the firm membrane of which enclosed a fluid content, and a solid smaller body inside this, which R. Brown had recognized as the nucleus in 1833. They compared the cell, as a microscopic individual, to an organic crystal, and thought it arose by a sort of crystallization in an organic medium (cytoblastema); in this the central nucleus would serve as starting-point like the nucleus of the crystal.

In the first twenty years (1839-59) of the cell theory it was a fixed principle that there were three essential parts of the cell. Firstly, there was the strong outer membrane, which was not only regarded as a protective covering, but also credited with a great deal of importance as an element in the building of the organism. In the second place, there was the fluid or semi-fluid content (the sap); and, thirdly, the firm nucleus enclosed in the sap. In order to give a clearer idea of the relative thickness and disposition of these parts, the cell was compared to a cherry or a plum. The soft flesh of this fruit (corresponding to the cell sap) can, with difficulty, be separated from the external firm skin or from the hard stone within. A great step in advance was made in 1860, when Max Schultze showed that the external membrane was an unessential and secondarily formed part of the cell. It is, as a fact, altogether wanting in many, especially young, cells of the animal body. They are naked cells without any membrane. The distinguished anatomist also proved that the so-called "cell sap"--the real body of the cell--is not a simple fluid, but a viscous, albuminous substance, the independent movements of which had long been known in the rhizopods, and which the first to study it carefully, Felix Dujardin, had described as _sarcode_ in 1835. Max Schultze further showed that this "sarcode" was identical with the "cell mucus" of the plant cells which Hugo Mohl had designated "protoplasm" in 1846, and that this living matter must be regarded as the real vehicle of the phenomena of life. As the membrane was now recognized to be non-essential, of secondary growth, and completely wanting in some cases, there remained only two essential parts of the cell--the outer soft cell body, consisting of protoplasm, and the inner firm nucleus, consisting of a similar substance called nuclein. The original naked cell was now like a cherry or plum without the skin. This new idea of the cell, formulated forty years ago, which I endeavored to confirm in my monograph on the radiolaria (1862), is now generally accepted, and the cell is defined as a granule or particle of protoplasm (= cytoplasm) enclosing a firm and definite nucleus (or caryon, consisting of caryoplasm).

This would be a good occasion to glance at the errors to which microscopic investigation and the conclusions based on it are liable. Although Kölliker in 1845, and Remak in 1851, had drawn attention to the existence of naked cells, and had compared their movements (for instance, in lymph-cells) to those of the protoplasm in plant-cells, the majority of the leading microscopists clung for twenty years to the dogma that every cell must have a membrane; the definite outline which even a naked cell must show in a different refracting medium was taken to be the sign of a special and anatomically separable membrane. It would be just as correct to talk of a protective membrane on a homogeneous glass ball; its outline is sharply defined. In the long controversy that "exact" observers sustained as to the presence or absence of a membrane, this optical error--the false interpretation of a sharp contour--counted for a good deal. It is much the same with other conflicts of "exact" observers who give their "certain observations" as facts, whereas they are really inferences from imperfect observations on which different interpretations may be put.

Forty years ago (1864) I tried in vain to detect a nucleus in the naked, living, mobile protoplasm of a few small rhizopod-like protists (protamœba and protogenes). Other observers, who afterwards studied similar unnucleated cells (Gruber, Cienkowski, and others), were no more successful. On the ground of these observations, which were often repeated afterwards, I formed the class of the _monera_--the simplest unnucleated organisms--in my _General Morphology_ in 1866, and pointed out their great importance in solving some of the chief problems of biology. This importance has been much enhanced of late, since the chromacea and bacteria have also been recognized as unnucleated cells. Bütschli has, it is true, raised the objection that their homogeneous plasma-body behaves, not as cytoplasm, but as caryoplasm (or nuclein), and so that these simplest plastids correspond, not to the cell-body, but to the nucleus of other cells. On this view the bacteria and chromacea are not cells without nuclei, but nuclei without cell-bodies. This idea agrees with my own in conceiving the plasma-body of the monera (apart from its molecular structure) as homogeneous and not yet advanced as far as the characteristic differentiation of inner nucleus and outer cell-body. Bearing in mind that these essential parts of the cell (in the view of most cytologists) are chemically related yet different from each other, we have three possible cases of the original formation of the nucleated cell from the unnucleated cytode: (i) The nucleus and cell-body have arisen by differentiation of a homogeneous plasm (monera); (2) the cell-body is a secondary growth from the primary nucleus; (3) the nucleus is a secondary development from the cell-body.

On the first view, which I hold, the plasm, or living matter, of the earliest organisms on the earth (which can only be conceived as archigonous monera) was a homogeneous _plasson_ or archiplasm--that is to say, a plasma-compound that was not yet differentiated into outer cytoplasm and inner caryoplasm. The rise of this chemical distinction--and the accompanying morphological division of cell-body and nucleus--was due to a phyletic differentiation; it was the outcome of a very early and most important division of labor. The hereditary matter gathered in the nucleus, the outer cell-matter controlling the intercourse with the external world. Thus, by this first ergonomy, the nucleus became the vehicle of heredity and the cell-body the organ of adaptation. Opposed to this view is the second, the hypothesis which the founder of the cell-theory, Schleiden, had put forward--that the nucleus is the original base of the cell, and the cell-body a secondary development from it. This opinion (which, in the main, corresponds to that of Bütschli) raises a number of difficulties; as does also the third hypothesis, that the unnucleated "protoplasm-body" (the outer cytoplasm-body) is the original formation, and that the nucleus arose secondarily by condensation and chemical modification of it. At the bottom, however, the difference between the three hypotheses on the primary cytogenesis is not as great as it seems at first sight. However, I am more inclined to adhere to the first; it supposes that the physiological and chemical differences between nucleus and cell-body, which afterwards became so important, were not originally present. The phenomena of caryolysis in indirect cell-division show us still how close are the relations of the two substances.

If the organic population of our planet has arisen naturally, and not by a miracle, as Reinke and other vitalists suppose, the earliest elementary organisms, produced by the chemical process of archigony (spontaneous generation), could not be real nucleated cells, but unnucleated cytodes of the type of the chromacea (_cf._ chapter ii.). The nucleated real cell, as Oscar Hertwig and others define it to-day, can only have arisen by phylogenetic differentiation of nucleus and cell-body from the simple cytode of the monera. In that case it is a matter of simple logic to distinguish the older cytode from the later cell. The two may then best be comprised (as I proposed in vain in 1866) under the name of "plastids" (formative principles)--that is, the elementary organism in the broader sense. But if it is preferred to call the latter _cells_ (in the broader sense), the wrong modern idea of the cell must be altered, and the nucleus-feature omitted from it. The cell is then simply the living particle of plasm, and its two stages of development must be described by other names. The unnucleated plastid might be called _primitive cell_ (protocytos), and the ordinary nucleated one the nuclear cell (caryocytos).

A long gradation of cellular organization leads from the simplest primitive cells (monera) to the highest developed protists. While no morphological organization whatever is discoverable in the homogeneous plasma-body of the chromacea and bacteria, we find a composition from different parts in the highly differentiated body of the advanced protophyta (diatomes, siphonea) and protozoa (radiolaria, infusoria). The manifold parts of the unicellular organism, developed by division of work in the plasm, discharge various functions, and behave physiologically like the organs of the multicellular histona. But as the idea of "organ" in the latter is morphologically fixed as a multicellular part of the body, made up of numerous tissues, we cannot call these similarly functioning parts "organs of the cell," and had better describe them as organella (or organoids).

The great majority of the protists are, in the developed condition, as actual individuals, equivalent morphologically to real nucleated cells. By means of adaptation to the most varied conditions and the inheritance of the properties thus acquired such a variety of unicellular forms has been evolved in the course of millions of years that we can distinguish thousands of living species, both of plasmodomous protophyta and plasmophagous protozoa. The number of known and named species is already as high as this in several distinct classes, as, for instance, in the diatomes of the primitive plants and the radiolaria of the primitive animals. These solitary living unicellulars, or "hermit-cells," may be called _monobia_.

Many other protists have abandoned this original solitary life; they follow their social instincts and form communities or colonies of cells (_cœnobia_). These are usually formed by the daughter-cells which arise from the cleavage of a mother-cell remaining united after the division, and so on with the succeeding generations which come from their repeated segmentation. The following are the chief forms of these cœnobia:

1. GELATINOUS CŒNOBIA.--The social cells secrete a structureless mass of jelly, and remain associated in the common gelatinous mass, without actual contact. Sometimes they are regularly, at other times irregularly, distributed in it. We find cœnobia of this kind even among the monera, such as the _zooglœa_ of many bacteria and chromacea. They are common among the protophyta and protozoa.

2. SPHERICAL CŒNOBIA.--The cell-community forms a sort of ball, the cells lying close together at its surface, touching each other or even forming a continuous layer; such are _holosphæra_ and _volvox_ among the protophyta, _magosphæra_ and _synura_ among the protozoa. The latter are particularly interesting because they resemble the _blastula_, an important embryological stage of the metazoa, of which the simple, epithelial cell-layer at the surface of the hollow sphere is called the _blastoderm_ (or germinal membrane).

3. ARBOREAL CŒNOBIA.--The cell-community takes the form of a small tree or shrub, the fixed cells secreting jelly-like stalks at their base and these forming branches. At the top of each stalk or branch is an independent cell; so in the case of the _gomphonema_ and many other diatomes, the _codonocladium_ among the flagellata, and the _carchesium_ among the ciliata.

4. CATENAL CŒNOBIA.--The cell-community forms a chain, the links of which (the individual cells) are joined in a row. We find chainlike cell-communities of this sort, or "articulated threads," even among the monera (_oscillaria_ and _nostic_ among the chromacea, _leptothrix_ among the bacteria). Among the diatomes we have the _bacillaria_, among the thalamophora _nodosaria_, as examples. Many of the lower protophyta (algaria and algetta) form the direct transition to the true algæ among the metaphyta, as the threadlike layer of the latter (for instance, _cladophora_) is only a higher development of the catenal cœnobium, with polymorphism of the co-ordinated cells. We may also regard these articulated multicellular threads as the first sketch for the formation of tissues in the metaphyta.

The stable communities of cells which make up the body of the histona, or multicellular plants and animals, are called tissues (_tela_ or _hista_). They differ from the cœnobia of the protists in that the social cells give up their independence, assume different forms in the division of labor, and subordinate themselves to the higher unity of the organ. However, it would be just as difficult to lay down a sharp limit between the cœnobia and the tissues as between the protists and the histona which possess them; the latter have been developed phylogenetically from the former. The original physiological independence of the cells which have combined to form tissues is more completely lost in proportion to the closeness of their combination, the complexity of their division of labor, and the differentiation and centralization of the tissue-organism. Hence the various kinds of tissue in the body of the histona behave like the various classes and professions in a state. The higher the civilization and the more varied the classes of workers, the more they are dependent on each other, and the state is centralized.

In the lower tissue-forming plants, the algæ and fungi, the plant-body has the appearance of a layer of cells, the tissues of which show little or no division of labor. In these _thallophyta_ there are none of the conducting or vascular fibres, the formation of which is of great importance in the higher plants in connection with their physiological function of circulation of the sap. These more advanced vascular plants comprehend the two great groups of ferns (_pteridophyta_) and flowering plants (_anthophyta_, or phanerogams). Their body is always composed of two chief organs, the axial stem and the lateral leaves. This is also the case with the mosses (_bryophyta_), which have no vascular fibres; they lie between the two chief groups of the non-vascular thallophyta and the vascular cormophyta. However, this histological and organological division of the two great groups of tissue-plants must not be pressed; there are many exceptions and intermediate forms. In general their manifold tissue-forms may be brought under two chief groups, which we may call primary and secondary. The primary tissues are the phylogenetically older and histologically simple "cell-tissues," such as we have in the thallophyta (algæ, fungi, and mosses); in these there are no conducting fibres, or, at least, only rudimentary ones. The secondary tissues are a later development from these; they form conducting and vascular fibres and other highly differentiated forms of tissue (cambium, wood, etc.). They make up the bodies of the more complex vascular plants, the ferns and flowering plants.

In the bodies of the tissue-animals we may similarly distinguish two chief groups of tissues, the primary and secondary. The former are phylogenetically and ontogenetically older than the latter. The primary tissues of the metazoa are the _epitelia_, simple layers of cells or forms of tissue directly derived from such (glands, etc.). Secondary tissues, evolved from the former by physiological change of work and morphological differentiation, are the _apotelia_; of these "derivative tissues" we may distinguish the three leading groups of connective tissue, muscular tissue, and nerve tissue. These three great groups of tissue in the animal world may be subdivided, like the plant groups, into lower and higher sub-sections. The cœlenteria (gastræads, sponges, cnidaria) are predominantly built up of epitelia, as are also the phyletically older group of the cœlomaria; in the vast majority of the latter, however, the great mass of the body is formed of apotelia, and they are subject to the most extensive differentiation. The embryo of all the metazoa consists solely of epitelia (the germ-layers) at first; apotelia are developed from these afterwards by differentiation of the tissues.

Comparative anatomy distinguishes in the multicellular body of the tissue-forming organisms a great number of different parts, which are regularly adapted to discharge definite vital functions, and have been most intricately developed in virtue of the division of labor. They are called "organs" in the stricter sense in opposition to the organella (or organoids) of the protists; the latter have, it is true, a similar physiological purport, but are not (being parts of a cell) equal to the former morphologically. The remarkable efficiency that we find in the structure of the various organs in view of the functions they have to discharge, and the regularity of their construction in the unity of the histon--in other words, their adaptive organization--is explained mechanically by the theory of selection, while the teleological hypotheses of dualistic biology (for instance, the "intelligent dominants" of Reinke) completely fail to account for their origin. The gradual advance of the organs and their physiological division of labor have many analogies in the two kingdoms of the histona. While at the lowest stages the simple organ represents only a separate individual piece of primitive tissue, we find special systems of organs and organic apparatus in the higher stages.

The idea of a particular system of organs is determined by the unity of one tissue which forms the characteristic element in the totality of the organs that belong to it. Of such systems in the kingdom of the metaphyta we have: the skin-system (with the tissue of the epidermis), the vascular system (with its conducting and vascular fibres), and the complementary tissue system (with the basic tissue). In the kingdom of the metazoa we may similarly distinguish: the skin-system (integument of the epidermis), the vascular system (with the mesenchyma-tissue of the blood and blood-vessels), the muscular system (with the muscle-tissue), and the nervous system (with the neurona of the nerve-tissue).

In contrast with the histological idea of a system of organs, we have the physiological conception of an apparatus of organs. This is not determined by the unity of the constituent tissue, but by the unity of the lifework that is accomplished by the particular group of organs in the histona. Such an apparatus of organs is, for instance, the flowers and the fruit developing therefrom in the phanerogams, or the eye or the gut of an animal. In these apparatus the most diverse organs and systems of organs may be associated for the fulfilment of a definite physiological task.

In the higher animals and plants we usually regard as the "real individual" (in the wider sense of the word) the tissue-forming organism made up of various organs; and we may here briefly and instructively call this the histonal individual (or, more briefly, the "histonal"). Botanists call this individual phenomenon among the metaphyta a sprout (_blastus_). Zoologists give the title of "person" (_prosopon_) to the corresponding unity among the animals. The two forms agree very much in their general features, and may be called "individuals of the second order," if we take the cells to be the first and the stock the third stage in the hierarchy of organic individuality. In comprising them here under the general head of histonals, or histonal individuals, I mean by this to designate the definite physiological unity of the multicellular and tissue-forming organism, as contrasted with the unicellular protist on the one hand, and the higher stem, made up of several histonals, on the other.

The plant-histonal, which Alexander Braun especially clearly marked out and described as the sprout, is found in two principal forms in the kingdom of the metaphyta--the lower form of the layer-sprout (_thallus_) and the higher form of the stalk-sprout (_culmus_). The thallus predominates in the lower and older sub-kingdom of the layer-plants (_thallophyta_), in the classes of the algæ and fungi; the culmus in the higher and younger sub-kingdom of the stalk-plants (_cormophyta_), in the classes of the mosses, ferns, and flowering plants. The culmus presents in general the characteristic form of an axial central organ, the stalk, with lateral organs, the leaves, attached to this at the sides, the former having an unlimited vertical growth and the latter an unlimited basal growth. The thallus does not yet show this important morphological division. There are, however, exceptions in both groups of the metaphyta. The large and highly developed _fucoidea_ among the algæ exhibit similar differentiations of organs to those we distinguish as stalk and leaves in the higher cormophyta. On the other hand, they are wanting in the lower liverworts, which form a thallus like many of the algæ; thus, for instance, the liverwort _riccia fluitans_ is just like the brown alga _dictyota dichotoma_. Other primitive liverworts (such as the _anthoceros_) have also a very simple thallus; but most of them have a separation of the thallus into an axial organ (stalk) and several lateral organs (leaves). In the distribution of labor among the leaves there then emerge the differences between the lower leaves, foliage leaves, higher leaves, and flower leaves. A simple poppy-plant (_papaver_) or a single-flowered _gentiana ciliata_, which has only one bloom at the top of its branchless stalk, is a good example of a highly developed culmus.

To the plant-sprout corresponds in the animal world the _person_. All the tissue-animals pass in the course of their embryonic development through the important stage of the _gastrula_, or "cup-shaped embryo." The whole body of the tissue-animal at this stage forms at first a simple gut-sac or gastric sac (the primitive gut), the cavity of which opens outward by a primitive mouth. The thin wall of the sac is formed by two superimposed layers of cells, the two primary germinal layers. This gastrula is the simplest form of the "person," and the two germinal layers are its sole organs.

The diverse animal forms which develop along different lines from this common embryonic form of the gastrula may be grouped into two sub-kingdoms, the lower (_cœlenteria_) and the upper (_cœlomaria_) animals. The former correspond in the simplicity of their structure in many respects to the thallophyta, and the latter to the cormophyta. Of the four stems of the cœlenteria (which have only a ventral opening and no gut-cavity) the gastræads remain at the gastrula stage, and the sponges are formed by multiplication of the same stems of gastræads. On the other hand, the cnidaria develop into higher radial (star-shaped) persons, and the platodes into lower bilateral persons. From the latter are derived the worms (_vermalia_), the common stem-groups of the five higher animal stems, the unarticulated mollusks, echinoderms, and tunicates, and the limb-forming articulates and vertebrates.

A large part of the physiological advantages and morphological perfection which the higher histona have, as contrasted with the lower, may be traced to the circumstance that the tissue-forming organism articulates--that is to say, divides on its long axis into several sections. With this multiplication of groups of organs there goes, as a rule, a more or less extensive division of work among them, a leading factor of higher development. In this point also we see the biogenetic parallelism between the two great groups of the tissue-plants and tissue-animals.

In the kingdom of the tissue-plants the articulated cormophyta rise high above the unarticulated thallophyta. While the articulation of the stem of the former proceeds and leaves are developed at the knots (_nodi_) between each two sections of the stalk, far greater play is offered to polymorphic differentiation than in the thallophyta, which are generally without this metamerism. The formation of the bloom in the flowering plants or phanerogams consists in a sexual division of labor among the thickly gathered leaves in a short section of a stem.

To the two groups of unarticulated and articulated sprouts in the kingdom of the tissue-plants correspond, in many respects, the two sections of the tissue-animals, the unarticulated and the articulated. The two stems of the articulates and vertebrates rise above all the other metazoa by the perfection of their organism and the variety of their functions. In the articulates the metamerism is chiefly external--an articulation of the body wall. In the vertebrates it mainly affects the internal organs, the skeleton, and the muscular system. The vertebration (articulation) of the vertebrates is not outwardly visible like that of the articulates. In both stems the articulation is similar in the lower and upper forms, as we find in the annelids and myriapods, the acrania and cyclostoma. On the other hand, the higher the organization the greater is the unlikeness of the members or articulated parts, as in the arachnida and insects, the amphibia and amniotes. The same antithesis is found in the lower and higher crustacea. This metamerism of the higher metazoa is of a motor character, having been acquired through the manner of movement of the lengthened body; but we find in some groups of the lower, and usually unarticulated, metazoa a propagative metamerism, determined by budding at the end; such is the strobilation of the chain-worms and the scyphostoma polyps. The individual metamera (parts) that are released from the end of the chain in these cases immediately show their individuality. This is also the case with many of the annelids, in which every member that is separated has the power to reproduce the whole chain of metamera.

The third and highest stage of individuality to which the multicellular organism attains is the stock or colony (_cormus_). It is usually formed by a permanent association of histonals that are produced by cleavage (imperfect segmentation or budding) from one histonal individual. The great majority of the metaphyta form complex plants in this sense. But among the metazoa we find this form of individuality only in the lower (and generally stationary) stages of development. Here also there is a striking parallelism of development between the two chief groups of the histona. At the lower stages of stock-formation there is equality of the social histonals. But in the higher grades they become unequally developed in the division of labor; and the greater the differences between them become, the greater is the centralization of the whole stock (as in the case of the siphonophora). We may therefore distinguish two principal forms of stocks--the homonomous and heteronomous, the one without, and the other with, division of labor among the histonals.

The history of civilization teaches us that its gradual evolution is bound up with three different processes: (1) Association of individuals in a community; (2) division of labor (ergonomy) among the social elements, and a consequent differentiation of structure (polymorphism); (3) centralization or integration of the unified whole, or rigid organization of the community. The same fundamental laws of sociology hold good for association throughout the entire organic world; and also for the gradual evolution of the several organs out of the tissues and cell-communities. The formation of human societies is directly connected with the gregariousness of the nearest related mammals. The herds of apes and ungulates, the packs of wolves, the flocks of birds, often controlled by a single leader, exhibit various stages of social formation; as also the swarms of the higher articulates (insects, crustacea), especially communities of ants and termites, swarms of bees, etc. These organized communities of free individuals are distinguished from the stationary colonies of the lower animals chiefly by the circumstance that the social elements are not bodily connected, but held together by the ideal link of common interest.

VIII

FORMS OF LIFE

Morphology--Laws of symmetry--Fundamental forms of animals and plants--Fundamental forms of protists and histona--Four chief classes of fundamental forms: (1) Centrostigma: vesicles (smooth vesicle and tabular vesicle); (2) Centraxonia: typical forms with central axis--Uniaxial (monaxonia, equipolar and unequipolar)--Transverse-axial (stauraxonia, double-pyramidal and pyramidal); (3) Centroplana: fundamental forms with central plane--Bilateral symmetry--Bilateral-radial and bilateral-symmetrical fundamental forms--Asymmetrical fundamental forms; (4) Anaxonia: irregular fundamental forms--- Causes of form-construction--Fundamental forms of monera, protists, and histona--Fundamental form and mode of life--Beauty of natural forms--Æsthetics of organic forms--Art forms in nature.

The infinite variety of forms which we observe in the realm of organic life not only delight our senses with their beauty and diversity, but also excite our curiosity, in suggesting the problem of their origin and connection. While the æsthetic study of the forms of life provides inexhaustible material for the plastic arts, the scientific study of their relations, their structures, their origin and evolution, forms a special branch of biology, the science of forms or morphology. I expounded the principles of this science in my _General Morphology_ thirty-eight years ago. They are so remote from the ordinary curriculum of education, and are so difficult to explain without the aid of numerous illustrations, that I cannot think of going fully into them here. In the present chapter I will only briefly describe those features of living things which relate to the difficult question of their ideal fundamental forms, the laws of their symmetry, and their relation to crystal-formation. I have treated these intricate questions somewhat fully in the last (eleventh) part of _Art-forms in Nature_. The hundred plates contained in this work may serve as illustrations of morphological relations. In the following pages the respective plates are indicated by the letters A-f, with the number of each.

The unity of the organic structure, which expresses itself everywhere in the fundamental features of living things and in the chemical composition and constructive power of their plasm, is also seen in the laws of symmetry in their typical forms. The infinite variety of the species may, both in the animal and plant worlds, be reduced to a few principal groups or classes of fundamental forms, and these show no difference in the two kingdoms (_cf._ plate 6). The lily has the same regular typical form as the hexaradial coral or anemone (A-f, 9, 49), and the bilateral-radial form is the same in the violet and the sea-urchin (clypeaster, A-f, 30). The dorsiventral or bilateral-symmetrical form of most green leaves is repeated in the frame of most of the higher animals (the cœlomaria); the distinction of right and left determines in each the characteristic antithesis of back and belly.

The distinction between protists and histons is much more important than the familiar division of organisms into plants and animals, in respect of their fundamental forms and their configuration. For the protists, the unicellular organisms (without tissue) exhibit a much greater freedom and variety in the development of their fundamental forms than the histons, the multicellular tissue-forming organisms. In the protists (both protophyta and protozoa) the constructive force of the elementary organism, the individual cell, determines the symmetry of the typical form and the special form of its supplementation; but in the histons (both metaphyta and metazoa) it is the plasticity of the tissue, made up of a number of socially combined cells, that determines this. On the ground of this tectological distinction we may divide the whole organic world into four kingdoms (or sub-kingdoms), as the morphological system in the seventh table shows.

In respect of the general science of fundamental forms (promorphology), the most interesting and varied group of living things is the class of the radiolaria. All the various fundamental forms that can be distinguished and defined mathematically are found realized in the graceful flinty skeletons of these unicellular sea-dwelling protozoa. I have distinguished more than four thousand forms of them, and illustrated by one hundred and forty plates, in my monograph on the _Challenger_ radiolaria [translated].

Only a very few organic forms seem to be quite irregular, without any trace of symmetry, or constantly changing their formless shape, as we find, for instance, in the amœbæ and the similar amœboid cells of the plasmodia. The great majority of organic bodies show a certain regularity both in their outer configuration and the construction of their various parts, which we may call "symmetry" in the wider sense of the word. The regularity of this symmetrical construction often expresses itself at first sight in the arrangement side by side of similar parts in a certain number and of a certain size, and in the possibility of distinguishing certain ideal axes and planes cutting each other at measurable angles. In this respect many organic forms are like inorganic crystals. The important branch of mineralogy that describes these crystalline forms, and gives them mathematical formulæ, is called crystallography. There is a parallel branch of the science of biological forms, promorphology, which has been greatly neglected. These two branches of investigation have the common aim of detecting an ideal law of symmetry in the bodies they deal with and expressing this in a definite mathematical formula.

The number of ideal fundamental forms, to which we may reduce the symmetries of the innumerable living organisms, is comparatively small. Formerly it was thought sufficient to distinguish two or three chief groups: (1) radial (or actinomorphic) types, (2) bilateral (or zygomorphic) types, and (3) irregular (or amorphic) types. But when we study the distinctive marks and differences of these types more closely, and take due account of the relations of the ideal axes and their poles, we are led to distinguish the nine groups or types which are found in the sixth table. In this promorphological system the determining factor is the disposition of the parts to the natural middle of the body. On this basis we make a first distinction into four classes or types: (1) the centrostigma have a _point_ as the natural middle of the body; (2) the centraxonia a straight line (axis); (3) the centroplana a plane (median plane); and (4) the centraporia (acentra or anaxonia), the wholly irregular forms, have no distinguishable middle or symmetry.

I. CENTROSTIGMATIC TYPES.--The natural middle of the body is a mathematical point. Properly speaking, only one form is of this type, and that is the most regular of all, the sphere or ball. We may, however, distinguish two sub-classes, the smooth sphere and the flattened sphere. The smooth sphere (_holospœra_) is a mathematically pure sphere, in which all points at the surface are equally distant from the centre, and all axes drawn through the centre are of equal length. We find this realized in its purity in the ovum of many animals (for instance, that of man and the mammals) and the pollen cells of many plants; also cells that develop freely floating in a liquid, the simplest forms of the radiolaria (_actissa_), the spherical cœnobia of the volvocina and catallacta, and the corresponding pure embryonic form of the _blastula_. The smooth sphere is particularly important, because it is the only absolutely regular type, the sole form with a perfectly stable equilibrium, and at the same time the sole organic form which is susceptible of direct physical explanation. Inorganic fluids (drops of quicksilver, water, etc.) similarly assume the purely spherical form, as drops of oil do, for instance, when put in a watery fluid of the same specific weight (as a mixture of alcohol and water).

The flattened sphere, or facetted sphere (_platnosphæra_), is known as an endospherical polyhedron; that is to say, a many-surfaced body, all the corners of which fall in the surface of a sphere. The axes or the diameters, which are drawn through the angles and the centre, are all unequal, and larger than all other axes (drawn through the facets). These facetted spheres are frequently found in the globular silicious skeletons of many of the radiolaria; the globular central capsule of many spheroidea is enclosed in a concentric gelatine envelope, on the round surface of which we find a net-work of fine silicious threads. The meshes of this net are sometimes regular (generally triangular or hexagonal), sometimes irregular; frequently starlike silicious needles rise from the knots of the net-work (A-f, 1, 51, 91). The pollen bodies in the flower-dust of many flowering plants also often assume the form of facetted spheres.

II. CENTRAXONIA TYPES.--The natural middle of the body is a straight line, the principal axis. This large group of fundamental forms consists of two classes, according as each axis is the sole fixed ideal axis of the body, or other fixed transverse axes may also be distinguished, cutting the first at right angles. We call the former uniaxial (_monaxonia_), and the latter transverse-axial (_stauraxonia_). The horizontal section (vertically to the chief axis) is round in the uniaxials and polygonal in the transverse-axial.

In the monaxonia the form is determined by a single fixed axis, the principle axis; the two poles may be either equal (_isopola_) or unequal (_allopola_). To the isopola belong the familiar simple forms which are distinguished in geometry as spheroids, biconvex, ellipsoids, double cones, cylinders, etc. A horizontal section, passing through the middle of the vertical chief axis, divides the body into two corresponding halves. On the other hand, many of the parts are unequal in size and shape in the _allopola_. The upper pole or vertex differs from the basal pole or ground surface; as we find in the oval form, the planoconvex lens, the hemisphere, the cone, etc. Both sub-classes of the monaxonia, the allopola (conoidal) and the isopola (spheroidal), are found realized frequently in organic forms, both in the tissue-cells of the histona and the independently living protists (A-f, 4, 84).

In the stauraxonia the vertical imaginary principal axis is cut by two or more horizontal cross-axes or radial-axes. This is the case in the forms which were formerly generally classed as regular or radial. Here also, as with the monaxonia, we may distinguish two sub-classes, isopola and allopola, according as the poles of the principal axis are equal or unequal.

Of the _stauraxonia isopola_ we have, for instance, the double pyramids, one of the simplest forms of the octahedron. This form is exhibited very typically by most of the acantharia, the radiolaria in which twenty radial needles (consisting of silicated chalk) shoot out from the centre of the vertical chief axis. These twenty rays are (if we imagine the figure of the earth with its vertical axis) distributed in five horizontal zones, with four needles each, in this wise: two pairs cross at right angles in the equatorial zone, but on each side (in north and south hemispheres) the points of four needles fall in the tropical zone, and the points of four polar needles in the polar circles; twelve needles (the four equatorial and eight polar) lie in two meridian planes that are vertical to each other; and the eight tropical needles lie in two other meridian planes which cross the former at an angle of forty-five degrees. In most of the acantharia (the radial acanthometra and the mailed acanthophracta)--there are few exceptions--this remarkable structural law of twenty radial needles is faithfully maintained by heredity. Its origin is explained by adaptation to a regular attitude which the sea-dwelling unicellular body assumes in a certain stage of equilibrium (A-f, 21, 41). If the points of the real needles are connected by imaginary lines, we get a polyhedrical body, which may be reduced to the form of a regular double pyramid. This typical form of the equipolar stauraxonia is also found in other protists with a plastic skeleton, as in many diatomes and desmidiacea (A-f, 24). It is more rarely found embodied in the tissue-cells of the histona.

Unequipolar stauraxonia are the pyramids, a fundamental form that plays an important part in the configuration of organic bodies. They were formerly described as regular or fundamental forms. Such are the regular blooms of flowering plants, the regular echinoderms, medusæ, corals, etc. We may distinguish several groups of them according to the number of the horizontal transverse axes that cut the vertical main axis in the middle.

Two totally different divisions of the pyramidal types are the regular and the amphithecta pyramids. In the regular pyramids the transverse axes are equal, and the ground-surface (or base) is a regular polygon, as in the three-rayed blooms of the iris and crocus, the four-rayed medusæ (A-f, 16, 28, 47, 48, etc.), the five-rayed "regular echinoderms," most of the star-fish, sea-urchins, etc. (A-f, 10, 40, 60), and the six-rayed "regular corals" (A-f, 9, 69).

The amphithecta (or two-edged) pyramids, a special group of pyramidal types, are characterized by having as their basis a rhombus instead of a regular polygon. We may, therefore, draw two imaginary transverse axes, vertical to each other, through the ground-surface, both equipolar, but of unequal length. One of the two may be called the sagittal axis (with dorsal and ventral pole), and the other the transverse axis (with right and left pole); but the distinction is arbitrary, as the two are equipolar. In this lies the chief difference from the centroplane and dorsiventral forms, in which only the lateral axis is equipolar, the sagittal axis being unequipolar. We find the bisected pyramid in a very perfect form in the class of the ctenophora (or comb-medusæ, A-f, 27), where it is quite general. The striking typical form of these pelagic cnidaria is sometimes called biradial, sometimes four-rayed and bilateral, and sometimes eight-rayed-symmetrical. Closer study shows it to be a rhombus-pyramid. The originally four-rayed type, which it inherited from craspedote medusæ, has become bilateral by the development of different organs to the right and left from those before and behind.

Similar rhombo-pyramidal forms to those of the ctenophora are also found in some of the medusæ and siphonophora, many of the corals and other cnidaria, and many flowers. The name "two-edged" which is given to this special type is taken from the ancient two-edged sword. Its chief axis is unequipolar, the handle being at the basic pole and the point at the verticle pole; but the two edges left and right are equal (poles of the lateral axis), and also the two broad surfaces (dorsal and ventral, joined by the sagittal axis).

III. CENTROPLANE TYPES.--The natural middle of the body is a plane, the median or chief plane (_planum medianum_ or _sagittale_); it divides the bilateral body into two symmetrical halves, the right and the left. With this is associated the characteristic antithesis of back (_dorsum_) and belly (_venter_); hence, in botany this type (found, for instance, in most green leaves) is called the dorsiventral, and in zoology the bilateral in the narrower sense. One characteristic of this important and wide-spread type is the relation of three different axes, vertical to each other; of these three straight axes (enthyni) two are unequipolar and the third equipolar. Hence, the centroplanes may also be called tri-axial (_triaxonia_). In most of the higher animals (as in our own frame) the longest of the three axes is the principal one (_axon principalis_); its fore pole is the oral or mouth pole, and its hinder pole is the aboral or caudal (tail) pole. The shortest of the three enthyni is, in our body, the sagittal (arrow) or dorsiventral axis; its upper pole is at the back and its lower pole at the belly. The third axis--the transverse or lateral axis--is equipolar, one pole being called the right and the other the left. The various parts which make up the two halves of the body have relatively the same disposition in each half; but absolutely speaking (namely, in relation to the middle plane) they are oppositely arranged.

Further, the centroplane or bilateral forms are also characterized by three vertical axes which may be drawn through each of the normal axes. The first of these normal planes is the median plane; it is defined by the chief axis and the sagittal axis, and divides the body into two symmetrical halves, the right and left. The second normal plane is the frontal plane; this passes through the chief axis and the transverse axis (which is parallel to the frontal surface in our body), and divides the dorsal half from the ventral half. The third normal plane is the cingular (waist) plane: this is defined by the sagittal and transverse axes. It divides the head half (or the vertical part) from the tail half (or the basal part).

The name "bilateral symmetry," which is especially applied to the centroplane and dorsiventral types, is ambiguous, as I pointed out in 1866 in an exhaustive analysis and criticism of these fundamental forms in the fourth book of the _General Morphology_. It is used in five different senses. For our present general purpose it suffices to distinguish two orders of centroplane types, the bilateral-radial and the bilateral-symmetrical; in the former the radial (pyramidal) form is combined with the bilateral, but not in the latter.

The bilateral-radial type comprises those forms in which the radial structure is combined in a very characteristic fashion with the bilateral. We have striking examples in the three-rayed flowers of the orchids (A-f, 74), the five-rayed blooms of the labiate and papilionaceous flowers, etc., in the plant world; and in the five-rayed "irregular" echinoderms, the bilateral sea-urchins (spatangida, clypeastrida, A-f, 30) in the animal world. In these cases the bilateral symmetry is recognizable at the first glance, as is also the radial structure, or the composition from three to five or more raylike parts (paramera), which are arranged bilaterally round a common central plane.

The bilateral-symmetrical type is general among the higher animals which move about freely. The body consists of two antithetic parts (_antimera_), and has no trace of radial structure. In the free moving, creeping, or swimming animals (vertebrates, articulates, mollusks, annelids, etc.) the ventral side is underneath, against the ground, and the dorsal side upward. This form is clearly the most useful and practical of all conceivable types for the movement of the body in a definite direction and position. The burden is equally distributed between the two sides (right and left); the head (with the sense organs, the brain, and the mouth) faces frontward and the tail behind. For thousands of years all artificial vehicles (carts on land and ships in water) have been built on this type. Selection has recognized it to be the best and preserved it, while it has discarded the rest. There are, however, other causes that have produced the predominance of this type in green leaves--the relation to the supporting stalk, to the sunlight that falls from above, etc.

Special notice must be taken of those bilateral forms which were originally symmetrical (by heredity), but have subsequently become asymmetrical (or of unequal halves), by adaptation to special conditions of life. The most familiar example among the vertebrates are the flat-fishes (_pleuronectides_), soles, flounders, turbots, etc. These high and narrow and flattened boney-fishes have a perfect bilateral symmetry when young, like ordinary fishes. Afterwards they form the habit of laying on one side (right or left) at the bottom of the sea; and in consequence the upper side, exposed to the light, is dark colored, and often marked with a design (sometimes very like the stony floor of the ocean--a protective coloring), while the side the flat-fish lies on remains without color. But, what is more curious, the eye from the under side travels to the upper side, and the two eyes lie together on one side (the right or left); while the bones of the skull and the softer parts of each side of the head grow quite crooked. Naturally, this ontogenetic process, in which a striking lack of symmetry succeeds to the early complete symmetry of each individual, can only be explained by our biogenetic law; it is a rapid and brief recapitulation (determined by heredity) of the long and slow phyletic process which the flat-fish has undergone for thousands of years in its ancestral history to bring about its gradual modification. At the same time, this interesting metamorphosis of the _pleuronectides_ gives us an excellent instance of the inheritance of acquired characteristics, as a consequence of constant œcological habit. It is quite impossible to explain it on Weismann's theory of the germ-plasm.

We have another striking example among the invertebrates in the snails (_gasteropoda_). The great majority of these mollusks are characterized by the spiral shape of their shells. This variously shaped, and often prettily colored and marked, snail's house is in essence a spirally coiled tube, closed at the upper end and open at the lower (or mouth): the mollusk can at any moment withdraw into its tube. The comparative anatomy and ontogeny of the snails teach us that this spiral shell came originally from a simple discoid or cylindrical dorsal covering of the once bilateral-symmetrical mollusk, by the two sides of the body having an unequal growth. The cause of it was a purely mechanical factor--the sinking of the growing visceral sac, covered with the shell, to one side; one part of the viscera contained in it (the heart, kidneys, liver, etc.) grew more strongly on one side than the other in consequence of this; and this was accompanied by considerable displacement and modification of the neighboring parts, especially the gills. In most snails one of the gills and kidneys and the ventricle of the heart corresponding to these have disappeared altogether, only those of the opposite side remaining; and the latter have moved from the right side to the left, or vice versa. The conspicuous lack of symmetry between the two halves of the body which resulted from this finds expression in the spiral form of the snail's shell. This remarkable ontogenetic metamorphosis also can be fully explained by a corresponding phylogenetic process, and affords a very fine instance of the inheritance of acquired characters.

There are also many examples of this asymmetry of bilateral forms in the plant world, such as the green foliage-leaves of the familiar begonia and the blooms of _canna_.

IV. THE CENTRAPORIA.--Few organic forms are completely irregular and without axes, as usually the attraction to the earth (geotaxis) or to the nearest object determines the special direction of growth, and so the formation of an axis in some direction or other. Nevertheless, we may instance as quite irregular the soft and ever-changing plasma-bodies of many rhizopods, the amœbinæ, mycetozoa, etc. Most of the sponges also--which we regard as stocks of gastræads--are completely irregular in structure; the most familiar example is the common bath-sponge.

An impartial and thorough study of organic forms has convinced me that their actual, infinitely varied configurations may all be reduced to the few typical forms I have described. Comparative anatomy and ontogeny further teach us that the countless modifying processes which have led to the appearance of the various species have acted by adaptation to different environments, habits, and customs, and give us, in conjunction with heredity, a physiological explanation of this morphological transformation. But the question arises as to the origin of these few geometrically definable types, and the cause of their divergence.

In this important and difficult question we find a great variety of opinions and a strong leaning to dualistic and mystic theories. Educated laymen, who have only a partial and imperfect acquaintance with the biological facts, think that they are justified here in appealing to a supernatural creation of forms. They contend that only a wise creator, following a rational and conscious design, could produce such structures. Even distinguished and informed scientists lean in this matter towards mystic and transcendental ideas; they believe that the ordinary natural forces do not suffice to explain these phenomena, and that at least for the first construction of these fundamental types we must postulate a deliberate creative thought, a design, or some such teleological cause, and therefore consciously acting final causes. So say Nägeli and Alexander Braun.

In direct opposition to this, I have ever maintained the view that the action of familiar physical forces--mechanical efficient causes--fully suffices to explain the origin and transformation of these fundamental types, as well as for all other biological and inorganic processes. In order to understand this monistic position thoroughly, and to meet the errors of dualism, we must bear in mind always the radical processes of growth which control all organic and inorganic configuration, and also the long chain of advancing stages of development, which lead us from the simplest protists, the monera, to the most advanced organisms.

The unicellular organisms exhibit the greatest variety from the promorphological point of view. In the single class of the radiolaria we find all imaginable geometrical types represented. This is seen in a glance at the one hundred and forty plates on which I have depicted thousands of these graceful little protozoa in my monograph (_Challenger Report_, vol. xviii.). On the other hand, the monera, at the lowest stage of organic life, the structureless organisms without organs that live on the very frontier of the inorganic world, are very simple. Especially interesting in this connection are the chromacea, which have hitherto been so undeservedly and so incomprehensibly neglected. Among the well-known and widely distributed chroococcacea, the chroococcus, cœlosphærium, and aphanocapsa are quite the most primitive of all organisms known to us--and at the same time the organisms that enable us best to understand the origin of life by spontaneous generation (archigony). The whole organism is merely a tiny, bluish-green globule of plasm, without any structure, or only surrounded by a thin membrane; its fundamental form is the simplest of all, the centraxial smooth sphere. Next to these are the oscillaria and nostochina, social chromacea, which have the appearance of thin, bluish-green threads. They consist of simple primitive (unnucleated) cells joined to each other; they seem often to be flattened into a discoid shape as a result of close conjunction. Many protists are found in two conditions, one mobile with very varied and changeable forms, and one stationary with a globular shape. But when the separate living cell begins to form a firm skeleton or protective cover for itself, it may assume the most varied and often most complicated forms. In this respect the class of the radiolaria among the protozoa, and the class of the diatomes among the protophyta (both of which have flinty shells), surpass all the other groups of the diversified realm of the protists. In my _Art-forms in Nature_ I have given a selection of their most beautiful forms (diatomes, A-f, 4, 84; radiolaria, A-f, 1, 11, 21, 22, 31, 41, 51, 61, 71, 95). The most remarkable and most important fact about them is that the artistic builders of these wonderful and often very ingenious and intricate flinty structures are merely the plastidules or micella, the molecular and microscopically invisible constituents of the soft viscous plasm (sarcode).

The configuration of the histona differs essentially from that of the protists, since in the case of the latter the simple unicellular body produces for itself alone the whole form and vital action of the organism, while in the histona this is done by the cell state, or the social combination of a number of different cells, which make up the tissue body. Hence the ideal type which we can always define in the actual histonal form has quite a different significance from that in the unicellular protists. In the latter we find the utmost diversity in the configuration of the independent living cells and the protective cover it forms; among the histona the number of fundamental forms is limited. It is true that the cells themselves which make up the tissues may exhibit a great variety in form and structure; but the number of the different tissues which they make up is small, and so is the number of ideal types exhibited by the organism they combine to form--the sprout (_culmus_) in the plant kingdom and the person in the animal kingdom. The same may be said of the stock (_cormus_) in both kingdoms--that is to say, of the higher individual unity which is constituted by the union of several sprouts or persons.

The two classes of fundamental forms which are especially found in the plant sprouts or the animal persons are the radial and bilateral. The one is determined by the stationary life, the other by free movement in a certain attitude and direction (swimming in water or creeping on the ground). Hence we find the radial form (as pyramidal) predominant in the blooms and fruits of the metaphyta, and the persons of the polyps, corals, and regular echinoderms. On the other hand, the bilateral or dorsiventral form preponderates in most free-moving animals; though it is also found in many flowers (papilionaceous and labial flowers, orchids, and others that are fertilized by insects). Here we have to seek the cause of the bilateralism in different features, in the relations with the insects, in the mode of their fastening to and distribution on the stalk (for the green foliage leaves), and so on.

The complex individuals of the first order, the stocks (_cormi_), are more dependent in their growth on the spatial conditions of their environment than the sprouts or persons; hence their typical form is generally more or less irregular, and rarely bilateral.

The interest which we take in natural and artistic forms, and which has for thousands of years prompted men to reproduce the former in the latter, depends for the most part, if not altogether, on their beauty--that is to say, on the feeling of pleasure we experience in looking at them. The causes of this pleasure and joy in the beautiful and the naturalness of its development are explained in æsthetics. When we combine this science with the results of modern cerebral physiology, we may distinguish two classes of beauty--direct and indirect. In direct or sensible beauty the internal sense-organs, or the æsthetic neurona or sense-cells of the brain, are immediately affected with pleasure. But in indirect or associational beauty these impressions are combined with an excitement of the phronetic neurona--the rational brain-cells which effect presentation and thought.

Direct or sensible beauty (the subject of sensual æsthetics) is the direct perception of agreeable stimuli by the sense-organs. We may distinguish the following stages of its perfection: 1. Simple beauty (the subject of primordial æsthetics); the pleasure is evoked by the direct sense-impression of a simple form or color. Thus, for instance, a wooden sphere makes an agreeable impression as compared with a shapeless piece of wood, a crystal as compared with a stone, a sky-blue or golden-yellow spot as compared with a greenish-blue or dull-yellow one (in music a simple pure bell-tone as compared with a shrill whistle). 2. Rhythmic beauty (the subject of linear æsthetics); the æsthetic sensation is caused by the serial repetition of some simple form--for instance, a pearl necklace, a chainlike community of monera (nostoc) or of cells (diatomes, A-f, 84, figs. 7 and 9): in music a tasteful series of simple notes. 3. Actinal beauty (the subject of radial æsthetics); the pleasure is excited by the orderly arrangement of three or more homogeneous simple forms about a common centre, from which they radiate; for instance, a regular cross or a radiating star, the three counter-pieces in the iris-bloom, the four paramera in the body of the medusa, the five radial-pieces in the star-fish. The familiar experience of the kaleidoscope shows how amply the simple radial constellation of three or more simple figures may delight our æsthetic sense (in music we have the simple harmony of several simultaneous notes). 4. Symmetrical beauty (the subject of bilateral æsthetics); the pleasure is caused by the relation of a simple object to its like, the mutual completion of two similar halves (the right and left parts). When we fold a piece of paper over an ink-stain in such a way that it is equally impressed on both halves of the fold, we get a symmetrical figure which makes an agreeable impression on our natural sense of space or equilibrium.

The æsthetic impressions in indirect associational beauty (the subject of associative or symbolical æsthetics) are not only much more varied and complex than those we have described, but they also play a much more important part in the life of man and the higher animals. The anatomic condition for this higher physiological function is the elaborate construction of the brain in the higher animals and man, and particularly the development of the special association-centres (thought-centres, reason-sphere) and their differentiation from the internal sense-centres. In this millions of different neurona or psychic cells co-operate, the sensual æstheta acting in conjunction with the rational phroneta, and thus, by complex associations of ideas, much higher and more valuable functions arise. We may indicate four chief groups of this associational or indirect beauty. 5. Biological beauty (the subject of botanical and zoological æsthetics): the various forms of organisms and their organs (for instance, a flower, a butterfly) excite our æsthetic interest by association with their physiological significance, their movements, their bionomic relations, their practical use, and so on. 6. Anthropistic beauty (the subject of anthropomorphic æsthetics): man, as "the measure of all things," regards his own organism as the chief object of beauty, either morphologically considered (beauty of the whole body and its various organs--the eyes, mouth, hair, flesh-tint, etc.), or physiologically (beauty of movements or positions), or psychologically (the expression of the emotions in the physiognomy). As man transfers to the objective world this personal gratification he experiences from self-consideration, and anthropomorphically regards other beings in the light of them, this anthropistic æsthetic obtains a far-reaching significance. 7. Sexual beauty (the subject of erotic æsthetics): the pleasure is caused by the mutual attraction of the sexes. The supreme importance of love in the life of man and most other organisms, the powerful influence of the passions, the sexual selection that is associated with reproduction, have evoked an infinite number of æsthetic creations in every branch of art relating to the antithesis of man and woman. The special pleasure which is caused by the bodily and mental affinities of the sexes can be traced phylogenetically to the cell-love of the two sexual cells, or the attraction of the sperm-cell to ovum. 8. Landscape beauty (the subject of regional æsthetics): the pleasure which is caused by the sight of a fine landscape, and that finds satisfaction in modern landscape-painting, is more comprehensive than that of any other æsthetic sensations. In point of space the object is larger and richer than any of the individual objects in nature which are beautiful and interesting in themselves. The varying forms of the clouds and the water, the outline of the blue mountains in the background, the woods and meadows in the middle-distance, and the living figures in the foreground, excite in the mind of the spectator a number of different impressions which are woven together into a harmonious whole by a most elaborate association of ideas. The physiological functions of the nerve-cells in the cortex which effect these æsthetic pleasures, and the interaction of the sensual æstheta with the rational phroneta, are among the most perfect achievements of organic life. This "regional æsthetics," which has to establish scientifically the laws of landscape beauty, is much younger than the other branches of the science of the beautiful. It is very remarkable that absolute irregularity, the absence of symmetry and mathematical forms, is the first condition for the beauty of a landscape (as contrasted with architecture, and the beauty of separate objects in nature). Symmetrical arrangement of things (such as a double row of poplars or houses) or radial figures (a flower-bed or artificial wood) do not please the finer taste for landscape; they seem tedious.

A comparative survey of these eight kinds of beauty in natural forms discovers a connected development, rising from the simple to the complex, from the lower to the higher. This scale corresponds to the evolution of the sense of beauty in man, ontogenetically from the child to the adult, phylogenetically from the savage to the civilized man and the art critic. The stem-history of man and his organs, which explains to us in anthropogeny the gradual rise from lower to higher forms by the interaction of heredity and adaptation, also finds an application in the history of æsthetics and ornamentation. It teaches us how feeling, taste, emotion, and art have been gradually evolved. On the other hand, we have corresponding to this evolutionary series the scale of the typical forms which lie at the root of the real forms of bodies both in nature and art.

SEVENTH TABLE

THE MORPHOLOGICAL SYSTEM OF ORGANISMS

Division of living things (plants and animals) into two kingdoms (protista and histona) on the ground of their cell-structure and body-structure.

┌──────────────────────────────────────────────────────────┐ │ First organic kingdom: UNICELLULAR, protista. │ │ │ │Organisms which as a rule remain unicellular │ │throughoutlife (_monobia_), less frequently they │ │form loose cell communities (_cœnobia_) by │ │repeated cleavage, but never real tissues. │ │ │ │ Sub─kingdom of the protista. │ ├───────────────────────────┬──────────────────────────────┤ │ A. PRIMITIVE PLANTS │ B. PRIMITIVE ANIMALS │ │ (protophyta). │ (protozoa). │ │ │ │ │ A. Character: │ B. Character: │ │ Plasmodomous. │ Plasmophagous. │ │ │ │ │ Unicellulars with │ Unicellulars with │ │ vegetal metabolism: │ animal metabolism: │ │ Carbon─assimilation. │ Albumin─assimilation. │ │ │ │ │ CHIEF GROUPS: │ CHIEF GROUPS: │ │ │ │ │ I. Phytomonera │ I. Zoomonera. │ │ │ │ │Protophyta without nucleus │ Protozoa without nucleus │ │ (monera) │ (monera). │ │ Chromacea │ Bacteria. │ │ │ │ │ II. Algariæ. │ II. Sporozoa. │ │ │ │ │Unicellular algæ with │ Nucleated protozoa without │ │nucleus, without ciliary │ mobile processes: Gregarinæ, │ │motion: Paulotomea, │ chytridinæ. │ │diatomea. │ │ │ │ │ │ III. Algettæ. │ III. Rhizopoda. │ │ │ │ │Unicellular algæ with │ Nucleated protozoa with │ │nucleus, and with ciliary │ pseudopodia: Labosa, │ │motion: Mastigota, │ radiolaria. │ │melthallia, siphonea. │ │ │ │ IV. Infusoria. │ │ │ │ │ │ Nucleated protozoa with │ │ │ cilia or lashes: │ │ │ Flagellata, ciliata. │ └───────────────────────────┴──────────────────────────────┘

┌────────────────────────────────────────────────────────────┐ │ Second organic kingdom: MULTICELLULAR, histona. │ │ │ │ Organisms which are only unicellular at the │ │ beginning of their existence, are later │ │ multicellular, and always form real tissues │ │ (_histobia_) by the firm conjunction of social cells. │ │ │ │ Sub─kingdom of the histona. │ ├────────────────────────────┬───────────────────────────────┤ │ C. TISSUE PLANTS │ D. TISSUE ANIMALS │ │ (metaphyta). │ (metazoa). │ │ │ │ │ C. Character: │ D. Character: │ │ Plasmodomous. │ Phasmophagous. │ │ │ │ │ Multicellulars with │ Multicellulars with │ │ vegetal metabolism: │ animal metabolism: │ │ Carbon─assimilation. │ Albumin─assimilation. │ │ │ │ │ CHIEF GROUPS: │ CHIEF GROUPS: │ │ │ │ │ I. Thallophyta. │ I. Cœlenteria │ │ │ (cœlenterata). │ │ Thallus─plants. Metaphyta │ │ │ with thallus: Algæ, mycetæ │ Metazoa without body │ │ (fungi). │ cavity and anus: Gastræada. │ │ │ Sponges, cnidaria, platodes. │ │ II. Mesophyta. │ │ │ │ II. Cœlomaria │ │ Median plants, with │ (bilaterals). │ │ prothallium: Mosses, ferns │ │ │ (muscinæ filicinæ). │ Metazoa with body cavity │ │ │ and anus (generally also │ │ │ blood─vessels). Vermalia, │ │ III. Anthophyta │ mollusca, echinoderma, │ │ (phanerogams). │ articulata, tunicata, │ │ │ vertebrata. │ │ Flowering plants, with │ │ │ blooms and seeds │ │ │ (spermophyta): │ │ │ Gymnosperms, angiosperms. │ │ │ │ │ │ │ │ │ │ │ │ │ │ └────────────────────────────┴───────────────────────────────┘

IX

MONERA

The simplest forms of life--Cell theory and cell dogma--Precellular organisms: monera, cytodes, and cells--Actual monera--Chromacea (cyanophyceæ)--Chromatophora--Cœnobia of chromacea: vital phenomena--Bacteria--Relations of the bacteria to the chromacea, the fungi, and the protozoa--Rhizomonera (protamœba, protogenes, protomyxa, bathybius)--Problematic monera--Phytomonera (plasmodoma) and zoomonera (plasmophaga)--Transition between the two classes.

In the study and explanation of all complex phenomena the first thing to do is to understand the simple parts, the manner of their combination, and the development of the compound from the simple. This principle applies generally to inorganic objects, such as minerals, artificially constructed machines, etc. It is also of general application in biological work. The efforts of comparative anatomy are directed to the comprehension of the intricate structure of the higher organisms from the rising scale of organization and life in the lower, and the origin of the former by historical development from the latter. The modern science of the cell (cytology), which has in a short time attained a considerable rank, pursues a method in opposition to this principle. The intricate composition of the unicellular organism, in many of the higher protists (such as the ciliata and infusoria) and many of the higher tissue-cells (such as the neurona) has led to the erroneous ascription of a highly complex organization to the cell in general. One would be justified in saying that of late the cell-theory has established itself in the dangerous and misleading position of a cell-dogma.

The modern treatment of the science, as we find it in numbers of recent works, even in some of the most distinguished manuals, and which we must resent on account of its dogmatism, culminates in something like the following theses:

1. The nucleated cell is the general elementary organism; all living things are either unicellular, or made up of a number of cells and tissues.

2. This elementary organism consists of at least two different organs (or, more correctly, organella), the internal nucleus and the outer cell-body (or cytoplasm).

3. The matter in each of these cell-organs--the caryoplasm of the nucleus and the cytoplasm of the body--is never homogeneous (or consisting of a chemical substratum), but always "organized," or made up of several chemically and anatomically different elementary constituents.

4. The plasm (or protoplasm) is, therefore, a morphological, not a chemical, unity.

5. Every cell comes (and has come) only from a mother-cell, and every nucleus from a mother-nucleus (_omnis cellula e cellula--omnis nucleus e nucleo_).

These five theses of the modern cell-dogma are by no means sound; they are incompatible with the theory of evolution. I have, therefore, consistently resisted them for thirty-eight years, and consider them to be so dangerous that I will briefly give my reasons. First, let us clearly understand the modern definition of the cell. It is now generally defined (in accordance with the second thesis) as being composed of two essentially different parts, the nucleus and the cell-body, and it is added that these organella differ constantly both in respect of chemistry, morphology, and physiology. If that is really so, the cell cannot possibly be the primitive organism; if it were, we should have a miracle at the beginning of organic life on the earth. The theory of natural evolution clearly and distinctly demands that the cell (in this sense) is a secondary development from a simpler, primary, elementary organism, a homogeneous cytode. There are still living to-day very simple protists which do not tally with this definition, and which I designated _monera_ in 1866. As they must necessarily have preceded the real cells, they may also be called "precellular organisms."

The earliest organisms to live on the earth, with which the wonderful drama of life began, can, in the present condition of biological science, only be conceived as homogeneous particles of plasm--biogens or groups of biogens, in which there was not yet the division of nucleus and cell-body which characterizes the real cell. I gave the name "cytodes" to these unnucleated cells in 1866, and joined them with the real nucleated cells under the general head of "plastids." I also endeavored to prove that such cytodes still exist in the form of independent monera, and in 1870 I described in my _Monograph on the Monera_ a number of protists which do not tally with the above definition.

Fifty years ago I made the first careful observations of living monera (_protamœba_ and _protogenes_), and described them in my _General Morphology_ (vol. i., pp. 133-5; vol. ii., p. xxii.) as structureless organisms without organs and the real beginnings of organic life. Soon afterwards, during a stay in the Canary Islands, I succeeded in following the continuous life-history of a related organism of the rhizopod type, which behaved like a very simple mycetozoon, but differed in having no nucleus; I have reproduced the picture of it in the first plate of my _History of Creation_. The description of this orange-red globule of plasm (_protomyxa aurantiaca_) appeared first in my _Monograph on the Monera_. Most of the organisms which I comprised under this name exhibited the same movements as true rhizopods (or sarcodina). It was afterwards proved of some of them that there was a nucleus hidden within the homogeneous particle of plasm, and that, therefore, they must be regarded as real cells. But this discovery was wrongly extended to the whole of the monera, and the existence of unnucleated organisms was denied altogether. Nevertheless, there are living to-day several kinds of these organisms without organs, some of them being very widely distributed. The chief examples are the chromacea and the bacteria, the former with vegetal and the latter with animal metabolism (or the former plasmodomous = plasma-forming, and the latter plasmophagous = plasma-feeding). On the ground of this important chemical difference, I distinguished two principal groups of the monera in my _Systematic Phylogeny_ twenty years ago--the phytomonera and the zoomonera, the former being unnucleated protophyta and the latter unnucleated protozoa.

Among living organisms the chromacea are certainly the most primitive and the nearest to the oldest inhabitants of the earth. Their simplest forms, the chroococcacea, are nothing but small structureless particles of plasm, growing by plasmodomism (formation of plasm) and multiplying by simple cleavage as soon as their growth passes a certain limit of individual size. Many of them are surrounded by a thin membrane or somewhat thicker gelatinous covering, and this circumstance had prevented me for some time from counting the chromacea as monera. However, I became convinced afterwards that the formation of a protective cover of this kind around the homogeneous particle of plasm may indeed be regarded from the physiological stand-point as a "purposive" structure, but at the same time may be looked upon, from the purely physical stand-point, as a result of superficial strain. On the other hand, the physiological character of these plasmodomous monera is especially important, as it gives us the simple key to the solution of the great question of spontaneous generation (or archigony, _cf._ chapter xv.).

The chromacea are to-day found in every part of the earth, living sometimes in fresh water and sometimes in the sea. Many species form blue-green, violet, or reddish deposits on rocks, stones, wood, and other objects. In these thin gelatinous plates millions of small homogeneous cytodes are packed close together. Their tint is due to a peculiar coloring matter (phycocyan), which is chemically connected with the substance of the plasma-particle. The shade of this color differs a good deal in the various species of chromacea (of which more than eight hundred have been distinguished); in the native species it is generally blue-green or sage-green, sometimes blue, cyanine blue, or violet. Hence the common name cyanophyceæ (_i.e._, blue algæ). It is incorrect, for two reasons; firstly, because only a part of these protophyta are blue, and, secondly, because they (as simple, primitive plants without tissue) must be distinguished from the real algæ (phyceæ), which are multicellular, tissue-forming plants. Other chromacea are red, orange, or yellow in color, as the interesting _trichodesmium erythræum_, for instance, the flaky masses of which, gathering in enormous quantities, cause at certain times the yellow or red coloring of the sea-water in the tropics; it is these that are responsible for the name "Red Sea" on the Arabian and "Yellow Sea" on the Chinese coast. When I passed the equator in the Sunda Straits on March 10, 1901, the boat sailed through colossal accumulations, several miles in width, of this trichodesmium. The yellow or reddish surface of the water looked as if it were strewn with sawdust. In the same way, the surface of the Arctic Ocean is often colored brown or reddish-brown by masses of the brown _procytella primordialis_ (formerly described as _protococcus marinus_).

It is clearly quite illogical to regard the chromacea as a class or family of the algæ, as is still done in most manuals of botany. The real algæ--excluding the unicellular diatomes and paulotomes, which belong to the protophyta--are multicellular plants that form a _thallus_ or bed of a certain form and characteristic tissue. The chromacea, which have not advanced as far as the real nucleated cell, are unnucleated cytodes of a lower and earlier stage of plant-life. If one would compare the chromacea with algæ or other plants at all, the comparison cannot be with their constituent cells, but merely with the chromatophora or chromatella, which are found in all green plant-cells, and form _part_ of their contents. To be more precise, these green granules of chlorophyll must be regarded as organella of the plant-cell, or separated plasma-formations which arise beside the nucleus in the cytoplasm. In the embryonic cells of the germs of plants and in their vegetation points the chromatophora are as yet colorless, and are developed, as solid, very refractive, globular, or roundish granules, from the firm layer of plasm which immediately surrounds the nucleus. Afterwards they are converted, by a chemical process, into the green chlorophyll granules or chloroplasts, which have the most important function in the plasmodomism or carbon-assimilation of the plant.

The fact that the green chlorophyll granules grow independently within the living plant-cell and multiply by segmentation is very important and interesting. The globular chloroplasts are constricted in the middle, and split into two equal daughter-globules. These daughter-plastids grow, and multiply in turn in the same way. Hence they behave within the plant-cell just like the free-living chromacea in the water. On the strength of this significant comparison, one of our ablest and most open-minded scientists, Fritz Müller-Desterro, of Brazil, pointed out in 1893 that we may see in every green vegetal cell a symbiosis between plasmodomous green and plasmophagous not-green companions (_cf._ my _Anthropogeny_, figs. 277 and 278, and in the text).

Many species of the simplest chromacea live as monobia (individually). When the tiny plasma globules have split into two equal halves by simple segmentation, they separate, and live their lives apart. This is the case with the common, ubiquitous chroococcus. However, most species live in common, the plasma granules forming more or less thick cœnobia, or communities or colonies of cells. In the simplest case (_aphanocapsa_) the social cytodes secrete a structureless gelatinous mass, in which numbers of blue-green plasma globules are irregularly distributed. In the _glœocapsa_, which forms a thin blue-green gelatinous deposit on damp walls and rocks, the constituent cytodes cover themselves immediately after cleavage with a fresh gelatinous envelope, and these run together into large masses. But the majority of the chromacea form firm, threadlike cell communities or chains of plastids (catenal cœnobia.) As the transverse cleavage of the rapidly multiplying cytodes always follows the same direction, and the new daughter-cytodes remain joined at the cleavage surfaces, and are flattened into discoid shape, we get stringlike formations or articulated threads of considerable length, as in the oscillaria and nostochina. When a number of these threads are joined together in gelatinous masses, we often get large, irregular, jelly-like bodies, as in the common "shooting-star jellies" (_nostoc communis_). They attain the size of a plum.

In view of the extreme importance which I attach to the chromacea as the earliest and simplest of all organisms, it is necessary to put clearly the following facts with regard to their anatomic structure and physiological activity:

1. The organism of the simplest chromacea is _not_ composed of different organella or organs; and it shows no trace of purposive construction or definite architecture.

2. The homogeneous tinted plasma granule which makes up the entire organism in the simplest case (_chroococcus_) exhibits no plasma structure (honeycomb, threads, etc.) whatever.

3. The original globular form of the plasma particle is the simplest of all fundamental types, and is also that assumed by the inorganic body (such as a drop of rain) in a condition of stable equilibrium.

4. The formation of a thin membrane at the surface of the structureless plasma granule may be explained as a purely physical process--that of surface strain.

5. The gelatinous envelope which is secreted by many of the chromacea is also formed by a simple physical (or chemical) process.

6. The sole essential vital function that is common to all the chromacea is self-maintenance, and growth by means of their vegetal metabolism, or plasmodomism (=carbon assimilation); this purely chemical process is on a level with the catalysis of inorganic compounds (chapter x.).

7. The growth of the cytodes, in virtue of their continuous plasmodomism, is on a level with the physical process of crystal growth.

8. The reproduction of the chromacea by simple cleavage is merely the continuation of this simple growth process, when it passes the limit of individual size.

9. All the other vital phenomena which are to be seen in some of the chromacea can also be explained by physical or chemical causes on mechanical principles. Not a single fact compels us to assume a "vital force."

Especially noteworthy in regard to the physiological character of these lowest organisms are their bionomic peculiarities, especially the indifference to external influences, higher and lower temperatures, etc. Many of the chromacea live in hot springs, with a temperature of fifty to eighty degrees centigrade, in which no other organism is found. Other species may remain for a long time frozen in ice, and resume their vital activity as soon as it thaws. Many chromacea may be completely dried up, and then resume their life if put in water after several years.

Next in order to the chromacea we have the bacteria, the remarkable little organisms which have been well known in the last few decades as the causes of fatal diseases, and the agents of fermentation, putrefaction, etc. The special science which is concerned with them--modern bacteriology--has attained so important a position in a short period--especially as regards practical and theoretical medicine--that it is now represented by separate chairs at most of the universities. We may admire the penetration and the perseverance with which scientists have succeeded, with the aid of the best modern microscopes and methods of preparation and coloring, in making so close a study of the organism of the bacteria, determining their physiological properties, and explaining their great importance for organic life by careful experiments and methods of culture. The bionomic or economic position of the bacteria in nature's household has thus secured for these tiny organisms the greatest scientific and practical interest.

However, we find that certain general views have been maintained by specialists in bacteriology up to our own time which are in curious contrast with these brilliant results. The biologist who studies the systematic relations of the bacteria from the modern point of view of the theory of descent is bewildered at the extraordinary views as to the place of the bacteria in the plant-world (as segmentation-fungi), their relations to other classes of plants, and the formation of their species. When we carefully consider the morphological properties that are common to all true bacteria and compare them with other organisms, we are forced to the conclusion that I urged years ago in various writings: the bacteria are not real (nucleated) cells, but unnucleated cytodes of the rank of the monera; they are not real (tissue-forming) fungi, but simple protists; their nearest relatives are the chromacea.

The individual organisms of the simplest kind, which bacteriologists call "bacteria-cells," are not real nucleated cells. That is the clear negative result of a number of most careful investigations which have been made up to date with the object of finding a nucleus in the plasma-body of the bacteria. Among recent exact investigations we must especially note those of the botanist Reinke, of Kiel, who sought in vain to detect a nucleus in one of the largest and most easily studied genera of the bacteria, the _beggiatoa_, using every modern technical aid. His conviction that this important cell-structure is really lacking is the more valuable, as it is very prejudicial to his own theory of "dominants." Other scientists (especially Schaudinn) have recently claimed, as equivalent to a nucleus in some of the larger bacteria, a number of very small granules, which are irregularly distributed in the plasm, and are strongly tinted under certain coloring processes. But even if the chemical identity of these substances which take the same color were proved--which is certainly not the case--and even if the appearance of scattered nuclein-granules in the plasm could be regarded as a preliminary to, or a beginning of, the differentiation of an individual, morphologically distinct nucleus, we should not yet have shown its independence as an organellum of the cell.

Nor is this any more proved from the circumstance that in some bacteria (not all) we find a severance of the plasm into an inner and outer layer, or a frothy structure with vacuole-formation, or a special sharply outlined membrane on the plastid. Many bacteria (but not all) have such a membrane, like the nearly related chromacea, and also the secretion of a gelatine envelope. Both classes have also in common an exclusively monogenetic reproduction. The bacteria multiply, like the chromacea, by simple segmentation; as soon as the structureless plasma-granule has reached a certain size by simple growth, it is constricted and splits into two halves. In the long-bodied bacteria (the rod-shaped bacilli) the constriction always goes through the middle of the long axis, and is, therefore, simple transverse cleavage. Many bacteria have also been said to multiply by the formation of spores. But these so-called "spores" are really permanent quiescent forms (without any multiplication of individuals); the central part of the plastid (endoplasm) condenses, separates from the peripheral part (exoplasm), and undergoes a chemical change which makes it very indifferent to external influences (such as a high temperature).

The great majority of the bacteria differ so little morphologically from the chromacea that we can only distinguish these two classes of monera by the difference in their metabolism. The chromacea, as protophyta, are plasmodomous. They form new plasm by synthesis and reduction from simple inorganic compounds--water, carbonic acid, ammonia, nitric acid, etc. But the bacteria, as protozoa, are plasmophagous. They cannot, as a rule, form new plasm, but have to take it from other organisms (as parasites, saprophytes, etc.); they decompose it by analysis and oxydation. Hence the colorless bacteria are without the important green, blue, or red coloring matter (phycocyan) which tints the plastids of the chromacea, and is the real instrument of the carbon-assimilation. However, there are exceptions in this respect: _bacillus virens_ is tinted green with chlorophyll, _micrococcus prodigiosus_ is blood-red, other bacteria purple, and so on. Certain earth-dwelling bacteria (_nitro-bacteria_) have the vegetal property of plasmodomism; they convert ammonia by oxydation into nitrous acid, and this into nitric acid, using as their source of carbon the carbonic acid gas in the atmosphere. They are thus quite independent of organic substances, and feed, like the chromacea, on simple inorganic compounds.

Hence the affinity between the plasmodomous chromacea and plasmophagous bacteria is so close that it is impossible to give a single safe criterion that will effectually separate the two classes. Many botanists accordingly combine both groups in a single class with the name of _schizophyta_, and within this distinguish as "orders" the blue-green chromacea as _schizophycæ_ (cleavage-algæ) and the colorless bacteria as _schizomycetes_ (cleavage-fungi). However, we must not take this division too rigidly; and the absolute lack of a nucleus and tissue-formation separates the chromacea just as widely from the multicellular tissue-forming algæ as the bacteria from the fungi. The simple multiplication by the halving of the cell, which is expressed in the name "cleavage-plants" (_schizophyta_), is also found in many other protists.

The number of forms that can be distinguished as species in the technical sense is very great in the case of the bacteria, in spite of the extreme simplicity of their outward appearance; many biologists speak of several hundred, and even of more than a thousand, species. But when we look solely to the outer form of the living plasma-granule, we can only distinguish three fundamental types: (1) Micrococci, or spherobacteria (briefly, cocci), globular or ellipsoid; (2) bacilli, or rhabdo-bacteria (also called eubacteria, or bacteria in the narrower sense), rod-shaped, cylindrical, and often twisted like worms (comma-bacilli); (3) spirilla, or spirobacteria, screw-shaped rods (vibriones when the screw is slight, and spirochæta when it has many coils). Besides this threefold difference in the forms of the cytodes, we have a ground of distinction in many bacilli and spirilla in the possession of one or more very thin lashes (flagella), which proceed from one of both poles of the lengthened plastid. The construction and vibration of these serves for locomotion in the swimming bacteria; but they are only found for a time in many species, and in many others are altogether wanting.

Since, then, neither the simple outer form of the bacterium-cytodes nor their homogeneous internal structure provides a satisfactory ground for the systematic distinction of the numerous species, their physiological properties are generally used for the purpose, especially their different behavior towards organic foods (albumin, gelatine, etc.), their chemical actions, and the various effects of poisoning and decomposition which they produce in the living organism. No bacteriologist now doubts that all the vital activities of the bacteria are of a chemical nature, and precisely on this account these microbes are of extreme importance. When we bear in mind how complicated are the relations of the various species of bacteria to the tissues of the human body, in which they cause the diseases of typhus, hypochondriasis, cholera, and tuberculosis, we are bound to admit that the real cause of these maladies must be sought in the peculiar molecular structure of the bacterium-plasm, or the particular arrangement of its molecules and the innumerable atoms (more than a thousand) which are, in a very loose way, made up into special groups of molecules. The chemical products of their mutual action are what we call ptomaines, which are partly very virulent poisons (toxins). We have succeeded in producing several of these poisonous matters in large quantities by artificial culture, and isolating them and experimentally ascertaining their nature; as, for instance, tetanin, which causes tetanus, typhotoxin, the poison of typhus, etc.

In thus declaring the action of bacteria to be purely chemical and analogous to that of well-known inorganic poisons, I would particularly point out that this very justifiable statement is a pure hypothesis; it is an excellent illustration of the fact that we cannot get on in the explanation of the most important natural phenomena without hypotheses. We can see nothing whatever of the chemical molecular structure of the plasm, even under the highest power of the microscope; it lies far below the limit of microscopic perception. Nevertheless, no expert scientist has the slightest doubt of its existence, or that the complicated movements of the sensitive atoms and the molecules and groups of molecules they make up are the causes of the vast changes which these tiny organisms effect in the tissues of the human and the higher animal body.

Moreover, the distinction of the many species of bacteria is of interest in connection with the general question of the nature and constancy of a species. Whereas formerly in biological classification only definite morphological characters, or definable differences in outer form or inner structure, were regarded as of any moment in the distinction of species, here, in view of the vagueness or total lack of these characters, we have to look mainly to the physiological properties, and these are based on the chemical differences in their hypothetical molecular structure. But even these are not absolutely constant; on the contrary, many bacteria lose their specific qualities by progressive culture under changed food-conditions. By a change in the temperature and the nutritive field in which a number of poisonous bacteria have been reared, or by the action of certain chemicals, not only the growth and multiplication are altered, but also the injurious effect they have on other organisms by the generation of poisons. This poisonous effect is weakened, and--what is most important--the weakening is transmitted by heredity to the following generations. On this is based the familiar process of inoculation, an admirable example of the inheritance of acquired characteristics.

As the bacteria are still often described as "cleavage-fungi" and classified along with the real fungi, we must particularly point out the wide gulf that separates the two groups. The real fungi (or _mycetes_) are metaphyta, their multicellular body (_thallus_) forming a very characteristic sort of tissue, the mycelium; this is composed of a number of interlaced and interwoven threads (or hyphens). Each fungus-thread consists of a row of lengthened cells, which have a thin membrane and enclose a number of small nuclei in the colorless plasm. Moreover, the two sub-classes of the real fungi, the ascomycetes and basimycetes, form peculiar fruit-bodies which generate spores (ascodia and basidia). There is no trace whatever of these real characteristics of the true fungus in the bacteria. Nor is it less incorrect to class them with the fungilli, the so-called unicellular fungi or phycomycetes (ovomycetes and zygomycetes); these form a special class of protists which has the closest affinity to the gregarinæ.

Like the closely related chromacea, many of the bacteria show a marked tendency to form communities or cell-colonies. These cell-communities arise, as elsewhere, from the fact that the individuals, which multiply rapidly by continuous cleavage, remain joined together. This may happen in two ways. When the social bacteria secrete large quantities of gelatine, and remain distributed in this, we have the _zooglœa_ (as in the case of the _aphanocapsa_ and _glœocapsa_ among the chromacea). If, on the other hand, the long-bodied bacilli remain fastened together in rows, we get the knotted threads of _leptothrix_ and _beggiatoa_ (which may be compared with the oscillaria). And, if these threads go into branches, we have _cladothrix_. Other cœnobia of bacteria have the appearance of disks, the cytodes dividing in a plane, usually in groups of four (as in _merismopedia_), or of cube-shaped packets when they are in all three directions of space (_sarcina_).

The two classes of bacteria and chromacea seem, in the present condition of our knowledge, on account of their simple organization, to be the simplest of all living things, real monera, or organisms without organs. Hence we have to put them at the lowest stage of the protist kingdom, and must regard the difference between them and the most highly differentiated unicellular beings (such as the radiolaria, ciliated infusoria, diatomes, or siphonea) as no smaller than the difference (in the realm of the histona) between a lower polyp (_hydra_) and a vertebrate, or between a simple alga (_ulva_) and a palm. But if the kingdom of the protists is badly divided, on the older rule, into a plant kingdom and an animal kingdom, the only discriminating mark we have left is the difference in metabolism; in that case we have to include the plasmophagous bacteria in the animal kingdom (as Ehrenberg did in 1838) and the plasmodomous chromacea in the plant kingdom. The remarkable class of the flagellata, which includes ciliated unicellulars of both groups, contains several forms which are only distinguished from the typical bacterium by the possession of a nucleus. If it is true that in some of the protists which were counted as bacteria a real nucleus has been detected, these must be separated from the others (unnucleated) and included in the nucleated flagellata.

The monera which I described in 1866, and on which I based the theory of the monera in my monograph, belong to a different division of the protists from the classes of bacteria and chromacea. These are the forms which I described as _protamœba_, _protogenes_, _protomyxa_, etc. Their naked mobile plasma-bodies thrust out pseudopodia, or variable "false feet," from their surface, like the (nucleated) real rhizopods (=sarcodinæ); but they differ essentially from the latter in the absence of a nucleus. Afterwards (in my _Systematic Phytogeny_) I proposed to separate these unnucleated rhizopods from the others, giving the name of _lobomonera_ (_protamœba_) to the amœba-like monera with flap-shaped feet, and the name of _rhizomonera_ (_protomyxa_, _pontomyxa_, _biomyxa_, _arachnula_, etc.) to the gromia-like, root-feet forming monera. However, of late years, real nuclei have been detected in each of these large monera, and so they have been proved to be true cells. This discovery was made possible by the improved modern methods of coloring the nucleus which I had not the use of thirty years ago in my first observations. On the strength of these recent discoveries many scientists claim that all the monera I described are true cells, and must have nuclei. This baseless assertion is much employed by the opponents of the theory of evolution in order to deny the existence of the monera altogether.

Of the genus of monera which we call protamœba I have given an illustration in my _History of Creation_ (tenth edition), which has been frequently reproduced. Several species (at least two or three) of this genus still exist, and are distinguished by the shape of their flap-formation and their method of motion. They resemble ordinary simple amœbæ, and only differ from these to any extent in the absence of a nucleus. The _protamœba primitiva_ seems to be pretty widely distributed; it has been found repeatedly by observers (Gruber, Cienkowski, Leidy, etc.) in inland waters. In the zoological demonstrations which I have given at the University of Jena for forty years, and in the course of which the lowly inhabitants of our fresh water are regularly examined with the microscope, the _protamœba primitiva_ has been found four or five times. It always had the same form, as I described it, moved about by the slow formation of flaps at its surface, multiplied by simple cleavage, and showed no trace of a nucleus in its homogeneous plasma-body even with the most careful application of the modern methods of tinting the nucleus. A larger number of very fine granules (microsoma) that were irregularly distributed in the plasm, and were more or less colored by nucleus-reagents, cannot be reckoned as clear equivalents of the nucleus in this or in similar cases; they are probably products of metabolism. The same may be said of the larger marine form of rhizomoneron, which A. Gruber has recently called _pelomyxa pallida_.

The large marine form of rhizomoneron to which Huxley gave the name of _bathybius Haeckelii_ in 1868, and as to the real nature of which many opinions have been expressed, seems, according to the latest investigation, not to have the significance ascribed to it. However, the much-discussed question of the bathybius is superfluous as far as our monera theory and the associated hypothesis of archigony (chapter xv.) are concerned, since we have now a better knowledge of the much more important monera-forms of the chromacea and bacteria.

In the case of some of the protists I described in my _Monograph on the Monera_, it is at present doubtful whether their plasma-body contains a nucleus or not, and, therefore, whether they are to be classed as true cells or cytodes. This applies especially to the forms which only happened to come under observation once, such as _protomyxa_ and _myxastrum_. In these obscure cases we must wait for fresh investigations and the application of the modern methods of tinting the nucleus. I may, however, point out, in passing, that these famous methods of nucleus-coloring give by no means the absolute certainty which is ascribed to them; there are other substances which take color in the same way as chromatin. As far as my monera theory is concerned, or the great general importance which I attach to these unnucleated living granules of plasm, it does not matter whether a nucleus is detected in these problematic monera or not. The chromacea alone--the most important of all monera--completely suffice to provide a base for the far-reaching theoretical conclusions which I draw from it.

At the close of these observations on the monera I will briefly recapitulate the weighty inferences which we can deduce from their simple organization. They serve as a solid foundation for the chief theses of our monistic biology; and they are inconsistent with the dualistic views of modern vitalists. In the first place, I emphasize the fact that the structureless plasm-body of the simple monera has no sort of organization and no composition from dissimilar parts co-operating for definite vital aims. Reinke's conscious "dominant"--as well as Weismann's mechanical "determinants"--have nothing to do here. The whole vital activity of the simplest monera, especially of the chromacea, is confined to their metabolism, and is therefore a purely chemical process, that may be compared to the catalysis of inorganic compounds. The simple formation of individuals in this primitive living matter is merely a question of the cleavage of plasma globules of a certain size (_chroococcus_); and their primitive multiplication (by simple self-division) is only a continued growth (analogous to that of the crystal). When this simple growth passes a certain limit, that is fixed by the chemical constitution, it leads to the independent existence of the redundant growth-products.

X

NUTRITION

Functions of nutrition--Assimilation and disassimilation--Plasmodoma and plasmophaga--Phytoplasm and zooplasm--Plasmodomism of plants--Chlorophyll granules and nitro-bacteria--Plasmophagism of fungi and animals--Metasitism (conversion of metabolism)--Nutrition of the monera (chromacea, bacteria, rhizomonera)--Nutrition of the protophyta and metaphyta (cell-plants and tissue-plants)--Nutrition of the metazoa--- Gastræa theory--Gastro-canal system of the cœlenteria (gastræads, sponges, cnidaria, platodes)--Nutrition of the cœlomaria (digestion, circulation, respiration, evacuation)--Saprositism--Parasitism--Symbiosis.

The wonder of life which we call, in the widest sense of the word, "nutrition" is the chief factor in the self-maintenance of the organic individual. It is always bound up with a chemical modification of the living matter, an organic metabolism (circulation of matter), and a corresponding circulation of force. In this chemical process plasm is used up, built up afresh, and once more disintegrated. The metabolism which lies at the root of this chemistry of food is the essential feature in the manifold processes of nutrition. A large part of the several nutritive processes are explained without further trouble by the known physical and chemical properties of inorganic bodies; for another part of them we have not yet succeeded in doing this. Nevertheless, all impartial physiologists now agree that it is possible in principle, and that we have no reason to introduce a special vital principle. All the trophic (nutritive) processes, without exception, are subject to the law of substance.

In all the higher plants and animals the chemical process of metabolism, with the stream of energy that accompanies it, is a very complex vital activity, in which many different functions and organs co-operate with the common aim of self-maintenance. As a rule, they are distributed in four groups--namely: (1) Intussusception of food and digestion: (2) distribution of the food in the body, or circulation; (3) respiration, or exchange of gases; and (4) excretion of unusable matter. In most of the histona, either tissue-plants or tissue-animals, a number of organs are differentiated for the accomplishment of these tasks. At the lower stages of life this division of labor is not found, the entire process of nutrition being accomplished by a single layer of cells (lower algæ, gastræads, sponges, lower polyps). In the protists, again, it is the single cell that does all these things itself; in the simplest cases, the monera, a homogeneous plasma-globule. As a long gradation uninterruptedly unites these lowest forms of nutrition with the more complicated forms, we must regard the latter no less than the former as physico-chemical processes.

When we take the whole of the metabolic functions in organisms together, we may look upon them as the outcome of two opposite chemical processes--on the one hand the building-up of living matter by taking in food (assimilation), and on the other the breaking-down of it in consequence of its vital activity (disassimilation). As in every case the plasm is the active living matter, we may say: _Assimilation_ (or plasma-production) consists in the conversion within the organism into the special plasm of the particular species of food that has been received from without; _disassimilation_ (or plasma-destruction) is the result of the work done by the plasm, which is the cause of its partial decomposition or breakdown. In both respects there is a striking difference between the two great kingdoms of organic nature. The plant kingdom is, on the whole, the agent of assimilation, forming new plasm by synthesis and reduction from inorganic matter. In the animal world, on the contrary, disassimilation preponderates, the plasm received being resolved by oxydation, and the actual energy taken out of it by analysis being converted into heat and motion. Plants are plasmodomous; animals, plasmophagous.

Of all the chemical processes the most important, because the most indispensable, for the origin and maintenance of organic life is the constant reconstruction of plasm. We give it the name of plasmodomism (_domeo_=to build up), or carbon-assimilation. Botanists have the habit of late of calling it briefly assimilation, and have thus caused a good deal of misunderstanding. The more common and older meaning of assimilation in animal physiology is, in the widest sense, the intussusception and preparation of the food received. But the carbon-assimilation in plants--what I call plasmodomism--is only the first and original form of plasma-production. It means that the plant is able, under the influence of sunlight, to form carbohydrates, and from these new plasm, out of simple inorganic compounds (water, carbonic acid, nitric acid, and ammonia) by synthesis and reduction. The animal is unable to do this. It has to take its plasm in its food from other organisms--plant-eaters directly, and animal-eaters indirectly. We therefore give the title of _plasmophagous_ to these animal "plasma-eaters." In working up the foreign plasm it has eaten, and converting it into its own specific form of plasm, the animal also accomplishes assimilation; but this animal albumin-assimilation is totally different from the vegetal carbon-assimilation. The fresh-formed animal plasm is then broken up by oxydation, and by this analysis the energy needed for the vital movements is obtained.

The physiological contrast which we thus find between the two principal forms of living matter, the synthetic plasm of the plant and the analytic plasm of the animal, is of great importance for the lasting maintenance of the whole organic world. It depends on a reversal of the molecular movement in the plasm, the intimate nature of which is just as little known to us as the chemical constitution of the albumins in general, and that of living albumin, the plasm, in particular. As I mentioned in chapter v., modern physiological chemistry has good reason to believe that the invisible albumin-molecule is, comparatively speaking, gigantic, and is composed of more than a thousand atoms. These are in such an unstable equilibrium, so complicated and impermanent an arrangement, that the slightest push or stimulus suffices to alter them and form a new kind of plasm. As a fact, the number and variety of kinds of plasm are immense. This is seen at once from the ontogenetic fact that the ovum and sperm-cell of each species (and each variety) have a specific chemical constitution. In reproduction this is transmitted to the offspring. But, setting aside these countless finer modifications, we may distinguish two chief groups of kinds of plasm: the phytoplasm of the plant, with the synthetic property of plasmodomism, and the zooplasm of the animal, which is destitute of this property, and so confined to plasmophagy.

The remarkable synthetic process of building up the plasm, to which we give the name of plasmodomism, or carbon-assimilation, usually needs as its first condition the radiant energy of sunlight. Every green plant-cell contains in its chlorophyll-granules so many tiny laboratories, their green plasm being able to form new plasm out of inorganic compounds under the influence of light. The water that is needed for this, besides nitrogenous compounds (nitric acid, ammonia), is drawn from the earth by the roots; the carbonic acid is taken from the atmosphere by the green leaves. The immediate products of the synthesis, due to the separation of the carbonic acid, is, as a rule, a non-nitrogenous starch-flour (_amylum_). This is further used for the composition of the nitrogenous albumin by an as yet unknown synthetic process, with the aid of nitrogenous mineral compounds. In this process of reduction the separated free oxygen is returned to the atmosphere. The carbohydrates that chiefly co-operate in this are glucoses and maltoses: the mineral substances, especially salts of potassium and magnesium, and compounds of these elements with nitric acid, sulphuric acid, and phosphoric acid. Iron is also found to be an important element in the process, though in a very small quantity. As a rule, the ferruginous chlorophyll can only form new plasm with the help of light-waves. The most important part of the spectrum for this purpose is that containing the red, orange, and yellow waves.

The chief factor in plasma-formation in the organic world is the photosynthesis, or ordinary carbon-assimilation by chlorophyll, the wonderful green matter that amounts to only a very small percentage (about one-tenth) of the weight of the chlorophyll-granules, and can be separated from their plasmatic substance by certain methods. Even when the plant has some other color than green the chlorophyll is still the real plasmodomous substance. Its green color is then masked by some other color--diatomin in the yellow diatomes, phycorhodin in the red rhodophyceæ, phycophæin in the brown phæophyceæ, and phyocyan in the blue-green chromacea or cyanophyceæ. The latter have an especial interest for us, because in the simplest specimens the entire organism is merely a globular bluish-green granule of plasm. Moreover, in the simplest forms of nucleated primitive plants (_algariæ_)--many of the so-called unicellular algæ--the metabolism is effected by a single grain of chlorophyll. There is usually a large number of them in the plasm of the plant-cells.

Another kind of plasm-synthesis, quite different from the ordinary plasmodomism by chlorophyll and sunlight has lately been discovered in some of the lowest organisms (by Heraeus, Winogradsky, and others). The nitro-bacteria (or nitromonades) are tiny monera (unnucleated cells) that live in complete darkness underground. Their globular colorless plasma-bodies contain neither chlorophyll nor nucleus. They have the remarkable capacity of forming carbohydrates, and from these plasm, by a peculiar synthesis out of purely inorganic compounds--water, carbonic acid, ammonia, and nitric acid. Pfeffer has called this carbon-assimilation, on account of its purely chemical nature, "chemosynthesis," in opposition to the ordinary photosynthesis by means of sunlight. There are also other bacteria (sulphur-bacteria, purple-bacteria, etc.) that show various peculiarities of metabolism. The nitro-bacteria must belong to the oldest monera, and represent a transition from the vegetal chromacea to the animal bacteria.

The extensive class of the fungi (or _mycetes_) resembles a part of the bacteria in regard to metabolism. These organisms are, it is true, generally regarded as plants, but they have not the capacity of the green, chlorophyll-bearing plants to supply themselves with carbon from the carbonic acid in the atmosphere. They have to take it from organic substances, such as albumin, carbohydrates, etc., like the animals. But while the animals have to derive their nitrogen from the latter, the fungi can obtain it from inorganic matter in the earth. Fungi cannot support life without the addition of organic compounds; but we can make them grow in a food solution consisting of sugar and purely inorganic nitrogenous salts. Thus they are on the border that separates the plasmodomous plants from the plasmophagous animals. Like the latter, the fungi have evolved from the plants through changed food conditions. We find this process even among the unicellular protists in the phycomycetes, which descend from the siphonea. In the same way the real multicellular fungi (ascomycetes and basimycetes) may be traced to the tissue-forming algæ.

All true animals have to derive their food from the plant kingdom, the vegetal feeders directly, and the flesh feeders indirectly, when they consume vegetal feeders. Hence the animals are, in a certain sense, as the older natural philosophy put it four hundred years ago, "parasites of the plant world." From the point of view of phylogeny, the animal kingdom is, therefore, clearly much younger than the plant kingdom. The development of the animals from the plants was determined originally by a change in the method of nutrition which we call metasitism.

The chemical modification of the living matter which is connected with the loss of plasmodomism--in other words, the conversion of the reducing phytoplasm into oxidizing zooplasm--must be regarded as one of the most important changes in the history of organic life. This "reversal of metabolism" is polyphyletic; it has been repeated many times in the course of biological history, and has taken place independently in very different groups of the organic world--whenever a plasmodomous cell or group of cells (=tissue) had occasion to feed directly on ready-made plasm, instead of giving itself the trouble of building it up out of inorganic compounds. We see this particularly among the unicellular protists in the independent ciliated cells. The longer plasmophagous flagellata, which are colorless, and have no chlorophyll (monodina, conoflagellata), closely resemble in form and movement the older plasmadomous and chlorophyll-bearing mastigota, from which they are descended (volvocina, peridinia); they only differ in the manner of nutrition. The colorless flagellata feed on ready-formed plasm, which they obtain either by means of their lashes or by a special cell mouth in their cell body. On the other hand, their ancestors, the green or yellow mastigota, form new plasm by photosynthesis like true cells. But there are also complete intermediate forms between the two groups--for instance, the chrysomonades and the gymnodinia; these may behave alternately as protozoa or protophyta. In the same way we can derive the phycomycetes by metasitism from the siphonea, the fungi from the algæ; and, finally, the process is also found in many of the higher parasitic plants (orchids, orobanches, etc.). (See under "Parasitism.")

As is the case with every other vital function, so for the function of metabolism we find a starting-point in the lowest and simplest group of the protophyta, the chromacea. In their oldest forms, the chroococcacea, the whole body is merely a blue-green, structureless, globular plasma particle, growing by means of its plasmodomous power, and splitting up as soon as it reaches a certain stage of growth. There the miracle of life consists merely of the chemical process of plasmodomism by photosynthesis. The sunlight enables the blue-green phytoplasm to form new plasm of the same kind out of inorganic compounds (water, carbonic acid, ammonia, and nitric acid). We may look upon this process as a special kind of catalysis. In this case there is absolutely nothing to be done by Reinke's "dominants," or conscious and purposive vital forces. There are, as yet, no differentiated physiological functions in these organisms without organs, and no anatomically distinct members; and so their one vital activity, growth, may very well be compared to the simple growth of inorganic crystals.

It has been pointed out repeatedly that the remarkable monera which now play so important a part in biology as bacteria stand, in many respects, quite apart from the ordinary vital phenomena of the higher organisms. This is especially true of their metabolism, which has the most striking peculiarities. Morphologically, many of the bacteria cannot be distinguished from their nearest relatives and direct ancestors, the chromacea, differing from them only in the absence of coloring matter in the plasm. Many of them are simple, globular, ellipsoid, or rod-shaped plasma particles, without any visible organization or movement. Others move about by means of one or more very fine lashes (like the flagellata). No real nucleus can be discovered in the structureless plasma body. The very fine granules which are found in some species, and the vacuole-formation that we see in others, may be regarded as products of metabolism; and the same may be said of the thin membrane or the thicker gelatinous envelope which many of the bacteria secrete. This makes all the more remarkable the peculiarity of their chemical constitution and the metabolism determined thereby. The nitro-bacteria we have mentioned previously are plasmodomous; the anaërobe bacteria (of butyric acid and tetanus) only flourish where oxygen is excluded; the sulphur bacteria (_beggiatoa_) secrete--by the oxydation of sulphuretted hydrogen--pure regulation sulphur in the form of round granules. The ferruginous bacteria (_leptothrix ochrocea_) store up oxyhydrate of iron (by the oxydation of carbonic protoxide of iron). The saprogenetic bacteria cause putrefaction, and the zymogenetic fermentation. Finally, we have the very interesting pathogenetic bacteria which cause the most dangerous diseases by the secretion of special poisons--toxins--festering, small-pox, tetanus, diphtheria, typhus, tuberculosis, cholera, etc. On account of their great practical importance, these bacteria have of late been taken over by a special branch of biology, bacteriology. But only a few of the many experts in this department have pointed out the extreme theoretical significance which these zoomonera have for the important questions of general biology. These structureless plasma bodies show unmistakably that their vital activity is a purely chemical phenomenon. Their great variety proves how manifold and complicated must be the molecular composition of the plasm, even in these simplest organisms.

The unicellular protophyta exhibit the same form of metabolism and plasmodomism as the familiar green cells of the tissue-plants; but in most of the protozoa we find special features of nutrition and plasmophagy. The great class of the rhizopods is distinguished by the fact that their naked plasma body can take in ready-formed solid food at any point of its surface. On the other hand, most of the infusoria have a definite mouth-opening in the outer wall of their unicellular body, and sometimes a gullet-tube as well. Besides this cell-mouth (_cytostoma_) we usually find also a second opening for the discharge of indigestible matter, a cell-anus (_cytopyge_).

Metabolism in the tissue plants (metaphyta) forms a long gradation from very simple to very complicated arrangements. The lowest and oldest thallophyta, especially the simplest algæ, are not far removed from the communities of protophyta, and, like these, are merely definitely grouped colonies of cells. The social cells which form their most rudimentary tissue are quite homogeneous, with no differentiation beyond that of sex. The thallus or bed-formation consists in the simplest specimens of plain or branched fine threads, consisting of rows or chains of homogeneous cells (so _conferva_ among the green, _ectocarpus_ among the brown, and _callithamnion_ among the red algæ). Other algæ (such as the ulva) form thin leaf-shaped forms of the thallus, a number of homogeneous cells lying side by side along a level. In the larger algæ compact tissue-bodies are formed, in which frequently firmer rows of cells exhibit the rudiments of fibres; and the thallus divides, as in the cormophyta, into root, stalk, and leaves. There is also a trophic differentiation, the fibres undertaking special functions of nutrition (the conduction of the sap). The same must be said of the mosses (_bryophyta_). Their lowest forms (_ricciadinæ_) are close akin to the algæ; the highest mosses (the _mnium_ and _polytrichum_, for instance) approach the cormophyta. Many botanists comprise these lower plants--algæ, fungi, and mosses--under the title of "cell-plants" (_cytophyta_), and oppose the higher plants--ferns and flowering-plants--to them as "vascular plants" (_angiophyta_), because they have complex fibres or sap vessels. This distinction has a phylogenetic significance similar to the division between cœlenteria and cœlomaria in the animal kingdom.

While most of the cell-plants either live in the water (algæ) or are very simply organized on account of their saprophytic or parasitic habits (fungi), the vascular plants mostly live on land, and have to adapt themselves to much more complicated conditions. Their nutrition is accordingly distributed among different functions, and special organs have been evolved to discharge them. This is equally true of the crytogam ferns (_pteridophyta_) and the phanerogam flowering plants (_anthophyta_). The most important later acquisition which distinguishes both groups from the lower cell-plants is the possession of vascular or conducting fibres. These organs for conducting water pass through the entire body of the vascular plant in the shape of long tubes, formed by the combination of rows of cells; the cells themselves die off, and their plasma content disappears. The stream of water that rises constantly in these tubes is taken up by the roots, conducted by the fibres to all parts, and given off (transpiration) by the pores of the leaves. But these pores also serve for the breathing of plants, being connected with the air-containing intercellular passages; through these air-spaces, which serve for the aëration of the higher plant-body, air and moisture can enter, and oxygen be given off in respiration. Finally, many of the vascular plants have special glands that serve for secretion (of oil, resin, etc.). In the higher flowering plants this division of work among the various digestive organs gives rise to a very complicated apparatus for nutrition. Among the many remarkable structures that have been developed in this way by adaptation to special conditions we may particularly note the organs for catching and digesting insects in the insect-eating plants, the European _drosera_ and _utricalaria_, and the tropical _nepenthas_ and _dionæa_.

The long scale of evolutionary forms which we find in the tissue animals (metazoa) leads up uninterruptedly from the simplest to the most elaborate physiological functions and a corresponding morphological complexity of organs. The two principal divisions of the metazoa are chiefly distinguished by the circumstance that in the cœlenteria one single system of organs, the gastro-canal system, discharges the whole (or most part) of the partial functions of nutrition; while in the cœlomaria they are usually distributed among four different systems of organs, each of which is made up of a number of organs. To an extent, we find once more in each great division characteristic types of organization. However, comparative ontogeny teaches us that all these various structures have been developed from one simple fundamental form, as I have shown in my theory of the gastræa (1872).

The older research into the origin of the nutritive apparatus in the metazoa--especially its chief part, the alimentary or gastric canal--had led to the erroneous belief that in several groups of the metazoa it owed its origin to very different growth-processes, and that particularly in the higher vertebrates (the amniotes) it was a comparatively late product of evolution. On the other hand, the comparative study of the embryology of the lower and higher animals led me thirty-four years ago to the opposite conclusion, that a simple gastric sac was the first and oldest organ of all the metazoa, and that all the different forms of it had been developed from this primitive type. I gave this view in my _Biology of the Sponges_ in 1872; and I developed and established it in my _Studies of the Gastræa Theory_ in 1873. In the latter book I also worked out the important conclusions that follow from this monistic reform of the theory of germinal layers for the phylogenetic natural classification of the animal kingdom. I began with the consideration of the simplest sponges (_olynthus_) and cnidaria (_hydra_). The whole body of these lowest and oldest of the cœlenteria is in essence nothing but a round, oval, or cylindrical gastric vesicle, a digestive sac, the thin wall of which consists of two simple layers of cells. The outer layer (the ectoderm or skin-layer) is the covering layer of the external skin (epidermis); it is the instrument of sensation and movement. The inner layer of cells (entoderm or gastric layer) serves for nutrition; it clothes the simple cavity of the sac, which admits the food by its opening and digests it. This opening is the primitive mouth (_prostoma_ or _blastoporus_), the inner cavity itself the primitive gut (_progaster_ or _archenteron_). I proved that there was the same composition in the young embryos or larvæ of many of the lower animals, and showed that the manifold and apparently very different embryonic form of all the higher animals may be reduced to the same common type. To this I gave the name of the "cup-embryo" or gastric larvæ (_gastrula_), and concluded, in virtue of the biogenetic law, that it is the palingenetic reproduction of a corresponding ancestral form (the _gastræa_) maintained until the present by heredity. It was not until much later (1895) that Monticelli discovered a modern gastræad (_pemmatodiscus_) which corresponds completely to this hypothetical ancestor (see the last edition of my _Anthropogeny_, fig. 287). The simplest living forms of the sponges (_olynthus_) and the cnidaria (_hydra_) only differ from this hypothetical primitive form of the gastræa by a few secondary and subsequently acquired features.

The classes of the lower animals which we comprise under the name cœlenteria (or cœlenterata in the widest sense) generally agree in having all the functions of nutrition accomplished exclusively (or for the most part) by a single system of organs, the gastro-canal or gastro-vascular system. From their common stem-group, the gastræads, three different stems have been evolved--the sponges, cnidaria, and platodes. All these cœlenteria have three features in common: (1) The gastric canal or tube has only one opening--the primitive mouth, which serves at once for admitting food and ejecting indigestible matter; there is no anus; (2) there is no special body-cavity (_cœloma_) distinct from the gastric tube; (3) there is also no trace of a vascular system. All cavities that are found in these lower animals besides the digestive gut-cavity are direct processes from it (with the exception of the nephridia in the platodes).

While the simple digestive gut is the sole organ of nutrition in the stem-group of the gastræads, we find other structures co-operating in the rest of the cœlenteria. The characteristic stem of the sponges is distinguished by the piercing of the wall of the gastric vesicle with several holes. Through these water pours into the body, bringing with it the small particles of food which are received and digested by the ciliated cells of the entoderm; the water emerges again by the mouth-opening (_osculum_). The best-known of the sponges is the common bath-sponge (_euspongia officinalis_), the horny skeleton of which we use daily in washing. In these and most other sponges the large, unshapely body is traversed by a number of branching canals, on which there are thousands of tiny vesicles, produced by the multiplication of a simple gastric vesicle of the primitive sponge (_olynthus_). Each of these ciliated chambers is really a tiny gastræa, a "person" of the simplest character (_cf._ chapter vii.). Hence we may regard the whole sponge-body as a gastræad-stock (_cormus_).

The large group of the cnidaria offers a long series of evolutionary stages, from very small and simple to very large and elaborate forms. Some of them remain at a very low stage, as does our common green fresh-water polyp (_hydra viridis_), which only differs from the gastræa by a few variations in tissue and the formation of a crown of feelers about the mouth. Most of the polyps form stocks (_cormi_), the individuals shooting out buds which remain joined to the mother animal. In these and all the other stock-forming animals the nutrition is communistic; all the food that the individuals get and digest is conducted by tubes to the common fund and equally distributed. In all the larger cnidaria the body-wall becomes thicker, and is traversed by branching gastro-canals; these convey the nutritive fluid to all parts of the body.

While the fundamental type in the cnidaria is radial (determined by the crown of radiating feelers or tentacles that surrounds the mouth), it is bilateral-symmetrical in the platodes or "flat-worms" (_plathelminthes_). In this animal-stem, moreover, the lowest forms, the platodaria (also called _cryptocœla_ and _acæla_) come very close to the gastræa. But most of the platodes are distinguished from the rest of the cœlenteria by the formation of a pair of nephridia (renal canals or water-vessels), thin tubes which, as excretory organs, remove from the body the unusable products of metabolism, the urine. Here we have a second organ of nutrition, the gut tube, added to the first. In the lower platodes this remains very simple. As a rule, a gullet tube (pharynx) is formed by the hollowing out of the mouth, as in the corals; and as in the case of the latter branched canals, which conduct the nutritive sap from the stomach to distant parts of the body, grow out of the stomach, in the larger coil-worms (_turbellaria_) and suction-worms (_trematodes_). On the other hand, the gut atrophies in the tape-worms (_cestodes_); as these parasites live in the intestines or other organs of animals, they can obtain their nutritive sap directly from them through the surface of the skin.

The more highly organized cœlomaria differ from the simpler cœlenteria chiefly by the greater complexity in the structure and functions of their apparatus of nutrition. As a rule, these functions are divided between four groups of organs, which are not yet differentiated in the cœlenteria--namely: 1, organs of digestion (gastric system); 2, organs of circulation (vascular system); 3, organs of breathing (respiratory system); and 4, organs of excretion (renal system). Moreover, in the cœlomaria the gastric canal has usually two openings, the mouth and the anus. Finally, they all have a special body-cavity (_cœloma_); this is quite separate from the gastric canal, which is suspended in it, and serves for the formation of the sexual cells. It is formed in the embryo by the hollowing out and cutting off of a pair of sacs (cœlom-pouches) from the gut near the mouth; the pouches touch, and then coalesce, as their division-walls break down. If a part of the dividing wall remains, it serves as mesentery to fasten the gut to the body-wall. The action of the four groups of alimentary organs remains very simple in the lowest and oldest cœlomaria, the worms (_vermalia_); but in the other higher animals, which have been evolved from these, they have very varied and often complicated features.

In the great majority of the cœlomaria the gastric system forms a highly differentiated apparatus, composed, as in man, of a number of different organs. The food is usually taken in by the mouth, ground up by the jaws or the teeth, and softened with saliva, which the salivary glands pour into the cavity of the mouth. From the mouth the pulpy food passes in swallowing into the gullet, which often has glandular appendages, and from this through the narrow esophagus into the stomach. This most important part of the alimentary apparatus is often divided into several sections, one of which (the masticating stomach) is armed with teeth and prepared for a further triturition of solid pieces, while the other (the glandular stomach) produces the dissolving gastric juice. The liquefied food (_chylus_) then passes into the small intestine (_ileum_), which has to absorb it, and is as a rule the longest section of the alimentary canal. A number of different digestive glands open into this intestine, the most important of them being the liver. The small intestine is often sharply distinguished from the large intestine (_colon_), the last large section of the alimentary canal; into this also a number of glands and blind intestines open. The last portion of it is called the _rectum_, and this removes the indigestible remnants of the food (_fæces_) through the anus.

This general plan of the alimentary system, which is common to most of the cœlomaria in its chief features, is very much modified in the various groups of these animals and adapted to their several conditions of nutrition. The simplest structures are found in many of the vermalia; the lowest forms of these, the rotifers, and especially the gastrotricha, still closely resemble their platode ancestors, the turbellaria. The higher type of animal-stems which have been evolved from them are partly distinguished by special structures. Thus the mollusks have a characteristic masticating apparatus; on their tongue there is a hard plate (_radula_) armed with a number of teeth, which grinds against a hard upper jaw, and so breaks up the food. In most of the articulates this work is done by side-jaws, which consist of hard rods and represent modified bones. The vertebrates and the closely related tunicates are distinguished by the conversion of the first sections of the alimentary canal into a characteristic respiratory apparatus (gills). But the construction of the various sections of the gastro-canal also varies a good deal in the small groups of the cœlomaria, as it depends to a great extent on the nature of the food and the conditions in which it is got and prepared. The largest expenditure of mechanical and chemical energy is needed for a voluminous solid vegetal diet. Hence the alimentary canal and its many appendages are longest and most complicated in the plant-eating snails, leaf-eating insects, and grass-eating ruminants. On the other hand, they are shortest and simplest in parasitic cœlomaria, which derive their fluid food already prepared from the contents of another animal's intestines. In these cases the gut may altogether atrophy; as in the _acanthocephala_ among the vermalia, the _entoconcha_ among the mollusks, and the _sacculina_ among the crustacea.

The greater the extent of the body, and the more complex the organization of the higher animals, the more necessary it is to have an orderly and regular distribution of the nutritive fluid to all parts. In the cœlenteria this work is accomplished by the gastric canals (side branches from the gut, opening into its cavity) but in the cœlomaria it is done much better by means of blood-vessels (_vasa sanguifera_). These canals do not communicate directly with the gastro-canal, but are formed independently of it in the surrounding parenchyma of the mesoderm. They take up the filtered and chemically improved food-fluid, which transudes through the intestinal walls, and conduct it in the form of blood to all parts of the body. This blood generally contains millions of cells, which are of great importance in metabolism. The blood-cells of the lower cœlomaria are usually colorless (leucocytes), while those of the vertebrates are mostly red (rhodocytes).

The circulation of the blood in most of the cœlomaria is effected by a heart, a contractile tube, formed by the local thickening of a skin-vessel, which contracts and beats regularly by means of its muscular bands. Originally two of these skin-vessels were developed in the abdominal wall--a dorsal vessel in the upper and ventral vessel in the lower wall (as in many of the vermalia). The heart is formed from the dorsal vessel in the mollusks and articulates, but from the ventral in the tunicates and vertebrates. The arteries are the vessels which conduct the blood from the heart; those which conduct it from the body to the heart are the veins. The finest branchlets of both kinds of vessels, which form the connecting link between them, are called capillaries; these immediately effect the interchange of matter in the tissues by osmosis. The blood-vessels co-operate very closely with the respiratory organs.

The interchange of gases in the organism, which we call breathing or respiration--the taking in of oxygen and giving out of carbonic-acid gas--does not require special organs in the lower animals. In these it is accomplished by epithelial cells, which clothe the surface of the body--the ectoderm of the outer skin layer and the entoderm of the inner gut-covering. As nearly all these cœlenteria live in the water, or (as parasites) in some fluid that contains air, and as these fluids are constantly pouring in and out of the body, the exchange of gases is accomplished at the same time. But in the higher animals this is rarely found, only in the small animals of simple construction (such as the rotifers and other vermalia, and the smallest specimens of the mollusca and articulata). The majority of these cœlomaria attain a considerable size, and so require special organs; these afford a larger surface for the exchange of gases in the limited space, and accomplish a very peculiar chemical work as the localized organs of respiration. They fall into two groups according to the nature of the environment; gills for breathing in water and lungs for breathing on land. The latter take the oxygen directly from the atmosphere, and the former from the water, in which atmosphere air is contained in solution.

The instruments of water-respiration which we call gills (_branchiæ_) are generally attenuated parts or processes of the outer skin or the inner gastric skin; hence we distinguish the two chief forms, external and internal gills. Both are richly provided with blood-vessels which bring the blood from the body for the purpose of aëration. Cutaneous or external gills are especially found in the vertebrates, in the form of threads, combs, leaves, pencils, tufts of feathers, etc., which are drawn out from the entoderm as local processes of the outer skin, and afford a wide surface for the interchange of gases between the body and the water. In the mollusca there are usually a pair of comb-shaped gills near the heart; in the articulates there are several pairs, repeated in the different segments of the body. Gastric or internal gills are peculiar to the vertebrates and the next-related tunicates, with a small group of the vermalia, the enteropneusta. In these the fore-gut or head-gut is converted into a gill-organ, the wall of which is pierced with gill-fissures; the water that has been taken in by the mouth passes away through the outer openings of these fissures. In the lower aquatic vertebrates (acrania, cyclostoma, and fishes) the gills are the sole organs of breathing; in the higher animals, that live in the air, they fall into disuse, and their place is taken by lungs. Nevertheless, heredity is so tenacious that we find from three to five pairs of rudimentary gill-clefts in the embryo right up to man, though they have long since ceased to have any function. This is one of the most interesting of the palingenetic facts that prove the descent of the amniotes (including man) from the fishes.

The group of the aquatic echinoderms has some very peculiar features of respiration. Their body possesses an extensive water-duct, which takes in the sea-water and returns it by special openings (skin-pores or madreporites). The many branches of these water-vessels or ambulacral vessels fill with water, especially the tiny feelers or feet, which extend from the skin in thousands; they serve at once for movement, feeling, and breathing. But many of the echinoderms have also special gills--the star-fish have small finger-shaped cutaneous gills on the back, the sea-urchins special leaf-shaped ambulacral gills, the sea-cucumbers internal gastric gills (tree-shaped branching internal folds of the rectum).

The organs of air-breathing are called, in general, lungs (_pulmones_). Like the organs of water-breathing, they are formed sometimes from the external and sometimes from the internal covering of the body. Cutaneous or external lungs are found in several groups of the vertebrates. Among the mollusks the land-dwelling lung-snails have acquired a lung-sac by change in the work of the gill cavity: among the articulata the lung-spiders and scorpions have two or more trachea-lungs; that is to say, cutaneous sacs, in which are enclosed fanwise a number of trachea-leaves. In the other air-breathing articulates (tracheata) we find, instead of these simple or branched, and often bushlike, air-tubes (_tracheæ_), which spread through the whole body and conduct the air direct to the tissues. They take the air from without by special air-holes in the skin (_stigmata_ and _spiracula_). The myriapods and insects generally have numbers of air-holes; the spiders only one or two, more rarely four, pairs. When these air-tube animals return to an aquatic life (as happens with the larvæ of various groups of insects), the outer air-holes close up, and new thread-shaped or leaf-shaped trachea-gills are formed, which take the air from the surrounding water by osmosis. The oldest and lowest tracheata are the primitive air-tube animals, or protracheata, and form the link between the older annelids and the myriapods. They have a number of clusters of short air-tubes distributed over the whole skin, and it is clear that these have been evolved from simple skin-glands by change of function.

Gastric or internal lungs are only found in the higher animals, to which we give the name of quadrupeds (or _tetrapoda_), the amphibia and amniotes, and their fishlike ancestors, the dipneusta. These internal lungs are sac-shaped folds of the fore-gut, formed originally from the swimming-bladder (_nectocystis_) of the fishes by change of function. This air-filled bladder, a sac-shaped appendage of the gullet, merely serves the purpose of a hydrostatic organ, by varying the specific weight, in the fishes. When the fish wishes to descend it contracts the bladder and becomes heavier; it rises to the top by inflating it again. The lungs were formed by the adaptation of the blood-vessels in the wall of the swimming-bladder to the interchange of gases. In the oldest living lung-fishes (_ceratodus_) it is still a simple sac (_monopneumones_=one-lunged); in the others the simple gullet-cavity divides early into a pair of sacs (_dipneumones_, two-lunged). The wind-pipe (_trachea_--not to be confused with the organ of the same name in the tracheata) is formed by the lengthening of their stalk and strengthening of it with cartilaginous rings. At the anterior end of the trachea we find already formed in the amphibia the larynx, the important organ of voice and speech.

The function of removing unusable matter is not less important to the organism than breathing. Just as breathing gets rid of the poisonous carbonic acid, so the kidneys remove fluid and solid excreta in the shape of urine; these are partly acid (uric acid, hippuric acid, etc.), partly alkaline (urea, guanine, etc.). In most of the cœlomaria special organs for removing these would be superfluous, as this is accomplished (like breathing) by the stream of water that is constantly passing through the whole body. But with the platodes we begin to find important excretory organs in the nephridia, a pair of simple and ramified canals which lie on either side of the gut, and open outward. These primitive renal canals are transmitted by the platodes to the vermalia, and by these to the higher stems of the cœlomaria. In the latter they generally open by special funnels into the inner body-cavity, which serves as first receptacle for the urine. Their outer opening sometimes (primarily) goes through the outer skin at the back (excretory pores), sometimes (secondarily) to the rectum, and so out through the anus. The oldest articulates, the annelids, have a pair of nephridia in each segment of the body; each renal canal, or segmental canal, consists of three sections, an inner funnel which opens into the body-cavity, a middle glandular section, and an external bladder that ejects the urine by contraction. The disposition of the renal system in the internally articulated vertebrates is very similar to this; but now complicated structures begin to appear, a pair of compact kidneys (_renes_), which are made up of a number of branching nephridia. Three generations of kidneys succeed each other, as phylogenetic stages of evolution--first the primary fore-kidneys (_protonephros_), in the middle the secondary primitive kidneys (_mesonephros_), and last the tertiary after-kidneys (_metanephros_). The latter are only reached in the three highest classes of vertebrates, reptiles, birds, and mammals. Mollusks also have a couple of compact kidneys. They are developed from a pair of nephridia, the funnels of which open internally into the heart-pouch (the remainder of the reduced body-cavity); at the back they open outward. The crustacea also have generally a pair of renal canals. On the other hand, the protracheata (the stem-forms of the air-tube animals) have segmental nephridia, a pair to each joint inherited from their annelid ancestors. The rest of the tracheata, the myriapods, spiders, and insects, have, instead of these, Malpighi tubes, funnel-shaped glands that arise from the entodermal rectum, sometimes one pair or less, sometimes a number in a cluster.

While most plants are purely plasmodomous, and most animals plasmophagous, there are nevertheless in both organic kingdoms a number of species (especially the lower) whose metabolism has assumed peculiar forms by their relations to other organisms. To this class belong especially the saprosites and parasites. By saprosites are understood those plants and animals which feed entirely or mostly on the corpses of other animals, or the decomposed matter which is unfit for the food of higher animals. Among the unicellular protists many of the bacteria, especially, belong to this class, and also many fungilla (_phycomycetes_); among the metaphyta the fungi (mycetes), and among the metazoa the sponges. I have already spoken of the many peculiarities of metabolism in the ubiquitous bacteria; while many of them cause putrefaction, they at the same time feed on the parts of other organisms which have died. The fungi feed for the most part on the decayed remains of plants and the products of putrefaction which accumulate on the ground. In this character of scavengers they play the same important part on land as the sponges do at the bottom of the sea. But a number of small groups of the higher plants and animals have, as a secondary habit, turned to saprositism. Among the metaphyta we have especially the monotropea (to which our native asparagus, _monotropa hypopitys_, belongs) and many orchids (_neottia_, _corallorhiza_). As they find their plasm directly in the decayed matter in the woods, they have lost their chlorophyll and green leaves. Among the metazoa many of the vermalia, and some of the higher animals, such as the rain-worm and many tube-dwelling annelids (the mud-eaters, _limicolæ_), etc., live on putrid matter. The organs which their nearest relatives use for obtaining, breaking up, and digesting food (eyes, jaws, teeth, digestive glands) have been entirely or mostly lost by these saprosites. Many of them form a transitional type to the parasites.

By parasites, in the narrower sense, we understand, in modern biology, only those organisms which live on others and derive their nourishment from them. They are numerous in all the chief divisions of the plant and animal kingdoms, and their modifications are of great interest in connection with evolution. No other circumstance has so profound an influence on the organism as adaptation to a parasitic existence. Moreover, there is no other section in which we can follow, step by step, the course of the degeneration which is caused, and show clearly the mechanical nature of the process. Hence the science of parasites--parasitology--is one of the soundest supports of the theory of descent, and provides an abundance of the most striking proofs of the much-contested inheritance of acquired characteristics.

Among the unicellular organisms, the bacteria are the most conspicuous instances of manifold adaptation to parasitic habits. As we count these unnucleated protozoa among the oldest and simplest organisms, and trace them directly by metasitism to the plasmodomous chromacea, it is very probable that they turned to parasitism very early in the history of life. Even a part of the monera (in which group we must place the bacteria on account of their lack of a nucleus) found it convenient and advantageous to prey on other protists and assimilate their plasm directly, instead of going through the laborious process of carbon assimilation themselves in the hereditary fashion. This is also true of the large class of the sporozoa or fungilla (_gregarinæ_, _coccidia_, etc.), real nucleated cells, which have adapted themselves in various ways to parasitic habits. Many of them live in the rectum, the cœlum, or other organs of the higher animals (the gregarinæ, especially in the articulates); others in the tissues (for instance, the sarcosporidia in the muscles of mammals, the coccidia and myxosporidia in the liver of vertebrates). A good many of them are "cell-parasites," and live inside the cells of other animals, which they destroy; such are the hœmosporidia, which destroy the blood-cells in man, and so cause intermittent fever.

Among the multicellular metaphyta it is particularly the fungi that have taken to parasitism in various ways. Many of them are, as is known, the most dangerous enemies of the higher animals and plants. The various species of fungi cause certain diseases by their poisonous (chemical) action on the tissues of their host. It is well known how our most important cultivated plants, the vine, potato, corn, coffee, etc., are threatened by fungoid diseases; and this is also true of many of the lower and higher animals. It is probable that the fungi have been evolved polyphyletically by metasitism from the algæ.

Among the higher metaphyta we find parasitism in many different families, especially orchids, rhinanthacea (_orobranche_, _lathraca_), convolvulacea (_cuscuta_), aristolochiacea, loranthacea (_viscum_, _loranthus_), rafflesiacea, etc. These various kinds of flowering-plants often grow to resemble each other by convergence (that is to say, by their common adaptation to parasitic life); they lose their green leaves, the plasmodomous chlorophyll of which is of no further use to them. Frequently rudimentary leaves are left on them in the form of colorless scales. For the purpose of clinging to the plants they live on, and penetrating into their tissues, they evolve special clinging apparatus (haustoria, suctorial cups, creepers). Their stalks and roots are also modified in a characteristic way. The whole productive force of these parasites is expended on their sexual organs; _rafflesia_ has the largest flowers there are, more than a yard in diameter.

Parasitism in the metazoa (in all groups) is even more frequent and interesting than in the metaphyta. The mollusks and echinoderms show the least disposition for it, and the platodes, vermalia, and articulates the most. Even among the gastræada, the common ancestral group of the metaphyta, we find parasites (kyemaria and gastremaria). The protection they find inside their hosts is probably the reason why these oldest of the metazoa have remained unchanged to the present day. Real parasites are not numerous among the sponges and cnidaria. But they are very numerous among the platodes. The suctorial worms (_trematodes_) live partly externally (as ectoparasites) on other animals and partly inside them (as endoparasites), and produce serious diseases in them. They have lost the vibratory coat of their free-living ancestors, the turbellaria, and acquired clinging apparatus instead. The tape-worms (_cestodes_), which live entirely in the interior of other animals, and are descended from the suctorial worms, have lost their gastro-canal; they are nourished by imbibition through the skin. The same degeneration is found in the itchworms (_acanthocephala_) among the vermalia, the parasitic snails (_entoconcha_) among the mollusks, and the root-crabs (_rhizocephala_) among the crustacea.

The class of crustacea affords the most numerous and most instructive examples of degeneration through parasitism, because in this class it is found polyphyletically in very different orders and families, and because their highly organized body shows every stage of degeneration together in the different organs. The free-living crustacea generally move about very rapidly and ingeniously; their numerous bones are well jointed and excellently adapted for the most varied methods of locomotion (running, swimming, climbing, digging, etc.); their organs of sense are highly developed. As these are no longer used when they take to parasitism, they atrophy and gradually disappear. The younger crustacea all proceed from the same characteristic form of the _nauplius_, and swim freely about; later, when they settle down to parasitic habits, their organs of sense and locomotion atrophy. As Fritz Müller-Desterro showed in his famous little work, _For Darwin_ (1864), forty years ago, the crustacea afford most luminous proofs of the theory of descent and selection, and of progressive heredity and the biogenetic law. These facts are the more important as the crab undergoes the same degeneration by parasitic habits in a number of different orders and families.

From parasitism we must entirely distinguish that intimate life-union of two different organisms which we called symbiosis or mutualism. Here we have an association of two living things for their mutual benefit, while the parasite lives entirely at the expense of his host. Symbiosis is found among the protista, being very wide-spread among the radiolaria. In the gelatinous envelope (_calymma_) which encloses the central capsule of their unicellular bodies we find a number of motionless yellow cells (_zooxanthella_) scattered. These are protophyta or (as it is said) "unicellular algæ" of the class of paulotomea (_palmellacea_). They receive protection and a home from the radiolaria, grow plasmodomously, and multiply by rapid segmentation. A large part of the starch-flour and the plasm which they form by carbon-assimilation goes as food directly to the radiolarium-host; the other part of the xanthella goes on growing and multiplying. Similar yellow zooxanthella or green zoochlorella are found as symbionta in the tissues of many animals. Our common fresh-water polyp (_hydra viridis_) owes its green color to the zoochlorella which live in great numbers on the ciliated cells of its entoderm (the digestive gut-epithelium). In general, however symbiosis is rarer in the metazoa than in the metaphyta. In the latter case it is the fundamental feature of a whole class of plants, the lichens. Each lichen consists of a plasmodomous plant (sometimes a protophyte, sometimes an alga) and a plasmophagous fungus. The latter affords a home, protection, and water to the green alga, which repays the service by providing food.

XI

REPRODUCTION

Reproduction and generation--Sexual and asexual reproduction--Superfluous growth--Monogony--Self-cleavage--Budding--Formation of spores--Amphigony--Ovum and sperm-cell--Hermaphrodite formation and separation of the sexes--Hermaphrodism and gonochorism of the cells--Monoclinism and diclinism--Monœcism and diœcism--Alternation of sex-division--Sexual glands of the histona--Hermaphroditic glands--Sexual ducts--Generative organs--Parthenogenesis--Pædogenesis--Metagenesis--Heterogenesis-- Strophogenesis--Hypogenesis--Hybridism--Generation of hybrids and the species--Graduation of forms of reproduction.

While nutrition secures the maintenance of the organic individual, reproduction insures that of the organic species, or the group of definite forms which we distinguish from others by the name "species." All individuals are more or less restricted in the duration of their lives, and die off after the lapse of a certain time. The succession of individuals, connected by reproduction and belonging to a species, makes it possible for the specific form itself to last for ages. In the end, however, the species is temporary; it has no "eternal life." After existing for a certain period, it either dies or is converted by modification into other forms.

The rise of new individuals by reproduction from parent organisms is a natural phenomenon with definite time-restriction. It cannot have continued from eternity on our planet, as the earth itself is not eternal, and even long after its formation was incapable of supporting organic life on its surface. This only became possible when the surface of the glowing planet had sufficiently cooled for liquid water to settle on it. Until this stage carbon could not enter into those combinations with other elements (oxygen, hydrogen, nitrogen, and sulphur) which led to the formation of plasm. As I intend to deal with this process of _archigony_, or spontaneous generation, in a special chapter, I leave it for the present, and confine myself to the study of _tocogony_, or parental generation.

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The various forms of tocogony, or the reproduction of living things, are generally divided into two large groups; on the one hand there is the simple form of asexual generation (monogony), and on the other the complex form of sexual generation (amphigony). In asexual generation the action of one individual only is needed, this providing a product of transgressive (redundant) growth which develops into a new organism. In sexual generation it is necessary for two different individuals to unite in order to produce a new being from themselves. This amphigony (or _generatio digenea_) is the sole form of reproduction in man and most of the higher animals. But in many of the lower animals and most of the plants we find also asexual multiplication, or monogony, by cleavage or budding. In the lowest organisms, the monera and many of the protists, fungi, etc., the latter is the only form of propagation.

Strictly speaking, monogony is a universal life-process; even the ordinary cell-cleavage, on which depends the growth of the histona, is a cellular monogony. Hence historical biology must say that monogony is the older and more primitive form of parental generation, and that amphigony was secondarily developed from it. It is important to emphasize this because, not only some of the older writers, but even some recent ones, regard sexual generation as a universal function of organisms, and declare that it dates from the very beginning of organic life.

The complex and frequently very intricate phenomena of sexual generation, as we find them in the higher organisms, become intelligible to us when we compare them with the simpler forms of asexual generation at the lowest stages of life. We then learn that they are by no means unintelligible and supernatural marvels, but natural physiological processes, which, like all others, may be traced to the action of simple physical forces. The form of energy which lies at the root of all tocogony is _growth_ (_crescentia_). And as this phenomenon is also the cause, in the form of gravitation, of the formation of crystals and other inorganic individuals, we do away with another of the boundaries which people would establish between organic and inorganic nature. Reproduction is a kind of nutrition and growth of the organism beyond the individual standard, building up a part of it into a whole. This _limit_ of individual size is determined for each species by two factors--the inner constitution of the plasm, which is inherited, and the dependence on the outer environment, which controls adaptation. When this limit has been passed, the transgressive growth takes the form of reproduction. Every species of crystal has also a definite limit of growth; when this is passed, new crystal-individuals are formed in the mother-water on the old individual, which grows no further.

Asexual or monogenetic tocogony (also called "vegetative multiplication") is always effected by a single organic individual, and so must be traced to its transgressive growth. When this affects the entire body as a total growth, the whole dividing into two or more equal parts, we call the monogenetic process division (or segmentation). But when the growth is partial, and affects only a part of the individual, or when this special part separates from the generating organism in the form of a bud (_gemma_), the process is called budding or gemmation (_gemmatio_). Hence the essential difference between the two forms of generation is that in division the parent disappears in its partial products (children); these are of the same age and form. But in budding the generating parent retains its individuality; it is larger and older than the young bud. This important difference between division and gemmation, which is often overlooked, holds good both for protists (unicellulars) and histona (multicellulars). The fact that in division the individual as such is destroyed is a sufficient refutation of Weismann's theory of the immortality of the unicellulars. (See above, and also the _Riddle_,