CHAPTER XXIV.

OBJECTIONS AGAINST, AND PROOFS OF THE TRUTH OF, THE THEORY OF DESCENT.

Objections to the Doctrine of Filiation.—Objections of Faith and Reason.—Immeasurable Length of the Geological Periods.—Transition Forms between Kindred Species.—Dependence of Stability of Form on Inheritance, and of the Variability of Form on Adaptation.—Origin of very complicated Arrangement of Organisation.—Gradual Development of Instincts and Mental Activities.—Origin of a priori Knowledge from Knowledge a posteriori.—The Knowledge requisite for the Correct Understanding of the Doctrine of Filiation.—Necessary Interaction between Empiricism and Philosophy.—Proofs of the Theory of Descent.—Inner Causal Connection between all the Biological Series of Phenomena.—The Direct Proof of the Theory of Selection.—Relation of the Theory of Descent to Anthropology.—Proofs of the Animal Origin of Man.—The Pithecoid Theory as an Inseparable Part of the Theory of Descent.—Induction and Deduction.—Gradual Development of the Human Mind.—Body and Mind.—Human Soul and Animal Soul.—A Glance at the Future.

If in these chapters I may hope to have made the Theory of Descent seem more or less probable, and to have even convinced some of my readers of its unassailable truth, yet I am by no means unconscious that, to most of them, during the perusal of my explanations, a number of objections more or less well founded must have occurred. Hence it seems absolutely necessary at the conclusion of our examination to refute at least the most important of these, and at the same time, on the other hand, once more to set forth the convincing arguments which bear testimony to the truth of the theory of development.

The objections which are raised to the doctrine of descent may be divided into two large groups: objections of faith and objections of reason. The objections of the first group originate in the infinitely varied forms of faith held by human individuals, and need not here be taken into consideration at all. For, as I have already remarked at the beginning of this book, science, as an objective result of sensuous experience, and of the striving of human reason after knowledge, has nothing whatever to do with the subjective ideas of faith, which are preached by a single man as the direct inspirations or revelations of the Creator, and then believed in by the dependent multitude. This belief, very different in different nations, only begins, as is well known, where science ends. Natural Science believes, according to the maxim of Frederick the Great, “that every one may go to heaven in his own fashion,” and only necessarily enters into conflict with particular forms of faith where they appear to set a limit to free inquiry and a goal to human knowledge, beyond which we are not to venture. Now this is certainly the case here in the highest degree, for the Theory of Development applies itself to the solution of the greatest of scientific problems—that of the creation, the coming into existence of things; more especially the origin of organic forms, and of man at their head. It is here certainly the right as well as the sacred duty of free inquiry, to fear no human authority, and courageously to raise the veil from the image of the Creator, unconcerned as to what natural truth may lie concealed beneath. The only Divine revelation which we recognise as true, is written everywhere in nature, and to every one with healthy senses and a healthy reason it is given to participate in the unerring revelation of this holy temple of nature, by his own inquiry and independent discovery.

If we, therefore, here disregard all objections to the Doctrine of Descent which may be raised by the priests of the different religious faiths, we must nevertheless endeavour to refute the most important of those objections which seem more or less founded on science, and which we grant might, at first sight, to a certain extent captivate us and deter us from adopting the Doctrine of Descent. Many persons seem to think the length of the periods of time required the most important of these objections. We are not accustomed to deal with such immense periods as are necessary for the history of the creation. It has already been mentioned that the periods, during which species originated by gradual transmutation, must not be calculated by single centuries, but by hundreds and by millions of centuries. Even the thickness of the stratified crust of the earth, the consideration of the immense space of time which was requisite for its deposition from water, taken together with the periods of elevation between the periods of depression, indicate a duration of time of the organic history of the earth which the human intellect cannot realize. We are here in much the same position as an astronomer in regard to infinite space. In the same way as the distances between the different planetary systems are not calculated by miles but by Sirius-distances, each of which comprises millions of miles, so the organic history of the earth must not be calculated by thousands of years, but by palæontological or geological periods, each of which comprises many thousands of years, and perhaps millions, or even, milliards, of thousands of years. It is of little importance how high the immeasurable length of these periods may be approximately estimated, because we are in fact unable with our limited power of imagination to form a true conception of these periods, and because we do not as in astronomy possess a secure mathematical basis for fixing the approximate length of duration in numbers. But we most positively deny that we see any objection to the theory of development in the extreme length of these periods which are so completely beyond the power of our imagination. It is, on the contrary, as I have already explained in one of the preceding chapters, most advisable, from a strictly philosophical point of view, to conceive these periods of creation to be as long as possible, and we are by so much the less in danger of losing ourselves in improbable hypotheses, the longer we conceive the periods for organic processes of development to have been. The longer, for example, we conceive the Permian period to have been, the easier it will be for us to understand how the important transmutations took place within it which so essentially distinguish the fauna and flora of the Coal period from that of the Trias. The great disinclination which most persons have to assume such immeasurable periods, arises mainly from the fact of our having in early youth been brought up in the notion that the whole earth is only some thousands of years old. Moreover, human life, which at most attains the length of a century, is an extremely short space of time, and is not suitable as a standard for the measurement of geological periods. Our life is a single drop in the ocean of eternity. The reader may call to mind the duration of life of many trees which is more than fifty times as long; for example, the dragon-trees (Dracæna) and monkey bread-fruit trees (Adansonia), whose individual life exceeds a period of five thousand years; and, on the other hand, the shortness of the individual life of many of the lower animals, for example, the infusoria, where the individual, as such, lives but a few days, or even but a few hours, contrasts no less strongly with human longevity. This comparison brings the relative nature of all measurement of time very clearly before us. If the theory of development be true at all, there must certainly have elapsed immense periods, utterly inconceivable to us, during which the gradual historical development of the animal and vegetable kingdom proceeded by the slow transformation of species. There is, however, not a single reason for accepting a definite limit for the length of these periods of development.

A second main objection which many, and more especially systematic zoologists and botanists, raise against the theory of descent, is that no _transition forms_ between the different species can be found, although according to the theory of descent they ought to be found in great numbers. This objection is partly well founded and partly not so, for there does exist an extraordinarily large number of transition forms between living, as well as between extinct species, especially where we have an opportunity of seeing and comparing very numerous individuals of kindred species. Those careful investigators of individual species who so frequently raise this objection are the very persons whom we constantly find checked in their special series of investigations by the really insuperable difficulty of sharply distinguishing individual species. In all systematic works, which are in any degree thorough, one meets with endless complaints, that here and there species cannot be distinguished because of the excessive number of transition forms. Hence every naturalist defines the limit and the number of individual species differently. Some zoologists and botanists, as I mentioned (vol. i. p. 276), assume in one and the same group of organisms ten species, others twenty, others a hundred or more, while other systematic naturalists again look upon these different forms only as varieties of a single “good” species. In most groups of forms there is, in fact, a superabundance of transition forms and intermediate stages between the individual species.

It is true that in many species the forms of transition are actually wanting, but this is easily explained by the principle of divergence or separation, the importance of which I have already explained. The circumstance that the struggle for existence is the more active between two kindred forms the closer they stand to each other, must necessarily favour the speedy extinction of the connecting intermediate forms between the two divergent species. If one and the same species produce diverging varieties in different directions, which become new species, the struggle between these new forms and the common primary form will be the keener the less they differ from one another; but the stronger the divergence the less dangerous the struggle. Naturally therefore, it is principally the connecting intermediate forms which will in most cases quietly die out, while the most divergent forms remain and reproduce themselves as distinct “new species.” In accordance with this, we in fact no longer find forms of transition leading to those groups which are becoming extinct, as, for example, among birds, are the ostriches; and among mammals, the elephants, giraffes, Semi-apes, Edentata, and Ornithorhyncus. The groups of forms approaching their extinction no longer produce new varieties, and naturally the species are what is called “good,” that is, the species are distinctly different from one another. But in those animal groups where development and progress are still active, where the existing species deviate into many new species by the formation of new varieties, we find an abundance of transition forms which cause the greatest difficulties to systematic naturalists. This is the case, for example, among birds with the finches; among mammals with most of the rodents (more especially with those of the mouse and rat kind), with a number of the ruminants and with genuine apes, more especially with the South American forms (Cebus), and many others. The continual development of species by the formation of new varieties here produces a mass of intermediate forms which connect the so-called “good” species, which efface their boundaries, and render their sharp specific distinction completely illusory.

The reason that this nevertheless does not cause a complete confusion of forms, nor a universal chaos in the structure of animals and vegetables, lies simply in the fact that there is a continual counteraction at work between progressive _adaptation_ on the one hand, and the _retentive_ power of _inheritance_ on the other hand. The degree of stability and variability manifested by every organic form is determined solely by the actual condition of the equilibrium between these two opposite functions. _Inheritance is the cause of the stability of species, adaptation the cause of their modification._ When therefore some naturalists say that, according to the theory of descent, there ought to be a much greater variety of forms, and others again, that there ought to be a much greater equality of forms, the former under-estimate the value of inheritance and the latter the value of adaptation. _The ratio of the interaction between inheritance and adaptation determines the ratio of the stability and variability of organic species_ at any given period.

Another objection to the theory of descent, which, in the opinion of many naturalists and philosophers is of great weight, is that it ascribes the origin of organs which act for a definite purpose to causes which are either aimless or mechanical in their operation. This objection seems to be especially important in regard to those organs which appear so excellently adapted for a certain definite purpose that the most ingenious mechanician could not invent a more perfect organ for the purpose. Such are, above all, the higher sense-organs of animals, the eye and ear. If the eyes and auditory apparatus of the higher animals alone were known to us, they would indeed cause great and perhaps insurmountable difficulties. How could we come to the conclusion that the extraordinarily great and wonderful degree of perfection and conformity to purpose which we perceive in the eyes and ears of higher animals, is in every respect attained solely by natural selection? Fortunately, however, comparative anatomy and the history of development help us here over all obstacles; for when in the animal kingdom we follow the gradual progress towards perfection of the eyes and ears, step by step, we find such a finely graduated series of improvement, that we can clearly follow the development of the most complex organs through all the stages towards perfection. Thus, for example, the eye in the lowest animal is a simple spot of pigment which does not yet reflect any image of external objects, but at most perceives and distinguishes the different rays of light. Later, we find in addition to this a sensitive nerve; then there gradually develops within the spot of pigment the first beginning of the lens, a refractive body which is now able to concentrate the rays of light and to reflect a definite image. But all the composite apparatus for the movement of the eye and its accommodation to variations of light and distance are still absent, namely, the various refractive media, the highly differentiated membrane of the optic nerve, etc., which are so perfectly constructed in higher animals. Comparative anatomy shows us an uninterrupted succession of all possible stages of transition, from the simplest organ to the most highly perfected apparatus, so that we can form a pretty correct idea of the slow and gradual formation of even such an exceedingly complex organ. The like gradual progress which we observe in the development of the organ during the course of individual development, must have taken place in the historical (phyletic) origin of the organ.

Many persons when contemplating these most perfect organs—which apparently were purposely invented and constructed by an ingenious Creator for a definite function, but which in reality have arisen by the aimless action of natural selection—experience difficulties in arriving at a rational understanding of them, which are similar to those experienced by the uncivilized tribes of nature when contemplating the latest complicated productions of engineering. Savages who see a ship of the line, or a locomotive engine for the first time, look upon these objects as the productions of a supernatural being, and cannot understand how a man, an organism like themselves, could have produced such an engine. Even the uneducated classes of our own race cannot comprehend such an intricate apparatus in its actual workings, nor can they understand its purely mechanical nature. Most naturalists, however, as Darwin very justly remarks, stand in much the same position in regard to the forms of organisms as do savages to ships of the line and to locomotive engines. A rational understanding of the purely mechanical origin of organic forms can only be acquired by a thorough and general training in Biology, and by a special knowledge of comparative anatomy and the history of development.

Among the remaining objections to the Theory of Descent, I shall here finally refer to and refute but one more, as in the eyes of many unscientific men it seems to possess great weight. How are we, from the Theory of Descent, to conceive of the origin of the mental faculties of animals, and more especially their specific expressions—the so-called instincts? This difficult subject has been so minutely discussed by Darwin in a special chapter of his chief work (the seventh), that I must refer the reader to it. We must regard instincts as essentially the habits of the soul acquired by adaptation, and transmitted and fixed by inheritance through many generations. Instincts are, therefore, like all other habits, which, according to the laws of cumulative adaptation (vol. i. p. 233) and established inheritance (vol. i. p. 216), lead to the origin of new functions, and thus also to new forms of the organs. Here, as everywhere, the interaction between function and organ goes hand in hand. Just as the mental faculties of man have been acquired by the progressive adaptation of the brain, and been fixed by continual transmission by inheritance, so the instincts of animals, which differ from them only in quantity, not in quality, have arisen by the gradual perfecting of their mental organ, that is, their central nervous system, by the interaction of Adaptation and Inheritance. Instincts, as is well known, are inherited, but experiences and, consequently, new adaptations of the animal mind, are also transmitted by inheritance; and the training of domestic animals to different mental activities, which wild animals are incapable of accomplishing, rests upon the possibility of mental adaptation. We already know a series of examples, in which such adaptations, after they had been transmitted through a succession of generations, finally appeared as innate instincts, and yet they have only been acquired from the ancestors of the animals. Inheritance has here caused the result of training to become instinct. The characteristic instincts of sporting dogs, shepherd’s dogs, and other domestic animals, and the natural instincts of wild animals, which they possess at birth, were in the first place acquired by their ancestors by adaptation. They may in this respect be compared to man’s “knowledge a priori,” which, like all other knowledge, was originally acquired by our remote ancestors, “a posteriori,” by sensuous experience. As I have already remarked, it is evident that “knowledge a priori” arose only by long-enduring transmission, by inheritance of acquired adaptations of the brain, out of originally empiric or experiential “knowledge a posteriori” (vol. i. p. 31).

The objections to the Theory of Descent here discussed and refuted are, I believe, the most important which have been raised against it; I consider also that I have sufficiently proved to the reader their futility. The numerous other objections which besides these have been raised against the Theory of Development in general, or against its biological part, the Theory of Descent in particular, arise either from such a degree of ignorance of empirically established facts, or from such a want of their right understanding, and from such an incapacity to draw the necessary conclusions, that it is really not worth the trouble to go further into the refutation. There are only some general points in regard to which, I should like, in a few words, to draw attention.

In the first place I must observe, that in order thoroughly to understand the doctrine of descent, and to be convinced of its absolute truth, it is indispensable to possess a general knowledge of the whole of the domain of biological phenomena. _The theory of descent is a biological theory_, and hence it may with fairness and justice be demanded that those persons who wish to pass a valid judgment upon it should possess the requisite degree of biological knowledge. Their possessing a special empiric knowledge of this or that domain of zoology or botany, is not sufficient; they must possess a _general insight into the whole series of phenomena_, at least in the case of one of the three organic kingdoms. They ought to know what universal laws result from the comparative morphology and physiology of organisms, but more especially from comparative anatomy, from the individual and the palæontological history of development, etc.; and they ought to have some idea of the deep _mechanical, causal connection_ between all these series of phenomena. It is self-evident that a certain degree of general culture, and especially a philosophical education, is requisite; which is, however, unfortunately by many persons in our day, not considered at all necessary. _Without the necessary connection of empirical knowledge and the philosophical understanding of biological phenomena, it is impossible to gain a thorough conviction of the truth of the Theory of Descent._

Now I ask, in the face of this first preliminary condition for a true understanding of the Theory of Descent, what we are to think of the confused mass of persons who have presumed to pass a written or oral judgment upon it of an adverse character? Most of them are unscientific persons, who either know nothing of the most important phenomena of Biology, or at least possess no idea of their deeper significance. What should we say of an unscientific person who presumed to express an opinion on the cell-theory, without ever having seen cells; or of one who presumed to question the vertebral-theory, without ever having studied comparative anatomy? And yet one may meet with such ridiculous arrogance any day in the history of the biological Theory of Descent. One hears thousands of unscientific and but half-educated persons pass a final judgment upon it, although they know nothing either of botany or of zoology, of comparative anatomy or the theory of tissues, of palæontology or embryology. Hence it happens, as Huxley well says, that most of the writings published against Darwin are not worth the paper upon which they are written.

We might add that there are many naturalists, and even celebrated zoologists and botanists, among the opponents of the Theory of Descent; but these latter are mostly old stagers, who have grown grey in quite opposite views, and whom we cannot expect, in the evening of their lives, to submit to a reform in their conception of the universe, which has become to them a fixed idea.

It is, moreover, expressly to be remarked, that not only a general insight into the _whole_ domain of biological phenomena, but also a philosophical understanding of it, are the necessary preliminary conditions for becoming convinced of and adopting the Theory of Descent. Now we shall find that these indispensable preliminary conditions are, unfortunately, by no means fulfilled by the majority of naturalists of the present day. The immense amount of empirical facts with which the gigantic advances of modern natural science have recently made us acquainted has led to a prevailing inclination for the special study of single phenomena and of small and narrow domains. This causes the knowledge of other paths, and especially of Nature as a great comprehensive whole, to be in most cases completely neglected. Every one with sound eyes and a microscope, together with industry and patience for study, can in our day attain a certain degree of celebrity by microscopic “discoveries,” without, however, deserving the name of a naturalist. This name is deserved only by him who not merely strives to _know_ the individual phenomena, but who also seeks to _discover_ their causal connection. Even in our own day, most palæontologists examine and describe fossils without knowing the most important facts of embryology. Embryologists, on the other hand, follow the history of development of a particular organic individual, without having an idea of the palæontological history of the whole tribe, of which fossils are the records. And yet these two branches of the organic history of development—ontogeny, or the history of the individual, and phylogeny, or the history of the tribe—stand in the closest causal connection, and the one cannot be understood without the other. The same may be said of the systematic and the anatomical part of Biology. There are even now, in zoology and botany, many systematic naturalists who work with the erroneous idea that it is possible to construct a natural system of animals and plants simply by a careful examination of the external and readily accessible forms of bodies, without a deeper knowledge of their internal structure. On the other hand, there are anatomists and histologists who think it possible to obtain a true knowledge of animal and vegetable bodies merely by a most careful examination of the inner structure of the body of some individual species, without the comparative examination of the bodily form of all kindred organisms. And yet here, as everywhere, the internal and external factors, to wit, Inheritance and Adaptation, stand in the closest mutual relation, and the individual can never be thoroughly understood without a comparison of it with the whole of which it is a part. To those one-sided specialists we should like in Goethe’s words to say:—

We must, contemplating Nature, Part as Whole, give equal heed to: Nought is inward, nought is outward, For the inner is the outer.[6]

And again:—

Nature has neither kernel nor shell, It is she that is All and All at once.[7]

What is even more detrimental to the general understanding of nature as a whole than this one-sided tendency, is _the want of a philosophical culture_, and this applies to most of the naturalists of the present day. The various errors of the earlier speculative nature-philosophy made during the first thirty years of our century, have brought the whole of philosophy into such bad repute with the exact empirical naturalists, that they live in the strange delusion that it is possible to erect the edifice of natural science out of mere facts, without their philosophic connection; in short, out of mere knowledge, without the understanding of it. But as a purely speculative and absolutely philosophical system, which does not concern itself with the indispensable foundation of empirical facts, becomes a castle in the air, which the first real experiment throws to the winds; so, on the other hand, a purely empirical system, constructed of nothing but facts, remains a disorderly heap of stones, which will never deserve the name of an edifice. Bare facts established by experience are nothing but rude stones, and without their thoughtful valuation, without their philosophic connection, no science can be established. As I have already tried to impress upon my reader, the _strong edifice of true monistic science_, or what is the same thing, the _Science of Nature, exists only by the closest interaction, and the reciprocal penetration of philosophy and empirical knowledge_.

This lamentable estrangement between science and philosophy, and the rude empiricism which is now-a-days unfortunately praised by most naturalists as “exact science,” have given rise to those strange freaks of the understanding, to those gross insults against elementary logic, and to that incapacity for forming the simplest conclusions which one may meet with any day in all branches of science, but especially in zoology and botany. It is here that the neglect of a philosophical culture and training of the mind, directly avenges itself most painfully. It is not to be wondered at that the deep inner truth of the Theory of Descent remains a sealed book to those rude empiricists. As the common proverb justly says: they cannot see the wood for the trees. It is only by a more general philosophical study, and especially by a more strictly logical training of the mind, that this sad state of things can be remedied. (Compare Gen. Morph. i. 63; ii. p. 447.)

If we rightly consider this circumstance, and if we further reflect upon it in connection with the empirical foundation of the philosophical theory of development, we shall at once see how we are placed respecting the oft-demanded _proofs of the theory of descent_. The more the doctrine of filiation has of late years made way for itself, and the more all thoughtful, younger naturalists, and all truly biologically-educated philosophers have become convinced of its inner truth and absolute necessity, the louder have its opponents called for actual proofs. The same persons who, shortly after the publication of Darwin’s work, declared it to be “a groundless, fantastic system,” an “arbitrary speculation,” an “ingenious dream,” now kindly condescend to declare that the theory of descent certainly is a scientific “_hypothesis_,” but that it still requires to be “_proved_.” When these remarks are made by persons who do not possess the requisite empirico-philosophical culture, nor the necessary knowledge in comparative anatomy, embryology, and palæontology, we cannot be much offended, and we refer them to the study of those sciences. But when similar remarks are made by acknowledged specialists, by teachers of zoology and botany, who certainly ought to possess a general insight into the whole domain of their science, or who are actually familiar with the facts of those scientific domains, then we are really at a loss what to say. Those who are not satisfied with the treasures of our present empirical knowledge of nature as a basis on which to establish the Theory of Descent, will not be convinced by any other facts which may hereafter be discovered; for we can conceive no circumstances which would furnish stronger or a more complete testimony to the truth of the doctrine of filiation than is even now seen, for example, in the well-known facts of comparative anatomy and ontogeny. I must here again direct attention to the fact, _that all the great and general laws, and all the comprehensive series of phenomena of the most different domains of biology can only be explained and understood by the Theory of Development_ (and especially by its biological part, the Theory of Descent), and that without it they remain completely inexplicable and incomprehensible. _The internal causal connection_ between them all proves the Theory of Descent to be the greatest _inductive law_ of Biology.

Before concluding, I will once more name all those series of inductions, all those general laws of Biology, upon which this comprehensive law of development is firmly based.

(1.) _The palæontological history of the development of organisms_, the gradual appearance and the historical succession of the different species and groups of species, the empirical laws of the palæontological change of species, as furnished to us by the science of fossils, and more especially the _progressive differentiation and perfecting_ of animal and vegetable groups in the successive periods of the earth’s history.

(2.) _The individual history of development of organisms_, embryology and metamorphology, the gradual changes in the slow development of the body and its particular organs, especially _the progressive differentiation and perfecting_ of the organs and parts of the body in the successive periods of the individual development.

(3.) _The inner causal connection between ontogeny and phylogeny_, the parallelism between the individual history of the development of organisms, and the palæontological history of the development of their ancestors, a connection which is actually established by the laws of _Inheritance_ and _Adaptation_, and which may be summed up in the words: ontogeny, according to the laws of inheritance and adaptation, repeats in its large features the outlines of phylogeny.

(4.) _The comparative anatomy of organisms_, the proof of the essential agreement of the inner structure of kindred organisms, in spite even of the greatest difference of external form in the various species; their explanation by the causal dependence of the internal agreement of the structure on _Inheritance_, the external dissimilarity of the bodily form on _Adaptation_.

(5.) _The inner causal connection between comparative anatomy and the history of development_, the harmonious agreement between the laws of the gradual development, _the progressive differentiation and perfecting_, as they may be seen in comparative anatomy on the one hand, in ontogeny and palæontology on the other.

(6.) _Dysteleology, or the theory of purposelessness_, the name I have given to the _science of rudimentary organs_, of suppressed and degenerated, aimless and inactive, parts of the body; one of the most important and most interesting branches of comparative anatomy, which, when rightly estimated, is alone sufficient to refute the fundamental error of the teleological and dualistic conception of Nature, and to serve as the foundation of the mechanical and monistic conception of the universe.

(7.) _The natural system of organisms_, the natural grouping of all the different forms of Animals, Plants, and Protista into numerous smaller or larger groups, arranged beside and above one another; the kindred connection of species, genera, families, orders, classes, tribes, etc., more especially, however, the _arboriform branching character of the natural system_, which is the spontaneous result of a natural arrangement and classification of all these graduated groups or categories. The result attained in attempting to exhibit the relationships of the mere forms of organisms by a tabular classification is only explicable when regarded as the expression of their actual _blood relationship_; _the tree shape of the natural system_ can only be understood as the actual _pedigree of the organisms_.

(8.) _The chorology of organisms_, the science of the local distribution of organic species, of their _geographical_ and _topographical dispersion over the surface of the earth_, over the heights of mountains and in the depths of the ocean, but especially the important phenomenon that every species of organism proceeds from a so-called “_centre of creation_” (more correctly a “_primæval home_” or “_centre of distribution_”); that is, from a single locality, where it originated but once, and whence it spread.

(9.) _The œcology of organisms_, the knowledge of the sum of the _relations of organisms to the surrounding outer world_, to organic and inorganic conditions of existence; the so-called “_economy of nature_,” the correlations between all organisms living together in one and the same locality, their adaptation to their surroundings, their modification in the struggle for existence, especially the circumstances of parasitism, etc. It is just these phenomena in “the economy of nature” which the unscientific, on a superficial consideration, are wont to regard as the wise arrangements of a Creator acting for a definite purpose, but which on a more attentive examination show themselves to be the necessary results of mechanical causes.

(10.) _The unity of Biology as a whole_, the deep inner connection existing between all the phenomena named and all the other phenomena belonging to zoology, protistics, and botany, and which are simply and naturally explained by a single common principle. This principle can be no other than the common derivation of all the specifically different organisms from a single, or from several absolutely simple, primary forms like the Monera, which possess no organs. The Theory of Descent, by assuming this common derivation, throws a clear light upon these individual series of phenomena, as well as upon their totality, without which their deeper causal connection would remain completely incomprehensible to us. The opponents of the Theory of Descent can in no way explain any single one of these series of phenomena or their deeper connection with one another. So long as they are unable to do this, _the Theory of Descent remains the one adequate biological theory_.

We should, on account of the grand proofs just enumerated, have to adopt Lamarck’s Theory of Descent for the explanation of biological phenomena, even if we did not possess Darwin’s Theory of Selection. The one is so completely and _directly proved_ by the other, and established by mechanical causes, that there remains nothing to be desired. The laws of _Inheritance_ and _Adaptation_ are universally acknowledged _physiological_ facts, the former traceable to _propagation_, the latter to the _nutrition_ of organisms. On the other hand, the _struggle for existence_ is a _biological_ fact, which with mathematical necessity follows from the general disproportion between the average number of organic individuals and the numerical excess of their germs. But as Adaptation and Inheritance in the struggle for life are in continual interaction, it inevitably follows that _natural selection_, which everywhere influences and continually changes organic species, must, by making use of _divergence of character_, produce new species. Its influence is further especially favoured by the active and passive _migrations_ of organisms, which go on everywhere. If we give these circumstances due consideration, the continual and gradual modification or transmutation of organic species will appear as a biological process, which must, according to causal law, of _necessity_ follow from the actual nature of organisms and their mutual correlations.

That even the _origin of man_ must be explained by this general organic process of transmutation, and that it is simply as well as naturally explained by it, has, I believe, been sufficiently proved in my last chapter but one. I cannot, however, avoid here once more directing attention to the inseparable connection between this so-called “theory of apes,” or “pithecoid theory,” and the whole Theory of Descent. If the latter is the greatest _inductive law_ of biology, then it of necessity follows that the former is its most important _deductive law_. They stand and fall together. As all depends upon a right understanding of this proposition, which in my opinion is very important, and which I have therefore several times brought before the reader, I may be allowed to explain it here by an example.

In all mammals known to us the centre of the nervous system is the spinal marrow and the brain, and the centre of the vascular system is a quadrupal heart, consisting of two principal chambers and two ante-chambers. From this we draw the general inductive conclusion that all mammals, without exception, those extinct, together with all those living species as yet unknown to us, as well as the species which we have examined, possess a like organization, a like heart, brain, and spinal marrow. Now if, as still happens every year, there be discovered in any part of the earth a new species of mammal, a new species of marsupial, or a new species of deer, or a new species of ape, every zoologist knows with certainty at once, without having examined its inner structure, that this species must possess a quadruple heart, a brain and spinal marrow, like all other mammals. Not a single naturalist would ever think of supposing that the central nervous system of this new species of mammal could possibly consist of a ventral cord with an œsophageal collar as in the insects, or of scattered pairs of knots as in the molluscs, or that its heart could be many-chambered as in flies, or one-chambered as in the tunicates. This completely certain and safe conclusion, although it is not based upon any direct experience, is a _deductive conclusion_. In the same way, as I have shown in a previous chapter, Goethe, from the comparative anatomy of mammals, established the general inductive conclusion that they all possess a mid jawbone, and afterwards drew from it the special deductive conclusion that man, who in all other respects does not essentially differ from other mammals, must also possess a like mid jawbone. He maintained this conclusion without having actually seen the human mid jawbone, and only proved its existence subsequently by actual observation (vol. i. p. 84).

The process of _induction_ is a logical system of forming conclusions _from the special to the general_, by which we advance from many individual experiences to a general law; _deduction_, on the other hand, draws a conclusion _from the general to the special_, from a general law of nature to an individual case. Thus the _Theory of Descent_ is, without doubt, a great _inductive law_, empirically based upon all the biological experience cited above; the pithecoid theory, on the other hand, which asserts that man has developed out of lower, and in the first place out of ape-like mammals, is a _deductive law_ inseparably connected with the general inductive law.

The pedigree of the human race, the approximate outlines of which I gave in the last chapter but one, of course remains in detail (like all the pedigrees of animals and plants previously discussed) a more or less approximate general hypothesis. This however does not affect the application of the theory of descent to man. Here, as in all investigations on the derivation of organisms, one must clearly distinguish between the general _theory_ of descent and the special _hypotheses_ of descent. The general _theory_ of descent claims full and lasting value, because it is an inductive law, based upon all the whole series of biological phenomena and their inner causal connection. Every special _hypothesis_ of descent, on the other hand, has its special value determined by the existing condition of our biological knowledge, and by the extent of the objective empirical basis upon which we deductively establish this particular hypothesis. Hence, all the individual attempts to obtain a knowledge of the pedigree of any one group of organisms possesses but a temporary and conditional value, and any special hypothesis relating to it will become the more and more perfect the greater the advance we make in the comparative anatomy, ontogeny, and palæontology of the group in question. The more, however, we enter into genealogical details, and the further we trace the separate off-shoots and branches of the pedigree, the more uncertain and subjective becomes our special _hypothesis_ of descent on account of the incompleteness of our empirical basis. This however does no injury to the general _theory_ of descent, which remains as the indispensable foundation for really profound apprehension of biological phenomena. Accordingly, there can be no doubt that we can and must, with full assurance, regard the derivation of man—in the first place, from ape-like forms; farther back, from lower mammals, and thus continually farther back to lower stages of the vertebrata down to their lowest invertebrate roots, nay, even down to a simple plastid—as a general _theory_. On the other hand, the special tracing of the human pedigree, the closer definition of the animal forms known to us, which either actually belong to the ancestors of man, or at least stand in very close blood relationship to them, will always remain a more or less approximate _hypothesis_ of descent, all the more in danger of deviating from the real pedigree the nearer it endeavours to approach it by searching for the individual ancestral forms. This state of things results from the immense gaps in our palæontological knowledge, which can, under no circumstances, ever attain to even an approximate completeness.

A thoughtful consideration of this important circumstance at once furnishes the answer to a question which is commonly raised in discussing this subject, namely, the question of scientific _proofs for the animal origin of the human race_. Not only the opponents of the Theory of Descent, but even many of its adherents who are wanting in the requisite philosophical culture, look too much for “signs” and for special empirical advances in the science of nature. They await the sudden discovery of a human race with tails, or of a talking species of ape, or of other living or fossil transition forms between man and the ape, which shall fill the already narrow chasm between the two, and thus empirically “prove” the derivation of man from apes. Such special manifestations, were they ever so convincing and conclusive, would not furnish the proof desired. Unthinking persons, or those unacquainted with the series of biological phenomena, would still be able to maintain the objections to those special testimonies which they now maintain against our theory.

The absolute certainty of the Theory of Descent, even in its application to man, is built on a more solid foundation; and its true inner value can never be tested simply by reference to individual experience, but only by a philosophical comparison and estimation of the treasures of all our biological experiences. The inestimable importance of the Theory of Descent is surely based upon this, that the theory follows of necessity (as a general inductive law) from the comparative synthesis of all organic phenomena of nature, and more especially from the triple parallelism of comparative anatomy, of ontogeny, and phylogeny; and the pithecoid theory under all circumstances (apart from all special proofs) remains as a special deductive conclusion which must of necessity be drawn from the general inductive law of the Theory of Descent.

In my opinion, all depends upon a right understanding of this _philosophical foundation of the Theory of Descent_ and of the _pithecoid theory_ which is inseparable from it. Many persons will probably admit this, and yet at the same time maintain that all this applies only to the _bodily_, not to the _mental_ development of man. Now, as we have hitherto been occupied only with the former, it is perhaps necessary here to cast a glance at the latter, in order to show that it is also subject to the great general law of development. In doing this it is above all necessary to recollect that body and mind can in fact never be considered as distinct, but rather that both sides of nature are inseparably connected, and stand in the closest interaction. As even Goethe has clearly expressed it—“matter can never exist and act without mind, and mind never without matter.” The artificial discord between mind and body, between force and matter, which was maintained by the erroneous dualistic and teleological philosophy of past times has been disposed of by the advances of natural science, and especially by the theory of development, and can no longer exist in face of the prevailing mechanical and monistic philosophy of our day. How human nature, and its position in regard to the rest of the universe, is to be conceived of according to the modern view, has been minutely discussed by Radenhausen in his “Isis,”(33) which is excellent and well worth perusal.

With regard to the origin of the human mind or the soul of man, we, in the first place, perceive that in every human individual it develops from the beginning, step by step and gradually, just like the body. In a newly born child we see that it possesses neither an independent consciousness, nor in fact clear ideas. These arise only gradually when, by means of sensuous experience, the phenomena of the outer world affect the central nervous system. But still the little child is wanting in all those differentiated emotions of the soul which the full-grown man acquires only by the long experience of years. From this graduated development of the human soul in every single individual we can, in accordance with the inner causal connection between ontogeny and phylogeny, directly infer the gradual development of the human soul in all mankind, and further, in the whole of the vertebrate tribe. In its inseparable connection with the body, the human soul or mind has also had to pass through all those gradual stages of development, all those various degrees of differentiation and perfecting, of which the hypothetical series of human ancestors sketched in a late chapter gives an approximate representation.

It is true that this conception generally greatly offends most persons on their first becoming acquainted with the Theory of Development, because more than all others it most strongly contradicts the traditional and mythological ideas, and the prejudices which have been held sacred for thousands of years. But like all other functions of organisms, the human soul must necessarily have historically developed, and the comparative or empirical study of animal psychology clearly shows that this development can only be conceived of as a gradual evolution from the soul of vertebrate animals, as a gradual differentiation and perfecting which, in the course of many thousands of years, has led to the glorious triumph of the human mind over its lower animal ancestral stages. Here, as everywhere, the only way to arrive at a knowledge of natural truth is to compare kindred phenomena, and investigate their development. Hence we must above all, as we did in the examination of the bodily development, compare the highest animal phenomena on the one hand with the lowest animal phenomena, and on the other with the lowest human phenomena. The final result of this comparison is this—that _between the most highly developed animal souls, and the lowest developed human souls, there exists only a small quantitative, but no qualitative difference_, and that this difference is much less than the difference between the lowest and the highest human souls, or than the difference between the highest and the lowest animal souls.

In order to be convinced of this important result, it is above all things necessary to study and compare the mental life of wild savages and of children.(32) At the lowest stage of human mental development are the Australians, some tribes of the Polynesians, and the Bushmen, Hottentots, and some of the Negro tribes. Language, the chief characteristic of genuine men, has with them remained at the lowest stage of development, and hence also their formation of ideas has remained at a low stage. Many of these wild tribes have not even a name for animal, plant, colour, and such most simple ideas, whereas they have a word for every single, striking _form_ of animal and plant, and for every single sound or colour. Thus even the most simple abstractions are wanting. In many of these languages there are numerals only for one, two, and three: no Australian language counts beyond four. Very many wild tribes can count no further than ten or twenty, whereas some very clever dogs have been made to count up to forty and even beyond sixty. And yet the faculty of appreciating number is the beginning of mathematics! Nothing, however, is perhaps more remarkable in this respect, than that some of the wildest tribes in southern Asia and eastern Africa have no trace whatever of the first foundations of all human civilization, of family life, and marriage. They live together in herds, like apes, generally climbing on trees and eating fruits; they do not know of fire, and use stones and clubs as weapons, just like the higher apes. All attempts to introduce civilization among these, and many of the other tribes of the lowest human species, have hitherto been of no avail; it is impossible to implant human culture where the requisite soil, namely, the perfecting of the brain, is wanting. Not one of these tribes has ever been ennobled by civilization; it rather accelerates their extinction. They have barely risen above the lowest stage of transition from man-like apes to ape-like men, a stage which the progenitors of the higher human species had already passed through thousands of years ago.(44)

Now consider, on the other hand, the highest stages of development of mental life in the higher vertebrate animals, especially birds and mammals. If, as is usually done, we divide the different emotions of the soul into three principal groups—sensation, will, and thought—we shall find in regard to every one of them, that the most highly developed birds and mammals are on a level with the lowest human beings, or even decidedly surpass them. The _will_ is as distinctly and strongly developed in higher animals as in men of character. In both cases it is never actually free, but always determined by a causal chain of ideas. (Compare vol. i. p. 237.) In like manner, the different degrees of will, energy, and passion are as variously graduated in higher animals as in man. The _affections_ of the higher animals are not less tender and warm than those of man. The fidelity and devotion of the dog, the maternal love of the lioness, the conjugal love and connubial fidelity of doves and love-birds are proverbial, and might serve as examples to many men. If these virtues are to be called “instincts,” then they deserve the same name in mankind. Lastly, with regard to _thought_, the comparative consideration of which doubtless presents the most difficulties, this much may with certainty be inferred—especially from an examination of the comparative psychology of cultivated domestic animals—that the processes of thinking, here follow the same laws as in ourselves. Experiences everywhere form the foundation of conceptions, and lead to the recognition of the connection between cause and effect. In all cases, as in man, it is the path of induction and deduction which leads to the formation of conclusions. It is evident that in all these respects the most highly developed animals stand much nearer to man than to the lower animals, although they are also connected with the latter by a chain of gradual and intermediate stages. In Wundt’s excellent “Lectures on the Human and Animal Soul,”(46) there are a number of proofs of this.

Now, if instituting comparisons in both directions, we place the lowest and most ape-like men (the Austral Negroes, Bushmen, and Andamans, etc.), on the one hand, together with the most highly developed animals, for instance, with apes, dogs, and elephants, and on the other hand, with the most highly developed men—Aristotle, Newton, Spinoza, Kant, Lamarck, or Goethe—we can then no longer consider the assertion, that the mental life of the higher mammals has gradually developed up to that of man, as in any way exaggerated. If one must draw a sharp boundary between them, it has to be drawn between the most highly developed and civilized man on the one hand, and the rudest savages on the other, and the latter have to be classed with the animals. This is, in fact, the opinion of many travellers, who have long watched the lowest human races in their native countries. Thus, for example, a great English traveller, who lived for a considerable time on the west coast of Africa, says: “I consider the negro to be a lower species of man, and cannot make up my mind to look upon him as ‘a man and a brother,’ for the gorilla would then also have to be admitted into the family.” Even many Christian missionaries, who, after long years of fruitless endeavours to civilize these lowest races, have abandoned the attempt, express the same harsh judgment, and maintain that it would be easier to train the most intelligent domestic animals to a moral and civilized life, than these unreasoning brute-like men. For instance, the able Austrian missionary Morlang, who tried for many years without the slightest success to civilize the ape-like negro tribes on the Upper Nile, expressly says: “that any mission to such savages is absolutely useless. They stand far below unreasoning animals; the latter at least show signs of affection towards those who are kind towards them, whereas these brutal natives are utterly incapable of any feeling of gratitude.”

Now, it clearly follows from these and other testimonies, that the mental differences between the lowest men and the animals are less than those between the lowest and the highest men; and if, together with this, we take into consideration the fact that in every single human child mental life develops slowly, gradually, and step by step, from the lowest condition of animal unconsciousness, need we still feel offended when told that the mind of the whole human race has in like manner gone through a process of slow, gradual, and historical development? Can we find it “degrading” to the human soul that, by a long and slow process of differentiation and perfecting, it has very gradually developed out of the soul of vertebrate animals? I freely acknowledge that this objection, which is at present raised by many against the pithecoid theory, is quite incomprehensible to me. On this point Bernhard Cotta, in his excellent “Geologie der Gegenwart,” very justly remarks: “Our ancestors may be a great honour to us; but it is much better if we are an honour to them!”(31)

Our Theory of Development explains the origin of man and the course of his historical development in the only natural manner. We see in his gradually ascensive development out of the lower vertebrata, the greatest triumph of humanity over the whole of the rest of Nature. We are proud of having so immensely outstripped our lower animal ancestors, and derive from it the consoling assurance that in future also, mankind, as a whole, will follow the glorious career of progressive development, and attain a still higher degree of mental perfection. When viewed in this light, the Theory of Descent as applied to man opens up the most encouraging prospects for the future, and frees us from all those anxious fears which have been the scarecrows of our opponents.

We can even now foresee with certainty that the complete victory of our Theory of Development will bear immensely rich fruits—fruits which have no equal in the whole history of the civilization of mankind. Its first and most direct result—the complete reform of _Biology_—will necessarily be followed by a still more important and fruitful reform of _Anthropology_. From this new theory of man there will be developed a new _philosophy_, not like most of the airy systems of metaphysical speculation hitherto prevalent, but one founded upon the solid ground of Comparative Zoology. A beginning of this has already been made by the great English philosopher Herbert Spencer.(45) Just as this new monistic philosophy first opens up to us a true understanding of the real universe, so its application to practical human life must open up a new road towards moral perfection. By its aid we shall at last begin to raise ourselves out of the state of social barbarism in which, notwithstanding the much vaunted civilization of our century, we are still plunged. For, unfortunately, it is only too true, as Alfred Wallace remarks with regard to this, at the end of his book of travels: “Compared with our wondrous progress in physical science and its practical applications, our system of government, of administering justice, of national education, and our whole social and moral organisation remains in a state of barbarism.”

This social and moral barbarism we shall never overcome by the artificial and perverse training, the one-sided and defective teaching, the inner untruth and the external tinsel, of our present state of civilization. It is above all things necessary to make a complete and honest return to Nature and to natural relations. This return, however, will only become possible when man sees and understands his true “place in nature.” He will then, as Fritz Ratzel has excellently remarked,(47) “no longer consider himself an _exception_ to natural laws, but begin to seek for what is lawful in his own actions and thoughts, and endeavour to lead a life _according_ to natural laws.” He will come to arrange his life with his fellow-creatures—that is, the family and the state—not according to the laws of distant centuries, but according to the rational principles deduced from knowledge of nature. Politics, morals, and the principles of justice, which are still drawn from all possible sources, will have to be formed in accordance with natural laws only. An _existence worthy of man_, which has been talked of for thousands of years, will at length become a reality.

The highest function of the human mind is perfect knowledge, fully developed consciousness, and the moral activity arising from it. “Know thyself!” was the cry of the philosophers of antiquity to their fellow-men who were striving to ennoble themselves. “Know thyself!” is the cry of the Theory of Development, not merely to the individual, but to all mankind. And whilst increased knowledge of self becomes, in the case of every individual man, a strong force urging to an increased attention to conduct, mankind as a whole will be led to a higher path of moral perfection by the knowledge of its true origin and its actual position in Nature. The simple religion of Nature, which grows from a true knowledge of Her, and of Her inexhaustible store of revelations, will in future ennoble and perfect the development of mankind far beyond that degree which can possibly be attained under the influence of the multifarious religions of the churches of the various nations,—religions resting on a blind belief in the vague secrets and mythical revelations of a sacerdotal caste. Future centuries will celebrate our age, which was occupied with laying the foundations of the Doctrine of Descent, as the new era in which began a period of human development, rich in blessings,—a period which was characterized by the victory of free inquiry over the despotism of authority, and by the powerful ennobling influence of the Monistic Philosophy.

FOOTNOTES:

[1] With the exception of a single specimen of the bones of a foot, preserved in the cabinet of Amherst College.—E. R. L.

[2] The primary stock of the Coniferæ divided into two branches at an early period, into the Araucariæ on the one hand, and the Taxaceæ, or yew-trees, on the other. The majority of recent Coniferæ are derived from the former. Out of the latter the third class of the Gymnosperms—the Meningos, or Gnetaceæ—were developed. This small but very interesting class contains only three different genera—Gnetum, Welwitschia, and Ephedra; it is, however, of great importance, as it forms the transition group from the Coniferæ to the Angiosperms, and more especially to the Dicotyledons.

[3] “Ueber ein Aequivalent der takonischen Schiefer Nordamerikas in Deutschland.”

[4] The English word “Insects” might with advantage be used in the Linnæan sense for the whole group of Arthropods. In this case the Hexapod Insects might be spoken of as the Flies.—E. R. L.

[5] Weisbach: “Novara-Reise,” Anthropholog. Theil.

[6] Müsset im Naturbetrachten Immer Eins wie Alles achten. Nichts ist drinnen, Nichts ist drauszen, Denn was innen, das ist auszen.

[7] Natur hat weder Kern noch Schale, Alles ist sie mit einem Male.

LIST OF THE WORKS REFERRED TO IN THE TEXT BY FIGURES, THUS—(1),

_The study of which is recommended to the Reader._

1. _Charles Darwin_, On the Origin of Species by means of Natural Selection; or, the Preservation of Favoured Races in the Struggle for Life. London, 1859. 5th Edition, 1869.

2. _Jean Lamarck_, Philosophie Zoologique, ou Exposition des Considérations relatives à l’histoire naturelle des animaux; à la diversité de leur organisation et des facultés, qu’ils en obtiennent; aux causes physiques, qui maintiennent en eux la vie et donnent lieu aux mouvemens, qu’ils exécutent; enfin, à celles qui produisent, les unes le sentiment, et les autres l’intelligence de ceux qui en sont doués. 2 Tomes. Paris, 1809.

3. _Wolfgang Goethe_, Zur Morphologie: Bildung und Umbildung organischer Naturen. Die Metamorphose der Pflanzen, 1790. Osteologie, 1786. Vorträge über die drei ersten Capitel des Entwurfs einer allgemeinen Einleitung in die vergleichende Anatomie, ausgehend von der Osteologie, 1786. Zur Naturwissenschaft im Allgemeinen, 1780-1832.

(Wolfgang Goethe, Contributions to Morphology: Formation and Transformation of Organic Natures. The Metamorphosis of Plants, 1790. Osteology, 1786. Lectures on the first three chapters of an Attempt at a General Introduction to Comparative Anatomy, beginning with Osteology, 1786. Contributions to the Science of Nature in general, 1780-1832.)

4. _Ernst Haeckel_, Generelle Morphologie der Organismen: Allgemeine Grundzüge der organischen Formenwissenschaft, mechanisch begründet durch die von Charles Darwin reformirte Descendenz-theorie. I. Band, Allgemeine Anatomie der Organismen, oder Wissenschaft von den entwickelten organischen Formen. II. Band, Allgemeine Entwickelungsgeschichte der Organismen, oder Wissenschaft von den entstehenden organischen Formen. Berlin, 1866.

(Ernst Haeckel, General Morphology of Organisms; General Outlines of the Science of Organic Forms based on Mechanical Principles through the Theory of Descent as reformed by Charles Darwin. Vol. I., General Anatomy of Organisms; or, the Science of Fully Developed Organic Forms. Vol. II., General History of the Development of Organisms; or, the Science of Organic Forms in their Origin. Berlin, 1866.)

5. _Louis Agassiz_, An Essay on Classification. Contributions to the Natural History of the United States. Boston. Vol. I., 1857.

6. _August Schleicher_, Die Darwin’sche Theorie und die Sprachwissenschaft. Weimar, 1863.

(August Schleicher, Darwin’s Theory and the Science of Language. Weimar, 1863.)

7. _M. J. Schleiden_, Grundzüge der wissenschaftlichen Botanik (die Botanik als inductive Wissenschaft). 2 Bände. Leipzig, 1849.

(M. J. Schleiden, Principles of Scientific Botany (Botany as an Inductive Science). 2 Vols. Leipzig, 1849. Translated by Edwin Lankester, M.D., F.R.S. London, 1849.)

8. _Franz Unger_, Versuch einer Geschichte der Pflanzenwelt. Wien, 1852.

(Franz Unger, Essay on the History of the Vegetable Kingdom. Vienna, 1852.)

9. _Victor Carus_, System der thierischen Morphologie. Leipzig, 1853.

(Victor Carus, System of Animal Morphology. Leipzig, 1853.)

10. _Louis Büchner_, Kraft und Stoff. Empirisch-naturphilosophische Studien in allgemein verständlicher Darstellung, Frankfort, 1855, 3 Auflage. 1867, 9 Auflage.

(Louis Büchner, Force and Matter. Studies in the Empirical Philosophy of Nature, treated popularly. Frankfort, 1855, 3rd Edition. 1867, 9th Edition.)

11. _Charles Lyell_, Principles of Geology. London, 1830. 10th Edition, 1868.

12. _Albert Lange_, Geschichte des Materialismus und Kritik seiner Bedeutung in der Gegenwart. Iserlohn, 1866.

(Albert Lange, History of Materialism, and a Criticism of its Importance at the Present Time. Iserlohn, 1866.)

13. _Charles Darwin_, Voyage of the _Beagle_. London.

14. _Charles Darwin_, The Variation of Animals and Plants under Domestication. 2 Vols. London, 1868.

15. _Ernst Haeckel_, Studien über Moneren und andere Protisten, nebst einer Rede über Entwickelungsgang und Aufgabe der Zoologie. Mit 6 Kupfertafeln. Leipzig, 1870.

(Ernst Haeckel, Studies on the Monera and other Protista, together with a Discourse on the Evolution and the Problems of Zoology. With 6 Copper-plates. Leipzig, 1870.)

16. _Fritz Müller_, Für Darwin. Leipzig, 1864.

(Fritz Müller, For Darwin. Translated by W. S. Dallas. London, Murray.)

17. _Thomas Huxley_, On our Knowledge of the Causes of the Phenomena of Organic Nature. Six Popular Lectures. London, Hardwicke, 1862.

18. _H. G. Brönn_, Morphologische Studien über die Gestaltungsgesetze der Naturkörper überhaupt, und der Organischen insbesondere. Leipzig und Heidelberg, 1858.

(H. G. Brönn, Morphological Studies on the Laws of Form of Natural Bodies in General, and of Organic Bodies in Particular. Leipzig and Heidelberg, 1858.)

19. _H. G. Brönn_, Untersuchungen über die Entwickelungsgesetze der organischen Welt während der Bildungszeit unserer Erdoberfläche. Stuttgart, 1858.

(H. G. Brönn, Investigations on the Laws of Development of the Organic World during the Time of the Formation of the Earth’s Crust. Stuttgart, 1858.)

20. _Carl Ernst Bär_, Ueber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion. 2 Bände. 1828.

(Carl Ernst Bär, On the History of the Development of Animals. Observation and Reflection. 2 Vols. 1828.)

21. _Carl Gegenbaur_, Grundzüge der vergleichenden Anatomie. Leipzig, 1859. 2 (Umgearbeitete) Auflage, 1870.

(Carl Gegenbaur, Outlines of Comparative Anatomy. Leipzig, 1859. 2nd (Revised) Edition, 1870.)

22. _Immanuel Kant_, Allgemeine Naturgeschichte und Theorie des Himmels, oder Versuch von der Verfassung und dem mechanischen Ursprunge des ganzen Weltgebäudes nach Newton’schen Grundsätzen abgehandelt. Königsberg, 1755.

(Immanuel Kant, General History of Nature and Theory of the Heavens; or, Essay on the Constitution and the Mechanical Origin of the whole Universe treated according to Newton’s Principles. Königsberg, 1755.)

23. _Ernst Haeckel_, Die Radiolarien. Eine Monographie. Mit einem Atlas von 35 Kupfertafeln. Berlin, 1862.

(Ernst Haeckel, The Radiolaria. A Monograph, with Atlas containing 35 Copper-plates. Berlin, 1862.)

24. _August Weismann_, Ueber den Einflusz der Isolirung auf die Artbildung. Leipzig, 1872.

(August Weismann, On the Influence of Isolation on the Formation of Species. Leipzig, 1872.)

25. _Ernst Haeckel_, Ueber die Enstehung und den Stammbaum des Menschengeschlechts. Zwei Vorträge in der Sammlung gemeinverständlicher wissenschaftlicher Vorträge, herausgegeben von Virchow und Holtzendorff. Berlin, 1868. 2 Auflage, 1870.

(Ernst Haeckel, On the Origin and the Pedigree of the Human Race. Two Lectures in the Collection of Popular Scientific Lectures, edited by Virchow and Holtzendorff. Berlin, 1868. 2nd Edition, 1870.)

26. _Thomas Huxley_, Evidences as to Man’s Place in Nature. Three Parts: 1. On the Natural History of the Man-like Apes. 2. On the Relations of Man to the Lower Animals. 3. On some Fossil Remains of Man. London, Williams & Norgate.

27. _Carl Vogt_, Vorlesungen über den Menschen, seine Stellung in der Schöpfung und in der Geschichte der Erde. 2 Bände. Giessen, 1863.

(Carl Vogt, Lectures on Man, his Place in Creation and in the History of the Earth. 2 Vols. Giessen, 1863.)

28. _Friedrich Rolle_, Der Mensch, seine Abstammung und Gesittung im Lichte der Darwin’schen Lehre von der Art-Entstehung, und auf Grund der neueren geologischen Entdeckungen dargestellt. Frankfurt-a-M., 1866.

(Friedrich Rolle, Man, his Derivation and Civilization, in the Light of Darwin’s Theory of the Origin of Species, based on Recent Geological Discoveries. Frankfort-a-M., 1866.)

29. _Eduard Reich_, Die allgemeine Naturlehre des Menschen. Giessen, 1865.

(Eduard Reich, The General Natural History of Man. Giessen, 1865.)

30. _Charles Lyell_, The Antiquity of Man. London, Murray.

31. _Bernhard Cotta_, Die Geologie der Gegenwart. Leipzig, 1866.

(Bernhard Cotta, The Geology of the Present Day.)

32. _Karl Zittel_, Aus der Urzeit. Bilder aus der Schöpfungsgeschichte. München, 1871.

(Karl Zittel, Primæval Times. Pictures from the History of Creation. Munich, 1871.)

33. _C. Radenhausen_, Isis. Der Mensch und die Welt. 4 Bände. Hamburg, 1863. 2 Auflage, 1871.

(C. Radenhausen, Isis. Man and the Universe. 4 Vols. Hamburg, 1863. 2nd Edition, 1871.)

34. _August Schleicher_, Ueber der Bedeutung der Sprache für die Naturgeschichte des Menschen. Weimar, 1865.

(August Schleicher, On the Importance of Language to the Natural History of Man. Weimar, 1865).

35. _Wilhelm Bleek_, Ueber den Ursprung der Sprache. Herausgegeben mit einem Vorwort von Ernst Haeckel. Weimar, 1868.

(Wilhelm Bleek, On the Origin of Language. Edited and with a Preface by Ernst Haeckel. Weimar, 1868.)

36. _Alfred Russel Wallace_, The Malayan Archipelago. London, Macmillan.

37. _Ernst Haeckel_, Ueber Arbeitstheilung in Natur- und Menschenleben. Sammlung gemeinverständlicher wissenschaftlicher Vorträge, herausgegeben von Virchow und Holtzendorff. 4 Serie. 1869. Heft 78.

(Ernst Haeckel, On Differentiation in Nature and in Human Life. A Collection of Popular Scientific Lectures, edited by Virchow and Holtzendorff. 4th Series. 1869. No. 78.)

38. _Hermann Helmholtz_, Populäre wissenschaftliche Vorträge. Braunschweig, 1871.

(Hermann Helmholtz, Popular Scientific Lectures. Brunswick, 1871.)

39. _Alexander Humboldt_, Ansichten der Natur. Stuttgart, 1826.

(Alexander Humboldt, Views of Nature. Stuttgart, 1826.)

40. _Moritz Wagner_, Die Darwin’sche Theorie und das Migrationsgesetz der Organismen. Leipzig, 1868.

(Moritz Wagner, Darwin’s Theory and the Law of the Migration of Organisms. Leipzig, 1868.)

41. _Rudolf Virchow_, Vier Reden über Leben und Kranksein. Berlin, 1862.

(Rudolf Virchow, Four Discourses on Life and Disease. Berlin, 1862.)

42. _Friedrich Müller_, Ethnographie (Reise der österreichischen Fregatte Novara. Anthropologischer Theil. 3 Abtheilung). Wien, 1868.

(Friedrich Müller, Ethnography (Voyage of the Austrian Frigate _Novara_. Anthropological Part. 3rd Part). Vienna, 1868.)

43. _Ludwig Büchner_, Die Stellung des Menschen in der Natur, in Vergangenheit, Gegenwart und Zukunft. Leipzig, 1870.

(Ludwig Büchner, Man’s Place in Nature in the Past, the Present, and the Future. Leipzig, 1870.)

44. _John Lubbock_, Prehistoric Times. London, 1867.

45. _Herbert Spencer_, A System of Philosophy. (1. First Principles. 2. Principles of Biology. 3. Principles of Psychology, etc.) London, 1867. 2nd Edition.

46. _Wilhelm Wundt_, Vorlesungen über die Menschen- und Thierseele. Leipzig, 1863.

(Wilhelm Wundt, Lectures on the Human and Animal Soul. Leipzig, 1863.)

47. _Fritz Ratzel_, Sein und Werden der organischen Welt. Eine populäre Schöpfungsgeschichte. Leipzig, 1869.

(Fritz Ratzel, Nature and Origin of the Organic World. A Popular History of Creation. Leipzig, 1869.)

48. _Charles Darwin_, The Descent of Man, and Selection in Relation to Sex. 2 Vols. London, 1871.

APPENDIX.

EXPLANATION OF THE PLATES.

PLATE FACING TITLE-PAGE.

_Developmental History of a Calcareous Sponge_ (Olynthus). Compare vol. ii. p. 140. The egg of the Olynthus (Fig. 9), which represents the common ancestral form of all Calcareous Sponges, is a simple cell (Fig. 1). From this there arises, by repeated division (Fig. 2), a globular, mulberry-like heap of numerous equi-formal cells (Morula, Fig. 3; vol. ii. p. 125.) As the result of the change of these cells into an outer series of clear ciliated cells (Exoderm) and an inner series of dark, non-ciliated cells (Entoderm), the ciliated larva, or Planula, makes its appearance. This is oval in shape, and forms a cavity in its centre (gastric cavity, or primitive stomach, Fig. 6 _g_, with an opening (mouth-opening, or primitive mouth, Fig. 6 _o_); the wall of the gastric cavity consists of two layers of cells, or germ-layers, the outer ciliated Exoderm (_e_) and the inner non-ciliated Entoderm (_i_). Thus arises the exceedingly important stomach-larva, or Gastrula, which reappears in the most different tribes of animals as a common larval form (Fig. 5, seen from the surface; Fig. 6, in long section. Compare, vol. ii. pp. 126 and 281). After the Gastrula has swum about for some time in the sea, it fastens itself securely to the sea-bottom, loses its outer vibratile processes, or cilia, and changes into the Ascula (Fig. 7, seen from the surface; Fig. 8, in long section; letters as in Fig. 6). This Ascula is the recapitulative form, according to the biogenetic fundamental law, the common ancestor of all Zoophytes, namely, the Protascus (vol. ii. pp. 129, 133). By the development of pores in the wall of the stomach and of three-rayed calcareous spicules, the Ascula changes into the Olynthus (Fig. 9.) In Fig. 9 a piece is cut out from the stomach-wall of the Olynthus in order to show the inside of the stomachal cavity, and the eggs which are forming on the surface (_g_). From the Olynthus the most various forms of Calcareous Sponges can develop. One of the most remarkable is the Ascometra (Fig. 10), a stock or colony from which different species, and in fact different generic forms, grow (on the left Olynthus, in the middle Nardorus, on the right Soleniscus, etc., etc.). Further details as to these most interesting forms, and their high importance for the Theory of Descent, may be found in my “Monograph of the Calcareous Sponges” (1872), especially in the first volume. (Compare vol. ii. pp. 160, 167).

PLATE I. (_Between pages 184 and 185, Vol. I._)

_History of the Life of the most Simple Organism_, a Moneron (Protomyxa aurantiaca). Compare vol. i. p. 184, and vol. ii. p. 53. The plate is a smaller copy of the drawing in my “Monographie der Moneren” (Biologische Studien, 1 Heft, 1870; Taf. 1), of the developmental history of the Protomyxa aurantiaca; I have there also given a detailed description of this remarkable Moneron (pp. 11-30). I discovered this most simple organism in January, 1867, during a stay in Lanzarote, one of the Canary Islands; and moreover I found it either adhering to, or creeping about on the white calcareous shells of a small Cephalopod (vol. ii. p. 162), the Spirula Peronii, which float there in masses on the surface of the ocean, or are thrown up on the shore. The Protomyxa aurantiaca is distinguished from the other Monera by the beautiful and bright orange-red colour of its perfectly simple body, which consists merely of primæval slime, or protoplasm. The fully developed Moneron is represented in Figs. 11 and 12, very much enlarged. When it is hungry (Fig. 11), there radiate from the surface of the globular corpuscule of plasm, quantities of tree-shaped, branching and mobile threads (pseudo-feet, or pseudopodia), which do not become retiformly connected. When, however, the Moneron eats (Fig. 12), the mucous threads become variously connected, form net-works and enclose the extraneous corpuscule which serves as food, which the threads afterwards draw into the interior of the Protomyxa. Thus in Fig. 12 (above on the right), a silicious and ciliated Whip-swimmer (Peridinium, vol. ii. pp. 51, 57), has just been caught by the extended mucous filaments, and has been drawn into the interior of the mucous globule, in which there already are several half digested silicious infusoria (Tintinoida), and Diatomeæ (Isthmia). Now, when the Protomyxa has eaten and grown sufficiently, it draws in all its mucous filaments (Fig. 15), and contracts into the form of a globule (Fig. 16 and Fig. 1). In this state of repose the globule secretes a simple gelatinous covering (Fig. 2), and after a time subdivides into a large number of small mucous globules (Fig. 3). These soon commence to move, become pear-shaped (Fig. 4), break through the common covering (Fig. 5), and then swim about freely in the ocean by means of a delicate whip-shaped process, like the Flagellata (vol. ii. p. 57, Fig. 11). When they meet a Spirula shell, or any other suitable object, they adhere to it, draw in their whip, and creep slowly about on it by means of form-changing processes (Figs. 6, 7, 8), like Protamœbæ (vol. i. p. 186, vol. ii. p. 52). These small mucous corpuscules take food (Figs. 9, 10), and attain their full grown form (Figs. 11, 12), either by simple growth or by several of them fusing to form a larger protoplasmic mass (Plasmodium, Figs. 13, 14).

PLATES II. AND III. (_Between pages 294 and 295, Vol. I._)

_Germs or Embryos of four different Vertebrate Animals_, namely, Tortoise (_A_ and _E_), Hen (_B_ and _F_), Dog (_C_ and _G_), and Man (_D_ and _H_). Figs. _A_, _D_, an early stage of development; Figs. _E_, _H_, a later stage. All the eight embryos are represented as seen from the right side, the curved back turned to the left. Figs. _A_ and _B_ are seven times enlarged, Figs. _C_ and _D_ five times, Figs. _E_ and _H_ four times. Plate II. exhibits the very close blood relationship between birds and reptiles; Plate III. that between man and the other mammals.

PLATE IV. (_Between pages 34 and 35, Vol. II._)

_The Hand, or Fore Foot, of nine different Mammals._ This plate is intended to show the importance of Comparative Anatomy to Phylogeny, in as much as it proves how the internal skeleton of the limbs is continually preserved by _inheritance_, although the external form is extremely changed by _adaptation_. The bones of the skeleton of the hand are drawn in white lines on the brown flesh and skin which surrounds them. All the nine hands are represented in the same position, namely the wrist (where the arm would be joined to it) is placed above, whilst the ends of the fingers or toes are turned downwards. The thumb, or the first (large) fore-toe is on the left in every figure; the little finger, or fifth toe is to the right at the edge of the hand. Each hand consists of three parts, namely (i.) the _wrist_ (carpus), composed of two cross rows of short bones (at the upper side of the hand); (ii.) the _mid-hand_ (metacarpus), composed of five long and strong bones (marked in the centre of the hand by the numbers 1-5); and (iii.) the five _fingers_, or _fore toes_ (digiti), every one of which again consists of several (mostly from two to three), _toe-pieces_, or _phalanges_. The hand of _man_ (Fig. 1), in regard to its entire formation, stands midway between that of the two large human apes, namely, that of the _gorilla_ (Fig. 2), and that of the _orang_ (Fig. 3). The fore paw of the _dog_ (Fig. 4), is more different, and the hand or breast fin of the _seal_ (Fig. 5) still more so. The adaptation of the hand to the movement of swimming, and its transformation into a fin for steering, is still more complete in the _dolphin_ (Ziphius, Fig. 6). The extended fingers and bones of the central hand here have remained short and strong in the swimming membrane, but they have become extremely long and thin in the _bat_ (Fig. 7), where the hand has developed into a wing. The extreme opposite of the latter formation is the hand of the _mole_ (Fig. 8), which has acquired a powerful spade-like form for digging, with fingers which have become extremely short and thick. What is far more like the human hand than these latter forms, is the fore paw of the lowest and most imperfect of all mammals, the Australian _beaked animal_ (Ornithorhynchus, Fig. 9), which in its whole structure stands nearer to the common, extinct, primary form of mammalia, than any known species. Hence man differs less in the formation of the hand from this common primary form than from the bat, mole, dolphin, seal, and many other mammals.

PLATE V. (_Between pages 84 and 85, Vol. II._)

_Monophyletic, or One-rooted Pedigree of the Vegetable Kingdom_, representing the hypothesis of the common derivation of all plants, and the historical development of the different groups of plants during the palæontological periods of the earth’s history. The horizontal lines denote the different smaller and larger periods of the organic history of the earth (which are spoken of in vol. ii. p. 14), and during which the strata containing fossils were deposited. The vertical lines separate the different main-classes and classes of the vegetable kingdom from one another. The arboriform and branching lines indicate, in an approximate manner, by their greater or less number and thickness, the greater or less degree of development, differentiation, and perfecting which each class probably attained in each geological period. (Compare vol. ii. pp. 82, 83.)

PLATE VI. (_Between pages 130 and 131, Vol. II._)

_Monophyletic, or One-rooted Pedigree of the Animal Kingdom_, representing the _historical growth of the six animal tribes_ during the palæontological periods of the organic history of the earth. The horizontal lines _g h_, _i k_, _l m_, and _n o_ divide the five large periods of the organic history of the earth one from another. The field _g a b h_ comprises the archilithic, the field _i g h k_, the palæolithic, the field _l i k m_ the mesolithic, and the field _n l o m_ the cenolithic period. The short, anthropolithic period is indicated by the line _n o_. (Compare vol. ii. p. 14.) The height of the separate fields corresponds with the relative length of the periods indicated by them, as they may approximately be estimated from the relative thickness of the neptunic strata deposited between them. (Compare vol. ii. p. 22.) The archilithic and primordial period alone, during which the Laurentian, Cambrian, and Silurian strata were deposited, was probably considerably longer than the four subsequent periods taken together. (Compare vol. ii. pp. 10, 20.) In all probability the two tribes of worms and Zoophytes attained their full development during the mid-primordial period (in the Cambrian system); the star-fishes and molluscs probably somewhat later (in the Silurian system); whereas the articulata and vertebrata are still increasing in variety and perfection.

PLATE VII. (_Between pages 146 and 147, Vol. II._)

_Group of Animal-Trees_ (_Zoophytes, or Cœlenterata_) _in the Mediterranean_. On the upper half of the plate is a swarm of swimming medusæ and ctenophora; on the lower half a few bunches of corals and hydroid polyps adhering to the bottom of the sea. (Compare the system of Zoophytes, vol. ii. p. 132, and on the opposite page their pedigree.) Among the adhering Zoophytes at the bottom of the ocean there is, below on the right hand, a large coral-colony (1), which is closely akin to the red precious coral (Eucorallium), and like the latter belongs to the group of corals with eight rays (Octocoralla Gorgonida); the single individuals (or persons) of the branching stock have the form of a star with eight rays, consisting of eight tentacles, which surround the mouth. (Octocoralla, vol. ii. p. 143.) Directly below and in front of it (quite below on the right), is a small bush of hydroid polyps (2), belonging to the group of bell-polyps, or Campanulariæ (vol. ii. p. 146). A larger stock of hydroid polyps (3), belonging to the group of tube-polyps, or Tubullariæ, rises, to the left, on the opposite side, with its long thin branches. At its base is spread a stock of silicious sponges (Halichondria) (4), with short, finger-shaped branches (vol. ii. p. 139). Behind it, below on the left (5), is a very large marine rose (Actinia), a single individual from the class of six-rayed corals (Hexacoralla, vol. ii. p. 143). Its low, cylindrical body has a crown of very numerous and large leaf-shaped tentacles. Below, in the centre of the ground (6), is a sea-anemone (Cereanthus) from the group of fourfold corals (Tetracoralla). Lastly, on a small hill on the bottom of the sea, there rises, on the right above the corals (1) a cup-polyp (Lucernaria), as the representative of the stalked-jellies. (Podactinaria, or Calycozoa, vol. ii. p. 144.) Its cup-shaped, stalked body (7) has eight globular clusters of small, knotted tentacles on its rim.

Among the _swimming Zoophytes_ which occupy the upper half of Plate VII., the hydromedusæ are especially remarkable, on account of their alteration of generation. (Compare vol. i. p. 206.) Directly above the Lucernaria (7) floats a small tiara jelly (Oceania), whose bell-shaped body has a process like a dome, the form of a papal tiara (8). From the opening of the bell there hangs a wreath of very fine and long tentacles. This Oceania is the offspring of a tube-polyp, resembling the adhering Tubularia below on the left (3). Beside this latter, on the left, swims a large but very delicate hair-jelly (Æquorea). Its disc-shaped, slightly arched body is just drawing itself together, and pressing water out of the cavity of the cup lying below (9). The numerous, long, and fine hair-like tentacles which hang down from the rim of the cup are drawn by the ejected water into a conical bunch, which towards the centre turns upwards like a collar, and is thrown into folds. Above, in the middle of the cavity of the cup, hangs the stomach, the mouth of which is surrounded by four lobes. This Æquorea is derived from a small bell-polyp, resembling the Campanularia (2). The small, slightly arched cap-jelly (Eucope), swimming above in the centre (10), is likewise derived from a similar bell-polyp. In these three last cases (8, 9, 10), as in the majority of the hydromedusæ, the alternation of generation consists in the freely swimming medusa (8, 9, 10), arising by the formation of buds (therefore by non-sexual generation, vol. i. p. 192), from adhering hydroid polyps (2, 3). These latter, however, originate out of the fructified eggs of the medusæ (therefore by sexual generation, vol. i. p. 195). Hence the non-sexual, adhering generation of polyps (I., III., V., etc.) regularly alternates with the sexual, freely swimming generation of medusæ (II., IV., VI., etc.). This alteration of generation can only be explained by the Theory of Descent.

The same remark applies to a kindred form of propagation, which is still more remarkable, and which I discovered in 1864, near Nice, in the Elephant-jellies (Geryonida), and called _allœogony_, or _allœogenesis_. In this case two completely distinct forms of medusa are descended from one another; the larger and more highly developed generation (11), Geryonia, or Carmarina, is six-rayed, with six foliated sexual organs, and six very movable marginal filaments. From the centre of its bell-shaped cup, like the tongue of a bell, hangs a long proboscis, at the end of which is the opening of the mouth and stomach. In the cavity of the stomach is a long, tongue-shaped bunch of buds (which on Plate VII. (_n_) is extended from the mouth on the left like a tongue). On this tongue, when the Geryonia is sexually ripe, there bud a number of small medusæ. They are, however, not Geryoniæ, but belong to an entirely distinct but very different form of medusa, namely, to the genus Cunina, of the family of the _Æginida_. This Cunina (12) is very differently constructed; it has a flat, semi-globular cup without proboscis, consists in early life of six divisions, later of sixteen, and has sixteen bag-shaped sexual organs, and sixteen short, stiff, and strongly curved tentacles. A further explanation of this wonderful allœogenesis may be found in my “Contributions to the Natural History of the Hydromedusæ.” (_Leipzig_, Englemann, 1865), the first part of which contains a monograph of the Elephant-jellies, or Geryonida, illustrated by six copper-plates.

Even more interesting and instructive than these remarkable relations are the vital phenomena of the _Siphonophora_, whose wonderful polymorphism I have frequently spoken of, and described in a popular manner in my lecture on “Differentiation in Nature and Human Life.”(37) (Compare vol. i. p. 270, and vol. ii. p. 140.) An example of this is given in Plate VII. in the drawing of the beautiful Physophora (13). This swimming stock or colony of hydromedusæ is kept floating on the surface of the sea by a small swimming bladder filled with air, which in the drawing is seen rising above the surface of the water. Below it is a column of four pairs of swimming bells, which eject water, and thereby set the whole colony in motion. At the lower end of the column of swimming bells is a crown-shaped wreath of curved spindle-shaped _sensitive polyps_, which also serve as a covering, under the protection of which the other individuals of the stock (the eating, catching, and reproductive persons) are hidden. The _ontogenesis_ of the Siphonophora (and especially of this Physophora), I first observed in Lanzerote, one of the Canary Islands, in 1866, and described in my “History of the Development of the Siphonophora,” and added fourteen plates for its explanation. (Utrecht, 1869). It is rich in interesting facts, which can only be explained by the Theory of Descent.

Another circumstance, which is also only explicable by the Theory of Descent, is the remarkable change of generation in the higher medusæ, the disc-jellies (Discomedusæ, vol. ii. p. 136), a representative of which is given at the top of Plate VII., in the centre (rather in the background), namely, a Pelagia (14). From the bottom of the bell-shaped cup, which is strongly arched and the rim of which is neatly indented, there hang four very long and strong arms. The non-sexual polyps, from which these disc-jellies are derived, are exceedingly simple primæval polyps, differing very little from the common fresh-water polyp (Hydra). The alternation of generation in these Discomedusæ has also been described in my lecture on Differentiation,(37) and there illustrated by the Aurelia by way of example.

Finally, the last class of Zoophytes, the group of comb-jellies (Ctenophora, vol. ii. p. 142), has two representatives on Plate VII. To the left, in the centre, between the Æquorea (9), the Physophora (13), and the Cunina (12), is a long and thin band like a belt (15), winding like a snake; this is the large and splendid _Venus’ girdle_ of the Mediterranean (Cestum), the colours of which are as varied as those of the rainbow. The actual body of the animal, which lies in the centre of the long belt, is very small, and constructed exactly like that of the _melon-jelly_ (Cydippe), which floats above to the left (16). On the latter are visible the eight characteristic fringed bands, or ciliated combs, of the ctenophora, and also two long tentacles which extend right across the page, and are fringed with still finer threads.

PLATES VIII. AND IX. (_Between pages 170 and 171, Vol. II._)

_History of the Development of Star-fishes_ (_Echinoderma_, or _Estrella_). The two plates exhibit their alternation of generation (vol. ii. p. 168), with an example from each of the four classes of Star-fishes. The sea-stars (Asterida) are represented by Uraster (_A_), the sea-lilies (Crinoida) by Comatula (_B_), the sea-urchins (Echinida) by Echinus (_C_), and finally, the sea-cucumbers (Holothuriæ) by Synapta (_D_). (Compare vol. ii. pp. 166 and 176.) The successive stages of development are marked by the numbers 1-6.

Plate VIII. represents the individual development of the first and non-sexual generation of Star-fishes, that is, of the _nurses_ (usually, but erroneously, called larvæ). These nurses possess the form-value of a simple, unsegmented worm-individual. Fig. 1 represents the egg of the four Star-fishes; and it, in all essential points, agrees with that of man and of other animals. (Compare vol. i. p. 297, Fig. 5.) As in man, the protoplasm of the egg-cell (the yolk) is surrounded by a thick, structureless membrane (zona pellucida), and contains a globular, cell-kernel (nucleus), as clear as glass, which again encloses a nucleolus. Out of the fertilised egg of the Star-fish (Fig. _A_ 1) there develops in the first place, by the repeated sub-division of cells, a globular mass of homogeneous cells (Fig. 6, vol. i. p. 299), and this changes into a very simple nurse, which has almost the same shape as a wooden shoe (Fig. _A_ 2-_D_ 2). The edge of the opening of the shoe is bordered by a fringe of cilia, the ciliary movements of which keep the microscopically small and transparent nurse swimming about freely in the sea. This fringe of cilia is marked in Fig. _A_ 2-_A_ 4, on Plate VII., by the narrow alternately light and dark seam. The nurse then, in the first place, forms a perfectly simple intestinal canal for nutrition, mouth (_o_), stomach (_m_) and anus (_a_). Later, the windings of the fringe of cilia become more complicated, and there arise arm-like processes (Fig. _A_ 3-_D_ 3). In sea-stars (_A_ 4) and sea-urchins (_C_ 4) these arm-like processes, which are fringed with cilia, afterwards become very long. But in the case of sea-lilies (_B_ 3) and sea-cucumbers (_D_ 4), instead of this, the fringe of cilia, which at first, through winding in and out, forms one closed ring, changes subsequently into a succession of separate ciliated girdles, one lying behind the other.

In the interior of this curious nurse there then develops, by a non-sexual process of generation, namely, by the formation of internal buds or germ-buds (round about the stomach), the second generation of Star-fishes, which later on become sexually ripe. This second generation, which is represented on Plate IX. in a fully developed condition, exists originally as a stock or cormus of five worms, connected at one end in the form of a star, as is most clearly seen in the sea-stars, the most ancient and original form of the star-fishes. The second generation, which grows at the expense of the first, appropriates only the stomach and a small portion of the other organs of the latter, but forms for itself a new mouth and anus. The fringe of cilia, and the other parts of the body of the nurse, afterwards disappear. The second generation (_A_ 5-_D_ 5), is at first smaller or not much larger than the nurse, whereas, by growth, it afterwards becomes more than a hundred times, or even a thousand times, as large. If the ontogeny of the typical representatives of the four classes of Star-fishes be compared, it is easily seen that the original kind of development has been best preserved in sea-stars (_A_) and sea-urchins (_C_) by inheritance, whereas in sea-lilies (_B_) and sea-cucumbers it has been suppressed according to the laws of abbreviated inheritance (vol. i. p. 212).

Plate IX. shows the fully developed and sexually mature animals of the second generation from the mouth side, which, in the natural position of Star-fishes (when creeping at the bottom of the sea), in sea-stars (_A_ 6) and sea-urchins (_C_ 6), is below, in sea-lilies (_B_ 6) above, and in sea-cucumbers (_D_ 6) in front. In the centre we perceive, in all the four Star-fishes, the star-shaped, five-pointed opening of the mouth. In sea-stars, from each arm there extend several rows of little sucking feet, from the centre of the under-side of each arm to the end. In sea-lilies (_B_ 6), each arm is split and feather-like from its base upwards. In sea-urchins (_C_ 6) the five rows of sucking feet are divided by broader fields of spines. In sea-cucumbers, lastly (_D_ 6), on the worm-like body it is sometimes only the five rows of little feet, sometimes only the feathery tentacles surrounding the mouth, from five to fifteen (in this case ten), that are externally visible.

PLATES X. AND XI. (_Between pages 174 and 175, Vol. II._)

_Historical Development of the Crab-fish_ (Crustacea).—The two plates illustrate the development of the different Crustacea from the nauplius, their common primæval form. On Plate XI. six Crustacea, from six different orders, are represented in a fully developed state, whereas on Plate X. the early nauplius stages are given. From the essential agreement between the latter we may, on the ground of the fundamental law of biogeny, with full assurance maintain the derivation of the different Crustacea from a single, common primary form, a long since extinct Nauplius, as was first shown by Fritz Müller in his excellent work “Für Darwin.”(16)

Plate X. represents the _early nauplius stages_ from the ventral side, so that the three pairs of legs, on the short, three-jointed trunk are distinctly visible. The first of these pairs of legs is simple and unsegmented, whereas the second and third pairs are forked. All three pairs are furnished with stiff bristles, which, through the paddling motion of the legs, serve as an apparatus for swimming. In the centre of the body, the perfectly simple, straight intestinal canal is visible, possessing a mouth in front, and an anal orifice behind. In front, above the mouth, lies a simple, single eye. All the six forms of nauplius entirely agree in all these essential characteristics of organization, whereas the six fully developed forms of Crustacea belonging to them, Plate XI., are extremely different in organisation. The differences of the six nauplius forms are confined to quite subordinate and unessential relations in regard to size of body, and the formation of the covering of the skin. If they could be met with in this form in a sexually mature condition, no zoologist would hesitate to regard them as six different species of one genus. (Compare vol. ii. p. 175.)

Plate XI. represents those fully developed and sexually mature forms of Crustacea, as seen from the right side, which have ontogenetically (hence also phylogenetically) developed out of the six kinds of nauplius. Fig. _A c_ shows a freely swimming fresh-water crab (Limnetis brachyurus) from the order of the Leaf-foot Crabs (Phyllopoda), slightly enlarged. Of all the still living Crustacea, this order, which belongs to the legion of Gill-foot Crabs (Branchiopoda), stands nearest to the original, common primary form of nauplius. The Limnetis is enclosed in a bivalved shell, like a mussel. Our drawing (which is copied from Grube) represents the body of a female animal lying in the left shell; the right half of the shell has been removed. In front, behind the eye, we see the two feelers (antennæ), and behind them the twelve leaf-shaped feet of the right side of the body, behind on the back (under the shell), the eggs. Above, in front, the animal is fixed to the shell.

Fig. _B c_ represents a common, freely swimming fresh-water crab (Cyclops quadricornis) from the order of Oar-legged crabs (Eucopepoda), highly magnified. In front, below the eye, we see the two feelers of the right side, the foremost of which is longer than the hinder one. Behind these are the gills, and then the four paddling legs of the right side. Behind these are the two large egg-sacks, which, in this case, are attached to the end of the hinder part of the body.

Fig. _C c_ is a parasitic Oar-legged crab (Lernæocera esocina), from the order of fish lice (Siphonostoma). These peculiar crabs, which were formerly regarded as worms, have originated, by adaptation to a parasitical life, out of freely swimming, Oar-legged crabs (Eucopepoda), and belong to the same legion (Copepoda, vol. ii. p. 176). By adhering to the gills on the skin of fish or other crabs, and feeding on the juice of these creatures, they forfeited their eyes, legs, and other organs, and developed into formless, inarticulated sacks, which, on a mere external examination, we should never suppose to be animals. On the ventral side only there exist, in the shape of short, pointed bristles, the last remains of legs which have now almost entirely disappeared. Two of these rudimentary pairs of legs (the third and fourth) are seen in our drawing on the right. Above, on the head, we see thick, shapeless appendages, the lower ones of which are split. In the centre of the body is seen the intestinal canal, which is surrounded by a dark covering of fat. At its posterior end is the ovary, and the cement-glands of the female sexual apparatus. The two large egg-sacks hang externally (as in the Cyclops, Fig. _B_). Our Lernæocera is represented in half profile, and is copied from Claus. (Compare Claus, “Die Copepoden-Fauna von Nizza. Ein Beitrag zur Characteristik der Formen und deren Abänderungen im Sinne Darwins.” Marburg, 1866).

Fig. _D c_ represents a so-called “duck mussel” (Lepas anatifera), from the order of the Barnacle crabs (Cirripedia). These crabs, upon which Darwin has written a very careful monograph, are, like mussels, enclosed in a bivalved, calcareous case, and hence were formerly (even by Cuvier) universally regarded as a kind of mussel, or mollusc. It was only from a knowledge of their ontogeny, and their early nauplius form (_D n_, Plate VIII.), that their crustacean nature was proved. Our drawing shows a “duck mussel” of the natural size, from the right side. The right half of the bivalved shell has been removed, so that the body is seen lying in the left half of the shell. From the rudimentary head of the Lepas there issues a long, fleshy stalk (curving upwards in our drawing); by means of it the Barnacle crab grows on rocks, ships, etc. On the ventral side are six pairs of feet. Every foot is forked and divided into two long, curved, or curled “tendrils” furnished with bristles. Above and behind the last pair of feet projects the thin cylindrical tail.

Fig. _E c_ represents a parasitic sack-crab (Sacculina purpurea) from the order of Root-crabs (Rhizocephala). These parasites, by adaptation to a parasitical life, have developed out of Barnacle crabs (Fig. _D c_), much in the same way as the fish-lice (_C c_), out of the freely swimming Oar-legged crabs (_B c_). However, the suppression, and the subsequent degeneration, of all of the organs, has gone much further in the present case than in most of the fish-lice. Out of the articulated crab, possessing legs, intestine, and eye, and which in an early stage as nauplius (_E n_, Plate VIII.), swam about freely, there has developed a formless, unsegmented sack, a red sausage, which now only contains sexual organs (eggs and sperm) and an intestinal rudiment. The legs and the eye have completely disappeared. At the posterior end is the opening of the genitals. From the mouth grows a thick bunch of numerous tree-shaped and branching root-like fibres. These spread themselves out (like the roots of a plant in the ground) in the soft hinder part of the body of the hermit-crab (Pagurus), upon which the root-crab lives as a parasite, and from which it draws its nourishment. Our drawing (_E c_), a copy of Fritz Müller’s, is slightly enlarged, and shows the whole of the sausage-shaped sack-crab, with all its root-fibres, when drawn out of the body upon which it lives.

Fig. _F c_ is a _shrimp_ (Peneus Mülleri), from the order of _ten-foot_ crabs (Decapoda), to which our river cray-fish, and its nearest relative, the lobster, and the short-tailed shore-crabs also belong. This order contains the largest and, gastronomically, the most important crabs, and belongs, together with the mouth-legged and split-legged crabs, to the legion of the stalk-eyed mailed crabs (Podophthalma). The shrimp, as well as the river crab, has in front, on each side below the eye, two long feelers (the first much shorter than the second), then three jaws, and three jaw-feet, then five very long legs (the three fore ones of which, in the Peneus, are furnished with nippers, and the third of which is the longest). Finally, on the first five joints of the hinder part of the body there are other five pairs of feet. This shrimp, which is one of the most highly developed and perfect crabs, originates (according to Fritz Müller’s important discovery) out of a nauplius (_F n_ Plate VIII.), and consequently proves that the higher Crustacea have developed out of the same form as the lower ones, namely, the nauplius. (Compare vol. ii. p. 175.)

PLATES XII. AND XIII. (_Between pages 200 and 201, Vol. II._)

_Blood relationship between the Vertebrata and the Invertebrata._ (Compare vol. ii. pp. 152 and 201.) It is definitely established by Kowalewski’s important discovery, which was confirmed by Kupffer, that the ontogeny of the lowest vertebrate animal—the Lancelet, or Amphioxus—agrees in all essential outlines completely with that of the invertebrate Sea-squirts, or Ascidiæ, from the class of Sea-sacks, or Tunicata. On our two plates, the ascidia is marked by _A_, the amphioxus by _B_. Plate XIII. represents these two very different animal-forms in a _fully developed_ state, as seen from the _left side_, the end of the mouth above, the opposite end below. Hence, in both figures the dorsal side is to the right, the ventral to the left. Both figures are slightly magnified, and the internal organisation of the animals is distinctly visible through the transparent skin. The full-grown ascidia (Fig. _A_ 6) grows at the bottom of the ocean, from whence it cannot move, and clings to stones and other objects by means of peculiar roots (_w_) like a plant. The full-grown amphioxus, on the other hand (Fig. _B_ 6), swims about freely like a small fish. The letters on both figures indicate the same parts: (_a_) orifice of the mouth; (_b_) orifice of the body, or porus abdominalis; (_c_) dorsal rod, or chorda dorsalis; (_d_) intestine; (_e_) ovary; (_f_) oviduct (same as the sperm-duct); (_g_) spinal marrow; (_h_) heart; (_i_) blind-sac of the intestine; (_k_) gill basket (respiratory cavity); (_l_) cavity of the body; (_m_) muscles; (_n_) testicle (in the ascidia united with the ovary into a hermaphrodite gland); (_o_) anus; (_p_) genital orifice; (_q_) well-developed embryos in the body cavity of the ascidia; (_r_) rays of the dorsal fin of the amphioxus; (_s_) tail-fin of the amphioxus; (_w_) roots of the ascidia.

Plate XII. shows the _Ontogenesis_, or the individual development of the _Ascidia_ (_A_) and the _Amphioxus_ (_B_) in five different stages (1-5). Fig. 1 is the egg, a simple cell like the egg of man and all other animals (Fig. _A_ 1 the egg of the ascidia, Fig. _B_ 1 the egg of the amphioxus). The actual cell-substance, or the protoplasm of the egg-cell (_z_), the so-called yolk, is surrounded by a covering (cell-membrane, or yolk-membrane), and encloses a globular cell-kernel, or nucleus (_y_), the latter, again, contains a kernel-body, or nucleolus (_x_); when the egg begins to develop, the egg-cell first subdivides into two cells. By another sub-division there arise four cells (Fig. _A_ 2, _B_ 2), and out of these, by repeated sub-division, eight cells (vol. i. p. 190, Fig. 4 _C_, _D_). By fluid gathering in the interior these form a globular bladder bounded by a layer of cells. On one spot of its surface the bladder is turned inwards in the form of a pocket (Fig. _A_ 4, _B_ 4). This depression is the beginning of the intestine, the cavity (_d_ 1) of which opens externally by the provisional larval-mouth (_d_ 4). The body-wall, which is at the same time the stomach-wall, now consists of two layers of cells—the germ-layers. The globular larva (Gastrula), now grows in length. Fig. _A_ 5 represents the larva of the ascidia, Fig. _B_ 5 that of the amphioxus, as seen from the left side in a somewhat more advanced state of development. The orifice of the intestine (_d_ 1) has closed. The dorsal side of the intestine (_d_ 2) is concave, the ventral side (_d_ 3) convex. Above the intestinal tube, on its dorsal side, the neural tube, the beginning of the spinal marrow, is being formed, its cavity still opens externally in front (_g_ 2). Between the spinal marrow and the intestine has arisen the spinal rod, or chorda dorsalis (Notochord) (_c_), the axis of the inner skeleton. In the larva of the ascidia this rod (_c_) proceeds along the long rudder-tail, a larval organ, which is cast off in later transformation. Yet there still exist some very small ascidiæ (Appendicularia) which do not become transformed and attached, but which through life swim about freely in the sea by means of their rudder-tail.

The ontogenetic facts which are systematically represented on Plate XII. and which were first discovered in 1867, deserve the greatest attention, and, indeed, cannot be too highly estimated. They fill up the gap which, according to the opinion of older zoologists existed between the vertebrate and the so-called “invertebrate” animals. This gap was universally regarded as so important and so undeniable, that even eminent zoologists, who were not disinclined to adopt the theory of descent, saw in this gap one of the chief obstacles against it. Now that the ontogeny of the amphioxus and the ascidia has set this obstacle completely aside, we are for the first time enabled to trace the pedigree of man beyond the amphioxus into the many-branching tribe of “invertebrate” worms, from which all the other higher animal tribes have originated.

If our speculative philosophers, instead of occupying themselves with castles in the air, were to give their thoughts for some years to the facts represented on Plates XII. and XIII., as well as to those on Plates II. and III., they would gain a foundation for true philosophy—for the knowledge of the universe firmly based on experience—which would be sure to influence all regions of thought. These facts of ontogenesis are the indestructible foundations upon which the monistic philosophy of future times will erect its imperishable system.

PLATE XIV. (_Between pages 206 and 207, Vol. II._)

_Monophyletic, or One-rooted Pedigree of the Vertebrate Animal tribe_, representing the hypothesis of the common derivation of all vertebrate animals, and the historical development of their different classes during the palæontological periods of the earth’s history. (Compare