Chapter 3

Finally, we find almost always in protoplasm other substances composed of carbon and hydrogen and oxygen which are called hydrocarbons, distinguished from carbohydrates by the fact that the number of oxygen atoms is less than half the number of hydrogen atoms. These substances are the fats and oils of various kinds, less powerful sources of energy than the proteins, but they contain more potential energy than the carbohydrates because they are more oxidizable.

Besides the characteristic substances of these three classes, protoplasm contains certain other chemical compounds, like the various salts of sodium, chlorine, magnesium and potassium, and a few others, which bring the list of chemical elements to the number twelve. We have already noted how strikingly small and restricted is the list of elements composing living matter as compared with the long array of eighty-odd different kinds of chemical atoms existing in the world as a whole.

But an astonishing result is reached through the brief analysis we have just made. It is this: we do not find _peculiar_ kinds of atoms which occur exclusively in living matter; the materials are exactly the same as those of the outer world. In short, the elements of both the organic and inorganic divisions of the universe prove to be the same. Carbon is carbon, whether it is part of the substance of a living brain cell, or black inert coal, or the glistening diamond, or an incandescent part of the fiery sun. Hydrogen is the same, whether it be a constituent of the ocean, of the air, or of the living muscle fiber. And so it is with all of the other elements of the living mechanism. This starts us upon a line of thought which leads to a significant conclusion, namely, that a living thing which seems so distinct and permanent is after all only a temporary aggregate of elements which come to it from the not-living world; existing for a time in peculiar combinations which render life possible, they pass incessantly away from the living thing and return to the inorganic world. Every breath we draw sends out particles which were at one time living portions of ourselves; every movement we make involves the destruction of living muscle cells, whose protoplasm breaks down into the ash and gas and fluid wastes which eventually return to the world of dead things. A tree loses its living leaves with each recurring season, and the antlers of the stag are lost annually, to be replaced anew. Indeed the major part of some organisms is itself actually dead. The bones and hair and nails of such an animal as a cat are almost entirely lifeless, even though they are integral and necessary portions of the organism as a whole. They are constructed by living protoplasm which has died in their making. Thus without going beyond the boundaries of the individual body, these substances have passed from the sphere of life, and are dead. The apparent gap on the other side between the lifeless and living world is equally imaginary, for our living substance is continually replenished and rebuilt from the elements of our dead foods. So, as Huxley says, a living organism is like a flame or a whirlpool, which is an ever changing though seemingly constant individuality. We look at a gas flame, and we see in the flame itself those particles of gas which have come through the pipe to be agitated violently in the higher temperature of the flame as they are oxidized or burnt. These particles immediately pass off as carbonic acid gas and water vapor which are no longer parts of the flame. A fountain is continually replenished by the water which is not-fountain, but which becomes for the time a part of the graceful jet, falling out and away as it leaves the fountain itself. Just so a living organism is an ever changing, ever renewed, and ever destroyed mass of little particles--the atoms of the inorganic world which combine and come to life for a time, but which return inevitably to the world of lifeless things. This is one of the most fundamental facts of biology. The independence of a living thing like a human being or a crustacean is a product of the imagination. How can we be independent of the environment when we are interlocked in so many ways with inorganic nature? Our very substance with its energies has been wrested from the environment; and as we, like all other living things, must replenish our tissues as we wear out in the very act of living, we cannot cease to maintain the closest possible relations with the environment without surrendering our existence in the battle of life.

From the foregoing discussion, it will be evident, I am sure, that there is ample justification for the biological dictum that a living individual is a mechanism. Not only is the organism composed always of cell units grouped mechanically in tissues and organs and organic systems; not only are the operations which make up its life constant and regular under similar conditions; not only is the whole creature mechanically connected with the inorganic world; but above all the whole activity of a biological individual is concerned necessarily and again mechanically with the acquisition of materials endowed with energy, which materials and energy are mechanically transformed into living matter and its life. Even though an organism is so much more complex than a locomotive, and so plastic, nevertheless, in so far as both are mechanisms, the conception of the evolution of the former may be much more readily understood through a knowledge of the historical transformation of the latter.

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What, now, is life? To most people "life seems to be something which enters into a combination of carbon and hydrogen and the other elements, and makes this complex substance, the protoplasm, perform its various activities." Nearly every one finds it difficult to regard life and vitality as anything but actuating principles that exist apart from the materials into which they enter, and which they seem to make alive. According to this general conception, "life is something like an engineer who climbs into the cab of the locomotive and pulls the levers which make it go," as health might supposedly be regarded as something that does not inhere in well-being, but gets into the body to alter it. But is this conception really justified by the facts of animal structure and physiology? Let us recall the steps of our analysis. The living organism is a collection of differentiated parts, the organs; the life of an organism is a series of activities of the several organic systems and organs. If we could take away one organ after another, there would be nothing left after the last part had been subtracted. In a similar manner, the activities of organs prove to be the combined activities of the tissue-cells, and again the truth of this statement will be clear when we imagine the result of taking away one cell after another from organisms like the frog or tree. When the last cell had been withdrawn, there would be nothing left of the frog's structure, and there would be no element of the frog's life. It is true that the particular way the tissue-cells are combined is of primary importance, but it is none the less true that the life of a cell is the kind of element out of which the life of even the most complex organism is built. And we have seen that the essential substance of a cell is a complex chemical compound we call protoplasm, whose elements are identical with chemical substances outside the living world. Is there any ground for supposing that the properties of protoplasm are due to any other causes than those which may be found in the chemical and physical constitution of protoplasm? In brief, is life physics and chemistry? Nowadays the majority of biologists believe that it is. Just as the properties of water are contributed by the elements hydrogen and oxygen which unite to form it, just so the marvelous properties of protoplasm are regarded as the inevitable derivatives of the combined properties of the various chemical elements which constitute protoplasm. Biologists have known for more than a century, since the work of Lavoisier and Laplace in 1780, that the fundamental process of the living mechanism is oxidation, and that this process is the same, as they said, for the burning candle and the guinea pig. Beginning with Woehler, in 1828, scores of students of physiological chemistry have duplicated the chemical processes of living matter, which were regarded as so peculiar to the living organism that they seemed to be due to the operation of a non-mechanical and vital cause. The investigator mentioned was the first to construct artificially from inorganic substances the nitrogen-containing ash product of the living organism called urea. Now hundreds of so-called organic compounds have been made synthetically and their number is added to week after week. Therefore, the biologist who finds that a physical and chemical analysis of some vital processes is possible, and that the analysis is being extended with astonishing rapidity, finds himself unable to regard protoplasmic activity as anything different in kind or category from the processes of physics and chemistry which go on in the world of dead things.

It is true that even at the present time some biologists are reluctant to accept the thoroughgoing mechanical interpretation of organic phenomena, partly because these are so complex that their ultimate constituents cannot be discerned, but more often on account of the apparently purposeful nature of biological processes. Some, indeed, have gone so far as to postulate something like consciousness which controls and directs the formation of protoplasm, and the exercise of its distinctive properties in the way of growth, reproduction, and embryonic development into the adapted adult. But the fact remains that wherever analysis has been possible the constituent elements of an organic process prove to be physical and chemical. Protoplasm differs from inorganic materials only in its complexity and in the properties which seem to owe their existence to this complexity. As Huxley points out, it is no more justifiable to postulate the existence of a vitalistic principle in protoplasm than it would be to set up an "aquosity" to account for the properties of water, or a "saltness" for the qualities of a certain combination of sodium and chlorine. We may not know how the elements produce the properties of the compound, but we do know that such properties are the invariable products of their respective constituents in combination. As far as the evidence goes, it tells strongly and invariably in favor of the mechanistic interpretation.

Under the present limitations, it is impossible to give this subject the further discussion it deserves. It is not our purpose to review the origin of life in times past, and the origin of living matter from inorganic constituents, though the subject is one of the most important in the field of cosmic evolution. We must begin with the living organism; and how the first one arose must be of less importance to us than the knowledge of its mechanical constitution and of its mechanical operation. Of far greater value is the realization that a living creature is not an independent thing, but that, on the contrary, it must hold the closest possible relations with the world of materials and energies constituting its environment. We must again insist upon the importance of that mechanical adjustment to the conditions of life which is the universal characteristic of plants and animals. It is the history of these creatures and the origin of their adapted conditions that we are called upon to study. We must scrutinize the nature of to-day to see if we can find evidence that evolution is true, and if we can discern the forces which, acting upon the living mechanism as man has dealt with machines, might bring the various species of the present day to their modern forms.

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We have now learned that evolution means a common ancestry of living forms that have come to differ in the course of time; our common reason has shown us also that organisms are in a true sense complicated chemical mechanisms adapted to meet the conditions under which they must operate. We come now to the evidences offered by the organic world that evolution is true and that natural forces control its workings. Clearly the examination of the matter of _fact_ is independent of the question of _method_. For just as the chemist may experiment with various substances to see if they will dissolve in water and not in alcohol before it is necessary or desirable for him to take up the further studies of the laws of solution, so reasonable grounds must be found for regarding evolution as true before passing to its method of accomplishment. And in the following discussions, the animals will be used almost exclusively, not because the study of plants fails to discover the same relations and principles, but because the better known animal series is more varied and extensive, and above all for the reason that the human organism arrays itself as the highest term of the animal series.

In the complete scheme adopted by most naturalists, five categories include the evidences bearing upon the fact of evolution. These are _Classification_; Comparative Anatomy, or _Morphology_; Comparative Development, or _Embryology; _Palæontology_, which comprises the facts provided by fossil relics of animals and plants of earlier geological ages; and _Geographical Distribution_. Each of these divisions includes a descriptive and analytical series of facts, whose characteristics are "explained" or summarized in the form of the general principles of the respective divisions. Such principles, taken singly and collectively, constitute the evidences of evolution.

The particular nature of any one of these categories, evolved in the development of science practically in the order stated, depends upon the special quality of an animal which it selects for comparison and organization in connection with other similar facts, and also in its own mode of viewing its facts. One and the same organism may present materials for two, three, or even all five of these divisions, for they are by no means mutually exclusive. For example, a common cat possesses certain definite characteristics which give it a particular place when animals more or less like it are grouped or classified according to their degrees of resemblance and difference, in small _genera_ of very similar forms, in larger _tribes_ or _orders_ of similar genera, and in more and more inclusive groups of these lesser divisions, such as the _classes_ and _phyla_, or main branches of the animal tree. The common cat and its relatives are even earlier to be regarded as anatomical subjects, and their thorough analysis belongs to comparative anatomy,--a name which explains itself. The purpose of this department of natural history is to explore the entire range of animal forms and animal structures, and to determine the degree of resemblance and difference exhibited by the general characters of entire organisms and by the special qualities of their several systems of organs. It provides the data from which classification selects those which indicate mutual affinities with greatest precision and surety. But its materials are _all_ the facts of animal structure, and because each and every known organism can be and must be studied, the investigator engaged in formulating the evidence of evolution has at his disposal all the data referring to the entire realm of animals. The data of embryology are likewise coextensive with the territory of the animal world, for we do not know of any form which does not change in the course of its life history. An adult cat is the product of a kitten which is itself the result of a long series of changes from earlier and simpler conditions. In so far as it deals with structures in the making, embryology is a study of anatomy, but as it is concerned primarily with all of the plastic remodeling which animals undergo during the production of their final forms, it is an independent study. Nevertheless we shall learn how intimate are the relations of these two divisions of zoölogy and how the evolutionary teachings of each body of fact support and supplement those of the other.

Palæontology searches everywhere among the deposits of earlier ages for links to be fitted into their proper sequence of time, from which it constructs the chain of diverse types leading down to the species of the present. A cat of to-day is therefore viewed in an entirely different connection, as the last term in a consecutive series of species. Forming alliances with geology, and even with physics and chemistry, this department of zoölogy endeavors to reconstruct the past from what it learns to-day about organisms and the conditions under which they live. Finally the observations that cats of various kinds do not occur everywhere in the world, but only in certain more or less restricted localities, belong to the subject of geographical distribution, and illustrate its nature.

Our task is to learn the teachings of these several divisions by recalling and putting together what we know already about the commonest animals, or noting what can be observed in a visit to a zoölogical garden and aquarium. On account of the present limitations of time, the subject of classification will be combined with comparative anatomy; embryology will be taken up together with these subjects; palæontology will be the main subject of the next discussion, which will include also a brief statement of the meaning of distribution. Then we will be prepared to study nature to see how evolution works.

II

THE STRUCTURE AND DEVELOPMENT OF ANIMALS AS EVIDENCE OF EVOLUTION

In order to become acquainted with the way the structures of animals provide evidences of evolution, it is by no means necessary to review the entire range of their forms, because research has discovered that the principles of relationship are universal among animals, and that any group of examples will demonstrate what is taught by comparative anatomy as a whole. The commonest creatures may serve us best in order that we may come to view evolution as a process that involves each and every living thing that we know, and not as something which belongs only to the remote and unknown past.

Let us begin with the common cat and the group of carnivora or flesh-eating animals to which it belongs. As we pass along the streets of the city, we will see many cats which differ in some details, though they resemble one another closely. While they vary somewhat in form, the range in this quality is not so noticeable as in the matter of color; some of them will be gray, some maltese, while others will be yellowish or black, and they will differ in the striped or spotted character of their coloration. We readily classify them all as "cats" in spite of their differences, because they are alike in so many ways that we have learned to associate as the distinguishing characteristics of these animals, and to label--"cat." The animals which we might see in a walk of several blocks may reasonably be regarded as offspring of the same pair of ancestors of a few years back, even though they are dissimilar. We all know that the kittens of one and the same litter vary: no two of them are ever exactly alike in color or disposition or voice or size, nor is any one identical with either of its parents, although it may be necessary to employ exact means of measuring them in order to demonstrate their variation. The fact of difference, then, is surely not inconsistent with even the closest ties of blood, and we do not need to go beyond the scope of daily observation to find that this is true in nature wherever we look.

Should we extend our observations so as to include the cats of Boston and Philadelphia and San Francisco, the animals would probably vary over a wider range, but they would be so similar to New York cats in their make-up that we would have no difficulty in regarding them and all the others of the United States as the descendants of a single pairs of ancestors, perhaps brought over in the "Mayflower." But why does this view seem justified? Because experience has taught us that the living things which resemble each other most closely are those which are most intimately bound by ties of blood and common heritage. It is "natural" for relatives to resemble one another more than persons not related, and for brothers and sisters to be more alike than cousins. Science does not refer to something outside everyday observation when it states that _the possession by two animals of a great body of similar characters beneath their minor differences is an indication of their common ancestry_.

Thus at the very outset our simple illustration establishes the most fundamental principle of comparative anatomy. Let us see how it works further. The Manx cat possesses an abbreviated tail, although in other respects it is practically the same as the familiar long-tailed form; the Angora and the Persian differ in having long hair. All of these animals are so much alike in so many respects, and so closely resemble the wild cats, that it is not unreasonable to regard them all as the descendants of the same original wild ancestors, and as the varying products of lines which branched out from the same stock in different directions and at different times. It is, in a word, their "cat-_ness_" which demonstrates their relationships. But common sense need not stop here. Guided by the facts of anatomical similarity, it convinces us that the dun-colored lion and puma, the striped tiger and the spotted leopard are simply cats of a larger growth whose remoter ancestry is one with that of the previously cited forms. Not until we explore and compare their several systems do we see how thoroughgoing is their uniformity in structural plan. And because reason justifies the view regarding the origin of domestic cats from wild ancestors, the evolution of all the various members of the cat tribe must be acknowledged. These animals exhibit a fundamental likeness, which, to employ a musical analogy, is the "theme" of "cat-_ness_," and they are so many variations of this theme.

The members of another tribe of the familiar carnivora display in their own way the same kind of evidences of relationship. The varieties of domesticated dogs differ far more widely among themselves than do common cats, yet their community of ancestry is demonstrated not only by structural resemblances, but also by the striking fact that forms as diverse as the greyhound and the fox terrier can be crossed. Here again there are wild forms, like the wolf and fox and jackal, so like the domesticated members of the dog tribe that we cannot fail to recognize a common "dog-_ness_" and its significance as evidence of the relationship in ancestry of all these animals.