The Doctrine of Evolution - Its Basis and Its Scope
by Henry Edward Crampton
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Set up and electrotyped. Published June, 1911. Reprinted December, 1912; September, 1916.

Norwood Press J.S. Cushing Co.—Berwick & Smith Co. Norwood, Mass., U.S.A.


The present volume consists of a series of eight addresses delivered as the Hewitt Lectures of Columbia University at Cooper Union in New York City during the months of February and March, 1907. The purpose of these lectures was to describe in concise outline the Doctrine of Evolution, its basis in the facts of natural history, and its wide and universal scope. They fall naturally into two groups. Those of the first part deal with matters of definition, with the essential characteristics of living things, and, at greater length, with the evidences of organic evolution. The lectures of the second group take up the various aspects of human evolution as a special instance of the general organic process. In this latter part of the series, the subject of physical evolution is first considered, and this is followed by an analysis of human mental evolution; the chapter on social evolution extends the fundamental principles to a field which is not usually considered by biologists, and its purpose is to demonstrate the efficiency of the genetic method in this department as in all others; finally, the principles are extended to what is called "the higher human life," the realm, namely, of ethical, religious, and theological ideas and ideals.

Naturally, so broad a survey of knowledge could not include any extensive array of specific details in any one of its divisions; it was possible only to set forth some of the more striking and significant facts which would demonstrate the nature and meaning of that department from which they were selected. The illustrations were usually made concrete through the use of photographs, which must naturally be lacking in the present volume. In preparing the addresses for publication, the verbal form of each evening's discussion has been somewhat changed, but there has been no substantial alteration of the subjects actually discussed.

The choice of materials and the mode of their presentations were determined by the general purpose of the whole course. The audiences were made up almost exclusively of mature persons of cultivated minds, but who were on the whole quite unfamiliar with the technical facts of natural history. It was necessary to disregard most of the problematical elements of the doctrine so as to bring out only the basic and thoroughly substantiated principles of evolution. The course was, in a word, a simple message to the unscientific; and while it may seem at first that the discussions of the latter chapters lead to somewhat insecure positions, it should be remembered that their purpose was to bring forward the proof that even the so-called higher elements of human life are subject to classification and analysis, like the facts of the lower organic world.

It may seem that the biologist is straying beyond his subject when he undertakes to extend the principles of organic evolution to those possessions of mankind that seem to be unique. The task was undertaken in the Hewitt Lectures because the writer holds the deeply grounded conviction that evolution has been continuous throughout, and that the study of lower organic forms where laws reveal themselves in more fundamental simplicity must lead the investigator to employ and apply those laws in the study of the highest natural phenomena that can be found. Another motive was equally strong. Too frequently men of science are accused of restricting the application of their results to their own particular fields of inquiry. As individuals they use their knowledge for the development of world conceptions, which they are usually reluctant to display before the world. It is because I believe that the accusation is often only too well merited that I have endeavored to show as well as circumstances permit how universal is the scope of the doctrine based upon the facts of biology, and how supreme are its practical and dynamic values.

It remains only to state that the present volume contains nothing new, either in fact or in principle; the particular form and mode of presenting the evolutionary history of nature may be considered as the author's personal contribution to the subject. Nothing has been stated that has not the sanction of high authority as well as of the writer's own conviction; but it will be clear that the believers in the truth of the analysis as made in the later chapters may become progressively fewer, as the various aspects of human life and of human nature are severally treated. Nevertheless, I believe that this volume presents a consistent reasonable view that will not be essentially different from the conceptions of all men of science who believe in evolution.














The Doctrine of Evolution is a body of principles and facts concerning the present condition and past history of the living and lifeless things that make up the universe. It teaches that natural processes have gone on in the earlier ages of the world as they do to-day, and that natural forces have ordered the production of all things about which we know.

It is difficult to find the right words with which to begin the discussion of so vast a subject. As a general statement the doctrine is perhaps the simplest formula of natural science, although the facts and processes which it summarizes are the most complex that the human intellect can contemplate. Nothing in natural history seems to be surer than evolution, and yet the final solution of evolutionary problems defies the most subtle skill of the trained analyst of nature's order. No single human mind can contain all the facts of a single small department of natural science, nor can one mind comprehend fully the relations of all the various departments of knowledge, but nevertheless evolution seems to describe the history of all facts and their relations throughout the entire field of knowledge. Were it possible for a man to live a hundred years, he could only begin the exploration of the vast domains of science, and were his life prolonged indefinitely, his task would remain forever unaccomplished, for progress in any direction would bring him inevitably to newer and still unexplored regions of thought.

Therefore it would seem that we are attempting an impossible task when we undertake in the brief time before us the study of this universal principle and its fundamental concepts and applications. But are the difficulties insuperable? Truly our efforts would be foredoomed to failure were it not that the materials of knowledge are grouped in classes and departments which may be illustrated by a few representative data. And it is also true that every one has thought more or less widely and deeply about human nature, about the living world to which we belong, and about the circumstances that control our own lives and those of our fellow creatures. Many times we withdraw from the world of strenuous endeavor to think about the "meaning of things," and upon the "why" and "wherefore" of existence itself. Every one possesses already a fund of information that can be directly utilized during the coming discussions; for if evolution is true as a universal principle, then it is as natural and everyday a matter as nature and existence themselves, and its materials must include the facts of daily life and observation.

Although the doctrine of evolution was stated in very nearly its present form more than a century ago, much misunderstanding still exists as to its exact meaning and nature and value; and it is one of the primary objects of these discussions to do away with certain current errors of judgment about it. It is often supposed to be a remote and recondite subject, intelligible only to the technical expert in knowledge, and apart from the everyday world of life. It is more often conceived as a metaphysical and philosophical system, something antagonistic to the deep-rooted religious instincts and the theological beliefs of mankind. Truly all the facts of knowledge are the materials of science, but science is not metaphysics or philosophy or belief, even though the student who employs scientific method is inevitably brought to consider problems belonging to these diverse fields of thought. A study of nervous mechanism and organic structure leads to the philosophical problem of the freedom of the will; questions as to the evolution of mind and the way mind and matter are related force the investigator to consider the problem of immortality. But these and similar subjects in the field of extra-science are beyond its sphere for the very good reason that scientific method, which we are to define shortly, cannot be employed for their solution. Evolution is a science; it is a description of nature's order, and its materials are facts only. In method and content it is the very science of sciences, describing all and holding true throughout each one.

The overwhelming importance of knowing about natural laws and universal principles is not often realized. What have we to do with evolution and science? Are we not too busy with the ordering of our immediate affairs to concern ourselves with such remote matters? So it may appear to many, who think that the study of life and its origin, and of the vital facts about plants and animals may be interesting and may possess a certain intellectual value, but nothing more. The investigation of man and of men and of human life is regarded by the majority as a mere cultural exercise which has no further result than the recording of present facts and past histories; but it is far otherwise. Science and evolution must deal with mere details about the world at large, and with human ideals and with life and conduct; and while their purpose is to describe how nature works now and how it has progressed in the past, their fullest value is realized in the sure guidance they provide for our lives. This cannot be clear until we reach the later portions of our subject, but even at the outset we must recognize that knowledge of the great rules of nature's game, in which we must play our parts, is the most valuable intellectual possession we can obtain. If man and his place in nature, his mind and social obligations, become intelligible, if right and wrong, good and evil, and duty come to have more definite and assignable values through an understanding of the results of science, then life may be fuller and richer, better and more effective, in direct proportion to this understanding of the harmony of the universe.

And so we must approach the study of the several divisions of our subject in this frame of mind. We must meet many difficulties, of which the chief one is perhaps our own human nature. For we as men are involved, and it is hard indeed to take an impersonal point of view,—to put aside all thoughts of the consequences to us of evolution, if it is true. Yet emotion and purely human interest are disturbing elements in intellectual development which hamper the efforts of reason to form assured conceptions. We must disregard for the time those insistent questions as to higher human nature, even though we must inevitably consider them at the last. Indeed, all the human problems must be put aside until we have prepared the way for their study by learning what evolution means, what a living organism is, and how sure is the evidence of organic transformation. When we know what nature is like and what natural processes are, then we may take up the questions of supreme and deep concern about our own human lives.

* * * * *

Human curiosity has ever demanded answers to questions about the world and its make-up. The primitive savage was concerned primarily with the everyday work of seeking food and building huts and carrying on warfare, and yet even he found time to classify the objects of his world and to construct some theory about the powers that made them. His attainments may seem crude and childish to-day, but they were the beginnings of classified knowledge, which advanced or stood still as men found more or less time for observation and thought. Freed from the strife of primeval and medieval life, more and more observers and thinkers have enlarged the boundaries and developed the territory of the known. The history of human thought itself demonstrates an evolution which began with the savages' vague interpretation of the "what" and the "why" of the universe, and culminates in the science of to-day.

What, now, is a science? To many people the word denotes something cold and unfeeling and rigid, or something that is somehow apart from daily life and antagonistic to freedom of thought. But this is far from being true. Karl Pearson defines science as organized knowledge, and Huxley calls it organized common sense. These definitions mean the same thing. They mean that in order to know anything that deserves confidence, in order to obtain a real result, it is necessary in the first place to establish the reality of facts and to discriminate between the true, the not so sure, the merely possible, and the false. Having accurate and verified data, scientific method then proceeds to classify them, and this is the organizing of knowledge. The final process involves a summary of the facts and their relations by some simple expression or formula. A good illustration of a scientific principle is the natural law of gravitation. It states simply that two bodies of matter attract one another directly in proportion to their mass, and inversely in proportion to the square of the distance between them. In this concise rule are described the relations which have been actually determined for masses of varying sizes and at different distances apart,—for snowflakes falling to the earth, for the avalanche on the mountain slope, and for the planets of the solar system, moving in celestial cooerdination.

Such a principle as the law of gravitation, like evolution, is true if the basic facts are true, if they are reasonably related, and if the conclusion is drawn reasonably from them. It is true for all persons who possess normal minds, and this is why Huxley speaks of science as "common sense,"—that is, something which is a reasonable and sensible part of the mental make-up of thinking persons that they can hold in common. The form and method of science are fully set forth by these definitions, and the purpose also is clearly revealed. For the results of investigation are not merely formulae which summarize experience as so much "conceptual shorthand," as Karl Pearson puts it, but they must serve also to describe what will probably be the orderly workings of nature as future experience unfolds. Human endeavor based upon a knowledge of scientific principles must be far more reliable than where it is guided by mere intuition or unreasoned belief, which may or may not harmonize with the everyday world laws. Just as the law of gravitation based upon past experience provides the bridge builder and the architect with a statement of conditions to be met, so we shall find that the principles of evolution demonstrate the best means of meeting the circumstances of life.

Evolution has developed, like all sciences, as the method we have described has been employed. Alchemy became chemistry when the so-called facts of the medievalist were scrutinized and the false were discarded. Astrology was reorganized into astronomy when real facts about the planets and stars were separated from the belief that human lives were influenced by the heavenly bodies. Likewise the science of life has undergone far-reaching changes in coming down to its present form. All the principles of these sciences are complete only in so far as they sum up in the best way the whole range of facts that they describe. They cannot be final until all that can be known is known,—until the end of all knowledge and of time. It is because he feels so sure of what has been gained that the man of science seems to the unscientific to claim finality for his results. He himself is the first to point out that dogmatism is unjustified when its assertions are not so thoroughly grounded in reasonable fact as to render their contrary unthinkable. He seeks only for truth, realizing that new discoveries must oblige him to amend his statement of the laws of nature with every decade. But the great bulk of knowledge concerning life and living forms is so sure that science asserts, with a decision often mistaken for dogmatism, that evolution is a real natural process.

* * * * *

The conception of evolution in its turn now demands a definite description. How are we to regard the material things of the earth? Are they permanent and unchanged since the beginning of time, unchanging and unchangeable at the present? We do not need Herbert Spencer's elaborate demonstration that this is unthinkable, for we all know from daily experience that things do change and that nothing is immutable. Did things have a finite beginning, and have they been "made" by some supernatural force or forces, personified or impersonal, different from those agencies which we may see in operation at the present time? So says the doctrine of special creation. Finally, we may ask if things have changed as they now change under the influence of what we call the natural laws of the present, and which if they operated in the past would bring the world and all that is therein to be just what we find now. This is the teaching of the doctrine of evolution. It is a simple brief statement of natural order. And because it has followed the method of common sense, science asserts that changes have taken place, that they are now taking place, and furthermore that it is unnecessary to appeal to other than everyday processes for an explanation of the present order of things.

Wherever we look we see evidence of nature's change; every rain that falls washes the earth from the hills and mountains into the valleys and into the streams to be transported somewhere else; every wind that blows produces its small or greater effect upon the face of the earth; the beating of the ocean's waves upon the shore, the sweep of the great tides,—these, too, have their transforming power. The geologists tell us that such natural forces have remodeled and recast the various areas of the earth and that they account for the present structure of its surface. These men of science and the astronomers and the physicists tell us that in some early age the world was not a solid globe, with continents and oceans on its surface, as now; that it was so very hot as to be semi-fluid or semi-solid in consistency. They tell us that before this time it was still more fluid, and even a mass of fiery vapors. The earth's molten bulk was part of a mass which was still more vast, and which included portions which have since condensed to form the other bodies of the solar system,—Mars and Jupiter and Venus and the rest,—while the sun remains as the still fiery central core of the former nebulous materials, which have undergone a natural history of change to become the solar system. The whole sweep of events included in this long history is called cosmic evolution; it is the greater and more inclusive process comprising all the transformations which can be observed now and which have occurred in the past.

At a certain time in the earth's history, after the hard outer crust had been formed, it became possible for living materials to arise and for simple primitive creatures to exist. Thus began the process of organic evolution—the natural history of living things—with which we are concerned in this and later addresses. Organic evolution is thus a part of the greater cosmic process. As such it does not deal with the origin of life, but it begins with life, and concerns itself with the evolution of living things. And while the investigator is inevitably brought to consider the fundamental question as to the way the first life began, as a student of organic forms he takes life for granted and studies only the relationships and characteristics of animals and plants, and their origins.

But even as a preliminary definition, the statement that organic evolution means natural change does not satisfy us. We need a fuller statement of what it is and what it involves, and I think that it would be best to begin, not with the human being in which we are so directly interested, nor even with one of the lower creatures, but with something, as an analogy, which will make it possible for us to understand immediately what is meant by the evolution of a man, or of a horse, or of an oak tree. The first steam locomotive that we know about, like that of Stephenson, was a crude mechanism with a primitive boiler and steam-chest and drive-wheels, and as a whole it had but a low degree of efficiency measured by our modern standard; but as time went on inventive genius changed one little part after another until greater and greater efficiency was obtained, and at the present time we find many varied products of locomotive evolution. The great freight locomotive of the transcontinental lines, the swift engine of the express trains, the little coughing switch engine of the railroad yards, and the now extinct type that used to run so recently on the elevated railroads, are all in a true sense the descendants of a common ancestor, namely the locomotive of Stephenson. Each one has evolved by transformations of its various parts, and in its evolution it has become adapted or fitted to peculiar circumstances. We do not expect the freight locomotive with its eight or ten powerful drive-wheels to carry the light loads of suburban traffic, nor do we expect to see a little switch engine attempt to draw "the Twentieth Century Limited" to Chicago. In the evolution, then, of modern locomotives, differences have come about, even though the common ancestor is one single type; and these differences have an adaptive value to certain specific conditions. A second illustration will be useful. Fulton's steamboat of just a century ago was in a certain true sense the ancestor of the "Lusitania," with its deep keel and screw propellers, of the side-wheel steamship for river and harbor traffic like the "Priscilla," of the stern-wheel flat-bottom boats of the Mississippi, and of the battleship, and the tug boat. As in the first instance, we know that each modern type has developed through the accumulation of changes, which changes are likewise adjustments to different conditions. The diversity of modern types of steamships may be attributed therefore to adaptation.

The several kinds are no more interchangeable than are the different forms of locomotives that we have mentioned. The flat-bottom boat of the Mississippi would not venture to cross the Atlantic Ocean in winter, nor would the "Lusitania" attempt to plow a way up the shallow mud-banked Mississippi. These products of mechanical development are not efficient unless they run under the circumstances which have controlled their construction, unless they are fitted or adapted to the conditions under which they must operate.

Evolution, then, means descent with adaptive modification. We must examine the various kinds of living creatures everywhere to see if they, like the machines, exhibit in their make-up similar elements which indicate their common ancestry in an earlier age, and if we can interpret their differences as the results of modifications which fit them to occupy different place in nature.

Two objections to the employment of these analogies will present themselves at once. The definition may be all very well as far as the machines are concerned, but, it may be asked, should a living thing like a horse or a dog be compared with the steamship or the locomotive? Can we look upon the living thing as a mechanism in the proper sense of the word? A second objection will be that human invention and ingenuity have controlled the evolution of the steamship and engine by the perfection of newer and more efficient parts. It is certainly true that organic evolution cannot be controlled in the same way by men, and that science has not yet found out what all the factors are. And yet we are going to learn in a later discussion that nature's method of transforming organisms in the course of evolution is strikingly similar to the human process of trial and error which has brought the diverse modern mechanisms to their present conditions of efficiency. This matter, however, must remain for the time just as it stands. The first objection, namely, that an organism ought not to be viewed as a machine, is one that we must meet immediately, because it is necessary at the very outset to gain a clear idea of the essentially mechanical nature of living things and of their relations to the conditions under which they live. It is only when we have such a clear understanding that we can profitably pursue the further inquiries into the evidence of evolution. Our first real task, therefore, is an inquiry into certain fundamental questions about life and living things, upon which we shall build as we proceed.

* * * * *

All living things possess three general properties which seem to be unique; these are a peculiar chemical constitution, the power of repairing themselves as their tissues wear out, and the ability to grow and multiply. The third property is so familiar that we fail to see how sharply it distinguishes the creatures of the organic world. To realize this we have only to imagine how strange it would seem if locomotives and steamships detached small portions of themselves which could grow into the full forms of the parent mechanisms. Equally distinctive is the marvelous natural power which enables an animal to re-build its tissues as they are continually used up in the processes of living; for no man-made, self-sustaining mechanism has ever been perfected. The property of chemical composition is believed by science to be the basis of the second and the third; but this matter of chemical constitution must take its proper place in the series of structural characters, which we shall discuss further on as we develop the conception of organic mechanism.

Whatever definition we may employ for a machine or an engine, we cannot exclude the living organism from its scope. As a "device for transforming and utilizing energy" the living organism differs not at all from any "dead" machine, however complex or simple. The greatest lesson of physiological science is that the operations of the different parts of the living thing, as well as of the whole organism itself, are mechanical; that is, they are the same under similar circumstances. The living creature secures fresh supplies of matter and energy from the environment outside of itself; these provide the fuel and power for the performance of the various tasks demanded of an efficient living thing, and they are the sources upon which the organism draws when it rebuilds its wasted tissues and replenishes its energies. The vital tasks of all organisms must be considered in due course, but at first it is necessary to justify our analogies by analyzing the structural characteristics of animals and plants, just as we might study locomotives in a mechanical museum before we should see how they work upon the rails.

Among the familiar facts which science reveals in a new light are the peculiarly definite qualities of living things as regards size and form. There is no general agreement in these matters among the things of the inorganic world. Water is water, whether it is a drop or the Pacific Ocean; stone is stone, whether it is a pebble, a granite block, or a solid peak of the Rocky Mountains. It is true that there is a considerable range in size between the microscopic bacterium at one extreme and the elephant or whale at the other, but this is far less extensive than in the case of lifeless things like water and stone. In physical respects, water may be a fluid, or a gas in the form of steam, or a solid, as a crystal of snow or a block of ice. But the essential materials of living things agree throughout the entire range of plant and animal forms in having a jellylike consistency.

But by far the most striking and important characteristic of living things is their definite and restricted chemical composition. Out of the eighty and more chemical elements known to science, the essential substance of living creatures is formed by only six to twelve. These are the simple and obvious characteristics of living things which are denoted by the word "organic." Everyone has a general idea of what this expression signifies, but it is important to realize that it means, in exact scientific terms,—constituted in definite and peculiar ways.

The living thing, then, possesses a definite constitution, which is a mechanical characteristic, while furthermore it is related to its surroundings in a hard and fast way. Just as locomotives are different in structure so that they may operate successfully under different conditions, so the definite characteristics of living things are exactly what they should be in order that organisms may be adjusted or fitted into the places in nature which they occupy. This universal relation to the environment is called adaptation. It is only too obvious when our attention is directed to it, but it is something which may have escaped our notice because it is so natural and universal. The trunk of a tree bears the limbs and branches and leaves above the ground, while the roots run out into the surrounding soil from the foot of the trunk; they do not grow up into the air. An animal walks upon its legs, the wings of a bird are just where they should be in order that they may be useful as organs of flight. And these mechanical adjustments in the case of living creatures occur for the same reason as in mechanisms like the steamship, which has the propeller at its hinder end and not elsewhere, and which bears its masts erect instead of in any other way.

The next step in the analysis of organisms reveals the same wonderful though familiar characteristics. The living organism is composed of parts which are called organs, and these differ from one another in structural and functional respects. Each of them performs a special task which the others do not, and each differentiated organ does its part to make the whole creature an efficient mechanism. The leg of the frog is an organ of locomotion, the heart is a device for pumping blood, the stomach accomplishes digestion, while the brain and nerves keep the parts working in harmony and also provide for the proper relation of the whole creature to its environment. So rigidly are these organs specialized in structure and in function that they cannot replace one another, any more than the drive wheels of the locomotive could replace the smokestack, or the boiler be interchanged with either of these. All of the organs are thus fitted or adjusted to a particular place in the body where they may most efficiently perform their duties. Each organ therefore occupies a particular place in an organic environment, so to speak. Thus the principle of adaptation holds true for the organs which constitute an organism, as well as for organisms themselves in their relations to their surroundings.

The various organs of living things are grouped so as to form the several organic systems. There are eight of these, and each performs a group of related tasks which are necessary for complete life. The alimentary system concerns itself with three things: it gets food into the body, or ingests; it transforms the insoluble foods by the intricate chemical processes of digestion; and it absorbs or takes into itself the transformed food substances, which are then passed on to the other parts of the body. It is hardly necessary to point out that the ingestive structures for taking food and preparing it mechanically lie at and near the mouth, while the digesting parts, like the stomach, come next, because chemical transformation is the next thing to be done; while finally the absorbing portions of the tract, or the intestines, come last. The second group of organs, like gills and lungs, supplies the oxygen, which is as necessary for life as food itself; this respiratory system also provides for the passage from the body of certain of the waste gases, like carbonic acid gas and water vapor. The excretory system of kidneys and similar structures collects the ash-waste produced by the burning tissues, and discharges this from the whole mechanism, like the ash hoist of a steamship. The circulatory system, made up of smaller and larger vessels, with or without a heart, transports and propels the blood through the body, carrying the absorbed foods, the supplies of oxygen, and the waste substances of various kinds. All of these four systems are concerned with "commissary" problems, so to speak, which every individual must solve for and by itself.

Another group of systems is concerned with wider relations of the individual and its activities. For example, the motor system accomplishes the movements of the various organs within the body, and it also enables the organism to move about; thus it provides for motion and locomotion. Systems of support, comprising bones or shells, occur in many animals where the other organs are soft or weak. Perhaps the most interesting of the individual systems of relation is the nervous system. The strands of its nerve fibers and its groups of cells keep the various organs of the body properly cooerdinated, whereas in the second place, through the sensitive structures at the surface of the body, they receive the impressions from the outside world and so enable the organism to relate itself properly to its environment. The last organic system differs from the other seven in that the performance of its task is of far less importance to the individual than it is to the race as a whole. It is the reproductive system, with a function that must be always biologically supreme. We can very readily see why this must be so; it is because nature has no place for a species which permits the performance of any individual function to gain ascendency over the necessary task of perpetuating the kind. Nature does not tolerate race suicide.

All organisms must perform these eight functions in one way or another. The bacterium, the simplest animal, the lowest plant, the higher plants and animals,—all of these have a biological problem to solve which comprises eight terms or parts, no more and no less. This is surely an astonishing agreement when we consider the varied forms of living creatures. And perhaps when we see that this is true we may understand why adaptation is a characteristic of all organisms, for they all have similar biological problems to solve, and their lives must necessarily be adjusted in somewhat similar ways to their surroundings.

Carrying the analysis of organic structure one step further, it is found that the various organisms are themselves complex, being composed of tissues. A frog's leg as an organ of locomotion is composed of the protecting skin on the outside, the muscles, blood vessels, and nerves below, and in the center the bony supports of the whole limb. Like the organs, these tissues are differentiated, structurally and functionally, and they also are so placed and related as to exhibit the kind of mechanical adjustment which we call adaptation. The tissues, then, in their relations to the organs are like the organs in their relations to the whole creature, i.e. adapted to specific situations where they may most satisfactorily perform their tasks.

Finally, in the last analysis, all organisms and organs and tissues can be resolved into elements which are called cells. They are not little hollow cases, it is true, although for historical reasons we employ a word that implies such a condition. They are unitary masses of living matter with a peculiar central body or nucleus, and every tissue of every living thing is composed of them.

The cells of bone differ from those of cartilage mainly in the different consistency of the substances secreted by the cells to lie between them; skin cells are soft-walled masses lying close together; even blood is a tissue, although it is fluid and its cells are the corpuscles which float freely in a liquid serum. Thus an organism proves to be a complex mechanism composed of cells as structural units, just as a building is ultimately a collection of bricks and girders and bolts, related to one another in definite ways.

Our analysis reveals the living creature in an entirely new light, not only as a machinelike structure whose parts are marvelously formed and coordinated in material respects, but also as one whose activities or workings are ultimately cellular in origin. Structure and function are inseparable, and if an animal or a plant is an aggregate of cells, then its whole varied life must be the sum total of the lives of its constituent cells. Should these units be subtracted from an animal, one by one, there would be no material organism left when the last cells had been disassociated, and there would be no organic activity remaining when the last individual cell-life was destroyed. All the various things we do in the performance of our daily tasks are done by the combined action of our muscle and nerve and other tissue cells; our life is all of their lives, and nothing more. The cell, then, is the physiological or functional unit, as truly as it is the material element of the organic world. Being combined with countless others, specialized in various ways, relations are established which are like those exhibited by the human beings constituting a nation. In this case the life of the community consists of the activities of the diverse human units that make it up. The farmer, the manufacturer, the soldier, clerk, and artisan do not all work in the same way; they undertake one or another of the economic tasks which they may be best fitted by circumstances to perform. Their differentiation and division of labor are identical with the diversity in structure and in function as well, exhibited by the cells of a living creature. We might speak of the several states as so many organs of our own nation; the commercial or farming or manufacturing communities of a state would be like the tissues forming an organ, made up ultimately of human units, which, like cells, are engaged in similar activities. As the individual human lives and the activities of differentiated economic groups constitute the life of a nation and national existence, so cell-lives make the living of an organism, and the expressions "division of labor" and "differentiation" come to have a biological meaning and application.

* * * * *

The cell, then, is in all respects the very unit of the organic world. Not only is it the ultimate structural element of all the more familiar animals and plants that we know, as the foregoing analysis demonstrates, but, in the second place, the microscope reveals simple little organisms, like Amoeba, the yeast plant and bacteria, which consist throughout their lives of just one cell and nothing more. Still more wonderful is the fact that the larger complex organisms actually begin existence as single cells. In three ways, therefore,—the analytic, the comparative, and the developmental,—the cell proves to be the "organic individual of the first order." As the ultimate biological unit, its essential nature must possess a profound interest, for in its substance resides the secret of life.

This wonderful physical basis of life is called protoplasm. It contains three kinds of chemical compounds known as the proteins, carbohydrates, and hydrocarbons. Proteins are invariably present in living cells, and are made up of carbon, hydrogen, nitrogen, sulphur, and usually a little phosphorus. The elements are also combined in a very complex chemical way. For example, the substance called haemoglobin is the protein which exists in the red blood cells and which causes those cells to appear light red or yellow when seen singly. Its chemical formula states the precise number of atoms which enter into the constitution of a single molecule as: C{600}H{960}N{154}FeO{179}. This is truly a marvelously complex substance when compared with the materials of the inorganic world, like water, for example, which has the formula H{2}O. And just as the peculiar properties of H{2}O are given to it by the properties of the hydrogen and the oxygen which combine to form it, just so, the scientist believes, the marvelous properties of protein are due to the assemblage of the properties of the carbon and hydrogen and other elements which enter into its composition.

It would be interesting to see how each one of these elements contributes some particular characteristic to the whole compound. The carbon atom, for example, is prone to combine with other atoms in definite varied ways, and the high degree of complexity which the protein molecule possesses may depend in greater part upon the combining power of its carbon elements. The nitrogen atom makes the protein an extremely volatile compound, so that the latter burns readily in the tissue cells; and the hydrogen and oxygen bring their specific characteristics to the total molecule. And furthermore, it is evident that the great complexity of this constituent, protein, gives to protoplasm its power of doing work, or, in a word, its power of living. In constructing it, much energy has been absorbed and stored up as potential energy, and so, like the stored-up energy in a watch spring or in gunpowder, this may be converted, under proper conditions, into the kinetic energy and the work of actual operation. On account of its peculiar and complex nature, it possesses great capacity for burning or oxidization, thus serving as a source of vital power. It burns in the living tissue just as coal oxidizes in the boiler of an engine; its atoms fly apart and unite with oxygen so as to satisfy their chemical affinities for this substance. If we could only see what happens to the protein molecule when it undergoes oxidization, we would witness a violent explosion, like that of a mass of gunpowder. And the astonishing fact is that this process is actually the same for the living molecule, for exploding gunpowder, and for the fuel which burns in the locomotive boiler. Does this mean that the essential process of what we call life is a chemical one? So it would seem on the basis of this fact alone, but a conclusion must be deferred until we reach a later point.

The second kind of substance which we find in protoplasm is the carbohydrate. A typical member of this group is common sugar, C_{6}H_{12}O_{6}; another sugar has the formula C_{12}H_{22}O_{11}. Starch is again a typical carbohydrate, and its formula is C_{6}H_{10}O_{5}, or some multiple of this. One sees at a glance that these substances agree in having twice as many hydrogen atoms as there are oxygen atoms, the same proportion that the hydrogen bears to the oxygen in the compound water,—a characteristic which makes it easy to remember the general constitution of carbohydrate as compared with the protein. The substances of this second class are obviously much less complex, both as regards the different kinds of atoms and in respect to the numbers of each kind that enter into the formation of a single molecule. Therefore the carbohydrates do not possess so much power or energy as the protein molecule; in short, they are not such good fuels for the living mechanism.

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; _Palaeontology_, 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 zooelogy and how the evolutionary teachings of each body of fact support and supplement those of the other.

Palaeontology 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 zooelogy 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 zooelogical 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; palaeontology 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.



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.

Extending our survey so as to include the other tribes of flesh-eaters, identical principles come to light. One is compelled to regard the polar and grizzly bears as obvious blood relatives of the brown bear, and even of the raccoon of our own territory. Instead of walking upon their toes like cats and dogs, these animals plant their feet flat upon the ground; and they agree in many other details of structure that place them together, but somewhat apart from the other tribes. The many kinds of seals and walruses and sea elephants form still another group displaying similar bodily characters, but differing more widely from the "cat theme" in these differences. They are all true carnivora, but in the course of their evolution they have progressively changed so as to be adapted to life in the water where they find their prey. The bones of the limbs are the same in number and arrangement as in the cat's limb, but the seal's anterior appendage or "arm" has altered in numerous ways so as to become an efficient flexible paddle, while the hind limbs have shifted posteriorly, very much as screw propellers have evolved in the history of steam vessels. How the members of the seal tribe have changed in their descent from purely terrestrial ancestors is partly explained by such intermediate animals as the otter. This form is adapted by its slender body and partly webbed feet to a semi-aquatic life; it seems to have halted at a point beyond which all of the seals have passed in their evolution.

Each one of these tribes by itself provides conclusive evidence of evolution, for it is most reasonable to regard the "theme" in every case as a product of common inheritance, while the variations of any theme are best understood as the results of adaptive changes in various directions. But the examples have disclosed a larger relation and a principle of wider scope, as indeed the assignment of all these tribes to the single natural group of the carnivora implies. These tribes are put together because comparative anatomy finds that the common characters of all cats are fundamentally like those of all dogs and bears and seals, and in these common qualities the carnivora differ from all other mammalia. Does this mean that the branches which bear respectively the various members of the several tribes are outgrowths of a single limb of the evolving animal tree? Science does not hesitate to give an affirmative answer, because, as in the case of the similar but varying domestic cats, no other explanation of tribal resemblance in structure seems so reasonable and natural.

So far the examples have been taken from one order of the highest class of backboned animals, called mammalia. When our survey is extended to other divisions of this class, additional laws of organic relationship are discovered. If in a series of evolving generations the line of modification proceeding from a terrestrial animal like a cat to semi-aquatic and marine types substantially like an otter and a seal should be carried further, it will inevitably lead to forms possessing characters such as those displayed by whales and the related porpoises, dolphins, and narwhals of the order cetacea. In their make-up all of these animals clearly possess the general characteristics of mammals, and they constitute collectively another limb which has sprung from the same stock as the carnivora, although at an earlier time. This we believe because of their plan of body and because their peculiar organization fits them even more perfectly than the seals for aquatic existence that is their only possible mode of life. In the case of the whales the bony framework of the fore limb is again like that of the cat's leg, although the whole structure is a flexible finlike paddle. The hind limb has disappeared as an efficient organ, but the significant fact is that small rudiments of hind limbs are present just where corresponding structures are placed in the seal. These vestiges cannot be reasonably accounted for, unless they are the degenerate hinder limbs of a remote four-footed ancestor. Furthermore the unborn whale possesses a complete coat of hair, which is afterwards replaced by blubber; but hair is a thatchlike coat to shed rain, as the way the hairs lie on a terrestrial mammal indicates. We are therefore forced to conclude that whales have originated from four-footed animals walking about on land, because no opposed explanation gives so reasonable an interpretation of the observed facts.

Another group of familiar animals materially reinforces the results already established. After what has been said, it will not be difficult to perceive the meaning of the resemblances among mice of the house and field, and of rats and rabbits and squirrels. All of them possess heavy curved gnawing teeth, or incisors, and lack the flesh-tearing or canine teeth. They agree in many other respects which distinguish them as a separate natural order of the mammals called the rodentia. Again we find a highly aberrant form in the flying squirrel, which leads toward an order with another plan of body. This animal is a true rodent, which lengthens its leap from branch to branch by means of a fold of skin stretching between its fore and its hind limbs. It is an animated aeroplane, and it shows in part how bats have originated. The wing of a bat is an elastic membrane stretching not only between the two legs of one side, but also between the greatly lengthened "fingers" of the fore limb. But the bones of arm, wrist, and fingers are almost precisely the same in number and relation as in walking forms. The fact that this peculiar wing adheres to a plan belonging to the anterior legs of walking or climbing types has no reasonable explanation save that of evolution.

The well-known group of hoofed animals, including horses and cattle, is also valuable for our present purposes, as well as in a later connection when the evidence of fossils is described. The elephant possesses five toes armed with well-developed nails or hoofs. A tapir has four or three toes, and it would seem that its ancestor had had five toes, of which one or two had been lost. A rhinoceros possesses three toes, and its foot is constructed internally like the elephant's with the outer elements absent. The horse comes last with one large toe and hoof, but on either side of the main bones of this digit are vestiges of what must have been toes in its ancestors. Among the even-toed forms the hippopotamus has four which reach the ground, with a vestige of a fifth, so this animal has apparently descended from a typical mammal with the full number along a different line from that taken by the odd-toed forms. A pig has a cloven hoof, made up of what we may call the third and fourth members of a series of five digits, but the second and fifth fingers and toes are present, though they are withdrawn from the ground so as to be no longer functional; this animal seems to have proceeded further along the same line taken by the hippopotamus. A deer, with still smaller rudiments at the sides of its double foot, leads in the comparative series to the camel with a cloven hoof devoid of any such relics.

We must pass with only brief mention the lower orders of mammalia, like the insect-eating forms to which armadillos and ant-bears belong. Of greater interest are the pouched mammals like the kangaroo and opossums, which live almost exclusively in the Australian realm. The kangaroo is endowed with a head somewhat like that of a goat, and well-developed hind legs that enable it to make leaps of astonishing length. Some of its relatives, such as the bandicoot, are like rats, or like bears, as in the case of the wombat. The Tasmanian wolf is another true marsupial, even though divergent adaptation has brought it to resemble the carnivora of the dog tribe in general appearance and in special structures like the teeth. Finally at the very bottom of the mammalian scale are two small forms living in the Australian faunal region. The duckbill or Ornithorhynchus is the better known animal, with its close fur, webbed feet, and flattened ducklike beak, while its only other near relative, the Echidna, is somewhat similar to the spiny hedgehog in external appearance. A unique peculiarity of these two forms is that they produce eggs much like those of reptiles and birds, and this fact, together with others of a structural nature, brings the whole group of mammals near to the lower classes of the Vertebrata.

Looking back on the several orders of mammals, it will be seen that the last mentioned are much less differentiated or specialized in their general organization. Above the level of the egg-layers and the pouched mammals, the higher orders branch out in different directions and reach up to various levels of the scale of animal organization.

The foregoing structural evidences of organic transformation in the past histories of cats and seals and whales insistently recall the analogies of the locomotive and the ship employed at the outset. All these animals, like the mechanical examples, have come to differ in their derivation from the same original parents, and their lines of descent have diverged so as to fit the products of evolutionary modification to diverse circumstances. Even the vestigial organs of animals have their counterparts in the machines. The cowcatcher was a large and important structure in the early days of railroading, but it has become relatively useless with the decrease of grade crossings and the construction of more complete lines of fence. The structure still persists, sometimes in a greatly reduced form. Even more obvious is the change of structure in the case of masts of vessels, which originally bore the sails for propelling the ship. When steam engines were employed to give motive power, masts did not disappear. They now provide the derrick supports of trading steamers; in battleships their function is changed to that of fighting tops and signal yards. Even the poles carried by canal boats to bear windmills must be regarded as the reduced vestiges of masts originally constructed to carry sails; and their adaptive evolution, like that of countless structures in animals, has been accomplished by degeneration.

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The birds are another class of backboned animals which exhibit identical principles of relationship. A heron has long legs and wide-spreading toes, which keep its body out of the water as it stalks about the marshes where it seeks its food; its bill is a long slender pincers. Compare it with an eagle; the latter has a short and heavily hooked beak to tear flesh, while its stout legs bear strongly curved talons to hold its struggling prey. Swimming birds like the swan and duck and loon possess feet which are constructed in general like those of the former examples, but they are webbed and shortened to serve as paddles. In the penguin we find a counterpart of the seal among mammals; its feathers are much reduced and its fore limbs are no longer wings enabling the animal to fly, but they are paddles which it uses when it swims in pursuit of fish. Finally the ostrich and wingless bird of New Zealand—the Apteryx—have wings that are useless vestiges, which, in the latter case, are hidden under the brushlike feathers covering the body. It is unnecessary to add more examples, for even these few illustrations establish exactly the same principles of relationship and evidences of evolution that are to be found in the series of mammalia.

Reptiles also are grouped, like the mammals and birds, as variations about a central theme. An ordinary lizard is perhaps the nearest in form to the remote ancestor from which all have sprung. Some lizards are long and very slender, with all four limbs of greatly reduced size. Others, which are still true lizards, have lost the hind limbs, or even all the legs, as in the "blind worms" of England. One step more, and an animal which has progressed further along a similar line of descent would be a snake. Just as whales as a group are derivable from forms which resemble types belonging to another order, so snakes as an order are to be regarded as more radically altered derivatives of some four-footed lizardlike creature. Alligators are very much like lizards in general form, and their order is a diverging branch from the same limb. Finally the evolution of turtles from the same ancestors is intelligible if we begin with a short stout animal like the so-called "horned toad" of Arizona, and proceed to the soft-shelled tortoise of the Mississippi River system; the establishment of a bony armor completes the evolution of the familiar and more characteristic turtle.

Frogs and salamanders constitute another lower class, called the amphibia, whose members are gilled during the earlier stages of development. An adult frog is essentially a salamander without a tail and with highly developed hinder limbs. The salamanders differ as regards the number of fishlike gill clefts that they all possess in their young stages, but which disappear entirely or in part during later life. In comparison with the lizard as a typical reptile, a salamander is more primitive in all of its inner organic systems, while in its nearly continuous body, with head and tail gradually merging into the trunk, it also displays a somewhat simpler form of body.

The fishes are the lowest among the common vertebrates, and they offer an abundance of independent testimony as to the truth of the principles of comparative anatomy. The common shark is perhaps the most fundamental form, with a hull-like body undivided into head, trunk, and tail, and from it have originated such peculiar variations as the hammerhead and skate. Among fishes with true bones, a cod or trout is the most typical in general features. Without ceasing to be true bony fishes, the trunk-fish and cow-fish are adapted by their peculiar characters of spine and armor plate to repel many enemies. The puff fish can take in a great amount of water, when disturbed, so as to become too large to be swallowed by some of its foes, illustrating another adaptive modification for self-defense. The wonderful colors and color patterns of the tropical fish of the reef, or of the open water forms like the mouse-fish of the Sargossa Sea, often render them more or less completely hidden from the foraging enemy. A flounder looks like a fish which was originally symmetrical, but which had come to lie flat on its side upon the bottom, whereupon the eye underneath had left its original place to appear on the upper surface. The difficult and unusual conditions of deep-sea existence have been met by fishes in two ways; some forms possess luminous frilled and weedlike fins, which lure their prey to within easy reach of their jaws, while others have enormous eyes, so as to make use of all possible rays of light in their pursuit of food organisms. But all of these diverse forms are true fishes, possessing a common heritage of structure which demonstrates their unity of origin.

The brief review of backboned animals has shown how comprehensive are the principles of relationship. The families and tribes of each order, such as the carnivora, are like branches arising from a single limb; the orders in their turn exhibit common qualities of structure which mean that they have grown from the same antecedents, while even the larger divisions or classes of mammals, birds, reptiles, amphibia, and fishes, possess a deep underlying theme whose dominant motif is the backbone, which proves their ultimate unity in ancestry. The greater and lesser branches have reached different levels, for the fish is clearly simpler in its make-up than the highly specialized bird. But the great fact is that structural evidences demonstrating the reality of genealogical affinities are displayed by the entire series of vertebrates; although they differ much or little in many or fewer respects they have one and the same ground-plan.

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The lower animals devoid of backbones, and therefore called invertebrates, are not so well-known except to the student of comparative anatomy, because they are not so often met with, and because they are usually very small or microscopic; but in many respects their importance to the evolutionist surpasses that of the vertebrates. Their structural plans are far more varied, and they range more widely from higher and relatively complicated organisms to the unitary one-celled animals. A knowledge of some of them is essential for our present purpose, which is to learn how sure is the basis for the principles of relationship and how complete is the structural evidence of evolution.

Worms are represented in the minds of most people by the common earthworm or sandworm. The body in either case is made up of a series of segments or joints which agree closely throughout the animal in external appearance and in internal constitution. A section of the digestive tract, a pair of nerve centers, two funnel-like tubes for excretion, and similar blood vessels occur in each portion.

Precisely similar features are displayed by the crustacea, which seem to be so different. Every one is familiar with the appearance of lobsters and crabs. Even in these animals the body is composed of segments, but these are not like one another, nor are they freely movable throughout the body. Five are fused in all crustacea to make a head; in lower members of the order the eight succeeding segments are free, but in the lobster they are joined together and united with the head. The hinder part of this animal is a long abdomen whose segments remain more primitive and independent. But in a crab, the whole plan has been modified by the shortening and broadening of the head-thorax, and by the reduction of the abdomen, which is also turned under the anterior part of the body. The internal organic systems are constructed upon a worm plan with modifications. Nearly every one of the segments bears one pair of appendages, which can be referred by their forked nature to the two-parted, oarlike flaps of sandworms, but the appendages of crustacea have departed from their prototypes in functional respects and in details of structure. They are variously feelers, jaws, legs, pincers, and swimming paddles, evolved to serve different purposes, just as the limbs of the vertebrates we have described have become variously arms, wings, flippers and paddles in apes, bats, seals, and whales.

Butterflies, beetles, bees, and grasshoppers seem at first sight to be entirely different, even though they agree in being more or less segmented. But all of them have heads with four pairs of appendages of the same essential plan, middle thoracic regions of three segments more or less united, bearing three pairs of legs and usually two pairs of wings, while the hinder part is a freely jointed abdomen without real limbs. In these respects the countless varieties of insects agree so that they also like crustacea of various kinds seem to have been derived from wormlike animals with more simply segmented bodies. Indeed spiders and scorpions and their relatives of the group arachnida prove for similar reasons to be derivatives of the same original stock, and own cousins of the insects.

In nearly every one of the invertebrate branches we find representatives which interest us chiefly because they appear to have reached their present condition by retrograde evolution. Barnacles are really crustacea, but they have lost their eyes as well as some other structures that are most useful in animals with a free existence, because they have adopted a fixed mode of life, which has also brought about the loss of the original freely jointed character of the body. A tapeworm as an example of internal parasites is an extremely degenerate form which lacks a digestive tract, because this is superfluous in an animal which lives bathed in the nutrient fluids of its host. Comparing it in other respects with other low wormlike creatures, it appears to be a relative of peculiar simple worms with complete organization and independence of life. All these degenerate forms enlarge our conception of adaptation by adding the essential point that progress is not always the result of evolution. Indeed we have learned this in the case of vestigial and rudimentary structures of higher forms like whales, and now we find that entire animals may degenerate as a result of changes no less adaptive than progressive modifications.

Passing by other invertebrate groups made up of species arranged like higher animals in smaller and larger branches according to their degree of fundamental similarity, we arrive at a place in the scale occupied by two-layer animals without the highly developed and clearly differentiated organic systems of the forms above. The fresh-water animal Hydra exemplifies the creatures of this level, where also we find sea-anemones and the soft polyps which form corals and coral reefs by their combined skeletons. Hydra is an animal to which we must return again and again as we study one or another aspect of organic evolution. In general form it is a hollow cylinder closed at one end, by which it attaches itself, while at the upper end, surrounded by a group of tentacles, is the mouth which leads to the central cavity. The wall of this simple body is composed of two layers of cells, between which there is a gelatinous layer rarely invaded by cells. The inner layer lines the central space into which food organisms are thrust by the tentacles, and it is concerned primarily with digestion. The outer layer comprises cells for protection and sensation primarily. Cells of both layers have muscular prolongations which by their operation enable the whole animal to change its form and to move from one place to another.

It may seem that such an animal is totally unlike any of the higher and more complex types. In certain respects, however, it is identical with the other forms inasmuch as it performs all of the eight biological tasks demanded by nature. It is also similar in so far as its inner layer, like the innermost sheet of cells in higher forms, is concerned with problems of taking and preparing food, while the protective outer layer resembles in function the outermost covering of all animals higher in the scale. Beyond these a still more fundamental agreement is found in its cellular composition.

At the lower end of the animal scale are organisms which consist of one cell and nothing more. Amoeba, to which we must refer again and again, is an example of this group which possesses an overwhelming importance to the comparative student because the origins of all the characteristics of animals higher in the scale are to be found within it. Amoeba itself is a naked mass of protoplasm, about 1/100 of an inch in diameter, enclosing a nucleus. Its form is not constant during activity, for fingerlike processes called pseudopodia are pushed out tentatively in many directions to be followed as circumstances direct by the materials of the whole cell body. Other protozoa differ in possessing constant forms, or in having constant vibratile processes, or shells of some kind, while in still other cases like individuals combine to make colonies which are more or less definite and permanent. Here at the very foot of the organic scale are found animals which seem to be entirely different from those above. Upon examination they, like Hydra, prove to be the same as regards the number and kind of functions they perform, but in structural regards their evolutionary relation to all higher animals is indicated solely by the fact that they are cells composed of protoplasm. Nevertheless the principle which states that resemblance means consanguinity still holds true, for cellular constitution is a unique possession of things of the living world,—something which demonstrates the common origin of all living things just as truly as the "cat-ness" of our first series of examples reveals for a smaller group the significance of likeness and the nature of the basic law of comparative anatomy.

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Employing a figure of speech, we have climbed down the animal tree from the higher regions where the mammals belong. Having reached the very foot of the trunk we are in a position to review and summarize the evidences which we have discovered all about us as we have descended. The various examples we have mentioned and the groups to which they belong clearly occupy different places in the scale which begins with the protozoa and extends upward to the most complicated and differentiated animals. Hydra takes its place above the protozoa for obvious structural reasons; worms belong to a still higher zone, surpassed by the more complex jointed animals like crustacea and insects. Far above these are the vertebrates, among which we have already demonstrated the occurrence of different grades of organization, from the fish up to the higher amphibia and reptiles, and beyond in two directions to the diverging birds and mammals. The basic characteristics of every group in a high position may be traced back to some one or another of the divisions at a lower level, so that the general sequence of the structural levels from low to high becomes intelligible as the order of their evolution.

To my mind the rudimentary and vestigial structures of animals are in themselves proof positive of a natural history of change. The few illustrations can be reinforced by countless examples offered by every group of living animals. If such structures have not evolved naturally by degenerating from more efficient counterparts in ancestors of earlier times, and if they have been specially created, they are utterly meaningless and their very existence is unreasonable. If common sense is to be employed, they demonstrate evolution.

Everywhere throughout the whole series animals place themselves in a treelike arrangement, for in their respective levels they occur like leaves at the ends of the lines of descent which have led up to them and which are comparable to the branches and limbs arising from the trunk of a tree. Thus the major and minor divisions of animals do not follow in the order of the rungs of a ladder, even though they must be assigned to different levels according to the complexity of their construction. The summary given above, namely, that the occurrence of lower and higher levels reveals an order of evolution, is amplified and not contradicted by the statement that the species of animals are group in a treelike arrangement. It is the task of the evolutionist, provided with all the facts of comparative anatomy and dealing only with the various species as separate leaves, so to speak, to reconstruct the now invisible but not unreal twigs and branches and limbs of the animal tree, and to show how they have diverged at one time or another as they have grown and spread to produce the species of the present day. This he may do in so far as he may find sufficient materials to enable him to employ the methods of comparative anatomy and the great natural principle established by this method—that essential likeness means consanguinity.

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No evidence of evolution could be more significant and interesting than the results provided by the comparative study of development. In the first place it is an obvious fact that every living thing changes in the course of its life-history, and if as an adult it occupies a high place in the animal scale, its embryological transformation is more elaborate and intricate than in the case of a lower form. Every one knows that organisms do develop, and yet I believe that few appreciate the tremendous significance of the mere fact that this is true, while still fewer are aware that the peculiar and characteristic early stages through which an animal passes in becoming an adult are even more striking than the fact of development itself. We shall learn something of these earlier conditions in the development of some of our most familiar animals, but at the outset nothing can be more important than an appreciation of the first great lesson of this department of natural history—namely that organic transformation is real and natural. We do not need to employ the methods of formal logic to know that in growing up a human infant undergoes the changes of childhood and adolescence, that kittens become cats, and that an oak tree is produced by an acorn, for we know these things directly by observing them. It is natural for development to take place under normal conditions, and if it does not, then something has interfered with nature. Inasmuch as "growing up" is accomplished by the alteration of an organic mechanism with one structure into an individual with a changed plan of body, it is in essence the actual process of evolution which the comparative study of grown animals of to-day demonstrates in the way we have learned. The study of animal structure discovers the process of evolution because the most reasonable interpretation of the similarities and minor differences exhibited everywhere by the various groups of animals is that descent with adaptive and divergent modification has taken place; the result is reached by inference, it is true, but by scientific and logical inference. With development it is otherwise. No reasoning is necessary to tell us that organic transformation is a real and a natural process. We see it everywhere about us and we ourselves have come to be what we are by a natural history of change. Can we consistently deny that it is possible for a species to alter in the long course of time when a few brief weeks are sufficient for the new-laid egg of the fowl to develop into a fledgling? Many indeed strain at the gnat of the longer process in the past when without hesitation they recognize the real and obvious fact of individual development in a brief period.

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