The Doctrine of Evolution Part 1

The Doctrine of Evolution.

by Henry Edward Crampton.

PREFACE

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 a.n.a.lysis 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 ill.u.s.trations 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 cla.s.sification and a.n.a.lysis, 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 circ.u.mstances 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 a.n.a.lysis 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.

I

EVOLUTION. THE LIVING ORGANISM AND ITS NATURAL HISTORY

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 a.n.a.lyst 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 cla.s.ses and departments which may be ill.u.s.trated 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 circ.u.mstances 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 a.s.signable 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 a.s.sured 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 cla.s.sify 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 cla.s.sified 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 cla.s.sify 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 ill.u.s.tration 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 ma.s.s, 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 ma.s.ses 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 coordination.

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 circ.u.mstances 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 a.s.sertions 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 a.s.serts, 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 a.s.serts 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 sh.o.r.e, 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 ma.s.s of fiery vapors. The earth's molten bulk was part of a ma.s.s 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 relations.h.i.+ps 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 a.n.a.logy, 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 circ.u.mstances. 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 ill.u.s.tration 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 steams.h.i.+p for river and harbor traffic like the "Priscilla," of the stern-wheel flat-bottom boats of the Mississippi, and of the battles.h.i.+p, and the tug boat. As in the first instance, we know that each modern type has developed through the acc.u.mulation of changes, which changes are likewise adjustments to different conditions. The diversity of modern types of steams.h.i.+ps 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 circ.u.mstances 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 a.n.a.logies 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 steams.h.i.+p 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 steams.h.i.+p 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 const.i.tution, 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 steams.h.i.+ps 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 const.i.tution 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 circ.u.mstances. 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 a.n.a.logies by a.n.a.lyzing 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,--_const.i.tuted in definite and peculiar ways_.

The living thing, then, possesses a definite const.i.tution, 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 steams.h.i.+p, 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 a.n.a.lysis 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 const.i.tute 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 pa.s.sed 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 pa.s.sage 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 steams.h.i.+p. 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 sh.e.l.ls, 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 coordinated, 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 astonis.h.i.+ng 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 a.n.a.lysis 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.