Chapter 9

The biologist employs the identical methods used by the geologist in working out the past history of the earth's crust. The latter observes the forces at work to-day, and compares the new layers of rock now being formed with the strata of deeper levels; these are so much alike that he is led to regard the constructive influences of the past as identical with those he can now watch at work. Similarly the biologist must first learn, as we have done, the principles of animal construction and development, and of other classes of zoölogical facts, and then he must turn his attention from the dead object of laboratory analysis to the workings of organic machines. The way an organism lives its life in dynamic relations to the varied conditions of existence, as well as the mutual physiological relations of the manifold parts of a single organism, reveal certain definite natural forces at work. Therefore his next task is to compare the results accomplished by these factors in the brief time they may be seen in operation with the products of the whole process of organic evolution, to learn, like the geologist in his sphere, that the present-day natural forces are able to do what reason says they have done in the past.

When the subject of inquiry was the reality of evolution, it was perhaps surprising to find that even the most familiar animals like cats and frogs provided adequate data for science to use in formulating its principles. So it is with the matter of method; it is unnecessary to go beyond the observations of a day or a week of human life to find forces at work, as real and vital as animal existence and organic life themselves. This is true, because evolution is true, and because the lives of all creatures follow one consistent law. Our task is therefore much more simple than most people suppose it to be; let us look about us and classify what we may observe, increasing our knowledge from the wide array of equally natural facts supplied by the biologist.

The analogies of the steamship and the locomotive proved useful at many times during the discussion of the fact of evolution, and even in the present connection they will still be of service. The evolution of these dead machines has been brought about by man, who, as an element of their environment, has been their creator as well as the director of their historical transformations. The result of their changes has been greater efficiency and better adjustment or adaptation to certain requirements fixed by man himself. The whole process of improvement has been one, in brief, of trial and error; new inventions have often been worthless, and they have been relegated to the scrap-heap, while the better part has been finally incorporated in the type machine. In brief, then, the important elements in the evolution of these examples have been three; first, _adaptation_, second, the _origination of new parts_, and third, the _retention of the better invention_.

Are the creatures of the living world so constituted that biological equivalents of these three essential elements of mechanical evolution can be found? Are organisms adapted to the circumstances controlling their lives, and are they capable of changing naturally from generation to generation, and of transmitting their qualities to their offspring? These are definite questions that bring us face to face with the fundamental problems relating to the dynamics or workings of evolution. We need not ask for or expect to find complete answers, for we know that it is impossible to obtain them. But we may expect to accomplish our immediate object, which is to see that evolution is natural. Our attention must be concentrated upon the three biological subjects of _adaptation_, _variation,_ and _inheritance_, and we must learn why science describes them as real organic phenomena and the results of natural causes.

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At the very outset, when the general characteristics of living things were considered, much was said on the subject of adaptation as a universal phenomenon of nature. It was not contended that perfection is attained by any living mechanism, but it was held that no place exists in nature for an organism that is incapable of adjusting itself to the manifold conditions of life. A _modus vivendi_ must be established and some satisfactory degree of adaptation must be attained, or else an animal or a species must perish. With this fundamental point as a basis, we look to nature for two kinds of natural processes or factors, first, those which may originate variations as _primary factors_,--the counterparts of human ingenuity and invention in the case of locomotive evolution,--and the _secondary factors_ of a preservative nature which will perpetuate the more adaptive organic changes produced by the first influences; it is clear that the latter are no less essential for evolution than the first causes for the appearance of variations.

The term "variation" is employed for the natural phenomenon of being or becoming different. It is an obvious fact that no child is ever exactly like either of its parents or like any one of its earlier ancestors; while furthermore in no case does an individual resemble perfectly another of its own generation or family. This departure from the parental condition, and the lack of agreement with others even of its closest blood-relatives, are two familiar forms of variation. As a rule, the degree to which a given organism is said to vary in a given character is most conveniently measured by the difference between its actual condition and the general average of its species, even though there is no such thing as a specimen of average nature in all of its qualities. In brief, then, variation means the existence of some differences between an individual and its parents, its fraternity, and, in a wider sense, all others of its species.

Passing now to the causes of variation, all of the countless deviations of living things can be referred to three kinds of primary factors; namely, the _environmental_, _functional_, and _congenital_ influences that work upon the organism in different ways and at different times during its life. We shall learn that the evolutionary values of these three classes are by no means equal, but we take a long step forward when we realize that among the things we see every day are facts demonstrating the reality of three kinds of natural powers quite able to change the characters of organic mechanisms.

The "environment" of an organism is everything outside the creature itself. In the case of an animal it therefore includes other members of its own kind, and other organisms which prey upon its species or which serve it as food, as well as the whole series of inorganic influences which first come to mind when the term is used. For example, the environment of a lion includes other lions which are either members of its own family, or else, if they live in the same region, they are its more or less active rivals and competitors. In the next place, other kinds of animals exist whose lives are intimately related to the lion's life, such as the antelopes or zebras that are preyed upon, and the human hunter to whom the lion itself may fall a victim. In addition, there are the contrasted influences of inorganic nature which demand certain adjustments of the lion's activities. Light and darkness, heat and cold, and other factors have their direct and larger or smaller effects upon the life of a lion, although these effects are less obvious in this instance than in the case of lower organisms.

The reality of variations due to the inorganic elements of the environment is everywhere evident. Those who have spent much time in the sun are aware that sunburn may result as a product of a factor of this class. The amount of sunlight falling upon a forest will filter through the tree-tops so as to cause some of the plants beneath to grow better than others, thus bringing about variations among individuals that may have sprung from the myriad seeds of a single parent plant. In times of prolonged drought, plants cannot grow at the rate which is usual and normal for their species, and so many variations in the way of inhibited development may arise.

Then there are the variations of a second class, more complex in nature than the direct effects of environment,--namely, the functional results of use and disuse. A blacksmith uses his arm muscles more constantly than do most other men, and his prolonged exercise leads to an increase of his muscular capacity. All of the several organic systems are capable of considerable development by judicious exercise, as every one knows. If the functional modifications through use were unreal, then the routine of the gymnasium and the schoolroom would leave the body and the mind as they were before. Furthermore, we are all familiar with the opposite effects of disuse. Paralysis of an arm results in the cessation of its growth. When a fall has injured the muscles and nerves of a child's limb, that structure may fail to keep pace with the growth of the other parts of the body as a result of its disuse. These are simple examples of a wide range of phenomena exhibited everywhere by animals and even by the human organism, demonstrating the plasticity of the organic mechanism and its modification by functional primary factors of variation.

But by far the greater number of variations seem to be due to the so-called congenital causes, which are sharply contrasted with the influences of the first and second classes. It is quite true that the influences of the third class cannot be surely and directly demonstrated like the others, but however remote and vague they themselves may appear to be, their effects are obvious and real, while at the same time their effects are to be clearly distinguished from the products of the other two kinds. Congenital factors reside in the physical heritage of an organism, and their results are often evident before an individual is subjected to environmental influences and before it begins to use its various organs. For example, it is a matter of common observation that a child with light hair and blue eyes may have dark-eyed and brown-haired parents. The fact of difference is a phenomenon of variation; the causes for this fact cannot be found in any other category than that comprising the hereditary and congenital influences of parent upon offspring. _How_ the effect is produced by such causes is less important in the present connection than the natural _fact_ of congenital variation. Science, however, has learned much about the causes in question, as we shall see at a later point.

Thus the first step which is necessary for an evolution and transformation of organic mechanisms proves to be entirely natural when we give only passing attention to certain obvious phenomena of life. The fact of "becoming different" cannot be questioned without indicting our powers of observation, and we must believe in it on account of its reality, even though the ultimate analysis of the way variations of different kinds are produced remains for the future.

Having learned that animals are able to change in various ways, the next question is whether variations can be transmitted to future generations through the operation of secondary factors. Long ago Buffon held that the direct effects of the environment are immediately heritable, although the mode of this inheritance was not described; it was simply assumed and taken for granted. Thus the darker color of the skin of tropical human races would be viewed by Buffon as the cumulative result of the sun's direct effects. Lamarck laid greater stress upon the indirect or functional variations due to the factors of use and disuse, and he also assumed as self-evident that such effects were transmissible as "acquired characters." This expression has a technical significance, for it refers to variations that are added during individual life to the whole group of hereditary qualities that make any animal a particular kind of organism. If evolution takes place at all, any new kind of organism originating from a different parental type must truly acquire its new characteristics, but few indeed of the variations appearing during the lifetime of an animal owe their origin to the functional and environmental influences, whose effects only deserve the name of "acquired characters" in the special biological sense.

In sharp contrast to Lamarckianism, so called,--although it did not originate in the mind of the noted man of science whose name it bears,--is the doctrine of natural selection, first proposed in its full form by Charles Darwin. This doctrine presents a wholly natural description of the method by which organisms evolve, putting all of the emphasis upon the congenital causes of variation, although the reality of other kinds of change is not questioned. But the contrast between Darwinism and the other descriptions of secondary factors can best be made after a somewhat detailed discussion of the former, which has gained the adherence of the majority of the naturalists of to-day. However, we must not pass on without pointing out that however much the explanations given by various men of science may differ, they all agree in expressly recognizing the complete naturalness of the secondary as well as of the primary factors of evolution.

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The doctrine of natural selection forms the best basis for the detailed discussion of the way evolution has come about in the past and how it is going on to-day. This is true because it was the first description of nature's program to carry conviction to the scientific world, and because its major elements have stood the test of time as no other doctrine has done. Much has been added to our knowledge of natural processes during post-Darwinian times, and new discoveries have supplemented and strengthened the original doctrine in numerous ways, although they have corrected certain of the minor details on the basis of fuller investigation.

At the outset it must be clearly understood that Darwin's doctrine is concerned primarily with the _method_ and not with the evidences as to the actual _fact_ of evolution. Most of those who are not familiar with the principles of science believe that Darwin discovered this process; but their opinion is not correct. The reality of natural change as a universal attribute of living things had been clearly demonstrated long before Darwin wrote the remarkable series of books whose influence has been felt outside the domains of biology and to the very confines of organized knowledge everywhere. The "Origin of Species" was published in 1859, and only the last of its fourteen chapters is devoted to a statement of the evidence that evolution is true. In this volume Darwin presented the results of more than twenty-five years of patient study of the phenomena of nature, utilizing the observations of wild life in many regions visited by him when he was the naturalist of the "Beagle" during its famous voyage around the world. He also considered at length the results of the breeder's work with domesticated animals, and he showed for the first time that the latter have an evolutionary significance. Because his logical assembly of wide series of facts in this and later volumes did so much to convince the intellectual world of the reasonableness of evolution, Darwin is usually and wrongly hailed as the founder of the doctrine. It is interesting to note in passing that Alfred Russel Wallace presented a precisely similar outline of nature's workings at about the same time as the statement by Darwin of his theory of natural selection. But Wallace himself has said that the greater credit belongs to the latter investigator who had worked out a more complete analysis on the basis of far more extensive observation and research.

The fundamental point from which the doctrine of natural selection proceeds is the fact that all creatures are more or less perfectly adapted to the circumstances which they must meet in carrying on their lives; this is the reason why so much has been said in earlier connections regarding the universal occurrence of organic adaptation. An animal is not an independent thing; its life is intertwined with the lives of countless other creatures, and its very living substance has been built up out of materials which with their endowments of energy have been wrested from the environment. Every animal, therefore is engaged in an unceasing struggle to gain fresh food and new energy, while at the same time it is involved in a many-sided conflict with hordes of lesser and greater foes. It must prevail over all of them, or it must surrender unconditionally and die. There is no compromise, for the vast totality we individualize as the environment is stern and unyielding, and it never relents for even a moment's truce.

To live, then, is to be adapted for successful warfare; and the question as to the mode of origin of species may be restated as an inquiry into the origin of the manifold adaptations by which species are enabled to meet the conditions of life. Why is adaptation a universal phenomenon of organic nature?

The answer to this query given by Darwinism may be stated so simply as to seem almost an absurdity. It is, that if there ever were any unadapted organisms, they have disappeared, leaving the world to their more efficient kin. Natural selection proves to be a continuous process of trial and error on a gigantic scale, for all of living nature is involved. Its elements are clear and real; indeed, they are so obvious when our attention is called to them that we wonder why their effects were not understood ages ago. These elements are (1) the universal occurrence of variation, (2) an excessive natural rate of multiplication, (3) the struggle for existence entailed by the foregoing, (4) the consequent elimination of the unfit and the survival of only those that are satisfactorily adapted, and (5) the inheritance of the congenital variations that make for success in the struggle for existence. It is true that these elements are by no means the ultimate causes of evolution, but their complexity does not lessen their validity and efficiency as the immediate factors of the process.

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Taking up the first proposition, we return to the subject of variation that has been discussed previously for the purpose of demonstrating its reality. The observations of every day are enough to convince us that no two living things are ever exactly alike in all respects. The reason is that the many details of organic structure are themselves variable, so that an entire organism cannot be similar to another either in material or in functional regards, while furthermore it would be impossible for an animal to be related to environmental circumstances in the same way as another member of its species unless it was possible for two things to occupy the same space at the same time! Individual differences in physical constitution are displayed by any litter of kittens, with identical parents; it needs only a careful examination to find the variations in the shape of the heads, the length of their tails, and in every other character. Sometimes the differences are less evident in physical qualities than in disposition and mental make-up, for such variations can be found among related kittens just as surely as among the children belonging to a single human family.

Not only do all organisms vary, but they seem to vary in somewhat similar ways. While modern investigations have thrown much light upon the relations between variations and their causes, of particular value in the case of the congenital phenomena, the greatest advance since Darwin's time consists in the demonstration by the naturalists who have employed the laborious methods of statistical analysis that the laws according to which differences occur are the same where-ever the facts have been examined. A single illustration will suffice to indicate the general nature of this result. If the men of a large assemblage should group themselves according to their different heights in inches, we would find that perhaps one half of them would agree in being between five feet eight inches and five feet nine inches tall. The next largest groups would be those just below and above this average class,--namely, the classes of five feet seven to eight inches and five feet nine to ten inches. Fewer individuals would be in the groups of five feet five to six inches and five feet ten to eleven inches, and still smaller numbers would constitute the more extreme groups on opposite sides of these. If the whole assemblage comprised a sufficient number of men, it would be found that a class with a given deviation from the average in one direction would contain about the same number of individuals as the class at the same distance from the average in the opposite direction. Taking into account the relative numbers in the several classes and the various degrees to which they depart from the average, the mathematician describes the whole phenomenon of variation in human stature by a concise formula which outlines the so-called "curve of error." From his study of a thousand men, he can tell how many there would be in the various classes if he had the measurements of ten thousand individuals, and how many there would be in the still more extreme classes of very short and very tall men which might not be represented among one thousand people.

It is not possible to explain why variation should follow this or any other mathematical law without entering into an unduly extensive discussion of the laws of error. The mathematicians themselves tell us in general terms that the observations they describe so simply by their formulæ follow as the result of so-called chance, by which they mean that the combined operation of numerous, diverse, and uncorrelated factors brings about this result, and not, of course, that there is such a thing as an uncaused event or phenomenon.

Whenever any extensive series of like organisms has been studied with reference to the variations of a particular character, the variations group themselves so as to be described by identical or similar curves of error. It is certainly significant that this is true for such diverse characters, cited at random from the lists of the literature, as the number of ray-flowers of white daisies, the number of ribs of beech leaves, and of the bands upon the capsules of poppies, for the shades of color of human eyes, for the number of spines on the backs of shrimps, and for the number of days that caterpillars feed before they turn into pupæ.

To summarize the foregoing facts, we have learned that variation is universal throughout the living world, and that the primary factors causing organic difference--the counterparts of human ingenuity in the case of dead mechanisms--are the natural influences of the environment, of organic physiological activity, and of congenital inheritance. These factors are accorded different values in the evolution of new species, as we may see more clearly at a later juncture, but the essential point here is that they are not unreal, although they may not as yet be described by science in final analytical terms.

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