The facts of geographical distribution const.i.tute the fifth division of zoology, and an independent cla.s.s of evidences proving the occurrence of evolution. This department of zoology a.s.sumed its rightful status only after the other divisions had attained considerable growth. Many naturalists before Darwin and Wallace and Wagner had noticed that animals and plants were by no means evenly distributed over the surface of the globe, but until the doctrine of evolution cleared their vision they did not see the meaning of these facts. As in the case of all the other departments of zoology the immediate data themselves are familiar, but because they are so obvious the mind does not look for their interpretation but accepts the facts at their face value. While the phenomena of distribution are no less fascinating to the naturalist, and no less effective in their demonstration of evolution, their comprehensive treatment would demand more s.p.a.ce than the whole purpose of the present description of organic evolution would justify. Thus a brief outline only can be given of the salient principles of this subject in order that their bearing upon the problem of species may be indicated.
Even as children we learn many facts of animal distribution; every one knows that lions occur in Africa and not in America, that tigers live in Asia and Malaysia, that the jaguar is an inhabitant of the Brazilian forests, and that the American puma or mountain lion spreads from north to south and from east to west throughout the American continents. The occurrence of differing human races in widely separated localities is no less familiar and striking, for the red man in America, the Zulu in Africa, the Mongol and Malay in their own territories, display the same discontinuity in distribution that is characteristic of all other groups of animals and of plants as well. As our sphere of knowledge increases, we are impressed more and more forcibly by the diversity and unequal extent of the ranges occupied by the members of every one of the varied divisions of the organic world. Another fact which becomes significant only when science calls our attention to it is the absence from a land like Australia of higher mammals such as the rabbit of Europe. The hypothesis of special creation cannot explain this absence on the a.s.sumption that the rabbit is unsuited to the conditions obtaining in the country named, for when the species was introduced into Australia by man, it developed and spread with marvelous rapidity and destructive effect. It may seem impossible that facts like these could possess an evolutionary significance, but they are actual examples of the great ma.s.s of data brought together by the naturalists who have seen in them something to be interpreted, and who have sought and found an explanation in the formularies of science.
The general principles of distribution appear with greatest clearness when an examination is made of the animals and plants of isolated regions like islands. The Galapagos Islands const.i.tute a group that has figured largely in the literature of the subject, partly because Darwin himself was so impressed by what he found there in the course of his famous voyage around the world in the "Beagle." They form a cl.u.s.ter on the Equator about six hundred miles west of the nearest point of the neighboring coast of South America. Although the lizards and birds that live in the group differ somewhat among themselves as one pa.s.ses from island to island, on the whole they are most like the species of the corresponding cla.s.ses inhabiting South America. Why should this be so? On the hypothesis of special creation there is no reason why they should not be more like the species of Africa or Australia than like those of the nearest body of the mainland. The explanation given by evolution is clear, simple, and reasonable. It is that the characteristic island forms are the descendants of immigrants which in greatest probability would be wanderers from the neighboring continent and not from far distant lands. Reaching the isolated area in question the natural factors of evolution would lead their offspring of later generations to vary from the original parental types, and so the peculiar Galapagos species would come into being. The fact that the organisms living on the various islands of this group differ somewhat in lesser details adds further justification for the evolutionary interpretation, because it is not probable that all the islands would be populated at the same time by similar stragglers from the mainland. The first settlers in one place would send out colonies to others, where independent evolution would result in the appearance of minor differences peculiar to the single island. In this manner science interprets the general agreement between the animals of the Azores Islands and the fauna of the northwestern part of Africa, the nearest body of land, from which it would be most natural for the ancestors of the island fauna to come.
The land-snails inhabiting the various groups of islands scattered throughout the vast extent of the Pacific Ocean provide the richest and most ideal material for the demonstration of the principles of geographical distribution. In the Hawaiian Islands snails of the family of Achatinellidae occur in great abundance, and like the lizards of the Galapagos Islands different species occur on the different members of the group. Within the confines of one and the same island, they vary from valley to valley, and the correlation between their isolation in geographical respects and specific differences on the other hand, first pointed out by Gulick, makes this tribe of animals cla.s.sical material. In Polynesia and Melanesia are found close relatives of the Achatinellidae, namely, the Partulae, which are thus in relative proximity to the Achatinellidae and not on the other side of the world. Furthermore, the Partulae are not alike in all of the groups of Polynesia where they occur; the species of the Society Islands are absolutely distinct from those of the Marquesas, Tonga, Samoan, and Solomon Islands, although they agree closely in the basic characters that justify their reference to a single genus. The geological evidence tells us that these islands were once the peaks of mountain ranges rising from a Pacific continent which has since subsided to such an extent that the mountain tops have become separate islands. Thus the resemblances between Hawaiian and Polynesian snails, and the closer similarities exhibited by the species of the various groups of Polynesia, are intelligible as the marks of a common ancestry in a widespread continental stock, while the observed differences show the extent of subsequent evolution along independent lines followed out after the isolation of the now separated islands. The principle may be worked out in even greater detail, for it appears that within the limits of one group diverse forms occupy different islands, evolved in different ways in their own neighborhoods; while in one and the same island, the populations of the different valleys show marked effects of divergence in later evolution, precisely as in the case of the cla.s.sic Achatinellidae of the Hawaiian Islands.
The broad and consistent principle underlying these and related facts is this: _there is a general correspondence between the differences displayed by the organisms of two regions and the degree of isolation or proximity of these two areas_. Thus the disconnected but neighboring areas of the Galapagos Islands and South America support species that resemble each other closely, for the reasons given before; long isolated areas like Australia and its surroundings possess peculiar creatures like the egg-laying mammals, and all of the pouched animals or marsupials with only one or two exceptions like our own American opossum,--a correlation between a geological and geographical discontinuity on the one hand and a peculiarity on the other that reinforces our confidence in the faunal evolutionary interpretation of the facts of distribution.
It is true that the various cla.s.ses of animals do not always appear with coextensive ranges. The barriers between two groups of related species will not be the same in all cases. A range like the Rocky Mountains will keep fresh-water fish apart, while birds and mammals can get across somewhere at some time. All these things must be taken into account in a.n.a.lyzing the phenomena of distribution, and many factors must be given due attention; but in all cases the reasons for the particular state of affairs in geographical and biological respects possess an evolutionary significance.
Having then all the facts of animal natural history at his disposal, and the uniform principles in each body of fact that demonstrate evolution, it is small wonder that the evolutionist seems to dogmatize when he a.s.serts that descent with adaptive and divergent modification is true for all species of living things. The case is complete as it stands to-day, while it is even more significant that every new discovery falls into line with what is already known, and takes its natural place in the all-inclusive doctrine of organic evolution. Because this explanation of the characteristics of the living world is more reasonable than any other, science teaches that it is true.
IV
EVOLUTION AS A NATURAL PROCESS
The purpose of the discussions up to this point has been to present the reasons drawn from the princ.i.p.al cla.s.ses of zoological facts for believing that living things have transformed naturally to become what they now are.
Even if it were possible to make an exhaustive a.n.a.lysis of all of the known phenomena of animal structure, development, and fossil succession, the complete bodies of knowledge could not make the evolutionary explanation more real and evident than it is shown to be by the simple facts and principles selected to const.i.tute the foregoing outline. We have dealt solely with the evidences as to the fact of evolution; and now, having a.s.sured ourselves that it is worth while to so do, we may turn to the intelligible and reasonable evidence found by science which proves that the familiar and everyday "forces" of nature are competent to bring about evolution if they have operated in the past as they do to-day.
Investigation has brought to light many of the subsidiary elements of the whole process, and these are so real and obvious that they are simply taken for granted without a suspicion on our part of their power until science directs our attention to them.
For one reason or another, those who take up this subject for the first time find it difficult to banish from their minds the idea that evolution, even if it ever took place, has been ended. They think it futile to expect that a scrutiny of to-day"s order can possibly find influences powerful enough to have any share in the marvelous process of past evolution demonstrated by science. The naturalists of a century ago held a similar opinion regarding the earth, viewing it as an immutable and unchanged product of supernatural creation, until Lyell led them to see that the world is a plastic ma.s.s slowly altering in countless ways. It is no more true that living things have ceased to evolve than that mountains and rivers and glaciers are fixed in their final forms; they may seem everlasting and permanent only because a human life is so brief in comparison with their full histories. Like the development of a continent as science describes it, the origin of a new species by evolution, its rise, culmination, and final extinction may demand thousands of years; so that an onlooker who is himself only a conscious atom of the turbulent stream of evolving organic life does not live long enough to observe more than a small fraction of the whole process. Therefore living species seem unchanged and unchangeable until a conviction that evolution is true, and a knowledge of the method of science by which this conviction is borne upon one, guide the student onwards in the further search for the efficient causes of the process.
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 cla.s.ses of zoological facts, and then he must turn his attention from the dead object of laboratory a.n.a.lysis 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 cla.s.sify what we may observe, increasing our knowledge from the wide array of equally natural facts supplied by the biologist.
The a.n.a.logies 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 sc.r.a.p-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 const.i.tuted that biological equivalents of these three essential elements of mechanical evolution can be found? Are organisms adapted to the circ.u.mstances 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.
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.
Pa.s.sing 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 cla.s.ses 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 compet.i.tors. 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 cla.s.s. 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 cla.s.s, 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 cla.s.ses. It is quite true that the influences of the third cla.s.s 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 pa.s.sing 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 a.n.a.lysis 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 a.s.sumed and taken for granted. Thus the darker color of the skin of tropical human races would be viewed by Buffon as the c.u.mulative 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 a.s.sumed 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 pa.s.s 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.
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 a.s.sembly 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 pa.s.sing 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 a.n.a.lysis 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 circ.u.mstances 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.
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 circ.u.mstances in the same way as another member of its species unless it was possible for two things to occupy the same s.p.a.ce at the same time! Individual differences in physical const.i.tution 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 a.n.a.lysis that the laws according to which differences occur are the same where-ever the facts have been examined. A single ill.u.s.tration will suffice to indicate the general nature of this result. If the men of a large a.s.semblage 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 cla.s.s,--namely, the cla.s.ses 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 const.i.tute the more extreme groups on opposite sides of these. If the whole a.s.semblage comprised a sufficient number of men, it would be found that a cla.s.s with a given deviation from the average in one direction would contain about the same number of individuals as the cla.s.s at the same distance from the average in the opposite direction. Taking into account the relative numbers in the several cla.s.ses 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 cla.s.ses if he had the measurements of ten thousand individuals, and how many there would be in the still more extreme cla.s.ses 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 formulae 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 pupae.
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 a.n.a.lytical terms.
We come now to the second element of the whole process of evolution, namely, what we may call overproduction or excessive multiplication. Like variation and so many other phenomena of nature, this is so real and natural that it escapes our attention until science places it before us in a new light. The normal rate of reproduction _in all species of animals_ is such that if it were unchecked, any kind of organism would c.u.mber the earth or fill the sea in a relatively short time. That this is universally true is apparent from any ill.u.s.tration that might be selected. Let us take the case of a plant that lives for a single year, and that produces two seeds before it withers and dies; let us suppose that each of these seeds produces an adult plant which in its turn lives one year and forms two seeds. If this process should continue without any interference, the twentieth generation after as many years would consist of more than one million descendants of the original two-seeded annual plant, provided only that each individual of the intervening years should live a normal life and should multiply at the natural rate. But such a result as this is rendered impossible by the very nature which makes annual plants multiply in the way they do. Let us take the case of a pair of birds which produce four young in each of four seasons. Few would be prepared for the figures enumerating the offspring of a single pair of birds at the end of fifteen years, if again all individuals lived complete and normal lives: at the end of the time specified there would be more than two thousand millions of descendants. The English sparrow has been on this continent little more than fifty years; it has found the conditions in this country favorable because few natural enemies like those of its original home have been met, and as a consequence it has multiplied at an astounding rate so as to invade nearly all parts of North America, driving out many species of song birds before it. About twenty years ago David Starr Jordan wrote that if the English sparrow continued to multiply at the natural rate of that time, in twenty years more there would be one sparrow to every square inch of the state of Indiana; but of course nature has seen to it that this result has not come about. A single conger-eel may produce fifteen million eggs in a single season, and if this natural rate of increase were unchecked, the ocean would be filled solid with conger-eels in a few years. Sometimes a single tapeworm, parasitic in the human body, will produce three hundred million embryos; the fact that this animal is relatively rare diverts our attention from the alarming fertility of the species and the excessive rate of its natural increase. Perhaps the most amazing figures are those established by the students of bacteria and other micro-organisms. Many kinds of these primitive creatures are known where the descendants of a single individual will number sixteen to seventeen millions after twenty-four hours of development under ordinarily favorable conditions. Though a single rodlike individual taken as a starting-point may be less than one five-thousandth of an inch in length, under natural circ.u.mstances it multiplies at a rate which _within five days_ would cause its descendants _to fill all the oceans to the depth of one mile_. This is a fact, not a conjecture; the size of one organism is known, and the rate of its natural increase is known, so that it is merely a matter of simple arithmetic to find out what the result would be in a given time.
Even in the case of those animals that reproduce more slowly, an overcrowding of the earth would follow in a very short time. Darwin wrote that even the slow-breeding human species had doubled in the preceding quarter century. An elephant normally lives to the age of one hundred years; it begins to breed at the age of thirty, and usually produces six young by the time it is ninety. Beginning with a single pair of elephants and a.s.suming that each individual born should live a complete life, only eight hundred years would be requisite to produce nineteen million elephants; a century or two more and there would be no standing room for the latest generation of elephants. It is only too obvious that such a result is not realized in nature, but it is on account of other natural checks, and not because the natural rate of reproductive increase is anything but excessive.
The third element of the process of natural selection is the struggle for existence which is to a large extent the direct consequence of over-multiplication. Because nature brings more individuals into existence than it can support, every animal is involved in many-sided battles with countless foes, and the victory is sometimes with one and sometimes with another partic.i.p.ant in the conflict. A survivor turns from one vanquished enemy only to find itself engaged in mortal combat with other attacking forces. Wherever we look, we find evidence of an unceasing struggle for life, and an apparently peaceful meadow or pond is often the scene of fierce battles and tragic death that escape our notice only because the contending armies are dumb.
A community of ants, often comprising more individuals than an entire European state, depends for its national existence upon its ability to prevail over other communities with which it may engage in sanguinary wars where the losses of a single battle may exceed those of Gettysburg. The developing conger-eels find a host of enemies which greatly deplete their numbers before they can grow even into infancy. An annual plant does not produce a million living offspring in twenty years because seeds do not always fall upon favorable soil, nor do they always receive the proper amount of sunlight and moisture, or escape the eye of birds and other seed-eating animals. These three ill.u.s.trations bring out the fact that there are three cla.s.ses of natural conditions which must be met by every living creature if it is to succeed in life. In detail, the struggle for existence is _intra-specific_, involving some form of compet.i.tion or rivalry among the members of a single species; it is _inter-specific_, as a conflict is waged by every species with other kinds of living things; and finally it involves an adjustment of life to _inorganic environmental_ influences. While it may seem unjustifiable to speak of heat and cold and sunlight as enemies, the direct effects produced by these forces are to be reckoned with no less certainty than the attacks of living foes.
The three divisions of the struggle for existence are so important not only in purely scientific respects, but also in connection with the a.n.a.lysis of human biology, that we may look a little further into their details, taking them up in the reverse order. Regarding the environmental influences, the way that unfavorable surroundings decimate the numbers of the plants of any one generation has already been noted, and it is typical of the vital situation everywhere. English sparrows are killed by prolonged cold and snow as surely as by the hawk. The pond in which bacteria and protozoa are living may dry up, and these organisms may be killed by the billion. Even the human species cannot be regarded as exempt from the necessity of carrying on this kind of natural strife, for scores and hundreds die every year from freezing and sunstroke and the thirsts of the desert. Unknown thousands perish at sea from storm and shipwreck, while the recorded casualties from earthquakes and volcanic eruptions and tidal waves have numbered nearly one hundred and fifty thousand in the past twenty-eight years. The effects of inorganic influences upon all forms of organic life must not be underestimated in view of such facts as these.