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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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 cumber the earth or fill the sea in a relatively short time. That this is universally true is apparent from any illustration 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 circumstances 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 assuming 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 participant 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 illustrations bring out the fact that there are three classes 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 competition 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 analysis 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.
In the second place, the vital struggle includes the battles of every species with other kinds of living things whose interests are in opposition. The relations of protozoa and bacteria, conger-eels and other fish, English sparrows and hawks, plants and herbivorous animals, are typical examples of the universal conflict in which all organisms are involved in some way. Again it is only too evident that human beings must participate every day in some form of warfare with other species. In order that food may be provided for mankind the lives of countless wild organisms must be sacrificed in addition to the great numbers of domesticated animals reared by man only that they may be destroyed. The wolf and the wildcat and the panther have disappeared from many of our Eastern states where they formerly lived, while no longer do vast herds of bison and wild horses roam the Western prairies. Because one or another human interest was incompatible with the welfare of these animals they have been driven out by the stronger invaders.