The Evolution Theory, Vol. 1 of 2

by August Weismann

To make this idea clear I must to some extent diverge.

It is a generally observed fact that, in every species of animals or of plants, more germs and more individuals are produced than grow to maturity, or become capable of reproduction. Numerous young individuals perish at an early stage, often because of unfavourable circumstances—cold, drought, damp, or through hunger, or at the hands of their enemies. When we ask which of the progeny perish early, and which survive to carry on the species, we are at first sight inclined to suppose that this is entirely a matter of chance; but this is just what Darwin disputed. It is not chance alone, it is, above all, the differences between individuals, which enable them to withstand adverse circumstances better or worse, and thus decide, according to his view, which shall perish and which shall survive. If this be so, then we have a veritable process of selection, and one which secures that the 'best,' that is, the most capable of resistance, survive to breed, being thus, so to speak, 'selected.'

It may be asked, however, why so many individuals must perish in youth, and whether it could not have been arranged that all, or at least most, should survive till they had reproduced. But this is an impossibility, unrealizable for this among other reasons, that organisms multiply in geometrical progression, and that their progeny would very soon exceed the limits of computability. This does not occur, for there is a limit set which they can in no case overstep,—which, indeed, as we shall see, they never reach—I mean the limits of space and food-supply. Every species, by the natural requirements of its life, is restricted to a particular habitat, to land or to water, but most are still more strictly limited to a definite area of the earth's surface, which alone affords the climate suited to them, or where alone the still more specialized conditions of their existence can be realized. Thus, for instance, the occurrence of a particular species of plant determines that of the animal which is dependent on it for its food-supply. If they could multiply unchecked, that is, without the loss of many of their progeny, every species would fill up its area of occurrence and exhaust the whole of its food-supply, and thus bring about its own extermination. This seems to be prevented in some way, for as a matter of fact it does not happen.

It may, perhaps, be imagined that this might be prevented by a regulation of the productivity of the species, and that those which have not a large area of distribution, or can only count on a relatively limited food-supply, have also a low rate of multiplication, but this is not the case; even the lowest rate of multiplication would very soon suffice to make any species fill up its whole available space and completely exhaust its food-supply. Darwin takes as an example the elephant, which only begins to breed at thirty years of age, and continues to do so till about ninety, but so slowly that in these sixty years only three pairs of young are produced. Nevertheless, in 500 years an elephant pair would be represented by fifteen millions of descendants, if all the young survived till they were capable of reproduction. A species of bird with a duration of life of five years, during which it breeds four times, producing and rearing four young each time, would in the course of fifteen years have 2,000 millions of descendants.

Thus, although the fertility of each species is, as a matter of fact, precisely regulated, a low rate of multiplication is not in itself sufficient to prevent the excessive increase of any species, nor is the quantity of the relevant food-supply. Whether this be very large or very small, we see that in reality it is never entirely used up, that, as a matter of fact, a much greater quantity is always left over than has been consumed. If increase depended only on food-supply, there would, for instance, be food enough in their tropical home for many thousand times more elephants than actually occur; and among ourselves the cockchafers might appear in much greater numbers than they do even in the worst cockchafer year, for all the leaves of all the trees are never eaten up; a great many leaves and a great many trees are left untouched even in the years when the voracious insects are the most numerous. Nor do the rose-aphides, notwithstanding their enormously rapid multiplication, ever destroy all the young shoots of a rose-bush, or all the rose-bushes of a garden, or of the whole area in which roses grow.

At the same time it must be noted, that the number of individuals in a species undoubtedly does bear some relation to the amount of the food-supply available; for instance, it is very low among the large carnivores, the lion, the eagle, and the like. In our Alps the eagles have become rarer with the decrease of game, and where one eagle pair make their eyrie they rule alone over a hunting territory of more than sixty miles, a preserve on which no others of the same species are allowed to intrude. If there were several pairs of eagles in such a preserve, they would soon have so decimated the food-supply that they would starve. On the other hand, numerous herbivores, e.g. chamois and marmots, live within the bounds of the pair of eagles' hunting grounds, since the food they require is present in enormously greater quantity.

While it is true that the number of individuals of a given species which live in a particular area is not exactly the same year in year out, being subject to small, and sometimes, as in the case of the aphides and cockchafers, to very great fluctuations, nevertheless we may assume that the average number remains the same, that in the course of a century, or, let us say, of a thousand years, the number of mature individuals inhabiting the particular area remains the same. This, of course, only holds true on the supposition that there has been no great change in the external conditions of life during this period. But before Man began to interfere with nature, these external conditions would remain uniform for much longer periods than we have assumed. Let us call the average number of individuals occurring on such a uniform area, the normal number of the species; this number will be determined in the first instance by the number of offspring that are annually brought forth, and secondly by the number that annually perish before reaching maturity. As the fertility of a species is a definite quantity, so also will its elimination be definite, or, as we may say, when the normal number under uniform conditions of life remains constant, the ratio of elimination will also remain constant. Each species is therefore subject to a perfectly definite ratio of elimination which remains on the average constant, and this is the reason why a species does not multiply beyond its normal number notwithstanding the great excess of the food-supply, and notwithstanding the fertility which, in all species, is sufficient to lead to boundless multiplication.

It is not difficult to calculate the ratio of elimination for a particular species, if one knows its rate of multiplication; for if the normal number remains constant, it follows that only two of all the offspring which a pair brings forth in the course of its life can attain to reproductive maturity, and that all the rest must perish.

Suppose, for instance, a pair of storks produced four young ones annually for twenty years, of these eighty young ones which are born within this period, on an average seventy-eight must perish, and only two can become mature animals. If more than two attained maturity the total number of storks would increase, and this is against the presupposition of constancy in the normal number. It is important, in reference to the fact on which we are now focusing our attention, that we should consider some other illustrations from the same point of view. The female trout yearly produces about 600 eggs; let us assume that it remains capable of reproduction for only ten years, then the elimination-number of the species will be 6,000 less two, that is, 5,998, for of the 6,000 eggs only two can become mature animals. But in the majority of fishes the ratio of extermination is enormously greater than this. Thus a female herring brings forth 40,000 eggs annually, the duration of life is estimated at ten years, and this means an elimination number of 400,000 less two, that is, 399,998. The carp produces 200,000 eggs a year, and the sturgeon two millions, and both species live long, and remain capable of reproduction for at least fifty years. But of all the 100 million eggs which are produced by the sturgeon, only two reach their full development and reproduce; all others perish prematurely.