46 THE EVOLUTION THEORY 



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,coo less two, 

 that is, 5,99^, 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 r 

 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. 



But even with these examples we have not reached the highest 

 elimination number, for many of the lower animals — not to speak 

 of many plants — produce an even greater number of offspring. 



