44 THE EVOLUTION THEORY 



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 organ- 

 isms 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 com- 

 pletely 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 re- 

 production. 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 2,0c o millions of 

 descendants. 



