THE DARWINIAN THEORY 45 



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, notwith- 

 standing 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 tru« 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 numher 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 



