NATURAL SELECTION 69 



consider their rate of increase. It is quite common for them to 

 have three broods in the year and we are not beyond the mark 

 in allowing them four in each brood. In order to avoid any 

 possible exaggeration we will assume that each pair has eight 

 young ones each season. At this rate, if there were no deaths, 

 there would in five years be six thousand two hundred and 

 forty-eight house-martins sprung from one pair. 



It is quite true that with many animals the rate of increase is 

 much slower. But if we take the least promising instances, the 

 results, when we work them out, are astonishing. The elephant 

 is believed to be the slowest breeder of all known animals. Yet 

 according to Darwin's familiar calculation, "after a period of 

 from seven hundred and forty to seven hundred and fifty years 

 there would be nearly nineteen million elephants alive, descended 

 from the first pair." l And in many species, and notably among 

 plants, the rate of multiplication is far more rapid than it is among 

 house-martins. Well known as these facts are, it is well to 

 emphasize them, because there has been a tendency of late among 

 many biologists to underrate the power of Natural Selection ; to 

 say, " It may account for some facts, but evolution must have 

 gone on, to a great extent, independently of it." I believe this 

 idea is partly due to failure to realise the enormous amount of 

 elimination that is constantly taking place. As a general rule a 

 species about maintains its numbers ; in some there is a tendency 

 to increase, in others to slowly diminish. And yet it is obvious 

 that if there were no natural check, any species would in no 

 great number of years people the whole world to the exclusion 

 of all others. Such a thing is impossible, since no species of 

 organism, except perhaps some of the very lowest, can live 

 isolated and independent of all other species. But to say this is 

 only to mention one of the natural checks that stand in the way 

 of unlimited increase. 



In September, in parts of the coast where sea-gulls congre- 

 gate, it is easy to convince oneself that the majority are immature 

 birds. They wear the garb of youth, and that though in 

 herring gulls (Larus argentatus), the species perhaps most 



1 Origin of Species, p. 51 (6th edition). 



