82 LIFE OF 



dozen species where another reckons only one. If such 

 divergence of opinion is possible between good observers, 

 it is evident that there is no sufficiently clear rule 

 for deciding what a species is, although for centuries 

 naturalists have laboured to establish them. If species 

 vary continually, and become modified, then this diffi- 

 culty is explained. 



But what is there in nature to answer to the breeder's 

 selection ? Here comes in Darwin's remarkable applica- 

 tion and amplification of Malthus's principle of population. 

 " Nothing is easier," he says, " than to admit in words the 

 truth of the universal struggle for life, or more difficult 

 at least I have found it so than constantly to bear this 

 conclusion in mind. Yet unless it be thoroughly en- 

 grained in the mind, I am convinced that the whole 

 economy of nature, with every fact on distribution, rarity, 

 abundance, extinction, and variation will be dimly seen 

 or quite misunderstood. We behold the face of nature 

 bright with gladness ; we often see superabundance of 

 food ; we do not see, or we forget, that the birds which 

 are idly singing round us mostly live on insects or seeds, 

 and are thus constantly destroying life ; or we forget how 

 largely these songsters, or their eggs, or their nestlings are 

 destroyed by birds and beasts of prey ; we do not always 

 bear in mind, that though food may be now superabun- 

 dant, it is not so at all seasons of each recurring year." 

 The proofs given of the enormous rate at which animals 

 and plants tend to increase in numbers are very striking ; 

 even the elephant, the slowest breeder of all animals, 

 would increase from one pair to fifteen millions in the 

 fifth century, if no check existed. 



