Darwin s Artificial and Natural Selection 139 



Darwin deals with instincts of animals in the same way as he 

 deals with their structures. After pointing out that instincts 

 are variable, and that the variations are hereditary, he pro- 

 ceeds to show how selection may act by picking out those 

 individuals possessing the more favorable instincts. In other 

 words, the theory of natural selection is applied to functions, 

 as well as to structure. Darwin makes use here also of the 

 Lamarckian factor of inheritance, and concludes that "in 

 most cases habit and selection have probably both occurred." 



A few examples will sufficiently serve to illustrate Darwin's 

 meaning. The first case given is that of the cuckoo, which 

 lays its eggs in the nests of other birds, where they are 

 hatched and the young reared by their foster-parents. The 

 starting-point for such a perversion of the ordinary habits of 

 birds is to be found, he thinks, in the occasional deposi- 

 tion of eggs in the nests of other birds, which has at times 

 been observed for a number of species. For instance, this 

 has been seen in the American cuckoo, which ordinarily builds 

 a nest of its own. It is recorded and believed to be true 

 that the young English cuckoo, when only two or three days 

 old, ejects from the nest the offspring of its foster-parents, 

 and this " strange and odious instinct " is supposed by Darwin 

 to have been acquired in order that the young cuckoo might get 

 more food, and that the young bird has acquired during succes- 

 sive generations the strength and structure necessary for the 

 work of ejection. This is of course largely speculative, and 

 it is by no means obvious that it was a greater benefit to the 

 cuckoo to have other birds rear its young than to do so itself. 

 We can equally well imagine, since this is the turn the argu- 

 ment takes, that the occasional instinct to deposit eggs in the 

 nests of other birds would be disadvantageous, and could not 

 have been acquired by the selection of a fluctuating instinct 

 of this sort. We have no right to assume, that because a 

 new habit has been acquired, that it is a more advantageous 

 one than the one that has been lost. AH that we can legit- 



