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 proceeds 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 deposition 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 successive 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 argument 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. All that we can legitimately infer is, that, although the normal instinct has been changed into another, the race has still been able to remain in existence. The same conclusion applies to the case of _Molothrus bonariensis_, cited by Darwin, and is here even more obvious:—