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. All that we can legit- 
