Again as in the case of corporeal structure, and conformably with my theory, the instinct of each species is good for itself, but has never, as far as we can judge, been produced for the exclusive good of others.
I can only assert, that instincts certainly do vary--for instance, the migratory instinct, both in extent and direction, and in its total loss.
The possibility, or even probability, of inherited variations of instinct in a state of nature will be strengthened by briefly considering a few cases under domestication.
But this instinct retained by our chickens has become useless under domestication, for the mother-hen has almost lost by disuse the power of flight.
I will select only three, out of the several which I shall have to discuss in my future work,--namely, the instinct which leads the cuckoo to lay her eggs in other birds' nests; the slave-making instinct of certain ants; and the comb-making power of the hive-bee: these two latter instincts have generally, and most justly, been ranked by naturalists as the most wonderful of all known instincts.
It is now commonly admitted that the more immediate and final cause of the cuckoo's instinct is, that she lays her eggs, not daily, but at intervals of two or three days; so that, if she were to make her own nest and sit on her own eggs, those first laid would have to be left for some time unincubated, or there would be eggs and young birds of different ages in the same nest.
207-9) gives a good illustration of an instinct growing wiser through experience.
1) That instinct requires no prevision of the biological end which it serves;
2) That instinct is only adapted to achieve this end in the usual circumstances of the animal in question, and has no more precision than is necessary for success AS A RULE;
3) That processes initiated by instinct often come to be performed better after experience;
4) That instinct supplies the impulses to experimental movements which are required for the process of learning;