They are fertile in statistics; but an interpretation of these statistics that furthers our purpose is still to seek.
But surely, it will be said, we can tell an instinct when we come across it, so uniform as it is, and so independent of the rest of the system.
Not at all.

For one thing, the idea that an instinct is apiece of mechanism, as fixed as fate, is quite out of fashion.

It is now known to be highly plastic in many cases, to vary considerably in individuals, and to involve conscious processes, thought, feeling and will, at any rate of an elementary kind.

Again, how are you going to isolate an instinct?
Those few automatic responses to stimulation that appear shortly after birth, as, for instance, sucking, may perhaps be recognized, since parental training and experience in general are out of the question here.

But what about the instinct or group of instincts answering to sex?
This is latent until a stage of life when experience is already in full swing.