By a continued process of this nature, I believe that the strange instinct of our cuckoo has been generated.

It has, also recently been ascertained on sufficient evidence, by Adolf Muller, that the cuckoo occasionally lays her eggs on the bare ground, sits on them and feeds her young.

This rare event is probably a case of reversion to the long-lost, aboriginal instinct of nidification.

It has been objected that I have not noticed other related instincts and adaptations of structure in the cuckoo, which are spoken of as necessarily co-ordinated.

But in all cases, speculation on an instinct known to us only in a single species, is useless, for we have hitherto had no facts to guide us.