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.

Until recently the instincts of the European and of the non- parasitic American cuckoo alone were known; now, owing to Mr.