Origin of Species Variorum

to lay their eggs in other birds nests, and thus be more successful in rearing their young. 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 that the cuckoo occasionally lays her eggs on the bare ground, sits on them and feeds her young; this rare and strange event evidently is 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 in the cuckoo, which are falsely spoken of as necessarily co-ordinated. But in all cases, speculation on an instinct known only in a single species, is useless, for we have no facts to guide us. Until quite recently the instincts of the European and of the non-parasitic American cuckoo alone were known; now, owing to Mr. E. Ramsay observations, we know something about three Australian species, which lay their eggs in other birds nests. The chief points ... are three: first, that the cuckoo, with rare exceptions, lays only one egg in a nest, so that the large and voracious young bird may receive ample food. Secondly, that the eggs are of remarkably small size, not exceeding those of the skylark,—a a bird about one-fourth as large as the cuckoo. That the small size of the egg is a real case of adaptation we may infer from the fact of the non-parasitic American cuckoo laying full-sized eggs. Thirdly, .. that the young cuckoo, soon after birth, has the instinct, the strength, and a properly shaped back for ejecting its foster-brothers, which then perish from cold and hunger. This, it has been boldly maintained, is a beneficent arrangement, in order that the young cuckoo may get sufficient food, and that its foster-brothers may perish before they have acquired much feeling.

Turning now to the Australian species; though these