Seite - 271 - in The Origin of Species

INSTINCTS OF THE CUCKOO 271 jay (Garrulus cristatus) ; and as both were nearly full feath- ered, there could be no mistake in their identification. I could also give several instances of various birds which have been known occasionally to lay their eggs in other birds' nests. Now let us suppose that the ancient progenitor of our Euro- pean cuckoo had the habits of the American cuckoo, and that she occasionally laid an egg in another bird's nest. If the old bird profited by this occasional habitthrough beingenabled to migrate earlier or through any other cause; or if theyoung were made more vigorous by advantage being taken of the mistaken instinct of another species than when reared by their own mother, encumbered as she could hardly fail to be by having eggs and young of different ages at the same time J then the old birds or the fostered young would gain an ad- vantage. And analogy would lead us to believe, that the young thus reared would be apt to follow by inheritance the occasional and aberrant habit of their mother, and in their turn would be apt to lay their eggs in other birds' nests, and thus be more successful in rearing their young. By a con- tinued process of this nature, I believe that the strange in- stinct of our cuckoo has been generated. It has, also, re- cently been ascertained on sufficient evidence, by Adolf Miiller, 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 in- stinct 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 instinctknown 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. Ramsay's observations, we have learnt something about three Australian species, which lay their eggs in other birds' nests. The chief points to be referred to are three: first, that the common cuckoo, with rare exceptions, lays only one egg in a nest, so that the large and voracious young bird receives ample food. Secondly, that the eggs are remarkably small, not exceeding those of the skylark,—a bird about one-fourth