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 habit through being enabled 

 to migrate earlier or through any other cause; or if the young 

 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 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. 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 



