Chap. VIIl.] INSTINCTS OF THE CUCKOO. 331 



have been known occasionally to lay their eggs in other 

 birds' nests. Now let us suppose that the ancient pro- 

 genitor of our European 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 ear- 

 lier 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; then the old birds or the fostered 

 young would gain an advantage. 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 continued pro- 

 cess 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 Miil- 

 ler, 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. Until recently the instincts 

 of the European and of the non-parasitic American 

 cuckoo alone were known; now, owing to Mr. Eamsay's 



