858 THE ORIGIN OF SPECIES 



he once found in Illinois a young cuckoo together with 

 a young jay in the nest of a Blue jay (Garrulus cris- 

 tatus); and as both were nearly full feathered, 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 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 earlier or through 

 any other cause; or if the young were made more vigor- 

 ous by advantage being taken of the mistaken instinct of 

 another species than when reared by their own mother, 

 incumbered 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 ad- 

 vantage. And analogy would lead us to believe that 

 the young thus reared would be apt to follow by inheri- 

 tance 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 ji 

 young. By a continued process of this nature, I believe 

 that the strange instinct of our cuckoo has been gen- 

 erated. It has, also, recently been ascertained on suffi- 

 cient evidence, by Adolf Miiller, that the cuckoo occa- 

 sionally lays her eggs on the bare ground, sits on them,,;! 

 and feeds her j^oung. This rare event is probably a'^ 

 case of reversion to the long-lost, aboriginal instinotiji 

 of nidification. ' 



It has been objected that I have not noticed other 

 related instincts and adaptations of structure in the 



I 



