Chap. YIIL] INSTINCTS OF THE CUCKOO. 331 



been known occasionally to lay their eggs in other birds' 

 nests. Now let us suppose that the ancient progenitor 

 ox 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 

 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 suc- 

 cessful 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 on sufficient evidence, by Adolf Muller, 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. Ramsay's 



