212 SPECIAL INSTINCTS [Chap. VIII. 



rited mental changes; in other cases compulsory habit has done 

 nothing, and all has been the result of selection, pursued both 

 methodically and unconsciously : but in most cases habit and 

 selection have probably concurred. 



Special Instincts. 



We shall, perhaps, best understand how instincts in a state of 

 nature have become modified by selection, by considering a few 

 cases. 1 will select only three,— namely, the instinct which leads 

 the cuckoo to lay her eggs in other birds' nests ; the slave-making 

 instinct of certain ants ; and the cell-making power of the 

 hive-bee. These two latter instincts have generally and justly 

 been ranked by naturalists as the most wonderful of all known 

 instincts. 



Instincts of the Cuckoo. — It is supposed by some naturalists that 

 the more immediate cause of the instinct of the cuckoo is, that she 

 lays her eggs, not daily, but at intervals of two or three days ; so 

 that, if she were to make her own nest and sit on her own eggs, 

 those first laid would have to be left for some time unincubated, or 

 there would be eggs and young birds of different ages in the same 

 nest. If this were the case, the process of laying and hatching 

 might be inconveniently long, more especially as she migrates at a 

 very early period ; and the first hatched young would probably 

 have to be fed by the male alone. But the American cuckoo is in 

 this predicament ; for she makes her own nest, and has eggs and 

 young successively hatched, all at the same time. It has been both 

 asserted and denied that the American cuckoo occasionally lays her 

 eggs in other birds' nests ; but I have lately heard from Dr. Merrell, 

 of Iowa, that he once found in Illinois a young cuckoo together 

 with a young jay in the nest of a Blue jay (Garrulus cristatus) ; 

 and as both were nearly fully 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 

 vigorous by advantage being taken of the mistaken instinct of 

 another species than when reared by their own mother, encum- 

 bered 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 



