2i 4 Instinct of the Cuckoo. Chap. vhi. 



two of the Australian cuckoos, when they lay their eggs in an open 

 nest, manifest a decided preference for nests containing eggs similar 

 in colour to their own. The European species apparently manifests 

 some tendency towards a similar instinct, but not rarely departs 

 from it, as is shown by her laying her dull and pale-coloured eggs 

 in the nest of the Hedge-warbler with bright greenish- blue eggs. Had 

 our cuckoo invariably displayed the above instinct, it would assu- 

 redly have been added to those which it is assumed must all have 

 been acquired together. The eggs of the Australian Bronze cuckoo 

 vary, according to Mr.Eamsay,to an extraordinary degree in colour; 

 so that in this respect, as well as in size, natural selection might 

 have secured and fixed any advantageous variation. 



In the case of the European cuckoo, the offspring of the foster- 

 parents are commonly ejected from the nest within three days after 

 the cuckoo is hatched ; and as the latter at this age is in a most 

 helpless condition, Mr. Gould was formerly inclined to believe that 

 the act of ejection was performed by the foster-parents themselves. 

 But he has now received a trustworthy account of a young cuckoo 

 which was actually seen, whilst still blind and not able even to 

 hold up its own head, in the act of ejecting its foster-brothers. One 

 of these was replaced in the nest by the observer, and was again 

 thrown out. With respect to the means by which this strange and 

 odious instinct was acquired, if it were of great importance for the 

 young cuckoo, as is probably the case, to receive as much food as 

 possible soon after birth, I can see no special difficulty in its having 

 gradually acquired, during successive generations, the blind desire, 

 the strength, and structure necessary for the work of ejection ; for 

 those young cuckoos which had such habits and structure best deve- 

 loped would be the most securely reared. The first step towards 

 the acquisition of the proper instinct might have been mere unin- 

 tentional restlessness on the part of the young bird, when somewhat 

 advanced in age and strength ; the habit having been afterwards 

 improved, and transmitted to an earlier age. I can see no more 

 difficulty in this, than in the unhatched young of other birds ac- 

 quiring the instinct to break through their own shells ; — or than in 

 young snakes acquiring in their upper jaws, as Owen has remarked, 

 a transitory sharp tooth for cutting through the tough egg-shell. 

 For if each part is liable to individual variations at all ages, and the 

 variations tend to be inherited at a corresponding or earlier age,— 

 propositions which cannot be disputed, — then the instincts and 

 structure of the young could be slowly modified as surely as those of 

 the adult ; and both cases must stand or fall together with the whole 

 theory of natural selection. 



