INSTINCTS OF THE MOLOTHRUS. 237 



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

 in the nest of the hedge-warbler with bright greenish-blue 

 eggs. Had our cuckoo invariably displayed the above in- 

 stinct, it would assuredly 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. Ram- 

 say, to an extraordinary degree in color; 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, while 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 cuckoos which had such habits and 

 structure best developed would be the most securely reared. 

 The first step toward the acquisition of the proper instinct 

 might have been mere unintentional restlessness on the part 

 of the young bird, when somewhat advanced in age and 

 strength ; the habit having been afterward improved, and 

 transmitted to an earlier age. I can see no more difficulty 

 in this than in the unhatched young of other birds acquiring 

 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 can- 

 not 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. 



