INSTINCT 217 



been of an advantage to this species to have laid eggs even smaller 

 than those now laid, so as to have deceived certain foster-parents, 

 or, as is more probable, to have been hatched within a shorter 

 period (for it is asserted that there is a relation between the size 

 of eggs and the period of their incubation), then there is no diffi- 

 culty in believing that a race or species might have been formed 

 which would have laid smaller and smaller eggs; for these would 

 have been more safely hatched and reared. Mr. Ramsay remarks 

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

 open nest, manifest a decided preference for nests containing eggs 

 similar in color to their own. The European species apparently 

 manifests some tendency toward a similar instinct, but not rarely 

 departs from 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 instinct, 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. Ramsay, 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 be- 

 lieve 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 im- 

 portance for the young cuckoo, as is probably the case, to receive 

 as much food as possible soon after birth, I can see no special diffi- 

 culty in its having gradually acquired, during successive genera- 

 tions, 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 



