360 THE ORIGIN OF SPECIES 



relation between the size of eggs and the period of their 

 incubation), then there is no difficulty 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 re- 

 marks 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 de- 

 gree 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 themselvei 

 But he has now received a trustworthy account of 

 young cuckoo which was actually seen, while still blim 

 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 





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