272 ORIGIN OF SPECIES 



as large as the cuckoo. That the small size of the egg is a 

 real case of adaptation we may infer from the fact of 

 the non-parasitic American cuckoo laying full-sized eggs. 

 Thirdly, that the young cuckoo, soon after birth, has the in- 

 stinct, the strength, and a properly shaped back for ejecting 

 its foster-brothers, which then perish from cold and hunger. 

 This has been boldly called a beneficial arrangement, in order 

 that the young cuckoo may get sufficient food, and that its 

 foster-brothers may perish before they had acquired much 

 feeling ! 



Turning now to the Australian species; though these birds 

 generally lay only one egg in a nest, it is not rare to find two 

 and even three eggs in the same nest. In the Bronze cuckoo 

 the eggs vary greatly in size, from eight to ten lines in length. 

 Now if it had been of an advantage to this species to have 

 laid eggs even smaller than those now laid, so as to have de- 

 ceived 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 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 remarks that 

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

 an open nest, manifest a decided preference for nests con- 

 taining 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 as- 

 suredly 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 ex- 

 traordinary 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 



