236 SPECIAL INSTINCTS. 



rare eveut is probably the case of reversion to the long-lost, 

 aboriginal instinct of nidification. 



It has been objected that I have not noticed other related 

 instincts and adaptations of structure in the cuckoo, which 

 are spoken of as necessarily co-ordinated. But in all cases, 

 speculation on an instinct known to us only in a single spe- 

 cies, is useless, for we have hitherto had no facts to guide us. 

 Until recently the instincts of the European and of the non- 

 parasitic American cuckoo alone were known ; now, owing 

 to Mr. Ramsay's observations, we have learned something 

 about three Australian species, which lay their eggs in other 

 birds' nests. The chief points to be referred to are three : 

 first, that the common cuckoo, with rare exceptions, lays 

 only one egg in a nest, so that the large and voracious young 

 bird receives ample food. Secondly, that the eggs are re- 

 markably small, not exceeding those of the skylark — a bird 

 about one-fourth 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 instinct, 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 beneficent arrange- 

 ment, 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 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 difficult}^ 

 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 pref- 

 erence for nests containing eggs similar in color to their 

 own. The European species apparently manifests some tend- 

 ency toward a similar instinct, but not rarely departs from 



