Chap. VIII.] INSTINCTS OF THE CUCKOO. 213 



believe, that the young thus reared would be apt to follow by inhe- 

 ritance the occasional and aberrant habit of their mother, and in 

 their turn would be apt to lay their eggs in other birds' nests, and 

 thus be more successful in rearing their young. By -a continued 

 process of this nature, I believe that the strange instinct of our 

 cuckoo has been generated. It has, also, recently been ascertained 

 on sufficient evidence, by Adolf Miiller, that the cuckoo occasionally 

 lays her eggs on the bare ground, sits on them, and feeds her young. 

 This rare event is probably a case of reversion to the long-lost, 

 aboriginal instinct of nidification. 



It has been objected that I have not noticed ether related instirxta 

 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 species, is useless, for we have hitherto 

 had no facts to guide us. Until recently the instincts of the Euro- 

 pean and of the non-parasitic American cuckoo alone were known : 

 now, owing to Mr. Bamsay's observations, we have learnt something 

 about three Australian species, which lay their eggs in other birds' 

 nests. The chief poiets 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 remarkably 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 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 gene- 

 rally 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 smallei 

 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 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. Bamsay remarks that 



