216 THE ORIGIN OF SPECIES 



occasional habit through being enabled to emigrate earlier or 

 through any other cause; or if the young were made more vigorous 

 by advantage being taken of the mistaken instinct of another spe- 

 cies than when reared by their own mother, encumbered as she 

 could hardly fail to be by having eggs and young of different ages 

 at the same time, then the old birds or the fostered young would 

 gain an advantage. And analogy would lead us to believe that the 

 young thus reared would be apt to follow by inheritance the occa- 

 sional 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 evi- 

 dence, by Adolf Muller, that the cuckoo occasionally lays her eggs 

 on the bare ground, sits on them and feeds her young. This rare 

 event 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 in- 

 stincts 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 in- 

 stincts 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 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 Amer- 

 ican 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 gen- 

 erally 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 



