66 Proceedings of the Royal Physical Society. 



(8.) Some corroboration is obtained from the character of 

 the American cuckoo. There seems no doubt that it is 

 occasionally parasitic, and it is interesting to note that 

 observers speak of its unnaturally careless indifference for 

 the fate of its young. The character in fact is less markedly 

 evil ; the occasional parasitism is just as intelligible as the 

 occasional *' reversion " of our cuckoo to ancestral habits, 

 to apparent affection, as well as observed incubation. 



(9.) In the cow-birds, again, where the habit occurs in 

 different species in different degrees of perfection (if the 

 term be admissible), the character is strikingly immoral. 

 In one species {Ifolothrus cadius), a nest may be simply 

 stolen, or the rightful nestlings may be thrown out, or actual 

 parasitism may occur as an exception. In M. canariensis, the 

 eggs may be dropped on the bare ground, or fifteen to twenty 

 from different parents may be lazily, and of course fatally, 

 huddled together in one nest. (Two cuckoo eggs are sometimes 

 found in one nest.) In M. pecoris, which is polygamous, the 

 crime has been evolved, and the habit is that of our cuckoo, 

 one egg being laid in each foster-nest. The important point 

 is the general immorality and reproductive carelessness which 

 in one species finds expression in an organised device. 



Conclusion. — The general character of the birds, the un- 

 social life, the selfish cruelty of the nestlings, and the lazy 

 parasitic habit have a common basis in the constitution. 

 The insatiable appetite, the small size of the reproductive 

 organs, the smallness of the eggs, the sluggish parturition, 

 the rapid growth of the young, the great preponderance of 

 males, the absence of true pairing, the degeneration of 

 maternal affection, are all correlated and largely explicable in 

 terms of the fundamental contrast between nutrition and 

 reproduction, between hunger and love. Similar unnatural 

 or immoral instincts in other birds, in mammals, and even in 

 the lower animals are explicable in similar terms. The 

 existing tlieory being deficient as an account both of the 

 origin and development of the parasitic instinct, the present 

 constitutional theory is proposed as complementary. The 

 parasitic habit is a natural outcrop of the general character 

 or constitution, only one expression of a dominant diathesis. 



