254 



THE EVOLUTION OF SEX. 



scious of being fooled, hatch the cuckoo egg among their own. 

 The nestling grows rapidly, and is a dog in the manger by birth. 

 Greedy and jealous, he (the pronoun is oftenest correct) soon asserts 

 his monopoly of nest and food and care, by the summary eviction 

 of the rightful tenants, whether they be still passive in ova or more 

 awkwardly assertive as nestlings. The result is the success of the 



stronger. 



Of this habit there are various explanations, but the prevalent one 

 regards it as only a special case of a universal method which favors 

 selfishness. Jenner was the first to emphasize what he regarded 

 as obvious advantages of the trick. The bird has but a short time 

 to stay in its breeding area, and much to do in that short time. 

 " Nature," he said, "has a call upon it to produce a numerous pro- 

 gency," and as it is at the same time advantageous to migrate 

 early, the gain of leaving the eggs to a succession of other birds 

 to incubate is manifest. Darwin supposed the habit to crop up as 

 a mere fortuitous variation, as it occasionally does in the normally 

 nesting American cuckoo. The result was an advantage to the 

 parent and also to the offspring; the former got away sooner, the 

 latter were better cared for. Those that learned the trick prospered, 

 those that did not were eliminated; and so, in virtue of its natural 

 or unnatural success, the device passed from being exceptional to 

 become universal, became in fact an inherited specific instinct. Com- 

 menting upon this, Romanes, in a surely somewhat sanguine passage, 

 says : ' ' We have here a sufficiently probable explanation of the raison 

 d' Hre of this curious instinct; and whether it is the true reason, or 

 the only reason, we are justified in setting down the instinct to the 

 creating influence of natural selection." 



But against the supposition that a mere freak has been fostered 

 by selection into a habit, it must be noticed that the trick, to be 

 successful, must be played with some care. It is hardly on a par 

 with the casual use made by a partridge of a pheasant's nest, or 

 by a gull of an eider duck's. Again, the advantages to the parent, 

 apart from that of trouble saved, are somewhat dubious. Food, 

 Macgilivray says, remains abundant, and the climate which does not 

 injure the young for two months longer could hardly incommode the 

 parents. Nor is the case improved outside the British area. To 

 suppose, on the other hand, that the advantage to the young has 

 formed the utilitarian basis, is involved in difficulties. We can 

 not suppose that the mother bird had or has a careful forethought 

 of the best for her offspring in sending them out to nurse. Nor 

 is it easy to see how the comfort of fostered youth will remain as 

 an impulse to the adult to do the like for her young in turn. The 



