242 Habit and Instinct. 



habit, informs me that though he has observed good cases 

 of matching — for example, a bluish egg in a hedge- 

 sparrow's nest, and a reddish one in that of a spotted 

 flycatcher — yet such cases are in his experience exceptional. 

 To my question: "Is colour assimilation the rule?" he 

 replies, " Emphatically, no." 



It is clear, then, that different observers are not agreed 

 as to the facts. It is possible that the habit in England 

 is different from that on the Continent; but further ob- 

 servations are needed to establish any such difference. In 

 any case Dr. Norton regards it as probable that the cuckoo, 

 having laid an egg on the ground, carries it in her bill, 

 and drops it into the first nest that she comes across. 

 There is little or no intelligence displayed in the whole 

 business. Dr. Norton informs me of a case in which the 

 cuckoo had deposited her egg in an old disused nest, and 

 of others in which the eggs had been dropped into nests 

 in holes from which the young cuckoo was too large to 

 escape. It would seem very doubtful whether intelligence 

 has played any decisive part in the establishment of 

 the cuckoo instinct. We must remember that there are 

 three associated facts or groups of facts. First, the re- 

 latively small size of the eggs, about one-third that of the 

 eggs of the American cuckoo ; secondly, the deposition of 

 these eggs in alien nests ; and thirdly, the ejection of other 

 nestlings by the young changeling whose broad shoulders 

 and hollow back seem specially adapted to this purpose. 

 Now, the first and third of these — the small size of the 

 eggs, and the ejection of other nestlings — cannot be re- 

 garded as due to intelligence, but may fairly be attributed 

 to natural selection. The parent bird would be unable to 

 carry large eggs in her bill, and these would be left on the 

 ground, and would remain unhatched, thus giving rise to 

 a steady process of elimination of large eggs. And the 



