120 A PHILOSOPHER WITH NATURE 



of means to an end which the habits of the cuckoo 

 afford. Many strange and ingenious theories, a 

 few examples of which have been given, have been 

 propounded to account for the instinct which appar- 

 ently leads the cuckoo to deposit her eggs in nests 

 containing others resembling them in appearance. 

 As a matter of fact, however, we have here only 

 another beautiful example of appropriate results 

 produced by natural selection. The great variation 

 in the cuckoo's eggs has been already referred to. 

 Anyone who has ever placed, as I have done, a 

 cuckoo's egg of the largest type in a nest with the 

 eggs of one of the smaller birds utilized, could not 

 help being struck with the incongruity of the 

 appearance. There would be little doubt in his 

 mind that if the cuckoo herself deposited her eggs 

 thus unsuitably, they must often not be hatched 

 out. That this happens sometimes at present is 

 not unlikely ; that it happened more often in the 

 past there can be little doubt. Mr. Nuttall relates 

 significant instances of the sagacity of the American 

 summer yellow bird in refusing to hatch the egg 

 of the cow-bird placed among her own. The strange 

 egg is sometimes broken, or being too large for 

 ejectment, it is enclosed in the bottom of the nest 

 and a new lining built over it, and the bird is said 

 sometimes to enclose even her own eggs in this 

 manner rather than hatch out that of the intruder. 

 Some selection of this kind must undoubtedly have 

 been going on in the case of the cuckoo's eggs for 

 an immense period. The eggs which had most 

 chance of being hatched out were always those most 

 closely resembling the eggs of the foster-parent. 

 But now comes the most curious part. Natural 



