38 Life History of Common Cuckoo. 



IV. 



Various theories have been advanced to account 

 for these extraordinary powers, habits, and instincts 

 in the cuckoo. Some have held that the individual 

 birds must have powers to modify the colouring of 

 the eggs to suit the nest into which they mean to 

 deposit theirs. The writer of the article in the Ency- 

 clopaedia Britannica rejects with scorn the idea that 

 individual cuckoos have the power to vary their eggs 

 in the least degree, not to speak of through such a 

 range, and he suggests the theory that, by heredity, 

 different sets of cuckoos come to lay always the same 

 coloured eggs and to place them unerringly in the 

 nests for which they are adapted, the eggs laid by the 

 same individual bird being, he holds, always the 

 same. 



He goes on to argue that were it not so, much of 

 the ingenuity shown by the cuckoo would be wasted, 

 as some birds are so much more easily imposed on in 

 this respect than others that it would in certain cases be 

 needless labour : " We know that certain birds resent 

 interference with their nests much less than others, 

 and among them it may be asserted that the hedge- 

 sparrow will patiently submit to various experiments. 

 She will brood with complacency the egg of a red- 

 breast {ErithacHs riibecula), so unlike her own, and 

 for aught we know to the contrary, may be colour- 

 blind. In the case of such a species there would be 



no need of anything more to secure success 



But with other species it may be, nay, doubtless is, 

 different." 



So here we really have what was a complete sliding 



