igiS. I by Birds of Eggs unlile their ou-n. 141 



classes of enemies. A similar general difference occurs in 

 the number of eggs composing the usual clutch — five in 

 England, three in south-east Africa ; and here again the 

 difference is probably dependent on some general diiFerence 

 in the conditions. That desertion is common, at any rate 

 in some species, as a result of the insertion of a Cuckooes 

 egg, is shown by Major Meiklejohn's quotation of the fact 

 that Wrens (in 150 cases noted by Walter), Willow- 

 Warblers, Wood- Warblers, and ChiffchafFs invariably desert 

 under these circumstances. Another mode of dealing with 

 the Cackoo's egg would seem to be to cover it with fresh 

 nesting material, so that it remains built into the lining. 



2. The history of the parasitic habit. — Wallace^s view, I 

 think, may be quickly dismissed. It was that, the colours of 

 small birds' eggs being protective — fitting in, as he supposed 

 them to do, with their chequered surroundings of light find 

 shade — a Cuckoo's egg, unlike the others in the nest, would 

 strike a discordant note ''and lead to the destruction of the 

 whole set. Those Cuckoos, therefore, which most frequently 

 placed their eggs among the kinds which they resembled, 

 would, in the long run, leave most progeny, and thus the very 

 frequent accord in colour might have been brought about " 

 (Darwinism, 2nd ed. p. 216). I have indicated elsewhere 

 Clbis,' Oct. 1916, pp. 531-532)* my view, which must, 

 1 think, be shared by field-naturalists generally, that it is 

 impossible to regard Hedge-Sparrows' and Song-Thrushs' 

 eggs, for example, as protectively coloured, and the survival 

 of the " discordant " Cuckoos' eggs accepted by Hedge- 

 Sparrows also tells against the theory. I have stated above 

 that in my experiments, the robbery, probably by enemies, 

 of nests containing discordant eggs was not more frequent 

 than that of nests without them. 



Darwin quotes the statement that some Cuckoos " mani- 

 fest a decided preference for nests containing eggs similar in 



* A correspondent regards it as my own view that the Song- Thrush's 

 egg is protectively coloured. Evidently I did not make it sufficiently 

 clear that I was merely stating- an illustration of Wallace's theory and 

 that I atrong-ly disagreed with him. 



