558 Mr. C. F. M. Sw3nnierton on 



expense^ the value of definite experiment as against uncon- 

 trolled stray observation. Above all it has been fascinating 

 again to watch natural selection at work. 



The result by no means affects the general explanation of 

 the distinctive element in the coloration of eggs as being for 

 recognition by enemies, unless it should be considered that 

 parasitism was once so rampant that distinctive coloration 

 had to be developed in order to enable parents to distinguish 

 their own eggs from those of Cuckoos that were following 

 them up in the matter of coloration. This would be an 

 alternative, if not very probable, explanation, and it may 

 certainly have acted as an important contributory factor in 

 a number of cases — in some of them towards the selection 

 of high distinctiveness, in others towards polymorphism 

 (as in Warblers and Weavers). Polymorphism in the host's 

 eggs would certainly reduce the individual Cuckoo's chances 

 of matching its egg, if (as seems likely) its choice is based 

 on recognition of foster parent and nest ; and I do not 

 know at present that polymorphism occurs in this definite 

 form outside of the birds that Cuckoos victimize. The 

 variability of the Guillemot's eg^ is rather a different thing, 

 and will doubtless be found to possess a special explanation 

 of its own. The explanation of the resemblances remains 

 unaffected. 



Nor is the view that the attacks of enemies may have 

 contributed to miraiciy in Cuckoo's eggs disproved, though 

 it is certainly rendered unnecessai'y and, at best, secondary. 

 Where the difference between the eggs exchanged was slight 

 no rejection took place, but I do not lay stress on this. A 

 case I shall refer to below suggests that further experiment 

 might have produced occasional cases of finer seleciion. 



"'Darkness of the nests" argument. — In the above-men- 

 tioned experiments Hyphantornis jamesoni regularly ejected 

 eggs not its own, and a Sitagra ocularia an e^^ of Hyphan- 

 tornis nigriceps of the spotted blue type. Still, neither's 

 nest can be regarded as dark. But the H. nigriceps also 

 ejected the Sitagra egg that was used in the above exchange. 

 This is highly important, for this Hyphantornis builds, at 



