NATURAL SELECTION 267 



studied for over a hundred years. The more important works 

 are given by Jourdain (1925) in his bibliography. 



The most striking feature of this phenomenon is that a 

 single species of Cuckoo (e.g. Cuculus canorus telephonus) may use 

 several different species as fosterers and in certain cases the 

 eggs of the Cuckoo resemble those of the various fosterers very 

 closely. It is claimed that the resemblance is brought about 

 by the rejection by the fosterer of such Cuckoo's eggs as do not 

 resemble its own. The salient facts, in so far as the selective 

 explanation is involved, are as follows : 



(/) In the first place, the instances of a species of Cuckoo 

 utilising various fosterers, to the eggs of which its own attain 

 a close resemblance, are well attested and striking. Moreover, 

 where the fosterer happens to show local or geographical varia- 

 tion, it often happens that the parasite's eggs follow the detail of 

 this very closely, as Stuart Baker has shown in the crows (1923). 



(2) The degree of resemblance is very diverse. At the one 

 end of the scale we find some fosterers (e.g. the Hedge Sparrow) 

 habitually accepting and brooding Cuckoo's eggs which do not 

 resemble their own (Jourdain, I.e. p. 641). At the other we 

 have the very striking and close resemblances seen, e.g., between 

 C. cuculus canorus and Emberiza cioides ciopsis. 



(3) The crucial question, as far as the mode of origin of 

 the i mimicry ' is concerned, is whether there is any evidence 

 of the rejection of unsuitably coloured eggs, and of a correlation 

 between the closeness of resemblance and the intensity of 

 rejection. That some fosterers do reject the Cuckoo's eggs is 

 certain. It is remarkable that so well informed a writer as 

 Cuenot should dismiss (1925, p. 344) as a fable the evidence 

 that such rejection takes place. Both Stuart Baker (1923) and 

 Jourdain (I.e.) assemble many instances, and point out that 

 the dissimilar Cuckoo's eggs are eliminated in three ways : 

 (a) by actual ejection from the nest, (b) by desertion, and (c) by 

 a new nest being built over' the parasitised one. What is true, 

 however, is that the incidence of rejection ' varies enormously.' 

 In some cases it is as low as 5 per cent. ; in others it is 80 

 to 100 per cent. Moreover, ' these rates are not necessarily 

 connected with the closeness of the mimicry or the reverse ' 

 (Jourdain, I.e. p. 652). What is not stated (and apparently 

 not studied) is that the rejected eggs are more dissimilar than 

 those which are tolerated. 



