268 THE VARIATION OF ANIMALS IN NATURE 



At first sight this seems to be a very strong presumptive 

 case for the occurrence of Natural Selection. There is, how- 

 ever, a general objection of some importance. Most authors 

 agree that the primitive non-parasitic Cuckoos laid white 

 eggs (Stuart Baker, 1923, pp. 278-9). If this is true, as Baker 

 points out, we would have to accept the probability that all 

 other birds' eggs were white at the time of the origin of the 

 parasitic habit, and that the colours of the Cuckoo's eggs 

 developed pari passu with those of the fosterers. If this were 

 not the case — i.e. if the Cuckoo's eggs w T ere white or some other 

 neutral colour and the fosterers' were multicoloured — we must 

 assume either that quite marked variations towards the colour 

 of the fosterers' eggs occurred or that even slight differences 

 were enough to influence rejection and acceptance of the 

 Cuckoo's eggs. This dilemma confronts us, of course, in all 

 selectionist arguments. It is true that we can plausibly 

 imagine that the primitive colour of the Cuckoo's egg was 

 some generalised one, a grey or a drab, and that it was gradually 

 assimilated towards various multicoloured types. But this 

 seems to us to place a very high strain on the potentiality for 

 variation in the Cuckoo's constitution. If the Cuckoo's eggs 

 were white and those of the fosterers multicoloured, it seems 

 most unlikely that they could have been assimilated by selec- 

 tion alone. We therefore seem driven back on Stuart Baker's 

 hypothesis of an evolution of the Cuckoo and the fosterers 

 pari passu from a stage when they all had white eggs. But 

 Baker holds that Cuckoos are relatively recent in their origin, 

 and the parasitic habit is still more modern. It is inconceiv- 

 able, however, that all the fosterers should have conveniently 

 remained in the condition of having white eggs until the 

 Cuckoos evolved. 



If we are to accept the colour of the non-parasitic Cuckoo's 

 eggs, which is white, as evidence as to that of the primitive 

 Cuckoos, it seems that we have two courses open to us : (a) to 

 argue that the fosterers' must also have been white at the 

 time of the origin of the habit, which is very unlikely, or (b) on 

 the assumption that the fosterers had multicoloured eggs, to 

 postulate either a very surprising degree of variation in the 

 Cuckoo, or some process (? optical stimulus) by which the 

 Cuckoo itself produced the right sort of variation, a resort 

 which admittedly involves just as many difficulties (e.g. the 



