19 1 7'] Letters, Extracts, and Notes. 457 



does not eject the foster egg when she is dealing with a nest 

 that ah-eady contains one only. Unfortunately we have no 

 knowledge of the state of incubation in any of these cases, 

 and so cannot tell which are unfinished clutches and which 

 are those despoiled by tlie laying Cuckoo. 



Major Meiklejohn does not refer to Mr. O. Latter^s tables 

 published a few years ago in ' Biometrika.^ I have not 

 access to these at present, but I believe it was shown there 

 that the relation in size between parasite and host-egg 

 though small is constant, whereas relation in colour is not. 

 Major Meiklejohn admits that the colour-resemblance has 

 been greatly overrated ; but on page 207 he postulates that 

 there is considerable selection by the foster-parents. Yet in 

 this country, where the Hedge-Sparrow is so commonly 

 parasitised, the Cuckoo never lays a blue egg. We cannot 

 explain this by calling it the Cuckoo's "^ instinct," because 

 it is the instinct that we are out to explain. In Germany 

 and Finland, where the blue type of egg is produced, it is 

 laid in the nest of the Redstart. But if these Redstarts 

 breed in closed situations as the British Redstarts do, it 

 must very often be impossible for either bird to distinguish 

 the colour of the eggs. As to the view on page 207 that it 

 is "• unlikely " that the male may influence the coloration, 

 surely in our present knowledge it is unsafe to say whicii 

 traits go by one sex or the other. The egg pigments are 

 products of a certain kind of metabolism in the parent, and 

 as we know that the father may profoundly influence the 

 metabolism of his female oftspring in other ways, I cannot 

 agree that it is " unlikely " that he does so in this. If he 

 does have such an influence, there would be a constant 

 tendency for the difl'erent " gentes" of Cuckoos to break up 

 owing to intercrossing. What are required are figures from 

 a district where only one type of egg is found, showing what 

 is the commonest foster-parent and what percentage of eggs 

 are laid in other nests. We also badly need records of nests 

 that are deserted owing to parasitism by a Cuckoo. The 

 chief criticism that one could make of Major Meiklejohn's 

 important suggestion that the egg may be carried to the 



SER. X. VOL. v. 2 II 



