146 Mr. C. F. M. Swynnerton on Rejections [Ibis, 



It seems to me rather that the original parasite is likely to 

 have laid eggs of approximately one type, like some of tlie 

 non-parasitic Cuckoos to-day, and that, whether this was the 

 case or not, the two conditions are likely to have alternated 

 one with the other in any given locality. The homoic condi- 

 tion will have given place to the heteroic where a dominant 

 species, hitherto the chief victim and model for mimicry, 

 has for any reason (including the over-success of the 

 Cuckoo) become relatively scarce, and where it takes several 

 species of birds to make up the population needed for the 

 consequent overflow on the Cuckoo's part. It (the homoic 

 condition) will be resumed again as one of these species 

 becomes abundant and more and more discriminating; for 

 the other types of Cuckoos' eggs, dependent for their con- 

 tinuance on the scarcer or less discriminatiug hosts, will 

 sooner or later, if the chief host be really abundant, come 

 under its inspection and be eliminated. At the same 

 stroke, obviously, will be eliminated the tendency to choose 

 other species as foster-parents, while instances through 

 difficulty in finding the right foster-parent will also be 

 reduced by the latter's abundance. 



In view of the fact that the dominant soft-billed birds are 

 different in different localities and that in some localities 

 there is no very marked dominance in numbers on the part 

 of any favourite species, it is easy to believe that "the eggs 

 of the Cuckoo (C. canorus) vary more in colouring and 

 markings than those of any other known species" (Key's 

 first conclusion, as quoted by Major Meiklejohn). The case 

 is readily comparable with what occurs in mimetic genera in 

 butterflies, such as Fseudacrcea and Euralia. This mention of 

 butterflies at once recalls the fact that in polj-morphic mimics 

 the inheritance has been practically proved by breeding 

 experiments to be INIendeliau. Further, whether the domi- 

 nant or the recessive form will be abundant depends on the 

 presence of the appropriate model. The hippocoon female 

 form (incomplete recessive) oi Papilio dardanus is abundant 

 at Chirinda (S. Rhodesia) and also in other places where 



