191^-] by Birds of Eggs unlike their own. 147 



its model, Amauris dominicanus, is abundant. The cenea 

 form (incomplete dominant) "^ is abundant at Natal, where 

 that Amauris model is nearly absent, but Amauris echeria 

 abundant. But in each case the scarce form is still kept up 

 in small numbers through the Mendelian relationship and 

 might replace the other form were a change in the numbers 

 of A, dominicanus to lead to a corresponding change in tlie 

 incidence of selection. Otlier female forms of the species 

 also occur, mostly mimetic, but one or two not mimicking 

 any pattern at present extant amongst models. 



The same principles seem likely to apply in the case of 

 the eggs o£ the Cuckoo. Here we have exactly the same 

 evidence of the dependence of particular forms on the 

 presence of particular models, the same local results from 

 changes in the relative abundance of particular models, the 

 same " mixed " and now non-mimetic forms, scarce or ap- 

 parently absent where some model dominates completely, 

 more abundant where this is not the case f. And the ap- 

 pearances of the case — the highly distinctive types obviously 

 duly segregated in generation after generation, — the neces- 

 sities of the case, and the analogy of the butterflies all 

 strongly suggest Mendelian inheritance. 



With the criticism of the theory that similarity of diet will 

 have produced resemblance between the Cuckoo's egg and 

 the host's, one cannot but agree ; but the view that par- 

 ticular foods may affect the coloration of the eggs of birds 

 is not to be summarily dismissed. Dr. Peringuiey told me, 

 in 1915, that ducks fed on acorns at the Cape laid black 

 eggs, and I was interested to see a black duck-egg a few 

 days later, shown me by Mr. Fitzsimons of the Port Elizabeth 



* Tliis conclusion is indiciited by the results, in the Fj generation, of 

 a series of matings I obtained recently between individuals of a pure 

 cenea strain (out of pupae sent me from Natal by Mr. E. E. Piatt) and 

 individuals of pure hippocoon parentage from Chirinda. 



t The facts here referred to constitute the real evidence for the 

 existence of mimicry in Cuckoos' eggs, Mr. Stuart Baker has stated 

 them very convincingly for the Asiatic Cuckoos in ' The Ibis' for July 

 1913. The fact that the size of the e^^ is not reduced where the usual 

 host lays a large egg is also not to be despised. 



l3 



