DIMORPHISM AND POLYMORPHISM 375 



interbreed, — as is actually the case at the junction of the dis- 

 tricts in which the two forms are respectively distributed, — the 

 double-determinants of the southern form will meet with single- 

 determinants of the northern form in the germ-plasm of the 

 offspring. The male descendants of such a cross would remain 

 unmodified, but the females would be either black or yellow 

 according to the power of transmission of the 'female' halves of 

 the determinants, or — as was observed by Edwards* — a com- 

 bination of both these colours. Such combinations might either 

 arise from the cross between a yellow female and a yellow male 

 of the dimorphic variety, or from that between a black female 

 and a yellow male of the monomorphic form ; for in the 

 dimorphic variety the germ-plasm contains double-determinants 

 in the males as well as in the females. If we suppose that 

 these crosses occur frequently, the number of females of an 

 intermediate form in the borderland of the two districts would 

 gradually increase, and might ultimately result in the production 

 of a constant intermediate female form. But if the males ex- 

 hibited a preference for the females corresponding to them, the 

 female forms would remain essentially distinct from one 

 another. This seems to be the case in PapUio turniis, at least 

 Edwards states that the intermediate forms are rare. 



The dimorphic and polymorphic females in many butterflies 

 may perhaps be looked upon as belonging to sexually diniorpJiic 

 local forms which have siibsequoitly spread and occasionally 

 crossed. In places where the varieties merely exist side by side 

 without interbreeding, each of them also contains either single- 

 or double-determinants, according to whether it is sexually 

 monomorphic or dimorphic ; but when interbreeding occurs, 

 the determinants of the two races come together, and then 

 several homologous double-determinants may even meet in the 

 same germ-plasm, — some in certain ids and some in others. 



In Papilio tnrnns the case is not quite so simple as I have 

 stated it : as a matter of fact, this species exhibits a double 

 di/norphisni, for the yellow females do not exactly resemble the 

 males, but differ considerably from them as regards the shade 

 of yellow and the pattern on the wings : — thus they possess an 

 orange-coloured eye-spot on the posterior wings, which is absent 

 in the male. We must therefore assume that double-deter- 



* W. H. Edwards, ' Butterflies of North America.' 



