70 Studies in the Theory of Descent. 



take effect upon the winter generation. In 

 such a case the summer characters would appear, 

 instead of remaining latent as formerly. In this 

 manner it may be imagined that at first but few, 

 and later more numerous individuals, approximate 

 to the summer form, until finally the dimorphism 

 entirely disappears, the new form thus gaining 

 ascendency and the species becoming once more 

 monomorphic. Such a supposition is indeed 

 capable of being supported by some facts, an 

 observation on A. Levana apparently contra- 

 dicting the theory having been already inter- 

 preted in this sense. I refer to the fact that 

 whilst some butterflies of the winter generation 

 emerge in October as Prorsa, others hibernate, 

 and appear the following spring in the Levana 

 form. The winter form of Pieris Napi also 

 no longer preserves, in the female sex, the 

 striking coloration of the ancestral form Bryonice, 

 a fact which may indicate the influencing of the 

 winter generation by numerous summer genera- 

 tions. The double form of the spring generation 

 of Papilio Ajax can be similarly explained by the 

 gradual change of alternating into continuous 

 heredity, as has already been mentioned. All 

 these cases, however, are perhaps capable of 

 another interpretation ; at any rate, the correct- 

 ness of this supposition can only be decided by 

 further facts. 



Meanwhile, even if we suppose the above ex- 



