56 Stiidies in the Theory of Descent. 



the fact that the summer forms have not originated 

 suddenly, but only in the course of numerous gene- 

 rations ; and when we further compare the two 

 seasonal forms in other species. Thus in Pieris 

 Napi the winter is distinguished from the summer 

 form, among other characters, by the strong black 

 dusting of the base of the wings. But we cannot 

 conclude from this that in the present case more 

 black pigment is produced in the winter than in the 

 summer form, for in the latter, although the base of 

 the wings is white, their tips and the black spots 

 on the fore-wings are larger and of a deeper black 

 than in the winter form. The quantity of black 

 pigment produced does not distinguish between 

 the two forms, but the mode of its distribution 

 upon the wings. 



Even in the case of species the summer form of 

 which really possesses far more black than the 

 winter form, as, for instance, Araschnia Levana, 

 one type cannot be derived from the other simply 

 by the expansion of the black spots present, since 

 on the same place where in Levana a black band 

 crosses the wings, Prorsa, which otherwise pos- 

 sesses much more black, has a white line. (See 

 Figs, i 9, Plate I.) The intermediate forms which 

 have been artificially produced by the action of 

 cold on the summer generation present a graduated 

 series, according as reversion is more or less com- 

 plete ; a black spot first appearing in the middle of 

 the white band of Prorsa, and then becoming en- 



