On the Seasonal Dimorphism of Butterflies. 95 



extent under our eyes. It cannot be proved with 

 certainty what the cause of this phenomenon may 

 be, but the conjecture is almost unavoidable that 

 it is closely related to the origin of the seasonal 

 dimorphism of butterflies, since both depend on 

 the alternating climatic influences of summer and 

 winter: it is most probable that these influences 

 have directly u brought about a shortening of the 

 period of development in summer. Thus we 

 have here a case of heterogenesis nearly related 

 to the seasonal dimorphism of butterflies in a 

 twofold manner first, because the cycle of gene- 

 rations is also in this case brought about by the 

 direct action of the external conditions of life ; 

 and secondly, the winter form is here also the 

 primary, and the summer form the secondary 

 one. 



In accordance with the idea first introduced into 

 science by Rudolph Leuckart, we have hitherto 

 understood heterogenesis to be only the alterna- 

 tion of dissimilar sexual generations. From this 

 point of view the reproduction of Leptodora can 

 be as little ascribed to heterogenesis as can that 

 of Aphis or Daphnia, although the apparent agamic 

 reproduction of the winter and a portion of the 

 summer generation is undoubtedly partheno- 



11 [Eng. ed. Recent researches on alternation of generation 

 in the Daphnoidea have convinced me that direct action 

 of external conditions does not in these cases come into con- 

 sideration, but only indirect action,] 



