On the Seasonal Dimorphism of B litter flies. 93 



insignificant characters become prominently 

 changed, characters which are without importance 

 for the welfare of the species ; while in true hetero- 

 genesis we are compelled to admit that useful 

 changes, or adaptations, have occurred. 



Heterogenesis may thus be defined either in 

 accordance with my proposal or in the manner 

 hitherto adopted, since it may be regarded as more 

 morphological than the cyclical succession of dif- 

 ferently formed sexual generations ; or, with Claus, 

 as the succession of different sexual generations, 

 " living under different conditions of existence " 

 a definition which applies in all cases to seasonal 

 dimorphism. Varying conditions of existence, in 

 their widest sense, are the result of the action of 

 different climates ; and a case has been made 

 known recently in which it is extremely probable 

 that the climatic differences of the seasons have 

 produced a cycle of generations by influencing the 

 processes of nutrition. This case is quite ana- 

 logous to that which we have observed in the 

 seasonal dimorphism of butterflies, but with the 

 distinction that the difference between the winter 

 and summer generations does not, at least entirely, 

 consist in the form of the reproductive adult, but 

 almost entirely in its ontogeny in the mode of 

 its development. A comparison of this case with 

 the analogous phenomenon in butterflies, may 

 be of interest. In the remarkable fresh- water 

 Daphnid, Leptodora hyalina Lillejeborg, it was 



