On the Seasonal Dimorphism of Butterflies. 69 



being apparent why, in any particular case, the 

 one or the other form of heredity should take 

 place. 



The foregoing explanation may obtain in the 

 case of sexual selection, in which it is not incon- 

 ceivable that certain characters may not be so 

 easily produced, or even not produced at all, 

 in one sex, owing to its differing from the other 

 in physical constitution. In the class of cases" 

 under consideration, however, it is not possible 

 that the inherited characters can be prevented 

 from being acquired by one generation owing 

 to its physical constitution, since this constitution 

 was similar in all the successive generations before 

 the appearance of dimorphism. The constitution 

 in question first became dissimilar in the two 

 generations to the extent of producing a change 

 of specific character, through the action of tem- 

 perature on the alternating broods of each year, 

 combined with cyclical heredity. If the law 

 of cyclical heredity be a general one, it must 

 hold good for all cases, and characters acquired 

 by the summer generation could never have been 

 also transmitted to the winter generation from 

 the very first. 



I will not deny the possibility that if alternating 

 heredity should become subsequently entirely sup- 

 pressed throughout numerous generations, a period 

 may arrive when the preponderating influence of 

 a long series of summer generations may ultimately 



