On the Seasonal Dimorphism of Butterflies. 2 9 



to respond to such influences, and that the dis- 

 position to abandon the ordinary course of 

 development is different in different individuals. 

 In no other way can it be explained that, in all 

 the experiments made with the first and second 

 generations of Prorsa, only a portion of the pupae 

 were compelled by cold to take the direction of 

 development of Levana^ arid that even from the 

 former only a few individuals completely reverted, 

 the majority remaining intermediate. 



If it be asked why in the corresponding experi- 

 ments with Pieris Napi complete reversion 

 always occurred without exception, it may be 

 supposed that in this species the summer form has 

 not been so long in existence, and that it would 

 thus be more easily abandoned ; or, that the 

 difference between the two generations has not 

 become so distinct, which further signifies that 

 here again. the summer form is of later origin. It 

 might also be finally answered, that the tendency 

 to reversion in different species may vary just as 

 much as in different individuals of the same 

 species. But, in any case, the fact is established 

 that all individuals are impelled by cold to com- 

 plete reversion, and that in these experiments it 

 does not depend so particularly upon the moment 

 of development when cold is applied, but that diffe- 

 rences of individual constitution are much more the 

 cause why cold brings some pupae to complete, and 

 others to partial, reversion, while yet others are 



