On the Seasonal Dimorphism of Butterflies. 5 



not capable of being regarded as pure seasonal 

 dimorphism, may well have been produced in part 

 by the same causes, as the subsequent investiga- 

 tion on the relation of seasonal dimorphism to 

 alternation of generation and heterogenesis will 

 more fully prove. 



Now what are these causes ? 



Some years ago, when I imparted to a lepidop- 

 terist my intention of investigating the origin of 

 this enigmatical dimorphism, in the hope of pro- 

 fiting for my inquiry from his large experience, I 

 received the half-provoking reply : " But there is 

 nothing to investigate : it is simply the specific 

 character of this insect to appear in two forms ; 

 these two forms alternate with each other in regular 

 succession according to a fixed law of Nature, and 

 with this we must be satisfied." From his point 

 of view the position was right ; according to the 

 old doctrine of species no question ought to be 

 asked as to the causes of such phenomena in par- 

 ticular. I would not, however, allow myself to be 

 thus discouraged, but undertook a series of investi- 

 gations, the results of which I here submit to the 

 reader. 



The first conjecture was, that the differences in 

 the imago might perhaps be of a secondary nature, 

 and have their origin in the differences of the cater- 

 pillar, especially with those species which grow 

 up during the spring or autumn and feed on dif- 

 ferent plants, thus assimilating different chemical 



