STUDIES IN THE THEORY OF DESCENT. 



part fc 



ON THE SEASONAL DIMORPHISM OF 

 BUTTERFLIES. 



I. 



THE ORIGIN AND SIGNIFICANCE OF SEASONAL 

 DIMORPHISM. 



THE phenomena here about to be subjected to a 

 closer investigation have been known for a long 

 period of time. About the year 1 830 it was shown 

 that the two forms of a butterfly (Araschnia) which 

 had till that time been regarded as distinct, in spite 

 of their different colouring and marking really be- 

 longed to the same species, the two forms of this 

 dimorphic species not appearing simultaneously 

 but at different seasons of the year, the one in 

 early spring, the other in summer. To this phe- 

 nomenon the term " seasonal dimorphism" was 

 subsequently applied by Mr. A. R. Wallace, an 

 expression of which the heterogeneous composition 



B 



