On the Seasonal Dimorphism of Butterflies. 67 



selves exactly in the same manner as do the 

 successive stages of development of the individual 

 in the other ; and we may conclude therefrom (as 

 has long been admitted on other grounds) that a 

 generation is, in fact, nothing else than a stage of 

 development in the life of a species. This appears 

 to me to furnish a beautiful confirmation of the 

 theory of descent. 



Now if, returning to questions previously solved, 

 the alternating action of cold in winter and warmth 

 in summer leads to the production of a winter and 

 summer form, according to the law of cyclical here- 

 dity, the question still remains : why do we not 

 find seasonal dimorphism in all polygoneutic 

 butterflies ? 



We might at first suppose that all species are 

 not equally sensitive to the influence of tempe- 

 rature : indeed, the various amounts of difference 

 between the winter and summer forms in different 

 species would certainly show the existence of 

 different degrees of sensitiveness to the modifying 

 action of temperature. But even this does not 

 furnish an explanation, since there are butterflies 

 which produce two perfectly similar 3 generations 



8 When Meyer-Dikr, who is otherwise very accurate, states 

 in his " Verzeichniss der Schmetterlinge der Schweiz," (1852, p. 

 207), that the winter and summer generations of P. JEgeria 

 differ to a small extent in the contour of the wings and in 

 marking, he has committed an error. The characters which 

 this author attributes to the summer form are much more appli- 



F 2 



