9 2 Studies in the Theory of Descent. 



tion of certain insignificant black spots on the 

 under side of the wing (P. Alexis female, and P. 

 Agestis) ; and this diagnosis proves sufficient, since 

 P. Alexis, which has the spots in a straight row, 

 has a different caterpillar to P. Agestis, in which 

 the central spot is nearer the base of the hind 

 wing ! 



For the reasons just given, I maintain that it 

 is neither justifiable nor useful to designate the 

 di- and polymorphism of butterflies as di- and 

 polychroism, and thereby to attribute but little im- 

 portance to these phenomena. 8 This designation 

 would be only justifiable if the differences of colour 

 were due to other causes than the differences of 

 form, using this last word in a narrow sense. But 

 it has been shown that the same direct action of 

 climate which originates new colours, produces 

 also in some species differences of form (contour of 

 wing, size, &c.) ; whilst, on the other hand, it has 

 long been known that many protective colours can 

 only be explained by the indirect action of ex- 

 ternal conditions. 



When I raise a distinction in the nature of the 

 changes between seasonal dimorphism and the re- 

 maining known cases of heterogenesis, this must be 

 taken as referring only to the biological or physio- 

 logical result of the change in the transformed 

 organism itself. In seasonal dimorphism only 



8 With reference to this subject, see the discussion by the 

 Belgian Entomological Society, Brussels, 1873. 



