On the Seasonal Dimorphism of Butterflies. 59 



extreme fidelity the most insignificant details of 

 marking are, in constant species of butterflies, 

 transmitted from generation to generation, a total 

 change of the kind under consideration cannot but 

 appear surprising, and we should not explain it by 

 the nature of the agency (warmth), but only by the 

 nature of the species affected. The latter cannot 

 react upon the warmth in the same manner that a 

 solution of an iron salt reacts upon potassium fer- 

 rocyanide or upon sulphuretted hydrogen ; the 

 colouring matter of the butterfly's wing which was 

 previously black does not become blue or yellow, 

 nor does that which was white become changed 

 into black, but a new marking is developed from 

 the existing one or, as I may express it in more 

 general terms, the species takes another course of 

 development ; the complicated chemico-physical 

 processes in the matter composing the pupa be- 

 come gradually modified in such a manner that, 

 as the final result, a new marking and colouring 

 of the butterfly is produced. 



Further facts can be adduced in support of the 

 view that in these processes it is the constitu- 

 tion of the species, and not the external agency 

 (warmth), which plays the chief part. The latter, 

 as Darwin has strikingly expressed it, rather per- 

 forms the function of the spark which ignites a 

 combustible substance, whilst the character of the 

 combustion depends upon the nature of the ex- 

 plosive material. Were this not the case, increased 



