52 Studies in the Theory of Descent. 



III. 



NATURE OF THE CAUSES PRODUCING CLIMATIC 

 VARIETIES. 



IT has been shown that the phenomenon of sea- 

 sonal dimorphism has the same proximate cause 

 as climatic variation, viz. change of climate, and 

 that it must be regarded as identical in nature 

 with climatic variation, being distinguished from 

 ordinary, or, as I have designated it, simple (mono- 

 morphic) climatic variation by the fact that, be- 

 sides the new form produced by change of climate, 

 the old form continues to exist in genetic con- 

 nexion with it, so that old and new forms alternate 

 with each other according to the season. 



Two further questions now present themselves 

 for investigation, viz. (i) by what means does 

 change of climate induce a change in the marking 

 and colouring of a butterfly ? and (2) to what ex- 

 tent does the climatic action determine the nature 

 of the change ? 



With regard to the former question, it must, in 

 the first place, be decided whether the true effect of 

 climatic change lies in the action of a high or low 

 temperature on the organism, or whether it may 



