IV WEISMANN'S EXPLANATION 127 



tion whicli I maintain give a satisfactory explanation. ]\Iy 

 works on the affinities of butterflies will bring to light 

 numbers of such species evidently produced by climatic 

 influences, wdiich are obviously connected but distinct from 

 one another. Here I have intentionally discussed the ex- 

 amples brought forward by Weismann, firstly, in order to 

 use his results for my argument as those of an impartial 

 witness ; and secondly, in order to introduce the agreements 

 and the differences between my explanations and his. To 

 these examples of the effect of climate on the formation of 

 butterflies, may be added that of Pararge Egeria, which in 

 South Europe appears in the variety Meione. Meione, how- 

 ever, is connected with Egeria by an intermediate form of 

 the Ligurian coast, wdiich does not produce a summer and 

 winter generation. 



In a separate chapter on " The Nature of the Causes of 

 Change" Weismann lays stress on the case of Polyommatus 

 riilseas as showing that climate, and not the duration of 

 development, determines the formation of climatic varieties. 

 But, he proceeds, the nature of the change essentially depends 

 on the organism itself, not on the warmth to which it is 

 exposed. It is not the quantity of the black pigment pro- 

 duced which distino'uishes the summer from the winter form, 

 but the mode of its distribution on the wdngs. Under the 

 influence of warmth arise quite different markings : starting 

 from the markings already present, quite new ones are 

 developed, or, "as I may express it in other w^ords, the 

 direction of evolution of the species is quite altered. The 

 complicated chemico-physical processes in the metabolism of 

 the dormant pupae are gradually altered until they result in 

 a new marking and colouring of the butterfly. That in these 

 processes the constitution of the species plays the chief j)art, 

 and not the external agent warmth ; that the latter rather per- 

 forms the function of the spark which, as Darwin appropriately 



