124 ACQUIRED CHARACTERS sec. 



Marcellus forty-five produced Marcellus imagines after four- 

 teen days, and five did not change until April of the next 

 year, and then into Telamonides ; of the pupae of each of the 

 three summer generations only a portion produced butterflies 

 after a short period (fourteen days) ; another much smaller 

 portion did not change till the following spring, and then into 

 the winter form. Weismann concludes from this that " here 

 there is no doubt that, not external conditions of various kinds, 

 but purely internal causes determine the retention of the 

 long-inherited direction of development, for all the larvae and 

 pupte of the many different broods w^ere exposed simul- 

 taneously to the same external influences." 



This statement, in my opinion, is probably to be thus 

 altered : The external conditions cause, as a rule, the repro- 

 duction of the parent form, but on some individuals the 

 summer warmth has not sufficient influence; they do not 

 develop before the winter, and the winter cold, i.e. the absence 

 of warmth and light, then determine the emergence of the 

 cold form. It is a question, therefore, obviously of the dif- 

 ferent sensitiveness of the individuals in regard to external 

 influences, a difference which is equally striking in the meta- 

 morphosis of the Axolotl, in wdiich also some individuals are 

 more prone than others to change into Amblystoma. 



In the highest degree noteworthy for my view^s, however, 

 is the fact already pointed out, that as in the Axolotl, so in 

 these butterflies, there are not a large number of transitional 

 forms in the metamorphosis side by side, but that in every 

 case when transitional forms do occur they form distinctly 

 separate stages. Only when the ancestral and the new form 

 live side by side and interbreed, as with Bryonise and Napi 

 in the Alps and the Jura, do the varieties show intermediate 

 forms. In Lapland, on the contrary, Bryonise is constant. 

 The intermediate form Porima is a great rarity in nature as 

 in experimental breeding. 



