240 THE EFFECT TEMPERATURE 



subsequently the summer form, of the common P. napi, 

 have probably arisen gradually from it through the in- 

 fluence of a warmer climate. Now Weismann found 

 that he was unable to convert bryonice into napi by the 

 action of warmth, though he could by the application of 

 cold readily change the summer form of napi into the 

 winter form. Similarly, also, the progeny of V. levana 

 are readily converted by cold into levana, but only ex- 

 ceptionally can the progeny of V. prorsa be converted 

 into prorsa. Now, as already mentioned, levana is 

 probably the ancestral form, and so, in both this case and 

 that of P. napi, cold readily produces what is probably 

 a phylogenetically older form, whilst warmth generally 

 has no effect. Certain observations by Merrifield * 

 also afford some support to Eimer's view, for he found 

 that " the capability of being turned during the pupal 

 period from one type partially into the direction of 

 the other exists in both the summer and the winter 

 type, but is much greater in the former than in the 

 latter." 



With regard to the critical period at which tempera- 

 ture especially exerts its influence, there is a general 

 consensus of opinion that it is confined to the pupal 

 stage, and in most cases also to the first part of this 

 stage. Dorfmeister f concluded that temperature 

 exerted its greatest influence during the change from 

 the larval into the pupal stage, or shortly afterwards. 

 Weismann J noticed that in V. prorsa-levana it acted 

 only at the beginning of the pupal stage. Standfuss, 



* Trans. Ent. Soc. 1892, p. 53. 



f Vide Eimer's " Organic Evolution," p. 131. 



j" Germ-Plasm," p. 402. 



