AND OF LIGHT. 239 



inclined to extend its scope to some of the other cases 

 as well. Thus Weismann formerly made use of it to 

 account for seasonal dimorphism, though now he rather 

 withdraws this opinion.* According to Fischer,t both 

 very low and very high temperatures are equally capa- 

 ble of determining reversion by acting simply as ex- 

 citants. A moderate elevation of the temperature, on 

 the contrary, may give rise to new modifications which 

 are not phylogenetic, but which actually occur in warm 

 climates. Dixey,$ arguing especially from Merrifield's 

 observations on V. atalanta, and Merrifield himself, 

 from these and other observations, have come to the 

 conclusion that reversion may be occasioned by ex- 

 posure both to high and to low temperatures, but that 

 the kind of effect produced is different in the two cases. 

 Eimer is of the opinion that only cold has the power of 

 causing a reversion to an ancestral form, the effect of 

 warmth being " evidently a direct effect." || In sup- 

 port of his views, he refers to Weismann's experiments 

 on Pieris napi (Green-veined White), and V. levana- 

 prorsa. The former butterfly occurs in a summer and a 

 winter form, the winter being the darker. There is 

 also a variety of P. napi, viz., bryonice, which is found 

 in the Swiss Alps and in the polar regions, and which 

 can be described as a very dark variety of the winter 

 form of P. napi. This bryonice is in all probability the 

 ancestral form of P. napi, whilst the winter form, and 



* The Entomologist, 1896, p. 240. 



f " Transmutationen der Schmetterlinge infolge Temperaturan- 

 derungen," Berlin, 1894. 

 \ Trans. Ent. Soc. 1893, p. 72. 

 Trans. Ent. Soc. 1894, p. 425. 

 || " Organic Evolution," p. 125. 



