262 CAUSES OF VARIATION 



longer and under much lower temperatures than we should at 

 first suppose. 



Substantive variation due to temperature ; color markings. 

 Early in the section it was remarked that the effects of tem- 

 perature are qualitative as well as quantitative. Without doubt 

 temperature exerts a controlling influence upon the color of 

 butterflies, as has been determined by a number of direct ex- 

 periments. 



For example, Vanessa levana and V.prorsa were long regarded 

 as distinct species. Levana is "characterized by a yellow-and- 

 black pattern on the upper side of the wings," while prorsa 

 "has black wings with a broad white transverse band and deli- 

 cate yellow lines running parallel to the margins." 1 Later this 

 was recognized as a case of "'-seasonal dimorphism" the yellow- 

 and-black levana being the spring brood and the darker prorsa 

 being the summer brood.; that is to say, levana, emerging in 

 the spring, breeds immediately, producing a summer brood 

 (prorsa)) and this brood in the same way gives rise to a genera- 

 tion which passes the winter in the chrysalis form, emerging in 

 the spring as levana. Thus these two " species " are produced 

 from the same stock, the difference being that one passes the 

 chrysalis stage in the summer, the other in the winter. 



That this difference is one of temperature was proved by 

 direct experiment. Dorfmeister 2 succeeded in producing prorsa 

 directly irom prorsa by the application of warmth to the pupae, 

 and " by the application of cold he obtained from levana not 

 the pure levana form, but one intermediate between it and 

 prorsa!' an intermediate occasionally observed in nature and 

 known as V. porima? 



Weismann, repeating the experiment, found that by using 

 lower temperatures levana could be produced directly from 

 levana, and he adds, " The converse experiment was also occa- 

 sionally successful, the pupae of the winter generation being 

 forced to assume the summer form by the influence of a higher 

 temperature during, or shortly after, pupation." 3 



1 Weismann, Germ Plasm, p. 379. 



2 Vernon, Variation in Animals and Plants, p. 233. 



3 Ibid. ; also Weismann, Germ Plasm, p. 379. 



