234 



THE EFFECT OF TEMPERATURE 



fluence of warmth is actually held by Elmer.* Accord- 

 ing to Weismann, however, the action, both for warmth 

 and cold, is an indirect one. The change of the prog- 

 eny of levana back to levana through the influence of 

 cold he attributes to reversion to the ancestral form, for 

 there is practically no doubt that levana is phylogeneti- 

 cally the older form of the two. He considers that 

 prorsa has slowly arisen through the gradual increase 

 in the warmth of the climate, or is a seasonally adaptive 

 form, and that its occasional production from the prog- 

 eny of prorsa is due to the high temperature unduly 

 stimulating the development of the prorsa " determi- 

 nants." 



A clearer case of the direct influence of warmth and 

 cold is afforded by Polyommatus phlceas, the Small Cop- 

 per butterfly. By exposure of the pupae to various 

 temperatures, Merrifield f obtained the following re- 

 sults : 



Here we see that the temperatures ranged from about 

 30 C., or 85 F., to just above the freezing point. The 



* " Organic Evolution," p. 122. 

 f Trans. Ent. Soc. 1893, p. 55. 



