AND OF LIGHT. 233 



Lepidoptera. It has been known for more than sixty 

 years that the two butterflies Vanessa levana and V. 

 prorsa, formerly regarded as different species, are but 

 seasonal forms of one and the same species. Thus V. 

 levana emerges in the spring, breeds immediately, and 

 produces adult V. prorsa progeny in the same summer. 

 The progeny of these insects pass the winter as chrysa- 

 lids, and emerge the next spring as V. levana. The 

 levana form is characterised by a yellow and black pat- 

 tern on the upper side of the wings, whilst the prorsa 

 form has black wings with a broad white transverse 

 band. The lower surfaces differ only slightly. 



It is a natural supposition that these changes of 

 colour marking are dependent in some way on tempera- 

 ture, and Dorf meister * proved that this is actually the 

 case. By the application of warmth to the pupae he 

 succeeded in producing prorsa out of the offspring of 

 prorsa, and by the application of cold he obtained from 

 levana not the pure levana form, but one intermediate 

 between it and prorsa. This intermediate form, which 

 has occasionally been observed in nature, is termed V. 

 porima. These experiments were repeated and ex- 

 tended by Weismann, and by employing a greater de- 

 gree of cold he succeeded in obtaining levana from 

 levana; but he found that prorsa was only exceptionally 

 reared from prorsa by the application of heat. 



The mode of action of the temperature is not so clear 

 as might at first sight be imagined. The simplest ex- 

 planation is to attribute the effect to the direct influ- 

 ence of the warmth and cold, and this view of the in- 



*Mitt. des naturwiss. Vereins fur Steiermark, 1864. See also 

 Elmer's " Organic Evolution," English Ed., p. 116, et seq. 



