vni] THE LIFE-STORY AND THE SEASONS 103 



intensive degree the characters of the spring brood. 

 This female laid eggs the caterpillars from which fed 

 and pupated. The pupae although kept through the 

 summer in a hothouse all produced typical bryoniae, 

 and none of these with one exception appeared until 

 the next year, for in the alpine and arctic regions 

 this species is only single-brooded. Weismann ex- 

 perimented also with a small vanessid butterfly, 

 Araschnia levana, common on the European con- 

 tinent, though unknown in our islands, which is 

 double (or at times treble) brooded, its spring form 

 (levana) alternating with a larger and more brightly 

 coloured summer form (prorsa). Here again by 

 refrigerating the summer pupae, butterflies were 

 reared most of which approached the winter pattern, 

 but it was impossible by heating the winter pupae 

 to change levana into prorsa. Experiments with 

 North American dimorphic species have given similar 

 results. Weismann argued from these experiments 

 that the winter form of these -seasonally dimorphic 

 species is in all cases the older, and that the butter- 

 flies developing within the summer pupae can be 

 made to revert to the ancestral condition by repeating 

 the low-temperature stimulus which always prevailed 

 during the geologically recent Ice Age. On the other 

 hand, a high temperature stimulus applied to one 

 generation of the winter pupae cannot induce the 

 change into the summer pattern, which has been 



