a batch for three months at the temperature of freezing water, he succeeded in completely changing every individual of the summer generation into the winter form.  The reverse of this experiment also was attempted by Weismann.  He took a female of bryoniae, an alpine and arctic variety of Pieris napi, showing in an 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 experimented also with a small vanessid butterfly, Araschnia levana, common on the European continent, 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 butterflies 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 evolved still more recently by slow stages, as the continental climate has become more genial.  In tropical countries where instead of an alternation of winter and summer, alternate dry and rainy seasons prevail, somewhat similar seasonal dimorphism has been observed among many butterflies.  Not a few forms of Precis, an African and Indian genus allied to our Vanessa, that had long been considered distinct species are now known, thanks to the researches of G.A.K.  Marshall (1898), to be alternating seasonal forms of the same insect.  The offspring when adult does not closely resemble the parent; its appearance is modified by the climatic environment of the pupa.  The experiments of Weismann just sketched in outline show at least that the same principle holds for our northern butterflies.

We are thus led to see from the life-story of such insects, that the course of the story is not rigidly fixed; the creature in its various stages is plastic, open to influence from its surroundings, capable of marked change in the course of generations.  And so the seasonal changes in the history of the individual from egg to imago point us to changes in the age-long history of the race.

CHAPTER IX