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