It is obviously a fact of fundamental importance to the theory of seasonal dimorphism, that the summer form can be readily changed into the winter form, whilst the latter cannot be changed into the summer form. I have thus far only made experiments on this subject with _A. Levana_, but the same fact appears to me to obtain for _P. Napi_. I did not, however, operate upon the ordinary winter form of _P. Napi_, but chose for this experiment the variety _Bryoniæ_, well known to all entomologists. This is, to a certain extent, the potential winter form of _P. Napi_; the male (Fig. 14, Plate I.) exactly resembles the ordinary winter form in the most minute detail, but the female is distinguished from _Napi_ by a sprinkling of greyish brown scales over the whole of the upper side of the wings (Fig. 15, Plate I.). This type, _Bryoniæ_, occurs in Polar regions as the only form of _Napi_, and is also found in the higher Alps, where it flies in secluded meadows as the only form, but in other localities, less isolated, mixed with the ordinary form of the species. In both regions _Bryoniæ_ produces but one generation in the year, and must thus, according to my theory, be regarded as the parent-form of _Pieris Napi_. If this hypothesis is correct--if the variety _Bryoniæ_ is really the original form preserved from the glacial period in certain regions of the earth, whilst _Napi_ in its winter form is the first secondary form gradually produced through a warm climate, then it would be impossible ever to breed the ordinary form _Napi_ from pupæ of _Bryoniæ_ by the action of warmth, since the form of the species now predominant must have come into existence only by a cumulative action exerted on numerous generations, and not _per saltum_. The experiment was made in the following manner: In the first part of June I caught a female of _Bryoniæ_ in a secluded Alpine valley, and placed her in a capacious breeding-cage, where she flew about among the flowers, and laid more than a hundred eggs on the ordinary cabbage. Although the caterpillars in the free state feed upon another plant unknown to me, they readily ate the cabbage, grew rapidly, and pupated at the end of July. I then brought the pupæ into a hothouse in which the temperature fluctuated between 12° and 24° R.; but, in spite of this high temperature, and--what is certainly of more special importance--notwithstanding the want of cooling at night, only one butterfly emerged the same summer, and that a male, which, from certain minute characteristic markings, could be safely identified as var. _Bryoniæ_. The other pupæ hibernated in the heated room, and produced, from the end of January to the beginning of June, 28 butterflies, all of which were exquisite _Bryoniæ_.