For let us assume that the omitted experiment had been performed--that pupæ of the summer brood were retarded in their development by cold until the following spring, and that every specimen then emerged in the perfect winter form, Levana. Such a result, taken in connexion with the corresponding experiment upon _Pieris Napi_, would warrant the conclusion that the direct action of a certain amount of cold (or of retardation of development) is able to compel all pupæ, from whichever generation derived, to assume the winter form of the species. From this the converse would necessarily follow, viz. that a certain amount of warmth would lead to the production of the summer form, _Prorsa_, it being immaterial from which brood the pupæ thus exposed to warmth might be derived. But the latter conclusion was proved experimentally to be incorrect, and thus the former falls with it, whether the imagined experiment with _Prorsa_ had succeeded or not.
I have repeatedly attempted by the application of warmth to change the winter into the summer form, but always with the same negative result. _It is not possible to compel the winter brood to assume the form of the summer generation._
_A. Levana_ may produce not only two but three broods in the year, and may, therefore, be said to be _polygoneutic_.[16] One winter brood alternates with two summer broods, the first of which appears in July, and the second in August. The latter furnishes a fourth generation of pupæ, which, after hibernation, emerge in April, as the first brood of butterflies in the form _Levana_.
I frequently placed pupæ of this fourth brood in the hothouse immediately after their transformation, and in some cases even during the caterpillar stage, the temperature never falling, even at night, below 12° R., and often rising during the day to 24° R. The result was always the same: all, or nearly all, the pupæ hibernated, and emerged the following year in the winter form as perfectly pure _Levana_, without any trace of transition to the _Prorsa_ form. On one occasion only was there a _Porima_ among them, a case for which an explanation will, I believe, be found later on. It often happened, on the other hand, that some few of the butterflies emerged in the autumn, about fourteen days after pupation; and these were always _Prorsa_ (the summer form), excepting once a _Porima_.