It might be expected that the same result could be more readily obtained with _A. Levana_, and fresh experiments were undertaken, in order that the pupæ might remain in the refrigerator fully two months from the period of their transformation (9-10th July). But the result obtained was the same as before--fifty-seven butterflies emerged in the hothouse[14] from September 19th to October 4th, nearly all of these approaching very near to the winter form, without a single specimen presenting the appearance of a perfect _Levana_, while three were of the pure summer form (_Prorsa_). Thus with _Levana_ it was not possible, by refrigeration and retardation of development, to change the summer completely into the winter form in all specimens. It may, of course, be objected that the period of refrigeration had been too short, and that, instead of leaving the pupæ in the refrigerator for two months, they should have remained there six months, that is, about as long as the winter brood remains under natural conditions in the chrysalis state. The force of this last objection must be recognized, notwithstanding the improbability that the desired effect would be produced by a longer period of cold, since the doubling of this period from four to eight weeks did not produce[15] any decided increase in the strength of the transformation. I should not have omitted to repeat the experiment in this modified form, but unfortunately, in spite of all trouble, I was unable to collect during the summer of 1873 a sufficient number of caterpillars. But the omission thus caused is of quite minor importance from a theoretical point of view. 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._