Should it not be possible to perfect the transformation, so that each individual should take the _Levana_ form? If the assumption of the _Prorsa_ or _Levana_ form depends only on the direct influence of temperature, or on the duration of the period of development, it should be possible to compel the pupæ to take one or the other form at pleasure, by the application of the necessary external conditions. This has never been accomplished with _Araschnia Prorsa_. As in the experiment already described, and in all subsequent ones, single specimens appeared as the unchanged summer form, others showed an appearance of transition, and but very few had changed so completely as to be possibly taken for the pure _Levana_. In some species of the sub-family _Pierinæ_, however, at least in the case of the summer brood, there was, on the contrary, a complete transformation. Most of the species of our ‘Whites’ (_Pierinæ_) exhibit the phenomenon of seasonal dimorphism, the winter and summer forms being remarkably distinct. In _Pieris Napi_ (with which species I chiefly experimented) the winter form (Figs. 10 and 11, Plate I.) has a sprinkling of deep black scales at the base of the wings on the upper side, while the tips are more grey, and have in all cases much less black than in the summer form; on the underside the difference lies mainly in the frequent breadth, and dark greenish-black dusting, of the veins of the hind wings in the winter form, while in the summer form these greenish-black veins are but faintly present. I placed numerous specimens of the summer brood, immediately after their transformation into chrysalides, in the refrigerator (0°-1° R.), where I left them for three months, transferring them to a hothouse on September 11th, and there (from September 26th to October 3rd) sixty butterflies emerged, the whole of which, without exception--and most of them in an unusually strong degree--bore the characters of the winter form. I, at least, have never observed in the natural state such a strong yellow on the underside of the hind wings, and such a deep blackish-green veining, as prevailed in these specimens (see, for instance, Figs. 10 and 11). The temperature of the hothouse (12°-24° R.) did not, however, cause the emergence of the whole of the pupæ; a portion hibernated, and produced in the following spring butterflies of the winter form only. I thus succeeded, with this species of _Pieris_, in completely changing every individual of the summer generation into the winter form.