I am only too conscious how entirely theoretical are these conjectures. It is very possible that observation of nature will render numerous corrections necessary. For instance, I have assumed that every species is able, when necessary, to adapt any one of its developmental stages to hibernation. Whether this is actually the case must be learnt from further researches; at present we only know that many species hibernate in the egg stage, others in the larval state, others as pupæ, and yet others in the perfect state. We know also that many species hibernate in several stages at the same time, but we do not know whether each stage of every species has an equal power of accommodation to cold. Should this not be the case the above conjectures would have to be considerably modified. To take up this subject, so as to completely master all the facts connected therewith, naturalists would have to devote their whole time and energy to the order Lepidoptera, which I have been unable to do. From the considerations offered, it thus appears that the phenomena of seasonal dimorphism may depend on extremely complex processes, so that one need not be surprised if only a few cases now admit of certain analysis. We must also admit, however, that it is more advantageous to science to be able in the first place to analyze the simplest cases by means of breeding experiments, than to concern oneself in guessing at cases which are so complicated as to make it impossible at present to procure all the materials necessary for their solution. [Illustration: Plate I. Aug. Weismann pinx. Lith. J.A. Hofmann, Würzburg. ] [Illustration: Plate II. Aug. Weismann pinx. Lith. J.A. Hofmann, Würzburg. ] EXPLANATION OF THE PLATES. PLATE I. Fig. 1. Male _Araschnia Levana_, winter form. Fig. 2. Female _A. Levana_, winter form. Fig. 3. Male _A. Levana_, artificially bred intermediate form (so-called _Porima_). Fig. 4. Female _A. Levana_, intermediate form (_Porima_), artificially bred from the summer generation, agreeing perfectly in marking with the winter form, and only to be distinguished from it by the somewhat darker ground colour. Fig. 5. Male _A. Levana_, summer form (_Prorsa_). Fig. 6. Female _A. Levana_, summer form (_Prorsa_). Figs. 7 to 9. Intermediate forms (_Porima_), artificially bred from the first summer generation. Figs. 10 and 11. Male and female _Pieris Napi_, winter form, artificially bred from the summer generation; the yellow ground-colour of the underside of the hind wings brighter than in the natural winter form. Figs. 12 and 13. Male and female _Pieris Napi_, summer form. Figs. 14 and 15. _Pieris Napi_, var. _Bryoniæ_, male and female reared from eggs. PLATE II. Fig. 16. _Papilio Ajax_, var. _Telamonides_, winter form. Fig. 17. _P. Ajax_, var. _Marcellus_, summer form. Fig. 18. _Plebeius Agestis_ (_Alexis_, Scop.), German winter form. Fig. 19. _P. Agestis_ (_Alexis_, Scop.), German summer form.