The species of _Araschnia_ through which the discovery of seasonal dimorphism was made, formerly bore the two specific names _A. Levana_ and _A. Prorsa_. The latter is the summer and the former the winter form, the difference between the two being, to the uninitiated, so great that it is difficult to believe in their relationship. _A. Levana_ (Figs. 1 and 2, Plate I.) is of a golden brown colour with black spots and dashes, while _A. Prorsa_ (Figs. 5 and 6, Plate I.) is deep black with a broad white interrupted band across both wings. Notwithstanding this difference, it is an undoubted fact that both forms are merely the winter and summer generations of the same species. I have myself frequently bred the variety _Prorsa_ from the eggs of _Levana_, and _vice versâ_. Since the discovery of this last fact a considerable number of similar cases have been established. Thus P. C. Zeller[3] showed, by experiments made under confinement, that two butterflies belonging to the family of the ‘Blues,’ differing greatly in colour and marking, and especially in size, which had formerly been distinguished as _Plebeius_ (_Lycæna_) _Polysperchon_ and _P. Amyntas_, were merely winter and summer generations of the same species; and that excellent Lepidopterist, Dr. Staudinger, proved the same[4] with species belonging to the family of the ‘Whites,’ _Euchloe Belia_ Esp. and _E. Ausonia_ Hüb., which are found in the Mediterranean countries. The instances are not numerous, however, in which the difference between the winter and summer forms of a species is so great as to cause them to be treated of in systematic work as distinct species. I know of only five of these cases. Lesser differences, having the systematic value of varieties, occur much more frequently. Thus, for instance, seasonal dimorphism has been proved to exist among many of our commonest butterflies belonging to the family of the ‘Whites,’ but the difference in their colour and marking can only be detected after some attention; while with other species, as for instance with the commonest of our small ‘Blues,’ _Plebeius Alexis_ (= _Icarus_, Rott.), the difference is so slight that even the initiated must examine closely in order to recognize it. Indeed whole series of species might easily be grouped so as to show the transition from complete similarity of both generations, through scarcely perceptible differences, to divergence to the extent of varieties, and finally to that of species. Nor are the instances of lesser differences between the two generations very numerous. Among the European diurnal Lepidoptera I know of about twelve cases, although closer observation in this direction may possibly lead to further discoveries.[5] Seasonal dimorphism occurs also in moths, although I am not in a position to make a more precise statement on this subject,[6] as my own observations refer only to butterflies.