Now, the differences in the most pronounced cases of seasonal dimorphism--for example, in _Araschnia Levana_--are much less manifest on the _under_ than on the _upper_ side of the wing. The explanation by adaptation is therefore untenable; but I will not here pause to confute this view more completely, as I believe I shall be able to show the true cause of the phenomenon. If seasonal dimorphism does not arise from the _indirect_ influence of varying seasons of the year, it may result from the _direct_ influence of the varying external conditions of life, which are, without doubt, different in the winter from those of the summer brood. There are two prominent factors from which such an influence may be expected--temperature and duration of development, i.e., duration of the chrysalis period. The duration of the larval period need not engage our attention, as it is only very little shorter in the winter brood--at least, it was so with the species employed in the experiments. Starting from these two points of view, I carried on experiments for a number of years, in order to find out whether the dual form of the species in question could be traced back to the direct action of the influences mentioned. The first experiments were made with _Araschnia Levana_. From the eggs of the winter generation, which had emerged as butterflies in April, I bred caterpillars, and immediately after pupation placed them in a refrigerator, the temperature of the air of which was 8°-10° R. It appeared, however, that the development could not thus be retarded to any desired period by such a small diminution of temperature, for, when the box was taken out of the refrigerator after thirty-four days, all the butterflies, about forty in number, had emerged, many being dead, and others still living. The experiment was so far successful that, instead of the _Prorsa_ form which might have been expected under ordinary circumstances, most of the butterflies emerged as the so-called _Porima_ (Figs. 3, 4, 7, 8, and 9, Plate I.); that is to say, in a form intermediate between _Prorsa_ and _Levana_ sometimes found in nature, and possessing more or less the marking of the former, but mixed with much of the yellow of _Levana_.