This interpretation of these experiments, which I offered some years ago, has been confirmed in several ways in regard to various other diurnal Lepidoptera. By employing a temperature as low as 8° C. in the case of fresh pupæ of various species of _Vanessa_ Standfuss and Merrifield, and especially E. Fischer, succeeded in getting great deviations in the marking and colour of the full-grown insects,--so-called aberrations, such as had previously been found only very rarely and singly under natural conditions. The deviations from the normal must undoubtedly be ascribed to the effect of cold, but it does not follow that they are new forms which have suddenly sprung into existence, as many have assumed without further experiment. Dixey, on the other hand, has attempted to establish, by a comparison of the different species of _Vanessa_, the phyletic development of their markings, and has found that these aberrations due to cold are more or less complete reversions to earlier phyletic stages. As regards the common small painted lady (_Vanessa cardui_), the small tortoise-shell butterfly (_Vanessa urticæ_), the 'Admiral' (_Vanessa atalanta_), the peacock (_Vanessa io_), and the large tortoise-shell (_Vanessa polychioros_), I can agree with this interpretation, and I do so the more readily because some years ago I suggested that the alternation of differently coloured generations of seasonally dimorphic Lepidoptera might be considered as a reversion. But this by no means excludes the possibility that other than atavistic aberrations may be produced by cold or heat. There is nothing against this theoretically. Yet we must not, without due consideration, compare these abruptly occurring variations to the sport-varieties of plants which we have already discussed; there is an important difference between the two sets of cases. In the Lepidoptera a single interference, lasting only for a short time, modifies the wing-marking, but in the plant varieties the visible appearance of the variation is preceded by a long period of preparatory change within the germ-plasm. This period required for the external influences to take effect was already recognized by Darwin, and it has recently been named by De Vries the 'premutation period.'