A proof of this may be found in the results of experiments made upon the little red-gold fire-butterfly (_Polyommatus phlæas_), to which I have briefly referred in a former lecture. This little diurnal butterfly of the family Lycænidæ has a wide distribution and occurs in two climatic varieties. In the far north and also in the whole of Germany the upper surface is red-gold with a narrow black outer margin, but in the south of Europe the red-gold has been almost crowded out by the black. I reared caterpillars in Germany from eggs of _P. phlæas_ found at Naples and exposed them directly after they had entered on the pupa-stage to a relatively low temperature (10° C.). Butterflies emerged which were not quite so black as those of Naples, but considerably darker than the German form. Conversely, German pupæ were exposed to greater warmth (38° C.), and these gave rise to butterflies which were rather less fiery gold and considerably blacker than the ordinary German form. If I had to repeat these experiments I should use a much lower temperature in the case of the cold experiments, because we now know from the experiments of Standfuss, E. Fischer, and Bachmetjeff, that most of the pupæ of diurnal butterflies can stand a temperature below zero for a considerable time; probably the results would be even more marked then. But even from the results of my former experiments we are justified in concluding that the blackening of the upper surface of the wing is really the direct result of the increased temperature during pupahood, and that the pure red-gold results from the lowered temperature. Similar experiments made by Merrifield with English _Phlæas_ pupæ agree exactly with mine. But we may conclude further from these experiments that both warmth and cold only give rise to slight variations in the individual pupæ, and that the pure red-gold of the northern form and the black of the southern are the result of a long process of inheritance and accumulation, in which the germ-plasm has been caused to vary in as far as the relevant determinants are concerned, so that these yield the respective northern and southern forms even in less extreme temperatures. As it is to be assumed that these determinants are present not only in the primordium of the wing in the pupa, but also in the germ-cells, both must be affected by the varying temperature, and, in accordance with the continuity of the germ-plasm, each variation of these determinants, however slight, would be continued in the next generation. It is thus intelligible that somatic variations like the blackening of the wings through warmth appear to be directly inherited and accumulate in the course of generations; in reality, however, it is not the somatic change itself which is transmitted, but the corresponding variation evoked by the same external influence in the relevant determinants of the germ-plasm within the germ-cells, in other words, in the determinants of the following generation.