Should any one be inclined to conclude, from my reversion experiments with seasonally dimorphic butterflies, that the secondary species when exposed to the same climate as that which produced it must revert to the primary, he forgets that this reversion to the winter form is nothing but a reversion--i.e., a sudden return to a primary form through peculiar laws of inheritance--and by no means a gradual re-acquisition of this primary form under the gradual influence of the primary climate. Reversion to the winter form occurs also through other influences, as, for instance, by high temperature. Reversions of this kind, depending on laws of heredity, certainly happen with those cases of transmutation which do not alternate with the primary form, as in seasonal dimorphism, but which occur continuously. They would, however probably be more quickly suppressed in such cases than in seasonal dimorphism, where the constant alternation of the primary and secondary forms must always maintain the tendency of the latter to produce the former. That the above conclusion is correct--that a secondary species, when exposed to the external conditions under the influence of which the primary form originated, does not again revert to the latter--is proved by experience with plants. Botanists[53] assure us “that cultivated races which become wild, and are thus brought back to their former conditions of life, do not become changed into the original wild form, but into some new one.” A second point which appears to me to be elucidated by seasonal dimorphism, is the origin of variability. It has already been prominently shown that secondary forms are for the most part considerably more variable than primary forms. From this it follows that similar external influences either induce different changes in the different individuals of a species, or else change all individuals in the same manner, variability arising only from the unequal time in which the individuals are exposed to the external influence. The latter is undoubtedly the case, as appears from the differences which are shown by the various individuals of a secondary form. These are always only differences of degree and not of kind, as is perhaps most distinctly shown by the very variable _A. Prorsa_ (summer form), in which all the occurring variations differ only by the _Levana_ marking being more or less absent, and, at the same time, by approximating more or less to the pure _Prorsa_ marking; but changes in a totally different direction never occur. It is likewise further evident, as has been mentioned above, that allied species and genera, and even entire families (_Pieridæ_), are changed by similar external inducing causes in the same manner--or, better, in the same direction.