We may explain these remarkable aberrations theoretically in the following way: The determinants of the wing-scales in the wing-primordium of the young pupa are influenced by the cold in different ways, some kinds of determinants being strengthened by it, others markedly weakened, even crippled so to speak, and in this way one colour-area spreads itself out more than is normal on the surface of the wing, and another less, while a third is suppressed altogether. That this disturbance of the equilibrium between the determinants leads usually to the development of a phyletically older marking pattern leads us to the conclusion that in the germ-plasm of the modern species of _Vanessa_ a certain number of determinants of the ancestors must be contained in addition to the modern ones. We might even inquire whether these were not better able to endure cold than their modern descendants, since their original possessors, the old species of the Ice age, were accustomed to greater cold, but this idea is contradicted by the experiments of E. Fischer, which go to show that the same aberrations are evoked by abnormally high temperature. That the old ancestral determinants are present in _different_ numbers in the germ-plasm of the modern species, I am inclined to infer from the fact that among a large number of experiments made by me in the course of several years the aberrations have always occurred in very different numbers in the different broods, although the greatest care was taken to have the conditions as nearly alike as possible; absolutely alike, of course, they never can be. But it would lead me too far if I were to enter on a detailed discussion of these cases, which have not yet been fully worked up; only one thing more need be mentioned, that is, that the aberrations induced by cold are to a certain extent transmissible. Standfuss first succeeded in making some aberrant specimens of _Vanessa urticæ_ reproduce, and from their eggs he procured butterflies which showed a much slighter deviation from the normal, which however was still so decided that it could not be regarded as due to chance. I myself succeeded in doing the same, but the deviation in this case was much slighter. But that these observed cases are rightly referred to the cold to which their parents had been subjected is proved by other observations recently published by E. Fischer. These refer to one of the Bombycidæ (_Arctia caja_), which flies by day, and accordingly has a gay and very definite marking and coloration. A large number of pupæ were exposed to cold at 8° C., and some of these resulted in striking and very dark aberrant forms (Fig. 129, _A_). A pair of these yielded fertilized eggs; in the progeny, which were reared at a normal temperature, there were among the much more numerous normal forms a few (17) which exhibited the aberration of the parents, though to a considerably less degree (Fig. 129, _B_).