Even in the case of species the summer form of which really possesses far more black than the winter form, as, for instance, _Araschnia Levana_, one type cannot be derived from the other simply by the expansion of the black spots present, since on the same place where in _Levana_ a black band crosses the wings, _Prorsa_, which otherwise possesses much more black, has a white line. (See Figs. 1-9, Plate I.) The intermediate forms which have been artificially produced by the action of cold on the summer generation present a graduated series, according as reversion is more or less complete; a black spot first appearing in the middle of the white band of _Prorsa_, and then becoming enlarged until, finally, in the perfect _Levana_ it unites with another black triangle proceeding from the front of the band, and thus becomes fused into a black bar. The white band of _Prorsa_ and the black band of _Levana_ by no means correspond in position; in _Prorsa_ quite a new pattern appears, which does not originate by a simple colour replacement of the _Levana_ marking. In the present case, therefore, there is no doubt that the new form is not produced simply because a certain pigment (black) is formed in larger quantities, but because its mode of distribution is at the same time different, white appearing in some instances where black formerly existed, whilst in other cases the black remains. Whoever compares _Prorsa_ with _Levana_ will not fail to be struck with the remarkable change of marking produced by the direct action of external conditions. The numerous intermediate forms which can be produced artificially appear to me to furnish a further proof of the gradual character of the transformation. Ancestral intermediate forms can only occur where they have once had a former existence in the phyletic series. Reversion may only take place completely in some particular characters, whilst in others the new form remains constant--this is in fact the ordinary form of reversion, and in this manner a mixture of characters might appear which never existed as a phyletic stage; but particular characters could certainly never appear unless they were normal to the species at some stage of phyletic development. Were this possible it would directly contradict the idea of reversion, according to which new characters never make their appearance, but only such as have already existed. If, therefore, the ancestral forms of _A. Levana_ (which we designate as _Porima_) present a great number of transitional varieties, this leads to the conclusion that the species must have gone through a long series of stages of phyletic development before the summer generation had completely changed into _Prorsa_. The view of the slow cumulative action of climatic influences already submitted, is thus confirmed.