In _A. Levana_ we saw that reversion occurred in very different degrees with different individuals, seldom attaining to the true _Levana_ form, and generally only reaching the intermediate form known as _Porima_. Now it would, at all events, be astonishing if with _P. Ajax_ the reversion were always complete, as it is precisely in this case that the tendency to individual reversion is so variable. I might, for this reason, suppose that one of the two winter forms, viz. the var. _Walshii_, is nothing else than an incomplete reversion-form, corresponding to _Porima_ in the case of _A. Levana_. Then _Telamonides_ only would be the original form of the butterfly, and this would agree with the fact that this variety appears later in the spring than _Walshii_. Experiments ought to be able to decide this.[22] The pupæ of the first three generations placed upon ice should give, for the greater part, the form _Telamonides_, for the lesser portion _Walshii_, and for only a few, or perhaps no individuals, the form _Marcellus_. This prediction is based on the view that the tendency to revert is on the whole great; that even with the first summer generation, which was the longest exposed to the summer climate, a portion of the pupæ, without artificial means, always emerged as _Telamonides_, and another portion as _Marcellus_. The latter will perhaps now become _Walshii_ by the application of cold. One would expect that the second and third generations would revert more easily, and in a larger percentage, than the first, because this latter first acquired the new _Marcellus_ form; but the present experiments furnish no safe conclusion on this point. Thus, of the first summer generation only seven out of sixty-seven pupæ hibernated, and these gave _Telamonides_; while of the second generation forty out of seventy-six, and of the third generation twenty-nine out of forty-two pupæ hibernated. But to establish safer conclusions, a still larger number of experiments is necessary. According to the experience thus far gained, one might perhaps still be inclined to imagine that, with seasonal dimorphism, external influences operating on the individual might directly compel it to assume one or the other form. I long held this view myself, but it is, nevertheless, untenable. That cold does not produce the one kind of marking, and warmth the other, follows from the before-mentioned facts, viz. that in _Papilio Ajax_ every generation produces both forms; and, further, in the case of _A. Levana_ I have frequently reared the fourth (hibernating) generation entirely in a warm room, and yet I have always obtained the winter form. Still, one might be inclined not to make the temperature _directly_ responsible, but rather the retardation or acceleration of development produced through the action of temperature. I confess that I for a long time believed that in this action I had found the true cause of seasonal dimorphism. Both with _A. Levana_ and _P. Napi_ the