The explanation of these facts is easily afforded by the foregoing theory. According to this, both the winter forms must be regarded as primary, and the _Marcellus_ form as secondary. But this last is not yet so firmly established as _Prorsa_, in which reversion of the summer generations to the _Levana_ form only occurs through special external influences; whilst in the case of _Ajax_ some individuals are to be found in every generation, the tendency of which to revert is still so strong that even the greatest summer heat is unable to cause them to diverge from their original inherited direction of development, or to accelerate their emergence and compel them to assume the _Marcellus_ form. It is here beyond a doubt that it is not different external influences, but internal causes only, which maintain the old hereditary tendency, for all the larvæ and pupæ of many different broods were simultaneously exposed to the same external influences. But, at the same time, it is evident that these facts are not opposed to the present theory; on the contrary, they confirm it, inasmuch as they are readily explained on the basis of the theory, but can scarcely otherwise be understood. If it be asked what significance attaches to the duplication of the winter form, it may be answered that the species was already dimorphic at the time when it appeared in only one annual generation. Still, this explanation may be objected to, since a dimorphism of this kind is not at present known, though indeed some species exhibit a sexual dimorphism,[20] in which one sex (as, for instance, the case of the female _Papilio Turnus_) appears in two forms of colouring, but not a dimorphism, as is here the case, displayed by both sexes.[21] Another suggestion, therefore, may perhaps be offered.