The first answer that can be given to this question is, that the cause of the different action produced by a similar agency can only lie in the _constitution_, i.e., in the _physical nature_ of the broods in question, and not in the external influences by which they are acted upon. Now, what is the difference in the physical nature of these respective broods? It is quite evident, as shown by the experiments already described, that cold and warmth cannot be the _immediate_ causes of a pupa emerging in the _Prorsa_ or _Levana_ form, since the last brood always gives rise to the _Levana_ form, whether acted on by cold or warmth. The first and second broods only can be made to partly assume, more or less completely, the _Levana_ form by the application of cold. In these broods then, a low temperature is the _mediate_ cause of the transformation into the _Levana_ form. The following is my explanation of the facts. The form _Levana_ is the original type of the species, and _Prorsa_ the secondary form arising from the gradual operation of summer climate. When we are able to change many specimens of the summer brood into the winter form by means of cold, this can only depend upon reversion to the original, or ancestral, form, which reversion appears to be most readily produced by cold, that is, by the same external influences as those to which the original form was exposed during a long period of time, and the continuance of which has preserved, in the winter generations, the colour and marking of the original form down to the present time.