On the under side all the males were of one type, the colours being very intense. There was considerably more red, both dark and pale, over the whole surface, than in a series of natural examples in which shades of brown and a bluish hue predominate. No change was observed in the females on the under side.
It appears that fourteen days were as effective in producing changes as a longer period. In fact, the most decided changes were found in the females exposed the shorter period. It also appears that with this species cold will produce change if applied after the chrysalis has hardened. The same experiments were attempted in 1878 with pupæ of Grapta Comma. They were put on ice at from ten minutes to six hours after forming, and subjected to a temperature of about 0°-1° R. for eighteen to twenty days, but every pupa was killed. Chrysalides of Papilio Ajax in the same box, and partly exposed very soon after pupation, were not injured. It was for this reason that none of the Interrogationis pupæ were placed in the box till six hours had passed.
It appears further that cold may change the markings on one part of the wing only, and in cases where it does change dark or dusky markings melanises them; or it may deepen the colours of the under surface (as in the females of the present experiment). The females in the above experiment were apparently most susceptible to the cold, the most decided changes having been effected in them.
The resulting butterflies were all of one form, although both might have been expected to appear under natural circumstances.
Dr. Weismann’s remarks on the foregoing experiments.—The author of the present work has, at my request, been good enough to furnish the following remarks upon Mr. Edward’s experiments with G. Interrogationis:—
The interesting experiments of Mr. Edwards are here principally introduced because they show how many weighty questions in connexion with seasonal dimorphism still remain to be solved. The present experiments do not offer a direct but, at most, only an indirect proof of the truth of my theory, since they show that the explanation opposed to mine is also in this case inadmissible. Thus we have here, as with Papilio Ajax, two out of the four annual generations mixed, i.e., consisting of summer and winter forms, and the conclusion is inevitable that these forms were not produced by the gradual action of heat or cold. When, from pupæ of the same generation which are developed under precisely the same external conditions, both forms of the butterfly are produced, the cause of their diversity cannot lie in these conditions. It must rather depend on causes innate in the organism itself, i.e., on inherited duplicating tendencies which meet in the same generation, and to a certain extent contend with each other for precedence. The two forms must have had their origin in earlier generations, and there is nothing against the view that they have arisen through the gradual augmentation of the influences of temperature.
In another sense, however, one might perceive, in the facts discovered by Edwards, an objection to my theory.
By the action of cold the form Umbrosa, which flies in June, was produced. Now we should be inclined to regard the var. Umbrosa as the summer form, and the var. Fabricii, which emerges in the autumn, hibernates in the imago state, and lays eggs in the spring, as the winter form. It would then be incomprehensible why the var. Umbrosa (i.e., the summer form) should be produced by cold.
But it is quite as possible that the var. Umbrosa as that the var. Fabricii is the winter form. We must not forget that, in this species, not one of the four annual generations is exposed to the cold of winter in the pupal state. When, therefore, we have in such cases seasonal dimorphism, to which complete certainty can only be given by continued observations of this butterfly, which does not occur very commonly in Virginia, this must depend on the fact that the species formerly hibernated in the pupal stage. This question now arises, which of the existing generations was formerly the hibernating one—the first or the last?
Either may have done so à priori, according as the summer was formerly shorter or longer than now for this species. If the former were the case, the var. Fabricii is the older winter form; were the latter the case, the var. Umbrosa is the original winter form, as shall now be more closely established.