Mr. Edwards thus explains the case under consideration:--“When _Phaon_, _Vesta_, and _Tharos_ were as yet only varieties of one species, the sole coloration was that now common to the three. As they gradually became permanent, or in other words, as these varieties became species, _Tharos_ was giving rise to several sub-varieties, some of them in time to become distinct and well marked, while the other two, _Phaon_ and _Vesta_, remained constant. As the climate moderated and the summer became longer, each species came to have a summer generation; and in these the resemblance of blood-relationship is still manifest. As the winter generations of each species had been much alike, so the summer generations which sprung from them were much alike. And if we consider the metropolis of the species _Tharos_, or perhaps of its parent species, at the time when it had but one annual generation, to have been somewhere between 37° and 40° on the Atlantic slope, and within which limits all the varieties and sub-varieties of both winter and summer forms of _Tharos_ are now found in amazing luxuriance, we can see how it is possible, as the glacial cold receded, that only part of the varieties of the winter form might spread to the northward, and but one of them at last reach the sub-boreal regions and hold possession to this day as the sole representative of the species. And at a very early period the primary form, together with _Phaon_ and _Vesta_, had made its way southward, where all three are found now.”
EXPERIMENTS WITH GRAPTA INTERROGATIONIS.[64]
[_Communicated by_ Mr. W. H. EDWARDS, _November 15th, 1879_.]
The experiments with this species were made in June, 1879, on pupæ from eggs laid by the summer form _Umbrosa_ of the second brood of the year, and obtained by confining a female in a bag on a stem of hop. As the pupæ formed, and at intervals of from six to twenty-four hours after pupation (by which time all the older ones had fully hardened), they were placed in the ice-box. In making this experiment Mr. Edwards had three objects in view. 1st. To see whether it was essential that the exposure should take place immediately after pupation, in order to effect any change. 2ndly. To see how short a period would suffice to bring about any change. 3rdly. Whether exposing the summer pupæ would bring about a change in the form of the resulting butterfly. Inasmuch as breeding from the egg of _Umbrosa_, in June, in a former year,[64] gave both _Umbrosa_ (11) and _Fabricii_ (6), the butterflies from the eggs obtained, if left to nature, might be expected to be of both forms. The last or fourth brood of the year having been found up to the present time to be _Fabricii_, and the 1st brood of the spring, raised from eggs of _Fabricii_ (laid in confinement), having been found to be wholly _Umbrosa_, the latter is probably the summer and _Fabricii_ the winter form. The two intervening broods, _i.e._ the 2nd and 3rd, have yielded both forms. This species hibernates in the imago state.