But, besides the true _P. Polytes_, there are several allied forms of females to be considered, namely, _P. Theseus_, Cr., _P. Melanides_, De Haan, _P. Elyros_, G. R. G., and _P. Romulus_, L. The dark female figured by Cramer as _P. Theseus_ seems to be the common and perhaps the only form in Sumatra, whereas in Java, Borneo, and Timor, along with males quite identical with those of Sumatra, occur females of the _Polytes_ form, although a single specimen of the true _P. Theseus_, Cr., taken at Lombock would seem to show that the two forms do occur together. In the allied species found in the Philippine ♀Islands (_P. Alphenor_, Cr., _P. Ledebouria_, Eschsch., ♀ _P. Elyros_, G. R. G.) forms corresponding to these extremes occur along with a number of intermediate varieties, as shown by a fine series in the British Museum. We have here an indication of how dimorphism may be produced; for let the extreme Philippine forms be better suited to their conditions of existence than the intermediate connecting links, and the latter will gradually die out, leaving two distinct forms of the same insect, each adapted to some special conditions. As these conditions are sure to vary in different districts, it will often happen, as in Sumatra and Java, that the one form will predominate in the one island, the other in the adjacent one. In the island of Borneo there seems to be a third form; for _P. Melanides_, De Haan, evidently belongs to this group, and has all the chief characteristics of _P. Theseus_, with a modified coloration of the hind wings. I now come to an insect which, if I am correct, offers one of the most interesting cases of variation yet adduced. _Papilio Romulus_, L., a butterfly found over a large part of India and Ceylon, and not uncommon in collections, has always been considered a true and independent species, and no suspicions have been expressed regarding it. But a male of this form does not, I believe, exist. I have examined the fine series in the British Museum, in the East India Company’s Museum, in the Hope Museum at Oxford, in Mr. Hewitson’s and several other private collections, and can find nothing but females; and for this common butterfly no male partner can be found except the equally common _P. Pammon_, a species already provided with two wives, and yet to whom we shall be forced, I believe, to assign a third. On carefully examining _P. Romulus_, I find that in all essential characters,—the form and texture of the wings, the length of the antennæ, the spotting of the head and thorax, and even the peculiar tints and shades with which it is ornamented,—it corresponds exactly with the other females of the _Pammon_ group; and though, from the peculiar marking of the fore wings, it has at first sight a very different aspect, yet a closer examination shows that every one of its markings could be produced by slight and almost imperceptible modifications of the various allied forms. I fully believe, therefore,