The next pair, _P. Theseus_, Cr., and _P. Antiphus_, Fab., have been united as one species both by De Haan and in the British Museum Catalogues. The ordinary variety of _P. Theseus_ found in Java almost as nearly resembles _P. Diphilus_, Esp., of the same country. The most interesting case, however, is the extreme female form of _P. Memnon_ (_P. Achates_, Cr.)[10], which has acquired the general form and markings of _P. Coon_, an insect which differs from the ordinary male _P. Memnon_, as much as any two species differ which can be chosen in this extensive and highly varied genus; and, as if to show that this resemblance is not accidental, but is the result of law, when in India we find a species closely allied to _P. Coon_, but with red instead of yellow spots (_P. Doubledayi_, Wall.), the corresponding variety of _P. Androgeus_ (_P. Achates_, Cram., 182, A, B,) has acquired exactly the same peculiarity of having red spots instead of yellow. Lastly, in the island of Timor, the female of _P. Œnomaus_ (a species allied to _P. Memnon_) resembles so closely _P. Liris_ (one of the _Polydorus_-group), that the two, which were often seen flying together, could only be distinguished by a minute comparison after being captured. Footnote 10: See Plate I. fig. 4. The last six cases of mimicry are especially instructive, because they seem to indicate one of the processes by which dimorphic forms have been produced. When, as in these cases, one sex differs much from the other, and varies greatly itself, it may happen that occasionally individual variations will occur having a distant resemblance to groups which are the objects of mimicry, and which it is therefore advantageous to resemble. Such a variety will have a better chance of preservation; the individuals possessing it will be multiplied; and their accidental likeness to the favoured group will be rendered permanent by hereditary transmission, and, each successive variation which increases the resemblance being preserved, and all variations departing from the favoured type having less chance of preservation, there will in time result those singular cases of two or more isolated and fixed forms bound together by that intimate relationship which constitutes them the sexes of a single species. The reason why the females are more subject to this kind of modification than the males is, probably, that their slower flight, when laden with eggs, and their exposure to attack while in the act of depositing their eggs upon leaves, render it especially advantageous for them to have some additional protection. This they at once obtain by acquiring a resemblance to other species which, from whatever cause, enjoy a comparative immunity from persecution.