From these facts I am led to conclude that we have here a variable form spread over an extensive area, and kept variable by the continual intercrossing of individuals, which would otherwise segregate into distinct and sharply defined races. The same area is inhabited by many species of birds common to all parts of it; and just as the birds of Ceram and Amboyna are almost all distinct species from those of New Guinea, so do we find those islands inhabited by the _Ornithoptera Priamus_, a well-marked and constant species, readily distinguishable in either sex from the inconstant forms of New Guinea proper. The same parallel holds in North Australia. Many New Guinea species of birds extend, with very slight variation, to the country about Cape York; but when we reach the Moreton Bay district all these have disappeared, and we find only true Australian species. So the variable forms of _O. Poseidon_ reach North Australia and Cape York, while in the Moreton Bay district we find the comparatively well-marked species _O. Richmondia_. Similar causes, whether geographical or climatal, have thus produced an analogous distribution in these widely separated groups of animals. 3. ORNITHOPTERA CRŒSUS, Felder. _O. Crœsus_, Feld. Wien. Ent. Monats., Dec. 1859. _O. Crœsus_, G. R. Gray, Proc. Zool. Soc. 1859, p. 424. _Hab._ Batchian (Moluccas) (_Wall._). Local form, _a._—_Male_: has the orange colour of the upper surface of a much deeper fiery-red hue; on the under surface, the black spots of the lower wings are nearer the margin, and the yellow spots below them are entirely absent; there is also a green line between the subcostal nervure and the margin; on the under surface of the fore wings the green patch in the discoidal cell extends to its base, and is reflexed in a narrow line along its upper edge. _Female_: differs still more from that sex in _O. Crœsus_; the white markings on all the wings are so large as almost to fill up the spaces between the veins, the lower part of the discoidal cell in both upper and under wings being also occupied with a whitish patch; the range of spots occupying the posterior margin are of a dusky yellow colour. _Hab._ Ternate (♂), Gilolo (♀) (_Wall._). This well-marked local form is no doubt peculiar to Gilolo and the small adjacent islands, as the original species is to Batchian.