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Show 514 MR. D. G. ELLIOT ON THE GENUS PTILOPUS. [May 7, have but little in common with the members of Ptilopus, the peculiarly notched first primary and bare face easily distinguishing them. GEOGRAPHICAL DISTRIBUTION. Probably no group in ornithology exhibits to a greater degree the effects upon the coloration of plumage arising from the physical causes incidental to an insular life than the one here considered as composing the genus Ptilopus. Some of these birds that have evidently had a common origin exhibit greater or less variations from each other, according to the position of their various habitats, sometimes of sufficient importance to constitute their possessors distinct species, at others only of that trivial kind that would at best but cause them to rank merely as varieties, the lapse of time during which the individuals have dwelt in their separate localities not having been sufficiently great, or the physical causes of climate, food, and soil not powerful enough to modify essentially their appearance from the typical form. It is therefore not surprising that we find some species inhabiting one island only, others dwelling upon several, though perhaps separated by miles of sea, while again in other localities uncertain forms are observed not changed sufficiently to authorize the bestowal upon them of a separate distinctive rank, but yet differing enough to show that a departure from the type and towards a distinctive independent form has been commenced, and will continue (unless the race should become exterminated) until the variety eventually blooms into a separate species possessing characters not found elsewhere. It is not to be supposed that the continents of which the islands of the Pacific and those of the Eastern Archipelago are the sole remains, were broken up simultaneously or always suddenly throughout their length and breadth ; but more probably the casualty happened at various periods and sometimes gradually. Therefore we may not be surprised at the apparently strange phenomenon that one species should inhabit various islands, between which are others containing totally distinct forms of the same genus. This may have been brought about in two probable ways. A species may have been widely dispersed over the continent; and when portions of this had disappeared beneath the waves, the fragments that remained above water at the outset were all occupied by the same species: but physical effects at intermediate points were of a different character from those at the extremes; and in course of time the birds dwelling on the intervening islands departed entirely from their types, while those most widely separated retained their original characters. Or it may, on the other hand, have been that on the breaking-up of the continent a district inhabited by a strictly local species (but one surrounded by a more widely disseminated and distinct species) had not been entirely submerged ; and this, all other circumstances being equal, would explain the fact that a distinct form should intrude itself on an island lying between others inhabited by a different one, the species with the greater range having been preserved at the extremes |