The geographical distribution of animals, with a study of the relations ...

;chap. x.J THE PAL^ARCTIC REGION. 185the supposed regional limits, or send one or two, out of a largenumber of species, into adjacent regions ;yet there is somereason to believe that the latter are really more important ascharacterising a zoological region than the former.In the caseof a single isolated species or genus we have a dying-out groupand we have so many cases of discontinuous species of suchgroups (of which Urotrichus in Japan and British Columbia,Eupetes in Sumatra and New Guinea are examples), that it isquite as probable as not, that any such isolated specieshas onlybecome peculiar to the region by the recent extinction of anallied form or forms in some other region. On the other hand,a genus consisting of numerous speciesranging over an entireregion or the greater part of one, is a dominant group, whichhas most likely been for some time extending its range, andwhose origin dates back to a remote period. The slight extensionof such a group beyond the limits of the region to whichit mainly belongs, isprobably a recent phenomenon, and in thatcase cannot be held in any degree to detract from its value asone of the peculiar forms of that region.The most numerous examples of this class, are those birds ofthe temperate regions which in winter migrate, eitherpartially, into adjacent warmer countries.wholly orThis migration mostlikely began subsequent to the Miocene period, during thatgradual refrigeration of the temperate zones which culminatedin the glacial epoch, and which still continues in a mitigatedform. Most of the genera, and many even of the species ofbirds which migrate southwards in winter, have therefore, mostlikely, always been inhabitants of our present Palsearctic andNearctic regions ;permanent residents during warm epochs, butonly able now to maintain their existence by migration inwinter. Such groups belong truly to the temperate zones, andthe test of this isthe fact of their not having any, or very few,representatives, which are permanent residents in the adjacenttropical regions.When there are such representative species, wedo not claim them as peculiar to the Northern regions.Bearingin mind these various considerations, it will be found that we havebeen very moderate in our estimate of the number of generaYol. I.—14