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

—;184 ZOOLOGICAL GEOGRAPHY. [part hi.among the far-wandering aquatic birds we have no less than fivegenera which are more especially Palsearctic,corn-crake, and Otis, the great bustard, being typicalOrtygometra,theexamples.We may add to these, several genera almost confined to thisregion, such as Garrulus (jays), Fringilla (true finches), Yunx(wrynecks) and some others ; so that in proportion to its totalgeneric forms a very large number are found to be peculiar orcharacteristic.This view, of the high degree of speciality of the Palsearcticregion, will no doubt be objected toby some naturalists, on theground that many of the genera reckoned as exclusivelyPalsearctic are not so, but extend more or less into other regions.It is well, therefore, to consider what principles should guide usin a matter of this kind, especially as we shall have to applythe same rules to each of the other regions. We may remarkfirst, that the limits of the regions themselves are, when notformed by the ocean, somewhat arbitrary, depending on theaverage distribution of a number of characteristic forms; andthat slight local peculiarities of soil, elevation, or climate, maycause the species of one region to penetrate more or lessdeeplyinto another. The land boundary between two regions will be,not a defined line but a neutral territory of greater or lesswidth, within which the forms ofboth regions will intermingleand this neutral territory itself will merge imperceptibly intoboth regions. So long therefore as a species or genus does notpermanently reside considerably beyond the possible limits ofthis neutral territory, we should not claim it as an inhabitantof the adjacent region. A consideration of perhaps more importancearises, from the varying extent of the range of a genus,over the area occupied by the region. Some genera are representedby single species existing only in a very limited areaothers by numerous species which occupy, entirely or verynearly, the whole extent of the region ;and there is every intermediategrade between these extremes. Now, the small localisedgenera, are always reckoned as among the best examplesof types peculiar to a region ;while the more wide-spread groupsare often denied that character if they extend a little beyond