184 ZOOLOGICAL GEOGRAPHY. [part hi. 



among the far-wandering aquatic birds we have no less than five 

 genera which are more especially Palsearctic, — Ortygometra, the 

 corn-crake, and Otis, the great bustard, being typical examples. 

 We may add to these, several genera almost confined to this 

 region, such as Garrulus (jays), Fringilla (true finches), Yunx 

 (wrynecks) and some others ; so that in proportion to its total 

 generic forms a very large number are found to be peculiar or 

 characteristic. 



This view, of the high degree of speciality of the Palsearctic 

 region, will no doubt be objected to by some naturalists, on the 

 ground that many of the genera reckoned as exclusively 

 Palsearctic are not so, but extend more or less into other regions. 

 It is well, therefore, to consider what principles should guide us 

 in a matter of this kind, especially as we shall have to apply 

 the same rules to each of the other regions. We may remark 

 first, that the limits of the regions themselves are, when not 

 formed by the ocean, somewhat arbitrary, depending on the 

 average distribution of a number of characteristic forms; and 

 that slight local peculiarities of soil, elevation, or climate, may 

 cause the species of one region to penetrate more or less deeply 

 into another. The land boundary between two regions will be, 

 not a defined line but a neutral territory of greater or less 

 width, within which the forms of both regions will intermingle ; 

 and this neutral territory itself will merge imperceptibly into 

 both regions. So long therefore as a species or genus does not 

 permanently reside considerably beyond the possible limits of 

 this neutral territory, we should not claim it as an inhabitant 

 of the adjacent region. A consideration of perhaps more im- 

 portance arises, from the varying extent of the range of a genus, 

 over the area occupied by the region. Some genera are repre- 

 sented by single species existing only in a very limited area ; 

 others by numerous species which occupy, entirely or very 

 nearly, the whole extent of the region ; and there is every inter- 

 mediate grade between these extremes. Now, the small local- 

 ised genera, are always reckoned as among the best examples 

 of types peculiar to a region ; while the more wide-spread groups 

 are often denied that character if they extend a little beyond 



