184 ZOOLOGICAL GEOGRAPHY. ‘ [PART III. 
among the far-wandering aquatic birds we have no less than five 
genera which are more especially Palzarctic,—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), Yuna 
(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 Palearctic 
region, will no doubt be objected to by some naturalists, on the 
ground that many of the genera reckoned as exclusively 
Palearctic 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 
