CHAP. X. | THE PALZARCTIC REGION. 185 
the supposed regional limits, or send one or two, out of a large 
number of species, into adjacent regions; yet there is some 
reason to believe that the latter are really more important as 
characterising a zoological region than the former. In the case 
of a single isolated species or genus we have a dying-out group ; 
and we have so many cases of discontinuous species of such 
groups (of which Urotrichus in Japan and British Columbia, 
Eupetes in Sumatra and New Guinea are examples), that it is 
quite as probable as not, that any such isolated species has only 
become peculiar to the region by the recent extinction of an 
allied form or forms in some other region. On the other hand, 
a genus consisting of numerous species ranging over an entire 
region or the greater part of one, is a dominant group, which 
has most likely been for some time extending its range, and 
whose origin dates back to a remote period. The slight exten- 
sion of such a group beyond the limits of the region to which 
it mainly belongs, is probably a recent phenomenon, and in that 
case cannot be held in any degree to detract from its value as 
one of the peculiar forms of that region. 
The most numerous examples of this class, are those birds of 
the temperate regions which in winter migrate, either wholly or 
partially, into adjacent warmer countries. This migration most 
likely began subsequent to the Miocene period, during that 
gradual refrigeration of the temperate zones which culminated 
in the glacial epoch, and which still continues in a mitigated 
form. Most of the genera, and many even of the species of 
birds which migrate southwards in winter, have therefore, most 
likely, always been inhabitants of our present Palearctic and 
Nearctic regions ; permanent residents during warm epochs, but 
only able now to maintain their existence by migration in 
winter. Such groups belong truly to the temperate zones, and 
the test of this is the fact of their not having any, or very few, 
representatives, which are permanent residents in the adjacent 
tropical regions. When there are such representative species, we 
do not claim them as peculiar to the Northern regions. Bearing 
in mind these various considerations, it will be found that we have 
been very moderate in our estimate of the number of genera 
Vou. L—14 
