chap, x.] THE PAL.® ARCTIC REGION. 2.H 
usual dividing line of the Palsearctic and Oriental regions. The 
causes of such a phenomenon are not difficult to conceive. Even 
now, that portion of the Palrearctie region between Western 
Asia and Japan is, for the most part, a bleak and inhospitable 
region, abounding in desert plateaus, and with a rigorous climate 
even in its most favoured districts, and can, therefore, support 
but a scanty population of snakes, and of such groups of 
insects as require flowers, forests, or a considerable period of 
warm summer weather ; and it is precisely these which are 
represented in Japan and North China by tropical forms. We 
must also consider, that during the Glacial epoch this whole 
region would have become still less productive, and that, as the 
southern limit of the ice retired northward, it would be followed 
up by many tropical forms along with such as had been driven 
south by its advance, and had survived to return to their 
northern homes. 
It is also evident that Japan has a more equable and probably 
moister climate than the opposite shores of China, and has also 
a very different geological character, being rocky and broken, 
often volcanic, and supporting a rich, varied, and peculiar vege- 
tation. It would thus be well adapted to support all the more 
hardy denizens of the tropics which might at various times 
reach it, while it might not be so well adapted for the more 
boreal forms from Mongolia or Siberia. The fact that a mixture 
of such forms occurs there, is then, Httle to be wondered at, but 
we may rather marvel that they are not more predominant, and 
that even in the extreme south, the most abundant forms of 
mammal, bird, and insect, are modifications of familiar Pahearctic 
types. The fact clearly indicates that the former land con- 
nections of Japan with the continent have been in a northerly 
rather than in a southerly direction, and that the tropical immi- 
grants have had difficulties to contend with, and have found the 
land already fairly stocked with northern aborigines in almost 
every class and order of animals. 
General Conclusions as to the Fauna of the P aloe arctic Re- 
gion . — From the account that has now been given of the fauna 
