CHAP. XII. ] THE ORIENTAL REGION. 363 
causes of this change were of two kinds. There was a great 
geographical and physical revolution effected by the elevation 
of the Himalayas and the Thibetan plateau, and, probably at 
the same time, the northward extension of the great Siberian 
plains. This alone would produce an enormous change of 
climate in all the extra-tropical part of Asia, and inevitably 
lead to a segregation of the old fauna into tropical and tem- 
perate, and a modification of the latter so as to enable it to 
support a climate far more severe than it had previously known. 
But it is almost certain that, concurrently with this, there was 
a change going on of a cosmical nature, leading to an alteration 
of the climate of the northern hemisphere from equable to 
extreme, and culminating in that period of excessive cold which 
drove the last remnants of the old sub-tropical fauna beyond 
the limits of the Palearctic region. From that time, the Oriental 
and the Ethiopian regions alone contained the descendants of many 
of the most remarkable types which had previously flourished 
over all Europe and Asia; but the early history of these two 
regions, and the peculiar equatorial types developed in each, 
sufficiently separate them, as we have already shown. The 
Malayan sub-region is that in which characteristic Oriental 
types are now best developed, and where the fundamental con- 
trast of the Oriental, as compared with the Ethiopian and 
Palearctic regions, is most distinctly visible. 
