The geographical distribution of animals, with a study of the relations ...

chap, xii.] THE ORIENTAL EEGION. 363causes of this change were of two kinds. There was a greatgeographical and physical revolution effected by the elevationof the Himalayas and the Thibetan plateau, and, probably atthe same time, the northward extension of the great Siberianplains. This alone would produce an enormous change ofclimate in all the extra-tropical part of Asia, and inevitablylead to a segregation of the old fauna into tropical and temperate,and a modification of the latter so as to enable it tosupport a climate far more severe than it had previously known.But it is almost certain that, concurrently with this, there wasa change going on of acosmical nature, leading to an alterationof the climate of the northern hemisphere from equable toextreme, and culminating in that period of excessive cold whichdrove the last remnants of the old sub-tropical fauna beyondthe limits of the Palaearctic region. From that time, the Orientaland the Ethiopian regions alone contained the descendants of manyof the most remarkable types which had previously flourishedover all Europe and Asia ; but the early history of these tworegions, and the peculiar equatorial types developed in each,sufficiently separate them, as we have already shown. TheMalayan sub-region is tLat in which characteristic Orientaltypes are now best developed, and where the fundamental contrastof the Oriental, as compared with the Ethiopian andPaleearctic regions, is most distinctly visible.