162 The Geographical Distribution of Animals. — [March, 
secondly, of the existing radical diversity of the Australian 
region from the rest of the eastern hemisphere. 
Owing to the much greater extent of the old Palzaretic region 
(including our Oriental) and the greater diversity of Mammalia 
it appears to have produced, we can have little doubt that here 
was the earliest seat of the development of the vertebrate type, 
and probably of the higher forms of insects and land-mollusks. 
Whether the Nearctic region ever formed one mass with it, or 
gnly received successive immigrations from it by northern land 
connections both in an easterly and westerly direction, we can- 
not decide; but the latter seems the most probable supposition. 
In any case, we must concede the first rank to the Palearctic 
and Oriental regions, as representing the most important part of 
what seems always to have been the great continent of the 
earth, and the source from which all the other regions were sup- 
plied with the higher forms of life.. These once formed a single 
great region which has been since divided into a temperate 
and a tropical portion, now sufficiently distinct, while the Neare- 
tic region has, by deterioration of climate, suffered a considera- 
ble diminution of productive area, and has in consequence lost a 
number of its more remarkable forms. The two temperate 
regions have thus come to resemble each other, more than they 
once did, while the Oriental retains more of the zodlogical aspect 
of the great northern regions of Miocene times. The Ethiopian 
form having been once an insular region, where lower types of 
vertebrates alone prevailed, has been so overrun with higher 
types from the old Palearctic and Oriental lands that it now 
rivals, or even surpasses, the Oriental region in its representation 
of the ancient fauna of the great northern continent. Both of 
our tropical regions of the eastern hemisphere possess faunas 
which are to some extent composite, being made up in different 
proportions of the productions of the northern and southern con- 
tinents, — the former prevailing largely in the Oriental, while 
the latter constitutes an important feature in the Ethiopian 
fauna. The Neotropical region has probably undergone great 
fluctuations in early times; but it was, undoubtedly, for long 
periods completely isolated, and there developed the Edentate 
type of mammals and the Formicaroid type of passerine birds 
into a variety of forms, comparable with the diversified marsu- 
pials of Australia, and typical Passeres of the eastern hemi- 
sphere. It has, however, received successive infusions of higher 
types from the north, which now mingle in various degrees with — i 
