160 The Geographical Distribution of Animals. [ March, 
that the Palearctic region of early Tertiary times was, for the 
most part, situated beyond the tropics, although it probably had 
a greater southward extension than at the present time. It cer 
tainly included much of North Africa, and perhaps reached far 
into what is now the Sahara, while a southward extension of its 
central mass may have included the Abyssinian highlands, where 
some truly Palearctic forms are still found. This is rendered 
probable by the fossils of Perim Island a little farther east, which 
show that the characteristic Miocene fauna of South Europe and 
North India prevailed so far within the tropics. There existed, 
however, at the extreme eastern and western limits of the region, 
two extensive equatorial land areas, our Indo-Malayan and West 
African sub-regions, both of which must have been united for 
more or less considerable periods with the northern continent. 
They would then have received from it such of the higher verte- 
brates as were best adapted for the peculiar climatal and organic 
conditions which everywhere prevail near the equator ; and these 
would be preserved, under variously modified forms, when they 
had ceased to exist in the less favorable and constantly deteriorat- 
ing climate of the north. At later epochs, both these equato- 
rial lands became united to some part of the great South African 
continent (then including Madagascar), and we thus have ex- 
plained many of the similarities presented by the faunas of these 
distant and generally very different countries. 
During the Miocene period, when a subtropical climate pre- 
vailed over much of Europe and Central Asia, there would be no 
such marked contrast as now prevails between temperate and 
- tropical zones; and at this time much of our Oriental region, 
perhaps, formed a hardly separable portion of the great Palæarc- 
tic land. But when, from unknown causes, the climate of Eu- 
rope became less genial, and when the elevation of the Hima- 
layan chain and the Mongolian plateau caused an abrupt differ- 
ence of climate on the northern and southern sides of that great 
mountain barrier, a tropical and a temperate region were neces- 
- sarily formed ; and many of the animals which once roamed over 
the greater part of the older and more extensive region now be- 
came restricted to its southern or northern divisions, respectively. 
Then came the great change we have already described (vol. 1. 
p. 288), opening the newly formed plains of Central Africa to 
the incursions of the higher forms of Europe, and following on 
this, a still further deterioration of climate, resulting in that 
marked contrast between temperate and tropical faunas, which 18 
