158 
ZOOLOGICAL GEOGRAPHY. 
[PART III. 
from it such of the higher vertebrates as were best adapted for 
the peculiar climatal and organic conditions which everywhere 
prevail near the equator ; and these would he preserved, under 
variously modified forms, when they had ceased to exist in 
the less favourable and constantly deteriorating climate of the 
north. At later epochs, both these equatorial lands became 
united to some part of the great South African continent (then 
including Madagascar), and we thus have explained 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 Palaearctic 
land. But when, from unknown causes, the climate of Europe 
became less genial, and when the elevation of the Himalayan 
chain and the Mongolian plateau caused an abrupt difference of 
climate on the northern and southern sides of that great moun- 
tain barrier, a tropical and a temperate region were necessarily 
formed ; and many of the animals which once roamed over the 
greater part of the older and more extensive region, now became 
restricted to its southern or northern divisions respectively. 
Then came the great change we have already described (vol. i. 
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 is now one 
of the most prominent features in the distribution of animal as 
well as of vegetable forms. 
It is not necessary to go into any further details here, as we 
have already, in our discussion of the origin of the fauna of the 
several regions, pointed out what changes most probably occurred 
in each case. These details are, however, to a great extent 
speculative; and they must remain so till we obtain as much 
knowledge of the extinct faunas and past geological history of 
the southern lands, as we have of those of Europe and North 
