210 ZOOGEOGRAPHICAL REGIONS 
Such isolation of portions of the earth’s surface is 
caused by the existence of barriers to the distribution 
of terrestrial animals. Necessarily these barriers must 
be supposed to have a definite date of origin, and 
this origin must always be supposed to be later in 
time than that of the ancestral stock of the group 
under consideration, but to precede the differentiation 
of the stock into many divisions as the result of the 
isolation. As an example of the origin in time of such 
a barrier let us take a progressive climatic change. 
The animals of temperate Europe and Asia show 
a marked difference from those of tropical Asia and 
of tropical Africa. To some extent this difference is 
doubtless due to the existence of transverse mountain- 
chains, such as the Himalayas and the Andes, but it 
is also largely due to the existence of a great band of 
deserts and wastes across Asia and Northern Africa, for 
both the mountains and the deserts prevent intermixing. 
Now there is much evidence to show that there has 
been a progressive desiccation in this region within 
geologically recent times. One effect has been to 
favour the development of the many steppe and desert 
animals of the region, but another has certainly been 
to separate an originally homogeneous fauna into three 
parts, and by isolating much of Africa on the one hand 
and the peninsula of India with Further India on the 
other, to favour evolution in these two areas, now pro- 
tected from the incursion of northern forms, save on 
a very limited scale. We have seen, for example, how 
scantily represented in temperate Europe and Asia 
are the abundant antelopes of Africa, and similarly 
how the numerous deer of the north are shut out from 
Africa by the, to them, impassable desert. 
To the zoogeographer, however, the greatest barrier 
