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 hmited scale. We have seen, for example, bow 

 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 



