70 DISTKIBUTION OF ANIMALS. [part i. 



must form a separate primary region for them. As a matter of 

 convenience the former plan seems the best ; and it is that 

 which is in accordance with our treatment of other intermediate 

 tracts which contain special forms of life. The great desert 

 zone, extending from the Atlantic shores of the Sahara across 

 Arabia to Central Asia, is a connecting link between the Palse- 

 arctic, Ethiopian, and Oriental regions, and contains a number 

 of " desert " forms wholly or almost wholly restricted to it ; but 

 the attempt to define it as a separate region would introduce 

 difficulty and confusion. Neither to the " desert " nor to the 

 " arctic " regions could any defined limits, either geographical 

 or zoological, be placed ; and the attempt to determine what 

 species or genera should be allotted to them would prove an 

 insoluble problem. The reason perhaps is, that both are essen- 

 tially unstable, to a much greater extent than those great masses 

 of land with more or less defined barriers, which constitute our 

 six regions. The Arctic Zone has been, within a recent geologi- 

 cal period, both vastly more extensive and vastly less extensive 

 than it is at present. At a not distant epoch it extended over 

 half of Europe and of North America. At an earlier date it 

 appears to have vanished altogether ; since a luxuriant vegeta- 

 tion of tall deciduous trees and broad-leaved evergreens 

 nourished within ten degrees of the Pole ! The great deserts 

 have not improbably been equally fluctuating; hence neither 

 the one nor the other can present that marked individuality 

 in their forms of life, which seems to have arisen only when 

 extensive tracts of land have retained some considerable sta- 

 bility both of surface and climatal conditions, during periods 

 sufficient for the development and co-adaptation of their several 

 assemblages of plants and animals. 



We must also consider that there is no geographical difficulty 

 in dividing the Arctic Zone between the two. northern regions. 

 The only debateable lands, Greenland and Iceland, are generally 

 admitted to belong respectively to America and Europe. 

 Neither is there any zoological difficulty ; for the land mam- 

 malia and birds are on the whole wonderfully restricted to their 

 respective regions even in high latitudes ; and the aquatic forms 



