70 DISTRIBUTION 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 Pale- 
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. Ata 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 
flourished 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 
