The geographical distribution of animals, with a study of the relations ...

70 DISTRIBUTION OF ANIMALS. [part r.must form a separate primary region for them. As a matter ofconvenience the former plan seems the best ; and it is thatwhich is in accordance with our treatment of other intermediatetracts which contain special forms of life. The great desertzone, extending from the Atlantic shores of the Sahara acrossArabia to Central Asia, is a connecting link between the Palsearctic,Ethiopian, and Oriental regions, and contains a numberof " desert " forms wholly or almost wholly restricted to it ; butthe attempt to define it as a separate region would introducedifficulty and confusion. Neither to the " desert " nor to the" arctic " regions could any defined limits, either geographicalor zoological, be placed ; and the attempt to determine whatspecies or genera should be allotted to them would prove aninsoluble problem. The reason perhaps is, that both are essentiallyunstable, to a much greater extent than those great massesof land with more or less defined barriers, which constitute oursix regions.The Arctic Zone has been, within a recent geologicalperiod, both vastly more extensive and vastly lessextensivethan it is at present. At a not distant epoch it extended overhalf of Europe and of North America. At an earlier date itappears to have vanished altogether ; since a luxuriant vegetationof tall deciduous trees and broad-leaved evergreensflourished within ten degrees of the Pole ! The great desertshave not improbably been equally fluctuating; hence neitherthe one nor the other can present that marked individualityin their forms of life, which seems to have arisen only whenextensive tracts of land have retained some considerable stabilityboth of surface and climatal conditions, during periodssufficient for the development and co-adaptation of their severalassemblages of plants and animals.We must also consider that there isno geographical difficultyin dividing the Arctic Zone between the two northern regions.The only debateable lands, Greenland and Iceland, are generallyadmitted to belong respectively to America and Europe.Neither is there any zoological difficulty ; for the land mammaliaand birds are on the whole wonderfully restricted to theirrespective regions even in high latitudes ; and the aquatic forms