212 ZOOGEOGRAPHICAL REGIONS 
exists between the animals, at least of the northern 
regions. We have already pointed out that the tundra 
animals of the east and west show very marked resem- 
blances to one another; that similarly there is con- 
siderable resemblance as regards both the plants and 
animals of the belt of coniferous forest in both hemi- 
spheres, and that only as we travel southwards does 
differentiation appear. There is much evidence, both 
geological and zoological, to show that not long ago there 
was free land communication between the Old and New 
Worlds in the northern hemispheres, across what is 
now the Bering Strait, as well as possibly across parts 
of the North Atlantic. Further, though parts of what 
are now the continents of Africa and of India were, 
in early Tertiary times, probably separated by stretches 
of sea from the land-masses to the north, and though 
they are now functionally separated from the great 
land-mass in the eastern temperate and frigid zones 
by belts of deserts and by transverse mountain-chains, 
yet, before the progressive formation of these belts of 
desert, there was apparently a time when it was 
physically possible for land animals to spread from 
the far north of Europe and Asia to the far south of 
Africa and India, and south-eastwards to parts of 
the Malay Archipelago. With this great eastern land- 
mass North America, as already suggested, was in 
free communication, though it seems to have been leng 
separated from South America. 
We thus have to consider as forming one great 
REALM of the earth’s surface the whole Eurasian con- 
tinent with parts of the Malay Archipelago, the con- 
tinent of Africa with the isolated region of Madagascar, - 
and North America. In this great land-mass evolu- 
tion, as we would expect from its great extent, has been 
