132 MOSTLY MAMMALS 
modified as regards coloration for the exigencies of a purely 
desert existence, and as it is also structurally very different 
from all its existing kindred, it must clearly be looked upon 
as a very ancient type, which commenced its adaptation to 
the surroundings of the Sahara ages and ages ago. The 
Arabian desert is the home of another species of oryx 
(O. beatrix), which, although more nearly allied to the 
East African beisa, is a much smaller and paler-coloured 
creature. In this case also there would seem little doubt 
that the period when this animal first took to a purely desert 
existence must have been extremely remote. 
But an even more striking instance is afforded by 
another antelope remotely connected with the gemsbok, 
which is an inhabitant of the Sahara and the Arabian 
desert, and is commonly known as the addax. It is an 
isolated creature, with no near relation in the wide world, 
and is easily recognised by its dirty white colour, shaggy 
mane, and long twisted horns. It must have branched off 
at a very remote epoch from the gemsbok stock, and 
affords almost conclusive evidence of the antiquity of the 
deserts it inhabits, as we have no evidence of the occurrence 
of allied extinct species in other countries. 
Some degree of caution is, however, necessary in drawing 
conclusions that all isolated desert animals have been 
evolved in the precise districts they now inhabit. A case 
in point is afforded by the saiga, a pale-coloured antelope 
without any very near kindred, inhabiting the steppes of 
Eastern Russia and certain parts of Siberia, where it is 
accompanied by the hopping Kirghiz jerboa (Alactaga). 
Now, since fossilised remains of both these very peculiar 
animals have been discovered in the superficial deposits of 
the south-eastern counties of England, it is a fair inference 
that physical conditions similar to those of the steppes 
