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 {Aladaga). 

 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 



