DESERTS AND THEIR INHABITANTS 133 



(which, by the way, are by no means true deserts) 

 obtained in that part of our own country at an earlier 

 epoch of its history. From their comparatively isolated 

 position in the zoological system, as well as from their 

 occurrence in the strata referred to, both these desert 

 animals evidently indicate very ancient types, and they 

 accordingly serve to show not only that the semi-desert 

 steppe area formerly had a much greater western extension 

 than at present, but probably also that the existing portion 

 of that area dates from a very remote epoch. Hence 

 they confirm the idea of the early origin of the present 

 deserts of the Old World and their inhabitants. 



It will be gathered from the foregoing that the deserts 

 and steppes of Africa and Asia possess a large number of 

 animals belonging either to species which have no very 

 near living relatives, or to altogether peculiar genera. In 

 the Arizona Desert of the Sonoran area of North America 

 it seems, however, to be the case that its fauna is largely 

 composed of animals much more nearly related to those 

 inhabiting the prairie or forest-lands of the adjacent 

 districts, of which, in many cases at any rate, they con- 

 stitute mere local races distinguished by their paler and 

 more sandy type of coloration. This is well exemplified 

 by the mule-deer, which in the Rocky Mountains is a 

 comparatively dark and richly coloured animal, but be- 

 comes markedly paler on the confines of the Arizona Desert, 

 assuming again a more rich coloration when it reaches the 

 humid extremity df the Californian peninsula. Most of 

 the North American mammals, indeed, acquire similar 

 pale tints as they reach the Arizona desert-tract, and a 

 practised naturalist can pick out with comparative ease 

 the specimens coming from this area from those of the 

 moister districts. 



