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 of 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. 
