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First soil, then plant and animal life, are thus in the last resort wholly dependent for their existence on the amount of rainfall. But in deserts, where rain seldom or never falls (except by accident) the first term in this series is altogether wanting. There can be no rivers, brooks or streams to wash down beds of alluvial deposit from the mountains to the valleys. Denudation (the term, though rather awful, is not an improper one) must therefore take a different turn. Practically speaking, there is no water action; the work is all done by sun and wind. Under these circumstances, the rocks crumble away very slowly by mere exposure into small fragments, which the wind knocks off and blows about the surface, forming sand or dust of them in all convenient hollows. The frequent currents, produced by the heated air that lies upon the basking layer of sand, continually keep the surface agitated, and so blow about the sand and grind one piece against the other till it becomes ever finer and finer. Thus for the most part the hollows or valleys of deserts are filled by plains of bare sand, while their higher portions consist rather of barren, rocky mountains or table-land.

The effect upon whatever animal or vegetable life can manage here and there to survive under such circumstances is very peculiar. Deserts are the most exacting of all known environments, and they compel their inhabitants with profound imperiousness to knuckle under to their prejudices and preconceptions in ten thousand particulars.

To begin with, all the smaller denizens of the desert–whether butterflies, beetles, birds, or lizards–must be quite uniformly isabelline or sand-coloured. This universal determination of the desert-haunting creatures to fall in with the fashion and to harmonise with their surroundings adds considerably to the painfully monotonous effect of desert scenery. A green plant, a blue butterfly, a red and yellow bird, a black or bronze-coloured beetle or lizard would improve the artistic aspect of the desert not a little. But no; the animals will hear nothing of such gaudy hues; with Quaker uniformity they will clothe themselves in dove-colour; they will all wear a sandy pepper-and-salt with as great unanimity as the ladies of the Court (on receipt of orders) wear Court mourning for the late lamented King of the Tongataboo Islands.

In reality, this universal sombre tint of desert animals is a beautiful example of the imperious working of our modern Deus ex machina, natural selection. The more uniform in hue is the environment of any particular region, the more uniform in hue must be all its inhabitants. In the arctic snows, for example, we find this principle pushed to its furthest logical conclusion. There, everything is and must be white–hares, foxes, and ptarmigans alike; and the reason is obvious–there can be no exception. Any brown or black or reddish animal who ventured north would at once render himself unpleasantly conspicuous in the midst of the uniform arctic whiteness. If he were a brown hare, for example, the foxes and bears and birds of prey of the district would spot him at once on the white fields, and pounce down upon him forthwith on his first appearance. That hare would leave no similar descendants to continue the race of brown hares in arctic regions after him. Or, suppose, on the other hand, it were a brown fox who invaded the domain of eternal snow. All the hares and ptarmigans of his new district would behold him coming from afar and keep well out of his way, while he, poor creature, would never be able to spot them at all among the white snow-fields. He would starve for want of prey, at the very time when the white fox, his neighbour, was stealing unperceived with stealthy tread upon the hares and ptarmigans. In this way, from generation to generation of arctic animals, the blacker or browner have been constantly weeded out, and the greyer and whiter have been constantly encouraged, till now all arctic animals alike are as spotlessly snowy as the snow around them.