The normal course of events where new land rises above the sea is something like this, as oceanic isles have sufficiently demonstrated.  The rock when it first emerges from the water rises bare and rugged like a sea-cliff; no living thing, animal or vegetable, is harboured anywhere on its naked surface.  In time, however, as rain falls upon its jutting peaks and barren pinnacles, disintegration sets in, or, to speak plainer English, the rock crumbles; and soon streams wash down tiny deposits of sand and mud thus produced into the valleys and hollows of the upheaved area.  At the same time lichens begin to spring in yellow patches upon the bare face of the rock, and feathery ferns, whose spores have been wafted by the wind, or carried by the waves, or borne on the feet of unconscious birds, sprout here and there from the clefts and crannies.  These, as they die and decay, in turn form a thin layer of vegetable mould, the first beginning of a local soil, in which the trusty earthworm (imported in the egg on driftwood or floating weeds) straightway sets to work to burrow, and which he rapidly increases by his constant labour.  On the soil thus deposited, flowering plants and trees can soon root themselves, as fast as seeds, nuts or fruits are wafted to the island by various accidents from surrounding countries.  The new land thrown up by the great eruption of Krakatoa has in this way already clothed itself from head to foot with a luxuriant sheet of ferns, mosses, and other vegetation.

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.