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 bro