DESERT ANIMALS. Men are apt to think of the vast tracts of absolutely treeless arid sand as uninhabited, because they are void of the. creatures known to the regions where humanity dwells. Yet these tracts are often teeming with life. On the shadowless expanse which affords no lurking-place, animals, adapt ing themselves to the exigencies of their life, often assume the pale tints of the sands whereon, by lying motionless, they may be over looked by their enemies. They not only de velop a protective coloring, but acquire certain other capabilities. They learn, for instance, to subsist on a minimum of water, or to store it within their bodies, some indeed, develop ing an ability to live altogether without direct water-supply. Many desert animals are said to aestivate, that is, to sleep throughout the summer, as animals of cold climate hibernate by lying dormant through the winter season. The desert snail, in order to protect itself from dessication, builds up a wall of mucus, some times with two or three layers, across the open ing of its shell to prevent evaporation of its moisture during the extremest heats. Small desert animals, like plain-dwellers, are burrow ers, not only because they can thus escape the fierce rays of the sun, but also because they are so protected against their enemies. Even some serpents burrow, and these are more virulently poisonous than corresponding species of a dif ferent habit. All the desert creatures, from the
snakes and lizards to the camels, are provided muscularly with the ability to shut out from their nostrils and eyes the sand that is blown by the powerful winds; and most of the insects (except the locusts) are practically wingless, so that these strong desert currents do not carry them away.
The large desert animals are swift, and their feet are adapted for the hot rocks and sand by being cushioned beneath with callous skin. The swiftness of these animals is indispensable to their preservation; for they must generally es cape their pursuers by flight, since their habitat affords no hiding-places; moreover, they have to travel long distances for both food and water. Examples of this may be found in the camel and ostrich (qq.v.).
The desert is undoubtedly the refuge of cer tain animals which have been driven by competi tion from the more desirable habitations, and which, having located in arid land, have adapted themselves to their unfavorable environment. That they are the unsuccessful and outcast rep resentatives of species living under better condi tions is substantiated by the fact that they are almost always closely related to the forms in the fauna of the green, fertile lands beyond the desert; the differences usually being only the changes necessitated by difference of habitat.