resembling in its hues the fine desert sand mixed with black grain pebbles. Another lizard of the Sahara (Trapelus ægypticus) possesses the same peculiarity in a higher degree. The property of changing color depends on the presence of certain dark cells in the tissue of the skin, called chromatophores or color-bearers, which, contracting, under reflex influences of the nervous system, permit the full display of the ground-color of the animal, or, expanding to a certain extent, overlie it.
The power of changing color also exists in insects, but less commonly. We more frequently find among them varieties which are distinguished by constantly different but always protective colors. Lefevre observed in the Libyan Desert curious praying crickets of the same species as to other marks, which were brown on a brown soil, and a hundred paces away, on white fossil shells and fragments of limestone, were correspondingly white. They resembled the background against which they stood so much that the French naturalist could not detect them except when they moved. They had other peculiarities, among them wings so contracted that they could not fly; a phenomenon which is sometimes met among insects and birds inhabiting large territories and islands where they are but little exposed to pursuit. They have disused flight with advantage, for only a good flier can keep his ground under the conditions that prevail in such places. A weak flier would be taken by the wind and carried off helpless to destruction.
Sand-fowl (Pterocles) are represented by fourteen species. in the Old World, and are spread from the deserts and steppes of central Asia and India through all continental Africa. They visit southern Europe as breeding-birds, crossing the Strait of Gibraltar into the Iberian Peninsula. Their home is never in wooded regions; the more barren, stony, and arid the land, the less the extent of water and swamps, or contrast of mountain and valley, the more agreeable it is to them. In such regions live these modest birds, on the little which the land affords them, often on the sparse halfa grass; yet they can be found in coveys of hundreds, in places where it seems a puzzle how anything can live. Only ability to move speedily from place to place can make this possible. None but accomplished fliers can exist under such circumstances, and then when gathered in large groups. "It is easy for them," says Brehm, who has observed them more closely than any other naturalist, "to execute a flight, before going to sleep, which would appear to us equal to a day's journey or more." At breeding-time the coveys separate into pairs, and live in this state for a considerable period. When the brood is hatched they are still confined to their household duties, and, not being able to roam around, many suffer for want of the food which their narrow domain does
