250 THE POPULAR SCIENCE MONTHLY. 



resembling in its hues the fine desert sand mixed with black grain- 

 pebbles. Another lizard of the Sahara (Trapelus cegypticus) pos- 

 sesses 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, ex- 

 panding to a certain extent, OA^erlie 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 con- 

 tracted that they could not fly ; a phenomenon which is sometimes 

 met among insects and birds inhabiting large territories and isl- 

 ands where they are but little exposed to pursuit. They have dis- 

 used 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 de- 

 struction. 



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 south- 

 ern 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 mod- 

 est 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 pos- 

 sible. None but accomplished fliers can exist under such circum- 

 stances, 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 breed- 

 ing-time the coveys separate into pairs, and live in this state for a 

 considerable period. When the brood is hatched they are still con- 

 fined to their household duties, and, not being able to roam around, 

 many suffer for want of the food which their narrow domain does 



