94 



also true of the lizards of tbe genera Plirynosoma^ HolbrooJcia, aud See- 

 loporus. Uutaenia and Sceloporus become metallic in tbe Mexican sub- 

 region, as is also the case with the Anoles. The North American species 

 of AnoUs does not display metallic luster, while a large part of those of 

 Mexico and a smaller proportion of those of the West Indies exhibit it. 



Another important influence in the modification of the life in ques- 

 tion is the amount of terrestrial and atmospheric moisture. In the 

 case of the Batrachia, this agent is as important as that of heat, since 

 a greater or less part of their life is, in most species, necessarily spent 

 in the water. The reptiles are less dependent on it, but, as their food 

 consists largely of insects, and as these in turn depend on vegetation 

 for sustenance, the modifying influence of moisture on their habits must 

 be very great. 



The Central region combines the disadvantages of low tempera- 

 ture, due to its elevation above the level of the sea, and of arid atmos- 

 phere ; hence its poverty in Batrachia and EeptiUa. There are but nine 

 species of both classes peculiar to it, while a few others enter from sur- 

 rounding areas. 



The distribution in the other regions is evidently dependent on the 

 same conditions. Thus the well-watered, forest-covered Eastern and 

 Austroriparian regions are the home of the salamanders, the frogs, 

 the tree-toads, and the turtles. The dry and often barren Sonoran and 

 Central regions abound in the lizards and the toads. The Pacific re- 

 gion, which is intermediate in climatic character, exhibits a combination 

 of the two types of life: it unites an abundant lizard-fauna with 

 numerous frogs and salamanders, while there is but one tortoise. 



Another character of the reptilian life of arid regions is to be seen 

 in a peculiarity of coloration. This, which has been already observed 

 by the ornithologists, consists of a pallor, or arenaceous hue of the 

 body, nearly corresponding with the tints of dry or sandy earth. This 

 prevails throughout the Batrachia and Keptilia of the Sonoran re- 

 gion, although it is often relieved by markings of brilliant color, of 

 which red is much the most usual. This peculiaiity doubtless results 

 immediately from the power of metachrosis, or color-change, possessed 

 by all cold-blooded Vertebrata, by means of which they readily assume 

 the color of the body on which they rest. That a prevalent color of 

 such bodies should lead to a habit of preference for that color is neces- 

 sary, and as such habits become automatic, the permanence of the color 

 is naturally established. 



