Introduction 



The more specialized a race of animals and the more per- 

 fectly it Is adapted to life in a given haunt, the more dependent 

 is its distribution on the physical features of a continent. The 

 Salientia, though members of a relatively generalized Class, are 

 in several respects most highly specialized; and they are illus- 

 trations of such perfect adaptation that they are helpless in the 

 face of any condition radically different from those of the chosen 

 environment. 



For instance, they are cold-blooded, and so cannot live in 

 extreme northern latitudes or in the highest altitudes, because of 

 the attendant decrease in metabolism with lowered tempera- 

 ture. (See p. 15.) To a large extent, they breathe through the 

 skin and absorb water through the skin, hence they must live 

 in moist situations. They have not endurance enough to mi- 

 grate even short distances over dry regions. Again, no Batra- 

 chian can live in salt water,^ and not only is salt poisonous to 

 them, but lime is, also. Therefore they can neither live in lime- 

 stone regions or in those impregnated with salt, nor can they 

 cross such regions. 



Thus the North American Salientia must be dependent on 

 the physical features of the North American continent. We 

 shall not expect to find them north of a line of 0° C, annual mean 

 temperature, because just north of this the ground remains frozen 

 the year through, except at the surface. We shall not, more- 

 over, expect to find as many species for some distance south of 

 this limit as there are in the southern part of the continent. We 

 shall not look for them in the bordering salt marshes, gulfs, and 

 bays, nor in Salt Lake nor on the salt plains of the West. There 

 must be relatively few of them on the dry Western plains, in the 

 Rocky Mountains, and in the desert and plateau regions of Cali- 

 fornia, Nevada, Utah, Arizona, Mexico, New Mexico, and Texas. 

 Also, the species living in these latter places must be the species 

 least dependent on moisture, namely the Toads; or those that live 

 in deep burrows, never leaving these burrows except during rain 

 (the Spadefoot Toads). We shall not expect to find tree frogs 

 (Hylidae) in treeless though fertile regions, such as the prairies of 

 the Middle West. However, this point is less certain, since the 

 condition is less vital. Moisture and other conditions being right, 

 tree frogs can adapt themselves to life on the ground, and may 



' Certain species can endure wati.' with i% salt; sea water has about 3.a%. 



36 



