93 



Tropidonotus valid us cclaeno. 

 Oharina bottae. 

 Lichanura trivirgata. 

 Lichanura myriolepis. 

 Licbanura roseofusca. 



Lacertilia. 



Pbyllodactylus unctus. 

 Pbyllodactylus xauti. 

 Cnemidopborus maximus. 

 Verticaria hyperythra. 

 Calbsaurus dracontoides. 

 CTta tbalassina. 

 Uta nigricauda. 

 Sceloporus clarkii zosteroinus. 

 Phrynosoma coronatum. 

 Cj'clura beniilopba. 



XI. — RELATION OF DISTRIBUTION TO PHYSICAL CAUSES. 



The first observation with regard to the Batrachian and Reptilian 

 fauna of North America is the usual one, viz, that the number of spe- 

 cific and generic types exhibits a rapid increase as we approach ♦^^he 

 tropics. Of the area inhabited by these forms of animals, less than one- 

 fourth is included in the three Southern regions — the Austroriparian, 

 the Sonoran, and the Lower Californian ; yet these contain more than 

 half of the entire number of species, and all but eight of the genera are 

 found in them. Of this number, forty-two genera, or one-third of the 

 total, is confined to within their boundaries. It is a truism directly 

 resulting from the very small production of animal heat by these ani- 

 mals, that temperature, and therefore latitude, lias the greatest influ- 

 ence on their life and distribution. This is exhibited in other ways than 

 in multiplication of forms. It is well known, that although plainly-col- 

 ored reptiles are not wanting in the tropics, brilliantly-colored species 

 are much more abundant there than in temperate regions. Although 

 the Eegnum Nearcticum does not extend into the tropics, its south- 

 ern districts are the habitat of most of the species characterized 

 by bright colors. This is most instructively seen in species having a 

 wide range. Such is the case with the southern subspecies of Besmatog- 

 nathus among salamanders, and Ryla among frogs. So with snakes of the 

 genera Crotalus, Caudisona, Ophibolus, Bascanium, and Eutaeniu. It is 



