INTRODUCTION. 643 



tion. Another important factor, to which Prof. Cope* has directed atten- 

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

 Amphibians, which spend nearly or all their life in water, and the 

 aquatic turtles and serpents, the dependence of the species upon this 

 element for distribution is sufficiently manifest. The well- watered eastern 

 border and the Mississippi Valley are the homes of the aquatic Reptilian 

 and Amphibian life, tvhile the dry and almost barren region from Mexico 

 to Arizona and Nevada is characterized by the predominance of Lizards, 

 Toads, and Snakes with an extraordinary development of the rostral 

 shield. The latter characteristic, seen in our Hog- nose Snake, probably 

 is in some way useful to the animal in removing the sand in which it 

 either burrows for concealment or seeks for food. A peculiar foot struc- 

 ture, or movable spines on the side of the leg, may find a similar explana- 

 tion, while the prolongation of the nostrils forward in our Trionychidx, 

 or Soft- shelled Turtles, is a character adapted to their habits of life, they 

 living buried in mud, and only bringing this proboscis to the surface to 

 accomplish the work of respiration. 



In a similar manner may be traced a relation between the powers of 

 endurance of these animals and the extent of their distribution. Thus 

 Amphibians will endure more cold than the Ophidians, and hence ex- 

 tend farther northward. In the writer's, and, so far as his knowledge 

 goes, other's efforts to keep serpents over the winter, they, if once frozen 

 stiff, invariably failed to resuscitate, but a frog, even when taken out of 

 the ice and gradually thawed, comes forth to an apparently new life. 

 The modes of progression, serpents being limbless, the scarcity or abun- 

 dance of food, the enemies of a species, and the method of reproduction, 

 have important bearings. It cannot be expected that snakes which pro- 

 pagate only when several years old, oviposit usually in the hotter parts 

 of summer, and then lay only a few eggs, should compete with the more 

 enduring frogs and toads, which have such a numerous progeny. Owing 

 to such causes one would expect what is actually found to be the case, 

 that the Amphibians are far more abundantly distributed over the earth 

 than are the Ophidians. 



In the Western Continent, Dr, Giinther has shown that we have two 

 apparently distinct creations, the one radiating from the Valley of the 

 Amazon; the other from that of the Mississippi. That these faunas 

 meet and mingle along Nerthern Mexico, Western Texas, Arizona, and 

 Nevada, is a fact abundantly attested. To these might perhaps be added 

 the mixed life of the Pacific region, and that radiating from the Mississippi 



*Proc. Am. Ass. Adv. Soi., 1875, B. p. 197. 



