INTRODUCTION. 643 
tion. Another important factor, towhich 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, while 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 Trionychide, 
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 wouldexpect 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 Northern 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. Sci., 1875, B. p. 197. 
