116 ORIGIN, ETC., OF THE FAUNA. 
from Guatemala and southern Mexico. The Boine are widely distributed in the 
Old World, but the majority are American, and the occurrence in Madagascar of 
species of the ‘Tropical American genera Corallus and Boa is curious; it is evident 
that these genera had formerly a much wider range. 
Many other genera of snakes have a wide distribution; thus the Crotalide have 
several genera common to Asia and America, and, although they extend southwards 
to the La Plata, there are no peculiar Neotropical genera. One species of Crotalus 
ranges from the United States to Argentina. 
Without going into further details, it may be said that, as compared with the Lizards, 
a larger proportion of the Central American genera are found also on the Mexican 
plateau and in the southern United States, although there are, of course, a number of 
neotropical genera that range northwards only to southern Mexico. 
The Amblycephalide are the only family of snakes that are strictly neotropical in 
America; there are three South American genera, one of these, Leptognathus, having 
several Central American species, the northernmost in Tehuantepec. The other two 
genera occur in South-Eastern Asia, and it is probable that the family was formerly 
a northern one. Indeed, it is not unlikely that the whole neotropical Ophidian fauna 
has been derived from immigrants from the north that did not begin to reach South 
America until the Miocene connection was established. ‘The distribution of the vast 
family Colubride seems to favour this view. 
REPTILIA, BATRACHIA, anp PISCES. 
Summary.—The mountains that fringe the Mexican Plateau form the boundary 
between the Neotropical and Nearctic Regions. True fresh-water fishes can neither 
cross seas nor travel overland from one river to another ; consequently their dispersal 
has been slow. ‘The nearctic and neotropical fishes are quite different, and belong to 
different families; the two faune scarcely overlap; in the Lerma System, on the 
southern part of the Mexican plateau, there are no neotropical fishes, the nearctic 
fishes are mostly generically distinct from those of the Rio Grande, and two endemic 
groups that are neither nearctic nor neotropical are the most important elements. 
For most Batrachians the sea is an impassable barrier, but they can migrate over- 
land and their dispersal may have been rapid. The nearctic and neotropical 
Batrachians are mostly distinct, but overlap to a considerable extent; a few genera 
(Bufo, Hyla) range throughout both regions. The rapidity of dispersal as compared 
with the fishes is exemplified by the holarctic group Urodela; one genus, Spelerpes, 
has penetrated far into South America, whereas only one nearctic fish, the ancient 
Lepidosteus, has reached Panama. The spreading of Amblystoma tigrinum. of the 
United States southwards to the border of the Mexican plateau is in striking contrast to 
the peculiarity of the fish-fauna of the southern half of the plateau, where no species 
