1905.} AMPHIBIANS AND REPTILES. 241 
which, like most Gerrhonotus, ave now restricted to the higher 
mountains. 
Lastly, a considerable number of Southern species ascend from 
the hot lowlands high up onto mountains which rise isolated, or 
which fringe the plateau. 
Of course it is difficult, perhaps premature, to generalise in this 
respect, and sharp lines cannot be drawn between these categories. 
Not the least cause is the vagueness or doubtful nature of many 
of the reported localities. For instance, Cope had various corre- 
spondents in Mexico, and some of the alleged localities are quite 
impossible. Peters had a good correspondent resident in Puebla 
City, but the specimens which now figure as ‘“‘ Puebla” came 
from anywhere in that State, which has the most perplexing, 
intricate boundaries, and contains altitudes from 3500 to less 
than 100 metres! “ Vera Cruz” is another snare to the unwary. 
Others have bought specimens, even collections, in Mexico City. 
I myself found in a shop at Orizaba several large glass vessels full 
of well-preserved snakes for sale, but I left them alone since 
nobody knew where they came from. Sumichrast lived for many 
years in Tehuantepec and he travelled widely, all over the 
Isthmus and beyond. The town is situated on a plain, about 
100 feet above the not distant sea; within a few hours’ ride are 
mountains, covered with pines, well above the Tierra Caliente, as 
typical of which every specimen labelled “Tehuantepec” is put 
down. 
I shall not, at least in this paper, go into the detail of the 
generic overlapping, a very important question. Suflice it to say, 
that Im many cases the species of a genus are so distributed that 
some are decidedly northern, living on the plateau, typical 
inhabitants of the Tierra Fria; another species lives in the ad- 
joining Tierra Templada, more often on the western than on the 
eastern slopes and descending more or less far into the lowlands ; 
while a third kind is confined to the typical tropical Tierra Caliente. 
Such cases are clearly illustrative of the evolution of species due 
to the prevailing physical conditions, especially when none of 
these species has a wide geographical range. 
Are we justified in calling a certain species ancient because it 
has a wide continuous range? For instance, Zropidonotus ordi- 
natus, Crotalus terrificus. It is rather doubtful, because these 
creatures are so indifferent to climatic conditions. With more 
right we consider those as ancient which have to be very par- 
ticular about their terrain, and which are now scattered, without 
the least chance of communication—as, for instance, Thorius, 
Chirotes, Heloderma, and other slow, or digging, creatures. 
3. Northern and Southern Immigration. 
In the following table the Mexican Amphibia and Reptiles are 
divided into a Northern or Nearctic and a Southern or Neotropical 
mass, according to their presumable ancestral home or cerrttre of 
Proc. Zoot. Soc.—1905, Vor. Il. No. XVI. 16 
