Mexican Amphibians and Reptiles. 691 
plateau and were divided into an Atlantic and a Pacific branch, which 
eversince, strangers in a new country, have remained separated from 
each other and are now more and more differentiating. A few, com- 
paratively few, of these Southerners have climbed onto the plateau 
and have found a footing there, whilst many more Northerners have 
descended from it into the Tierra caliente, some by the Atlantic, 
others by the steeper Pacific slope, and these also, separated as their 
descendants are, demonstrate the old maxim that Si duo faciunt 
idem non est idem. 
But there is a third element, neither Neotropical nor Nearctiec, 
namely the old indigenous Sonoran stock, an assembly difficult to 
discern and to handle. We shall not be far out, if we assume 
1) that an older Sonoraland extended much further West, including 
not only what is now the narrow peninsula of Lower California at 
a time when Mexico to the East of the Western Sierra Madre 
was not yet in existence, but also land of which the Tres Marias 
and the Revilla Gigedo Islands are remnants. 2) That most of the 
present Nearctic fauna and flora are compounds of those of the old 
Sonoraland, and the Eastern Appalachian land-complex, with further 
Northern additions. 3) "That whatever the extent of old Sonoraland, 
it must have had a coast, therefore comprising some Lowland con- 
ditions, probably tropical; it must also have been standing out very 
high because so much of the present Mexico to the East of it is 
covered to great depths with continental debris which must have 
come from somewhere in the West. In fact the whole plateau 
between the Eastern and Western Sierras Madres has a deep bottom 
of. cretaceous Limestone, like the Eastern Sierra itself, and this 
immense depression has been filled and levelled up with comminuted 
debris, burying all but the outstanding tops of isolated mesas, hills 
or whole ridges of mostly marine origin, which nearly everywhere 
form the many-tinted far off boundaries of otherwise seemingly. 
interminable plains. 4) That this Sonoraland may have been connected 
with South America, but this point we need not argue out; suffice 
it to say that a land-connexion existed between North and South 
America within the eretaceous period, and that it is equally certain 
that this connexion did not pass over the present bulk of Mexico. 
The following investigations are to a great extent a continuation 
of those published in Proc. zool. Soc. 1905. There about 250 species 
of Mexican Amphibia and Reptiles were dealt with, because I was 
