Mexican Amphibians and Reptiles. 705 
to change) or more plastic, i. e. more easily changed by new con- 
ditions, than are the Southerners. This seems contradictory! 
The principle implied here may easily become a fruitful source 
of misunderstanding. It has only to be insinuated that accommodation 
or adaptation implies plastieity. So it does, and yet I may adapt 
myself to new environment without changing perceptibly, whilst to 
another person the same shift of environment may mean either death 
or profound, although healthy change. One has a strong constitution, 
the other’s is plastic, and it is not difficult to guess, which of the 
two will in the long run be the more successful immigrant. So far 
as their offspring is concerned, the result is a foregone conclusion. 
The same considerations apply to our Mexican genera. If a Southern 
lot of individuals cannot spread into more Northern climes without 
undergoing constitutional changes, they will in the long run only 
stock the conquered country with species new to it, but these species 
themselves being new the original genus is enlarged. But if the 
Northerners are so strong that they remain what they were, the 
country’s fauna alone is altered by their addition, but not the genus 
which has merely extended its range. Curiously enough this same 
drama is now being enacted physically and politico-economically bet- 
ween Mexicans and Americans. 
The above calculations would be highly satisfactory if they did 
not result in the contradictory dilemma that the Northern group is 
both conservative and progressive. This is a strong indication that 
this group is not homogeneous, but is composed of two perhaps rather 
different elements, namely bona fide Northern forms, which for want 
of a better name are here called Nearctics, and Sonorans. 
There can be but little doubt that the Sonorans as the aboriginal 
genera are from the beginning composed of both Hotcountry and 
Upland forms. Whatever the old Sonoraland, the nucleus of Mexico, 
was like, it must have comprised coasts, therefore a hot zone 
although not necessarily low lands, whilst we know that its eleva- 
tion must have been considerable. But it is only a vague surmise 
that owing to is long North-South extension its original fauna may 
have been divisible into Northern and Southern forms. What can 
we gather about the old Sonoran fauna? 
In the list are mentioned 29 Sonorans, out of a total of 
52 Northerners. 
As Old Sonorans I have treated Doa, the Crotalinae, Heloderma, 
Chirotes, the large genera Unemidophorus and Sceloporus, Phrynosoma 
