Mexican Amphibians and Reptiles. 707 
of those “Northern” species which occeur well within the Tropies and 
they comprise all those “Northerners” which are confined within the 
Hotlands or at least within the first 4000 feet. On the other hand 
they contribute only 2 of the total 7 “Northern” species which are 
restrieted to the cold zone from 7000' upwards. As about one fourth 
of the Sonorans have an amplitude of at least 7000 feet, and since 
they comprise the two species of Sceloporus which have the largest 
altitudinal range on record the Sonorans are certainly accommodating; 
but they are also rather plastic, having produced species for the 
hot, temperate and cold zones respectivey. However we must bear 
in mind that the old Sonoran fauna contained most likely some Hot- 
country stock and that this may account for the number of tropical 
forms, so that only tbose have to be reckoned as “modified” species 
which are now restricted to the cold zone. To judge from their 
general distribution and behaviour, the Sonorans are the product of 
a warm and dry climate, of the Pacific type. As aborigines they 
must have been longer in the country and therefore had the best chances 
of adapting themselves to its changes, and two genera, Onemidophorus 
and Sceloporus have indeed become the leading and most characte- 
ristic Mexican reptiles. 
The mean altitude of the 29 Sonoran species is 4400 feet, 
against 5700 of the Nearcties. It may seem rather ridiculous to 
make such calculations, but these results are not at all bad, because they 
place the average level of the Nearctics at the transitional border 
between the conventional Tierra templada and Tierra fria, and that of 
the Sonorans exactly in the middle of the T. templada. The mean 
for the Southerners is 3000 feet which is rather higher than expected, 
but as natives of the Hotlands can extend their range far by ascent 
only, every case of great amplitude must increase the average level 
whilst a species starting form midlevel may have a much greater 
amplitude and yet not affect its average level. 
It stands to reason that the genera and species of a group, 
which like the Southerners have all their affinities with Neotropical 
forms, should, as colonists of Mexico, feel most at home in the 
Hotlands; and these countries are the first which the immigrants would 
have to enter in their northward spreading. It is equally reasonable 
to expect that the Nearctic forms, bred in Northern climes, 
will find a new congenial home on the higher grounds; that, in fact, 
they will compensate the loss of latitude by increased altitude, 
10° of latitude mean, theoretically, a change of 13,5° F (7,5° C), 
