DISTRIBUTION OF PLANTS IN SOUTHERN MEXICO. 439 
conclusion does not at all apply о ће distribution of Amphibia and Reptiles, 
two groups which I have submitted to a detailed scrutiny *. 
Tierra caliente with 173 species. Areal density 434 square miles. 
Tierra templada „ 100 ,, » „ 850 , " 
‘Tierra fria » 86 " » » 290 ., " 
Above 10,000 ft. „ 10 " " " 30 ., » 
The total number of species in Southern Mexico is 250. 
The actual number of species decreases from the tropics to the higher 
levels, just as we might expect ; and the fewer species of the cool regions, 
just over half of those in the tropics, being condensed upon one-third of 
space, show naturally a much larger areal density. This is also not sur- 
prising, and it agrees with the plants. But the temperate belt possesses only 
100 species; it is therefore comparatively by far the poorest of the three 
zones. Т have been able to sift the total of 250 species into two categories. 
I. Southern or hot-country species. 
4 5] 
Restricted to hot countries — ............ 44 per cent. 
Ascending into temperate countries ... 16 » 
Ascending into cool zone ............ .. 5 , 
JI. Northern or cool-country species. 
Restricted to cool or cold zone ......... 17 per cent. 
Descending into temperate zone  ...... » 
Descending into hot countries ......... " 
The remaining 6 per cent.—or, allowing for the diffieulty of sorting 
intermediate species, at most 10 per cent., 
restricted to the temperate belt. This surprisingly small number—only 
about 15 to 20 species—does not represent the Sonoran or truly endemic 
stock, but is for the most part composed of Southern ascendants or Northern 
descendants, which, so to speak, have stuck at this level and there have been 
modified into the respective species. 
are species which seem to be 
# The “Distribution of Mexican Amphibians and Reptiles,” Proc. Zool. бое. 1905, 
ii. pp. 191-244 (with tabulated lists and 6 diagrams to llustrate the contours of Mexico at 
different geological ages). 
