1905. ] AMPHIBIANS AND REPTILES, 239 
plateau, which by climate and every other physical feature is a 
direct continuation of the more northern countries. Hence the 
imperceptible change from Arizona, New Mexico, and Texas 
southwards. The political frontier between Mexico and the 
United States is no boundary whatever for our purposes. 
For northern animals and plants the drier climate, not so much 
the annual mean temperature, of the plateau suggests this as a 
natural limit, but not a few northern forms, even the same species, 
have adapted themselves to life in the hot lowlands and have 
extended their range far south, even into South America. With 
the original natives of the latter continent, conditions are different. 
They could spread easily through Central America, but arrived in 
South Mexico the wedge of the plateau divides them into an 
Atlantic and a Pacific mass. They can go a long way north, and 
are still in Tierra Caliente, like the countries whence they came. 
But a sifting takes place. The Atlantic lowlands are hot and 
moist, whilst the Pacific slopes and much narrower lowlands are 
hot and rather dry, the dryness increasing rapidly towards the 
north. To people such divergent countries implies a severe sifting 
of the immigrants, or the necessity of changing, by adaptation to, 
or by, the new surroundings. 
This is well illustrated by the gradual change, from species to 
species, of essentially northern into slightly less northern, into 
almost tropical forms of the same genus; or, since a genus is 
most cases an imaginary abstract, of the same group of closely 
allied creatures. Still further south that particular genus comes 
in most cases to an end, ‘There may be a species or two which 
form outposts, straggling on, perhaps in actual process of successful 
adaptation ; however, after all the genus has found its mit. But 
it is there not met by the outposts of the southerners; they in 
their turn stand much further north. If it were otherwise, there 
would be a real boundary line, with a kind of neutral zone between 
North and South, and this neutral zone should contain compara- 
tively few species and genera. Emphatically this is not the case. 
Thé two faunas overlap broadly ; they commingle, except on the 
plateau, which seems to be a much more effective barrier to the 
southerners than is the descent from the plateau into the hot 
lowlands to the northern creatures. It seems to be easier for 
xerophile northern genera, and even species, to go south and to 
adapt themselves to life in a more equably hot and decidedly 
moister country with luxurious vegetation, than for hygrophile 
southerners to do the reverse. 
Be it noted, however, that this applies only to those terrestrial 
northerners which can adapt themselves to arboreal life; rattle- 
snakes cannot doit. Speaking broadly, xerophiles are essentially 
humivagous; hygrophiles either live on the ground which is 
rich in humus, grass, or herbaceous tangle and underwood, or 
they are arboreal. 
A favourite way of adaptation is arboreal life, whereby the 
xerophiles escape inundations, accumulation of humus, debris, 
