1905.] AMPHIBIANS AND REPTILES. 239 



plateau, which by climate and every other physical feature is a 

 direct continuation of the more northern covmtries. 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 diner 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 Tieri'a 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 siun^oundings. 



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 in 

 most cases an imaginai-y 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 limit, 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. 

 The two faunas overlap broadly ; they commingle, except on the 

 plateau, which seems to be a much more eflfective 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, aiid 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 do it. Speaking broadly, xerophiles are essentially 

 humivagous ; hygrophiles either live on the gi'ound 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, 



