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Show 1 9 0 5 .] AMPHIBIANS AND REPTILES, 2 3 9 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 in 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 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 comparatively 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 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 ; rattlesnakes cannot do it. Speaking broadly, xeropliiles 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, |
