Snakes vary greatly in their habits: there are snakes which live on the ground, hiding under stones, leaves, or brush, and in rock crevices; other snakes live chiefly in the water, making their holes in the bank below water-mark; others live in trees, and some others almost entirely underground. To this tribe surrounded by enemies, nature has given abundant means of self-preservation,--swiftness of motion; instinct of instant flight; poison, sometimes deadly to its enemies; vast size and strength, as in some boas and pythons, and especially colorations, which closely imitate their surroundings. Thus ground snakes have, especially on the back and sides, brown, gray, black, and white in spots and markings, which together look much like earth, twigs, pebbles, and dead leaves, so that the snake can scarcely be distinguished from its bed or home. Tree snakes, and snakes that live in grass or mosses, are usually green, marbled, or streaked with black and brown. Snakes from the marshes or muddy banks are nearly black; among the abundant flowers and gayly colored shrubs of the tropics, snakes take very brilliant hues. Snakes of temperate climates are also often tricked out with scarlet, or white or orange stripes and bands. Snakes are largest, most numerous, and most brightly colored in hot countries. Like all the cold-blooded vertebrates, they delight in hot sunshine, and will lie motionless, basking for hours. In taking a position for this sun-bath, they select a place where they will not be conspicuous. I have often noticed that while a gray snake will stretch itself out in the sun on a bare rock, a green snake never chooses such a post. Instead, the green snake extends itself on a bed of moss, or close, short grass, or if slim and light of body, will crawl up the midrib of a big leaf, or the stalk of a fern frond, where its color is scarcely discernible from the vegetation about it. A dark brown or black snake seems to know that it is tolerably secure lying upon a fallen, decaying tree-trunk. So a gray, or black and white snake, may often be seen sunning himself on a fence-rail, or on the top of a stone wall. There seems to be a common notion that because a creature is a snake, it must be dangerous and poisonous. This is quite wrong, for in point of fact, very few of the numerous serpent families are venomous. The greater number of their species are quite harmless. One of the most dangerous of all snakes, the boa-constrictor, does not kill its prey by poisoning, but by crushing it. In general, the head of the harmless snake is rounded and oblong, while the poisonous species have heads flat and triangular.