Centuries and centuries passed on, the sea and air gradually changed, the temperature of the elements no longer remained the same, and thus by degrees a new ocean and a new atmosphere were formed, uncongenial to the nature of those huge reptiles. Thus they have been effaced from the roll of living things, and some petrified remains alone bear testimony to their former existence. The most powerful saurians of the present day--the crocodile the gavial and the alligator--have left to the dolphins, the sharks, and other monstrous or swiftly-swimming cetaceans and fishes the dominion of the seas, and now merely infest the rivers and swamps of the tropical zone. The lizards also have long since retired from the scene where they once abounded, and the ocean at present harbours no other reptiles in its bosom than turtles and sea-snakes. Most of the animals belonging to this class are either dangerous or of a disgusting appearance. Few creatures are objects of such universal abhorrence as the crocodile--the very type of brutal cold-blooded ferocity; as the venomous snake--the emblem of perfidy and ingratitude; or as the loathsome, but innocent toad, to which, on account of its ugliness, noxious properties have been ascribed which the poor animal does not possess. The frogs, lizards, and turtles alone seem to have escaped this general detestation, either from their more active habits, or their well-known harmlessness, or their various utility to man. The anatomy of the turtle offers many points of interest; its vertebræ, ribs, and breast-bone growing together so as to form a bony envelope round the whole animal. This harness is covered by the skin, which in its turn is bedecked with large scales, while all the muscles and other soft parts are enclosed in the inner cavity. Only the head, feet, and tail protrude through openings between the upper and under carapace, and these can, by the land tortoises at least, be withdrawn entirely under the former. This is the only protection which Nature has afforded these animals against their enemies, for they have neither swiftness of flight, nor any offensive weapon at their command. But as soon as anything suspicious approaches, they conceal themselves under their massive cover, and oppose to every attack by tooth or nail the passive resistance of an impenetrable shield. Most of their enemies find it, besides, no easy task to turn them on their back, as many species attain a very considerable weight, so that their mere bulk constitutes a good defence. It might be supposed that this protection could only avail for a short time, as the want of air must soon force the animal to stretch its head out of its hiding-place, and this indeed would be the case, if kind Nature had not taken her measures against this emergency, by giving the creature a _cold_ blood, so that it can remain a very long time without breathing; long enough, at least, to tire the patience of the most obstinate foe.