especially such forms as the alligator and crocodile and the ancient marine saurians, in reality we should think of them as primarily land-inhabiting animals, some of which have gradually taken to aquatic life aud in the course of time become more and more confined to a watery sphere. Aquatic turtles show this most decidedly, an extreme being found in the soft-shelled turtle, which is not only a constant resident in water, but has become possessed of special organs for respiration in water, so that air-breathing is scarcely necessary. Birds that live in the water have taken the same convenient highway, and many of them have traveled it from the time of their toothed ancestors down to the present. Penguins have gone so far along this road that their wings can no longer serve for flight, while loons and grebes and auks are on the way. Gulls, albatrosses and petrels may go far out o'er billowy wave hundreds of miles from land. Ducks and geese and swans have struck the trail, and snipe, stork and heron, crane and flamingo have rolled up their trousers and are wading in. Even the fish-hawk, the osprey and the eagle find it worth while to look beneath the wave.
The aquatic habits of the beaver furnish a most remarkable example of this, and if we could trace his acquisition of this habit from the time when he must have been an ordinary terrestrial rodent with neither a paddle-tail nor a web foot, we should certainly find an interesting career. The musk-rat has not gone so far and can not reach the same goal, as he has flattened his tail in the wrong direction.
The whale—that giant of the seas—largest of mammals and indeed of all animals, has out-traveled all his relatives in reaching out into the great ocean, but we can not possibly conceive the whale to have come from any other source or to have other ancestor than a land-inhabiting mammal. We get glimpses of the mile posts he has passed in the structures shown by the manatee, the walrus, the sea lions and others. Not that these constitute in any sense his ancestral line, for that was far back in time and so far largely a lost history. But along such stages we must believe his ancestors to have passed. The manatee in its way is as strictly aquatic as the whale but hugs the shore or river mouths.
The hippopotamus is well on the way, and I would digress here to call attention to the remarkable similarity in adaptation of the sense organs of this animal to those of the alligator and crocodile. Note that the eyes, ears and nostrils are almost exactly in the same plane and so situated that they may all be above the surface of the water, while practically no part of the head or body may be visible, an admirable adjustment to avoid detection from foes or for protection against flies, mosquitoes, etc. Seals, walruses, sea lions, sea otters and, in less degree, the polar bear, the common otter and mink, all show aquatic habit fixed or growing, and even the small boy at his favorite swim-
