The seals then, while they are in all main points constructed like land animals, have gained many advantages, not by having new parts, but by the old ones becoming so modified as to make them admirably fitted for a watery life; and when we add that they have large eyes well adapted for seeing under water, keen ears with little or no outer ear, which would be useless, but a very acute hearing apparatus within, and nostrils which will close firmly and keep the air in and the water out when they dive, we must acknowledge that they make good use of all parts of their body. Indeed, their breathing apparatus is the most curious of all, for they can remain under water sometimes for twenty minutes, and meanwhile the circulation of their blood is probably controlled by large reservoirs in the veins, which prevent it going back to the heart and lungs till it can be purified by fresh breath. Now, if all these changes from a land to a water-frequenting animal have been made gradually, we shall expect to find some forms less altered than others, and so it is. The Walrus, which is not a seal, but a creature with a thick hide having no fur and only a few scattered hairs upon it, and long tusks in his mouth, is much more of a land-animal than the seals. He passes a great part of his life sauntering along on the low shores of the Arctic seas, digging up mussels, cockles, and clams with his long canine teeth or tusks; and in accordance with this we find that his hind legs are much freer than even those of the sea-lions, for the skin binding them to his body is broader and his hips are stronger, so that, as he throws his front flippers forwards, he can also throw out his feet and walk on all fours in a strange straddling manner. He is remarkably fierce and strong, and Captain Scoresby caught one once in the act of killing and eating a large narwhal, so that they are evidently not afraid of attacking even large animals. The walrus is even said to stand at bay on shore and fight his great destroyer the polar bear, throwing up his head so as to strike forcibly with his sharp tusks, but in this battle he is generally defeated. His tusks alone would suggest that he lives a good deal on land exposed to dangers, for his more aquatic relations the seals are without tusks, and though their teeth are sharp enough, and they fight among themselves, yet their way of escaping the great tyrant of the ice-fields is to slip into the water.