Not very many years ago dugongs were found in large herds, sometimes consisting of two or three hundred individuals, and were so tame that they would even permit themselves to be touched without attempting to escape. But they have been killed in great numbers for the sake of their hides and a valuable oil which is extracted from their bodies, so that nowadays it seldom happens that more than two or three are seen together. A full-grown dugong is generally from seven to eight feet long, and measures about six feet round the body. The Australian dugong is said to attain a length of fourteen feet. CHAPTER XIV THE RODENT ANIMALS The group of the rodents is the largest of all the tribes of mammals, for it contains more than a thousand different animals. Indeed, nearly one third of all the mammals in the world belong to this very important division. TEETH OF THE GNAWERS The word rodent signifies gnawing, and is given to these creatures because their front teeth are specially formed for the purpose of gnawing hard substances. You know, of course, how long and sharp the front teeth of a rat or a mouse are, and how easily these animals can nibble their way through a stout piece of board. Well, all the rodent animals have these teeth formed in just the same way. And when we come to examine them we find that they are beautifully suited to their purpose. You would think that as they are so constantly in use, these teeth would quickly be worn down to the gums, wouldn't you? Ours would, if we employed them in the same way. But then, in the rodent animals, these teeth never stop growing, so that as fast as they are worn from above they are pushed up again from below. Sometimes this fact leads to a very singular result. It happens now and then that a rodent animal meets with an accident and breaks off one of its front teeth. Now these teeth, remember, cannot be used unless they have one another to work against, just as the blades of a pair of scissors cannot be used unless they have one another to cut against. So, you see, when one tooth is broken short off, the opposite tooth in the other jaw becomes useless. It has nothing to work against. So it is no longer worn away from above. But of course it still goes on growing. So before very long it projects in front of the other teeth. Still it continues to grow, and in course of time its natural curve brings it round in a semicircle, with the point toward the face. And at last, if it is a lower tooth, it pierces first the flesh of the forehead and then the skull beneath it, and enters the brain and kills the animal; while, if it happens to be an upper tooth, the point curls round under the chin and at length prevents the poor creature from opening its mouth, so that it dies miserably of starvation! It seems impossible, doesn't it? Yet in museums there are skeletons of hares and rabbits which have been killed in this singular way by one of their own front teeth. HOW THE TEETH ARE KEPT SHARP