The grebe, denied the power to rise easily and even, to ran about on land without considerable effort, is, however, splendidly adapted to its water life, and the rapidity of its motions places it near the head of the higher active creatures,--with the woodchuck near the opposite extreme.
Throughout the depths of the sea, silence, as well as absolute darkness, prevails. The sun penetrates only a short distance below the surface, at most a few hundred feet, and all disturbance from storms ceases far above that depth, Where the pressure is a ton or more to the square inch, it is very evident that no sound vibration can exist. Near the surface it is otherwise. The majority of fishes have no lungs and of course no vocal chords, but certain species, such as the drumfish, are able to distend special sacs with gas or air, or in other ways to produce sounds. One variety succeeds in producing a number of sounds by gritting the teeth, and when the male fish is attempting to charm the female by dashing round her, spreading his fins to display his brilliant colours, this gritting of the teeth holds a prominent place in the performance, although whether the fair finny one makes her choice because she prefers a high-toned grit instead of a lower one can only be imagined! But vibrations, whether of sound or of water pressure, are easily carried near the surface, and fishes are provided with organs to receive and record them. One class of such organs has little in common with ears, as we speak of them; they are merely points on the head and body which are susceptible to the watery vibrations. These points are minute cavities, surrounded with tiny _cilia_ or hairs, which connect with the ends of the nerves.
The ears of the frogs and all higher animals are, like the tongue-bone and the lower jaw, derived originally from portions of gills, which the aquatic ancestors of living animals used to draw the oxygen from the water. This is one of the most wonderful and interesting changes which the study of evolution has unfolded to our knowledge.
The disproportionate voices are produced by means of an extra amount of skin on the throat, which is distensible and acts as a drum to increase the volume of sound. In certain bullfrogs which grow to be as large as the head of a man, the bellowing power is deafening and is audible for miles. In Chile a small species of frog, measuring only about an inch in length, has two internal vocal sacs which are put to a unique use. Where these frogs live, water is very scarce and the polliwogs have no chance to live and develop in pools, as is ordinarily the case. So when the eggs are laid, they are immediately taken by the male frog and placed in these capacious sacs, which serve as nurseries for them all through their hatching and growing period of life. Although there is no water in these chambers, yet their gills grow out and are reabsorbed, just as is the case in ordinary tadpoles. When their legs are fully developed, they clamber up to their father's broad mouth and get their first glimpse of the great world from his lower lip. When fifteen partly developed polliwogs are found in the pouches of one little frog, he looks as if he had gorged himself to bursting with tadpoles. To such curious uses may vocal organs be put.