Because scales, while they afford protection to the bodies of fish, are conveniently adapted to their motions; and as the scales present no surface to obstruct their passage through the water, as hair or feathers would do, they evidently form the best covering for the aquatic animal.
1056. Why do fishes float in streams (when they are not swimming) with their heads towards the stream?
Because they breathe by the transmission of water over the surface of their gills, the water entering at the mouth, and passing over the gills behind. When, therefore, they lie motionless with their heads to the stream, they are in that position which naturally assists their breathing process.
1057. Why have fishes air-bladders?
Because, as the density of water varies greatly at different depths, the enlargement or contraction of the bladder regulates the relation of the specific gravity of the body of the fish to that of the water in which it moves.
1058. Why have whales a very large development of oily matter about their heads?
Because their heads are thereby rendered the lighter part of their bodies, and a very slight exertion on the part of the animal will bring its head to the surface to breathe air, which it constantly requires.
1059. Why have birds that swim upon water web-feet?
Because the spreading out of the toes of the bird brings the membrane between the toes into the form of a fin, or water-wing, by striking which against the water, the bird propels itself along.