The fins of fishes are to them, _what wings and tails are to birds_, enabling them to rise in the fluid in which they live by the _reaction of the motions of the fins upon its substance_. [Verse: "Speak to the earth, and it shall teach thee; and the fishes of the sea shall declare unto thee. Who knoweth not in all these that the hand of the Lord hath wrought this."--JOB XII.] 1054. _Why are the fins of fishes proportionately so much smaller than the wings of birds?_ Because there is less difference between the _specific gravity_ of the body of a fish, and the water in which it moves, than between the body of a bird, and the air on which it flies. The fish, therefore _does not require such an expanded surface to elevate or guide it_. 1055. _Why have fishes scales?_ 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_. [Verse: "And Jesus saith unto him, The foxes have holes, and the birds of the air have nests; but the son of man hath not where to lay his head."--MATTHEW XIII.] 1060. _Why have birds that swim and dive short legs?_ Because long legs would greatly _impede their motions in the water_, by becoming repeatedly entangled in the weeds, and by striking against the bottom. _Waders_, however, require long legs because they have to move about through the _tall vegetation of marshy borders_. [Illustration: Fig. 69.--STILT-PLOVER AND DUCK.] 1061. _Why have the feet of the heron, cormorant, &c., deep rough notches upon their under surface?_