We do not find in the fish the elegant weakness of the reptile and the insect, so slender that in those parts one can cut through them as through a thread; his segments are within, and well protected. He uses them for contractile power, but does not, as the less perfect reptile and insect do, expose them to external injury.
Like the crustaceæ, the fish prefers strength to beauty, and for this end has no neck; head and trunk form one mass. Admirable principle of strength, which enables him, in cleaving through so yielding an element as water, to strike, at will, with a thousand fold more force than is necessary, and then his motion is as the flight of an arrow or the flash of lightning!
The interior bone, single in the Seiche, is in the fish at once one and multiple; one for force of unity, multiple for elasticity, enabling the muscles alternately to contract and expand, and thus create swift motion. Marvellous, really marvellous is that formation of the fish, so solid without and contractile within, that inward keel to which are attached the motor muscles which work with an alternating shock. Exteriorly, he exposes only his auxiliary oars, short fins which are but little in danger, being strong, slippery, and sharp to wound, or to scrape. How superior in all this is the fish, to the Poulpe and the Medusa, which present to all comers soft flesh, a tempting morsel for the crustaceæ or the porpoise.
This true son of the water, gliding and mobile as his mother, glides by means of his mucus, cleaves with his head, impelled by his contractile muscles, and finally, with his strong fins rows and steers.
The least of these powers would suffice, but he unites them all; a perfect model and absolute type of swift motion.
Even the bird is less mobile, seeing that he has to perch. He is fixed for the night, but the fish, never; even asleep, he still floats.
So extremely mobile, he at the same time is in the highest degree strong and lively. Wherever there is water, there is the fish: he is the universal creature of the globe. In the loftiest lakes of Asia and of the Cordilleras, where the atmosphere is so rarefied that no other creature can endure it, the fish lives and thrives. It is the red fish of the Gudgeon species, which thus looks down upon all the earth. In like manner, in the great depths, beneath the most enormous weights, live the Herring and the Cod. Forbes, who divides them into ten superposed beds or stages, finds them all inhabited, and in the lowest of all, supposed to be so dark, he finds a fish provided with eyes so admirable that he finds sufficient light in that which seems to us the uttermost darkness of night.
There is yet another privilege of the fish. Many species, as Salmon, Shad, Eels, Sturgeon, &c., can live equally in fresh water or sea water, and regularly migrate from one to the other. Many families of fish include both sea fish and fresh water fish, as for instance, the Thornback.