第17章

There is reason, too, for winged creatures having feet, but fish none.The former have their home in the dry medium, and cannot remain always in mid air; they must therefore have feet.Fish on the contrary live in the wet medium, and take in water, not air.Fins are useful for swimming, but feet not.And if they had both they would be non-sanguineous.There is a broad similarity between birds and fishes in the organs of locomotion.Birds have their wings on the superior part, similarly fish have two pectoral fins; again, birds have legs on their under parts and near the wings; similarly, most fish have two fins on the under parts and near the pectorals.Birds, too, have a tail and fish a tail-fin.