used as fins only. "Every variety of motion peculiar to land, air, and water-navigating animals, as such, is imitated by others which take to the elements in question, secondarily or at intervals." It is probably true, however, that no animal which lives indiscriminately in two media attains the highest development for traveling in or upon either. In such cases the maximum speed is not attained. Those animals, says the author, which swim the best, walk, as a rule, with difficulty, and vice versa, as the movements of the auk and the seal, in and out of water, amply testify. It is evident that all the supposed gaps between typical forms for locomotion are bridged by forms intermediate, and the author's position is fully sustained, that walking, swimming, and flying, are essentially the same.
Fig. 2.
The Little Penguin, adapted exclusively for swimming and diving. In this quaint bird the wing forms a perfect screw, and is employed as such in swimming and diving.
Before entering upon the question of the movement and functions of specialized organs for locomotion, attention is invited to the interesting statement that, however wonderful and beautiful, in its way, the bony skeleton maybe, it is after all only an adjunct to locomotion, and of motion in general—that all the really essential movements of an animal occur in the soft parts. "The osseous system is therefore to be regarded as secondary in importance to the muscular, of which it may be considered a differentiation. Instead of regarding the muscles as adapted to the bones, the bones ought to be regarded as adapted to the muscles. Bones have no power either of originating or perpetuating motion. This begins and terminates in the muscles."
The bones are the passive organs of locomotion, in the movement of which muscular force is expended. In land animals, as a rule, the bones are harder and more elastic than in aquatic species. The cartilaginous and spongy bones of many fishes would be ill suited to bear the strains and shocks of terrestrial progression.
The velocity with which a limb may be moved will depend upon the acuteness of the angles of its several bones. Hence the fleetness of many animals, in which the angles formed by the bones are acute. This is well shown in the skeleton of a deer, of which Fig. 3 is an ex-
