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Show 322 MR. W. WOODLAND ON TIIE [Apr. 21, to the height through which upheaval is effected, and inversely proportional to the period of time occupied by upheaval. This statement in its application to terrestrial locomotion provides the clue for the solution of the present problem. With the exception of aquatic organisms, which exist in a medium so similar in density to themselves that the influence of gravity is not felt, all animals which fly, crawl, leap, or run are subject to this primary condition of self-support imposed by the earth's attraction. In Aves the body is not entirely unsupported, the resistance of the air to the large expanse of body yielding considerable aid in this respect, and the small amount of self-sustainment needed can be provided by the minor elevations imparted by individual strokes of the wing. The reactions between the wing and the air are of necessity small, owing to the mobility of the latter substance, and hence, in spite of the great activity of birds, no great accelerations are imparted to the body -the locomotion is not impulsive. The conditions affecting terrestrial locomotion are exceedingly unlike. Here, as before remarked, the powers of resistance possessed by the substances of organism and substratum both being of high degree, reaction between the two is correspondingly great. In Reptilia and (excepting the saltatory Anura referred to below) terrestrial Amphibia, however, the forces involved in locomotion are not conspicuous for their intensity, owing both to the fact that the period of upheaval is prolonged (no sudden impact occurring between the limbs and the earth) and the small amount of elevation effected by the action of the limbs, these two features resulting from the conformation of the body and the general inactivity of the animal. In Mammalia, on the other hand, the period occupied by contact of the limbs with the earth is extremely brief and the height of elevation considerable; and hence, though the angulation of the limbs tends to diminish concussion, the organisation is subject to the intense strains and stresses resulting from the enormous forces generated during locomotion. Consider the gallop of a typical Ungulate or Carnivore. The elongated trunk possessing two pairs of limbs, each pair being in an opposite phase of motion compared with the other, it follows that the two halves of the trunk will alternately be uplieaved through a considerable distance on contact with the earth being made by their respective pairs of limbs, and depressed in the interval which exists between successive contacts. That is, the pendulous swing of each pair of extremities is accompanied by an elevatory impulse at the centre of the arc each describes, and these impulses effect the upheavals of the respective halves of the body through the distances which they have fallen in the time required for each pair of limbs to describe twice the length of its appropriate path. Considering the mode of action of either pair of limbs, and beginning at the horizontal stage of the trunk's position when the half of the body is being depressed under the influence of gravity, this continues to descend until the |
