322 MR. W. WOODLAND ON THE [ 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 con- 
siderable 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 eleva- 
tion 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 lmbs 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 alter- 
nately be upheaved through a considerable distance on contact 
with the earth being made by their respective paurs of limbs, and 
depressed in the interval which exists between successive contacts. 
That is, the pendulous swing of each pair of extremities is accom- 
panied by an elevatory impulse at the centre of the are 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 
