MOTION. 
lation at which the trunk and thorax are flexed 
upon each other; fand g, the centres of force 
of the anterior and posterior parts of the body ; 
when the body is curved preparatory to the 
leap, it takes the position a’ x’ b’, and the centre 
of gravity will be at o’ ; when the body returns 
to its former position a x b, the two centres 
of force come in contact with the ground in 
J and g; the forces with which they strike the 
ground acting perpendicularly to a b, impart to 
the centre of gravity o a velocity in the same 
direction, but the reaction of the ground gives 
to the body two impulses equal and opposite 
to the forces d f and e g; the linesf dand ge 
not passing through the centre of gravity o, 
one part only of the force raises the body above 
the ground, the other part of the force pro- 
duces a rotation of the body around the centre 
of gravity ; the force g e being much nearer 
the centre than fd, and their velocity of rota- 
tion being in the direct ratio of their distances 
from the point 0, the motion produced by g e, 
being opposite to that of fd, is destroyed by 
a part of the latter, and the remainder of fd 
will give the body a rotatory motion in the 
direction of f.f'h: by means of this motion 
the insect is enabled to reverse its position.* 
The larve of insects are variously organized ; 
for example, that of the Tyrophaga casei, when 
reparing to leap, bends itself into a circle, 
ringing its head and tail in contact, and 
fixing its two mandibles in the cavities of its 
anal tubercles; it then contracts its body into 
an oblong figure, so that the two halves are 
parallel to each other. This being accom- 
plished, it suddenly unlocks its head, and ex- 
tends its body with such force, that the re- 
action of the surface on which it rests propels 
the body into the air to the distance of several 
inches. The leaping of fishes and serpents has 
been already mentioned in Section IV. Birds 
walk, run, and leap like Man. The Incessores 
and other short-legged Birds usually move on 
solids by a succession of leaps. 
In most quadrupeds the propelling force of 
the leap is produced by the extension of the 
posterior extremities alone. When a consi- 
derable elevation of the body is the object to be 
attained, the centre of gravity is, previously to 
the leap, thrown back and lowered by the 
flexion of the hip, knee, and ankle-joints of the 
hind legs; the fore-legs are then raised from 
the ground by the extensor muscles of the 
trunk, the axis of which, before parallel to 
the horizon, is now inclined at a considerable 
angle to it. At thismoment the extensor mus- 
cles of the posterior extremities are suddenly 
* According to Kirby and Spence (loco cit. vol. ii. 
p- 314), the elater makes a double movement pre- 
paratory to the leap, by first drawing out the spine 
from its sheath, making it re-enter, and then 
causing it to fly out with a spring in the moment of 
leaping. This, however, appears to be a very in- 
correct view of what takes place. Mr. Darwin 
does not consider that sufficient stress has been 
laid on the elasticity of the spine, and that the 
sudden spring of the elater could not have resulted 
from muscular contraction alone, without some me- 
chanical contrivance. Bs of the Voyage of 
the Beagle, vol. iii. p. 35.) 
477 
contracted, and the leg is extended with suffi- 
cient velocity to elevate the trunk of the animal 
above the ground. At the end of the leap, 
the anterior feet first reach the ground simul- 
taneously, and then the two hind feet in a 
similar manner; the head being brought be- 
tween the two fore-legs, in order to throw the 
centre of gravity as far back as possible. The 
direction of the force of propulsion depends on 
the position of the propelling legs, and on the 
angle of inclination of the trunk to the plane 
of motion ;—the intensity on the quantity of 
force impressed by the extension of the legs. 
In those quadrupeds which constantly move 
by a succession of leaps, the length of the 
posterior legs greatly exceeds that of the ante- 
rior; but in those wherein the length of the 
four legs is nearly equal, the leap is attended 
with so great an expenditure of muscular action 
that itis only resorted to on particular occasions. 
Borelli* is of opinion that the power of 
leaping is greatest in those animals in which 
the extremities of the bones of the legs de- 
scribe (in proportion to their masses) ares of 
greatest circles, because they must move with 
greater velocity in the same time; in other 
words, that those animals which have the 
greatest relative length of the posterior legs 
leap to the greatest height. Nor doves this mili- 
tate against the analysis of Straus, according to 
which we have seen that animals of the same 
kind which have the greatest length of leg 
occupy a proportionably long time in springing. 
In the Bull-frog amongst the Batrachia, in 
the Kangaroo amongst the Marsupialia, and in 
the Jerboa amongst the Rodentia, we find the 
greatest disproportion in the length of the ante- 
rior and posterior extremities ; the length of the 
latter predominating so greatly, that they walk 
and trot with difficulty; but when pursued, the 
Jerboa can leap nine feet at each step, and repeat 
these leaps so rapidly, that it is said the Cos- 
sacks, though mounted on the fleetest horses, 
cannot overtake them. The Kangaroo reposes on 
the hind legs and tail, which form a triangular 
base, leaving the arms free for prehension. In- 
dependently of the Jerboa, other Rodentia, 
such as the Hare and Rabbit, are also furnished 
with lengthened posterior extremities, by which 
they possess considerable power of springing. 
As the mass of animals increases in a greater 
ratio than the force of their muscular system, 
the large Proboscidie are almost incapable of 
leaping, but the solidungulous Pachydermata 
are well organized for leaping, as well as many 
of the Ruminantia, such as the Deer and Ante- 
lope. In these two orders the lengthened cal- 
caneum and metatarsal bones contribute chiefly 
to assist the muscles in the spring. In the 
Carnivora, we find the ankle-joint possessing the 
same structure as in the Ruminantia and Pa- 
chydermata. The geometrical relation of their 
osseous and muscular systems is such as to 
confer on them great power in making a 
spring; that of the Tiger is well known. The 
leaping of the Cheiroptera and Quadrumana 
* Quod longiores sunt vectes extremi crurum, saltus 
majores fiunt, See Part. prim. Prop. 176, p, 211. 
