MOTION. 



4?7 



lation at which the trunk and thorax are flexed 

 upon each other ; ./'and 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 

 ,/'andg; 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 /'and e g ; the lines/' (/ and g e 

 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/'rf, and their velocity of rota- 

 tion being in the direct ratio of their distances 

 from the point o, the motion produced by g e, 

 being opposite to that of J'd, is destroyed by 

 a part of the latter, and the remainder of J'd 

 will give the body a rotatory motion in the 

 direction of ff h : by means of this motion 

 the insect is enabled to reverse its position.* 

 The larvae of insects are variously organized ; 

 for example, that of the Tyrophaga casei, when 

 preparing to leap, bends itself into a circle, 

 bringing 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 this moment 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. (Journal of the Voyage of 

 the Beagle, vol. iii. p. 35.) 



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 gieat an expenditure of muscular action 

 that it is 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) arcs 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 does 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 Jeiboa 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 Proboscidiav are almost incapable of 

 leaping, but the solidungulous Pachydennata 

 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 



* Quo longiores sunt vectes extremi crurum, salt us 

 majores fiunt. See Part. prim. Piop. 17t>, p. 211. 



