On comparing the inferior extremities of man with the legs of birds, or the posterior extremities of quadrupeds, say the horse or deer, we find that the bones composing them are not so obliquely placed with reference to each other, neither are the angles formed by any two bones so acute. Further, we observe that in birds and quadrupeds the tarsal and metatarsal bones are so modified that there is an actual increase in the number of the angles themselves. In the extremities of birds and quadrupeds there are four angles, which may be increased or diminished in the operations of locomotion. Thus, in the quadruped and bird (fig. 4, p. 21, and fig. 24, p. 47), the femur forms with the ilium one angle (_a_); the tibia and fibula with the femur a second angle (_b_); the cannon or tarso-metatarsal bone with the tibia and fibula a third angle (_c_); and the bones of the foot with the cannon or tarso-metatarsal bone a fourth angle (_d_). In man three angles only are found, marked respectively _a_, _b_, and _c_ (figs. 26 and 27, pp. 55 and 59). The fourth angle (_d_ of figs. 4 and 24) is absent. The absence of the fourth angle is due to the fact that in man the tarsal and metatarsal bones are shortened and crushed together; whereas in the quadruped and bird they are elongated and separated. As the speed of a limb increases in proportion to the number and acuteness of the angles formed by its several bones, it is not difficult to understand why man should not be so swift as the majority of quadrupeds. The increase in the number of angles increases the power which an animal has of shortening and elongating its extremities, and the levers which the extremities form. To increase the length of a lever is to increase its power at one end, and the distance through which it moves at the other; hence the faculty of bounding or leaping possessed in such perfection by many quadrupeds.[34] If the wing be considered as a lever, a small degree of motion at its root produces an extensive sweep at its tip. It is thus that the wing is enabled to work up and utilize the thin medium of the air as a buoying medium. [34] “The posterior extremities in both the lion and tiger are longer, and the bones inclined more obliquely to each other than the anterior, giving them greater power and elasticity in springing.”