PROGEESSION ON THE LAND. 



51 



organized for walking and running on solids, can also swim. 

 Mr. Darwin, that most careful of all observers, informs us 

 that ostriches take to the water readily, and not only ford 

 rapid rivers, but also cross from island to island. They swim 

 leisurely, with neck extended, and the greater part of the 

 body submerged. 



Locomotion in Man. — The speed attained by man, although 

 considerable, is not remarkable. It depends on a variety of 

 circumstances, such as the height, age, sex, and muscular 

 energy of the individual, the nature of the surface passed 

 over, and the resistance to forward motion due to the presence 

 of air, whether still or moving. A reference to the human 

 skeleton, particularly its inferior extremities, will explain why 

 the speed should be moderate. 



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 meta- 

 tarsal 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 (6) ; 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-meta- 

 tarsal 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 elon- 

 gated and separated. 



As the speed of a limb increases in proportion to the num- 

 ber and acuteness of the angles formed by its several bones, it 

 is not difficult to understand why man should not be so swift 



