334 THE CORRESPONDENCE BETWEEN THE [chap. 



The modifications with regard to the anterior extremity 

 are that the humerus may be quite horizontal, or its distal 

 end may incline upwards, or, as is much more frequently the 

 case, it may incline somewhat downwards, so that the dorsal 

 surface is posterior and the ventral surface anterior; the fore- 

 arm in the ordinary resting position may be quite vertical, 

 or inclined with its upper end backwards ; the whole of the 

 manus may rest entirely on the ground, as in the so-called 

 " plantigrade " or rather " palmigrade " animals, or the 

 proximal part, the tarsus and metatarsus, may be raised and 

 placed more or less vertically, the limb resting either on all 

 or only on the terminal phalanges, according to the com- 

 pleteness of the " digitigrade " mode of progression. 



Similar modifications occur in the hind limb. The 

 femur is usually inclined with its distal end downwards, so 

 that the dorsal or "extensor" surface is anterior, and the 

 ventral or " flexor " surface posterior. In the Elephants it 

 is very nearly vertical. In most animals which occasionally 

 assume the upright position, as the Kangaroos and some 

 Rodents, the femur is ordinarily inclined upwards at its 

 distal extremity, so that the knee is above the acetabulum, 

 and the pelvis slung as it were between the two hind limbs. 

 In Man, on the other hand, in standing or walking the 

 femur is nearly vertical with the distal ends downwards, and 

 the pelvis is supported on the top of the limbs. 



The positions of the limbs which are quite exceptional 

 are those of certain aquatic animals. 



In the Cetacea (Fig. h) none of the segments of the 

 anterior limb undergo any deflection from the primitive 

 straight condition, nor is there any rotation of the bones of 

 the forearm. The only changes which take place are a partial 

 rotation backwards from the shoulder, and a slight turning 

 downwards of the preaxial border. In the Sirenia and the 



