368 THE CORRESPONDENCE BETWEEN THE [CHAP. 



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 completeness 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 fore-arm. The only changes which take place are a partial 

 rotation backwards from the shoulder, and a slight turning 

 downwards of the pre-axial border. In the Sirenia and the 

 Seals, there is a slight bend at the elbow and the wrist, 

 but little or no rotation of the fore-arm. 



In the hind limb of the Seal (Fig. i) there is very little 

 flexure at the joints, and the whole limb is turned backwards 

 instead of forwards from the hip, and at the same time 

 rotated on its axis, so that the pre-axial border becomes 



