3o6 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



time. The mechanical character of that use has not 

 been identical. It is of two principal kinds, viz. : im- 

 pact and longitudinal strain. These two forms of en- 

 ergy move in directions opposite to each other ; the 

 one as compression in the direction of the length of 

 the bone ; the other, as a stretching in the direction of 

 the length of the bone. Both processes alike appear 

 to have stimulated growth in the direction of the length 

 of the bone. 



The increase in the length of the legs has not been 

 always due to increase in length of the same segment. 

 In a majority of the higher mammals, the increase has 

 been principally in the foot, and especially in the meta- 

 podials and digits, producing digitigradism. In the 

 forms which have remained plantigrade, the femur 

 (Proboscidia), or femur and tibia (Quadrumana), or 

 all three segments (Tarsius), have been the seat of the 

 elongation. We can again trace these especial elonga- 

 tions to special uses. In animals which leap, the dis- 

 tal segments of the limbs are elongated; in those 

 which do not leap, but which merely run or walk, it is 

 the proximal segments of the limbs which are elon- 

 gated. 



Animals which run by leaping are divided into 

 those which run and leap with all fours, as Diplarthra ; 

 and those which run and leap with the posterior limbs 

 only, as the jerboas and kangaroos. In both types, 

 the distal segments of the hind limb are elongated, and 

 in the Diplarthra, those of the fore limb also. 



Animals which do not leap in progression (ele- 

 phants, Quadrumana, bears) are always plantigrade, 

 and have very short feet, but elongate thighs, and, 

 mostly, tibias. 



These facts show that those elements which receive 



