KINETOGENESIS. 367 



cessities of such limbs are twofold : first, to serve as 

 supports when at rest or in progression ; second, to be 

 applied to the body in protection from enemies, or in 

 aiding the functions of feeding, reproduction, etc. 

 The first function requires principally mobility at the 

 point of connection with the body. The second, flexi- 

 bility at some point on the shaft of the limb. The two 

 kinds of movements in question would conserve two 

 principal points of flexure, and these would be for the 

 fore limb, just what we find, the shoulder and elbow 

 joints ; and for the hind limbs, the hip and knee joints. 

 The two median joints are directed in opposite ways, 

 the elbow backwards and the knee forwards. This 

 diversity is clearly due to the diverse positions of the 

 functioning regions. The opposite extremities of the 

 alimentary canal, the posterior including the exits of 

 the urogenital organs, requires that the fore limbs 

 should bend forwards, and the posterior limbs back- 

 wards. And the constantly recurring necessity for the 

 exercise of these flexures must necessarily have devel- 

 oped the appropriate articulations in preference to all 

 others. The terminal flexure, that of the wrist or 

 ankle, has been evidently due to a similar mechanical 

 cause, viz., the flexure due to pressure of the weight 

 of the body on the terminal segments when in contact 

 with earth. The distal segments are the most slender 

 in all types, and least able to maintain a linear direc- 

 tion under pressure, hence, they have flexed easily and 

 thus the line of separation between leg and foot had 

 its origin. The ankle and wrist in the Batrachia Uro- 

 dela is still a mere flexure. 



Mr. Herbert Spencer has endeavored to account for 

 the origin of the segmentation of muscles into myo- 

 tomes, and the division of the sheath of the notochord 



