REPRESENTATIVE LAMARCKIANS 255 



while between these grooves a keel of the tibia de- 

 scends to fill a corresponding groove of the astragalus. 

 Such a joint as this can be broken by force, but it 

 cannot be dislocated. Now, in all bones, the external 

 walls are composed of dense material, while the cen- 

 tres are spongy and comparatively soft. The first 

 bone of the foot (astragalus) is narrower, from side 

 to side, than the tibia which rests upon it. Hence the 

 edges of the dense side-walls of the astragalus fall 

 within the edges of the dense side-walls of the tibia, 

 and they have pressed into the more yielding material 

 that forms the end of the bone, and causing bone ab- 

 sorption, pushed it upward, thus allowing the side- 

 walls of the tibia to embrace the side-walls of the as- 

 tragalus. Now, this is exactly what would happen if 

 two pieces of plastic dead material, similarly placed, 

 should be subjected to a continual pounding in the 

 direction of their length. And in view of the facts 

 already cited we cannot ascribe any other immediate 

 origin to it in the living material. 



"The same active cause that produced the two 

 grooves of the lower end of the leg produced the 

 groove of the middle of the upper end of the astraga- 

 lus. Here we have the yielding lower end of the tibia 

 resting on the equally spongy material of the middle 

 of the astragalus. There is here no question of the 

 hard material cutting into soft, but simply the result 

 of continuous concussion. The consequence of con- 



