ORIGIN OF VARIATIONS. 159 



as resulting from various applications of pressure, 

 strain, and shock. The characteristic structure of the 

 joints is chiefly due to pressure. As is well known, 

 constant pressure prevents growth, as is shown in 

 the artificially compressed heads and feet of some 

 races of people. Let us suppose the development 

 of a typical ball and socket joint from the apposi- 

 tion of the undifferentiated surfaces of two bones. 

 We would have a surface of a highly movable bone 

 opposed to a large surface of a relatively immovable 

 bone. The permanent tension of the muscles keeps 

 up a constant pressure of one bone upon the other. 

 The movement of one bone, which we will suppose 

 to be a long bone, brings different parts of its end 

 surface in opposition to the larger surface of the 

 other bone ; but whichever way the long bone points, 

 its end presses against the same spot on the other 

 bone. That part of the immovable bone would 

 therefore be checked in its growth as compared 

 with the surrounding part, or its waste would not 

 be repaired, and a depression would result at that 

 point, forming a socket. Suppose the movable bone 

 has at first opposed a sharp point to the immovable 

 bone, — in that case, that point of the movable bone 

 would be under constant pressure, and by the stop- 

 page of growth would tend to become blunt and 

 rounded. Wherever, through roughness in the first 



