ORIGIN OF VARIATIONS. 161 



in most cases we would find only a slightly raised 

 ridge. An illustration of the effects of pressure 

 and pull on the bones may be found in the flattened 

 shape of the ribs, caused by the pressure of the in- 

 ternal organs and the external muscle and skin, and 

 by the pulling of the intercostal muscles. At the 

 back, where the spinal muscles arise from the ribs, 

 their shape is nearly round in cross-section. The 

 spines themselves may be regarded as chiefly due 

 to the steady pull of the symmetrical dorsal muscles. 

 The effects of a pulling upon growth are more 

 easily seen on the softer parts of the body, as the 

 elongation of the lobes of the ears and of the lips 

 caused by the strain of the ornaments of savages. 

 The mere absence of a pressure in some cases causes 

 growth. Thus the removal of one of the molar 

 teeth causes the growth of a protuberance of flesh 

 from the inner wall of the cheek into' the empty 

 space. We cannot, of course, trace out with accu- 

 racy the causes of the various effects which we see 

 in the shaping of the skeleton and flesh, but we know 

 that these forces which I have mentioned do act, 

 and that their action is extremely complicated. 

 How powerful their action is, may be readily seen 

 by instituting on a growing limb, or even a full- 

 grown one, some abnormal conditions of pressure 

 and pull. It is well known that there would be a 



