NATURAL SELECTION 



that the shape of the whole head will be altered. 

 This increased weight of the head, and the in- 

 creasingly violent activity of the muscles which 

 move the jaws, entails a greater strain upon the 

 vertebrae which support the head, and upon 

 the cervical muscles which move it from side to 

 side. The heightened nutrition of these bones 

 and muscles will add to their weight, so that 

 the shoulders and chest will be affected. There 

 will be a tightening of the tendons, and proba- 

 bly a pergsptible alteration in the relative lengths 

 of the different bones and muscles throughout 

 the anterior part of the body ; and these changes, 

 altering the animal's centre of gravity, will in- 

 evitably cause other compensating changes in 

 the rest of the body. The legs, shoulders, and 

 haunches will be modified. Alterations in the 

 weights bearing upon the chest will affect the 

 growth of the lungs and the aeration of the 

 blood. And the stomach, intestines, and va- 

 rious secreting glands will respond to the re- 

 quirements of all these nutritive changes. While, 

 lastly, such deep-seated variations cannot fail to 

 influence the nervous system of the animal, 

 and to modify somewhat its temperament and 

 its modes of life. 



To illustrate the effects of use and disuse, let 



us reconsider the antelopes, of whom natural 



selection has so long preserved the swiftest and 



most quickly frightened individuals that they 



23 



