IB COSMIC PHILOSOPHY. [pt. xi 



siiJed and emphasized by other changes indirectly produced 

 by correlation of growth, and also by what is called the law 

 of use and disuse. By correlation of growth, or internal 

 equilibration, we mean the effect produced upon any part of 

 the organism by change in a related or neighbouring part. 

 Let us suppose that it becomes advantageous to some feline 

 animal, like the ancestor of the lion, to have large and power- 

 ful jaw^s. Since no two of our leonines would have jaws of 

 exactly the same size and strength, natural selection would 

 preserve all the strong-jawed individuals, while the weak- 

 jawed individuals w^ould succumb in the struggle for life. In 

 the course of many generations our race of leonines would 

 possess on the average much larger and stronger jaws than at 

 the period at which we began to consider it. But greater 

 weight of jaw entails increased exertion of the muscles 

 which move the jaw, so that these muscles, receiving more 

 and more blood, will become permanently increased in size 

 and power. The portions of the skull into which the jaw- 

 bones fit will likewise receive an extra strain, and will con- 

 sequently increase in rate of nutrition and grow to a larger 

 size, so that the shape of the whole head will be altered. 

 This increased weight of the head, and the increasingly 

 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 

 probably a perceptible 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 inevitably 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 luuiis and the aeration 



