542 THE EVOLUTION OF LIFE. 



fited in the struggle for life, by the slight decrease in these 

 comparatively-small hones? No functional superiority pos- 

 sessed by a small jaw over a large jaw in civilized life, can 

 be named as having caused the more frequent survival of 

 small-jawed individuals. The only advantage accompanying 

 smallness of jaw, is the advantage of economized nutrition; 

 and this cannot be great enough to further the preservation 

 of those distinguished by it. The decrease of weight in the 

 jaw and co-operative parts, which has arisen in the course 

 of thousands of years, does not amount to more than a few 

 ounces. This decrease has to be divided among the many 

 generations which have lived and died in the interval. Let 

 us admit that the weight of these parts diminished to the 

 extent of an ounce in a single generation (which is a large 

 admission) ; it still cannot be contended that the having to 

 carry an ounce less in weight, and to keep in repair an ounce 

 less of tissue, could sensibly affect any man's fate. And if it 

 never did this — nay, if it did not cause a frequent survival of 

 small-jawed individuals where large-jawed individuals died; 

 natural selection could neither cause nor aid diminution of 

 the jaw and its appendages. Here, therefore, the decreased 

 action which has accompanied the growth of civilized habits 

 (the use of tools and the disuse of coarse food), must have 

 been the sole cause at work. Through direct equilibration, 

 diminished external stress on these parts has resulted in 

 diminution of the internal forces by which this stress is met. 

 From generation to generation, this lessening of the parts 

 consequent on functional decline has been inherited. And 

 since the survival of individuals must always have been 

 determined by more important structural traits, this trait 

 can have neither been facilitated nor retarded by natural 

 selection. 



§ 167. Eeturning from these extensive classes of facts for 



jaw-bones are powerfully developed, and large in proportion to the cra- 

 nium."] 



^ 



i 



