60 tse descent of man. 



No doubt these means of change often co-operate; thus 

 when certain muscles, and tlie crests of bone to which 

 they are attached, become enlarged by habitual use, this 

 shows that certain actions are habitually performed and 

 must be serviceable. Hence the individuals which per- 

 formed them best would tend to survive in greater numbers. 



The free use of the arms and hands, partly the cause 

 and partly the result of man's erect position, appears to 

 have led in an indirect manner to other modifications of 

 structure. The early male forefathers of man were, as 

 previously stated, probably furnished with great canine 

 teeth; but as they gradually acquired the habit of using 

 stones, clubs, or other weapons for fighting with their 

 enemies or rivals they would use their jaws and teeth less 

 and less. In this case the jaws, together with the teeth, 

 would become reduced in size, as we may feel almost sure 

 from innumerable analogous cases. In a future chapter 

 we shall meet with a closely parallel case in the reduction 

 or complete disappearance of the canine teeth in male 

 ruminants, apparently in relation with the development of 

 their horns; and in horses in relation to their habits of 

 fighting with their incisor teeth and hoofs. 



In the adult male anthropomorphous apes, as Eiiti- 

 meyer* and others have insisted, it is the effect on the 

 skull of the great development of the jaw-muscles that causes 

 it to differ so greatly in many respects from that of man, 

 and has given to these animals ^' a truly frightful physi- 

 ognomy." Therefore, as the jaws and teeth in man's pro- 

 genitors gradually become reduced in size, the adult skull 

 would have come to resemble more and more that of exist- 

 ing man. As we shall hereafter see, a great reduction of 

 the canine teeth in the males would almost certainly affect 

 the teeth of the females through inheritance. 



As the various mental faculties gradually developed 

 themselves the brain would almost certainly become larger. 

 No one, I presume, doubts that the large proportion which 

 the size of man's brain bears to his body, compared to the 

 same proportion in the gorilla or orang, is closely connected 

 with his higher mental powers. We meet with closely 

 analogous facts with insects, for in ants the cerebral gan- 

 glia are of extraordinary dimensions, and in all the Hyme- 



*"Die Grenzen der Tliierwelt, eine Betrachtung zu Darwin's 

 Lelire," 1868, s. 51. 



