THE DESCENT OR OBIOIN OF MAN 79 



the human skull are the result of his erect position"; and 

 these processes are absent in the orang, chimpanzee, etc., 

 and are smaller in the gorilla than in man. Various other 

 structures, which appear connected with man's erect posi- 

 tion, might here have been added. It is very difficult to 

 decide how far these correlated modifications are the result 

 of natural selection, and how far of the inherited efiects of 

 the increased use of certain parts, or of the action of one 

 part on another. No doubt these means of change often 

 co-operate; thus when certain muscles, and the crests of 

 bone to which they are attached, become enlarged by habit- 

 ual use, this shows that certain actions are habitually per- 

 formed and must be serviceable. Hence the individuals 

 which performed 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 rela- 

 tion to their habit of fighting with their incisor teeth and 

 hoofs. 



In the adult male anthropomorphous apes, as Etitimeyer" 

 and others have insisted, it is the effect on the skull of the 



Review," Oct., 1868, p. 428. Owen ("Anatomy of Vertebrates," voL iL, 

 J866, p. 551) on the mastoid processes in the higher apes. 



" "Die Grenzen der Thlerwelt, eine Betrachtung zu Darwin's Lehre," 

 1868. 8. 61. 



