138 THE DESCENT OF MAN. [Pakt I. 



the spine peculiarly curved and the head fixed in an altered 

 position, and all these changes have been attained by man. 

 Prof. Schaafthausen 67 maintains that "the powerful mastoid 

 processes of 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 might here have been 

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

 tion. It is very difficult to decide how far all these cor- 

 related modifications are the result of natural selection, 

 and how far of the inherited effects of the increased use of 

 certain parts, or of the action of one part on another. No 

 doubt these means of change act and react on each other : 

 thus when certain muscles, and the 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 num- 

 bers. 



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 progenitors 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, they would have used their jaws and teeth less 

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

 would have become reduced in size, as we may feel 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 rumi- 



67 " On the Primitive Form of the Skull," translated in ' Anthropo- 

 logical Review,' Oct. 18G8, p. 428. Owen (' Anatomy of Vertebrates,' 

 rol. ii. 18G6, p. 551) on the mastoid processes in the higher apes. 



