Chap. IV. MANNER OF DEVELOPMENT. 143 



runs with a sidelong shambling gait, but more commonly 

 progresses by resting on its bent hands. The long- 

 armed apes occasionally use their arms like crutches, 

 swinging their bodies forward between them, and some 

 kinds of Hylobates, without having been taught, can 

 walk or run upright with tolerable quickness ; yet they 

 move awkwardly, and much less securely than man. 

 We see, in short, with existing monkeys various grada- 

 tions between a form of progression strictly like that of 

 a quadruped and that of a biped or man. 



As the progenitors of man became more and more 

 erect, with their hands and arms more and more modi- 

 fied for prehension and other purposes, with their feet 

 and legs at the same time modified for firm support 

 and progression, endless other changes of structure 

 would have been necessary. The pelvis would have 

 had to be made broader, the spine peculiarly curved 

 and the head fixed in an altered position, and all these 

 changes have been attained by man. Prof. Schaaff- 

 hausen 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 orangj chim- 

 panzee, &c, and are smaller in the gorilla than in man. 

 Various other structures might here have been specified, 

 which appear connected with man's erect position. It 

 is very difficult to decide how far all these correlated 

 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 



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

 logical Review,' Oct. 1868, p. 428. Owen (' Anatomy of Vertebrates,' 

 vol. ii. 1SUG, p. 551) on the mastoid processes in the higher apes. 



