Chap. IV. 
MANNER OF DEVELOPMENT. 
143 
progresses by resting on its bent bands. 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 orang, 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. 1866, p. 551) on the mastoid processes in the higher apes. 
