Man's Erect Position 119 
of lungs and thorax, as a result of living constantly at high al- 
titudes. 
Such special forms of variation as arrests of development (micro- 
cephalism) and reversion to lower forms are next discussed. Darwin 
himself felt! that these subjects are so nearly related to the cases 
mentioned in the first chapter, that many of them might as well have 
been dealt with there. It seems to me that it would have been better 
so, for the citation of additional instances of reversion at this place 
rather disturbs the logical sequence of his ideas as to the conditions 
which have brought about the evolution of man from lower forms. 
The instances of reversion here discussed are microcephalism, which 
Darwin wrongly interpreted as atavistic, supernumerary mammae, 
supernumerary digits, bicornuate uterus, the development of ab- 
normal muscles, and so on. Brief mention is also made of correlative 
variations observed in man. 
Darwin next discusses the question as to the manner in which 
man attained to the erect position from the state of a climbing 
quadruped. Here again he puts the influence of Natural Selection in 
the first rank. The immediate progenitors of man had to maintain a 
struggle for existence in which success was to the more intelligent, 
and to those with social instincts. The hand of these climbing 
ancestors, which had little skill and served mainly for locomotion, 
could only undergo further development when some early member of 
the Primate series came to live more on the ground and less among 
trees. 
A bipedal existence thus became possible, and with it the 
liberation of the hand from locomotion, and the one-sided develop- 
ment of the human foot. The upright position brought about 
correlated variations in the bodily structure; with the free use of 
the hand it became possible to manufacture weapons and to use 
them; and this again resulted in a degeneration of the powerful 
canine teeth and the jaws, which were then no longer necessary for 
defence. Above all, however, the intelligence immediately increased, 
and with it skull and brain. The nakedness of man, and the absence 
of a tail (rudimentariness of the tail vertebrae) are next discussed. 
Darwin is inclined to attribute the nakedness of man, not to the 
action of natural selection on ancestors who originally inhabited 
a tropical land, but to sexual selection, which, for aesthetic reasons, 
brought about the loss of the hairy covering in man, or primarily in 
woman. An interesting discussion of the loss of the tail, which, 
however, man shares with the anthropoid apes, some other monkeys and 
lemurs, forms the conclusion of the almost superabundant material 
which Darwin worked up in the second chapter. His object was to 
1 Descent of Man, p. 54. 
