(microcephalism) and reversion to lower forms are next discussed.
Darwin himself felt[93] 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 abnormal 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 development 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
conclus