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 forefathers
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 or rivals, they would
use their jaws and teeth less and less. In this case, the jaws,
together with the teeth, would become reduced in size, as we may feel
almost 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 ruminants, apparently in
relation with the development of their horns; and in horses, in
relation to their habit of fighting with their incisor teeth and hoofs.
In the adult male anthropomorphous apes, as Rutimeyer (77. Die Grenzen
der Thierwelt, eine Betrachtung zu Darwin s Lehre, 1868, s. 51.), and
others, have insisted, it is the effect on the skull of the great
development of the jaw-muscles that causes it to differ so greatly in
many respects from that of man, and has given to these animals a truly
frightful physiognomy. Therefore, as the jaws and teeth in man s
progenitors gradually become reduced in size, the adult skull would
have come to resemble more and more that of existing man. As we shall
hereafter see, a great reduction of the canine teeth in the males would
almost certainly affect the teeth of the females through inheritance.
As the various mental faculties gradually developed themselves the
brain would almost certainly become larger. No one, I presume, doubts
that the large proportion which the size of man s brain bears to his
body, compared to the same proportion in the gorilla or orang, is
closely connected with his higher mental powers. We meet with closely
analogous facts with insects, for in ants the cerebral ganglia are of
extraordinary dimensions, and in all the Hymenoptera these ganglia are
many times larger than in the less intelligent orders, such as beetles.
(78. Dujardin, Annales des Sciences Nat. 3rd series, Zoolog., tom.
xiv. 1850, p. 203. See also Mr. Lowne, Anatomy and Phys. of the Musca
vomitoria, 1870, p. 14. My son, Mr. F. Darwin, dissected for me the
cerebral ganglia of the Formica rufa.) On the other hand, no one
supposes that the intellect of any two animals or of any two men can be
accurately gauged by the cubic contents of their skulls. It is certain
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