But before entering upon the precise question of the amount of
difference between the Ape's brain and that of Man, it is necessary
that we should clearly understand what constitutes a great, and what a
small difference in cerebral structure; and we shall be best enabled to
do this by a brief study of the chief modifications which the brain
exhibits in the series of vertebrate animals.
The brain of a fish is very small, compared with the spinal cord into
which it is continued, and with the nerves which come off from it: of
the segments of which it is composed--the olfactory lobes, the cerebral
hemisphere, and the succeeding divisions--no one predominates so much
over the rest as to obscure or cover them; and the so-called optic lobes
are, frequently, the largest masses of all. In Reptiles, the mass of
the brain, relatively to the spinal cord, increases and the cerebral
hemispheres begin to predominate over the other parts; while in Birds
this predominance is still more marked. The brain of the lowest
Mammals, such as the duck-billed Platypus and the Opossums and
Kangaroos, exhibits a still more definite advance in the same
direction.
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