uld 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. The
cerebral hemispheres have now so much increased in size as, more or
less, to hide the representatives of the optic lobes, which remain
comparatively small, so that the brain of a Marsupial is extremely
different from that of a Bird, Reptile, or Fish. A step higher in the
scale, among the placental Mammals, the structure of the brain acquires
a vast modification--not that it appears much altered externally, in
a Rat or in a Rabbit, from what it is in a Marsupial--nor that the
proportions of its parts are much changed, but an apparently new
structure is found between the cerebral hemispheres, connecting them
together, as what is called the 'great commissure' or 'corpus callosum.'
The subject requires careful re-investigation, but if the currently
received statements are correct, the appearance of the 'corpus callosum'
in the placental mammals is the greatest and most sudden modification
exhibited by the brain in the whole series of vertebrated animals--it is
the greatest leap anywhere made by Nature in her brain work. For the
two halves of the brain being once thus knit together, the progress of
cerebral complexity is traceable through a complete series of steps from
the lowest Rodent, or Insectivore, to Man; and that complexity consists,
chiefly, in the disproportionate development of the cerebral hemispheres
and of the cerebellum, but especially of the former, in respect to the
other parts of the brain.

In the lower placent