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.
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