95 
to hide the representatives of the optic lobes, which remain 
comparatively small, so that the brain of a Marsupial is ex- 
tremely 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 isin 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 cerebel- 
lum, but especially of the former, in respect to the other parts 
of the brain. 
In the lower placental mammals, the cerebral hemispheres 
leave the proper upper and posterior face of the cerebellum 
completely visible, when the brain is viewed from above, but, 
in the higher forms, the hinder part of each hemisphere, sepa- 
rated only by the tentorium (p. 99) from the anterior face of 
the cerebellum, inclines backwards and downwards, and 
grows out, as the so-called “ posterior lobe,” so as at length 
to overlap and hide the cerebellum. In all Mammals, 
each cerebral hemisphere contains a cavity which is termed 
the ‘ventricle’ and as this ventricle is prolonged, on the one 
hand, forwards, and on the other downwards, into the sub- 
stance of the hemisphere, it is said to have two horns or 
‘cornua,’ an ‘anterior cornu,’ and a ‘descending cornu.’ 
When the posterior lobe is well developed, a third prolonga- 
