TO THE LOWER ANIMALS 277 



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 com- 

 posed 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 Kan- 

 garoos, 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 w r hat 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 modi- 

 fication exhibited by the brain in the whole series of verte- 

 brated 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 develop- 



