TO THE LOWER ANIMALS. 113 



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 appar- 

 ently new structure is found between the cerebral hemi- 

 spheres, connecting them together, as what is called the 

 ' great commissure ' or ' corpus callosum.' The subject 

 requires careful re-investigation, but if the currently re- 

 ceived statements are correct, the appearance of the ' cor- 

 pus 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 Insect- 

 ivore, to Man ; and that complexity consists, chiefly, in 

 the disproportionate development of the cerebral hemi- 



