ii VERTEBRATA: BRAINS. 131 



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 Mam- 

 mals, such as the duck-billed Platypus and the 

 Opossums and Kangaroos, exhibits a still more 

 definite advance in the same direction. The cere- 

 bral 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, Eeptile, or Fish. A 

 step higher in the scale, among the placental Mam- 

 mals, 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 hemi- 

 spheres, connecting them together, at what is 

 called the " great commissure " or " corpus cal- 

 losum." The subject requires careful re-investi- 

 gation, but if the currently received statements 

 are correct, the appearance of the " corpus cal- 

 losum ,J 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 



