132 MAN AND THE LOWER ANIMALS n 



callosum." The subject requires careful re-in- 

 vestigation, but if the currently received state- 

 ments are correct, the appearance of the "corpus 

 callosum" in the placental mammals is the 

 greatest and most sudden Codification 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 con- 

 sists, 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. 



In the lower placental mammals, the cerebra. 

 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. 137) 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 

 hi.le 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 substance of the hemi- 



