or THE CLASS MAMMALIA. 



19 



adaptive principle, in the Cetacea ; but, in the rest of the sub-class, 

 with the exception of the Elephants, they pass out of the abdomen, 

 and the Gyrencephalous quadrupeds, as a general rule, have a 

 scrotum. The vidva is externally distinct from the anus. With 



Fig. 4. — Chimpanzee. 



Fig. 3. 



the exception, again, of the Elephants, the blood from the head 

 and anterior limbs is returned to the right auricle by a single 

 precaval trunk. The mammalian modification of the Vertebrate 

 type attains its highest physical perfections in the GyrencepJiala, 

 as manifested by the bulk of some, by the destructive mastery of 

 others, by the address and agility of a third order. And, tlirough 

 the superior psychological faculties — ran adaptive intelligence pre- 

 dominating over blind instinct — which are associated with the 

 higher development of the brain, the Gyrencephala afford those 

 species which have ever formed the most cherished companions 

 and servitors, and the most valuable sources of wealth and power, 

 to Mankind. 



In Man the brain presents an ascensive step in development, 

 higher and more strongly marked than that by which the pre- 

 ceding subclass was distinguished from the one below it. Not 

 only do the cerebral hemispheres (figs. 5 & 6, a) overlap the olfac- 

 tory lobes and cerebellum, but they extend in advance of the one, 

 and further back than the other (fig. 6, c) . Their posterior deve- 

 lopment is so marked, that anatomists have assigned to that part 

 the character of a third lobe ; it is peculiar to the genus Homo, and 



2* 



