Paleontology. 



195 



construction of teeth, which has already been depicted 

 as regards the particular case of the Horse-family (Fig. 

 83), is no less apparent in the pedigree of all the 

 other mammalia, wherever the palaeontological history 

 is sufficiently intact to serve as a record at all. 



FIG. 87. Ideal section through all the above stages. (After Le Conte.} 



Lastly, as regards the skull, casts of the interior 

 show that all the earlier mammals had small brains 

 with comparatively smooth or unconvoluted surfaces ; 

 and that as time went on the mammalian brain 

 gradually advanced in size and complexity. Indeed 

 so small were the cerebral hemispheres of the primitive 

 mammals that they did not overlap the cerebellum, 

 while their smoothness must have been such as in this 

 respect to have resembled the brain of a bird or reptile. 

 This, of course, is just as it ought to be, if the brain, 

 which the skull has to accommodate, has been gradually 

 O 2 



