24 



MAMMALIAN DENTITION 



find ourselves already at the parting of the ways where the 

 various orders of Mammals diverge. We must then realize that 

 all Mammals are members of one great tribe and that we can- 

 not consider the origin of one family or even of one species 

 without referring eventually to the ancestors of others. 



Mammalian history covers a colossal period of time. Not a 

 century, not a millennium but eons are occupied in the evolution 

 of the mammalian class. During this period incalculable in 



Fig. 1. — Zoological regions on north polar projection. (After Matthew.) The 

 unshaded areas around the continents indicate the extent of the continental shelf and 

 thus show the greatest possible land connections permitted by the theory of Isostatic 

 Balance. 



human thought the earth has seen considerable changes; 

 periods of heat and cold have followed each other ; the ground 

 has sunk beneath the sea and risen again above it; yet, and this 

 is very important, the main confines of land and water in all 

 probability have remained approximately the same. If one 

 turn to an ordinary map of the world on Mercator's pro- 



