GEOGRAPHIC DISTRIBUTION 



259 



malian evolution and the continued definite direction and dif- 

 ferences of velocity in the development of characters have 

 been discovered. 



General Succession of Mammalian Life in North 



America 



In Upper Cretaceous and Pakeocene time we find that the 

 northern hemisphere is covered with an archaic adaptive radi- 

 ation of mammals distinguished 

 by the extremely small size of 

 the brain and clumsy mechanics 

 of the skeleton. Of these the 

 carnivorous forms radiate into a 

 number of families adapted to a 

 great variety of feeding and lo- 

 comotor habits which are anal- 

 ogous to the families of existing 

 Carnivora. Similarly the hoofed 

 mammals (Condylarthra, Am- 

 blypoda) divide into swift- 

 footed (cursorial) and heavy- 

 footed (graviportal) forms, the 

 latter including the Amblypoda 

 {Coryphodon and Dinoccras) . 

 From surviving members of this 

 archaic adaptive radiation of small-brained mammals there arise 

 all the stem forms of the orders existing to-day, which almost 

 without exception have now been traced back to the close of 

 Eocene time, namely, the ancestors of the whales, of the modern 

 families of carnivores, insectivores, bats, lemurs, rodents, and 

 the edentates (armadillos and ant-eaters). Especially remark- 

 able is the discovery in the Lower Eocene of the ancestors of 



Fig. 124. Two Stages in the Early 

 Evolution of the Ungulates. 



Pantolambda {A), an archaic Palaeocene 

 form which transforms into Coryphodon 

 (B), a Lower Eocene form of increased 

 size, with greatly enlarged head, ab- 

 breviated tail, and defensive tusks. 

 This transformation occupied a period 

 estimated at 500,000 years, nearly one- 

 sixth of Tertiary time. Restorations 

 in the American Museum of Natural 

 Historv, bv Osborn and Knight. 



