96 THE AGE OF MAMMALS 



is in the struggle or competition wliicii we observe between two great divisions 

 of mammals, which are so distinct in their affinities and evolutionary stages 

 that they may almost be set apart as two groups. These are: 



I. Archaic primitive mammals, partly descended from ancestors of great 

 antiquity in the Age of Reptiles; mostly without modern descendants. 



II. Mammals with modern descendants and relationships, chiefly compris- 

 ing ancestors of existing families. 



The gradual djdng out, or extinction, of the archaic in competition with 

 the modern types thus affords a second important means of dividing the 

 Cajnozoic into faunal phases. A third means of distinguishing the faunal 

 phases, and one which lends variety to the subject, is that, especially during 

 periods of separation, several families independently evolve in Europe and 

 North America respectively, without interchange by migration. 



Through these three means we may clearly divide the Csenozoic into 

 seven great faunal phases, as follows: 



I. First Faunal Phase, Basal Eocene, archaic maimnals only are known, 

 (p. 102). 



II. -Second Faunal Phase, Lower Eocene, archaic and modern mammals 

 intermingled (p. 112). 



III. Third Faunal Phase, Lower to Upper Eocene. Europe and North 

 America separated (p. 138). 



IV. Fourth Faunal Phase, Oligocene. Archaic mammals extinct. Europe 

 and North America reunited (p. 178). 



V. Fifth Faunal Phase, Miocene. African mammals reach Europe and 



North America. Europe invaded from Asia (p. 242). 

 VI. Sixth Faunal Phase, Middle Pliocene. North and South America 



reunited (p. 304). 

 VII. Seventh Faunal Phase, Pleistocene. Widespread extinction. Fresh 

 invasion of America by European mammals (p. 374). 



h. The archaic mammals. — Nature deals in transitions rather than in 

 sharp lines. We cannot circumscribe the archaic mammals sharply, nor 

 be sure as yet that some of them did not give direct descent to certain of the 

 modernized mammals. Yet the mammals of the Basal Eocene of both Eu- 

 rope and North America are altogether of very ancient type ; they exhibit 

 many primitive characters, such as extremely small brains, simple, triangular 

 teeth, five digits on the hands and feet, prevailing plantigradism. They 

 are to be collectively regarded as the first grand attempts of nature to estab- 

 lish insectivorous, carnivorous, and herbivorous groups, or unguiculates 

 and ungulates. The ancestors or centers of these adaptive radiations 

 date far back in the Age of Reptiles. At the beginning of the Eocene we 

 find the lines all separated from each other but not as yet very highly special- 

 ized. The speciahzation and divergence of these archaic mammals con- 



