138 THE AGE OF MAMMALS 



many genera of mammals, a faunal community which will probably be 

 enriched rather than diminished by future discoveries in France, we now 

 enter a long period of disunion. This disunion culminates in there being at 

 the close of the Eocene only five families of mammals common to western 

 Europe and the Rocky Mountains, namely, Lophiodontidae, Dichobunidse, 

 Homacodontidse, Equidse, Hysenodontidae, while at the same time eleven 

 families of mammals are known in Europe which have not been discovered 

 in America, and thirteen families are known in America which have not 

 been discovered in Europe. We can thus hardly avoid the conclusion that 

 there was a prolonged period of geographic or climatic isolation between 

 the two regions, constituting a very distinct faunal phase, as follows: 



///. THIRD FAUNAL PHASE — GRADUAL DIVERGENCE BETWEEN 

 MAMMALS OF NORTH AMERICA AND THOSE OF WESTERN 

 EUROPE, AND LITTLE EVIDENCE OF FAUNAL INTERCHANGE. 

 NO EVIDENCE OF FURTHER NORTHERN OR EURASIATIC MIGRA- 

 TION IN NORTH AMERICA. DESCENDANTS OF THE ARCHAIC AND 

 MODERNIZED MAMMALS SLOWLY EVOLVING AND COMPETING 

 WITH ONE ANOTHER DURING THE LOWER AND MIDDLE EOCENE. 

 GRADUAL ELIMINATION OF THE ARCHAIC MAMMALS. CON- 

 TINUATION IN NORTH AMERICA OF SIMILAR CONDITIONS OF 

 ENVIRONMENT. 



As to life in the mountain region, attention may be especially directed 

 to the evidence of uniform and favorable environmental conditions and per- 

 sistent evolution throughout the Middle and Upper Eocene periods. The 

 changes are those of progressive modification and adaptation rather than of 

 breaks in the balance of nature by migration or extinction. Both the archaic 

 and the modernized mammals increase in size and in variety. The changes 

 are, moreover, specific rather than generic. At the close of the Lower 

 Eocene, or Coryphodon Zone, just considered, we have observed the elimi- 

 nation of the phenacodonts, coryphodonts, palaeonictids, arctocyonids; 

 no further very striking elimination occurs. The surviving archaic mammals 

 appear to flourish and increase, especiall^'^ in size and muscular power. 



It is noteworthy that in this long period, and even up to the summit of 

 the American Eocene, only three new or previously unknown families of 

 mammals make their appearance. These are the ancestral camels (Camel- 

 idse), the oreodonts (Oreodontidse) , and the primitive armadillos (Edentata 

 Dasypoda). In western Europe there is by contrast the repeated appear- 

 ance of new families from some easterly and northerly sources. 



Correlation. — Both in Europe and in North America a rich mam- 

 malian fauna opens the Middle Eocene and pursues a quite independent 

 evolution in the Old and New Worlds during the entire Middle and Upper 



