THE EOCENE PEROID 787 



The Land Animals 1 

 The undifferentiated nature of the early Eocene mammals. It 



is difficult to carry our conceptions of mammals back to the Eocene 

 prototypes, without carrying with them distinctions which did not 

 then exist. The earliest Eocene mammals were much more prim- 

 itive than those of the Middle Eocene, and their rapid convergence 

 backward seems to point to some set of conditions which caused a 

 rapid advance of the class at this time whatever their previous his- 

 tory. The coming into a new domain of rich and varied conditions 

 whether by immigration or indigenous development, may be safely 

 included among these conditions. 



The mammals of the earliest Eocene, as now understood, in- 

 cluded several vaguely differentiated groups, in which existing 

 orders were foreshadowed rather than represented. The herbi- 

 vores were foreshadowed by the Condylarthra, and the carnivores 

 by the Creodonta; but the two were not sharply differentiated. 

 Both were five-toed plantigrades, whose phalanges had horny 

 coverings that were neither hoofs nor claws. Ancestral edentates, 

 insectivores, rodents, and lemuroids seem to have been represented 

 in an obscure fashion. 



The evolution of the mammals was so rapid that before the 

 close of the Eocene, the Herbivora (Ungulata), Carnivora, Edentata, 

 Insectivora, Rodentia, Quadrumana, Cetacea, and Sirenia, and 

 probably the Cheiroptera were distinctly defined (see p. 945). 

 None of the present genera, however, are known as early as the 

 Eocene. When it is recalled that the name Eocene was founded 

 on the presence of some few species of living invertebrates, the great 

 difference between the stage of their evolution and that of the mam- 

 mals may be realized. In general the mammalian faunas of the 

 early Eocene were closely similar to those of Western Europe, 

 while in the middle and late Eocene there seems to have been 

 faunal separation from Europe. 2 



The main herbivore line. While the condylarths and creodonts 

 were" near each other at the beginning of the period, the hoofed 



1 For references to important literature on the American Tertiary Mam- 

 malia, see the authors' larger work, Vol. Ill, p. 228. 



2 Osborn, Bull. 361, U. S. Geol. Surv. 



