THE EOCENE PERIOD. 229 



seems to point to some set of conditions which caused an exceptionally 

 rapid deployment of the great class at this stage, whatever their previous 

 history had been. 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 very earliest Eocene placentals, so far as they can be inter- 

 preted from the remains in the basal Eocene (Puerco beds of America 

 and Cernaysian of France), constituted an assemblage of groups quite 

 vaguely differentiated, in which the present orders were rather fore- 

 shadowed than distinctly expressed. The present great groups of 

 herbivores were foreshadowed by the Condylarthra, and the carnivores, 

 by the Creodonta, but these were not sharply distinguished, both 

 classes being five-toed plantigrades, the ends of whose phalanges were 

 armed with horny coverings that were neither quite hoofs nor claws. 

 Thus the first stages of the now pronounced division into the ungulates 

 and the unguiculates were only obscurely indicated. So obscure are 

 the relationships of the ancestral edentates, the Ganodonta, that they 

 have only been recognized recently through the critical studies of Wort- 

 man and Osborn. The insectivores were not more definitely charac- 

 terized, and Eocene genera were referred to the order Insectivora and 

 later withdrawn by the early paleontologists, because of their uncer- 

 tain limitations and imperfect differentiation. The definition of the 

 ancestral lemuroids was equally imperfect. All these orders seem, how- 

 ever, to have been represented in this obscure fashion. The rodents 

 have not been recognized in the Puerco beds, though present in the 

 Wasatch. 



But so rapid was the early evolution that before the close of the 

 Eocene, the Herbivora (Ungidata), Camivora, Edentata, Insectivora, 

 Rodentia, Quadra mana, Cetacea, and Sirenia, and probably the Cheirop- 

 tera were distinctly defined. Progress was even made in the evolution 

 of some of the suborders and families. It seems to have been a most 

 remarkable instance of rapid evolution. 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 species of living 

 invertebrates, the great difference between the stage of evolution of the 

 invertebrates and of the placentals may be realized. 



From this general view we may turn to some of the salient facts rela- 

 tive to the evolution of the several orders. 



