TERTIARY MAMMALS. 131 



Seeing that throughout the whole of the Secondary 

 period, with the possible exception of a few lowly insecti- 

 vores, there is no evidence of the existence of any mammals 

 belonging to the higher placental type (under which are 

 included all living representatives of the class save the 

 marsupial and egg-laying groups), the reader will naturally 

 inquire when such higher forms first made their appear- 

 ance. We answer, with the first dawn of the Tertiary 

 period ; for in the very lowest Eocene strata both of France 

 and the United States there are found, side by side with 

 small mammals allied to Plagiaulas and the marsupials 

 of the Jurassic and Cretaceous, others, which, though still 

 of small size, were evidently placentals. And it is very 

 remarkable that this first definite appearance of the higher 

 forms of mammalian life should, so far as we know, have 

 been contemporaneous with the disappearance of so many 

 gigantic types of extinct reptiles, such as the dinosaurs, 

 the fish-lizards, and the plesiosaurs, which seem to have 

 reached the end of their term of existence at or about the 

 close of the Secondary period. 



Most of these early Tertiary mammals had molar teeth 

 carrying three cusps arranged in a triangle, like their 

 marsupial forerunners of the Secondary epoch, from 

 which, indeed, they were probably derived ; and at this 

 comparatively early epoch the orders of mammals were 

 but very imperfectly differentiated from one another, it 

 being frequently difficult to decide which were carnivores 

 and which were ungulates. A few stages later differentia- 

 tion of ordinal types, accompanied by a great increase in 

 the bodily size of their representatives, had, however, taken 

 place ; and by the close of the Eocene period, as exemplified 

 by the higher deposits of the Paris basin, most of the 

 present orders of mammals were well defined. Thence, 

 through the succeeding Miocene and Pliocene epochs, 

 there went on a continual evolution of mammalian life, 

 resulting in the production of giant forms like the elephant 

 and the rhinoceros, and also characterized by the develop- 

 ment of specially modified types like the horse and the ox, 

 which differ so widely from their five- toed ancestors. 

 During the same epochs antlers were developed in the 



