318 



HISTORICAL GEOLOGY 



the evolution of the Mammals during the Cenozoic was the gradual 

 increase in the relative sizes of the brains. The accompanying 



sketches graphically illustrate 

 this fact (Fig. 199). 



Mammals comprise the 

 highest of all animals and are 

 all characterized by suckling 

 the young. For convenience 

 of discussion, they may be 

 divided into three groups as 

 follows : (1) Monotr ernes or egg- 

 laying forms, such as the mod- 

 ern Spiny Ant-eater; (2) Mar- 

 supials (e.g. Opossum and 

 Kangaroo) or those giving 

 birth to imperfectly formed 

 young, which are then carried 

 by the mother in a pouch 

 (marsupium); and (3) Placen- 

 tals (e.g. Dog, Horse, and 

 Man) or those giving birth to 

 well-formed young which, in 

 the prenatal condition, are 

 attached to the mother by the 

 placentum. So far as known, 

 only Monotremes and Mar- 

 supials existed during the Mes- 

 ozoic, but during the Tertiary 

 they were ver}^ subordinate to 

 the Placentals, and today they 

 are comparatively rare. The 

 Cenozoic was, (and is) there- 

 fore, very decidedly the "Age 

 of Placental Mammals." 



Because of the vast wealth 



of material concerning Tertiary 



Mammals, we can do no more, in our brief survey, than to refer to a 



few of the more interesting and better known evolutionary features. 



Generalized Mammals of the Early Tertiary. — Although 



Mammals were the dominant animals even in the early Tertiary, 



Fig. 199 

 Sketches to illustrate increase in size 

 of brains of Mammals from the 

 Eocene to the present. A, Eocene 

 Uintatherium; B, Miocene Bronto- 

 therium; C, modern Horse, E quits. 

 (After Marsh, from Shimer's "In- 

 troduction to the Study of Fossils," 

 courtesy of The Macmillan Com- 

 pany.) 



