CENOZOIC ERA: AGE OF MAMMALS 595 



generalized, which also gave .rise to the hoofed mammals, such as 

 the horse, elephant, and ox. They were, in other words, mammals 

 with such indefinite characters that, by the modification of their 

 organs, their descendants could be developed into animals widely 

 different in form and habits, such as the lion and the dog, the seal 

 and the whale. Some of the members of the generalized carnivores 

 (Creodonta) were larger, others smaller, than a fox. The largest 

 form of the Eocene (Pachyaena) was the size of a small bear and had 

 unusually blunt teeth, which are thought to indicate that it lived on 

 decaying flesh. 



The primitive carnivores (Creodonta) (Fig. 537) lived through the 

 Eocene into the Oligocene, when they became extinct. Those that 

 passed into the latter epoch attained not only their greatest bodily 

 size, but their greatest brain capacity as well. This bears out the 

 general rule that the brains of surviving races are, upon the whole, 

 larger than those of declining races. However, we shall find in our 

 later study that certain tribes with well-developed brains, as for 

 example certain rhinoceroses (TeleoCeras) and elephants (Mastodon), 

 failed to survive. The reason for such extinction is usually, though 

 not always, to be found in the failure of other organs to develop to 

 meet new conditions. 



Marine Mammals. — Perhaps nothing shows the rapid evolution 

 of mammals in the Tertiary better than the appearance early in 

 the Eocene of whales perfectly adapted to marine existence, which 

 were not descended from the marine reptiles of the Mesozoic, but 

 from land mammals. Whether mammals gradually acquired an 

 aquatic habit because of the abundance of fish which they voluntarily 

 and habitually sought, or whether they were forced to find new food 

 on account of the competition on the land, it is not possible to state 

 (see also marine reptiles, p. 552), but probably in the one way or the 

 other, whales, porpoises, sea lions, and other animals arose. These 

 marine mammals were not descended from a common ancestor, but 

 some (manatee) are thought to have been derived from the same 

 stock as the elephant, some (whales) from carnivores, and some 

 (seals), possibly, from the same stock as the bear. 



Zeuglodon. — For many years enormous vertebrae have been 

 found in the Eocene deposits of the Gulf coast, the largest of which 

 measure 15 to 18 inches in length and weigh 50 to 60 pounds in the 

 fossil condition. They belong to marine mammals to which the 

 name Zeuglodon (Greek, zeugle, yoke, and odont-, tooth) has been given 



