MAMMALIA 345 



latter half of the Eocene period as the time when the modernized 

 mammals arose. Some types evidently arose considerably later. 



Migrations. — The spread of the modernized mammals must have 

 been southward. This must have been so for two reasons: first, that 

 was the only possible direction in which a group originating in the 

 north could migrate; and second, because the increasing cold which 

 culminated in the first glacial epoch, must have driven the majority 

 of the mammals out of the northern regions. A few of the hardiest 

 types still find these regions habitable. The migration occurred in 

 several great waves, probably due to the alternating periods of cold 

 and warm climate in the north. The groups least tolerant of cold 

 probably migrated southward first and went farthest south; among 

 these first migrants were probably the insectivores and primates; 

 these were probably followed by the perissodactyls (horses and tapirs) 

 and, somewhat later, by the true carnivores, especially the cat-like 

 forms. The bears and rodents remained longer than the rest and 

 still live well toward the north. To-day the modernized mammals 

 have a world-wide distribution except in the oceanic islands, which 

 they have no means of reaching. 



Mammals of the Present 

 Brief Classification 



Class Mammalia. "Beasts," "quadrupeds," "animals." Warm- 

 blooded, hair-clad vertebrates with mammary glands. 



Sub-Class I. Prototheria. — Egg-laying mammals. 

 Order 1. Monotremata. 



Family 1. Ornithorhynchidae. 

 " 2. Echidnidae. 



Sub-Class II. Eutheria.—Yiviparous mammals. 



Division I. Didelphia (Metatheria). — Marsupials. 



Order 1. MarsupiaUa. Mammals that usually carry 

 the young in a pouch; usually no placenta. 

 Sub-Order 1. Polyprotodontia. 

 Sub-Order 2. Diprotodontia. 



Division II. Monodelphia (Placental mammals). Young 

 never carried in a pouch; a true placenta, 

 which nourishes the unborn fetus. 



