THE EVOLUTION OF THE MAMMALIA 457 



Amblypoda, large, clumsy, premature rhinoceros-like forms 

 such as Uintatherium, and which soon went extinct. 



In the later Eocene divergent evolution has progressed, and 

 it is very interesting to notice that a number of the Orders of 

 Mammals have become differentiated, and that these are not 

 yet split up into the various families. The Creodonta had given 

 rise to the Carnivora, which branched out into the Pinnipedia 

 or seals, and Fissipedia or dogs, cats, bears, civets, and badgers. 

 The Rodentia came off from near the primitive Insectivora, as 

 did also the Primates (Lemuroids and Tarsioids) and the 

 Edentates. The Perissodactyla or odd- toed Ungulates emerged 

 from a stock intermediate between Condylarthra and Insecti- 

 vora, and blossomed out into the huge Titanotheres which soon 

 went extinct, the horses, tapirs, and rhinoceroses. The even- 

 toed Ungulates or Artiodactyla emerged from some form 

 between the Creodonta and the Insectivora, and, apart from a 

 number of short-lived groups, radiated out into the pigs and 

 hippopotamuses on the one hand, and the camels, antelopes, 

 deer, cattle, and giraffes on the other. Related to the Ungulates 

 are the conies (Hyracoidea) and the elephants (Proboscidea). 

 The whales (Cetacea) are regarded as having arisen from a 

 stock related to the Creodonta, and the Sirenia may have a 

 common descent with the Proboscidea. South America 

 became inhabited by a peculiar collection of archaic forms 

 which were all doomed to extinction, but of which some such 

 as the Thoatheria had evolved into a very remarkable imitation 

 of the horses. The Edentata include armadillos, sloths, and 

 ant-eaters. 



The Cheiroptera or bats are related to the Insectivora, 

 while the Dermoptera have affinities with the Insectivora and 

 Primates. In some cases sufficient fossil forms are known 

 from successive strata to enable lines of descent to be traced 

 with considerable precision. This applies especially to the 

 horses, the camels, and the elephants. The evolution of the 

 horses from Eohippus (Eocene) through Mesohippus (Oligo- 

 cene), Miohippus (Miocene), Pliohippus (Pliocene) to Equus, 

 was accompanied by a progressive increase in size, lengthening 

 of the teeth which become " rootless,'' development of ridges 



