78 THE AGE OF MAMMALS 



First, the anatomy of these mammals points back to the introduction into 

 AustraUa of some small arboreal opossum or Didelphys-Vikc forms as 

 the source of the wonderful adaptive radiation of the marsupials of this 

 continent. Whence this Didelphys-\\ke form came, whether from Asia 

 or from Antarctica, is unknown, and why the placental insectivorous forms 

 did not enter the continent at the same time is also a mystery. The present 

 imperfect palseontological evidence favors the entry of marsupials into Aus- 

 tralia by way of South America and Antarctica, but it must be remembered 

 that this turns upon the fact that our knowledge of pre-Oligocene mammal 

 life of Asia is entirely a blank. The affinities of the Australian mammalian 

 life with that of South America consist in the common presence of both 

 the polyprotodont or carnivorous forms, allied to the existing "Tasmanian 

 wolf" (Thylacinus) , and of the small diprotodont herbivorous forms (epanor- 

 thids, Ccenolestes), very remotely allied to the kangaroos. 



South America or Neogcea, A Theater of Evolution Equal to that of Africa 



South America appears to have had late Cretaceous or early Eocene 

 connections through Antarctica with Australia on the south, and with the 

 great northern radiation of mammals of the northern hemisphere through 

 North America. This constituted its original supply of mammalian life, 

 from which sprang a grand and peculiar adaptive radiation after the connec- 

 tions both with Australia and with North America were cut offthrough either 

 geographic or climatic barriers. There is no satisfactory evidence of connec- 

 tion at any time with the mammalian life of Africa except in very late Plio- 

 cene times through migration by way of North America. The twelve orders 

 of mammals which evolved in South America from these original sources of 

 supply include three (primates, rodents, and odontocetes) which appar- 

 ently arrived later than the remainder. 



Mammalian Orders of Pre-Pliocene Times, South America 



Marsupialia Condjdarthra 



Insectivora (identification doubtful) 



(rare) Toxodontia 

 Rodentia of the suborder Hystricomorpha Astrapotheria 



only Litopterna 



Edentata Pyrotheria 



Anthropoidea ^ (Platyrhini) Cetacea (Odontoceti) 



Conspicuous by their absence from the pre-Pliocene formations of South 

 America are the following orders : 



Chiroptera Artiodactyla 



Tillodontia Perissodactyla 



' It is important to note that the South American monkeys are widely distinct from the 

 Old World or catarrhine monkeys and apes. 



