INTRODUCTION 65 



form, still remains as one of the great problems of the Age of Mammals 

 which has not been thoroughly worked out. The solution turns upon 

 pahpog(H)graiihy, or the past relations of the continents and islands to each 

 other, but many pala^og(^ograi)hic problems in turn appeal to the past and 

 present distribution of animals and plants. The existence of a great 

 southern continent, Antarctica, for example, is just begimiing to be de- 

 monstrated through geography and geology;^ it was first indicated through 

 the facts of palaeontology, zoology, and botany. 



Thus we may first briefly consider the distril)ution of mammalian or- 

 ders in the past, and then the palceogeography, or former distribution of 

 land surfaces, migrating tracts, etc. 



World-wide distribution during the Age of Rejdiles. — Our comparison 

 begins with Eocene times, or the first period of the Age of Mammals. We 

 find proofs from the very outset not only that all the great continents were 

 richly supplied with mammalian life, but that during the preceding Age 

 of Reptiles important migrations and interchanges of mammalian life had 

 taken place, establishing a sort of cosmopolitan distribution of the more 

 primitive forms, especially of the Insectivora. These generalized stem 

 forms of mammals were widely distributed. We discover (compare p. 64) 

 during the latter part of the Age of Reptiles and very early in the Age of 

 Mammals evidences of connections between the following areas: 



Australia, deriving its marsupials either from the north through Asia 

 and the northern continents, or from the south through Antarctica and 

 South America. 



South America, sharing a community of marsupial life with Australia 

 either through Antarctica on the south or through North America, Europe, 

 and Asia on the north, and of placental life with (?) North America. 



North America, sharing a community of life in the orders Marsupialia, 

 Edentata, and probably Condylarthra and (?) Amblypoda on the south with 

 South America, and on the north with Europe and probaljly Asia. 



Africa, showing evidence of a community of mammalian life with 

 Europe on the north and Asia on the northeast, but as yet no evidence of 

 connection either with the mammalian life. of Australia or the early mam- 

 malian life of South America. 



Mammals of the Northern Hemisphere. Holarctica the Grand Center of Evo- 

 lution 



As regards the grand centers of origin, Riitimeyer in his classic paper of 

 1867 set forth what is now known as the Bi-polar theory.^ Other authors 

 have advocated the North Polar theory; others again the South Polar 

 theory (Ameghino). 



' The recent British Antarctic expeditions confirm the pioneer geographic work of Wilkes, 

 and report the discovery of sedimentary deposits and of fossilized wood. 



- Kutimeyer, Ueber die Hcrkunft unserer Thierwelt. Basel und Genf, 1867. 



F 



