DEPOSITS OF THE TERTIARY ERA 121 



have originated in the far distant past and are now scattered 

 here and there in isolated colonies, their present range indi- 

 cates former conditions of land and water. By comparing 

 the discontinuous distribution of such old groups in the dif- 

 ferent continents, we come to certain conclusions as to the 

 conditions of the continents during the time when their range 

 was continuous. We have already become acquainted with 

 several genera and species showing discontinuous distribu- 

 tion. I argued, in the first and fourth chapters, mainly from 

 distributional evidence, that North America was connected 

 with north-western Europe and with north-eastern Asia in 

 Pliocene times. We know of relict genera and species which 

 represent the life and the geographical conditions of still 

 more remote stages of the Tertiary Era, namely, the Miocene, 

 Oligocene and Eocene, and even periods of the secondary or 

 Mesozoic JEra. We thus possess in our recent fauna an invalu- 

 able adjunct to palaeontological research. That this state- 

 ment is not a mere assumption will be amply demonstrated in 

 the next few chapters. 



Returning to Professor Osborn's researches, he directs al- 

 tention to the striking diminution of European types in the 

 last stages of the Eocene deposits. This, he argues, might 

 have been due to the existence of prolonged geographic or 

 climatic barriers between the two continents. In Oligocene 

 times the faunal community with western Europe once more 

 becomes closer. It is important to note, Professor Osborn re- 

 marks, that many American lower Oligocene types are repre- 

 sented by more primitive forms of European upper Eocene and 

 partly of north African types. This stage is followed by a long 

 period of independent evolution and partial extinction of the 

 same fauna to the close of the lower Miocene age. About the 

 middle of the Miocene Period another profound change in 

 the mammals of North America occurs. This is mainly due to 

 the sudden appearance of a large number of new forms of 

 African and Eurasiatic origin, such as the elephants, which 

 are believed to have come from Africa, and the rhinoceroses 

 and the true ruminant animals, which are supposed to have 

 had their home in Asia or Europe. These North American 

 middle Miocene deposits contain animals which first appear 

 in the lower Miocene of Europe. Hence there is distinct evi- 



