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 Era. 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 at¬ 
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- 
