THE TERTIARY PERIOD 445 



we have mammals of the forest (for example, squirrels), of the 

 plains (antelopes), of the marshes (beavers), of the air (bats), 

 of the ocean (whales), and many more. 



Interestingly enough, as the mammals adopted these modes 

 of life, they often took on in a degree the form and appearance 

 of their reptilian predecessors. To appreciate this one has 

 only to compare the bat with the pterosaur (Fig. 435), the 

 porpoise with the fish reptiles (Fig. 421), and the rhinoceros 

 with the heavy dinosaurs (Fig. 433). 



Migrations of the Tertiary mammals. There are to-day 

 some very peculiar things about the distribution of certain 

 animals which are explained only when we study the fossils 

 from the Tertiary formations. The camels are now found 

 in Asia and Africa, and also in the Andes Mountains of South 

 America. In Tertiary times, as the fossils show us, they 

 roamed widely over western North America as well, and it 

 seems probable that they migrated thence to Eurasia by way 

 of Alaska at a time when that peninsula was less submerged 

 than now and enjoyed a warmer climate. Later they died 

 out in North America. This is but an instance of many 

 migrations by which the mammals of Eurasia and America 

 mingled during the Tertiary period. 



Some islands were so isolated by water that they could 

 not be reached by the mammals which originated in 

 the larger continents. Australia is a case in point. 

 There we find almost none of our familiar higher mammals, 

 but instead a host of peculiar marsupials, among which 

 are kangaroos, wombats, and opossums. It is known that 

 these marsupials are most closely related to animals that 

 lived in Europe in the Mesozoic era, but died out there earty 

 in the Tertiary period. The inference is that Australia has 

 been isolated from the other lands since perhaps the Creta- 

 ceous period, and that during the Tertiary period her peculiar 

 mammals evolved along their own lines without that inter- 

 ference which comes from sharp competition with the more 

 progressive higher animals. 

 B. & B. GEOL. 26 



