THE NEARCTIC REGION 171 



Section VII. — The Past History of the Nearctic 

 Mammal-fauna 



During the last twenty years the wonderful discoveries 

 of American palaeontologists have thrown a flood of light, 

 not only on the past history of the Nearctic Region, but 

 also on the evolution of many of the mammalian groups 

 themselves. It is, therefore, very necessary, when review- 

 ing the geographical distribution of the present mammalian 

 fauna, to shortly recapitulate the more important results 

 and conclusions arrived at from their writings. 



A very useful and comprehensive summary of this 

 work will be found in a paper by Professor Zittel (3), lately 

 published in the Geological Magazine. The beds which 

 contain the remarkably perfect remains above alluded to 

 are found only in the western part of North America. 

 Here, apparently, there existed throughout the Tertiary 

 Epoch a series of great fresh-water lakes, on the sides and 

 the bottoms of which were formed an almost continuous 

 series of deposits with the remains of the animals of the 

 surrounding districts embedded in them. The great 

 interest of these discoveries lies in the fact that we can 

 here trace the gradual formation and evolution of several 

 of the mammalian orders as they at present exist. In the 

 oldest beds the mammals resemble one another so closely 

 that it is often impossible to assign them very definitely 

 to any of the existing orders, although the germs of the 

 commencing distinctive characters can even here be 

 traced. 



In the later beds the various groups gradually 

 differentiate themselves, until in the most modern of 



