SUCCESSIVE MAMMALIAN FAUNAS 207 



characteristic in that continent. In North America they have 

 been found only in Mexico and the southern United States. 



The many and great chmatic changes which took place in 

 the Pleistocene led to very extensive migrations of mammals 

 from one part of the continent to another, as the conditions of 

 temperature and moisture changed. In Interglacial stages, 

 when the climate was much ameliorated, southern species 

 spread far to the north, as when the fMastodon ranged into 

 Alaska, and the Manatee, or Sea-Cow, of Florida waters, came 

 up the coast to New Jersey, while the increasing cold of on- 

 coming glaciation caused a reverse movement and drove 

 northern and even Arctic forms far to the south. Thus, the 

 Musk-Ox, the Caribou and the northern fMammoth came 

 south beyond the Ohio and the Potomac, and the Walrus was 

 found on the South Atlantic coast. It is these migrations which 

 give such a mixed character to the Pleistocene faunas from the 

 climatic point of view, as it is often very difficult to correlate 

 or synchronize the fossiliferous deposits with the Glacial and 

 Interglacial stages, though this has been definitely accomplished 

 in several very important instances. 



The latest of the Pleistocene faunas is less completely 

 known than those of the earlier and middle portions of the 

 epoch, for but few localities have yet been discovered with 

 any extensive series of fossils. As worked out by Osborn, 

 this fauna coincided with the last Glacial stage and was a greatly 

 reduced and impoverished assemblage as compared with those 

 of the middle and lower Pleistocene, though it is not safe to 

 argue that all the animals not found in this fauna were already 

 extinct, for the known list is still far too short to be entirely 

 representative. The American jAIastodon {fMastodon ameri- 

 canus, see p. 196) was still abundant in the forested regions and 

 was apparently able to withstand severe winter temperatures, 

 as certainly was the fMammoth (Elephas fprimigenius, see 

 p. 196), which was so abundant in the coldest part of Siberia 

 and which extended south to the Potomac, presumably at this 



