SUCCESSIVE MAMMALIAN FAUNAS 193 



Attention was long ago directed to the fact that the tops of 

 high mountains support a flora and fauna which, on the low- 

 lands, will be found only hundreds, or even thousands, of miles 

 to the northward. The plants which grow on the summits 

 of the White Mountains of New Hampshire recur in Labrador, 

 but not in the intervening area ; the vegetation and animals 

 of the high Alps are those of the Arctic regions, and many 

 similar instances might be cited. Hooker and Darwin were 

 the first to find a highly probable explanation of this curious 

 phenomenon by referring it to the climatic changes of the Pleis- 

 tocene epoch. During the last period of cold and glaciation, 

 the northern plants and animals were driven far to the south 

 and occupied the lowlands along the ice-front and well beyond 

 it ; when milder conditions gradually returned, the northern 

 forms not only retreated northward, but also ascended the 

 mountains, as the latter were freed from ice, and thus became 

 cut off as isolated colonies. The general explanation of ''dis- 

 continuous distribution" (see p. 138) is thus always the same, 

 viz., that the intervening regions were once occupied by the 

 forms now so widely separated, which, for one reason or another, 

 have vanished from the connecting areas. 



I. Quaternary Faunas 



North America. — The Quaternary faunas of North America 

 are extremely difficult to correlate and place in chronological 

 order, because, for the most part, they are found in locally 

 restricted areas, such as tar-pools, bogs, caverns and similar 

 places. Professor Osborn has, however, succeeded in making 

 an admirable arrangement, which, though it will doubtless 

 be corrected and expanded by future research, represents 

 a most important advance. Of the general problem he says : 

 ''The study of the mammals of the Quaternary has by no means 

 progressed so far in America as in Europe ; it will be many 

 years before the faunistic succession can be worked out with 

 such chronologic accuracy and precision as has at last been 



