432 THE ICE AGE IF FORTH AMERICA. 



Wherefore the high, and not the low, latitudes must be 

 assumed as the birthplace of our present flora, and the present 

 arctic vegetation is best regarded as derivative of the temperate. 

 This flora, which, when circumpolar, was as nearly homogene- 

 ous round the high latitudes as the arctic vegetation is now, 

 when slowly translated into lower latitudes, would preserve its 

 homogeneousness enough to account for the actual distribution 

 of the same and similar species round the world, and for the 

 original endowment of Europe with what we now call Ameri- 

 can types. It would also vary or be selected from by the in- 

 creasing differentiation of climate in the divergent continents 

 and on their different sides in a way which might well account 

 for the present diversification. From an early period the sys- 

 tem of the winds, the great ocean-currents (however they may 

 have oscillated north and south), and the general proportions 

 and features of the continents in our latitude (at least, of the 

 American Continent), were much the same as now, so that 

 species of plants, ever so little adapted or predisposed to cold 

 winters and hot summers, would abide and be developed on 

 the eastern side of continents, therefore in the Atlantic United 

 States and in Japan and Manchuria : those with preference 

 for milder winters would incline to the western sides ; those 

 disposed to tolerate dryness would tend to interiors or to re- 

 gions lacking summer rain. So that, if the same thousand 

 species were thrust promiscuously into these several districts, 

 and carried slowly onward in the way supposed, they would 

 inevitably be sifted in such a manner that the survival of the 

 fittest for each district might explain the present diversity. 



Besides, there are resif tings to take into the account. The 

 Glacial period or refrigeration from the north, which at its 

 inception forced the temperate flora into our latitude, at its 

 culmination must have carried much or most of it quite beyond. 

 To what extent displaced, and how far superseded by the vege- 

 tation which in our day borders the ice, or by ice itself, it is 

 difficult to form more than general conjectures, so different 

 and conflicting are the views of geologists upon the Glacial 

 period. But upon any, or almost any, of these views it is safe 

 to conclude that temperate vegetation, such as preceded the 

 refrigeration, and has now again succeeded it, was either thrust 



