SEQUOIA AND ITS HISTORY. 225 



subarctic regions in pliocene times, and that it had 

 been gradually pushed southward as the temperature 

 lowered and the glaciation advanced, even beyond its 

 present habitation ; that plants of the same stock and 

 kindred, probably ranging round the arctic zone as the 

 present arctic species do, made their forced migration 

 southward upon widely different longitudes, and re- 

 ceded more or less as the climate grew warmer ; that 

 the general difference of climate which marks the east- 

 ern and the western sides of the continents the one 

 extreme, the other mean was doubtless even then 

 established, so that the same species and the same sorts 

 of species would be likely to secure and retain foothold 

 in the similar climates of Japan and the Atlantic 

 United States, but not in intermediate regions of 

 different distribution of heat and moisture ; so that 

 different species of the same genus, as in Torreya, or 

 different genera of the same group, as redwood, Taxo- 

 dium, and Glyptostrobus, or different associations of 

 forest-trees, might establish themselves each in the 

 region best suited to the particular requirements, 

 while they would fail to do so in any other. These 

 views implied that the sources of our actual vegetation 

 and the explanation of these peculiarities were to be 

 sought in, and presupposed, an ancestry in pliocene or 

 earlier times, occupying the higher northern regions. 

 And it was thought that the occurrence of peculiar 

 North American genera in Europe in the Tertiary 

 period (such as Taxodium, Carya, Liquidambar, sassa- 

 fras, Negundo, etc.) might be best explained on the as- 

 sumption of early interchange and diffusion through 

 North Asia, rather than by that of the fabled Atlantis. 



