SEQUOIA AND ITS HISTORY. 157 



grew warmer ; that the general difference of climate which 

 marks the eastern 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 Red- 

 wood, Taxodium, and Glyptostrobus, or different associations 

 of forest trees, might establish themselves each in the region 

 best suited to their 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 ances- 

 try in pliocene or still earlier times, occupying the higher 

 northern regions. And it was thought that the occurrence of 

 peculiarly North American genera in Europe in the tertiary 

 period (such as Taxodium, Carya, Liquidambar, Sassafras, 

 Negundo, etc.), might be best explained on the assumption of 

 early interchange and diffusion through north Asia, rather 

 than by that of the fabled Atlantis. 



The hypothesis supposed a gradual modification of species 

 in different directions under altering conditions, at least to 

 the extent of producing varieties, sub-species, and representa- 

 tive species, as they may be variously regarded ; likewise the 

 single and local origination of each type, which is now almost 

 universally taken for granted. 



The remarkable facts in regard to the eastern American 

 and Asiatic floras which these speculations were to explain 

 have since increased in number, more especially through the 

 admirable collections of Dr. Maximowicz in Japan and adja- 

 cent countries, and the critical comparisons he has made and 

 is still engaged upon. 



I am bound to state that, in a recent general work l by 

 a distinguished European botanist, Professor Grisebach, of 



1 " Die Vegetation der Erde nach ihrer klimatischen Anordnung." 

 1871. 



