SEQUOIA AND ITS HISTORY. 150 
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! by 
a distinguished European botanist, Professor Grisebach, of 
1 “Die Vegetation der Erde nach ihrer klimatischen Anordnung.” 
1871. 
