228 THE GENESIS AND MIGRATIONS OF PLANTS 



species. They are separated by oceans and by belts of land 

 occupied by plants which have not been obliged to migrate. 

 Thus, while the flora of the Eastern United States resembles 

 that of China and Japan, that of California and Oregon is 

 distinct from both, and represents a belt of old species retained 

 in place by the continued warmth of the Pacific shore, and the 

 continuous extension of the American continent to the south 

 affording them means of retreat in the Glacial age. Were the 

 plants of China and Eastern America enabled to return to the 

 Arctic, they would then reunite into one flora. Gray compares 

 the process of their separation to the kind of selection which 

 might be made by a botanical distributor who had the whole 

 collection placed in his hands, with instructions to give one 

 species of each genus to Europe, to Eastern Asia, and to 

 Eastern America; and if there was only one species in a 

 genus, or if one remained over, this was to be thrown into one 

 of the regions, with a certain preference in favour of America 

 and Asia. This remarkable kind of geographical selection 

 opens a wide field not only for thought, but for experiment on 

 the actual relationship of the representative species. There is 

 a similar field for comparison between the trees of Georgia in 

 latitude 30° to 35°, and the same species or their representa- 

 tives as they existed in Cretaceous times in the latitudes of 

 50° and 60°. The two floras, as I know from actual com- 

 parison, are very similar. 



One word may be said here as to use of fossil plants in 

 determining geological time. In this I need only point to 

 the fact of my having defined in Canada three Devonian 

 floras, a Lower, Middle, and Upper, and that Mr. Whiteaves, in 

 his independent study of the fossil fishes, has vindicated my 

 conclusions. There are also in Nova Scotia three distinctive 

 sub-floras of the Lower, Middle, and Upper Carboniferous.^ I 



* Transactions Royal Society of Canada, 1883 to 1891. 



