﻿444 ON THE BOTANY OF JAPAN. 



Atlantic United States, or of a new Eschscholtzia, Platystemon, or Calais west of the 

 Rocky Mountains, would excite no surprise. A converse discovery, or the detection of 

 any of these genera in a remote region, would excite great surprise. The discovery of 

 numerous closely related species thus divided between two widely separated districts 

 might not, in the present state of our knowledge, suggest former continuity, migration, 

 or interchange ; but that of identical species peculiar to the two inevitably would. 



Why should it? Evidently because the natural supposition is that individuals of 

 the same kind are descendants from a common stock, or have spread from a common 

 centre ; and because the progress of investigation, instead of eliminating this precon- 

 ception from the minds of botanists, has rather confirmed it. Every other hypothesis 

 has derived its principal support from diflB.culties in the application of this. A re- 

 view of what has been published upon the subject of late years makes it clear that 

 the doctrine of the local origin of vegetable species has been more and more accepted, 

 although, duiing the same period, species have been shown to be much more widely 

 dispersed than was formerly supposed. Facts of the latter kind, and the conclusions 

 to which they point, have been most largely and cogently brought out by Dr. Hooker, 

 and are among the very important general results of his extensive investigations. 

 And the best evidence of the preponderance of the theory of the local origin of 

 species, — notwithstanding the great increase of facts which at first would seem to 

 tell the other way, — is furnished by the works of the present De Candolle upon geo- 

 graphical botany. This careful and conscientious investigator formerly adopted and 

 strenuously maintained Schouw's hypothesis of the double or multiple origin of species. 

 But in his great work, the Geographie Botanique Raisonnee, published in the year 

 1855, he has in effect discarded it, and this not from any theoretical objections to that 

 view, but because he found it no longer needed to accoUnt for the general facts of dis- 

 tribution. This appears from liis qualified, though dubious, adherence to the hy- 

 pothesis of a double origin, as a dernier resort, in the few and extraordinary cases which 

 he could hardly explain in any other way. His decisive instance, indeed, is the occur- 

 rence of the Eastern American Phryma leptostachya in the Himalaya Mountains. 



The facts presented in the present memoir effectually dispose of this subsidiary 

 hypothesis, by showing that the supposed single exception belongs to a not uncommon 

 case. Indeed, so many species are now known to be common to Eastern and Northern 

 Asia and Eastern North America, — some of them occurring also in Northwestern 

 America and some not, — and so many genera are divided between these two regions, 

 that the antecedent improbability of such occurrence is done away, and more cases of 

 the kind may be confidently expected. However others may regard them, it is clear 



