134 ESS A YS. 



but that of identical species peculiar to the two inevitably 

 would. 



Why should it? Evidently because the natural supposi- 

 tion 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 preconception from the minds of botanists, has rather 

 confirmed it. Every other hypothesis has derived its prin- 

 cipal support from difficulties in the application of this. A 

 review 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, 

 during 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 ex- 

 tensive investigations. And the best evidence of the prepon- 

 derance of the theory of the local origin of species — notwith- 

 standing the great increase of facts which at first would seem 

 to tell the other way — is furnished by the works of the pres- 

 ent De Candolle upon geographical 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 objec- 

 tions to that view, but because he found it no longer needed 

 to account for the general facts of distribution. This 

 appears from his qualified though dubious adherence to the 

 hypothesis of a double origin, as a dernier ressort, in the few 

 and extraordinary cases which he could hardly explain in any 

 other way. His decisive instance, indeed, is the occurrence 

 of the eastern American Phryma leptostachya in the Hima- 

 laya Mountains. 



The facts presented in the present memoir effectually dis- 

 pose of this subsidiary hypothesis, by showing that the sup- 

 posed single exception belongs to a not uncommon case. 



