154 ESSAYS. 
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 ihvestigations. 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 ‘ Géographie 
Botanique Raisonnée,” 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. 
