22 J. D. Hooker, Introductory Essay to the Flora of Tasmania, 
These and a multitude of analogous facts have led to the study. 
of two classes of agents, both of which may be reasonably sup- 
posed to have had a powerful effect in determining the distribu- 
tion of plants; these are changes of climates, and changes in 7 
d 
the relative positions and elevations of lan 
26. Of these, that most easy of direct application is the effect 
of humidity in extending the range of species into regions char- 
acterized by what would otherwise be to them destructive tem- _ 
peratures. 
ji 
have, in the ‘ Antarctic Flora,’ shown that the distribution | 
ascend the humid extratropical mountains of Eastern Ben 
in our speculations on the or range of species under 
isti when to this is added that the 
average Tange in altitude of each Himalayan tropical and tem- 
perate and alpine species of flowering plant is 4000 feet, which 
27. ‘To explain more fully the present distribution of species 
and genera in area, I have recourse to those arguments which 
survived great relative changes of sea and land. is doctrine, 
which I in that Essay endeavored to expand by a study of the 
