828 The American Naturalist. [September, 
geographical distribution of animals. Making allowance for the 
greater age of plant life and the facility with which seeds are 
carried over barriers not easily crossed by animals, it is plain that 
the same laws have governed in the one case as in the other. 
In the second place, although the theory of dispersal of each 
species from a single centre, occupied by its own ancestral form, 
has been found to harmonize better with the facts thus far ob- 
served than any other, the application of this principle, simple and 
intelligible in itself, is beset with practical difficulties, owing to the 
complicated relations of the various agencies involved. 
It seems perfectly plain, for example, that changes of climatic 
conditions have had much to do with the present distribution of 
plants in both hemispheres, but just how much it is hard to tell; 
and, in the same way, the extent to which ordinary means of dis- 
persal, such as wind, water, etc., have operated can hardly be de- 
termined with precision. 
To illustrate: when we find in Eastern Asia our own gold- 
thread, blue cohosh, twin-leaf and mandrake, poison ivy and 
prickley ash, Mayflower, snowberry, partridge-berry, and a host 
of other either identical or equivalent species, and find all these 
absent from Europe, we feel no hesitation in taking these facts in 
connection with the paleontological evidence in assuming that the 
changes of climate during the glacial epoch have been largely, 
we might fairly say chiefly, the physical factors involved ; but when 
we find, to follow Hooker’s enumeration, fifty and seventy-five New 
‘Zealand plants indigenous to Northern Europe, thirty-eight com- 
mon to Australia, Northern Europe, and Asia, about fifty of those 
of Terra-del-Fuego in North America and Europe, and close rela- 
tives of other European species on the island of Fernando Po 
and the mountains of Abyssinia, it is by no means easy to account 
for it all. 
Much is still required, from different sources, in order to the 
future advantageous study of the whole question. It is hardly 
necessary to say that notwithstanding the very extensive collec- 
tions of plants that already exist in numerous herbaria, the first 
condition of the comprehensive study of any one of der with 
reference to its distribution is the gathering of still more of its 
