428 CORRESPONDENCE. [1857, 
generic, and how many words will you allow each to 
consist of ? 
Give me your names through some family, say 
Ranunculacee. If I can see my way clear, I shall 
follow your lead, or cause it to be followed on an occa- 
sion which will soon be presented. 
I wish I had known of Clitoria Mariana-acuminata, 
ete., in time to add it to my list in the last number of 
“ Silliman’s Journal ;” a copy of the article was sent — 
to Dr. Hooker by post last week. I will send more, 
from my extras, presently. 
Tam quite prepared for what you say about inter- 
change of species of United States and Europe taking 
place via Asia, instead of across the Atlantic ; but you 
will see there are a few, besides aquatics (Subularia, 
Eriocaulon, ete.), which would seem to have taken the 
shorter cut. 
As respects identical species, interchange is the 
only thing that, on our views of what a species is, will 
explain the occurrence of the same species here and 
there. But as to genera, I do not yet feel free to 
assume an interchange, or a former continuity of land, 
between two widely separated regions on account of 
their having identical genera or closely related species. 
I see no reason why cognate species may not have 
been originally given to most widely separated sta- 
tions ; and, as to the facts of association, can we say 
more than this, that the species of a genus are apt 
to be confined to one part of the world? Are there 
not too many cases to the contrary to warrant our 
suspecting former continuity of two remote districts 
on account of common genera? Peculiar genera, 
such as Torreya, Hlicium, Philadelphus, Astilbe, ete., 
divided between Japan and the United States of 
