very interesting to
me. Your "indefinite" answers are perhaps not the least valuable part;
for Botany has been followed in so much more a philosophical spirit
than Zoology, that I scarcely ever like to trust any general remark
in Zoology without I find that botanists concur. Thus, with respect to
intermediate varieties being rare, I found it put, as I suspected, much
too strongly (without the limitations and doubts which you point out) by
a very good naturalist, Mr. Wollaston, in regard to insects; and if it
could be established as true it would, I think, be a curious point.
Your answer in regard to the introduced plants not being particularly
variable, agrees with an answer which Mr. H.C. Watson has sent me in
regard to British agrarian plants, or such (whether or no naturalised)
[as] are now found only in cultivated land. It seems to me very odd,
without any theoretical notions of any kind, that such plants should not
be variable; but the evidence seems against it.

Very sincere thanks for your kind invitation to the United States: in
truth there is nothing which I should enjoy more; but my health is not,
and will, I suppose, never be strong enough, except for the quietest
routine life in the country. I shall be particularly glad of the sheets
of your paper on geographical distribution; but it really is unlikely in
the highest degree that I could make any suggestions.

With respect to my remark that I supposed that there were but few
plants common to Europe and the United States, not ranging to the
Arctic regions; it was founded on vague grounds, and partly on range of
animals. But I took H.C. Watson's remarks (1835) and in the table at the
end I found that out of 499 plants believed to be common to the Old and
New World, only 110 did not range on either side of the Atlantic up to
the Arctic region. And on writing to Mr. Watson to ask whether he knew
of any plants not ranging northward of Britain (say 55 deg) which were
in common, he writes to me that he imagines there are very few; with Mr.
Syme's assistance he found some 20 to 25 species thus circumstanced,
but many of them, from one cause or other, he considered doubtful. As
examples, he specifies to me, with doubt, Chrysosplenium oppositifolium;
Isnardia palustris; Astragalus hypoglottis; Thlaspi alpestre; Arenaria
verna; Lythrum hyssopifolium.

I hope that you will be inclined to work out for your next paper, what
number, of your 321 in common, do not range to