To Asa Gray 24 August 
Down Bromley Kent
My dear Dr. Gray
I am much obliged for your letter,1 which has been 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 & doubts which you point out) by a very good naturalist, Mr. Wollaston, in regard to insects;2 & if it could be established as true it wd., I think, be a curious point.—3 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 Agragrian plants,4 or such (whether or no naturalised) 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 U. States: in truth there is nothing which I should enjoy more; but my health is not, & 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 Geograph. Distrib; 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 & U. States, not ranging to the Arctic Regions; it was founded on vague grounds, & partly on range of animals. But I took H. C. Watsons Remarks (1835) & in the table at the end I found that out of 499 plants believed to be common to the Old & new worlds, only 110, did not range on neither side of the Atlantic up to Arctic region.5 And on writing to Mr. Watson, to ask whether he knew of any plants not ranging northward of Britain (say 55o) which were in common,6 he writes to me that he imagines there are very few; with Mr Syme’s assistance he found some 20–25 species thus circumstanced, but many of them, from on〈e〉 cause or other, he considered doubtful.7 As 〈exa〉mples, he specifies to me, with doubt, Chrysopleniu〈m〉 oppositifolium; Isnardia palustris; Astragalus Hypoglottis; 〈Thl〉aspi 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 Arctic Regions.8 Such plants seem exposed to such much greater difficul〈 〉 in diffusion.—
Very many thanks for all your kindness & answers to my questions, & believe me. Yours very sincerely & obliged | Ch. Darwin
If anything shd. occur to you on variability of naturalised or agragrian plants; I hope that you will be so kind as to let me hear, as it is a point, which interests me greatly.—
Rarity of intermediate varieties.
Variability of introduced plants.
Ranges of plants common to Europe and U. S.