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 Arctic regions.

Such plants seem exposed to such much greater difficulties in diffusion.

Very many thanks for all your kindness and answers to my questions.
P.S .-- If anything should occur to you on variability of naturalised or agrarian plants, I hope that you will be so kind as to let me hear, as it is a point which interests me greatly.
LETTER 330.

ASA GRAY TO CHARLES DARWIN.