To Asa Gray   14 July [1856]¹

My dear D^r. Gray

You have been so very kind in giving me information of the greatest use to me;² that I venture to trouble you with a question, which cannot cause you much trouble.— I have been reading a paper by you on plants on mountains of Carolina, (in London Journal of Botany)³ in which you state that most are the same with the plants of the N. States & Canada.— Now what I want to know is, whether the Alleghenies are sufficiently continuous so that the plants could travel from the north in the course of ages thus far south?⁴ I remember Bartram makes the same remark with respect to several trees on the Occone M^ts.,—not that I know where these Mountains are.—⁵

How does your memoir on Geograph. Distrib. get on?⁶ I do heartily wish it was now published; for I have been trying to make out how many plants are common to Europe, which do not range up to the Arctic shores, & they seem to be very few.—

I have just thought of one other question, connected with my subject, which I cannot resist asking.— I have seen it remarked by entomologists, that it often happens that the intermediate varieties connecting together two varieties (& thus showing that such are varieties) are less common or numerous in individuals, than the two varieties themselves.⁷ If you can enlighten me on this head I sh^d. be very much obliged. I am inclined to think there must be some truth in it; otherwise varieties would not be so well marked as they often are.— I wrote some time ago, a troublesome letter, in which I begged for information on the amount of variability of your naturalised & your agragrian plants, as compared with other species of the same genera.—⁸

But I know I have been scandalously troublesome to you.— Can you forgive me? & believe me, Yours truly obliged | Ch. Darwin