To John Denny   22 July 1872

My dear Sir

I thank you for yr kind & interesting letter, which has not been too long for me by a single word.¹ In my opinion the peltatum case, after you have made some more experiments, w^d be worth communicating by itself to the Linn. Soc. But I also strongly think that the other cases w^d be well worth giving, with a statement that the parentage of the supposed vars. was not known. You c^d state that you c^d perceive no marked difference between the 2 sets of vars. which are mutually sterile; & it w^d add greatly to the value of yr paper, if you c^d get some botanist, whose name is known, to compare the vars & to say whether he c^d detect any difference which appeared of specific value.² Please to remark how strange a fact it is that forms, to all appearance mere vars, sh^d be mutually sterile, whilst other vars are mutually fertile.

I dare say not one of yr experiments has been vitiated by insects not having been excluded; but you might like to hear that the pollen of a pre-potent form will fertilize a stigma, already thickly coated with its own pollen: I know this experimentally, & in the case of Primula the second dose of pollen was put on after an interval of 24 hours—³ After a grain of pollen has remained on the stigma for about half a day, & all the contents have passed down the pollen-tube, I have found the detection of the shrunken envelope of the grain very difficult.

With my best hopes for the success of y^r future observations believe me my dear Sir | yours very faithfully | Ch. Darwin

P.S. I was quite delighted to see the action of the Hort. Soc^y. about D^r Hooker.—⁴