To Miles Joseph Berkeley 7 April 
Down Farnborough Kent
My dear Sir
I hope that you will forgive my troubling you & begging a great favour.— I saw the other day at Kew your very curious specimen of Peas affected by pollen,1 as described by Wiegmann2 & Gærtner.—3 I am going to plant in 2 or 3 days a bed with 41 reputed varieties of Peas to see how far they really differ (& of course I shall observe whether any of the Peas are affected by chance hybridisation)4 & I want very much a dozen of the curious brown Peas, or Pois sans Parchemin. I have sent to London for this var. but have today received a white smallish Pea, called the Sugar Pea. Now I write to know whether you happen to have a few of the brown peas, & could spare me a dozen to plant, & if you can whether you wd. be so kind as to send me them per Post.—
I have been reading straight through Gærtner which for me, a very poor German scholar has been a gigantic task.5 As I know from your excellent abstract that you have read it carefully,6 I shd. very much like to know (& my notes from your Abstract are hardly copious enough for me to tell) whether you were at all puzzled by any expressions on the fertility of Hybrids: he is never weary of stating that hybrids, though fertile, invariably produce a less number of seeds than the pure parents; yet every now & then there occur expressions which make me suspect that the author has misgivings on this head: moreover he always ranks lessened fertility as a less important character in Hybrids than variability &c &c.— It has struck me that it wd. be not a little difficult to ascertain the normal number of seeds in so many pure species, & with a strong impression on the less fertility of Hybrids, a person might easily deceive himself.— In the table7 the surprising number of times, he gives, (i a), as the degree of fertility, so very close to (K), & yet the fewness of times, he gives actually (K), (& nevertheless shows how beautifully graduated the sterility in all the lower degrees is) strikes me as rather odd.—8
Lastly I cannot understand how it is, that he does give (K) or normal fertility in some few cases in the Table (as in Lobelia cardanalifulgens & reverse; Lychnis diurna & vespertina; malva mauritiano-sylvestris; Verbascum Lychnitis albo-pyramidatum), yet in the text seems to disregard these examples;—at least he often repeats (as at p. 102) that fertility in hybrids never equals that of the pure species.
If this point occurred to you, & if (as is very probable) the subject has not gone out of your mind, I shd. be extremely glad to hear what you think. Though in truth I have much cause to apologise for writing at such length; & though it is but a poor excuse, I assure you I had no intention whatever of asking your opinion on this head when I began to write.
Pray believe me | My dear Sir | Yours very sincerely | Charles Darwin
Did any of your Peas cross each other without artificial aid? Wiegmann’s & some of Gærtner’s Peas having crossed of themselves seems to me an extraordinary part of this extraordinary case.9
Have you ever corresponded with Gærtner, & do you know his address?10 If I can get no light from you, or from relooking over whole book, I shd very much like to venture to write, & ask him on the above point, & indeed on two or three others.—11 What an admirable Book it appears: but did you observe what endless errors in the References.—
Asks for a pea variety for an experiment.
Discusses C. F. v. Gärtner’s results [in Bastarderzeugung im Pflanzenreich (1849)]. Criticises Gärtner’s belief that hybrids are always less fertile than their parents.
Asks about MJB’s experiments.
Please cite as
Darwin Correspondence Project, “Letter no. 1662,” accessed on 3 December 2016, http://www.darwinproject.ac.uk/DCP-LETT-1662