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 26 October 2016, http://www.darwinproject.ac.uk/DCP-LETT-1662