This letter is remarkable in two ways. First, Darwin describes here the results of experiments similar to those conducted by Mendel—actually, the very experiments that had led Mendel to the formulation of Mendelian heredity. Darwin came pretty close to discovering the Mendelian 3:1 ratio by himself. After he crossed the common snapdragon (having bilateral symmetry) with the peloric (star-shaped) form, the first generation of offspring were all of the common type, and the second had eighty-eight common to thirty-seven peloric (a ratio of 2.4:1). Second, Darwin points out the obvious fact that the simple observation that all offspring are either male or female, rather than some intermediate hermaphrodite, in itself argues against “paint-pot” blending! So the evidence of the proper form of heredity was right there in front of Darwin’s eyes. As he had already remarked in The Origin: “The slight degree of variability in hybrids from the first cross or in the first generation, in contrast with their extreme variability in the succeeding generations, is a curious fact and deserves attention.” Note also that the entire Darwin-Wallace correspondence above took place before the publication of Jenkin’s review. All the same, even though Darwin came tantalizingly close to Mendel’s discovery, he did not grasp its all-encompassing generality, and he failed to recognize its vital importance for natural selection.

To fully understand Darwin’s attitude toward particulate heredity, there are a few other nagging questions that need to be resolved. Gregor Mendel read the seminal paper describing his experiments and his theory of genetics—“Versuche über Pflanzen-Hybriden” (“Experiments in Plant Hybridization”)—to the Brünn (Moravia) Natural History Society in 1865. Is it possible that Darwin read that paper at some point? Were his letters to Wallace in 1866 inspired (to some extent at least) by Mendel’s work rather than representing his own insights? If he had read Mendel’s paper, why didn’t he see that Mendel’s results provided the definitive answer to Jenkin’s criticism?

Intriguingly, no fewer than three books published between 1982 and 2000 alleged that copies of Mendel’s paper had been found in Darwin’s library, and a fourth book (published in 2000) even claimed that Darwin had supplied Mendel’s name for inclusion in the Encyclopaedia Britannica, under an entry of “hybridism.” Obviously, if this last claim were shown to be true, it would mean that Darwin was fully aware of Mendel’s work.

Andrew Sclater of the Darwin Correspondence Project at Cambridge University responded definitively to all of these questions in 2003. As it turns out, Mendel’s name (as an author) does not appear even once in the entire list of books and articles owned by Darwin. This is not surprising, given that Mendel’s original paper appeared in the somewhat obscure proceedings of the Brünn Natural History Society, to which Darwin had never subscribed. Furthermore, Mendel’s work languished almost unread for thirty-four years, until its rediscovery in 1900, when the botanists Carl Correns of Germany, Hugo de Vries of Holland, and Erich von Tschermak-Seysenegg of Austria published supporting evidence independently. Nevertheless, two of the books that were in Darwin’s possession did refer to Mendel’s work. In Darwin’s book The Effects of Cross and Self Fertilisation in the Vegetable Kingdom, he even cited one of those books: Hermann Hoffmann’s Untersuchungen zur Bestimmung des Werthes von Species und Varietät (Examinations to Determine the Value of Species and Variety), published in 1869. However, Darwin never cited Mendel’s work, nor did he annotate any mention of Mendel in Hoffmann’s book. Again, this is hardly surprising, since Hoffmann himself did not comprehend the significance of Mendel’s work, and he summarized Mendel’s conclusions by the rather low-key statement “Hybrids possess the tendency in the succeeding generations to revert to the parent species.” Mendel’s pea experiments were mentioned in another book owned by Darwin: Die Pflanzen-Mischlinge (The Plant Hybrids), by Wilhelm Olbers Focke. Figure 7 shows the title page, on which Darwin wrote his name. As I’ve seen with my own eyes, this book had an even less distinguished fate: The precise pages describing Mendel’s work remained uncut in Darwin’s copy of the book! (In old bookbinding, pages were connected at the outer edges and had to be cut open.) Figure 8 shows a picture of Darwin’s copy, made at my request, displaying the uncut pages. However, had Darwin read those pages, he would not have been much enlightened, since Focke failed to grasp Mendel’s principles.

One question still remains: Did Darwin indeed suggest Mendel’s name to the Encyclopaedia Britannica? Sclater left no doubt as to the answer: No, he did not. Rather, when asked by the naturalist George Romanes to read a draft on hybridism for the Britannica and to provide references, Darwin sent him his copy of Focke’s book (with the uncut pages), suggesting to Romanes that the book could “aid you much better than I can!”