The effects of cross and self fertilisation in the vegetable kingdom, published on 10 November 1876, was the result of a decade-long project to provide evidence for Darwin’s belief that ‘‘Nature thus tells us, in the most emphatic manner, that she abhors perpetual self-fertilisation’ (Orchids, p. 359). In his book, On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of intercrossing (1862), and in several papers on plants with two or three different forms of flowers, Darwin had focused on the anatomical and physiological adaptations that promoted intercrossing and limited or prevented self fertilisation in flowering (angiosperm) plants. The research for those works had been limited to studying the structure of flowers and the physiological effects of different forms of pollen. Although many plants that Darwin observed had flowers with adaptations to prevent self fertilisation, many of these were nevertheless fertile with their own pollen. He set out to compare several generations of cross and self fertilised plants, comparing germination rates, growth, and constitutional vigour. Although Darwin was no stranger to long months and years of research on other projects, Cross and self fertilisation would become one of his most sustained and detailed studies, encompassing plants from as many families as he could obtain from his wide network of correspondents.

‘The difference … is highly remarkable’

In September 1866, Darwin announced to the American botanist Asa Gray, ‘I have just begun a large course of experiments on the germination of the seed & on the growth of the young plants when raised from a pistil fertilized by pollen from the same flower, & from pollen from a distinct plant of the same or of some other variety. I have not made sufficient experiments to judge certainly, but in some cases the difference in the growth of the young plants is highly remarkable’ (To Asa Gray, 10 September [1866]). By early December, the French botanist Édouard Bornet had provided seeds of some varieties of poppy (Papaver) that always bred true even when grown together for several years (To Édouard Bornet, 1 December 1866). Darwin began a series of experiments, reporting back to Bornet in August 1867 that all but one of the varieties bred true (To Edouard Bornet, 20 August [1867]). It was only after a new season of experiments that Darwin would confirm that this poppy shed its pollen immediately after the flower opened, resulting in self fertilisation; his earlier results were thus called into question since even those flowers to which he applied foreign pollen had probably already been self fertilised. This case highlighted the complexity of the research: simply preventing or allowing insect access to flowers was only the tip of the iceberg.

Darwin next focused on the California poppy (Eschscholzia californica). Fritz Müller, writing from Brazil in December 1866, noted that plants of this poppy growing in his garden were completely self-sterile, on which he commented, ‘This complete infertility with own pollen could hardly have remained unnoticed, had it existed in all individuals of such a common garden plant. Perhaps in the case of my plants it can be attributed to their cultivation in divergent climatic conditions’ (From Fritz Müller, 1 December 1866). Darwin’s interest was piqued and he described the case as ‘extremely curious’ (To Fritz Müller, [late December 1866 and] 1 January 1867). The following year, his experiments showed that plants of this species produced seed when self-fertilised, although fewer than crossed plants. Darwin sent some of these seeds to Müller, hoping that he would ‘raise a plant, cover it with a net, & observe whether it is self-fertile; at the same time allowing several uncovered plants to produce capsules’ (To Fritz Müller, 30 January [1868]). Müller, in turn, sent seeds from his plants to Darwin and both men continued to experiment, observing changes in the degree of self-fertility over subsequent generations. In June 1869, Müller remarked, on receiving a new batch of seeds from Darwin, ‘that it was ‘curious to see, on what trifling circumstances fertility sometimes depends’ (From Fritz Müller, 15 June 1869). By May 1870, Darwin reported that he was ‘rearing crossed & self-fertilized plants, in antagonism to each other, from your semi-sterile plants, so that I may compare their comparative growth with that of the offspring of English fertile plants’ (To Fritz Müller, 12 May 1870).

From a fairly early stage in his experimental programme, Darwin began to pay more attention to the conditions that might affect his results. In March 1867, he told his close friend Joseph Hooker, ‘The only fact which I have lately ascertained, & about which I dont know whether you w^d care, is that a great excess of, or very little pollen produced not the least difference in the average number, weight, or period of germination in the seeds of Ipomœa. I remember saying the contrary to you & M^r Smith at Kew. But the result is now clear from a great series of trials. On the other hand seeds from this plant, fertilised by pollen from the same flower, weigh less, produce dwarfer plants, but indisputably germinate quicker than seeds produced by a cross between two distinct plants’ (To J. D. Hooker, 17 March [1867]). He noted another factor in a letter to Gray, remarking, ‘I am going on with my trials of the growth of plants raised from self-fertilised & crossed seeds, & begin now to suspect that the wonderful difference in growth & conststitutional vigour occurs only with exotic plants which have been raised by seed during many generations in England, but which are not properly visited by insects & so have been rarely crossed’ (To Asa Gray, 15 April [1867]). One of these ‘exotics’ was the sweet pea (Lathyrus odoratus), and in October 1867, Darwin wrote to James Moggridge to ask him to observe whether spontaneous crossing of different varieties of this plant occurred in the south of France where Moggridge lived for part of the year (To J. T. Moggridge, 1 October [1867]). Darwin was beginning to suspect that the insects which could transfer pollen in sweet peas simply did not exist in Britain.

During a visit to Darwin in May 1866, Robert Caspary, a specialist in aquatic plants, had discussed his observations on Euryale ferox, an Asian water lily that produced both open flowers above water and closed ones below. Caspary later counted seeds from the unopened self-fertilised flowers as well as those from artificially cross-fertilised ones, finding that more seed was produced by the former (From Robert Caspary, 18 February 1868). Darwin eagerly requested seed from both cross and self-fertilised plants in order to ‘compare their power of growth’ (To Robert Caspary, 25 February [1868]).  By this time he had already recorded results with several generations of some more common garden plants like morning glory (Ipomoea purpurea) and monkey flower (Mimulus luteus). He added these preliminary findings to his new book, The variation of plants and animals under domestication (Variation 2: 128-9), which was published on 30 January 1868.

In April 1868, Darwin informed George Bentham, ‘I am experimenting on a very large scale on the difference in power of growth between plants raised from self fertilised & crossed seeds; and it is no exaggeration to say that the difference in growth & vigour is sometimes truly wonderful.’ Visitors were astonished by his plants, he told Bentham, adding, ‘I always supposed until lately that no evil effects w^d be visible until after several generations of self fertilisation; but now I see that one generation sometimes suffices; & the existence of dimorphic plants & all the wonderful contrivances of orchids are quite intelligible to me’ (To George Bentham, 22 April 1868). A month later, he had another set of remarkable experimental results: he had found mignonette (Reseda odorata) was absolutely sterile with pollen from same plant in spite of the fact that stamens bent upward to shed pollen on stigmas of the same flower. ‘How utterly mysterious it is’, he reported to Hooker, ‘that there sh^d be some difference in ovules & contents of pollen-grains (for the tubes penetrate own stigma) causing fertilisation when these are taken from any two distinct plants, & invariably leading to impotence when taken from the same plant!’ (To J. D. Hooker, 21 May [1868]) Pollen tubes, or rapidly elongating vegetative cells enclosing the sperm, provide a conduit through the style to the ovary of a flower; they are triggered to elongate when the pollen touches the stigmatic surface. Darwin was able to discern that penetration of the pollen tubes did not necessarily result in fecundation.

‘The essence of sexual reproduction’

In an article on plant sexual relations, Müller, who sent the publication to Darwin, reported that he was surprised at the lessened fertility when he pollinated plants using pollen from other plants of the same species growing close by. He speculated that these plants might have grown from seeds of the same ‘mother plant’ and that this close relationship had lessened the fertility of the offspring (F. Müller 1868b, p. 629). Darwin urged further experimentation. ‘I am convinced that if you can prove that a plant growing in a distant place under different conditions is more effective in fertilization than one growing close by, you will make a great step in the essence of sexual reproduction’, he told Müller (To Fritz Müller, 28 November 1868). In March 1869, Müller reported results of experiments with a new species of Abutilon found by him on the Rio Capivary (now Capivari), a small tributary of the Rio Grande, in Bahia, Brazil. Not only were the plants self sterile, but plants raised from seeds from the same pod were mutually sterile (From Fritz Müller, 14 March 1869). ‘The case of the Abutilon sterile with some individuals is remarkable’, Darwin replied, adding that he had sown seeds of this plant sent by Müller (To Fritz Müller, 18 July [1869]). Darwin sent specimens of plants he raised from this seed to Hooker, who named it Abutilon darwinii. ‘I am glad to hear the Abutilon is a new species, & I am honoured by its name’, Darwin told Hooker, ‘It offers an instance, of which I have known others, of being during the early part of the flowering season quite sterile with pollen from the same plant, though fertile with the pollen of any other plant, though later in the season it becomes capable of self-fertilisation’ (To J. D. Hooker, 23 July [1871]). Darwin also informed Müller of this fact. It had taken only one generation for the plant to go from self-sterility in its native Brazilian setting to moderate self-fertility in his hothouse (To Fritz Müller, 2 August [1871]).

By late 1871, Darwin was already planning to publish these experimental results. He told the Italian botanist Federico Delpino, ‘Next summer or autumn I hope to publish a long Essay, the result of 5 or 6 years work, on the comparative growth, vigour & fertility of crossed & self-fertilised plants during several successive generations’ (To Federico Delpino, 22 November 1871). Delpino replied that he looked forward to the appearance of the work, noting its practical importance for agriculture and horticulture (From Federico Delpino, 5 December 1871). When Darwin began writing in February 1873, he asked Hooker for names of families of several genera as well as  guidance with choosing which taxonomic system to follow (To J. D. Hooker, 17 February 1873). Despite also working on experiments with insectivorous plants such as Drosera, Darwin had been able to concentrate solely on cross and self-fertilised plants, as he explained to Gray, ‘I worked last summer hard at Drosera, but could not finish till I got fresh plants, & consequently took up the effect of crossing & self-fertilising plants & am got so interested that Drosera must go to the dogs till I finish with this & get it published’ (To Asa Gray, 11 March [1873]).

In April 1873, the publisher John Murray announced in the Athenæum  that a book by Darwin with the title ‘The Evil Effects of Interbreeding in the Vegetable Kingdom’ was forthcoming. Darwin begged him not to advertise it again, warning, ‘Heaven knows when it will be ready. The announcement took me quite by surprise. I am already plagued by foreign Translators, Reviewers, &c.’ (To John Murray, 4 May [1873]). In reply to his German translator Julius Carus, who wrote in early May, Darwin stated, ‘M^r Murray announced my next book without my knowledge & I was vexed about it, for it is only half-written, & I have no idea when it will be published’ (To J. V. Carus, 8 May [1873]). Hermann Müller also wrote from Germany, informing Darwin ‘The book you are now about to write will indeed be of the highest interest to me and probably will decide in a great measure my further working’ (From Hermann Müller, 10 June 1873). Darwin, in turn, had found Müller’s book on the fertilisation of flowers of great interest and had told Müller, ‘The whole discussion seems to me quite excellent, and it has pleased me not a little to find that in the rough M.S. of my last chapter, I have arrived on many points at nearly the same conclusions that you have done, though we have reached them by different routes’ (To Hermann Müller, 30 May 1873). Although Darwin had completed a first draft of his book, the publication would be delayed for years while he finished his work on insectivorous plants. In the meantime, he continued his experiments on further generations of crossed and self fertilised plants.

‘We must turn to the vegetable kingdom’

In June 1873, Delpino informed Darwin that gardeners in Florence kept varieties of sweet peas separated to avoid crossing (From Federico Delpino, 18 June 1873). Darwin was intrigued. ‘I am very glad to hear about Lathyrus odoratus; for here in England the vars. never cross, & yet are sometimes visited by Bees’, he told Delpino (To Federico Delpino, 25 June [1873]). Darwin’s suspicion that sweet peas were cross fertilised in their native setting was confirmed; the English environment lacked the large solitary bees that visited these flowers in their native Mediterranean setting. Although he continued his crossing experiments through the early summer, by August 1873, Darwin decided to shift focus back to Drosera. He informed Carus that his next book would be on this and other insectivorous plants and not on ‘the evil effects of Interbreeding’ (To J. V. Carus, 2 August [1873]).

In September, Darwin wrote a long letter to Nature commenting on a seemingly unrelated phenomenon, the existence of complemental males in some species of barnacles. Darwin noted that the most interesting occurrence of these tiny reduced males was not with females, but with hermaphrodites and added, ‘We must turn to the vegetable kingdom for anything analogous to this; for, as is well known, certain plants present hermaphrodite and male individuals, the latter aiding in the cross-fertilisation of the former’. The comparison was extended to the more typical hermaphrodite species of both barnacles and plants; Darwin added that ‘the good depends on the crossed individuals having been exposed to slightly different conditions of life’ (To Nature, 20 September [1873]). Just as the free-swimming barnacle larvae could come from different parts of their environment before settling as adults forever fixed in close proximity to others, so pollen from widely separated flowers could be transported to their sedentary recipients.

Darwin remained optimistic regarding the publication of his results, telling Fritz Müller that he hoped to publish in a year and that his results would ‘prove what great vigour is given to seedling plants by the crossing of their parents’ (To Fritz Müller, 25 September 1873). But by March 1874, some doubts seemed to have arisen when he told Carus, ‘My next book, (if I live & have strength to complete it) will be on the advantages of Crossing Plants, & this will include all my papers on Dimorphic & Trimorphic plants with new & related matter. (To J. V. Carus, 19 March [1874]). A year later, Darwin still planned to add his reworked papers on heterostyled plants, but told Carus, ‘I have to add new researches on this subject. (To J. V. Carus   7 February 1875). In fact, Darwin had planned a new set of experiments for the summer, as he informed Gray when asking for seeds of Nesaea verticillata (swamp loosestrife), ‘to raise seedlings from illegitimate unions to see if the seedlings are sterile like true Hybrids & like the illeg. offspring of Lythrum; for the fact seems to me all important.’ (To Asa Gray, 30 May [1875]). In earlier papers on plants with two or more stylar forms of flowers, Darwin had referred to unions where pollen from one form had been applied to the same form as ‘illegitimate’. He had discovered that some forms were absolutely sterile with their own pollen while others had varying degrees of fertility. Darwin had briefly discussed this North American relative of purple loostrife (Lythrum salicaria) in his 1864 paper, ‘Three forms of Lythrum salicaria’. There are no notes to indicate that Darwin ever carried out these experiments, but as he had just finished correcting proof sheets of his book Insectivorous plants, he probably decided to devote one more summer to research on crossed and self fertilised plants.

‘There will always be men who dispute and differ’

Thomas Meehan had been a vocal opponent of Darwin’s views on crossing, and his paper, ‘Are insects any material aid to plants in fertilization?’ (Meehan 1875) prompted Darwin to inform him that he had ‘begun to prepare for press observations continued for 10 years on the effects of crossing plants, & I think that these will convince you & every one else that it is a great advantage to plants to intercross’ (To Thomas Meehan, 3 October 1875). Hermann Müller had also read Meehan’s work and told Darwin, ‘I am the more glad to hear that you have begun putting your notes thereabout together, as newly some Italian and American botanists have done their best work in order to obscure this matter’ (From Hermann Müller, 23 October 1875). The Italian botanists were identified by Müller as Nicola Pedicino and Orazio Comes, who had published papers on fertilisation in which they merely inferred from observations on self fertilising plants that crossing was of little importance (Pedicino 1875; Comes 1875). Darwin was philosophical, telling Müller, ‘There will always be men who dispute and differabout everything that is discovered; and we may take comfort by remembering that even the sexuality of plants was disputed for half a century after Kölreuter’s papers’ (To Hermann Müller, 26 October 1875). Darwin’s copy of Johann Kölreuter’s paper describing his experiments and observations regarding the sex of plants in the 1760s (Kölreuter 1761–6) is heavily annotated.

Darwin had no doubts about the significance of his findings in a broader context. He told his long-time supporter Ernst Haeckel, ‘It is really wonderful what an effect pollen from a distinct seedling plant which has been exposed to different conditions of life, has on the offspring, in comparison with pollen from the same flower or from a distinct individual but which has been long subjected to the same conditions. The subject bears on the very principle of Life, which seems almost to require changes in the conditions’ (To Ernst Haeckel, 13 November 1875). He added on a darker note, ‘What I shall do in Future if I live, Heaven only knows: I ought perhaps to avoid general and large subjects as too difficult for me with my advancing years, and I suppose enfeebled brain’. At this point, Darwin still planned to publish his earlier papers in the same book (To J. V. Carus, 25 December 1875).

As Darwin continued to write up the results of his experiments comparing growth over several generations in several different species, he decided that some sort of comparative table of the average heights would be useful. He asked his mathematician son George whether it would be ‘an easy calculation to give the average or mean height of the 33 self-fertilised plants, the 34 crossed plants being still taken as 100.? I sh^d. rather like to know what the general mean is of all the crossed & self-fertilised plants which I measured; but I want more particularly, because in half-a-dozen cases the self-fertilised plants exceeded in height the crossed, & I desire to know whether this mean excess equals the mean excess of the crossed over the self-fertilised’ (To G. H. Darwin, 8 January [1876]). George explained the difficulties of lumping different species together, questioning whether the procedure would be ‘wholly justifiable’, but then decided it would be ‘worth while to give a mean not of all the plants which were measured, but a mean of the means, assuming for the moment that all of equal value.’ (From G. H. Darwin, [after 8 January 1876]). It was his cousin, the statistician Francis Galton, who provided a statistical table and report on Darwin’s results. Darwin thanked Galton for ‘the immense labour’ he had taken and promised to publish the report in the introduction to the book (To Francis Galton, 13 January [1876]).

Joseph Henry Gilbert (1817–1901) of the Rothamsted Experimental Station, Harpenden (Biodiversity Heritage Library)

The fact that he was now writing in earnest did not deter Darwin from setting out on yet another experimental aspect of his work. In February 1876, he wrote to the agricultural chemist Joseph Gilbert, describing experiments with common garden plants self fertilised and grown under constant conditions in which the flower colour often changed in each generation. Darwin suspected ‘that the cause of variation must lie in different substances absorbed from the soil by these plants when their powers of absorption are not interfered with by other plants with which they grow mingled in a state of nature’ (To J. H. Gilbert, 16 February 1876). Darwin wanted to try to remove all nutrients from soil before adding different substances to see how each affected variability. Gilbert responded with a recipe for treating soil to remove nutrients (From J. H. Gilbert, 4 March 1876). In June 1876, Darwin had supposedly nutrient-free and natural soil samples analysed by Edward Frankland to see how the samples differed (To Edward Frankland, [before 6 June 1876]). The project proved to be too complex and Francis Darwin later recalled, ‘the research was ultimately abandoned.’ (LL 3: 342).

‘I am convinced that the book is of value’

By August 1876, the book had gone to press and Darwin told Gray, ‘This will complete all that I shall ever do on this subject’ (To Asa Gray, 9 August 1876). As Darwin began correcting proofsheets, he wrote to his publisher, ‘The greater part of the book is extremely dry & the whole on a special subject. Nevertheless I am convinced that the book is of value, and I am convinced that for many years copies will be occasionally sold. Judging from the Sale of my former books and from supposing that some persons will purchase it to complete the set of all my works, I would suggest 1,500’ (To R. F. Cooke, 16 September 1876). In the meantime, a happy event, the birth of Darwin’s first grandchild, a son born to Amy and Francis Darwin on 7 September, suddenly turned to tragedy when Amy died only four days later. Francis returned to Wales where his wife was to be buried, but still continued to help his father going over proofs. Darwin sent a chapter for review but advised, ‘I earnestly beg you not to strain your mind & return the sheets marked “uncorrected” if, as I expect, you find it too much for you’ (To Francis Darwin, 16 September [1876]). Francis must have found some solace in work, for Darwin soon told him, ‘You have worked excellently at my Proof-sheets, but I have gone through (for it is hard work) only about a quarter of them, & as yet have accepted all, though some slightly modified’ (To Francis Darwin, 20 September [1876]). Darwin continued to send work, encouraging his son, ‘Your corrections are very good & very useful’ (To Francis Darwin   25 September [1876]).

At the end of September 1876, Darwin sent sheets to his German translator, noting, ‘I sent by this morning’s Post the 4 first perfect sheets of my new book, the title of which you will see on first page, & which will be published early in November’ (To J. V. Carus, 27 September 1876). The title had now changed from that first advertised (‘The evil effects of intercrossing in the vegetable kingdom’) to the more sober and cautious The effects of cross and self fertilisation in the vegetable kingdom. When Darwin told correspondents about his forthcoming book, he warned them not to read most of it, advising, ‘The book is a very dull one, but I think has some value. All the first chapters are mere record of experiments.—The latter chapters alone worth reading. (To Otto Zacharias, 5 October [1876]). Hermann Müller, in contrast, wrote that the book would be of the ‘highest interest’ and hoped it would silence writers like Meehan, Pedecino, and Comes (From Hermann Müller, 4 October 1876).

Gray was impatient for a copy and asked for proofsheets if the book had not yet been released (From Asa Gray, 12 October 1876). Darwin sent the sheets, apologised for his writing style, and added, ‘Please observe that the 6 first chapters are not readable, & the 6 last very dull. Still I believe that the results are valuable. If you review the book, I shall be very curious to see what you think of it, for I care more for your judgment than for that of almost anyone else’ (To Asa Gray, 28 October 1876). Gray reassured him, ‘I have as yet read only the introduction. This is far from dull. The dullness you deprecate I may find in the details of experiments and statistical matter’, adding, ‘It is most amusing to read what you write of “licking a horrid bad style into intelligible English”. Over here we are accustomed to hear your style spoken and written of, as being as faultless as your temper’ (From Asa Gray, 12 November 1876).

The book was published on 10 November 1876. Within days, Darwin received a statement of all his book sales and pronounced himself ‘well satisfied’, adding, ‘I am rejoiced about the Fertilisation as I did not expect that more than 6 or 700 would sell.’ (To John Murray, 15 November 1876). In fact, Murray sold 1100 copies of Cross and self fertilisation at his annual trade sale dinner. Recipients of presentation copies were mostly positive in their reaction to the book. Hildebrand wrote that ‘a peep into it has shown me what I could not expect otherwise, that it is again of very great value for science’ (From Friedrich Hildebrand, 6 December 1876). After reading the book, Hildebrand was even more impressed, noting, ‘Surely the most important matter is, that you have proved the benefits derived not only from mere crossfertilization, but from fertilization between individuals, that are not related nearly, and have grown under different conditions of life. This explains the high value of the adaptation for wide dissemination of plants.’ (From Friedrich Hildebrand, 18 January 1877). Hermann Müller enthused that Darwin’s references to Müller’s own book on fertilisation were, ‘the highest reward I am capable of imagining and will be to me the most efficacious encouragement for further work’ (From Hermann Müller, 6 December 1876). Alphonse de Candolle noted the importance of Darwin’s research and concluded that plant physiology had nothing as satisfying, also marvelling at Darwin’s perseverance over such a long series of experiments (From Alphonse de Candolle   16 December 1876). One critical review came from Alfred Wallace, who complained, ‘I am afraid this book will not do much or anything to get rid of the one great objection, that the physiological characteristic of species, the infertility of hybrids, has not yet been produced’ (From A. R. Wallace, 13 December 1876). No reply to this letter has been found, but Darwin had long rejected the idea that sterility was a test for physiological species, based on his research on different stylar forms of flowers that showed sterility could exist when pollen from one form was applied to the same form of flower.

Most published reviews that appeared were also positive, but George Henslow, in his review in Gardeners’ Chronicle, criticised Darwin’s statistics. Darwin wrote to the journal, ‘I hope that any reader who is interested in the subject will not take Mr. Henslow’s interpretation of my statements without consulting my book’ (To Gardeners’ Chronicle, 19 February [1877]). In contrast, as Hooker told Darwin, ‘Dyer is full of your Cross & Self Fertilization & about to review it for “Nature”— he gloats over it' (From J. D. Hooker, 27 January 1877). Darwin was especially pleased with Gray’s review, and told him, ‘Your abstract of my book is inimitably good. You have given everything,—you have quite eviscerated it’ (To Asa Gray, 18 February [1877]). By mid-March 1877, the edition was ‘nearly exhausted’ (From R. F. Cooke, 16 March 1877). In November 1877, Murray suggested stereotyping the book, but Darwin refused as he wanted to make corrections for a new edition. On 11 December, Darwin sent corrected sheets to the publisher, noting ‘the last 100 pages will have to be repaged & the index a little altered’ (To R. F. Cooke, 11 December [1877]). These changes were necessitated by the addition of a long note discussing Wilhelm Rimpau’s observations on the predominance of self-sterile individuals in the populations of rye and wheat that he had studied (From A. W. Rimpau, 10 December 1877). By the end of February 1878, Murray was ready to print the second edition, after which the book would be stereotyped (From R. F. Cooke, 26 February 1878). In the end, the ‘unreadable’ and ‘dull’ book had proved far more popular than Darwin ever imagined.