At the start of 1869, Darwin was hard at work making changes and additions for a fifth edition of On the origin of species (Origin). By the close of the previous year, he had taken stock of the criticisms that had surfaced since the fourth edition appeared at the end of 1866 and had told his cousin William Darwin Fox, 'My work will have to stop a bit for I must prepare a new edit. of that everlasting origin, & I am sick of correcting' (Correspondence vol. 16, letter to W. D. Fox, 12 December [1868]). He may have resented the interruption to his work on sexual selection and human evolution, but he spent forty-six days on the task. As he remarked to his best friend, the botanist Joseph Dalton Hooker, 'If I lived 20 more years, & was able to work, how I^d. have to modify the "Origin", & how much the views on all points will have to be modified.- Well it is a beginning, & that is something' (letter to J. D. Hooker, [22 January 1869]).

Much of the remainder of the year was spent researching and revising chapters for The descent of man, and selection in relation to sex (Descent), and gathering additional material on emotional expression. Yet the scope of Darwin's interests remained extremely broad, and many letters throughout the year touched on subjects such as South American geology, barnacle morphology, insectivorous plants, and earthworms, subjects that had exercised Darwin for decades, and that would continue to occupy him for many years to come.

Revising Origin

Carl von Nägeli and perfectibility

Darwin's most substantial addition to Origin was a response to a critique of natural selection by Carl Wilhelm von Nägeli, a Swiss botanist and professor at Munich (Nägeli 1865). Darwin had considered Nägeli's paper important enough to have a translation made for his personal use. While not entirely dismissing natural selection, Nägeli had assigned it an auxilliary role, second to his own theory of perfectibility ('Vervollkommnung'), which he posited as the principal engine of change in the development of species. Darwin correctly assessed Nägeli's theory as a major challenge requiring a thorough and thoughtful response; indeed, over the next twenty years, Nägeli's critique inspired many to reassess their support for natural selection (see Cittadino 1990, pp. 122-3, 128-30).

Nägeli had argued that natural selection could only operate on 'functional' characteristics, while his perfectibility principle could account for changes in most morphological features (Nägeli 1865, p. 29). Darwin sent a manuscript of his response (now missing) to Hooker, remarking: 'I should be extremely obliged if you would read it over, and see whether I have made any blunders, as is very likely to be the case' (letter to J. D. Hooker, 13 January 1869). Hooker went straight to a crucial point: 'I do not quite like the starting by shirking the question of what is a "Morphological character" … all characters i.e. all departures from a given structure are & must be morphological'. The comment highlights Darwin's apparent confusion about Nägeli's separation of features into functional and 'purely morphological'. The modern reader may well share Darwin's uncertainty, but Nägeli evidently believed the distinction was clear-cut. For example, he argued that while the function of leaves could be modified by natural selection, their arrangement, which he claimed had no value in terms of 'usefulness', could only be altered by his perfectibility principle (Nägeli 1865, pp. 28-9). In further letters, Hooker tried to provide Darwin with botanical examples he could use to challenge Nägeli; Darwin made full use of Hooker's assistance and many of his examples feature in the seven pages of the new edition that countered Nägeli's thesis (Origin 5th ed., pp. 151-7).

Fleeming Jenkin and problems of heredity

Another important criticism that Darwin sought to address in the fifth edition was that of the engineer Henry Charles Fleeming Jenkin. Darwin had been very impressed by Jenkin's 1867 review, which argued that any variation in an individual, however favourable, would not be preserved within a breeding population. Such variations, according to the prevailing theory of blending inheritance that Jenkin and Darwin both shared, would tend to be lost in intercrossing. Darwin tried to resolve this problem by emphasising variability within the breeding population as a whole; if a sufficient number of individuals possessed the favourable trait, then its perpetuation would be assured (Origin 5th ed., pp. 103-4). The terminology that Darwin and others employed in these matters ('individual differences', 'single variations') was sometimes ambiguous, however, and gave rise to misunderstandings. 'I must have expressed myself atrociously', Darwin wrote to Alfred Russel Wallace on 2 February, 'I meant to say exactly the reverse of what you have understood … I always thought individual differences [i.e., differences between individuals] more important, but I was blind & thought that single variations [i.e., variations that occurred in a single individual] might be preserved much oftener than I now see is possible or probable' (see also letter to A. R. Wallace, 22 January [1869], and letter from A. R. Wallace, 30 January 1869).

James Croll, ice ages, and the age of the earth

A further modification to the new edition of Origin was the result of correspondence between Darwin and the geologist James Croll. In the previous year, Croll had discussed topics such as the age of the earth and evidence of glaciation in North America, but it was his theory of alternate ice ages that piqued Darwin's interest the most. He wrote, 'this conclusion, which you have arrived at from physical considerations, explains so well whole classes of facts in distribution, that I must joyfully accept it; indeed I go so far as to think that your conclusion is strengthened by the facts in distribution' (letter to James Croll, 31 January [1869]). Darwin had argued (Origin, pp. 377-8) that plant species would migrate towards the equator during an ice age and that temperate species would survive at higher altitudes while tropical ones would inhabit lower levels. He had been forced to retrench that position following criticism from his friend Hooker, by admitting that the survival of tropical species was a difficulty for his theory (Origin 4th ed., pp. 450-1). Croll's theory, simply stated, proposed that ice ages alternated between hemispheres, so that a warmer, non-glaciated hemisphere where tropical species could survive would always exist. In Origin 5th ed., pp. 450-61, Darwin accounted for the survival of tropical species using Croll's theory.

In the same letter to Croll, Darwin had expressed another worry: 'I am greatly troubled at the short duration of the world according to Sir W. Thompson, for I require for my theoretical views a very long period before the Cambrian formation' (letter to James Croll, 31 January [1869]). Croll could not supply Darwin with an estimate of the age of the earth much greater than that calculated by William Thomson, but he did point out, 'As regards determining the age of the earth's crust from the secular cooling of the globe I am not altogether satisfied with the plan. It would no doubt do if we had proper data to go by, but don't think we have got that yet' (letter from James Croll, 4 February 1869).

Darwin did not directly challenge Thomson's estimate, but he added more on the process of subaerial denudation based on recent work of Croll, Andrew Crombie Ramsay, William Whitaker, and others (Origin 5th ed., pp. 352-4). Later in the year he was pleased to see Thomson's work challenged by both Thomas Henry Huxley and Wallace. He confided to Huxley, 'I find that the few sentences which I have sent to press in the Origin about the age of the world will do fairly well, though if I had read you first, perhaps I^d have been less deferential towards [Thomson]' (letter to T. H. Huxley, 19 March [1869]).

Towards Descent

Once Darwin had completed revisions of the 'everlasting old Origin' (letter to Asa Gray, 1 June [1869]), he was able to return to work on Descent. He continued to receive material on sexual selection in various species from Britain and overseas. The dog-breeder George Cupples worked hard on Darwin's behalf, sending a steady stream of information on the proportion of the sexes in sheep, cattle, horses, and dogs, and circulating Darwin's queries to various contacts. As one Scottish sheep-farmer wrote: 'We all know very well who Mr Darwin is, although some of us do not think ourselves bound to agree with him in all his theories, if I could assist him however in his researches I would willingly do so' (letter from Robert Elliot to George Cupples, 21 June 1869).

Details on mating behaviour and sexual characteristics continued to flood in, occasionally causing difficulties. The entomologist Frederick Smith, whom Darwin had asked to study the musical activities of insects, reported that one male field cricket had annoyed some London neighbours with his noisy courting of the female in the garden (letter from Frederick Smith, 8 October 1869). Albert Günther, assistant in the zoology department at the British Museum in London, supplied extensive data on differences between male and female fish, reptiles, and amphibians, while Roland Trimen in South Africa and John Jenner Weir in London sent more information on male and female butterflies, supplementing that received the previous year. Darwin also continued to receive assistance from his long-time correspondent, the pigeon and poultry fancier William Bernhard Tegetmeier, who sent him samples of juvenile plumage in chickens, as well as live chickens and eggs of various breeds. Darwin's cook evidently agreed to supervise the hatching of the eggs, though she doubted her ability to recognise the different varieties (letter to W. B. Tegetmeier, 25 February [1869]).

The data contined to accumulate, and towards the end of the year Darwin complained: 'I am weary of everlasting males & females, cocks & hens.-' (letter to J. D. Hooker, 13 November [1869]). Yet completion of the work was in sight. He was now polishing his chapters on sexual selection, and beginning to choose the artists to make engravings for Descent.

Researching emotion

In 1869, Darwin still expected that Descent would include his extensive research on the expression of emotions in humans and other animals. He received more observations on infants, and on indigenous peoples in India and Africa. The American botanist Asa Gray and his wife, Jane Loring Gray, who spent part of the year in England and visited the Darwins in August, travelled to Egypt, and Jane furnished Darwin with several answers to his questionnaire: 'Passing slowly a common country cargo boat, the old man on board stood looking at us, with brow wrinkled & mouth compressed & upper lip raised- An expression, as I read it, of dislike & contempt-almost hatred-' (from Asa Gray and J. L. Gray, 8 and 9 May [1869]).

James Crichton-Browne and mental patients

In addition to infants and non-Europeans, a group that particularly interested Darwin in his research were persons diagnosed as insane, for he believed that they expressed passions without reserve. Darwin initially contacted the physician Henry Maudsley, who had worked for some years in medical asylums. Maudsley forwarded Darwin's queries to James Crichton-Browne, the director of the West Riding Pauper Lunatic Asylum in Wakefield. Crichton-Browne sent a lengthy reply, adding: 'Should the sort of information which I have sent prove of any service to M^r. Darwin I can supply him with much more of the same description' (enclosure to letter from Henry Maudsley, 20 May 1869). Darwin had often complained of the difficulty of studying expression, and was always keen to find astute observers. Darwin wrote enthusiastically to Crichton-Browne on 22 May: 'I do not know how to thank you enough for your MS. observations on expression. They contain exactly and fully the information which I wanted; and besides being of the greatest use to me, are most interesting and so graphic as to be almost painful'.

Thus began what would become Darwin's most detailed and lengthy correspondence on the subject of expression. Crichton-Browne had ambitions to make the asylum at Wakefield a centre of scientific research, and toward this end had begun to make pioneering use of photography to record the conditions of his patients, and to aid in diagnosis. Darwin was already familiar with the work of the French physiologist Guillaume Benjamin Amand Duchenne, who had used photography to capture the expression of persons whose facial muscles had been stimulated by an electrical current. Darwin sent Crichton-Browne his copy of Duchenne's photographic album, and began to receive, in turn, a large collection of photographs of the Wakefield patients, together with lengthy descriptions. One photograph of a woman with erect hair formed the basis of a woodcut that Darwin would later use in Expression of the emotions in man and animals (Expression), published in 1872, more than a year after Descent.

Natural selection and humans: differences with Wallace

But even as Darwin's research on expression and sexual selection added strength to his views on the continuity between humans and the animal world, others were growing less certain.

As you expected I differ grievously from you, & I am very sorry for it

On 24 March, Wallace wrote to Darwin about a forthcoming article in the Quarterly Review: 'I venture for the first time on some limitations to the power of natural selection. [These] are the expression of a deep conviction founded on evidence which I have not alluded to in the article but which is to me absolutely unassailable'. In the article, Wallace claimed that certain human structures and higher capacities - a large brain, the delicate movements of the hand, sophisticated powers of language - could not have evolved through natural selection, because they conferred no advantage in the lower stages of human development. Savages, he asserted, had no need for them and made no use of them, and yet they were responsible for all of the higher achievements of human culture and civilization. Such features had only emerged, according to Wallace, through the agency of 'a Power which has guided the action of [natural] laws in definite directions and for special ends' ([Wallace] 1869b, pp. 393-4).

Darwin was astonished by Wallace's assertions: 'If you had not told me I^d have thought that they had been added by some one else. As you expected I differ grievously from you, & I am very sorry for it. I can see no necessity for calling in an additional & proximate cause in regard to Man' (letter to A. R. Wallace, 14 April 1869). More remarkable still were Wallace's reasons for believing in the providential origin and destiny of humans. Responding to Darwin's reply, Wallace wrote: 'My opinions on the subject have been modified solely by the consideration of a series of remarkable phenomena, physical & mental, which I have now had every opportunity of fully testing, & which demonstrate the existence of forces & influences not yet recognised by science. This will I know seem to you like some mental hallucination' (18 April 1869).

Since his marriage to Annie Mitten in 1866, Wallace had become involved in the movement known as 'spiritualism', which had gained currency among the Victorian middle classes, and had attracted some interest from members of the scientific community as well. Wallace would later become an outspoken proponent of spiritualism, which he viewed as a wholly natural phenomenon, subject to scientific investigation and explanation. Despite their increasingly fundamental disagreements, Darwin and Wallace continued to display deep mutual respect and cordiality in their letters and publications. Darwin lavished praise on Wallace's long-awaited book, The Malay Archipelago: the land of the orang-utan, and the bird of paradise (Wallace 1869a; letter to A. R. Wallace, 22 March [1869]), and scolded him for again being too modest about his co-authorship of the theory of descent by natural selection: 'you are the only man I ever heard of who persistently does himself an injustice & never demands justice' (letter to A. R. Wallace, 14 April 1869).

Proceeding on all fronts

While much of Darwin's attention in 1869 was focused on questions of human evolution and sexual selection, he continued to pursue his broad interests in geology, zoology, and botany. He revisited subjects that had occupied him early in his career, such as the geological structures of the South American cordillera (letter to Charles Lyell, 20 May 1869), and fossil discoveries in Patagonia and Wales (letter from T. H. Huxley, 7 May 1869, letter from W. B. Dawkins, 17 July 1869). He exchanged letters with Carl Friedrich Claus in Marburg, who was working on barnacle morphology and embryology and examining some of the same species that Darwin had investigated in depth (letter from C. F. Claus, 6 February 1869). In a letter to the Gardeners' Chronicle, he responded to the gardener and journalist David Taylor Fish in order to confirm the view expressed in his 1840 paper 'Formation of mould' of the astonishing role of earthworms in the formation of the soil (letter to Gardeners' Chronicle, 9 May [1869]).

In March, Darwin received specimens of the primitive and rare insectivorous plant Drosophyllum lusitanicum that had been painstakingly collected by William Chester Tait in Portugal. Darwin wanted to compare the insect-catching mechanism of Drosophyllum with that of Drosera (the sundew), a genus that he had studied in the early 1860s (letter to W. C. Tait, 12 and 16 March 1869). This research contributed to Insectivorous plants, published in 1875.

Prompted by a request from a French student, Louis Rérolle, to translate On the various contrivances by which British and foreign orchids are fertilised by insects (Orchids), Darwin decided to add new material on the subject that he had been acquiring since its publication in 1862. Darwin asked his son William to examine the British orchid Epipactis palustris on the Isle of Wight in order to better ascertain its manner of pollination. William's contribution, and those of many others, were acknowledged in the separate publication that resulted from these researches, 'Notes on the fertilization of orchids'. Darwin was now easily able to observe many more orchids himself, as he had four greenhouses providing various degrees of temperature and humidity.

A growing international reputation

The year was marked by numerous requests for translations and foreign editions, showing both Darwin's growing international reputation and the continuing controversy over his work. The fifth edition of Origin, published in June, formed the basis for a new German edition (Bronn and Carus trans. 1870), prepared by Julius Victor Carus, who had worked on the previous German edition (Bronn and Carus trans. 1867), as well as on the German translation of Variation (Carus trans. 1868).

The French translation proved more difficult, for Darwin learned that a third French edition of Origin had already been prepared without his consent and without regard for either the fourth or the fifth English editions (see letter from Victor Masson, 29 September 1869). The work had been undertaken, like the previous French editions, by Clémence Auguste Royer, and it had a new preface with Royer's own evolutionary views and critical commentary (Royer trans. 1870). Darwin complained to Hooker, 'Besides her enormously long & blasphemous preface to 1^st Edit, she has added a 2^d Preface, abusing me like a pick-pocket for pangenesis, which of course has no relation to the Origin- Her motive being, I believe, because I did not employ [her] to translate "Domestic Animals"' (letter to J. D. Hooker, 19 November [1869]). Angered by these proceedings, Darwin arranged for another publisher, Charles Reinwald, and another translator, Jean Jacques Moulinié, to bring out a new French edition, incorporating his latest revisions (Moulinié trans. 1873). Reinwald and Moulinié had been engaged to produce the French edition of Variation (Moulinié trans. 1868), and CD now extended his permission for them to bring out his next book (Descent).

As European and North American publishers sought to secure the rights to Darwin's books, both past and future, Darwin began to consider his own rights as an author in a period when international copyright laws were only just emerging. 'I am as yet quite ignorant on the subject,' he wrote to Moulinié on 23 October, 'yet it strikes me that I ought to receive some payment from the Publisher in Paris for the right of Translation; & I intend to make enquiries whether this is ever done.'

I can truly say that I look at the publication of yr essay as one of the greatest honours ever conferred on me

But the translation that was dearest to Darwin's heart was probably the one he commissioned and paid for himself: William Sweetland Dallas's edition of Fritz Müller's Für Darwin (Dallas trans. 1869). The book, an explication of Darwinian evolution through the natural history of crustaceans, had greatly impressed Darwin when it first appeared in 1864, and the ensuing correspondence with Müller was one of the most important of Darwin's entire career. When Darwin read the translation, he wrote to Müller in Brazil on 18 March: 'I can truly say that I look at the publication of yr essay as one of the greatest honours ever conferred on me . . . . What an admirable illustration it affords of my whole doctrines! A man must indeed be a bigot in favour of separate acts of creation, if he is not completely staggered after reading y^r essay'.

The work received a number of reviews, including one in the Athenaeum that quipped: 'It is to be feared that Dr. Müller's moral code is rather Crustacean than Christian'. Müller confessed to his brother Hermann that he thought this a rather good joke, although to Darwin he was more apologetic: 'I am sorry to learn … that some expressions of mine have shocked the religious feelings of one of the reviewers. I should indeed have suppressed or modified some passages, before offering my book to English readers.-' (letter from Fritz Müller, 15 June 1869).

Darwinism and religion

More religious controversy over Darwinism broke out at the annual meeting of the British Association for the Advancement of Science, held in Exeter in August 1869. The controversy was directed in part at Darwin's most outspoken British supporter, Thomas Huxley, whose address 'The physical basis of life', published earlier in the year, was attacked in a series of papers by Anglican clergymen in the biology section of the meeting. Hooker described the session with some enthusiasm: 'The "punctum saliens" of the whole meeting was decidedly Huxley's answer to D^r M^cCann. He literally poured boiling oil over the bumptious man' (letter from J. D. Hooker, 7 September 1869). Huxley playfully groused that as usual Darwin's 'abominable heresies' got him into all sorts of trouble: 'Three parsons set upon you-and if you were the most malicious of men you could not have wished them to have made greater fools of themselves than they did' (letter from T. H. Huxley, 28 September 1869).

Problems of design and purposefulness in nature had been alluded to in a more thoughtful criticism of Darwin's work, an article by the Italian botanist Federico Delpino. The article addressed specifically Darwin's hypothesis of heredity, 'pangenesis', as presented in the second volume of Variation. Delpino raised many technical objections to Darwin's hypothesis, but also described it as 'monistic' and therefore as denying 'liberty' to the organism. Delpino's criticism was noted in a letter from Thomas Henry Farrer, who had been reading some of Delpino's botanical works for his views on dichogamy: 'In one of the books he assails you as a fatalist, and wanders into regions of free will & necessity, into which … I do not care to follow him' (letter from T. H. Farrer, 9 October 1869). Farrer ventured to summarise Delpino's position in the form of a poem, titled 'The biological teleologist', composed he said 'during a sleepless hour':

'The animal can comprehend/ A purpose, and attain an end:/ Whilst in its turn the humbler plant/ Can feel and satisfy a want-/ And thus we find throughout the line/ Freedom and Will and High Design-'

(letter from T. H. Farrer, 13 October 1869).

Darwin was sufficiently impressed by Delpino's criticism of pangensis, however, to have it translated and published in a British journal. Detailed discussion of pangenesis had been scarce in scientific literature, and the appearance of Delpino's review in Scientific Opinion allowed Darwin to publicly defend his provisional hypothesis in the form of a letter to the same journal: 'I can speak from recent experience, that he who has to consider complex cases of inheritance, as limited either separately or conjointly by sex, age, and season, with the inherited characters themselves and the form of inheritance liable to change from crossing and variability, will be able to disentangle the phenomena much more clearly, if he admits for the time our hypothesis with all its imperfections' (letter to Scientific Opinion, [before 20 October 1869]).

The new scientific journals

Scientific Opinion, launched towards the end of 1868, was one of several periodicals begun in these years, expanding and broadening the forums in which Darwinism was discussed. John Murray brought out the first issue of the Academy in September 1869. He described its aims to Darwin on 18 September: 'to convey early & trustworthy intelligence to English Readers on Literature Science & Art & to establish, if possible, a higher tone of Criticism than that now prevailing'. Here Murray was alluding particularly to the Athenaeum, the leading journal of its type for some time, which had carried highly unfavourable reviews of Darwin's works in recent years. 'I heartily wish your Periodical all success', Darwin wrote to Murray, 'I wish it had been weekly, as then perhaps it would have killed the Athenæum by a lingering death, & that to me would have been a pleasing sight' (letter to John Murray, [after 18 September 1869]).

Much more influential in the long term, however, was Nature, the first issue of which appeared in November. Hooker was initially disappointed with it, but added, 'there is plenty of room for a good journal of the kind', and made a number of concrete suggestions to its publisher, Macmillan (letter from J. D. Hooker, 14 November 1869). Darwin was uniformly pleased. 'I like all scientific periodicals', he wrote to Hooker, regretting only that Nature did not review more foreign articles (letter to J. D. Hooker, 19 November [1869]).

New correspondents

Yet despite his enthusiasm for scientific journals, Darwin's most important medium of communication remained correspondence. His network of contacts and collaborators continued to grow and diversify in 1869. A gardener to the king of Prussia, Adolf Reuter, sent beautiful sketches of some curious examples of botanical reversion: 'as a small stone to [Darwin's] great building' (letter from Adolf Reuter, 23 September 1869). The physiologist William Thierry Preyer enclosed a paper on the struggle for existence recently given in a successful series of popular lectures on Darwinism in Bonn. Darwin's photograph was requested by the director of the national museum in Zagreb, Spiridion Brusina, to adorn diplomas of a new natural history society in Croatia (letter from Spiridion Brusina, 29 April 1869). The German zoologist Anton Dohrn solicited Darwin's support for a new zoological station on the Mediterranean coast; the enterprise would result in the establishment of the marine station at Naples, which remains an important institution to this day (letter from Anton Dohrn, 30 December 1869).

I had longed once again to set foot on summit of a mountain

In his reply to Dohrn, Darwin remarked: 'You ask about my Health.- I cannot say much in its praise; but as long as I live the life of a hermit I am able to work some hours daily' (letter to Anton Dohrn, 4 January 1870). Darwin's health was generally much the same as it had been since his last period of prolonged illness in 1864 and 1865, although a particularly low spell in May prompted him to accompany his family to Barmouth in Wales, for an extended holiday. As often happened with Darwin, the enforced leisure left him feeling more tired, and he groaned to Hooker: 'it seems as soon as the stimulus of mental work stops, my whole strength gives way' (letter to J. D. Hooker, 22 June [1869]).

Darwin also found it discouraging to reside in the vicinity where, in earlier years, he had energetically collected insects and studied geology: 'I have been much disappointed at gaining no strength & failing to climb even a hill, & I had longed once again to set foot on summit of a mountain.-' (letter to T. H. Huxley, 9 July [1869]).

Earlier in the year, Darwin had suffered a more serious setback when he was thrown by his horse. Having been advised in 1866 by the doctor Henry Bence Jones to go riding for his health, Darwin had faithfully followed the prescription. Henrietta Emma Darwin wrote to her brother George on 10 April (DAR 245: 291) about the incident:

We've had a very unpleasant event this week. The immaculate Tommy has thrown Father. they were cantering over Keston Common when Tommy tripped & fell bang down-so completely head over heels that his ears & the pommel of the saddle were the two parts muddied. Father of course imitated Tommy's movements & wd not have been hurt at all if tommy had not hit him a fearful blow in the back. This numbed his back & prevented his getting up, but as it was a frequented place someone soon came to his help & he was taken into a house & lay down on a sofa for a bit- after ½ an hour Tommy was caught & as the fly Father ordered was very long in coming he got on Tommy & was led home. he came in m. exhausted & in considerable pain.

Despite the seriousness of the accident, Darwin suffered no broken bones, and though in considerable pain, was soon 'able to hobble about the room a little'. News of the incident spread gradually through Darwin's network, and letters of concern were received for months afterwards.

Francis Galton: Hereditary genius and Pangenesis

In the Christmas season, Darwin spent some of his evenings listening to Emma read aloud from a new book by Darwin's half-cousin, Francis Galton. The work, Hereditary genius: an inquiry into its laws and consequences, described the inheritance of mental ability through generations of particular families. The chapter on 'men of science' contained an entry on the Darwins. Galton traced the pedigree of achievement from Darwin's paternal grandfather, Erasmus, to two of Darwin's sons (George and Leonard), who had recently excelled in their examinations. Darwin himself was described briefly as 'the illustrious modern naturalist; author of the "Theory of Natural Selection"'.

Darwin was full of praise for the book: 'I have only read about 50 pages … but I must exhale myself, else something will go wrong in my inside'. He did express some reserve about Galton's arguments for inheritance as the deciding factor in success: 'You have made a convert of an opponent in one sense, for I have always maintained that, excepting fools, men did not differ much in intellect, only in zeal & hard work; & I still think there is aneminently important difference' (letter to Francis Galton, 23 December [1869]).

Only a few weeks earlier, Galton had enlisted Darwin's assistance in investigating the laws of inheritance through experiments on rabbits (letter from Francis Galton, 11 December 1869). This was the beginning of a series of systematic attempts to test Darwin's theory of pangenesis. Galton's research on the transmission of characters would form the basis of extensive correspondence in the next few years, and would lead in turn to the development of his own distinct theory of heredity, which diverged substantially from Darwin's own. Darwin advised and encouraged the younger man in his scientific work, much as he had done with his own sons. Reflecting on Darwin's pivotal role as mentor, and on the inspirational example of Darwin's own work, Galton wrote, 'I always think of you in the same way as converts from barbarism think of the teacher who first relieved them from the intollerable burden of their superstition … the appearance of your "Origin of Species" formed a real crisis in my life … and was the first to give me freedom of thought' (letter from Francis Galton, 24 December 1869).