In 1862, Charles Darwin published his landmark study on the different forms of flower in Primula ; he coined the term distyly and subsequently expanded his studies to other species, including those with tristyly. Darwin is widely recognized as the first to study pin and thrum flowers in Primula , and to provide an explanation for the functional significance of the two floral morphs. Our laboratory is pursuing the genes that underpin floral heteromorphy in Primula , work influenced by Darwin's observations. One day, while appreciating a print of Primula vulgaris from William Curtis’ Flora Londinensis , I was struck by the fact that I was looking at images of dimorphic Primula flowers captured in a late‐1700s copper‐plate engraving that predated Darwin's observations by over 70 yr. This realization triggered a journey into archives of botanical texts, herbals and florilegia from the 16 th to 19 th Centuries, and correspondence archives, in search of earlier documents that could have influenced Darwin and the origins of an idea. Darwin was not the first to observe floral heteromorphy in Primula , but he was the first to realize the significance of the two floral morphs. Darwin's insight and exposition of purpose have underpinned all consequent work on the subject.

I. Introduction – the germination of an idea On Monday 7 May 1860, Darwin wrote to his friend and intellectual confidant J. D. Hooker with some thoughts on pollination, and a description of observations he had made that morning on two forms of flower in cowslips and primroses which showed ‘balancement of long and short pistils and stamens’ (Darwin, 1860a). This is perhaps the first written reference to Darwin's work on heterostyly in Primula. In this letter, Darwin describes both forms of flower and says ‘this I have somewhere seen noticed, I think by Henslow’ (Darwin, 1860a), referring to John Stevens Henslow, his former Cambridge tutor and mentor. Later that week, on Friday 11 May, Darwin wrote to Hooker again on a different matter with a footnote stating that he had examined more cowslips and primroses and that these consistently produced two forms of flower (Darwin, 1860b). On the following Monday, 14 May, Darwin wrote to Henslow (Darwin, 1860d) to thank him for his support against attacks and criticisms elicited by publication of On the Origin of Species (Darwin, 1859) the previous year. As a postscript he recalls Henslow's observations on the different pistil lengths in cowslips and primroses and speculates from his own observations that they, and auriculas, are perhaps dioecious; there is no documented reply. That same day, he wrote again to Hooker stating that the evidence on cowslips was becoming ‘clearer and clearer’ (Darwin, 1860c). On the following Wednesday, 17 May, he wrote again to Henslow (Darwin, 1860e) recounting various detailed points made in his earlier letter to Hooker and asserted that all primroses and cowslips ‘exist in two forms of about equal numbers’, which appeared to be male or female. For one form, the presumed female, he describes low stamens, small oblong pollen, and a long style with rough stigmatic surface above the anthers. In the second form, the presumed male, he describes high anthers, large rounded pollen, a short style and a smoother stigma beneath the anthers. These five letters, spanning 10 days in May 1860, reveal Darwin's key observations, and the germination of an idea that underpins his subsequent realization of the significance of the two forms of flower. On 21 November 1861, Darwin read his paper ‘On the two forms, or dimorphic condition, in species of Primula, and on their remarkable sexual relations’ at the Linnaean Society; the article was published in March the following year (Darwin, 1862). Despite his fascination with primroses, Darwin did not restrict his studies to these species and subsequently published further works on heterostylous Linum (Darwin, 1863b) and Lythrum (Darwin, 1864) as well as his Different Forms of Flowers book (Darwin, 1877).

II. Developments since Darwin In the 153 yr since Darwin's original paper (Darwin, 1862), there has been important progress towards understanding heterostyly in Primula and other species. We know from genetic analysis that heterostyly in Primula is controlled by a diallelic S locus (Bateson & Gregory, 1905); plants producing flowers with a long style, low anthers and small pollen are recessive at this locus; those with a short style, high anthers and large pollen carry a dominant S allele (Bateson & Gregory, 1905; Ernst, 1928; Dowrick, 1956; Lewis & Jones, 1992). We also know from studies of homostyle plants that the S locus comprises a co‐adapted linkage group of genes (Ernst, 1928, 1936c; Pellow, 1928; Haldane, 1933; Dowrick, 1956; Lewis & Jones, 1992; Richards, 1997) and it has been proposed that recombination within the locus results in long or short homostyle flowers (Dowrick, 1956; Lewis & Jones, 1992), although Ernst originally suggested that these forms arise by mutation (Ernst, 1936b). Darwin showed that reciprocal crosses between the two forms were required for abundant seed set and that within‐morph, or illegitimate, crosses were not as productive as crosses between morphs (Darwin, 1862). Darwin's observation that pin and thrum plants are found in equal numbers in the progeny of legitimate crosses (Darwin, 1862), and his observations on the numbers of progeny types arising from illegitimate crosses (Darwin, 1877), before recognition of the significance of Mendel's work (Mendel, 1866; Bateson, 1902; Moore, 2001), are consistent with the subsequent demonstration that thrums behave as heterozygotes for a dominant S allele and pins as recessive homozygotes (Bateson & Gregory, 1905). Darwin, not surprisingly, speculated on the evolutionary origins of heterostyly (Darwin, 1877), and proposed that the selection and establishment of reciprocal herkogamy preceded the evolution of self‐incompatibility. He considered the initial and primary function of heterostyly as an adaptation to promote the insect‐mediated reciprocal transfer of pollen between floral morphs, and regarded floral heteromorphy as a mechanism to enhance the amount and accuracy of pollen transfer from anther to stigma between the two forms of flower (Darwin, 1877). He saw no advantage to a system that rendered a plant unreceptive to pollination by half the population, and considered the self‐incompatibility (SI) of within‐morph crosses as an incidental consequence of the adaptation of male and female organs for reciprocal interaction. Subsequ‐ent studies have further explored the sporophytic SI system (Golynskaya et al., 1976; Heslop‐Harrison et al., 1981; Shivanna et al., 1981, 1983; Wedderburn & Richards, 1990) which functions to minimize the negative effects of self‐pollination, but also acts as a barrier to within‐morph crosses. Alternative scenarios for the evolution of the morphological and physiological aspects of heterostyly have been proposed, suggesting either that SI and heterostyly evolved together (Mather & De Winton, 1941) or that SI preceded the establishment of dimorphic flowers (Charlesworth & Charlesworth, 1979b; Piper & Charlesworth, 1986; Charlesworth, 2006). Others have concurred with aspects of Darwin's interpretation that reciprocal herkogamy was the key initial step and that cross‐pollination was the selective force for the evolution of heterostyly (Lloyd & Webb, 1992a,b). Ornduff directly measured pollen flow by measuring pollen load on stigmas in a population of Primula vulgaris but found only limited support for Darwin's hypothesis of reciprocal pollen transfer because of high levels of self pollen on pin and thrum stigmas (Ornduff, 1979). A subsequent study using emasculated flowers which removed the opportunity for contamination by self pollen (Piper & Charlesworth, 1986) enabled the demonstration of significantly greater inter‐morph than intra‐morph pollen transfer to the stigma surface. Such studies, and more recent consideration of them, have been interpreted as providing support for Darwin's proposal that the evolution of heterostyly has been driven by the promotion of out‐crossing rather than avoidance of selfing (Lloyd & Webb, 1992b). Further insight into earlier and contemporary thinking on the evolution and function of heterostyly can be found in a number of reviews (Charlesworth & Charlesworth, 1979a,b; Ganders, 1979; Barrett, 1992a; Lloyd & Webb, 1992a,b; Barrett & Shore, 2008). Others have considered the evolutionary relationships of members of the Primulaceae (Conti et al., 2000; Mast et al., 2001; Mast & Conti, 2006; Schmidt‐Lebuhn et al., 2012) and recent advances using classical and molecular genetics approaches in Primula (Manfield et al., 2005; McCubbin et al., 2006; Li et al., 2007, 2008, 2010, 2015; Cocker et al., 2015), and other heterostylous species (Matsui et al., 2004; Yasui et al., 2004; Labonne et al., 2008, 2009, 2010; Labonne & Shore, 2011; Ushijima et al., 2012), have made progress towards identifying genes involved in floral heteromorphy. The recent publication of a draft assembly of the Primula veris genome (Nowak et al., 2015) has confirmed the linkage of previously characterized S‐linked genes (Li et al., 2007, 2008, 2010) and identified other S‐linked contigs. This information, together with an integrated genetic and physical map of the P. vulgaris S locus (Li et al., 2015) and the completion of a genome sequence for P. vulgaris (P. M. Gilmartin et al., unpublished), will facilitate identification of the key genes underpinning floral heteromorphy in Primula. The various studies on floral heteromorphy in Primula spanning a century and a half have been summarized in numerous papers and reviews on heterostyly (Charlesworth & Charlesworth, 1979a; Ganders, 1979; Barrett, 1992b, 2010; Lewis & Jones, 1992; Richards & Barrett, 1992; Charlesworth, 2006; Barrett & Shore, 2008; McCubbin, 2008; Owens & Miller, 2009; Weller, 2009), including ‘Evolution and function of heterostyly’ (Barrett, 1992a), the only monograph published since Darwin on heterostyly. Some publications have recognized the historical context of Darwin's findings, and one study revealed the significance of Henslow's influence on Darwin's thinking (Kohn et al., 2005), but the majority have focused on work that has been published by and after Darwin, although some studies reference van Dijk's scholarly contribution (van Dijk, 1943). In his Historical Perspective on Heterostyly, Ornduff (1992) focused on Darwin’ contributions and the influence of his contemporaries; he highlights van Dijk's reference to observations by Clusius in the 16th Century, but does not directly examine published accounts of heterostyly in the intervening 280 yr before Darwin's 1862 publication (Darwin, 1862). In this review, I explore the origins of observations of floral heteromorphy by reviewing work cited by Darwin (1877), the original texts discovered by van Dijk (1943), and various early herbals and florilegia dating back to the 16th Century in an attempt to document the transition from botanical observation of flower form to Darwin's insight into the significance of heterostyly in Primula (Darwin, 1862). This review is not intended to address the evolution or functional significance of heterostyly, which have been extensively reviewed by others and provide contemporary updates on Darwin's observations and interpretations. Table 1 presents a comparison of the various and multiple names of Primula species, and their relevant authorities, discussed in this review. In some cases these names predate the Linnaean binomial system (Linnaeus, 1735, 1753), and in others they reflect name changes as the Linnaean system became adopted and precedence established (Brummitt & Meikle, 1993). Table 1. Synopsis of Primula species names used by different authors Current binomial P. vulgaris Huds.1 P. veris (L.) Hill2 P. elatior (L.) Hill2 P. farinosa L.3 P. scotica Hook4 P. auricula L.3 Common name Primrose Cowslip Oxlip Birdseye Primula Scottish Primula Auricula Nomenclature used by Linnaeus3 P. veris var. acaulis P. veris var. officinalis P. veris var. elatior P. farinosa P. auricula Names used by Curtis 7 P. acaulis P. officinalis P. elatior P. farinosa L. P. scotica Names used by Persoon8 P. acaulis (L.) Hill2 P. odorata Gilib.5 P. inodora Hill2 Pre‐Linnaean descriptive names Descriptive names include Primula veris sylvestris flora pallido9 (primrose) and Primula veris pallido flore elatior9 (cowslip); others which cannot be aligned to current species names include Auricula Ursi III10 , Auricula Ursi minima V10 , and Bear's Ears11 Horticultural varieties P. acaulis is sometimes used for commercial primrose varieties; and P. × polyantha Mill.6 for polyanthus Auricula

III. The evolution of Darwin's thinking on heterostyly Although Darwin observed that the two forms of Primula flower contained both pistils and stamens, he initially interpreted the differences as transitions towards dioecy through a reduction in the size and significance of the male structures in one form, and the female structures in the other. In his correspondence with Henslow (Darwin, 1860e), he made the comparison to dioecious holly (Ilex aquifolium), in which ‘the Male plant has anthers but no pollen’, in an attempt to explain the differences between the two forms of Primula flower. By the following year, Darwin had resolved the conundrum and recognized that these forms were not transitions to dioecism, but represented a different breeding system which he called distyly. In a letter to Asa Gray at Harvard in 1861 (Darwin, 1861), Darwin states that both forms of flower are hermaphrodite, and that ‘The pollen of A is fitted for stigma of B & conversely’. His letter includes a diagram of pin and thrum flowers (Fig. 1a). Darwin's subsequent paper at the Linnaean Society (Darwin, 1862), and his landmark book, The Different Forms of Flowers on Plants of the Same Species (Darwin, 1877), both include a now‐familiar image (Fig. 1b) produced by the illustrator W. H. Fitch of long‐ and short‐styled P. veris flowers. This image was prepared from Darwin's hand‐drawn sketch (Fig. 1c). The original sketch was sold at auction by Christie's in London on 11 November 1998 (Christie's, 1998) for £8625, and again on 15 July 2004 at Bonhams, London, this time fetching £7170 (Bonhams, 2004). Like the sketch he sent to Gray, Darwin's image shows long stamen filaments rather than differentiating between the two forms based on the point of anther attachment (Fig. 1c). Fitch's image in the published work (Fig. 1b), however, depicts the two forms of flower with astounding accuracy. Figure 1 Open in figure viewer PowerPoint Primula images. (a) Darwin's hand‐drawn sketch of thrum (B) and pin (A) Primula flowers from his letter to Asa Gray on 16 September 1861; the anther and stigma are labelled (Darwin, 1861 Primula veris flowers that was used to illustrated Darwin's publications (Darwin, 1862 1877 Primula flower: short stamens or pin‐headed (left) and long stamens (right); pistil, calyx stamens and corolla are labelled (Christie's, 1998 Darwin'simages. (a) Darwin's hand‐drawn sketch of thrum () and pin (flowers from his letter to Asa Gray on 16 September 1861; the anther and stigma are labelled (Darwin,). (b) Woodcut print engraved by W. H. Fitch showing the long‐styled form (pin) and short‐styled form (thrum) offlowers that was used to illustrated Darwin's publications (Darwin,). (c) Darwin's hand‐drawn sketch of the two forms offlower: short stamens or pin‐headed (left) and long stamens (right); pistil, calyx stamens and corolla are labelled (Christie's,). (a) Reproduced with permission from The Asa Gray Library, Harvard University, USA; (b) photograph from the original by the author; (c) reproduced with permission from Christie's London and Bridgman Art Library, UK. Following publication of his Primula paper (Darwin, 1862), Darwin exchanged notes on heterostyly in Primula and Linum with Freidrich Hildebrand in Bonn. In a letter dated 10 November 1863 (Hildebrand, 1863a), Hildebrand summarizes his observations on Primula sinensis, but is unable to comment on experiments on Primula officianalis (elatior) and P. veris as these were ‘destroyed by children in the botanical gardens’ (Hildebrand, 1863a). This event may explain the focus of Hildebrand's subsequent publications on heterostyly in P. sinensis (Hildebrand, 1863b, 1864), and contrasts with Darwin's experience described in a letter to Hooker, that his children had helped gather 522 flower stalks for his studies (Darwin, 1860c). Darwin also corresponded extensively with John Scott, Head Gardener at the Royal Botanic Gardens, Edinburgh and communicated Scott's comprehensive manuscript describing heterostyly in several Primula and other species to the Linnaean Society (Scott, 1864). These various correspondences helped Darwin further develop his thoughts on heterostyly which were included, with numerous other studies on floral dimorphism, in his treatise Different Forms of Flowers (Darwin, 1877). Additional insight into the interactions between Darwin and his contemporaries on heterostyly can be found in Ornduff's earlier review (Ornduff, 1992). In both his original publication (Darwin, 1862) and subsequent book (Darwin, 1877), Darwin used the terms pin‐headed and thrum‐eyed to distinguish the floral morphs, although in the original paper (Darwin, 1862) thrum is, as noted by (Ornduff, 1992), misspelt as thumb. Pin‐headed flowers are so called after the appearance of the long style and globular stigma which resembles the head of a pin inserted into the flower. In his Different Forms of Flowers book, Darwin defines thrum, according to Johnson's dictionary, as the ends of weaver's threads (Darwin, 1877). The fact that these terms were in use in 1861 when Darwin read his paper at the Linnaean Society (Darwin, 1862) suggests that the two forms of flower were already familiar before Darwin described them. A potential insight into earlier observations on heteromorphy appears on page 43 of Different Forms of Flowers (Darwin, 1877), where Darwin cites observations by Kerner on heteromorphy in Primula auricular in 1835; 27 yr before Darwin's original publication. A search for these references, however, revealed referencing errors; the articles actually appeared in 1875 (Kerner, 1875a,b,c,d) and do not therefore reveal precedence on Darwin's observations. Darwin was also aware of homostyle variants of cultivated Auricula (Darwin, 1862) and had observed homostyles in P. veris × P. vulgaris hybrids, but was surprised to hear (Darwin, 1863a) of a long‐homostyle native cowslip described in a letter from John Scott on 21 May 1863 (Scott, 1863). It seems that Darwin regarded homostyly as a consequence of hybridization and cultivation; we now recognize that homostyles can occur by mutation (Ernst, 1936b) or proposed recombination within the S locus (Dowrick, 1956; Lewis & Jones, 1992). Different Forms of Flowers (Darwin, 1877) contained detailed observation on various homostyled plants. Perhaps one of the earliest illustrations of a long homostyle, which predates Darwin's and Scott's correspondence, comes from Alcide D'Orbingy's Dictionnaire Universel d'Histoire Naturelle (D'Orbingy, 1849) (Fig. 2). Another image published in Edward Step's Favourite Flowers of Garden and Greenhouse (Step, 1897) depicts both thrum and what appears to be a short‐homostyle flower; however, the accompanying text describes both long‐ and short‐styled dimorphic flowers; the image therefore appears to reveal an engraving or printing error (Fig. 3). Figure 2 Open in figure viewer PowerPoint Primula, engraved by Vc. Fournier, from Charles D'Orbigny's Dictionnaire Universel d'Histoire Naturelle published in 1849, showing a dissected long‐homostyle flower on the left as well as seed capsules and seed. Stigma and anthers are visible in the corolla mouth of the intact flowers (D'Orbingy, 1849 Long homostyle (1849). Hand‐coloured copper‐plate print of polyanthus, engraved by Vc. Fournier, from Charles D'Orbigny'spublished in 1849, showing a dissected long‐homostyle flower on the left as well as seed capsules and seed. Stigma and anthers are visible in the corolla mouth of the intact flowers (D'Orbingy,). Figure 3 Open in figure viewer PowerPoint Primula variabilis from Edward Step's Favourite Flowers of Garden and Greenhouse published in 1897 showing the whole plant with thrum flowers and dissected thrum flower at lower left, and dissected apparent short homostyle at lower right (Step, 1897 Apparent short homostyle (1897). Polyanthusfrom Edward Step'spublished in 1897 showing the whole plant with thrum flowers and dissected thrum flower at lower left, and dissected apparent short homostyle at lower right (Step,). In his paper, Darwin did not refer to earlier studies on distyly in Primula (Darwin, 1862), although he did cite Asa Gray's work on different floral morphs in the Rubiaceae (Gray, 1856). However, in Different Forms of Flowers, Darwin not only credits the contributions of Gray, Hildebrand and Scott, but dedicates the volume to Gray (Darwin, 1877). In this book, he also recognizes the contributions of John Scott, by referring to his Primula observations on several occasions, and acknowledges Hildebrand's use of the term heterostyly as superior to his own term, distyly (Darwin, 1877). At the same time, he rejects Gray's proposal for the term heterogonous (Gray, 1877; Ornduff, 1992), as the term heterostyled had ‘by this time become accepted in various countries’ (Darwin, 1877). In Different Forms of Flowers, Darwin clearly recognizes the intellectual contributions of others and also cites earlier observations on heterostyly in Primula species, and the water violet Hottonia palustis, through reference to Sprengel's description of heterostyly in Hottonia from 1793, and to Persoon as having first observed heterostyly in Primula in 1794, as cited by von Mohl (1863). As these cases were not cited in the original Primula paper (Darwin, 1862), it can be assumed that he only became aware of them in the years between the two publications; these earlier observations cannot therefore have influenced his original thinking.

IV. Influences on Darwin's thinking What did perhaps influence Darwin's thinking on Primula was the unpublished work of Henslow (Kohn et al., 2005), who had drawn both forms of flowers in 1826 (Fig. 4), 34 yr before Darwin's letters of discovery. As revealed by his 1860 correspondence with Henslow (Darwin, 1860c) and Hooker (Darwin, 1860a), Darwin did recall the earlier description, possibly from his botany studies with Henslow as a student at Cambridge between 1829 and 1831 (Kohn et al., 2005). Henslow was, however, not the first to document the two forms of flower; three earlier representations exist. The first is in Francoise‐Pierre Chaumeton's Flore Medicale, illustrated by Pierre Jean Francoise Turpin, and published in eight volumes between 1814 and 1820. In Volume 5, published in 1818 (Chaumeton & Turpin, 1818), Turpin presents images of pin and thrum flowers of P. veris. However, the text only describes the pin form and the figure legend refers only to the thrum as ‘provenant d'un autre individu’ (from another individual). The different forms of flower were clearly recognized and documented (Chaumeton & Turpin, 1818) but the significance and relevance were not (Fig. 5). Henslow's 1826 drawings are therefore predated by Turpin's in 1818. A second example appears in Wilibald Artus’ Hand‐Atlas sammtlicher medicinisch‐pharmaceutischer Gewachse illustrated by Kirchner (Artus & Kircher, 1848) which shows a dissected P. veris pin flower and a second corolla without style but anthers in the thrum position (Fig. 6). The text refers to flowers having either high or low anthers, but only refers to the long style. An even earlier representation of pin and thrum flowers can be found in William Curtis’ Flora Londinensis (Curtis, 1777–1798a); it is this image (Fig. 7) that triggered this review. It is perhaps surprising that Darwin was not familiar with Curtis’ publication. Figure 4 Open in figure viewer PowerPoint Primula images (1826). Hand‐drawn sketch by John Stevens Henslow of var. (varieties) in cowslip Prim. Off. (Primula veris, formerly P. officianalis) showing dissected and intact flowers of pin (above) and thrum (below). It is dated 18 April 1826 and signed J.S.H. (Kohn et al., 2005 Henslow'simages (1826). Hand‐drawn sketch by John Stevens Henslow of var. (varieties) in cowslip. (, formerly) showing dissected and intact flowers of pin (above) and thrum (below). It is dated 18 April 1826 and signed J.S.H. (Kohn.,). Reproduced with permission from Cambridge University Botanic Gardens, UK. Figure 5 Open in figure viewer PowerPoint Primula veris) (1818). Hand‐coloured copper‐plate print, engraved by Pierre Turpin, from Volume 5 of Françoise‐Pierre Chaumeton's Flore Médicale published in 1818 of a cowslip (P. veris) plant. Dissected pin (1) and thrum (2) flowers are illustrated. Seed capsules (3, 4 and 5) and seed (6 and 7) are also shown (Chaumeton & Turpin, 1818 Turpin's cowslip () (1818). Hand‐coloured copper‐plate print, engraved by Pierre Turpin, from Volume 5 of Françoise‐Pierre Chaumeton'spublished in 1818 of a cowslip () plant. Dissected pin (1) and thrum (2) flowers are illustrated. Seed capsules (3, 4 and 5) and seed (6 and 7) are also shown (Chaumeton & Turpin,). Figure 6 Open in figure viewer PowerPoint Primula veris) (1848). Hand‐coloured copper‐plate print, engraved by F. Kirchner for Wilibald Artus’ Hand‐Atlas sämmtlicher medicinisch‐pharmaceutischer Gewächse published in 1848 of a cowslip (P. veris) plant (A). A dissected pin flower (B) is illustrated with ovary (a) and anthers (b) labelled. A dissected anther (1) and cross‐section of the stigma (2) and various stages of seed capsule (3, 4, 5 and 6) are shown. The dissected corolla and anthers (C) represent a thrum flower. Intact and dissected seeds (7, 8, 9 and 10) are also shown (Artus & Kircher, 1848 Kirchner's cowslip () (1848). Hand‐coloured copper‐plate print, engraved by F. Kirchner for Wilibald Artus’published in 1848 of a cowslip () plant (A). A dissected pin flower (B) is illustrated with ovary (a) and anthers (b) labelled. A dissected anther (1) and cross‐section of the stigma (2) and various stages of seed capsule (3, 4, 5 and 6) are shown. The dissected corolla and anthers (C) represent a thrum flower. Intact and dissected seeds (7, 8, 9 and 10) are also shown (Artus & Kircher,). Figure 7 Open in figure viewer PowerPoint Primula vulgaris from Curtis’ Flora Londinensis (1777–1798). Hand‐coloured copper‐plate print, engraved by Sydenham Edwards for William Curtis’ Flora Londinensis published between 1777 and 1798. This image of Primula acaulis (P. vulgaris) published on 1 March 1791 shows pin stigmas in the mouths of the intact flowers. The images at the bottom left show a calyx (1) and a dissected thrum flower showing the corolla (2) and high anthers (3). The images at bottom right show a dissected pin flower with corolla (2) and low anthers (4). An enlarged anther (5) and carpel with ovary (6), long style (7) and stigma (8) are also shown (Curtis, 1777–1798a from Curtis’(1777–1798). Hand‐coloured copper‐plate print, engraved by Sydenham Edwards for William Curtis’published between 1777 and 1798. This image of) published on 1 March 1791 shows pin stigmas in the mouths of the intact flowers. The images at the bottom left show a calyx (1) and a dissected thrum flower showing the corolla (2) and high anthers (3). The images at bottom right show a dissected pin flower with corolla (2) and low anthers (4). An enlarged anther (5) and carpel with ovary (6), long style (7) and stigma (8) are also shown (Curtis,). Curtis uses the Linnaean binomial system and follows his Systema Sexuale classification system (Linnaeus, 1735) based on reproductive organ number; this focus on reproductive structures perhaps explains the depiction of dissected flowers alongside the whole plants in Flora Londinensis. Curtis’ copper‐plate image of P. vulgaris (Fig. 7) shows the whole plant and dissected flowers that show both forms of flower. The accompanying text describes the two forms and names them as pin‐eyed and thrum‐eyed. This is perhaps the earliest record of these terms, nearly 100 yr before Darwin used them. Flora Londinensis is considered one of the foremost 18th Century illustrated floras (Walpole, 1976) and was published in 72 parts each comprising six plates, and is typically dated between 1777 and 1798 (Walpole, 1976), although the first parts were released in 1775 (Stevenson et al., 1961; Nelson, 1980). The complete work comprises 432 plates depicting 435 species in six fasciculi grouped into two volumes; each fasciculus contains 12 parts with 72 plates, and each volume comprises three fasciculi. The serial publication over 23 yr, the limited number of intact copies, and the order of plates in the different bound copies obscure dates for individual plates. However, an 18th Century review (Schrank, 1793), published contemporaneously with Curtis’ work, includes an ordered list of plates for Flora Londinensis. Without Schrank's list it would not be possible to date individual plates. In Flora Londinensis, Primula farinosa, P. officianalis (veris) and P. acaulis (vulgaris) are included in Volume 2 of the first edition and indexed and bound as Plates 14, 15 and 16, respectively, in the final sixth fasciculus (Curtis, 1777–1798c; Stevenson et al., 1961). Plate 16, P. acaulis (vulgaris), was published on 1 March 1791; Plate 15, P. officianalis (P. veris), was published by 10 April 1793, and P. farinosa was published some time during 1795. Plates depicting P. elatior and Primula scotica were not included in the first edition (Stevenson et al., 1961) but were included in the later Hooker edition (Curtis et al., 1815–1828). Despite clear descriptions and images of pin and thrum flowers of the primrose (Curtis, 1777–1798a), other Primula species are not depicted with both forms of flower and the text does not refer to them. Images show P. veris and P. farinosa as thrums (Curtis, 1777–1798a); the second edition shows P. elatior as a pin and P. scotica as a homostyle with dissected flowers showing high anthers and a long style (Curtis et al., 1815–1828). Flora Londinensis was edited and republished as an expanded ‘Hooker Edition’ in 1815 by George Graves and William J. Hooker (Curtis et al., 1815–1828; Walpole, 1976); W. J. Hooker was J. D. Hooker's father; at the time of his correspondence with Darwin, J. D. Hooker was Assistant Director at Kew (Desmond, 2007). The lack of reference to Flora Londinensis in their correspondence suggests that neither (J. D.) Hooker nor Darwin was aware of Curtis’ description and images of pin and thrum Primula flowers. Although J. D. Hooker may not have been aware, his father certainly was; in the later edition, Hooker edited Curtis’ original P. acaulis (vulgaris) text with a footnote discussing flower stalk length in comparisons to P. elatior (Curtis et al., 1815–1828). There is a similar familial record of Curtis’ Flora Londinensis in Darwin's family. His grandfather Erasmus Darwin corresponded directly with William Curtis in November 1781 expressing his delight with the Flora Londinensis ‘which he had taken ever since it was published’ (Darwin, 1781), although Erasmus is curiously not listed as one of the founding subscribers (Curtis, 1777–1798b). Although Erasmus would no doubt have seen Curtis’ depiction of the two forms of Primula flower, these did not influence his various writings. Primroses and cowslips are poetically introduced without description in his Botanic Garden (Darwin, 1791a,b), although he does describe ‘the adhesions of stamen filaments’ to the inner corolla in what was probably a pin flower in his Phytologia (Darwin, 1800). On page 106 of his 1787 translation of Linnaeus’ Families of Plants, the Primula entry describes a pin flower (Linnaeus, 1787). Erasmus’ failure to highlight the two forms of flower is perhaps surprising given his interest in ‘the loves of the plants’ (Darwin, 1791b). Erasmus died 7 yr before his grandson Charles was born so perhaps Darwin was unaware of his grandfather's enthusiasm for Curtis’ Flora Londinensis. In those elements of Darwin's library donated to the Botany Department in Cambridge by his son Francis, there is no record for Flora Londinensis; there is, however, a record of Volumes 1 and 2 of another of William Curtis’ publications, the Botanical Magazine (Rutherford, 1908). In these two volumes, the only Primula species illustrated is Primula villosa (Vol. 1, Plate 14) (Curtis, 1790), but the image does not show any flower detail that could have influenced Darwin.

V. Darwin and the historical landscape of botanical illustration It is surprising that neither Darwin nor Hooker was aware of Flora Londinensis and the engravings and descriptions of pin and thrum flowers that it contains. Henslow was, however, aware of the images therein, as revealed in his article ‘On the specific identity of the Primrose, Oxlip, Cowslip and Polyanthus’ (Henslow, 1830). In this article he concludes, based on his own observations, and those of Reverend William Herbert, of the mixture of primrose, cowslip and oxlip progeny obtained from a single flowering umbel of ‘an highly manured Red Cowslip’ (Herbert, 1822), that the plants are merely varieties of the same, rather than distinct species (Henslow, 1830). It is perhaps surprising, given his experimental approach to hybrid crosses, that Herbert did not recognize the two forms of flower. Henslow cites Hooker's edition of Curtis’ Flora Londinensis as providing an example of Primula plants with single and compound scapes, possibly influenced by Hooker's footnote, but he does not refer to the pin and thrum flowers illustrated there. The clear intellectual focus at the time on whether primroses, cowslips and oxlips represented a single or multiple species may perhaps explain why the details of heterostyly were overlooked. Henslow had, however, by this date illustrated the two forms of flower, but had not published them (Kohn et al., 2005); had he done so he may well have cited Curtis’ prior observations of pin and thrum P. vulgaris flowers in Flora Londinensis. The contemporaneous English Botany with copper‐plate engravings by James Sowerby and text by James Edward Smith was published between 1790 and 1813 (Sowerby & Smith, 1790–1813). It is curious that Primula species illustrated in English Botany are not shown with both forms of flower; perhaps more surprising given the fact that Sowerby contributed engravings to both publications (Stevenson et al., 1961), although the Primula engravings in the Flora Londinensis are attributed to Sydenham Edwards not James Sowerby (Curtis, 1777–1798a; Stevenson et al., 1961). Of the five species described in English Botany, the P. vulgaris and P. veris images show only whole flowers illustrated as thrums. The dissected flower images for P. elatior and P. farinosa show pin and thrum flowers, respectively. For P. scotica, the intact flower shows both anthers and stigma, suggesting a long homostyle (Sowerby & Smith, 1790–1813). The majority of 18th and 19th Century botanical illustrations depict intact, not dissected, flowers. Such images do not typically permit distinction between pin and thrum flowers, unless the mouth of the flower is clearly visible. In pre‐Darwinian texts this may reveal a lack of appreciation of the two floral forms, but in some later Victorian flower books it may reflect a more aesthetic, artistic or poetic view of the importance of flowers (Seaton, 1985), as opposed to seeing them as botanical specimens of reproductive importance. An example is John Ruskin's Proserpina (Ruskin, 1888) in which he illustrates the ‘Four stages in the Young Life of a Primrose’ showing four stages of flower bud development (Fig. 8). Ruskin uses intact flowers which obscure any reproductive detail, perhaps reflecting attitudes of the era or, as proposed by Smith (2006), as a statement of rejection of Darwin's view of the significance and importance of reproductive structures. The primrose images in Ruskin's Proserpina (Ruskin, 1888) probably provide the earliest representation of Primula flower bud development. Figure 8 Open in figure viewer PowerPoint Primula vulgaris) flowers (1888). Print from a wood engraving by Arthur Burgess of four stages of primrose flower development drawn by John Ruskin for his Proserpina published in 1888 (Ruskin, 1888 Ruskin's developing primrose () flowers (1888). Print from a wood engraving by Arthur Burgess of four stages of primrose flower development drawn by John Ruskin for hispublished in 1888 (Ruskin,). Reproduced with permission from Cambridge University Library, UK. In Frederick Edward Hulme's Familiar Wild Flowers (Hulme, 1878), Primula images show only pin P. vulgaris, P. veris and P. elatior. Similarly, Anne Pratt's earlier books Wild Flowers (Pratt, 1852) and The Flowering Plants of Britain’ (Pratt, 1855–1866) do not distinguish between the two forms, although the former does include a detailed description of stamen and pistil position, with an illustration of a dissected pin primrose flower; text in the latter book describes the location of stamens in Primula as within the tube of the corolla. The illustration from The Flowering Plants of Britain (Supporting Information Fig. S1) shows flowers of the five British Primula species, depicting cowslip as thrum, oxlip as pin, P. farinosa and P. scotica with high anthers, and, curiously, primrose as a long homostyle; a thrum Hottonia palustris is also included (Pratt, 1855–1866). Notable exceptions to the depiction of intact flowers in 18th Century texts come from three works with high‐quality images of dissected Primula flowers which clearly display pin or thrum forms without reference to their significance. These are Elizabeth Blackwell's Curious Herbal (Blackwell, 1737‐1739), The Flora Danica (Oeder, 1761–1883) and Johannes Zorn's Icones Plantarum Medicinalium (Zorn, 1780). The Curious Herbal was originally published weekly as four images and a text page, and subsequently republished in 1750 and 1757 (Blackwell, 1757). The first edition shows intact P. vulgaris and P. veris flowers (Blackwell, 1737‐1739); the 1757 edition, however, provides much greater anatomical detail for P. veris (Fig. S2) with dissected thrum flowers (Blackwell, 1757). Zorn's (1780) image of a dissected P. veris flower reveals details of a pin flower (Fig. S3). In the Flora Danica, P. vulgaris and P. elatior are depicted as pins, and P. veris as a thrum (Oeder, 1761–1883). The P. vulgaris image (Fig. S4) was published in c. 1765, and the P. veris and P. elatior images date from c. 1767 (Oeder, 1761–1883). The lack of consistency between illustrators in presenting images of whole flowers or dissected flowers in 18th and 19th Century herbals, florilegia and flower books suggests that the differences in floral organ arrangements were not recognized as important, although the botanical accuracy captured by Blackwell (1737‐1739), by Zorn (1780) and in Floral Danica (Oeder, 1761–1883) shows significant attention to this detail. Looking back to even earlier representations of Primula flowers, 17th Century herbals and florilegia typically used wood‐block prints which lacked the quality of resolution required to capture detail of floral architecture (Gerard, 1597; Besler, 1613; Parkinson, 1629). However, at the end of the 17th Century, copper‐plate printing was replacing wood‐block printing (Arber, 1912) and this new technology enabled fine details of floral form to be captured. One such example is Hortus Floridus by Crispin van de Pass the Younger, which contains copper‐plate images of plants drawn ‘true to life’ (van de Passe, 1614) and arranged by the four seasons. Plate 7 of the ‘Spring’ section of Hortus Floridus depicts two mutant forms of P. veris, one showing double flowers, the other Hose in Hose flowers (van de Passe, 1614). We now know that Hose in Hose is linked to the S locus (Ernst, 1936a; Webster & Grant, 1990; Li et al., 2010); however, there is insufficient detail in the image to determine whether the Hose in Hose plant depicted is a pin or a thrum. In Plate 8 of Hortus Floridus, van de Pass presents two forms of Auricula Ursi; both images show anthers in the mouth of the flower (Fig. S5), and these are possibly the earliest illustration of a Primula thrum flower (van de Passe, 1614). Hortus Floridus was published in two parts; the Altera Pars (the other part) (van de Passe, 1605) is not arranged by season as in Hortus Floridus, but presents plants in different arrangements as food or medicinal plants. Although Altera Pars is sometimes regarded as an appendix to Hortus Floridus, it probably predates it, although the exact publication date, c. 1605, is not clear (Savage, 1923; Gerard, 1996). Plate 27 of Altera Pars depicts a P. vulgaris plant with the stigma in the mouth of the flower; this image (Fig. S6), along with the copper‐plate images of pin flowers in de Reneaulme's Historiae Plantarum Plantae (Fig. S7) (de Reneaulme, 1611), may be the earliest images of Primula pin flowers.

VI. The historical work cited in Darwin's Different Forms of Flowers book By 1877, when Different Forms of Flowers was published, Darwin had discovered that heterostyly had been observed and documented previously, both in Primula (Persoon, 1794) and in Hottonia (Sprengel, 1793). Darwin had clearly read Sprengel's work on Hottonia which precisely describes some flowers as having ‘anthers located within the corolla tube, and the style extended above, and others with stamens longer than the corolla tube and shorter styles’ (Sprengel, 1793). He even notes Sprengel's sagacity (his term) (Darwin, 1877) in suggesting that this is not just fortuitous but a device of Nature, although unlike Darwin, Sprengel does not speculate on the reason for the differences (Sprengel, 1793). For Primula, Darwin cites Persoon's work as ‘according to Von Mohl’ in the Botanical Zeitung in 1863 (von Mohl, 1863), the year after his Primula paper (Darwin, 1862). Von Mohl is aware of Darwin's preceding paper and cites Darwin's work; he also provides an important link to the earlier work of Persoon (von Mohl, 1863). Review and translation of Persoon's original description of Primula flowers, which appeared in Latin in Pauli Usteri's Annalen der Botanick in 1794 (Persoon, 1794), is interesting for two reasons. First, it describes the different forms of flower in three species of Primula: Primula inodora (elatior), P. odorata (veris) and P. acaulis (vulgaris). In his description of P. inodora, Persoon defines two forms of flower: prominula (prominent), with an exerted pistil and anthers inserted into the tube, and latitans (hiding), with anthers in the throat of the flower and a shorter pistil. His description of the two forms of P. odorata, as exserta (protruding) and abscondita (hidden), also refers to the style length, and he describes the ‘situs staminium & pistilli diversa longitudo’ – ‘different positions of the stamens and pistil lengths’. For the descriptions of P. acaulis (vulgaris), which he discusses alongside P. odorata (veris), he uses the anther height rather than the style length to distinguish the two forms, or varieties (Var.) of flower: Var. α Antheris prominulis (anthers prominent) and Var. β Antheris latitantibus (anthers hiding). The second and most striking element of Persoon's descriptions comes from his reference to Curtis’ Flora Londinensis in relation to P. odorata (veris), and after his own full description of the two forms of flower – ‘Ex observatione Curtisii’ – ‘from the observations of Curtis’ (Persoon, 1794). Given the date of Persoon's article, he must have been referring to the first edition of Flora Londinensis (Curtis, 1777–1798a) in which pin and thrum flowers of P. acaulis (vulgaris) were described and illustrated (Fig. 7). Persoon published his observations in Annalen der Botanick in 1794 (Persoon, 1794), 3 yr after the primrose was illustrated by Curtis in 1791 (Stevenson et al., 1961); Schrank's review of Flora Londinensis also appeared in Annalen der Botanick a year before Persoon's (Schrank, 1793). It is therefore likely that Persoon had seen Schrank's earlier article and that this may have prompted him to explore Flora Londinensis, if he was not already aware of it, where he discovered Curtis’ precedence for the description of pin and thrum flowers. Flora Londinensis reads: While we are thus describing the varieties to which this plant is subject, it may not be amiss to observe that the stamina also vary greatly in their situation, being sometimes found low down in the tube of the blossom, sometimes at its mouth, in the former instance the pistil which varies also in length shows its round stigma, and with its attendant style looks like a pin stuck in the centre of the flower; such flowers in the Polyanthus are termed pin‐eyed, while those in which the anthers close the mouth of the tube, are called thrum‐eyed, and this latter appearance in the opinion of the florist is an essential requisite in a good flower. The text fromreads: Curtis’ point on the ‘requisite of a good flower’ is further borne out by an article in the first edition of Gardener and Practical Florist (Unknown, 1843) on the properties of the Polyanthus, which states: ‘The tube should be nearly filled up with the six anthers, which are technically called the thrum, and the flowers should not exhibit the pistil.’ A footnote reveals: ‘Some Polyanthus show the pistil, and are called pin‐eyed; these are considered worthless’ (Unknown, 1843). Clearly, early gardeners did not recognize the mutual dependence of the two forms and the importance of pins for reproductive success. It is interesting that Darwin uses the exact same terms as Curtis, pin‐eyed and thrum‐eyed, in his first Primula paper (Darwin, 1862). Although Persoon recognized and cited observations by Curtis as his primary source of the descriptions of two forms of flower (Persoon, 1794), he did not use the terms pin and thrum, even though these were already in use by 1791 (Curtis, 1777–1798a). Had Darwin pursued von Mohl's reference (von Mohl, 1863) back to Persoon's original article (Persoon, 1794), he would have made the link to Curtis’ earlier observations, and been able to provide the reference that is conspicuously absent in Different Forms of Flowers (Darwin, 1877). With the discovery of Curtis's Flora Londinensis as the source of Persoon's descriptions, and Schrank's review which may have alerted Persoon to Flora Londinensis, it might seem that this closes the chapter on the origins of the earliest descriptions of two forms of flower in Primula, but this is not the case; the history of floral heteromorphy goes back even further.

VII. The insights of Clusius and de Reneaulme and the influence of Linnaean thinking In 1943, van Dijk (1943) published a remarkable analysis of the Latin texts of Carolus Clusius (Clusius, 1583, 1601) and Latin and Greek descriptions by Pauli de Reneaulme (1611) to provide a historical perspective on the origins of observations on floral heteromorphy, details of which have been overlooked by citation of van Dijk without retrospective analysis or interpretation of the original texts that he uncovered. Perhaps this lack of analysis is a consequence of publication of van Dijk's monumental analysis in French, with no subsequent English version of this or the 16th Century texts that he cites. However, translation of van Dijk's original work (van Dijk, 1943) reveals his outstanding depth of perception into the 16th Century botanical world. Van Dijk's narrative states that he was ‘by chance’ reading Rariorum Plantarum Historia (Clusius, 1601) when the descriptions of differences between long and short style forms of Primula caught his attention (van Dijk, 1943). He pursued his research back to 1583 and an earlier publication, Rariorum Aliquot Stirpum, per Pannoniam, Austriam, & vicinas quasdam provincias observatarum Historia (Clusius, 1583). Within these two volumes, similar Latin text describes the different forms of Primula flower (Clusius, 1583, 1601). Although Clusius divides his descriptions into Primula and Auricula, he recognizes their relatedness. Linnaeus would similarly classify these plants 170 yr later, and link them with Hottonia palustris in his Systema Naturae (Linnaeus, 1735). Molecular studies have since confirmed and advanced the validity of this classification and evolutionary relationships between species (Conti et al., 2000; Mast et al., 2001). Clusius grouped P. veris with P. farinosa but differentiated these from Auricula Ursi (Bear's Ears), which were popular with horticulturalists, then as now. In describing Auricula Ursi, he describes one variety as ‘stilo inter stamina nonnumquam prominente, interdum autem nullo’ – ‘sometimes a prominent style between the stamens, sometimes not’ – and refers to a prominent style in another recognized variety, Auricula Ursi IIII carnie colors flora (Clusius, 1583). Clusius also recognized another variety, Auricula Ursi minima V, with ‘nullo prominente stilo’ – ‘without a prominent style’ (Clusius, 1583). What Clusius was describing in 1583 were different floral morphs, not distinct varieties. It would therefore seem that he noted the different forms of flower in different varieties and species, but did not make the connection between the two forms within one variety or species (van Dijk, 1943). Van Dijk takes this interpretation a stage further and expresses his surprise that Clusius, who was clearly focused on detailed observation, did not describe the short style within the corolla tube of thrum flowers and concludes that Clusius did not dissect the flowers (van Dijk, 1943). Had he done so, he could have provided us with the earliest description of the two forms of flower 279 yr before Darwin's paper (Darwin, 1862). In Historia Plantarum Rariorum, Clusius also noted an association between flower colour and floral morph. ‘Illud autem hoc flora in obervavi, ut intensius rubeat, pistillum sive Stilum prominentem Habeat, qumedmodum nonnullarum Primularum flores: at dilutior minimal’ (Clusius, 1601); which, translated from van Dijk's French (van Dijk, 1943), reads ‘The darker red flowers possess a prominent pistil, as found in flowers of different Primulas, but the paler flowers do not’. Van Dijk was not aware of colour associated with floral form in contemporary literature (van Dijk, 1943). However, genetic studies in the early 1900s on P. sinensis had provided one of the first examples of linkage in plants, namely, linkage between the S locus and the B locus, which controls flower colour (Gregory, 1911; Bridges, 1914; Altenburg, 1916; Gregory et al., 1923; De Winton & Haldane, 1933, 1935). Kurian and Richards subsequently identified two flower pigment loci in P. vulgaris that co‐segregate with the S locus (Richards, 1997). Although Clusius did not recognize the significance of the two forms of flowers, he did recognize them in different species, and observed linkage of flower colour to floral morph (Clusius, 1601). Perhaps the first recognition of the relevance of different flower forms comes from Pauli de Reneaulme's Specimen Historiae Plantarum (de Reneaulme, 1611). Like Clusius (1583), de Reneaulme attempted to classify plants by morphology, in contrast to Besler (1613) and van de Passe (1614) who focused their groupings on seasons and flowering time; before Linnaeus (1735), classification was complex. As noted by van Dijk (1943), de Reneaulme focused on single Greek or Latin words to describe plants, rather than long descriptive names favoured by Besler (1613), Clusius (1601) and van de Passe (1614). De Reneaulme defined Primula as Phlosmikos (ΦΛΟΜΙΣΚΟΣ) and described two forms: a, Makrostylos (ΜΑΚΡΟΣΤΨΛΟΣ) (long style) and b, Anostemon (ΑΝΩΣΤΗΜΩΝ) (short style). He described the short stamens and long style in the Makrostylos form a, and the long style and lower stamens of the Anostemon form b (de Reneaulme, 1611); Persoon subsequently used similar terms, variety α and variety β, to distinguish the two forms (Persoon, 1794). Perhaps Persoon was not only familiar with Curtis’ work but also recognized de Reneaulme's classification system. Another revelation comes from de Reneaulme's description of the two forms of Primula; he states ‘Minutas istas persequi differentias non est vanum. Deus siquidem & natura non frustra distinxere’ – ‘These minute details are not in vain for God and Nature distinguish these for a reason’. Sprengel voiced a similar view in relation to Hottonia (Sprengel, 1793), as did Darwin when he provided an explanation and rationale for the two forms of flower (Darwin, 1877), but in Darwin's case with a focus on the role played by natural selection. Van Dijk takes his analysis further and contrasts de Reneaulme's conviction that these differences are not without reason with Linnaeus’ view that ‘Varietates levissimas non curat botanicus’ – ‘the botanist is not concerned with slight variations’ (Linnaeus, 1792). Intriguingly, Linnaeus uses Primula as an example where, in paraphrased translation, ‘flower enthusiasts focus on small floral details that no sane Botanist would consider important’ (Linnaeus, 1792). Van Dijk speculates that it is this difference in attention to detail and the dominance of Linnaean thinking during the 18th and 19th Centuries that led to the failure to recognize the significance of such important floral morphologies. It seems remarkable that a classification system based on the Systema Sexualis (Linnaeus, 1735) could obscure the relevance of floral heteromorphy by using the number, rather than the defining differences between reproductive structures, as the basis for classification. The recognition of two forms of flower by Clusius, de Reneaulme, Curtis, Sprengel, Persoon, Henslow and then Darwin demonstrates not only that careful observation can reveal the varietates levissimas or small differences that Linnaeus rejected, but highlights the importance of seeking to understand the reasons for these differences. This is what Charles Darwin did where others had only observed and not sought to explain. In his autobiography Darwin wrote: ‘no little discovery of mine ever gave me so much pleasure as making out the meaning of heterostyled flowers’ (Darwin, 1887). Although Darwin was not able to test his hypothesis on the meaning of heterostyly, subsequent ecological studies have, and the current availability of molecular genetic tools and resources now provides the opportunity to identify the genes underpinning the ‘balancement of long and short pistils and stamens’ that Darwin first noted in Primula in the spring of 1860.

Acknowledgements I thank the following for images and copyright permissions. Fig. 1(a), The Asa Gray Library, Harvard University; Fig. 1(c), Christie's, London and The Bridgeman Art Library; Fig. 4, Cambridge University Botanical Garden; Figs 8 and S7, Cambridge University Library; Fig. S5, the John Innes Foundation, Norwich. All other images were photographed from the author's personal collection. Access to historical texts and Darwin‐related correspondence was possible thanks to The Darwin Correspondence Project (http://www.darwinproject.ac.uk/), the Biodiversity Heritage Library (http://www.biodiversitylibrary.org/), the Internet Archive (http://archive.org/), Botanicus at the Missouri Botanical Garden Library (http://www.botanicus.org/), and the Digital Library Royal Botanic Garden, Spain (http://bibdigital.rjb.csic.es/ing/index.php). I acknowledge online resources provided by GoogleTranslate (https://translate.google.com/) and BabelFish (http://www.babelfish.com/). I am grateful to Luke Pitcher, Oxford University for advice on translation of a particularly troublesome passage from Linnaeus. I would also like to thank the anonymous reviewers for their helpful and constructive suggestions.

Supporting Information Please note: Wiley Blackwell are not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing material) should be directed to the New Phytologist Central Office. Filename Description nph13558-sup-0001-FigS1-S7.pdfapplication/PDF, 451.5 KB Fig. S1 Anne Pratt's posy of Primula species (1860). Fig. S2 Elizabeth Blackwell's cowslip (P. veris) (1757). Fig. S3 Johannes Zorn's cowslip (P. veris) (1780). Fig. S4 Primula vulgaris from Flora Danica (1761–1883). Fig. S5 Crispin van de Passe's Auricula (1615). Fig. S6 Crispin van de Passe's primrose (c. 1605). Fig. S7 Pauli de Reneaulme's cowslip (1611). Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.