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Convened & chaired by Professor Ian Wallace
Professor John S Parker
University of Cambridge
22 February 2006
The history of science is punctuated by great figures, who have come to represent their disciplines as if they were the single originators. Darwin stands as the beacon of the modern theory of evolution, which was supposedly revealed to his untutored mind on his almost mystical voyage on board HMS Beagle. The reality is far different. Darwin embarked on his world trip equipped with a deep appreciation of the nature of species, of hybridity, of the search for laws governing nature, all given to him by his Cambridge professor, mentor and life-long friend, John Stevens Henslow. In this lecture we explore the science of Henslow in order to gain a clearer understanding of Darwin’s intellectual heritage and how the instruction he received influenced Darwin’s path to the Origin of Species.
John Henslow was born in Rochester, Kent in 1796. From his infancy he was drawn to natural history and during his school days he studied entomology and its illustration. On family holidays he collected marine animals for the British Museum, specialising in molluscs. Henslow came to St. John’s College, Cambridge in 1814 and graduated in mathematics. Here he developed an interest in mineralogy and began a lifetime friendship with geologist Adam Sedgewick. Henslow carried out several geological surveys. In 1822, a paper on the geology of Anglesey appeared in Volume 1 of the Transactions of the Cambridge Philosophical Society. After his graduation, Henslow supported himself by teaching mathematics and by practical demonstrating in Chemistry. In 1822, Henslow was appointed to the Chair of Mineralogy. In 1821, he began the serious study of botany with his young College friend Leonard Jenyns (whose sister Harriet he married in 1823). They resolved to start a dried plant collection of the whole British flora. Henslow’s intention, however, was very different from that of Jenyns. Jenyns was a committed and avid collector of everything in natural history but Henslow’s collecting was focussed on addressing what he considered the major question of biology – the nature of species. Henslow studied variation as the key to this problem. He arranged his herbarium sheets to display the variation present within and between populations; 3709 of these sheets in Cambridge have recently been analysed.
On the death of Professor Martyn in 1825 the Chair of Botany at Cambridge was conferred on Henslow. Aged 29, Henslow held two Chairs but after two years he relinquished Mineralogy and remained Professor of Botany until his death in 1861.
Henslow developed his herbarium by his own collecting in Cambridgeshire his family at home and on holiday, his friends such as Leonard Jenyns from Cambridge, and a network of collaborators across the country. The growth of Henslow’s herbarium can be established due to his meticulous annotation of sheets. Henslow recorded species, place of collection, date of collection and name of collector, in a set order. Where plants were obtained from different places, numbers were added to identify them. He combined disparate plant collections to define species, in a practice he referred to as ‘collation’. Collated sheets show variation in size, leaf shape and branching patterns. None of the leading botanists of his day - Balfour at Edinburgh University, Hooker at Glasgow University - carried out collation. The result was was ‘A Catalogue of British Plants, arranged according to the Natural System’ (1829). Darwin used this list in 1829, 1830, and 1831 each time he attended Henslow’s 5-week course in the summer term.
Henslow’s herbarium sheets mostly show variation in plant size. He was also fascinated by sudden changes of form termed ‘monstrosity’, now ascribed to major mutations or developmental abnormalities. Henslow published papers on monstrosities in Adoxa, Reseda and Acer and sought ‘the laws that govern nature’. Further insights came from the study of hybrids. Henslow analysed a sterile spontaneous hybrid Digitalis and its two parents, a paper considered a model study by Henslow’s contemporaries. Henslow proposed that studies of 100 different hybrids would reveal the laws of heredity.
Henslow used his artistic ability to great effect. When a new Professor he set about the preparation of 70 elephant-folio drawings to illustrate his lectures. Henslow’s lecture course in 1828 was greeted with enthusiasm due to the excellence of these illustrations. William Darwin Fox wrote to his cousin Charles Darwin describing them and advising him not to miss them. Charles took Henslow’s course three times – the only lectures he attended at Cambridge. Some of these illustrations were discovered a few years ago in Cambridge occupying forty-five artist’s folios.
The genus Primula was central to Henslow’s understanding of varieties and species. Around Cambridge three species are found – P. vulgaris (primrose), P. veris (cowslip), and P. elatior (oxlip) as well as hybrids known as false oxlips. Henslow considered these a single species with three varieties. Some plants were both primrose-like (single flowers on long stems) and oxlip/cowslip-like (with umbels of flowers) suggesting varieties of a single species, and noted changes from one to the other in his cultivation experiments. Primula studies occupied Henslow for years. Darwin repeated Henslow’s Primula experiments 20 years later to gain botanical evidence for species instability, rather than stability.
During 1826 Henslow observed and drew flower-forms in Primula now referred to as ‘pin’ and ‘thrum’. This phenomenon was ‘discovered’ by Charles Darwin in 1860, who then recalled having been shown it by Henslow during his student days.
Henslow was an experimental botanist and needed living material for students for research. Some could be obtained from nature but botanic gardens could provide much greater scope. The University Botanic Garden was founded on a small site near the centre of Cambridge in 1762 for the instruction of medical students. Henslow inherited this run-down Botanic Garden ‘unsuited for modern experimental botany’. He identified a 40-acre greenfield site for a new botanic garden south of Cambridge in 1830.
This new Botanic Garden was based around a tree collection. Sufficient trees remain to deduce the ideas behind the plantings, and remarkably the three herbarium themes herbarium – variation, monstrosity and hybridisation – are all represented. On the Main Walk extreme morphological variants of the Pinus nigra group are planted opposite each other. P. nigra ssp nigra is a densely short-needled form from central Europe while opposite is the much-branched long-needled P. nigra ssp salzmannii from Spain. A group of three Fagus sylvatica (beech) illustrate monstrosity – a standard tree, a weeping form, and a fern-leaved beech. Hybrids are shown by Platanus and Quercus groups.
Darwin came to Christ’s College, Cambridge in 1828 to read theology after a year reading medicine at Edinburgh University. He spent time hunting, eating and collecting beetles. Darwin enrolled for Henslow’s course in 1829, with repeats in 1830 and 1831. The course was supplemented by walks Henslow led around Cambridge. During these walks Henslow expounded on ‘everything worthy of note in natural history’. Darwin was a constant attender and was known to the dons as ‘the man who walks with Henslow’.
Henslow sent Darwin to gain experience of field geology to Snowdonia with Adam Sedgewick in 1831. Darwin returned home via Barmouth where he collected three specimens of Matthiola sinuata for Henslow who subsequently ‘collated’ them. At home, Darwin found a letter from Henslow informing him that he was to be offered a position as naturalist on board HMS Beagle. Henslow stated that he deemed Darwin to be the most suitable naturalist he knew to undertake such a trip, while warning him that he was not yet ‘a finished naturalist’. Darwin took with him on the voyage a complete theory of the nature of species given him by Henslow. This Henslovian view centred on the relationship between varieties and species, exemplified by Primula. During the voyage, Darwin began to move from species stability towards species instability: ‘varieties’ thus became incipient species.
This shift towards an evolutionary view was especially influenced by his visit to the Galapagos Islands. He collected plants (including ‘monstrosities’) for Henslow, who was particularly interested in the floras of islands and their endemic species. Darwin’s plant records are meticulous – name, date, location – just as Henslow had demonstrated. This contrasts with his bird specimens most of which were not localised. These plants were a present for his creationist mentor Henslow. For us, however, they represent evolutionary radiation of species on isolated oceanic islands.
Darwin’s intellectual development under Henslow gave him the mental framework for approaching the natural world, through observation and through experiment. The interaction between Henslow and Darwin continued until Henslow’s death in 1861, soon after the publication of the Origin of Species. The influences of our youth are profound and, as a historian of Cambridge Science remarked, in this case ‘had there been no Henslow at Cambridge, there might have been no Darwin’. Darwin himself recorded his admiration for Henslow’s character as well as his science in his memoir of 1862: ‘his moral attributes rise in pre-eminence over his science’. Henlsow’s son-in-law Joseph Hooker records that he ‘had more influence over my life and conduct than any other man, so good, so calm, so wise… in all situations in life’. Henslow was truly a great and influential man.