The Royal Society of Edinburgh

Lecture

Henslow’s Legacy, Darwin’s Inheritance

Professor John Parker

23 November 2009

Report by Steve Farrar

Before he was a guru, was a disciple. John Parker revealed the scientific legacy of John Stevens Henslow that gave his favoured student the intellectual context and tools necessary to rewrite the rules of life.

The assumption that underlies most of the musings on Charles Darwin in this, the 200th year since his birth, is that the author of stepped aboard HMS Beagle in ignorance, only to stride back ashore almost six years later fully equipped to deduce the theory of evolution by natural selection. John Parker’s research into the life and work of the Cambridge Professor of John Henslow suggested the truth was somewhat different. Great teachers often feature in the development of great people. For Darwin, that teacher was Henslow, a man whose name has been barely mentioned amid all the bicentennial debate, yet without whom the word Darwinism would not have made it into the dictionary.

John Stevens Henslow, 1796–1861, was born in Rochester, Kent. Educated in Camberwell, he was admitted to St John’s College, Cambridge, to study mathematics, although his interests and pursuits were much broader. Henslow excelled at everything and his rise was meteoric. In 1822, a geological field map of Anglesey made Henslow’s name in scientific circles and the following year, just five years after graduating in mathematics, he was elected Professor of . Two years later, he was also elected Professor of Botany.

Henslow’s early research interests sound diverse, Professor Parker admitted. He explored the of complex regions and produced the first geological field map of the . He probed the mathematical underpinning of the new science of crystal structures. He carried out the first dissection of a mollusc in the British Isles and considered its whole life cycle, taking what might be described as an ecological approach. He contributed marine biology specimens to the Natural History Museum.

But it was his passion for plants that commanded the most energy. The earliest Henslow botanic specimen that Professor Parker has been able to confirm came from his undergraduate years. It was woad, gathered and pressed in 1816. Even then, Henslow showed a consistent and systematic approach to collecting and recording. That approach let him create an unprecedented botanic database when, in 1820, he decided he would collect the entire flora of the British Isles.

Henslow developed a remarkable network of 110 naturalists and enthusiasts from Shetland to the Channel Islands. Even his maiden aunts joined the project. Ultimately, he collected about 7,000 specimens himself, while his collaborators added a further 8,000, ensuring he was able to cover the 1,200 species of what Henslow considered native species.

But this was no taxonomic exercise. Henslow was driven by a desire to understand how plants developed, grew and behaved. Professor Parker explained that he was fixated with variation. The specimens in his herbarium, still kept in Cambridge, make this obsession clear. When, for example, he collected a tuft of moss from the Gog Magog hills outside Cambridge in 1821, he dissected and presented a full range of size variants. Henslow carried out the world’s first population study on a mathematical basis to define living species by the patterns of variation that they showed in nature. He looked at the natural world like a landscape of mountains. Each peak represented a species and was defined through patterns of variation.

But this landscape was not unspoiled. Henslow realised that these patterns were disturbed by what he called monstrosity – isolated variants that produced unusual numbers of leaves, grew to strange proportions or exhibited some other abnormality. For Henslow, these monsters were the key to understanding plant development and they feature appropriately in his herbarium. Sadly, the tools used to explore ‘evo-devo’ today were not available in the 1820s and so he had little prospect of developing such work further.

He did, however, have a way to explore the limits of a species – hybridisation. If he could hybridise two plants and produce fertile offspring, he knew he was dealing with different varieties not species. He used this experimental criterion to test his ideas – in this sense Henslow was an experimental population biologist. He also believed that this approach gave him a way, if he could accumulate sufficient data, to establish the laws of heredity. Professor Parker noted that Henslow was wrong to make this assertion and, indeed, that mistake would manifest itself as a major weakness in Darwin’s laws.

But it is the scope of Henslow’s project that is most significant. His 14-year botanical research programme tried to understand the nature of species, all built on a bedrock of systematically collected specimens.

Among those specimens in Henslow’s herbarium are some he collected with some of his students on a fieldtrip to Gamlingay in May 1830. And with him that day was a young man who would become Henslow’s most devoted student, Charles Darwin. Darwin had arrived at Cambridge in 1828 after dropping out of studying medicine at Edinburgh. His despairing family had sent him to get an ordinary degree in theology to prepare him for a life as an Anglican minister. The young man, however, attended only one series of lectures – those given by Henslow.

By all accounts, Henslow was an inspirational lecturer, pioneering the use of illustrations, field trips and regular Saturday nature rambles. Darwin became hooked. He attended Henslow’s lecture series not once but for three years, never missed a nature ramble, enthusiastically participated in field trips and joining his teacher’s social evenings with some of the greatest names in science. So devoted was this student that to other dons he became known simply as “the man who walks with Henslow”. It seems, though, that the admiration was mutual. Henslow saw an innate brilliance in the failed medic.

In 1831, Henslow was asked to recommend someone to go on a geological surveying expedition around the world. His response was remarkable – he put forward an unknown, someone who was, in his opinion, “the best person I know for that position”. So Darwin joined the crew of the Beagle. Before he went, the teacher sent his protégé on a preparatory geological field trip to Snowdonia with his colleague, the geologist . He also asked Darwin to bring him back some specimens of night-scented stock for the herbarium. This is the oldest known Darwin specimen, arranged to show patterns of variation as Henslow demanded.

Professor Parker noted that Darwin set off not only with the skills his teacher had taught him but also his ideas about how to understand variation. He promptly set about collecting population samples and started to see all sorts of patterns within them. He also collected monstrous forms that Henslow believed to be a key feature in natural selection. And he sent his specimens back to Cambridge, where most still reside.

On 6 November, 1835, while Darwin was somewhere between the Galapagos Islands and Tahiti, Henslow read extracts of his correspondence to a suitably impressed audience at the Cambridge Philosophical Society. He had them printed and published. Darwin had left Cambridge as a ‘nobody’ with an ordinary degree in theology. But thanks to Henslow’s efforts, he returned as a respected scientist.

The teacher had helped shape the scientific context of his student and he was now able to help him gain the necessary level of recognition to pursue his own research. Henslow’s contribution to Darwin’s work was far greater than a letter of recommendation to sail on the Beagle. Professor Parker confidently described Henslow as “Darwin’s mentor and creator”.

QUESTIONS

A retired chemical engineer asked how Henslow knew that a particular specimen’s size was due to variation rather than just age. Professor Parker explained that when the plant flowered it was considered mature.

A retired molecular biologist asked how Henslow responded to Darwin’s On the Origin of Species. Professor Parker said Henslow remained a firm and vocal supporter of Darwin despite his own creationist beliefs. He gave his last lecture at Cambridge the year after the book’s publication and died shortly after. Since 1839, Henslow had lived in Suffolk, where he was minister to the parish of Hitcham. Characteristically, he did many remarkable things in the village, including founding its school, providing allotments for landless labourers and helping start the agrichemical industry by pioneering the use of as a fertiliser. Henslow retained his chair at the university, though was far less involved in botanical research. But in the last letter Henslow wrote, in 1860, he commented on how much he enjoyed Origin and that he was sure Darwin was correct. Nevertheless, the church minister could not help concluding that he wondered whether that was all there was to life.

A former zoologist asked why Henslow did not correct Darwin’s mathematics in relation to his work on speciation. Professor Parker said Darwin knew he was a poor mathematician but Henslow was in poor health for the last 10 years of his life and could not help his former student.

A vote of thanks was proposed by Professor David Ingram, Honorary Professor, Universities of Edinburgh and Glasgow, who said: “Through your eloquence you have brought to life a great man on whose shoulders Charles Darwin was lifted to become the iconic figure that he is today.”

Pictured: One of Henslow’s specimens, showing a full spectrum of variety for a single species, Moenchia erecta. The plants were collected at Gamlingay in May, 1830, with the assistance of a young student, Charles Darwin.

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