John Stevens Henslow (6 February 1796 – 16 May 1861) was an English clergyman, botanist and geologist.
Henslow first came to Cambridge to study mathematics but it was his geological work in field studies of the Isle of Man and Anglesey that brought him fame. By the age of 27 he was elected Professor of Mineralogy and two years later became Professor of Botany at the University of Cambridge, a post he was to hold until his death in 1861. Henslow’s research focused on the nature of species and between 1821 and 1835 he compiled a Herbarium of 3,654 sheets containing specimens from over 10,000 plants. He was renowned for his new teaching techniques. He used his own illustrations in lectures, introduced practical science classes, and led walks around the Cambridge district as he taught natural history. He fostered independent discovery and utilized unusual field trips for his students.
In order to persuade farmers to apply scientific methods to their operations, Henslow gave public lectures on the fermentation of manure and wrote newsletters for publication in local newspapers. During the potato famine (1845-46) in Ireland, he showed stricken farmers how to extract starch from rotten potatoes. In 1837, Henslow accepted the rectorship of the neglected parish of Hitcham in Suffolk, where we was to remain for the rest of his life.
And he mentored Charles Darwin and recommended him to Captain FitzRoy as aspiring scientist on the HMS Beagle voyage.
Henslow received numerous letters and botanical specimens from Darwin and published part of Darwin’s scientific findings. Henslow thus helped establish Darwin’s reputation as a botanist, geologist and fossil collector.
Their friendship was lifelong. In 1859, Charles Darwin published his Origin of Species and send a copy to Henslow, his “dear old master in natural history” acknowledging that “ I fear, however, that you will not approve of your pupil in this case.” – And he was right: Henslow did not share Darwin’s view on the evolutionary history of living beings via natural selection, as he said in a letter to Joseph Dalton Hooker in May 1860:
“I do not disguise my own opinion that Darwin has pressed his hypothesis too far—but at the same time I assert my belief that his Book is […] the ‘Book of the Day’”
Henslow did defend, though, his former pupil:
“I stuck up for Darwin as well as I could, refusing to allow that he was guided by any but truthful motives, and declaring that he himself believed he was exalting & not debasing our views of a Creator, in attributing to him a power of imposing laws on the Organic World by which to do his work, as effectually as his laws imposed upon the inorganic had done it in the Mineral Kingdom”
Henslow died on 16 May 1861, just two years after the Origin of Species had been published. In a contribution to a biography of Henslow, Charles Darwin describes his friendship with him in the following words:
“I have not as yet mentioned a circumstance which influenced my whole career more than any other. This was my friendship with Professor Henslow. Before coming up to Cambridge, I had heard of him from my brother as a man who knew every branch of science, and I was accordingly prepared to reverence him. He kept open house once every week when all undergraduates, and some older members of the University, who were attached to science, used to meet in the evening. I soon got, through Fox, an invitation, and went there regularly. Before long I became well acquainted with Henslow, and during the latter half of my time at Cambridge took long walks with him on most days; so that I was called by some of the dons “the man who walks with Henslow;” and in the evening I was very often asked to join his family dinner. His knowledge was great in botany, entomology, chemistry, mineralogy, and geology. His strongest taste was to draw conclusions from long-continued minute observations. His judgment was excellent, and his whole mind well balanced; but I do not suppose that any one would say that he possessed much original genius. He was deeply religious, and so orthodox that he told me one day he should be grieved if a single word of the Thirty-nine Articles were altered. His moral qualities were in every way admirable. He was free from every tinge of vanity or other petty feeling; and I never saw a man who thought so little about himself or his own concerns. His temper was imperturbably good, with the most winning and courteous manners; yet, as I have seen, he could be roused by any bad action to the warmest indignation and prompt action.”
- John Stevens Henslow, in the public domain
- Photo taken in September 2015, at the Adam Sedgwick Museum in Cambridge
- Commemorative plaque at the entrance to the Botanic Garden in Cambridge