Professor John Stevens Henslow, 1796-1861
John Henslow was the Professor of Botany at Cambridge University during Darwin’s undergraduate years from 1828 to 1831. He was a highly influential and respected scientist as well as a great teacher. Henslow had a remarkable breadth of knowledge and experience and was called “the man who knows everything” by one of Darwin’s fellow-students. He was a dedicated naturalist from a child and entered Cambridge University to study Mathematics. He spent much of his time, however, on zoological investigations, particularly of snails, and developed interests in mineralogy and geology.
After graduation, to earn a living, Henslow tutored students in maths and worked as practical class assistant to the Professor of Chemistry. During this time he also carried out geological field studies of the Isle of Man and Anglesey, which were subsequently published and led to his recognition as an able scientist.
Henslow was appointed at the age of 26 to the Chair of Mineralogy at Cambridge and two years later was awarded the Chair of Botany as well. Although he gave an annual lecture course in mineralogy for four years, his main research from 1821 onwards was botanical. He resigned as Professor of Mineralogy in 1827 and devoted himself solely to botany from then on.
In his research, Henslow actively set out to define the nature of species through observations and experiments on plants. He approached this through a detailed analysis of variation in natural populations, and showed how hybridisation could be used to test whether plants belonged to the same species or to different species.
As well as carrying out high-quality research, Henslow was also a great innovator in undergraduate teaching. He introduced beautiful illustrations into his lecture course and, for the first time at Cambridge, gave students the opportunity to make observations and carry out experiments themselves, as well as taking them out on “herborizing excursions”. Charles Darwin attended these exciting courses on botany in 1829, 1830 and again in 1831. Henslow also led informal walks around Cambridge for anyone who cared to join him. Darwin never missed one of these, and became known in the University as “the man who walks with Henslow”.
Henslow and Darwin became close friends, despite their difference in age and position. When the opportunity arose, in 1831, for a position on the world voyage on HMS Beagle, Henslow put forward Darwin’s name to Captain Fitzroy as the “best person I know worthy of observing, recording and collecting everything…in natural history”.
Professor John Henslow’s Herbarium
John Henslow started his herbarium in 1821, when he began to collect samples of all the plants of Britain. By the time Darwin joined Henslow’s botanical class in 1829, this herbarium consisted of over 10,000 British plants, with some European and South American plants too. The plant collection continued to grow rapidly during Darwin’s student days at Cambridge and Charles himself contributed to it, along with over 100 other collaborators scattered across Britain.
Uniquely, Henslow’s herbarium was developed for his research programme. He focussed on the variation found in populations of plants in nature as a way of defining species. Henslow organised his samples by a process he called “collation”. A “collated” herbarium sheet contains two or more plants arranged carefully on the sheet, each meticulously labelled with the species name, the date and place of collection, and the name of the collector.
If we look at variation in overall size, it is particularly easy to see how Henslow arranged his plant specimens. Frequently, he placed his plants with the largest in the centre surrounded on each side by smaller ones, giving a bell-shaped pattern on the sheet. Sometimes, however, he placed the largest at one edge, with the rest diminishing regularly in size across the sheet. Using the patterns of variation he produced, Henslow then identified the species of the British flora for himself. He published his conclusions for the benefit of his Cambridge students. Darwin used this book each year in the undergraduate classes on botany given by Henslow.