Charles Darwin and J.S. Henslow

From Oliver Sack’s book The River of Consciousness:

About Darwin: “He grew up in a botanical family—his grandfather Erasmus Darwin had written a long, two-volume poem called The Botanic Garden, and Charles himself grew up in a house whose extensive gardens were filled not only with flowers but with a variety of apple trees crossbred for increased vigor. As a university student at Cambridge, the only lectures Darwin consistently attended were those of the botanist J.S. Henslow, and it was Henlslow, recognizing the extraordinary qualities of his student, who recommended him for a position on the Beagle.

“It was to Henslow that Darwin wrote very detailed letters full of observations about the fauna and flora and geology of the places he visited. (These letters, when printed and circulated, were to make Darwin famous in scientific circles even before the Beagle returned to England.) And it was for Henslow that Darwin, in the Galapagos, made a careful collection of all the plants in flower and noted how different islands in the archipelago could have different species of the same genus.

Darwin's Mentor: John Stevens Henslow 1796–1861’ by S.M. Walters & E.A. Stow

Darwin and Henslow. The growth of an idea. London: Bentham-Moxon Trust: Nora Barlow, ed.

From Barlow’s book: Darwin's theory of the origin of species by natural selection was profoundly influenced by his observations made during the voyage of H.M.S. Beagle from 1831 to 1836. That he joined this surveying voyage at all was due to his close friendship with John Stevens Henslow, Professor of Botany at Cambridge, who proposed his name for the post to the Admiralty. During the Beagle's slow and intricate course, due to the needs of the survey, Darwin corresponded frequently with Henslow about the collections, geological, zoological and botanical, which he was sending back to Cambridge. The letters show vividly the impact of these first travels upon his mind and the growth of a dedicated sense of direction. From extreme diffidence, under Henslow's guidance, he soon gained a reliance in his own powers of deductive thought. Later, on his return to England, his ideas outdistanced Henslow's, but their friendship never ceased. The later letters show how  much he was limiting his discussion to avoid offence to his old friend. Nevertheless, this collection will establish beyond doubt the importance of Henslow's influence and support in the early years: it was a vital friendship and it laid the foundation upon which all Darwin's subsequent work was to rest.

I fully believe a better man never walked this earth.

—Charles Darwin to J.D. Hooker
18th May, 1861

Darwin described meeting Henslow as the one circumstance "which influenced my career more than any other."