Darwin and Hooker: Azaleas and Rhododendrons, Part I
Duncan M. Porter
Department of Biology
Virginia Polytechnic Institute & State University
Darwin Correspondence Project
University Library, Cambridge, England
Joseph Dalton Hooker, naval assistant surgeon, botanist to the Geological Survey, Assistant Director and Director of the Royal Botanic Gardens, Kew, and friend of Darwin, is well known for his studies on Rhododendron. Charles Darwin, medical and theology student, naturalist, evolutionist, and country squire, on the other hand, is not. However, a reading of Darwin's correspondence, notes, notebooks, and publications reveals his interests in the genus and the aid that Hooker gave Darwin in his pursuit of these interests.
The two met in 1839 in London, but they only began to correspond in November 1843. They soon became fast friends, so much so that Hooker was the first to know of Darwin's belief in the transmutation of species, revealed to him in a letter of Jan. 11, 1843 (32).1 Throughout their long friendship, Hooker acted as a sounding board for Darwin's ideas and speculations. Thus, it is not surprising that about 10 percent of the 15,000 letters of the Darwin correspondence known to be extant are between these two eminent scientists.
Charles Darwin is famous for writing down his every thought, a habit with both assets and liabilities. On the one hand, not every reference is consequential or even necessarily correct; some of the notes are ephemera as opposed to matured thoughts. On the other hand, this very habit, as exemplified in the letters, gives us a view of the scientist in action.
Rhododendrons were grown at the Darwin family home, the Mount, in Shrewsbury, England. Charles wrote to his sister Caroline from London in May or June 1837 that "Catherine [another sister] tells me to say that the Rhododendrons went by Jelby's Van for Shrewsbury" (3). Robert Waring Darwin, their father, had both a garden and a greenhouse at the Mount.
On Oct. 31,1847, Darwin wrote to his wife Emma from his father's home in Shrewsbury: "we all here perfectly understand why so many laurels must be dug up, perhaps you would like the Azalia & one of the Deodars for Elizabeth" (5). Sarah Elizabeth Wedgwood was Emma's sister and lived in Hartfield, Sussex. Further evidence that azaleas were grown at Down House, the Darwins' home at Downe, Kent, comes from a May 1848 letter that Charles wrote to Emma from Shrewsbury: "What a very good girl you are to write me such very nice letters, telling me all I like to hear; though you have not mentioned the 2 new azaleas" (5).
Later, Hooker sent rhododendrons to Darwin from Kew. On April 7,1855, Darwin advised Hooker: "I wrote this morning to thank for the Rhododendrums" (6). In the postscript to his letter of June 10, 1855, Darwin wrote to Hooker that "your Rhododendrums are shooting nicely" (6).
These azalea and rhododendron specimens surely are amongst those that Darwin utilized for the observations and experiments discussed in the rest of this paper. These observations and experiments are approached chronologically by theme (ecology, crossing, flowering, etc.) below. The most interesting of Darwin's remarks on Rhododendron that I have found are included.
Darwin's comments on Rhododendron begin with an entry in one of his Transmutation Notebooks (2), a series of notebooks begun in 1837 to record his notes on evidence for evolution. These notes resulted both from his voluminous reading and from personal observation.
March 12th  - It is difficult to believe in the dreadful but quiet war of organic beings going on in the peaceful woods & smiling fields - we must recollect the multitudes of plants introduced into our gardens (opportunities of escape for foreign birds & insects) which are propagated with very little care -& which might spread themselves, as well as our wild plants, we see how full nature, how firmly each holds it place - When we hear from authors that in the Pyrenees, that the Rhododendron ferrugineum begins at 1600 metres precisely & stops at 2600 & yet know that plant can be cultivated with ease near London - What makes the line, as trees in Beagle channel - it is not elements - we cannot believe in such a line - it is other plants - a broad border of killed trees would form fringe - but there is a contest & a grain of sand turns the balance.
Darwin returned to discussing competition in plants in On the Origin of Species. His reference to the Beagle Channel refers to an observation made in Tierra del Fuego in 1833 while he was on the voyage of the Beagle (31):
At the height of above 1400 feet I found dwarf Beech trees (about a foot high), in sheltered corners the main line of separation between the trees and grass is perhaps 2 or 300 feet lower. Within the Beagle channel this line was so horizontal and wound round in the values in so straight a direction as to resemble the high water mark on a beach.
Over the years, J.D. Hooker supplied Darwin with much information on the ecology and distribution of Rhododendron. In "Darwin's Notes Arising from Conversations with Joseph Dalton Hooker," written in December 1844, Darwin noted "a Rhododendron on summit of Ceylon & Java - " (4). In a letter to Darwin of February 1845, Hooker mentions the genus when discussing the distribution of the flora along the Ob (Obi) river in western Siberia (4).
Hooker spent 1848-51 on a plant collecting trip to the Himalayas. The published journal of his trip was dedicated to Darwin. In one of his long letters to Darwin from India, Hooker wrote on Feb. 3, 1849: "We floundered along through Snow & dwarf Rhododendrons to a spot at 13000 feet" (5). From Sikkim, Hooker wrote to Darwin on June 24, 1849, when camped at 12,000 feet: "vegetation all a scrub of rhodods." He compares it to what he saw when he was in Tierra del Fuego on the voyage of Erebus and Terror in 1842: "here however the blaze of Rhod flowers & various colored jungle proclaims a differently constituted region in a naturalists eye & twenty species here, to one there, always are asking me the vexed question, where do we come from?" (5).
Darwin answered on Oct. 12,1849: "Two days after receiving your letter, there was a short leading notice about you, in Gardener's Chronicle, in which it said that you have discovered a noble crimson rose & 30 Rhododendrons: I most heartily congratulate you on these discoveries..." (5).
On Nov. 26, 1850, Hooker wrote to Darwin from East Bengal of Rhododendron occurring at low elevations in the area (5).
In a letter to G.H.K. Thwaites, superintendent of the Peradeniya botanic garden in Ceylon, Darwin wrote on March 8,1856: "I should be particularly glad to see in print or M.S. some particulars in regard to the species from different elevations, which show different degrees of capacity for cultivation at a new level: Hooker has published a similar case in regard to the Himmalaya Rhododendrums" (7).
From mid-1856 until mid-1858, when he began writing On the Origin of Species, Darwin was working on a long manuscript he called his "big book" and which he titled Natural Selection. Darwin continued to add to this manuscript during the 1860s, but I will treat it in this paper as though quoted parts were written during 1856-58. The first entry to mention the ecology of Rhododendron was in a footnote to: "In plants several cases are on record of the same individual or all its seedlings changing in a few generations, without the aid of selection, the tint of its flowers when brought from its native home into our gardens." The footnote reads: "Dr. Hooker on the Climate & Vegetation of the Sikkim Himalaya p. 49 in regard to a Rhododendron..." (34).
In a few more pages Darwin continues, "Dr. Hooker states that he has found a great difference in the hardiness of individuals of several Himalayan plants, depending upon the height at which the seeds were gathered...& so there is a great difference with the Rhododendron arboreum according to the height at which the seeds have been collected." Hooker's note was, "The common scarlet Rhododendron of Nepal and the Northwest Himalayas is tender, but seedlings of the same species from Sikkim, whose parents grew at a greater elevation, have proved perfectly hardy."
Hooker wrote to Darwin in early April 1859: "I think there are plenty of examples of best marked vars [varieties] on verges of range...In fact the phenomenon is so common I did not think it necessary to quote any example but Rhododendron & that only as conspicuous illustration" (8).
Some of this information that Darwin had been gathering on the ecology of Rhododendron was used in his On the Origin of Species (13):
We have reason to believe that species in a state of nature are limited in their ranges by the competition of other organic beings quite as much as, or more than, by adaptation to particular climates. But whether or not the adaptation be generally very close, we have evidence, in the case of some few plants, of their becoming, to a certain extent, naturally habituated to different temperatures, or becoming acclimatised: thus the pines and rhododendrons, raised from seed collected by Dr. Hooker from trees growing at different heights on the Himalaya, were found in this country to possess different constitutional powers of resisting cold
As we saw in the Introduction, Hooker had supplied Darwin with plants of some of his Himalayan rhododendrons, and Darwin planted them at Down House. To Hooker, Darwin wrote on May 24-25,1861: "Two of your Himalayan Rhododendrons have flowered; R. formosum (I think) splendid dark crimson & R. Thompsoni (?) more like an Azalea-orange shading into Reddish orange" (11). Darwin's second publication of information on Rhododendron ecology, in Variation Under Domestication also mentioned flower colour. In the chapter "Direct and Definite Action of the External Conditions of Life," Darwin wrote: "The Rhododendron ciliatum produced at Kew flowers so much larger and pale-coloured than those which it bears on its native Himalayan mountain, that Dr. Hooker would hardly have recognized the species by the flowers alone. Many similar facts with respect to the colour and size of flowers could be given.
1 References will be listed at the conclusion of the article.
Part II of "Darwin and Hooker: Azaleas and Rhododendrons" will be continued in the next issue of the Journal.