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Darwin Correspondence Project

From J. D. Hooker   21 February 1866


Feby 21/66.

Dear old Darwin

I have been atrociously busy & am so still—but have all along been itching to have a shot at you. I hear you are disgustingly well & entitled to no sort of consideration— How I should chuckle when well, if I were an habitual invalid, & feel I was cheating my life & old time out of what they would have witheld if they could. Your restored health has brought peace to me— long may it last. The Busks come here today for the night & the Lyells meet them at dinner— I hope Lady L. will take sufficiently to Mrs. Busk who is no end of times better than some of the Lyells friends.1 All the same I cannot add that she is a special favorite of mine.

As to this Agassian affair,2 I wish I could explain to you my crude notions as to glacial period & your position towards it & the Universe at large3

I suppose I hold this doctrine, that there was a glacial period, but that it was not one of universal cold; because I think that the existing distributer of glaciers is sufficiently demonstrative of the proposition, that by comparatively slight redispositions of sea & land & perhaps axis of globe, you may account for all the leading Palæontological phenomena upon which the glacial period is established, & more than all the purely geological & existing Botanical phenomena by which it is supported. Remember this, that the extent of the glacial action is deduced from 3 phenomena, 1. glacial blocks boulders &c. a purely geological argument, that does not carry the action very far south;—2 Palæontology—which carries the action further & wider:—3 existing distribution of existing plants & animals, which carries its action across every latitude & according to you simultaneously in every longitude. 4

Now the value of these phenomena differs greatly.— the Geological is irrefragable— the Palæontological is open to the objection, that the glacial organisms may have been suited to warmer distribution then than now—or that under a less struggle for existence they had a wide climatic distribution. The third phenomenon is unsupported by the 1st & 2d. & is open to the further objections that the tropical alpine dispersion of glacial plants may be accounted for by the action of birds winds &c, that the phenomena of tropical vegetation, whether as to its extent, variety, or distribution, is against it,5 & that the Physicists deny its possibility on grounds as good as those of existing plants.

You attach little importance to the Physicists objection;6—I attach great;—because in the main, physical phenomena regulate the production & distribution of organisms; & because the your theory in contrast to it is a crude one not in harmony with the fact, that the relations of Life to conditions are intricate, & the results can only be accounted for after exploring a Labyrinth of conflicting facts. This is why I call yours a sledge-hammer hypothesis. I know no phenomena so subtle or so difficult to attach their true value to, as this of the distribution of Arctic & temperate types over tropical mts.7 To account for it at all, requires a far greater amount of Geographical change than you are disposed to admit.—eg in the case of Panama, which must have been occupied by mountains at least 4 or 5000 feet higher than at present; whilst the dispersion of temperate forms from Japan to Tasmania & from Algeria to the Cape without intermediate mts requires what every botanist will consider an extinction of thousands of tropical genera & even orders. You are surely illogical when you found on the distribution of a very few temperate genera & species, a glacial extension that does violence to a host of tropical species genera & orders.8 You are illogical when you deny to the physicist the right to maintain as an argument in favor of this absence of a colder period, the phenomena of tropical distribution, & claim that of temperate in your own justification.—

The question resolves itself into this—putting aside the physicists has there been time enough since the glacial epoch, to have repeopled the tropics with its forms, & to have distributed them as they are now distributed.— if yes, then why has so little change been produced in the same time in temperate latitudes. But here you may ask, is the tropical differentiation greater than the temperate & is tropical distribution of types more general than temperate. I think it is, but to answer that one must see how many orders genera are common in each Zone to the several continents. & balance results.

Your argument drawn from the fact, that a geologist had a right to insist on elevation before the means of elevation were shown,9 is not applicable in this case. I do not deny your universal cold period because you cannot show how the cold was brought about, but because the Physicists profess to prove that the earth must have been hotter if anything at your Epoch—10 It is not as if it were an open question whether the earth were then hotter or colder or equal in temperature. if it were so I would go along with you a great way.

With regard to tropical orchids requiring cool treatment,11 that is no argument, they are cool climate species & were killed before by the heat— no tropical orchid is cultivable in a cool house even when it is never exposed to frost.

I want to get to Down soon but have little prospect, just now. except for a Sunday soon12

Ever Yr affec | J D Hooker

CD annotations

2.1 As to] opening square bracket ink
End of letter: ‘(Not very important general arguments)’ pencil


Hooker refers to George and Ellen Busk, and Charles and Mary Elizabeth Lyell. Both families lived in Harley Street (Post Office London directory), but Mary Lyell did not invite the Busks to social gatherings (see Correspondence vol. 13, letter from J. D. Hooker, [2 June 1865] and n. 16).
Hooker refers to reported claims that Louis Agassiz had found evidence of former glacial action in the Amazon basin in Brazil. See letter from C. J. F. Bunbury to Charles Lyell, 3 February 1866, and the letters to Charles Lyell, 7 February [1866] and 15 February [1866]. See also letter from C. J. F. Bunbury to Charles Lyell, 20 February 1866.
Hooker’s comments were apparently prompted by reading CD’s letter to Charles Lyell, 15 February [1866]; that letter was in turn prompted by CD’s reading a letter from Hooker to Lyell (see letter to Charles Lyell, 15 February [1866] and n. 2). For the controversies surrounding Agassiz’s work on global cooling, glaciation, and the implications of possible glaciation in the Amazon basin, see the letter from C. J. F. Bunbury to Charles Lyell, 3 February 1866 and n. 8.
Hooker drew a sketch map of Europe and America, showing his estimate of the southern limit of the glacial sea, in a memorandum enclosed with his letter to CD of 9 November 1856 (Correspondence vol. 6). For a modern view of the geographical extent of the last ice age, see Imbrie and Imbrie 1979, pp. 12–13. On estimating the extent of glaciation by geological and biological methods in the nineteenth century, see, for example, Rudwick 1969. For a contemporary theory of the influence of the relative positions of land and sea on terrestrial temperature, see C. Lyell 1853, 1: 105–10; and for the theories about the variation in the tilt of the earth’s axis, see Imbrie and Imbrie 1979, pp. 69–75. During his Brazilian expedition, Agassiz claimed to have gathered evidence of glaciation from erratic blocks and palaeontology (see, for example, Hartt 1870, pp. 28–31, 467–72). CD explained the present distribution of species in part by their migration north and south along broad longitudinal belts that had formerly been colder from pole to pole (see the letter to Charles Lyell, 15 February [1866], n. 9).
In an earlier paper on the temperate flora of Cameroon and Nigeria, Hooker had juxtaposed CD’s theory of temperate species moving overland to the tropics at the onset of the glacial period (and their colonisation of northern latitudes or high tropical altitudes on the return of warm conditions) with the explanation of transport of seeds by birds and winds (J. D. Hooker 1863, pp. 180–1). CD’s lightly annotated copy of J. D. Hooker 1863 is among the unbound journals in the Darwin Library–CUL. On the existence of temperate plants on tropical mountains, see also n. 7, below. On the survival of tropical vegetation around the formerly cold equator, see n. 8, below.
CD’s view of the distribution of plants (see n. 4, above), which relied on a cold period, was not compatible with the opinion of contemporary physicists that the temperature had then been warmer than CD had believed (see letter to Charles Lyell, 15 February [1866] and n. 4). For a perspective on contemporary understanding of the physics of climate change, see Fleming 1998.
Hooker had earlier expressed concern about the difficulty of explaining the presence of European genera among the plants on tropical mountains (see Correspondence vol. 11, letters from J. D. Hooker, [7 May 1863] and n. 11, and [24 May 1863] and n. 8).
For earlier discussions of the problem of the survival of tropical plants during a mundane glacial period, see Correspondence vol. 11, letter to J. D. Hooker, 17 March [1863] and nn. 15–18, and letter from J. D. Hooker, [24 March 1863]. See also the section on ‘Dispersal during the glacial period’ in Origin, pp. 365–82.
The prevailing theories of climate change, based on contemporary physics, are reviewed in Croll 1864; they were not compatible with CD’s views of geographical distribution (see also n. 6, above). More recently, James Croll had proposed a succession of ice ages affecting each hemisphere alternately (see, for example, Croll 1865a, pp. 270–1); in the hemisphere that was not glaciated, Croll argued that winter temperatures, in particular, would have been higher than at the present day (Croll 1865a, pp. 271, 436).
Hooker’s next recorded visit to Down was from Saturday 24 March to Monday 26 March 1866 (Emma Darwin’s diary (DAR 242)).


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 29 vols to date. Cambridge: Cambridge University Press. 1985–.

Croll, James. 1864. On the physical cause of the change of climate during geological epochs. Philosophical Magazine 4th ser. 28: 121–37.

Fleming, James Rodger. 1998. Charles Lyell and climatic change: speculation and certainty. In Lyell: the past is the key to the present, edited by Derek J. Blundell and Andrew C. Scott. London: Geological Society.

Hartt, Charles Frederick. 1870. Thayer Expedition: scientific results of a journey in Brazil by Louis Agassiz and his travelling companions: geology and physical geography of Brazil. Boston, Mass.: Fields, Osgood, & Co.

Imbrie, John and Imbrie, Katherine Palmer. 1979. Ice Ages: solving the mystery. London and Basingstoke: Macmillan Press.

Origin: On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. By Charles Darwin. London: John Murray. 1859.

Post Office London directory: Post-Office annual directory. … A list of the principal merchants, traders of eminence, &c. in the cities of London and Westminster, the borough of Southwark, and parts adjacent … general and special information relating to the Post Office. Post Office London directory. London: His Majesty’s Postmaster-General [and others]. 1802–1967.

Rudwick, Martin John Spencer. 1969. The glacial theory. History of Science 8: 136–57.


Had Busks and Lyells to dinner.

Examines and criticises evidence for CD’s hypothesis that the glacial period was not one of universal cold. Physicists deny its possibility.

Letter details

Letter no.
Joseph Dalton Hooker
Charles Robert Darwin
Sent from
Source of text
DAR 102: 59, 62–4
Physical description
ALS 8pp †

Please cite as

Darwin Correspondence Project, “Letter no. 5013,” accessed on 17 April 2024,

Also published in The Correspondence of Charles Darwin, vol. 14