From Charles Lyell   1 March 1866

53 Harley Street

March 1st./66.

My dear Darwin

Please sign the memorial & return it by return of post as no time is to be lost.1 I have had Hooker’s letter copied.2 I feel sure that the whole globe must at times have been superficially cooler— Still I think that during extreme excentricity the sun would make great efforts to compensate in perihelion for the chill of a long winter in aphelion in one hemisphere & a cool summer in the other—3

I think you will turn out to be right in regard to meridianal lines of mountain chains by which the migrations across the equator took place while the was contemporaneous tropical heat of certain low lands, where plants requiring heat & moisture were saved from extinction, by the heat of the earth’s surface, which was stored up in perihelion being prevented from radiating off freely into space by a blanket of aqueous vapour caused by the melting of ice & snow.4 But though I am inclined to profit by Croll’s maximum excentricity for the glacial period, I consider it quite subordinate to geographical causes or the relative position of land & sea & the abnormal excess of land in polar regions.5 It is a vast subject & if one applies to the astronomer there is no end to the number of uncertain data which it has never been worth their while to calculate for their own purposes. The worst of all the uncertainties is that which relates to the temperature of space6

believe me | ever most truly yrs | Cha Lyell

1
The memorial has not been identified, but may be that submitted by CD and others to W. E. Gladstone, 14 May 1866 (see this volume).
2
Lyell refers to the letter from J. D. Hooker, 21 February 1866. CD had forwarded part of the letter to Lyell, having been told by Joseph Dalton Hooker that Lyell wished to see it (see letter from J. D. Hooker, [26 or 27 February 1866], and letter to J. D. Hooker, [28 February 1866] and n. 3).
3
For CD’s belief that the entire earth was cooler during the glacial period, see the letter to Charles Lyell, 7 February [1866] and n. 12. Hooker had disagreed with CD on this and related topics in his letter to CD of 21 February 1866. The effects on global climate of aphelion and perihelion, eccentricity of the earth’s orbit, and other astronomical phenomena are discussed in chapter 13 of C. Lyell 1867–8.
4
Lyell refers to CD’s argument in Origin, pp. 377–8, that temperate plants may have crossed through the tropical regions near the equator during a glacial period by means of mountain ranges, which would have remained cooler, while tropical plants were preserved in the greater heat of the low-lying areas. This would account for the existence of similar species in both the northern and southern temperate zones. For some years, Hooker challenged CD on the problem of the survival of tropical plants in ‘so very cool a greenhouse’ as would be required to allow temperate plants to cross the equator, suggesting that the distribution of plants and animals could be better explained by land-bridges and continental extension (see Correspondence vol. 11, letter to J. D. Hooker, 5 March [1863] and n. 17). In the fourth edition of Origin, CD admitted that the survival of tropical species was a difficulty for his theory (Origin 4th ed., pp. 450–1), but this text was not retained in later editions (see Peckham ed. 1959, pp. 599–603). CD later explained the survival of tropical plants according to James Croll’s theory of ice ages that alternated between hemispheres, and during which the temperature of the non-glaciated hemisphere rose (Origin 5th ed., pp. 450–61; see also n. 5, below). Lyell, however, had invoked alternate meridional belts of cooler and warmer temperature to explain the survival of species during the glacial period (C. Lyell 1863, pp. 366–7).
5
Croll had recently published a paper relating the occurrence of ice ages to the eccentricity of the earth’s orbit and to the longitude of the perihelion (Croll 1866a). For more on changes in global temperature, including the significance of Croll’s work to CD’s theory of the migration of plants during the glacial period, see the letter to Charles Lyell, 7 February [1866] and n. 12, and the letter from J. D. Hooker, 21 February 1866 and n. 10; see also the letter to J. D. Hooker, [28 February 1866] and n. 7. Lyell explained former changes in climate in relation to geographical and astronomical causes in C. Lyell 1867–8, chapters 12 and 13, respectively. For a summary of Lyell’s position on former climate changes, see Ospovat 1977 and Fleming 1998.
6
CD wrote of uncertainties about the temperature of space, in the light of recent contributions by physicists and astronomers to the understanding of glaciation, in the letter to J. D. Hooker, [28 February 1866].

Please cite as “DCP-LETT-5024,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-5024