To James Croll   31 January [1869]1

Down. | Bromley. | Kent. S.E.

Jany 31

My dear Sir

Tomorrow I will return registered your book, which I have kept so long.

I am most sincerely obliged for its loan, & especially for the MS. without which I shd have been afraid of making mistakes.2

If you require it the M.S shall be returned. Your results have been of more use to me than I think any other set of papers which I can remember.

Sir C. Lyell who is staying here is very unwilling to admit the greater warmth of the S. hemisphere during the glacial period in the N;3 but, as I have told him, this conclusion, which you have arrived at from physical considerations, explains so well whole classes of facts in distribution, that I must joyfully accept it; indeed I go so far as to think that your conclusion is strengthened by the facts in distribution.4 Your discussion on the flowing of the great ice-cap southward is most interesting.5

I suppose that you have read Mr Moseley’s recent discussion on the force of gravity being quite insufficient to account for the downward movement of glaciers:6 if he is right, do you not think that the unknown force may make more intelligible the extension of the great northern ice cap. Notwithstanding your excellent remarks on the work which can be effected within a million years, I am greatly troubled at the short duration of the world according to Sir W. Thompson, for I require for my theoretical views a very long period before the Cambrian formation.7 If it wd not trouble you I shd like to hear what you think of Lyell’s remarks on the magnetic force which comes from the sun to the earth; might not this penetrate the crust of the earth & then be converted into heat.8 This wd give a somewhat longer time during which the crust might have been solid; & this is the argument on which Sir W. Thompson seems chiefly to rest. You seem to argue chiefly on the expenditure of energy of all kinds by the sun, & in this respect Lyell’s remark wd have no bearing.

My new edition of the “origin” will be published, I suppose in about two months, & for the chance of yr liking to have a copy, I will send one.9

With my very sincere thanks for all your kind assistance | I remain | yours very faithfully | Charles Darwin

I wish that you would turn your astronomical knowledge to the consideration whether the form of the globe does not become periodically slightly changed, so as to account for the many repeated ups & downs of the surface in all parts of the world.— I have always thought that some cosmical cause would some day be discovered

LS(A)

1
The year is established by the relationship between this letter and the letter from James Croll, 2 December 1868 (Correspondence vol. 16).
2
Croll had sent CD a manuscript summarising his views on climate change, together with a book containing all of his published papers up to that time and possibly some unpublished work. Croll had evidently paginated the contents of the book and referred to the material by this system of pagination. Likely sources of some, but not all, of the material have been identified (see Correspondence vol. 16, letters from James Croll, 2 December 1868 and [2 December 1868]). CD had informed Croll that he wanted to keep the book until Christmas so that George Howard Darwin could read some of the papers (Correspondence vol. 16, letter to James Croll, 4 December [1868]).
3
According to Emma Darwin’s diary (DAR 242), Charles Lyell arrived at Down on 29 January 1869. See C. Lyell 1867–8, 1: 268–73.
4
CD refers to Croll’s theory of alternate ice ages in northern and southern hemispheres. See Correspondence vol. 16, letter from James Croll, [2 December 1868]. In Origin, pp. 377–8, CD argued that temperate plants could 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. Joseph Dalton Hooker had challenged the explanation, arguing it would require ‘so very cool a greenhouse’ for temperate plants to cross the equator that tropical species would not survive, and suggested the distribution 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). CD here alludes to the fact that Croll’s theory of alternate ice ages would solve the difficulty by allowing for a warmer non-glaciated hemisphere where tropical species could survive. In Origin 5th ed., pp. 450–61, CD accounted for the survival of tropical species using Croll’s theory.
5
In his letter of [2 December 1868] (Correspondence vol. 16), Croll referred CD to his book of papers for explanations of the flow of ice over North America. The references have not been identified (see n. 2, above).
6
At the 7 January 1869 meeting of the Royal Society of London, Henry Moseley read a paper, ‘On the mechanical possibility of the descent of glaciers by their weight only’ (Moseley 1869a). An abbreviated version of the paper appeared in the 13 January 1869 issue of Scientific Opinion (Moseley 1869b).
7
The physicist William Thomson (later Lord Kelvin) had calculated the age of the earth based on its cooling from a molten ball. He concluded that the earth could not possibly have existed with its crust in a similar state as at present for the length of time proposed by geologists (see W. Thomson 1865). In Origin 5th ed., p. 379, CD noted that based on Thomson’s calculation of the age of the earth and Croll’s estimation that 60 million years had passed since the Cambrian period, the time left for the pre-Cambrian period (140 million years) was ‘hardly sufficient for the development of the varied forms of life which certainly existed towards the close of the Cambrian period’.
8
In C. Lyell 1867–8, 2: 232, Lyell had suggested that the magnetic force of the sun was one of the principal means by which heat lost by conduction into space might be restored to the planet.
9
The fifth edition of Origin was published in May 1869 (Publishers’ Circular, 1 July 1869, p. 386).

Manuscript Alterations and Comments

2.1 especially] interl in CD’s hand
5.8 force] in CD’s hand above del ‘power’
8.1 I wish … discovered 8.4] in CD’s hand

Please cite as “DCP-LETT-6585,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-6585