To James Croll   24 November 1868

Down | Bromley, Kent, S.E.

Nov. 24, 1868

Dear Sir,

I have read with the greatest interest the last paper which you have kindly sent me.1 If we are to admit that all the scored rocks throughout the more level parts of the United States result from true glacier action it is a most wonderful conclusion, and you certainly make out a very strong case; so I suppose I must give up one more cherished belief.2 But my object in writing is to trespass on your kindness and ask a question, which I dare say I could answer for myself by reading more carefully as I hope hereafter to do, all your papers, but I shall feel much more confidence in a brief reply from you. Am I right in supposing that you believe that the glacial periods have always occurred alternately in the Northern and Southern Hemispheres, so that the erratic deposits which I have described in the S. parts of America, and the glacial work in New Zealand could not have been simultaneous with our glacial period.3 From the glacial deposits occurring all round the Northern Hemisphere, and from such deposits appearing in S. America to be as recent as in the north, and lastly, from there being some evidence of the former lower descent of glaciers all along the Cordilleras, I inferred that the whole world was at this period cooler.4 It did not appear to me justifiable without distinct evidence to suppose that the North and South glacial deposits belonged to distinct epochs, tho’ it would have been an immense relief to my mind if I could have assumed that this had been the case. Secondly, do you believe that during the glacial period in one hemisphere, the opposite hemisphere actually becomes warmer, or does it merely retain the same temperature as before? I do not ask these questions out of mere curiosity, but I have to prepare a new edition of my origin of Species, and am anxious to say a few words on this subject on your authority.5 I hope that you will excuse my troubling you.

Pray believe me, yours sincerely, | Charles Darwin.

Copy

1
CD refers to the final part of Croll 1868 (see letter to James Croll, 19 September 1868 and n. 1).
2
CD probably refers to his belief that erratic boulders and rocks had been scored by drifting icebergs (see Origin, p. 366). Croll argued that icebergs lacked both the necessary pressure and steadiness of motion to account for the striations in rock (Croll 1868, pt 3, pp. 367–8). In Origin 5th ed., p. 443, CD changed the sentence on erratic boulders and rocks, omitting the reference to icebergs.
3
In the margin of page 18 of his offprint of the third instalment of Croll 1868 (see p.379), CD had written, ‘Hence glacial period of Patagonia & New Zealand at a different period— Ask Croll’. CD had described South American erratic deposits in ‘Distribution of the erratic boulders’.
4
For CD’s belief that the entire earth was cooler during the glacial period, see Correspondence vol. 14, letter to Charles Lyell, 7 February [1866] and n. 12. See also Origin, pp. 369–80.
5
In Origin, pp. 377–8, CD argued that temperate plants could have crossed through the tropical regions near the equator during a global glacial period, while tropical plants were preserved in a few warm 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. 6, letters from J. D. Hooker, 4 August 1856 and 9 November 1856). 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 ice ages that alternated between hemispheres 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. For more on Croll’s theory of ice ages, see letter from James Croll, [2 December 1868].

Please cite as “DCP-LETT-6473,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-6473