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Darwin Correspondence Project

From E. L. Garbett to the Athenæum   19 July 1869

7, Mornington Road,

July 19, 1869.

Either of my last methods, applied to Mr. Darwin’s supposition of twin births alone at intervals of 30 years, would give the same results as the series 1, 1, 2, 4, 7, 13, &c. (continued by the scale 2+0+0-1), whose terms quadrupled give, as Mr. George1 observes, the elephants alive, on that supposition, in the decade after each bearing year.2 In the periods of 20 years, however, between these exceptional decades, their numbers are only the doubles of these terms. The general term of this progression being 1:839, shows the period of doubling to be about 34 years, while, on my truer supposition of single births at intervals of 12 years, the increase is about a sixteenth faster. The mistake that led me at first to a doubling in 23 years may be the same as the original calculator’s. At least it would fully account for his result.



George Howard Darwin.
See letter to Athenæum, 7 [July] 1869 and n. 2.


Athenæum. 1844. A few words by way of comment on Miss Martineau’s statement. No. 896 (28 December): 1198–9.


More on the problem of the increase of elephants (see 6806f and 6820).

Letter details

Letter no.
Edward Lacy Garbett
Sent from
London, Mornington Road, 7
Source of text
Athenæum, 24 July 1869, p. 115

Please cite as

Darwin Correspondence Project, “Letter no. 6836F,” accessed on 2 December 2020,

Also published in The Correspondence of Charles Darwin, vol. 17