Ɛpsilon

From H. B. Dobell   6 October 1871

84, Harley Street, | Cavendish Square, W.

Oct 6 1871

Dear Mr Darwin

An idea has occurred to me with reference to Species, which my knowledge is insufficient to test. It may be of great value or none, and as I have never seen it suggested before, I feel that my best plan is to communicate it to you, who will, at once, be able to test its truth & estimate its importance, and if it of any value you will, I know, give me the full credit of the suggestion.—

—It is this, that the distinctive mark of species may be the duration of pregnancy, incubation, or germination, under identical conditions of life.1

So far as I am aware the average duration of pregnancy is quite independent of external conditions of life; with eggs & seeds I believe it is different & in these the test is not so easily applied, but still it might be effected by artificially securing identical conditions.

As I said before, my knowledge is too limited to allow me to test the value of the idea and I therefore convey it to you in case it should be worth anything.

Believe me | Yours truly | Horace Dobell

Ch. Darwin Esq | &c

1
Dobell and CD had corresponded in 1863 and 1864 on regeneration of limbs, heredity, and other subjects (see Correspondence vols. 11 and 12). In his review of Origin, Thomas Henry Huxley stated that a significant if inconclusive test of whether two individuals represented distinct ‘physiological’ species was to attempt to hybridise them, since distinct species would often either be infertile inter se or produce infertile offspring, whereas varieties of the same species would give rise to fertile progeny ([T. H. Huxley] 1860a, pp. 552–5). The lack of any positive evidence ‘that any group of animals has, by variation and selective breeding, given rise to another group which was even in the least degree infertile with the first’ was, he argued, the weakest point in CD’s hypothesis of natural selection (ibid., pp. 567–8).

Please cite as “DCP-LETT-7989,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-7989