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Darwin Correspondence Project

To Alfred Newton   2 January 1877

Down, | Beckenham, Kent. | Railway Station | Orpington. S.E.R.

Jan 2. 1877

Dear Newton,

Many thanks for telling me of the complex cross among the wag-tails.1 I am surprised at so much close interbreeding not checking their propagation. I do not suppose that I shall ever have strength to work up my data on Hybridism, so I think I will not write to Mr Monk2

Pray believe me yours | very sincerely | Charles Darwin


Newton’s letter has not been found. Wagtails are birds of the genus Motacilla.
In 1873, John Jenner Weir had informed CD about fertile hybrids of two species of wagtails in the aviary of Thomas James Monk (see Correspondence vol. 21, letter from J. J. Weir, 13 November 1873). Thomas Henry Huxley’s critique of natural selection as a mechanism for creating new species had served as an important impetus to CD’s investigations into cross and hybrid sterility. Huxley’s position was based on the widely accepted view that sterility of hybrids was an essential criterion of species. For a summary of CD and Huxley’s earlier discussions on the topic, and some of CD’s unpublished notes on hybridity, see Correspondence vol. 10, Appendix VI. CD added new material on hybrid sterility to Origin 4th ed., and concluded, ‘the physiological test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of specific distinction’ (ibid., p. 323).


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

Origin 4th ed.: On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. 4th edition, with additions and corrections. By Charles Darwin. London: John Murray. 1866.


Thanks AN for telling him of the complex cross among wagtails. CD is surprised that so much close interbreeding does not check their propagation.

CD does not suppose he will ever have strength to work up his data on hybridism, so he will not write to Mr Monk.

Letter details

Letter no.
Charles Robert Darwin
Alfred Newton
Sent from
Source of text
Cambridge University Library (MS Add. 9839/1D/63)
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 10766,” accessed on 22 September 2021,