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 For a summary of CD and Huxley’s earlier discussions on the topic, and some of CD’s unpublished notes on hybridity, see Correspondence vol. 10, Appendix VI.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).