The recipient is conjectured from the fact that the letter was pasted into Pryor’s copy of Origin 6th ed.; the date is conjectured from the relationship between this letter and the letter to Francis Darwin, [after 21 January 1871] (Correspondence vol. 19).

In Journal of researches (1860), p. 146, CD described the niata cattle in Argentina which, because of their peculiarly shaped jaw, were unable to graze on the twigs of trees and reeds during severe droughts when grass was unavailable. He used them as an example again in Variation 2: 226, as an instance of natural selection operating only at long intervals and in special circumstances. In Origin 6th ed., p. 177, CD mentioned them in response to St George Jackson Mivart’s objection that natural selection was unable to explain the incipient stages of useful structures. Mivart’s arguments were partly based on the neck of the giraffe (see Mivart 1876, pp. 24–9). Pryor was evidently planning to write a review of Mivart’s On the genesis of species (Mivart 1876; see Correspondence vol. 19, letter to Francis Darwin, [after 21 January 1871]).