The date is established by the reference in the letter in which this quotation is contained to CD’s theory of natural selection, which was first published in book-form on 24 November 1859 (Origin).

This quotation is from a letter sent to Francis Darwin by Mathias Mull after CD’s death. Mull writes: I possess a very interesting note from your late but ever-to-be revered father, in response to one from me, in which I drew his attention to a couplet in the eleventh Sonnet of Shakespeare, as so applicable, I thought, or so expressive of his principle of “the survival of the fittest.” It is this: | “Let those whom Nature hath not made for store, | … barrenly perish.” | The reply was this— “I write one line to thank you for the reference, which is curious and new to me.” | I think it well worth publication with the couplet, and if you see no objection I will send it to one of the London papers. | I am, | yours faithfully, | M. Mull. No published version of the extract has been found. The term ‘survival of the fittest’ was coined by Herbert Spencer in 1864 as an alternative to ‘natural selection’ (see Correspondence vol. 14, letter from A. R. Wallace, 2 July 1866 and n. 5).