Haeckel had been on a lecture tour popularising evolution in February and March 1878 (Krauße 1987, p. 133). He gave a lecture in Vienna on 22 March 1878 on cell souls and soul cells (Haeckel 1878). The letter from Scherzer has not been found.

Rudolf Carl Virchow had given an address to the Assembly of German Naturalists and Physicians titled ‘The liberty of science in the modern state’ in Munich in September 1877; an English version of the text was published in Nature, 22 November 1877, pp. 72–4; 29 November 1877, pp. 92–4; 6 December 1877, pp. 111–13. Virchow was responding to Haeckel’s address at the same meeting titled ‘The present position of the evolution theory’, which advocated that evolution be taught in schools (Haeckel’s address was published in English in Nature, 4 October 1877, pp. 492–6). Virchow held that the German nation would regard naturalists favourably only if they exercised moderation with respect to personal speculation, especially in relation to the theory of descent. He therefore advocated that the theory of evolution should not be taught in schools. These issues were part of Kulturkampf (culture struggle), the power struggles surrounding the role of the Catholic Church in the emerging secular nation state; Virchow’s address delighted the religious right, while secular liberals felt betrayed (Hopwood 2015, p. 137).

Arthur Lane was the son of CD’s hydropathic doctor and friend Edward Wickstead Lane. Scherzer and his wife, Julie Karoline Scherzer, had visited Lane’s hydropathic establishment in Richmond, Surrey, in late 1876 or in 1877 (see Correspondence vol. 24, letter to Karl von Scherzer, 24 December 1876). The envelope that contained CD’s letter was addressed to Scherzer at a hotel called The Mansion in Richmond, Surrey; Scherzer was possibly being treated at Lane’s hydropathic establishment at Ham, near Richmond, in April 1878.