The conjectured recipient is identified on the basis that CD had formerly sent samples of microscopic material to Christian Gottfried Ehrenberg via Bunsen, who was Prussian ambassador in London, 1841–54. See Correspondence vol. 3, letters to C. G. Ehrenberg, 21 May [1845], 29 October [1845], and letter to J. D. Hooker, [29 March or 5 April 1846].

The year is based on the watermark ‘Towgood 1853’ and the Down Bromley Kent address, which was used for only a brief period (July–December) in 1853. CD began to use it regularly in 1856, but the stationery of that year consistently has a Towgood 1855 watermark. See also n. 4, below.

Christian Gottfried Ehrenberg had corresponded with CD during the years 1844–6 when CD was writing South America (see Correspondence vol. 3). His microscopical analysis of Pampas mud and other specimens collected by CD helped to identify the nature of the geological deposits. In March 1853 Ehrenberg read two papers at the Akademie der Wissenschaften zu Berlin on the microscopical organisms he had found in CD’s Galápagos volcanic rock and mud specimens (Ehrenberg 1853a, pp. 178–9, and 1853b, pp. 180–94).

Bunsen was officially recalled to Berlin in April 1854 and left England in July of that year in order to spend the summer months at his home in Charlottenberg, near Bonn (Bunsen ed. 1868, 1: 330, 355).