To Fritz Müller   1 January 1874

Down, | Beckenham, Kent.

Jan 1. 1874

My dear Sir

No doubt I owe to your kindness two pamphlets received a few days ago, which have interested me in an extraordinary degree.1 It is quite new to me what you shew about the effects of relationship in hybrids; that is to say, as far as direct proof is concerned. I felt hardly any doubt on the subject, from the fact of hybrids becoming more fertile when grown in numbers in nursery gardens, exactly the reverse of what occurred with Gärtner.2 The paper on Termites is even still more interesting, & the analogy with Cleistogene flowers is wonderful.3 The manner in which you refer to my chapter on crossing is one of the most elegant compliments which I have ever received.4

I have directed to be sent to you Belt’s Nicuaragua, which seems to me the best Nat. Hist. book of travels ever published.5 Pray look to what he says about the leaf-carrying ants storing the leaves up in a minced state to generate mycelium, on which he supposes that the larvæ feed.6 Now could you open the stomachs of these ants & examine the contents so as to prove or disprove this remarkable hypothesis?

With hearty admiration of all that you publish, I remain | my dear Sir | yours sincerely | Ch. Darwin

LS

1
Müller had sent copies of his papers ‘Bestaubungsversuche an Abutilon-Arten’ (Pollination experiments on Abutilon species; F. Müller 1871–3), and the first part of ‘Beiträge zur Kenntniss der Termiten’ (Contributions to the understanding of termites; F. Müller 1873–5). CD’s annotated copies are in the Darwin Pamphlet Collection–CUL.
2
Müller began experiments to study the fertility of crosses in species of Abutilon (Indian mallow) in 1869 and reported his initial results to CD in a letter of 18 October 1869 (Correspondence vol. 17). In F. Müller 1871–3, pp. 29–30, Müller criticised Karl Friedrich von Gärtner’s conclusions on the fertility of hybrids, noting that Gärtner had made both theoretical and methodological errors. In his heavily annotated copy of Gärtner 1849, CD had also noted flaws in Gärtner’s experimental design and pointed out theoretical errors in his discussion of hybrid fertility (see Marginalia 1: 267–9).
3
In the first part of his study of termites (F. Müller 1873–5, pp. 333–9), Müller had described two sexual forms of both male and female termites in a newly discovered species Calotermes canellae (now Glyptotermes canellae). Winged forms had well-developed genitalia, while soldiers of both sexes had smaller, rudimentary organs. Cleistogamic flowers are modified forms in which the flower never opens and only self-fertilisation is possible. CD discussed cleistogamy in Forms of flowers, pp. 310–45.
4
In F. Müller 1871–3, p. 450, Müller noted that Gärtner’s observation about fertile hybrids losing their fertility over subsequent generations was cited by Marie Jean Pierre Flourens (Flourens 1864, p. 101) as proof of a sharp division between species and varieties. Müller contrasted Flourens’s conclusion with CD’s observation in Origin 4th ed., p. 295: Darwin hat bereits mit gewohntem Scharfblick die Vermuthung ausgesprochen, dass diese vielfach beobachtete Abnahme der Fruchtbarkeit Folge sei nicht der Bastardnatur, sondern zu enger Inzucht und freue mich in den hier mitgetheilten Beispielen verminderter Fruchtbarkeit und völliger Unfruchtbarkeit als Folge zu enger Inzucht bei Abutilon-Bastarden einen neuen Beleg für die Richtigkeit der Vermuthung Darwin’s bieten zu können. [With his usual perspicacity, Darwin has already argued that this frequently observed decrease of fertility is the result not of hybridity, but of too close inbreeding, and I am happy to be able to offer a new proof of the correctness of Darwin’s contention in the examples presented here of reduced fertility and complete sterility as the result of too close inbreeding in Abutilon hybrids.]
5
Thomas Belt’s The naturalist in Nicaragua (Belt 1874) was published in December 1873 (Publishers’ circular, 18 December 1873, p. 1062).
6
Belt described chambers in the ant nest filled with a spongy substance, which, he discovered, was composed of a minute white fungus growing on small pieces of leaves. He also observed deserted chambers filled with refuse leaf particles and hypothesised that the ants ate not the leaves, but the fungus (Belt 1874, pp. 80–1).

Please cite as “DCP-LETT-9223,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-9223