Francis Darwin, in Life and letters of Charles Darwin, wrote of Fritz Müller
They never met, but the correspondence with Müller, which continued to the close of my father's life, was a source of very great pleasure to him. My impression is that of all his unseen friends Fritz Müller was the one for whom he had the strongest regard.
Fritz Müller, in a letter to Ernst Krause written shortly after Darwin’s death, expressed his feelings: “For many years, I was used to telling Darwin all my little natural science experiences, and receiving suggestions and hints from him. The wonderful qualities of his character, this total freedom from any petty conceit, this temperateness in judgement, this ungrudging, often too ready acknowledgement of others had to endear him to everyone; for me personally he always demonstrated such heartfelt and, as I discovered in the case of our flood, such self-sacrificing goodwill, that I can easily say, I considered him to be a second father.”
Fritz (Johann Friedrich Theodor) Müller was born in the small Thuringian town of Windischholzhausen in 1822, the eldest son of a natural history loving pastor. After completing secondary school in Erfurt he studied pharmacy for a year before beginning his university career in 1841 at Berlin, where he studied mathematics and natural sciences. As was typical at the time, he spent three semesters at Greifswald before returning to Berlin to complete his studies and where, under the supervision of Johannes Müller, he received his PhD in 1844. Müller returned to Erfurt and began a year as a probationary teacher at the Erfurt Gymnasium, but left after six months because he was unwilling to be hampered by the Prussian state’s restrictions on freedom of belief. Four years of medical study at Greifswald followed, during which time he finally broke with Christianity altogether. He completed all the requirements for a medical degree but one: he refused to swear an oath, and as a result he could not qualify as a doctor. This decision had profound personal and professional consequences. Most of his family broke off direct contact; only his two brothers, Hermann and August, maintained close relations with him. In 1849, he took a job as a private tutor; by this time he had a family of his own and had begun to consider emigrating. The plan was realised in 1852 when he, his brother August, and their families emigrated to the Blumenau Colony in Santa Catarina, Brazil.
After four years of living a frontier existence, Müller left the mainland and got a job in Destêrro (now Florianópolis) on Santa Catarina Island in the new provincial lycée that opened in 1857, where he taught mathematics and later physics, chemistry, and natural history. It was during this ten-year period that he read Darwin’s On the origin of species, having received a copy of the German edition in 1861 from his good friend, Max Schlultze, formerly a fellow student at Greifswald and by this time professor of anatomy at Bonn. It was in large part through his contact with Schultze that Müller’s own original research was given a wider audience in the German scientific community and eventually came to Darwin’s attention. Earlier in 1861, Müller had discovered a new group of parasitic Crustacea, the Rhizocephala, while studying juvenile stages in various crustaceans, and, inspired by his reading of Origin, decided to write a work on the subject of crustacean morphology and development that would, as he told his brother Hermann, provide important evidence in support of Darwin’s theory.
The book, simply titled Für Darwin (for Darwin), appeared in 1864, and Darwin received a copy from Müller. Darwin found the German too difficult to understand, so engaged the childrens’ German governess, Camilla Ludwig, to translate the book, and had it read to him in 1865 while he was still suffering from an illness that affected his stomach and gave him frequent dizzy spells. This same illness had prevented Darwin from working on his follow-up book to Origin (Variation of animals and plants under domestication), but he was still able to carry on botanical research and in June 1865 published a work on ‘Climbing plants’ in a double issue of the Journal of the Linnean Society (Botany).
Having now read Müller’s book, Darwin initiated a correspondence with Müller that would last right up to his death. In spite of the fact that he was addressing a complete stranger, Darwin’s tone in this first letter was already collegial; he was clearly impressed both with Müller’s technical skill as well as his analytical abilities. Among other things, he asked Müller to settle a point concerning the nature of a barnacle organ, drew Müller’s attention to a barnacle that he thought might be a link to the Rhizocephala, mentioned parallels between Müller’s discovery of two forms of males in certain species of crustaceans with his own work on floral dimorphism, and informed him that he had sent a copy of ‘Climbing plants’, hoping Müller would have received it.
Müller had not only received and read ‘Climbing plants’, he had already written to Darwin describing several genera of climbing plants he collected and promising more information after studying them. In two further letters, he was able to provide so much new information that after consulting his friend Joseph Dalton Hooker, Darwin sent off the letters for publication in the Journal of the Linnean Society (Botany). The pattern was set for the rest of their correspondence. With each work Darwin sent, he received valuable supplementary material on many areas of research from Müller, and encouraged him to publish his own observations in English as well as German periodicals.
Müller had returned to the mainland in 1867, once again homesteading and living off a meagre income from the state for his work in natural history; at one point he feared that he would have to give up scientific pursuits altogether. Thanks to a change in government, Müller was appointed to the position of travelling naturalist for the national musem in October 1876, and was therefore able to continue his scientific work. He is probably best known for his elaboration of the theory of insect mimicry which bears his name; his early paper on the topic was translated by Raphael Meldola into English and published in the Transactions of the Entomological society of London in 1879 on Darwin’s suggestion. Müller continued to produce a range of scientific work, but in 1891 lost his official position when the government decided that all its naturalists had to be based at the museum in Rio de Janiero, a move that resulted in the wholesale dismissal of travelling naturalists. He refused offers of help from abroad but continued to work and publish until his death in 1897.