The Countenance of Nature Transformed, 1861–1869
The Countenance of Nature Transformed, 1861–1869
Abstract and Keywords
In reaction to Darwin’s On theOrigin of Species, which he read in October 1861, Müller reorganized his ongoing research on crustacea. One result was his Für Darwin, published in 1864, utilizing detailed crustacean research. Müller applied Darwin’s theory of natural selection to crustaceans, yielding some novel predictions, some of which he subsequently verified. Müller showed that crustacean embryology provided a partial guide to crustacean genealogy, demonstrated that genealogical thinking resolved puzzles in the chaotic taxonomy of crustaceans, defended Darwin’s theory against several common objections, and demonstrated its superiority to special creationist alternatives. Darwin responded to Für Darwin with high praise and instigated an English translation of the book. He initiated correspondence with Müller, in which they discussed applications and extensions of Darwin’s theories and directions for further fieldwork. Für Darwin initially drew a generally tepid reception, with severe reviews from some critics. The chapter closes by comparing Müller’s evaluations of Darwin’s theory with those of Ernst Haeckel and August Weismann.
Müller Encounters On the Origin of Species
According to his annotation on the flyleaf, Müller received Heinrich Bronn’s German translation of Darwin’s Origin in 1861.1 Inside the back cover is the sticker of the Hamburg bookseller Robert Kittler, whom Müller called “my bookseller,”2 but it certainly came at the initiation of Schultze, who arranged an annual shipment of books, reprints, and equipment, although sometimes long delayed in transit.3 The precise date of arrival is not known, but Müller had already read at least part of the Origin by 30 October, when he wrote his first reaction to his father and stepmother: “Darwin’s book on the origin of species in the animal and plant kingdoms has given me, and still gives me, much to think about.”4 Given the speed with which he read later books by Darwin, and unencumbered by pioneer farming, he had probably read it all by that time. Müller was already familiar with Darwin’s Journal of Researches, but through the Origin he would have recognized his common ground with this distant Englishman, most importantly in Darwin’s rejection of all things supernatural. They only made contact, however, four years later.
In mid-November Müller told Schultze that he was “settling ever more into Darwin’s views and finding meaning and connection for a mass of formerly unintelligible facts.”5 In December he told Schultze that for several weeks he had been occupied with the Crustacea, especially “with a subject [development] that seems to me of the greatest importance for a natural genealogical arrangement of that class and for its morphology.”6 The impetus surely came from Darwin, who had written, “descent is the hidden (p.59) bond of connection which naturalists have sought under the term of the Natural System. On this idea of the natural system being … genealogical in its arrangement, … we can understand the rules which we are compelled to follow in our classification.”7
In the winter of 1861 Müller had found the immature stages of the Porcellana crabs that he had been seeking when distracted by the Rhizocephala,8 and in November he submitted his first manuscript on crab development. “Even among crabs there is metamorphosis, and the youngest stages are Cyclops-like, [followed by] a Zoëa-like stage … [and] finally a Mysis-like form, this last seeming by its three pairs of claws to point the way toward the [prawn] genus Penaeus.” He was beginning to find developmental stages that were traversed in common by members of different crustacean groups (figure 3.1). “The Crustacea overwhelm me with so many questions, and prawn larvae hold such promise for my hope of finding in their development the key to a definitive natural classification, that I will probably not be free of the group for years!”9 And indeed he was not.
(p.60) During the summer of 1861–62 Müller pursued his hope of clarifying crustacean relationships in support of Darwin, although “using larvae fished from the sea to compare the development of animals that pass through a long series of different forms is a most time-consuming and difficult task.”10 “Difficult” because it was hard to keep immature stages alive in an aquarium long enough to follow metamorphosis from beginning to end, making inference necessary for connecting juveniles with adults. In addition to accepting Darwin’s view that genealogy was the key to classification, Müller also saw natural selection as the principal motive force in organic change.
Response to the Origin: Für Darwin
Although Müller thought about “publishing some general observations in favor of Darwin’s theory of natural selection,” he quickly decided that the theory would best be proven by applying it to “specific circumstances” to see if it brought “clarity and order to an apparent chaos.” He hoped that such an approach would benefit the theory more than by the “usual deductions, with which … only those [with] the same general viewpoint can be expected to agree.”11 He had therefore already settled on three important components of the book that was forming in his head: evidence in favor of natural selection, development as a key to genealogy, and the use of genealogy in classification. In June 1862 Schultze told him about Louis Agassiz’s 1860 criticism of Darwinism, but Müller, though much interested, said that it did not change his “favorable opinion of Darwin.” Nevertheless, keeping an open mind, he added: “I agree with you that the matter is not yet ripe for a decision and that the first concern is to gather facts.”12
In December 1862, having heard with delight that his brother Hermann had accepted Darwin’s theories, Fritz finally laid out a plan suggested by the Origin, “namely to attempt to draw up a genealogy of the Crustacea” that would show “in what sequence the various living forms diverged from the ancestral form and what different stages they passed through.” For example, starting at the top of the genealogy with prawns, which he believed to be the most recently divergent group, he would attempt to determine the structure of the “common ancestor of all prawns,” and then do the same down through increasingly inclusive groups: the Macrura (lobsters, crayfish, and prawns), Decapoda (Macrura plus crabs), Podophthalmata (decapods plus other stalk-eyed crustaceans), and finally, at the bottom, (p.61) the common ancestral form of all higher Crustacea. “If Darwin’s theory is correct,” Müller had no doubt that “all Crustacea had descended from a nauplius,” because that was the form of the hatchling in Rhizocephala, barnacles, copepods, and other lower groups, and even in some of the highest Crustacea, for example a prawn, Penaeus, as he had himself discovered.13 Fritz told Hermann that he would “in time” explain how questions of relationship such as these could be answered with the help of development; that was to be the subject of the last part of Müller’s book.14 Müller’s verbal “diagram” was probably based on Darwin’s figure in the Origin showing descent with branching divergence from ancestors.15 The two differ, however, in that Darwin’s diagram was theoretical, while Müller’s referred to real organisms.
During 1863, as revealed by the details of crustacean anatomy and development that filled his letters to Schultze, Müller was “gathering facts” for the book that would be his major work in favor of Darwin. The first part of the manuscript went in September of that year to Schultze, who responded warmly with suggested alterations, and in January 1864 Müller told him that his “extremely and undeservedly favorable opinion of my attempt to bring some new facts for Darwin into the field has boosted my courage” to finish the “half-sent work.”16 On 5 February 1864 Müller dispatched the remainder of the manuscript, with last revisions and additions, and also gave Schultze a list of recipients of free copies, if the publisher were to offer them.
The list included Carl Gegenbauer (1826–1903), comparative anatomist, Jena, and Carl Claus (1835–1899), authority on crustaceans, Marburg, both Darwinists; Franz von Leydig (1821–1908), microscopist, Tübingen; Adolph Eduard Grube (1812–1880), zoologist, Breslau; Rudolf Leuckart (1822–1898), zoologist, Gieβen; Eduard Oscar Schmidt (1823–1886), specialist in sponges and a friend from Greifswald, then in Graz; Henri Milne-Edwards (1800–1885), preeminent authority on crustaceans, Paris; “and above all Darwin. Also [Charles] Spence Bate, whom I must not forget.”17 Bate (1819–1889) was an English dentist and naturalist interested in the development and morphology of Crustacea. Müller’s list included prominent crustaceologists, friends, and others with whom he had been corresponding and who had helped him in their specialties, and of course the man in whose support the book was written. Müller later asked that copies of the book be sent to his father and to Alexander Agassiz at Harvard University, with whom he was already in a friendly argument about the validity of Darwinism. He had (p.62) told Agassiz about the impending book and was “curious what he will say about it.”18 The letters to Schultze also reveal Müller’s debt to his old friend for comments on his manuscripts and help in sending them on for publication. Unfortunately, none of Schultze’s letters has been found.
Für Darwin appeared around the middle of 1864, and Müller received his copy in November, when he thanked Schultze for guiding the manuscript through publication.19 On the title page is the quotation from Otto F. Müller that Fritz had used in his dissertation twenty years before, avowing that everything was his own work and was based on repeated observations. The aptness of that epigraph is confirmed by the wealth of original observations in the book, as well as in Müller’s previous publications and in his letters, especially to Schultze, in the early 1860s.
The book now out, Schultze suggested a sequel, and Müller wrote optimistically that “there is certainly no lack of material to follow up with a second volume during my next vacation. Only one thing is lacking: time!”20 In 1865 he reiterated his desire to aid in the recognition of Darwin’s theory “with a wealth of further original observations.”21 He told Schultze that he had suggested to Haeckel a new journal to promote Darwinism. “There is a whole swarm of embryonic articles buzzing around in my head,” he wrote, but if Haeckel couldn’t do it, Müller might prepare that second volume of Für Darwin as a stimulus. Candidly, he hoped that it “would turn out some-what less impossible to read than the first, with its special carcinological content.”22
The book was reviewed favorably in 1865 by the German zoologist Carl E. Adolf Gerstaecker (1828–1895), at that time curator of the Zoological Museum of Humboldt University.23 “Among the ever increasing number of young scientists who support the Darwinian theory of the origin of species by natural selection,” wrote Gerstaecker, “Fritz Müller clearly occupies one of the most distinguished places by his brilliant as well as weighty treatise ‘Für Darwin.’” Unlike most of Darwin’s followers, “the author has done research to confirm the correctness of those views directly through a series of observations of the class Crustacea, or at least to provide them with weighty support.” Gerstaecker then summarized Müller’s principal points. Another review, favorable but unsigned, in Geneva early in 1865 was translated in the Annals and Magazine of Natural History later that year. Neither of those reviews was by a specialist in Crustacea, but the one in the Annals and Magazine gave a clear-eyed summary and brought Müller’s book to the attention of English readers.24 Bate devoted nine pages to a review in the first (p.63) volume of the Zoological Record, but although he was a specialist in Crustacea he missed nearly all of Müller’s points and seriously misrepresented him in others from a poor understanding of German and an unfamiliarity with the Darwinian topics that Müller addressed. “Independent of any interest [in] the arguments of Dr. Müller, some of which his opponents may not be inclined to accept,” Bate concluded that “every student of carcinology must study it with interest,” but his review would have left his readers confused and in the dark.25
Contact with Darwin and Translation of Für Darwin
Darwin himself learned of the work when the publisher’s copy arrived, and in November 1864 he told Haeckel that the German was difficult and that he had employed a translator to help. But it was only in August 1865, after a long illness, that Darwin wrote an enthusiastic appreciation to Müller in the first of a flow of letters between the two that continued until Darwin’s death in 1882:
And now you must permit me to thank you cordially for the great interest with which I have read it. You have done admirable service in the cause in which we both believe. Many of your arguments seem to me excellent, & many of your facts wonderful…. Your observations on Classification and Embryology seem to me very good and original. They shew what a wonderful field there is for enquiry on the development of Crustacea, and nothing has convinced me so plainly what admirable results we shall arrive at in Natural History in the course of a few years. What a marvellous range of structure the Crustacea present, & how well adapted they are for your enquiry!26
Darwin had already struck on the “facts” and “arguments” that formed the title of the English translation four years later. Müller received Darwin’s letter on 8 October.27 He responded the next day with his hypothesis of the link between barnacles and rhizocephalans, which he had sent to Schultze in 1864 to be included in Für Darwin. He told Schultze that he was “on the track of a solution to a problem that has cost [him] much pondering … namely how root-headed cirripedes were derived from barnacles,” and gave a preliminary solution,28 but the passage was too late for inclusion in the book and appeared only in the English translation in 1869.29
In September 1865 Darwin wrote: “Does it not often strike you that Natural (p.64) History is rendered extremely interesting by such views as we both hold.”30 To which Müller answered: “To be sure! Ever since I read your book On the Origin of Species and was converted to your point of view, many facts which I used to view with indifference have become quite remarkable; others which formerly seemed meaningless oddities have acquired great significance, and thus has the countenance of all Nature been transformed.”31 The contrast between Müller’s correspondence and publications written before he encountered Darwin late in 1861, and the contents of Für Darwin as well as later publications and correspondence, tell us precisely how that “countenance” had been transformed.
Louis Agassiz’s criticism of Darwin that Schultze reported to Müller must have been Agassiz’s review of the second edition of the Origin (1860), first in an American journal and later that year in volume 3 of Agassiz’s Contributions to the Natural History of the United States, in two places in the chapter on “Acalephs in General.”32 In it Agassiz rejected every detail of the Origin, repeating his belief that everything we see proves the truth of intelligent thought as the causative agent. The review was in a discussion of “individuality,” and Agassiz demurred from Darwin on the individuality of everything in the taxonomic hierarchy except that of an individual organism. But he had many other objections, including Darwin’s reading of the fossil record and the worthlessness of artificial selection as a model for natural selection, and he misunderstood the significance of Darwin’s intermediate species as evidence of transitions. Some of these reservations are understandable for the time—he could not have known what the fossil record would reveal in the future. But historian of science Jon Roberts’s belief that Agassiz was “unwilling to defer to religious orthodoxy in arriving at conclusions concerning the structure, history, and processes of the natural world” misrepresents the religious, though not biblical, premise that underlay his conclusions, whether “orthodox” or not. Roberts also suggests that Agassiz’s “opposition to a priori conclusions and his adamant insistence that explanations of phenomena should be determined only after the most careful evaluation of evidence would seem to be in accordance with the best tradition of tough-minded empiricism.”33 But what else is Agassiz’s belief in the certainty of “intelligent thought” but an a priori conclusion? Müller, too, insisted on the primacy of evidence, but when he discovered Darwin his first thought was to test the theories, not to impose on them any “a priori conclusions.”
Louis Agassiz assailed Darwin for being all conjecture and no facts, but (p.65) he apparently never saw Müller’s book, either in its German original, of which his son Alexander had a copy, or in the translation of 1869. In fact, he seems to have been unaware of Müller’s existence. Müller’s book was thick with the kinds of facts that should have appealed to him, and even if he were unwilling to follow its arguments, he should have recognized that the book provided much that Darwin’s “abstract” could not give.
Over the years following his encounter with Darwin and the publication of Für Darwin and its translation, Müller found many examples of “meaning and connection” for those “formerly unintelligible facts.” One of these was a new caddis fly species that passed its immature stages in the stagnant rainwater pools of bromeliads high in the forest. It differed in a suite of structural and behavioral peculiarities from a closely related caddis fly that Müller found breeding in local forest streams; Müller recognized the peculiarities as adaptations to the unusual conditions of the bromeliad pools and could easily ascribe them to natural selection. Here was a set of correlated adaptations that distinguished two closely related species, a demonstration of evolution generating species diversity.34
Before he encountered Darwin, Müller used morphological similarities as the principal criteria for determining taxonomic relationships, although he was certainly aware of other criteria that might be useful. For example, he wrote in 1859 that further observations on development would be needed to reveal the systematic position of two new species of jellyfish.35 In Für Darwin he described the problem of bringing the diversity of Crustacea into a genealogical tree in order “to form some idea of the probable structure of their ancestors.” Existing systematic works were of little use, because they “laid more weight upon the characters separating the genera, families and orders, than upon those which unite the members of each group.” Once again he said that a “thorough knowledge of development was indispensable” but recognized how little was known of the subject.36 Of course, those who, like Louis Agassiz, held a creationist view of organic diversity would not have thought of genealogy anyway, but Müller never had creationist views, and although he lacked a formal taxonomic framework, he was already receptive to Darwin’s suggestion in the Origin of development as a key to taxonomy.
In a paper submitted in June 1863, Müller contrasted Darwin’s view with an extreme alternative and showed that Darwin had provided him with an appropriate framework for making taxonomic judgments. He described the peculiar structural details of a chelate (clawed) isopod, an unusual member (p.66) of that group of Crustacea, and asked: What would be the significance of those peculiarities in determining an archetype (either the fundamental body plan, or the earliest ancestor) of isopods? That, he answered, would depend on one’s view of systematics. “He who views species as unvarying creations shot out together … in a flash in each of the hundreds of repeated creations, and who constructs the archetype of each … group from the common characters of the majority of its members, will naturally see the chelate isopods as most deviant from the isopod type [that is farthest from the archetype].”37 Müller refers here to Cuvier’s creationist view that the members of each group of animals were created on a unique and unvarying body plan, and in this context the archetype would be deduced from the most common plan and would have nothing to do with ancestry. “On the other hand,” Müller continued, “he who with Darwin considers that the goal of systematics is the setting up of family trees of animals and plants, and who for that reason considers the archetype of a group to be its common ancestor, will … be inclined to the view that among all isopods of the present day the chelate isopods, with their [“unisopod-like”] movable eyes and their zoea-respiration, are closest to the isopod archetype.”38
Darwin told Haeckel in 1865 that he intended to have Müller’s book translated, but he took action only in 1868. William S. Dallas (1824–1900), an English naturalist and the translator of Müller’s papers in the Annals and Magazine of Natural History, suggested that it would sell well and offered to do it.39 Darwin immediately told Hermann Müller about the plan and, being assured that his brother would not object, made the arrangements and wrote the details to Fritz. He told him that Dallas thought it “important to have some few corrections or additions, in order to account for a translation appearing so long after the original.”40 Having heard from Hermann about the plan, however, Fritz had already sent Darwin six English additions, including his theory about the evolution of Rhizocephala from barnacles.
He also included a memo with his argument that the “so-called ‘complete metamorphosis’ of insects was not inherited from the primitive ancestor of all insects, but acquired at a later period.”41 Müller’s use of the terms “acquired” and “inherited” has often been misunderstood, but this passage makes the distinction clear. The metamorphosis of shrimps was “inherited,” because it was found in more primitive crustaceans, but that of insects was a set of adaptations recently “acquired,” because it was not found in putative ancestral insects. Müller thought that many subtle characters, like wing (p.67) venation in butterflies, were of deep ancestry (inherited), whereas mimetic resemblances, for example, had evolved (been acquired) recently. His usage was certainly influenced by Darwin, who wrote in the Origin that “on the view that the natural system is founded on descent with modification, … characters … showing true affinity … have been inherited from a common parent” (Müller’s “inherited” characters).42 Convergent or adaptive characters would not show “true affinity,” because they were adaptations to special circumstances (Müller’s “acquired” characters) and “may involve only a limited portion of the genetic endowment of a group and thus be less informative than the general habitus.”43 Müller did not mean that they were “acquired” by use in an individual and then transmitted to its offspring in the traditional Lamarckian sense. Some authors, however, have reacted reflexively to Müller’s “acquired characters” as though the phrase were preceded by “inheritance of.”44
Dallas completed his translation, “a difficult job,” in late November 1868,45 and the publisher brought out a thousand copies of Facts and Arguments for Darwin in mid-March 1869 (figure 3.2).46 The book was a specialized work; the running head throughout was “History of Crustacea,” and all but three chapters were almost completely restricted to that class of organisms. Müller himself recognized its difficulty for the average reader, and a decade later fewer than seven hundred had been sold.47
In Russia, Für Darwin was translated by the young and promising evolutionary embryologist N. D. Nozhin (1841–1866), who had gone to Naples in 1863 to look for morphological links between invertebrates and vertebrates and was a close friend of the embryologist V. O. Kovalevskii (1842–1883).48 Nozhin’s translation was “generally acclaimed as the first successful effort to combine meticulous embryological research with Darwinian transformism … [and] marked the first scientific effort to base embryological research on Darwin’s theoretical principles.”49 Darwin’s Origin had just appeared in a well-received Russian translation, and Für Darwin fell on fertile soil because of the strong Russian tradition of embryology, from von Baer through Kovalevskii and Élie Mechnikov (1845–1916). Nozhin’s translation had no input from Müller, who was apparently unaware of it, and the same was true of a French translation that appeared in 1882–83.50
Favorable English reviews appeared in The Academy and Scientific Opinion and in The Annals and Magazine of Natural History.51 The last reviewer confessed that Für Darwin, “although highly appreciated in Germany, has made but little progress in this country,” and hoped that the translation (p.68)
(p.69) would make “this admirable little treatise” better known to English naturalists, although the reader did need “Darwinian proclivities” to benefit from reading it. The Athenaeum’s reviewer, John Leifchild (1780–1862), however, was highly critical.52 The value of the book’s facts could be judged only by specialists in the Crustacea, Leifchild declared, especially those applied to the evolution of Crustacea. Müller’s hope, at the end of the book, that he had “succeeded in convincing unprejudiced readers that Darwin’s theory furnishes the key of intelligibility for the developmental history [evolution] of the Crustacea”53 was for Leifchild a conclusion from comparatively few facts, and once again only specialists could judge that. The reviewer showed that his premises were different from Müller’s when he cited Müller’s explanation of the convergent evolution of air breathing in terrestrial crabs and said that any “unprejudiced reader” would be convinced of “special provision” (special creation) rather than highly improbable developmental evolution.54 Leifchild’s sharpest criticism, however, was of Müller’s “sneers” against Louis Agassiz. “It is to be feared that Dr. Müller’s moral code is rather Crustacean than Christian. Nor does it enforce his argument to say that ‘to read [Agassiz’s] remarkable writing we need the spectacles of Faith, which seldom suit eyes accustomed to the microscope.’”55
Müller received the reviews from Darwin on 11 June 1869 and wrote to his brother Hermann the next day that The Athenaeum’s reviewer “took great offense at my unchristian expressions as unworthy of a ‘scientific writer.’ The witticism [about his moral code] is not so bad and amused me greatly.”56 Müller’s “unchristian expressions” come as no surprise from a man who had declared himself an atheist fifteen years before he encountered the Origin. The reviewer, along with nearly all his contemporaries, including most of those who accepted Darwin’s basic message, carried at least a remnant of religious faith, which made it difficult for them to read Darwin dispassionately. Müller was a conspicuous exception.
Aside from the reviewers, two of Darwin’s important correspondents commented on the book. Alfred Russel Wallace received a copy from Darwin and wrote that he had read it “with great pleasure,” but whether because he was so ignorant of aquatic animals or had expected too much, he thought that Müller had not “put either his facts or his arguments” as well as Darwin would have done. “If no one else makes a similar observation I must impute it to my own dulness in matters of minute anatomy & embryology.”57
(p.70) Joseph Dalton Hooker (1817–1911, director, Royal Botanic Gardens, Kew, 1865–85, and Darwin’s principal correspondent) found the first ninety-six pages “very dull, not remembering enough of Crustacea, but Chapters X on Principles of Classification & XI on Evolution strike me as being remarkably good in matter,” if not in manner. Hooker added that he had often had the same thoughts about embryogenic resemblance.58 The crustacean details that fascinated Darwin as an authority on barnacles were clearly an impediment to both Wallace and Hooker, but Müller’s train of thought is sometimes hard to follow.
Facts and Arguments for Darwin
Facts and Arguments is a faithful translation of Für Darwin, with the exception of two major additions and a few small ones, and for the convenience of English-speaking readers I have therefore used it in considering the content and import of the book.
Müller’s intent was to establish a genealogical tree of a specific group of animals as a test of Darwin’s theories. He saw three possible results: first, it might lead to “irreconcilable and contradictory conclusions”; second, it might be “successful to a greater or less extent”; or third, it might be frustrated “without settling the question either way,” but even then it would be worthwhile. He chose Crustacea for three reasons: first, nearly all of the class was available around Desterro; second, only in the Crustacea, especially in the parasitic ones where the mode of life was understood as arrested development, had the transformation of species been spoken of as a matter of course;59 and third, the taxonomy of the Crustacea was “satisfactorily established,” unlike that of the annelids and acalephs, with which Müller was also acquainted.60 Some reviewers of the book thought, with some reason, that it was primarily about Crustacea as such and would have to be judged by specialists in that field, but there is no question that Müller’s intentions were far broader and that the Crustacea were simply the means to an end.
Müller doubted at first that he had time or strength, not to mention the relevant literature in his isolated place of residence, to complete the job, but its practicability grew as “fresh observations daily” made him “more favourably inclined towards the Darwinian theory.” He was, he said, “chiefly influenced” by Darwin’s statement that “Whoever is led to believe that species are mutable, will do a good service by conscientiously expressing his conviction.”61 Finally, he thanked Darwin for “instructions and suggestions” (p.71) and said that he was throwing “this sand-grain with confidence into the scale against ‘the load of prejudice by which this subject is overwhelmed,’ without troubling [himself] as to whether the priests of orthodox science will reckon [him] amongst the dreamers and children in knowledge of the laws of nature.”62 Darwin’s emotions can hardly be imagined when he received the book unheralded in 1864 and these pages were read to him by a translator.
Having recognized that the consequences proceeding from a “false supposition” will eventually lead to “absurdities and palpable contradictions,” Müller said that he went through a “period of tormenting doubt,” searching for such contradictions. He found none, however, nor was he aware of “any necessary consequences of Darwin’s hypotheses” having been shown to be contradicted by anyone else. If the “most profound students of the animal kingdom are amongst Darwin’s opponents,” he wrote, they should easily have “crush[ed] him long since beneath a mass of absurd and contradictory inferences,” but they had not, and that alone was in Darwin’s favor.
Darwin’s opponents had made a major criticism of the absence of intermediates between species in the fossil record, a problem that Darwin explained by the improbability of preservation, as well as by natural selection, but Müller thought the criticism of little significance, drawing an analogy with the difficulty that one has tracing development through “larvae fished from the sea” while vainly seeking “transitional forms” which he knew “to be swarming around him in thousands.”63
Müller offered two examples of potential contradictions growing out of Darwin’s hypotheses and then showed that they actually supported Darwin. The first, of which Müller gave further details later in the book, concerned species of marine crabs that “remain for a long time out of water” and have special adaptations for opening their gill cavities to air. Müller found in allied families two terrestrial crabs that were very similar in “all the essential conditions of their structure,” except the adaptations for air breathing, and because the families must have separated at a “much earlier period than the habit of leaving the water,” the adaptations could not have been inherited from a common ancestor, according to Darwin’s genealogical view. Since the adaptations are functionally, but not structurally, similar in the two species, they must be of independent origin, what we now call convergent. It would be an embarrassment for Darwin’s hypothesis if the adaptations in those unrelated crabs were just the same, that is to say inherited from a common ancestor (figure 3.3).64 (p.72)
For his second case, Müller used a real example to test Darwin’s theoretical view of classification. Darwin proposed that “[if] the natural system is founded on descent with modification,” characters showing affinity between species are those that had been inherited from a common ancestor, and “all true classification is genealogical.”65 Müller applied that principle of ancestor-descendant relationships to a genus of crustaceans in order to unravel its evolutionary history, and he showed how Darwin’s approach could clear up an ambiguity in the distribution of certain characters among its species.
Müller knew five species of the amphipod genus Melita (exilii, etc.) in which one of the second pair of feet had “a small hand of usual structure, and the other an enormous clasp-forceps” which was lacking in other species of Melita (palmata, etc.) (figure 3.4). The clasp-forceps was so unusual (p.73)
among amphipods, and so accordant in the five species, that Müller was sure that they had a common ancestor distinct from that of the rest of the genus. But one of the five species, M. fresnelii, differed from the other four, which lack a “secondary flagellum” on the anterior antennae; however, the presence of the secondary flagellum was considered a generic character and would have placed M. fresnelii in a new genus.
To sort out the relationships, Müller constructed two genealogical diagrams. One of them (figure 3.5a) has the clasp-forceps evolving once, as an innovation in the common ancestor of M. fresnelii and the other four possessing it. The alternative (figure 3.5b) has the clasp-forceps evolving twice, first in M. fresnelii at the same time that it acquired the secondary flagellum, and again at the point where the other four species with a clasp-forceps split from the rest of the genus without it. Müller argued that the first alternative was preferable because the clasp-forceps is “so unusual among Amphipods” that the species having it must have descended from “common ancestors belonging to them alone among known species,” and because the secondary flagellum occurs haphazardly among amphipods and is sometimes present early in life and lost at maturity.66 Müller based his preference in part on the principle of maximum parsimony, choosing the pattern that requires the fewest steps. (p.74)
Müller was in essence recognizing the value in classification of what are now called shared derived characters, or synapomorphs.67 Even with Darwin’s genealogical approach, however, classification was at the time based on similarities among taxa, and the use of shared derived characters for determining phylogenetic relationships among living species was Müller’s innovation.68 Müller used the occurrence of a secondary flagellum in many other amphipod genera as an “outgroups comparison,” one of the ways of resolving cladistic ambiguities.69 His branch diagram is of the type now known as a cladogram, in which organisms are clustered in clades based on the branching order (cladogenetic splitting of lineages),70 but the method was fully developed only in the mid-twentieth century, and Müller’s discussion is fragmentary by modern standards.71
Müller next turned to the predictive power of Darwin’s theory. It would be a “positive triumph,” he wrote, “if far-reaching conclusions founded upon it should subsequently be confirmed by [previously unsuspected] facts,” and he presented two such facts of his own that confirmed those “far-reaching (p.75) conclusions.”72 The lower crustaceans were known to hatch as a simple nauplius larva, a form not at that time known in higher Crustacea, but Müller hypothesized from his studies of crustacean development that if “higher and lower Crustacea were at all derivable from common [ancestors], the former also must once have passed through Nauplius-like [stages].” That prediction was fulfilled on 13 December 1861, after he read Bronn’s translation, when he discovered the nauplius larva of a prawn, Penaeus, one of those higher crustaceans. His paper on the subject was completed in March 1862 and published the following year.73 In 1864 he wrote: “I must admit that this discovery gave me the first decided turn in Darwin’s favour.”74
Müller also hypothesized that because stalk-eyed Crustacea (crabs, prawns, etc.) and sessile-eyed Crustacea (amphipods and isopods) share some important characteristics, those characteristics must be inherited from “the same ancestors.” And since most of the living stalk-eyed Crustacea pass through zoea-like stages, the same must also have been true of the immediate ancestors of the Amphipoda and Isopoda, or at least of their common ancestry with stalk-eyed Crustacea. That assumption was “very hazardous,” however, because no one had found a zoea in amphipods or isopods, but Müller had recently learned of a species of the amphipod genus Tanais that had “many of the most essential peculiarities of the Zoëae,” which he thought evidence enough to confirm his hypothesis. “For these discoveries, … science is less indebted to a happy chance than immediately to Darwin’s theory.”75
Darwin believed that although species arise through selection among many small variations, they “come to be tolerably well-defined objects, and do not at any one period present an inextricable chaos of varying and intermediate links,” because natural selection, among other things, “constantly tends … to exterminate the parent-forms and the intermediate links.”76 Bronn’s “first and most material objection against the new theory” in the critical chapter that he added to his translation of the Origin was that Darwin had not explained how tiny variations “[accumulating] in various directions could produce varieties and species which stand out from the primary form [and from one another] clearly and sharply.”77 Bronn “would gladly discover some external or internal principle which should compel the variations of each species to advance in one direction, instead of permitting them in all directions, to explain why many species do not coalesce by means of intermediate forms.”78
(p.76) Darwin lacked a specific case of the process that he theorized, and Müller stepped in with an analogical example in Tanais dubius, the same amphipod that had already served in support of Darwin. He discovered that it had two male forms, one with “powerful, long-fingered, and very mobile chelae [pincers]” and a modest number of olfactory filaments, the other with “short thick chelae” but “a far greater number of olfactory filaments.” Müller saw this double dimorphism as two alternative adaptive states, “claspers” that grasped the females more firmly, and “smellers” that detected them more easily, with intermediates being eliminated by natural selection. In answer to Bronn’s objection Müller wrote that “only a few directions stand open in which the variations are at the same time improvements, and in which therefore they can accumulate and become fixed; whilst in all others, being either indifferent or injurious, they will go as likely as they come.”79 With some prescience, he recognized the “randomness” of variation and how natural selection would trim it. Bronn missed the point that natural selection was Darwin’s “external principle.” Müller took a swipe at “those who regard the ‘plan of creation’ as the ‘free conception of an Almighty intellect,’” to whom the two kinds of males will “appear to be a mere caprice of the Creator, … inexplicable either from the point of view of practical adaptation, or from the ‘typical plan of structure.’” For Müller, of course, Darwin’s theory gave nature “meaning and significance.”80
In chapter 5, Müller gave full details of aerial respiration in terrestrial crabs and contrasted the “old school” explanation with a Darwinian one. Why do most members of different crab families have so many similarities, and yet the terrestrial species have different special adaptations? The “old school” could answer only from the “theologico-teleological standpoint, from which the mode of production of an arrangement is supposed to be explained if its ‘adaptation’ to the animal can be demonstrated.” Müller saw no need to refute that theory of adaptation, because nothing essential had been “added to the many admirable remarks … upon this subject since the time of Spinoza,” but he credited Darwinian theory with having “deprived those considerations of usefulness [and] of their mystical supremacy.”81 With Darwin, the special adaptations would have arisen independently in each family by natural selection.
In the following four chapters, Müller gave results from his own studies of development in all the major groups of Crustacea and discussed aspects of early development that showed the imprint of natural selection, including differences in metamorphosis in marine and aquatic species, (p.77) and the retrograde metamorphosis of parasitic isopods, amphipods, and barnacles.82 These contributed a body of examples that he enlisted in the last two chapters of the book on phylogenetic reconstruction and evolution.
By the turn of the nineteenth century, embryology had revealed similarities between the embryos of higher animals and the adults of lower ones that called for an explanation. In 1811 Johann Friedrich Meckel (1781–1833) proposed that there were “parallels between the embryonic stages of higher animals and the adults of lower animals.” Using examples of human organs that he thought had developed through stages like those of adult crustaceans, fishes, and so forth, Meckel concluded that “the analogy between the human embryo and lower animals is unmistakable.”83 He later wrote that during the development of a higher animal, it “essentially passes through the permanent organic stages [that is adult stages] that lie below it,” a statement of recapitulation.84 The observations of Karl Ernst von Baer (1792–1876) between 1816 and 1828, however, led him to different conclusions about parallelism and recapitulation. In his “History of Development” of 1828 von Baer proposed his own law concerning the “relationship between the development of a single embryo and its taxonomic position.”85 The third part of the law refuted parallelism: “Each embryo of a definitive animal form, instead of passing through the other definitive forms, departs increasingly from them.” And in the fourth part he refuted recapitulation: “Fundamentally, therefore, the embryo of a higher animal is never equivalent to another animal form, but only to its embryo.”86
Darwin was also impressed by similarities in embryos, but with his evolutionary theories he interpreted them as showing genealogical relationship, not merely taxonomic position, as the pre-Darwinists had done. To them, taxonomy had nothing to do with history but was merely a useful ordering of organisms. “Community in embryonic structure reveals community of descent,” Darwin wrote in the Origin, and thinking of barnacles, whose correct placement among the Crustacea was a triumph of that approach, he added: “It will reveal this community of descent, however much the structure of the adults may have been modified and obscured.” He overlooked, or perhaps merely ignored, von Baer when he wrote that “ancient and extinct forms of life should resemble the embryos of their descendants—our existing species.” He believed, however, that this would be demonstrable only in those cases where the ancestral form has not been obliterated, either by the evolution of modified development of early stages, (p.78) or by the loss of early steps.87 He recognized, for example, that early developmental stages in insects were less informative for revealing genealogy because they had often “been adapted for special lines of life” and might therefore be very different in different groups.88 In these restrictions Darwin anticipated the argument in Müller’s last two chapters. Von Baer was not an evolutionist; Darwin and Müller were, and they realized that life histories (individual development) could evolve.
Darwin favored a genealogical basis for a natural system of classification and thought that embryonic structure would be better for that purpose than adult structure. He based that conclusion in part on his observations of varieties of pigeons. The characteristic differences among breeds were in some cases so great that the breeds would “be ranked in distinct genera, had they been natural productions.” Yet in nearly every breed those characteristic differences were hardly detectable in the young.89 Breeders had selected for the adult characters alone, and he reasoned that if natural selection were modifying several new species “in accordance with their diverse habits,” it would act principally on adult variation, and that the young of the new species would “tend to resemble each other much more closely than do the adults.”90
Müller saw a problem for Crustacea in that last statement: “Perhaps someone else [could], even without Darwin, find the guiding clue through the confusion of developmental forms, now so totally different in the nearest allies, now so surprisingly similar in members of the most distant groups.” There was no “general plan,” he wrote, “or typical mode of development of the Crustacea,” in any of its taxonomic divisions, and he recalled a few examples, including the presence and absence of complex metamorphosis and differences in limb development in related genera.91 He could not yet account for all of that “confusion” himself, but natural selection for special adaptations was clearly part of the story. Darwin’s comment about pigeon breeders selecting for adult characters alone could easily be turned around in cases where (natural) selection was working on immature characters. That appears to be the direction in which Müller was turning.
In chapter 10, before turning to development and what meaning it might have for classification and evolution, Müller launched an extended philippic against Agassiz’s views of classification, taking statements from Agassiz and Gould’s Principles of Zoology and pointing out ambiguities in Agassiz’s doctrinaire treatment.92 Two examples follow.
(p.79) (§ 318) “The organs of the body are formed in the sequence of their organic importance; the most central always appear first.” No, says Müller, that proposition is “a priori indemonstrable, since it is impossible either in general, for any particular animal, to establish a sequence of importance amongst equally indispensable parts.”93
(§ 321) “The peculiarities which appear earlier should be considered of higher value than those which appear subsequently.” If those peculiarities are given precedence, Müller responded, then where the structure of the adult leads to one taxonomic placement and that of the larva to another, the latter must decide the point. Giving a few examples, Müller thought that even the “most zealous embryomaniac would probably shrink from this course.”94
Müller closed chapter 10 with a partial and absurd classification of the Crustacea based upon Agassiz’s last quoted statement, and in a passage the irony of which Bate missed in his review of Für Darwin, he hailed how “brilliantly the naturalness of such an arrangement [forces] itself upon us.” Müller turned a line from Agassiz’s criticism of Darwin back on its author: “No theory, however plausible it may be, can be admitted in science, unless it is supported by facts.”95 For Müller, it was Agassiz’s interpretations rather than his facts that were the problem.
Müller now turned to the “more congenial” look at development from a Darwinian view. The contents of chapter 11, “The Progress of Evolution,” are probably the best-known part of the book. Darwin spoke of “descent with modification” in chapter 13 of the Origin, “Mutual Affinities of Organic Beings,” but Müller thought that Darwin’s conclusions needed further treatment. The question was how “modification” proceeded. He asked how and when developing animals might progress as they deviated from their parents in the production of new species and other categories. Some changes might be expressed earlier in life, some later, and some only in adulthood, and he formulated a general statement on alternative modes of progress: “Descendants … reach a new goal, either  by deviating sooner or later whilst still on the way towards the form of their parents, or  by [continuing] without deviation, but then … advanc[ing] still farther.”96 The first mode will predominate where the descendants of common ancestors form a more or less homogeneous group, as in amphipods, crabs, or birds. Here the development of “descendants can only agree with that of their ancestors up to [the] point at which their courses separate, … [and adult structure] (p.80) will teach us nothing.” The second mode would be evinced where adults of some animals have characters resembling the early developmental stages of other animals. In this case, the developmental stages of the ancestors are also traversed by their descendants, and “so far as [speciation] depends on this mode,” the evolutionary history of the species will be mirrored in its development. The rapidly passing changes in the embryo and the larvae will provide a “more or less complete and more or less true picture” of the evolutionary transformations by which a species has “struggled up to its present state.”97
Müller was speaking of recapitulation, but because the “historical record preserved in development is effaced if metamorphosis is simplified and is falsified by natural selection for adaptations in free-living larvae,” he warned against placing too much confidence in it for evolutionary reconstructions.98 His skepticism was apparent when he mentioned only two examples of this second mode of progress. One was a general example, the gradual increase in the segment number of annelids during individual development, to which he added a caveat restricting such “supplementary progress” to the “addition of new segments and limbs.”
The other was a specific example in a tube-dwelling annelid that he had observed three years before but only now appreciated (figure 3.6). The tube dweller was living on the wall of one of his glass vessels and had at first three pairs of laterally barbed branchial filaments, as were found in a recognizable genus. In the course of a few days one of the filaments was transformed into an opercular peduncle, still barbate, suggesting a different genus, and a few days later the opercular peduncle had lost its lateral barbs and become an operculum that could close up the tube against intruders and made the animal recognizable as a member of still a third genus, Serpula, to which it was assigned. Müller asked of Agassiz’s school what purpose there would be in having the lateral filaments of the opercular peduncle “sprout forth merely for the sake of an invariable plan of structure, when they must be immediately retracted again as superfluous.” It would, he suggested, be “evidence rather of childish trifling or dictatorial pedantry, than of infinite wisdom.”99 That last phrase had undergone a transformation, from a touch of mysticism in 1844 (Fritz’s letter to his brother Hermann) to sheer mockery.
(p.82) insects—beetles, Lepidoptera, and others—was an acquired metamorphosis, unlike the inherited metamorphosis of prawns. There had been “[adult] insects before larvae and pupae,” he said, but nauplii and zoeae “far earlier than [adult] Prawns.”
In his last chapter, Müller applied the principles just enunciated to evolution in the Crustacea. He held up the prawn Penaeus as the “animal, which [among] the higher Crustacea … furnishes the truest and most complete indications of its primitive history.” Most complete because of the long series of immature stages, and truest because those stages are ecologically more alike than in most other stalk-eyed Crustacea, and because their peculiar zoëal characters seem not to be “late-acquired advantages” transferred back to an earlier stage, nor to be special adaptations acquired by natural selection.100 He concluded that the ancestor of all Malacostraca (the higher Crustacea, including crabs, lobsters, prawns, amphipods, and isopods) must “once have passed through a similar development,” although in most of them the original course “starting from the Nauplius, [had been] nearly effaced.”
The transfer of characters back to an earlier stage, as envisioned by Müller, would be the result of the loss of at least parts of a complex metamorphosis by natural selection. For example, if several stages of development are compressed, the result is that several new pairs of limbs sprout simultaneously. His examples of “special adaptations” included the enormously long spines on the carapace of the zoeae of crabs and Porcellanae, which make it difficult for a predator to eat them; and the chelae on the feet of the young Brachyscelus, a pelagic amphipod living on jellyfish, which make it easier for the amphipod to get a grip in rough water.101
Müller assumed that it was when crabs, isopods, and amphipods arose from a common ancestor that their development became simplified, and he credited Darwin’s theory with making this intelligible. When circumstances favoring a group of animals cause it to spread and diverge “into forms adapting themselves to new and various conditions of existence,” that greater variability will favor simplified development, “which is almost always advantageous; moreover, exactly at this period, during adaptation to new circumstances.” Müller was thinking, for example, of prawns or crabs as they evolved into fresh water. That process would be more difficult if they retained the long series of larval forms of their marine ancestors.102
Müller was sure that he could construct a general picture of the “primitive (p.83) history” of the Crustacea, even lacking many facts, but “those who were not familiar with the class” would accept it naively, while those who knew Crustacea well would recognize a few uncertainties and conclude that even the well-founded facts were merely “floating in air.” It would require “lengthy, dry explanations” to avoid those misconceptions, and he was thus deterred.103
Müller’s explanation of how the parasitic Rhizocephala, the “most abnormal” group of Crustacea, might have evolved from barnacles, which missed inclusion in Für Darwin, is perhaps the most lucid passage in the book.104 Barnacles attach themselves to their (usually solid) substrates by means of cement-ducts, while the Rhizocephala (“root-headed cirripedes”) penetrate the skin of their fleshy hosts with a set of “roots” that proliferate through the host’s body and rob it of nutrients. The points of attachment of these structures are located in both groups at the site of the larval antennae that they replace at maturity, and for that reason Müller recognized them as homologous. For natural selection to convert cement-ducts to absorptive “roots,” he proposed that some ancestral barnacle had “once upon a time selected the soft ventral surface of a Crab … for its dwelling place” and that its cement-ducts might have penetrated through the soft skin, giving it a more secure hold when the host molted. The cement-ducts would now be bathed in the body fluids of the host, and an “interchange must necessarily have been set up between the materials dissolved in these fluids and the contents of the cement-ducts,” an interchange that would nourish the (evolving) parasite. The cement-ducts could now be modified by natural selection into efficient (absorptive) organs, and the many characteristic features of the barnacle, including its feeding structures, would be lost.
Darwin had studied living and fossil barnacles intensively between 1846 and 1854, had published monographs on their taxonomy and evolutionary biology, and was the world’s authority on the group. In Darwin’s monograph on living barnacles Müller recognized what appeared to be the sort of intermediate he had in mind: Anelasma, a pedunculated barnacle “which lives upon Sharks … [and seems] to be in the best way to lose its [feeding] cirri and buccal [mouth] organs.” The thick peduncle “is immersed in the skin of the Shark [and] is beset with much-ramified, hollow filaments, which ‘penetrate the Shark’s flesh like roots.’”105
Darwin was delighted by Müller’s story: “I have re-read many parts [of your book], especially that on cirripedes, with the liveliest interest. I had (p.84) almost forgotten your discussion on the retrograde development of the Rhizocephala. What an admirable illustration it affords of my whole doctrine! A man must indeed be a bigot in favour of separate acts of creation if he is not staggered after reading your essay; but I fear that it is too deep for English readers, except a select few.”106 Both editions of Müller’s book were considered difficult reading, but Darwin was unique among those “select few” in appreciating it as Müller intended it to be. Müller closed the book by asking that his deficiencies not be laid to Darwin’s plan but to the “clumsiness of the workman who did not know how to find the proper place for every portion of his material.”107
Controversy over Müller’s Penaeus Nauplius
Müller’s discovery in 1863 of a nauplius larva that he claimed to belong to the prawn genus Penaeus was criticized because the nauplius was collected swimming freely in the sea, and Müller could not get the eggs of Penaeus to hatch in captivity and therefore could not be certain of his identification. The identity of that nauplius was a central truth in his book, and, sensitive to critics whose numbers had grown, Müller reinforced his reasoning in a paper written in 1877 and published the following year. “Spence Bate expressed to me doubts on the connection of the young forms described by me. Properly, one should never refer larvae to definite grown-up animals unless obtained from the parent,” and Müller admitted that his nauplius “might possibly not be the larva of Penaeus at all.” But he was convinced that it was and went over the old ground again, asking whose larva it could be if not that of Penaeus.108
Within a month Bate himself published a long critique of Müller’s claim of fifteen years before. “Fritz Müller’s high reputation as an accurate observer and philosophic naturalist induced carcinologists to accept his statement,” although Bate thought at the time that Müller should identify a few more links in the developmental chain to validate his claim. Bate reviewed recently published papers on crustacean development and concluded that Müller’s nauplius might be the larva of some other crustacean, but that there is “every reason to believe that it is not the young of any known prawn, and there is no evidence to determine its relation to Penaeus.”109 In a letter to the editor of the Annals, October 1878, Bate clarified his last remark: “As far as our knowledge extends, it has not been proved to be so.”110 And so (p.85) matters stood until 1900, when “the final proof that the Peneid egg gives rise to a Nauplius was only given” by Francesco Monticelli and Salvatore Lo Bianco.111 Methods of rearing larvae from the egg were developed in the twentieth century, and larval stages could then be linked directly with their adults. For example, a paper published in 1961 illustrates all of the many larval forms of a Penaeus species and vindicates Müller’s conjectures of 1863. Ironically, the author could not even then obtain all of the post-naupliar stages from eggs and had to rely in part on fishing larvae from the sea.112
Other Responses to the Origin
Müller’s response to the Origin has been treated in detail because it is nearly unique in its originality, and because it was for a long time not easy to find and is difficult to read. No other naturalist questioned and tested Darwin’s theories so thoroughly and came to accept them on his own evidence. But others also answered Darwin’s plea in the Origin that “anyone who is inclined to the view that species are variable will through conscientious confession do his scientific convictions a service.” Only Hermann Müller came close to emulating his brother. After studying mosses, he responded with a paper on detailed studies of “difficult” moss species, “Facts from Bryology in Favor of Darwin.”113
Haeckel’s response appeared in his monograph of the Radiolaria in 1862. He described the morphology and biology of a large number of species in this group of single-celled marine protozoans (protists) and proposed a “natural system” of classification based on the characteristics of their siliceous skeletons. He had found “numerous intermediate forms” that connected natural groups and “sometimes [made] their systematic separation difficult,” and he thought them particularly noteworthy because they might contribute to understanding the “gradual development of organic beings from a common ancestral form.”114 He answered Darwin’s plea by announcing his conviction, based on the Radiolaria, that “species are variable and that all organisms have a real genealogical relationship.”
But Haeckel took the cautious view that there might be “more error in Darwin than truth,” and he had “reservations about sharing every trend in Darwin’s views and hypotheses.” He suggested that in addition to “such indisputably important principles as natural selection, the struggle for existence, the relations of organisms to each other, the divergence of characters,” (p.86) and others, there may be “many other equally important [but unknown] principles, similarly or even more causally active,” although he had none in mind. He also criticized Darwin for failing to offer a “clue about the emergence of the original organism.”115 These remarks are in a footnote to a passage about the numerous transitional forms between radiolarian families that could help in working out their common ancestry, and Haeckel also had evidence of transitions between species and clearly wanted to add what he could to support Darwin’s “brilliant theories.” But with the admitted limitations of his material (vast though it seems) he had rather little to say and did not put Darwin’s theories to any test.116 His assessment of Darwin was respectful, but it was empty of the facts, arguments, and testing that made Für Darwin such a masterful response to the Origin. In brief, Haeckel did little more than call on Müller’s book in support of Darwin.
August Weismann (1834–1914), evolutionist and developer of the germ plasm theory, went a step further in his lecture on the correctness of the Darwinian theory in 1868. He had the hindsight of nine years during which all of Darwin’s points had been subjected to views pro and con, but he used few of his own observations as a test and endorsed Müller and Für Darwin as his principal source of support for Darwin’s “transmutation hypothesis” against special creation. The hypothesis, he said, had stimulated renewed attention to development, and the success of that approach was “best shown by F. Müller’s excellent investigations in Crustacea … in which this outstanding researcher succeeded in tracing the developmental history [evolution] of that class.”117 Otherwise, even Weismann largely rehearsed Darwin’s own arguments.
(2) FM to Oscar Schmidt, 31 July 1862, Letters, 38.
(3) Information from Dr. Harald Lorenzen; and in FM to MS, 27 July 1860, Letters, 20, where FM tells of receiving “last year’s shipment of books” in March.
(6) FM to MS, 17 December 1861, Letters, 30.
(8) FM to MS, 1 August 1861, Letters, 25.
(12) FM to MS, 13 June 1862, Letters, 35.
(14) FM to HM, 16 December 1862, Letters, 40–41.
(17) FM to MS, 5 February 1864, Lorenzen and Höxtermann 2004, 109–10.
(p.263) (18.) FM to MS, 21 May 1864, paraphrased in Lorenzen and Höxtermann 2004, 111.
(20) FM to MS, 10 July 1864, Lorenzen and Höxtermann 2004, 111.
(22) FM to MS, 12 December 1865, Lorenzen and Höxtermann 2004, 111–12.
(26) CD to FM, 10 August 1865, CCD, 13:212–13.
(27) Müller annotated forty-three of Darwin’s fifty-eight letters with date of receipt and/or response.
(28) FM to MS, 13 March 1864, Letters, 51–52.
(30) CD to FM, 20 September 1865, CCD, 13:234.
(31) FM to CD, 5 November 1865, CCD, 13:295, 472, re-translated by the author.
(40) CD to FM, 16 March 1868, CCD, 16:271.
(45) W. S. Dallas to CD, 27 November 1868, CCD, 16:865.
(47) In 1882, at Darwin’s death, 247 copies were left, and in 1889 there were still 29 on hand. Ledgers, John Murray, Ltd.
(56) FM to HM, 12 June 1869, Letters, 157–58.
(57) ARW to CD, 24 March 1869, CCD, 17:153–54.
(58) Hooker to CD, 17 July 1869, CCD, 17:319.
(104) This addition and five smaller ones are in FM to CD, 22 April 1868, CCD, 16:436–39 and 933–35 (translation).
(106) CD to FM, 18 March 1869, CCD, 17:138.