what unexpected observation helped lead darwin to develop the theory of natural selection?

Abstruse

Charles Darwin's contemporaries repeatedly criticized him for advocating a theory that was unsupported by the geological record. It is well known that Darwin responded to these criticisms by attributing the absence of transitional forms to the fact that the geological record is incomplete. Often overlooked, nonetheless, is the fact that he seemed to agree out some hope that fossilized transitional forms might eventually be uncovered. Several transitional form candidates including the famous London Archaeopteryx were discovered after the Origin was first published, only these specimens are scarcely mentioned in subsequent editions of the text. I examine Darwin's treatment of these animals and suggest that his views on biological classification likely prevented him from citing their existence every bit prove in favor of his theory. I conclude by attempting to demonstrate that Darwin desired ane very specific type of paleontological evidence: a graded succession of forms.

One of the most serious impediments to the acceptance of the evolutionary theory Darwin developed in the Origin of Species was the failure of the geological record to evidence to the being of the many transitional forms predicted past his business relationship. Darwin was well aware of this difficulty and attempted to preempt his critics past issuing a series of pessimistic arguments that were intended to demonstrate that the fossil record is necessarily incomplete. He famously claimed, for case, that the geological record is "a history of the world imperfectly kept" of which we "possess the last volume lone, relating only to ii or three countries" (Darwin 1959, p. 519). Although it is well known that Darwin relied on this type of reasoning to defend his theory (see, e.g., Herbert and Norman 2009, pp. 142–143), his optimistic remarks about the geological tape have often been overlooked. These assertions are best described as a series of subtle hints that the theory might somewhen be supported past fossil prove. In the inaugural edition of the Origin—published as On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life—Darwin was careful to point out that nosotros "continually forget how big the world is, compared with the area over which our geological formations have been carefully examined" (Darwin 1959, p. 507). Two years later, in the 3rd edition, he added that "in all cases positive paleontological bear witness may be implicitly trusted," but "negative evidence is worthless" (Darwin 1959, p. 507)

These remarks lend credence to the seemingly self-­evident truth that Darwin would have welcomed the discovery of transitional forms. Interestingly enough, before the Origin made it to press, a number of transitional-form candidates had been described, and in the intervening years betwixt the beginning and 6th editions, several celebrated specimens, including the London Archaeopteryx, were discovered. Surprisingly, however, these animals received little or no mention in the Origin. This largely unexplored historical fact will be the focal point of this commodity. My primary aim will be to demonstrate that Darwin's views on classification likely forced him to downplay the importance of the specimens in question. I conclude by suggesting that the optimistic remarks about the fossil record found in the Origin are all-time interpreted as expressions of hope that a graded succession of forms might eventually be unearthed.

Neglected fossil evidence

Today, Archaeopteryx lithographica is regarded as one of the paradigmatic instances of a transitional form (Houck et al. 1990, Schad 1993, Shipman 1998), but to many nineteenth-century researchers, this designation would have seemed inappropriate. Shortly after the London specimen (figure 1) was unearthed in 1861, the German language zoologist Andreas Wagner (1862) publicly denied the transitional status of the species sight unseen, because he feared that the Darwinians would "employ the new discovery as an exceedingly welcome occurrence for the justification of their strange views upon the transformations of animals" (p. 266). One year later, Richard Owen (1863) drew a similarly anti-Darwinian conclusion about the specimen, arguing that "the best-­adamant parts of its preserved structure declare it unequivocally to be a Bird, with rare peculiarities indicative of a distinct order in that class" (p. 46). Owen'due south interpretation glossed the reptilian features of Archaeopteryx, and every bit such, his description, like Wagner'south before, disallowed the ­possibility that the specimen might be used to ­support Darwin's theory. Although the truth of evolution was ­established long ago, it is interesting to note that there is withal some debate as to whether Archaeopteryx should be classified as a bird or a not-avian dinosaur (see, eastward.chiliad., Witmer 2011).

Effigy 1.

Delineation of the London Archaeopteryx from the Catalogue of the Fossil Birds in the British Museum (Natural History), 1891. It is described every bit "ane of the nearly valuable specimens in the Museum" (p. 362). Courtesy of the Biodiversity Heritage Library (www.biodiversitylibrary.org).

Effigy ane.

Delineation of the London Archaeopteryx from the Catalogue of the Fossil Birds in the British Museum (Natural History), 1891. Information technology is described as "i of the nearly valuable specimens in the Museum" (p. 362). Courtesy of the Biodiversity Heritage Library (world wide web.biodiversitylibrary.org).

The technical aspects of Owen's analysis were somewhen challenged by T. H. Huxley in an 1868 paper titled "Remarks upon Archaeopteryx lithographica." In this piece, Huxley highlighted a host of difficulties with Owen's description, many of which are attributable to the fact that he incorrectly inferred that the animal'southward ventral side was facing upwards. Speaking on Owen's error, Huxley (1868) remarked:

It is obviously impossible to compare the bones of one creature satisfactorily with those of another, unless it is clearly settled that such is the dorsal and such is the ventral aspect of a vertebra, and that such a bone of the limb-arches, or limbs, belongs to the left, and such some other to the correct side. Identical animals may seem quite different, if the bones of the same limbs are compared nether the impression that they vest to opposite sides; and very unlike bones may appear to be similar, if those of opposite sides are placed in juxtaposition. (p. 244)

Contrary to popular lore, Huxley did not immediately suggest that the existence of Archaeopteryx provided back up for Darwin'due south theory (Desmond 1984, 1997, Shipman 1998). His movement toward such a position was a gradual procedure that began with his famous "On the Animals which are About Nearly Intermediate betwixt Birds and Reptiles" (1868b). Huxley (1868b) began this newspaper past arguing that reptiles and birds might be more closely related than is commonly supposed, and and so presented a serial of data-driven arguments that led to the decision, "in by times, birds more like reptiles than whatsoever now living, and reptiles more like birds than any now living, actually did exist" (p. 364). Archaeopteryx, said Huxley, represents the almost reptile-similar bird, and the dinosaur Compsognathus longpipes can be understood as the nearly bird-like dinosaur (see Herbert and Norman 2009, pp. 150–152). The implication, of course, is that these species constitute two of the transitional forms that link the avian and reptilian lines.

Although Huxley'south assessment of the evidentiary value of Archaeopteryx was somewhat uncharacteristically tempered, other Darwinians such equally Hugh Falconer and Friedrich Rolle (1863) were quick to announce that the animal provided support for the idea that species are transmutable. Falconer'south (1863) excitement about Archaeopteryx was credible in a letter of the alphabet addressed to Darwin in which he announced, "Had the Solenhofen [sic] quarries been commissioned—past august command—to turn out a strange being à la Darwin—it could not accept executed the behest more than handsomely—than in the Archaeopteryx." In his answer to Falconer, Darwin (1863a) remarked, "I particularly wish to hear about the wondrous Bird; the case has delighted me, considering no grouping is and then isolated every bit Birds." As noted past Herbert (2005, p. 333), he would later echo these sentiments in a letter of the alphabet sent to James Dana (Darwin 1863b). Publicly, however, he rarely discussed the beast. Darwin mentioned Archaeopteryx in the 4th edition of the Origin, merely in doing so, he was careful to admit Owen'due south administrative description of the London specimen. The just original statement he made well-nigh the animal is that its beingness shows us "how niggling we as yet know of the former inhabitants of the world" (Darwin 1959, p. 509), and information technology goes without saying that this is hardly a assuming assertion virtually the specimen'southward evidentiary value.

By the time the 5th and sixth editions of the Origin were beingness prepared, Huxley had published his clarification of the London specimen, and Owen's assay was largely discredited. This being then, Darwin was all but handed a gift that could have been used to eternalize the brownie of his theory. Surprisingly, nevertheless, he chose to add together a single sentence about the beast to the last editions of his book:

Fifty-fifty the wide interval betwixt birds and reptiles has been shown by Professor Huxley to be partially bridged over in the most unexpected fashion, by, on the one hand, the ostrich and extinct Archaeopteryx, and on the other hand, the Compsognathus, one of the Dinosaurians—the group that includes the most gigantic of all terrestrial reptiles. (Darwin 1959, p. 540)

Darwin never explicitly stated that Archaeopteryx should be regarded as a transitional form, and what makes his timidity on this matter then perplexing is the fact that critics repeatedly criticized him for advocating a theory that was unsupported past paleontological show. Louis Agassiz (1860), for case, objected to his theory on the grounds that it was not supported by "a single fact to show that individuals change, in the course of fourth dimension, in such a manner every bit to produce at last species unlike from those known earlier" (p. 144). Similarly, Wagner (1862) declared that Darwin's views "must be at once rejected as fantastic dreams," because neither he nor whatever of his followers had supported the evolutionary hypothesis with fossil bear witness that showed "the intermediate steps by which the transition of some 1 living or extinct animal from one form into another was effected" (p. 267). Darwin could have chosen upon the pessimistic arguments mentioned earlier to blunt the strength of these criticisms, but information technology seems that a straightforward appeal to Archaeopteryx or one of the other transitional form candidates known at the time would accept done a not bad deal to advance the standing of his theory. Compsognathus received less attention than Archaeopteryx in the Origin, and other potential transitional forms such as the amphibian Archegosaurus and the dinosaur Hypsilophodon receive no mention whatsoever.

Darwin's neglect of Archegosaurus and Hypsilophodon is noteworthy because the published analyses of these animals were anything but anti-evolutionary. Richard Owen's (1859) description of Archegosaurus, for instance, almost begs for a Darwinian interpretation, because he stated that it is an "intermediate gradation" that "conducts the march of evolution from the fish proper to the labyrinthodont blazon" (p. 154). Similarly, Huxley (1870) wrote of Hypsilophodon that it "affords unequivocal evidences of a farther stride towards the bird" and adds that "if just the pubis and the ischium … had been discovered, they would have been unhesitatingly referred to Aves" (p. 28). The fact that these transitional-form candidates were non discussed in the Origin is important considering information technology reveals that Darwin's treatment of Archaeopteryx was not anomalous. His apparent lack of interest in the London specimen is role of a blueprint of interpretation that, I volition now suggest, is made comprehensible by carefully scrutinizing his views on biological classification.

Darwin on species and varieties

Understanding Darwin's views on species can be challenging. There is fairly widespread agreement (Ghiselin 1969, Beatty 1992, Ereshefsky 2010) that he rejected the reality of the species category, pregnant he regarded the Linnaean rank of species as a useful heuristic rather than a genuine division in the natural earth. However, some (eastward.m., Stamos 1996 and especially 2007) have argued that Darwin believed in the reality of species. Passages such equally the following excerpt from an 1856 letter to Joseph Hooker have been of particular importance in the debate on this issue:

Information technology is really laughable to see what different ideas are prominent in various naturalists minds, when they speak of "species" in some resemblance is everything & descent of little weight—in some resemblance seems to go for nil & Creation the reigning idea—in some descent the primal—in some sterility an unfailing test, with others not worth a farthing. It all comes, I believe, from trying to define the ­undefinable. (Darwin 1856)

Here, Darwin seems to be suggesting that the dream of determining the necessary and sufficient conditions for membership in the species category is unrealizable. Like remarks can exist found in the Origin, in which he suggested that nosotros should "treat species in the aforementioned manner as those naturalists treat genera, who admit that genera are merely bogus combinations made for the sake of convenience" (Darwin 1959, pp. 754–755). Regardless of whether he did or did not believe in the reality of the species category, remarks such equally these are important for our purposes because they provide valuable insight into his views almost the procedure of biological classification.

In the Origin, Darwin (1959) stated rather unambiguously that at that place are no key differences betwixt species and varieties, remarking, "I look at the term species, as one arbitrarily given for the sake of convenience to a ready of individuals closely resembling each other, and that it does not substantially differ from the term variety, which is given to less singled-out and more than fluctuating forms" (p. 136). Crucially, however, he did not believe that the process of determining whether a particular specimen should be ranked as a species or a variety is entirely haphazard:

[I]n those cases in which intermediate links have not been found betwixt doubtful forms, naturalists are compelled to come up to a determination by the amount of difference between them, judging by illustration whether or not the amount suffices to raise one or both to the rank of species. Hence the amount of difference is one very important criterion in settling whether two forms should be ranked as species or varieties. (Darwin 1959, pp. 140–141)

This passage is noteworthy because it reveals that Darwin believed the chore of classifying "hundred-to-one forms" is carried out by examining known gradations of form. That is, when we attempt to make up one's mind whether a specimen belongs to a particular species, the judgment we reach will likely be based on morphological comparisons with previously described members of the species. If the investigator believes that the unidentified specimen is sufficiently similar to known varieties, it will be classified every bit a member of the described species. If it is non deemed sufficiently like, it volition be treated as a new species. With this in mind, we can plow our attention back to transitional forms and, in particular, how Darwin would go about classifying them.

Darwin on transitional forms

Casually speaking, a transitional or intermediate course is a species in which a quantitative trait of interest represents an approximate morphological median betwixt two other species. If a hypothetical transitional form is labeled B, nosotros should await it to exist similar to a species A in some regards, only in other ways, information technology should resemble a species C. This is the way transitional forms were understood during the nineteenth century and, arguably, the same way we tend to conceive them today. The fact that transitional forms should have been coveted by Darwin appears obvious, in role considering he held that evolutionary alter is a very gradual process. As Eldredge (2015) has recently discussed, Darwin believed that species initially grade continuous populations. Over time, however, populations begin to diverge geographically, leading to subpopulations being subjected to different selective pressures. The resulting adaptations initially produce "well-marked varieties" and, eventually, new species. Specialist forms with extreme morphologies are favored over generalist ancestral types, and as fourth dimension passes, the latter are eventually driven to extinction. If, as Darwin suggested, evolutionary modify occurs in this gradual manner rather than through rapid saltations, the gaps we find in the fossil tape must take formerly been filled by morphological intermediates. Because his detractors repeatedly criticized him for failing to produce evidence that transitional forms one time existed, why did he non attempt to rebut these charges by pointing to the existence of an animate being such as Archaeopteryx?

Ane reason is that it would be extremely difficult for someone with his views virtually nomenclature to declare that a particular specimen is transitional. Darwin's thoughts on intermediate forms are most fully developed in the following selection from the Origin, which deserves to be quoted at length:

It is earth-shaking to recall that naturalists accept no golden rule by which to distinguish species and varieties; they grant some little variability to each species, simply when they meet with a somewhat greater amount of deviation between any ii forms, they rank both equally species, unless they are enabled to connect them together by close intermediate gradations. And this from the reasons just assigned we tin seldom hope to issue in whatever one geological section. Supposing B and C to be two species, and a third, A, to be constitute in an underlying bed; even if A were strictly intermediate between B and C, information technology would simply be ranked equally a tertiary and distinct species, unless at the aforementioned time it could be most closely continued with either one or both forms by intermediate varieties. Nor should it be forgotten, as before explained, that A might be the actual progenitor of B and C, and yet might non at all necessarily be strictly intermediate between them in all points of structure. So that we might obtain the parent-species and its several modified descendants from the lower and upper beds of a formation, and unless we obtained numerous transitional gradations, we should not recognise their relationship, and should consequently be compelled to rank them all as distinct species. (Darwin 1959, p. 499)

Co-ordinate to Darwin, members of any given species vary in certain ways, but the amount of variation nosotros allow within a taxon is finite. Again, this means that if the organism under consideration cannot be connected to previously known varieties, it volition be classified as a new species.

If nosotros follow Darwin and take that there is no touchstone standard past which species can be distinguished from varieties, we are forced to maintain that most classifications are made in the style simply described. However, if systematic analyses are carried out exclusively past ways of comparing with known forms, the task of classifying paleontological specimens becomes very difficult. As Darwin mentioned in conjunction with his pessimistic arguments about the geological record, fine gradations of form are non constitute because (a) fossilization is extremely rare and (b) phenomena such as erosion and the movement of the Earth's chaff are constantly destroying the geological strata. To sympathise why the incompleteness of the geological tape fabricated the identification of transitional forms hard for Darwin, it is necessary to annotation that he regarded well-marked varieties as incipient species. Speaking on this matter in the Origin, he plainly asserted that "the only distinction between species and well-marked varieties is, that the latter are known, or believed, to be connected at the nowadays day past intermediate gradations, whereas species were formerly thus connected" (Darwin 1959, p. 754).

Given this equivalence principle, the procedure of determining whether a item fossil should be classified as a transitional form is like to the procedure past which a specimen is judged to exist a species or a multifariousness. According to Darwin, the transitional designation can only exist conferred upon a specimen if information technology can be connected to two seemingly singled-out lineages by means of a series of intermediate varieties. Given that such varieties are non preserved in the fossil record, he was unable to course Archaeopteryx, Compsognathus, or whatsoever of the other fossils that have been discussed as transitional forms. Just as an unidentified specimen of contemporary origin that cannot be linked to known varieties will be classified as a new species, a paleontological specimen that cannot be linked to purported ancestral and descendent types by means of varieties will exist treated equally a new monotypic lineage. Therefore, information technology tin can be said that the incompleteness of the geological record and Darwin's views nigh species and nomenclature conspired to foreclose him from supporting his theory past citing the existence of certain well-known transitional-form candidates.

Positive paleontological evidence

At this bespeak, enough has been said about Darwin'southward treatment of the transitional-class candidates (see figure two for a summary of the relevant events), but ane very important question remains unanswered: Given that he was clearly reluctant to support his view by appealing to these specimens, why did Darwin (1959) suggest that "positive paleontological evidence may exist implicitly trusted?" One plausible respond to this question can be establish by noting that all of the animals discussed so far happened to be separated from the lineages they were supposed to unite by large morphological and temporal gaps. There is some indication that the positive evidence Darwin desired was a graded succession of forms. Prove of this type would have done little to appease a persistent critic who relishes in playing the missing-link game, and, of class, Darwin himself did not demand to exist persuaded of the truth of his theory. However, such a discovery would have undoubtedly helped to sway objective researchers who had previously been unconvinced past the data and accompanying discussion in the Origin.

Figure 2.

A timeline of events, 1859–1880. Portraits courtesy of Wikimedia Commons.

Figure 2.

A timeline of events, 1859–1880. Portraits courtesy of Wikimedia Eatables.

The fact that Darwin hoped paleontological evidence would be discovered is shown in his praise of the American paleontologist Othniel Marsh'south work on horses (1874) and Cretaceous toothed birds (1880). The former research provided quantitative evidence of the evolutionary development of the equus caballus from Orohippus, the multi-toed mount equus caballus of the Eocene, to single-toed Quaternary animals that closely resemble the modernistic domestic horse (figure 3). What made Marsh'south work on birds noteworthy is the fact that he managed to acquire the remains of over one hundred animals from two very different genera. Because these animals were toothed, and varied in morphology within and across genera, they provided important new insight into the evolutionary history of the birds, and their human relationship to dinosaurs. In a letter to Marsh, Darwin (1880) made his thoughts on the importance of these discoveries absolutely clear: "I received some time agone your very kind note of July 28th, & yesterday the magnificent volume [Marsh, 1880]. I take looked with renewed admiration at the plates, & volition shortly read the text. Your piece of work on these one-time birds & on the many fossil animals of Northward. America has afforded the best back up to the theory of evolution, which has appeared inside the final 20 years." The fact that Darwin explicitly stated that Marsh's piece of work provided the "best support for the theory of evolution" seems to demonstrate that he placed an enormous amount of value on evidence of paleontological succession. Each equine form Marsh discovered could exist quantitatively continued to an bequeathed and descendent form with minimal difficulty, and as well, in conjunction with previously known specimens such as Archaeopteryx, the birds he discovered provided preliminary insight into how the transition from the ancestral dinosaurian to the derived avian form actually occurred. The crucial point is that in both cases, the animals he described graded into ancestral and descendent lineages without a substantial amount of theorize needed to fill up in the gaps.

Figure 3.

Image from Marsh's 1874 paper on fossil horses of North America, depicting the transition from a multi-toed to single-toed status. Courtesy of the Biodiversity Heritage Library (world wide web.biodiversitylibrary.org).

Figure three.

Image from Marsh'southward 1874 paper on fossil horses of North America, depicting the transition from a multi-toed to single-toed condition. Courtesy of the Biodiversity Heritage Library (www.biodiversitylibrary.org).

Like Darwin, Huxley (1877) was quick to praise Marsh, remarking in his American Addresses that his horse research provided "demonstrative bear witness of evolution" (p. 90). As I mentioned earlier, Huxley eventually suggested that specimens such as Archaeopteryx and Hypsilophodon could be used to support the theory of evolution, merely he often seemed hesitant to use isolated forms as evidence in favor of the Darwinian hypothesis. Although his work was well received past prominent evolutionists of the era, Marsh'due south findings only became widely known in the last quarter of the nineteenth century, and as such, one does not find mention of his studies in the Origin. However, we have skillful reason to believe that Darwin regarded Marsh'southward discoveries every bit positive evidence that could exist used to gainsay the accuse that the theory of evolution was unsupported past the geological record. It seems that Darwin did not rely on fossil prove to support his theory in the Origin simply because the isolated specimens known at the time were not the type of show he sought.

I thank Jim Secord, Alex Rosenberg, Sarah Roe, Bob Kallal, and the three anonymous BioScience referees for helpful comments on before drafts of this article. Audience members at the Museum of Natural History in Toulouse also provided valuable suggestions. This research was supported by grants from the Danish–American Fulbright Commission and Duke University.

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