Charlesm
Birth & Death of Species by Niles Eldredge American Museum of Natural History
http://darwin.amnh.org/article.php?id=2
[Moderator: February 12, 2012 is the 203rd birthday of Charles Darwin. This article is part of an archival website [ http://darwin.amnh.org ] which the curators at the American Museum of Natural History have posted -
Whether you are a student or a researcher, our
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In these documents, you can trace the
development of Darwin as a thinker and you will
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inspired observer, and a determined
experimenter. You will also find Darwin the
shrewd reader, attuned to his cultural context,
and the strategic writer, ever reconsidering and
revising.
Happy Birthday Charles Darwin!]
Charles Darwin opened the Origin of Species (1859) alluding to the patterns of species replacement in both time and in space that he had observed and documented so fully while on the Beagle-the very patterns which 'seemed to me to throw some light on the origin of species'. Darwin's 'Geology Notes' recording his paleontological discoveries- especially when read in conjunction with his 'Zoology Notes'-bear out Darwin's contention that he was keenly interested throughout his nearly three-year stay in southern South America, both in how extinct species seemed to be replaced by similar living species, and (as he put it in his Autobiography) 'by the manner in which closely allied animals replace one another in proceeding southward over the continent.' Indeed, in his first truly open remarks on evolution- written in the Red Notebook after arriving back in England, Darwin explicitly equated patterns of horizontal, geographic replacement with those seen vertically through time in the fossil record: 'The same kind of relation that common ostrich bears to Petisse.extinct Guanaco to recent.'-alluding to the parapatric distributions of the two living rhea species and the supposition that the fossil he collected at Port St. Julian in january 1834 (Macrauchenia-erroneously identified by Richard Owen as an extinct camel) was replaced by the guanaco in the living fauna.
Darwin's contemporaneous scientific notes and letters reveal that he was focusing all along primarily on endemic, 'closely allied' species. And he was fortunate that his earliest paleontological discoveries were in sediments sufficiently young that the species he collected seemed to him to be either the same as those still living in the same place-or instead were closely related ('allied') extinct species that were replaced by similar modern counterparts. The Paleozoic brachiopods (Geol. Note references) he later discovered on the Falkland Islands, as well as the Mesozoic ammonoids he collected in the Andes (Geol notes references) belong to extinct groups only remotely related to the living fauna.
Darwin's very first stop in the Beagle journey was in January, 1832, at St. Jago in the Cape Verde Islands. Discovering a band of fossiliferous limestone along a beach, Darwin said of the fossils 'To what a remote age does this in all probability call us back & yet we find the shells themselves & their habits the same as exist in the present sea.' (Geological Notes reference) Darwin was comparing his fossils with the modern equivalent species from the very beginning of his voyage.
But it was to be his next encounter with fossils-in September and October of 1832-which was to prove pivotal in the young Darwin's scientific life. Bahia Blanca is a large embayment along the Argentinean coastline, just north of the point where the pampas give way to the southerly reaches of Patagonia. There Darwin discovered two outcrops within approximately 20 miles of one another: Punta Alta (since destroyed) and Monte Hermoso. Both bore the bones of fossil mammals-at Punta Alta, associated with the fossil shells of what, once again, appeared to Darwin to belong to the very same species still living in the offshore waters. The Punta Alta mammals, in contrast to the invertebrates, all seemed to be extinct (though Darwin has in these notes a report of a possible still-living species of ground sloth-a note he later deleted).
Darwin wrote a series of entries on the geology and fossils of these two localities in his Geological Notes (DAR reference); he also wrote an important letter to his mentor J.S. Henslow, drafted earlier, but not dated and mailed until November 24, in which he also described his finds (Darwin letter ref). At Punta Alta, Darwin had found the bones of a number of large mammals which appeared to him to be allied to modern sloths and armadillos-all 'edentates' endemic to the Americas.
Of a portion of a large bony carapace Darwin collected, he wrote to Henslow: '.in the same formation I found a large surface of the osseous polygonal plates.'. Immediately I saw them I thought they must belong to an enormous Armadillo, living species of which genus are so abundant here.' And though Darwin knew that other naturalists thought these 'osseous polygonal plates' to have been embedded in the skin of giant ground sloths (whose bones Darwin also found at Punta Alta)-his first reaction was much closer to the truth, for the glyptodonts whose carapaces he was finding were in fact a form of giant armadillo. When he found additional fossil carapaces as the journey progressed, he sometimes called them 'armadillos' and sometimes 'Megatherium'-(giant ground sloth-as in the footnote cited below).
Yet it was the bones of a much smaller mammal-a rodent- that Darwin unearthed at Monte Hermoso which, at intervals throughout the journey, seemed to fascinate Darwin most. He thought they were the remains of a cavy (which he sometimes called an 'agouti'), a smaller now- extinct species closely related to the living Patagonian cavy. As Darwin wrote in his Geological Notes of his Monte Hermoso finds:
I could perceive traces of 4 or 5 distinct animals:
two of which certainly belonged to the Rodentia. One
must have been allied to the Agouti; the tarsi &
Metatarsi belong to an animal less than the present
common inhabitant, Cavia patagonica.
(b)The Agoutis are all proper to S. America; & none
have hitherto been found in a fossil state:- To
conclude with the organic remains I have shown that
some of the bones probably belong to the Edentata. &
that the osseous plates are supposed to belong to
the Megatherium. [(b) refers to a footnote
transcribed below.]
Though the anatomist Richard Owen was later to identify these bones as belonging to a species of tuco-tuco- belonging to another endemic group of South American rodents-it is nonetheless what Darwin himself thought of the cavy/agouti while on board the Beagle that truly matters in tracing the development of his thinking.
Darwin worried that the outcrops at Punta Alta and Monte Hermoso may not represent the same geological age; the fossils were basically different, the bones at Monte Hermoso were black and the sediments themselves were different. In two of his letters to Henslow (the original Nov 24, 1832 letter, and another written in 1834 from the Falkland Islands), he begged Henslow to take care to preserve the numbers he had associated with the fossils-as 'it would be curious to prove some of the same genus [he was referring to the 'agouti'] coexisted with the Megatherium.' Earlier, assuming the beds were indeed more or less the same age, Darwin had written in his Geology Notes (:71v-the footnote (b) to his passage on the agouti):
It is interesting to observe that this tribe of
animals [the Agoutis-inserted], which is now
peculiar to S. America, should in this epoch when
the Megatherium flourished, also be present-showing
that with the extinction of one genus, that of
others did not follow.
Not only do not all mammalian genera living at the same time and place become extinct at the same time, but some species can be seen to be replaced by another of the same genus in the living fauna-the unspoken implication of Darwin's remark.
Darwin revisited Bahia Blanca a year later-and here his notes reveal his doubts about the temporal correlation of the beds of Punta Alta with those of Monte Hermoso. The Punta Alta fossils (he writes at the end of his Bahia Blanca appendix) can 'prove nothing,' and though the Monte Hermoso fossils show the 'coevality of certain animals,' they cannot prove anything 'with reference to other formations.' Yet Darwin seems to have lost these doubts as the trip wore on (cf. his comment to Henslow about showing his interest in proving that the cavy and megatherium are the same age) and, indeed, in the first edition of the Journal of Researches (1839, pp. 104-105) he asserts that all the fossil mammals he collected in South America co-existed in the same, very recent, geological epoch.
Darwin's two known essays written while aboard the Beagle-Reflection Upon Reading My Geological Notes (1834-DAR XXX) and February 1835 (DAR xxx)-develop and integrate Darwin's thoughts on the geological development of eastern South America, as well as his paleontological observations. Darwin could see no evidence of external, environmental change that might account for the extinction of species (a point over which he differed from the opinions of Charles Lyell). In the February 1835 Essay, he opens with a discussion of extinction, and then developing the idea that species have innate longevities and differential lifespans- leading to a staggered (he uses the word 'gradual') extinction of different species. These ideas were introduced by the Italian geologist Giambattista Brocchi in 1814-and discussed (though gently dismissed) by Lyell in Volume 2 of his Principles of Geology. He also, apparently for the first time, writes of the 'births' as well as the 'deaths' of species-and suggests that new species must be born not long after the deaths of the species they replace:
If the existence of species is allowed, each
according to its kind, we must suppose deaths to
follow at different epochs, & then successive births
must repeople the globe or the number of its
inhabitants has Varied exceedingly at different
periods.- A supposition in contradiction to the
fitness, which the Author of Nature has now
established.'
In other words, diversity does not oscillate all
that much through time-as Lyell himself insisted,
though Darwin attributes it as a law determined by
the Author of Nature; therefore new, replacement
species must appear promptly to 'repeople' the
globe.
Darwin once again mentions his Bahia Blanca cavies-the extinct fossil species and its living replacement. For it is clear that this is his one good example of an extinct species being replaced by a different species now living in the modern fauna.
In February 1835, Darwin is clearly discussing the pattern of deaths and successive births of closely allied species-what might be expected to be observed if there were some natural process underlying the orderly birth and death of species. Some (e.g. Brinkman, 2009; Eldredge, 2009) conclude that it is with his fossil evidence-and in particular with his cavies-that Darwin first entertained the notion of evolution. That his explicitly evolutionary passages in the Red Notebook (1836-by now off the Beagle) echo the contents of February 1835 almost exactly (including the innate longevities of species and the patterns of replacement) suggests that, while in South America on the Beagle, Darwin had indeed been using his paleontological experiences (and concurrent observations on geographic replacement) in a conscious exploration of the very idea of evolution.