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
goal is to offer you digital access to the
primary evidence for the birth and maturation of
Darwin's attempts to explore and explain the
natural world.
In these documents, you can trace the
development of Darwin as a thinker and you will
meet Darwin as a keen-eyed collector, an
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.
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