Read Eight Little Piggies Online
Authors: Stephen Jay Gould
Since Lamarck’s argument centers upon an explanation for why creatures still living yield no evidence of their continued vitality, we should not be surprised that he ignored the Cenozoic gap entirely. We must assume that trigonians spent the entire Cenozoic safe in the bosom of Neptune, full fathom five hundred or more, and unrecorded in a fossil archive of shallow-water sediments.
Charles Darwin, leading evolutionist of the next generation, selected yet another feature of living trigonians—their geographic distribution—to bolster a different theme dear to his view of life. Darwin’s creationist opponents, as we shall see, rendered the history of life as a series of static faunas and floras separated by episodes of sudden extirpation and renewal. To rebut this catastrophist credo, and to advance his own distinctive and uncompromisingly gradualist view of nature, Darwin argued that the extinction of a group should be as smooth and extended as its origin. A group should peter out, dwindle slowly, decrease steadily in numbers and geographic range—not die in full vigor during an environmental crisis. What better evidence than a family once spread throughout the world in stunning diversity but now confined to one small region and one single species. In his private essay of 1844, precursor to the
Origin of Species
(1859), Darwin wrote: “We have reason to believe that…the numbers of the species decrease till finally the group becomes extinct…. The
Trigonia
was extinct much sooner in Europe, but now lives in the seas of Australia.”
Darwin followed Lamarck in dismissing the Cenozoic gap as an artifact of our imperfect fossil record (I can imagine no other option for an evolutionist committed to genealogical connection). But Darwin was explicit where Lamarck had been silent. Darwin also tried to accentuate the positive by arguing that the rarity of such long gaps strongly implied their artificial status. He wrote in the
Origin of Species:
A group does not reappear after it has once disappeared; or its existence, as long as it lasts, is continuous. I am aware that there are some apparent exceptions to this rule, but the exceptions are surprisingly few, so few that…the rule strictly accords with my theory.
Creationists, meanwhile, looked at
Trigonia
from the other side. They treasured the Cenozoic gap and found nearly everything else puzzling. The major creationist thinkers tended to agree that life’s history had been episodic—a series of stages separated by sudden, worldwide paroxysms that removed the old and set a stage for the new. But they divided into two camps on the issue of progress. Did each new episode improve upon the last? Was God, in other words, learning by doing? Or had life maintained a fairly consistent complexity throughout its episodic history? Progressionists and nonprogressionists found different messages in
Trigonia
.
James Parkinson, England’s leading progressionist (though he switched allegiances later on), chose
Trigonia
as a premier example in his
Organic Remains of a Former World
(1811). He read the Cenozoic gap literally, extracting the congenial message that life’s history features a series of creations not connected by ties of genealogy and physical continuity.
But
Trigonia
also presented a special problem for Parkinson. He argued that each successive episode of creation had been marked “with increasing excellence in its objects,” thus matching in all ways but one the Mosaic progression from chaos to Adam as described in Genesis. “So close indeed is this agreement, that the Mosaic account is thereby confirmed in every respect except as to the age of the world” (a problem then resolved by an allegorical interpretation of God’s six creative “days”). Now a
Trigonia
, as some folks say about roses, is a
Trigonia
(subtleties evident to the professional eye aside). Why should a modern shell with radial ribs alone be “better” than a fossil representative with radials and concentrics? Why are the modern versions superior, as Parkinson’s theory of progressive creation required? Parkinson was evidently troubled. In the summary statement to his three-volume work, he devoted more space to
Trigonia
than to any other genus. He clutched at the one available straw, but clearly without conviction. At least the modern trigonians are different. We don’t know why, but different must be better:
This shell, although really of this genus, is of a different species from any shell, which has been found in a fossil state. So that none of the species of shells of this genus, which are known in a fossil state, have, in fact, been found in any stratum above the hard chalk [the Cretaceous, or last period of dinosaurs], or in our present seas.
Louis Agassiz, most able of all creationists, followed Parkinson’s personal route in reverse. He began as an advocate of progress in each successive creation and ended by defending the earliest of God’s creatures as fully up to snuff (largely because he despised Darwinism with such passion and felt that any admission of progress would bolster the evolutionary cause). For him, therefore, the apparent lack of improvement in modern trigonians posed no problem, while the Cenozoic gap brought nothing but pleasure and confirmation. In the major pre-Darwinian work on these clams, his
Mémoire sur les trigonies
(1840), Agassiz argued explicitly that a Cenozoic gap, if conclusively affirmed, would effectively disprove evolution (quite a cogent claim, by the way):
The absence of
Trigonia
in Tertiary [Cenozoic] strata is a very important fact for discussions of the origin and relationships of species of different epochs; for if it could one day be shown that
Trigonia
never existed throughout the entire duration of Tertiary time, it would no longer be possible to maintain the principle that species of a genus living in successive geological epochs are derived from each other.
But Agassiz well understood the discomforting uncertainty of negative evidence. Find one nasty, ugly little Cenozoic trigonian tomorrow, and the entire argument collapses. So Agassiz decided to cover his rear and disclaim: No Cenozoic trigonian is dandy, but future discovery of a Cenozoic trigonian would prove nothing. God may, after all, ordain temporal continuity among a group of related, created forms.
Although his passage may be an exercise in special pleading, it also contains one of the most succinct and eloquent defenses ever written for the Platonic version of creationism.
Although I now invoke this fact [the Cenozoic gap] to support my conviction that the different species of a genus are not variants of a single type…the discovery of a Tertiary trigonian would still not demonstrate, to my eyes, that the relationship among species of a genus is one of direct descent and successive transformation of original types…. I certainly do not deny that natural relationships exist among different species of a genus; on the contrary, I am convinced that species are related to each other by bonds of a higher nature than those of simple direct procreation, bonds that may be compared to the order of a system of ideas whose elements, developed at different times, form in their union an organic whole—although the elements of each time period also appear, within their limits, to be finished products.
In summary, as Darwin’s revolution dawned in 1859, the supposedly pure and simple little fact of modern trigonians stood neither as arbiter nor slayer of theories but as touted support for all major conflicting and contradictory views of life—for evolution by Lamarckian and Darwinian agencies, and for creationism in both progressionist and directionless versions. How can something so important be so undecisive, unless Huxley’s heroic vision of raw empiricism triumphant rarely describes the history of ideas or even the progress of science? Percepts may not create and drive concepts, but concepts are not intractable and immune to perceptual nudges either. Thought and observation form a wonderfully complex web of interpenetration and mutual influence—and the interaction often seems to get us somewhere useful.
The
Trigonia
story has a natural ending that should be conventional and happy, but doesn’t quite work in the expected way. The resolution is not hard to guess, since Darwin’s vision has prevailed. The elusive Cenozoic trigonian was found in Australian rocks—at just the right time, in 1865, when nascent evolutionism needed all the help it could get.
H. M. Jenkins, a minor figure in British geology, explicitly defended Darwin in describing the first Cenozoic trigonians. He interpreted the happy closure of the Cenozoic gap as a clear vindication of Darwin’s characteristic attitude toward the fossil record and as direct support for evolution. Darwin viewed the fossil record as riddled with imperfections—“a history of the world imperfectly kept…of this history we possess the last volume alone…. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines” (
Origin of Species
, 1859). Gaps, as the old saying goes, represent absence of evidence, not evidence of absence. Jenkins wrote, linking the newly discovered Cenozoic trigonian to this fundamental Darwinian prediction:
Every paleontologist believes that, when a genus of animals is represented by species occurring in strata of widely different ages, it must have been perpetuated by some one or more species during the whole of the intervening period…. The only rational meaning that has ever been attached to this presumed general law…is that the perpetuation of the genus…has been due to “descent with modification.”
Trigonia subundulata
[the formal name for the Cenozoic trigonian] is one of the links hitherto wanting; first, in explanation of the existence of the genus
Trigonia
in the Australian seas of the present day; and secondly, as showing that the great gap which before existed in its life-history was…simply a consequence of the imperfection of our knowledge of the geological record.
Finally, a personal confession in closing: This essay has been an exercise in self-indulgence and expiation. I put together the trigonian story at the very beginning of my professional career (when I was barely big enough to pay full price at the stadium). I published a rather poor account in a technical journal in 1968 (frankly, it stunk).
I got part of the story right. I did recognize that everyone managed to slot the living trigonian into his system and that simple, single facts did not (at least in this case) undo general theories. But I got the end all wrong because the traditional, Huxleyan view still beguiled me. I told the happy ending because I read Jenkins’s quote and took it at face value—as an evolutionary prediction fulfilled and an empirical vindication provided. I forgot (or hadn’t yet learned) a cardinal rule of scholarly detection: Don’t only weigh what you have; ask why you don’t see what you ought to find. Negative evidence is important—especially when the record is sufficiently complete to indicate that an absence may be genuine.
I now read the Cenozoic discovery quite differently, because I have confronted what should have happened but didn’t. If Darwin’s vindication required a set of new, clean, pristine, unexpected facts, then why didn’t the Cenozoic trigonian inspire a wave of rejoicing? Darwin had predicted the discovery; Agassiz had invested much hope in its nonexistence.
Sure, Jenkins said the right things in his article; I quoted them and regarded my task as complete. But the key to the story lies elsewhere—in the nonevents. Jenkins wrote a two-and-a-half-page note in a minor journal. No one else seemed to notice. Darwin never commented, though the
Origin of Species
still had several editions to run.
Trigonia
did not become a textbook example of evolution triumphant. Most curiously, Jenkins did not find the Cenozoic trigonian. It was unearthed by Frederick McCoy, an eminent leader of Australian science, the founder and head of the Museum of Natural History and Geology in Melbourne. He must have known what he had and what it meant. But he didn’t even bother to publish his description. I should have taken my clue from the opening lines of Jenkin’s paper, but I passed them by:
The very interesting discovery of a species of
Trigonia
in the Tertiary deposits of Australia has in England remained entirely in the background, and I have been several times surprised at finding students of Tertiary paleontology, generally
au courant
with the progress of their special branch of science, unacquainted with the circumstance. Its importance, in a theoretical point of view, is beyond all question, hence the deep interest always exhibited by those to whom I have spoken on the subject.
I had, in short, succumbed to the view I was questioning. I had recognized that the original discovery of the living trigonian upended no theory, but I had let the Cenozoic fossil act as a Huxleyan nasty fact because Jenkins had so presented it. But when we consider what the Cenozoic trigonian did
not
provoke, we obtain a more general and consistent account of the entire affair. The living trigonian changed no theory, because it could fit (however uncomfortably) with all major views of life. The Cenozoic trigonian did not prove evolution either, because Agassiz’s position of retreat was defensible (however embarrassing) and because evolution was too big a revolution to rely critically on any one datum.
Trigonia
didn’t hurt, but a multitude of fish were frying, and one extra clam, however clean and pretty, didn’t bring the meal to perfection.
In a case that has become a cliché in our language, Sherlock Holmes solved a mystery by noting no bark and inferring that no dog had been present. Nonevents matter, not only new and nasty facts. Which reminds me: I must have looked at a thousand periwinkles this morning. Still no lefties. Maybe someday.
Postscript
Circumstances demand that this essay receive the best, the most indisputable, of all conceivable epilogues—though it must occur at my expense. The factual correction of error may be the most sublime event in intellectual life, the ultimate sign of our necessary obedience to a larger reality and our inability to construct the world according to our desires. For science, in particular, factual correction holds a specially revered place for two reasons: first, because we define the enterprise as learning more and more about an external reality; second, because we know in our hearts that we can be as stubborn and resistant to change as petty bureaucrats and fundamentalist preachers—and undeniable factual correction therefore becomes a kind of salvation from our own emotional transgressions against a shared ideal. Nothing, therefore, can be quite so joyful, quite so appropriate for a finale to this book, as a “nasty, ugly little fact” that, Samsonlike, brings down a conceptual edifice.