Branching Through a Wormhole

Natural History, March, 1999 by Stephen Jay Gould

Lamarck's Ladder Collapses

In 1800 the French zoologist Jean-Baptiste Lamarck changed the science of biology forever by presenting the first public account of his theory of evolution at the Museum d'Histoire Naturelle. More than a half century before Charles Darwin published On the Origin of Species, Lamarck proposed that modern species had descended from common ancestors over immense periods of time. In Part One of this essay, the author discussed Lamarck's original model, which consisted of two vectors--an upward ladder of progression, modified by the lateral pull toward local adaptations. Part Two demonstrates how Lamarck's study of invertebrates eventually compelled him to abandon that system and to arrive, well heft)re Darwin, at the paradigm of a branching bush or tree life--Eds.

PART TWO

V. AN ODYSSEY OF WORMS

I have always considered it odd (and redolent either of arrogance or parochialism) when a small minority divides the world into the two wildly unbalanced categories of itself versus all others and then defines the big category as an absence of the small--as in my grandmother's taxonomy for Homo sapiens: Jews and non-Jews. Yet our conventional classification of animals follows the same strategy by drawing a basic distinction between vertebrates and invertebrates--when only about forty thousand of more than a million named species belong to the relatively small lineage of vertebrates.

On the venerable principle that bad situations can always be worse, we can gain some solace by noting the even greater imbalance devised by the founder of modern taxonomy, Carolus Linnaeus. At least we now recognize vertebrates as only part of a single phylum, while most modern schemes divide invertebrates into some twenty to thirty separate phyla. But in his Systema naturae of 1758, the founding document of modern zoological nomenclature, Linnaeus identified only six basic animal groups: four among vertebrates (mammals, birds, reptiles, and fishes) and two for the entire realm of invertebrates (Insecta, for insects and their relatives, and Vermes, literally "worms," for nearly everything else).

When Lamarck became professor of invertebrates at the Museum d'Histoire Naturelle in 1793 (with an official title in a Linnaean straitjacket as professor of insects, and worms), he already recognized that reform demanded the dismemberment of Linnaeus's "wastebucket" category of Vermes. (Wastebucket actually ranks as a semitechnical term among professional taxonomists, a description for inflated groups that become receptacles for heterogeneous bits and pieces that most folks would rather ignore--as in Linnaeus's relegation of all "primitive" animals to the category of "worms," ranking far beneath the notice of specialists on vertebrates.)

In his 1801 book, Systeme des animaux san vertebres, Lamarck identified the hodgepodge of Linnaeus's Vermes as the biggest headache and impediment in zoology:

   The celebrated Linnaeus, and almost all other naturalists up to now, have
   divided the entire series of invertebrate animals into only two classes:
   insects and worms. As a consequence, anything that could not be called an
   insect must belong,  without exception, to the class of  worms.

By the time Lamarck wrote his most famous book, Philosophic zoologique, in 1809, his frustration had only increased; he called Linnaeus's class of worms "une espece de chaos dans lequel les objets tres disparates se trouvent reunis (a kind of chaos where very disparate objects have been united together)." He then blamed the great man himself for this sorry situation: "The authority of this scientist carried such great weight among naturalists that no one dared to change this monstrous class of worms." (By writing cette classe monstrueuse, I am confident that Lamarck meant to attack the sheer numbers of included genera, not the moral status, of Linnaeus's Vermes). Lamarck therefore began his campaign of reform by raiding Vermes and gradually adding the extracted groups as novel phyla to his newly named category of invertebrates. In his first museum lecture course in 1793, he had already expanded the Linnaean duality to a ladder of progress with five rungs--mollusks, insects, worms, echinoderms, and polyps (corals and jellyfish)--by liberating three new phyla from the wastebucket of Vermes.

This reform accelerated in 1795, when Georges Cuvier arrived at the museum and began to study invertebrates as well. The two men collaborated in friendship at first, and they surely operated as one mind on the key issue of dismembering Vermes. Thus, during almost every annual course of lectures, Lamarck continued to add phyla, extracting most new groups from Vermes but some from the overblown Linnaean Insecta as well. In year 7 of the French revolutionary calendar (1799), he established the Crustacea (for marine arthropods, including crabs, shrimps, and lobsters), and in year 8 (1800), the Arachnida (for spiders and scorpions). Lamarck's invertebrate classification of 1801 therefore featured a growing ladder of progress, now bearing seven rungs. In 1809 he presented a purely linear sequence of progress for the last time in his most famous book, Philosophie zoologique. His tall and rigid ladder now contained fourteen rungs, as he had added the four traditional groups of vertebrates atop a list of invertebrate phyla that had just reached double digits (see Lamarck's chart, page 84).