Leonardo’s Mountain of Clams and the Diet of Worms (14 page)

Dana buttressed each of his two controlling ideas with a definite biological theory of his own construction. In attempting to explain why and how the history of life should
feature a vector of progress as the fundamental thrust of created change through time, Dana invented an influential notion that he named “cephalization,” or increasing domination of the head. Crustaceans are segmented animals, and their classification largely depends upon the form and number of appendages on the head segments. (In ancestral arthropods, each body segment presumably bore a pair
of legs. In later evolution, these appendages often become concentrated and specialized. Many paired organs of crustaceans and other modern arthropods—antennae, mouthparts, swimming paddles, claspers used as external genitalia—evolved from modified legs. Insect and crustacean mouthparts, especially when greatly magnified in TV nature specials, look particularly weird and, to use the modern vernacular,
yucky, because we correctly perceive them as a bunch of little legs waving about—and legs just don’t protrude from mouths on a “proper” vertebrate model!) Dana, under the controlling influence of his progressivist worldview, therefore chose to arrange the diversity of modern crustaceans in sequences of primitive to advanced, as defined by complexity and domination of the head and its appendages.
He then extended this principle of “cephalization” to all of animal life.

Dana first published his theory of cephalization in the mid-1850s, before learning about Darwin’s ideas. But he wrote his four major articles on the subject between 1863 and 1866 (all published in the
American Journal of Science
, a periodical that he edited).

I find the theory of cephalization wonderfully fascinating and
a bit mad. If I could write a volume, rather than an essay, on this subject, I would gladly discuss each of Dana’s sixteen criteria (many with several subdivisions) for the supposedly objective measure of precise degrees in organic “highness” and “lowness” based on the character of the head and its relative domination of the body. I would also demonstrate the self-serving (for his worldview, not
his persona) and “fluid” nature of these criteria—scarcely objective in any reasonable sense, but obviously constructed in order to validate Dana’s a priori desires, and almost wantonly fudged or changed whenever an apparent exception arose.

For example, one criterion states that progress can be measured by position of the head and brain along the body axis—the farther forward the better. Thus,
Dana regards whales as “low” mammals because they have so much mouth and snout in front of the brain. But as soon as he gets stuck because a group he wants to regard as primitive happens to place a brain right up front, he just shifts his criteria. For example, he wants to place millipedes and centipedes below insects, but these many-legged arthropods situate their head and brain right at the front
end. So he pronounces this particular head as weak, although located in the “right” place—as measured by minimal domination over the rest of the body because “lowly” legs proliferate behind. Dana writes: “The head is here strictly at the anterior extremity; but the cephalic force has so feeble control, that the joints multiply behind.” Dana even admits, in several passages, that his criteria
may be inconsistent, but he then argues that such complexity requires ever more subtle interpretations, and ever more experienced interpreters, to make the system work!

Each of Dana’s papers attempts a summary definition of cephalization as the dominant motor of life’s progressive history. His “last fling” on the subject, a final article in 1876 after a ten-year hiatus, offers this account:

Although Dana may have developed this concept of cephalization from his work on crustacea, and although he claimed an objective status for his theory, any thorough reading will reveal that he cobbled his theory together in order to
proclaim the message that controlled his worldview: the centrality and domination of human life in the cosmos. Why choose cephalization as the criterion of progress, except as a way of exalting our own big head, located right at the top, with anterior limbs freed from the lowly task of locomotion and dedicated instead to the head’s service (as I type this essay with my fingers, but only curl my toes
uselessly under my chair)? Dana wrote:

To buttress his second dominating idea of created type, Dana developed an idiosyncratic taxonomy within a popular pre-Darwinian genre that evolution would soon render incoherent—a numerological system, with a fixed number of subgroups in each larger group. Dana favored either two or four subgroups
as the key to correct classification.

Historians of taxonomy have often argued—quite incorrectly—that the development of evolution inspired little change in the structure of classification, for the order that had once been attributed to God could easily be shifted to evolution without any alteration of content. This claim supposedly carries the message that theories should be viewed as mental
constructions sitting lightly upon nature—or, to put the argument another way, that nature’s evident factuality must be rendered in the same manner by any chosen mode of explanation. In either formulation, the role of theory, or worldview, gets demoted and the proper balance between inside theory and outside nature becomes distorted by deemphasizing the internal aspect of a truly intricate and equal
pairing.

But this common claim must be rejected. Evolution made a world of difference in classification. The major groups may have retained their definitions (arthropods are arthropods, and vertebrates vertebrates, whether created by God or developed by evolution), but a hundred other significant details had to change because the geometry of evolution differs fundamentally from the structure
of created systems. Numerological schemes like Dana’s disappeared forever as soon as Darwinism triumphed—and most modern taxonomists don’t even know that such systems ever existed (and therefore fail to appreciate the power of evolution to alter the practice of their profession), because numerology suffered such a complete and sudden death. If God made all species, and their order reflects the nature
of his thought, then why not search for an arcane numerological system that might embody divine wisdom? But if organisms are tied together by genealogy on an evolutionary tree of life, then success or failure becomes a question of contingent history, and no rationale for fixed numbers of subgroups within groups can possibly be devised.

When Dana worked by twos, he divided each group into a “typical”
class defining the essence, and a “hemitypical” class specifying a departure. Thus, for example, he regards terrestriality as typical for vertebrates (don’t ask me why, for fishes came first—but I do suspect an a priori desire to define the group containing humans as typical). The twofold division of vertebrates therefore contrasts Tetrapoda (all terrestrial forms) as typical, with Pisces
(fishes) as hemitypical.

When Dana worked by four, he specified three degrees of typicality in descending order—alphatypic, betatypic, and gammatypic—with a fourth group as a true departure named “degenerative.” In this version, mammals are alphatypic (for standing tall), birds betatypic (lovely creatures, but not on defining terra firma), reptiles gammatypic (as slitherers on the ground), and
fishes degenerative (as living in the “wrong” place for vertebrates).

In a revealing article, titled “Thoughts on Species” (including mineralogical as well as biological entities), and published in 1857 as Darwin composed his magnum opus, Dana defended his numerological system as embodying an unchangeable, Platonic, and universal truth:

He even argued that the human soul requires fixed numbers, both to avoid despondency by perceiving order, and to adore God even more. (This passage also expresses Dana’s hostility
to any notion of graded evolutionary transition between groups.)

Darwin’s candid reaction to Dana’s two theories of cephalization and numerological taxonomy
provides a striking illustration of the unbridgeable discordance between their worldviews. On February 17, 1863, in the interval between Dana’s key letter on evolution and Darwin’s emotional reply, Darwin also wrote to his guru and confidant Charles Lyell about his unhappiness with Dana’s paper on classification of mammals by principles of cephalization and numerical order. Darwin, with his usual
insight, nails Dana for constructing a system whose absurdity would be apparent to anyone not firmly committed to ranking humans as the crown of creation. Dana’s whole scheme, Darwin correctly notes, becomes one long, forced rationale for human centrality:

(Darwin’s valid complaint provides another example of how theory can control the arrangement of nature. Dana used the functional status of the fore-limb as a key character for making major taxonomic divisions among birds because, under his theory of cephalization,
the head controls use of the forelimbs, and their status therefore defines the degree of domination by the head. Darwin, in the last sentence of his letter to Lyell, regards such a scheme as absurd because different uses of forelimbs, in his evolutionary version of life, must be interpreted as immediate adaptations to varying modes of life, not as fundamental divisions on the genealogical
tree of birds.)

As a chief ingredient in the mythology of science, the accumulation of objective facts supposedly controls the history of conceptual change—as logical and self-effacing scientists bow before the dictates of nature and willingly change their views to accommodate the growth of empirical knowledge. The paradigm for such an idealistic notion remains Huxley’s famous remark about “a
beautiful theory, killed by a nasty, ugly little fact.” But single facts almost never slay worldviews, at least not right away (and properly so, for the majority of deeply anomalous observations turn out to be wrong; every fact about the revolution of the earth can be paired with a hundred claimed observations for cold fusion, perpetual motion, or the transmutation of gold).

Rather, at least
for a first approach, anomalous facts get incorporated into existing theories, often with a bit of forced stretching to be sure, but usually with decent fit because most worldviews contain considerable flexibility. (How else could they last so long, or be so recalcitrant to overthrow?) The best test for the power of a worldview to order and interpret facts—and, therefore, an excellent illustration
of the fascinating and intricate interaction between theory and data in science—arises when someone discovers an absolutely pristine and unanticipated bit of novel information. Fortunately, I practice a profession—paleontology—particularly well supplied with superb test cases, for nothing can be quite so out-of-the-blue as a newly discovered fossil. Therefore, if we consider a discovery that, in
modern hindsight, points unambiguously toward the validity of a new worldview, we achieve our ideal test case: if everyone bows to the fact immediately, and accepts the implied reconstruction of nature, then Huxley’s dictum triumphs. But if most members of the old guard manage to embrace the new fact comfortably enough within their conventional worldview, then major changes of theory in science require
a more complex push involving social context as well as factual impetus.

In the early 1860s, as Darwin and Dana debated evolution in their letters, the best possible example of an unanticipated fact burst upon the scene—the discovery of
Archaeopteryx
, not only the oldest bird but also, apparently, beautifully intermediate between reptiles and birds in its retention of teeth, reduced coating of
feathers, and basically reptilian anatomy. Score one knockout blow for evolution.