Free Full House by Stephen Jay Gould Page A

to see the magnificent series of fossil horses that Marsh had gathered in the American West. Marsh convinced him that the American series formed a true evolutionary main line, with the European offshoots as disconnected side branches. Huxley had to scramble, for he had promised to give a lecture in New York, just a few weeks later, on fossil horses—and he now had to revise his story completely.
    Marsh agreed to help with these quick changes, and he prepared a famous chart for Huxley’s use in the New York lecture (reproduced here as Figure 8). This figure, among the most celebrated in the history of science, shows two of the three major trends in our classic tale: (1) reduction in number of toes, from four on the front feet and three behind in the earliest horses (bottom of the figure), to three functional toes, to a central toe with two shortened side toes, to a single toe with two side splints as vestiges of former toes (modern horses at top of the figure); (2) steady increase in the height of molar teeth (fifth column of specimens) with elaboration in the convolutions of their cusps (columns six and seven). Marsh chose to draw all his specimens at the same size, and therefore didn’t show the third and most evident trend of marked increase in bulk from the initial stage (which he described as cat-sized, though fox terriers have since triumphed as a canonical metaphor—see Gould, 1991) to the massive Clydesdale of today. Later versions showed all three coordinated trends, as in the best-known figure by the next generation’s paleontological leader, William D. Matthew, first published early in our century in a pamphlet by the American Museum of Natural History, still for sale in the museum shop during my youth in the 1950s, and endlessly reproduced all along. (One copy, for example, appeared in the textbook used by John Scopes to teach evolution to the schoolchildren of Dayton, Tennessee—a source, therefore, for W. J. Bryan’s fulminations at Scopes’s famous "monkey" trial—"no more repulsive doctrine was ever proclaimed by man.") This version arranges the specimens in stratigraphic order next to a geological column and shows all trends of size, toes, and teeth (Figure 9).

    FIGURE 8 A famous chart on the evolution of horses prepared by O. C. Marsh for T. H. Huxley’s New York lecture. Note the linear march to progress in all characters.

    FIGURE 9 W. D. Matthew’s linear and progressive evolution of horses plotted in stratigraphic order, showing increasing body size, decreasing number of toes, and increasing height of teeth.
    In some legitimate though limited sense, these trends are true. The first horses, technically called Hyracotherium (though I love the informal, if taxonomically incorrect, name eohippus, or "dawn horse"), were small, and did have four toes in front, three behind, and low-crowned teeth. The standard story for the advantages of these trends—probably also basically correct—points to a switch in habitat from browsing in forested areas (where many toes hug the soft ground and low-crowned teeth can manage the leafy vegetation) to grazing on plains (where hooves are superior on the hard terrain, and strong, high-crowned teeth deal better with tough grasses and their substantial content of silica. Grasses first evolved in the midst of equine evolution, thus promoting these trends by opening up an extensive new habitat.) In a strictly join-the-dots sense, we do make a correct statement about genealogy when we connect the point for Hyracotherium with the point for modern Equus (the only living genus of horses, including eight species—three zebras, four donkeys and asses, and Old Dobbin, or Equus caballus, representing true horses alone).
    So far, so good—but (as I shall show) so very limited, and so misleading. The lineage of Hyracotherium to Equus represents only one pathway through a very elaborate bush of evolution that waxed and waned in a remarkably complex pattern through the last 55