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by Stephen Jay Gould


  Two points stand out in this epitome for the last third of equine history. First we note a primary signal of branching, branching, and more branching. Where, in this forest, could anyone identify a main trunk? The bush has many tips, though all but one, the genus _Equus_, are extinct. Each tip can be connected to a last common ancestor by a labyrinthine route, but no paths are straight, and all lead back by sidestepping from one event of branching speciation to another, and not by descent down a ladder of continuous change. If you venture an argument that the pathway to modern _Equus_ should be viewed as a main line because the genus still lives and once spread (by its own devices, and not by human transport) over all major continents, I reply that _Equus_ died out over most of its range, including the North American fatherland, and that all modern species derive from Old World remnants. Second, I think that any unbiased observer must identify decline as the major feature of equine evolution during the last 10 million years—the very period when traditional ladder models proclaim perfection and fine-tuning of the distinctive trend to a single hoofed toe, with side toes reduced to vestigial splints. An average of sixteen contemporaneous species lived in North America alone from about 15 million to about 8 million years ago—until, to invoke Agatha Christie's famous image, they died one by one—and then there were none.

  Rearguard defenders of the ladder might reply that I have been discussing only the last (and admittedly bushy) third of equine evolution. What about the first 40 million years, shown as tolerably linear even on MacFadden's arborescent picture (Figure 10)? This earlier period has been the chief domain for friends of linearity. Even G. G. Simpson, who began the transition to bushy thinking in his wonderful 1951 book, _Horses_, and who drew the first famous arborescent diagram of equine phylogeny (a less bushy ancestor of MacFadden's version, reproduced here), defended the basic linearity of this earlier record. "The line from _Eohippus_ [_Hyracotherium_] to _Hypohippus_," he wrote (1951, page 215), "exemplifies a fairly continuous phyletic evolution." Simpson especially emphasized the supposedly gradual and continuous transformation from _Mesohippus_ to _Miohippus_ near the top of this sequence (see Figure 10 for all names and times):

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  The more progressive horses of the middle Oligocene . . . are placed by convention in a separate genus, _Miohippus_. In fact _Mesohippus_ and _Miohippus_ intergrade so perfectly and the differences between them are so slight and variable that even experts find it difficult, at times nearly impossible, to distinguish them clearly.

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  The enormous increase in fossil evidence since Simpson's time has allowed paleontologists Don Prothero and Neil Shubin (1989) to falsify this view, and to introduce extensive bushiness into this last stronghold of the ladder, as predicted by the theory of punctuated equilibrium (see Eldredge and Gould, 1972; Gould and Eldredge, 1993). Prothero and Shubin made four major discoveries in this early segment of equine history that Simpson had designated as the strongest case for a gradual sequence of linear transformation—the transition from _Mesohippus_ to _Miohippus_.

  First, the two genera can be sharply distinguished by features of the footbones, previously undiscovered. _Mesohippus_ does not grade insensibly into _Miohippus_. (Previous claims had been based on teeth, the best preserved parts of mammalian skeletons. The genera cannot be distinguished on dental evidence—the major criterion available to Simpson.)

  Second, _Mesohippus_ does not evolve to _Miohippus_ by insensible degrees of gradual transition. Rather, _Miohippus_ arises by branching from a _Mesohippus_ stock that continues to survive long afterward. The two genera overlap in time by at least 4 million years.

  Third, each genus is itself a bush of several related species, not a rung on a ladder. These species often lived and interacted in the same area at the same time. One set of strata in Wyoming, for example, has yielded three species of _Mesohippus_ and two of _Miohippus_, all contemporaries.

  Fourth, the species of these bushes tend to arise with geological suddenness, and then to persist with little change for long periods. Evolutionary change occurs at the branch points themselves, and trends are not continuous marches up ladders, but concatenations of increments achieved at nodes of branching on evolutionary bushes. Prothero and Shubin write,

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  This is contrary to the widely held myth about horse species as gradualistically varying parts of a continuum, with no real distinctions between species. Throughout the history of horses, the species are well-marked and static over millions of years. At high resolution, the gradualistic picture of horse evolution becomes a complex bush of overlapping, closely related species.

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  In other words, bushiness now pervades the entire phylogeny of horses.

  2. Plausible alternative histories would have yielded a very different and not nearly so attractive story. The substitution of bushes for ladders certainly calls into question, but does not necessarily falsify, the conventional lockstep view of transitions to fewer toes, larger bodies, and higher-crowned teeth. After all, older branches of a bush need not endure for long, and their early removal would leave no ancient vestiges to compromise a trend by persistent variation. If all the early branches die, and all the later twigs bear "progressive" features, then the tree becomes "modernized" throughout—and we may fairly talk of a pervasive trend. If all small horses die early, if no three-toed horses survive into the regime of one-toed _Equus_, then we may justly speak of general trends to increased size and a single hoof—and the old marching order from _Hyracotherium_ to _Equus_ might be defended as a fair epitome of real directionality (while still subject to criticism for neglecting the equally important pattern of waxing and waning diversity). In such a world, the objections that I have raised would be carping and trivial. Yes, we could still emphasize that many pathways run through the bush, and that _Hyracotherium_ to _Equus_ marks only one lineage—but it all pathways pass through the same sequence to larger size and fewer toes, then any one will show the genuine trend, and we shouldn't be too critical if convention favors one case over all others.

  This last-ditch defense of equine progress cannot be sustained. The conventional trends are by no means pervasive (though their relative frequency does increase through the bush, albeit in a fitful way). Several late lineages negate the most prominent trends, and a different outcome for the history of horses—perfectly plausible in our world of contingency (see Gould, 1989)—would have compelled a radically altered tale.

  Consider just one arresting scenario. Contrary to the usual view that horses increase inexorably in body size, MacFadden (1988) studied all ancestral-descendant pairs of species that he could identify with confidence in the equine bush. Of twenty-four such pairs, he found that five, or more than 20 percent, showed a _decrease_ in size. Dwarfing has been a common and persistent phenomenon, repeated throughout the history of horses. Even the first genus, _Hyracotherium_, included periods of size decrease during its geological history (see Gingerich, 1981).

  The most recent, and most profound, trend to dwarfing occurred in a North American genus appropriately named _Nannippus_ (or dwarfed horse). Simpson writes of this remarkable genus (1951, page 140): "Some of the late specimens were miniatures no higher than a small Shetland pony and considerably more slender. These graceful creatures had long, thin legs and feet, and the general form probably suggested a small gazelle more than an ordinary horse."

  Now suppose that _Nannippus_ had survived as the only living member of the Equidae, and _Equus_ had died or never arisen. How would we then tell the story of horses in our biased mode of running steamrollers over one pathway through the bush and calling the resulting line canonical. I hear you crying "foul." You say that _Nannippus_ was a funny little side branch and _Equus_ a powerful main line—so I must be playing verbal games with a story that could never have occurred. Not so; my tale is plausible, but just unrealized. _Nannippus_ showed substantial geographic breadth and geological depth. The genus lived in the United States and Central America, arose more than 10 million y
ears ago, and failed to survive by only a whisker, becoming extinct only about 2 million years ago (MacFadden and Waldrop, 1980). Four species have been described (MacFadden, 1984), and their range of some 8 million years greatly exceeds the longevity of _Equus_ (see Figure 11). If you say that _Equus_ had a greater chance because this modern genus spread from an American homeland into Eurasia and Africa, while _Nannippus_ never colonized the Old World, I reply that _Equus_ became entirely extinct throughout its hemisphere of origin, and therefore only survived by a whisker itself. Suppose that _Nannippus_ had migrated and _Equus_ stayed at home?

  What would be left of our vaunted horse story if _Nannippus_ had survived, and _Equus_ died? We wouldn't be advertising any drive to increased size because _Nannippus_, though a dwarfed descendant of larger ancestors, isn't much bigger than the original _Hyracotherium_. We wouldn't be getting very excited about reduction in toes either, because _Nannippus_ still sported three on each foot (though the side toes were reduced), whereas the original _Hyracotherium_ had four toes on the front feet and three behind (not five on each limb, as commonly misconstrued). We would be left, in fact, only with the trend to increased crown height of the molar teeth—and here we could gloat, because _Nannippus_ chewed with the relatively tallest teeth of any horse in history, including modern _Equus_. But then, tooth height has never provided much of a draw for museums or textbook diagrams, and the conventional story rests upon reduction of toes and growth of body. In short, if _Nannippus_ had survived and _Equus_ died, we wouldn't be telling any famous story about horses at all. The equine bush would become just another anonymous part of the rich mammalian record, known to specialists and unadvertised to the public. Yet nothing would he different but the substitution of one twig for another at the very end of a rich history.

  3. Modern horses are not only depleted relative to horses of the past; on a larger scale, all major lineages of the Perissodactyla (the larger mammalian group that includes horses) are pitiful remnants of former copious success. Modern horses, in other words, are failures within a failure—about the worst possible exemplars of evolutionary progress, whatever such a term might mean.

  Mammals are ranked into some twenty major divisions, called orders. Horses belong to the order Perissodactyla, or odd-toed ungulates—large, herbivorous animals with an odd number of toes on each foot. (The other major ungulate order, called Artiodactyla, contains creatures with an even number of toes on each foot. Each of these orders represents a genuine evolutionary unit traceable to a common ancestor, not an artificial construct devised only by counting toes.) The perissodactyls are a small and depleted order, with only three surviving groups, and seventeen species _in toto_—horses (eight species), rhinoceroses (five species), and tapirs (four species).

  If you become overly sanguine and insist that you won't demerit this group for limited modern diversity because the three kinds of survivors fascinate us so much, I can only recommend a deeper geological look and the famous lamentation of David over Saul and Jonathan: "How are the mighty fallen in the midst of the battle." Perissodactyls were once the giants of mammalian life; we now honor a few straggling ghosts in our zoos because they intrigue us, and because one species has made such a profound difference in human history.

  The rhinocerotoids were once among the most abundant and varied of all mammalian groups. Their extensive ecological range included small and sleek running forms no larger than a dog (the hyracodontmes), rotund river-dwellers that looked like hippos (teleoceratines), an array of dwarfed forms, and the largest land mammals that ever lived—the giant indricotheres, including the all-time size champion _Paraceratherium_ (often called _Baluchitherium_), which stood eighteen feet tall at the shoulder and browsed on treetops (see Prothero, Manning, and Hanson, 1986; Prothero and Schoch, 1989; Prothero, Guerin, and Manning, 1989). The five modern species, all looking much alike, all Old World, and all endangered, form a sad remnant of former glory. The same story may be told for horses, with their decline from sixteen to zero Old World species; and for tapirs, with their modern Asian and South American remnants of a former worldwide spread.

  Moreover, the three living lineages include only a fraction of former perissodactyl diversity, for several major groups have been lost entirely—including, most spectacularly, the large-bodied and prominently horned titanotheres of early Tertiary times, and the chahcotheres, with their powerful digging claws.

  Steady perissodactyl decline has been matched by a reciprocal rise to dominance of the contrasting artiodactyls, once a small group in the shadow of ruling perissodactyls, and now the most abundant order, by far, of large-bodied mammals. The perissodactyls survive as three twiggy vestiges. Artiodactyls are the lords of largeness—cattle, sheep and goats, deer, antelopes, pigs, camels, giraffes, and hippos. Need any more be said? Horses are remnants of a remnant, yet their story provides our false icon of progress—life's little joke. Antelopes represent the most vigorous family in an expanding and dominant group—but who has ever seen a picture of this group's astonishing success? Antelopes are examples of nothing in our museums and textbooks.

  I therefore submit that the history of any entity (a group, an institution, an evolutionary lineage) must be tracked by changes in the variation of all components—the full house of their entirety—and not falsely epitomized as a single item (either an abstraction like a mean value, or a supposedly typical example) moving on a linear pathway. As a final footnote to life's little joke, I remind readers that one other prominent (or at least parochially beloved) mammalian lineage has an equally long and extensive history of conventional depiction as a ladder of progress—yet also lives today as the single surviving species of a formerly more copious bush. Look in the mirror, and don't be tempted to equate transient domination with either intrinsic superiority or prospects for extended survival.

  Part Three

  THE MODEL BATTER: EXTINCTION OF 0.400 HITTING AND THE IMPROVEMENT OF BASEBALL

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  Stating the Problem

  During my lifetime, two events clearly stand out above all others as milestones in the history of batting in baseball: Joe DiMaggio's fifty-six-game hitting streak (see page 32), and Ted Williams's seasonal batting average of 0.406. Unfortunately, I missed them both because I was too busy gestating during the season of their joint occurrence in 1941. Boston Red Sox manager Joe McCarthy had offered to let Williams sit out the meaningless doubleheader of the season's last day (the Yankees had clinched the pennant long before). Williams's average stood at 0.3995, and would have rounded up to an even 0.400. No one had hit 0.400 for ten years, since New York Giants first baseman Bill Terry reached 0.401 in 1930. Ted couldn't bear to back in. He played both games, went 6 for 8, and finished the season at 0.406. No one has hit 0.400 since then (closest calls include George Brett at 0.390 in 1980. Rod Carew at 0.388 in 1977, and Ted Williams himself at 0.388, sixteen years later in 1957, the season of his thirty-ninth birthday). So I'm still waiting to see for myself what life _in utero_ denied to my conscious understanding—and I'm not getting any younger.

  Between 1901, when the American League began and Nap Lajoie hit 0.422, and 1930, when Terry hit 0.401, batting 0.400, while always honored, cannot be called particularly rare. League-leading averages exceeded 0.400 in nine of these thirty years, and seven players (Nap Lajoie, Ty Cobb, Shoeless Joe Jackson, George Sisler, Rogers Hornsby, Harry Heilmann, and Bill Terry) reached this apogee, three times each for Cobb and Hornsby. (Hornsby's 0.424 in 1924 tops the charts, while three players exceeded 0.400 in 1922—Sisler and Hornsby in the National League, Cobb in the American. I am, by the way, omitting the even more common nineteenth-century averages in excess of 0.400, because differing rules and practices in baseball's professional infancy make comparison difficult.) Then the bounty dried up: the thirties were a wasteland (despite high league averages during this decade, as I shall show later); Williams reached his lonely pinnacle in 1941; since then, zip.

  If philately attracts perforation counters, and sumo wrestling f
avors the weighty, then baseball is the great magnet for statistical mavens and trivia hounds. Consider baseball's virtues for the numerically minded: Where else can you find a system that has operated with unchanged rules for a century (thus permitting meaningful comparison throughout), and has kept a complete record of all actions and achievements subject to numeration? Moreover, unlike almost any other team sport, baseball's figures are records of individual achievements, not elusive numbers that may be assigned to a single player, but really record some aspect of team play—for baseball is a congeries of contests between two individuals: hitter versus pitcher, runner versus fielder. Thus, records assigned to players of the past can be read as their personal achievement, and can be compared directly with the same measures of modern performers. No wonder, then, that the largest organization of scholarly fans, the Society for American Baseball Research, is so numerically minded, and has contributed, through its acronym, a new word to our language: sabermetrics, for the statistical study of sporting records.

  Humans, as I have argued, are trend-seeking creatures (perhaps I should say "storytelling animals," for what we really love is a good tale—and, for reasons both cultural and intrinsic, we view trends as stories of the best sort). We are therefore driven to scan the charts of baseball records for apparent trends—and then to devise stories for their causes. Remember that our cultural legends include two canonical modes for trending: advances to something better as reasons for celebration, and declines to an abyss as sources of lamentation (and hankering after a mythical golden age of "good old days"). Since 0.400 hitting is both so noticeable and so justly celebrated, and since its pattern of decline and disappearance so clearly embodies the second of our canonical legends, no other trend in baseball's statistical history has attracted such notoriety and engendered such lamentation.

 

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