The Great Ideas Today 1983 0852294115

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Angel

Family

Animal

Fate

Aristocracy

Form

Art

God

Astronomy

Good and Evil

Beauty

Government

Being

Habit

Cause

Happiness

Chance

History

Change

Honor

Citizen

Hypothesis

Constitution

Idea

Courage

Immortality

Custom and Convention

Induction

Definition

Infinity

Democracy

Judgment

Desire

Justice

Dialectic

Knowledge

Duty

Labor

Education

Language

Element

Law

Emotion

Liberty

Eternity

Life and Death

Evolution

Logic

Experience

Love

:.i f r?:?

Man

Reasoning

Mathematics

Relation

Matter

Religion

Mechanics

Revolution

Medicine

Rhetoric

Memory and Imagination

Same and Other

Metaphysics

Science

Mind

Sense

Monarchy

Sign and Symbol

Nature

Sin

Necessity and Contingency

Slavery

Oligarchy

Soul

One and Many

Space

Opinion

State

Opposition

Temperance

Philosophy

Theology

Physics

Time

Pleasure and Pain

Truth

Poetry

Tyranny

Principle

Universal and Particular

Progress

Virtue and Vice

Prophecy

War and Peace

Prudence

Wealth

Punishment

Will

Quality

Wisdom

Quantity

World

Among Greeks of the classical period, the paradigm of science was mathematics and mathematical astronomy. In this detail from Raphael's "The School of Athens." Euclid is surrounded by ag roup of eager followers as he explains so me rare mathematical truth

The

1983

Encyclopaedia Bntannica, Inc. CHICAGO AUCKLAND • GENEVA • LONDON • MANILA • PARIS • ROME • SEOUL* SYDNEY • TOKYO • TORONTO

© 1983 by Encyclopaedia Britannica, Inc. Copyright under International Copyright Union. All rights reserved under Pan American and Universal Copyright Conventions by Encyclopaedia Britannica, Inc. No part of this work may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the publisher. Excerpt from “Asides on the Oboe” by Wallace Stevens from The Collected Poems of Wallace Stevens. Copyright 1954 by Alfred A. Knopf, Inc. Reprinted by permission of Alfred A. Knopf, Inc. Library of Congress Number: 61-65561 International Standard Book Number: 0-85229-411-5 International Standard Serial Number: 0072-7288 Printed in the U.S.A.

EDITOR

MortimerJ. Adler EXECUTIVE EDITOR

John Van Doren Consulting Editor Otto Bird

Editorial Assistants Cynthia L. Rutz Elizabeth Sack

ART DIRECTOR

Cynthia Peterson Picture Editor La Braviajenkins

Illustrator JohnL. Draves

Art Staff Paul Rios

DIRECTOR, COPY AND COMPOSITION

J. Thomas Beatty MANAGER, COPY DEPARTMENT

Anita Wolff Elizabeth A. Blowers

Copy Staff Stephen Isaacs

Julian Ronning

MANAGER, COPY CONTROL

Mary C. Srodon Copy Control Staff Marilyn Barton Mayme R. Cussen MANAGER, COMPOSITION DEPARTMENT

Dorajeffers Composition Staff Griselda Chaidez JohnKrom,Jr. MaryaSavich Van Smith Danette Wetterer

DIRECTOR, EDITORIAL COMPUTER SYSTEMS

Melvin Stagner MANAGER, COMPUTER OPERATIONS

Ronald Laugeman Computer Operators Arnell Reed Michael Schramm MANAGER, PROGRAM DEVELOPMENT

Walter Markovic, Jr.

EDITORIAL ADMINISTRATION

Philip W. Goetz, Editor-in-Chief Margaret Sutton, Executive Editor Bruce L. Felknor, Director of Yearbooks Robert Dehmer, Executive Director, Production Verne Pore, Director of Budgets and Controller ENCYCLOPAEDIA BRITANNICA, INC.

Robert P. Gwinn, Chairman of the Board Charles E. Swanson, President

A NOTE ON REFERENCE STYLE

In the following pages, passages in Great Books of the Western World are referred to by the initials ‘GBWW,’ followed by volume, page number, and page section. Thus, ‘GBWW, Vol. 39, p. 210b’ refers to page 210 in Adam Smith’s The Wealth of Nations, which is Volume 39 in Great Books of the Western World. The small letter ‘b’ indicates the page section. In books printed in single column, ‘a’ and ‘b’ refer to the upper and lower halves of the page. In books printed in double column, ‘a’ and ‘b’ refer to the upper and lower halves of the left column, ‘c’ and ‘d’ to the upper and lower halves of the right column. For example, ‘Vol. 53, p. 210b’ refers to the lower half of page 210, since Volume 53, James’s Principles of Psychology, is printed in single column. On the other hand, ‘Vol. 7, p. 210b’ re¬ fers to the lower left quarter of the page, since Volume 7, Plato’s Dialogues, is printed in double column. Gateway to the Great Books is referred to by the initials ‘GGB,' fol¬ lowed by volume and page number. Thus, ‘GGB, Vol. 10, pp. 3957’ refers to pages 39 through 57 of Volume 10 of Gateway to the Great Books, which is James’s essay, “The Will to Believe.” The Great Ideas Today is referred to by the initials ‘GIT,' followed by the year and page number. Thus ‘GIT 1968, p. 210’ refers to page 210 of the 1968 edition of The Great Ideas Today.

Contents

Preface PART ONE

Current Developments in the Arts and Sciences Evolution of Life: Evidence for a New Pattern Steven M. Stanley Mathematics in Our Time Felix £. Browder

PART TWO

PART SIX

112 137

182

Special Features Dante the Thinker: Poetry and Philosophy Otto Bird The Child as Reader Clifton Fadiman

PART FIVE

55

Editorial Essay The Idea of Civil Police

PART FOUR

2

Reconsiderations of Great Books Reflections on Galen Douglas Buchanan Ptolemy, Copernicus, and Kepler Owen Gingerich

PART THREE

vi

204 236

Reviews of Recent Books Braudel’s Mediterranean Charles Van Doren Scott Buchanan’s So Reason Can Rule Ramsey Clark

289

Note to the Reader

302

266

Additions to the Great Books Library Astronomia Nova Johannes Kepler (selected chapters translated by Owen Gingerich and William Donahue) Phaedra Jean Racine The Birth of Tragedy Friedrich Nietzsche

306 342 396

Preface

T

his year again, as for several years past, we omit the symposium discussion of a topic of current interest with which issues of The Great Ideas Today used to begin—omit it for the reason, among others, that as the contents of the book have to be determined many months in advance of publication, in order that contributors may have the time they need to write the sort of articles we request of them, we can no longer be confi¬ dent, given the pace of change in this rapidly spinning world, that any topic selected for discussion will actually be current by the time the book appears. So we now leave such currency to publications that are more quickly assembled and more frequently printed than is possible with an annual such as ours. If we seem nevertheless to have landed in the very midst of a timely controversy with the first of two essays that comprise our review of developments in the arts and sciences—an essay on evolution by Steven M. Stanley of the Johns Hopkins University—it is because any consider¬ ation of that subject is likely to suggest a response to the Creationist teachings that were enacted into law in certain parts of the country not long ago as curricular requirements, before the courts threw them out. In fact, Professor Stanley has little if anything to say about such teachings. His essay is evidence rather of the continuing revision of Darwinian theory which is going on in science, a revision not in Darwin’s basic hy¬ pothesis, of which there can be no doubt (evolution is perhaps the most overwhelmingly substantiated theory that empirical science has ever come up with), but in the manner and the pace according to which the emergence (and disappearance) of species has been accomplished. The correction of our understanding of this which now seems possible is set forth with great clarity and persuasiveness by Professor Stanley. A second article in this section of the volume provides at long last what readers of The Great Ideas Today have more than once indicated they would like to see, which is an account of mathematics in our time. The problem with such an undertaking, which is that any serious discussion of mathe¬ matics must to some extent be mathematical, and that, if it is, it will be too difficult to grasp, has happily been overcome by Felix Browder, who is chairman of the department of mathematics at the University of Chicago. Professor Browder discusses his subject in a manner that, if not always VI

easy to follow, is no harder than it requires, and as in addition he has conceived his task in terms that justify a lengthy survey of the history of mathematics, readers will find themselves doubly informed. Two essays that will serve as rereadings of the Great Books follow these articles. One is a discussion of Galen by Douglas Buchanan, himself a physician and longtime reader of Great Books, who was invited by the editors to become a contributor on the strength of an earlier paper he had written on Galen as a physiologist, which is incorporated into the longer essay that appears here. A second piece devoted to Great Books authors is Owen Gingerich’s account of the astronomers who are to be found in volume 16 of GB11 II—that is, Ptolemy, Copernicus, and Kepler—whose writings Professor Gingerich, a contributor to previous issues of The Great Ideas Today, has taught for many years in his classes at Harvard. Readers who feel intimidated by the scientific works in the GBWW will be reas¬ sured by the discussion of these writings, which is intended by Professor Gingerich for the nonspecialist, and which may serve for anyone as the ideal introduction to the materials with which it deals. This year’s issue also offers an editorial essay, the work of the staff of the Institute for Philosophical Research, working with the Institute’s Di¬ rector, MortimerJ. Adler, on the subject of civil police. These are the only kind of police which are appropriate to a civil society, and although arguably they are indispensable to the maintenance of any civil govern¬ ment, by curious omission they did not exist until recent times. In addition, there are two essays that may be regarded as special fea¬ tures. One is a discussion by Otto Bird, our consulting editor, of the relation between poetry and philosophy as they appear in The Divine Comedy. The other is an essay by Clifton Fadiman on children as readers of books—that is, on the character of children’s imagination, their taste, and their literary perception. Mr. Fadiman, who is as near an authority on the subject as anyone who is not a child can presume to be, has added a list of children’s books which have established themselves as favorites since they first appeared, and endeavors to suggest why that is so. In still another part of the volume, two recent books which have been thought of interest to readers of The Great Ideas Today are discussed by contributors both of whom have appeared in previous issues. First is Charles Van Doren, who wrote last year on The Mind’s /, edited by Doug¬ las R. Hofstadter and Daniel C. Dennett; on Brainstorms, by Daniel C. Dennett; and on Mind Design edited by John Haugeland. Mr. Van Doren is here concerned with Fernand Braudel’s depiction of the Mediterranean world in the age of Philip the Second, a massive work by one of the most celebrated and, Mr. Van Doren suggests, among the greatest of living historians. Braudel’s is not history as most people understand the term, however, but something by comparison very rich and strange, and those who have not yet made the acquaintance of his works will be tempted to do so, we think, by this account of him. VII

A second review essay is by Ramsey Clark, the former United States At¬ torney General, now a practicing lawyer much concerned with public is¬ sues, who discusses a volume of essays by Scott Buchanan, one of which appeared in an earlier Great Ideas Today (GIT 1972, pp. 4-21). Buchanan, who served on the editorial advisory board of Great Books of the Western World, wrote most of these essays while a fellow of the Center for the Study of Democratic Institutions in Santa Barbara, California. This col¬ lection of his papers on law and politics is the first of several volumes that his friends and former students intend to bring out of his writings, which have long been out of print. Three works that may be regarded as additions to the Great Books library which The Great Ideas Today has accumulated over the years finish out this issue of the annual. The first consists of the introduction and a number of samples from Kepler’s New Astronomy, a work of 1609 in which Kepler demonstrated the ellipsoid pattern of the planets’ motion around the sun, in particular that of Mars. His marvelous introduction to this book, as well as certain sample chapters from it, are offered here in a new translation, portions of which are by Professor Gingerich and others by William Donahue. Kepler’s work is followed by what is generally considered the best, as it was also the last, of the tragedies written by Racine during the great peri¬ od of French classical drama, his Phaedra. Based on the Hippolytus of Euripides (GBWW, vol. 5, pp. 225-236), it is a play that recounts the guilty passion of Phaedra, queen of Athens, for Hippolytus, the son of Theseus, king of the city and her husband. Racine makes Phaedra the central figure of the story, whereas for Euripides, as for the Greeks generally, it was rather Hippolytus, if not Theseus himself. And finally, for the first time in The Great Ideas Today, there is a book by Nietzsche, The Birth of Tragedy. This is an essay on the drama of the Greeks which is celebrated for its analysis of the conflict between what it calls Ap¬ ollonian and Dionysian elements in Greek culture and its perception of the harsh character of the Greek tragic vision (as distinct from the seren¬ ity that Matthew Arnold, for example, believed the plays contained). The Birth of Tragedy remains an essay of great power and interest, although its thesis is no longer accepted as wholly adequate or even entirely correct. It was Nietzsche’s first book, and one that, given its development, is easier to read than his subsequent writings, which are set in his more familiar aphoristic style, can ever be.

viii

PART ONE

Current Developments in the Arts and Sciences

Evolution of Life: Evidence for a New Pattern Steven M. Stanley

Steven M. Stanley is a professor of paleobiology at the Johns Hopkins University. He did his undergraduate work at Princeton University and received his Ph.D. from Yale in 1968. He began his scientific career studying living organisms—bivalve mollusks—and later applied his conclusions to the interpretation of their evolution as recorded in the fossil record. In the early 1970s he began to undertake more general studies, interpreting trends, rates, and patterns of evolution from the fossils he studied. This further led him to explore ways of testing the punctuational model of evolution and examining its consequences. Recently he has been interested in patterns of extinction, especially the causes of mass extinctions of marine life. In 1973 Professor Stanley received the Maryland Academy of Science’s Allan C. Davis Medal for the outstanding young scientist of the year. He was also awarded the Schuchert Award by the Paleontological Society in 1977 for contributions to teaching and research before the age of forty. He has written many scientific articles in his field as well as two recent books: Macroevolution: Pattern and Process (1979) and a less technical volume, The New Evolutionary Timetable: Fossils, Genes, and The Origin of Species (1981).

F

or decades, most evolutionary biologists have assumed that the many species populating the world around them are in a process of slow but substantial evolutionary change. The prevailing idea has been that spe¬ cies continually evolve to meet changes in the environment, and that this evolution, over long intervals of time, produces large-scale restructuring of most species. This traditional view of large-scale evolution has become known as the gradualistic view, and it can be traced back to Charles Dar¬ win. Darwin expressed the gradualistic view many times and in many dif¬ ferent ways in his great book On the Origin of Species. Perhaps his most elo¬ quent expression of the view is in the following passage (Darwin, 1859, p. 84; cf. GBWW, Vol. 49, p. 42): It may metaphorically be said that natural selection is daily and hourly scrutinising, throughout the world, the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages. . . .

Natural selection was, of course, the mechanism that Darwin proposed to account for evolution. It amounts to a difference among the individuals of a population in contribution of descendants to future generations. The kinds of individuals that, in time, come to predominate in a population are those that, by virtue of long life or high reproductive rate, produce relatively large numbers of fertile offspring. Darwin asserted that as long as new kinds of individuals appear in each generation of an established species, there is raw material available for the biological transformation of the species in a slow but ultimately profound way. Also conveying this gradualistic view of evolution is the only illustra¬ tion to be found in On the Origin of Species (reproduced here as figure 1). This diagram represents a tree of life, and in presenting it, Darwin estab¬ lished a metaphor for evolutionary branching that has become familiar to all educated people of the Western world. Actually, what Darwin plotted was not so much a tree as a bush, whose branches had grown diagonally upward in many directions. Each branch represents what is known as a lineage, or a line of evolutionary descent. On a graph such as this, the 3

Figure 1. The only illustration that Darwin published in On the Origin of Species. This is the first portrayal of the "tree of life’’ in the era of modern evolutionary biology. The diagonal lines represent biological lineages, or lines of evolutionary descent, with the vertical axis representing time and the horizontal axis representing one or more biological traits. Note that the origin of a new lineage is not associated with rapid evolutionary divergence (a new lineage does not branch from its ancestral lineage at an especially low angle on the graph).

vertical axis represents time and the horizontal axis, one or more biologi¬ cal traits that are evolving—often one or more aspects of size or shape. Thus, the angle of a branch depicts the rate of evolution within that branch: the more horizontal the branch, the more rapid the evolution. An important aspect of Darwin’s hypothetical tree of life is that its branches did not originate by means of unusually rapid evolutionary change. No branch departed suddenly from its parent branch along a nearly horizontal path and later bent upward to record a subsequent his¬ tory of much slower change. This point relates to a major source of con¬ troversy in the field of evolutionary biology today. Many evolutionists, particularly ones specializing in the study of fossils, now argue that most evolutionary change is associated with the rapid ori¬ gins of species. There are two facets to this so-called punctuational view of evolution. One is the idea that most evolution occurs when one species buds rapidly from another; the second is the idea that once a species has budded from another and taken firm hold of a place in nature—once it is populous and widely distributed—it experiences relatively little further evolution. The budding of one species from another is known as specia-

4

Steven M. Stanley

tion. The occunence of speciation has, of course, long been recognized. At issue is what it actually achieves in the evolutionary history of a group of animals or plants. Does speciation simply add a new lineage that departs slowly in some new evolutionary direction? Or does it often entail sudden evolutionary change? The persistence of Darwin s gradualistic view, which downplays the role of speciation, can be seen in the writing of prominent evolutionists of the twentieth century. The modern synthesis emerged in the 1940s, after three decades of controversy about the mechanism of evolution, as a reaf¬ firmation of Darwinism in the context of modern genetics. The new movement took its name from the title of a book, Evolution, The Modern Synthesis, written by Julian Huxley in 1942. Here Huxley wrote: Species-formation constitutes one aspect of evolution; but a large fraction of it is in a sense an accident, a biological luxury, without bearing upon the major and continuing trends of the evolutionary process.

Theodosius Dobzhansky, the foremost experimental geneticist of the mid-twentieth century, likewise attributed little importance to rapidly divergent speciation. In an article published in 1972, he finally began to see the matter differently, and in this article he described the slow, rather than rapid, divergence of new species from their ancestors not as the certain but merely as the “usual, and by now orthodox, view.” Other biologists and paleontologists had occasionally expressed skep¬ ticism of the orthodox view—some in mild ways and some with radical al¬ ternatives. The radical claims were rejected out of hand, and even the mild views were crushed beneath the weight of the gradualistic establishment. A crucial issue in the emergence of the gradualistic view of evolution related to the quality of the fossil record. When Darwin formulated his gradualistic view during the middle part of the last century, he was quite naturally compelled to look to the fossil record for confirmation. When he did so, he was sorely disappointed. His hope had been to find laid out before him in the stratigraphic column sequences of populations docu¬ menting the gradual transformation of species. What he encountered in¬ stead were discrete species that showed virtually no change in shape be¬ tween their first and last appearances. Darwin responded to this situation by challenging the quality of fossil evidence. He devoted many pages of On the Origin of Species to arguments that the fossil record was woefully in¬ complete. Among his passages on this subject was the following: The noble science of Geology loses glory from the extreme imperfection of the record. The crust of the earth with its embedded remains must not be looked at as a well-filled museum, but as a poor collection made at hazard and at rare intervals.

5

Evolution of Life

It is notable that Darwin did not reach this conclusion by careful scruti¬ ny of the fossil record. On the other hand, careful scrutiny by a lone scientist in Darwin’s day would hardly have yielded the detailed picture that we are developing at the present time. Not only are we reaping the harvests of thousands of expert paleontologists whose collective efforts have spanned many decades; we have at our disposal good estimates of the absolute ages of fossils, based on measurements of the radioactive de¬ cay of naturally occurring chemical elements. Darwin was quite honest in admitting that he had not shown the fossil record to be incomplete on the basis of independent evidence but had deduced the poor quality of the record from its failure to support his gradualistic picture of evolution. In On the Origin of Species, he wrote: But I do not pretend that I should ever have suspected how poor was the record in the best preserved geological sections, had not the absence of innumerable transitional links between the species . . . pressed so hardly on my theory.

That Darwin devoted so much of his book to a negative evaluation of the quality of fossil evidence testifies to his great concern about the problem. He confessed that the failure of the fossil record to document gradual evolution was “probably the gravest and most obvious of all the many objections which may be urged against my views.” There was, in fact, a way of resolving the difficulty that Darwin chose not to pursue. This would have been to propose that most evolutionary change has been concentrated in sudden branching events within the tree of life—to accept the idea that species have formed rapidly and then have experienced relatively little evolutionary change, even if they have sur¬ vived for millions of years thereafter.

The new challenge It is, in fact, increased faith in the fossil record that has led to the chal¬ lenge that during the past decade has been leveled at the gradualistic view of evolution. “Faith” is perhaps a poorly chosen word, because the new ideas have an empirical foundation. They are rooted in fossil data. In 1954, Ernst Mayr suggested that the sudden appearance of evolu¬ tionary novelties in the fossil record might reflect the actual pattern of evolution rather than the extreme imperfection of the record. Mayr, a biologist, did not arrive at these ideas by studying the fossil rec¬ ord, however. His ideas were based on observations of living species of birds. Mayr noted that many species of birds seemed to have evolved rapidly and recently from small populations of parent species—popula¬ tions situated near the peripheries of the geographic ranges of the parent

6

Time

Steven M. Stanley

Figure 2. Idealized trees of life depicting the punctuational model of evolution (left) and the gradualistic model (right). The gradualistic tree resembles the tree published by Darwin, in that branching events do not characteristically accelerate evolution; they merely establish new pathways for gradual evolution. The punctuational tree, In contrast, depicts a pattern in which many branching events are associated with accelerated evolution, whereas once established, new branches (species) evolve very slowly; the result is that most evolution is associated with branching events.

species. Mayr reasoned that, in general, local speciation events might be the source of most major evolutionary transitions. More recently, the idea that most evolution is concentrated in specia¬ tion events (branching events by which the tree of life grows) has been formalized as the punctuational model of evolution. The alternative view, that most evolution takes place by the slow transformation of well-estab¬ lished, populous species, represents the gradualistic model The two models entail quite different shapes for the tree of life (figure 2). In effect, the punctuational view transforms the traditional tree into something resem¬ bling a saguaro cactus. It is primarily paleontologists who, during the past decade, have tested this idea, championed it, and explored its consequences. While their case rests in part on biological arguments, like those of Ernst Mayr, paleonto¬ logical data form its primary foundation. Most compelling is evidence that large numbers of species have existed for vast stretches of geologic time. This and other lines of evidence will be outlined below, but first, three points about the punctuational model deserve mention. First, the punctuational view of the fossil record sees that record as being much more complete than Darwin and many others have envi-

7

Evolution of Life

sioned, but still not perfect. The record is faithful enough to demonstrate that very minor evolutionary change is the rule for large, well-established species; on the other hand, the record is generally not good enough to document geologically rapid change—change accomplished in less than a few tens of thousands of years—within a small population restricted to a local area. Fossil evidence that well-established species are stable forces us to conclude that small populations must be the sites of most evolution. This constitutes almost total reliance on negative evidence, but many sci¬ entific tests are similarly based. If there are only two alternatives (in this case, that most change occurs in populous species or that most change occurs in small populations), then the elimination of one alternative renders the other almost certainly valid. Actually, there is a third alterna¬ tive for the pattern of evolution, which entails the sudden transformation of an entire large species—a sort of rebirth, but arguments will be ad¬ vanced below to the effect that dramatic events of this type are unlikely to play a major role in evolution. Second, it should be understood that not all speciation events produce marked change. We have clear evidence that many species have budded off from others with little alteration. Pairs of species known as sibling species, for example, consist of two nearly identical members, one of which has usually evolved from the other. The punctuational view simply holds that speciation events accomplish major changes and that such changes account for most evolution in the history of life. Third, the punctuational view does not rule out a dominant role for natural selection as the agent of evolutionary change. Natural selection, as proposed by Darwin, has two aspects. What the process amounts to is the contribution of different kinds of individuals to the genetic composi¬ tion of future generations. One aspect has to do with survival through the reproductive period of life: the longer an individual lives, the more off¬ spring it is likely to engender. The other aspect has to do with rate of production of offspring: even an individual having only an average life span can contribute a disproportionately large number of offspring to the next generation if it produces them at a high rate. The best of both worlds in natural selection is living an unusually long time and being unusually fecund in the process. Of course, processes other than natural selection could still play major roles in the rapid emergence of new species within small populations. The point is simply that the punctuational model by no means demands that we abandon traditional Darwinian natural selec¬ tion as the prevailing mechanism of change.

The test of adaptive radiation One way of investigating whether the gradual transformation of estab¬ lished species can have been responsible for most evolution in the history 8

Steven M. Stanley

of life is to look to the place where most evolution has occurred: adaptive radiation. Adaptive radiation is the rapid proliferation of new taxa (spe¬ cies, genera, and groups of higher rank) from some ancestral group. “Ra¬ diation is an apt metaphor for the expansion of many new forms of life from a common source. The fossil record provides clear evidence that most major evolutionary transitions occur within adaptive radiations. One of the most spectacular of these took place not long ago, by geologi¬ cal standards, and its relatively recent occurrence leaves its history well displayed in the fossil record. This large-scale event is also of interest to us because we are one of its most conspicuous products. It is the adaptive radiation of the class Mammalia, which gave the most recent geological era, the Cenozoic, its nickname—the Age of Mammals. Before primitive mammals began their great adaptive radiation, they had been in existence for about 150 million years—a long stretch of time even to a geologist. They had, however, remained small and inconspicu¬ ous; the largest were no larger than a house cat, and many species may have been nocturnal. There is little question that during this long interval before their modern radiation the mammals’ great evolutionary potential was held in check by the remarkable success of the dinosaurs. The dino¬ saurs had got off to an evolutionary head start on the mammals, and during their reign on Earth they undoubtedly suppressed the mammals through both competitive interference and predation. But about 65 mil¬ lion years ago, for reasons that are still debated, the dinosaurs disap¬ peared, and the mammals inherited the Earth. The ensuing adaptive radiation of the mammals accomplished a great deal in a very short interval of geological time. Within about 12 million years—by the end of Early Eocene time—there existed most of the orders of mammals that have ever come into being (figure 3). Most striking in their degree of evolutionary divergence from the ancestral forms, which generally resembled small rodents, were the order that includes the whales and the order that includes the bats. In fact, it appears that each of these two groups evolved from small terrestrial ancestors in much less than 12 million years. The test of adaptive radiation asks a simple question: During an inter¬ val when major evolutionary changes were wrought in the history of a group (during an adaptive radiation), how long were individual species in existence? If a species was gradually remodeled by evolution, it might have accumulated enough changes before extinction to deserve a new name. Because such an evolving lineage represents a continuum, its division by a scientist into two or more species is a matter of arbitrary judgment. What is important in the present context is simply that any set of populations that is grouped as a species encompasses only a small amount of variation in form. If a gradual transformation of species was responsible for most of the evolution that occurred in an adaptive radia¬ tion, like the great radiation of modern mammals, then many individual 9

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Figure 3. Diagram depicting the adaptive radiation of mammals during the Cenozoic era, or Age of Mammals. A number of groups of small mammals lived during the Mesozoic era, but it was not until the dinosaurs suffered extinction that the mammals began their major adaptive radiation. The vertical bars of the upper part of the diagram represent individual orders of mammals that existed during the Cenozoic era. The width of a bar indicates the number of genera within an order. Most Cenozoic orders were already present by Early Eocene time. (After P. D. Gingerich.)

Steven M. Stanley

species could only have existed for very short intervals in comparison to the span of time during which the major evolutionary changes of the radi¬ ation took place. The enormous changes that ushered in the Age of Mammals were achieved in less than 12 million years. As it turns out, species were quite stable during this interval. The Big Horn Basin of Wyoming offers the best documentation of their stability. Here the arid climate of today leaves bare for our scrutiny thick sedimentary deposits of the Early Eo¬ cene age (figure 4). During Early Eocene time, the mammalian radiation was in full swing. Within this interval, which lasted no longer than about 5 million years and perhaps as little as 3.5 million, more than a score of new families appeared on Earth, and hundreds of new genera (a genus— plural, genera—is the taxonomic category below the family and above the species). Deposits of the Big Horn Basin accumulated in front of the new¬ ly forming Rocky Mountains, and the continuous supply of sediment from the mountains, resulted in an unusually complete stratigraphic rec¬ ord. The rich fossil faunas here represent life of moist, well-vegetated habitats within a subtropical climatic zone. How long were individual species surviving in the Big Horn Basin region during Early Eocene time, when so many new kinds of mammals were appearing on Earth? Figure 5 reveals that large numbers of species lived for one or two or even three million years without evolving enough to deserve new names. Each of the species represented in this diagram is known from many successive stratigraphic intervals, and yet, as each is traced upward through time, its preserved teeth and bones undergo changes so minor as to be undetectable by experts. The species whose stratigraphic ranges appear in figure 5 form the Willwood fauna, named for the Willwood Formation, which is the se¬ quence of sediments in which the species have been fossilized. The Willwood fauna must represent a large portion of the Early Eocene fauna of North America. In other words, it constitutes a good statistical sample of what species were doing, in an evolutionary sense, while many new kinds of mammals were appearing on Earth. Thus, it appears that we must appeal to rapid evolution within small, localized populations whose history is not to be found in the fossil record if we are to account for the major evolutionary transformations of the great mammalian adaptive radiation. Once formed, species were quite stable—far more stable than a decade ago almost any reputable paleonto¬ logist or biologist would have predicted. The story told by the Big Horn Basin fossils, whose record is unrivaled in quality by faunas elsewhere in the world, offers strong testimony in favor of the punctuational model of evolution. Though lacking fossil records as complete as the record of the Willwood mammal fauna, many other groups of animals and plants offer simi¬ lar evidence of great geological stability. 11

Figure 4. Early Eocene sediments exposed in badlands of the Big Horn Basin of Wyoming. These sediments, which accumulated under warm, moist conditions, contain what may be the finest readily accessible record of ancient mammal species in the world, and these species survived for long stretches of geologic time without undergoing appreciable evolutionary change. (Photo courtesy of the American Museum of Natural History.)

Figure 5. The recognized stratigraphic ranges of sixty-nine species of fossil mammals in the central part of the Big Horn Basin of Wyoming. Time runs horizontally in the diagram, and the bar at the top of the diagram shows the time scale, with uncertainty indicated by the dashed portion. The entire interval depicted here represents the Early Eocene Age, which lasted at least 3.5 million years and at most 5 million years. The range of each species is shown by a horizontal bar, along which vertical ticks indicate occurrence at particular levels. Arrows at the ends of some bars indicate known occurrence of a species in older or younger sediments at other localities. Species known from only one or two samples are not included in the diagram, but the many species that are plotted might all exhibit substantial evolutionary change and yet none does. In fact, virtually no evolution can be detected in any of the species lineages, even though most span between one and three million years. (After D. Schankler and R. T. Bakker.)

2 MILLION

YEARS

HAPLOMYLUS- ECTOCION RANGE LOWER Phenacodus primaevus Oxyoena gu/o Di dymictis leptomy/us Vi verrovus sp. 2 Vi verrovu s sp. 3 Phenacodus intermedius Esthonyx spa tut orius Esthonyx granger/' Coryphodon sp. I An ocodon ursidens Oxyoena intermedia Oxyaeno forc/poto Oxyoena ultimo Tritemnodon strenua Didymictis curtidens Didymictis ly si ten sis M/oc/s exiguus Vutpavus australis Vossacyon promicrodon

^

ZONE UPPER

BUNOPHORUS INTERVAL ZONE

^

^

-~P i—0

Hyopsodus loti dens or miticu/us Esthonyx Pisuleafus Diacodexis robustus Hyrocotherium sp. 2 Phenacodus vortmoni ^ ^ Phenacodus brochypternus ^ Hyopsodus loomisi or minor Hyrocotherium sp. I P/ogiomene multicusp is Phenoco/emur praecox ^ | Pachyoena ossifrogo | Pa c hya en a gracilis ) Paloeonictis occidentalis W Arfia op isthotoma J Tritemnodon sp. I Uintacyon rud/s ^ Ectocion os born ion us ^ M Hap/omy/us speirionus Aphe Use us wapitien sis Aphe/iscus ms/d/osus Tetonoides te n u/ c u/us Tetonius homunculus Cf. Kn ightomys depressus Cf. Da wsonomys minor }

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"Pelycodus" ralstoni Pelycodus mckennai MM Pelycodus tr/gonodus Prodiacodon touricinerei M | Thryptacodon onttquus Protimnocyon otavus Po/aeosinopo veterrimo Didelphodus absarokoe Apatemys chord ini or bellulus Mac roc ra n ion nitens Poromys cf. excovotus MM* Paromys cf. copei k Ectogonus simplex Prototomus sp. I MM* Po loeono don ignavus Viverrovus cant/us M Diacodexis metsiocus *m** Coryphodon sp 2 fe Phenacolemur simonsi N Apatemys J