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  • Vertebrates

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  • S i x t h e d i t i o n

    VertebratesComparative Anatomy, Function, Evolution

    Kenneth V. Kardong, Ph.D.Washington State University

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  • KARDONG: VERTEBRATES: COMPARATIVE ANATOMY, FUNCTION, EVOLUTION, SIXTH EDITION

    Published by McGraw-Hill, a business unit of The McGraw-Hill Companies, Inc., 1221 Avenue of the Americas, New York, NY 10020. Copyright 2012 by The McGraw-Hill Companies, Inc. All rights reserved. Previous editions 2009, 2006, and 2002. No part of this publication may be reproduced or distributed in any form or by any means, orstored in a database or retrieval system, without the prior written consent of The McGraw-Hill Companies, Inc., including,but not limited to, in any network or other electronic storage or transmission, or broadcast for distance learning.

    Some ancillaries, including electronic and print components, may not be available to customers outside theUnited States.

    This book is printed on recycled, acid-free paper containing 10% postconsumer waste.

    1 2 3 4 5 6 7 8 9 0 QDB/QDB 1 0 9 8 7 6 5 4 3 2 1 0

    ISBN 9780073524238MHID 0073524239

    Vice President & Editor-in-Chief: Marty LangeVice President EDP/Central Publishing Services: Kimberly Meriwether DavidPublisher: Janice Roerig-BlongMarketing Manager: Heather WagnerProject Manager: Melissa M. LeickDesign Coordinator: Brenda A. RolwesCover Designer: Studio Montage, St. Louis, MissouriPhoto Research Coordinator: Lori HancockCover Images: Blue-Footed Booby: Photodisc/Getty Images RF; Blue Poison Dart Frog: Digital Vision/GettyImages RF; Fossil Skeleton of the Earliest Bird, Archaeopteryx: Getty Images RF; Allosaurus Skeleton Skull, Jaws,and Teeth: National Geographic/Getty Images RF; Silky Shark: CORBIS RF; Raptor Dinosaur: Alamy RF.Buyer: Susan K. CulbertsonMedia Project Manager: Balaji SundararamanCompositor: S4Carlisle Publishing ServicesTypeface: Goudy 10/12Printer: Quad Graphics

    All credits appearing on page or at the end of the book are considered to be an extension of the copyright page.

    Library of Congress Cataloging-in-Publication Data

    Kardong, Kenneth V.Vertebrates : comparative anatomy, function, evolution / Kenneth V. Kardong. 6th ed.

    p. cm.ISBN-13: 9780073524238ISBN-10: 0073524239

    1. VertebratesAnatomy. 2. VertebratesPhysiology. 3. Anatomy,Comparative. 4. VertebratesEvolution. I. Title.

    QL805.K35 2012571.3'16dc22

    2010050475

    www.mhhe.com

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  • Dedicated with pleasure and gratitude to

    T. H. Frazzetta

    who, like me, remembers fondly

    Richard C. Snyder

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  • CHAPTER one

    INTRODUCTION 1

    CHAPTER two

    ORIGIN OF CHORDATES 48

    CHAPTER three

    THE VERTEBRATE STORY 82

    CHAPTER four

    BIOLOGICAL DESIGN 128

    CHAPTER five

    LIFE HISTORY 161

    CHAPTER six

    INTEGUMENT 212

    CHAPTER seven

    SKELETAL SYSTEM: THE SKULL 240

    CHAPTER eight

    SKELETAL SYSTEM: THE AXIALSKELETON 294

    CHAPTER nine

    SKELETAL SYSTEM: THEAPPENDICULAR SKELETON 325

    CHAPTER ten

    THE MUSCULAR SYSTEM 372

    CHAPTER eleven

    THE RESPIRATORY SYSTEM 413

    CHAPTER twelve

    THE CIRCULATORY SYSTEM 451

    CHAPTER thirteen

    THE DIGESTIVE SYSTEM 503

    CHAPTER fourteen

    THE UROGENITAL SYSTEM 545

    CHAPTER fifteen

    THE ENDOCRINE SYSTEM 592

    CHAPTER sixteen

    THE NERVOUS SYSTEM 625

    CHAPTER seventeen

    SENSORY ORGANS 671

    CHAPTER eighteen

    CONCLUSIONS 714

    vi

    Brief Contents

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  • Preface xv

    CHAPTER oneINTRODUCTION 1

    COMPARATIVE VERTEBRATE MORPHOLOGY 1Designs of Students 2Vertebrate DesignForm and Function 3Grand Design 3

    HISTORICAL PREDECESSORSEVOLUTION 3The Process behind the Change 4Linnaeus 4Naturalists 5J-B. de Lamarck 5

    Acquired Characteristics 6Upward to Perfection 7

    Natural Selection 7A. R. Wallace 7Charles Darwin 8Critics and Controversy 9

    HISTORICAL PREDECESSORSMORPHOLOGY 10Georges Cuvier 10Richard Owen 11

    WHY ARE THERE NO FLYING ELEPHANTS? 14

    MORPHOLOGICAL CONCEPTS 14Similarities 14Symmetry 15Segmentation 16

    EVOLUTIONARY MORPHOLOGY 18Function and Biological Role 18Preadaptation 18Evolution as Remodeling 20

    PHYLOGENY 20Of Bean Stalks and Bushes 20Simplification 22Patterns of Phylogeny 23Grades and Clades 23

    PALEONTOLOGY 29Fossilization and Fossils 29Recovery and Restoration 31From Animal to Fossil 34Dating Fossils 34

    Stratigraphy 36Index Fossils 36Radiometric Dating 37Geological Ages 38

    TOOLS OF THE TRADE 40The Question 40The Function 41The Biological Role 45

    OVERVIEW 47

    CHAPTER twoORIGIN OF CHORDATES 48

    CHORDATE PHYLOGENY 48

    CHORDATE CHARACTERISTICS 51Notochord 52Pharyngeal Slits 52Endostyle or Thyroid Gland 53Dorsal and Tubular Nerve Cord 53Postanal Tail 54Chordate Body Plan 54

    PROTOCHORDATES 54Hemichordata 55

    EnteropneustaAcorn Worms 56Pterobranchia 59Hemichordate Phylogenetic Affinities to Chordates 60Hemichordate Phylogenetic Affinities to Echinoderms 60

    Cephalochordata 61Urochordata 66

    AscidiaceaSea Squirts 67Larvacea (Appendicularia) 70Thaliacea 73

    Overview of Protochordates 73

    vii

    Contents

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  • CHORDATE ORIGINS 74Chordates from Annelids and Arthropods 75Chordates from Echinoderms 75

    Auricularian Hypothesis 76Larval Echinoderm to Chordate Tadpole 77

    Chordate Origins and Phylogeny 77

    OVERVIEW 80

    CHAPTER threeTHE VERTEBRATESTORY 82

    INTRODUCTION 82Innovations 83

    Vertebral Column 83Head 84

    Origin of Vertebrates 84Step 1: Prevertebrate 84Step 2: Agnathan 85Step 3: Gnathostome 85

    Vertebrate Classification 86

    AGNATHANS 86Living Agnathans 86

    Myxinoidea 86Petromyzontida 88

    Early Vertebrate Fossils 89Conodonts 89Ostracoderms 90Pteraspidomorpha 92Other Ostracoderms (Osteostracans, Anaspids, Thelodonts) 92

    Overview of Agnathan Evolution 93

    GNATHOSTOMES 94Placodermi 94Chondrichthyes 95

    ElasmobranchiiSharks and Rays 96HolocephaliChimaeras 97

    TELEOSTOMI 97Acanthodii 97Osteichthyes 98

    Actinopterygii 99Sarcopterygii 101

    Overview of Fish Phylogeny 104

    TETRAPODS 104Primitive Tetrapods 104

    Labyrinthodonts 104LissamphibiaModern Amphibians 106

    Urodela (Caudata) 107Salientia (Anura) 108

    Gymnophiona (Apoda) 108Lepospondyls 108

    AMNIOTES 108Stem-Amniotes 110Sauropsids 111

    Mesosaurs 111Reptilia 111

    Synapsida 118Pelycosauria 120Therapsida 120Mammalia 122

    OVERVIEW 126

    CHAPTER fourBIOLOGICAL DESIGN 128

    INTRODUCTION: SIZE AND SHAPE 128

    SIZE 129Relationships among Length, Area, and Volume 129Surface Area 133Volume and Mass 133

    SHAPE 134Allometry 134Transformation Grids 134

    ON THE CONSEQUENCES OF BEING THE RIGHT SIZE 137

    BIOMECHANICS 137Fundamental Principles 138

    Basic QuantitiesLength, Time, and Mass 138Units 139Derived QuantitiesVelocity, Acceleration, Force, and Relatives 139Reference Systems 140Center of Mass 140Vectors 140

    Basic Force Laws 141Free Bodies and Forces 141Torques and Levers 142Land and Fluid 144

    Life on Land: Gravity 144Life in Fluids 145

    Machines 147Strength of Materials 148

    Loads 149Biological Design and Biological Failure 149

    Tissue Response to Mechanical Stress 151Responsiveness of Bone 151

    BIOPHYSICS AND OTHER PHYSICAL PROCESSES 156Diffusion and Exchange 156

    Pressures and Partial Pressures 156Countercurrent, Concurrent, and CrosscurrentExchange 156

    viii Contents

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  • Optics 158Depth Perception 158Accommodation 158

    OVERVIEW 159

    CHAPTER fiveLIFE HISTORY 161

    INTRODUCTION 161

    EARLY EMBRYOLOGY 163Fertilization 163Cleavage 164

    Amphioxus 164Fishes 165Amphibians 165Reptiles and Birds 165Mammals 166Overview of Cleavage 167

    Gastrulation and Neurulation 167Amphioxus 169Fishes 169Amphibians 171Birds and Reptiles 171Mammals 173

    ORGANOGENESIS 176Histogenesis 177Epithelium 177

    Covering and Lining Epithelium 179Glandular Epithelium 180

    Connective Tissues 180General Connective Tissues 181Special Connective Tissues 181

    Bone Development and Growth 182Endochondral Bone Development 183Intramembranous Bone Development 184Comparative Bone Histology 186Bone Remodeling and Repair 186Joints 187

    Neural Crest and Ectodermal Placodes 189

    EXTRAEMBRYONIC MEMBRANES 190Reptiles and Birds 190Mammals 191

    Eutherian Placenta 192Other Placentae 193

    OVERVIEW OF EARLY EMBRYONICDEVELOPMENT 195

    DEVELOPMENT OF THE COELOM AND ITSCOMPARTMENTS 195

    MATURATION 197Metamorphosis 197

    Heterochrony 198Peramorphosis 199Paedomorphosis 199

    ONTOGENY AND PHYLOGENY 201Biogenetic Law 201Von Baers Law 202Overview of the Biogenetic Laws 202Hox Genes and Their Kingdoms 204

    Egg to Adult 204Shaping Up: Positions and Parts 204Evolutionary Significance 204

    Epigenomics 205Induction 206Phylogeny 206

    OVERVIEW 210

    CHAPTER sixINTEGUMENT 212

    EMBRYONIC ORIGIN 213

    GENERAL FEATURES OF THE INTEGUMENT 213Dermis 213Epidermis 215

    PHYLOGENY 216Integument of Fishes 216

    Primitive Fishes 217Chondrichthyes 217Bony Fishes 218

    Integument of Tetrapods 219Amphibians 219Reptiles 220Birds 221Mammals 226

    SPECIALIZATIONS OF THE INTEGUMENT 232Nails, Claws, Hooves 232Horns and Antlers 233Baleen 235Scales 235Dermal Armor 235Mucus 236Color 236

    OVERVIEW 237

    CHAPTER sevenSKELETAL SYSTEM: THE SKULL 240

    INTRODUCTION 241

    CHONDROCRANIUM 241Embryology 241

    Contents ix

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  • SPLANCHNOCRANIUM 243Embryology 243Origin of Jaws 245Types of Jaw Attachments 246

    DERMATOCRANIUM 247Parts of the Dermatocranium 248

    Dermal Bone Series 248

    OVERVIEW OF SKULL MORPHOLOGY 249Braincase 249Jaws 252Hyoid Apparatus 252

    CRANIAL KINESIS 252

    PHYLOGENY OF THE SKULL 254Agnathans 254

    Early Vertebrates 254Ostracoderms 254Cyclostomes 254Gnathostomes 254Fishes 255Early Tetrapods 262Primitive Amniotes 263Modern Reptiles 267Birds 271Synapsids 273

    OVERVIEW OF SKULL FUNCTION AND DESIGN 283Prey Capture 284

    Feeding in Water 284Feeding in Air 286

    Swallowing 287

    OVERVIEW 287Cranial Neural Crest 287Emergence of Mammals 288Evolutionary Modifications of Immature Forms:Akinesis in Mammals 290Composite Skull 291

    CHAPTER eightSKELETAL SYSTEM: THE AXIALSKELETON 294

    INTRODUCTION 294BASIC COMPONENTS 295

    Vertebrae 295Regions of the Vertebral Column 295Centra 295

    Ribs 298Sternum 299Gastralia 299

    EMBRYONIC DEVELOPMENT 301Fishes 301Tetrapods 302

    PHYLOGENY 304Fishes 304

    Agnathans 304Gnathostomes 304

    Tetrapods 309Early Tetrapods 309Amniotes 313

    FORM AND FUNCTION 315Fluid Environment 315Terrestrial Environment 316Design of Vertebrae 318

    Direction of the Neural Spine 318Height of the Neural Spine 318

    Regionalization of the Vertebral Column 319

    OVERVIEW 322

    CHAPTER nineSKELETAL SYSTEM: THEAPPENDICULAR SKELETON 325

    INTRODUCTION 325

    BASIC COMPONENTS 326Fins 326Limbs 326

    ORIGIN OF PAIRED FINS 327Gill-Arch Theory 327Fin-Fold Theory 328Embryonic Development of Tetrapod Limbs 330

    PHYLOGENY 331Fishes 331

    Agnathans 331Placoderms 331Chondrichthyans 333Acanthodians 334Bony Fishes 334

    Tetrapods 336Pectoral Girdle 336Pelvic Girdle 339Manus and Pes 339

    EVOLUTION OF THE APPENDICULARSYSTEM 346

    Dual Origin of the Pectoral Girdle 346Adaptive Advantage of Lobe Fins 347Onto the Land 347

    FORM AND FUNCTION 348Swimming 349Terrestrial Locomotion 350

    Early Gaits 350Early Modes of Locomotion 350

    x Contents

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  • Cursorial Locomotion 353Aerial Locomotion 358

    Origin of Bird Flight 367Fossorial Locomotion 369

    OVERVIEW 370

    CHAPTER tenTHE MUSCULAR SYSTEM 372

    INTRODUCTION 372ORGANIZATION OF MUSCLES 373

    Classification of Muscles 373Skeletal Muscle 374Cardiac Muscle 375Smooth Muscle 375

    Structure of Skeletal Muscles 375Tendons 376Basis of Muscle Contraction 376

    Resting and Active Muscle 376Molecular Mechanisms of Contraction 376

    MUSCLE FUNCTION 377Muscle Fibers 377

    Tension-Length Curves for a Single Muscle Fiber 377Properties of Muscle Fibers 377

    Muscle Organs and Fibers 379Whole Muscle Force Generation 379Tension-Length Curves for a Whole Muscle 380Graded Force 380Cross-Sectional Area 383Fiber Orientation 383Velocity of Shortening 385Distance of Shortening 385

    Bone-Muscle Lever Systems 385Sequencing of Muscle Actions 387Overview of Muscle Mechanics 388Muscle Actions 388Muscle Homologies 390

    EMBRYONIC ORIGIN OF MUSCLES 391Postcranial Musculature 392

    Appendicular Musculature 392Axial Musculature 393

    Cranial Musculature 393Jaw and Pharyngeal Musculature 393Extrinsic Eye Muscles 393

    COMPARATIVE ANATOMY 394Postcranial Musculature 394

    Axial Musculature 394Appendicular Musculature 397

    Cranial Musculature 405Branchiomeric Musculature 405Hypobranchial Musculature 408

    OVERVIEW 411

    CHAPTER elevenTHE RESPIRATORYSYSTEM 413

    INTRODUCTION 413RESPIRATORY ORGANS 416

    Gills 416Gas Bladders 416

    Lungs 416Swim Bladders 416

    Cutaneous Respiratory Organs 417Accessory Air-breathing Organs 417Breathing and Embryos 418

    VENTILATORY MECHANISMS 421Cilia 421Muscular Mechanisms 421

    Water Ventilation: Dual Pump 421Air Ventilation: Buccal Pump 422Air Ventilation: Aspiration Pump 423

    PHYLOGENY 424Agnathans 424Elasmobranchs 426Bony Fishes 427Overview of Fish Respiration 427

    Gills 427Lungs and Swim Bladders 428

    Amphibians 430Amphibian Larvae 430Amphibian Adults 432

    Reptiles 433Mammals 435

    Ventilation 435Gas Exchange 437

    Birds 437

    FORM AND FUNCTION 438Patterns of Gas Transfer 438Rates of Gas Transfer 443Breathing in Water 444Breathing in Air 444

    EVOLUTION OF RESPIRATORYORGANS 444

    Acid-Base Regulation 444Ventilation 446

    Ciliary Pumps 446Muscular Pumps 446

    Water-to-Land Transition 446Air-breathing Organs 446Advantages of Movement to Land 448Air-breathing Mechanisms 448

    Bird Lungs and Air Sacs 449

    OVERVIEW 450

    Contents xi

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  • CHAPTER twelveTHE CIRCULATORYSYSTEM 451

    INTRODUCTION 451

    CARDIOVASCULAR SYSTEM 452Blood 452Arteries, Veins, and Capillaries 452

    Arteries 453Hemodynamics of Circulation 453Veins 454Microcirculation 454

    Single and Double Circulation 455Embryonic Development of the Cardiovascular System 456Phylogeny of the Cardiovascular System 457

    Arterial Vessels 461Venous Vessels 466

    Hearts 473Basic Vertebrate Heart 473Fishes 475Amphibians 478Reptiles 480Birds and Mammals 489

    Cardiovascular System: Matching Design to Environmental Demands 490

    Accessory Air-breathing Organs 491Diving Birds and Mammals 491Heart Flow 492Ontogeny of Cardiovascular Function 492Fetal Circulation in Placental Mammals 492Changes at Birth 492Heat Transfer 494

    LYMPHATIC SYSTEM 496Lymphatic Vessels 496Lymphatic Tissue 498Form and Function 498

    OVERVIEW 499

    CHAPTER thirteenTHE DIGESTIVE SYSTEM 503

    INTRODUCTION 503Preview 503

    COMPONENTS OF THE DIGESTIVE SYSTEM 504Buccal Cavity 504

    Boundaries 504Palate 505Teeth 506Tongue 516

    Pharynx 517Alimentary Canal 520

    Esophagus 521Stomach 521Intestines 523Cloaca 525Specializations of the Alimentary Canal 525Vascularization of the Gastrointestinal Tract 527Fishes 527Tetrapods 528

    Associated Glands of Digestion 531Oral Glands 531Liver 533Pancreas 533

    FUNCTION AND EVOLUTION OF THE DIGESTIVESYSTEM 535

    Absorption 535Feces 535Mechanical Breakdown of Food 536

    Mastication 536Gizzards 536

    Chemical Breakdown of Food 536Foregut Fermentation 537Hindgut Fermentation 540

    Foregut versus Hindgut Fermenters 540Size and Fermentation 541

    Digesting Toxins 542Feeding and Fasting 542

    OVERVIEW 543

    CHAPTER fourteenTHE UROGENITALSYSTEM 545

    INTRODUCTION 545

    URINARY SYSTEM 545Structure of the Mammalian Kidney 545Embryonic Development 547

    Nephrotome to Nephric Tubules 547Tripartite Concept of Kidney Organization 548

    Kidney Phylogeny 551Fishes 551Tetrapods 552

    Kidney Function and Structure 553Excretion: Removing the Products of NitrogenMetabolism 553Osmoregulation: Regulating Water and Salt Balance 555

    Evolution 562Preadaptation 562Origin of Vertebrates 562

    xii Contents

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  • REPRODUCTIVE SYSTEM 563Structure of the Mammalian Reproductive System 563Embryonic Development 563

    Gonads and Gametes 563Reproductive Tracts 565Overview 565

    Female Reproductive System 567Ovary 567Genital Ducts 567Oviduct 569Uterus 570

    Male Reproductive System 572Testis 572Genital Ducts 572Copulatory Organs 576

    Cloaca 582Urinary Bladder 586Function and Evolution 588

    Potency and Fertility 588External and Internal Fertilization 588Delays in Gestation 589

    OVERVIEW 589

    CHAPTER fifteenTHE ENDOCRINE SYSTEM 592

    SURVEY OF ENDOCRINE ORGANS 592Thyroid Gland 592

    Structire and Phylogeny 592Function 594

    Ultimobranchial Body and Parathyroid Gland 596Ultimobranchial Body 596Parathyroid Gland 597Form and Function 598

    Adrenal Gland 598Structure and Phylogeny 598Function 601

    Pancreatic Islets 602Structure and Phylogeny 602Function 602

    Pituitary Gland 604Structure 604Phylogeny 606Function 607

    Gonads 610Pineal Gland 610Secondary Endocrine Organs 611

    Gastrointestinal Tract 611Kidneys 612

    ENDOCRINE COORDINATION 613Mammalian Reproduction 613

    Male 613Female 613

    Metamorphosis in Frogs 616Fundamentals of Hormonal Control 620

    Functional and Structural Linkage 620Target Tissue Responses 620

    The Endocrine System and the Environment 621

    EVOLUTION 622

    OVERVIEW 624

    CHAPTER sixteenTHE NERVOUS SYSTEM 625

    INTRODUCTION 625Types of Cells within the Nervous System 625

    Neuroglia 625Neurons 625

    Transmission of Information 626Neurosecretory Cells 628

    PERIPHERAL NERVOUS SYSTEM 628Spinal Nerves 629Cranial Nerves 630Evolution 637Functions of the Peripheral Nervous System 638

    Spinal Reflexes 638The Autonomic Nervous System 641

    CENTRAL NERVOUS SYSTEM 645Embryology 646Spinal Cord 647

    Spinal Reflexes 647Spinal Tracts 650

    Brain 652Phylogeny 652Form and Function 654Functional Associations of Parts of the Central Nervous System 666

    Limbic System 666

    OVERVIEW 669

    CHAPTER seventeenSENSORY ORGANS 671

    INTRODUCTION 671

    COMPONENTS OF A SENSORY ORGAN 672

    GENERAL SENSORY ORGANS 672Free Sensory Receptors 672Encapsulated Sensory Receptors 673Associated Sensory Receptors 673

    Proprioception 673

    Contents xiii

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  • Mechanisms of Perceiving Stimuli from General Sensory Receptors 674

    SPECIAL SENSORY ORGANS 674Chemoreceptors 674

    Nasal Passages 675Vomeronasal Area 677Mouth 678

    Radiation Receptors 679Photoreceptors 681Infrared Receptors 691

    Mechanoreceptors 693Lateral Line System 694Vestibular Apparatus 695Auditory System 696Functions of the Ear 701

    Electroreceptors 709Structure and Phylogeny 709Form and Function 709

    Additional Special Sensory Organs 713

    OVERVIEW 713

    CHAPTER eighteenCONCLUSIONS 714

    INTRODUCTION 714

    STRUCTURAL ANALYSIS 717

    FUNCTIONAL ANALYSIS 718How Does It Work? 718Functional Coupling, Functional Compromise 719Multiple Functions 720Performance 721

    ECOLOGICAL ANALYSIS 722

    EVOLUTIONARY ANALYSIS 722Historical Constraints 722Primitive and Advanced 722Diversity of Type/Unity of Pattern 723Mosaic Evolution 725Morphology and Modules 725

    MODE AND TEMPO OF EVOLUTION 727Remodeling 728Embryonic Changes 729Hox Genes 729Evolutionary Significance 730

    THE PROMISE OF VERTEBRATE MORPHOLOGY 730

    APPENDIX A

    VECTOR ALGEBRA 731

    APPENDIX B

    INTERNATIONAL UNITS (SI) 733

    APPENDIX C

    COMMON GREEK AND LATIN COMBINING FORMS 736

    APPENDIX D

    CLASSIFICATION OF CHORDATES LINNAEAN 740

    CLASSIFICATION OF CHORDATES CLADISTIC 743

    GLOSSARY 744CREDITS 758INDEX 764

    xiv Contents

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  • xv

    If you are a student coming to the study of vertebrates forthe first time, several introductory remarks may be helpful,especially on how this textbook will support your work. First,the discipline of vertebrate biology is diverse and inclusive.It brings together themes from molecular biology, genes andgenomes, evolution and embryology, biomechanics, andexperimental physiology, and it incorporates continuing andastonishing new fossils into the vertebrate story. Much ofwhat you have met in earlier courses you will meet againhere in an integrated way.

    Second, to unify these themes, I have again written andrevised this sixth edition within the unifying framework ofform, function, and evolution. The first few chapters set thisup, and the subsequent chapters treat vertebrates system bysystem. You may notice that each of these subsequent chaptersbegins with a discussion of morphology, followed by a discus-sion of function and evolution. Each chapter is therefore self-containedform, function, evolution.

    Third, as a student you likely enter this course aftersome background in the sciences, perhaps expecting to equipyourself with practical knowledge useful later in professionalschools or in health-related careers. Certainly this course, inpart, delivers such practical information. But because verte-brate morphology is an integrative discipline, it bringstogether physiology, embryology, behavior, and ecology andalso deploys modern methods of systematics and new findsin paleontology. Consequently, you will move beyondmemorizing facts in isolation or as an end in themselves,and instead begin to meet and understand larger concepts.What may come as a surprise is that many theories, espe-cially evolutionary theories within vertebrate biology, arestill unsettled and unresolved, inviting a new idea or freshapproach open to anyone. This is one of the reasons I haveincluded various controversies, and support your efforts tobecome engaged in the thinking and scientific process.

    For faculty who have used this textbook before, youwill find it retains a familiar and inviting organization withthe science updated and the student support enhanced. Forthose coming to this textbook for the first time, you willnotice that the morphology receives generous treatmentwithin a phylogenetic context. But, today we expect ourstudents to develop academic and professional skills beyond

    just facility with anatomical terminology. In general, weexpect our students to develop skills in critical thinking and afacility with scientific concepts. Each of us will find our ownway of composing a course in vertebrate morphology thatserves such course objectives. This textbook was written tosupport such course objectives as individual instructors buildtheir courses. It is flexible. One need not move through inthe same order presented here, but chapters can be assignedin the order suited to the organization of ones own course.Because each chapter integrates form, function, and evolutionpertinent to that system, each chapter is coherent withinitself. Although discussed in earlier editions, let me repeatthe specific strategy built into this textbook to improvestudent success and to help them develop skills in criticalthinking and conceptual understanding.

    For the Student

    A number of practical features within the textbook enhanceits usefulness for students. It is richly illustrated with figuresthat include new information and provide fresh perspective.Each chapter opens with an outline. Important conceptsand major anatomical terms are boldfaced. Cross refer-ences direct students to other areas of the text where theycan refresh their understanding or clarify an unfamiliarsubject. Each chapter concludes with a chapter overview,which draws attention to some of the concepts developedwithin the chapter. Box Essays are included along the wayin most chapters. Their purpose is to present subjects orhistorical events that students should find interesting and,perhaps from time to time, even fun. A glossary of defini-tions is included at the end of the book.

    In addition to its practical features, the textbook alsouses selected topics within vertebrate morphology, function,and evolution to develop student skills in critical thinkingand mastery of concepts within a coherent framework.

    Critical Thinking

    Within the sciences, critical thinking is the ability to marshalfactual information into a logical, reasoned argument. Espe-cially if accompanied by a laboratory, a course in vertebrate

    Preface

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  • morphology delivers hands-on experience with the anatomyof representative animals. Students can be directly engagedin the discovery of vertebrate form. But they can be encour-aged to go beyond this. Instructors can lead students intolarger issuesHow does it function? How did it evolve? Forexample, early on in the textbook, students are introduced toTools of the Trade, the methods by which we empiricallyexamine how parts work and how we can place organismswithin a phylogenetic context. After a discussion of basicmorphology, each chapter discusses how these systems workand how they evolved.

    I have deliberately included new, neglected, orcompeting views on function and evolution. Many of theseideas come from Europe, where they have been known fora long time. Personally, I find many of these ideascompelling, even elegant. Others strike me, frankly, as thinand unconvincing. Despite my own skepticism, a fewcontrary ideas are included. My purpose is to get studentsto think about issues of form, function, and evolution.

    Several theories on the evolution of jaws arediscussed, as are several theories of the origin of paired fins.Often students expect that today we have the finalanswers. Students implore, Just tell me the answer. Thedebate about dinosaur physiology is a wonderful opportu-nity to show students the ongoing process of scientificinvestigation. Most have seen the Hollywood films andexpect the issue settled. But we know that science is aprocess of refinement, challenge, and sometimes revolu-tionary change. One Box Essay sets forth the early case fordinosaur endothermy. That debate spawned further investi-gation that now returns to challenge such a view ofdinosaurs as hot-blooded beasts. The second Box Essayon dinosaur endothermy presents this newer and contraryevidence, and thereby showcases how, even in extinctanimals, it is possible to test hypotheses about their physi-ology, morphology, and lifestyles.

    Concepts

    Vertebrate morphology also helps develop an appreciationand understanding of the scientific concepts that unitebiology and reflect on how science works. As John A.Moore put it, science is a way of knowing (Moore,American Zoologist, 1988). Comparative morphology throwsinto clear relief differences and similarities betweenorganisms. The concepts of homology, analogy, andhomoplasy help us understand the basis of these compara-tive features. Many of the concepts were birthed in thenineteenth century and have grown into the guidingthemes of biology today. Evolution, defined as descent withmodification through time, is one of the foundationconcepts in biology. Vertebrate morphology provides ashowcase of adaptive change on the basic vertebrate bodyplan. But evolution is change in a highly integratedorganism, a connected system of parts and their functions.This too was recognized within the nineteenth century,

    suggesting constraints on evolutionary modification. Verte-brate morphology provides compelling examples of how anintegrated organism might evolve. For example, a remark-able fossil record documents an undeniable change in jawarticulation within synapsids, seeing the two participatingbones (articular, quadrate) of basal synapsids replaced bytwo different bones in derived groups, including mammals.Fossil intermediates between the two conditions mark theanatomical changes, but they also suggest how functionalchanges, which must accompany evolving systems, alsochange without disrupting performance.

    Within many vertebrate systems, the close coupling ofform and function with lifestyle is illustrated. Built on a basicvertebrate plan, the tetrapod locomotor system illustrates theclose relationship between limbs and axial skeleton, and thetype of locomotionflight, cursorial, burrowing. The cardio-vascular system, especially in organisms that exploit waterand air, illustrates the close relationship between vascularmorphology and the physiological flexibility that permits.The basic concepts of form, function, and adaptive evolutionparade before us as we move from system to system in verte-brate morphology.

    Evolution proceeds most often by remodeling, modi-fication of a basic underlying plan, not by all new construc-tion. This is illustrated in the skeletal system, as well aswithin the cardiovascular (aortic arches) system.

    Organizational Strategy and Rationale

    I have written this book within the unifying framework ofform, function, and evolution. These are common themesthat run throughout. The vertebrate groups are organizedphylogenetically, and their systems discussed within such acontext. Morphology is foremost, but I have developed andintegrated an understanding of function and evolution intothe discussion of anatomy of the various systems. The firstfive chapters prepare the way.

    Chapter 1 introduces the discipline, evaluates theintellectual predecessors to modern morphology, definescentral concepts, and alerts students to misunderstandingsthey may unknowingly bring with them to the study ofevolutionary processes. Chordates and their origins arecovered in chapter 2. Considerable attention is given to theneglected protochordates and their evolution. This sets thestage for an extended discussion of the cast of characters inthe vertebrate radiation, which occupies us for the remainderof the book, beginning next in chapter 3. Here we discussvertebrates, their origins, and basic taxonomic relationships.Chapter 4 introduces basic concepts of biomechanics andbiophysics, preparing for their use later in understandingaspects of vertebrate design. Chapter 5 includes a summaryof descriptive embryology and concludes with a discussionof the role embryonic processes play in vertebrate evolu-tionary events.

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  • The remaining chapters develop each major system.Besides carrying overall themes, each chapter internallyfollows a consistent organization. Each begins with a basicintroduction to the morphology, and then proceeds todiscuss function and evolution. This way, the overallthemes are repeated in each chapter, bringing consistencyof presentation to each chapter and coherence throughout.

    New and Expanded in the Sixth Edition

    Remarkable and innovative research continues to enrichthe discipline of vertebrate biology. Much of this is addedto this new edition.

    Feathers. We now know that the regeneration offeathers is a much more complex process than previouslythought, thanks to new research. The inductive interac-tion between skin dermis and epidermis deep within thefeather follicle establishes a zone of cell proliferationproducing the feather proper, and a patterning zone wherefates of newly formed cells are established in a remarkablyintricate system. Feathers evolved before birds. This meansthat these skin specializations addressed biological rolesbefore they addressed flight. This new description offeathers therefore opens up a new perspective on this majorevolutionary event. This is discussed in the chapter onintegument (chapter 6) with new supportive illustrations.

    Cardiac Shunt. The hearts of living amphibians andreptiles permit a right-to-left shunting of blood, therebybypassing a trip to the lungs, but instead blood high in CO2heads out directly to systemic tissues. This cardiac shuntwas thought to be important during diving, where lungsquickly become depleted of oxygen and little physiologicalbenefit attended sending blood to the lungs. This may stillbe true, but new and speculative research suggests another,or an additional, explanation for the shunt. This bloodbypassing the lungs may bring CO2 to digestive organsprocessing a meal and thereby increase effectiveness, espe-cially in ectothermic vertebrates. This new insight isdiscussed and illustrated in the circulatory system chapter(chapter 12).

    Evo-Devo. I have built on the genetic section onevolution and development (chapter 5) introduced inearlier editions. This has included additional illustrationsand revised accompanying text. Examples throughout showhow master control genes (Hox genes) and developmentalgenes preside over the construction of the vertebrate bodyand its various systems. In the concluding chapter, Iemphasize how these special genetic gene sets provide thebasis for major evolutionary changes.

    Phylogenetic Relationships. Thanks to continuing use ofimproved genetic and morphological data sets, phylogeneticrelationships are becoming better resolved, and naturalgroups are emerging from this analysis with better clarity.This is the basis for revision in chapter 3, but these updatedphylogenies are carried forward throughout the book.

    Turning over Chordates. New developmental genetics,discussed in the previous edition, informs us that theimmediate chordate ancestors flipped over, reversing dorsaland ventral surfaces. That view seems to hold still andtherefore remains the surprising basis of the chordate bodyplan today.

    Updated and Revised. Countless changes and revisionsthroughout this new edition have been made, some major,some small. These changes have corrected misinformation,updated information, and often better clarified an explana-tion. For this I am indebted to students, reviewers, andcolleagues for bringing these suggestions to my attention.

    Serving the Student. Features of the textbook have beenfurther expanded to make its presentation more clear andinviting. The use of color brightens these sections of thebook. Color has also been used to better correlate andcompare structures between figures in these chapters. Wherefeasible, within color signatures, for example, I have addedmore color to the illustrations. Many illustrations are new,revised, or relabeled to improve clarity. For example, besidesthose illustrations mentioned earlier, new/revised figuresillustrate an updated full skeleton of Ichthyostega, pectoralgirdle evolution, air bladder evolution, and cardiovascularblood flow; and various changes have been made in figureselsewhere. Scientific references are available to the students,online, if they would like to follow up or read more about aparticular subject. The accompanying laboratory dissectionguide (authored with E. J. Zalisko) is closely cross-referencedto this textbook. In addition, selective functional laborato-ries are available, online, to provide students with firsthandexperience of working between the anatomy and its func-tional and evolutionary significance.

    Serving Instructors. This sixth editionnew, revised,updatedcan serve as reference and resource support forthe course you put together on vertebrates. In addition tothis, resources are available to you online. The functionallaboratories may be downloaded and used as they supple-ment your course. PowerPoint images, chapter by chapter,are available online along with additional images fromMcGraw-Hill that can be used to compose lectures andlaboratory presentations.

    Supplements

    Comparative Vertebrate Anatomy: A Laboratory Dissection Guide

    Newly revised, Comparative Vertebrate Anatomy: A Labora-tory Dissection Guide, Sixth Edition, by Kenneth V. Kardongand Edward J. Zalisko, is now available. At the end of thisdissection guide, the authors include a Student Art Note-book. This notebook is a reprinted collection of the mostimportant and commonly used dissection figures in thecurrent edition of the laboratory manual. It addresses a frus-tration inherent in most dissection guides, especially when

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  • comparing homologous systems between representativeanimals, of having to flip between text and distantly placedillustrations. This laboratory manual weaves the functionaland evolutionary concepts from this textbook, Vertebrates:Comparative Anatomy, Function, Evolution, into themorphological details of the laboratory exercises. Usingicons, the laboratory manual identifies cross references tothis textbook, so students can quickly move from the dissec-tion guide to this textbook to consult the expandedtreatment of function and evolution. Each chapter of thedissection guide first introduces the system, makes compar-isons, and demonstrates common themes in the animalsystems. Then the written text carefully guides studentsthrough dissections, which are richly illustrated. Anatom-ical terms are boldfaced and concepts italicized. The dissec-tion guide is written so that instructors have the flexibilityto tailor-make the laboratory to suit their needs.

    Website for Vertebrates: ComparativeAnatomy, Function, Evolution, Sixth Edition

    A website for this textbook, available at www.mhhe.com/kardong6e, includes further useful information upon whichinstructors can depend and students can consult. Here canbe found the functional laboratories, helpful in a linkedlaboratory if available, or helpful selectively in lecture. End-of-chapter selected references, giving students a start intothe literature, are located here. Instructors can also accessprintable pages of illustrations that can be used as trans-parency masters, lecture handouts, or incorporated intoPowerPoint presentations.

    Biology Digitized Video Clips

    McGraw-Hill is pleased to offer digitized biology videoclips on DVD! Licensed from some of the highest-qualityscience video producers in the world, these brief segmentsrange from about five seconds to just under three minutesin length and cover all areas of general biology, from cellsto ecosystems. Engaging and informative, McGraw-Hillsdigitized biology videos will help capture students interestwhile illustrating key biological concepts and processes.Includes video clips on mitosis, Darwins finches, amoebalocomotion, tarantula defense, nematodes, bird/waterbuffalo mutualism, echinoderms, and much more! ISBN:978-0-07-312155-0 (MHID: 0-07-312155-X)

    Electronic Textbook

    CourseSmart is a new way for faculty to find and revieweTextbooks. Its also a great option for students who areinterested in accessing their course materials digitally andsaving money. CourseSmart offers thousands of the mostcommonly adopted textbooks across hundreds of coursesfrom a wide variety of higher education publishers. It is theonly place for faculty to review and compare the full text of

    a textbook online, providing immediate access withoutthe environmental impact of requesting a print copy. AtCourseSmart, students can save up to 50% off the cost of aprint book, reduce their impact on the environment, andgain access to powerful web tools for learningincludingfull text search, notes and highlighting, and email tools forsharing notes between classmates. www.CourseSmart.com

    Acknowledgments

    I am indebted to reviewers, students, and colleagues whohave generously shared with me their suggestions toimprove this edition of the textbook. My hope is that thesecolleagues will see, if not their point of view, at least theirinfluence within this edition, and accept my sincere thanksfor their thoughtful suggestions and criticisms. For theirspecial help I recognize:

    C. G. FarmerUniversity of Utah

    T. H. FrazzettaUniversity of Illinois

    Ira F. GreenbaumTexas A & M University

    Christine M. JanisBrown University

    Jon M. MallattWashington State University

    Stephen M. SecorUniversity of Alabama

    Tamara L. SmithWestridge School

    Keith W. SockmanUniversity of North Carolina at Chapel Hill

    Amanda StarnesEmory University

    Jeanette WynekenFlorida Atlantic University

    It has been a special pleasure for me to work withseveral especially supportive and helpful colleagues. Inparticular, I note the extensive help of Christine M. Janisin several difficult chapters, as well as the patient and espe-cially informative education I received on regeneratingbird feathers from P. F. A. Maderson and W. J. Hillenius.

    For answering my queries, supplying me with theircritical thoughts, and/or for earlier participation in this andprevious editions, I gratefully recognize the following: Neil F. Anderson, Miriam A. Ashley-Ross, Ann CampbellBurke, Walter Bock, Warren W. Burggren, AnindoChoudhury, Michael Collins, Mason Dean, Alan Feduccia,Adrian Grimes, Linda Holland, Marge Kemp, William T. Maple, Jessie Maisano, David N. M. Mbora, David O. Norris, R. Glenn Northcutt, Kathryn Sloan Ponnock,

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  • Michael K. Richardson, Timothy Rowe, John Ruben, J.Matthias Starck, James R. Stewart, Billie J. Swalla, StevenVogel, Alan Walker, and Bruce A. Young.

    It is again a pleasure to work with an artist as accom-plished and knowledgeable as L. Laszlo Meszoly (HarvardUniversity), who contributed beautiful new figures to thisedition.

    I am indebted to the patient, able, and supportivepeople at McGraw-Hill who were so important in bringingthis revised sixth edition along. As on earlier editions,

    Margaret Horn was indispensible as Developmental Editorand Sue Dillon as my favorite copy editor. I thank againthe McGraw-Hill field staff who link the summary effortof all who helped in this revision to faculty and studentswho use it. In turn, these field reps return your commentsof what you do and do not like, and thereby aid in theimprovement of this textbook, making it a shared workin progress.

    To friends and family I remain grateful and thank themfor their support during various editions of this textbook.

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    1

    COMPARATIVE VERTEBRATE MORPHOLOGYDesigns of StudentsVertebrate DesignForm and FunctionGrand Design

    HISTORICAL PREDECESSORSEVOLUTIONThe Process behind the ChangeLinnaeusNaturalistsJ-B. de Lamarck

    Acquired CharacteristicsUpward to Perfection

    Natural SelectionA. R. WallaceCharles DarwinCritics and Controversy

    HISTORICAL PREDECESSORSMORPHOLOGYGeorges CuvierRichard Owen

    WHY ARE THERE NO FLYING ELEPHANTS?

    MORPHOLOGICAL CONCEPTSSimilaritiesSymmetrySegmentation

    EVOLUTIONARY MORPHOLOGYFunction and Biological RolePreadaptationEvolution as Remodeling

    PHYLOGENYOf Bean Stalks and BushesSimplificationPatterns of PhylogenyGrades and Clades

    PALEONTOLOGYFossilization and FossilsRecovery and RestorationFrom Animal to FossilDating Fossils

    StratigraphyIndex FossilsRadiometric DatingGeological Ages

    TOOLS OF THE TRADEThe QuestionThe FunctionThe Biological Role

    OVERVIEW

    Introduction

    Comparative Vertebrate MorphologyComparative morphology deals with anatomy and its signif-icance. We focus on animals, in particular vertebrate ani-mals, and the significance these organisms and theirstructure may hold. The use of comparison in comparativemorphology is not just a convenience. It is a tool. Compari-son of structures throws similarities and differences into bet-ter relief. Comparison emphasizes the functional and

    11C H A P T E R

    evolutionary themes vertebrates carry within their struc-tures. Comparison also helps formulate the questions wemight ask of structure.

    For example, different fishes have different tail shapes.In the homocercal tail, both lobes are equal in size, makingthe tail symmetrical (figure 1.1a). In the heterocercal tail,found in sharks and a few other groups, the upper lobe is

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  • elongated (figure 1.1b). Why this difference? The homocer-cal tail is found in teleost fishessalmon, tuna, trout, andthe like. These fishes have a swim bladder, an air-filled sacthat gives their dense bodies neutral buoyancy. They neithersink to the bottom nor bob to the surface, so they need notstruggle to keep their vertical position in the water. Sharks,however, lack swim bladders, and so tend to sink. Theextended lobe of their heterocercal tail provides lift duringswimming to help counteract this sinking tendency. So, thedifferences in structure, homocercal versus heterocercal, arerelated to differences in function. Why an animal is con-structed in a particular way is related to the functionalrequirements the part serves. Form and function are cou-pled. Comparison of parts highlights these differences andhelps us pose a question. Functional analysis helps answerour question and gives us a better understanding of animaldesign. Functional morphology is the discipline that relatesa structure to its function.

    Comparative analysis thus deploys various methods toaddress different biological questions. Generally, compara-tive analysis is used either in a historical or a nonhistoricalcontext. When we address historical questions, we examineevolutionary events to work out the history of life. For exam-ple, on the basis of the comparison of characters, we mayattempt to construct classifications of organisms and theevolutionary phylogeny of the group. Often such historicalcomparisons are not restricted to classification alone butcenter on the process of evolution behind morphologicalunits, such as jaws, limbs, or eyes.

    When we make nonhistorical comparisons, as is fre-quently the case, we look outside an evolutionary context,with no intention of concluding with a classification or

    elucidation of an evolutionary process. Nonhistoricalcomparisons are usually extrapolative. For example, bytesting a few vertebrate muscles, we may demonstrate thatthey produce a force of 15 N (newtons) per square cen-timeter of muscle fiber cross section. Rather than testingall vertebrate muscles, a time-consuming process, we usu-ally assume that other muscles of similar cross section pro-duce a similar force (other things being equal). Thediscovery of force production in some muscles is extrapo-lated to others. In medicine, the comparative effects ofdrugs on rabbits or mice are extrapolated to tentative usein humans. Of course, the assumed similarities uponwhich an extrapolation is based often do not hold in ouranalysis. Insight into the human female reproductive cycleis best obtained if we compare the human cycle with thosein higher primates because primate reproductive cycles,including the human one, differ significantly from those ofother mammals.

    Extrapolation allows us to make testable predictions.Where tests do not support an extrapolation, science is wellserved because this forces us to reflect on the assumptionsbehind the comparison, perhaps to reexamine the initialanalysis of structures and to return with improved hypothe-ses about the animals or systems of interest. Comparisonitself is not just a quick and easy device. The point to empha-size is this: Comparison is a tool of insight that guides ouranalysis and helps us set up hypotheses about the basis of ani-mal design.

    Designs of StudentsSuch philosophical niceties, however, usually do not enticestudents into their first course in morphology. Most studentsfirst venture into a course in vertebrate morphology on theirway into some other profession. Customarily, morphologycourses prepare students headed into technical fields such ashuman medicine, dentistry, or veterinary medicine. In addi-tion, morphology is important to taxonomists who use thestructure of animals to define characters. In turn, these char-acters are used as the basis for establishing relationshipsbetween species.

    Morphology is central to evolutionary biology as well.Many scientists, in fact, would like to see a disciplinedevoted to the combined subject, namely, evolutionarymorphology. Evidence of past evolutionary changes isinscribed in animal structure. Within the amphibian limbare the structural reminders of its fish-fin ancestry; withinthe wing of a bird are the evidences of its derivation from thereptilian forelimb. Each modern group living today carriesforward mementos of the evolutionary course traveled by itsancestors. For many biologists, a study of the morphologicalproducts of the past gives insight into the processes that pro-duced them, insight into the natural forces that drove evo-lutionary changes, and insight into the limitations ofevolutionary change.

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    (a) Homocercal tail

    (b) Heterocercal tailPectoralfin

    FIGURE 1.1 Homocercal and heterocercal fish tails.Form differs because function differs. (a) Sweeping, side-to-sidemovements of the homocercal tail, common in fishes with neutralbuoyancy, drive the body forward. (b) Swimming strokes of theheterocercal tail propel the fish forward, and motion of the longextended upper lobe imparts an upward lift to the posterior endof the fish. Sharks, which are a good deal denser than water, needthe upward forces provided by the extended lobe of the tail tocounteract a tendency to sink.

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  • Vertebrate DesignForm and FunctionMorphology offers more than charitable assistance to otherdisciplines. The study of morphology provides its own plea-sure. It raises unique questions about structure and offers amethod to address these questions. In brief, vertebrate mor-phology seeks to explain vertebrate design by elucidating thereasons for and processes that produce the basic structuralplan of an organism. For most scientists today, evolutionaryprocesses explain form and function. We might hear it saidthat the wings of birds, tails of fishes, or hair of mammalsarose for the adaptive advantages each structure provided,and so they were favored by natural selection. Certainly thisis true, but it is only a partial explanation for the presence ofthese respective features in bird, fish, and mammal designs.The external environment in which an animal design mustserve certainly brings to bear evolutionary pressures on itssurvival, and thus on those anatomical features of its designthat convey adaptive benefits.

    Internal structure itself also affects the kinds of designsthat do or do not appear in animals. No terrestrial vertebraterolls along on wheels. No aerial vertebrate flies through theair powered by a rotary propeller. Natural selection alonecannot explain the absence of wheels in vertebrates. It isquite possible to imagine that wheels, were they to appear incertain terrestrial vertebrates, would provide considerableadaptive advantages and be strongly favored by naturalselection. In part, the explanation lies in the internal limi-tations of the structure itself. Rotating wheels could not benourished through blood vessels nor innervated with nerveswithout quickly twisting these cords into knots. Wheels andpropellers fall outside the range of structural possibility invertebrates. Structure itself contributes to design by the pos-sibilities it creates; evolution contributes to design by thefavored structures it preserves. We must consult both struc-ture and evolution to understand overall design. That is whywe turn to the discipline of morphology. It is one of the fewmodern sciences that addresses the natural unity of bothstructure (form and function) and evolution (adaptationand natural selection). By wrapping these together in anintegrated approach, morphology contributes a holisticanalysis of the larger issues before contemporary biology.Morphology is concerned centrally with the emergent prop-erties of organisms that make them much more than thereduced molecules of their parts.

    Grand DesignVertebrate design is complex, often elegant, and sometimesremarkably precise. To many early-day morphologists, thiscomplexity, this elegance, and this precision implied thedirect intervention of a divine hand in guiding the produc-tion of such sophisticated designs. However, not everyonewas convinced. After all, towering mountain ranges alsooffer spectacular vistas but do not require recourse to divineintervention to explain them. Plate tectonics offers a natu-ral explanation. Under pressure from colliding tectonic

    plates, the Earths crust crumples to produce these ranges.With knowledge, scientific explanations uncover the mys-teries that shroud geological events.

    Similarly, biology has found satisfying natural expla-nations to replace what were once assumed to be directdivine causes. Modern principles of evolution and struc-tural biology offer a fresh approach to vertebrate designand an insight into the processes responsible for producingthat design. Just as processes of plate tectonics help geolo-gists understand the origin of the Earths surface features,structural and evolutionary processes help biologistsunderstand the origin of plant and animal life. Life onEarth is a product of these natural processes. Humans arenot exempt nor are we given special dispensation fromthese processes. Like our fellow vertebrates, humans tooare products of our evolutionary past and basic structuralplan. The study of morphology, therefore, brings us anunderstanding of the integrated processes that forged us.To understand the processes behind our design is to under-stand the product, namely, humans themselves, both whatwe are and what we can become.

    But, I am getting ahead of the story. We have not hadan easy intellectual journey in reaching the clarity of mor-phological concepts we seem to enjoy at the moment. Theprinciples were not always so obvious, the evidence notalways so clear. In fact, some issues prevalent over 100 yearsago remain unresolved. The significance of underlying struc-ture to the evolution of design, central to much of biologyearly in the nineteenth century, is only recently being reex-amined for its potential contribution to modern morphol-ogy. Morphology has often been internally beset by unhappycontentions between those scientists centered on structureand those centered on evolution. To some extent, the fun-damental principles of both structure and evolution havegrown from different intellectual sources and different intel-lectual outlooks. To understand this, we need to examinethe historical development of morphology. Later in thischapter, we examine the intellectual roots of theories aboutstructure. But first, lets look to the intellectual roots of the-ories about evolution.

    Historical PredecessorsEvolution

    The concept of evolution is tied to the name CharlesDarwin (figure 1.2). Yet most persons are surprised to learnthat Darwin was not first, nor was he ever foremost, in pro-posing that organisms evolve. In fact, the idea of changethrough time in animals and plants dates back to ancientschools of Greek philosophy. Over 2,500 years ago, Anaxi-mander developed ideas about the course of change fromfishlike and scaly animals to land forms. Empedocles saworiginal creatures come together in oddly assembled wayshumans with heads of cattle, animals with branches liketrees. He argued that most perished, but only those creatureswho came together in practical ways survived. Even at their

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  • best, these armchair views are more poetic than scientific, soit would be an exaggeration to characterize this Greek philo-sophical thought as a practical predecessor of modern evolu-tionary science. Nevertheless, the idea of evolution existedlong before Darwin, thanks to these Greek philosophers.

    The Process behind the ChangeWhat the Englishman Charles Darwin contributed was notthe idea that species evolve. Rather, Darwin proposed theconditions for and mechanism of this evolutionary change.He proposed three conditions:

    First, if left unchecked, members of any species increasenaturally in number because all possess a high reproductivepotential. Even slow-breeding elephants, Darwin pointed out,could increase from a pair to many millions in a few hundredyears. We are not up to our rooftops in elephants, however,because as numbers increase, resources are consumed at anaccelerating rate and become scarce. This brings about condi-tion two, competition for the declining resources. In turn, com-petition leads to condition three, survival of the few. Darwintermed the mechanism now determining which organismssurvive and which do not natural selection, natures way ofweeding out the less fit. In this struggle for existence, thosewith superior adaptations would, on average, fare better andsurvive to pass on their successful adaptations. Thus, descentwith modification resulted from the preservation by naturalselection of favorable characteristics.

    As simple as this sounds today, Darwins insight was pro-found. He performed no decisive experiment, mixed no chem-icals in test tubes, ground no tissue in a blender. Rather,Darwins insight arose from observation and reflection. The

    controversy over evolutionary processes emerges at one ofthree levelsfact, course, mechanismand asks a differentquestion at each level. The first level addresses the fact of evo-lution and asks if organisms change through time. Did evolu-tion occur? The fact that evolution has occurred is today wellestablished by many lines of evidence, from gene changes tothe fossil record. But this does not mean that all controversiesover evolution are comfortably settled. At the next level, wemight ask: What course did evolution then take? For example,anthropologists who study human evolution usually agree onthe fact that humans did evolve, but they often disagree, some-times violently, over the course of that evolution. Finally wecan ask: What mechanism produced this evolution? At thisthird level in the evolutionary debate, Darwin made his majorcontribution. For Darwin, natural selection was the mecha-nism of evolutionary change.

    Verbal scuffles over the fact, course, and mechanism ofevolution often become prolonged and steamy becauseopponents ask questions at different levels and end up argu-ing at cross-purposes. Each of these questions had to be set-tled historically as well to bring us to an understanding of theevolutionary process. Historians have taken much notice ofthe violent public reaction to Darwins ideas on evolution, areaction spurred by their challenge to religious convention.But what of the scientific climate at that time? Even in sci-entific circles, opinion was strongly divided on the issue oftransmutation of species, as evolution was termed then.The issue initially centered around the fact of evolution. Dospecies change?

    LinnaeusForemost among the scientists who felt that species werefixed and unchangeable was Carl von Linn (17071778), aSwedish biologist who followed the custom of the day bylatinizing his name to Carolus Linnaeus, by which he is mostrecognized today (figure 1.3). Linnaeus devised a system fornaming plants and animals, which is still the basis of mod-ern taxonomy. Philosophically he argued that species wereunchangeable, created originally as we find them today. Forseveral thousand years, Western thought had kept companywith the biblical view, namely, that all species resulted froma single and special act of divine creation, as described inGenesis, and thereafter species remained unchanged.

    Although most scientists during the 1700s sought toavoid strictly religious explanations, the biblical view of cre-ation was a strong presence in Western intellectual circlesbecause it was conveniently at hand and meshed comfort-ably with the philosophical arguments put forth by Linnaeusand those who argued that species were immutable (unchang-ing). However, it was more than just the compatibility ofGenesis with secular philosophy that made the idea ofimmutable species so appealing. At the time, evidence forevolution was not assembled easily, and the evidence avail-able was ambiguous in that it could be interpreted both ways,for or against evolution.

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    FIGURE 1.2 Charles Darwin (18091882), about30 years old and three years back from his voyage aboardH.M.S. Beagle. Although The Origin of Species was still just a fewnotebooks in length and several decades away from publication,Darwin had several accomplishments behind him, including hisaccount of The Voyage of the Beagle, a collection of scientificobservations.At this time, he was also engaged to his cousinEmma Wedgwood, with whom he would live a happy married life.

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  • NaturalistsToday we understand the perfected adaptations of animalsthe trunks of elephants, the long necks of giraffes, thewings of birdsas natural products of evolutionarychange. Diversity of species results. To scientists of an ear-lier time, however, species adaptations reflected the careexercised by the Creator. Diversity of plant and animalspecies was proof of Gods almighty power. Animated bythis conviction, many sought to learn about the Creatorby turning to the study of what He had created. One of theearliest to do so was the Reverend John Ray (16271705),who summed up his beliefs along with his natural historyin a book entitled The Wisdom of God Manifested in theWorks of the Creation (1691). He tackled the tricky ques-tion of why the Divine made obnoxious creatures. To par-aphrase Ray, consider lice: They harbor and breed inclothes, an effect of divine providence, designed to determen and women from sluttishness and sordidness, and toprovoke them to cleanliness and neatness. WilliamPaley (17431805), archdeacon of Carlisle, also articu-lated the common belief of his day in his book NaturalTheology; or Evidences of the Existence and Attributes of the

    Deity Collected from the Appearances of Nature (1802). LouisAgassiz (18071873), curator of the Museum of Compara-tive Zoology at Harvard University, found much public sup-port for his successful work to build and stock a museum thatcollected the remarkable creatures that were this worldsmanifestations of the divine mind (figure 1.4). For most sci-entists, philosophers, and laypeople, there was, in the bio-logical world of species, no change, thus no evolution. Evenin secular circles of the mid-nineteenth century, intellectualobstacles to the idea of evolution were formidable.

    J-B. de LamarckAmong those taking the side of evolution, few were asuneven in their reputation as Jean-Baptiste de Lamarck(figure 1.5a). Most of his life, Lamarck lived on the border ofpoverty. He did not even hold the equivalent of a professor-ship at the Jardin du Roi in Paris (later the Museum NationaldHistoire Naturelle; figure 1.5b). Abrupt speech, inclinationto argument, and strong views did little to endear Lamarck tohis colleagues. Yet his Philosophie Zoologique, generally dis-missed when published in 1809 as the amusing ruminationsof a poet, eventually established the theory of evolutionarydescent as a respectable scientific generalization.

    Lamarcks ideas spoke to the three issues of evolutionfact, course, and mechanism. As to the fact of evolution,Lamarck argued that species changed through time. Curiously,he thought that the simplest forms of life arose by spontaneousgeneration; that is, they sprang ready-made in muck frominanimate matter but thereafter evolved onward and upward

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    FIGURE 1.3 Carolus Linnaeus (17071778).ThisSwedish biologist devised a system still used today for namingorganisms. He also firmly abided by and promoted the view thatspecies do not change.

    FIGURE 1.4 Louis Agassiz (18071873) was born inSwitzerland but came to his second and permanent home in theUnited States when he was 39. He studied fossil fishes and wasfirst to recognize evidence of the worldwide ice ages, episodes ofglaciation in Earths history. He founded the Museum of ComparativeZoology at Harvard University. Although brilliant and entertainingin public and in anatomical research, Agassiz remainedunconvinced of Darwinian evolution to the end of his life.

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  • into higher forms. As to the course of evolution, he proposeda progressive change in species along an ascending scale, fromthe lowest on one end to the most complex and perfect(meaning humans) on the other. As to the mechanism of evo-lution, Lamarck proposed that need itself produced heritableevolutionary change. When environments or behaviorschanged, an animal developed new needs to meet thedemands the environment placed upon it. Needs alteredmetabolism, changed the internal physiology of the organism,and triggered the appearance of a new part to address theseneeds. Continued use of a part tended to develop that part fur-ther; disuse led to its withering. As environments changed, aneed arose, metabolism adjusted, and new organs were cre-ated. Once acquired, these new characteristics were passed onto offspring. This, in summary, was Lamarcks view. It has beencalled evolution by means of the inheritance of acquired charac-teristics. Characters were acquired to meet new needs andthen inherited by future generations.

    While a debt is owed Lamarck for championing evo-lutionary change and so easing the route to Darwin, he alsocreated obstacles. Central to his philosophy was an inad-vertent confusion between physiology and evolution. Anyperson who begins and stays with a weight-lifting programon a regular basis can expect to see strength increase andmuscles enlarge. With added weight, use (need) increases;therefore, big muscles appear. This physiological response islimited to the exercising individual because big muscles arenot passed genetically to offspring. Charles Atlas, ArnoldSchwarzenegger, and other bodybuilders do not pass newlyacquired muscle tissue to their children. If their childrenseek large muscles, they too must start from scratch withtheir own training program. Somatic characteristicsacquired through use cannot be inherited. Lamarck, how-ever, would have thought otherwise.

    Unlike such physiological responses, evolutionary res-ponses involve changes in an organism that are inheritedfrom one generation to the next. We know today that suchcharacteristics are genetically based. They arise from genemutation, not from somatic alterations due to exercise ormetabolic need.

    Acquired CharacteristicsLamarcks proposed mechanism of inheritance of acquiredcharacteristics failed because it confused immediate physio-logical response with long-term evolutionary change. Yetmost laypeople today still inadvertently think in Lamarckianterms. They mistakenly view somatic parts arising to meetimmediate needs. Recently, an actor/moderator of a televi-sion nature program on giraffes spoke what was probably onthe minds of most viewers when he said that the origin of thelong neck helped giraffes meet the needs of reaching tree-top vegetation. Environmental demands do not reach intogenetic material and directly produce heritable improve-ments to address new needs or new opportunities. Bodybuild-ing changes muscles, not DNA. That route of inheritablemodification does not exist in any organisms physiology.

    The other side of the Lamarckian coin is disuse, loss ofa part following loss of a need. Some fishes and salamanderslive in deep caves not reached by daylight. These specieslack eyes. Even if they return to the light, eyes do not form.Evolutionarily, the eyes are lost. It is tempting to attributethis evolutionary loss of eyes to disuse in a dark environ-ment. That, of course, would be invoking a Lamarckianmechanism. Contrary to Lamarcks theory, somatic traits arenot inherited.

    Because it comes easily, it is difficult to purge aLamarckian explanation from our own reasoning. We fallautomatically and too comfortably into the convenient habitof thinking of parts as rising to meet needs, one creating theother. For Darwin, and for students coming to evolution freshtoday, Lamarcks theory of acquired characteristics impedesclear reasoning. Unfortunately, Lamarck helped popularizean erroneous outlook that current culture perpetuates.

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    FIGURE 1.5 (a) J-B. de Lamarck (17441829) workedmost of his scientific life at the Musum National dHistoireNaturelle (b). His academic position gave him a chance topromote the idea that species change.

    (a)

    (b)

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  • Upward to PerfectionThe proposed course of evolution championed by Lamarckalso remains an intellectual distraction. The concept of thescale of nature (Latin, scala naturae) goes back to Aristotleand is stated in various ways by various philosophers. Itscentral theme holds that evolving life has a direction begin-ning with the lowest organisms and evolving to the highest,progressively upward toward perfection. Evolutionists, likeLamarck, viewed life metaphorically as ascending a ladderone rung at a time, up toward the complex and the per-fected. After a spontaneous origin, organisms progressed upthis metaphorical ladder or scale of nature through thecourse of many generations.

    The concept of a ladder of progress was misleadingbecause it viewed animal evolution as internally driven in aparticular direction from the early, imperfect, soft-bodiedforms up toward perfected humans. As water runs naturallydownhill, descent of animals was expected to run naturally tothe perfected. Simple animals were not seen as adapted intheir own right but rather as springboards to a better future.The scale of nature concept encouraged scientists to view ani-mals as progressive improvements driven by anticipation of abetter tomorrow. Unfortunately, remnants of this idea stilllinger in modern society. Certainly humans are perfected inthe sense of being designed to meet demands, but no more sothan any other organism. Moles and mosquitoes, bats andbirds, earthworms and anteaters all achieve an equally perfectmatch of parts-to-performance-to-environmental demands. Itis not the benefits of a distant future that drive evolutionarychange. Instead, the immediate demands of the current envi-ronment shape animal design.

    The idea of perfection rooted in Western culture isperpetuated by continued technological improvements. Webring it unnoticed, like excess intellectual baggage, intobiology where it clutters our interpretation of evolutionarychange. When we use the terms lower and higher, we risk per-petuating this discredited idea of perfection. Lower animalsand higher animals are not poorly designed and betterdesigned, respectively. Lower and higher refer only to orderof evolutionary appearance. Lower animals evolved first;higher animals arose after them. Thus, to avoid any sugges-tion of increasing perfection, many scientists prefer toreplace the terms lower and higher with the terms primitiveand derived to emphasize only evolutionary sequence ofappearance, early and later, respectively.

    To Lamarck and other evolutionists of his day, nature gotbetter and animals improved as they evolved up the evolu-tionary scale. Thus, Lamarcks historical contribution to evo-lutionary concepts was double sided. On the one hand, hisideas presented intellectual obstacles. His proposed mecha-nism of changeinheritance of acquired characteristicsconfused physiological response with evolutionary adaptation.By championing a flawed scale of nature, he diverted attentionto what supposedly drove animals to a better future rather thanto what actually shaped them in their present environment.On the other hand, Lamarck vigorously defended the view

    that animals evolved. For many years, textbooks have beenharsh in their treatment of Lamarck, probably to ensure thathis mistakes are not acquired by modern students. However, itis also important to give him his place in the history of evolu-tionary ideas. By arguing for change in species, Lamarck helpedblunt the sharp antievolutionary dissent of contemporarieslike Linnaeus, gave respectability to the idea of evolution, andhelped prepare the intellectual environment for those whowould solve the question of the origin of species.

    Natural SelectionThe mechanism of evolution by means of natural selectionwas unveiled publically by two persons in 1858, although itwas conceived independently by both. One was CharlesDarwin; the other was Alfred Wallace. Both were part ofthe respected naturalist tradition in Victorian England thatencouraged physicians, clergymen, and persons of leisure todevote time to observations of plants and animals in thecountryside. Such interests were not seen as a way to passidle time in harmless pursuits. On the contrary, observationof nature was respectable because it encouraged intercoursewith the Creators handiwork. Despite the reason, the resultwas thoughtful attention to the natural world.

    A.R.WallaceAlfred Russel Wallace, born in 1823, was 14 years youngerthan Darwin (figure 1.6). Although following the life of anaturalist, Wallace lacked the comfortable economic cir-cumstances of most gentlemen of his day; therefore, heturned to a trade for a livelihood. First he surveyed land forrailroads in his native England, and eventually, following hisinterest in nature, he took up the collection of biologicalspecimens in foreign lands to sell to museums back home. Hissearch for rare plants and animals in exotic lands took him tothe Amazon jungles and later to the Malay Archipelago inthe Far East. We know from his diaries that he was impressedby the great variety and number of species to which his trav-els introduced him. In early 1858, Wallace fell ill while onone of the Spice Islands (Moluccas) between New Guineaand Borneo. During a fitful night of fever, his mind recalleda book he had read earlier by the Reverend Thomas Malthusentitled An Essay on the Principle of Population, as It Affects theFuture Improvement of Society. Malthus, writing of humanpopulations, observed that unchecked breeding causes popu-lations to grow geometrically, whereas the supply of foodgrows more slowly. The simple, if cruel, result is that peopleincrease faster than food. If there is not enough food to goaround, some people survive but most die. The idea flashedto Wallace that the same principle applied to all species. Inhis own words written some years later:

    It occurred to me to ask the question,Why do somedie and some live? And the answer was clearly, thaton the whole the best fitted lived. From the effects ofdisease the most healthy escaped; from enemies, the

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  • strongest, the swiftest, or the most cunning; fromfamine, the best hunters or those with the bestdigestion; and so on.

    Then I at once saw, that the ever presentvariability of all living things would furnish thematerial from which, by the mere weeding out ofthose less adapted to the actual conditions, thefittest alone would continue the race.

    There suddenly flashed upon me the idea of thesurvival of the fittest.

    The more I thought over it, the more I becameconvinced that I had at length found the long-sought-for law of nature that solved the problem of theOrigin of Species.

    (Wallace, 1905)

    Wallace began writing that same evening and withintwo days had his idea sketched out in a paper. Knowing thatDarwin was interested in the subject, but unaware of how farDarwins own thinking had progressed, he mailed the man-uscript to Darwin for an opinion. The post was slow, so thejourney took four months. When Wallaces paper arrived outof the blue with its stunning coincidence to his own ideas,Darwin was taken by complete surprise.

    Charles DarwinUnlike Wallace, Charles Darwin (18091882) was born intoeconomic security. His father was a successful physician, andhis mother part of the Wedgwood (pottery) fortune. Hetried medicine at Edinburgh but became squeamish duringoperations. Fearing creeping idleness, Darwins father redi-rected him to Cambridge and a career in the church, butDarwin proved uninterested. At formal education, heseemed a mediocre student. While at Cambridge, however,

    his long-standing interest in natural history was encouraged byJohn Henslow, a professor of botany. Darwin was invited ongeological excursions and collected biological specimens. Upongraduation, he joined as de facto naturalist of the governmentsH.M.S. Beagle over the objections of his father, who wished himto get on with a more conventional career in the ministry.

    He spent nearly five years on the ship and explored thecoastal lands it visited. The experience intellectually trans-formed him. Darwins belief in the special creation of species,with which he began the voyage, was shaken by the vast arrayof species and adaptations the voyage introduced to him. Theissue came especially to focus on the Galpagos Islands offthe west coast of South America. Each island contained itsown assortment of species, some found only on that particu-lar island. Local experts could tell at sight from which of theseveral islands a particular tortoise came. The same was trueof many of the bird and plant species that Darwin collected.

    Darwin arrived back in England in October 1836 andset to work sorting his collection, obviously impressed by thediversity he had seen but still wedded to misconceptionsabout the Galpagos collection in particular. He had, forinstance, thought that the Galpagos tortoise was introducedfrom other areas by mariners stashing reptilian livestock onislands to harvest during a later visit. Apparently Darwin dis-missed reports of differences among the tortoises of eachisland, attributing these differences to changes that attendedthe animals recent introductions to new and dissimilar habi-tats. However, in March of 1837, almost a year and a half afterdeparting the Galpagos, Darwin met in London with JohnGould, respected specialist in ornithology. Gould insistedthat the mockingbirds Darwin had collected on the three dif-ferent Galpagos Islands were actually distinct species. Infact, Gould emphasized that the birds were endemic to theGalpagosdistinct species, not just varietiesalthoughclearly each was related to species on the South Americanmainland. It seemed to have suddenly dawned on Darwinthat not only birds, but plant and tortoise varieties, were dis-tinct as well. These tortoises geographically isolated on theGalpagos were not only derivatives of ancestral stocks butnow distinct island species.

    Here then was the issue. Was each of these speciesof tortoise or bird or plant an act of special creation?Although distinct, each species also was clearly related tothose on the other islands and to those on the nearby SouthAmerican mainland. To account for these species, Darwinhad two serious choices. Either they were products of a spe-cial creation, one act for each species, or they were the nat-ural result of evolutionary adaptation to the different islands.If these related species were acts of special divine creation,then each of the many hundreds of species would represent adistinct act of creation. But if this were so, it seemed odd thatthey would all be similar to each other, the tortoises to othertortoises, the birds to other birds, and the plants to otherplants on the various islands, almost as if the Creator ran outof new ideas. If, however, these species were the naturalresult of evolutionary processes, then similarity and diversity

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    FIGURE 1.6 Alfred Russel Wallace (18231913) in histhirties.

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  • would be expected. The first animal or plant washed orblown to these oceanic islands would constitute the com-mon stock from which similar but eventually distinct speciesevolved. Darwin sided with a natural evolution.

    But Darwin needed a mechanism by which such evolu-tionary diversification might proceed, and at first he had noneto suggest. Not until his return to England did Darwins expe-riences from the Galpagos Islands and throughout his voyagecrystallize. Two years after his return, and while in the midst ofwriting up his results of other studies from the Beagle, Darwinread for amusement the essay on population by Malthus, thesame essay Wallace would discover years later. The significancestruck Darwin immediately. If animals, like humans, out-stripped food resources, then competition for scarce resourceswould result. Those with favorable adaptations would fare best,and new species incorporating these favored adaptations wouldarise. Here then I had at last got a theory by which to workwrote Darwin. In a moment of insight, he had solved thespecies problem. That was 1838, and you would think theexcitement would have set him to work on papers and lectur-ing. Nothing of the sort happened. In fact, four years lapsedbefore he wrote a first draft, which consisted of 35 pages in pen-cil. Two years later, he expanded the draft to over 200 pages inink, but he shoved it quietly into a drawer with a sum of moneyand a sealed letter instructing his wife to have it published ifhe met an untimely death. A few close friends knew what hehad proposed but most did not, including his wife with whomhe otherwise enjoyed a close and loving marriage. This wasVictorian England. Science and religion fit hand and glove.

    Darwins delay testifies to how profoundly he understoodthe larger significance of what he had discovered. He wantedmore time to gather evidence and write the volumes hethought it would take to make a compelling case. Then in June1858, 20 years after he had first come upon the mechanism ofevolution, Wallaces manuscript arrived. Darwin was dumb-founded. By coincidence, Wallace had even hit upon some ofthe same terminology, specifically, natural selection. Mutualfriends intervened, and much to the credit of both Wallace andDarwin, a joint paper was read in the absence of both beforethe Linnaean Society in London the following month, July1858. Wallace was, as Darwin described him, generous andnoble. Wallace, in deep admiration, later dedicated hisbook on the Malay Archipelago to Darwin as a token of per-sonal esteem and friendship. Oddly, this joint paper made nostir. But Darwins hand was now forced.

    Critics and ControversyDarwin still intended a thick discourse on the subject of nat-ural selection but agreed to a shorter version of only500 pages. This was The Origin of Species, published at theend of 1859. By then word was out, and the first edition soldout as soon as it appeared.

    Largely because he produced the expanded case forevolution in The Origin of Species, and because of a contin-ued series of related work, Darwin is remembered more than

    Wallace for formulating the basic concept. Darwin broughta scientific consistency and cohesiveness to the concept ofevolution, and that is why it bears the name Darwinism.

    Science and religion, especially in England, had beentightly coupled. For centuries, a ready answer was at handfor the question of lifes origin, a divine explanation, asdescribed in Genesis. Darwinism challenged with a naturalexplanation. Controversy was immediate, and in some rem-nant backwaters, it still lingers today. Darwin himself retiredfrom the fray, leaving to others the task of public defense ofthe ideas of evolution.

    Sides quickly formed. Speaking before the EnglishParliament, the future prime minister Benjamin Disraelisafely chose his friends: The question is thisIs man an apeor an angel? My lord, I am on the side of the angels.

    Despite the sometimes misguided reactions, two criti-cisms stuck and Darwin knew it. One was the question ofvariation, the other the question of time. As to time, thereseemed not to be enough. If the evolutionary events Darwinenvisioned were to unfold, then the Earth must be very oldto allow time for life to diversify. In the seventeenth century,James Ussher, Archbishop of Armagh and Primate of AllIreland, made an honorable effort to calculate the age of theEarth. From his biblical studies of who begot whom and fromhistorical dates available at the time, Ussher determined thatthe first day of Creation began in 4004 B.C. on SaturdayOctober 22, at nightfall. A contemporary, Dr. John Lightfoot,vice-chancellor at Cambridge University, estimated furtherthat humans were created five days later, at 9:00 in themorning, presumably Greenwich mean time. Many took thisdate as literally accurate, or at least as indicative of therecent origin of humans, leaving no time for evolution fromapes or angels. A more scientific effort to age the Earth wasmade by Lord Kelvin, who used temperatures taken in deepmine shafts. Reasoning that the Earth would cool from itsprimitive molten state to present temperatures at a constantrate, Kelvin extrapolated backward to calculate that theEarth was no more than 24 million years old. He did notknow that natural radioactivity in the Earths crust keeps thesurface hot. This fact deceptively makes it seem close intemperature and thus in age to its molten temperature at firstformation. The true age of the Earth is actually several bil-lion years, but unfortunately for Darwin, this was not knownuntil long after his death.

    Critics also pointed to inheritance of variation as aweak spot in his theory of evolution. The basis of hereditywas unknown in Darwins day. The popular view held thatinheritance was blending. Like mixing two paints, off-spring received a blend of characteristics from both par-ents. This view, although mistaken, was taken seriously bymany. It created two problems for Darwin. From where didvariation come? How was it passed from generation to gen-eration? If natural selection favored individuals with supe-rior characteristics, what ensured that these superiorcharacteristics were not blended and diluted out of exis-tence in the offspring? If favored characters were blended,

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  • they would effectively be lost from view and natural selec-tion would not work. Darwin could see this criticism c