Transcriber's note: A few typographical errors have been corrected: they are listed at the end of the text. Text enclosed by underscores is in italics (_italics_). A carat character is used to denote superscription. A single character following the carat is superscripted (example: X^1). Similarly an underscore represents a subscript (_sk_4_ has a subscript 4 and is in italics). Page numbers enclosed by curly braces (example: {25}) have been incorporated to facilitate the use of the Index. * * * * * THE ORIGIN OF VERTEBRATES BY WALTER HOLBROOK GASKELL M.A., M.D. (CANTAB.), LL.D. (EDIN. AND McGILL UNIV.); F.R.S.; FELLOW OF TRINITY HALL AND UNIVERSITY LECTURER IN PHYSIOLOGY, CAMBRIDGE; HONORARY FELLOW OF THE ROYAL MEDICAL AND CHIRURGICAL SOCIETY; CORRESPONDING MEMBER OF THE IMPERIAL MILITARY ACADEMY OF MEDICINE, ST. PETERSBURG, ETC. LONGMANS, GREEN, AND CO. 39 PATERNOSTER ROW, LONDON NEW YORK, BOMBAY, AND CALCUTTA 1908 _All rights reserved_ CONTENTS PAGE INTRODUCTION 1 CHAPTER I THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM Theories of the origin of vertebrates--Importance of the central nervous system--Evolution of tissues--Evidence of Palæontology-- Reasons for choosing Ammocoetes rather than Amphioxus for the investigation of this problem--Importance of larval forms-- Comparison of the vertebrate and arthropod central nervous systems--Antagonism between cephalization and alimentation-- Life-history of lamprey, not a degenerate animal--Brain of Ammocoetes compared with brain of arthropod--Summary 8 CHAPTER II THE EVIDENCE OF THE ORGANS OF VISION Different kinds of eye--Simple and compound retinas--Upright and inverted retinas--Median eyes--Median or pineal eyes of Ammocoetes and their optic ganglia--Comparison with other median eyes--Lateral eyes of vertebrates compared with lateral eyes of crustaceans-- Peculiarities of the lateral eye of the lamprey--Meaning of the optic diverticula--Evolution of vertebrate eyes--Summary 68 CHAPTER III THE EVIDENCE OF THE SKELETON