-• • SAM THE EVOLUTION OF MAN. THE EVOLUTION OF MAN A POP [-LAS EXPOSITION OF THE PRINCIPAL POINTS OF HUMAN ONTOGENY AND PHYLOGENY. FROM THE GERMAN OF ERNST IIAECKEL, PROFESSOR IX TflE f-N'lVERSITY OF JEXA, AUTHOR OF ';THE HISTORY OF CREATION," ETC. IX TWO VOLUMES. VOL. II. NEW YORK: D. APPLETON AND COMPANY, 72 FIFTH AVENUE. 1897. Authorized Edition. CONTENTS OF VOL. II. Lia*. of Plates x List of Woodcuts List of Genetic Tables xvii CHAPTER XV. THE DURATION OF HUMAN TRIBAL HISTORY. Comparison of Ontogenetic and Phylogenetic Periods of Time. — Dora- tion of Germ -history in Man and in Different Animals. — Extreme Brevity of the Latter in Comparison with the Immeasurable Long Perioda of Tribal Histoiy. — Relation of this Rapid Ontogenetio Modification to the Slow Phylogenetic Metamorphosis. — Estimate of the Past Duration of the Organic World, founded on the Relative Thickness of Sedimentary Rock-strata, or Neptunian Formations. — The Five Main Divisions in the Latter : I. Primordial, or Archilithic Epoch. II. Primary, or Palaeolithic Epoch. III. Second- ary, or Mesolithic Epoch. IV. Tertiary, or Caenolithic Epoch. V. Quaternary, or Anthropolithic Epoch. — The Relative Duration of the Five Epochs.— The Results of Comparative Philology as Explaining the Phylogeny of Species. — The Inter-relations of the Main Stems and Branches of the Indo-Germanio Languages are Analogous to the Inter-relations of the Main Stems and Branches of the Vertebrate Tribe. — The Parent Forms in both Cases are Extinct. — The Most Important Stages among the Human An- cestral Forms. — Monera originated by Spontaneous Generation. — Necessity of Spontaneous Generation ... ... VI CONTENTS. CHAPTER XVL THE ANCESTRY OF MAN. I. FROM THE MONERA TO THE GASTR-iEA. PA0I Relation of the General Inductive Law of the Theory of Descent to the Special Deductive Laws of the Hypotheses of Descent. — Incom- pleteness of the Three Great Records of Creation : Palaeontology, Ontogeny, and Comparative Anatomy. — Unequal Certainty of the Various Special Hypotheses of Descent. — The Ancestral Line of Men in Twenty -two Stages : Eight Invertebrate and Fourteen Verte- brate Ancestors. — Distribution of these Twenty-two Parent-forms in the Five Main Divisions of the Organic History of the Earth. — First Ancestral Stage : Monera. — The Structureless and Homo- geneous Plasson of the Monera. — Differentiation of the Plasson into Nucleus, and the Protoplasm of the Cells. — Cytods and Cells as Two Different Plastid-forms. — Vital Phenomena of Monera. — Organisms without Organs. — Second Ancestral Stage : Amoebaa. — One-celled Primitive Animals of the Simplest and most Un- differentiated Nature.— The Amoeboid Egg-cells.— The Egg is Older than the Hen. — Third Ancestral Stage : Syn-Amceba, Ontogeneti- cally reproduced in the Morula. — A Community of Homogeneous Amoeboid Cells. — Fourth Ancestral Stage : Planasa, Ontogenetically reproduced in the Blastnla or Plannla. — Fifth Ancestral Stage: Gastrrea, Ontogenetically reproduced in the Gastrula and the Two- layered Germ-disc. — Origin of the Gastrasa by Inversion (invagi- natio) of the Planaea. — Haliphysema and Gastrophysema. — Extant Gastncada . 84 CHAPTER XVIL THE ANCESTRAL SERIES OF MAN. II. FROM THE PRIMITIVE WORM TO THE SKULLED ANIMAL. The Four Higher Animal Tribes are descended from the Worm Tribe. — The Descendants of the Gastraea; in one direction the Parent Form of Plant-Animals (Sponges and Sea-Nettles), in the other the Parent Form of Worms. — Radiate form of the former, Bilateral form of the latter. — The Two Main Divisions of the Worms, Acoelomi and Ccelomati : the former without, the latter with, a Body Cavity and Blood-vessel System. — Sixth Ancestral Stage : Archelminthes, moat nearly allied to Turbellaria. — Descent of the CONTENTS. Vll PACK Coelomati from the Acoelomi. — Mantled Animals (Tunicata) and Chorda- Animals (Chordonia) .—Seventh Stage: Soft-Worms (Scale, cida). — A Side Branch of the latter : the Acorn-Worm (Balano- glossus). — Differentiation of the Intestinal Tube into Gill-intes- tine and Stomach-intestine. — Eighth Stage : Chorda-Animals (Chor- donia). — Ascidian Larva exhibits the Outline of a Chorda-Animal. — Construction of the Notochord. — Mantled Animals and Verte- brates as Diverging Branches of Chorda-Animals. — Separation of Vertebrates from the other Higher Animal Tribes (Articulated Animals, Star-Animals, Soft-bodied Animals). — Significance of tho Metameric Formation. — Skull-less Animals (Acrania) and Skulled Animals (Craniotd) . — Ninth Ancestral Stage : Skull-less Animals. — Amphioxus and Primitive Vertebrate. — Development of Skulled Animals (Construction of the Head, Skull, and Brain). — Tenth Ancestral Stage : Skulled Animals, allied to the Cyclostomi (Myxi- noidce and Fetromyzonidee) ... ... ... ... ... 71 CHAPTER XVIII. THE PEDIGREE OF MAN. III. FROM THE PRIMITIVE FISH TO THE AMNIOTIO ANIMAL. Comparative Anatomy of the Vertebrates. — The Characteristic Qualities of the Double-nostrilled and Jaw-mouthed : the Double-Nostrils, the Gill-arch Apparatus, with the Jaw-arches, the Swimming- bladder, the Two Pairs of Limbs. — Relationship of the Three Groups of Fishes : the Primitive Fishes (Selachii), the Ganoids (Ganoides), the Osseous Fishes (Teleostei) .— Dawn of Terrestrial Life on the Earth.— Modification of the Swimming-bladder into the Lungs. — Intermediate Position of the Dipnensta between the Primitive Fishes and Amphibia. — The Three Extant Dipneusta (Prctcpterus, Lepidosiren, Ceratodus). — Modification of the Many- toed Fin of the Fish into the Five-toed Foot. — Causes and Effects of the latter. — Descent of all Higher Vertebrates from a Five-toed Amphibian. — Intermediate Position of the Amphibians between the Lower and Higher Vertebrates. — Modification or Metamorphosis of Frogs. — Different Stages in Amphibian Metamorphosis. — The Gilled Batrachians (Proteus and Axolotl) .— The Tailed Batrachians (Salamanders and Mud-fish). — Frog Batrachians (Frogs and Toads).— Chief Group of the Amnion Animals, or Amniota (Reptiles, Birds, and Mammals). — Descent of all the Amniota from a Common viii CONTENTS. PAG! Lizard-like Parent-form (Protamnion) . — First Formation of the Allantois and of the Amnion. — Branching of the Amnion Animals in Two Lines : on the one side, Keptiles (and Birds), on the other side, Mammals ... ... ... ... ... ... 107 CHAPTER XIX. THE PEDIGREE OF MAN. IV. FROM THE PRIMITIVE MAMMAL TO THE APE. The Mammalian Character of Man.— Common Descent of all Mammals from a Single Parent-form (Promammalian). — Bifurcation of the Amnion Animals into Two Main Lines : on the one side, Rep- tiles and Birds, on the other, Mammals. — Date of the Origin of Mammals : the Trias Period. — The Three Main Groups or Sub- classes of Mammals : their Genealogical Relations. — Sixteenth Ancestral Stage : Cloacal Animal s (Monotremat a, or Ornithodelphia) . — The Extinct Primitive Mammals (Promammalia) and the Extant Beaked Animals (Ornithostoma) . — Seventeenth Ancestral Stage: Pouched Animals (Marsupialia, or DidelpTiia) . — Extinct and Extant Pouched Animals. — Their Intermediate Position between Mono- tremes and Placental Animals. — Origin and Structure of Placental Animals (Placentalia, or MonodelpTiia) . — Formation of the Pla- centa.— The Deciduous Embryonic Membrane (Decidua). — Group of the Indecidua and of the Deciduata. — The Formation of the Decidua (vera, serotina, reflexa) in Man and in Apes. — Eighteenth Stage: Semi-apes (Prosimtce). — Nineteenth Stage : Tailed Apes {Menocerca) . — Twentieth Stage : Man-like Apes (Anthropoides). — Speechless and Speaking Men (Mali. Homines) ,.. ... 140 CHAPTER XX. THE HISTORY OF THE EVOLUTION OF THE EPIDERMIS AND THE NERVOUS SYSTEM. Animal and Vegetative Organ-systems — Original Relations of these to the Two Primary Germ -layers. — Sensory Apparatus. — Constituents of Sensory Apparatus : originally only the Exoderm, or Skin-layer ; afterwards, the Skin-covering specialized from the Nerve-system. — Double Function of the Skin (as a Covering and as Organ of CONTENTS. IX TAGS Touch). — Outer Skin (Epidermis) and Leather-skin (Corium). — Appendages of the Epidermis : Skin-glands (Sweat-glands, Tear- glands, Sebaceous Glands, Milk-glands); Nails and Hair. — The Embryonic Wool-covering. — Hair of the Head and of the Beard. — Influence of Sexual Selection. — Arrangement of the Nerve-system. — Motor and Sensory Nerves. — Central Marrow : Brain and Dorsal Marrow.— Constitution of the Human Brain : Large Brain (Cere- brum) and Small Brain (Cerebellum). — Comparative Anatomy of the Central Marrow. — Germ-history of the Medullary-tube. — Sepa- ration of the Medullary-tube into Brain and Dorsal Marrow. — Modification of the Simple Brain-bladder into Five Consecutive Brain-bladders: Fore-brain (Large Brain, or Cerebrum), Twixt- brain ("Centre of Sight"), Mid-brain (" Four Bulbs "), Hind-brain (Small Brain, or Cerebellum), After-brain (Neck Medulla). — Various Formation of the Five Brain-bladders in the various Vertebrate Classes. — Development of the Conductive Marrow, or "Peripherie Nervous System" ... ... ... ... ... ... 190 CHAPTER XXL DEVELOPMENT OF THE SENSE-ORGANS. Origin of the most highly Purposive Sense-organs by no Preconceived Purpose, but simply by Natural Selection. — The Six Sense-organs and the Seven Sense-functions. — All the Sense-organs originally Developed from the Outer Skin-covering (from the Skin-sensory Layer). — Organs of the Pressure Sense, the Heat Sense, the Sexual Sense, and the Taste Sense. — Structure of the Organ of Scent.— The Blind Nose-pits of Fishes.— The Nasal Furrows change into Nasal Canals. — Separation of the Cavities of the Nose and Mouth by the Palate Roof.— Structure of the Eye.— The Primary Eye Vesicles (Stalked Protuberances from the Twixt-brain).— Inversion of this Eye Vesicle by the Crystalline Lens, separated from the Horn-plate. — Inversion of the Vitreous Body. — The Vas. cular Capsule and the Fibrous Capsule of the Eyeball. — EyelidB. — Structure of the Ear. — The Apparatus for Perception of Sound : Labyrinth and Auditory Nerve. — Origin of the Labyrinth from . the Primitive Ear Vesicles (by Separation from the Horn-plate). — Conducting Apparatus of Sound : Drum Cavity, Ear Bonelets, and Drum Membrane. — Origin of these from the First Gill-opening and the Parts immediately round it (the First and Second Gill- arch). — Rudimentary Outer Ear. — Rudimentary Muscles of the Ear-shell ... 233 * CONTENTS. CHAPTER XXII. DEVELOPMENT OF THE ORGANS OF MOTION. PAOB The Motive Apparatus of Vertebrates. — These are constituted by tho Passive and Active Organs of Motion (Skeleton and Mu?>les). — The Significance of the Internal Skeleton of Vertebrates. — Struc- ture of the Vertebral Column. — Formation and Number of the Vertebrao. — The Ribs and Breast-bone. — Germ-history of the Verte- bral Column. — The Notochord. — The Primitive Vertebral Plates.— The Formation of the Metamera. — Cartilaginous and Bony Verte- brae.— Intervertebral Discs. — Head-skeleton (Skull and Gill-arches). —Vertebral Theory of the Skull (Goethe and Oken, Huxley and Gegenbaur). — Primitive Skull, or Primordial Cranium. — Its Forma- tion from Nine or Ten Coalescent Metamera. — The Gill-arches (Ribs of the Head).— Bones of the Two Pairs of Limbs.— Develop- ment of the Five-toed Foot, adapted for Walking, from the Many- toed Fin of the Fish.— The Primitive Fin of the Selachians (ArcMpterygium of Gegeubaur). — Transition of the Pinnate into the Semi-pinnate Fin. — Atrophy of the Rays or Toes of the Fins. — Many-fingered and Five-fingered Vertebrates. — Comparison of the Anterior Limbs (Pectoi-al Fins) and the Posterior Limbs (Ventral Fins).— Shoulder Girdle and Pelvis Girdle.— Germ-history of the Limbs. — Development of the Muscles ... ... ... 273 CHAPTER XXIII. DEVELOPMENT OF THE INTESTINAL SYSTEM. The Primitive Intestine of the Gastrula. — Its Homology, or Morpho- logical Identity in all Animals (excepting the Protozoa). — Survey of the Structure of the Developed Intestinal Canal in Man. — The Mouth-cavity.— The Throat (pharynx) .— The Gullet (oesophagus).— The Wind-pipe (trachea) and Lungs. — The Larynx. — The Stomach. — The Small Intestine. — The Liver and Gall-bladder. — The Ventral Salivary Gland (pancreas). — The Large Intestine. — The Rectum. — The First Rudiment of the Simple Intestinal Tube. — The Gastrula of the Amphioxus and of Mammals. — Separation of the Germ from the Intestinal Germ Vesicle (Gastrocystis) . — The Primitive Intes- tine (Protogaster) and the After Intestine (Metagaster) . — Secondary Formation of the Mouth and Anus from the Outer Skin. — Develop- ment of the Intestinal Epithelium from the Intestinal-glandular Layer, and of all other parts of the Intestine from the Intestinal- fibrous Layer. — Simple Intestinal Pouch of the Lower Worms. — CONTENTS. XI PAGE Differentiation of the Primitive Intestinal Tube into a Respiratory and a Digestive Intestine. — Gill-intestine and Stomach-intestine of the Amphioxns and Ascidian. — Origin and Significance of the Gill- openings. — Their Disappearance. — The Gill-arches and the Jaw- Skeleton. — Formation of the Teeth. — Development of the Lungs from the Swim-bladder of Fish. — Differentiation of the Stomach. — Development of the Liver and Pancreas. — Differentiation of the Small and Large Intestines. — Formation of the Cloaca ... ... 311 CHAPTER XXIV. DEVELOPMENT OF THE VASCULAR SYSTEM. Application of the Fundamental Law of Biogeny. — The Two Sides. — Heredity of Conservative Organs. — Adaptation of Progressive Organs. — Ontogeny and Comparative Anatomy complementary of each other. — New "Theories of Evolution" of His. — The "En- velope Theory " and the " Waste-rag Theory." — Main Germ and Supplementary Germ. — Formative Yolk and Nutritive Yelk. — Phy- logenetic Origin of the latter from the Primitive Intestine.— Origin of the Vascular System from the Vascular Layer, or Intestinal- fibrous Layer. — Phylogenetic Significance of the Ontogenetic Suc- cession of the Organ-systems and Tissues. — Deviation from the Original Sequence ; Ontogenetic Heterochronism. — Covering Tissue. — Connective Tissue. — Nerve-muscle Tissue. — Vascular Tissue. — Relative Age of the Vascular System. — First Commencement of the Latter; Coeloma.— Dorsal Vessel and Ventral Vessel of Worms. — Simple Heart of Asciclia. — Atrophy of the Heart in the Am- phioxus.— Two-chambered Heart of the Cyclostoma. — Arterial Arches of the Selachii. — Double Auricle in Dipneusta and Am- phibia.— Double Ventricle in Birds and Mammals. — Arterial Arches in Birds and Mammals. — Germ-history (Ontogeny) of the Human Heart.— Parallelism of the Tribal-history (Phylogeny) ... ...318 CHAPTER XXV. DEVELOPMENT OF THE URINARY AND SEXUAL ORGANS. Importance of Reproduction. — Growth. — Simplest Forms of Asexual Reproduction : Division and the Formation of Buds (Gemmation). — Simplest Forms of Sexual Reproduction: Amalgamation of Two Differentiated Cells ; the Male Sperm-cell and the Female Egg-cell. — Fertilization. — Source of Love. — Original Hermaphroditism ; Xli CONTENTS. PMI Later Separation of the Sexes (Gonochorism). — Original Develop, ment of the Two Kinds of Sexual Cells from the Two Primary Germ-layers. — The Male Exoderm and Female Entoderm. — Develop, ment of the Testes and Ovaries.— Passage of the Sexual Cells into the Coelom. — Hermaphrodite Rudiment of the Embryonic Epi- thelium, or Sexual Plate. — Channels of Exit, or Sexual Ducts. — Egg-duct and Seed-duct. — Development of these from the Primitive Kidney Ducts. — Excretory Organs of worms. — " Coiled Canals " of Ringed Worms (Annelida). — Side Canals of the Amphioxus. — Primitive Kidneys of the Myxinoides. — Primitive Kidneys of Skulled Animals (Craniota). — Development of the Permanent Secondary Kidneys in Amniota. — Development of the Urinary Bladder from the Allantois. — Differentiation of the Primary and Secondary Primitive Kidney Ducts. — The Miillerian Duct (Egg-duct) and the Wolffian Duct (Seed-duct), — Change of Position of the Germ-glands in Mammals. — Formation of the Egg in Mammals (Graafian Pol- licle). — Origin of the External Sexual Organs. — Formation of the Cloaca.— Hermaphroditism in Man ... ... ... ... 388 CHAPTER XXVI. RESULTS OF ANTHROPOGENY. Review of the Germ-history as given. — Its Explanation by the Funda- mental Law of Biogeny. — Its Causal Relation to the History of the Tribe. — Rudimentary Organs of Man. — Dysteleology, or the Doc- trine of Purposelessness. — Inheritances from Apes. — Man's place in the Natural System of the Animal Kingdom. — Man as a Vertebrate and a Mammal. — Special Tribal Relation of Men and Apes. — Evidences regarding the Ape Question. — The Catarhina and the Platyrhina. — The Divine Origin of Man. — Adam and Eve. — History of the Evolution of the Mind. — Important Mental Differences within a Single Class of Animals. — The Mammalian Mind and the Insect Mind. — Mind in the Ant and in the Scale-louse (Coccus). — Mind in Man and in Ape.— The Organ of Mental Activity: the Central Nervous System. — The Ontogeny and Phylogeny of the Mind. — The Monistic and Dualistic Theories of the Mind.— Heredity of the Mind. — Bearing of the Fundamental Law of Biogeny on Psychology. — Influence of Anthropogeny on the Victory of the Monistic Philo- sophy and the Defeat of the Dualistic. — Nature and Spirit. — Natural Science and Spiritual Science. — Conception of the World reformed by Anthropogeny ... ... ... ... ... ... 432 NOTBS. Remarks and References to Literature ... ... 459 INDEX ... ... ... 491 LIST OF PLATES. Plate XII. (between p. 130 and p. 131). The Australian Mud- fish (Ceratodus Fosteri) ... ... ... Explanation 118 Plate XIII. (between p. 130 and p. 131). The Mexican Axolotl (Siredon pisciformis) and the European Land-salamander (Salamandra maculatd) ... ... ... Explanation 129 Plate XIV. (between p. 180 and p. 181). Four Catarhines (Chimpanzee, Gorilla, Orang, Negro) ... Explanation 181 Plate XV. (between p. 188 and p. 189). Pedigree of Man Explanation 184 LIST OF WOODCUTS. FIGURE PAGE 1 65. Moneron (Protamceba) . 46 164. Bathybins, primitive slime 49 165. Monerula of Mammal . 51 166. Cytula of Mammal . . 51 167. Amoeba .... 53 168. Amoeboid egg-cell . . 53 169. Original egg-cleavage . 55 170. Mulberry -germ (Morula) . 55 171. Germination of Monoxenia 57 172. 173. Magosphaera . . 60 174-179. Gastrula of various animals ... 65 180, 181. Haliphysema . . 67 182, 183. Ascula of a Sponge . 68 184, 185. A Gliding-worm (Rhaldoccelum) . . 80 186. Acorn-worm (Balanogloa- sus) .... 86 187. Append icularia . .90 188. Ascidia . . . . 90 189. Amphioxus ... 91 190. Lamprey (Petromyzon) . 103 191,192. Shark (Selachii) . 113 193. Larval Salamander . . 127 li>-t. Larva". Frog (Tadpole) . 127 FIGURE 195, 196. Beaked Animal (Orni. thorhynchus) and ita skeleton 197. Pouched Animal (Marsu- pial) with young . 198. Human egg- membranes . 199. Semi-ape (Lori) 200. Human germ with its 201. Human uterus, navel-cord, and embryo . . 202. Head of Nose-ape . 203. Tailed Ape (Sea-cat) 204. Skeleton of Gibbon 205. Skeleton of Orang-outang 206. Skeleton of Chimpanzee . 207. Skeleton of Gorilla . 208. Skeleton of Man . 209. Gastrnla of Gastrophy. sema .... 210. Germ-layers of Earth- worms 211. Nerve-system of Gliding. worm .... 212 Human skin.covering 148 152 158 16G 167 175 175 178 178 178 178 178 198 198 198 200 LIST OF WOODCUTS. XV FIGUKB 213. Epidermis cella 214. Tear-glands . 215. 216. Milk-glands PAGE 201 202 203 217, 218. Central marrow of human embryo . . 210 219. Human brain . . . 212 220. „ „ . 213 221-223. Lyre-shaped embryo Chick . . . .218 224-226. The five brain-blad. ders of the human germ 220 227. The five brain-bladders of Craniota . . . 222 228. Brain of Shark- . . 222 229. Brain of Frog . . .222 230. Brain of Eabbit . . 224 231. Nose of Shark . . 241 232-236. Development of the face in embryo Chick . 243 237. Nose and month cavities . 246 238-240. The face in the human embryo . . 247 241. Human eye . . . 250 242. Development of the eyes 253 213. „ „ 256 244. Human auditory passage 260 245. Human auditory labyrinth 263 246-248. Development of the ear .... 264 249. Primitive skull with ear- vesicles . . .264 250. Rudimentary ear-muscles 270 251. 252. Human skeleton . 279 253. Human vertebral column 280 254. Neck-vertebra . . 281 255. Breast-vertebra . . 281 256. Lumbar-vertebra . . 281 FIGTJBB 257. Portion of notochord FAG1 286 258-260. Growth of the primi. tive vertebral series £p embryo Chink . . 288 261. Longitudinal section of breast-vertebra . . 290 262. Transverse section of same 291 263. Intervertebral disc . . 291 264. Human skull . . .292 265. Head skeleton of Primi. tive Fish . . .296 266. Primitive skull of Man . 297 267. Skeleton of fin of Cwatodus 302 268. Skeleton of fin of Acan- thiax . . . .302 269. Skeleton of fin of Primi- tive Fish . . . 302 270. Skeleton of hand of Frog 302 271. Skeleton of hand of Gorilla 302 272. Skeleton of human hand . 302 273. Skeleton of hand of Mam- mal . . .306 274. Gastrula of Olynthus . 313 275. Human stomach . , 317 276. Gastrnla of Arnphioxus . 321 277. Gastrula of Mammal . 321 278. 279. Human germ with yelk-sac and allantois . 321 280. Intestine of Turlellaria . 327 281. Intestine of Ascidia . 327 282. Intestine of Amphioxua . 328 283. Scales of Shark . . 332 284. 285. Intestine of embryo Dog with the intestinal glandg . . . .334 286. Intestine with allantois . 338 287. Intestine of human germ 339 xvi LIST OF WOODCUTS. FIGUBB PAGE 288. Liver of human germ . 342 289. Nail-tissue . . .362 290. Intestinal epithelium . 362 291. Jelly-like tissue . . 363 292. Cartilaginous tissue . 363 293. Neuro-niuscular cells . 364 294. Nerve-tissue . . . 364 295. Muscle-tissue . . .364 296. Vascular tissue . . 365 297. Blood-cells . . . 36p 298. Blood-vessels of a Worm . 371 299. Head with blood-vessels of Fish. . . .375 300-302. Arterial arches . 377 303-306. „ „ . 378 307-310. Development of the heart . . . .380 311-314. Development of the heart . . . .882 315. Transverse section through Haliphysema . 393 FIGUBB PAGF 316. Rudiments of Urogenitalia 400 317. Primitive kidney of Bdello. stomn . . . .406 318. Earliest primitive kidney rudiments . . . 408 319. 320. Primitive kidneys of Mammala . . . 409 321. Development of nrogeni- tal system . . . 414 322,323. „ „ 415 324-326. „ „ 416 327. Female sexual organs of Beaked Animal (Orni~ thorhynchiis) . . 418 328. Change of position of both kinds of sexual glands in human beings . . 420 329. Development of the human external sexual organs 422 330. Human egg-folliclee . 426 LIST OF GENETIC TABLES. TABU! PAGB XII. Systematic Survey of palseontological periods ... 11 XIII. Systematic Survey of palaeontological formations ... 12 XIV. Systematic Survey of the thickness of the forma- tions ... ... ... ... ... 19 XV. Pedigree of Indo-Germanic languages ... ... 23 XVI. Systematic Survey of the most important stages in the animal ancestral line of Man ... ... 44 XVIL Systematic Survey of the five first stages in the evolution of Man (phylogenetic, ontogenetic, sys- tematic) ... ... ... ... ... 70 XVIII. Systematic Survey of the phylogenetic system of the animal kingdom ... ... ... ... 92 XIX. Monophyletic pedigree of the animal kingdom ... 93 XX. Systematic Survey of the phylogenetic system of Vertebrates... ... ... ... ... 120 XXI. Monophyletic pedigree of Vertebrates ... ... 121 XXII. Systematic Survey of the periods of human tribal history 184 XYTTI, Systematic Survey of the phylogenetic system of Mammals, founded on the Gastrsea Theory ... 187 XXIV. Monophyletic pedigree of Mammals ... ... 188 XXV. Pedigree of Apes ... ... ... ... 189 XXVI. Systematic Survey of the organ-systems of the human body 194 XXVIL Systematic Survey of the phylogenetic history of the human skin-covering ... ... ... 229 XXVIIL Systematic Survey of the phylogenetic history of the human nerve-system ... ... ... 230 34 Xviii LIST OF GENETIC TABLES. TABW pAQ, XXIX. Systematic Survey of the ontogeny of the skin and nerve systems ... ... ... ... 232 XXX. Systematic Survey of the phylogeny of the human nose ... ... ... ... ... 249 XXXI. Systematic Survey of the ontogeny of the human eye ... ... ... ... ... 258 XXXII. Systematic Survey of the phylogeny of the human ear ... ... ... ... ... 267 XXX TIL Systematic Survey of the ontogeny of the human ear 268 XXXIV. Systematic Survey of the constitution of the human skeleton ... ... ... ... ... 278 XXXV. Systematic Survey of the phylogeny of the human skeleton ... ... ... ... 309 XXXVI. Systematic Survey of the constitution of the human intestinal system ... ... ... ... 330 XXXVII. Systematic Survey of the phylogeny of the human intestinal system ... ... ... ... 346 XXXVIII. Systematic Survey of the sequence, according to age, of the human tissue-groups (phylogenetic sequence of the tissues) ... ... ... 366 XXXTX. Systematic Survey of the sequence, according to age, of the human 'organ-systems (phylogenetic sequence of the organs) ... ... ... 367 XTi, Systematic Survey of the phylogeny of the human vascular system ... ... ... ... 384 XLL Systematic Survey of the phylogeny of the human heart 385 XLII. Systematic Survey of the homologies of Worms, Articulated Animals (Arthropoda), Soft-bodied Animals (Mottusca), and Vertebrates ... ... 387 XLIIL Systematic Survey of the phylogeny of the human urinary and sexual organs ... ... ... 428 XLTV. Systematic Survey of the homologies of the sexual organs in the two sexes of Mammals ... ... 431 THE EVOLUTION OF MAN. CHAPTER XV. THE DURATION OF HUMAN TETBAL HISTORY. Comparison of Ontogenetic and Phylogenetic Periods of Time. — Duration of Germ-history in Man and in Different Animals. — Extreme Brevity of the Latter in Comparison with the Immeasurable Long Periods of Tribal History. — Relation of this Eapid Ontogenetic Modification to the Slow Phylogenetic Metamorphosis. — Estimate of the Past Duration of the Organic World, founded on the Eelative Thickness of Sedimentary Rock-strata, or Neptunian Formations. — The Five Main Divisions in the Latter : I. Primordial, or Archilithic Epoch. II. Primary, or Paleolithic Epoch. III. Secondary, or Mesolithic Epoch. IV. Tertiary, or Ca3nolithic Epoch. V. Quaternary, or Anthropolithic Epoch. — The Relative Duration of the Five Epochs.— The Results of Comparative Philology as Explaining the Phylogeny of Species. — The Inter-relations of the Main Stems and Branches of the Indo-Germanic Languages are Analogous to the Inter-relations of the Main Stems and Branches of the Vertebrate Tribe.— The Parent Forms in both Cases are Extinct. — The Most Important Stages among the Human Ancestral Forms. — Monera originated by Spontaneous Generation. — Necessity of Sponta- neous Generation. "In vain as yet has it been attempted to draw an exact line of demarcation between historic and prehistoric times ; the origin of man and the period of his first appearance pass back into indefinable time ; the so-called archaic age cannot be sharply distinguished from the present age. This is the fate of all geological, as of all historical periods. The periods which we dis. tiiiguish are, therefore, more or less arbitrarily defined, and, like the div iskras 2 THE EVOLUTION OF MAN. in systematic natural history, can only serve to bring the subject of oar study better before as and to render it more manageable ; but not to mark real distinctions between different things." — BERNHAED COTTA (1866). OUR comparative study of the Anatomy and Ontogeny of the Amphioxus and the Ascidian has afforded us aid, the value of which can hardly be over-estimated, towards acquiring a knowledge of human Ontogeny. For in the first place we have in this way filled up, as regards Anatomy, the wide chasm which in all previous systems of the animal kingdom existed between Vertebrates and Inverte- brates ; and in the second place, in the germ-history of the Amphioxus we have recognized primordial phases of de- velopment, which have long disappeared from the Ontogeny of Man, and which have been lost in accordance with the law of abridged heredity. Of special importance among theso phases of development is the Archigastrula, the ori- ginal, genuine Gastrula-form which the Amphioxus has retained up to the present time, and which re-appears in the same form in low invertebrate animals of the most diverse classes. The germ-history of the Amphioxus and the Ascidian has, therefore, so far perfected our direct knowledge of human genealogy, that, notwithstanding the incompleteness of our empiric knowledge, there is no essential gap of any great moment in the pedigree. We may, therefore, at once proceed to our task, and, aided by the ontogenetic and comparative -anatomical materials at our command, may reconstruct the main outlines of human Phylogeny. The immense importance of the direct application of the funda- mental biogenetic law of the causal connection between Ontogeny and Phylogeny now becomes evident But, before TIME REQUIRED FOR THE DEVELOPMENT OF MAN. 3 beginning this task, it will be well to note a few other general facts which may enable us better to understand the phenomena we are about to study. Firstly, it may not be out of place to insert a few remarks as to the duration of time during which Man was developing from the animal kingdom. The first thought that occurs to the mind when we consider the facts in question, is of the immense difference between the duration of the germ-history of Man on the one hand, and of his tribal history on the other. The brief period in which the Ontogeny of the human individual takes place, bears no proportion to the infinitely long period required for the Phylogeny of the human tribe. The human individual requires nine months for its perfect development from the fertilized egg-cell to the moment at which it is born and quits the mother's body. The human embryo, therefore, passes through the whole course of its development in the brief space of 40 weeks (usually in ^exactly 280 days). Each man is really older by this period than is usually assumed. When, for example, a child is said to be 9£ years old, he is in reality 10 years old. For individual existence does not begin at the moment of birth, but at the moment of fertilization. In many other Mammals the duration of the embryonic development is the same as in Man, e.g., the Ox. In the Horse and the Ass it is somewhat lunger, viz., from 43 to 45 weeks; in the Camel it is 13 months. In the largest Mammals the embryo requires a much longer time for its complete formation within the maternal body; in the Rhinoceros, for instance, 1^ year, in the Elephant 90 weeks. In the latter case, therefore, gestation lasts more than twice as long as in Man — for 4 THE EVOLUTION OF MAN. nearly a year and three quarters. In the smaller Mammals, the duration of embryonic development is, on the contrary, much shorter. The smallest Mammals, the Harvest Mice, develop fully in 3 weeks ; Rabbits and Hares in 4 weeks ; Rats and Marmots in 5 weeks ; the Dog in 9, the Pig in 17, the Sheep in 21, and the Stag in 36 weeks. Development is yet more rapid in Birds. The Chick, under normal con- ditions of incubation, requires only 3 weeks, or just 21 days for its full development. The Duck, on the other hand, takes 25, the Turkey 27, the Peacock 31, the Swan 42, and the New Holland Cassowary 65 days. The smallest of all Birds, the Humming-bird, quits the egg after the twelfth day. It is, therefore, evident that in Mammals and in Birds the duration of development within the egg-membranes stands in a definite relation to the size of body attained by each vertebrate species. But the latter is not the sole determin- ing cause of the former. There are many other circum- stances which influence the duration of individual develop- ment within the membranes of the egg.126 In all cases, however, the duration of the Ontogeny appears infinitely brief when compared with the enormous, the infinitely long period during which the Phylogeny, or gradual development of the ancestral series, took place. This period is not to be measured by years and centuries, but by thousands and millions of years. Many millions of years must indeed have elapsed while the most perfect vertebrate organism, Man, gradually developed from the primaeval one-celled ancestral organism. The opponents of the development theory, who regard this gradual develop- ment of Man from lower animal forms, and his original descent from a one-celled primitive animal as incredible, DURATION OF HUMAN GERM-HISTORY. 5 io not reflect that the very same marvel actually recurs before our eyes in the short space of nine months during the embryonic development of each human individual The same series of multifariously diverse forms, through which our brute ancestors passed in the course of many millions of years, has been traversed by every Man during the first 40 weeks of his individual existence within the maternal body. All changes in organic forms, all metamorphoses of animal and plant forms, appear to us all the more remark- able and all the more wonderful in proportion as they occur more rapidly. When, therefore, our opponents pronounce that the past development of the human race from lower animal forms is incredible, they must regard the embryonic develop- ment of the human individual from the simple egg-cell as far more wonderful in comparison. This latter process — the ontogenetic modification — which takes place before our eyes, must appear more wonderful than the phylogenetic modifi- cation, in proportion as the duration of the tribal history exceeds that of the germ-history. For the human embryo must pass through the whole process of individual develop- ment, from the simple cell up to the many-celled perfect Man, with all his organs, in the brief space of 40 weeks. On the other hand, we may assign many millions of years for the accomplishment of the analogous process of phyloge- netic development — the development of Man's ancestors from the simplest one-celled form. As regards these phylogenetic periods, it is impossible to fix approximately their length in hundreds or in thousands of years, or to establish any absolute measure of their duration. But the researches of geologists have long, since 6 THE EVOLUTION OF MAN. enabled us to estimate and compare the relative durations of the various periods of the earth's organic history. The most direct standard for determining the relative duration of geological periods is afforded by the thickness of the so- called Neptunian strata or sedimentary rock, i.e., all those strata which have been deposited, as mud, at the bottom of the ocean, or under fresh water. These stratified sedi- mentary rocks — limestone, clay, marl, sandstone, slate, etc. — which constitute the great mass of mountain-chains, and which are often several thousand feet in thickness, afford us data for estimating the relative lengths of the various periods of the earth's history. For the sake of completeness, I must say a few words as to the development of the earth as a whole, briefly indicating a few of the more prominent facts relating to this matter. At the very outset we are confronted with the weighty fact, that life originated on our planet at a certain definite period. This is a proposition that is no longer gainsaid by any competent geologist. We now know for certain that organic life upon our planet actually began at a certain time, and that it did not exist there from eternity, as some have supposed. The indisputable proofs of this are furnished, on the one hand, by physico-astronomical cos- mogeny ; on the other, by the Ontogeny of organisms. Species and tribes, like individuals, do not enjoy a perpetual life.127 They also had a beginning. The time which has elapsed since the origin of life upon the earth up to the present time (and with this period of time alone we are here concerned) we call the " history of the organic earth," as distinguished from the " history of the inorganic earth " which embraces the period before the origin of organic life THE FIRST DEVELOPMENT OF ORGANIC LIFE. ? With regard to the latter, we first obtained clear ideas from the natural philosophical researches and computations of the great critical philosopher, Immanuel Kant, and on this point I must refer the reader to Kant's " Allgemeine Natur- geschichte und Theorie des Himmels " and to the numerous Cosmogonies which treat the subject in a popular style. We cannot here dwell upon questions of this kind. The organic history of the earth could begin only when water in fluid drops existed upon its surface. For the very existence of all organisms, without any exception, depends on water in the fluid state, their bodies containing a con- siderable amount of the same. Our own body, in its fully developed state, contains in its tissues 70 per cent, of water and only 30 per cent, of solid matter. The amount of water is still greater in the body of the child, and is greatest of all in the embryo. In early stages of development the human embryo contains more than 90 per cent, of water, and not' 10 per cent, of solid matter. In low marine animals, especially in the Medusae, the body contains even more than 99 per cent, of water, and not even one per cent, of solid matter. No organism can exist and perform its vital functions without water. Without water there is no life. Water in the fluid state, which is, therefore, in- dispensable for the existence of life, could not, however, appear upon the earth until after the temperature of the surface of the fiery globe had sunk to a certain point. Before this it existed only in the form of steam. As soon, however, as the first drop of water in a fluid state was precipitated by cooling from the envelope of steam, it began its geological action, and from that time to this it has effected continual changes in the modification of the hard 8 THE EVOLUTION OF MAN. crust of the earth. The result of this unceasing work of the water, which in the form of rain and hail, of snow and ice, of rushing torrent and surging wave crumbles and dis- solves the rocks, is the formation of ooze. As Huxley says, in his excellent "Lectures on the Causes of the Phenomena of Organic Nature," the most important fact in the past history of our earth is ooze, and the question as to the history of the past ages of the world resolves itself into a question as to the formation of ooze. All the stratified rocks of our moun- tainous formations were originally deposited as ooze at the bottom of the waters, and only afterwards hardened into solid stone. As has already been said, it is possible, by bringing together and comparing the various rock-strata from many places on the surface of the earth, to obtain an approximate conception of the relative ages of these various strata. Geologists have long agreed that there is an entirely definite historical sequence of the various formations. The various groups of strata which lie one over another correspond to successive periods in the earth's organic history, during which they were deposited in the shape of mud at the bottom of the sea. Gradually this mud was hardened into solid rock. The latter, by alternate upheaval and depres- sion of the surface of the earth, was lifted above the water, and assumed the form of mountains. Four or five main periods in the earth's organic history, answering to the larger and smaller groups of these sedimentary rock-strata, are usually distinguished. These main periods are sub- divided into numerous subordinate or lesser periods. From twelve to fifteen of the latter are usually assumed. (Cf. Tables XII and XIII., pp. 11, 12.) The relative thick- GEOLOGICAL TIME. 9 ness of the various groups of strata affords the means of approximately estimating the relative length of these various divisions of time. Of course we cannot say, "In a hun- dred years on the average a stratum of a certain thick- ness (say two inches) is deposited, and therefore a rock- stratum of a thousand feet in thickness is 600,000 years old." For different rock-formations of equal thick- ness may have occupied periods of very various length in their deposition and consolidation. From the thickness of the formation we may, however, approximately judge of the relative length of the period during which it was formed. Of the four or five main periods of the earth's organic history, our acquaintance with which is indispensable for our Phylogeny of the human race, the first and oldest is known as the Primordial, Archizoic, or Archilithic Epoch. If we estimate the total thickness of all the sedimentary strata as averaging about 130,000 feet, then 70,000 feet belong to this first epoch — more than one half. From this and other circumstances we may conclude that the corresponding Primordial or Archilithic Epoch must alone have been con- siderably longer than the whole long period between the close of the Archilithic and the present time. Probably the Primordial Epoch was much longer than might appear from the ratio of 7 : 6, which we have given. This Epoch is divided into three sub-periods, known as the Laurentian, Cambrian, and Silurian, corresponding to the three principal groups of sedimentary rock-strata which constitute the Archilithic rocks. The enormous length of time required for the forma- tion at the bottom of the primordial sea of these gigantic strata, of over 70,000 feet in thickness, must, at all events, IO THE EVOLUTION OF MAN. have been many millions of years. During that time there came into existence by spontaneous generation the oldest and simplest organisms — those in which life began upon our planet — viz., the Monera. From these, one-celled plants and animals first developed — the Amoebae and many kinds of Protista. During this same Archilithic Epoch, also, all the invertebrate ancestors of the human race developed from these one-celled organisms. We draw this conclusion from the fact that towards the close of the Silurian period a few remains of fossil Fishes are already to be found, viz., Selachians and Ganoids. These are, however, much more highly organized and of later origin than the lowest Vertebrates (the Amphioxus), or than the various skull-less Vertebrates allied to Amphioxus, which must have lived during this time. The latter must necessarily have been preceded by all the invertebrate ancestors of man. Hence we may characterize this entire epoch as the " age of man's invertebrate ancestors;" or, with special reference to the oldest representatives of the Vertebrate tribe, as the " age of Skull-less Animals." During the whole Archilithic Epoch the inhabitants of our planet consisted exclusively of aquatic forms ; at least, no remains of terrestrial animals or plants dating from this period have as yet been found. A few remains of land-dwelling organisms which are some- times referred to the Silurian Period, are Devonian. The Primordial Epoch was followed by the Palaeolithic, Palaeozoic, or Primary Epoch, which is also separable into three sub-periods : the Devonian, the Carboniferous, and the Permian. During the Devonian Period the Old Red Sand- stone, or Devonian system was formed ; during the Car- boniferous, those great beds of coal were deposited which TABLE XII. Systematic Survey of the Palaeontological Periods, or the Greater Divisions in the History of the Organic Earth. I. First Epoch : The Archilithic, or Primordial Epoch. (Age of Skull-less Animals aud Seaweed Forests.) 1. The Older Archilithic Epoch or Laurentian Period 2. The Middle Archilithic Epoch „ Cambrian Period. 3. The Later Archilithic Epoch „ Silurian Period. II. Second Epoch : The Palaeolithic, or Primary Epoch. (Age of Fishes and Fern Forests.) 4. The Older Palaeolithic Epoch or Devonian Period. 6. The Middle Palaeolithic Epoch „ Coal Period. 6. The Later Palaeolithic Epoch „ Permian Period. III. Third Epoch : The Mesolithic, or Secondary Epoch. (Age of Eeptiles and Pine Forests, Coniferas.) 7. The Older Mesolithic Epoch or Triassic Period. 8. The Middle Mesolithic Epoch „ Jurassic Period. 9. The Later Mesolithic Epoch „ Chalk Period. IV. Fourth Epoch : The Caenolithic, or Tertiary Epoch. (Age of Mammals and Leaf Forests.) 10. The Older Caenolithic Epoch or Eocene Period. 11. The Middle Caenolithic Epoch „ Miocene Period. 12. The Later Caenolithic Epoch „ Pliocene Period. V. Fifth Epoch : The Anthropolithic, or Quaternary Epoch. (Age of Man and Cultivated Forests.) 13. The Older Anthropolithic Epoch or Ice Age, Glacial Period. 14. The Middle Anthropolithic Epoch „ Post Glacial Period. 15. The Later Anthropolithic Epoch „ Period of Culture. (The Period of Culture is the Historic Period, or Period of Tradition.) TABLE XIII. Systematic Survey of the Palaeoutological Formations, or the Fossiliferous Strata of the Earth's Crust. Rock-Groups. Systems. Formations. Synonyms of Formations. V. Quaternary Group, XIV. Recent [ 36. Present Upper alluvial or (Alluvium) 1 35. Becent Lower alluvial Anthropolithic ( A nthropozoi c) XIII. Pleistocene (Diluvium) f 34. Post glacial I 33. Glacial Upper diluvial Lower diluvial groups of strata IV. Tertiary Group, XII. Pliocene (New tertiary) XI. Miocene 1 32. Arvernian > 31. Sub-Appenine ] 30. Falunian Upper pliocene Lower pliocene Upper miocene Csenolithic (Caenozoic) groups of strata (Middle tertiary) X. Eocene (Old tertiary) 1 29. Limburgian !28. Gypsum 27. Nummulitio 26. London clay Lower miocene Upper eocene Middle eocene Lower eocene 25. White chalk Upper cretaceous III. Secondary 'IX. Cretaceous. 24. Green sand 23. Neocomian 22. Wealden Middle cretaceous Lower cretaceous The Kentish Weald Group, 121. Portlandian Upper oolite or Mesolithic 20. Oxfordian 19. Bath Middle ool;te Lower oolite (Mesozoic) groups of strata 18. Lias 17. Keuper Lias formation Upper trias \ VII. Trias 16. Muschelkalk Middle trias 15. Bunt cr sand Lower trias VI. Permian 14. Mountain Upper Permian IL Primary (New red sand-- stone) limestone (Zechstcin) 13. Bed sandstone Lower Permiai (jTOV.p, or I 12. Carboniferous Upper carbonifer- Palseoli »hic V. Carboniferous sandstone ous (Palaeozoic) (Coal) j 11. Carboniferous Lower carbonifer- groups of strata IV. Devonian | limestone 10. Pilton ous Upper Devonian (Old red sand- • 9. Ilfracomhe Middle Devonian etoue) | 8. Linton Lower Devonian T . rimordial 7. Ludlow Upper Silurian III. Silurian 6. Weulock Middle Silurian jroup, 5. Llandeilo Lower Silurian Archilithio (Archizoic) II. Cambrian 4. Potsdam 3. Longmynd Upper Cambrian Lower Cambrian groups of strata I. Laurentian 2. Labrador 1. Ottawa Upper Lauren tian Lower Laurentiau GEOLOGICAL PEBIODS. 13 supply us with our principal fuel; in the Permian, the New Red Sandstone, the Magnesian Limestone (Zechstein), and the Cupriferous Slate were formed. The approxi- mate thickness of this entire group of strata is esti- mated at 42,000 feet at most; some geologists make it somewhat more, others considerably less. In any case, this Palaeolithic Epoch, taken as a whole, is consider- ably shorter than the Archilithic, but yet is considerably longer than all the following Epochs taken together. The strata deposited during this Primary Epoch supply fossil animal remains in great abundance; besides numerous species of Invertebrates we find also very many Verte- brates— Fishes preponderating. As early as the Devonian, and even during the Carboniferous and the Permian Periods, there existed so great a number of Fishes, espe- cially Primitive Fishes (Sharks) and Ganoids, that we may designate the entire Palaeolithic Period as the Age of Fishes. The Palaeozoic Ganoids especially are represented by a large number of forms. But even during this period some Fishes began to accustom themselves to living upon the land, and thus gave rise to the Amphibian class. Even in the carboniferous system we find fossil remains of Amphibia — the earliest terrestrial and air-breathing animals. In the Permian Period the variety of these Amphibia becomes greater. To- wards its close the first Amnion-animals, the tribal ancestors of the true higher Vertebrate classes, seem first to appear. These are a few lizard-like animals, of which the Protero- saurus from the Cupriferous Slate at Eisenach is the best known. The appearance of the most ancient Amnion Animals (Amniota), to which the common parent-form of 14 THE EVOLUTION OF MAN. Reptiles, Birds, and Mammals must have belonged, seems in fact to be referred by these oldest reptilian remains back to the close of the Palaeolithic Epoch. During this Epoch the ancestors of the human race must accordingly have been represented, first by true Fishes, then by Mud-Fishes (Dipneusta) and Amphibia, and finally by the oldest Amnion Animals, the Protamnia. After the Palaeolithic Epoch comes a third main division of the earth's organic history, known as the Mesolithic, or Secondary Epoch. This is again distinguished into three subdivisions — the Triassic, the Jurassic, and the Cretaceous Periods. The approximate thickness of the strata-groups, formed during these three Periods from the beginning of the Triassic down to the end of the Cretaceous Period, amounts in all to about 15,000 feet, not one half the thick- ness of the Palaeolithic deposits. During this Epoch a very great and varied development took place in all divisions of the animal kingdom. In the vertebrate tribe especially a number of new and interesting forms developed. Among Fishes the Osseous Fishes (Teleostei) now first appear. But the Reptiles surpass all others both in numbers and in diversity of species — the most remarkable and the most familiar forms being the gigantic extinct Dragons (Dino- saurians), the Sea-Dragons (Halisaurians), and the Flying Lizards (Pterosaurians). In reference to this predominance of the reptilian class this time is known as the age of reptiles. But the class of Birds also developed during this period, undoubtedly originated from a branch of the lizard-like Reptiles. This is shown by the similar embryology of Birds and of Reptiles, by their Comparative Anatomy, and also by the fact that we know of fossil birds with toothed jaws FAUNA OF THE GEOLOGICAL PERIODS. 15 and with lizard's tail, belonging to this period (Odon- tornis ArcfiCBOpteryx). Finally, it was during this period that there appeared upon the scene that most perfect and, for us, most important vertebrate class, the mammalian class. The oldest fossil remains of these have been found in the most recent Triassic strata, viz., molar teeth of a small insectivorous Pouched Animal (Marsupial). Numer- ous remains occur somewhat later in the Jura system, and! a few in the chalk. All the remains of Mammals from this; Mesolithic Epoch with which we are acquainted belong to the low Pouched Animal division ; and among these were undoubtedly the ancestors of Man. On the other hand, not a single undisputed relic has yet been discovered throughout all this period of one of the higher Mammals (Placentalia). This last division, of which Man is a member, did not develop till later, in the immediately subsequent Tertiary Epoch. The fourth main division of the history of the organic earth, the Tertiary, Csenozoic, or Csenolithic Epoch, was of much shorter duration than the preceding. For the strata deposited during this period are in all only about 3000 feet in thickness. This Epoch, also, is divided into three sub- divisions, known as the Eocene, Miocene, and Pliocene Periods. During these periods the most diverse develop- ment of the higher classes of plants and animals took place and the fauna and flora of our globe now approached nearer and nearer to their present character. The most highly developed class of animals, that of Mammals, now attained pre-eminence. This Tertiary Epoch may, therefore, be called the age of Mammals. The most perfect section of this class, the Placental Animals, among which is Man, 35 1 6 THE EVOLUTION OF MAN. now first appeared. The first appearance of Man — or to speak more correctly — the development of man from the most nearly allied ape-form, dates probably either from the Miocene or the Pliocene Period, — from the middle or the latest section of the Tertiary Epoch. Perhaps, as is assumed by others, Man strictly so-called, i.e., Man gifted with language, first .developed from- the speechless man-like Apes, in the subsequent Anthropolithic Age. At all events, the perfect development and distribution of the various races of Man dates from the fifth and last main division of the organic history of the earth, and hence this Epoch has been called the Anthropolithic, or Anthro- pozoic, and also the Quaternary Epoch. It is true that, in the present imperfect state of our pakeontological and prehistoric knowledge, we cannot solve the problem as to whether the development of Man from the nearest allied Ape-forms took place in the beginning of this Anthropolithic Epoch, or as early as the middle or towards the close of the preceding Tertiary Epoch. This much, however, is certain, that the true development of human culture dates only from the Anthropolithic Epoch, and that this latter con- stitutes only an insignificantly small section of the entire enormous period of time occupied in the development of the organic earth. When we reflect upon this, it appears absurd to speak of the brief span of man's period of cul- ture as "the world's history." This so-called History of the World does not amount approximately to even one- half per cent, of the length of those enormous periods which have passed away from the beginning of the earth's organic history down to the present time. For this World's THE "AGE OF MAN." 17 History, or more correctly, History of People, is itself only the latter half of the Anthropolithic Epoch, while oven the first half of this Epoch must be reckoned as a prehistoric period. Hence this last main period, reaching from the close of the Cienolithic Epoch to the present time, can only be called the "age of man," inasmuch as the diffusion and differentiation of the different species and races of man, which have so powerfully influenced all the rest of the organic population of the globe, took place during its course. Since the awakening of the human consciousness, human vanity and human arrogance have delighted in regarding Man as the real main-purpose and end of all earthly life, and as the centre of terrestrial Nature, for whose use and service all the activities of the rest of creation were from the first defined or predestined by a "wise providence." How utterly baseless these presumptuous anthropocentric conceptions are, nothing could evince more strikingly than a comparison of the duration of the Anthropozoic or Quater- nary Epoch with that of the preceding Epochs. For even though the Anthropolithic Epoch may embrace several hun- dreds of thousands of years, how small is this time when compared with the millions of years that have elapsed since the beginning of the world's organic history down to the first appearance of the human race ! If the entire duration of the organic history of the earth, from the generation of the first Monera down to the present day, is divided into a hundred equal parts, and if then, corresponding with the relative average thickness of the intervening strata-systems, the respective percentages are 1 8 THE EVOLUTION OF MAN. assigned to the relative durations of the five main divisions or Epochs, the latter will be found to be about as follows : — I. Archilithic, or archizoic (primordial) Epoch . . 53.6 II. Palaeolithic, or palaeozoic (primary) Epoch . . 32 1 III. Mesolithic, or mesozoic (secondary) Epoch . . 11.5 IV. Cifinolithic, or cenozoic (tertiary) Epoch . . .2.3 Y. Anthropolithio, or anthropozoic (quaternary) Epoch . 0.5 Total ... 100.0 The relative durations of the five main epochs of the earth's organic history, are yet more clearly seen in the opposite Table (XIV.), in which the relative thicknesses of the strata systems deposited within these Epochs is repre- sented on a scale corresponding to their actual depths. This table shows that the period of the so-called History of the World forms but an inconsiderable span in comparison with the immeasurable duration of those earlier epochs during which Man did not exist upon this planet. Even the great Csenozoic Epoch, the so-called Tertiary Epoch, during which the Placenta! Animals, the higher Mammals, developed, includes but little more than two per cent, of the whole enormous duration of the organic history of the world.128 And now before we turn to our proper phylogenetic task ; before, guided by our knowledge of ontogenetic facts and by the fundamental law of Biogeny, we attempt to trace step by step the history of the palasontological evo- lution of our animal ancestors, let us turn aside for a short time into another and apparently very different and very remote department of science, a general review of which will make the solution of the difficult problems which now rise before us very much easier. The science is thai ( 19 ) TABLE XIY. Systematic Survey of the Neptunian fossiliferons strata of the earth with reference to their relative sectional thickness (130,000 feet circa). IV. Csenolithic Strata, circa 3000 feet. Pliocene, Miocene, Eocene. III. Mesolithic Strata, Deposits of the Secondary Epoch, circa 15.COO feet. IX. Chalk System. VIII. Jurassic System. VII. Triassic System II. Palaeolithic Strata. Deposits of the Primary Epoch, circa 42,000 feat. VI. Permian System. V. Coal System. IV. Devonian System. I. Archilithic Strata- Deposits of the Primordial Epoch, circa 70,000 feet. III. Silurian System, circa 22,000 feet. II. Cambrian System, circa 18,000 feet. I. Laurentian System, circa 80,000 feet. 2O THE EVOLUTION OF MAN. of Comparative Philology. Ever since Darwin, by the theory of Natural Selection, infused new life into Biology, and raised the fundamental question of development in every branch of science, attention has frequently and from very different quarters been called to the remarkable parallelism, which exists between the evolution of the various human languages and the evolution of organic species. The comparison is quite justifiable and very instructive. Indeed it is hardly possible to find an analogy better adapted to throw a clear light on many obscure and difficult facts in the evolution of species, which is governed and directed by the same natural laws which guide the course of the evolution of language. All philologists who have made any progress in their science, now unanimously agree that all human languages have developed slowly and by degrees from the simplest rudiments. On the other hand, the strange proposition which till thirty years ago was defended by eminent au- thorities, that language is a divine gift, is now universally rejected, or at best defended only by theologians and by people who have no conception of natural evolution. Such brilliant results have been attained in Comparative Philology that only one who is wilfully blind can fail to recognize the natural evolution of language. The latter is necessarily evident to the student of nature. For speech is a physio- logical function of the human organism, developing simul- taneously with its special organs, the larynx and the tongue, and simultaneously with the functions of the brain. It is, therefore, quite natural that in the history of the evolution of languages, and in their whole system, we should find the same correlations as in the history of the evolution of organic species and in their whole system. The various METHOD OF COMPARATIVE PHILOLOGY. 21 larger and smaller groups of speech-forms, which are distin- guished in Comparative Philology as primitive languages, fundamental languages, parent languages, derived languages, dialects, patois, etc., correspond perfectly in their mode of development with the various larger and smaller groups of organisms classed in systems of Zoology and Botany as tribes, classes, orders, families, genera, species, and varie- ties of the animal and vegetable kingdoms. The relations between these various systematic groups, or categories, are in both cases identical; moreover, evolution follows the same course in one case as in the other. This instructive comparison was first elaborated by one of the most eminent of German philologists, one who, unfortunately, died pre- maturely— August Schleicher, not only a philologist but also a learned botanist. In his more important works, the Com- parative Anatomy and evolutionary history of languages is treated by the same phylogenetic method which we employ in the Comparative Anatomy and evolutionary history of animal forms. He has especially applied this method to the Indo-Germanic family of languages ; and in his little treatise on " The Darwinian Theory and the Science of Language" ("Die Darwin'sche Theorie und die Sprach- wissenschaft "), he illustrated it by means of a synoptical pedigree of the Indo-Germanic family of languages.12* If the formation of the various branch languages which have developed from the common root of the primitive Indo-Germanic tongue is studied with the aid of this pedi- gree, a very clear idea of their Phylogeny will be acquired. At the same time it becomes evident how entirely analogous is the evolution of the greater and lesser groups of the \rertebrates, which have sprung from the one common root- 22 THE EVOLUTION OF MAN. form of the primitive Vertebrates. The primitive Indo- Germanic root-tongue first separated into two chief stems : the Slavo-Germanic and the Ario-Romanic main-trunks. The Slavo-Germanic then branched into a primitive German and a primitive Slavo-Lettic tongue. Similarly, the Ario- Romanic split up into a primitive Arian and a primitive Grseco-Romanic language (p. 23). If we continue our examination of this pedigree of the four primitive Indo- Germanic languages, we find that the primitive Germanic tongue divided into three chief branches — a Scandinavian, a Gothic, and a Teutonic branch. From the Teutonic branch proceeded, on the one hand, High German, and, on the other hand, Low German, to which latter belong the various Friesinn, Saxon, and Low German dialects. Similarly, the Slavo-Lettic tongue developed first into a Baltic and a Slavonic language. From the Baltic spring the Lettic, Lithuanian, and Old Prussian dialects. The Slavic, on the other hand, give rise, in the South-east, to the Russian and the South Slavic dialects, and, in the West, to the Polish and Czech dialects. Turning now to the other main stem of the Indo- Germanic languages and its branches — the primitive Ario- Romanic — it is found to develop with the same luxuriance. The primitive Gneco-Romanic language gave rise, on the one hand, to the Thracian language (Albanian Greek), and on the other, to the Italo-Keltic. From the latter in turn sprung two divergent branches — in the South, the Italian branch (Romanic and Latin), and in the North, the Keltic, from which arose all the different British (Old British, Old Scottish, and Irish"* and Gallic tongues. The numerous Iranian and Indian dialects branched out in the same way from the primitive Arian language. TABLE XV. Pedigree of the Indo-Germanic Language*. Lithuanians Ancient Prussians Letts Baltic Races Serbians Polish Czechs I J I West Sclaves Kussians South Sclave Anglo -Saxons Low Germans High Germans South-east Sclaves Sclaves Scandinavians Goths Germans Primitive Germans Eclavo-Letts Bomans Latins Ancient British Ancient Scotch Irish Gaels Gnu Is Brittaues* Italians Kelts Sclavo Germans Italo-Kelts Altanese Greeks Primitive Thracians Indians Iranians I | Ariaua Groeoo-Romans Ario-Bomans Indo-Gernians 24 THE EVOLUTION OF MAN. A close study of this pedigree of the Indo-Germamc languages is, in many respects, of great interest. Com- parative Philology, to which we are indebted for our know- ledge of this subject, thus shows itself to be a true science — a natuial science. It, indeed, long ago anticipated in its own province the phylogenetic method with the aid of which we now attain the highest results in Zoology and and in Botany. And here I cannot refrain from remarking how much to the advantage of our general culture it would be if the study of languages (which is undoubtedly one of the most powerful means of culture) were comparatively prosecuted; and if our cut and dried Philology were re- placed by a living, many-sided, comparative study of lan- guages. The latter stands in the same relation to the former as the living history of organic evolution does to the lifeless classification of species. How much deeper would the interest taken in the study of language by the students in our schools be, and how much iaore vivid would be the results if even the first elements of Comparative Philology were taught instead of the distasteful composi- tion of Latin exercises in Ciceronian style ! I have entered with this detail into the " Comparative Anatomy " and the history of the evolution of languages, because it is unsurpassed as a means of illustrating the Phylogeny of organic species. We find that in structure and in development these primitive languages, parent languages, derived languages, and dialects, correspond exactly like the classes, orders, genera, and species of the animal kingdom. The " natural system " is in both cases phylogenetic. Just as Comparative Anatomy, Ontogeny, and Palaeontology afford certain proof that all Vertebrates, MAN DESCENDED FROM EXTINCT FORMS. 2 5 whether extinct or extant, are descended from a common ancestral form, so does the comparative study of the dead and living Indo-Germanic language absolutely convince us that all these languages have sprung from a common origin. This monophyletic view is unanimously adopted by all linguists of importance who have studied the question, and who are capable of passing a critical judgment upon it.130 The point, however, to which I would specially call your attention in this comparison between the various branches, on the one hand, of the Indo-Germanic language, and, on the other, of the vertebrate tribe, is that the direct descendants must never be confounded with the collateral lines, nor the extinct with the extant forms. This mistake is often made, and results in the formation of erroneous notions of which our opponents often take advantage in order to oppose the whole theory of descent. When, for instance, it is said that human beings are descended from Apes, the latter from Semi-apes, and the Semi-apes from Pouched Animals (Marsupialia), very many people think only of the familiar living species of these different mammalian orders, such as are to be found stuffed in our museums. Now, our opponents attribute this erroneous view to us, and, with more craft than judgment, declare the thing impossible ; or else they ask us as a physiological exporiment to transform a Kangaroo into a Semi-ape, this into a Gorilla, and the Gorilla into a Man. Their demand is as childish as the conception on which it is founded is erroneous ; for all these extant forms have varied more or less from their common parent-form, and none of them are capable of pro- ducing the same divergent posterity which were really produced thousands of years ago by that parent-form.131 26 THE EVOLUTION OF MAN. There is no doubt that Man is descended from an extinct mammalian form, which, if we could see it, we should certainly class with the Apes. It is equally certain that this primitive Ape in turn descended from an unknown Semi -ape, and the latter from an extinct Pouched Animal. But then it is beyond a doubt that it is only in respect of essential internal structure, and on account of their similarity in the distinctive anatomical characters of the order, that all these extinct ancestral forms can be spoken of as members of the yet extant mammalian orders. In external form, in generic and specific characters, they must have been more or less — perhaps even greatly — different from all living repre- sentatives of the orders to which they belonged. For it must be accepted as a quite universal and natural fact in phylogenetic evolution that the parent-forms themselves, with their specific characters, became extinct at a more or less distant period. Those extant forms which come nearest to them, yet differ from them more or less, perhaps even very essentially. Hence in our phylogenetic researches and in our comparative view of the still living divergent descendants all we can undertake to do is to determine how far the latter depart from the parent-form. We may quite confidently assume that no single older parent-form has reproduced itself without modification down to our time. We find this same state of things on comparing various extinct and living languages, which have sprung from one common primitive tongue. If, from this point of view, we examine the genealogical tree of the Indo-Germanic languages, we may conclude, on d priori grounds, that all the earlier primitive languages, fundamental languages, and ancestral languages, from which the living dialects are COMPARATIVE METHODS OF PHILOLOGY AND ZOOLOGY. 27 descended in the first or second degree, have been extinct for a longer or shorter period. And this is the case. The Ado-Romanic and the Sclavo-Germanic tongues have long been altogether extinct, as are also the primitive Arian and Grseco -Romanic, the Sclavo-Lettic, and primitive Germanic languages. Some even of the languages descended from these have also long been dead, and all those of the Indo- Germanic branch which are yet extant, are akin only in so far as they are divergent descendants of common parent- forms. Some have diverged from this ancestral form more, others less. This easily demonstrable fact very well illustrates analogous facts in the descent of vertebrate species. Phy- logenetic " Comparative Philology," as a powerful ally, sup- ports ph}Tlogenetic " Comparative Zoology." The former can, however, adduce far more direct evidence than the latter, because the palaeontological materials of Philology, the ancient monuments of extinct tongues, have been far better preserved than the palaeontological materials of Comparative Zoology, the fossil bones of vertebrates. The more these analogous conditions are considered, the more convincing is their force. "We shall presently find that we can trace back the genealogical line of Man, not only to the lower Mammals, but even to the Amphibia, to the shark-like Primitive Fishes, and even far below these, to the skull-less Vertebrates allied to the Amphioxus. It must be remembered this does not mean that the living Amphioxus, Shark, or Amphibian accu- rately represent the outward appearance of the parent-forms of which we speak. Still less does it mean that the Amphioxus, or the Shark of our day, or any extant species 28 THE EVOLUTION OF MAN. of Amphibian is an actual parent-form of the highei Ver- tebrates and of Man. On the contrary, this important assertion must be clearly understood to mean, that the living forms, which have been mentioned, are side branches, which are much more nearly allied, and similar to the extinct common parent-forms, than any other known animal forms. In their internal characteristic structure they remain so similar to the unknown parent-forms, that we should class them both in one order, if we had the latter before us in a living state. But the direct descendants of the primitive forms have never remained unmodified. Hence it is quite impossible that among the living species of animals we should find the actual ancestors of the human race in their characteristic specific forms. The essential and charac- teristic features, which more or less closely connect living forms with the extinct common parent-forms, are to be found in the internal structure of the body, not in the external specific form. The latter has been much modified by adaptation. The former has been more or less retained by heredit}'. Comparative Anatomy and Ontogeny indisputably prove that Man is a true Vertebrate, so that the special genealo- gical line of Man must of course be connected with that of all those Vertebrates which are descended from the same common root. Moreover, on many definite grounds, sup- plied by Comparative Anatomy and Ontogeny, we must assume only one common origin for all Vertebrates — a monophyletic descent. Indeed, if the theory of descent is coirect, all Vertebrates, Man included, can only have descended from a single common parent-form — from a single primitive Vertebrate species. The genealogical line of the Vertebrates, therefore, is also that of Man. AMCEBOID ANCESTOKS. 2Q Our task of ascertaining a pedigree of Man thus widens into the more considerable task of constructing the pedigree of all the Vertebrates. This is connected, as we learned from the Comparative Anatomy and Ontogeny of the Amphioxus and of the Ascidian, with the pedigree of the Invertebrate animals, and directly with that of the Worms, while no con- nection can be shown with the genealogy of the indepen- dent tribes of the Articulated Animals (Arthropoda), Soft- bodied Animals (Mollusca), and Star-animals (Echinoderma). As the Ascidian belongs to the Mantled Animals (Tunicata), and as this class can only be referred to the great Worm tribe, we must, aided by Comparative Anatomy and Onto- geny, further trace our pedigree down through various stages to the lowest forms of Worms. This necessarily brings us to the Gastrsea, that most important animal form in which we recognize the simplest conceivable prototype of an animal with two germ-layers. The Gastnea itself must have ori- ginated from among those lowest of all simple animal forms, which are now included by the name of Primitive Animals (Protozoa). Among these we have already considered that primitive form which possesses most interest for us — the one-celled Amoeba, the peculiar significance of which depends on its resemblance to the human egg-celL Here we have reached the lowest of those impregnable points, at which the value of our fundamental law of Biogeny is directly found, and at which, from the embryonic evolutionary stage, we can directly infer the extinct parent-form. The amoeboid nature of the young egg-cell, and the one-celled condition in which each Man begins his existence as a simple parent- cell or cytula-cell, justify us in affirming that the oldest ancestors of the human race (as of the whole animal kingdom) were simple amoeboid cells. 30 THE EVOLUTION OF MAN. Here arises another question : " Whence, in the begin- ning of the organic history of the earth, at the commence- ment of the Laurentian period, came the earliest Amoebae ? " To this there is but one reply. Like all one-celled organ- isms, the Amcjebae have originally developed only from the simplest organisms know to us, the Monera. These Monera, which we have already described, are also the simplest con- ceivable organisms. Their body has no definite form, and is but a particle of primitive slime (plasson) — a little mass of living albumen, performing all the essential functions of life, and everywhere met with as the material basis of life. This brings us to the last, or perhaps the first question in the history of evolution — the question as to the origin of the Monera. And this is the momentous question as to the prime origin of life — the question of spontaneous generation (generatio spontanea or cequivoea). We have neither time, nor indeed have we any occasion, to discuss at length the weighty question of spontaneous generation. On this subject I must refer you to my " History of Creation," and, especially, to the second book of the Generelle Morphologic, and to the discussion on Monera and spontaneous generation in my " Studien iiber Moneren und andere Protista." 132 I have there stated my own views on this important subject in very great detail. Here I will only say a few words on the ob- scure question as to the first origin of life, and will answer it so far as it concerns our radical conception of the history of organic evolution. In the definite, limited sense in which I maintain spontaneous generation {gene- ratio spontanea) and assume it as a necessary hypo- thesis in explanation of the first beginning of life upon SPONTANEOUS GENERATION. 31 the earth, it merely implies the origin of Monera from inorganic carbon compounds. When animated bodies first appeared on our planet, previously without life, there must, in the first place, have been formed, by a process purely chemical, from purely inorganic carbon combinations, that very complex nitrogenized carbon compound which we call plasson, or " primitive slime," and which is the oldest material substance in which all vital activities are embodied. In the lowest depths of the sea such homogeneous amorphous protoplasm probably still lives, in its simplest character, under the name of Bathybius.137 Each individual living particle of this structureless mass is called a Moneron. The oldest Monera originated in the sea by spontaneous generation, just as crystals form in the matrix. This assumption is required by the demand of the human understanding for causality. For when, on the one hand, we reflect that the whole inorganic history of the earth proceeds in accordance with mechanical laws and without any intervention by creative power, and when, on the other hand, we consider that the entire organic history of the world is also de- termined by similar mechanical laws ; when we see that no supernatural interference by a creative power is needed for the production of the various organisms, then it is certainly quite inconsistent to assume ^uch supernatural creative, interference for the first production of life upon our globe. At all events we, as investigators of nature, are bound at least to attempt a natural explanation. At present, the much agitated question of spontaneous generation appears very intricate, because a large number of very different, and in part quite absurd, conceptions are included under the term "spontaneous generation," and 32 THE EVOLUTION OF MAN. because some have supposed that the problem could be solved by means of the crudest experiments. The doctrine of spontaneous generation cannot be experimentally refuted. For each experiment with a negative result merely proves that under the conditions (always very artificial) supplied by us, no organism has been produced from inorganic combina- tions. Neither can the theory of spontaneous generation be experimentally proved unless great difficulties are over- come ; and even if in our own time Monera were produced daily by spontaneous generation — as is very possible — yet the absolute empiric proof of this fact would be extremely difficult — indeed, in most cases impossible. He, however, who does not assume a spontaneous generation of Monera, in the sense here indicated, to explain the first origin of life upon our earth, has no other resource but to believe in a supernatural miracle ; and this, in fact, is the questionable standpoint still taken by many so-called " exact naturalists," who thus renounce their own reason. Sir William Thomson has indeed tried to avoid the necessary hypothesis of spontaneous generation by assuming that the organic inhabitants of our earth originally de- scended from germs which proceeded from the inhabitants of other planets, and which, with fragments of the latter, with meteorites, accidentally fell on to the earth. This hypothesis has met with much applause, and was even supported by Helmholtz. Friederich Zoellner, an acute physicist, has, however, refuted it in his excellent natural- philosophical work " Ueber die Natur der Cometen," a critical book containing most valuable contributions to the history and theory of knowledge.127 Zoellner has plainly shown that the hypothesis is unscientific in two respects — MONERA ALONE PRODUCED BY" SPONTANEOUS GENERATION. 33 firstly, in point of logic, and secondly, in its scientific tenor (p. xxvi). At the same time lie rightly shows that the hypothesis of spontaneous generation, in the sense which we have defined, is the " condition necessary to the conceiv- ability of nature in accordance with the laws of causality." In conclusion, I repeat, with emphasis, that it is only in the case of Monera — of structureless organisms without organs — that we can assume the hypothesis of spontaneous generation. Every differentiated organism, every organism composed of organs, can only have originated from an undifferentiated lower organism by differentiation of its parts, and consequently by Phylogeny. Hence, even in the production of the simplest cell we must not assume the pro- cess of spontaneous generation. For even the simplest cell consists of at least two distinct constituent parts ; the inner and firmer kernel (nucleus), and the outer and softer cell-substance or protoplasm. These two distinct parts can only have come into being by differentiation of the homogeneous plasson of a moneron and of a cytod. It is for this very reason that the natural history of Monera is of the highest interest ; for it alone can remove the principal difficulties which beset the question of spontaneous genera- tion. The extant Monera do afford us organless and struc- tureless organisms, such as must have originated by spon- taneous generation at the first beginning of organic life upon the earth.133 CHAPTER XVI. THE ANCESTRY OF MAN. I. FROM THE MONEEA TO THE Rolatioti of the General Inductive Law of the Theory of Descent to ttio Special Deductive Laws of the Hypotheses of Descent. — Incompleteness of the Three Great Records of Creation : Palaeontology, Ontogeny, and Comparative Anatomy. — Unequal Certainty of the Various Special Hypotheses of Descent. — The Ancestral Line of Men in Twenty-two Stages : Eight Invertebrate and Fourteen Vertebrate Ancestors. — Distri- bution of these Twenty-two Parent-forms in the Five Main Divisions of the Organic History of the Earth. — First Ancestral Stage : Monera. — The Structureless and Homogeneous Plasson of the Monera. — Differen- tiation of the Plasson into Nucleus, and the Protoplasm of the Cells. — Cytods and Cells as Two Different Plastid-forms.— Vital Phenomena of Monera. — Organisms without Organs. — Second Ancestral Stage : Amoebae. — One-celled Primitive Animals of the Simplest and most Un- cl i fferentiated Nature. — The Amoeboid Egg-cells. — The Egg is Older than the Hen. — Third Ancestral Stage : Syn-Amoeba, Ontogenetically repro- duced in the Morula. — A Community of Homogeneous Amoeboid Cells.— Fourth Ancestral Stage: Planaea, Ontogenetically .reproduced in the Blastula or Planula. — Fifth Ances'ral Stnge : Gnstraea, Ontogenetically reproduced in the Gastrula and the Two-layered Germ-disc.— Orierir- of the Gastraea by Inversion (invaginatio) of the Planaea. — Haliphysema and Gastrophysema. — Extant Ga^traeads. " Now, very probably, if the course of evolution proves to be so very simple, it will be thought that the whole matter is self-evident, and that research is hardly required to establish it. But the story of Columbus and the egg is daily repeated ; and it is necessary to perform the experiment INDUCTIVE AND DEDUCTIVE METHODS. 35 for one's self. How slowly progress is made in the knowledge even of self- evident matters, especially when respectable authorities disagree, I myself have experienced sufficiently." — K.AKL ERNST BAEK (1828). GUIDED by the fundamental law of Biogeny and by the sure records of creation, we now turn to the interesting tarfk of examining the animal parent-forms of the humait race in their proper sequence. To ensure accuracy, we must first become acquainted with the various mental operations which we shall apply in this natural-philosophical research. These operations are partly of an inductive, partly of a deductive nature: partly conclusions from numerous particular experiences to a general law ; partly conclusions from this general law back to particular ex- periences. Tribal history as a whole is an inductive science ; for the whole theory of descent, as an indispensable and most essential part of the whole theory of evolution, is entirely founded on inductions. From all the biological incidents in plant life, in animal life, and in human life, we have derived the certain inductive conception that the whole of the or- ganic inhabitants of our globe originated in accordance with one single law of evolution. To this law of evolution, La- marck, Darwin, and their successors gave definite form in the theory of descent. All the interesting phenomena ex- hibited by Ontogeny, Palaeontology, Comparative Anatomy, Dysteleology, Chorology, the (Ekology of organisms, all the important general laws, which we infer from multitudinous phenomena of these different sciences, and which are most intimately connected together, are the broad inductive data from which is drawn the most extensive inductive law of Biology, Because the innate connection between all 36 THE EVOLUTION OF MAN. these infinitely various groups of phenomena in these dif- ferent departments becomes explicable and comprehensible solely through the theory of descent, therefore this theory of evolution must be regarded as an extensive inductive law. If we now really apply this inductive law, and with its help seek to discover the descent of individual organic species, we must necessarily form phylogenetic hypotheses, which are of an essentially deductive nature, and which are inferences from the general theory of descent back to indi- vidual particular cases. These special deductive conclusions are, however, in accordance with the inexorable laws of Logic, as justifiable, as necessary, and as indispensable in our department of knowledge as the general inductive conclusions of which the whole theory of evolution is formed. The doctrine of the animal parent-forms of man- kind is ako a special deductive law of this kind, which is the logical conclusion from the general inductive law of the theory of descent.134 As is now very generally acknowledged, both by the adherents of and the opponents of the theory of descent, the choice, in the matter of the origin of the human race, lies between two radically different assumptions : We must either accustom ourselves to the idea that all the various species of animals and plants, Man also included, ori- ginated independently of each other by the supernatural process of a divine "creation," which as such is entirely removed from the sphere of scientific observation— .-or we are compelled to accept the theory of descent in its entirety, and trace the human race, equally with the various animal and plant species, from an entirely simple primaeval parent- form. Between these two assumptions there is no third FAITH OR SCIENCE. 37 ;ourse. Either a blind belief in creation, or a scientific theory of evolution. By assuming the latter, and this is the only possible natural-scientific conception of the universe, we are enabled, with the help of Comparative Anatomy and Ontogeny, to recognize the human ancestral line with a certain approximate degree of certainty, just as is more or less the case with respect to all other organisms. Our previous study of the Comparative Anatomy and Ontogeny of Man, and of other Vertebrates, has made it quite clear that we must first seek the pedigree of mankind in that of the vertebrate tribe. There can be no doubt that (if the theory of descent is correct) Man has developed as a true Vertebrate, and that he originated from one and the same common parent-form with all other Vertebrates. This special deduction must be regarded as quite certain, correct- ness of the inductive law of the theory of descent being of course first granted. No single adherent of the latter can raise a doubt about this important deductive conclusion. We can, moreover, name a series of different forms of the vertebrate tribe, which may be safely regarded as the repre- sentatives of different successive phylogenetic stages of evolution, or as different members of the human ancestral line. We can also prove with equal certainty that the vertebrate tribe as a whole originated from a group of low invertebrate animal forms; and among these we can again with more or less certainty recognize a series of members of the ancestral chain. We must, however, at once expressly say that the cer- tainty of the different hypotheses of descent, which are founded entirely on special deductive inferences, is very unequal. Several of these conclusions are already fully 38 THE EVOLUTION OP MAN. established ; others, on the contrary, are most doubtful ; in yet others, it depends upon the subjective proportion of the knowledge of the naturalist and on his capability of draw- ing conclusions, what degree of probability he will accoiJ to them. It is, at all events, necessary thoroughly to dis- tinguish between the absolute certainty of the general (inductive) theory of descent, and the relative certainty of the special (deductive) hypothesis of descent. We can never in any case prove the whole ancestral line of an- cestors of an organism with the same certainty with which we regard the theory of descent as the only scientific expla- nation of the organic forms. On the contrary, the special proof of all separate parent-forms must always remain more or less incomplete and hypothetical That is quite natural For all the records of creation upon which we rely are in a great measure incomplete, and will always remain incomplete ; just as in the case of Comparative Philology. Above all, Palaeontology, the most ancient of all records of creation, is in the highest degree incomplete. We know that all the petrifications with which we are acquainted form but an insignificantly small fragment of the whole number of animal forms and plant forms which have ever existed. For each extinct species obtained by us in a petrified condition, there are at least a hundi'ed, probably thousands of extinct species which have left no trace of their existence. Phis extreme and most deplorable defectiveness of the palsoon- tological record of creation, upon which it is impossible to insist too strongly, is very easily accounted for. The very conditions under which organic remains become petrified necessitate it. It is also partly explicable as the result of INCOMPLETENESS OF THE BIOLOGICAL RECORDS. 39 an imperfect knowledge in this department. It must be remembered, that far the greater proportion of the rock strata which constitute the mountain masses of the surface of the earth is not yet unfolded to us. Of the count- less petrifications which are hidden in the huge moun- tain chains of Asia and Africa, we know but a few small samples. Part of Europe and of North America has alone been more minutely explored. The whole of the petri- factions accurately known and in our collections do not amount to a hundredth part of those which really exist in the crust of the earth. In this respect we may, therefore, expect a rich harvest of discoveries in the future. But, in spite of this, the palaeontological record of creation (for reasons which I have amply explained in Chapter XV. of my "Natural History of Creation") will always remain extremely incomplete. Not less incomplete is the second, most important recoid of creation, that of Ontogeny. For the Phytogeny of the individual it is the most important of all. Yet, it also has its great defects, and often leaves us in the lurch. In this matter, we must distinguish quite clearly between palm- genetic and kenogenetic phenomena, between the original, inherited evolution and the later, vitiated evolution. We must never forget that the laws of abridged and vitiated heredity frequently disguise the original course of evolution beyond recognition. The reproduction of the Phytogeny in the Ontogeny is but rarely tolerably complete. The earliest and most important stages of germ-history are usually the most abridged and compressed. The youthful evolutionary forms have in turn often adapted themselves to new conditions, and have thus been modified The 4-0 THE EVOLUTION OF MAN. struggle for existence has excited an equally strong modify- ing influence upon the various independent and yet un- developed young forms, as upon the developed and mature forms. Therefore, in the Ontogeny of the higher animal forms, the Phylogeny has been very greatly limited by Keno- genesis ; as a rule, only a blurred and much vitiated picture of the original course of evolution of their ancestors now lies before us in the Ontogeny. Only with great precaution and judgment dare we infer the tribal history directly from the germ-history. Moreover, the germ-history itself is known to us only in the case of very few species. Lastly, the highly important record of creation afforded by Comparative Anatomy is unfortunately very incomplete, and for the simple reason, that the number of extant animal species forms but a very small fragment of the whole number of different animal forms that have existed from the beginning of the organic history of the world to the present time. The total sum of the latter may safely be estimated at several millions. The number of those animals the organization of which has at present been investigated by Comparative Anatomy is very small in pro- portion. The more extended investigations of the future will, here also, open up unexpected treasures. In view of this evident and natural incompleteness of the most important records of creation, we must of course take good care, in the tribal history of Man, not to lay too great weight on single known animal forms, nor with equal certainty to consider all the stages of evolution which come under our consideration, as parent-forms. On the contrary, in hypothetically arranging our ancestral line, we must take good care to remember that the single hypothetical UNEQUAL VALUE OF THE "ANCESTRAL STAGES." 4! parent-forms are of very diverse values in relation to the certainty of our knowledge. From the few remarks which, while speaking of the Ontogeny, we made as to the corre- sponding phylogenetic forms, it will have been understood that some germ-forms may with certainty be regarded as reproductions of corresponding parent-forms. We recog- nized the human egg-cell and the parent-cell which results from the impregnation of the latter as the first and most important form of this kind. From the weighty fact that the egg of the human being, like the egg of all other animals, is a simple cell, it may be quite certainly inferred that a one-celled parent-form once existed, from which all the many-celled animals, Man in- cluded, developed. A second very significant germ-form, which evidently reproduces a primaeval parent-form, is the germ-vesicle (Blastula), a simple hollow sphere, the wall of which con- sists of a single cell-stratum. A third extremely import- ant form in germ-history, which may be quite safely and directly referred back to the tribal history, is the true Gas- trula. This most interesting larval form already exhibits the animal body composed of two germ-layers, and fur- nished with the fundamental primitive organ, the intestinal canal Now, as the same two-layered germ-condition, with the primitive rudiment of the intestinal canal, is common to all the other animal tribes (with the single exception of the Primitive Animals, Protozoa), we may certainly from this infer a common parent-form of similar construction to the Gastrula, the Gastrtea. Equally important in their bearing on the Phylogeny of Man, are the very important ontoge- netical form conditions which correspond to certain Worms, 42 THE EVOLUTION OF MAN. Skull-less Animals (Acrania), Fishes, etc., etc. On the other hand, between these quite certain and most valuable phylo- guuetic points, great gaps in our knowledge unfortunately exist, with which we shall again and again meet, and which are satisfactorily explained by reasons which have already been named, especially by the incompleteness of Palaeon- tology, of Comparative Anatomy, and Ontogeny. In the first attempts to construct the human ancestral line, which I made in my Generelle Morphologic, and in the " Natural History of Creation," I arranged first ten, and, later, twenty-two different animal forms, which, with more or less certainty, may be regarded as the animal an- cestors of the human race, and which must be looked upon as in a sense the most important stages of evolution in the long evolutionary series from the one-celled organisms up to Man.185 Of these twenty to twenty -two animal forms, about eight fall within the older division of the Inverte- brates, while twelve to fourteen belong to the more recent Vertebrate division. How these twenty-two niost important parent-forms in the human ancestral line are distributed through the five main periods of the organic history of the earth, is shown in the following Table (XVI.). At least half of these twenty-two stages of evolution (that is, the eleven oldest ancestral forms) are found within the Archilithic Epoch, within that first main period of the organic history of the earth, which includes the larger half of the latter, and during which probably only aquatic organisms existed. The eleven remaining parent-forms fall within the four remaining main Epochs : three within the Palaeolithic Epoch, three within the Mesolithic Epoch, and four within the Caenolithic Epoch. In the last, the Anthropolithic Age, Man already existed. THE TWENTY-TWO ANCESTRAL STAGES. 43 If we would now undertake the difficult attempt to dis- ct»vei the phylogenetic course of evolution of these twenty- two human ancestral stages from the very commencement of life, and if we venture to lift the dark veil which covers the c]dost secrets of the organic history of the earth, we must undoubtedly seek the first beginning of life among those wonderful living beings which, under the name of Monera, we have already frequently pointed out as the simplest known organisms. They are, at the same time, the simplest conceivable organisms; for their entire body, in its fully developed and freely moving condition, consists merely of a small piece of structureless primitive slime or plasson, of a small fragment of that extraordinarily important nitro- genous carbon compound, which is now universally esteemed the most important material substratum of all the active phenomena of life. The experiences of the last ten years particularly have convinced us with more and more cer- tainty that wherever a natural body exhibits the active phenomena of life, nutrition, propagation, spontaneous movement, and sensation, a nitrogenous carbon compound, belonging to the chemical group of albuminous bodies, is always active, and represents the material substance through which these vital activities are effected. Whether, in a monistic sense, we conceive the function as the direct effect of the formed material substance, or, in a dualistic sense, we regard " Matter and Force " as distinct, it is at least certain that, hitherto, no living organism has been observed in which the exercise of vital activities was not inseparably connected with a plasson-body. In the Monera, the simplest con- ceivable organisms, the whole body consists merely of plasson, corresponding to the " primitive slime " of earlier natural philosophy. ( 44 ) TABLE XVI. Systematic Survey of the most Important Stages in the Animal Ancestral Line of Man. M N = Boundary between the Invertebrate and the Vertebrate Ancestors. Ejxtcht of the Organic llifto y <>f the Kaith. Geological Periodt oftke Organic ffittory of the Earth. Animal Ancestral Stage* of Man. Nearest Living /lelatioe* of thf. A ncestral Stagei. 1. Monera I Balhybius (Monera) j Protamoeba 2. Oldest Amcebas i Simple Annrba? ) (Automata) 3. Amoaboid Societies 1 j Morula larva? 4. Ciliated planulaa (I'lana-adif) Blast ula larva; 6. Primitive Intes- ( tinal animals J. Gastrula larva1 A-rchilithic 1. Laurentian Period (Gastrtpadiv) 6. Primitive Worms / Gliding Worms or s 2. Cambrian Period (Archelminthes) \ (Tarbtllaria) Primordial Epoch 3. Silurian Period 1. Soft-worms (Scolecida) i? Between the glid- ing worms and the Sea-squirts 8. Chorda animals (Churdonia) I Sea-squirts (Ascidia) 1 (Appendiculari'i) 9. Skull-less animals ! Lancelots (A crania) (Amphioxi) 10. Hound-mouths j Lampreys (Cyclo'tomi) 11. Primitive Fishes j (Petromi/zonta) 4 Sharks (Stlackii) ) (Squalacei) Devonian Period E och Pcrnlian Period 12. Salamander Fishes (Uipneutta) 4> Tailcd A LTriassic Period Mesolithic or •{ 8. Jurassic Period Secondary 15. Primitive Am- (Protamnia) 16. Primitive Mam- Mud fish ' ( I'rotoptera) Siren (1'roteus) and Axolotl (Siredonj Water-newt (Triton) 1 Between Tailed Amphibians and Beaked animals Beaked animals Epoch ir Pouched Animab Pouched Rats ^ (Marsupialia) \ (l)idelphyes) 18. Semi-Apes l Lori (Stenopg) IV. Caenolithio or 10 Eocene Period 11. Miocene Period iProsimin-) \ 19. Tailed Narrow- i nosed Apes / 20. Men-like Apes or t Maki (I.emur) Nose Apes Holy Apes Gorilla, Chimpau- Tertiary Epoch 12. Pliocene Period Tail-less Narrow- nosed Apes. ( 21. Speechless Men or t zee, Orang, Gibbon Cretins or Micro- Ape-like Men ( cephaH V. Quaternary 13. Diluvial Period , ^ Men ^^ ^ 14. Alluvial Period I speech j Australians a'«l Papuans MONERON AND MORULA. 45 The soft slimelike plasson-substance of the body of the Moneron is commonly called " protoplasma," and identified with the cell-substance of ordinary animal and plant cells. As, however, Eduard van Beneden, in his excellent work upon the Gregarinae, first clearly pointed out, we must, strictly speaking, distinguish thoroughly between the plasson of cytods and the protoplasm of cells. This dis- tinction is of special importance in its bearing on the history of evolution. As was before incidentally mentioned, we must assume two different stages of evolution in those ele- mentary organisms, which, as formative particles or plastids, represent organic individuality of the first order. The older and lower stage is that of the cytods. in which the whole body consists of but one kind of albuminous substance, of the simplest plasson or formative material The more recent and higher stage is that of cells, in which a separation or differentiation of the original plasson into two different kinds of albuminous substances, into the inner cell-kernel (nucleus), and the outer cell-substance (protoplasma}, has already taken place. The Monera are the simplest permanent cytods. Their entire bod.y consists merely of soft, structureless plasson. FLowever thoroughly we examine them with the help of the most delicate chemical reagents and the strongest optical instruments, we yet find that all the parts are completely homogeneous. These Monera are, therefore, in the strictest sense of the word, " organisms without organs ; " or even, in a strictly philosophical sense, they might not even be called " organisms," since they possess no organs, since they are not composed of various particles. They can only be called organisms, in so far as they are capable of exercising the 46 THE EVOLUTION OF MAN. organic phenomena of life, of nutrition, reproduction, sensa- tion, and movement. If we tried to construct, d priori, the simplest conceivable organism, we should always be com- pelled to fall back upon such a Monera. Although in all real Monera the body consists merely of such a small living piece of plasson, yet, among the Monera, which have been observed in the sea and in fresh water, we have been able to distinguish several dif- ferent genera and species, varying in the mode in which their tiny bodies move and reproduce. In the ways in which movement is accomplished very noticeable differences exist. FIG. 163. — A Moneron (Protamosba) in the act of reproduction : A, the whole Moneron, which moves, like the ordinary Amoaba, by means of variable processes ; B, a contraction round its circumference parts it into two halves ; C, the two halves separate, and each now forms an independent individual (much enlarged). In some Monera, especially in the Protamceba (Fig. 163), the formless body, during its movements, invariably de- velops only a few, short, and blunt processes, which project like fingers, slowly altering their form and size, but never branching. In other Monera, on the other hand (e.g., Protoinyxa, Myxastrum), very numerous, long, fine, and generally thread-like processes arise from the surface of the movable body, and these branch irregularly, inter- THE BATIIVDIUS. 47 twining their free moving ends, so as to foim a net. Huge masses of such slime-nets crawl upon the deepest bottom of the sea (Bathybius, Fig. 164). Within these soft slime- like plasson-nets slow currents continually pass. Such a Moneron may be fed with finely pulverized colouring matter (for instance, carmine or indigo powder), if this powder is scattered in the drop of water under the micro- scope, in which the Moneron is contained. The grains of colouring matter at first adhere to the surface of the slimy body, and then gradually penetrate, and are driven about in irregular directions. The separate smallest par- ticles, or molecules, of the Moneron-body, called " plas- tidules," 136 displace each other, change their relative positions, and thus effect a change in the position of the absorbed particles of colouring matter. This change of position, at the same time, proves positively that a hidden delicate structure does not exist. It might be argued that the Monera are not really structureless, but that their organization is so minute that, in conseo^ence of the in- adequate power of our magnifying glasses, it is invisible. This objection is, however, invalid, for by the experiment of feeding, we can, at any moment, prove the entrance of foreign, formed, small bodies into the different parts of the body of the Moneron, and that these are irregularly driven about in all directions. At the same time we see that the changeable network of threads, formed by the branching of the protoplasmic threads and the coalescence of the con- iluent branches, alter their configuration every moment; just as has long been known to occur in the thread-nets of the protoplasm in the interior of the plant-cells. The Monera are, therefore, really homogeneous and structureless; 48 THE EVOLUTION OF MAN. each part of the body is every other part. Each part can absorb and digest nourishment ; each part is excitable and sensitive ; each part can move itself independently ; and lastly, each part is capable of reproduction and regene ra- tion. The reproduction of Monera always occurs asexually. In the Protamoaba (Fig. 163), each individual, after it has grown to a certain size, simply separates into two pieces. Round the circumference of the body a contraction arises, as in cell-division. The connection between the two halves continually becomes more slender (5), and finally parts in the middle. Thus, in the simplest possible way, two new individuals proceed by self-division from one quite simple individual ((7). Otl.er Monera, after they have grown to a certain size, gather themselves together into a spherical form. The globular protoplasmic body exudes a jelly-like protecting envelope, and a breaking-up of the whole plasson-ball takes place within this covering ; it breaks either into four pieces (Vampyrella), or into a large number of smaller globules (Protomonas, Protomyxa ; cf. Plate I. in the " Natural History of Creation "). After a time, these globules begin to move, split the integument by their movement, and emerge; after which they float about by means of a long, thin, thread-shaped process. Each again passes by simple growth into the mature form. Thus, it is possible to distinguish different genera and species of Monera, on one hand, by the form of the different processes of the body, and, on the other hand, by the different kind and manner of reproduction. In the appendix to my monograph of the Monera I enumerated eight genera and sixteen species (" Biol. JStudien," vol. L p. 182). The THE MOXKRON AND B.VTHYB1F8. 49 most remarkable of all Monera is the Bathybius, which was discovered by Huxley in 1868 (Fig. 1G4-). This wonderful JMoneron lives in tlie deepest parts of the sea, especially iu Pia. 164. — Bathybius Hoeckelii (Huxley). A small piece of the formlpss and contiuually changing plasson-uet of this Munerou i'roui the Atlantic Uceaii. the Atlantic Ocean, and in places covers the whole lloor of the sea in such masses, that the fine mud on the latter consists, in great measure, of living slime. The protoplasm in these formless nets does not seem differentiated at all; each little piece is capable of forming an individual. The active amoeboid movements of these formless pieces of plasson, which were first observed by the English zoologists Carpenter and Wyville Thomson, have recently been again observed by the German Arctic voyager, Emil Bessels, in the Bathybius of the coast of Greenland.137 The origin and importance of these huge masses of living, formless plasson-bodies in the lowest depths of the 50 THE EVOLUTION OF MAN. sea. raises many different inquiries and thoughts. Spon- taneous generation, especially, is naturally suggested by the Bathybius. We have already found that, for the origin of fin;t Monera upon our globe, the assumption of spontaneous generation is a necessary hypothesis. We shall be all the more inclined to confirm it now that, in the Monera, we have recognized those simplest organisms, the origin of which bv spontaneous generation, in the present condition of our science, no longer involves very great difficulties. For the Monera actually stand on the very boundary between organic and inorganic natural bodies.138 Next to the simple cytod-bodies of the Monera, as the second ancestral stage in the human pedigree (as in that of all other animals), comes the simple cell, that most undifferen- tiated cell-form, which, at the present time, still leads an independent solitary life, as the Amoeba. For the first and oldest process of organic differentiation, which affected the homogeneous and structureless plasson-body of the Monera, caused the separation of the latter into two diilerent sub- stances ; an inner firmer substance, the kernel, or nucleus, and an outer, softer substance, the cell-substance, or protopluwna. By this extremely important separative pro- cess, by the differentiation of the plasson into nucleus and protoplasm, the organized cell originated from the structure- less cytod, the nucleolated from the non-nucleolated plastid. That the cells which first appeared upon the earth origin- ated in uiis manner, by the differentiation of the Monera, i« a conception which in the present condition of histological knowledge seems quite allowable ; for we can even yet. directly observe this oldest histological process of differ- entiation in Ontogeny. It will be rememberod that in tiio THE MONKHTLA. 5! e^g-cell of animals, either before or after fertilization, the original kernel disappeared. We explained this phenomenon as a reversion or atavism, and assumed that the egg-cell. in accordance with the law of latent heredity, first fails back into the kernel-less, cytod stage (Fig. 165). It is onjy after fertilization is accomplished that a new cell-kernel arises in this cytod, which thus becomes the parent-cell (Cytula, Fig. 166). The transitory kernel-less cytod-con- dition, intermediate between the egg-cell and the parent- cell, is an interesting germ-form, because, in accordance with the fundamental law of Biogeny, it reproduces the original, oldest parent-form of the Moneron ; we therefore call it the Monerula. (Cf. vol. i. pp. 178-183.) FIG. 185.— Monerula of Mammal (Rabbit). The fertilized egg-cell after the loss or the nucleus is a simple ball of protoplasm (d). The outer covering of the latter is formed by the modified zona pellucida (*) together with a mucous layer (h) secreted on to the outside of the latter. In this a few s^erm.cells are still visible (s). Fi«. 166.— Parent.cell (Cy*«?a) of a Mammal (Babbit): *, parent. k,rnel; n, nudeolus of the latter; p, protoplasm of the parent-cell ; z, modified zona pellucida ; s, sperm-cells ; h, outer albuminous covering. 52 THE EVOLUTION OF MAN. We have already explained the one-celled germ-form, which we see in the original egg-cell and the parent-cell which is originated by the fertilization of the egg-cell, as the reproduction of a one-celled parent-form, to which we ascribed the organization of an Amoeba (cf. Chap. VI.). For the Amoeba, as it yet lives widely distributed in the fresh and salt waters of the globe, must be regarded as the most undilferentiated and most original of the various one-celled Piimitive Animals. As the immature primitive egg-cells (which as "primitive eggs" or Protova are found in the ovary of animals) are indistinguishable from ordinary Amoebae, we are justified in pointing to the Amceba as the one-celled phylogenetic form, which, in accordance with the 'fundamental law of Biogeny, is at the present time yet reproduced in the ontogenetic primitive condition of the " Amoeboid egg-cell." As evidence of the striking cor- respondence of the two cells, it was incidentally men- tioned that in the case of some Sponges the real eggs of these animals were formerly described as parasitic Amoebae. Large one-celled Amceba-like organisms were seen creeping about in the interior of the Sponge, and were mistaken for parasites. It was only afterwards that it was discovered that these "parasitic Amoebae" (Fig. 1G8) are really the eggs of the Sponge, from which the young Sponges develop. These egg-cells of the Sponge are, however, so like the true common Amoebse (Fig. 167) in size and structure, in the nature of their nuclei and in the characteristic form of movement of their continually changing false-feet (pseudo- podia), that, unless their source is known, it is impossible to distinguish them. This phylogenetic explanation of the egg-cell and its 53 reference to the prima?val ancestral form of the Amoeba, directly enables us to give a definite answer to the old hu- morous riddle : Which was first, the egg or the hen ? We can FIG. 167. — A crawling Amoeba (much enlarged). The whole organism has the form-value of a simple naked cell and moves about by means of change- able processes, which are extended from the protoplasmic body and again drawn in. In the inside is the bright-coloured, roundish cell-kernel or nucleus. FIG. 168.— Egg-cell of a Chalk-Sponge (Olynthus). The egg-cell creeps about in the body of tlie Sponge by extending variable processes, like those of the ordinary Amoeba. now very simply answer this Sphinx-question, with which our opponents try to shake or even to refute the Theory of Evolution. The egg existed much earlier than the hen. Of course it did not exist in the form of a bird's egg, but as an undifferentiated amosboid cell of the simplest form. The egg existed independently during thousands of years as a simplest one-celled organism, as the Amoeba. It was only after the descendants of these one-celled Primitive Animals had developed into many-celled animal forms, and after these had sexually differentiated, that the egg, in the present physiological sense of the word, originated from the amoe- 54 THE EVOLUTION OF MAN. boid cell. Even then, the egg was first a Gastnea-egg, then a Worm-egg, then an Acrania-egg, then a Fish-egg, an Am- phibian-egg, a Reptile-egg, and lastly, a Bird-egg. The egg of the Bird, as it now is, is a most complex historical pro- duct, the result of countless processes of heredity, which have occurred in the course of many millions of years.139 The fact that this primitive egg-form, as it first appears in the ovary of the most dissimilar animals, is always of one form, an undifferentiated cell, of the simplest amoeboid character, has already been pointed out as an especially important phenomenon. In this earliest young condition, immediately after the individual egg-cell has originated in consequence of a separation of the cells of the maternal ovary, no essential difference is recognizable in the egg-cells of the most dissimilar animals. (Of. Fig. 10, vol. i. p. 134.) It is not till later, when the primitive egg-cells, or the primitive eggs (protowi), have absorbed different kinds of nutritive yelk, and have surrounded themselves with variously formed coverings, and in other ways differentiated — it is not till they have in this way changed into after-eggs (metova), that those of different classes of animals can usually be distinguished. These peculiarities of the developed after- egg, the mature egg, are naturally to be considered as only secondarily acquired, by adaptation to the different con- ditions of existence .both of the egg itself and of the animal which forms the egg. The two first and oldest ancestral forms of the human race, which we have now considered, the Moneron and the Amoeba, are, considered from a morphological point of view, simple organisms and individuals of the first order, Plastids. All subsequent stages in the ancestral chain arc. on the- PRIMORDIAL EGO-CLEAVAGE. 55 other band, compound organisms or individuals of higher order — social aggregations of a number of cells. The earliest of these, which, under the name of Synamoebse, we must rank as the third stage of our pedigree, are quite simple societies of all homogeneous imdifferentiated cells ; amoeboid communities. To be certain as to their nature and origin, we need only trace the ontogenetic product of the parent-cell step by step. After the cytula (Fig. 166) has originated, by the re-formation of a cell-kernel, from FIG. 169. — Original or primordial egg-cleavage. The parent-cell, cr cytula, which resulted from the fertilization of the egg-cell, first breaks up, by a continuous and regular process of division, into two cells (.4), then into four (7?), then into eight (C), and, lastly, into very numerous cleavage- cells (D). the Morula (Fig. 165), the parent-cell breaks up, by repeated division, into numerous cells. We have already minutely examined this important process of egg-cleavage, and have found that all the various modes of the latter are modifications of a single mode, that of original or primordial cleavage. (Of. Chap. VI1L, p. 188.) In the Ver- tebrate line this palingenetic form of egg-cleavage has been accurately re- 56 THE EVOLUTION OF MAN. tained to the present time only by the Amphioxus. while all other Vertebrates have assumed a modified kenogenetic form of cleavage. (Cf. Table III., vol. i. p. 241.) The latter certainly originated at a later period than the former, and the egg-cleavage of the Amphioxus is, therefore, extremely interesting (vol. i. p. 442). In this the parent-cell first parts into two similar cells, the two first cleavage-cells (Fig. 169, A}. From these, by continuous division, arise 4, 8, 16, 32, 64 cells, etc., etc. (Fig. 169). The final result of this primordial cleavage was, we found, the formation of a globular mass of cells, which was entirely composed of homo- geneous, undifferentiated cells of the simplest character (Figs. 170, and 171, -£"). On account of the resemblance which this globular mass of cells bears to a mulberry or blackberry, we called it the " mulberry-germ," or morula. This "morula" evidently at the present day shows us the many-celled animal body in the same entirely simple primitive condition in which, in the earlier Laurentian primitive epoch, it first originated from the one-celled amoeboid primitive animal form. The morula reproduces, in accordance with the fundamental law of Biogeny, the ancestral form of the Synamceba. For the first cell-com- munities, which then formed, and which laid the first foundation of the higher many-celled animal body, must have consisted entirely of homogeneous and quite simple amceboid cells. The earliest Amoebas lived isolated hermit lives, and the amceboid cells, which originated from the division of these one-celled organisms, must also have long lived isolated and self-dependent lives. Gradually, however, by the side of these one-celled Primitive Animals, small amoeboid communities arose, owing to the fact that the OERM I NATION OF A CORAL. 58 THE EVOLUTION OF MAN. FIG. 171.— Girin imition of a coral (Munoxmia Darwin it) : H, parent-cell (cytula); C, two cleavage-cells; D, four cleavage, cells ; R, mulberry -germ (marula) ; F, vesicular germ (blastula) ; G, vesicular gorm in section; H, infolded vesicular germ in section ; I, gastrula in longitu- dinal sect;ou; K, gastrula, or cup-germ, seen from the outside. kindred cells which originated through division remained united. The advantages which these first cell -societies had in the struggle for existence over the solitary hermit cell must have favoured their progression, and have encouraged further development. Yet even at the present time several genera of Primitive Animals live in the sea and in fresh water, and permanently represent these primitive cell- communities in their simplest form. Such, for instance, are several species of Cystophrys described by Archer, the Rhizopods described by Richard Hertwig under the name of Microgromia sociali's, and the Labyrintkulce which were discovered by Cienkowski; formless masses of homogeneous and quite simple cells.140 In order to recognize the ancestors of the human races which developed first phylogenetically from the Syn- aiiweba, we need only continue to trace the ontogenetic modification of the Amphioxus-morula in the next stages. The first thing noticed is that a watery fluid collects within the solid globular cell-mass, and the cells are forced together and driven out to the periphery of the body (Fig. 171, F, G; Plate X. Fig. 9). The solid mulberry-germ thus changes into a simple hollow globe, the wall of which is formed of a single cell-stratum. This cell-stratum \ve called the germ-membrane (blastodei-ma), and the hollow globe the germ -membrane vesicle (blast ula. or btaxto THE BLASTULA. 59 The interesting blastula germ-form is also of great sig- nificance, for the modification of the mulberry-germ into the germ-membrane vesicle takes place in the same way in a great many animals of very dissimilar tribes ; for instance, in many Plant-animals and Worms, in the Ascidians, in many Star-animals (Echinoderma) and Soft-bodied Animals (Mollusca), and also in the Amphioxus. In those animals, however, in the ontogeny of which there is no real palin- genetic blastula, this deficiency is evidently only the result of kenogenetic causes, of the formation of a nutritive yelk i, and of other conditions of embryonic adaptation. We may therefore assume that the ontogenetic blastula is the repro- duction of a primaeval phylogenetic ancestral form, and that all animals (with the exception of the lower Primitive Animals) have originated from a common parent-form, the structure of which was essentially that of a germ-mem- brane vesicle. In many lower animals, the evolution of the blastula takes place not within the egg-coverings, but out- side this, free in water. Very soon after this, each cell of the germ-membrane begins to extend one or more movable, hair-like protoplasmic processes; owing to the fact that these cilia or whips vibrate in the water the whole body swims about (Fig. 171, F). This vesicular larva, the body- wall of which forms a cell-stratum, and which rotates and swims by means of the united vibrations of the cilia, has, ever since the year IS 47, been called the planula, or ciliated larva. This designation, is, however, used by different zoologists in dilJ'erent senses, and the gastrula, of which we shall speak presently, has, especially, often been confused with the planula. It is, therefore, more convenient to call the tine planula- form the blastula. 60 THE EVOLUTION OF MAN. Various kinds of Primitive Animals, which yet exist both in the sea and in fresh water, are formed essentially like the blastula, and which, in a certain sense, may be con- sidered as permanent or persistent blastula-forms, hollow vesicles, the wall of which is formed of a single stratum of ciliated homogeneous cells. These Planseads, or Blastseads, as they may be called, are formed in the very mixed society of the Flagellatre, especially the Volvoces (for instance, Synurd). I noticed in September, 1869, on the Island Gis-Oe, on the coast of Norway, another very interesting form, which I named Magospkcera planula (Figs. 172, 173). The fully developed body of this forms a globular vesicle, the wall of which is composed of from thirty to forty vibratory homo- geneous cells, and which swims about freely in the sea. Aftei FIG. 172.— The Norwegian Flimmer-ball (Magosphcera planula), swim, ming: by means of its vibratile fringes ; seen from the surface. FIG. 173. — The same, in section. The pear-shaped cells are seen bound together in the centre of the gelatinous sphere by a thread-like process Each cell contains both a kernel and a contractile vesicle. FLIMMER-LARV.E. 6l having reached maturity the society dissolves. Each sepa- rate cell still lives a while independently, grows, and changes into a crawling Amoeba. This afterwards assumes a globu- lar form, and encases itself by exuding a structureless integument. The cell now has just the appearance of a common animal egg. After it has remained for a time in this quiescent state, the cell breaks up, by means of con- tinued division, first into 2, then into 4, 8, 16, 32 cells. These again arrange themselves so as to form a globular vesicle, put forth cilia, and bursting the encasing integu- ment, swim about in the same Magosphcera-form froro which we started. This accomplishes the entire life-history of this remarkable Primitive Animal.141 If we compare these permanent blastula-forms with the freely swimming Flimmer-larvse or planula-condition, of similar structure, of many other lower animals, we may with certainty infer therefrom the former existence of a primaeval and long-extinct parent-form, the structure of which was essentially like that of the planula or blastula. We will call this the Plansea, or Blastaea. The whole body, in its fully developed condition, consisted of a simple hollow globe, filled with fluid or structureless jelly, the wall of which formed a single stratum of homogeneous cells, covered with cilia. Many different kinds and species of Plansea-like Primitive Animals must certainly have existed and formed a distinct class of Protozoa, which we may call Flimmer-swimmers (Planceada). A remarkable proof of the natural philosophical genius with which Karl Ernst Baer penetrated into the deepest secrets of the history of animal evolution, is that, as early as the year 1828 (ten years before the cell-theory was established), he guessed the significance 62 TI1E EVOLUTION OF MAN. of the blastosphfera, and, truly prophetically, insisted upon it in his classical " Entwickelungsgeschichte der Thiere " (vol. i p. 223). The passage in question says : " The furthut back we go in evolution, the more do we find a corre- spondence in very different animals. This leads us to the question : Are not all animals in the beginning of their evolution essentially alike, and is there not a primary form common to all ? As the germ is the undeveloped animal itself, it is not without reason that it is asserted that the simple vesicular form is the common primitive form from which all animals, not only ideally, but also historically, develop." This latter sentence has not only ontogenetic, but also phylogenetic significance, and is all the more note- worthy because the blastula of the most diverse animals, and the constitution of its wall of a single cell-stratum, was not then known. And yet Baer, in spite of the extreme deficiency of his empiric grounds, ventured the bold state- ment: "At their first appearance all animals are perhaps alike, and are merely hollow globes." Next to the primaeval ancestral form of the Plansea, as the fifth stage in the human pedigree, is the Gastroea, a form which arises from the Planoea. Of all ancestral forms this, as we have already shown, is of pre-eminent philosophical significance. Its former existence is certainly proved by the very important gastrula, which is met with as a transitory gi.rm-stage in the ontogeny of the most various animals (Fig. 171, /, K). We found that the gastrula, in its original, palingenetic form, is a globular, oval or oblong-round body, with one axis which has a simple cavity with one opening (at one pole of the axis). This is the primitive intestinal cavity with its m^uth-openinj. The intestinal wall consists THE GASTR.EA. 63 of two cell-strata, which are, in fact, the two primary germ- layers, the animal skin-layer, and the vegetative intestinal layer. The ontogenetic origin of the gastrula from the blastula at the present day affords us trust worthy intelligence as to the phylogenetic origin of the Gastnea from the Planaoa. We found that on one side of the globular germ-membrane vesicle a groove-like depression begins, and this inversion (invaginatio) becomes continually deeper (Fig. 171, II}. At last it is so great, that the outer, inverted part of the germ- membrane, or blastoderm, attaches itself closely to the inner, uninverted portion (Fig. 171, /). Now, if guided by this ontogenetic process, we wish to conceive the phylogenetic origin of the Gastroea in accordance with the fundamental law of Biogeny, we must imagine that the one-layered cell- society of the globular Planaea began, especially at one point of its surface, to absorb nourishment. At the nutritive point on the surface of the ball a groove-like depression was gra- dually formed by natural selection. The groove, which was at first quite shallow, in course of time became continually deeper. The function of nourishing, of absorption- of nutriment, and digestion, was soon limited to the cells which lined the groove, while the other cells undertook the function of movement and covering. Thus originated the first division of labour among the originally homogeneous cells of the Planaea. The first result of this earliest histological differentia- tion was the distinction of two different kinds of cells ; within the hollow the nutritive cells, without, on the sur- face, the motive or locomotive cells. The distinction of the two primary germ-layers was thus caused. The inner cells 38 64 THE EVOLUTION OF MAN. of the hollow formed the inner or vegetative layer, accom- plishing the functions of nutrition ; the outer cells of the covering formed the outer or animal layer, exercising the functions of locomotion and covering the body. This first and oldest process of differentiation is of such funda- mental significance that it deserves the deepest thought. When we consider that the body of the human being, with all its different parts, and also the body of all other higher animals, originates from these two simple primary germ-layers, we cannot over-estimate the phylogenetic significance of the gastrula. For in the quite simple primi- tive intestine, or the primitive intestinal cavity of the gastrula and its simple mouth-opening, the first real organ of the animal body, in a morphological sense, is gained ; the earliest genuine organ, from which all the other organs have differentiated at a later period. The whole body of the gastrula is really only a "primitive intestine." We have already pointed out the remarkable agreement between the palingenetic gastrula-forms of animals of the most diverse classes; of Sponges (Fig. 174, A}, Polyps, Corals (Fig. 171, /), Medusae, Worms (Fig. 175, B} Star- animals (Eckinoderma, O), Articulated Animals (Artkro- poda, D), Soft-bodied Animals (Mollusca, E), and Verte- brates (F}. All these various forms of the palingenetic gastrula are much alike, and are only distinguished by such unessential and subordinate peculiarities, that the systematic zoologist, in his " natural system," could only represent them as different species of a single genus. The various kenoge- netic gastrula-forms which have been described were also referable to that original palingenetic form (vol. i. p. 231). The gastrula proved to be a germ-form common to all classes of DEVELOPMENT OF THE GASTR^EA. 65 animals, with the exception of the Protozoa. This highly important fact justifies the inference in accordance with the fundamental law of Biogeny, that the various ancestral FIG. 174. Fio. 179. FIG. 174— (A) Gastrula of a Zoophyte (Gastrophysema), Haeckel. FIG. 175. — (B) Gastrula of a Worm (Arrow -worm, Sagitta). After Kowa- levsky. FIG. 176.— (C) Gastrula cf an Echinoderm (Star-fish, Uraster). After Alexander Agassiz. FIG. 177._(D) Gastrula of an Arthropod (Primitive Crab, NaupUus). FIG. 178.— (E) Gastrula of a Mollusc (Pond-snail, Limnoeus). After Kail Rabl. FIG. 179._(F) Gastrula of a Vertebrate (Lancelet, Ampldoius) After Kowalevskv. 66 THE EVOLUTION OF MAN. linos of all these classes of animals have developed phylo- genetically from the same parent-form. This most signifi- cant primaeval parent-form is the Gastraea. The Gastrea was at any rate already present in the sea during the Laurentian period, and by means of its vibratory fringe hurried about in the water, just like the yet extant free-moving ciliated gastrulse of this age. Pro- bably the primaeval Gastraea, which has been extinct f . r many millions of years, differed from the living gastrula of the present day only in some unessential point. On grounds derived from Comparative Anatomy and Ontogeny, the explanation of which would lead us too far, we may assume that the Gastraea had already acquired sexual re- production, and did not only propagate its species asexually (by division — bud-formation or spore-formation), as was probably the case with the four preceding ancestral stages. Presumably, single cells of the primary germ-layers as- sumed the character of egg-cells, others that of fertilizing seed-cells. (Cf. Chapter XXV.) This hypothesis is founded on the fact that sexual reproduction is yet met with in the same simple forms in the lowest Plan t- Animals (Zoophyt a), especially in the Sponges. Two small animal forms are especially interesting in their bearing on this aspect of the Gastraea theory. They have as yet been little observed, but of all extant animals they are most nearly allied to the primaeval Gastroea, and may therefore be called "the Gastraeads of the present day."142 One of these animals, Haliphysema (Figs 180 and 181), has been described by Bowerbank as a Sponge ; the other, Gastropkysema, by Carter as a Rhizopod (as " Squa- mulina "). The entire mature body of the developed person EXTANT GASTR^EADS. 67 of Haliphysema forms a most simple, cylindrical or egg- shaped pouch, the wall of which consists of two cell-strata. The cavity of the pouch is the stomach -cavity, and the F'os. 180, 181. — TTaliphysema primordiale, an extant Gas trsoa- form. Fig. 180. External view of the whole spindle-shaped animal (attached by its foot to seaweed). Fig. 181. Longitudinal section of the same. The primitive intestine (d) opens at its npper end in the primitive month (m). Between the whip-cells (g) lie amoaboid eggs (e). The skin-layer (h) below is encrusted with grains of sand, above with sponge-spicnles. opening at the top is the mouth-opening (Fig. 181, m). The two cell-strata forming the wall of the pouch are the 68 THE EVOLUTION OF MAN. two primary germ -layers. These most simple Plant-Animals differ from the gastrula principally in the fact that the former are attached by one end (that opposite to the mouth- opening) to the bottom of the sea, while the latter are free. Moreover, the cells of the skin-layer are coalescent and have included many foreign bodies, such as sponge-spicules, sand-grams, etc., which serve to support the body-wall (Fig. 180). The intestinal layer, on the other hand, -con- sists merely of a stratum of ciliated cells (Fig 181, d). When the Haliphysema is sexually mature, individual cells of its entoderm assume the character of female egg-cells ; on the other hand, individual cells of its exoderm become male seed-cells ; the fertilization of the former by the latter Fros. 182, 183.— Vscnla of a Sponge (OlyntJw*) . Fijy. 182, from the out- side; Pig 183. in longitudinal sfcHon : g, primitive intestine ; o, primitive month ; i, intestinal layer ; e, skin-layer. REPRODUCTION IN THE GASTRJ5ADS. 69 takes place directly through the stomach-cavity. A true palingenetic gastrula (Fig. 174) develops, just as in the Monoxenia (Fig. 171), from the fertilized egg. This swims about for a time in the sea, then attaches itself, and in this state resembles a simple young-form, which occurs in the course of the evolution of many other Plant- Animals, and which is called the ascula (Figs. 182, 183). In consequence of the absorption of foreign bodies by the exoderm, it becomes the Haliphysema. When we consider that there is no other important difference between the free-swimming gastrula and this attached, simplest Plant-animal, we are fairly justified in stating that in the simplest form of Gastrsea sexual repro- duction must have taken place in the same way. In the Gastrseads, just as in Plant-animals, both kinds of sexual cells — egg-cells and sperm-cells — must have formed in the same person; the oldest Gastrseads must, therefore, have been hermaphrodite. For Comparative Anatomy shows that hermaphroditism, that is, the union of both kinds of sexual cells in one individual, is the oldest and original con- dition of sexual differentiation ; the separation of the sexes (Gonochorismus) did not originate till a later period. TABLE XVII. Systematic Survey of the five earliest evolntionary stages of the Human An. cestral Lino, compared with the five earliest stages of Individual and of Systematic Evolution. Form-Value Of the five earliest pt.iges of the animal body. Phylogeny. The five earliest st-igos in the evolu- tion of the tribe. Ontogeny. The five earliest stages in the evolu- tion of the germ. The System. The five earliest stages in the animal system. 1. 1. 1. 1. First Stage. Monera. Monernla. Monera. A quite simple cytod (a non-nucleated plos- tid). The oldest animal Monera (originating by spontaneous gene- A non - nucleated animal-egg (after fer- lilization and alter Protamceba, Bathy. bins, uiid other extant Monera. ration). loss of he genn- vesicle). : i. • 1 2. Second Stc,(je. Amoeba. Cytula. Amoeba. A simp e roll (a Oldest anii lal Amceba. A nucle ated, ferti- Extant Amoeba. nucleated p ustid). li/oilaninia -egg ("Iirst cleavage g obule ")• { . 2 j . 5 . Third Staye. Synamoeba. Morula. Labyrinthula. A quite Bimplc iig- The olde st iiggrpfra- " JluHier •y-germ." A mass of similar. grogation . >f simple, tionufaniu alAuiiBbse. A globu ar mass of one - celled primitive similar cellf , cleavage-cc 11s. anim.ils. 4 . '. . * . 4 Fourth Stn-fie. Planeea. Blastula. Magosphaera. A simple h jllow clob". An anii ial hollow A hollow elobo, the A hollow globe, the filled with liquid. ,lie|glol)e, the wall of which cunsNts 1 which con wall of sists of a wall of which consists of a single Btrainm wall of which consists of a single stratum of of a single stratum of sincle Ft rutum of of homoge icons colls homogeneous ciliated humogeneou s colls. ciliated eel Is (the I'lanu a of lower cells. (bias too.) «nim ils). (Mus/o. pVm,.) . £ { i , Fifth Stage. Gastraea. Gastrula. Haliphysema. A hollow body, with Parent-f >rm of in- Iniestii a1 larva. A quite s mple pl.mt- a single ax s, the wall tostinal a li nials, or A pirn pi ; Intestinal nnimal An unuriiru- of which consists of different cell-strata; M- tazoa. Simple pri- mitive intestine with cavity with a mouth- opening The bodv- lated unlaxial person, the body-wall of wl.ich with an ope ling at one primitive i iiiutli. The wall is fon nod by the consists of the exod-'i H pole of the » *U. body-wall Is formed two prim ary germ- and the entudenn. by the ex oderm and layers". thu eutodcrm. CHAPTER XVII. THE ANCESTRAL SERIES OF MAN. II. FKOM THE PRIMITIVE WOEM TO THE SKULLED ANIMAL. The Four Higher Animal Tribes are descended from the Worm Tribe.— Tlie Descendants of the Gastraea ; in one direction the Parent Form of Plant- Animals (Sponges and Sea-Nettles), in the other the Parent Form of Worms. — Radiate form of the former, Bilateral form of the latter. — The Two Main Divisions of the Worms, Accelomi and Ccelomati: the former without, the latter with, a Body Cavity and Blood vessel System. — Sixth Ancestral Stage : Archelminthes, most nearly allied to Turbelluria. — Descent of the Coelomati from the Acrelorni. — Mantled Animals (Tunicata) and Chorda- Animals (Chordonia). — Seventh Stage: Soft- Worms (Scolecida). — A Side Branch of the latter: the Acorn- Worm (liulanoglossiLs). — Differentiation of the Intestinal Tube into Gill-intes- tine and Stomach-intestine. — Eighth Stage : Chorda-Animals (Chor- donia).— Ascidian Larva exhibits the Outline of a Chorda-Animal. — Construction of the Notochord. — Mantled Animals and Vertebrates as Diverging Branches of Chorda- Animals. — Separation of Vertebrates from the other Higher Animal Tribes (Articulated Animals, Star- Animals, Soft-bodied Animals). — Significance of tho Metameric Formation. — Skull-less Animals (Acrania) and Skulled Animals (Cran iota) .—Ninth Ancestral ^ t:ige : Sknll-less Animals. — Amphioxus and Primitive Verte- brate.— Development of Skulled Animals (Construction 01' the Head, Skull, and Brain).— Tenth Aucestial Stage: Skulled Animals, allied to the Cyclostomi (Myxinuidov and 1'etromyzonidce), " Not like the gods am I ! Full well I know ; But like the worm which in the dust must go, And, finding in the dust his life and weal, IB crushed and buried by the traveller's heel.— 72 THE EVOLUTION OF MAN. Why dost thon grin at me, thou hollow skull P As though of old thy brain, like mine, was vexed, Had looked to find bright day, but in the twilight dull, In search for truth, was sad and sore perplexed ! " GOETHB. BOTH in prose and in poetry man is very often compared to a worm. " A miserable worm," " a poor worm," are common and almost compassionate phrases. If we cannot detect any deep phylogenetic reference in tins zoological metaphor, we might at least safely assert that it contains an unconscious comparison with a low condition of animal development which is interesting in its bearing on the pedigree of the human race. For there is no doubt that the vertebrate tribe, in common with those of the other higher classes of animals, have developed phylogenetically from that multiform group of lower invertebrate animals which are now called Worms. However closely we limit the zoological significance of the word " Worm," it yet remains indubitable that a large number of extinct Worms must be reckoned among the direct ancestors of the human race. The group of Worms (Vermes) is much more limited in the Zoology of the present day, than was the same class in the older Zoology, which followed the system of Linnseus. It, however, yet includes a great number of very diverse lower animals, which, phylogenetically, we may regard as the few last living twigs of an immense spreading tree, the trunk and main branches of which have for the most part long since died off. On the one side, among the widely divergent classes of Worms, are found the parent- forms of the four higher tribes of animals, the Molluscs, Star-animals, Articulates, and Vertebrates ; on the other side. DEVELOPMENT OF WORMS AND PLANT-ANIMALS. 73 several comprehensive groups and also single isolated genera of Worms are to be regarded as root-suckers which have sprouted directly from the rest of the primaeval family-tree of the Worms. Some of these suckers have evidently changed but little from the long-extinct parent-form, the Primitive Worm (Prothelmis), which is immediately con- nected with the Gastrsea. Comparative Anatomy and Ontogeny clearly and sig- nificantly prove that the Gastrsea must be regarded as the direct ancestor of this Primitive Worm. Even now, a gastrula develops from the egg of all Worms after its cleavage. The lowest and most imperfect Worms retain throughout life an organization so simple that they are but little raised above the lowest Plant-animals, which are also immediate descendants of the Gastrsea, and which also yet develop directly from the gastrula. If the genealogical relation of these two lower animal tribes, the Worms and the Plant-animals, is closely examined, it becomes evident that the most probable hypothesis of their descent is, that the two originated, as independent branches, directly from the Gastrsea. On the one side, the common parent-form of the Worms developed from the Gastrsea ; as, on the other side, did the common parent-form of the Plant-animals. (Of. Tables XVIII. and XIX.) The tribe of Plant-animals (Zoophytes, or Ccdentcrata] now comprehends, on the one side, the main class of Sponges (Spongice} ; on the other, the main class of the Sea nettles (AcalephcB) ; to the former belong the Gastraeads and Poriferse, to the latter the Hydroid-polyps, the Medusae, Ctenophorse, and Corals. From the Comparative Anatomy and the Ontogeny of these we may infer, with great pro- 74 THE EVOLUTION OF MAN. Lability, that all these Plant-animals descend from o common and very simple parent-form, the structure of which resembled that of the ascula in essential points (Figs. 182, 183, p. 68). The uniaxial outline of the ascula and the gastrula is usually retained by the Sponges, while in most Sea-nettles (AcalepJice) transverse axes have been differentiated in the course of further evolution, thus giving rise to a characteristic radiate structure with a pyramidal general outline. In distinction from this predominant radiate outline of Plant-animals, a marked bilateral general outline is de- veloped from the first in the second offshoot from the gastrula, in the Worms. As the radiate form is marked by adaptation to an adherent mode of life, so is the bilateral form by adaptation to certain definite acts of free loco- motion. The constant direction and carriage of the body which would be maintained in this mode of free locomotion, conditioned the two-sided, or bilateral outline of the symmetrical Worms. Even the parent-form of the latter, the Primitive Worm (Prothelmis) must have acquired this character, and thus have become distinguished from the uniaxial parent-form of the Plant-animals. In this simple mechanical impetus, in the defined free locomotion of the Worms, on the one hand, and in the stationary mode of life of the earliest Plant-animals on the other, we must look for the efficient cause which produced in the one the bi- lateral or two-sided, in the other the radiate outline of the body. The former, the bilateral outline, has been inherited by the human race from the Worms. Except through the Gastraea, the common parent-form of Plant-animals and Worms, the human race is, therefore. THE WORMS AS ANCESTORS OF MAN. 75 not related to the Plant-animals. It will be our next task to consider more closely the pedigree of Man in so far as it coincides with that of the Worms. Let us examine how far the Comparative Anatomy and Ontogeny of Worms justify us in looking among the latter for primaeval ancestors of Vertebrates, and therefore of Man. For this end we must first consider the zoological system of Worms. In accord- ance with the most recent investigations of the Comparative Anatomy and Ontogeny of Worms, we divide (without reference to the many and various peculiarities of the numerous separate classes, which in this place do not interest us) the whole mass of forms within this tribe into two large main groups. The first main group, which we call Bloodless Worms (Acoslomi), comprehends the earlier division of the lower Worms, which have no true body-cavity, no system of blood-vessels, no heart, no blood, — in short, none of the parts connected with this organ - system. The second main group, on the contrary, called Blood-worms (Ccdomati), are distinguished from the former by the possession of a true body-cavity, and also by the presence of a blood-like fluid, which fills this cavity ; most of them also develop special blood-vessels, which again cause further correlated advances in structure. The relation of these two main groups of Worms is very evi- dently phylogenetic. The Acoelomi, which are very nearly allied to the Gastrsea and the Plant-animals, are to be regarded as an earlier and lower group, from which the more recent and higher division of the Coelornati developed, perhaps towards the end of the Laurentian Period. We will first carefully examine the lower group of Worms, the Acoelomi, among which we must look for the 76 THE EVOLUTION OP MAN. sixth ancestral stage of the human race, the stage imme- diately following the gastrula. The name " Accelomi " signifies " Worms without a body-cavity, or coeloma," and therefore without blood, or vascular system. The extant Acoelomi are generally included in a single class, which, on account of their flattened bodies, are called Flat-worms (Plathdmintlies). To this class belong the Gliding- worms (Turbellaria), which live independently in the water; also the parasitic intestinal Sucking-worms (Trematoda), and the Tape-worms (Cestoda), which have become yet more degraded by parasitism. The phylogenetic relations of the three forms of Flat- worms are very evident ; the Sucking- worms originated from the free Gliding-worms by adaptation to a parasitic mode of life ; and, by a yet more completely parasitic life, the Tape-worms originated from the Sucking- worms. These are striking examples of the gradually increasing degeneration of the most important organs. In addition to these well-known extant Flat-worms, great numbers of other Acoelomi must have lived during the Archilithic Epoch, which in general form were very much like those of the present day, but were, in some respects, yet more simply organized, and were, in their lowest stages of development, immediately connected with the Gastneads. The whole of these lowest Acoelomi, among which the common parent-form of the whole Worm tribe (the Prothelmis) must have been, may be classed as "Primi- tive Worms " (Archelminthes). The two classes of the Acoelomi, the Primitive Worms and the Flat-worms, represent in their external form the simplest bilateral condition of the animal body. The body is a simple oval, usually somewhat flattened, with- BLOODLESS WORMS. 77 out any appendage (Figs. 184, 185). The dorsal side of the leaf-like body differs from the ventral side, on which the Worm creeps. Accordingly, even in these most simple Worms there are the three definite axes which mark the bilateral type-form, and which re-occur in the human body and in that of all higher animals : (1) a longitudinal axis (main axis), which passes from front to rear; (2) a lateral axis, passing from right to left ; and (3) a sagittal axis, passing from the dorsal to the ventral surface. (Gf. vol. i. p. 257.) This so-called symmetrical or " bilateral" arrangement of the outline of the body is simply the mechanical result of adaptation to a creeping form of loco- motion, during which one end of the body is always directed forwards. The geometric outline of the gastrula, as of the ascula, has but one axis with unequal poles (Monaxonia diplopola). The typical outline of Worms, as of Vertebrates, is, on the contrary, bilateral, with tranverse axes (Stau- raxonia dipleura).14* The whole outer surface of the Gliding-worms (Turbel- laria) is covered, as in the gastrula, with a thick, fine ciliated coat ; that is, with a fur-like covering of extremely fine and close microscopic hairs, which are direct processes of the uppermost cells of the epidermis, and maintain an uninterrupted whirling or vibratory motion (Fig. 184, /). The constant vibrations of these cilia cause a continued current of water over the surface of the body. Fresh water is constantly conve}red to the surface of the skin by this current, thus permitting respiration in its simplest form (skin- respiration). A similar ciliated covering, just as is seen in the extant Gliding-worms of our fresh-water seas, pre- sumably covered our extinct ancestors of the Primitive 78 THE EVOLUTION OF MAN. Worm group, the Archelminthes. They inherited this ciliated dress directly from the Gastraea. If we now make various vertical sections (longitudinal and transverse) through the simple body of the Gliding- worms (and that of the Archelminthes which are certainly very closely allied to the former), we soon discover that their internal structure is considerably higher than that of the Gastraeads. We first observe that the two primary germ- layers (inherited from the Gastraaa) have differentiated into several cell-strata. The skin-layer and the intestinal layer have each split into two strata. The four secondary germ- layers, which are thus produced, are the same that we found resulted from the first differentiation of the two primary germ-layers in the embryo of the Vertebrate also. (Gf. the transverse sections through the larval Amphioxus and Earth-worm, Figs. 50 and 51, p. 236, and Plate IV. Fig. 2; Plate V. Fig. 10.) The highly important histological differentiation of these four secondary germ-layers led directly to further organolo- gical processes of differentiation, by which the organism of the Primitive Worms was soon considerably raised above that of the Gastrseads. In the latter there was really, in a morphological sense, but a single organ, the primitive intes- tine, with its mouth-opening. The whole body was nothing but an intestinal canal ; the intestinal wall was at the same time the wall of the body. Of the two cell-layers, forming this intestinal wall, the inner accomplished the functions of nutrition, the outer those of motion and covering. As some of the cells of the primary germ-layers developed into egg-cells, and others into sperm-cells, these layers also performed the function of reproduction. In the GLIDING-WORMS. 79 Primitive Worms, however, simultaneously with the forma- tion of the secondary germ-layers, these various functions also began to be distributed to various organs, which detached themselves from the original main organ, the primitive in- testine. Special organs originated for reproduction (sexual glands), for secretion (kidneys), for motion (muscles), and for sensation (nerves and sense-organs). In order to obtain an approximate picture of the sim- plest form in which all these various organs first appeared in the Primitive Worms, it is only necessary to examine the most imperfect forms of Gliding-worms (Turbellaria), as they exist at the present time in salt and fresh water. They are mostly very small and insignificant Worms of the simplest form, many being scarcely a millimetre or a few millimetres in length. In the simplest species of Gliding- worms the greater part of the oval body is occupied by the intestinal canal. This is a very regularly shaped pouch with an opening, re- presenting both mouth and anus (Fig. 184, m). At the anterior section of the intestinal tube, which is separated as a throat (pharynx, sd\ the fibrous layer is very thick, a thick muscular layer. Immediately outside the intestinal- fibrous layer lies the skin-fibrous layer, which in most worms appears as a large skin-muscle sac. Above the throat in Gliding-worms a nerve system of the simplest form is already visible in front, a pair of small nerve- knots, or ganglia, which from their position are called the " upper throat ganglia," or " brain " (Fig. 185, g). Delicate nerve-threads (n) pass from this to the muscles and to the ciliated skin-sensory layer. A pair of quite simple eyes (au) and nose-pits (no) are to be found in a few Gliding- worms. The Flat-worms are also universally provided with 39 8o THE EVOLUTION OF MAN. a pair of simple kidney-canals ( " excretory organs " ), in the form of two long, thin, glandular tubes, which traverse the right and left sides of the intestine and open at the hinder end of the body (Fig. 184, wni). We found that the FIG. 184. — A simple Gliding-worm (Rhabdoccelum): m, month; sd, throat- epithelinm; sm, throat-muscles; d, stomach-intestine; nc, kidney ducts; nm, opening of the kidneys ; an, eye ; na, nose-pit. FIG. 185. — The same Gliding-worm, showing the remaining organs : g, brain; au, eye; na, nose-pit; n, nerves; ft, testes ; £ , male opening j ?> female opening ; e, ovary ; /, ciliated outer-skin. STRUCTURE OF THE GLIDING-WORMS. 8 1 two primitive kidney canals in the vertebrate embryo also appeared at a very early period, shortly after the first differentiation of the middle germ-layer (mesoder'nia). The appearance of these at so early a period shows that the kidneys are very important primordial organs. It also shows their universal existence in all Flat- worms ; for even the Tape- worms, which, in consequence of the adoption of a parasitic mode of life, have lost the intestine, yet have the two secreting primitive kidneys, or " excretory ducts." The latter seem, therefore, to be older and of greater physiologi- cal importance than the blood-vessel system, which is wholly wanting in the Flat- worms. The sexual organs appear in many of the Gliding- worms in a very complex form ; while in others their form is very simple. Most of them are hermaphrodites ; that is, each individual worm has both male and female sexual organs. In the simplest forms we find a testis in the anterior part (Fig. 185, A), a single or double ovary behind (a). One of these simplest existing Acrelomi, such as we find among the lowest Rhab- docoela, may give us an approximate idea of the structure of the Primitive Worm, which forms the sixth stage in the human pedigree. These ancestors of the human race, which, on account of their general organization, must be placed among the Bloodless Worms (Accelomi), must have been represented during the Archilithic Epoch by a large number of various Worm forms. The lowest of these must have been directly connected with the Gastraeads (the fifth ancestral stage); the most highly developed must, on the other hand, have been directly connected with the Coelomati (the seventh stage). As, however, our present knowledge of the Comparative 82 THE EVOLUTION OF MAN. Anatomy and Ontogeny of the Acoelomi is very fragmen- tary, and much too imperfect to enable us to point with certainty to the series of the various stages, we will not attempt a detailed arrangement of them. We will turn instead to the seventh stage in the human pedigree, which belonged to the multiform group of the Blood-bearing Worms (Coelomati), The great organic advance in structure by which the Blood-bearing worms, or Coelomati, developed from the older Bloodless Worms, or Accelomi, consisted in the for- mation of a body-cavity (coeloma), and of a nutritive juice filling the latter, the first blood. All the lower animals with which we have yet occupied ourselves in our Phy- logeny, all the Primitive Animals and Plant-animals, are, like the Accelomi, bloodless and without a body -cavity. In the formation of a special vascular system, the earliest Cuelomati made a very great advance. Much of the com- plexity in the organic structure in the four higher tribes of animals is based on the differentiation of the vascular system, which they have inherited from the Blood-bearing Worms. The first development of a true body-cavity (cceloma) is referable to the separation of the two fibrous layers ; to the formation of a spacious cavity between the outer skin- fibrous layer and the inner intestinal-fibrous layer. In the tissure-like gaps, which formed between the two germ-layers, a juice collected, which penetrated through the intestinal wall. This juice was the first blood, and the gaps between the two germ-layers formed the first rudiment of the body- cavity. The union of these gaps formed the simple ccelom, the large cavity, containing blood or lymph, which plays so BLOOD-BEARING WORMS. 83 important a part in all the higher animals as the receptacle of the very extensive intestines. The formation of this coelom, and of the blood-vessels developed in connection with it, exercised a very great influence on the further evolution of the animal organization. The most important result was, that it allowed the conveyance of rich nutritive juices to those parts of the body lying near the circumference, and developing at a considerable distance from the intes- tinal canal. The intimate correlation, or reciprocity of the parts, necessarily occasioned, in direct connection with the progressive development of the blood-vessel system, many other important advances in the structure of the body of the Blood-bearing Worms. Just as among the Acoelomi, so also among the Coelomati, the pedigree of our race must have passed through a large number of diverse ancestral stages. But among extant Coelomati (which form but a very small fraction of this once multiform group), there are but very few Worms which can with certainty be regarded as nearly allied to the long- extinct ancestors of Man. In this respect, but a single class of Coelomati is really of prominent importance ; these are the Mantled Animals (Tunicata), to which belong the Ascidia already known to us. Our careful examination of the structure and germ-history of the Ascidian and the Amphioxus have shown the extreme importance of these very interesting animal forms. (Cf. Chapters XIII. and XIV.) That examination fully justifies us in asserting that among the ancestors of the Vertebrates (and therefore of Man) there was an unknown extinct ccelomate species, to which the nearest allied form among extant aniuals is the Appendicularia (Fig. 187), of which we have already 84 THE EVOLUTION OF MAN. spoken, and the tailed Ascidian larva. We will fur the present call this kind of Worm, which was primarily dis- tinguished by the possession of a notochord, the Chorda- animal (Chordonium). The Ascidians on the one hand, and the Vertebrates on the other, developed, as two diverging branches, from these Chorda-animals. The common parent- form of the Chorda-animals themselves was a coelomate form, which finally must have descended from the Acoelomi, and from the Archelminthes. Many connecting intermediate forms must, of course, have existed between these two groups of Worms, between the Primitive Worms and the Chorda-animals. Unfortunately, however, zoological knowledge is at present especially im- perfect with regard to these important intermediate forms of the multiform Worm tribe. For very evident reasons, none of these Worms could leave fossil remains. For, like the great majority of other Worms, they had no hard parts in their bodies. Most even of the known fossil Worms are worthless, for they tell us little or nothing of the most im- portant structural features of the soft body. Fortunately, however, we can in great measure satisfactorily fill the con- siderable palseontological gap in this part of our pedigree, with the help of the Comparative Anatomy and Ontogeny of Worms. If, on the one hand, we examine the structure and mode of development of the lower Worms from the Gliding- Worms (Turbellaria), and, on the other hand, the Anatomy and Ontogeny of the Ascidians, it is not difficult, step by step, to re-construct in imagination the connecting inter- mediate forms, and to insert a series of extinct ancestral forms between the Acrelomi and the Chordonia. This series of forms under the name of Soft-worms (Scolecida) DEVELOPMENT OF CHORDA-ANIMALS. 85 we will consider as the seventh stage in the human pedigree. A.n examination of the Comparative Anatomy of the various Scolecid forms, which we might perhaps distinguish here, would lead us much too far into the difficult details of the Comparative Anatomy and Ontogeny of the Worms. For our purpose it seems more important to call attention to those phylogenetic advances, by means of which the organization of the earliest Blood-bearing Worms was in the end elevated to that of the Chorda-animals. The Com- parative Anatomy and Ontogeny of the Gliding-worms and of the Ascidians justify us in giving special weight to the significant differentiation of the intestinal canal into two distinct divisions ; into an anterior division (the gill-intes- tine), which accomplishes respiration, and a posterior divi- sion (the stomach-intestine), which accomplishes digestion. As in Gastrseads and Primitive Worms, so also in the Ascidian larva, the intestinal canal is at first a simple pouch-like body, provided merely with a mouth-opening. A second opening, the anus, does not develop till a later period. Gill- openings afterwards appear in the anterior section of the intestinal canal, by which the whole anterior intestine is transformed into a gill-body. This remarkable arrange- ment is, as we found, quite peculiar to Vertebrates, and, except in the Ascidians, occurs nowhere else. Among extant Worms there is, however, a single isolated and very remark- able Worm form, which in this respect may be regarded as distantly allied to the Ascidia and to Vertebrates, and perhaps as an off-shoot from the Soft-worms (Scolecida), This is the so-called "Acorn- worm" (Balanoglossus, Fig, 186), which lives in the sand of the sea-shore. The in 86 THE EVOLUTION OF MAN. fcoresting points connecting this with Ascidians and the Skull-less Animals (Acrania) were first accurately observed and explained by Gegenbaur. Al- though this singular Balanoglossus is in many other respects peculiar in its organization, so that Gegen- baur rightly ranked it as the re- presentative of a special class (Enteropneusta), yet the structure of the anterior section of the in- testinal tube is exactly similar to that of Ascidians and Skull-less Animals (&), a gill body, the walls of which are pierced on either side by gill-openings and are supported by gill-arches. Now, although the Acorn- worm in other points of its structure may differ very con- siderably from those extinct Soft- worms (Scolecidce), which we must regard as direct ancestors of our race, and as intermediate links between the Primitive Worms FIG. 186. — A young Acorn -worm (Bal- anoglossus). (After Alexander Agassi z.) r, acorn-like proboscis ; h, collar ; fe, gill openings and gill-arches of the anterior in- testine, in a long row one behind another on each side ; d, digestive posterior intes- tine, filling the greater part of the body- cavity ; v, intestinal vessel, lying between two parallel folds of skin ; a, anus. SOFT-WORMS. 87 and the Chorda-animals, yet, in virtue of this characteristic structure of the gill-intestine, it may be considered a re- motely allied collateral line of the Soft-worms. The development of an anus (Fig. 186, a) at the end opposite to the mouth, is also a considerable advance in the struc- ture of the intestine. The further development of the blood-vessel system in the Acorn-worm also indicates a marked advance. In the ciliary surface of the skin, on the contrary, it recalls the Gliding- worms. The sexes are separated, while our scolecid ancestors were probably hermaphrodite.145 From a branch of the Soft- worms, the group of Chorda- animals (Chordonia), the common parent-group of the Mantle-animals and Vertebrates also developed. The process which primarily led to the development of this important group of the ccelomati, was the formation of the inner axial skeleton (the notochord, or chorda dorsalis), which at the present day we find permanently retained in its simplest form in the lowest Vertebrate, the Amphioxus. We saw that this notochord is already found in the tailed and free-swimming larva of the Ascidian (Plate X. Fig. 5). The chorda does, indeed, serve specially as a support for the rudder-like tail of the larval Ascidian, but its anterior extremity passes in between the intestinal and medullary tubes within the actual body of the larva. A transverse section of this larva therefore shows that arrangement of the most important organs which is characteristic of the vertebrate type : in the centre is the firm notochord, which supports the other organs and serves especially as a base and point of attachment for the motive trunk muscles ; above this notochord, on the dorsal side, is the central 88 THE EVOLUTION OF MAN. nervous system in the form of a medullary tube ; below, on the ventral side, is the intestinal tube, the anterior half of which is a respiratory gill-intestine, its posterior half a digestive stomach-intestine. It is true that the free- swimming larva of the extant Ascidian possesses this typical vertebrate character only for a short time ; it soon relin- quishes its free roving mode of life, puts off its oar-like tail with the notochord, adheres to the bottom of the sea, and then undergoes that very great retrogression, the surprising final result of which we have already observed (Chapters XIII. and XIV.). Nevertheless, the Ascidian larva, in its very transitory evolution (for a brief space), affords us a picture of the long extinct Chordona-form, which must be regarded as the common parent-form of Mantle-animals and Vertebrates. There is even yet extant a small and insignificant form of Mantle-animal which throughout life retains the structure of the Ascidian larva with its oar- like tail and its free-swimming mode of life, and which reproduces itself in this form. This is the remark- able Appendicularia (Fig, 187), which we have already examined. If we ask ourselves what conditions of adaptation could possibly have had so remarkable a result as the develop- ment of the notochord, and the modification of a branch of the Soft-worms into the parent-form of the Chorda- animals, we may with great probability answer, that this result was effected by the habituation of the creeping Soft-worm to a swimming mode of life. By energetic and continued swimming movements, the muscles of the trunk would be greatly developed, and a strong internal point of attachment would be very favourable to this muscular THE ASCIDIANS. 89 activity. A support of this kind might arise by enlarge- ment and concrescence of the germ-layers along the longi- tudinal axis of the body; and the differentiation of an independent bony cord from this axial cord gave rise to the notochord. (C£ Fig. 88, 89, vol. i. pp. 300, 301.) In corre- lation to the formation of this central notochord, the simple nerve-ganglia, lying over the throat in the Soft-worms, lengthened into a long nerve-cord, reaching from front to rear, above the notochord ; in this way, the medullary tube originated from the " upper throat ganglia." As we have already minutely considered the great significance of the Ascidians (Fig. 188) in this respect, as well as their close relations to the Amphioxus (Fig. 189), we will not tarry longer over this point now. I will repeat, that we must by no means regard the Ascidian as the direct parent-form of the Amphioxus and of the other Vertebrates. On the contrary, we assert that, on the one hand the Ascidians, and on the other the Ver- tebrates, have both descended from one unknown Worm form, which has long been extinct ; the nearest relatives of this among existing animals are the Ascidiar^ larvae and the Appendicularia (Fig. 187). This unknown common parent-form must have belonged to the group of Chorda-animals, which we pointed out as the eighth ancestral stage in the human pedigree.146 Although we cannot form an entirely satisfactory idea as to all points of external and internal structure of this Chorda-animal, there is no doubt that, like its near relatives, the Mantle-animals, and like the preceding ancestral stage represented by the Soft-worms and Primitive Worms, it must be classified in the natural system of the animal 9O THE EVOLUTION OF MAN. kingdom as a genuine Worm. The difference between it and other genuine Worms cannot have been greater than is Fio. 187. — Appendicalaria, seen from the left side: m, mouth; ^gill- intestine ; o, oesophagus ; v, stomach ; a, anus ; n, nerve-ganglia (uppei throat-knots) ; g, ear-vesicle ; /, ciliated groove under the gill ; h, heart | t, teetes ; e, ovary ; c, notochord ; a, tail. FIQ. 188. — Structure of an Ascidian (seen from the left, as in Fig. 153 and Fig. 14, Plate XI.) : sb, gill-sac ; v, stomach ; i, large intestine ; c, heart; t, testes; vd, seed duct ; o, ovary; o', matured eggs in the bod"y cavity. (After Milue Edwards,) THE AMPHIOXUS. the difference between the extant Tape-worms and Ringed Worms (Annelida). Moreover, in a certain sense we may regard the extant Appendicularia as a last remnant of the Chordonia class. We have now studied the most import- ant animal forms which occur in the pedigree of the human race, and which, in the zoo- logical system, must be classed among the Worms. In leaving this lower class, and tracing our ancestry henceforth exclusively within the vertebrate tribe, we at once leave behind the great majority of animal forms, which branched off from the worm tribe in entirely different directions. When, in a previous chapter (IX.), the vertebrate nature of man was proved, it was incidentally mentioned that the very great majority of animals are in no way directly allied to our tribe. The parent-forms of the three other higher Animal tribes, the Articulated Animals (Arthroyoda), Star-animals (Echinoderma), and Soft-bodied Animals (Mollusca}, like the vertebrate tribe, originated from the FIG. 189. — Lancelot (Amphinxus lanceolatus), twice tho actual size, seen from the left (the longitudinal axis is represented vertically, the month turned up- ward, the tail downward, as in Plate XI. Fig. 15) : B, month-opening, surrounded by cilia ; 6, anal open- ing ; c, rentral opening (Porus dbdominalis) ; d, gill- body ; et stomach ; /, liver-coacum ; g, large intes- tine; h, ccelom: t, hotochord (under it the aorta); k, arches of the aorta ; I, main gill-artery ; m, swellings on its branches; n, hollow vein; o, intestinal vein. TABLE XVIII. Systematic Survey of the Phylogenetic System of the Animal Kingdom, founded on the Gastrasa Theory and the Homology of the Germ-layers. Tribes «r Phyla, of the Animal Kingdom. Main Classes or Brandies of the Animal Kingdom. Classes of the Animal Kingdom. Systematic !fam«e of the Claitet. FIRST SUB.KINGDOM : PRIMITIVE ANIMALS (Protozoa). Animala without germ-lay ers, intestine, or true tissues. A. ( L Kpg-animals ( £ Monera J- Mropneust» 19. Star- worms 19. Gephvrea y20. Ringed-worms 20. Annelida D. VII. Headless shell-fish / 21. Lamp-shells 21. Spirobranchla Snft-hnTiirli Acephala 122. Mussels 22. LamellibranchU animals Mollusca VIII. Head-bearing shell-fish Eucephala I 23. Snails | 24. Cuttle* 23. Cochlides 24. Cephalopoda •p ( IX. Ringed -arms / 25. Sea-stars 25. Asterida „ , ,»• . ,1 Colobrachia, 5tar=(ammals ( x Armies I 26. Sea-lilies / 27. Sea-urchins 26. Crinoida 27. Echlnida Echinoderma ( ijpobrachia 1 28. Sea-cucumber* 23. Holothuriae P. f XI. Gill-breathers { 29. Crabs 29. Crustacea animals ] HI- Tube-b eathers Arthropoda ( rmcheata I 30. Spiders < 31. Centipedes 132. Flies 30. Arachnida 31. Myriopoda 32. Insecta Q. Vertebrate animals Vertebrate XIII. Skull-less ( 33. Tube-hearta (Lance- 33. Leptocardia Acrania \ lets) XIV. Single-nostrilled ( 34. Round-mouths 34. Cyclostoma Jtonorrhina \ (l^ampreys) •KV Amninn lou 1 35. Fishes 35. Pisces "•ZSST {j-isjasu asssa: xv,. AJJU-JJ^U I g; *£»- g; K'» Amnwta 1 _,„_ M.in,maU 40- Mammali* ( 93 ) TABLE XIX. Monophyletic Pedigree of the Animal Kingdom, founded on the Gastreea Theory and the Hoinology of the Germ-layers.*4 m Vertebrates ^"S ll Veitebrata Articulates Soft-bodied Animals !J £o Arthropoda Mollusca 1? '•£ ^ Star-animals 18 a S II Echiiioderma III i! v 1 _^s s|l Ccelomati. tl (Wormt with body-cavity) H :|| Plant-animals i P Zoophyta (Ctet. nterata) Flat Worms Plathelmimlies • 8 ^ ° •*H n .. Sea-nettles 'S Ill Sponges (Acalephce) ^ J Spongiae Accelomi (Worms wit/tout body-cavity) !J S Jsa'o Protascus 1 «• i-l o Proth elmia c^ g |fs Gastraea radialis Gastraa bilateralte ^ 1 s § _ (stationary) (trawling) £ ^ U 'J3 fj?l Gastraea 3] 1 (Ontogeny: dastrula) i Primitive Animate jL Protozoa > ^ Cil ata S^Q "^ m Plana?ada 1 -S C (Ontogeny: Blastula) Acinetae 'c .S g S) 1 ^ ?o^ O Infusoria •| .S •§ H GregarinEe j |!|o Synamoeba i _j Amabina l^ll (Ontogeny: Morula) li|i /mo3b88 £if (Ontogeny: Ci/tvki) • Moner a. 5 (Ontogeny: Alvncruiati 94 THE EVOLUTION OF MAN. worm tribe ; but the parent-forms of the three former belong to worm-groups quite distinct from that of the Chordonia. It is only far down at the common root of the group of Coelomati, that we assume a common source for these various tribal forms. (Of. Tables XVIII. and XIX,) It is especially necessary to remember that there is no direct blood-relationship between Vertebrates and Articu- lated Animals. The Articulated Animals (Arthropoda), to which the most comprehensive of all classes of animals, that of Insects, and also the Spiders, Centipedes, as well as the Crabs, or Crustaceans, belong, are descendants of articulated Worms, the nearest allies of which are the extant Ringed Worms (Annelida). The tribe of Star-animals (Echinoderma), which includes the Star-fishes, Sea-lilies, Sea-urchins, and Sea-cucumbers, must also have descended from similar articu- lated Worms.118 The parent-form of the Soft-bodied Animala (Mollusca), whiclr include the Cuttles, Snails, Mussels, and Lamp-shells, must also be sought among the Worms. But the Coelomati, from which these three higher animal tribes originated, differed entirely in character from the Chorda- animals. Unlike the latter, they never developed a noto- chord. In them, the anterior section of the intestinal tube was never modified into a gill-body with gill-openings; nor were the upper throat-ganglia developed into a medullary tube. In a word, in Articulated Animals, Star-animals, and Soft-bodied Animals, as well as in their ancestors among the Blood-bearing Worms, the typical structural peculiari- ties which are exclusively characteristic of the vertebrate tribe and of their immediate invertebrate progenitors, were never present. Thus the great majority of all animals are DEVELOPMENT OF VERTEBRATES FROM INVERTEBRATES. 95 in no way the subject of our further investigations, which are only concerned with the Vertebrates, The development of the Vertebrates from the Inverte- brates most nearly related to them, the Chorda- Animals, occurred millions of years ago, during the Archilithic Epoch. (See Table XII., p. 11.) This is unmistakably shown by the fact that the most recent sedimentary rock-strata which were deposited during that immense period of time, the higher layers of the Upper Silurian formation, contain remains of fossil Fishes (Primitive Fishes, Selachii). As these Fishes, although they belong to the lowest stage of the Skulled Animals (Craniota), yet possess a compara- tively high organization, and as they must necessarily have been preceded by a long progressive series of lower Skull- less Vertebrates, we must attribute the origin of the oldest Skull-less Animals (Acrania) from the Chorda-animals to a much earlier part of the Archilithic Epoch. Therefore, not only all the invertebrate ancestors of our race, but also the earliest form of our vertebrate progenitors must have developed in that primordial time, which includes the Laurentian, Cambrian, and Silurian Periods. (Cf. Tables XIII., XIV., and XVI, pp. 12, 19, 44.) Unfortunately, Palaeontology can give us absolutely no information with regard either to the structure of our oldest vertebrate ancestors, or to the time of their appearance ; for their bodies were as soft and as destitute of hard parts capable of fossilization, as were the bodies of all our preceding invertebrate ancestors. It is, therefore, not surprising, but quite natural, that we find no fossil remains of the former in the Archilithic formations. The Fishes in which the soft cartilaginous skeleton was partly 40 96 THE EVOLUTION OF MAN. modified into hard bone, are the earliest Vertebrates capable of leaving petrified records of their existence and structure. Fortunately, this want is more than counterbalanced by the much more important testimony of Comparative Anatomy and Ontogeny, which henceforth form our safest guides within the Vertebrate pedigree. Thanks to the classic researches of Cuvier, Johannes Muller, Huxley, and especially of Gegenbaur, we are in possession of such extensive and instructive records of creation in this most important branch of tribal history, that we can prove at least the more significant features in the development of our Vertebrate ancestors, with the most gratifying certainty. The characteristic peculiarities by which Vertebrates in general are distinguished from all Invertebrates, engaged our attention some time ago, when we examined the structure of the ideal Primitive Vertebrate (Figs. 52-56, p. 256). The most prominent characters were as follows: (1) the formation of the notochord between the medullary and intestinal tubes; (2) the differentiation of the intestinal tube into an anterior gill-intestine and a posterior stomach-intestine; (3) the inner articulation, or formation of metamera. The Verte- brates share the first two qualities with the larval Ascidians and with the Chorda-animals ; the third quality is entirely peculiar to them. Accordingly, the most important struc- tural advance, by which the earliest vertebrate forms origin- ated from the most nearly allied Chorda-Animals, consisted in an internal metameric structure. This showed itself first most distinctly in the articulation of the muscular system, which broke up on the right and left into a series of consecutive muscular plates. At a later period the .articulation declared itself prominently in the skeleton, and CLASSIFICATION OF VERTEBRATES. 97 nervous and blood-vessel systems. As we have already seen, this process of articulation, or metameric formation, must essentially be regarded as terminal germination. Each distinct trunk-segment, or metameron, represents an individual. Thus the Vertebrates with their internal segmentation stand in a similar relation to their inarticulate Invertebrate ancestors, the Chorda Animals, as do the out- wardly segmented Ringed Worms (Annelida) and Articu- lated Animals (Arthropoda) to the simple inarticulate Worms from which they originated. The tribal history of Vertebrates is rendered much more intelligible by the natural classification of the tribe which I proposed first in my Gcnerelle Morphologic (18GG), and afterwards improved in many ways in " The Natural History of Creation" (Chap. XX., p. 192, etc.). In accordance with that, existing Vertebrates must be divided into at least eight classes, as follows : — SYSTEMATIC SURVEY OF THE EIGHT CLASSES OF VERTEBRATES. A. Skull-less (Acrania) 1. Tube-hearted 1. Leptocardia / a. Single-nostrillcd (Jfonorhina) 2. Round-mouths 2. Cyclostoma ( , [. (3. Fish. -8 3 Pisces R sv u<*i 1 ( Amnion-less 4 4. Mud-fi>hes 4. I)li>neusta Ctaiiiuta \ h' Double-no-trilljd I Anamnia (5. Amphibians 6. Amphibia I mfMi'l"'-* \ ir. ,6. Rrpiiles 6. Reptilia f WithAmnion \ 7. TVrds 7. Avea \ \ Amniuta, (». JJummala 8. Mammalia The whole Vertebrate tribe may primarily be divided into the two main sections of the Skull-less and the Skulled Vertebrates. Of the earlier and lower section, that of the Skull-less (Acrania), the Amphioxus is alone extant. To the more recent and higher section, the Skulled (Cra- niota), belong all other existing Vertebrates up to Man. The 98 THE EVOLUTION OF MAN. Craniota branched off from the Acrania, as these did from the Chorda Animals. Our exhaustive study of the Compara- tive Anatomy and Ontogeny of the Ascidian and the Amphioxus have already afforded proof of this relation. (Cf. Chapters XIII. and XIV., and Plates X. and XL with the explanations.) I will only repeat, as the most important fact, that the Amphioxus develops from the egg in exactly the same way as the Ascidian. In both, the original Bell- gastrala (Figs. 4 and 10) originates in an exactly similar manner, by primordial cleavage from the simple parent-cell (Figs. 1 and 7). From this originates that remarkable larva, which develops a medullary tube on the dorsal side of the intestinal tube, and between the two a notochord. At a later period, both in the Ascidian and in the Amphioxus, the intestinal tube differentiates into an anterior gill-intestine and a posterior stomach-intestine. In accordance with the fundamental principle of Biogeny, from these very important facts we may deduce the following statement of great phylo- genetic importance : the Amphioxus, the lowest Vertebrate form, and the Ascidian, the most nearly allied Invertebrate form, have both descended from one single extinct Worm form, which must have possessed the essential structure of the Chorda Animals. The Amphioxus, as has already been often shown, is of extreme importance ; not only because it thus fills the great gap between the Invertebrates and the Vertebrates, but also because it represents^ at the present time, the typical Vertebrate in its simplest form ; and because it directly affords the best standpoint from which tc examine the gradual historic evolution of the whole tribe. If the structure and germ-history of the Amphioxus were un- THE AMPHIOXUS AS THE ANCESTOR OF MAN. 99 known to us, the whole subject of the development of the Vertebrate tribe, and thus of our own race, would be enveloped in an impenetrable veil. The accurate anatomical and ontogenetic knowledge of the Amphioxus, attained during the last few years, has alone pierced that heavy veil, formerly supposed to be impenetrable. If the Amphioxus is compared with the developed Man or any other of the higher Vertebrates, a great number of striking dissimilarities will be seen. The Amphioxus has no specialized head, no brain, no skull, no jaws, no limbs ; it is without a central- ized heart, a developed liver and kidneys, a jointed vertebral column ; every organ appears in a much simpler and more primitive form than in the higher Vertebrates and in Man. (Of. Table X., vol. i. p. 466.) And yet, in spite of all these various deviations from the structure of other Vertebrates, the Amphioxus is a genuine, unmistakable Vertebrate ; and if, instead of the developed Man, the human embryo at an early period of its Ontogeny is compared with the Amphioxus, we shall find perfect parallelism between the two in all essential points. (Of. Table IX., vol. i. p. 465.) This highly important parallelism justifies the conclusion that all the Skulled Animals (Craniota) have descended from a common primaeval parent-form, the structure of which was essentially that of the Amphioxus. This parent- form, the earliest Primitive Vertebrate, possessed the peculiar characters of the Vertebrates, and yet was without all those important peculiarities that distinguish the Skulled Animals from the Skull-less. Although the Amphioxus ap- pears peculiarly organized in many respects, and although it may not be regarded as an unmodified descendant of the Primitive Vertebrate, yet it must have inherited from the IOO THE EVOLUTION OF MAN. latter the distinguishing characteristic features already mentioned. We cannot, therefore say that the Amphioxus is the progenitor of the Vertebrates ; but we may certainly say that the Amphioxus of all known animals is nearest allied to this progenitor ; both belong to the same limited family group, to the lowest Vertebrate class, that of the Skull-less Animals (Acrania). In the human pedigree, this group forms the ninth stage of the ancestral chain, the first among Vertebrate ancestors. From this Skull-less group was developed the Amphioxus on the one side, and on the other the parent-form of the- Skulled Animals (Craniota). The comprehensive group of the Skulled Animals includes all known Vertebrates, with the single excep- tion of the Amphioxus. All these Skulled Animals possess a distinct head, inwardly specialized from the trunk, and this contains a skull, enclosing a brain. This head also carries three of the higher sense-organs, which aro partially wanting in the Skull-less Animals (nose, ears, and eyes). At first, the brain appears in a very simple form, as an anterior bladder-like extension of the medullary tube (Plate XI. Fig. 16, m^. This, however, is soon distributed by several tranverse grooves — first into three, and afterwards into a series of five consecutive brain-bladders. In the formation of the head, skull, and brain, together with the higher sense-organs, lies the most essential advance mado by the skulled parent-form beyond its skull-less ancestors. Other organs, however, also soon rose to a higher grade of development; a compact centralized heart appeared, a more perfect liver and kidneys; and in other directions also important advance was made. The Skull-less Animals may be primarily subdivided SKULL-LESS ANIMALS. 10 1 into two differing main sections, that of the Single-nostrils (Monorhina), and that of the Double-nostrils (Amphirhina). Of the former there are but very few extant forms, which are called Round-mouths (Cyclostoma). These are, however, of great interest, because in their whole structure they are intermediate between the Skull-less Animals and the Double- nostrils (Amphirhina). Their organization is much higher than that of the Skull-less Animals, much lower than that of the Double-nostrils ; they thus form a very welcome phylogenetic link between those two divisions. We may therefore represent them as a special, tenth stage in the human ancestral series. The few existing species of the class of Round-mouths are distributed into two different orders, which are distinguished as the Hags and the Lampreys. The Hags (Myxinoides) have long, cylindrical, worm-like bodies. Linnaeus classed them among Worms, but later zoologists have placed them, sometimes among the Fishes, sometimes Amphibians, and again with Molluscs. The Hags live in the sea and are usually parasitic on Fishes, into the skin of which they penetrate by means of their round sucking mouths and their toothed tonguea They are occasionally found in the body -cavity of Fishes — for example, of the Cod and Stur- geon— having penetrated to the interior in their passage through the skin. The second order, that of the Lampreys (Petromyzontes), includes those well-known " Nine eyes," common at the seaside; the little river Lamprey (Petro- myzon fluviatilis) and the large sea Lamprey (Petromyzon marinus, Fig. 190). The animals included in the two groups of the Myxi- noides and the Petromyzontes, are called Round-mouths IO2 THE EVOLUTION OF MAN. (Cyclo8toma),fTom the fact that their mouth forms a circular or semi-circular opening. The upper and under jaws., which appear in all the higher Vertebrates, are completely wanting in the Round-mouths, as in the Amphioxus. Ali other Vertebrates are therefore distinguishable from them as "Jaw-mouthed" (Gnathostomi). The Round-mouths may also be called " Single-nostrils " (Monorhina), because they have but a single nasal tube, while the Gnathostomi are all furnished with a pair of nasal cavities, a right and a left nose-cavity (" Double-nostrilled," Amphirhina). But in addition to these peculiarities, the Jaw-mouths are also distinguished by many other remarkable structural arrange- ments, and are further removed from the Fishes than the latter are from Man. They must, therefore, evidently be regarded as the last remnant of a very old and very low class of Vertebrates, which are far below the structural stage of a genuine Fish. To mention here briefly only the most important, the Round-mouths are entirely with- out any trace of limbs. Their slimy skin is quite naked and smooth, without scales. They are wholly destitute of a bony skeleton. The inner skeleton axis is a very simple inarticulate notochord, like that of the Amphioxus. In the Lampreys alone a rudimentary articu- lation is indicated by the fact that upper arches appear in the vertebral tube proceeding from the notochord sheath, At the anterior end of the chorda a skull is developed in its very simplest form. From the notochord sheath pro- ceeds a small soft-membraneous skull capsule, which becomes partly cartilaginous: this capsule encloses the brain. The important apparatus of the gill-arches, the tongue-bone, etc., which is inherited by all Vertebrates LAMPREYS. from Fishes to Man, is wholly wanting in the Round-mouths. They have, indeed, a superficial, cartilaginous gill-skeleton, but this is of quite different morphological significance. On the other hand, in them we meet, for the first time, with a brain, that important mental organ, which has been transmitted from the Single-nostrils up to Man. It is true that in the Round-mouths the brain appears merely as a very small and comparatively insignificant swelling of the spinal chord ; at first a simple bladder (Plate XL Fig. 16, m^, which afterwards separates into five consecu- tive brain-bladders, as in the brains of all Double-breathers. These five simple primitive brain-bladders, which reappear in a similar form in the embryos of all higher Vertebrates, from Fishes up to Man, and which undergo a very complex modification, remain in the Round-mouths, in a very low and undifieren- tiated stage of development. The histological elementary structure of the nervous system is also much more imperfect than in other Verte- brates. While in the latter the organ ot hearing always has three semi-circular canals, in the Lampreys it has but two, and in the Hags but one. In most other points also, the organization of the Round-mouths is FIG. 190. — The large Sea-lamprey (Petromyzon mari. TIMS), much reduced in size. A series of seven gill-open- ings are visible below the eye. 104 THE EVOLUTION OF MAN. much simplei and more imperfect, as, for instance, in the structure of the heart, the circulatory system, and the kidneys. In them, as in the Amphioxus, the anterior portion of the intestinal canal does, indeed, form respiratory gills ; but these respiratory organs are developed in a very peculiar way : in the form of six or seven little pouches, or sacs, which lie on both sides of the anterior intestine and communicate with the throat (pharynx) by inner openings, and by outer ones with the external skin. This is a very peculiar formation of the respiratory organs, quite cha- racteristic of this class of animals. They have therefore been called the " Pouch -gills " (Marsupobranchii). The absence of one very important organ found in the Fishes, the swimming-bladder, from which the lungs of the higher Vertebrates have developed, should be especially noticed. In their germ-history, as in their whole anatomical struc- ture, the Round-mouths present many peculiarities. They are even peculiar in the unequal cleavage of the egg, which most nearly approaches that of the Amphibians (Fig. 31, voL i p. 203). This results in the formation of a Hood- gastrula, like that of Amphibians (Plate II. Fig. 11). From this develops a very simple organized larval form, which is closely allied to the Amphioxus, and which, for that reason, we examined and compared with the latter (voL i. p. 428, and Plate VIII. Fig. 16). The gradual germ-evolution of these larvae of the Round-mouths explains very clearly and unmistakably the gradual evolution of the Skulled from the Skull-less class of Vertebrates. At a later period, from this simple Lamprey larva is developed a blind and tooth- less larval form, which is so very different from the mature Lamprey that, until twenty years ago, it was generally BOUND-MOUTHS. IO5 described as a peculiar form of fish under the name of Ammocostes. By a further metamorphosis this blind and toothless Ammoccetes is transformed into the Lamprey with eyes and teeth (Petromyzori).ul Summing up all these peculiarities in the structure and embryology of the Round-mouths, we may assert that the oldest Skulled Animals, or Craniota, diverged in two lines ; one of these lines has continued up to the present time but little modified; it is represented by the Cyclostoma, or Monorhina, forming a collateral line which has made but little progress, but has remained at a very low stage of development. The other line, the direct line in the pedigree of the Vertebrates, advanced in a straight line to the Fishes, and by new adaptations attained many important improve- ments. In order rightly to appreciate the phylogenetic signi- ficance of interesting remnants of primaeval groups of animals, such as the Round-mouths, it is necessary to study minutely their various peculiar characters philosophically and with the aid of Comparative Anatomy. A careful distinction must be drawn between the hereditary cha- racters which have been accurately transmitted to the present day by heredity from common, primaeval ancestors, now extinct, on the one hand ; and, on the other, those special adaptive peculiarities which the existing remnant of that primaeval group have, in the course of time, gained secondarily by adaptation. To the latter class belong, for example, in the Round-mouths, the peculiar formation of the single nostril and the round sucking mouth ; as well as special structural arrangements of the epidermis and the pouch-shaped gills. But, on the other hand, to the IO6 THE EVOLUTION OF MAN. former class of characteristics, which alone have any phylo- genetic significance, belong the primitive formation of the vertebral column and the brain, the absence of the swim- ming-bladder, of jaws, limbs, etc. In the animal system, the Round-mouths are usually classed among Fishes ; but that this is quite incorrect is apparent from the simple fact that, in all important and prominent structural peculiarities, they are further removed from the Fishes than the Fighes are from the Mammals and from Man. CHAPTER XVIII. THE PEDIGREE OF MAN. III. FROM THE PRIMITIVE FISH TO THE AMXIOTIC ANIMAL. Comparative Anatomy of the Vertebrates. — The Characteristic Qualities of the Double-nostrilled and Jaw-mouthed : the Double-Nostrils, the Gill- arch Apparatus, with the Jaw-arches, the Swimming-bladder, the Two Pairs of Limbs. — Relationship of the Three Groups of Fishes : the Pri- mitive Fishes (Setac/iti), the Ganoids (Ganoides), the Osseous Fishes (Teleostei).— Dawn of Terrestial Life on the Earth.— Modification of the Swimming-bladder into the Lungs. — Intermediate Position of the Dipneusta between the Primitive Fishes and Amphibia. — The Three Extant Dipneusta (Protopterus, Lepidosiren, Ceratodus) — Modification of the Many-toed Fin of the Fish into the Five-toed Foot.— Causes and Effects of the latter. — Descent of all Higher Vertebrates from a Five-toed Amphibian. — Intermediate Position of the Amphibians between the Lower and Higher Vertebrates. — Modification or Metamorphosis of Frogs. — Different Stages in Amphibian Metamorphosis. — The Gilled Batrachians (Prof ens and Axolotl) .—The Tailed Batrachians (Salaman- ders and Mud -fish). —Frog Batrachians (Frogs and Toads).— Chief Group of the Amnion Animals, or Amniota (Reptiles, Birds, and Mammals). — Descent of all the Amniota from a Common Lizard-like Parent-form (Prof a mn ton). — First Formation of the Allantois and of the Amnion. — Branching of the Amnion Animals in Two Lines : on the one aide, Reptiles (and Birds), on the other side, Mammals. " The imagination is an indispensable faculty ; for it is that which, by forming new combinations, occasions important discoveries. The naturalist needs both the discriminating power of abstract reason, and the generalizing power of the imagination, and that the two should be harmoniously inter. IO8 THE EVOLUTION OF MAN. related. If the proper balance of these faculties is destroyed, the natnralist is hnrried into chimerical fancies by his imagination ; while the same gift leads the gifted naturalist of sufficient strength of reason to the most important discoveries." — JOHANNES MULLEB (1831). THE further we proceed in human tribal history, the nar- rower does that part of the animal kingdom become within which we must look for extinct ancestors of the human race. At the same time, the evidence as to the history of the evolution of our race given by what we have called the records of creation, the evidence of Ontogeny, of Compara- tive Anatomy, and of Palaeontology, grows constantly more extensive, complete, and trustworthy. It is therefore natural that Phylogeny should assume a more definite form the nearer we approach the higher and the highest stages of the animal kingdom. Comparative Anatomy especially has done far more for our knowledge of these higher stages of evolution in the animal kingdom than for the lower. This important science, which aims at a true philosophy of organic forms, has made greater progress in the Vertebrate tribe than in any section of the Invertebrate. Cuvier, Meckel, and Johannes Muller had already laid a deep and extensive foundation ; and now the Comparative Anatomy of Vertebrates has recently been powerfully advanced by the admirable inves- tigations of Owen and Huxley, and, especially, has been perfected to such a degree by the unsurpassed labours of Gegenbaur, that it now forms one of the strongest supports of the Theory of Descent. Relying on the evidence thus furnished, we can now, with a great degree of certainty, recognize the most important outlines of the series of stages and the ramifications of the Vertebrate pedigree. PKIMITIVE FISHES. IOQ That part of the animal kingdom with which wo are now concerned has become so narrow, even before we have left the Archilithic Epoch, that but a single one of the seven tribes of the animal kingdom forms the object of our study. Even within this tribe we have passed the lowest steps, and have risen above the Skull-less (Acrania} and Double-nostrilled Vertebrates (Monorhina), to the class of Fishes. The latter are the first of the great main division of Vertebrates distinguished by mouths with jaws and by double nostrils (Amphirhina, or Gnathostoma). From Fishes we start again, as from that class of Vertebrates which are indubitably shown by Comparative Anatomy and Ontogeny to be the ancestral class of all higher Vertebrates, all Am- phirhina. Of course no existing Fish can be regarded as the direct parent-form of the higher Vertebrates. But it is equally certain that from a common extinct Fish-like parent-form we may trace all those Vertebrates from Fibhes up to Man, which are included under the name of Am- phirhina. If this primaeval parent-form were extant, we should undoubtedly describe it as a genuine Fish and class it among Fishes. Fortunately, the Comparative Anatomy and Classification of the Fishes has been so far advanced (thanks to the labours of Johannes Muller and Gegenbaur) that we can very clearly distinguish these most important and interesting genealogical relations. In order correctly to understand the human pedigree within the Vertebrate tribe, it is very important to bear in mind the distinguishing characteristics, separating Fishes and all the other Double-nostrils (Amphirhina) from Single-nostrilled and Skull-less Animals (Monorhina and Acrania). These very distinguishing characteristic marks [TO THE EVOLUTION OF MAN. Fishes have in common with all other Double-nostrils up to Man, and it is on this parallelism that we found our claim of relationship to Fishes. (Of. Table X., vol. L p. 466.) The following characters of the Double-nostrils must be especially indicated as the systematic anatomical features of the highest importance : (1) the double structure of the nose ; (2) the internal gill-arch apparatus, together with the jaw-arches ; (3) the swimming-bladder, or lungs ; and (4) the two pairs of limbs. As to the nasal structure, on which is based the distinc- tion of the Single-nostrils (Monorhina) from the Double- nostrils (Amphirhind), it is certainly significant that even in Fishes the earliest rudiment of the nose consists of two en- tirely distinct lateral grooves or pits in the outer surface of the head, just as is the case in the embryo of Man and of all higher Vertebrates. On the other hand, in Single-nostrils and Skull-less Vertebrates the first rudiment of the nose is, from the first, a single pit in the centre of the forehead region. No less important is the higher development of the skeleton of the gill-arch and of the jaw apparatus connected with it, as it occurs in all Double-nostrils from Fishes to Man. It is true that the primitive modification of the anterior intestine into the gill-intestine, which occurs even in Ascidians, is developed in all Vertebrates from one simple rudiment; and in this respect the gill-openings, which in all Vertebrates and also in Ascidians pierce the wall of the gill-intestine, are quite characteristic. But the external framework of the gills, which in all Skull-less and Single- nostrilled Animals (Acraniota and Monorhina) supports the gill-body, is displaced in all Double-nostrils (Amphi- rhina) by an internal gill-skeleton which replaces the former DOUBLE-NOSTRILS AND SINGLE-NOSTRILS. Ill This internal gill-support consists of a consecutive series of cartilaginous arches, which are situated between the gill- openings within the wall of the throat (pharynx), and extend round the throat. The foremost of these pairs of gill-arches changes into the jaw-arch (maxillary arch), which gives rise to the upper and lower jaws. A third essential character by which all Double-nostrils are well distinguished from all those lower Vertebrates which we have already considered, is the formation of a blind sac which protrudes from the anterior portion of the intestinal canal, and which in the Fishes becomes the air- filled swimming-bladder (Plate V. Fig. 13, hi). As this organ, in proportion as it contains a greater or less quantity of air, or in proportion as this air is more or less compressed, imparts a higher or lower specific gravity to the Fish, it acts as a hydrostatic apparatus. By this means the Fish can rise or sink in the water. This swimming-bladder is the organ from which the lung of higher Vertebrates has developed. The fourth and last main character of Double- nostrils is the presence of two pairs of extremities or members in the primitive arrangement of the embryo ; a pair of fore limbs, which in Fishes are called pectoral fins (Fig. 191, v}, and a pair of hind limbs, which in Fishes are called ventral fins (Fig. 191, ft). The Comparative Anatomy of these fins is of supreme interest, because they contain the rudiments of all those parts of the skeleton which, in all the higher Vertebrates up to Man, form the skeleton or support of the extremities of the fore and hind limbs. In Skull-less and Single-nostrilled Animals there is, on the contrary, no trace of these extremities. In addition to these four most important main characters of the Amphi- 41 112 THE EVOLUTION OF MAN. rhina, we might further mention the presence of a sym- pathetic nerve-system, a spleen, a ventral salivary gland ; organs which are not represented in the lower Vertebrates already considered. All these important parts have trans- mitted themselves from Fishes up to Man, and from this circumstance alone it is evident how wide a chasm sepa- rates the Fishes from the Skull-less and Single-nostrilled Animals (Acraniota and Monorhina). Fishes and Man possess all these characters in common (Table X.). Turning now to consider the Fish class in greater detail, we may divide it primarily into three main groups, or sub- classes, the genealogies of which are evident. The first and most ancient group is that of the Primitive Fishes (Selachii), the best-known extant representatives of which are the members of the much-varied orders of Sharks and Rays (Figs. 191, 192). These are followed by a series of further developed Fish forms, by the sub-class of Mucous Fishes (Ganoides). The greater number of these have long been extinct, and only very few living representatives are known ; these are the Sturgeon and Huso of European seas, the Polypterus of African, and the Lepidosteus and Amia of American rivers. The earlier abundance of forms belong- ing to this interesting group is, however, proved by the abundance of their fossil remains. From these Mucous Fishes originated the third sub-class, that of the Osseous Fishes (Teleostei), to which belong most extant Fishes, espe- cially nearly all our river fish. Comparative Anatomy and Ontogeny very clearly show that the Ganoids sprang from the Selachii, just as the Teleostei sprang from the Ganoids. But, on the other hand, a second side-line, or rather the main ascending line of the Vertebrate tribe, EMBRYOS OF SHARKS FIG. 191. FIG. 192. 114 THE EVOLUTION OF MAN. FIG. 191. — Embryo of a Shark (Scymnus lichia), seen from ventral side : i', pectoral fins (in front of these five pairs of gill-openings) ; h, ventral fins ; a, anal opening ; s, tail fin ; k, external gill-tufts ; d, yelk-sac (the greater part of this has been removed) ; g, eye ; n, nose ; m, mouth fissure. FIG. 192. — Developed Man-shark (Carcliarias melanopterus), seen from the left side : rl first, rt second dorsal fin; s, tail fin; a, anal fin; v, pectoral fins ; /(, ventral fins. developed in another direction from the Primitive Fishes; this line leads upward through the Dipneusta group to the important class of Amphibia. This significant relationship between the three groups of Fishes has been placed beyond all doubt by the re- searches of Gegenbaur on the subject The lucid discussion on the " systematic position of the Selachii " which that author inserted in the introduction to his classic study of the "head skeleton of the Selachii," must be regarded as definitely proving this important relation.148 In Primitive Fishes (Selachii}, however, the scales (skin appendages) and the teeth (jaw appendages) are identical in formation and structure, while in the other two groups of Fishes (Mucous and Osseous Fishes) these organs have already become distinct and differentiated. Moreover, in Primitive Fishes, the cartilaginous skeleton (the vertebral column and the skull, as well as the members) is of the simplest and most primitive nature, of which the bony skeletons of Mucous and Osseous Fishes must be regarded as a modification. It is true that in certain respects (in the structure of the heart and of the intestinal canal) Mucous Fishes fully coincide with Primitive Fishes, and differ from Osseous Fishes. But a comparative review of all the anatomical relations plainly shows that the Mucous Fishes constitute a connecting group between Primitive and MUD-FISHES. 115 Osseous Fishes. The Primitive Fishes (Selachii) form the most ancient and original group of Fishes. From these, in one direction, all other Fishes have developed ; the Mucous Fishes first, which, at a much later period (in the Jurassic, or the Chalk Period), gave rise to the Osseous Fishes. In another direct'on, the Primitive Fishes gave rise to the parent-forms of the higher Vertebrates, directly to the Dipneusta, and thus to Amphibians. Regarding the Selachii as forming the eleventh stage in our pedigree, these would be followed by the Dipneusta group as the twelfth stage, and by the Amphibian group as the thirteenth stage. The advance effected in the development of the Mud- fishes (Dipneusta) from the Primitive Fishes is of great mo- ment, and is connected with a very noticeable change, which took place in the beginning of the Palaeozoic, or Primary Period in organic life as a whole. For the very numerous fossil remains of plants and animals which are now known to belong to the first three epochs of the history of the earth — to the Laurentian, the Cambrian, and the Silurian Periods, are exclusively those of aquatic plants and animals. From this paloeontological fact, taken in connection with certain weighty geological and biological considerations, we may infer, with tolerable certainty, that at that time no land animals yet existed. During the whole of the enormous Archizoic Period — during many millions of years — the living population of our globe were all water-dwellers : a very remarkable fact, when it is remembered that this period embraces the larger half of the entire organic history of the earth. The lower animal tribes are even now exclusively, or with very few exceptions, aquatic. But during the Archizoic, or Primordial Epoch, the higher animal tribes Il6 THE EVOLUTION OF MAN. continued exclusively adapted to aquatic habits of life. It was not till later that they adopted a land life. The earliest fossils of terrestrial animals occur in the Devonian strata, which were deposited in the beginning of the second great division of the earth's history (the Palaeozoic Epoch). They increase greatly in number in the deposits of the Coal and Permian Periods. Even in these early formations many terrestrial and air-breathing species, both of the Arthro- pod and of the Vertebrate tribe, occur ; while their aquatic ancestors of the Silurian Period breathed nothing but water. This physiologically significant modification of the mode of respiration is the most influential change that affected the animal organism in the transition from water to dry land. In the first place it caused the development of an air-breathing organ, the lung, the water-breathing gills having previously acted as respiratory organs. Simul- taneously, however, it effected a remarkable change in the circulation of the blood and in the organs connected with this ; for these are always most closely correlated with the respiratory organs. In addition to these, other organs also, either in consequence of more remote correlation with the respiratory organs, or in consequence of new adaptations, were more or less modified. Within the Vertebrate tribe it was undoubtedly a branch of the Primitive Fishes (Selachii) which, during the De- vonian Period, made the first successful effort to accustom itself to terrestrial life and to breathe atmospheric air. In this the swimming-bladder was especially of service, for it succeeded in adapting itself to respiration of air, and so became a lung. The immediate consequence of this was the modification of the heart and nose. While true Fishes EVOLUTION OF MUD-FISHES. 1 1/ have only two blind nose-pits on the surface of the head, these now became connected with the mouth-cavity by an open passage. A canal formed on each side, leading directly from the nose-pit into the mouth-cavity, and thus even while the mouth-opening was closed the necessary atmo- spheric air could be introduced into the lungs. While, moreover, in all true Fishes the heart consists simply of two compartments, an auricle, which receives the venous blood from the veins of the body, and a ventricle, which forces this blood through an arterial expansion into the gills, the auricle, owing to the formation of an incomplete partition wall, is now divided into a right and a left half The right auricle alone now received the venous blood of the body, while the left auricle received the pulmonic venous blood passing from the lungs and the gills to the heart. The simple blood -circulation of the true Fishes thus became the so-called double circulation of the higher Ver- tebrates ; and this development resulted, in accordance with the laws of correlation, in further progress in the structure of other organs. The vertebrate class, which thus first adapted itself to the habit of breathing air, and which originated from a branch of the Selachii, are called Mud-fishes (Dipneusta), or Double-breathers, because, like the lowest Amphibia, they retain the earlier mode of breathing through the gills, in addition to the newly acquired lung-respiration. This class must have been represented by numerous and diverse genera during the Palaeolithic Epoch (during the Devonian, Carboniferous, and Permian Periods). As, however, the skeleton is soft and cartilaginous, like that of the Selachii, they naturally left no fossil remains. The hard teeth of Il8 THE EVOLUTION OF MAN. single genera (Ceratodus) could alone endure ; these occur, for instance, in the Trias. At the present time there are only three extant genera of this whole class : Protopterus annectens, in the rivers of tropical Africa (White Nile, Niger, Quillimane, etc.) ; Lepidosiren paradoxa, in tropical South America (in the tributaries of the Amazon); and Ceratodus Fosteri, in the swamps of Southern Australia (Plate XII.).149 This wide distribution of the three isolated descendants of the class is alone sufficient to prove that they are the last remnants of a group which was formerly very widely developed. The whole structure of their bodies shows that the group to which they belong forms tho transition between Fishes and Amphibia. The direct tran- sitional structure between the two classes is so clearly expressed in the whole organization of these curious animals, that zoologists yet dispute whether the Dipneusta are Fishes or Amphibia. Some well-known zoologists still class them among Amphibia, while they are usually placed among Fishes. In fact, the characters of both the classes are so united in the Dipneusta that the answer to the question as to their nature depends entirely upon the mean- ing attached to the terms "Fish" and "Amphibian." In their mode of life they are true Amphibia. During the tropical winter, in the rainy season, they swim in the water like Fishes and inhale water through the gills. During the dry season they burrow in the mud as it dries up, and during that period breathe air through lungs, like Am- phibians and higher Vertebrates. In this two-fold respira- tion they do, it is true, coincide with the lower Amphibia, and stand far above Fishes. Yet, in most other characters they more nearly resemble the latter, and stand below the EXTANT MUD-FISHES. lig former. Their external appearance is entirely like that of Fishes. The head of the Dipneusta is not distinct from the trunk. The skin is covered with large fish-scales. The skeleton is soft, cartilaginous ; its development has been arrested at a very low stage, just as in the lower Primitive Fishes. The notochord is retained entire. The two pairs of limbs are very simple fins of primitive structure, like those of the lowest Primitive Fishes. The structure of the brain, of the intestinal tube, and the sexual organs, is also as in Primitive Fishes. The Dipneusta, or Mud-fishes, have, there- fore, by heredity, accurately retained many features of a lower organization derived from our primaeval Fish ancestors, while their adoption of the habit of breathing air through lungs introduced a great advance in the vertebrate organi- zation. Moreover, the three extant Mud-fishes differ a good deal from one another in important points of structure. The Australian Mud-fish (Ceratodus), which was first described at Sidney in 1870 by Gerard Kreffib, and which attains a length of six feet, appears in an especial degree to represent a primaeval and very conservative animal form (Plate XII.). This is especially true of the structure of its simple lung, and of its fins, which contain a pinnate skeleton. In the African Mud-fish (Protopterus), on the contrary, and in the American form (Lepidosiren) the double lung is present, as in all higher Vertebrates ; nor is the fin-skeleton pinnate. In addition to the internal gills, Protopterus has also ex- ternal gills, which are wanting in Lepidosiren. Those unknown Dipneusta, which were among our direct ancestors, and which formed the connecting link between the Selachii ( 120 ) TABLE XX. Systematic Survey of the Phyfogenetic Classification of Vertebrates. I. Shull=Ics0 (Acrania), or 8Tubr=f) cart ell (Leptocardia). Vertebrates without a specialized head, skull, brain, or centralized heart. 1. Amphioxida [I. Animals fcritf) skulls (Craniota) and with ccntraltjcti Ijrarts (Fachycardia), Vertebrates with specialized head, with skull and brain, and with a centralized heart. Ifain-classes of the Skulled Animals. Clowes of the Skulled Aniriala. Sub-classes of the Skulled Animals. Systematic X(KM of the Sub-classes. 2. Single* fiostrillcU • Monorhina [1. Eound months Cyclcstoma 2. Hags, or Mucous Fish 3. Lampreys 2. Hyperotreta (Myxinoida) 3. Hyperoartia (Pctromyzontia) III. Fishes Pisces 4. Primitive Fish 5. Ganoid Fish 4. Selachii 5. Ganoides 6. Osseous Fish 6. Teleostei 3. ffon* amntonate IV. Mud-fishes ( 7. Single-lunged Dipneustn \ 8. Double-lunged 7. Monopneumonee 8. Dipneumoues Anamnia (9. Mailed Batra- 9. Phractamphibia chians 10. Naked Batra- 10. Lissamphibia chians Amniota 11. Lizards 11. Lacertilia 12, Snakes 12. Ophidia L3. Crocodiles 13. Crocodilia VI. Eeptiles 14 Tortoises 14. Chelonia Beptilia 15. Sea-dragons 15. Ilalisauria 16. Dragons 16. Dinosauria 17. Flying Eeptiles 17. Ptcrosauria 18. BeakedAnimals 18. Anomodonta VH. Birds f JjJ- A \ £"• Long-tailed 19. 20. SaururaB Carinataa Aves |21 Bush-tailed 21. Eatitss ,22. CloacalAnimals 22. Monotrema 1 •'• • VIII. Mammals I **• Pouched Ani- •nnla 23. Marsupialia mais iL/UJUfHatlCl I &A Placenta! Ani- 24. Placenttlia TABLE XXI. Pedigree of Vertebrates. (Cf. Plate XV.) Osseous fish Telcostei Ganoid fish Ganoides Single-lnnged M onop nenmones Double-lunged Dipneumones 8. fflatnmals Mammalia Mud -fish Protopteri 7. Birds Aves 6. Reptiles Repti'ia I Smrn'on^nitnals Amniota I 5. Batrarljtans Amphibia Dipneusta Primitive fishes Selachii 8. Fishes Pisces Amphirhina 1. Tube-hearted Sea-squirts Leptocardia Ascidice | Thalic I Jflantlrti Sntmalu Tunicata Acrania Fcrtcbratea Vertebra ta Lampreys Hags Pctromygontes Myxinoides J _ L 2. Eound-mouths Cylostoma Monorhina Craniota Cfjorta^nitnals Chordoiiia Worms Vermes 122 THE EVOLUTION OF MAN. and the Amphibians, were doubtless in many respects different from their three direct descendants of the present time, but in the most essential characters they must have coincided with the latter. Unfortunately, the germ-history of the three surviving Mud -fishes is as yet entirely un- known; probably at some future time it will afford us further important information as to the tribal history of the lower Vertebrates and so of our ancestors. Very important information of this kind has been supplied by the next Vertebrate class, that of the Batra- chians (Amphibia), which is directly connected with tho Dipneusta, from which it originated. To this class belong the Axolotl, Salamanders (Plate XIII.), Toads, and Frogs. Formerly, after the example of Linnaeus, all Reptiles (Lizards, Snakes, Crocodiles, and Tortoises) were also classed among Amphibia. But these animals are of a far higher organiza- tion, and in the most important characters of their ana- tomical structure are more nearly allied to Birds than to Amphibians. The true Amphibia, on the other hand, are more nearly allied to the Double-breathers and to Primitive Fishes : they are also much older than Reptiles. Even as early as the Carboniferous Period numerous very highly developed Amphibia (some of large size) were extant, whereas the earliest Reptiles first appear only towards the close of the Permian Period. In all probability the Amphibia were developed from Double-breathers at an even earlier period — during the Devonian Period. The extinct Amphibia, of which fossil remains have been preserved from that most ancient Primaeval Epoch — and these are especially numerous in the Trias — were distinguished by a large bony coat of mail overlying the skin (like that of the Crocodile), while most BATRACHIAXS. 123 of the yet extant Amphibians have a smooth and slippery skin. The latter, also, are on an average smoother than the former, and must be regarded as their stunted posterity. Among the Amphibia of the present time we are, therefore, unable to find any forms that are directly referable to the pedigree of the human race, or that are to be re- garded as ancestors of the three higher Vertebrate classes ; yet, in important points of their internal anatomical struc- ture, and especially in their germ-development, they cor- respond so closely with us, that we are justified in affirming that between the Double-breathers (Dipneusta) on the one hand, and the three higher Vertebrate classes (grouped together as Amniota) on the other, there existed a series of extinct intermediate forms which, if we had them before us, we should class among Amphibia. The whole organization of the extant Amphibia represents a transitional group of this kind. In the important matters of respiration and circulation of the blood, they are still closely allied to the Double-breathers, although in other respects they rise above the latter This is especially true with respect to the ad- vanced structure of their lirnbs or extremities. The latter here for the first time appear as feet with five digits. The thorough researches of Gegenbaur have shown that the fins of Fishes, concerning which very erroneous views were pre- viously held, are feet with numerous digits ; that is to say, the several cartilaginous or osseous rays, many of which occur in every Fish-fin, correspond to the fingers or digits on the limbs of higher Vertebrates. The several joints of each ray correspond to the several joints of each digit. In the Double- breathers the fin yet retains the same structure as in Fishes, and it was only gradually that the five-toed form of foot, 124 THE EVOLUTION OF MAN. which occurs for the first time in Amphibians, was developed from this multi-digitate form. This reduction in the number of the digits from ten to five occurred in those Dipneusta which must be regarded as the parent-forms of the Amphibia, probably as early as the latter half of the Devonian Period — or, at latest, in the immediately subsequent Carboniferous Period. Several fossil Amphibia with five digits have already been found in the strata of the latter period. Fossil foot- prints of the same animals are very numerous in the Trias (Cherotheriun). The great significance of the five digits depends on the fact that this number has been transmitted from the Amphibia to all higher Vertebrates. It would be impossible to discover any reason why in the lowest Amphibia, as well as in Reptiles and in higher Vertebrates up to Man, there should always originally be five digits on each of the anterior and posterior limbs, if we denied that heredity from a common five-fingered parent-form is the efficient cause of this phenomenon : heredity can alone account for it. In many Amphibia, certainly, as well as in many higher Vertebrates, we find less than five digits. But in all these cases it can be shown that separate digits have retrograded, and have finally been completely lost. The causes which effected the development of the five- fingered foot of the higher Vertebrates in this Amphibian parent-form from the many-fingered foot, must certainly be found in the adaptation to the totally altered functions which the limbs had to discharge during the transition from an exclusively aquatic life to one which was partially terrestrial. While the many-fingered fins of the Fish had previously served almost exclusively to propel the body FINS AND FINGERS. 125 through the water, they had now also to afford support to the animal while creeping upon land. This effected a modification both of the skeleton and of the muscles of the limbs. The number of fin rays was gradually lessened, and was finally reduced to five. These five remaining rays now, however, developed more vigorously. The soft cartilaginous rays became hard bones. The rest of the skeleton also became considerably more firm. The move- ments of the body became not only more vigorous, but also more varied. The separate portions of the skeleton system, and consequently those of the muscular system also, became more and more differentiated. Owing to the intimate correlation of the muscular to the nervous system, the latter also naturally made marked progress in point of function and structure. We therefore find that the brain is very much more developed in the higher Amphibia than in Fishes, in Mud-fishes, and in the lower Amphibia. The organs which are most modified in consequence of an amphibious mode of life are, as we have already seen in the Double-breathers (Dipneusta), those of respiration and of the circulation of the blood. The first advance in organization necessitated by the transition from aquatic to terrestrial habits of life was, of course, the formation of an air-breathing organ, a lung. This developed directly from the swimming-bladder which these animals had inherited from the Fishes. At first the function of this organ would be quite subordinate to the more ancient organ, used for the respiration of water, the gills. Hence we find that the lowest Amphibia, the Gilled Amphibia, like the Dipneusto spend the greater part of their lives in the water, and that accordingly they breathe water through gills. It is only 126 THE EVOLUTION OF MAN. for brief intervals that they rise to the surface of the water or creep out of the water on to the land ; and at these times they breathe air through lungs. Some, however, of the Tailed Amphibians, the Axolotl and the Salamander, live exclusively in the water only when young, and afterwards usually remain on land. In the adult state they breathe only air through lungs. This is also the case with the most highly developed Amphibians, the Frog-amphibia (Frogs and Toads) ; some of the latter have even entirely lost the gilled larval form.153 The same is true of a few small snake-like Amphibia, the Ceeciliee, which, like earth-worms, live in the ground. The high degree of interest attached to the natural history of the Amphibian class is especially due to the fact that they hold a position exactly intermediate between the higher and the lower Vertebrates. While the lower Am- phibia are in their whole organization directly allied to the Dipneusta and the Fishes, living mostly in the water and respiring water through gills, the higher Amphibia are no less directly related to the Amnion Animals, for, like the latter, they live mostly on land, and breathe air through lungs. But when young the higher forms resemble the lower, and only attain their own higher degree of development after undergoing complete modification. The individual germ-history of most higher Amphibians still accurately reproduces the tribal history of the whole class ; and the various stages of modification which were necessitated in certain low Vertebrates by the transition from aquatic to terrestrial habits during the Devonian or Carboniferous Period, are still to be seen every spring in each Frog as it develops from the egg in our ditches and pools. IMPORTANCE OF AMPHIBIA. 127 Like the Tailed Salamanders (Fig. 193), each common Frog emerges from the egg in a larval form, totally different from that of the full-grown Frog (Fig. 194). The short Fro. 193. — Larva of Spotted Land-Newt (Salamandra maculata), from the ventral side. In the centre a yelk-sac yet protrudes from the intestine. The external gills are prettily branched and tree-like. The two pairs of limbs are yet very small. FIG. 194. — Larva of the Common Grass-Frog (Rana temporaries), a so- called tadpole : m, mouth ; n, a pair of suction cups used in clinging to stones ; d, skin-fold, which gives rise to the gill-roof ; behind are the gill-openings, from which the gill branches protrude ; s, tail-muscles ; /, skin-fold of the tail, forming a float. trunk is produced into a long tail, which in form and struc- 42 125 THE EVOLUTION OF MAN. bure resembles the tail of a Fish (s). At first it has no limbs. Respiration is accomplished solely by gills, which are at first external (&) and afterwards internal. Corre- spondingly, the heart is also of the same form as in the Fishes, and consists of only two compartments — an auricle, which receives the venous blood of the body, and a ven- tricle, which drives it through the arterial bulb into the gilla Numbers of these fish-like Frog larvae, or " tadpoles," as they are called, swim about every spring in all ponds and pools, using their muscular tails for propulsion, just as is done by Fishes and larval Ascidians. The remarkable transformation of the fish-like form into that of the Frog does not take place till after the tadpole has grown to a certain size. From the throat grows a closed sac which develops into a pair of large sacs; these are the lungs. The simple chamber of the heart is divided into two auricles, owing to the formation of a partition wall, and simul- taneously considerable changes of structure occur in the main arterial trunks. Previously all the blood passed from the heart-chamber through the aorta arches into the gills ; but only part of it now passes to the gills, while another part passes through the newly formed lung arteries into the lungs. From the lungs arterial blood returns into the left auricle of the heart, while the venous blood of the body collects in the right auricle. As both of the auricles open into the simple ventricle, the latter contains mixed blood. The fish-like form has now passed into the Dipneusta form. During the further course of modification the gills, with the gill- vessels, are entirely lost, and respiration is now per- formed by the lungs alone. Yet later, the long tail is also GILLED BATRACHIANS. I2Q rejected, and the Frog now leaps about on the land on legs which have sprouted in the mean time.150 This remarkable metamorphosis of the higher Amphibia is very instructive in its bearing on Man's ancestral history, and is especially interesting owing to the fact that the various groups of extant Amphibia have remained stationary at various stages of their tribal history, which, in accord- ance with the fundamental law of Biogeny, are reproduced in this germ-history. First, there is a very low order of Amphibia, the Gilled Batrachians (Sozobranchia), which, like Fishes, retain their gills throughout life. To this order belong, among others, the well-known blind " Olm " of the Adelsberg Cave (Proteus anguineus), the Mud-eel of South Carolina (Siren lacertina), and the Axolotl of Mexico (Sire- don pisciformis; Plate XIII. Fig. 1). All these Gilled Batrachians are fish -like animals with long tails, and in point of respiratory organs and of circulation of the blood they remain throughout life stationary at the Dipneusta stage. They possess both gills and lungs, and can either respire water through the gills or air through the lungs, as occasion requires. In another order, the Salamanders, the gills are lost during metamorphosis, and in the adult state air only is breathed through lungs. This order bears the name of Tailed Batrachians (Sozura) because they retain the tail throughout life. To this order belong the common Water- Newts (Triton) which swarm in all ponds during the summer, and the black, yellow-speckled Land-Salamanders (SaLamandra) found in damp woods (Plate XIII. Fig. 2). The latter are among the most remarkable of our indigenous animals, sundry anatomical characters proving them to be very ancient and highly conservative Vertebrates.151 A I3O THE EVOLUTION OF MAN. few Tailed Batrachians retain the gill-opening in the side of the neck, though the gills themselves are lost (Meno- poma). If the larvae of the Salamanders (Fig. 193) and Tritons are compelled to remain in water, and not allowed to get on land, they may, under favourable conditions, be made to retain their gills. In this fish-like condition they become sexually mature, and will throughout life remain compulsorily in the lower stage of development of the Gilled Batrachians. The opposite experiment was made some years ago in the case of the Mexican Gilled Batra- chian, the fish-like Axolotl (Siredon pisciformis; Plate XIII. Fig. 1). This animal had previously been regarded as a permanent Gilled Batrachian, remaining throughout life in this fish-like condition. But of the hundreds of these animals kept in the Jardin des Plantes at Paris, a few individuals, for some unknown reason, crept to land, lost their gills, and changed into a form closely allied to that of the Salamander (Ainblystoma, Fig. 2). In this state they became sexually mature.152 This phenomenon, which at first excited a lively interest, has since been repeatedly observed with care. Zoologists regarded the fact as some- thing peculiarly wonderful, though each spring every common Frog and Salamander passes through the same modification. In these animals we can in the same way follow each step in the significant metamorphosis of the aquatic and gill-respiring animal into the terrestrial and lung-respiring animal. That which thus takes place in tho individual during germ-evolution, took place in the same way in the whole class during the course of its tribal history. The metamorphosis which takes place in the third order ri.ATE XII. PLATE XIII. Fig, 2 Fig. I. Siredon pisciformis. Fig. 2. Salamandra maculata. TAILED BATRACHIAXS AND FROG BATRACHIANS. 13 1 of Amphibia, the Frog Batrachians (Batrackia, or Anura), is yet more complete than in the Salamanders. To these belong all the various kinds of Toads, Water-frogs, Tree- frogs, etc. In the course of transformation these lose not only the gills, but also the tail, which drops off in some t;ases earlier, in others later. In this respect the various species differ somewhat from one another. In most Frog Batrachians the larvae drop the tail very early, and the tail-less frog-like form subsequently grows considerably larger. Other species, on the contrary, as,. for instance, the Pseudes paradoxus of Brazil, as also an European Toad (Pelo- batesfuscus) remain for a very long time in the fish form, and retain a lengthy tail till they have almost attained their full size; hence, after their metamorphosis is com- pleted, they appear much smaller than before. The opposite extreme is seen in some Frogs but recently brought under notice, which have lost the whole of their historic meta- morphoses, and in which no tailed and gilled larva emerges from the egg, but the perfect Frog, without tail or gills. These Frogs inhabit isolated oceanic islands, the climate of which is very dry, and which are often for a con- siderable length of time without fresh water. As fresh water is indispensable for gill-respiring tadpoles, these Frogs have adapted themselves to this local deficiency and have entirely relinquished their original metamorphosis, e.g., Hylodes martinicensis.158 The ontogenetic loss of gills and tail in Frogs and Toad* can of course only be phylogenetically explained as owing to the fact that these animals have descended from long- tailed salamander-like Amphibians. This is also proved beyond doubt by the Comparative Anatomy of the two 132 THE EVOLUTION OF MAN. groups. This remarkable transformation is, in other respects also, of general interest, as throwing a flood of light upon the Phylogeny of the Tail-less Apes and of Man. Man's ances- tors were also long-tailed gill-breathing animals, resembling Gilled Batrachians, as is irrefutably demonstrated by the tail and the gill arches in the human embryo. During the Palaeozoic Epoch, and probably in the Car- boniferous Period, there is no doubt that the Amphibian class embraced a series of forms which must be regarded as direct ancestors of Mammals, and so of Man. On grounds derived from Comparative Anatomy and Ontogeny, we must not, however, look for these Amphibian ancestors of ours — as might perhaps be supposed — among the Tail-less Frog Batrachians, but only among the lower Tailed Amphibians. We can with certainty point to at least two extinct Batra- chian forms as direct ancestors of Man, as the thirteenth and fourteenth stages in our pedigree. The thirteenth ancestral form must have been closely allied to the Double- breathers (Dipneusta), must, like these, have possessed per- manent gills, but must have been already characterized by having five digits on each foot ; and were they still living we should place them in the group of Gilled Batrachians, with the Proteus and the Axolotl (Plate XIII. Fig. 1). The fourteenth ancestral form, on the other hand, must indeed have retained the long tail, but must have lost the gills, and hence the nearest allied forms among extant Tailed Batra- chians would be the Water-Newts and Salamanders (Plate XIII. Fig. 2). Indeed, in the year 1725 the fossil skeleton of one of these extinct Salamanders (closely allied to the present giant Salamander of Japan) was described by the Swiss naturalist, Scheuchzer, as the skeleton of PRIMITIVE AMNION ANIMALS. 133 a fossil Man dating from the Deluge! ("Homo diluvii testis." 154) As the vertebrate form occurring in our pedigree imme- diately after these Batrachian ancestors — and, therefore, as the fifteenth stage — let us now examine a lizard-like animal, of which no fossil remains have been obtained, and which is not even proximately represented in any extant animal form, but the former existence of which we may infer with the utmost certainty from certain comparative anatomical and ontogenetical facts. This important animal form we will call the Protamnion, or Primitive Amniotic animal. All Vertebrates higher than the Amphibia — that is, the three classes of Reptiles, Birds, and Mammals — are so essentially distinct in their whole structure from all the lower Verte- brates which we have as yet considered, and, on the other hand, have so much in common, that we may class them together in one group as Amnion Animals (Amniota). It is only in these three classes of animals that we find that remarkable envelope of the embryo known as the amnion. (Of. voL i. p. 386.) The latter must probably be regarded as a kenogenetic adaptation, as caused by the sinking of the embryo into the yelk -sac.155 All known Amnion Animals, all Reptiles, Birds, and Mammals (Man included), coincide in so many important points of organization and development that we are fully justified in asserting their common descent from a single parent form. If the testimony of Comparative Anatomy and Ontogeny is entirely unquestionable in any point, it is certainly so here. For all the special peculiarities and characters, which appear accompanying and following the formation of the amnion, and which we found in the 134 THE EVOLUTION OF MAN. development of the human embryo; all the many peculiari- ties in the development of the organs which we shall presently notice in detail; and, finally, the chief special arrangements of the internal structure of the body in all fully developed Amnion Animals; all these so clearly demon- strate the common origin of all Amnion Animals from a single extinct parent-form, that it is impossible to conceive their origin as poly phyle tic, and that they originated from several independent parent-forms. This unknown common parent-form is the Primitive Amnion Animal (Protam- nion). In external appearance the Protamnion was most probably an intermediate form between the Salamanders and the Lizards. It was probably during the Permian Period that the Protamnion originated ; perhaps at the beginning, perhaps at the close of that period. This we know from the fact that the Amphibia did not attain their full development till the Carboniferous Period, and that toward the close of the Permian the first fossil Reptiles make their appearance — or, at least, fossils (Proterosaurus, Rhopalodori) which must in all probability be referred to lizard-like Reptiles. Among the great and pregnant modifications of the vertebrate organization determined during this period by the develop- ment of the first Amnion Animals from salamander-like Amphibians, the three following are especially important : the total loss of water-breathing gills and modification of the gill-arches into other organs ; the formation of the allantois, or primitive urinary sac; and, finally, the develop- ment of the amnion. The total loss of the respiratory gills must be regarded •as one of the most prominent characters of all Amnion EVOLUTION OF AMNION ANIMALS. 135 Animals. All these, even such as live in the water, e.g.-. whales, respire only air through lungs, never water through gills. While all Amphibians, with very few exceptions, in the young state retain their gills for a longer or shorter period, and breathe through gills for some time (if not always), from this point gill-respiration entirely ceases. Even the Protamnion must have entirely ceased to breathe water. The gill-arches, however, remain, and develop into very different organs (partly rudimentary); into the various parts of the tongue-bone, into certain portions of the jaw apparatus, the organ of hearing, etc. But no trace of gill-leaves, of real respiratory organs on the gill-arches, are ever found in the embryo of Amnion Animals. With this total loss of the gills is probably connected the formation of another organ, which we have already described as occurring in human Ontogeny ; this is the allantois, or primitive urinary sac. (See vol. i. p. 379.) In all probability the urinary bladder of the Dipneusta is to be regarded as the first beginning of the allantois. Even in the American Mud-fish (Lepidosireri) we find an urinary bladder, which grows from the lower wall of the posterior extremity of the intestine, and serves as a receptacle for the renal secretions. This organ has been inherited by the Amphibia, as may be seen in any Frog. But it is only in the three higher Vertebrate classes that the allantois attains a special development ; in these it protrudes at an early period from the body of the embryo, forming a large sac filled with liquid, and traversed by a considerable number of large blood-vessels. This sac also discharges a portion of the nutritive functions. In the higher Mammals and in Man the allantois afterwards forms the placenta. T36 THE EVOLUTION OF MAN. The formation of the amnion and the allantois, together with the total loss of the gills and the exclusive adoption of lung-respiration, are the most important characters by which all Amnion Animals are distinguished from the lower Vertebrates which we have been considering. In addition to these there are a few subordinate characters which are constantly inherited by Amnion Animals, and are altogether wanting in animals without an amnion. One striking em- bryonic character of the Amnion Animals is the great curva- ture of the head and neck of the embryo. In the Anamnia the embryo is from the first either nearly straight, or else the whole body is bent in a sickle-shaped curve corre- sponding to the curvature of the yelk sac, to which the embryo is attached by its ventral surface; but there are no marked angles in the longitudinal axis (Plate VI. Fig. F). In all Amnion Animals, on the contrary, the body is very noticeably bent at an early age, so that the back of the embryo is much arched outwards, the head pressed almost at right angles against the breast, and the tail inclined on to the abdomen. The tail extremity, as it bends inwards, approaches so near to the frontal side of the head, that the two often nearly touch (Plates VI. and VII.). This striking triple curvature of the embryonic body, which has already been considered when we studied the Ontogeny of Man, and in which we distinguished the skull-curve, neck-curve, and tail-curve (vol. i. p. 371), is a characteristic peculiarity common to the embryos of all Reptiles, Birds, and Mammals. But in the formation of many internal organs also, an advance is observable in all the Amnion Animals which ranks them above the highest of the non-amnionate forms. Above all, a partition wall forms MAN AS AN AMNION ANIMAL. 137 within the simple ventricle of the heart, dividing it into a right and a left ventricle. In connection with the complete metamorphosis of the gill-arches, a further development of the organ of hearing takes place. A considerable advance is also noticeable in the development of the brain, the skele- ton, the muscular system, and other parts. Finally, the reconstruction of the kidneys must be regarded as a most important modification. In all the lower Vertebrates as yet considered, we have found the primitive kidneys, which appear very early in the embryos of all higher Vertebrates up to Man, acting as a secretory or urinary apparatus. In Amnion Animals, however, these early primitive kidneys lose their function at an early period of embryonic life, and it is assumed by the permanent " secondary kidneys," which grow out of the terminal portion of the primitive kidney ducts. Looking back at the whole of these characters of Amnion Animals, it is impossible to doub" that all animals of this group, all Reptiles, Birds, and Mammals, had a common origin, and constitute a single main division of kindred forms. To this division belongs our own race. In his whole organization and germ-history Man is a true Amnion Animal, and, in common with all other Amniota, has descended from the Protamnion. Although this whole group originated at the end, or perhaps even in the middle, of the Palaeozoic Epoch, it did not attain its full de- velopment and its full perfection till the Mesozoic Epoch. The two classes of Birds and Mammals then first appeared. Nor did the Reptilian class develop in its full variety until the Mesozoic Epoch, which is, therefore, called the "Age of Reptiles." The unknown and extinct Protamnion, the 138 THE EVOLUTION OF MAN. parent-form of tue entire group, must have been very nearly allied to the Reptiles in its whole organization, even though it cannot be regarded as a true Reptile in the present meaning of the term. Of all known Reptiles, certain Lizards are most nearly allied to the Protamnion ; and in the outward form of its body we may imagine the latter as an intermediate form between the Salamander and the Lizard.156 The Comparative Anatomy and Ontogeny of the Am- nionate group clearly explains its genealogy. The group which directly descended from Protamnion gave rise to two divergent branches. The first of these, which will in future receive our whole attention, forms the Mammalian group. The other branch, which assumed an entirely different course of progressive development, and which is connected with the mammalian branch only as the root, is the compre- hensive group constituted by Reptiles and Birds. The two latter forms may be classed together as Monocondylia, or Sauropsides. The common parent-form of these is an extinct lizard-like Reptile. From this, the Serpents, Croco- diles, Tortoises, Dragons, etc. — in short, all the various forms of the Reptilian group — developed in different directions. The remarkable group formed by the Birds also developed directly from an offshoot of the Reptilian group, as is now definitely proved. Down to a late time the embryos of Reptiles and of Birds are yet identical, and even later they are in some respects surprisingly similar. (See Plate VI. Fig. T and (7.) In their entire organization the resemblance between the two is so great that no anatomist now denies that the Birds originated from Reptiles. The Mammalian line is connected at its roots with the Reptilian line, but GENEALOGY OF AMNION ANIMALS. 139 afterwards diverged entirely from the latter, and developed in an entirely peculiar direction. The highest result of the development of the Mammalian line is Man, the so-called " Crown of Creation," CHAPTER XIX. THE PEDIGEEE OF MAN. IV. FEOM THE PRIMITIVE MAMMAL TO THE APE. The Mammalian Character of Man.— Common Descent of all Mammals from a Single Parent-form (Promammalian). — Bifurcation of the Am nion Animals into Two Main Lines : on the one side, Reptiles and Birds, on the other, Mammals. — Date of the Origin of Mammals : the Trias Period. — The Three Main Groups or Sub-classes of Mammals : their Genealogical Relations. — Sixteenth Ancestral Stage : Cloacal Animals (Monotremata, or Ornithodelphia). — The Extinct Primitive Mammals (Promammalia) and the Extant Beaked Animals (Ornithostoma) . — Seventeenth Ancestral Stage : Pouched Animals (Marsupialia, or Didel- phia). — Extinct and Extant Pouched Animals. — Their Intermediate Position between Monotremes and Placental Animals. — Origin and Structure of Placental Animals (Placentalia, or Monodelphia). — Forma- tion of the Placenta. — The Deciduous Embryonic Membrane (Decidua). — Group of the Indecidua and of the Deciduata. — The Formation of the Decidua (vera, serotina, reflexa) in Man and in Apes. — Eighteenth Stage: Semi-apes (Prosimios). — Nineteenth Stage : Tailed Apes (Meno- cerca). — Twentieth Stage : Man-like Apes (Anthropoides). — Speechless and Speaking Men (Mali. Homines). " A century of anatomical research brings ns back to the conclusion of Linnaeus, the great lawgiver of systematic zoology, that man is a member of the same order as the apes and lemurs. Perhaps no order of mammals presents us with so extraordinary a series of gradations as this, leading ns insensibly from the crown and summit of the animal creation down to creatures from which there is but a step, as it seems, to the lowest, smallest, and least intelligent of the placental mammalia. It is as if nature herself "MAN'S PLACE IN NATURE. 141 had foreseen the arrogance of man, and with Roman severity had provided that his intellect, by its very triumphs, should call into prominence the slaves, admonishing the conqueror that he is but dust." — THOMAS HUXLEY (1863). AMONG those zoological facts which afford us points of support in researches into the pedigree of the human race, the position of Man in the Mammalian class is one of the most important and fundamental. Much as zoologists have long disagreed in their opinions as to Man's particular place in this class, and especially in their ideas of his relation to the most nearty related group, that of the Apes, yet no naturalist has ever doubted that Man is a genuine Mammal in the whole structure and development of his body. Every anatomical museum, every manual of Comparative Anatomy, affords proof that the structure of the human body shares all those peculiarities which are common to all Mammals, and by which the latter are definitely distinguished from all other animals. Now, if we examine this established anatomical fact phylogenetically, and in the light of the Theory of Descent, we arrive immediately at the conclusion that Man is of a common stock with all the other Mammals, and springs from a root common to them. The various characteristics in which all Mammals coincide, and in which they differ from all other animals, are, moreover, of such a kind, that a polyphyletic hypothesis appears in a special degree inad- missible in their case. It is inconceivable that all existing and extinct Mammals have sprung from several different and originally separate root-forms. We are compelled, if we in any way acknowledge the Theory of Evolution, to assume the monophyletic hypothesis, that all Mammals, 142 THE EVOLUTION OF MAN. including Man must be traced from a single common mam- malian parent-form. This long extinct primaeval root-form and its immediate descendants — which differ from each other hardly more than do several species of one genus — we will call Primitive Mammals (Promammalia). As we have already seen, this root-form developed from the ancient parent- form of the Primitive Amnion Animals, in a direction wholly different from that followed by the Keptile group, which afterwards gave rise to the more highly developed class of Birds. The differences which distinguish Mammals on the one side, from Reptiles and Birds on the other, are so important and characteristic, that we may quite safely as- sume a bifurcation of this kind in the vertebrate family tree. Reptiles and Birds — which we classed together as Monocon- dylia, or Sauropsida — coincide entirely, for instance, in the characteristic structure of the skull and brain, which is strikingly dissimilar from that of the same parts in Mam- mals. In Reptiles and Birds, the skull is connected with the first cervical vertebra (the atlas) by a single joint-process (condyle) of the occipital bone ; in Mammals, on the con- trary (as in Amphibians), the condyle is double. In the former, the under jaw is composed of many parts, and is connected with the skull by a peculiar bone of the jaw (the square bone) so as to be movable ; in the latter, on the contrary, the lower jaw consists of but two bone-pieces, which are directly attached to the temporal bone. Again, the skin of the Sauropsida (Reptiles and Birds) is covered with scales or feathers, that of the Mammals with hair. The red blood-cells of the former are nucleated, those of the latter non-nucleated. The eggs of the former are very large, are provided with a large nutritive yelk, and undergo REPTILES AND MAMMALS. 143 discoidal cleavage resulting in a Disc-gastrula ; the eggs of the latter are very small, and their unequal cleavage results in the formation of a Hood-gastrula. Finally, two charac- ters entirely peculiar to Mammals, and by which these are distinguished both from Birds and Reptiles and "from all other animals, are the presence of a complete diaphragm, and of the milk-glands (mammce\ by means of which the new-born young are nourished by the milk of the mother. It is only in Mammals that the diaphragm forms a transverse partition-wall across the body-cavity (coeloma), completely separating the chest from the ventral cavity. (Cf. Plate V. Fig. 16 2.) It is only among Mammals that the mother nourishes the young with her milk ; and the whole class are well named from this. These important facts in Comparative Anatomy and Ontogeny clearly show that the tribe of Amnion Animals (Amniota) bifurcated from the very first into two main diverging lines ; on the one side, the Reptilian line, from which the Birds afterwards developed ; on the other side, the Mammalian line. The same facts also prove as indu- bitably that Man originated from the latter line. For Man, in common with Mammals, shares all the characteristics we have mentioned, and is distinguished by them from all other animals. And, finally, these facts indicate as certainly those advances in vertebrate structure by which one branch of the Primitive Amnion Animals developed into the parent- form of Mammals. The most prominent of these advances were (1) the characteristic modification of the skull and brain ; (2) the formation of a covering of hair ; (3) the com- plete development of the diaphragm ; and (4) the formation of the milk-glands and the adaptation to the suckling of 43 144 THE EVOLUTION OF MAN. the young. Intimately connected with these, other im- portant structural modifications gradually occurred. The period at which these important advances, whicli laid the first foundation of the Mammalian class, took place, may most probably be placed in the first part of the Mesolithic, or Secondary Epoch, in the Triassic Period. For the oldest known fossil remains of Mammals occur in sedimentary rock -strata of the most recent deposits of the Triassic Period, in the upper Keuper. It is possible, indeed, that the parent-forms of Mammals may have appeared earlier (perhaps even at the close of the Palaeo- lithic Epoch, in the Permian Period). But no fossil remains of Mammals belonging to that period ar.e as yet known. Throughout the Mesolithic Epoch, throughout the Triassic, Jurassic, and Calcareous Periods, fossil remains of Mammals are very scarce, and indicate a very limited development of the whole class. During this Mesolithic Epoch, Reptiles play the chief part, and Mammals are of quite secondary importance. It is, however, especially significant and interesting, that all mammalian fossil remains of the Mesozoic Epoch belong to the older and inferior division of Pouched Animals (Marsupialia), a few probably even to the yet older division of the Cloacal Animals (Mono- trema). Among them, no traces of the third and most highly developed division of the Mammals, the Placental Animals, have as yet boen found. The last, to which Man belongs, are much more recent, and their fossil remains do not occur till much later — in the succeeding Csenolithic Epoch ; in the Tertiary Period. This palceontological fact is very significant, because it harmonizes perfectly with that order of the development of Mammals which is un- THE THREE MAMMALIAN GROUPS. 145 mistakably indicated by Comparative Anatomy and Onto- geny. These show that the whole Mammalian class is divisible into three main groups, or sub-classes, corresponding to three successive stages of phylogenetic evolution. These three stages, which consequently represent three important ancestral stages in the human pedigree, were first dis- tinguished in the year 1816 by the celebrated French zoologist, Blainville, who named them, according to the different structure of the female organs of reproduction, Ornithodelpkia, Didelphia, and Monodelphia (&A0i»c, which, being interpreted, is uterus). It is not, however, only in the varied structure of the sexual organs that these three classes differ from one another, but in many other respects also, so that we x;an safely maintain the important phylogenetic statement: The Monodelphia, or Placental Animals, have descended from the Didelphia, or Pouched Animals; and the latter, again, have descended from the Cloacal Animals, or Ornithodelphia. Accordingly we have now to consider, as the sixteenth ancestral stage in the human pedigree, the oldest and lowest main group of Mammals; the sub-class of the Cloacal Animals (Monotremata, or Ornithodelphia}. They are so named in consequence of the cloaca, which they have in common with the other lower Vertebrates. This so-called cloaca is the common excretory channel for the excrement, the urine, and the sexual products (Fig. 327). For, in these Cloacal Animals, the urinary duct and the sexual canals yet open into the posterior parts of the intestine, while in all other Mammals they are wholly separated from the rectum and anus, and open by a special orifice 146 THE EVOLUTION OF MAN. urogenitalis}. The urinary bladder in the Monotremes also opens into the cloaca, and is separate from the two urinaiy ducts (Fig. 327, vo) ; in all other Mammals the Latter open directly into the urinary bladder. The structure of the milk -glands, by means of which all Mammals suckle their new-born young for a time, is also quite peculiar in the Cloacal Animals. In them the milk gland has no nipple which the young animal can suck ; there is only a peculiar sieve-like place in the skin, perforated with holes through which the milk passes out, and from which the young animal has to lick it. For this reason they have also been called Nipple-less Mammals (Amosta*). Again, the brain of the Cloacal Animals has remained at a much lower stage of development than that of any other Mammal The fore-brain, or cerebrum, is so small that it does not overhang the hind-brain, or cerebellum. In the skeleton (Fig. 196), the structure of the shoulder girdle, as well as of other parts, is remarkable, differing entirely from the same part in other Mammals, and resembling rather those of the lower Vertebrates, especially Reptiles and Amphibians. Like the latter, the Cloacal Animals have a well-developed coracoid bone (coracoideum'), a strong bone uniting the shoulder-blade with the breast-bone. In all other Mammals the coracoid bone (as in Man) has degene- rated, has coalesced with the shoulder-blade, and appears only as an insignificant process of the latter. These and many other less striking peculiarities prove beyond doubt that the Cloacal Animals occupy the lowest rank among Mammals, and represent a direct intermediate form between the Protamnia and other Mammals. All these marked Am- phibian characters must have been present in the parent EXTANT CLOACAL ANIMALS. 147 form of the whole vertebrate class, in the Primitive Mammal, by which they must have been inherited from the Primitive Amnion Animals. During the Triassic and Jurassic Periods, the sub-class of the Cloacal Animals seems to have been represented by many Primitive Mammals of very varied form. At present it is represented only by two isolated members, which are grouped together as the Beaked Animal family (Orni- thostoma). Both of these are confined to Australia and the neighbouring island of Van Diemen's Land, or Tasmania ; both are becoming less numerous year by year, and will soon be classed, with all their blood relations, among the extinct animals of our globe. One of these forms passes its life swimming about in rivers, and builds subterranean dwellings on the banks: this is the well-known Duck- billed Platypus (Ornithorhynchus paradoxus) : it is web- footed, has a thick, soft skin, and broad, flat jaws, which very much resemble a duck's bill (Figs. 195, 196). The other form, the Porcupine Ant-eater (Echidna hystrix), much resembles the Ant-eaters, in its mode of life, in the cha- racteristic form of its slender snout, and in the great length of its tongue ; it is covered with prickles, and can roll itself up into a ball like a hedgehog. Neither of these extant Beaked Animals possesses true bony teeth, and, in this point, they resemble the Toothless Mammals (Edentata). The absence of teeth, together with other peculiarities of the Ornithostomata, is probably the result of comparatively recent adaptation. Those extinct Cloacal Animals which embraced the parent-forms of the whole Mammalian class, the Promammalia, must certainly have been provided with a developed set of teeth, inherited from Fishes.157 Some THE EVOLUTION OF MAN. FKI. 195. — The Duck-Lilled Platy pus (Ornithorhynchus paradoxus). FIG. 196. — Skeleton of Platypus. POUCHED ANIMALS. 149 small single molars, found in the uppermost strata of the Keuper formation in England and Wurtemberg, and which are the oldest known vertebrate remains, probably belong to these primaeval Promammalia. These teeth, by their form, indicate species that lived on insects ; the species has been called Microlestes antiquus. Teeth belonging to another closely allied Primitive Mammal (Dromatherium silvestre) have recently been discovered in the North American Trias. On the one hand, the still extant Beaked Animals, and, on the other, the parent-forms of the Pouched Animals (Mar- supialia, or Didelphia), must be regarded as representing two distinct and divergent lines of descent from the Pro- mammalia. This second Mammalian sub-class is very interesting as a perfect link between the two other sub- classes. While the Pouched Animals, on the one side, retain many of the characters of the Cloacal Animals, they also, on the other, possess many placental characters. A few characters are quite peculiar to Pouched Animals alone ; such, for instance, is the structure of the male and female sexual organs, and the form of the lower jaw The dis- tinctive feature of the latter in these Pouched Animals is a peculiar hook-shaped bony process, passing inward hori- zontally from the angle of the lower jaw. As neither Cloacal Animals nor Placental Animals have this process, tais structure is alone sufficient to distinguish the Pouched Animals (Marsupiaiia). Nearly all the known mammalian fossils from the Jurassic and Cretaceous formation are lower jaws. Our whole knowledge of numerous mesolithic mam- malia, the former existence of which would otherwise never have been known, is solely derived from their fossilized I$O THE EVOLUTION OF MAN. lower jaws, no fragment of the rest of their bodies having been reserved. According to the logic usually applied to palaeontology by the " exact " opponents of the theory of evolution, the inference drawn from this fact would be that these Mammals had no bones except lower jaws. The remarkable circumstance is, after all, very easily accounted for. The lower jaw of Mammals being a solid and excep- tionally hard bone, but very loosely attached to the skull, it is easily detached from the carcase as the latter is carried down by some river, and, falling to the bottom, is retained in the mud. The rest of the carcase is carried on further, and is gradually destroyed. As all the mammalian lower jaws found, in England, in the Jurassic strata of Stonesfield and Purbeck, exhibit this peculiar process characteristic of the Pouched Animals (Marsupialia), we may infer, from this palieontological fact, that they belonged to Marsupials. No Placental Animals appear to have existed during the Mesolithic Epoch. At least no fossil remains, undoubtedly belonging to these and dating from that epoch, are known. The extant Pouched Animals, the most generally known of which are the gramnivorous Kangaroos and the carni- vorous Pouched Rats, display very considerable difference in their organization, in the form of their bodies and in size, and in many respects correspond to the several orders of Placental Animals. The great majority of them lira in Australia, in New Holland, and in a few of the Australian and South Asiatic islands; some few species occur in America. On the other hand, there is no longer a single indigenous Pouched Animal on the continents of Asia, of Africa, or of Europe. The case was very different during the Mesolithic, and also during the earlier Cteriolithic Epochs EXTANT POUCHED ANIMALS. 1 5 1 The Neptunian deposits of these epochs in all quarters of the globe, and even in Europe, contain abundant marsupial remains in great variety, some of them being of very large size. From this we may infer that the extant Pouched Animals are but the last remnant of a group which was once much more widely developed, and which was dis- tributed over the whole surface of the globe. During the Tertiary Period, these succumbed in the struggle for life with the stronger Placental Animals, and the survivors were gradually driven back by the latter into their present restricted area From the Comparative Anatomy of the extant Pouched Animals, very important conclusions may be drawn as to their phylogenetic intermediate position between Cloacal Animals and Placental Animals. The incomplete develop- ment of the brain, especially of the fore-brain (cerebrum), the possession of marsupial bones (ossa marsupialia), the simple structure of the allantois (which does not as yet develop a placenta), with many other characters, have been inherited by the Pouched Animals from Cloacal Animals. On the other hand, they have lost the independent coracoid bone (os corucoideum) attached to the shoulder girdle. A more important step consists in the fact that a cloaca is no longer formed ; the cavity of the rectum, together with the anal opening, is separated by a partition wall from the urinary and sexual opening (sinus urogenitalis}. Moreover, all Pouched Animals develop special nipples on the milk-glands, which are sucked by the young after birth. These nipples project into the cavity of a pouch, or marsupium, in the ventral side of the mother. This pouch is supported by a couple of marsupial bones. In it the young, which are 152 THE EVOLUTION OF MAN born in a very imperfect condition, are carried by the mother for a long time ; until, in fact, they are completely developed (Fig. 197). In the large Giant Kangaroo, which FIG. 1U7. — The Crab-eating Pouched Rat (Philander cancrivorus). A female with two young in its pouch. (After Brehm.) THE POUCHED ANIMALS AS ANCESTORS OF MAN. 153 attains the height of a man, the embryo develops in the uterus but for a month ; it is then bora in a very incomplete condition, and attains all its further development in the mother's pouch, where, for about nine months, it remains attached to the milk-glands' All these and other characters 'especially the peculiar structure of the internal and external sexual organs of the male and female) clearly show that the whole sub-class of the Pouched Animals (Marsupialia) are a single group, which originated from the promammalian branch. From a branch of these Pouched Animals (perhaps from several branches) the parent-forms of the higher Mammals, the Placental Animals, .afterwards sprang. Hence we must reckon a whole series of Pouched Animals among the an- cestors of the human race ; and these constitute the seven- teenth stage in the human pedigree.158 The remaining stages of our ancestral line, from the eighteenth to the twenty-second, all belong to the group of Placental Animals (Placentalia). This very highly de- veloped group of Mammals, the third and last, came into the world at a considerably later period. No single known fossil, belonging to any portion of the Secondaiy or Meso- lithic Epoch, can be referred with certainty to a Placental Animal, while we have plenty of placental fossils dating from everj part of the Tertiary or Csenolithic Epoch. From this palseontological fact we may provisionally infer that the third and last main division of Mammals did not develop from the Pouched Animals until the beginning of the Urenolithic Epoch, or, at the earliest, till the close of the Mesolithic Epoch (during the Chalk Period). In our survey of geological formations and periods (pp. 12, 19) we found 154 THE EVOLUTION OF MAN. how comparatively short this whole Tertiary or Granolithic Epoch was. Judging from the relative thicknesses of the various strata-formations we were able to say that this whole period, during which Placental Animals first appeared, and assumed their respective fo'rms, amounted at most to about three per cent, of the entire duration of the organic history of the earth, (Of. p. 18.) All Placental Animals are distinguished from the two lower Mammalian groups already considered, from the Cloacal Animals and Pouched Animals, by many prominent peculiarities. All these characters are present in Man ; a most significant fact. For on the most accurate comparative anatomical and ontogenetical researches, we may base the irrefutable proposition that Man is in every respect a true Placental Animal ; in him are present all those peculiarities in the structure and in the development of the body which distinguish Placental Animals from the lower Mammalian groups, and at the bame time from all other animals. Among these characteristic peculiarities the higher develop- ment of the brain, the organ of the mind, is especially prominent. The fore-brain, or large brain (cerebrum) is much more highly developed in these than in lower animals. The body (corpus callosum), which, like a bridge, connects the two hemispheres of the fore-brain, attains its full development only in Placental Animals ; in the Pouched Animals and Cloacal Animals it exists merely as an insigni- icant rudiment. It is true that in their brain structure the lowest of the Placental Animals yet resemble Pouched Animals very nearly; but within the Placental group we can trace a continuous series of progressive stages in the development of the brain, ascending quite gradually from PLACENTAL ANIMALS. 15? the lowest stage to the very highly developed mind-organ of the Monkey and of Man. (Cf. Chapter XX.) The human mind is but a more highly developed ape-mind. The milk-glands of Placental Animals, as of Marsu- pials, are provided with developed nipples; but the pouch in which the immature young of the latter are carried about and suckled is never present in the former. Nor are the marsupial bones (ossa marsupialia) present in Pla- cental Animals ; these bones, which are embedded in the abdominal wall, and rest on the anterior edge of the pelvis, are common to Pouched Animals and Cloacal Animals, ori- ginating from a partial ossification of the tendons of the inner oblique muscle of the abdomen. It is only in a few beasts of prey that insignificant rudiments of these bones are found. The hook-shaped process of the lower jaw, which characterizes Pouched Animals, is also entirely wanting in Placental Animals. The character, however, which especially distinguishes Placental Animals, and which has justly given its name to the entire sub-class, is the development of the placenta, or vascular cake. We have already spoken of this organ, in describing the development of the allantois in the human embryo (vol. i. p. 882). The urinary sac or allantois, that peculiar bladder which grows out of the posterior portion of the intestinal canal, is, we found, formed at an early stage in the human embryo just as in the germs of all other Amnion Animals. (Cf. Figs. 132-135, vol. i. p.<377-380.) The thin wall of this sac consists of the same two layers, or skins, as the wall of the intestine itself ; internally of the intestinal-glan- dular layer, and externally of the intestinal-fibrous layer. The cavity of the allantois is filled with fluid ; this primi- 156 THE EVOLUTION OF MAN. tive urine must be chiefly the product of the primitive kidneys. The intestinal fibrous layer of the allantois is traversed by large blood-vessels which accomplish the nutri- ment and, especially, the respiration of the embryo ; these are the navel-vessels, or umbilical vessels (vol. i. p. 400). In all Reptiles and Birds the allantois becomes an immense sac, which encloses the embryo with the amnion, and which does not coalesce with the outer covering of the egg (ehoriori). In Cloacal Animals (Monotreinata) arid Pouched Animals (Marsupialia) the allantois is also of this nature. It is only in Placental Animals that the allantois develops into that very peculiar and remarkable formation, called the placenta, or "vascular cake." The nature of the placenta is this : the branches of the blood-vessels which traverse the wall of the allantois, penetrate into the hollow tufts of the chorion, which are inserted into corresponding depressions in the mucous membrane of the maternal uterus. As this mucous membrane is also abundantly supplied with blood- vessels, which conduct the mother's blood into the uterus, and as the partition between these maternal blood-vessels and the embryonic vessels in the chorion-tufts soon becomes extremely thin, a direct exchange of substance is soon de- veloped between the two sets of blood-vessels, which is of the utmost importance for the nutrition of the young Mammal. The maternal blood-vessels do not, however, pass directly (anastomosis) into the blood-vessels of the embryonic chorion-tufts, so that the two kinds of blood simply mix, but the partition between the two sets of vessels becomes so thin, that it permits the passage of the most important food-materials, freed from unnecessary matter (transudation, or diosmosis). The larger the embryo THE PLACENTA. 157 prows in Placental Animals, and the longer it remains in the maternal uterus, the more necessary does it become that special structural arrangements should meet the increased consumption of food. In this point there is a very striking difference between the lower and the higher Mammals. In Cloacal Animals and Pouched Animals, in which the embryo remains for a comparatively brief time in the uterus, and is born in a very immature condition, the circulation as it exists in the yelk-sac and in the allantois suffices for nutrition, as in birds and reptiles. In Placental Animals, on the contrary, in which gestation is very protracted, and the embryo remains much longer in the uterus, there attaining its full development within its investing membranes, a new ap- paratus is required to convey a direct supply of richer nutritive matter ; and this is admirably effected by the development of the placenta. In order rightly to understand and appreciate the for- mation of this placenta and its important modifications in different Placental Animals, we must once more glance at the external coverings of the mammalian egg. The outermost of these was originally, and during the cleavage of the egg and the first formation of the axial portion of the germ, formed by the so-called zona pelludda, and" by the thick albuminous covering deposited externally on the latter (Fig. 19, Fig. 21, z, h, vol. i. p. 178). We called these two outer coverings, which afterwards amalgamate, the prochorion. This prochorion very soon disappears (in man perhaps in the second week of develop- ment), and is replaced by the permanent outer egg-mem- brane, the chorion. The latter, however, is simply the serous membrane, which, as we have already seen, is the 158 THE EVOLUTION OF MAN. product of the outer germ-layer of the germ-membrane vesicle. (See vol. i. p. 401, and Fig. 139, 4, 5, sh, p. 385.) This is at first a very smooth, thin membrane, surrounding the entire egg, as a closed spherical vesicle, and consisting of a single layer of exoderm cells. The chorion, however, be- comes very soon studded with a number of little protuber- ances or tufts (Fig. 139, 5, chz). These fit themselves into indentations in the mucous membrane of the uterus, and thus secure the egg to the wall of the latter. The tufts are, however, not solid, but hollow, like the fingers of a glove. Like the whole chorion, these hollow tufts consist of a thin layer of cells belonging to the horn-plate. They very soon attain an extraordinary development, growing and branching rapidly. In the spaces between them, new FIG. 198. — Egg-coverings of the human embryo (diagram- matic) : m, the thick fleshy wait of the uterus ; plu, placenta, the inner stratum (phi') of which has extended processes between the chorion-tufts (chz) (clif, tufted, chl, smooth cho- rion) ; a, amnion ; ah, amnion cavity ; as, amnion sheath of the navel-cord (passing down into the navel of the embryo, which is not represented here) ; d^111 ^ h ) Whales Cetacea VilUplacentalia morp III. (b) , 7- Pseudo-hoofed , Rock Conie8 Lamnvngia MammaTs'with Chdopkora. ( *^Ph"'t» I'roboscidaa Decidua, with J /"Land Beasts of Carniwra Girdle Placenta 8. Beast8 of prey ) prcy III. Third Sub-class of Decidtmta Carnassia. } Maiine Beasts of Zonoplacentalia \ { prey Pinnipedia fflammala: { y f Fingered animals teptolactyla >'lacental Mammals (I'lacevtalia, 9. Srrni-apes ) Long-tooted Prosimits "\ Flying Lemur \ Lemurs Macrotarsi PtentiplKura Brackytarsi W5 III. (c) Placental Mammals with 10. Gnawing Ani- j ^j^p^^68 Sciuromorpha Myomorpha Hystrick'imorpha Lagomorpha. Decidua, with 11. Toothless f Digging animals Effodientia Discoid Placenta Edentata \ Sloths Bradypoda, Ueciduata DiiCuplacentalia 12. Insect-eaters t With Ccecum Insectivora \ Without Ccecum Menotyphla Lipotyphla i 13. Flying Animals j Flying Foxes Cliiroptera ( Bats Pterncynet Nycterida • . 14. Aoes 4 FLit-nosed Platyrhivai i'iwuB \ Narrow-nosed Apes fatarhina ( 'S3 ) ABLE XXIV. Pedigree of Mammals. Elephants Proboscidea Rork Coiiies ' A .mungia I False-hocfed Chelophora. Flat -nosed Apes Piaty rhinos (Snafotng Animals s * Ecdentia 3pcs Fingered animals Slmiae Leptodacty'a iti.tn Homines Bota Man-like Apes Nycterides Anthropoides \ | Flying Foxea Narrow-nosed Apes Pterocynes Catarhince JFIging ^Lnimalg CLiroptera Sea beasts of prv.y Pinnipedia Lermira ttrnchytarsi I Toothless Edentata Land beasts of prey Carnivora Flving Lemnrs | I'tenopleura 33rasts of $3rrg I Carnassia I Insect-eat era I Long- footed Insectirora Macrotarsi I I Sjoofrt 'animal* Ungulata Semi-apes Prosimice Dccttiuous Sinimab DeciduaU Indeciduata I ilfubivorous Pouched Animals Marsupialia botanojihaga Placentalia Carnivorous Pouched Anim: Is Marsupialia lieaked Animals Ornithostoma Marsupiaiia Pr niitive Mamiriala Pr< /mammalia OTlanrnl Snhnals Monotrema PEDIGREE OF MAN. Primitive Mammala j Beaked Animals. ( (Promaromalia) Primitive Fisties (Selachu) Jawless Animals lostoma) L?*S.fnt* Skull-less Animals (Acrania) TABLE XXV. Pedigree of Apes. Gorilla Chimpanzee Gorilla Engeco Sfriran lamltkr apes fHan Homo sliftr frian Alalus Oran^-Ontang Satyna Gibbon Hylobate* astatic £Han4ikr Aathropoides Silk Apes Hapalida Clutch-tails Labidocerca I I, Flap-tails cerca Tall Apes Seinnopithecus Nose A pee Sea Cat Cercop it hecus Baboons Cynocephalvs 3pes of flrfo Flat-nosed Platyrhinae dn'Irt Sprs Menocerca Qpts of ©III Narrow-nused Catarhinaa Simiae Prosimias CHAPTER XX. THE HISTORY OF THE EVOLUTION OF THE EPIDERMIS AND THE NERVOUS SYSTEM. Auimal and Vegetative Organ-systems — Original Relations of these to the Two Primary Germ-layers. — Sensory Apparatus. — Constituents of Sensory Apparatus: originally only the Exoderm, or Skin-layer; after- wards, the Skin-covering specialized from the Nerve-system. — Double Function of the Skin (as a Covering and as Organ of Touch). — Outer Skin (Epidermis) and Leather-skin (Cerium). — Appendages of the Epi- dermis : Skin-glands (Sweat-glands, Tear-glands, Sebaceous Glands, Milk-glands); Nails and Hair. — The Embryonic Wool-covering. — Hair of the Head and of the Beard. — Influence of Sexual Selection. — Arrange- ment of the Nerve-system. — Motor and Sensory Nerves. — Central Marrow : Brain and Dorsal Marrow.— Constitution of the Human Brain : Large Brain (Cerebrum) and Small Brain (Cerebellum). — Comparative Anatomy of the Central Marrow. — Germ-history of the Medullary-tube. — Separation of the Medullary-tube into Brain and Dorsal Marrow. — Modification of the Simple Brain-bladder into Five Consecutive Brain, bladders : Fore-brain (Large Brain, or Cerebrum), T wixt-brain (" Centre of Sight "), Mid-brain (" Four Bulbs "), Hind-brain (Small Brain, or Cere- bellum), After-brain (Neck Medulla).— Various Formation of the Five Brain-bladders in the various Vertebrate Classes. — Development of the Conductive Marrow, or " Peripherie Nervous System." " Hardly any part of the bodily frame, then, could be fonnd better salculated to illustrate the truth that the structural differences between Man and the highest Ape are of less value than those between the highest THE DEVELOPMENT OF THE ORGANS. 191 and the lower Apes, than the hand or the foot, and yet, perhaps, there is one organ which enforces the same conclusion in a still more striking manner — and that is the brain."— Man's Place in Nature, p. 94 (1863). "As if to demonstrate, by a striking example, the impossibility of erecting any cerebral barrier between Man and the Apes, Nature has provided us, in the latter animals, with an almost complete series of gra- dations, from brains little higher than that of a Rodent to brains little lower than that of Man."— Hid. p. 96. OUR investigations, up to the present, have shown us how the whole human body has developed from an entirely simple beginning, from a single simple cell. The whole human race, as well as the individual man, owes its origin to a simple cell. The one-celled parent-form of the former is, even yet, reproduced in the one-celled germ-form of the latter. In conclusion, we must glance at the evolutionary history of the separate parts which constitute the human body. In this matter, I must, of course, restrict myself to the most general and important outlines ; for a detailed study of the evolutionary history of the separate organs and tissues would occupy too much space, and would demand a greater extent of anatomical knowledge than the generality of my readers are likely to possess. In considering the develop- ment of the organs, and of their functions, we will retain the method previously employed, except that we will consider the germ-history and the tribal history of the various parts of the body in common. In the history of the evolution of the human body as a whole we have found that Phylogeny everywhere serves to throw light on the obscure course of Ontogeny, and that the clew afforded by phylogenetic con- tinuity alone enables us to find our way through the labyrinth of ontogenetic facts. We shall experience exactly the same fact in the history of the development of the separate 46 £Q2 THE EVOLUTION OF MAN. organs ; but I shall be compelled to explain the ontogenetio and the phylogenetic origin of the organs simultaneously; for the further we penetrate into the details of organic development, and the more minutely we study the origin of the separate parts, the more clearly do we see how inseparably the evolution of the germ is connected with that of the tribe. The Ontogeny of the organs is intelligible and explicable only through their Phylogeny ; just as the germ-history of the entire body (the "person") is rendered intelligible only by the history of the tribe. Each germ- form is determined by a corresponding ancestral form. This is as true of the parts as of the whole. In endeavouring, with the help of this fundamental law of Biogeny, to obtain a general view of the main features in the development of the separate organs of man, we must, in the first place, consider the animal, and then the vegetative organ-systems of the body. The first main group of organs, the animal organ-systems, is formed by the sensory apparatus, together with the motor apparatus. To the former belong the skin-covering, the nervous system, and the organs of the senses. The motor apparatus consists of the passive organs of movement (the skeleton) and the active organs (the muscles). The second main group of organs, the vegetative organ-system, is formed by the nutritive and the repro- ductive apparatus. To the nutritive apparatus belongs especially the intestinal canal with all its appendages, together with the vascular and renal systems. The repro- ductive apparatus includes the various sexual organs (the germ-glands, germ-ducts, organs of copulation, etc.). In earlier chapters (IX. and X.) it has been stated that the animal organ-systems (the instruments of sensation and ANIMAL AND VEGETATIVE OEGAN-SYSTEM& 193 of movement) proceed especially from the outer primary germ-layer, from, the skin-layer. The vegetative organ- systems, on the other hand (the instruments of nutrition and reproduction), proceed principally from the inner primary germ-layer, from the intestinal layer. This radical contrast between the animal and the vegetative spheres of the body is, it is true, by no means absolute either in man or in the higher animals ; on the contrary, many separate parts of the animal apparatus (e.g., the intestinal nerve, or sympathetic) originated from cells which have proceeded from the ento- derm ; and, on the other hand, a large part of the vegetative apparatus (e.g., the mouth-cavity, and probably the greater part of the urinary and sexual organs) is formed of cells which are originally derived from the exoderm. Moreover, in the bodies of all the more highly developed animals, the most heterogeneous parts are so intermixed and blended that it is often extremely difficult to assign its true source to each one of the constituent parts. But, on the whole, we may assume as a certain and important fact, that in Man, and in all high animals, the greater part of the animal organs must be referred to the skin-layer, or exoderm ; the greater part of the vegetative organs to the intestinal layer, or entoderm. For this reason, Baer called the former the animal germ-layer, the latter, the vegetative germ-layer (Of. vol. i. pp. 53 and 196). Of course, in making this important assumption, we pre-suppose the correctness of Baer's view, according to which the skin-fibrous layer (the "flesh stratum " of Baer) must have originated (phylogenetically) from the exoderm, and, on the other hand, the intestinal- fibrous layer (Baer's "vascular layer") from the entoderm. This influential view, which is yet much disputed, is, we ( 194 ) TABLE XXVI. Systematic Surrey of the Organ- Systems of the Human Body. (N.B. — The origin of the separate organs from the four secondary germ, layers is indicated by the Roman numerals (I.-IV.) : I. Skin-sensory layer) II. Skin-fibrous layer; III. Intestinal-fibrous layer; IV. Intestinal-gland, ular layer.) fl. Skin-covering Outer skin Epidermis, I. (J.ierma) Leather skin Cerium, II. a. Central nerve- < Brain system I Spinal marrow Encephalon i T Medulla spinalis / L i A. Brain nerves Nervi cerebrates, I. + 11 > Sensory Apparatus system intestinal nervea iNervi spinalcs, II. Sympatheticus,ll.+ III \ Sensorium 1 Organ of touch (skin) Org tactus \ Org. gustus Oig. olfactus }•!. +11 Org. visus 1 j ( Organ of hearing (ear) Org. auditus ) a & /*. Muscle j system ( Sk|n mnscle9 Mnsculi cutane \ < Motive organs) \Ske M. skeleti Apparatus 1 6 skeleton-system I Vertebral column Locemotorium [ (passive motive^ Skull Vertebrarium Cranium V organs) ( Limb skeleton Sk. extremitatum / ft. Intestinal system ( Digestive organ (Coster) \ Respiratory organ 0. digestlva )1TT , IV 0.resi»irutoria)ln- + IV- aj €. (Body cavity Lymph vessels Coeloma, II. + III. Vasa lympha- J Nutritive . Apparatus Autritoritm (Organa circa- •{ UitionU") Blood vessels I Heart tica 5.II. + III. V. sanRiiifera ) Cor, III. ft 9. Renal system j R!f "^v* ducts ((Organa urinaria) ^^y bUo4r Renes \ T ..... TI Ureteroa ) L CO + " Urocystis, III. + IV. e Sexual glands (I. Ovary) Gonades (I,Ovaria)III. + IV.(?) § D. (11. Testes) Sexual ducts (11. Tesus)!. + 11. (.-) Gonophori , VEGET Reproductive Apparatus Pnpagatorium 10 Sexual organs ^ (I- Oviduct) (1. Oviduc- I tus) Jl. (?)+!! (11. Sperma- j due tus) / Copulatory organs Copulativa 1 (I. Sheath) (I. Vagina) \ I + II. yi. i'eot»> (II. Penis) > THE SENSORY APPARATUS. 195 think, securely founded on the Gastrula— that most impor- tant of all the germ-forms of the animal kingdom — which we find recurs in similar form in the germ-history of the most different classes of animals. This significant germ- foiTQ points unmistakably to a parent-form (the Gastrsea) common to all animals, the Protozoa alone excepted ; in this long extinct parent-form the entire body of the animal consisted throughout life of the two primary germ-layers, as is yet the case, for a short time, in the Gastrula. In the Gastrsea the simple skin-layer did actually represent all the animal organs and functions, and the simple intestinal layer, on the other hand, all the vegetative organs and functions ; potentially, this is even yet the case in the Gastrula. In studying the development of the first important part of the animal sphere, the sensory apparatus, or sen- sorium, we shall now find how well adapted this Gastrsea Theory is to explain, not only in a morphological but in a physiological sense, the most important facts in the history of evolution. This sensory apparatus consists of two very distinct parts, having, apparently, nothing in common : in the first place, the external skin-covering (Derma), together with its appendages, the hair, nails, sweat-glands, etc. ; and, secondly, the nervous system, situated internally. The latter includes the central nervous system (brain and spinal chord), the peripheric brain-nerves and medullary nerves, and finally, the organs of sense. In the fully developed vertebrate body these two main constituents of the sensorium are entirely separate ; the skin lying entirely cxternallv on the body, while the central nervous system is within, and quite separate from the former. The two are connected merely by a portion of the peripheric nerve- 196 THE EVOLUTION OF MAN. system and of the sense-organs. And yet, as we already know from the germ-history of man, the latter is developed from the former. Those organs of our body which discharge the highest and most perfect functions of animal life — those of sensation, volition, thought — in a word, the organs of the psyche, of mental life — arise from the external skin- covering. This remarkable fact, considered in itself alone, seems so wonderful, inexplicable, and paradoxical, that the truth of the fact was simply long denied. The most trustworthy embryo- logical observations were met with the erroneous statement that the central nerve-system develops, not from the outer germ-layer, but from a special cell-layer lying underneath this. The ontogenetic fact would not, however, yield ; and, now that Phylogeny has thrown light on the subject, the fact seems perfectly natural and necessary. When we reflect on the historic evolution of mind and sense activities, we must necessarily conceive the cells, which accomplish these, as originally situated on the outer surface of the animal-body. Such externally placed elementary organs could alone directly receive and deal with impressions from the outer world. Afterwards, under the influence of natural selection, the complex cell-masses which had become especially " sensitive " gradually withdrew into the shelter of the interior of the body, and there laid the first foundations of a central nervous organ. As differentiation advanced, the distance and distinction between the external skin- covering and the central nervous system detached from this, became continually greater, and finally the two were per- manently connected merely by the conductive peripheric nerves. DERMIC ORIGIN OF THE SENSORY ORGANS. 197 This view is fully confirmed by the results of Comparative Anatomy. Comparative Anatomy shows that many lower animals possess no nervous system, although, in common with higher animals, they exercise the functions of sensation, volition, and thought. In the Primitive Animals (Protozoa), which do not even form germ-layers, of course the nervous system, like the skin-covering, is wanting. Even in the second main division of the animal kingdom — in the Metazoa or Intestinal Animals — there is at first no nervous system. The functions of these are performed by the simple cell- layer of the exoderm, which the lower Intestinal Animals have inherited directly from the Gastraea (Fig. 209, e). This is the case in the lowest Plant Animals (Zoophyta), the Gas- tra3ads, Sponges, and the lowest Hydroid Polyps, which are but little higher than the Gastrseads. Just its all the vege- tative functions of these are performed by th>3 simple intes- tinal layer, so all the animal functions are discharged by the equally simple skin-layer. The simple cell stratum of the exoderm is, in these, skin-covering, motive apparatus, and nervous system simultaneously. Most probably the nervous system was also wanting in a large proportion of those Primitive Worms (Archelminthes) which were developed directly from the Gastrseads. Even those Primitive Worms in which the two primary germ-layers, had already split into the four secondary germ-layers (Plate V. Fig. 10), seem not to have possessed a nervous system distinct from the skin. The skin-sensory layer must, even in these long-extinct Worms, have been at once skin-covering and nerve-system. But already in the Flat Worms (Platel- minthes), and especially in the Gliding Worms (Turbellaria) which of all existing forms approach nearest to the Primitive 198 THE EVOLUTION OF MAN. Worms, we find an independent nerve-system, distinct and separate from the outer skin-covering. This is the " upper FIG. 209. — Gastrnla of Gastrophyseina (Gastrsead-class). FIG. 210. — Transverse section through an embryonic Earth-worm : hf, skin-sensory layer; hm, skin-fibroas layer; p, Marrow- plates. Fig. 223, with sixteen pairs of primitive vertebrae. The brain consists of five bladders: v, fore-brain ; z, twixt-brain; m. mid-brain ; h, hind-brain ; n, after-brain, o, Eye. vesicles ; gr, ear. vesicles ; c, heart 5 dv, yelk-veins ; mp, marrow-plate, uiv, primitive vertebrae. DEVELOPMENT OF THE BRAIN. 2 19 quite internally, the upper edges of the primitive vertebral plates, which penetrate, from right and left, in between the horn-plate and the medullary tube, uniting above the latter, and thus completely embedding it in a closed canal. As Gegenbaur most aptly remarks, " This gradual embedding in the interior of the body must be regarded as an incident acquired in connection with progressive differentiation, and with the consequent higher capacity, by which the most important organ of the system is secured in its interior." To every thoughtful and unprejudiced man it must appear an extremely important and pregnant fact, that our mental organ, like that of all other Skulled Animals (Cra- niota), commences in the same way and in exactly the same simple form in which this organ remains for life in the lowest Vertebrate, the Amphioxus (vol. i. p. 420, Fig. 151; Plate XI. Fig. 15, m). In the Cyclostomi, that is, in the stage above the Acrania, the anterior extremity of the cylindrical medullary tube begins to extend, at an early period, in the form of a pear-shaped bladder, which is the first distinct rudiment of a brain (Plate XI. Fig. 16, m-,). For the central medulla of Vertebrates thus first distinctly differentiates into its two main sections, the brain (m^) and the spinal marrow (m2). The first faint indication of this important differentiation is discoverable in the Amphioxus, perhaps even in the Ascidian larva (Plate X. Fig. 5). The simple bladder-like form of the brain, which is retained for a considerable time in the Cyclostomi, also appears at first in all higher Vertebrates (Fig. 221, hfy. In the latter, however, it soon disappears, in consequence of the separation of the simple brain-bladder, by transverse contractions of its circumference, into several consecutive 22O THE EVOLUTION OF MAN. parts. Two of these contractions first appear, and con- sequently the brain forms three consecutive bladders (Fig. 3 *• '• FIGS. 224-226. —Central mar row of human embryo in the seventh week, two cm. long. (After Koelliker.) Fig. 226, view of the whole embryo from the dorsal side ; the brain and dorsal marrow laid bare. Fig. 225, the brain and npper part of the dorsal marrow from the left side. Fig. 224, the brain from above : v, fore-brain ; z, twixt-brain ; m mid-brain ; h, hind-brain ; n, after-brain. 222, v, m, fi). The first and third of these three primitive bladders then again separate by transverse contractions, each into two parts, and thus five consecutive bladder-like divisions are formed (Fig. 223 : cf. also Plate V. Figs. 13-16 ; Plates VI. and VII., second cross-line). These five fundamental brain-bladders, which re-occur in the same form in the embryos of all the Skulled Animals (Craniota), were first clearly recognized by Baer, who understood their true importance and distinguished them, according to their rela- tive positions, by very appropriate names, which are still in general use : I., fore-brain (v) ; II., twixt-brain (0); IIL, mid- brain (m) ; IV., hind-brain (A) , and V., after-brain (n). In all Skulled Animals, from the Cyclostomi to Man, the same parts, although in very various forms, develop from these five original brain-bladders. The first bladder, the fore-brain (protopsyche, v), forms by far the largest part of the so-called " great brain " (cerebrum) ; it forms the two great hemispheres, the olfactory lobes, the streaked bodies (corpora striata), and the cross-piece (corpus callosum), together with the " arch " (fornix). From the second THE BRAIN IN SKULLED ANIMALS. 221 bladder, the twixt-brain (deutopsyche, z,} proceed primarily the " centres of sight " and the other parts which surround the so-called " third brain-ventricle," also the " funnel " (infundibidum), the " cone " (conarium), etc. The third bladder, the mid-brain (mesopsyche, m), furnishes the small group of the " four bulbs," together with the " aqueduct of Sylvius." From the fourth bladder, the hind-brain (meta- psyche, h}, the greater part of the so-called " little brain " (cerebellum) develops; the central "worm" (vermis), and the two lateral "small hemispheres." The fifth bladder, finally, the after-brain (epipsyche, TI), forms the neck- marrow, or the "elongated marrow" (medulla oblongata), together with the rhomboid groove, the pyramids, olives, etc. The very highest importance must certainly be ascribed to the fact, seen in Comparative Anatomy and Ontogeny, that the brain is originally formed in exactly the same way in the embryos of all Skulled Animals (Craniota), from the lowest Cyclostomi and Fishes, to Apes and Man. In all, the first rudiment of the brain is a simple bladder-like expansion at the anterior extremity of the medullary tube. In all, the five bladders are formed from this simple bladder- like expansion, and in all, these five primitive brain- bladders develop into the permanent brain, with its many complex anatomical arrangements, which afterwards appear in such extremely diverse forms in the various vertebrate classes. On comparing the mature brain of a Fish, an Amphibian, a Reptile, a Bird, and a Mammal, it is hardly conceivable that the several parts of these forms, so ex- tremely different, both internally and externally, may be traced back to one common condition. And yet, all these various brains of Craniota have originated from exactly the 222 THE EVOLUTION OF MAN. same rudimentary form. We need only compare the em- bryos of these various classes of animals at corresponding stages of development, in order to assure ourselves of this fundamental fact. (Cf. Plates VI. and VII., second cross- line.) JL FIG. 227.— Brains of three embryonic Skulled Animals in vertical longi- tudinal sections: A, of a Shark (Heptanchus) ; B, of a Snake (Coluber); C, of a Goat (Capra) ; a, fore-brain ; b, twixt- brain ; c, mid-brain ; d, hind-brain ; e, after-brain; s, primitive fissure of the brain. (After Gegenbanr.) FIG. 228. — Brain of a Shark (ScyUium) from the dorsal side : g, fore- brain ; h, olfactory bulbs of the fore-brain, which send the large olfactory nerves to the large nose capsules (o) ; d, twixt-brain ; 6, mid-brain (behind it, the insignificant rudiment of the hind-brain) ; a, after-brain. (After Bnsch.) FIG. 229. — Brain and dorsal marrow of a Frog : A, from the dorsal side ; B, from the ventral side ; a, olfactory bulbs, in front of the fore-brain (?>) ; i, funnel at the base of the twixt-brain ; c, mid-brain ; d, hind-brain ; s, rhomboid groove in the after-brain ; m, dorsal marrow (very short in the frog) ; m', root-processes of the spinal nerves ; t, fibre at the end of the dorsal marrow. (After Gegenbaur.) COMPARATIVE VIEW OF BRAIN DEVELOPMENT. 223 A thorough comparison of the corresponding stages of development in the brain in the various Skulled Animals (Craniota) is very instructive. If it is applied to the whole series of skulled classes, the following extremely interest- ing facts soon become evident : in the Cyclostomi (Myxi • noides and Petromyzontes), which, as we have seen, are the lowest and earliest Skulled Animals, the whole brain remains for life at a very low and primitive stage of development, through which the embryos of the other Skulled Animals pass very rapidly; the five original sections of the brain are visible throughout life in an almost unmodified form. But even in Fishes, an essential and important transformation of the five bladders takes place ; it is evidently from the brain of the Primitive Fishes (Selachii ; Fig. 228), that, on the one side, the brain of the other Fishes, and on the other, the brain of the Amphibians and also of the higher Vertebrates, must.be traced. In Fishes and Amphibians (Fig. 229), the central part, the mid-brain, and also the fifth section, the after-brain, are especially developed, while the first, second, and fourth sections remain far behind. In the higher Vertebrates, the exact reverse is the case, for in these the first and fourth sections, the fore and hind brains, develop pre-eminently ; on the other hand, the mid-brain remains very small, and the after-brain is also much smaller. The greater part of the " four-bulbs " is covered by the large brain (cerebrum) and the after-brain by the small brain (cerebellum). Even among the higher Vertebrates themselves, numerous grada- tions occur in the structure of the brain. From the Am- phibians upward, the brain, and with it the mental life, develops in two different directions, of which the one is 48 224 THE EVOLUTION OF MAI*. carried out in Reptiles and Birds, the other in Mammals. The latter are especially distinguished by the very charac- teristic development of the first section, the fore-brain. In FIG. 230.— Brain of Rabbit: A, from the dorsal side; P, from the Yentral side; lo, olfictory lobules; L, fore-brain ; h, hypophysis at the base of the twixt-brain ; III., mid-brain ; IV., hind-brain; V., after-brain; 2, optic nerve ; 3, motor nerve of the eye ; 5-8, fifth to eighth nerves of the brain. In A, the upper surface of the right large hemisphere (I.) is removed, so that the streaked bodies (corpora, striata) can be seen in its side chamber (ventriculua lateralis). (After Gegenbaur.) Mammals alone (Fig. 230) does this "great brain" develop to such an extent, that it eventually covers all the other parts of the brain from above. There are also remarkable differences in the relative positions of the brain-bladdera In the lower Skulled Animals the five brain-bladders are at first situated one behind the other in the same plane. If the brain is re- garded from the side, a straight line may be drawn through all the five bladders. But in the three higher vertebrate classes, in the Aomion Animals (Amniotd), a noticeable curving of the rudimentary brain takes place, simultaneously BRAIN CURVATURE. 22 5 with the head and neck curving of the whole body, owing to the fact that the whole upper dorsal surface of the brain grows much faster than the lower ventral surface. The result is that the brain is so curved that its parts are after- wards situated thus : the fore-brain lies quite in front and below, the twixt-brain somewhat higher and over it, while the mid-brain lies highest of all and projects furthest for- ward; the hind-brain is situated lower, the after-brain yet further back and below. This disposition occurs only in the three classes of the Amniota, in Reptiles, Birds, and Mammals. (Of. Plates I, VI., and VII) Though, in general features of growth, the brains of Mammals correspond with those of Birds and Reptiles, yet striking differences very soon appear between the two. In Birds and Reptiles (Plate VI. Figs. H and 0), the mid- brain (m) and the central part of the hind-brain develop considerably. In Mammals, on the other hand, these parts remain small, and instead, the fore-brain begins to grow so rapidly that it covers the other bladders from in front and above. As it constantly grows further back, it even- tually covers the whole of the rest of the brain above, and also encloses the central part from the sides. This process is of the greatest importance, because this fore-brain is the organ of the higher mental activities, — because in it are accomplished those functions of the nerve-cells, the sum of which is generally designated as the mind, or the "spirit" in the narrower sense. The highest activities of the animal body, the wonderful manifestations of consciousness, the complex phenomena of the activities of thought, have their seat in the fore-brain. It is possible to remove the great hemispheres of a Mammal, piece by piece, without killing 226 THE EVOLUTION OF MAN. the animal, thus proving that the higher mental activities, consciousness and thought, conscious volition and sensation, may be destroyed one by one, and finally entirely anni- hilated. If the animal thus treated is artificially fed, it may be kept alive for a long time ; for the nourishment of the entire body, digestion, respiration, the circulation of the blood, secretion, in short, the vegetative functions, are in no way destroyed by this destruction of the most important mental organs. Conscious sensation and voluntary motion, the capacity for thought and the combination of the various higher mental activities, have alone been lost. This fore-brain, the source of all these most wonderful nervous activities, reaches that high degree of perfection only in the higher Placental Animals (Placentalia) ; a fact which explains very clearly why the higher Mammals so far excel the lower in intellectual capacity. While the "mind" of the lower Placental Animals does not exceed that of Birds and Reptiles, we find among the higher Placentalia an uninter- rupted gradation up to Apes and Man. Accordingly, their anterior brains show surprising differences in the degree of perfection. In the lower Mammals, the surface of the great hemispheres (the most important part) is entirely smooth and even. The fore-brain, too, remains so small that it does not even cover the mid-brain above (Fig. 230). One stage higher, and this latter is indeed entirely covered by the excessive growth of the fore-brain ; but the hind-brain remains free and uncovered. At last, in Apes and in Man, the fore-brain covers the hind-brain also. A similar gradual advance may also be traced in the development of the peculiar furrows and protuberances which are so charac- teristically prominent on the surface of the large brain CONVOLUTIONS OF BRAIN. 227 (cerebrum) of higher Mammals (Figs. 219, 220). If the brains of the various mammalian groups are compared with reference to these convolutions and furrows, it appears that theii gradual development is entirely proportionate with the development of the higher intellectual activities. Much attention has recently been devoted to this particular branch of the Anatomy of the brain, and very striking individual differences have been found even within the human race. In all human individuals distinguished by peculiar ability and great intellect, these swellings and furrows on the surface of the great hemispheres exhibit a much greater development than in common average men; while in the latter, again, they are more developed than in Cretins and others of unusually feeble intellect. There are also similar gradations in the internal structure of the fore- brain in Mammals. The great cross-piece (corpus callosum), especially, the bridge between the two great hemispheres, is developed only in Placental Animals. Other arrange- ments, for example, in the structure of the lateral cavities, which seem primarily to be peculiar to Men as such, re- appear only in the higher species of Apes. It was long believed that Man had some entirely peculiar organs in the great brain (cerebrum), which are wanting in all other animals. But close comparison has shown that this is not the case, but that rather the characteristic qualities of the human brain exist in a rudimentary state even in the lower Apes, and are developed to a greater or less degree in the higher Apes. Huxley, in his important and much -quoted book, " Evidence as to Man's Place in Nature " (1863), has shown, most convincingly, that within the Ape-series the differences in the formation of the brain are greater between the 228 THE EVOLUTION OF MAN. higher and lower Apes than between the higher Apes and Man. This statement is, indeed, equally true of all the other parts of the body. But the fact that it is true of the central marrow is especially important. This does not become fully evident unless these morphological facts are considered in connection with the corresponding physio- logical phenomena; until we consider that every mental activity requires for its complete and normal exercise the complete and normal condition of the corresponding brain- structure. The extremely complex and perfect active phenomena within the nerve -cells, summed up in the word "mental life," can no more exist without their organs in the vertebrates, including man, than can the circulation of the blood without a heart or blood. As, however, the central marrow of Man has developed from the same medullary tube as in all other Vertebrates, so also must the mental life of Man have had the same origin. All this is of course true of the conductive marrow, or the so-called "peripheric nervous system." This consists of the sensitive nervous fibres which convey the impressions of sensation from the skin and the organs of the senses in a centripetal direction to the central marrow ; as well as of the motor nervous fibres, which, reversely, convey the movements of volition from the central marrow, in a cen- trifugal direction to the muscles. By far the greater part of these peripheric conductive nerves originates from the skin-fibrous layer, by peculiar local differentiation of the rows of cells into the respective organs. The membranous coverings and blood-vessels of the central marrow are identical in origin with the greater part of the conductive marrow; these membranous coverings ORIGIN OF THE FUNCTIONS OF THE BRAIN. 22Q are the inner membrane (pia mater\ the central membrane (meninx arachnoides), and the outer membrane (dura mater). All these parts are developed from the skin-fibrous layer. TABLE XXVII. SYSTEMATIC SURVEY OF THE MOST IMPORTANT PERIODS IN THE PHYLOOENY or THE HUMAN SKIN-COVERINGS. I. First Period : Skin of Gastrceads. The entire skin-covering (inclnding the nervous system, not yet differ- entiated from it) consists of one simple layer of ciliated cells (exoderm, or primary skin-layer); as it is at the present day in the gastrula of the Amphioxus. II. Second Period: Skin of Primitive Worms. The simple exoderm of the Gastraead has thickened and split into two distinct layers, or secondary germ-layers : the skin-sensory layer (rudiment of the horn-plate and nerve-system) and the skin-fibrous layer (rudiment of the leather skin (corium), the muscle-plate and the skeleton-plate. The skin is potentially both covering and miiid. III. Third Period : Skin of Chordonia. The skin-aensory layer has differentiated into the horn-plate (epidermis), and the central marrow (upper throat ganglia) separated from it ; the latter elongates into a medullary tube. The skin-fibrous layer has differentiated into the leather plate (corium) and, below this, the skin-muscular pouch (as in all Worms). IV. Fourth Period : Skin of Acrania, The horn-plate yet forms a simple epidermis. The leather-plate is fully differentiated f rum the muscle and skeleton plates. 230 THE EVOLUTION OF MAN. V. Fifth Period : STcin of Cyclostoma. The outer-skin remains a simple, soft mncons layer of cells, but forma one-celled glands (cup-cells). The leather-skin (corium) differentiates into cutis and aub-cutis. VI. Sixth Period : Skin of Primitive Fishes. The outer skin is still simple. The leather skin forms placoid scales or small bony tablets, as in the Selachii. VII. Seventh Period : Skin of Amphibia. The outer skin differentiates into an outer horn-layer, and au inner mucoua layer. The ends of the toes are covered with horny sheaths (first rudiments of claws or nails). VIII. Eiyhth Period : Skin of Mammals. The outer skin forms the appendages characteristic of Mammals only j hair, and sebaceous, sweat, and milk glands. TABLE XXVIII. SYSTEMATIC SURVEY OF THE MOST IMPORTANT PERIODS IN THE PHYLOGEKT OF THE HUMAN NERVOUS SYSTEM. I. First Period : Medulla of Gastrceads. Tho nerve system is not yet distinct from the skin, and, together with the latter, is represented by the simple cell-stratnm of the exodenr., or primary skin-layer; as it is at the present day in the gastrula of the Amphioxus. II. Second Period : Medulla of Primitive Worms. The central nerve system is yet, at first, a part of the skin-sensory layer, and afterwards consists of a throat medulla, a simple nerve-ganglion lying •above the throat ; as it is now in the lower Worms : the upper throat iganglion. SURVEY OF HUMAN NERVOUS SYSTEM. 23! ITT, Third Period : Medulla of Chordonia. The central nerve system consists of a simple medullary tube, an elongation of the upper throat ganglion, which is separated from the intes- tine by a notochord (chorda dorsalis). IV. Fourth Period : Medulla of Acrania. The simple medullary tube differentiates into two parts : a head, and a dorsal part. The head medulla resembles a small, pear-shaped, simple swelling (the primitive brain, or first rudiment of the brain) on the anterior extremity of the long cylindrical spinal marrow. V. Fifth Period : Medulla of Cyclostoma. The simple, bladder-like rudiment of the brain divides into five con- eecntive brain-bladders of simple structure. VI. Sixth Period : Medulla of Primitive Fishes. The five brain-bladders differentiate into a form similar to that now permanently retained by the Selachii. VII. Seventh Period : Medulla of Amphibia. The differentiation of the five brain-bladders progresses to that structure which is now characteristic of the brain in Amphibia. VIII. Eighth Period : Medulla of Mammals. The brain attains the characteristic peculiarities distinctive of Mammals. The following may be distinguished as subordinate stages of development ; 1, the brain of Monotremes ; 2, the brain of Marsupials ; 3, the brain of Semi-apes ; 4, the brain of Apes ; 5, the brain of Man-like Apes ; 6, the brain of Ape-men ; and 7, the brain of Man. C 232 TABLE XXIX. Systematic Survey of the Evolution of the Skin-covering and Nerve System. XXIX. A. Survey of the Evolution of tke Skin-covering. Skin (D rma, or Integvinentum) 1 Horn-layer of the outer / Hair skin Nails (Stratum cornmrn) | Sweat glands Mucous layer of the 1 Tear glands outer skin Sebaceous glands (Stratum mueosum) ( Milk glands Leather-skin pillae of taste and nerves of the Uather skin XXIX. B. Survey of the Evolution of the Central Marrow. / ( Great hemispheres I-'emirplirrrce cerebri 1 Olfactory lobules lobi olfactorii Lateral chambers Ventriculi lateralet Streaked bodies Corrora striata Arch Fornix Cross piece Corpus callosum Central Marrow, (•Centre of sipht II. Twist-brain J Thbirr?i_cbamber °f toe Thalami nptici Yentriculus tertius Central Nerve (Deutopsyche) *•, pine'al ^ [Funnel Conariim tnfundibul-un. System (I'tyche. or Medulla Central**). TTT MM Knin fFonrbutbs III. Mi^ora'n -1 Aqueduct of Sylvius Corpus bigeminui* Aqitfrductus Fylvii Pedunculi cerebri Product of the Skin-sensory TV Hi A h 1 ("Small hemispheres Hemitph(erse shells," spongy bony structures, over which the olfactory mucous membrane spreads. The first brain nerve, the olfactory nerve, with its delicate branches, passes FIGS. 238, 239.— Upper part of the body of a human embryo (16 mm. in length) during the sixth week : Fig. 238, from the left side ; Fig. 239, from the front. The origin of the nose in two lateral halves, originally separate, is still plainly visible. The nose and upper lip are disproportionately great in comparison with the rest of the face, especially with the lower lip. (After Kollman.) FIG. 240. — Face of a human embryo of eight weeks. (After Ecker.) Cf. Frontispiece, Plate I. Fig. Mi— Mm, 248 THE EVOLUTION OF MAN. from the large brain through the roof of both nasal cavities into the cavities, and extends over the olfactory mucous membrane. At the same time, by inversion of the nasal mucous membrane, the minor cavities of the nose, which are afterwards filled with air, and which communicate directly with the two nasal cavities, arise (frontal cavities, cavities of the sphenoid bone, jaw cavities, etc.). In this special stage of development they occur only in Mammals.171 The external nose is not developed until long after all these essential internal parts of the olfactory organ have been formed. The first trace in the human embryo appears at the end of the second month (Figs. 238-240). Any human embryo during the first month shows that originally there is no trace of the external nose. It afterwards grows out from the anterior nasal portion of the primitive skull. The form of nose which is characteristic of Man does not appear till a period far later. Much stress is usually laid on the shape of the external nose as a noble organ, occurring exclusively in Man; but there are Apes which have very human noses, as, for instance, the Nosed Ape already mentioned. On the other hand, the external nose, the fine shape of which is so extremely important to the beauty of the facial structure, possesses in certain inferior races of Man a shape anything but beautiful. In most Apes the external structure of the nose remains undeveloped. Especially remarkable is the important fact already cited that it is only in the Apes of the Old World, in the Cata- rhines, that the nasal partition wall (septum) remains as small as it is in Man; in Apes of the New World it widens considerably at the base, so that the nostrils open outwards iiii, p. 175). ( 249 ) TABLE XXX. SYSTEMATIC SURVEY OT THE CHIEF PHYLOGENETIC STAGES or TUB HUMAN NOSE. First Stage : Nose of the earlier Primitive Fishes. The nose is formed by a pair of simple skin-grooves (nose-pits) in the outer surface of the head (like those which are now permanently retained by the lower Selachians). Second Stage : Nose of the more recent Primitive Fishes. Each of the two blind nasal grooves becomes connected by a furrow (nasal-furrow) with one end of the month (as is yet permanently the case in the higher Selachians). Third Stage : Nose of the Dipneusta. The two nasal furrows change, in consequence of the coalescence of their edges, into closed canals (primary nose-canals), which open at their front ends, within the soft edges of the lip, into the primary mouth-cavity ; as is yet permanently the case in the Dipneusta and the earlier lower Amphibia (Sozobrancliia). Fourth Stage: Nose of Amphibia. The inner openings of the nasal canals penetrate further back into the primary mouth-cavity, so that they are surrounded by hard bony portions of the jaw (as is yet permanently the case in the higher Amphibia). Fifth Stage: Nose of the Protamnia. The primitive mouth-cavity, into which both nasal canals open, separates, in consequence of the formation of a horizontal partition (the palate-roof), into an upper nasal cavity and a lower (secondary) mouth-cavity. The formation of the spongy bones of the nose commences (as in the earlier Amnion Animals). Sixth Stage : Nose of the earlier Mamma!*. The simple nose-cavity separates, in consequence of the development of a vertical partition wall (the "plough," vomer), into two distinct nose-cavities, each of which is occupied by one of the nasal canals (as is yet the case in all Mammals). The spongy nose-bones differentiate. Seventh Stage : Nose of the more recent Mammals. Within both nose-cavities the development of the spongy bones proceeda further, and an external nose begins to form. Eighth Stage : Nose of the Catarhine Apes, The internal and the external nose attain the full development ex- clusively characteristic of Catarhine Apes and of Man. 250 THE EVOLUTION OF MAN. The history of the development of the eye is equally remarkable and instructive. For although the eye, owing to its exquisite optical arrangement and wonderful struc- ture, is one of the most complex and most nicely adapted organs, yet it develops, without a preconceived design, from a very simple rudiment in the outer skin-covering. FIG. 241. — The human eye in transverse section: a, protective membrare (sclerotica) ; l>, horn membrane (cornea) ; c, outer membrane (conjunctiva) ; d, circular veins of iris ; e, vascular membrane (chornidea) ; /, ciliary muscle ; g, corona ciliaris ; h, rainbow membrane (iris) ; i, optic nerve (n. opticus) ; k, anterior limit of the retina ; I, crystalline lens (lens crystal- Una); m, inner cover of the horn membrane (water membrane, membrana Descemeti); n, pigment membrane (pigmentosa) ; o, retina; p, "petits-ca.n&\;" q, yellow spot of the retina. (After Helmholtz.) When fully developed, the human eye is a globular capsule (the eyeball, bulbus, Fig. 24-1). This lies in the THE EYE. 251 bony orbit of the skull, surrounded by protective fat and by motor muscles. The greater part of this eyeball is occupied by a semi-fluid, clear gelatinous substance, the vitreous body (corpiis vitreum). The crystalline lens (Fig. 241, I) is embedded in the anterior surface of the vitreous body. It is a lentil-shaped, bi-convex, transparent body— the most important of the light-refracting media of the eye. Among these media is, in addition to the lens and vitreous body, the aqueous humour (humor aqueus, at ra, in Fig. 241), in front of the lens. These three pellucid, light-refracting media — the vitreous body, the crystalline lens, and the aqueous humour — by which the rays of light, incident on the eye, are refracted and concentrated, are enclosed in a firm globular capsule consisting of several different membranes, comparable with the concentric layers of an onion. The outer and thickest of these forms the white protective membrane of the eye (sclerotica, a). It consists of firm, compact white connective tissue. In front of the lens a circular, very convex, transparent plate, re- sembling a watch glass, is inserted in the white protective membrane ; this is the horny membrane (cornea, 6). On its outer surface the horny membrane is covered by a very thin coating of outer skin (epidermis) ; this coating is called the connecting membrane (conjunctiva}; it extends from the homy membrane over the inner surface of both eyelids — the upper and lower folds of skin which on closing the eyes are drawn together over them. At the inner corner of our eye there is, as a sort of rudimentary organ, the remnant of a third (inner) eyelid, which, as the " nic- titating membrane," is highly developed in the lower Vertebrates (vol. i. p. 11 0). Below the upper eyelid are lodged 252 THE EVOLUTION OF MAN. the tear-glands, the secretion of which keeps the surface of the eye smooth and clean. Directly under the protective membrane is a delicate dark-red, highly vascular membrane, the vascular mem- brane (choroidea, e), and within this the retina (o), which is a dilatation of the optic nerve (i). This latter is the second brain nerve. It extends from the " centre of sight " (the second brain-bladder) to the eye, penetrates the outer coats of this, and then extends, as the retina, between the vascular membrane (choroidea} and the vitreous body (corpus vitreum). Between the retina and the vascular membrane lies another very delicate membrane, which is commonly, but wrongly, considered as part of the latter. This is the black pigment membrane (pigmentosa, lamina pigmenti, ri), or the " black carpet " (tapetum nigrum). It consists of a single layer of beautiful hexagonal cells accurately joined together and filled with black pigment granules. This pigment membrane lines, not only the inner surface of the actual vascular membrane, but also the pos- terior surface of its anterior muscular prolongation, which, as a Circular ring-like membrane, covers the edge of the lens, and prevents the penetration of lateral rays. This is the well-known "rainbow membrane" (iris, Ji), which is differently coloured in different persons (blue, gray, brown, etc.). This " rainbow membrane " is the limit towards the front of the vascular membrane. The round hole in the iiis is the pupil, through which the rays of light pass into the interior of the eye. Where the iris proceeds from the edge of the actual vascular membrane, the latter is much thickened and forms a beautiful ciliated crown (corona c'diaris, m- brane of the lens Capsula vaswtosa lentis cryttalliiM 11. Folds of the leather 11. Eyelids /'o/p«6t'jb ^ skin (CO/-IUHI) ,12, 13. Vascular mem- 12. Vascular mem- Choroidta 1 brane of the eye- brane D. ball (capsula vas- 13. Rainbow mem- Iris Products of the J cuZoso buifri)) brane Skull-plate ] 14, 15. Fibrous membrane 14. Protective mem- Sclerotic* THE NICTITATING MEMBRANE. 259 The most important fact in this remarkable process of eye-development is the circumstance that the optic nerve, the retina, and the pigment-membrane originate from a part of the brain, from a protuberance of the twixt-brain, while the crystalline lens, the most important refracting medium, develops from the outer skin (epidermis). From the outer skin — the horny lamina — originates also the delicate connecting membrane (conjunctiva) which after- wards envelopes the outer surface of the eyeball The tear- glands proceed, as branched processes, from the conjunctiva (Fig. 214, p. 202). All the other parts of the eye originate from the skin-fibrous layer; the vitreous body and the vascular lens-capsule from the leather-plate, the choroid coat with the iris, and the protective membrane (sclerotica) with the horny membrane (cornea) from the head-plates. The outer protective organs for the eye, the eyelids, are merely simple folds of skin, which, in the human embryo, appear in the third month. In the fourth month the upper eyelid adheres to the lower, and the eye then remains covered by them till birth. (Plate VII. Fig. M ill., R in., etc.) The two eyelids usually again separate shortly before birth, but sometimes not till after. Our skulled ancestors had, in addition to these, a third eyelid, the nictitating membrane, which was drawn over the eye from the inner corner. Many Primitive Fishes (Selachii) and Amnion Animals yet retain this. In Apes and in Man it has atrophied, and only a small remnant of it exists in the inner corner of the eye as the " crescent-shaped fold," as a useless " rudi- rnontary organ." (Cf. voL i. p. 109.) Apes and Man have also lost the " Harder gland," opening below the nictitating membrane, which appears in other Mammals, and in Birds, Reptiles, and Amphibians. 26O THE EVOLUTION OF MAN. The ear of Vertebrates develops in many important points similarly to the eye and nose, but yet in other respects very differently.172 The organ of hearing of the developed human being resembles that of other Mammals in all essential particulars, and is especially similar to that of Apes. As in the latter, it consists of two principal parts, an apparatus for the conveyance of sound (external and middle ear) and an apparatus for producing the sensation of sound (internal ear). The outer ear opens in the ear-shell (concha PIQ. 244. — Auditory organ of man (left ear, seen from the front; natnrnl size) : a, ear-shell : b, external ear-canal ; c, drum, or tympanic membrane ; d, cavity of drum ; e, ear-trumpet; /, g, li, the three ear honeleta (/, hammer ; g, anvil ; h, stirrup) ; t, ear-pouch (utriculus) ; k, the three semi-circular canals ; I, ear-sac (sacculus) ; m, snail (cochlea) ; n, auditory nerve. auris), situated at the side of the head (Fig. 224-, a). From this the outer ear-canal, which is usually about an inch long, leads to the inside of the head (6). The inner end of this THE EAR. 26l tube is closed by the well-known tympanic membrane or drum (tympanum) ; a thin membrane of oval form (c), placed in a vertical position, but slightly inclined. This membrane separates the outer ear-canal from the so-called cavity of the drum (cavum tympani). This is a small cavity enclosed in the petrous part of the temporal bone, which is filled with air and connected by a special tube with the mouth-cavity. This tube is somewhat longer, but much narrower than the outer ear-canal ; it leads inward and forward in an oblique direction from the inside wall of the tympanum and opens behind the inner nostrils (or Choana) into the upper part of the cavity of the throat (pharynx). This canal is called the Eustachian tube (tuba Eustachii). It equalizes the pressure of the air in the tympanic cavity, and the outer atmospheric air which enters by the ear canal. Both the Eustachian tube and the tympanic cavity are lined by a thin, mucous membrane, which is a direct continuation of the mucous membrane of the throat. Within the tympanic cavity are the three bonelets of the ear, which, from their characteristic shape, are called the hammer, the anvil, and the stirrup (Fig. 244 /, g, fi). The hammer (/) lies furthest outward, just within the tympanic membrane; the anvil (g} is wedged in between the two others, above the hammer, and further in than the hammer ; and, lastly, the stirrup (K) lies next to the anvil toward the inside, and touches with its base the outer wall of the internal ear, or the auditory sac. All these parts of the middle and external ear belong to the sound-conducting apparatus. Their principal office is to convey the waves of sound from without through the thick side-wall of the head, to the internal ear. In Fishes these parts are entirely unre- 262 THE EVOLUTION OF MAN. presented. In them, the sound-waves are conveyed direct^ through the wall of the head itself to the internal ear. The inner apparatus, that which produces the sensation of sound, receiving the sound-waves thus conveyed to it, consists in Man, as in all other Vertebrates (with the single exception of the Amphioxus), of a closed auditory sac filled with fluid, and of an auditory nerve, the ends of which are distributed over the wall of this sac. The vibrations of the waves of sound are conveyed by that medium to these nerve-ends. In the auditory fluid (endolympK), which fills the labyrinth, and opposite the places at which the auditory nerves enter, are some small stones, composed of a mass of microscopic calcareous crystals (otoliths). The organs of hearing of most Invertebrates have essentially the same construction. In them, also, it usually consists of a closed sac filled with fluid, containing otoliths, and having the auditory nerve distributed over its wall. But while in Invertebrates the auditory vesicle is usually of a very simple spherical or oval form, in all Amphirhina, on the contrary, that is, in all Vertebrates above the Fishes up to Man, it is distinguished by a very characteristic and singular form known as the auditory labyrinth. This thin membra- nous labyrinth is enclosed in a bony envelope of the same form, the osseous labyrinth (Fig. 245), which lies within the petrous bone of the skull. The labyrinth in all Amphirhina is divided into two sacs. The larger sac is called the auditory pouch (utriculus), and has three curved appendages, called the semi-circular canals (c, d, e) ; the smaller sac is called the auditory sac (sacculus), and is connected with a peculiar appendage, which in Man and the higher Mammals is distinguished by a spiral form, like the shell of a snail, and DEVELOPMENT OF THE EAR. 263 hence is called the " snail " (cochlea, 6). On the thin wall of this delicate membranous labyrinth, the auditory nerve, which passes from the after-brain to the labyrinth, is dis- tributed in a very complex manner. It divides into two main branches, the nerve of the cochlea, and the nerve of vestibule, for the remaining part of the labyrinth. The former seems specially to determine the quality of the sound heard, the latter its quantity. The nerve of the cochlea FIG. 245. — The bony labyrinth of the human ear (loft side) : a, vestibule ; b, cochlea ; c, upper semi- circular canal; d, posterior semi-circular canal; e, outer semi-circular canal ; fjenestra ovalis ; g, fenestra rotunda. (From Meyer.) tells us the pitch and quality of sounds, the nerve of the vestibule their strength. The first rudiment of this extremely complex organ of hearing is very simple in the human embryo, as in those of all other Skulled Animals (Craniota) ; it is a groove-like depression of the outer skin (epidermis'). At the back of the head, near the after-brain, at the upper end of the second gill-opening, a little wart-like thickening of the horn-plate arises on each side (Figs. 2-46, A, fl ; 248, g\ This deepens into a small groove, and separates from the outer-skin, just as does the lens of the eye. (Cf. p. 253.) A small vesicle filled with fluid, the primitive ear-vesicle, is thus formed on each side, immediately below the horn-plate of the back part of the head ; this is also called the " primary laby- rinth " (Plates VI. and VII.). As this separates from its original site, the horn-plate, and grows inward and down- ward in the skull, it changes from a globular to a pear- shaped form (Figs. 246, B, Lv ; 249, o). The outer part has 264 THE EVOLUTION OF MAN. elongated into a thin stalk, which at first opens outward in a narrow canal. (Cf. Fig. 137,/, vol. i. p. 382.) This is called the appendage of the labyrinth recessus labyrintki,Yig. 246, lr). FIG. 216. — Development of the ear-labyrinth of a Chick, in five con- secutive ttages (A-E) (cross-sections through the rudimentary skull) : /?, ear-groove ; Iv, ear-vesicle ; Ir, labyrinth appendage ; r., rudiment of the cochlea ; csp, hind semi-circular canal ; cse, outer semi-circular canal ; jr, jugular vein. (After Reissner.) FIGS. 247, 248. — Head of an embryonic Chick, on the third day of incuba- tion : 247 in front, 248 from the right ; n} rudimentary nose (olfactory groove) ; I, rudimentary eye (ocular groove) ; g, rudimentary ear (auditory groove) ; v, fore-brain ; gl, eye-slit ; o, process of the upper jaw ; u, process of the lower jaw of the first gill-arch. (After Koelliker.) FIG. 249. — Primitive brain of human embryo of four weeks, in vertical section, and the left half observed from within : v, z, m, h, r>, the five grooves of the skull cavity in which the five brain bladders are situated (fore, twixt, mid, hind, and after brains) ; o, primary, pear-shaped auditory vesicle (showing through) ; a, eye (showing through) ; no, optic nerve ; p, canal of the hypophysis; t, central skull-pieces. (From Koelliker.) DEVELOPMENT OF THE EAR. 265 In lower Vertebrates, this develops into a peculiar cavity filled with calcareous crystals, which in some Primitive Fishes (Selachii) remains permanently open, and open? above on the skull (ductus endolymphaticus). In Mam- mals, on the contrary, the appendage of the labyriuth atrophies. In these, it is of interest only as a rudimentary organ, which has no longer any physiological significance. Its useless remnant traverses the osseous wall of the petrous bone in the form of a narrow canal, and is called the aque- duct of the vestibule (aquceductus vestibuli). Only the inner and lower part (extended like a bladder) of the detached ear-vesicle develops into the differentiated and extremely complex structure which is afterwards known as the " secondary labyrinth." This vesicle separates at a very early stage into an upper, larger section, and a lower, smaller section. The former gives rise to the ear- pouch (utriculus) with the three semi-circular canals; from the latter proceeds the ear-sac (sacculus) with the "snail" (cochlea, Fig. 246, c). The three semi-circular canals originate as simple pocket-like processes from the ear- pouch (Fig. 246, E, cse and csp). In the centre of each of these processes, the two walls coalesce, and separate them- selves from the utricle, while their extremities still commu- nicate with its cavity. In all Double-nostrils (Amphirhina) there are three semi-circular canals, as in Man, while of the Cyclostoini the Lampreys have but two, and the Myxinoides but one (p. 103). The highly-developed structure of the "snail" (cochlea), which is one of the most delicate and admirable products of adaptation in the mammalian body, originally develops very simply as a bottle-like process from the ear-sac (sacculus). As Hasse has shown, the 266 THE EVOLUTION OF MAN. various stages in its ontogenetic development still exist permanently side by side in the ranks of the lower Verte- brates.178 Even in Monotremes the snail-like spiral curving of the cochlea is not present ; it is exclusively characteristic of the other Mammals and Man. The auditory nerve (nervus acusticus), or the eighth brain-nerve, — one of the main branches of which distributes itself over the " snail " (cochlea), the other over the other parts of the labyrinth, — is, as Gegenbaur has shown, the sensory dorsal branch of a spinal brain-nerve, the motor ventral branch of which is the motor nerve of the facial muscles (nervus facialis). Phylogenetically it has, there- fore, originated from an ordinary skin-nerve, and is, conse- quently, of wholly different origin from the optic and olfactory nerves, which represent the two direct processes of the brain. In this respect the organ of hearing differs essentially from the organs of sight and of smell. The auditory nerve originates from the cells of the head-plate ; therefore, from the skin-fibrous layer. From this also develop all the membranous, cartilaginous, and bony cover- ings of the ear-labyrinth. The development of the apparatus for the conveyance of sound, situated in the middle and external ear of Mammals, is entirely distinct from that of the apparatus of auditory sensation. It must be regarded, phylogenetically as well as ontogenetically, as an independent, secondary formation, which only afterwards connects itself with the primary internal ear. Its development is, however, not less in- teresting, and is equally clearly explained by Comparative Anatomy. In all Fishes, and in the yet lower Vertebrates, there is no special apparatus for the conveyance of sound, ( 26; ) TABLE XXXII. SYSTEMATIC SURVEY oir THE CHIEF STAGES IN THE DEVELOPMENT of THE HUMAN EAB. I. First Stage. The auditory nerve is an ordinary sensitive skin-nerve, which, during the differentiation of the horn-plate, appears at a certain point on the skin of the head. H. Second -Stage. The differentiated place of the horn-plate, at which the anditory nerve appeared, forms a small special anditory groove in the skin, which has an outer orifice in the appendage called the " labyrinth." III. Third Stage. The auditory groove has detached itself from the horn-lamina, and forms a small closed anditory vesicle filled with fluid. The •'labyrinth-appendage" becomes rudimentary (Aquceductus vestibulf). IV. Fourth Stage. The auditory vesicle differentiates into two connected parts, the ear- pouch (utriculus) and the ear-sac (sacculus). Each of the two vesicles receives a special main branch of the auditory nerve. V. Fifth Stage. Three semi-circular canals grow from the ear-pouch (as in all Amphi- rhino). VI. Sixth Stage. The "snail" (cochlea) grows from the ear-sac in Fishes and Amphibia; it is very insignificant, and is only developed as an independent part in the Amniota. VII. Seventh Stage. The first gill-opening (the blow-hole of Selachians) changes into the tympanic cavity and the Eustachian tube ; the former is externally closed by the tympanic membrane (Amphibia). VIII. Eighth Stage. The small bones of the ear (ossicula auditus) (the hammer (malleus) and anvil (incus) from the first gill-arch, the stirrup (stapes) from the second) develop from parts of the first and second gill-arches. IX. Ninth Stage. The external ear is developed, together with the bony ear-canal. The shell of the ear is pointed and movable (as in most lower Mammals). X. Tenth Stage. The ear-shell, with its muscles, becomes disused and a rudimentary organ. It is no longer pointed, but, on the contrary, has a curved rim with a small ear- flap (as in Anthropoid Apes and Men). 268 TABLE XXXIII. Systematic Snrvey of the Development of the Human Ear. L Survey of the parts of the Internal Ear. (Apparatus perceptive of sound.) Products of the « Horn-plate 1. Stalk of the primary ear-vesicle 2, 3. Upper part of the p. iuiary ear-vesicle 1. Aqueduct of the vestibule (Ouc- tii.i endolym- pkaticus) 2. Ear-pouch a. Three semi-circu- lar, or curv. d canals Aqntrductui vestibuli a. Jiecestui lahyrinllii Utriculus Canutes temi-circu- laret 4, 5. Lower part of the primary ear-vesicle 4. Ear-sac 6. "The snail" Sacculva Codtlea )6. Auditory nerve 6. Auditory nerve Nervus acusticui 7. Bony covering of the ™nthbranOUS lab^' 8. Bony covering of the whole internal ear 7. Osseous labyrinth 8. " The stony bone " Labyrinthui asms II. Survey of the parts of the Intermediate and External Ear. (Apparaius for the conveyance of sound.) 19. Inner part of the first gill-opening 9. Eustachian tube Tuba Eustachii • 10. Central pirt of the first gill-opening 11. Closed part of the first gill-opening 10. Tympanic cavity (Interior of the drum) 11. Tympanic mem- brane (Head of the drum) Cavum tympani Membrand ry.'iijum' 12. Upper part of the becoud gill-arch 12. Stirrup (First bonelet of the ear) Stapet Products of the first two Gil] -arches 13. Upper part of the first gill-arch 14. Central part of the first gill-arch 13. Anvil (Second bonelet of the ear) 14. Hammer (Third bonelet of the ear) Incus Prod act of the J15. Tympanic circle Head-plate 1 (Annutu* <3f»»i«»fc««) 15. Bony outer audi- tory passage Jtfeatus cuditurius vtteut /16. Circular membranous f fold at the closed part Product of the J of V" flr8t *m- 16. Ear-shell Ooncha auria Skin^jovpring 17. Rudimentary ear- JMuenH< muscles DEVELOPMENT OF TUB EAR. 269 no external and middle ear ; in these animals there is only a labyrinth, an internal ear, situated within the skull. The tympanic membrane, its cavity, and all the connected parts are unrepresented. The middle ear first develops in the Amphibian class, in which a tympanic membrane, a tym- panic cavity, and an Eustachian tube are first found. All these essential parts of the middle ear develop from the first gill-opening, with its surrounding parts, which in the Pri- mitive Fishes (Selackii} remains through life as an open blow-hole, situated between the first and second gill-arches. In the embryos of higher Vertebrates it closes in the centre, the point of concrescence forming the tympanic membrane. The remaining outer part of the first gill-opening is the rudiment of the outer ear-canal. From the inner part originates the tympanic cavity, and further inward, the Eustachian tube. In connection with these, the three bone- lets of the ear develop from the first two gill-arches; the hammer and anvil from the first, and the stirrup from the upper end of the second gill-arch.174 Finally, as regards the external ear, the ear-shell (concha aims), and the outer ear-canal, leading from the shell to the tympanic membrane — these parts develop in the simplest way from the skin-covering which borders the outer orifice of the first gill-opening. At this point the ear-shell rises in the form of a circular fold of skin, in which cartilage and muscles afterwards form (Fig. 238, p. 247). This organ is also limited to Mammals. Among them, it is originally wanting only in the lowest division, in the Beaked Animals, (Monotrema}. In the others, on the contrary, it appears in very different stages of development and partly also of atrophy. The ear-shell has atrophied in most aquatic 270 THE EVOLUTION OF MAN. Mammals. Most of these have even lost it entirely ; this is so, for example, in the Sea-cows and Whales, and most Seals. On the other hand, in the great majority of Pouched Animals (Marsupialia) and Placental Animals (Placentalia), the ear-shell is well developed, receives and concentrates the waves of sound, and is provided with a highly-developed muscular apparatus, by means of which it can be turned freely to all sides, and at the same time can be changed in form. Every one must have noticed how strongly and freely our domestic Mammals, Horses, Cows, Dogs, Rabbits, etc., can " prick " their ears, erect them and turn them in different directions. Most Apes yet retain the power of doing this, and our ancient Ape progenitors could also do it. The more FIG. 250. Rudimentary ear-muscles on the human skull : o, upward muscle (TO. attollens) ; b, forward muscle (m. attrahens) ; c, backward muscle (m. retrahens) ; d, larger muscle of the helix (m. helicis major) ; e, smaller muscle of the helix (m. helicis minor) ; /, muscle of the tragus (m. tragicus) ; •1, muscle of the antitragus (m. antitragicus). (After H. Meyer.) recent Ape ancestors, common to Men and to the Anthropoid Apes (Gorilla, Chimpanzee, etc.), discontinued the habit of moving their ears, and hence the motor muscles gradually THE EAR IN MAN AND APES. 2/1 became rudimentary and useless. We still, however, possess them (Fig. 250). A few individual men can even move their ears forward or backward a little by the use of the forward muscle (6) and the backward muscle (c) ; and by long practice these motions can be gradually increased. On the other hand, no man is able to erect the ear-shell by the upward muscle (a), or to change its form by the little inner muscles of the ear (d, e, f, g). These muscles, which were very useful to our ancestors, have become entirely un- important to us. This is equally true of Anthropoid Apes. We also share only with the higher Anthropoid Apes — the Gorilla, Chimpanzee, and Orang — the characteristic form of our human ear-shell, especially the rolled edge, the helix, and the ear-flap. The lower Apes, like all other Mammals, have pointed ears without the helix, and without ear-flaps. Darwin has, however, shown that in some men a short, pointed process, not occurring in most individuals, is per- ceptible at the upper part of the folded rim of the ear. In some few individuals, this process is very well developed. It can only be explained as the remnant of the original point of the ear which, in consequence of the folding of the edge of the ear, has been bent forward and inward. (Of. the similarly folded ear in the embryo of the Pig and Cow, Plate VII. Fig. H m. and C in.) On carefully comparing the ear-shells of Man and of the various Apes in this particular, we find that they form a connected series of retrograde steps. In the common catarhine ancestors of tho Anthropoids and of Man, this retrogression began with the folding down of the ear-shell. In consequence of this, the ear-edge was formed on which that significant corner appears, the last trace of the free prominent point of the ear 51 2/2 THE EVOLUTION OF MAN. in our older Ape ancestors. Thus it is possible even here, with the help of Comparative Anatomy, to trace this human organ from the similar but more highly-developed organ of the lower Mammals, with certainty. At the same time, Com- parative Physiology shows us that this organ is of more Dr less high physiological value to the latter, while in Anthropoids and Man it is a useless rudimentary organ. Men with their ears cut off can hear as well as they did before. The conveyance of sound is not affected by the loss of the ear-shell. This explains the great diversity in the form and size of the ear-shell in different persons ; it shares this high degree of variability with other rudimentary organs.175 CHAPTER XXII. DEVELOPMENT OF THE ORGANS OF MOTION. The Motive Apparatus of Vertebrates. — These are constituted by the Passive and Active Organs of Motion (Skeleton and Mnscles). — The Significance of the Internal Skeleton of Vertebrates. — Structure of the Vertebral Column. — Formation and Number of the Vertebrae. — The Ribs and Breast-bone. — Germ-history of the Vertebral Column. — The Noto- chord.— The Primitive Vertebral Plates.— The Formation of the Meta- mera. — Cartilaginous and Bony Vertebrae. — Intervertebral Discs. — Head-skeleton (Skull and Gill-arches) .—Vertebral Theory of the Skull (Goethe and Oken, Huxley and Gegenbaur). — Primitive Skull, or Primordial Cranium. — Its Formation from Nine or Ten Coalescent Metamera,— The Gill-arches (Bibs of the Head).— Bones of the Two Pairs of Limbs. — Development of the Five-toed Foot, adapted for Walking, from the Many-toed Fin of the Fish. — The Primitive Fin of the Selachians (Archipterygium of Gegenbaur). — Transition of the Pinnate into the Semi-pinnate Fin. — Atrophy of the Rays or Toes of the Fins. — Many-fingered and Five-fingered Vertebrates. — Com- pnn'son of the Anterior Limbs (Pectoral Fins) and the Posterior Limbs (Ventral Fins).— Shoulder Girdle and Pelvis Girdle.— Germ-history of the Limbs. — Development of the Muscles. " In forming his estimate of my entire theory, the reader may begin with the details and examine the fundamental facts on which I base my con- clusions. But it is equally necessary to connect the detached facts, and estimate their bearing on the whole. He who in the world of organisms sees only disconnected existences, in which some organic similarities appear as 274 THE EVOLUTION OF MAN. accidental coincidences, will remain a stranger to the results of this investigation ; not merely because he does not comprehend the conclu- sions, but principally because the significance of the facts on which they are grounded, escapes him. A fact in itself is no more a scientific result, than a mere collection of facts is a science. That which makes a science of these facts, is their combination by that organizing mental faculty which determines the relations of the facts to each other." — KAKL GKUKXBAUK (1872). AMONG those features of the organization which are specially characteristic of the vertebrate tribe as such, the peculiar arrangement of the motive apparatus, or " locomotorium," undoubtedly occupies a principal place. As in all the higher animals, the active organs of motion, the muscles, form the most important part of this apparatus ; these are the fleshy bands which, by means of their peculiar contrac- tibility, of their power of contracting and shortening, move the various parts of the body, and thus change the position of the entire body. The arrangement of these muscles is, however, entirely peculiar in Vertebrates, and differs from the arrangement common to all Invertebrates. In most lower animals, especially in Worms, we find that the muscles form a simple, thin flesh -layer immediately below the outer skin-covering. This " skin-muscle pouch " is most intimately connected with the skin itself, and the same feature occurs in the tribe of the Soft-bodied Animals (MollusccC). In the great group of the Articulated Animals 1 'Arthropoda), in the Crab, Spider, Centipede, and Insect classes, we also find a similar feature, but with the difference that in these the skin-covering forms a hard coat of mail ; an inflexible skin-skeleton, formed of chitine, and often of carbonated chalk. This outer chitinous coat of mail is jointed in a great variety of ways both on the trunk and THE SKELETON. 2/5 on the limbs of Articulated Animals, and the muscular system, the contractile fleshy bands of which are attached to the inside of the chitinous tubes, is correspondingly jointed in an extremely varied manner. The case is exactly revered in Vertebrates. In these alone an internal hard skeleton develops; an inner cartilaginous or bony frame to which the fleshy muscles are externally attached, and in which they find a firm support. This bony frame forms a combined lever-apparatus, a passive apparatus of motion. The hard parts of this, the arms of the lever, or the bones, are moored against each other by the active movable muscular bands, as by hawsers. This admirable locomotive apparatus, and especially its firm central axis, the vertebral column, is quite peculiar to Vertebrates, on account of which the whole group has long been called that of Vertebrates. This internal skeleton, notwithstanding the similarity of its first rudiment, has, however, developed so variously and characteristically in the different vertebrate classes, and in the higher classes forms so complex an apparatus, that Comparative Anatomy finds one of its richest mines in this feature. This was recognized as long ago as the beginning of the century by the older Natural Science, which at once seized these very welcome materials with peculiar pleasure. That science also, which is now called in the higher and more philosophical sense, " Comparative Anatomy," has reaped its richest harvest from this field. The Comparative Anatoriy of the present day has studied the skeleton of Vertebrates more thoroughly, and revealed the laws of its formation more successfully, than has been the case with any other system of organs of the animal body. Here the well-known and oft-quoted passage, in which Goethe 2?6 THE EVOLUTION OF MAN. summed up the general result of his investigations in Mor- phology is especially appropriate : " All forms have a resemblance ; none is the same as another, And their chorus complete points to a mystical law." * Now that, by the Theory of Descent, we have discovered this " mystical law," have solved this " sacred enigma," now that we can explain the similarity of forms by Heredity, and their dissimilarity by Adaptation, we can find no weapon in the whole rich arsenal of Comparative Anatomy which defends the truth of the Theory of Descent more powerfully than the comparison of the internal skeletons of the various Vertebrates. We may, therefore, expect A priori that such comparison is of special importance in our History of the Evolution of Man. The inner vertebrate skeleton is one of those organs as to the Phylogeny of which Comparative Anatomy affords us conclusions far more important and deeper than those to be gained from its Ontogeny.176 More than any other system of organs, the internal skeleton of Vertebrates, when studied comparatively, clearly and immediately impresses the observer with the necessity of the phylogenetic connection between these allied and yet very varied forms. A thoughtful comparison of the bony frame of Man with that of other Mammals, and of these again with that of lower Vertebrates, is alone sufficient to afford conviction of the true tribal relationship of all Vertebrates. All the separate parts of which this bony frame is composed appear in other Mammals, in a great * «' Alle Gestalten Bind ahnlich, doch keine gleichet der andernj Und so deutet dor Chor auf ein gehehnes Gesetz." IMPORTANCE OF THE SKELETON. 2// variety of forms indeed, but yet in the same characteristic arrangement and relative position ; and if the comparison of the anatomical conditions of the skeleton is carried out below Mammals, we can prove that a direct and uninter- rupted connection exists throughout between these various forms which are apparently so utterly unlike, and can finally be traced from a most simple, common, fundamental form. These facts alone must fully convince every ad- herent of the Theory of Development that all Vertebrates, including Man, must be traced from a single common parent-form, from a Primitive Vertebrate ; for the mor- phological features of the inner skeleton, and of the mus- cular system which stands in the closest correlative rela- tions to it, are of such a kind that it is quite impossible to conceive a polyphyletic origin, a descent from several different root-forms. It is impossible, on mature reflection, to accept the theory that the vertebral column with its various appendages, or the skeleton of the limbs with their variously differentiated parts, could have originated on several occasions during the course of the earth's history, and that, consequently, the various Vertebrates must be referred in various lines of descent from Invertebrates. Indeed, it is exactly in this point that Comparative Anatomy and Ontogeny irresistibly drive us to the monophyletic conclusion, that the human race is a very recent offshoot of the same great single trunk, from branches of which all other Vertebrates have also sprung. In order to obtain a view of the outlines of the develop- ment of the human skeleton, we must first take a general survey of its arrangement in the developed Man. (Cf. Table XXXIV. and Fig. 251, the human skeleton from the TABLE XXXIV. Systematic Survey of the Arrangement of the Human Skeleton. A. Central Skeleton, or Axial Skeleton. Spine. A. a. Vertebral Bodies and Upper Arches. 1. Skill I 1 o. Pre-vertebral skull (Cranium) j 26. Vertebral skull f 1 Neck vertebrae 2. Vortehral ) 12 Che»t „ column ^ 5 Hip „ (foliimna | 5 Vertebrae of the sacrum vtrtebralis) (_ 4 „ n „ tail (coccyx) A.b. 7-ou-er Vertebral Archri. 1. Products of the gill- Producta araitoi arches brancltUlium 2. Ribs and breast- Costa el sternum bone B. Bones connecting the Extremities. B.a. Bones connectinrj the Anterior I.imbs: Jiones of the Shoulder. 1. Shoulder-blade Rapuia (2. 1'rimiiivc key-bon* l'r> coinroidesf) (3 Ravi-nione C<»acuid<:* f) 4. Collar-bone, or key-bone Clavtcula B.b. £on«* connecting the Aoirer /.imbt: O* iliu C/s ii»f Os uut 1. Intestinal bone 2. I'uliic bone 3. ilip-boue C. Jointed Skeleton of the Limbs. C.&. Skeleton of the Pore Limbs. I. FIRST DIVISION: UI-PER ARM. 1. Upptv arm bone Uumerus II. SKCOKD DIVISION: L^WKR ABM. 2. S|ioke-bone Kadiut 3. Kll-bone I'lna III. THIKD DrviaiON : HAND. C.b. Skeleton of the Hind Limit. I. FIKST DIVISION: THIGH. 1. Thigh-bone Femur II. SECOHD DIVISION : LEO. 2. Shin-bone Tibia 3. Calf-bone fibula III. THIRD DIVISION : F«x.r. III. A. AVrirt Carput III. Ankle Taraus Original parti. Modified r.iru. < rl-innl pares. Modified parti !a. Radical = Scaphoifcum a Tibial 1 i>. Intenucdhnn = l.nnatam U' Intermedium J — Astragalus c. Dinar = Triquet, urn \ c FlbuUr — Calcaneus LJ. Central — Intermedium f] Id. Central = Xaeiculart ,e Carpal I. /. . II. = 'frapezoiiiu i/: Tarsal I. II. - \f01M ^ )g. „ III. '*. „ iv. + v. — Caj'itatum = llamatum It „ HI. „ IV. + V. = Cubo1det III. B. Palm of the Hand Metacarput (5) in. B. Sole of the Foot Metatarsus ^5) 111. C. Five Fingers Vigiti in. C. Five Toes Digiti (14 bones Phalanges') (14 bones riuilanges) HUMAN SKELETON. FIG. 251. FIG. 252. 280 THE EVOLUTION OF MAN. FIG. 253.— Human vertebral colnmn (in an up- right position ; from the right side). (After H. Meyer.) right side (without arms); Fig. 252, the entire skeleton from the front.) In Man, as in all other Mammals, the skeleton is primarily distinguishable into the axial skeleton, or spine, and the skeleton of the appendages, or the bony frame of the limbs. The spine consists of the vertebral column and of the skull ; the latter being the pecu- liarly modified anterior part of the former. The ribs are the appendages of the vertebral column; the tongue-bone (os linguae), the lower jaw, and the other products of the gill-arches, are those of the skull. The skeletons of the two pairs of limbs, or ex- tremities, are composed of two different parts : of the bony frame of the actual, pro- minent extremities, and of the inner girdle skeleton, by which the limbs are attached to the vertebral column. The girdle skele- ton of the arms (or fore limbs) is the shoulder girdle ; the girdle skeleton of the legs (or the hind limbs) is the pelvic girdle. The bony vertebral column in human beings (columna vertebralis, or vertebra- rium, Fig. 253) is composed of thirty-three or thirty-four circular pieces of bone, which lie one behind the other (one above the other in the usual upright position of man). These bones (vertebrae) are sepa- rated from each other by elastic cushions, DEVELOPMENT OF THE SKELETON. 28 1 uliO intervertebral discs (ligamenta intervertebralia), and at the same time, are connected by joints, so that the entire vertebral column forms a firm and solid axis, which is, however, flexible and elastic, capable of moving freely in all directions. In the various regions of the trunk, the vertebrae differ in form and connection, so that the following are distinguished in the human vertebral column, beginning from above : seven neck- vertebrae, twelve breast- vertebne, five lumbar-vertebrae, five cross -vertebras, and four to five tail-vertebrse. The uppermost, those directly in FIG. 254. — Third neck-vertebra of man. FIG. 255. — Sixth breast. vertebra of man. Fi«. 256. — Second lumbar-vertebra of man. contact with the skull, are the neck-vertebrae (Fig. 254), and are distinguished by a hole found in each of the two lateral processes. There are seven neck-vertebrae in Man, as in nearly all other Mammals, whether the neck is long, as in the Camel and the Giraffe, or short, as in the Mole and the Hedgehog. The fact that the number of these neck-vertebrae is always seven, — and there are but few exceptions (explicable by adaptation),— is a strong argu- ment for the common descent of all Mammals ; it can only be accounted for as a strict transmission from a common 282 THE EVOLUTION OF MAN. parent-form, from some Promammal which had seven neck- vertebrae. If each animal species had been a distinct crea- tion, it would have been far more to the purpose to have furnished the long-necked Mammalia with a larger, and the short-necked with a smaller number of neck-vertebrae. The neck-vertebrae are immediately followed by those of the breast or thorax, which, in Man and most other Mammals, number twelve or thirteen (usually twelve). Attached to the sides of each breast-vertebra (Fig. 255) is a pair of ribs — long curved processes of bone lying in and supporting the wall of the thorax. The twelve pairs of ribs, with the connecting intercostal muscles and the breast-bone (sternum) constitute the breast body (thorax, Fig. 252, p. 279). In this elastic and yet firm thorax lie the double lung, and between the two halves of this, the heart. The chest-vertebrae are followed by a short but massive section of the vertebral column, formed by five large vertebrae. These are the lumbar- vertebra (Fig. 256), which bear no ribs and have no perforations in their lateral processes. Next comes the cross-bone (sacrum), which is inserted between the two halves of the pelvic girdle. This cross-bone consists of five fixed and amalgamated cross-vertebroa. Last comes a small rudimentary tail-vertebral column, the rump-bone (coccyx). This bone consists of a varying number (usually four, more rarely three or five) of small aborted vertebrae; it is a useless rudimentary organ, retaining no physiological sig- nificance either in Man or in the Tail-less Apes or Anthro- poids. (Gf. Figs. 204-208.) Morphologically it is, however, very interesting, as affording incontrovertible evidence of the descent of Man and of Anthropoids from Long-tailed Apes. For this assumption affords the only possible TUE VERTEBRA. 283 explanation of this rudimentary tail. In the human embryo, indeed, during the earlier stages of germ-history, the tail projects considerably. (Of. Plate VII. Fig. M 11., and Figs. 123, s, 124, s, vol. i. p. 370.) It afterwards becomes adherent, and is no longer externally visible. Yet traces of the aborted tail- vertebrae, as well as of the rudimentary muscles, which formerly moved them, persist throughout life. According to the earlier anatomists the tail in the female human being has one vertebra more than that of the male (four in the latter, five in the former).177 Chest Crost Jfeck or tho- or •mil Number of Vertebra in various Catarhini. sacral Verts.- Tdal. bras. Vei te- brce. bra. Verte- bra brce. Man (Fig. 208) 7 12 5 5 4 33 Tail- Gran" (Fi0. — Lyre-shaped germ-shield of a Chick, in three consecutive stages of development ; seen from the dorsal side ; enlarged about twenty times. Fig. 258, with six pairs of primitive vertebrae. The brain is a sim- ple bladder (hb). The spinal furrow from x remains wide open ; behind, at z, it is much enlarged, mp, Marrow-plates ; gp, side-plates ; y, limit be- tween the pharynx cavity (ah) and the head-intestine (vd). Fig. 259, with ten pairs of primitive vertebrae. The brain has separated into three bladders : v, fore-brain ; m, mid-brain ; h, hind-brain ; c, heart ; dv, yelk- veins. The spinal fnrrow is still wide open (2). mp, Marrow-plates. Fig. 260, with sixteen pairs of primitive vertebrae. The brain has separated PRIMITIVE VERTEBRAE. 289 into five bladders: v, fore-brain; z, twixt-brain; m, mid-brain; h, hind- brain ; n, after-brain ; a, eye-vesicles ; g, ear-vesicles ; c, heart ; dv, yelk- veins ; mp, marrow-plate ; uw, primitive vertebra. developed from an inarticulate worm-form by terminal budding, so the many-membered vertebrate body has originated from an inarticulate parent-form. The nearest extant allies of this parent-form are the Appendicularia (Fig. 162) and the Ascidian (Plate XI. Fig. 14). As has been repeatedly pointed out, this primitive vertebral, or metameric structure has a very important bearing on the higher morphological and physiological de- velopment of Vertebrates. (Cf. vol. i. p. 346.) For the articu- lation is by no means confined to the vertebral column, but equally affects the muscular, nervous, vascular, and other systems. As is shown by the Amphioxus, the metameric structure appeared much earlier in the muscular than in the skeleton system. Each so-called primitive vertebra is in fact far more than the mere rudiment of a future verte- bra. In each primitive vertebra exists the rudiment of a segment of the dorsal muscles, of a pair of spinal nerve- roots, etc. Only the inner portion — that which lies directly next to the notochord and the medullary tube — is employed, as the skeleton-plate, in the formation of actual vertebra. We have already seen how these true vertebrae develop from the skeleton-plate of the primitive vertebrae or metamera. The right and left lateral halves of each primitive vertebra, originally separate, unite. The ventral edges, meeting below the medullary tube, surround the chord and thus form the rudiments of the vertebral bodies ; the dorsal edges, meeting above the medullary tube, form the first rudiments of the vertebral arches. (Cf. Figs. 95-98, and Plate IV. Figs 3-8.) 290 THE EVOLUTION OF MAN. In all Skulled Animals (Craniota), most of the soft; undifferentiated cells which originally constitute the skeleton-plate, afterwards change into cartilage ceUs, which secrete a firm, elastic "intercellular sub- stance," and thus produce cartilaginous tissue. Like most other parts of the skeleton, the rudimentary vertebrae soon pass into a cartilaginous condition, and, in the higher Vertebrates, the cartila- ginous tissue is afterwards replaced by the rigid bony tissue with its peculiar radiate bone-cells (Fig. 5, vol. i. p. 120). The original axis of the vertebral column, the notochord, is more or less compressed by the cartilaginous tissue which grows vigorously round it In lower Verte- brates (i.e., in Primitive Fishes) a more or less considerable portion of the noto- chord remains within the vertebral bodies. In Mammals, on the contrary, it disappears almost entirely. In the human embryo, even at the end of the second month, the notochord is seen only as a thin thread which passes *th rough the axis of the thick cartilaginous ver- tebral column (Fig. 261, ch). In the cartilaginous vertebral bodies themselves, which afterwards ossify, the thin remnant of the notochord (Fig. 262, cJi) soon disappears entirely. A remnant remains, however, throughout life in the elastic " intervertebral discs" which develop, from the skeleton plate, between each pair of vertebral bodies (Fig. 261, K). In a new-born child, a large, pear-shaped cavity, filled with a gelatinous cell-mass, is visible in each intervertebral disc FIG. 261.— Tl.ree breast-vertebrte of a human embryoof eight weeks, in lateral lon- gitudinal section : v, cartilaginous vertebral bodies; li, iiiterverte- bral discs ; ch, noto- chord. (After Koel- liker.) EVOLUTION OF THE NOTOCHORD. 2QI (Fig. 263, a). This " gelatinous nucleus " of the elastic ver- tebral disc becomes less sharply defined, but persists throughout life in all Mammals, while in Birds and Rep- tiles, even the last remnant of the notochord vanishes. FTG. 262. — A brenst-vertebraof the same embryo in lateral cross-section: cv, cartilaginous vertebral bodies ; ch, notochord ; pr, square process ; a, vertebral arch (upper) ; c, upper end of rib (lower arch). (After Koelliker.) FIG. 263. — Intervertebral disc of new-born child in cross-section : a, remnant of the notochord. (After Koelliker.) When the cartilaginous vertebrae afterwards ossify, the first deposit of bone-substance (the first "bone-nucleus") in the vertebral bodies is formed immediately round the rem- nant of the notochord, and soon completely displaces the latter. A special bone kernel or nucleus is then formed in each half of the cartilaginous vertebral arch. It is not till after birth that the ossification progresses so far that the three bone-nuclei approach each other. The two bony halves of the arch unite during the first year, but it is not till much later, till between the eighth and the twelfth year, that they unite with the bony vertebral body. The bony skull (cranium), which must be regarded as 2Q2 THE EVOLUTION OF MAN. the foremost, peculiarly modified section of the vertebral column, develops in an exactly similar manner. Just as, in the spinal column, the vertebral canal envelopes and pro- lects the dorsal marrow, so the skull forms a bony covering round the brain; and, as the brain is merely the anterior, peculiarly differentiated portion of the dorsal marrow, we might conclude on d priori grounds, that the bony envelope of the brain is a peculiar modification of that of the dorsal marrow. It is true, that if the developed human skull (Fig. 264) is considered by itself, it is impossible to under- stand how it can be merely the modified anterior portion of the vertebral column. It is a complex, capacious bony structure, consisting of no less than twenty bones, differing widely in form and size. Seven of these skull-bones constitute the spacious case which encloses the brain, and in which we distinguish the strong, massive floor of the skull (basis cranii) below, and the boldly arched roof of the skull F.o. 264,-Hnman sknii, (farnix cranii} above. The other from the right side. thirteen bones form the " facial skull," which especially provides the bony envelopes of the higher sense-organs, and at the same time as the jaw- skeleton, encircles the entrance to the intestinal canal. The lower jaw (usually regarded as the twenty-first skull-bone) is jointed to the skull-floor, and behind this, embedded in the roots of the tongue, we find the tongue- bone, which, like the lower jaw, has originated from the gill-arches, together with a portion of the lower arch, which originally developed as " skull-ribs " from the ventral side of the skull-floor. VERTEBRAL THEORY OF THE SKULL. 2Q3 Although, therefore, the developed skull of the higher Vertebrates, in its peculiar form, its very considerable size, and its complex structure, seems to have nothing in common with ordinary vertebrae, yet the old comparative anatomists at the close of the eighteenth century correctly believed that the skull is originally merely a series of modified vertebrae. In 1790, Goethe "picked up out of the sand of the Jews' burying-ground among the downs near Venice, a dismembered skull of a sheep; he at once per- ceived that the face bones (like the three vertebrae of the back of the skull) are also derivable from vertebrae." And, in 1806, Oken (without knowing of Goethe's discovery), at Ilsenstein, on the way to the Brocken, " found a beautifully bleached skull of a hind; the thought flashed through him, It is a vertebral column ! " 179 For the last seventy years, this celebrated " Vertebral Theory of the Skull " has interested the most prominent zoologists ; the most important representatives of Compara- tive Anatomy have exercised their ingenuity in attempting to solve this philosophical skull-problem ; and the question has engaged attention in yet wider circles. It was not till 1872 that the solution was found, after seven years of labour, by the comparative anatomist, who, both in the wealth of his real empirical knowledge and in the pro- fundity of his philosophic speculations, surpasses all other students of this science. Karl Gegenbaur, in his classic "Researches in the Comparative Anatomy of Vertebrates" (third part), showed that the skull skeleton of the Selachii is the only record which affords definite proof of the verte- bral theory of the skull. Earlier comparative anatomists erred in starting from the developed mammalian skull, and 294 THE EVOLUTION OF MAN. in comparing the several component bones with the separate parts of vertebrae ; they supposed that in this way they could prove that the developed mammalian skull consists of from three to six original vertebrae. The hindmost of these skull -vertebras was, according to them, the occipital bone. A second and a third vertebra were represented by the sphenoid bone, with the parietal bones, and by the frontal bone, etc. The elements of anterior skull vertebrae were even supposed to exist in the face bones. In opposi- tion to this view, Huxley first called attention to the fact that in the embryo this bony skull originally develops from a simple cartilaginous vesicle, and that in this simple cartilaginous " primitive skull " not the slightest trace of a constitution of vertebrate parts is visible. This is equally true of the skulls of the lowest and most ancient Skulled Animals (Craniota), the Cyclostomi and the Selachii. In these the skull retains throughout life the form of a simple cartilaginous capsule — of an inarticulate " primitive or primordial skull." If the older skull-theory, as it was accepted from Goethe and Oken by most comparative anatomists, were correct, then in these lowest Skulled Animals especially, and in the embryos of the higher Skulled Animals, the constitution of the " primitive skull " by a series of " skull-vertebrae " would be very clearly evident. This simple and obvious consideration, first duly em- phasized by Huxley, indeed overturns the famous " Verte- brate Theory of the Skull," as held by the older comparative anatomists. Yet the entirely correct fundamental idea holds good, i.e., the hypothesis that the skull develops from the anterior portion of the spinal column by differentiation and peculiar modification, just as the brain develops from HUXLEY'S SKULL THEORY. 295 the anterior portion of the dorsal marrow. But the true mode of empirically establishing this philosophic hypothesis was yet to be discovered ; and this discovery we owe to Gegenbaur.180 He was the first to employ the phylogenetin method, which, in this as in all morphological questions, leads most surely and quickly to the result. He showed that the Primitive Fishes (Seladiii, Figs. 191, 192, p. 113), as the parent-forms of all Amphirhina, yet retain per- manently in their skull-structure that form of primordial skull, from which the modified skull of the higher Verte- brates, and therefore that of Man, has developed phylo- genetically. He also pointed out that the gill-arches of the Selachii show that their primordial skull was originally formed of a considerable number — at least nine or ten — primitive vertebrae, and that the brain-nerves, which branch from the base of the brain, entirely confirm this. These brain-nerves — with the exception of the first and the second pairs (the olfactory and the optic nerves) — are merely modi- tied spinal nerves, and, in their peripheric distribution, essentially resemble the latter. The Comparative Anatomy of these brain-nerves is one of the strongest arguments for the newer vertebral theory of the skull. It would lead us too far aside if we were to enter into the particulars of this ingenious theory of Gegenbaur, and I must content myself with referring to the great work already quoted ; in it the theory is fully demonstrated by empirical and philosophical arguments. The same author has given a brief abstract in his " Outlines of Comparative Anatomy" (1874), the study of which it is impossible to recommend too highly. In this work Gegenbaur indi- cates as original "skull-ribs," or "lower arches of skull- 296 THE EVOLUTION OF MAN. vertebrae," in the selachian skull (Fig. 2G5), the following pairs of arches : I. and II. are two lip cartilages, of which the anterior (a) consists only of an upper, and the inferior (be) of an upper and a lower piece; III., the jaw-arch, which also consists of two pieces on each side, — viz., the primitive upper jaw (os palato-quadratum, o) and the FIQ. 265. — Head skeleton of a Primitive Fish: u, uose-groove; eth, region of the sieve-bone ; orb, eye-cavity ; la, wall of ear-labyrinth ; occ, occipital region of the primitive skull ; cv, vertebral column ; a, front ; be, hind lip- cartilage ; o, primitive upper jaw (palato quadraturii) ; u, primitive lower jaw; 11., tongue-arch ; II1.-V11I., first to sixth gill-arches. (After Gegen- baur.) primitive lower jaw (it); IV., the tongue arch (II.), and V. to X., six true gill arches, in the stricter sense of that term (1II.-VIIL). The anatomical features of these nine or ten skull-ribs, or " lower vertebral arches," and of the brain nerves distributed over them, show that the apparently simple, cartilaginous "primordial skull" of the Primitive Fishes originally develops from an equal number (nine at the least) of primitive vertebrae. The base of the skull is formed by the vertebral bodies ; the roof of the skull by the upper vertebral arches. The coalescence and amalgamation of these into a single capsule is, however, so ancient, that EVOLUTION OF THE SKULL. 297 their primordial separate condition now appears effaced by the action of the "law of abridged heredity," and is no longer demonstrable in the Ontogeny. In the human primitive skull (Fig. 2GG), and in that of all higher Vertebrates, which has been modified, phyloge- netically, from the primitive skull of the Selachii, five con- secutive divisions are visible at a certain early period of development; these one might be tempted to refer to five FIG. 266.— Primitive skull of human embryo of four weeks ; vertical section, the left half seen from the inside : v, z, m, h, n, the five grooves in the skull cavity, in which lie the five brain-bladders (fore-brain, twixt-brain, mid-brain, hind- brain, after-brain) ; o, pear-shaped pri- mary ear- vesicle ; a, eye ; no, optic nerve ; p, canal of the hypophysist; t, central part of the cranial basis. (After Koelliker.) original primitive vertebrae ; they are, however, merely the result of adaptation to the five primitive brain- bladders, and, like the latter, they rather correspond to a larger number of metarnera. Tli3 fact that the primitive verte- brate skull is a much modified and profoundly transformed organ, and by no means a primitive structure, is also evi- dent in the circumstance that its rudiment, originally a soft membrane, commonly assumes the cartilaginous state only at its base and on the sides, while it remains membranous at tlie skull-roof. Here the bones of the later bony skull develop in the soft membranous rudiment as an external bony roof, without a previous intermediate cartilaginous state, as in the base of the skull. Thus a great part of the skull-bones originally developed as roof-bones from the 298 THE EVOLUTION OF MAN. leather-skin (c. 1R4^. 'I'he (c-uch-bones, which distinguish Monotremes and Marsni iala, disappear. XI. Eleventh Period: Skeleton nfthe Anthropoid Apes (Figs. 204-208, p. 179). The skeleton acquires the peculiar development shared by Man ex- clusively with the Anthropoid Apes. CHAPTER XXIII. DEVELOPMENT OF THE INTESTINAL SYSTEM. The Primitive Intestine of the Gastrula. — Its Ilomolngy, or Morphological Identity in all Animals (excepting the Protozoa). — Survey of the Structure of the Developed Intestinal Canal in Man. — The Mouth, cavity. — The Throat (pharynx). — The Gullet (oesophagus).— The Wind- pipe (trachea) and Lungs. — The Larynx. — The Stomach. — The Small Intestine. — The Liver and Gall-bladder. — The Ventral Salivary Gland (pancreas). — The Large Intestine. — The Rectum. — The First Rudiment of the Simple Intestinal Tube. — The Gastrula of the Amphioxus and of Mammals. — Separation, of the Germ from the Intestinal Germ Vesicle (Gastrocystis).— The Primitive Intestine (Protogaster) and the After Intestine (Metnga^ter). — Secondary Formation of the Mouth and Anus from tlie Outer Skin.— Development of the Intestinal Epithelium from the Intestinal-glandular Layer, and of all other parts of the Intestine from the Intestinal-fibrous Layer. — Simple Intestinal Pouch of the Lower Worms. — Differentiation of the Primitive Intestinal Tube into a Respiratory and & Digestive Intestine.— Gill-intestine and Stomach- ]nfestine of the Amphioxus and Ascidian. — Origin and Significance of tlie Gill-openings. — Their Disappearance.— The Gill-arches and the Jaw- skeleton. — Formation of the Teeth. — Development of the Lungs from the Swim-bladder of Fish. — Differentiation of the Stomach. — Development of the Liver and Pancreas. — Differentiation of the Small and Large Intestines. — Formation of the Cloaca. "Cautious people require ns to confine ourselves to gathering materials, and to leave it to posterity to raise a scientific structure from those materials ; because only in that way can we escape the ignominy of having the theories we believed in overthrown by the advance of knowledge. The anreasonableness of this demand is apparent enough from the fact that. 312 TIIK EVOLUTION OF MAN. Comparative Anatomy, like every other science, is endless; and therefore the endlessness of the accumulation of materials would never allow men, if they complied with this dem.-md, to reap any harvest from this field. But, further than this, history teaches as clearly, that no age in which scientific inquiry has been active, has been able so to deny itself, as, setting the goal of it* researches in -the futnre, to refrain from drawing conclusions for itseK from its larger or smaller treasury of observations, and from trying to fill tin gaps with hypotheses. It would, indeed, be a hopeless proceeding, if, in oi'cier to avoid lo--.in.fe any part of our possessions, we should refuse tc acquire any possessions whatever." — KARL ERNST RAKR (1N19). AMONG the vegetative organs of the human body, to the development of which we now turn our attention, the intes- tinal canal is the most important. For the intestinal tube is the oldest of all the organs of the animal body, and carries us back to the earliest time of organological differ- entiation, to the first period of the Laurentian Epoch. As we have already seen, the result of the first division of labour in the homogeneous cells of the earliest many-celled animal body must have been the formation of a nutritive intestinal canal. The first duty and the first need of every organism is self-support. This task is accomplished by the two functions of nutrition and of the covering of the body. When, therefore, in the primaeval collection of homogeneous cells (Synamcebium), of the phylogenetic existence of which we yet have evidence in the ontogenetic developmental form of the mulberry-germ (Morula), the several members of the community began to divide the work of life, they were first obliged to engage in two separate tasks. One half modified into nutritive cells, enclosing a digestive cavity, the intestinal canal ; the other half, on the contrary, developed into covering cells, forming the outer cover- ing of this intestinal canal, and, at the same time, of the whole body. Thus arose the first two germ-layers : the PRIMITIVE INTESTINAL CANAL. 313 inner, nutritive, or vegetative layer, and the outer, covering, or animal layer. If we try to construct for ourselves an animal body of the simplest conceivable form, possessing such a primitive intestinal canal, and the two primary germ-layers forming its wall, the result is necessarily the very remarkable germ-form of the gastrula, which we have shown to exist in wonderful uniformity throughout the whole animal FIG. 274. — Gastrnla of a Chalk-sponge (Olynthus) : A, from ontside ; B, in longitudinal section through the axis ; g, primitive intestine ; o, primi tive mouth ; *, intestinal Ir.vi-r, or entoderm ; e, skin-layer, or exoderm. series : in the Sponges, Sea-nettles (Acalephce), Worms, Soft-bodied Animals (Mottueca), Articulated Animals (A rt/iro- podti), and Vertebrates (Figs. 174-179, p. 65). In all these various animal tribes the gastrula reappears in the same entirely simple form (Fig. 274). Its whole body is really merely the intestinal canal ; the simple cavity of the body, the digestive intestinal cavity, is the primitive intestine 314 THE EVOLUTION OF MAN. (protogaster, The pancreas lies somewhat further back and more to the left (Fig. 16, p). The small intestine is so long that it has to lie in many folds in order to find room in the limited space of the ventral cavity ; these coils are the bowels. They are divided into an upper intestine, called the empty intestine (jejunum}, and a lower, the crooked intestine (ilium). In this latter part lies that part of the small intestine at which, in the embryo, the yelk-sac opens into the intestinal tube. This long, thin intestine then passes into the large intestine, from which it is separated by a peculiar valve. Directly behind this " Bauhinian valve" the first part of the large intestines forms a broad pouch- like expansion, the blind intestine (caecum), the atrophied extremity of which is a well-known rudimentary organ, the vermiform process (processus vermiformis). The large intestine (colon) consists of three parts , an ascending part on the right, a transverse central part, and a descending part on the left. The latter finally curves like an S, called the "sigmoid flexure," into the last part of the intestinal canal, above the rectum, which opens at the back by the anus (Plate V. Fig. 16, a). Both the large intestine and the small intestine are furnished with numerous glands, most of them very small, and which seci'ete mucous and other juices. Along the greater part of its length the intestinal canal is attached to the inner dorsal surface of the ventral cavity, or to the lower surface of the vertebral column. It is fastened by means of the thin, membranous plate, called the mesentery, which develops directly under the notochord 54 32O THE EVOLUTION OF MAN. from the intestinal-fibrous layer, at the point where this curves into the outer lamina of the side-layer, into the skin-fibrous layer (Plate IV. Fig. 5, g). The curving-point was distinguished as the middle-plate (Fig. 99, rap). The mesentery is, at first, very short (Plate V. Fig. 14,^); but it soon lengthens considerably at the central part of the intes- tinal canal, and takes the form of a thin, transparent, membranous plate, which has to be the more extended the further the folds of the intestine diverge from the place where they are first attached to the vertebral column. The blood-vessels, lymphatic vessels, and nerves which enter the intestinal canal traverse this mesentery. Although, therefore, the intestinal canal, in the adult human being forms an extremely complex organ, and though it shows in its details so many intricate and delicate structural arrangements, — into which we cannot enter here, — this entire structure has developed, historically, from that simplest form of primitive intestine which was possessed by our gastrsead ancestors, and which the extant gastrula now exhibits. We have already shown (in Chapter VIII.) that the peculiar Hood-gastrula (AmpJti- gastrula) of Mammals (Fig. 277) may be referred back to the original Bell-gastrula (Archi gastrula) form, which, among Vertebrates, is now accurately retained solely by the Amphioxus (Fig. 276 ; Plate X. Fig. 10). Like the latter, the gastrula of Man and of all Mam- mals must be regarded as the ontogenetic reproduction of that phylogenetic evolution-form which we call the Gastraea, and in which the whole body of the animal is intestine. The peculiar form and mode in which the complex DEVELOPMENT OF THE INTESTINAL CANAL. 321 human intestinal canal develops from the simple gastrula and which is similar to that in other Mammals, can there- fore be only correctly understood when it is considered in the light of Phylogeny. We must, accordingly, distinguish FIG. 276. — Archigastrnla of AmphioTUS (in longitudinal eectkn) : d, primitive intestine ; o, primitive month ; t, intestinal layer ; e, skin-layer. FIG. 277. — Amphigastrula of Mammal (in longitudinal section). The primitive intestine (d) and primitive mouth (o) are filled up by the cells of the intestinal layer (i) ; e, skin-layer. between the original primary intestine ("the primitive intestine, or protogaster ") of the Skull-less Animals (Acrania), and the differentiated or secondary intestine ("after intestine, or metagaster"} of the Skulled Animals (Craniota). The intestine of the Amphioxus (the repre- sentative of the Acrania) forms no yelk-sac, and develops, palingenetically, from the entire primitive intestine of the gastrula. The intestine of the Skulled Animals, on the other hand, has a modified, kenogenetic form of evolution, and differentiates at a very early period into two different parts : into the permanent seconda^ intestine, which alone 322 THE EVOLUTION OF MAN. gives rise to the various parts of the differentiated intestinal system, and the transient yelk-sac, which serves only as a storehouse of materials for the building of the embryo. The yelk-sac attains its greatest development in Primitive Fishes (Selachii), Bony Fishes (Teleostei), Reptiles, and Birds. In Mammals, and especially in Placental Animals, it is atrophied. The peculiar intestinal development of the Cyclostomi, Ganoids, and Amphibia must be regarded as an intermediate form, between the palingenetic intestinal development of the Skull-less animals, and the kenoge- netic intestinal development of the Amnion Animals (Am- niota).™ We have already seen in what a peculiar way the development of the intestine takes place ontogenetically in the human embryo and in that of other Mammals. Imme- diately from the gastrula of these originates a globular intestinal germ-vesicle (gastrocystis), filled with fluid (Figs. 72, 73, vol. i. p. 289). In the wall of this is formed the lyre-shaped germ-shield, on the lower side of which, along the middle line, appears a shallow groove, the first rudi- ment of the future, secondary intestinal tube. This intestinal groove grows constantly deeper, and its edges curve toward each other, to grow together at last and form a tube (Fig. 100, vol. i. p. 333). The wall of this secondary intestinal tube consists of two membranes of the inner, intestinal -glandular layer, and of the outer, intestinal- fibrous layer. The tube is completely closed at the ends, having only an opening in the centre of the lower wall, by which it is connected with the intestinal germ-vesicle (Plate V. Fig. 14). The latter, in the course of development, becomes continually smaller, as the intestinal canal continues DEVELOPMENT OF THE INTESTINAL CANAL. 323 to grow larger and more perfect. While, at first, the intes- tinal tube appears only as a little appendage on one side of the great intestinal germ-vesicle (Fig. 278), the remnant of the latter afterwards forms only a very inconsiderable appen- dage of the great intestinal canal. This appendage is the yelk-sac, or navel-vesicle. It entirely loses its importance, and at length disappears, while the intestinal canal is finally closed at the original central opening, where it forms the so-called intestinal navel (Fig. 94, vol. i. p. 312). It has also been said that this simple cylindrical intestinal tube, in Man as in all Vertebrates, is at first entirely closed at both ends (Plate V. Fig. 14), and that the two permanent openings of the intestinal canal — at the anterior extremity, the mouth, at the posterior, the anus — form only second- arily, and from the outer skin. At the fore end, a shallow mouth-furrow originates in the outer skin, and this grows toward the blind, anterior end of the head intestinal c^ity, into which it finally breaks. In the same way a shal- low furrow for the anus is formed behind in the skin, and this soon grows deeper, and grows toward the blind posterior end of the pelvic intestinal cavity, with which it finally unites. At both extremities there is, at first, a thin partition between the outer skin-furrow and the blind end of the intestine, and this disappears when the opening is made.188 Directly in front of the anus the allantois grows out of the posterior intestine ; this is the important embryonic appendage which develops, in Placental Animals, and only in these (thus in Man too) into the placenta (Figs. 278, 279, 1 ; Plate V. Fig. 14, al}. In this more developed form — repre- sented in the diagram (Fig. 94, 4, vol. i. p. 312) — the intestinal 324 THE EVOLUTION OF MAN. canal of Man, like that of all other Mammals, now forms a slightly-curved, cylindrical tube, which has an opening at both ends, and from the lower wall of which depend two sacs; the anterior navel-bladder, or yelk-sac, and the pos- terior allantois, or primitive urinary sac. Microscopic observation shows that the thin wall of this simple intestinal tube and of its two bladder-like append- ages is composed of two distinct cell-strata. The inner, which coats the entire cavity, consists of larger, darker cells, FIG. 278. — Human embryo of the third week, with the amnion and allantois. The great globular yelk-sac is below, the bladder-like allantois on the right ; there are as yet no limbs. The germ, with its appendages, is enclosed in the tufted membrane (chorion). FIG. 279. — Human embryo, with amnion and allantois, in the fourth week. (After Krause.) The amnion (w) lies pretty close to the body. The greater part of the yelk-sac (d) has been torn away. Behind this the allan- tois appears as a small pear-shaped bladder. Arms (/) and legs (I) are already commenced : v, fore-brain ; z, twixt-brain ; m, mid-brain ; ft, h.'nd- brain ; ?i, after-brain; a, eye; k, three gill-arches; c, heart; s, tail. RUDIMENT OF THE INTESTINAL CANAL. 325 and is the intestinal-glandular layer. The outer stratum consists of lighter, smaller cells, and is the intestinal fibrous- layer. The cavities of the mouth and the anus are the only exceptions to this, because they originate from the outer skin. The inner cell-coating of the entire mouth-cavity is therefore furnished, not by the intestinal glandular-layer, but by the skin-sensory layer, and its muscular lower layer, not by the intestinal-fibrous layer, but by the skin-fibrous layer. This is equally true of the wall of the anal cavity (Plate V. Fig. 15). If the question be asked, what relation these component germ-layers of the primitive intestinal wall bear to the infinitely varied tissues and organs which we afterwards find in the developed intestine, the answer is extremely simple. The relations of these two layers to the formation and differentiation of the tissues of the intestinal canal with all its parts, may be condensed into a single sentence : The intestinal epithelium, that is, the inner, soft cell-stratum which coats the cavities of the intestinal canal and of all its appendages, and which directly accomplishes the nutritive process, develops solely from the intestinal-glandular layer ; on the contrary, all other tissues and organs belong^ ing to the intestinal canal and its appendages, proceed fioru the intestinal-fibrous layer. From this latter, therefore, originates the entire outer covering of the intestinal tube and its appendages ; the fibrous connective tissue and the smooth muscles which compose its fleshy skin ; the carti- lages which support these, for example, the cartilage of the larynx and of the trachea ; the numerous blood and lymph vessels which absorb nutrition from the wall of the intestine; in short, everything belonging to the intestine, with the 326 THE EVOLUTION OF MAN. exception of the intestinal epithelium. From the intestinal- fibrous layer originates also the entire mesentery with all the adjacent parts, the heart, the large blood-vessels of the body, etc. (Plate V. Fig. 16). Let us now turn aside for a moment from this original rudimentary intestine of Mammals, in order to institute a comparison between it and the intestinal canal of those lower Vertebrates and Worms, which we have learned to recognize as the ancestors of Man. In the simplest Gliding- worm, or Turbellaria (Rhabdoicelum, Fig. 280), we find a very simple intestinal form. As in the gastrula, the intes- tine in these Worms is a simple pouch with a single open- ing, which latter acts both as mouth and anus (m). The intestinal pouch has, however, differentiated into two sec- tions, an anterior throat-intestine (sd) and a posterior stomach-intestine (d). This differentiation becomes more important in the Ascidia (Fig. 281) and in the Amphioxus (Fig. 282), which connects the Worms with the Vertebrates. In these two animal forms the intestine is essentially identical ; the anterior portion forms the respiratory gill- intestine, the posterior forms the digestive stomach-intes- tine. In both it develops, palingenetically, directly from the primitive intestine of the gastrula (Plate XI. Figs. 4, 10). But the original mouth-opening of the gastrula, or the primitive mouth, afterwards closes, and in its place is formed the later anus. In the same way, the mouth- opening of the Amphioxus and of the Ascidian is a new formation, as is the mouth-opening of Man, and generally, of all Skulled Animals (Craniota). The secondary forma- tion of the mouth of the Lancelot is connected, as may be conjectured with some probability, with the formation ol EARLY FORMS OF THE INTESTINAL CANAL. 327 tho gill-openings, which appear directly behind it on the intestine. The front portion of the intestine has thus FIG. 280.— A simple Gliding Worm (Rhabdoc&um) m, month; sd, throat- epithelium ; sin, throat muscle-mass ; d, stomach-intestine ; nc, renal ducts ; /, ciliated outer-skin ; nm, openings of the latter ; em, eye ; na, nose-pit. FIG. 281.— Structure of an Ascidian (seen from the left side, as in Plate XI. Fig. 14). The dorsal side is turned toward the right, the ventral side to the left; the mouth-opening (o) is above; at the opposite, tail end, the ascidian has become adherent. The gill-intestine (?>?•), perforated by many openings, extends into the stomach-intestine. The terminal intestine opens through the anus (a) into the gill-cavity (cl), from which the excre- ment is passed out with the respirated water through the gill-pore, or cloacal opening («') ; m, mantle. (After Gegenbaur.) 328 THE EVOLUTION OF MAN. become a respiratory organ. I have already pointed out how characteristic this adaptation is of Vertebrates and Mantle Animals (Tunicata, p. 87). The phylogenetic origin of the gill-openings in- dicates the beginning of a new epoch in the tribal history of Vertebrates. The most important process we meet with in the further ontogenetic development of the intestinal canal in the human embryo, is the origin of the gill-openings. At the head of the human embryo, the wall of the throat very early unites with the outer wall of the body, and four openings then form on the right and left sides of the neck, behind the mouth, and these lead directly from without into the throat-cavity. These openings are the gill-openings, and the par- titions separating them are the gill-arches (Figs. 116-118, vol. i. p. 356; Plates I. and V., Fig. 15, ks~). These embryonic for- mations are very interesting ; for they show FIG. 282. — Lancelot (Amphioxus lanceolatvs), double the natural size, seen from the left side (the longi- tudinal axis is perpendicular, the mouth end above, the tail end below (as in Plate XI. Fig. 15) : o, mouth- opening, surrounded by bristles; b, anal opening; c, gill-pore (porus branchialis) ; d, gill-body; e, stomach; /.liver; 17, small intestine ; h, gill-cavity ; t', notochord, below which is the aorta; A% aortal arch; ?, main stem of the gill-artery ; m, swellings on the branches of the latter; n, hollow vein; o, intestinal vein. that all the higher Vertebrates when in a very young state, ORIGIN OF THE GILL-OPENINGS. 329 reproduce, in accordance with the fundamental principle of Biogeny, the same process which was originally of the greatest importance to the development of the whole verte- brate tribe. This process was the differentiation of the Intestinal canal into two sections : an anterior, respiratory part, the gill-intestine, which serves only for breathing, and a posterior, digestive part, the stomach-intestine, which serves only for digestion. As we meet with this very characteristic differentiation of the intestinal tube into two, physiologically, very distinct main sections, not only in the Amphioxus, but also in the Ascidian and the Appen- dicularia, we can safely conclude that it also existed in our common ancestors, the Chorda Animals (Chordonia), especially as even the Acorn Worm (Balanoglossus) has it (Fig. 186, p. 86). All other Invertebrate Animals are entirely without this peculiar arrangement. The number of the gill-openings is still very large in the Amphioxus, as in Ascidians and in the Acorn Worm. In the Skulled Animals it is, on the contrary, very much lessened. Fishes mostly have from four to six pairs of gill- openings. In the embryos of Man and the higher Verte- brates also, only three or four pairs are developed, and these appear at a very early period. The gill-openings are perma- nent in Fishes, and afford a passage to the water which has been breathed in through the mouth (Figs. 191, 192, p. 113 ; Plate V. Fig. 13, ks). On the other hand, the Amphibians lose them partially, and all the higher Vertebrates entirely. In thelatter, only a single vestige of the gill-openings remains, the remnant of the first gill-opening. This changes into a part of the organ of hearing; from it originates the outer ear-canal, the tympanic cavity, and the Eustachian tube. ( 330 ) TABLE XXXVI. Srstematic Survey of the Development of the Human Intestinal System. N.B.— The parts marked thus f are processes from the iiitestinal tube. , Mouth-opening Kima orit _g Lips jMhia ** Jaws Maxilla &Z Teeth Jitntes Tongue T..nguebone OK liijuidft !i t Olivary gUnds Glaiididtr salivales 5 I. First main section of the Solt palate ^ Uvula Vrlunt palatinum Uvula. li Intestinal System: the Respiratory Intestine 2. Nose-cavitv (Cavum naif) Nose canal t Jaw cavities f Frontal cavities f EthiDoid cavity Jtfeatui narium Sinus Maxillarct Sinus f ran tales Sinus etltmoidales V ( Z I te tinal System: Digestive Intestine (*^r> (t YHk-sac'C or"enavel- blacldor) 1 Crooked intestine Jejunum ( Veficiila. umbiliea- lit) Heum £ < 11 1 = (Stomach Intes- Large intestine Colon -^ ^ tine). t Blind intestine Ccecum * 5 PKPIOCASTKR. (Tract a* 7 Posterior Intestine (Epigasttr) f V ermiform pri'cess Df Ithe ccecum Rectum Anal opening frocestut vermi- form is fectum Anut 11 8. Urinary Intestine (i'rogasttr) ( (f Primitive urinary sac t Urinary tube l-j-Uiiuarj bladder Allantois) Urethra "I THK MOUTH-SKELKTOX. 331 We havre already considered this remarkable formation, and will only call attention once more to the interesting fact that the human middle and external ear is the last remnant of the gill-opening of a Fish. The gill-arches, also, which separate the gill-openings, develop into very various parts. In Fishes they remain permanently as gill-arches, carrying the respiratory gill-tufts ; so also in the lowest Amphibia ; but in the higher Amphibia they undergo various modifica- tions in the course of development, and in all the three higher vertebrate classes, thus also in Man, the tongue-bone (os hyoides) and the bonelets of the ear originate from the gill-arches. (Of. Plates VI. and VII.) From the first gill-arch, from the centre of the inner surface of which the muscular tongue grows, proceeds the rudimentary jaw-skeleton ; the upper and lower jaws which enclose the cavity of the mouth and carry the teeth. The Acrania and Monorhina are entirely destitute of these important parts. They first appear in the genuine Fishes, and have been transmitted by these to the higher Vertebrates. The original formation of the human mouth- skeleton, of the upper and lower jaws, can thus be traced back to the earliest Fishes, from which we have inherited them. The teeth originate from the outer skin-covering which covers the jaws ; for, as the formation of the whole mouth-cavity takes place from the outer germ-layer, the teeth must, of course, also have developed originally from the skin- layer. This can be actually proved by close microscopic examination of the most delicate structural features of the teeth. The scales of Fishes, especially of Sharks, are, in this respect, exactly similar to their teeth (Fig. 283). Thus the human teeth, in their earliest origin, are modified fish- 332 THE EVOLUTION OF MAN. scales.188 On similar grounds we must regard the salivary glands, which open into the mouth-cavity, as really outer- skin (epidermic) glands, which have not developed, like the other intestinal glands, from the intestinal-glandular layer of the intestinal canal, but from the outer skin, from the horn-plate of the outer germ-layer. It is evident that, as the mouth develops in this way, the salivary glands must be placed genetically in the same series with the sweat, sebaceous, and milk glands of the epidermis. The human intestinal canal is therefore quite as simple in its original formation as the primitive intestine of the gastrula. It also resembles that of the lowest Worms. / It then differentiates into two sec- tions, an anterior gill-intestine, and a posterior stomach-intestine, like the intestinal canal of the Lancelet and the Ascidian. By the develop- ment of the jaws and gill-arches it is modified into a true Fish- intestine. Afterwards, however, the gill-intestine, which is a memorial of the Fish-ancestors, as such, is entirely lost. The parts that remain Shark (Centrcphorus calceus). take a wholly different form ; but Ou each rhomboid bone-tablet, notwithstanding that the anterior lying in the leather-skin, rises . , , a small, three-cornered tooth, section of our intestinal canal thus (After Gegenbaur.) surrenders entirely its original form of gill-intestine, it yet retains its physiological func- tion as a respiratory intestine ; for the extremely in- FIG. 283. — Scales THE BREATHING APPARATUS. 333 teresting and remarkable discovery is now made that even the permanent respiratory organ of the higher Vertebrates, the air-breathing lungs, has also developed from this anterior section of the intestinal canal. Our lungs, together with the wind-pipe (trachea) and the larynx, develop from the ventral wall of the anterior intestine. This entire great breathing-apparatus, which occupies the greater part of the chest (thorax) in the developed Man, is at first merely a very small and simple vesicle or sac, which grows out from the intestinal canal immediately behind the gills, and soon separates into two lateral halves (Figs., 284, c, 285, c ; Plate V. Figs. 13, 15, 1C, lu}. This vesicle occurs in all Vertebrates except in the two lowest classes, the Acrania and Cyclostomi. In the lower Vertebrates, however, it develops, not into lungs, but into an air-filled bladder of considerable size, occupying a great part of the body-cavity (coeloma'), and which is of quite a different significance from the lungs. It serves, not for breathing, but as an hydrostatic apparatus: for vertical swimming movements it is the swimming-bladder of Fish ; but the lungs of Man and of all other air-breathing Vertebrates develop from the same simple bladder-like appendage of the anterior intestine, which, in Fishes, becomes the swimming-bladder. Originally this sac also has no respiratory function, but serves only as an hydrostatic apparatus, augmenting or diminishing the specific gravity of the body. Fishes, in which the swimming-bladder is fully developed, are able to compress it, and thus to condense the air contained in it. The air sometimes also escapes from the intestinal canal through an air-passage which connects the swimming- bladder with the throat (pharynx}, and is expelled through 334 THE EVOLUTION OF MAX. the mouth ; in this way the circumference of the swim- ming-bladder is diminished, and the fish becomes heavier and sinks. When the animal is again about to ascend, the swimming-bladder is distended by remitting the com- FIG. 284. — Iiitestine of an embryonic Dog (which is represent^., in Fig. 137, vol. i. p. 382 ; after Bischoff), from the ventral side : a, gill-arches (four pairs); b, rudimentary throat and larynx; c, lungs ; d, stomach ;/, liver ; g, walls of the opened yelk-sac, into which the central intestine opens by a wide aperture ; h, rectum. FIG. 285. — The same intestine, seen from the right side : a, lungs ; b, stomach ; c, liver ; d, yelk-sac ; e, rectum. pressing force. This hydrostatic apparatus begins to be transformed into a respiratory organ in the Mud-fishes (Dipneusta), the blood-vessels in the wall of the swim- ming-bladder no longer merely separating air, but also inhaling fresh air, which has come in through the air- passage. This process is fully developed in all Amphibia. The original swimming-bladder here generally becomes a EVOLUTION OF THE LUNGS. 335 lung, and its air-passage a wind-pipe. The amphibian lung has been transmitted to the three higher vertebrate classes, and even in the lowest Amphibia the lung on either side is as yet a very simple, transparent, thin-walled sac — as, for instance, in our common Water-Newts, or Tritons, and very like the swimming-bladder of Fishes. The Amphibia have, it is true, two lungs, a right and a left; but in many Fishes also (in the ancient Ganoids) the swim- ming-bladder is double, the organ being divided into a right and a left half. On the other hand, the lung of the Ceratodus is single (p. 119). The earliest rudiment of the lung in the human embryo and in the embryo of all higher Vertebrates is also a simple, single vesicle, which does not separate till afterwards into a pair of halves — the right and the left lung. At a later period, the two vesicles grow con- siderably, occupy the greater part of the chest cavity, and lie one on each side of the heart ; even in Frogs we find that the simple sac, in the course of its development, is transformed into a spongy body of a peculiar, froth -like texture. This lung-tissue develops as a tree-like, branched gland, bearing berry-like appendages. The process by which the lung-sac was attached to the anterior intestine, which was originally very short, lengthens, by simple growth, into a long thin tube ; this tube is the wind-pipe (trachea) ; it opens above into the throat (pJcarynx), and below divides into two branches which pass into the two lungs. In the wall of the wind-pipe ring-shaped cartilages develop, which keep the whole distended ; at the upper end of this wind-pipe, below its entrance into the throat, the larynx, the organ of voice and speech, develops. The larynx occurs even in Amphibia in very various stages of development, and with the aid of 55 33^ THE EVOLUTION OF MAN. Comparative Anatomy we can trace the progressive develop- ment of this important organ from its very simple rudiment in the lower Amphibia up to the complex and vocal appara- tus represented by the larynx of Birds and Mammals. Though these organs of voice, speech, and air-respiration develop so differently in the various higher Mammals, they yet all arise from the same simple original rudiment — from a vesicle which grows out of the wall of the anterior intestine. We have thus satisfied ourselves of the interest- ing fact that both the respiratory apparatus of Vertebrates develop from the fore part of the intestinal canal ; first, the primary and more primitive water-respiring apparatus, the gill-body, which is altogether lost in the three higher vertebrate classes ; and, afterwards, the secondary and more recent air-breathing apparatus, which acts in Fishes only as a swimming-bladder, but as a lung from the Dipneusta upwards. We must say a few words about an interesting rudi- mentary organ of the respiratory intestine, the thyroid gland (tkyreoidea), the large gland situated in front of the larynx, and below the so-called " Adam's apple," and which, especially in the male sex, is often very prominent ; it is produced in the embryo by the separation of the lower wall of the throat (pharynx). This thyroid gland is of no use whatever to man ; it is only aesthetically interesting, because in certain mountainous districts it has a tendency to enlarge, and in that case it forms the " goitre " which hangs from the neck in front. Its dysteleological interest is, however, far higher ; for as Wilhelm Miiller of Jena has shown, this useless and unsightly organ is the last remnant of the "hypobranchial groove," which we have THE STOMACH. 337 already considered, and which, in the Ascidia and in the Amphioxus, traverses the middle of the gill-body, and is of great importance in conducting the food into the stomach (vol. i. p. 420; Plate XI. Figs. 14-16, 2/).189 The second main section of the intestinal canal, the stomach or digestive intestine, undergoes modifications no loss important than those affecting the first main section. On tracing the further development of this digestive section of the intestinal tube, we again find a very complex and composite organ eventually produced from a very simple rudiment. For the sake of rendering the matter more intelligible, we may distinguish the digestive intestine into three parts : the fore intestine (with the gullet and stomach) ; the middle intestine, the gall-intestine (with the liver and pancreas) ; the empty intestine (jejunum"), and crooked intestine (ileus] ; and the hind intestine (large intestine and rectum). Here we again find protuberances or appendages of the originally simple intestinal tube which change into veiy various structures. We have already discussed two of these appendages — the yelk-sac, which protrudes from the middle of the intestinal tube (Fig. 286, c), and the allantois, which grows out of the last portion of the pelvic intestine as a large sac-like protuberance (w). The protuberances from the middle of the intestine are the two great glands which open into the duodenum, the liver (k) and the ventral salivary gland. Immediately behind the bladder-like rudiment of the 1'ings (Fig. 286, 1) comes that portion of the intestinal tube which forms the most important part of the digestive apparatus, viz., the stomach (Figs. 284, d, 285, 6). This sac- 338 THE EVOLUTION OF MAX. shaped organ, in which the food is especially dissolved and digested, is not so complex in structure in the lower Verte- brates as in the higher. Thus, for instance, in many Fishes, it appears as a very simple spindle-shaped expansion at the FIG. 286. — Longitudinal section through an embryonic Chick on the fifth day of incubation : d, intestine ; o, mouth ; a, anus; I, lungs; h, livei ; q, mesentery ; v, auricle of heart ; k, ventricle of heart ; 6, arterial arches ; .'.aorta; c, yelk-sac; m, yelk-duct; u, allantois; r, stalk of allantois; n, :m m i< in ; IP, amnion-cavity ; s, serous membrane. (After Baer.) beginning of the digestive section of the intestine, which latter passes from front to rear in a straight line under the spinal column in the central plane of the body. In Mam- mals the rudiment of this organ is as simple as it thus is permanently in Fishes . but at a very early period the various parts of the stomach-sac begin to develop unequally. As the left side of the spindle-shaped pouch grows much more vigorously than the right, and as, at the same time, DEVELOPMENT OF THE STOMACH. 339 there occurs a considerable obliquity of its axis, it soon assumes an oblique position. The upper end lies more to the left and the lower end more to the right. The anterior end extends so as to form the long narrow canal of the gullet ((Esophagus) ; below the latter, the blind-sac of the stomach (fundus) bulges out to the left, and thus the later form of the stomach is gradually developed (Fig. 287, e ; Fig. 275, p. 317). The axis, which was originally verti- FIG. 287. — Human embryo of five weeks, from the ventral side ; opened (enlarged). The breast wall, abdominal wall, and liver, have been removed. 3, external nasal process ; 4, upper jaw ; 5, lower jaw ; a, tongue ; v, right, v', left ventricle of heart ; o', left auricle of heart ; b, origin of aorta ; 6' b" b'", 1st, 2nd, 3rd aorta-arches; c c' c", hollow vein ; oe, lungs (y, lung-arteries) ; e, stomach ; m, primitive kidneys (j, left yelk- vein ; s, pylorns ; a, right yelk- artery; n, navel-artery; it, navel- vein ); x, yelk-duct ; i, terminal intestine ; 8, tail; 9, fore-limb; 9', hind-limb. (After Coste.) cal, now inclines from a higher point on the left to a lower on the right, and continually acquires a more transverse direction. In the outer stratum of the stomach- wall, and from the intestinal-fibrous layer, develop the strong muscles which perform the powerful digestive movements. In 34O THE EVOLUTION OF MAN. the inner stratum, on the contrary, innumerable minoT glands develop from the intestinal-glandular layer. These are the peptic glands, which supply the most important digestive fluid — the gastric juice. At the lower extremity of the pouch of the stomach a valve develops, which, as the pylorus, separates the stomach from the small intestine (Fig. 275, d). The disproportionately long middle intestine, or small intestine, now develops below the stomach. The develop- ment of this section is very simple, and is essentially caused by a very rapid and considerable longitudinal growth. Originally this section is very short, straight, and simple ; but immediately below the stomach a horseshoe bend, or loop, begins to appear at a very early period in the intestinal canal, simultaneously with the separation of the intestinal tube from the yelk-sac and with the development of the mesentery. (Cf. Plate V. Fig. 14, g, and Fig. 136, vol. i. p. 381.) Before the abdominal wall closes, a horseshoe-shaped loop of intestine (Fig. 136, ra) protrudes from the ventral opening of the embryo, and into the curve of this the yelk-sac or navel- bladder opens (n). The thin, delicate membrane which secures this intestinal loop to the ventral side of the vertebral column, and occupies the inside of this horseshoe curve, is the first rudiment of the mesentery (Fig. 286, g). The most prominent part of the loop into which the yelk-sac opens (Fig. 287, x), and which is afterwards closed by the intestinal navel, represents that part of the small intestine which is afterwards called the crooked intestine (Ueurri). Soon a very considerable growth of the small intestine is observ- able ; and in consequence, this part has to coil itself in many loops. The various parts of the small intestine which wo THE SMALL INTESTINE. 34! have yet to distinguish differentiate later in a very simple way; these are the gall-intestine (duodenum), which is next to the stomach, the long empty intestine (jejunum) which succeeds, and the last section of the small intestine, the crooked intestine (ileum). The two large glands which we have already named, the liver and the ventral salivary gland, grow out, as protuber- ances, from the gall-intestine, or duodenum. The liver first appears in the form of two small sacs, situated right and left just behind the stomach (Figs. 284, /, 285, c).. In many low Vertebrates the two livers remain quite separate for a long time (in the Myxinoides for life), and coalesce only imper- fectly. In higher Vertebrates, on the other hand, the two livers coalesce more or less completely at an early period, and constitute one large organ. The intestinal-glandular layer, which lines the hollow, pouch-like rudiment of the liver, sends a number of branched processes into the investing intestinal-fibrous layer; as these solid processes (rows of gland-cells) again branch out, and as their branches coalesce, the peculiar netted structure of the developed liver is produced. The liver-cells, as the secreting organs which form the bile, all originate from the intestinal-glandular layer. The fibrous mass of connective tissue, which joins this great cellular network into a large compact organ, and which invests the whole, comes, on the other hand, from the intestinal-fibrous layer. From the latter originate also the great blood-vessels which traverse the entire liver, and the innumerable netted branches of which are interlaced with the network of the liver-cells. The gall-ducts, which traverse the entire liver, collecting the bile and discharging it into the intestine, originate as intercellular passages along 342 THE EVOLUTION OF MAN. the axis of the solid cell-cords ; they all discharge into the two primitive main gall or biliary ducts, which originate from the base of the two original protuberances of the intestine. In Man, and in many other Vertebrates, these two ducts afterwards unite, and form one simple gall-duct, which discharges into the ascending portion of the gall- intestine. The gall bladder originates as a hollow pro- tuberance of the right primitive liver duct. The growth of the liver is at first exceedingly rapid ; in the human embryo, even in the second month, it attains such dimen- sions that during the third month it occupies by far the largest part of the body-cavity (Fig. 288). At first, both FIG. 288. — Chest and abdominal viscera of a human embryo of twelve weeks, in natural size. (After Koelliker.) The head ia omitted ; the chest and abdominal walla removed. The greater part of the abdominal cavity is occupied by the liver, from an opening in the centre of which the blind- intestine (ccecum, v), with the worm appendage, protrudes. Above the diaphragm the heart is visible in the centre, with the small lun^s on the right and left. halves are equally well developed ; afterwards the left half lies considerably behind the right. In consequence of the asymmetrical developm-ent and alteration in the position of the stomach and other abdominal viscera, the whole of the liver is eventually forced over on to the right side. Although the growth of the liver is, afterwards, not so excessive, even at the end of gestation, it is comparatively much larger in the embryo than in the adult. In the latter, its weight THE LARGE INTESTINE. 343 in proportion to that of the whole body is as 1:86; in the former, as 1 : 18. The physiological significance of the liver during embryonic life — which is very great — depends espe- cially on the part it plays in the formation of blood, and less on its secretion of bile. From the gall-intestine, immediately behind the liver, grows another large intestinal gland, the ventral -salivary gland, or pancreas. This organ, which occurs only in Skulled Animals, also develops as a hollow sac-shaped protuberance of the intestinal wall. The intestinal-glan- dular layer of the latter sends out branching shoots, which afterwards become hollow. The ventral-salivary gland, just like the salivary glands of the mouth, develops into a large and very complex gland shaped like a bunch of grapes. The outlet of this gland (ductus pancreaticus), through which the pancreatic juice passes into the gall-intestine, seems to be at first simple and single ; afterwards it is often double. The last section of the intestinal tube, the terminal intestine or large intestine (epigaster), in mammalian embryos, is, at first, a very simple, short, and straight tube, opening posteriorly through the anus. In the lower Ver- tebrates it retains this form throughout life. In Mammals, on the other hand, it grows to a considerable size, coils, and differentiates into different sections, of which the foremost and longest is called the colon, the shorter and hinder the rectum. At the commencement of the former a valve (valvula Bauhini) forms, which divides the large intestine from the small intestine ; behind appears a pouch-like protuberance, which grows larger and becomes the blind- intestine (ccecitm) (Fig. 288, v). In plant-eating Mammals 344 THE EVOLUTION OF MAN. this becomes very large, while in those which eat flesh it remains very small, or is entirely aborted. In Man, as in most Apes, the beginning of the blind intestine alone becomes wide ; its blind end remains very narrow, and afterwards appears only as a useless appendage of the former. This " vermal appendage " is interesting in dys- teleology as a rudimentary organ. Its only importance in Man consists in the fact that now and then a raisin-stone, or some other hard, indigestible particle of food becomes lodged in its narrow cavity, causing inflammation and suppuration, and, consequently, killing individuals other- wise perfectly healthy. In our plant-eating ancestors this rudimentary organ was larger, and was of physiological value. Finally, we must mention another important appendage of the intestinal tube; this is the urinary bladder (uro- cystis) with the urinary tube (urethra), which in develop- ment and in morphological character belong to the intestinal system. These urinary organs, which act as receptacles and excretory passages for the urine secreted by the kidneys, originate from the inner part of the allantois-stalk. The allantois develops, as a sac-like protuberance, from the anterior wall of the last section of the intestine (Fig. 286, u). In the Dipneusta and Amphibia, in which this blind-sac first appears, it remains within the body-cavity (coeloma), and acts entirely as a urinary bladder. In all Amniota, on the other hand, it protrudes considerably out of the body- cavity of the embryo, and forms the large embryonic " primitive urinary sac," which, in higher Mammals, forms the placenta. At birth this is lost; but the long allantois- stalk (r) remains, its upper portion forming the central navel THE URINARY BLADDER. 345 band of the urinary vesicle (ligamentum vesico-umbilicale medium), a rudimentary organ which extends as a solid cord from the top of the urinary bladder to the navel. The lower part of the allantois-pedicle (the " urachus ") remains hollow, and forms the urinary bladder. At first, in Man, as in the lower Vertebrates, this organ discharges into the last section of the posterior intestine, and there is, there- fore, a true " cloaca," receiving both urine and excrement ; but, among the Mammals, this cloaca is permanent only in the Cloacal Animals, or Monotremes, as in Birds, Reptiles, and Amphibia. In all other Mammals (Marsupialia and Placentalia) a transverse partition forms at a later period, and separates the urinary-sexual aperture in front from the anal aperture behind. (Cf. Chapter XXV.) ( 346 ) EXPLANATION OF PLATE I.— (FRONTISPIECE ) DEVELOPMENT OF THE FACE. The twelve figures in Plate I. represent the faces of fonr different Mammals in three distinct stages of individual evolution: Hi-Mm that of Man, Bi-Bui of the Bat, Ci-Cni of the Cat, Si-Sin of the Sheep. The three different stages of evolution have been chosen to correspond as far as possible ; they have been reduced to about the same size, and are seen from in front. In all the figures the letters indicate the same : a, eye ; v, fore- brain ; m, mid-brain ; s, frontal process ; fe, nose-roof ; o, upper jaw process (of the first gill-arch) ; u, lower jaw process (of the first gill-arch) ; h, second gill-arch ; d, third gill-arch ; r, fourth gill-arch ; g, ear-fissure (remains of the front gill-opening) ; z, tongue. (Cf. Plates VI. and VII., Figs. 232-236, p. 243 ; also Figs. 123, 124, vol. i. p. 370.) TABLE XXXVII. SYSTEMATIC SURVEY or THE MOST IMPORTANT PERIODS IN TITB PHYLOGENY OF THE HUMAN INTESTINAL SYSTEM. I. First Period: Intestine of Gaatraea (Figs. 274-277; Plate V. Figs. 9, 10). The whole intestinal system is a simple pouch (primitive intestine), the simple cavity of which has one orifice (the primitive mouth). II. Second Period : Intestine of the Scolecida (Plate V. Fig. 11). The simple intestinal tube widens in the middle into the stomach, and icquires, at the end opposite to the primitive mouth, a second opening (primitive anus) ; as in the lower Worms. III. Third Period : Intestine of Chorda Animals (Fig. 281 ; Plate V. Fig. 12). The intestinal tube differentiates into two main sections — the respiratory intestine with gill-openings (gill-intestine) in front, the digestive intestine with stomach-cavity (stomach-intestine) behind ; as in Aocidia. SURVEY OF IIl'.MAN INTESTINAL SYSTEM. 347 IV. Fourth Period : Intestine of Skull-less Animals (Acranta) (Fig. 282; Plate XI. Fig. 15). The gill-streaks appear between the gill-openinps of the respratory iutestine ; a liver blind-sac grows from the stomach-pouch of the digestive intestine ; as in the Amphioxus. V. Fifth Period : Intestine of Cyclostoma (Plate XI. Fig. 16). The thyroid gland develops from the ciliated groove on the base of the gills (hypobranchial groove). A compact liver-gland develops from the liver bliud-sac. VI. Sixth Period: Intestine of Primitive Fishes (p. 114). Cartilaginous gill-arches appear between the gill-openings. The fore- most of these form the lip-cartilages and the j:iw-skek ton (upper and lower jaw). The swimming-bladder grows from the pharynx. The ventral-salivary gland appears near the liver, as in Selachii. VII. Sei-mth Period: Intestine of Dipneusta (p. 118). The swimming-bladder modifies into the lungs. The mouth-cavity becomes connected with the nose-cavity. The urinary bladder grows from the last section of the intestine, as in L,epidosiren. VIII. Eighth Period : Intestine of Amphibia (p. 126). The gill-openings close. The gills are lost. The larynx originates from the upper end of the trachea. IX. Ninth Period: Intestine ofMonotremes (p. 145). The primitive mouth and nasal cavity is separated by the horizontal palate-roof into the lower mouth-cavity (food passage) and the upper nose- cavity (air passage); as in all Aniniou Animals. X. Tenth Period: Intestine ofMarsv^'al* (p. 149). The existing cloaca is separated by a partition wall into an anterior urinary-sexual aperture and a posterior anal aperture. XI. Eleventh Period: Intrxtine of Catarhine Aprs (p. 176). AH parts of the intestine, and especially the teeth-apparat-i.-., acqniro the charuoteribtic development common to Alan and Catarhiue Apes. CHAPTER XXIV. DEVELOPMENT OF THE VASCULAR SYSTEM. Application of the Fundamental Law of Biogeny. — The Two Sides. — Heredity of Conservative Organs. — Adaptation of Progressive Organs. — Ontogeny and Comparative Anatomy complementary of each other. — New "Theories of Evolution" of His. — The ''Envelope Theory" and the " Waste-rag Theory." — Main Germ and Supplementary Germ. — Forma, tive Yelk and Nutritive Yelk. — Phylogenetic Origin of the latter from the Primitive Intestine.— Origin of the Vascular System from the Vascular Layer, or Intestinal-fibrous Layer. — Phylogenetic Significance of the Ontogcnetic Succession of the Organ-systems and Tissues. — Deviation from the Original Sequence ; Ontogenetic Heterochronism. — Covering Tissue. — Connective Tissue. — Nerve-muscle Tissue. — Vascular Tissue. — Relative Age of the Vascular System. — First Commencement of the Latter; Coeloma. — Dorsal Vessel and Ventral Vessel of Worms. — Simple Heart of Ascidia. — Atrophy of the Heart in the Amphioxus. — Two-chambered Heart of the Cyclostoma. — Arterial Arches of the Selacliii. — Double Auricle in Dipneusta and Amphibia. — Double Ven. tricle in Birds and MammaK — Arterial Arches in Birds and Mammals. Germ-history (Ontogeny) of the Human Heart. — Parallelism of the Tribal-history (Phylogeny). " Morphological comparison of the adult conditions should naturally precode the study of the earliest conditions. Only in this way can the investigation of the history of development proceed in a definite direction ; it is thus provided, as it were, to see each step in the formative process in its true relation with the condition which is finally to be reached. Treat- ment of the history of development without preparatory study is only too APPLICATION OP THE LAW OF BIOGENY. 349 likely to lead to groping in the dark ; and it not infrequently leads to the most unfortunate results— far inferior to those which might be established beyond question without any study of the history of development."— ALEXANDEK BUAUN (1872). IN applying to Organogeny the fundamental law of Bio- geny, we have already afforded some conception of the degree in which we may follow its guidance in the study of tribal history. The degree differs greatly in the different organ-systems ; this is so, because the capacity for trans- mission on one side, and the capacity for modification on the other, vary greatly in the different organs. Some parts of the body cling tenaciousl}7 to the inherited germ-history ; and, owing to heredity, accurately retain the mode of evolution inherited from primaeval animal ancestors ; other parts of the body, on the contrary, exhibit very small capacity for strict heredity, and have a great tendency to assume new kenogenetic forms by adaptation, and to modify the original Ontogeny. The former organs represent, in the many-celled community of the human organism, the con- stant or conservative; the latter, on the contrary, the changeable or progressive element of evolution. The mutual interaction of both elements determines the course of his- torical evolution. Only to the conservative organs, in which Heredity pre- ponderates over Adaptation, in the course of tribal evolu- tion, can we directly apply the Ontogeny to the Phylogeny, and can infer, from the palingenetic modification of the germ-forins, the primseval metamorphosis of the tribal forms. In the progressive organs, on the contrary, in which Adap- tation has acquired the ascendency over Heredity, the original course of evolution has, usually, been so changed, 35O THE EVOLUTION OF MAN. vitiated, and abbreviated, in the course of time, that we can gain but little certain information as to the tribal- history from the kenogenetic phenomena of their germ- history. Here, therefore, Comparative Anatomy must come to our help, and it often affords much more important and trustworthy disclosures as to Phylogeny than Ontogeny is able to impart. It is, therefore, most important, if the fundamental law of Biogeny is to be correctly and critically applied, to keep its two sides continually in view. The first half of this fundamental law of evolution enables us to use Phylogeny, as it shows us how to gain an approximate knowledge of the history of the tribe from that of the germ : the germ-form reproduces, by Heredity, the corre- sponding tribal form (Palingenesis). The other half of the law, however, limits this guiding principle, and calls attention to the foresight with which it must be employed ; it shows us that the original reproduction of the Phylogeny in the Ontogeny has been in many ways altered, vitiated, and abbreviated, in the course of millions of years. The germ-form has deviated, by Adaptation, from the corre- sponding tribal form i^Kenogenesis) ; the greater this devia- tion, the more are we compelled to employ Comparative Anatomy in the study of Phylogeny. Perhaps in no other system of organs of the human body is this so greatly the case as in the vascular system (vas- cular, or circulatory apparatus), the development of which we will now examine. If we attempted to infer the original structural features of our older animal ancestors solely from the phenomena which the individual develop- ment of these organ-systems, in the embryo of Man and of other high Vertebrates, exhibit, we should obtain wholly HIS ON THE VASCULAR SYSTEM. 351 erroneous views. By many influential embryonic adap- tations, among which the development of an extensive nutritive yelk must be regarded as the most important, the original course of development of the vascular system ha« been so altered, vitiated, and abbreviated, in the higher Vertebrates, that no, or very little, trace of many of the most important phylogenetic features are retained in the Ontogeny. Such explanation as is afforded by the latter would be entirely useless to us if Comparative Anatomy did not lend its aid, and afford us the clearest guidance in our search for tribal history. Comparative Anatomy is, therefore, especially important in helping us to understand the vascular system, and, equally, the skeleton system, so that, without its guidance, it is unsafe to take a single step in this difficult field. Positive proof of this assertion can be gained by studying the complex vascular system as explained in the classical works of Johannes Miiller, Heinrich Rathke, and Karl Gegenbaur. An equally strong negative proof of the asser- tion is afforded by the ontogenetic works of Wilhelm His, an embryologist of Leipsic, who has no conception of Com- parative Anatomy, nor consequently, of Phylogeny. In 1868, this industrious but uncritical worker published cer- tain comprehensive " Studies of the First Rudiment of the Vertebrate Body," which are among the most wonderful pi eductions in the entire literature of Ontogeny. As the author hopes to attain a " mechanical " theory of develop- ment by means of a most minute description of the germ- history of the Chick alone, without the slightest reference to Comparative Anatomy and Phylogeny, he falls into errors which are unparalleled in the whole literature of 352 THE EVOLUTION OF MAN. Biology, rich as this unfortunately is in errors. Only in the magnificent germ-history of the Bombinator by Alexander Goette is incomprehensible nonsense and derision of every reasonable causal connection in evolution more nakedly set forth. (Of. vol. i. pp. 65, G6.) His announces, as the final result of his investigations. " that a comparatively simple law of growth is the only essential in the first process of evolution. All formation, whether it consist in fission of layers, or in the formation of folds, or in complete articula- tion, results from this fundamental law." Unfortunately the author does not say in what this all-embracing " law of growth" really consists; just like other opponents of the theory of descent who substitute a great " law of evolution," without telling anything of its nature. From the study of the ontogenetic works of His, on the ol er hand, it soon becomes evident that he conceives form-constructing " Mother Nature " merely as a kind of clever dressmaker ; by cutting out the germ-layers in various ways, by bend- ing, folding, pulling, and splitting them, this clever semp- stress easily brings into existence the various forms of animal species, by " development "(!). The bendings and foldings especially play the most important part. Not only the differentiation of head and trunk, of right and left, of central stem and periphery, but also the rudiment of the limbs, as also the articulation of the brain, the sense-organs, the primitive vertebral column, the heart, and the earliest intestines, can be shown, with convincing necessity (!) to be mechanical results of the first development of folds. Most grotesque is the mode in which the dressmaker proceeds in forming the two pairs of limbs. Their first form is deter- mined by the crossing of four folds bordering the body, HIS ON TIIE VASCULAR SYSTEM. 353 ' like the four corners of a letter." Yet this wonderful " envelope theory " of the vertebrate limbs is surpassed by the " waste-rag theory " (Hollen-lappen Theorie) which His gives of the origin of the rudimentary organs : " Organs (like the hypophysis and the thyroid gland) to which no physiological part has yet been assigned, are embryonic remnants, comparable to the clippings, which in the cutting of a dress cannot be entirely avoided, even by the most economical use of the material " (!). Nature, therefore, in cutting out, throws the superfluous rags of tissue into the waste heap. Had our skull-less ancestors of the Silurian age had any presentiment of such aberrations of intellect of their too speculative human descendants, they would certainly have preferred relinquishing possession of the hypobranchial groove on the gill-body, instead of trans- mitting it to the extant Amphioxus, and of leaving a remnant of it to us, in the equally unsightly as useless thyroid gland. (Of. p. 336). It will probably be thought that the ontogenetic " dis- coveries " of His, which appear in a doubly comical light in consequence of the accompanying display of mathematical calculations, can only have occasioned momentary amuse- ment in critical scientific circles. Far from it ! Immedi- ately after their appearance, they were not only much praised as the beginning of a new " mechanical " era in Ontogeny, but they have even yet numerous admirers and adherents, who seek to spread the scientific errors of His as far as possible. On this account, I have felt myself obliged to point out emphatically the complete falsity of these views. The vascular system affords especial occasion for this ; for among the most important advances which Hu? 354 THE EVOLUTION OF MAN. claims to have caused by his new conception of genrv history, is, according to him, his discovery that " the blood and tissue of the connective substance" (that is to say, the greatest part of the vascular system) " do not originate from the two primary germ-layers, as do all the other organs, but from the elements of the white yelk." The latter is designated as " supplementary yelk, or parablast," to distinguish it from the "main-germ, or archiblast" (the germ-disc composed of the two primary germ -layers). The whole of this artificial development theory of His, and above all the unnatural distinction between the supple- mentary and the main germ, collapses like a card house when the Anatomy and Ontogeny of the Amphioxus, that invaluable lowest Vertebrate, is contemplated, which alone can elucidate the most difficult and darkest features in the development of the higher Vertebrates, and thus also of Man. The gastrula of the Amphioxus alone overthrows the whole artificial theory ; for this gastrula teaches us that all the various organs and tissues of complete Verte- brates originally developed entirely from the two primary germ-layers. The developed Amphioxus, like all other Vertebrates, has a differentiated vascular system and a skeleton of "connective substance tissues" extending throughout its body, and yet there is in this case no "sup- plementary germ " from which these tissues can originate thus, contrasting with the other tissues. The larva; of the Amphioxus, arising from the original bell-gastrula (archigastrula), in its further development, throws the most important rays of light also upon the diffi- cult history of development of the vascular system. In the first place, it answers the very important question, which THE VASCULAR SYSTEM. 355 we have already frequently indicated, as to the origin of the four secondary germ-layers ; it clearly shows that the skin-fibrous layer originates from the exoderm, the intes- tinal-fibrous layer, on the contrary, in an analogous manner, from the entoderm of the gastrula ; the cavity thus caused between the two fibrous layers is the first rudiment of the body-cavity, or the coelom (Figs. 50, 51, vol. i. p. 236). As the Amphioxus larva thus shows that the fission of the layers is the same in the lowest Vertebrates as in the Worms, it at the same time represents the phylogenetic connection be- tween the Worms and the higher Vertebrates. As, more- over, the primitive vascular stems in the Amphioxus originate in the intestinal wall, and in this, as in the em- bryos of all other Vertebrates, proceed from the intestinal- fibrous layer, proof is afforded us that the earlier embryolo- gists were right in calling the latter the vascular layer. Finally, the Comparative Ontogeny of the different verte- brate classes further convinces us that the vascular layer is originally everywhere the same. The vascular system in Man, as in all Skulled Animals, forms a complex apparatus of cavities, which are filled with juices, or fluids, containing cells. The vessels play an important part in the nourish- ment of the body ; some of them conduct the nutritive blood fluid round in the different parts of the body (blood- vessels) ; some collect the wasted juices and discharge them from the tissues (lymph-vessels). With the latter, the great "serous cavities" are also connected, especially the body-cavity, or coeloma. The heart, acting as a centre of motion for the regular circulation of the juices, is a strong muscular pouch, which contracts in regular pulsations, and is provided with valves, like those of a pump apparatus 356 THE EVOLUTION OF MAN. This constant and regular circulation of the blood alone makes the complex change of substance with the higher animals possible. Important as is the vascular system in the more highly developed and differentiated animal body, it is not, however, an apparatus as indispensable to animal life as is generally supposed. In the older theory of medicine the blood was regarded as the real source of life, and " humoral pathology" referred most diseases to " corrupt blood-mixture." Simi- larly, the blood plays the most important part in the pre- vailing, obscure conception of Heredity. Just as half-blood, pure blood, etc., etc., are yet common phrases, so it is widely believed that the transmission, by Heredity, of definite morphological and physiological characters from the parent to the child "lies in the blood." That this customary notion is entirely false, is easily seen from the fact that, neither in the act of procreation is the blood of the parents directly transmitted to the procreated germ, nor does the embryo acquire blood at an early period. As we have already seen, not only the separation of the four secondary germ-layers, but also the beginning of the most impor- tant organs, takes place, in the embryos of all Vertebrates, before the rudiment of the vascular systems, of the heart and blood, is formed. In accordance with this ontogenetic fact, we must, from a phylogenetic point of view, regard the vascular system as the most recent, the intestinal system, on the contrary, as the oldest formation of the animal body. The origin of the vascular system is, at least, much later than that of the intestinal system. If the fundamental law of Biogeny is rightly appreciated, it is possible, from the ontogenetic sequence, in which the various organs of the AGE OF THE VASCULAR SYSTEM. 357 animal body consecutively originate in the embryo, approxi- •Thately to infer the phylogenetic sequence, in which these organs gradually developed, one after the other, in the ancestral line of animals. In the " Gastraea theory " I made the first attempt to establish the phylogenetic significance of the ontogenetic sequence of the organ-systems; but it must be remarked that this sequence is not always iden- tical in the higher animal tribes. In Vertebrates, and therefore also in our own ancestral line, the organ-systems may be ranged according to age, in something like the following order : I. The skin-system (A} and the intestinal system (B~). II. The nerve (C) and muscular systems (Z>). III. The kidney system (#). IV. The vascular system (F). V. The skeleton system (£). VI.. The sexual system (H). (Cf. Table XXXIX, p. 367.) In the first place, the gastrula proves that in all animals with the exception of the Primitive Animals (Protozoa), — therefore, in all Intestinal Animals (Metazoa), — two primary organ-systems originally arose simultaneously and first; these were the skin-system (skin-covering) and the intes- tinal system (stomach-pouch). The first is represented, in its earliest and simplest form, by the skin-layer or exoderm, the latter by the intestinal layer or entoderm of the Gastraea. As we can ascribe the same origin, and, therefore, also the same morphological significance, to these two primary germ- layers in all Intestinal Animals, from the simplest Sponge to Man, the homology of these two layers seems sufficient proof of the above assumption. Immediately after the differentiation of the two primary germ-layers, an inner or outer skeleton develops in many lower animals (e.g., in Sponges, Corals, and other Plant 358 THE EVOLUTION OF MAN. Animals). In the ancestors of Vertebrates, the development of the skeleton did not take place till much later, in the Chorda Animals (Ckordonia). In them, after the sKa- system and the intestinal system, two other organ-systems simultaneously arise ; these are the nervous and the mus- cular systems. The way in which these two organ-systems which mutually condition each other, developed simulta- neously and independently, in reciprocal action and yet in opposition to each other, was first explained by Nicholaus Kleinenberg in his excellent monograph on the Hydra, the common fresh-water Polyp.190 In this interesting little animal, single cells of the skin-layer send fibre-shaped pro- cesses inward, which acquire the power of contraction, the capacity, characteristic of the muscles, of contracting in a constant direction. The outer, roundish part of the exo- derm cell remains sensitive and acts as the nervous element, the inner, fibre-shaped part of the same cell becomes con- tractile, and, incited to contraction by the former part, acts as the muscular element (Fig. 293). These remarkable neuro-muscular cells thus still unite in a single individual of the first order the functions of two organ-systems. One step further; the inner, muscular half of the neuro-muscular cell (Fig. 293, m) acquires its own nucleus, and separates from the outer, nervous half (n), and both organ-systems have their independent element of form. The fission of the muscular skin-fibrous layer from the nervous skin-sensory layer in embryonic Worms confirms this important phylo- genetic process (Figs. 50, 51, vol. i. p. 236). These four organ-systems, which have been mentioned, were already in existence, when an apparatus developed, tertiarily, in the human ancestral line, which, at first THE KIDNEYS. 359 sight, seems of subordinate significance, but which proves; by its early appearance in the animal series and in the embryo, that it must be very ancient and, consequently, of great physiological and morphological value. This is ' the urinary apparatus, or kidney system, the organ-system which secretes and removes the useless fluids from the body. We have already seen how soon the primitive kidneys appear in the embryo of all Vertebrates, long before any trace of the heart is discoverable. Correspondingly, we also find a pair of simple primitive kidney ducts (the so-called excretory ducts or lymphatic vessels) almost universally diffused in the Worm tribe, which is so rich in forms. Even the lowest classes of Worms, which have as yet neither body-cavity nor vascular system, are furnished with these primitive kidneys (Fig. 280, nc, p. 327). It was only in the fourth place, after the kidney system, that the vascular system developed in our invertebrate ancestors; this is plainly shown in the Comparative Anatomy of Worms. The lower Worms (Accelomi) possess no part of the vas- cular system, no body-cavity, no blood, no heart, and no vessels ; this is the case, for example, in the comprehensive group of the Flat Worms (Plathelminthes),t}\e Gliding Worms (Turbellaria), the Sucking Worms (Trematoda), and the Tape Worms. In the higher Worms, which are therefore called Ccelomati, a body-cavity (codoma), filled with blood, first begins to form j and, side by side with this, special blood-vessels then also develop. These features have been transmitted from the Ccelomati to the four higher animal tribes. These organ-systems are common to Vertebrates and to the three higher animal tribes, the Articulated Animals 360 THE EVOLUTION OF MAN. (Arthropoda), the Soft-bodied Animals (MollvRca), and the Star Animals (Echinoderma), and we may, therefore, infer that they have all acquired these, as a common inheritance from the Ccelomati ; but we now meet with a passive apparatus of movement, the skeleton system, which, in this form, is exclusively peculiar to Vertebrates. Only the very first rudiment of this, the simple notochord, is found in Ascidia, which are the nearest invertebrate blood-relations of Vertebrates. We infer from this, that the common ancestors of both, the Chorda Animals, did not branch off from the Worms till a comparatively late period. The notochord is, it is true, one of those organs which appear at a very early period in the vertebrate embryo ; but this is clearly due to an ontogenetic heterochronisin, to displace- ment in time in the germ-history, that is, a gradual dis- arrangement in the original phylogeiietic sequence, caused by embryonic adaptation. On Comparative Anatomical grounds it may safely be assumed, that the first origin of the skeleton system did not precede, but followed that of the kidney system and of the vascular system, although Ontogeny appears to indicate the contrary. Last of all the organ-systems, the sexual system finally developed, in the sixth place, in our ancestors ; of course it must be understood that this was last, in the sense that the sexual apparatus acquired the independent form of a special organ-system subsequently to all the other organs. The simplest form, that of reproductive cells, is certainly very ancient. Not only the lowest Worms and Plant Animals propagate sexually, but this was also probably the case in the common parent-form of all Metazoa, in the Gastrjea ; but in all these low animals, the reproductive cells do not AGE OF THE TISSUES. 361 constitute special sexual organs in a morphological sense; they are rather, as we shall soon see, simple component parts of other organs. Like the organ-systems of the human body, the tissues, \vhich compose these systems, are of different ages and of vaiying morphological value. As we were justified in J rawing an inference as to the phylogenetic sequence in age of the organ-systems, from the ontogenetic sequence in which they successively appear in the embryo, so are we justified in inferring the order in which the tissues originated during the course of tribal history, from the sequence of the stages in germ-history. The result of this is a phylogenetic classification (Table XXXVIII.) of the tissues of the human body, similar to that of the organs (Table XXXIX., p. 367). The tissues of the human body, arising by division of labour, the separation and the connection of the component cells, may be distributed, with reference to their develop- ment, in the four following distinct groups : — 1, covering- tissue (epithelium)-, 2, connective tissue (connectivum); 3, nerve and muscular tissue (neuro-musculum') ; and 4, vas- cular tissue (vasaliuiri). Of these, in accordance with the Gastraea theory, we must regard the covering-tissue as the oldest and most original form, as the actual primary 01 primitive tissue ; the three other main forms must, on the other hand, be considered as secondary or derived forms, which developed at a later period from the covering-tissue ; the connecting-tissue first, then the neuro-muscular, and lastly the vascular tissue. The oldest and most original form of tissue is, un- doubtedly, the covering-tissue (epithelium}, the cells of 362 THE EVOLUTION OF MAN. which are arranged in a simple strata-like way, and extend over the outer and inner surface of the body as a protective and secreting cover. This is proved by the simple fact that the formation of the tissues of the animal body begins with the formation of the gastrula, and that the latter itself consists solely of two simple epithelial strata, of the skin-layer (Fig. 274, e), and of the intestinal-layer (i). Histologically, the two primary germ-layers are simple epithelia. When these, afterwards, separate into the four secondary germ-layers, the skin-sensory layer becomes the outermost of the external coverings (dermal-epithelium) ; the intestinal-glandular layer becomes the innermost of the internal coverings (gastral-epithelium). The tissue of the outer skin and of all its appendages, such as nails (Fig. 289), FIG. 289. — Tissue of the nails (flattened epithelium) : a-e, cells of the upper strata ; /, g, cells of the lower strata. FIG. 290. — Tissue of the covering of the small intestine (columnar epithelium) : a, side view of three cells (with thicker, porous borders) ; b, surface view of four cells. (After Frey.) hairs, skin-glands, etc., arise from the skin-sensory layer. (Cf. Table XXIX., p. 232.) The inner covering of the intes- tinal tube and of its intestinal glands originates, on the other hand, from the intestinal-glandular layer (Fig. 290). TISSUES. Connective tissue (connectivum) must be regarded as forming, in order of phylogenetic age, the second main group of tissues. This is morphologically characterized by the intercellular substance, which develops between the FIG. 291. — Jelly-like tissue from the vitreous body of an embryo of four months (round cells as jelly-like intercellular substance). FIG. 292. — Cartilage-tissue of the fibrous or netted cartilage of the ear- shell : a, cells ; b, intercellular mass ; c, fibres in the latter. (After Frey. ) cells, physiologically, by the double part which it plays, as connecting substance and as complementary substance between the other tissues, as an inner supporting substance and as a protective covering for the inner organs. Of the numerous forms and varieties of connective tissue, we regard the jelly-like tissue (Fig. 291 : Fig. 6, vol. i. p. 126), the fatty tissue, and the chorda tissue as the earlier ; the fibrous, cartilaginous (Fig. 292), and bone-tissue (Fig. 5, vol. i. p. 126) as the more recent formations. All these various forms of connective tissue are products of the middle germ-layer, or mesoderm ; or, more accurately, of the two fibrous layers, of which the skin-fibrous layer is originally derived from the exoderm, the intestinal-fibrous layer from the entoderm. The nerve-muscular tissue (neuro-musculum) is of much more recent origin than the connective tissue. If epithelial tissue represents a primary period in tribal history, and THE EVOLUTION OF MAN. 364 connective tissue a secondary period, then we may cha- racterize a third, much later period, by nerve-muscle tissue. FIG. 294. FIG. 293. — Nerve-muscle tissue. Three cells from Hydra : nt outer, nervous ; m, inner, muscular part of the cells. (After Kleinenberg.^ FIG. 294. — Nerve-tissue (from a spinal nerve knot) : a, anterior, I, posterior root of the spinal nerve ; d, e, fibrous nerve-stem ; f,g,h,i, nerve cells in ganglion (/, unipolar, g, h, bipolar cells) ; k, I, nerve fibres. (After Frey.) FIG. 295. — Muscle-tissue. Three pieces of striped muscle fibre (a). Iit- terfibrons fat-cells (b). (After Frey.) For while in the lowest Plant Animals the body consists merely of covering tissue, and while in many other TISSUES. 365 Zoophytes a middle layer of connective tissue develops between the two primary germ-layers, it is only in the most highly developed Plant Animals that muscle and nerve tissue is formed. As has already been said, the latter first appeared as a common nerve and muscle tissue (neuro- musculum, Fig. 203 ; cf. p. 358). It was only afterwards that the muscle-tissue (Fig. 295) separated from the nerve- tissue (Fig. 294). The greater part of the nerve-tissue is derived from the skin-sensory layer, the greater part of the muscle-tissue from the skin-fibrous layer. Vascular tissue (vasalium) must be regarded as forming FIG. 296. — Va?cular tissue (vasalium). A hair- vessel from the mesentery : a, vascular cells ; 6, the kernels of these (" endothelium "). FIG. 297- — Red blood cells (corpuscles) of various Vertebrates (equally magnified): 1, Human; 2, Camel; 3, Pigeon; 4, Proteus (p. 129); 5, Water- salamander (Triton) ; 6, Frog ; 7, Fish (Cobitis) ; 8, Lamprey (Petromyzori) ; a, surface view ; b, edge view. (After Wagner.) ( 366 ) TABLE XXXVITT. Systematic Survey of the Sequence, according to Age, of the Hainan Tissue-groups. (Phylogenetic Classification of Vertebrate Tissues.) FIRST GROUP: PRIMARY TISSUES (Epithelium). 1. FIRST HISTOLOGICAL STAGE OF EVOLUTION. I. Covering-tissue (Epithelium). A. Skin-covering tissue (EpMelium de Intestinal covering tissue (Epittel. pastroU). ( 1. Real intestinal epithelium Intestinal layer, or Kntoderm, of Gastrula-j 2. Epithelium of the intestinal glands (afterwards the intestinal-glandular layer) ( 3. ( Karliest site of origin of egg-cell ;) SECOND GROUP: SECONDARY TISSUES. (All derived from the Covering-tissue, or Epithelium.) 2. SECOND HISTOLOGICAL STAGE OF EVOLUTION. II. Coimoctive-tissue (Comuvtleiun). II. C. Filling-up tissue (Tda conjunctiva) isuf.cr 1^ Fattv1ttaueWU* [surrounding] connec.ive tissue) \ 3' Jgjgjj ™ua II. Z>. Supporting tissue (Tito skeleta.is) (firmer f *' CarUlurh^'r tissue Supporting] connective tissue) } 6; Bone.'tfSMIr III. 8. Nerve-tissue (TVZa nertxa). Original outi-r portion of the nerve- muscle cells of the Exoderm III./1. Muscle-tissue (Tela muscularis) Original inner portion of the nerve-muscle cells of the Exodurm 3. THIRD HISTOLOGICAL STAGE OP EVOLUTION. III. Nerve-muscle Tissue (Neuro-musculum). 1 Nerve-cells ( 1. o. Peripherie nerve-cells (Hod-cells ol (Ganglion-cells) ^ > ASfSSSBSSftuW* lh) i 2. a. Sheath less nerve-fibres (p.ile, or 2. Nerve-fibre- ) medulla-less fibres) (Nerve-tubes) } 2. 6. Sheathed nerve-fibres (dark fibres (. with medulla) 1. One-celled muscle- fl. a. Smooth contractile fibre-colls fibres (16. Striped contractile fibre-cell* 2. Many-celled muscle- J 2. a. Smooth muscle-ma.-ses fibres ( 2. b. Striped muscle-masses 4. FOUUTH HISTOLOGICAL STAGE OP EVOLUTION. IV. Vascular Tissue (Vasalium). . 1. a. Exocoelarium (Parietal Co?lom-epl- rinm I thelium) (and secondary site of *^ P.i.h, ) origin of the sperm-cells?) lh!mm * el"Ul ' \ 1 b. Endoccelarium (Visceral ccelom-epi- thelium) (and secondary site of \ IV. G. Vascular lining tissue (Ikla vasalis). Inner wall-covering of the Coelom system origin of the egg-cells ?) (2.0. Endothelinm of the lymph-vessels ( 2 b~ Kndothelium of the blood-vessels . Lymph-tissue (TWa I lymphaticd). Liquid I 1. Lymph (Colourless blood-cells and fluid intercellular substance) contents of the Cut-lorn 1 2. Bluod (Ked blood-cells and fluid intercellular sub tance) system Hum) ( 367 ) TABLE XXXIX. Syptomntic Survey of the Sequence, according to Age, of the Tin-man Organ-systems. (Phylogenetic Classification of Vertebrate Organs.) (On the right are given the Ancestral Stages, in which the respective organs probably first appeared.) 1. FIRST STAGE IN THE EVOLUTION OF ORGANS. I. Skin and Intestinal Systems. The two Systems appear first, and simultaneously, in the Gastread ancestors. l A I. Simple exoderm Gostra-ud* I. A. Skin-system l A 2. Outer skin (Skin-sensory layers and ) ,.. (OftUm* de> male) \ leather skin (Skin-fii runs l.t'yei ) f n l \ A 3. Outer skin, with hairs, glands, eic. Mammals B 1. Simple entoderm Gusliaiud* I. B. Intestinal syste B2. Intestinal epithelium (Intestinal-glan- ) dular layer) and inles inal muscular [ Worms skin (Intestinal-fibrous layer) ) S3. Gill-intestine and stomach-intestine Chorda-.uiinials 2. SECOND STAGE IN THE EVOLUTION OF ORGAN& II. Nerve and Muscle Systems. The two Systems appear first, and simultaneously, in the Primitive Worm ancestors. ,T _ „ tCl. Upper throat ganglia Primitive Worn s II. C. Nerve-system c 2 ^^ medullary tube Chord*-anim.,ls (Systen eum) | c 3. Brain and spinal marrow Monorhina IT n MHS.-IP svstpm (/)l. Skin-muscle pouch Primitive Wo. ms ^ 8. THIRD STAGE IN THE EVOLUTION OF OKOAMS. III. Kidney and Vascular Systems. The two Systems first appear, one after the other, In the Soft-worm ancestors (.<*o?«cuio). !E 1. Primitive kidney canala Scolecida A'2. Sopnental canals Acrania? E3. 1'iimitive kidneys Monorhina £4. Permanent kidneys Protamnia f F 1. Simple ccelom Scolecida III. F. Vaocular system 1 /•' 2. Dorsal and ventral vessels Worms (Systema vascalare) ~]F3. Heart (part of the ventral vessel) Chorda-an:mal.<< I f 4. Heart, with auricle and ventricle Monorhina 4. FOURTH STAGE IN THE EVOLUTION OF ORGANS. IV. Skeleton and Sexual Systems. The two Sybtems first appear, one after the other, in the ChorJonia-ancestors. (Gl. Simple notochord Chorda-an'mals rV.(7. SkPloton-systera }G2. (.Cartilaginous (.rimitive skull Monorhina (Systeina slceUiare) ] G 3. Gill-arches, ribs, limbs Sebchii I. C 4. Limbs, with five digits Amphibia fHl. Simple hermaphrodite glands Chorda-animal* IV. //. Sexual system J H2. Distinct testes and ovaries Acrania (Systema sexuale) "j 7/3. Seed-duct and oviduct Selachil (lU. Phallus (penis, clitoru) Protamnia 57 368 TTTE EVOLUTION OF MAN. the most recent group of tissues, that which originated last. Under this name are included those epithelial-like tissues which line the closed inner cavities of the body (the ccelom, chest-cavity, ventral cavity, heart-cavity, blood-vessels, etc (Fig. 296). In addition to this vascular carpet (endo- theliuin), the liquids containing cells, which fill these cavities (lymph, blood, serum, etc.), must be classed with this tissue (Fig. 297)- All these tissues may be grouped as vasalia. His wrongly ascribed to them a quite different, " parablastic " origin (from the nutritive yelk); they are, however, products of the intestinal-fibrous layer (and partly, perhaps, of the skin-fibrous layer). As the co3loma and the whole vascular system is of more recent phylogenetic origin, its peculiar tissues must also be more recent. This phylogenetic explanation of the ontogenetic suc- cession of the tissues and of the organ systems arising from them, appears to me to be satisfactorily proved by Com- parative Anatomy, and by the Gastrsea theory. If it is correct, it discloses an interesting glimpse into the entirely various age of the most important constituent parts of our body. The human skin and intestine are, according to this, many thousands of years older than the muscles and nerves; these again are much more ancient than kidneys and blood- vQgsels, and the latter, finally, are many thousands of years older than the skeleton and the sexual organs. The com- mon view, that the vascular system is one of the most important and original organ-systems, is, therefore, erro- neous ; it is as false as the assumption of Aristotle that the heart is the first part to form in the incubated chick. On the contrary, all lower Intestinal Animals show plainly that the historic evolution of the vascular system did not RUDIMENTARY VASCULAR SYSTEM. 369 begin till a comparatively late period. Not only all Plant Animals (Sponges, Corals, Hydropolyps, Medusas), but also all lower Worms (Acoelomi), are entirely destitute of vascular system. In both groups, the fluid acquired by digestion is conveyed directly from the intestinal tube, through processes of this latter (the gastro-canals), into the different parts of the body. It is only in the intermediate and higher Worms that the vascular system first begins to develop, in consequence of the formation of a simple cavity (coeloma), or of a system of connected spaces, round the intestinal tube, in which cavities the nutritive fluid (blood) exuded through the intestinal wall, collects. In the human ancestral line we meet with this first rudiment of the vascular system in that group of Worms which we spoke of as Soft Worms (Scolecida ; p. 85). The Soft Worms, as we said, formed a seiies of intermediate stages bjtween the lowest bloodless Primitive Worms (ArchelmintJies) and the Chorda- worms (Ckordonia), which are already provided with a vascular system and a noto- chord. The vascular system must have begun, in the older Scolecida, with a very simple ccelom, a "body-cavity," filled with blood, and which surrounded the intestinal tube. Its origin was probably due to the accumulation of nutritive fluid in a cleft between the intestinal-fibrous la}Ter and the skin-fibrous layer. A vascular system in this simplest form is yet found in the Moss-polyps (Bryozoa) in the Wheel-animalcule (Rotatoria), and in other lower Worms. The inner, visceral, part of the wall of the ccelom is, naturally, formed by the intestinal-fibrous layer (endo- coelar), the outer, parietal, part by the skin-fibrous layer (oxocodar). The ccelom fluid, collected between the two, 37O THE EVOLUTION OF MAN. may contain detached cells (lymph-cells) from either fibrous layer. A first advance in the development of this most primi- tive vascular system was accomplished by the formation of canals or blood-conducting tubes, which developed, inde- pendently of the coeloma, in the intestinal wall, that is, in the intestinal-fibrous layer of the wall. These real blood- vessels, in the stricter sense, appear in very different form in Worms of the intermediate and higher groups ; sometimes they are very simple, sometimes very complex. Two primordial " primitive vessels " must be regarded as representing that form, which probably formed the first of the more complex vascular system of Vertebrates ; these are a dorsal vessel, which passes from front to back along the middle line of the dorsal wall of the intestine, and a ventral vessel which passes, in the same direction, along the middle line of the ventral wall Both at the front and at the back these two vessels are linked together by a loop sur- rounding the intestines. The blood enclosed in the two tubes is driven forward by the peristaltic contraction of this. The further development of this simplest rudimentary blood-vessel system is evident in the class of the Ringed Worms (Annelida), in which we find it in very various stages of development. In the first place, many trans1 • verse connections probably arose between the dorsal and ventral vessels, so as to encircle the intestine (Fig. 298). Other vessels then penetrated into the body-wall and branched, so as to conduct blood to this part. As in those ancestral Worms, which we have called Chordonia, the front section of the intestine changed into a gill-body, these THE VASCULAR SYSTEM. 371 ascular loops, within the wall of this gill-body, which passed from the ventral vessel to the dorsal vessel, became modified into respiratory gill- vessels. Even at the present day, the organization of the remarkable Acorn-worm (Balanoglossus) exhibits a similar condition of gill-circula- tion (Fig. 186, p. 86). A further important advance is exhibited, among extant Worms, in the Ascidia, which must be regarded as the nearest blood-rela- tions to our primitive Chordonia ancestors. In these we find, for the first time, a real heart, that is, a central organ of tJte circula- tion of the blood, by the pulsating contractions of the muscular wall of which the blood is driven forward in the vascular tubes. The heart appears here in the simplest form, as , a spindle-shaped pouch which passes at both S/ ends into a main vessel (Fig. 188, c. p. 90; /""Si Plate XL Fig. 14, Itz). The original position FIG. 298.— Blood-vessel system of a Ringed Worm (Saeiiuris) ; front section : d, dorsal vessel ; v, ventral vessel ; c, transverse connection between the two (en. larged like a heart). The arrows indicate the direction of ^ f the blood current. (After Gegenbaur.) of the heart on the ventral side, behind the gill-body of the Ascidian, plainly shows that it originated in a local dilation of a section of the ventral vessel. The alternating direc- tion of the movements of the blood, which has already been mentioned, is remarkable ; the heart expels the blood alter- nately through the anterior and through the posterior end. This is very suggestive, because in most Worms the blood 3/2 THE EVOLUTION OF MAN. in the dorsal vessel moves from back to front, while in Vertebrates, on the contrary, it flows in the opposite direc- tion, from front to back. As the heart of the Ascidian constantly alternates between these two opposite directions, it exhibits permanently, to a certain extent, the phylogenetic transition between the older direction of the dorsal blood- current toward the front in Worms, and the newer direc- tion of the same toward the rear in Vertebrates. As in the more recent Chorda Animals, which gave rise to the Vertebrate tribe, the newer direction became permanent, the two vessels which proceeded from the two ends of the heart-pouch, acquired a constant signifi- cance. The front section of the ventral vessel, since then, has steadily conducted the blood from the heart, acting, consequently, as an artery ; the hinder section of the ventral vessel, on the contrary, leads the blood, circulating in the body, back into the heart, and must, therefore, be called a vein. In reference to their relation to the two sections of the intestine, we may speak of the latter, more accurately, as the intestinal vein, and of the former as the gill-artery. The blood contained in both vessels, which alone fills the heart also, is venous blood ; that is, containing much carbonic acid. On the other hand, the blood which flows from the gills into the dorsal vessel is there re- furnished with oxygen ; is arterial blood. The most delicate branches of the arteries and veins pass into each other, within the tissue, through a network of extremely fine neutral hair-vessels or capillaries (Fig. 296). If we now turn from the Ascidia to the nearest allied form, the Amphioxus, we are immediately surprised by an apparent retrogression in the development of the vascular DEVELOPMENT OF THE VASCULAR SYSTEM. 373 system. The Amphioxus, as has been stated, has no real heart ; but the blood is circulated in its vascular system by the main vascular stems themselves, which contract and pulsate along their whole length. (Cf. Fig. 151, vol. i. p. 420.) A. dorsal vessel (aorta), situated over the intestine, absorbs the arterial blood from the gills and propels it through the body. The venous blood, in its return, collects in a ventral vessel (intestinal vein), situated under the intestine, and thus returns to the gills. Numerous vascular gill-arches, which accomplish respiration, and absorb oxygen from the water and emit carbonic acid, unite the ventral vessel with the dorsal vessel before. As, in Ascidia, that section of the ventral vessel which also forms the heart in Skulled Animals (Craniota), is already fully developed into a simple heart-pouch, we must regard the absence of the latter in the Amphioxus as the result of retrogression, as a reversion, in these Acrania, to the older form of vascular system, as it exists in Scolecida and many other Worms. We may assume that those Acrania which actually formed part of our ancestral line did not share this relapse, but rather inherited the one-chambered heart from the Chordonia and transmitted it directly to the older Skulled Animals (Craniota). The Comparative Anatomy of Skulled Animals clearly exhibits the further phylogenetic development of the blood- vessel system In the lowest stage of this group, in the Cyclostoma (p 102), we first meet with a real lymph-vessel system, side by side with the blood-vessel system, a system of canals which collect the colourless fluid flowing from the tissues, and conduct it to the blood-current. Those lymph- \vliich absorb the milky, nutritive fluid, obtained 374 THE EVOLUTION OF MAN. directly by digestion, from the intestinal wall, and conduct it to the blood-current, are distinguishable as chyle-vessels, or "milky juice vessels." While the chyle, or milky juice, in consequence of the great amount of fat globules which it contains, appears milk white, the real lymph is colour- less. The chyle, as well as the lymph, contain the same colourless amoeboid cells (Fig. 9, vol. i. p. 132), which are also distributed in the blood as colourless blood-cells (corpuscles) ; the latter contains, in addition, the much greater quantity of red blood-cells (corpuscles), which gives the blood of Skulled Animals its red colour. The distinction, common to all Craniota, between lymph-vessels, chyle-vessels, and blood-vessels, is to be regarded as the result of a division of labour which took place between different portions of an original unitary, primitive blood-vessel system (or hsemo- lymph system). The heart, the central organ of the circulation of the blood, which exists in all Craniota, also exhibits an advance in structure, even in the Cyclostoma. The simple spindle- shaped heart-pouch is separated into two divisions, or chambers, which are divided by two valves (Plate XI. Fig. 16, kv, hk}. The posterior division, the fore chamber (atrium, hv), absorbs the venous blood from the veins of the body, and discharges it into the anterior division, the chamber, or main chamber (ventriculus, hit). From here it is propelled by the gill-artery stem (the foremost section of the ventral vessel) iuto the gills. In Primitive Fishes (Seluchii), an arterial stalk (bulbvs arteriosus), separated by valves, originates, as a distinct section, from the foremost end of the ventricle. It forms the enlarged, hindmost end of the gill-artery stem (Fig. DEVELOPMENT OF THE VASCULAR SYSTEM. 375 299, abr). From each side of this, from five to seven gill- arteries proceed ; these rise between the gill-openings (s) to the gill-arches, encircle the throat, and combine above into a common aorta-stem, the continuation of which, passing backward above the intestine, corresponds to the dorsal vessel of Worms. As the arched arteries distribute themselves in a respiratory capillary net over the gill- arches, they thus contain venous blood in their lower part (as arterial gill-arches), and arterial blood in their upper part (as aorta-arches). The points at which separate aorta- arches unite, which occur on the right and left sides, are called aorta-roots. Of an originally greater number of aorta- arches, only five pairs are retained, and from these five (Fig. 300), in all higher Vertebrates, the most im- portant parts of the arterial system develop. Pro. 299. — Head of an embryonic Fish, with the rudiment and the blood-vessel system ; seen from the left side : dc, Cuverian duct (point of uuion of the front and hind main veins) ; sv, venous sinus (enlarged terminal portion of the Cnverian duct) ; a, auricle ; v, main chamber ; abr, gill-artery stem; s, gill-openings (between the arterial arches); ad, acrtn ; c\ heid-artery (carotis) ; n, nose-groove. (After Gegenbaur.) The appearance of the lungs, connected with the respi- ration of air, which first occurs in the Dipneusta, is most important in the further developcrnent of the arterial 3/6 THE EVOLUTION OF MAN. system. In Dipneusta, the auricle of the heart separates into two halves by the formation of an incomplete partition. Only the right auricle now absorbs the venous blood of the body-veins. The left auricle, on the other hand, absorbs the arterial blood of the lung- veins ; both auricles dis- charge in common into the simple ventricle, in which the two kinds of blood mingle, and are then propelled through the arterial stalk into the arterial arches. From the last of these latter spring the lung-arteries (Fig. 301, p)-, these convey a part of the mixed blood into the lungs, while the remainder is driven through the aorta into the body. From the Dipneusta upward, we trace a progressive development of the vascular system, which finally leads, with the loss of gill respiration, to a complete separation of the two parts of the double circulatory system. In Am- phibia, the partition between the two auricles becomes complete. In their young form, these yet retain gill- respiration and the circulatory system as in Fishes, and the heart contains only venous blood ; at a later period, the lungs, with their vessels, are developed also, and the main chamber of the heart then contains mixed blood. In Pro- tamnia and Reptiles, the main chamber and the arterial stalk belonging to it begin to separate, by the formation of a longitudinal partition, into two halves, and this partition becomes complete in the higher reptiles on the one side, in the parent-form of Mammals on the other. The right half of the heart alone now contains venous blood, the left half only arterial, as in all Birds and Mammals. The right auricle receives venous blood from the body-veins, and the right ventricle propels this through the lung-arteries into the lungs; from there it returns as arterial blood through DOUBLE CIRCULATORY SYSTEM. 377 the lung-veins to the left auricle, and is driven through the left ventricle into the body-arteries. Between the lung- arteries and lung-veins is situated the capillary system of the lesser, or lung-circulation ; between the body-arteries and the body-veins lies the capillary system of the greater, or body-circulation. Only in the two highest Vertebrate as FIG. 300.— The five arterial arches of Sknlled Animals (1-5) in their original form : a, arterial stalk ; a", main stem of the aorta ; c, head- artery (carotis, anterior continuation of the aorta-roots). (After Ratbke.) FIG. 301 —The five arterial arches of Birds ; the light portions of the rudiment disappear; only the dark parts are permanent. Letters as in Fig. 300 : s, arteries of the clavicula (sub-clavian) ; p, lung-artery ; p', branches of the same. (After Rathke.) FIG. 302.— The five arterial arches of Mammals. Letters as in Fig. 301 : v, vertebral artery ; 6, Botalli's duct (open in the embryo, afterwards closed). (After Rathke.) classes, in Birds and Mammals, is this complets separation of the two courses of the circulation perfect. Moreover, this separation has taken place in the two classes independently of each other, as is shown by the unequal development of the aortas. In Birds, which are descended from Reptiles, 378 THE EVOLUTION OF MAN. the right half of the fourth arterial arch has become the permanent arterial arch (arcus aortce, Fig. 301). On the other hand, the latter has developed from the left half of the same arch (Fig. 302) in Mammals, which are directly descended from the Protamnia. On comparing the arterial system in the various classes of the Skulled Animals (Craniota) in its matured condition, it appears in very various forms, and yet it develops, in all, from the same primitive form. This development takes place in man exactly as in other Mammals ; especially is the modification of the five arterial arches precisely the same in both cases (Figs. 303-308). At first, only a single pair of FIGS. 303-306. — Metamorphosis of the five arterial arches in the human embryo (diagram after Kathke) : ta, arterial stalk ; 1, 2, 3, 4, 5, the arterial arches from the first to the fifth pair ; ad, main stem of the aorta ; aw, roots of the aorta. In Fig. 303, three of the arterial arches are given ; in Fig. 304, the whole five (those indicated by dots are not yet developed); in Fig. 305, the first two have again disappeared ; in Fig. 306, the permanent arterial stems are represented. The dotted parts disappear, s, Sub-claviau artery ; v, vertebral artery ; a», axillary artery ; c, carotid artery (c't outer, c", inner carotis) ; p, pulmonary artery (lung-artery). arches develop, and these lie on the inner surface of the first pair of gill-arches (Figs. 147-150, vol. i. pp. 395-398 ; Fig. 30o). A second and a third pair of arches then develop DEVELOPMENT OF THE HEART. 379 behind the first, and these are situated on the inner suiface of the second and third gill-arches. At length, a fourth and a h'fth pair appear behind the third (Fig. 304); but while the latter are developing, the first two are again disappear- ing by growing together (Fig. 305). The permanent main arteries develop only from the three posterior arterial arches (3, 4, 5, in Fig. 304), the lung-arteries from the last (p; Fig. 306). (Cf. with this Fig. 302.) The human heart also (Fig. 314) develops exactly like that of other Mammals. We have already considered the first prin- ciples of its germ-history (vol. i. pp. 392-395, Figs. 143-147), which essentially corresponds with its Phylogeny.191 We saw that the very first rudiment of the heart is a spindle-shaped thickening of the intestiral-fibrous layer in the ventral wall of the head-intestine (Fig. 143, df). This spindle-shaped formation then becomes hollow, forms a simple pouch, and separates from the place at which it originated, so that it then lies freely in the cardiac cavity (Figs. 145, 146). This pouch bends into the form of an S (Fig. 144, c), and, at the same time, turns spirally on an imaginary axis, so that the posterior part lies on the dorsal surface of the anterior part The combined yelk-veins open into its posterior extremity ; from the anterior extremity proceed the arterial arches (Fig. 150, voL i. p. 398). This first rudiment of the human heart, which encloses a very simple cavity, corresponds to the heart of the As- cidians, and must be regarded as a reproduction of the heart of the Chordonia ; it now, however, separates into two, and then three parts, thus exhibiting for a very brief period the heart-structure of the Cyclostoma and of Fishes. The spiral turn and curve of the heart increases, and, simultaneously, 380 THE EVOLUTION OF MAN. two shallow transverse indentations of the circumference appear, which externally mark the three sections (Figs. 307, 308). The anterior section, which is turned toward the FIG. 307.— Heart of an embryonic Rabbit, from behind : a, yelk-veins ; b, auriculae ; c, auricle (atriuni) ; d, ventricle; e, artery-stalk; /, base of the three pairs of arterial arches. (After Bischoff.) FIG. 308.— Heart of the same embryo (Fig. 307), from the front: r, yelk- veins ; a, auricle ; ca, auricular canal ; I, left ventricle ; r, right ventricle ; ta, artery-stalk. (After Bischoff.) FIG. 309. — Heart and head of an embryonic Dog, from the front: a, fore-brain; b, eyes; c, mid-brain; d, primitive lower jaw; e, primitive upper jaw ; /, gill-arches ; g, right auricle ; h, left auricle ; i, left ventricle ; If, right ventricle. (After Bischoff.) FIG. 310. — Heart of the same embryo, from behind : a, entrance of tho yelk-veins ; b, left auricular process ; c, right auricular process ; d, auricle ; e, auricular canal ; /, left ventricle ; g, right ventricle ; h, artery-stalk. (After Bischoff.) ventral side, and from which the aortal arches spring, reproduces the arterial stalk (bulbus arteriosus') of the Selachii. The central section is the rudiment of a simple chamber, or ventricle (ventriculus) ; and the posterior section, the one turned toward the dorsal side, into which the yelk-veins open, is the rudiment of a simple auricle DEVELOPMENT OF THE HEART. 381 (atrium). The latter, like the simple auricle of the heart of the Fish, forms a pair of lateral protuberances, the heart ears, or auricular appendages (auricula, Fig. 307,6); and hence the indentation between the auricle and ventricle is called the auricular canal (canalis auricularis, Fig. 308, ca\ The heart of the human embryo is now a complete Fisl. heart. Corresponding exactly with the Phylogeny of the human heart (Table XLI ), its Ontogeny exhibits a gradual tran- sition from the Fish heart through the Amphibian heart to the Mammalian heart. The most important step in this advance is the formation of a longitudinal partition, im- perfect at first, afterwards complete, by which all the three sections of the heart are separated into a right (venous) and a left (arterial) half. (Of. Figs. 309-314.) The auricle (atrium] is thus divided into a right and a left auricle, each of which acquires its respective auricular process ; the body- veins discharge into the right auricle (ascending and de- scending vena cavce, Fig. 311, c, Fig. 313, c) ; the left auricle receives the lung- veins. Similarly, a superficial "inter- ventricular furrow" (sulcus interventricularis, Fig. 312, s) appears at an early period on the main chamber of the heart, the external expression of the internal partition, by the formation of which the ventricle is divided into two chambers, a right (venous) and a left (arterial) ventricle. Finally, a longitudinal partition forms, in a similar way, in the third section of the primitive heart, which so much resembles that of a Fish, in the arterial stalk, which is also externally indicated by a longitudinal furrow (Fig. 312, a/). This separates the cavity of the artery-stalk into two lateral halves ; the main lung artery, which opens into the 382 THE EVOLUTION OF MAN. FIG. 311. FIG. 313. FIG. 31t. FIG. 311. — Hear,t of a human enibrvo of four weeks ; 1, from the front ; 2, from the back ; 3, open, and with the upper half of the auricle removed ; a', left auricular process ; a", right auricular process ; v', left ventricle ; v", right ventricle ; an, artery-stalk ; c, upper hollow vein (rena cava) (cd, right, c.s, left); s, rudiment of the partition, between the chambers. (After Koelliker.) Flo. 312. — Heart of a human embryo of six weeks, from the front: r, right ventricle ; t, left ventricle ; s, furrow between the two ventricles ; ta, artery-stalk; of, furrow on its surface; at the right and left are the two large auricular processes of the heait. (After Ecker.) FIG. 313. — Heart of a human embryo of eight weeks, from behind : a', left auricular process ; a", right auricular process ; r', left ventricle ; v", right ventricle ; cd', right upper vena cava ; cs, left upper vena cava ; ci, lower vena cava. (After Koelliker.) FIG. 314. — Heart of human adult, perfectly developed, from the front, in its natural position : a, right auricular process (below it, the right ventricle) ; b, left auricular process (below it, the left ventricle); C, upper vena cava; V, lung-veins ; P, lung-artery ; d, Botalli's duct ; A, aorta. (After Meyer.) right ventricle, and the aorta-trunk, which opens into the left ventricle. Not until all these partitions are complete, is the lesser, or lung-circulation, entirely distinct from the POSITION OF THE RUDIMENTARY HEART. 383 greater, or body-circulation ; the right half of the heart is the centre of motion for the former, the left half for the latter. (Of. Table XLI.) In the human embryo, and in all other Amniota, the heart originally lies far forward on the lower side of the head, as in Fishes it remains permanently near the throat. Afterwards, with the advancing development of the neck and chest, the heart continually moves further back, until at last it is situated in the lower part of the breast between the lungs. At first its position is symmetrical, in the central plane of the body, so that its longitudinal axis corre- sponds with that of the body (Plate IV. Fig. 8). In most Mammals it retains this symmetrical position permanently ; but in the Apes the axis begins to incline obliquely, and to move the apex of the heart to the left side. This inclination is carried furthest in the Man-like Apes; in the Chim- panzee, Gorilla, and Orang, which also resemble Man in this oblique position of the heart. The germ-history of all other parts of the vascular system, like that of the heart, point out many and valuable facts re- garding the history of our descent. But as an accurate know- ledge of the complex arrangement of the entire vascular system of Man and other Vertebrates is required, in order to follow the matter sufficiently far to make it intelligible, we cannot here enter into any further detail.192 Moreover, many important features in the Ontogeny of the vascular system, especially in regard to the derivation of its various parts from the secondary germ-layers, are as yet very obscure and doubtful This is true, for example, of the question as to the origin of the coelom-epithelium — that is, of the cell-layer coating the body-cavity. Probably there is an important phylogenetio 58 384 THE EVOLUTION OF MAN. distinction between the exoccelar, or the parietal coelom- epithelium, which originates Irom the skin-fibrous layer, and the endoccelar, or the visceral ccelom-epithelium, which is derived from the intestinal-fibrous layer. The former is, perhaps, connected with the male germ-epithelium (the rudiment of the testes), the latter with the female germ- epithelium (the rudiment of the ovary). (Of. Chapter XXV.) TABLE XL. SYSTEMATIC SURVEY OP THE MOST IMPORTANT PERIODS IN THE PHYLOGENT OF THE HUMAN VASCULAR SYSTEM. I. First Period : Vascular System of the earlier Scolecida. Between the skin-covering and the intestinal wall is formed a simple body-cavity (cceloma), or a perienteric cavity (as in the extant Bryozoa and other Ccelomati). II. Second Period : Vascular System of the more recent Scolecida. The first real blood-vessels form in the intestinal wall (in the intestinal- fibrous layer), a dorsal vessel in the central line of the dorsal side of the intestinal tube, and a ventral vessel in the central line of its ventral side. The two vessels are connected by several circular vessels, encircling the intestine. III. Third Period : Vascular System of the earlier Chordonia. By the modification of the anterior half of the intestine into a gill- intestine, the anterior section of the ventral vessel becomes a gill-artery, and the anterior section of the dorsal vessel a gill-vein ; between the two a gill capillary network develops. IV. Fourth Period : Vascular System of the more recent Chordonia, The portion of the ventral vessel, lying immediately behind the gill. intostine, enlarges to a simple heart-pouch (Ascidian). PHYLOGENY OF THE HUMAN HEART. 385 V. Fifth Period : Vascular System of the Acrania. The ventral vessel (intestinal vein) forms, round the developing liver. sac, the first rudiment of a vena portae system. VI. Sixth Period : Vascular System of the Cyclostomi. The single-chambered heart divides into two chambers ; a posterior ventricle, and an anterior auricle. The lymph-vessel system develops side by side with the blood-vessel system. VII. Seventh Period : Vascular System of the Primitive Fishes, or Selachii. From the anterior section of the main chamber of the heart arises an artery-stalk or trunk, from which five (?) pairs of arterial arches proceed. VIII. Eighth Period : Vascular System of the Mud-fishes. From the last (fifth) pair of arterial arches the lung-arteries develop, as in the Dipneusta. IX. Ninth Period : Vascular System of Amphibia. The gill-arches gradually disappear with the gills. The right and left aortal arches remain. X. Tenth Period : Vascular System of Mammals. The separation of the greater from the lesser circulation is complete. The right aortal arch unites with Botalli's duct. TABLE £LL SYSTEMATIC SURVEY OP THE MOST IMPORTANT PERIODS IN THE PHTLOOINT OF THE HUMAN HEART. I. First Period : Heart of Chordonia. The heart forms a simple spindle-shaped enlargement of the ventral vessel, with an alternating blood-current '(as in Ascidia). II. Second Period : Heart of Acrania. The heart is like that of Chordonia, but the blood-current acquires a constant direction, passing only from back to front. (Retrograded in Amphioxns.) 386 THE EVOLUTION OF MAN. III. Third Period : Heart of Cyclostoma. The heart divides into two chambers, a posterior auricle (atrium) and an anterior ventricle (ventriculus). IV. Fourth Period : Heart of Primitive Fishes. From the anterior section of the ventricle is differentiated an arterial stalk (bulbus arteriosus), as in all Selachii. V. Fifth Period : Heart of the Mud-fishes. The anricle divides, by an imperfect and interrupted partition, into a right and a left half, as in Dipneusta. VI. Sixth Period : Heart of Amvhibia. The partition between the right and left auricles becomes complete, as in the higher Amphibia. VII. Seventh Period : Heart of Protamnia. The main chamber of the heart divides, by an incomplete partition, into a right and a left half, as in Reptiles. VIII. Eighth Period : Heart of Monotrema. The partition between the right and left ventricles becomes complete, as in all Mammals. IX. Ninth Period : Heart of Marsupials. ^ The valves between the auricles and ventricles (atrio-ventricular valves), together with the connecting filaments and papillary muscles belonging to them, are differentiated from the muscular masses of Monotremes. X. Tenth Period : Heart of Jpes. The main axis of the heart, lying in the central line of the body becomes oblique, so that the apex is turned to the left, as in Apes and TABLE XLII. Systematic Survey of those Primitive Organs which must probably be regarded as homologous in Worms, Articulated Animals, Soft-bodied Animals, and Vertebrates.1" Worms (ren*«). Articulated Animals (Arthropoda). Soft-bodied Animals (JfoUiwca). Vertebrates (Vertebrata). I. Products of the Differentiation of the Skin-sensory Layer. I. Outer skin (Epidermis) 2. Brain (upper throat- ganglia) 2. Excretory organs (water - vessels, segmented organs] 1. Chitinous skin (Hypodermis) 2. Brain (^upper throat- ganglia) 3. Shell-glands of the Crustacean (trachea of the Tracheata?) 1. Outer skin (Epidermis) 2. Brain (upper throat- ganglia) 3. Rudimentary kid- neys (Primitive kidneys) 1. Outer skin (Epidermis) 8. Medullary tube (an- terior part) 3. Primitive kidney- ducts (Proture- teres) and seg- mental organs II. Products of the Differentiation of the Skin-fibrous Layer. 4. Leather-skin (Cbnum) (together with the circular muscle- pouch ?) 6. Longitudinal muscle-pouch 6. Exoccelar innermost cell-layer of the body-wall (also male germ-plate?) 4. Leather-skin (Rudiment) 6. Trunk-muscles 6. Exoccelar Innermost cell-layer of the body-wall (also male germ-plate?; 4. Leather-skin (Corium) (together with the muscles of the skin?) 5. Inner trunk-muscles 6. Exoccelar parietal epithelium of the ccelom (also male germ-plate?) 4. Leather-skin (Oorium) (together with the muscular layer of the skin ?) 5. Side ttunk-muscles 6. Exoccelar parietal epithelium of the ccelom (also male germ- plate ?) III. Products of the Differentiation of the Intestinal-fibrous Layer. 7. Body-cavity (Caloma) 7. Body-cavity (CcOorna) 7. Body-cavity (Ccelama) 7. Pleuro-peritoneal cavity 8. Endocoelar outer- 8. Endoccelar outer- 8. Endoca-lar visceral 8. Kndoccelar visceral most cell-layer of the intestinal wall (together with the female most cell-layer of the intestinal wall (together with the female epithelium of the coelom (together with the female germ-plate?) epithelium of the ccelom (together with the female germ-plate?) germ-plate ?) germ-plate?) 9. Dorsal vessel 9. Heart 9. Chamber of the 9. Aorta (primordial) heart (and main 10. Ventral vessel 10. 10. —** 10. Heart (and gill- artery) 11. Intestinal wall (ex- 11. Intestinal wall (ex- 11. Intestinal wall (ex- 11. Intestinal wall (ex- cept the epithe- cept the epithe- cept the epithe- cept the epithe- lium) lium) lium) lium) IV. Products of the Differentiation of tlie Intestinal-glandular Layer. 12. Intestinal epithe Hum 12. Intestinal epithe- lium 12. Intestinal epithe- lium 12. Intestinal epithe- lium CHAPTEE XXV. DEVELOPMENT OF THE URINARY AND SEXUAL ORGANS. Importance of Reproduction. — Growth. — Simplest Forms of Asexual Repro- duction: Division and the Formation of Buds (Gemmation). — Simplest Forms of Sexual Reproduction: Amalgamation of Two Differentiated Cells ; the Male Sperm-cell and the Female Egg-cell.— Fertilization. — Source of Love. — Original Hermaphroditism ; Later Separation of the Sexes (Gonochorism). — Original Development of the Two Kinds of Sexual Cells from the Two Primary Germ-layers.— The Male Exoderm and Female Entoderm. — Development of the Testes and Ovaries. — Passage of the Sexual Cells into the Ccelom. — Hermaphrodite Rudiment of the Embryonic Epithelium, or Sexual Plate. — Channels of Exit, or Sexual Ducts. — Egg-duct and Seed-duct. — Development of these from the Primitive Kidney Ducts. — Excretory Organs of Worms. — " Coiled Canals " of Ringed Worms (Annelida). — Side Canals of the Amphioxus. — Primitive Kidneys of the Myxinoides. — Primitive Kidneys of Skulled Animals (Craniota). — Development of the Permanent Secondary Kidneys in Amniota. — Development of the Urinary Bladder from the Allantois. — Differentiation of the Primary and Secondary Primitive Kidney Ducts. — The Miillerian Duct (Egg-duct) and the Wolffian Duct (Seed-duct). — Change of Position of the Germ-glands in Mammals. — Formation of the Egg in Mammals (Graafian Follicle). — Origin of the External Sexual Organs. — Formation of the Cloaca. — Hermaphroditism in Man. " Tho most important truths in Natural Science are discovered, neither by the mere analysis of philosophical ideas, nor by simple experience, but , testes (h', lower, h", upper testis-cord) ; o, ovary ; o', lower ovary-cord ; i, groin-cord of the primitive kidney ; d, diaphragm-cord of the primitive kidney ; n, permanent kidneys (below these the 8-shaped urine-duct ; between the two the rectum) j v, urine- bladder ; a, navel-artery. (After Kolliker.) The most interesting facts in reference to this remark- able development of the primitive kidney ducts and their union with the sexual glands are exhibited in Amphibia (Figs. 321-323). The first rudiment of the primitive kidney ducts and their differentiation into the Mlillerian and DEVELOPMENT OF THE HUMAN KIDNEY. 417 Wolffian ducts is identical in both sexes, as is the case in the embryos of Mammals (Fig. 321, C, Fig! 324). In the female Amphibia the Miillerian duct on each side develops into a large ovary (Fig. 322, od), while the Wolffian duct acts permanently as a urinary duct (u). In the male, on the contrary, the Mullerian duct persists only as a rudimentary organ, without functional significance, as Rathke's cana) (Fig. 323, c) ; the Wolffian duct serves, in this case also, as a urinary duct, but also as a sperm or seed duct, the seminal tubes (ve) from the testes (t) entering the upper part of the primitive kidneys, and there uniting with the urinary canals. In Mammals these conditions, persistent in Amphibia, are rapidly traversed by the embryo in an early period of its development (Fig. 321, C}. The primitive kidneys, which in noii-amnionate Vertebrates persist throughout life as the urine-secretory organ, are superseded by the secondary kidneys. The actual primitive kidneys disappear almost entirely in the embryo at an early period, leaving but small traces. In the male Mammal the supplementary testis (epididymis) develops from the upper part of the primitive kidney ; in the female the same part gives rise to a useless rudimentary organ, the supplementary ovary (parovarium) In the female Mammal the Mullerian ducts undergo very considerable changes. The actual ovaries develop only from its upper part; the lower part widens out into a spindle- shaped pouch, with a thick, fleshy wall, within which the fertilized egg develops into the embryo. This pouch is the womb (uterus). At first the two uteri are perfectly separate, and open on each side of the urine-bladder (vu) into the cloaca, as is yet permanently the case in the lowest living Mammals, the Beaked Animals (Ornithostoma) ; THE EVOLUTION OF MAN. but even in Pouched Animals (Marsupialia) a connection forms between the two Mullerian ducts, and in Placental Animals they coalesce below with the rudimentary Wolffian ducts, forming with them a single "sexual cord" (funiculus geni- talis). But the original indepen- dence of the two parts of the uterus, and of the two vagina canals which proceed out of their lower extremities, persists in many lower Placental Animals, while in the higher members of the same group, these organs gradually coalesce to form one single organ. FIG. 327.— Female sexual The process of Coalescence ad- organs of a Beaked Animal vances steadily from below (or (Ornithorhynchvs, Figs. 195, _ , . 196): o, ovaries; t, oviduct; from behind) upwards (or for- u, uterus ; sug, urinary sexual wards). While in many Gnawing cavity (sinus vrogenitaUs); the AnimaJfl (Rodentia,e.g., Hares and two parts of the uterus open into .this at u' •. d, cloaca. Squirrels) two separate uteri open (After Gegenbaur.) into the vagina canal which has already become simple, in other Gnawing Animals, as also in Beasts of Prey, Whales, and Hoofed Animals (Ungulata), the lower halves of the two uteri are already coalescent, their upper halves (the so-called horns, "cornua") remaining distinct (" uterus bicornis "). In Bats and Semi-apes these upper horns are very short, while the unified lower part becomes longer. Finally, in Apes, as in Man., the cohesion of the two parts is complete, one simple pear-shaped uterus- pouch alone remaining, and into this the oviducts open on each side. POSITION OF THE HUMAN SEXUAL ORGANS. 419 In the male Mammal also, a similar coalescence of the lower portion of the Miillerian and Wolffian ducts takes place. In this case also, these ducts form a single " sexual cord " (Fig. 325, 7ros) = man, and genea (yevea) = Evolution history. There is no especial Greek word for " the history of evolution;" in its place is used either yevca (= de- scent), or yove'ia, (= generation). If goneia is preferred to genea, the word must be written Anthropogony. The word " Anthropogony," used first by Josephus, means, however, only " the generation of man." Genesis (yo/eu\oi') = tribe, and genea (yevea) = history of evolution. The phylon includes all organisms connected by blood, which are descended from a common typical parent-form. Phylogeny includes Palaeontology and Genealogy. — " Generelle Morphologie," vol. ii. p. 305. 6 (i. 6). Biogeny (Greek) = the history of the evolution of organisms or of living natural bodies in the widest sense. (Genea tu bin.) /3ios = life. 7 (i. 6). The fundamental law of Biogeny. Cf. my "General History of the Evolution of Organisms" (" Generelle Morphologie," 1866, vol. ii.), p. 300 (Essays on the causal connection of biogenetic and phyletic evolution) ; also the " Monograph of Chalk Sponges " (" Monographic der Kalkschwamme," 1872, vol. i. 471); also my " Natural History of Creation," 8 (L 10). Palingenesis (Greek) = original evolution, from palingenesia (•TraAiKyeveo'ia.) = new-birth, renewal of the former course of evolution. Therefore, Palingeny = inherited history (from TraXiv = reproduced, and yevca=history of evolution). 9 (i. 10). Kenogenesis (Greek) = modified evolution, from kenos (/cevo's) = strange, meaningless ; and genea (yevea) = history of evolution. The modifications introduced into Palingenesis by Kenogenesis are vitiations, strange, meaningless additions to the original, true coarse of evolution. Kenogeny = vitiated history. 10 (L 12). Latin definition of the fundamental law of NOTES. 461 Biogeny : " Ontogenesis snmmarlum vel recapitulatio cst pLy- logeneseos, tanto integriua, quanto hereditate palingenesis con- servatur, tanto minus integrtim, qnanto adaptatione kenogenesis introducitur." Cf. my "Aims and Methods of Recent History of Evolution " (" Ziele und Wege der Heutigen Entwickelungs- geschichte," p. 77. Jena, 1876). 11 (i. 17). Mechanical and purposive causes. Mechanical natural philosophy assumes that throughout nature, in organic as well as in inorganic processes, only non-purposive, mechanical, necessarily-working causes exist (causce efficientes, mechanism, causality) On the other hand, vitalistic natural philosophy asserts that the latter are at work only in inorganic processes, which in certain other, purposive, special causes are at work, conscious or purposive causes, working for a definite end (causce finales, Vitalism, Teleology). (Cf. " Generelle Morphologic," vol. i. p. 94) 12 (i. 17). Monism and Dualism. Unitary philosophy, or Monism, is neither extremely materialistic nor extremely spirit- ualistic, but resembles rather a union and combination of these opposed principles, in that it conceives all nature as one whole and nowhere recognizes any but mechanical causes. Binary philosophy, on the other hand, or Dualism, regards nature and spirit, matter and force, inorganic and organic nature as distinct and independent existences. (Cf. vol. ii. p. 456.) 13 (i. 20). Morphology and Physiology. Morphology (as the science of forms) and Physiology (as the science of the functions of organisms) are indeed connected, but co-ordinate sciences, independent of each other. The two together constitute Biology, or the " science of organisms." Each has its peculiar methods and aids. (Cf. " Generelle Morphologic," voL i. pp. 17-21.) 14 (i. 24). Morphogeny and Physiogeny. Biogeny, or the " history of the evolution of organisms," up to the present time has been almost exclusively Morphogeny. Just as this first opens the way to a true knowledge of organic forms, so will Physiogeuy afterwards make a true recognition of functions 462 NOTES. possible, by discovering their historic evolution. Its future promises to be most fruitful. Cf. " Aims and Methods of the Recent History of Evolution" (" Ziele und Wege der Heutigen Eritwickelungsgeschichte," pp. 92-98. Jena, 1876). 15 (i. 27). Aristotle. Five books on the generation and evolution of animals (jrepl £cou>v yeveVeos). 16 (i. 28). Parthenogenesis. On " virginal generation," or the "immaculate conception" of Invertebrates, especially of Articulated Animals (Crustacea, Insecta, etc.), see Siebold. "Remarks on Parthenogenesis among Arthropoda " (" Beitrage zur Parthenogenesis der Arthropoden." Leipzig, 1871). Georg Seidlitz, " Parthenogenesis and its Relation to other Forms of Generation in the Animal Kingdom " (" Die Parthenogenesis und ihr Verhiiltniss zu den iibrigen Zeugungs-Arten im Thierreich." Leipzig, 1872). 17 (i. 34). The Prefonnation-theory. This theory is, in Germany, usually called " Evolutions-theorie," in distinction from the " Epigenesis-theorie." As, however, in England, France, and Italy, the latter is, on the contrary, usually called the theory of evolution, evolution and epigenesis being used as synonymous terms, it appears better to call the former " the theory of pre- formation." Recently Kolliker has called his " theory of hetero- genous generation " " Evolutionism " (note 47). Cf. preface, p xxx. 18 (i. 37). Alfred Kirchhoff, "Caspar Friedrich Wolff, hia Life and Teaching in the Science of Organic Evolution." — " Jenaische Zeitschrift fur Naturwissenschaft," 1868, vol. iv. p. 193. 19 (i. 43). Part of the writings left by Wolff have not yet been published. His most important works are -the dissertation for the degree of doctor, Theoria generations (1759), and his model treatise " de formatione intestinorum " (on the formation of the intestinal canal). — "Nov. Comment. Acad. Sc. Petropol.," xii. 1768; xiii. 1769. Translated into German by Meckel. Halle, 1812. 20 (i. 51). Christian Pander, " Historia metamorphoseos, quam NOTES. 463 ovum incubatum pripribus quinque diebus subit." Vicebcrgi, 1817. (Dissertatio inauguralis.) "Contributions toward the history of the evolution of the chick within the egg." (" Beitrage zur Entwickelungsgeschichte des Hiihnchens im Eie." Wurz- burg, 1817.) 21 (i. 52). Karl Ernst Baer, "On the Evolution of Animals. Observations and Reflections" (" Ueber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion." 2 vols. K6nigsbergr 1827-1837). In addition to this chief work, cf. " Story of the Life and Writings of Dr. Karl Ernst Baer, told by himself " ("Nachrichten iiber Lebon und Schriften des Dr. Karl Ernst Baer, mitgetheilt von ihm selbst." Petersburg, 1865). 22 (i. CO). Albert Kolliker. His "History of the Evolution of Man and the Higher Animals " (" Entwickelungsgeschichte des Menschen und der hoheren Thiere "). The 2nd (corrected) edition, 1876, contains (pp. 28-40) a catalogue of ontogenetic literature. On the newer contributions to this, cf. the "Jahresberichte iiber die Leistungen und Fortschritte der Medicin " (Berlin), by Virchow and Hirsch (the " History of Evolution," by Waldeyer) ; also the " Jahresberichte fiber die Fortschritte der Anatomie und Physiologic," by Hofmann and Schwalbe (Leipzig) ; the "History of Evolution," by R. Hertwig and Nitsche. Most of Kowalev- sky's researches are contained in the " Memoircs de l'Acad6mie imperiale de St. Petersburg " (from the year 18G6). Others are published in Max Schultze's "Archiv fur mikroskopische Anatomie," and in other periodicals. 23 (i. 60). Theodor Schwann, " Microscopic Researches into the Identity in Structure and Growth of Plants and Animals " (" Mikroskopische Untersuchungen xiber die Uebercinstimmung in der Structur und Wachsthnm der Thiere und Pflanzen." Berlin, 1839). 24 (i. 69). Ernst Haeckel, the Gastrsea Theory, phylogenetic classification of the animal kingdom and homology of the germ- layers. — " Jenaische Zeitschrift fur Naturwissenschaft," vol. viii. 1874, pp. 1-56. 25 (i. 75). Ernst Haeckel, "The History of Creation." London, 1876. 63 464 NOTE8. 26 (i. 81). Fritz Schultze, "Kant and Darwin." A con- tribution to the history of the science of evolution. Jena, 1875. 27 (i. 81). Immanuel Kant, " Critique of Teleological Rea- son " ("Kritik der teleologischen Urtheilskraft "). 1790. §74 and § 79. Cf . also my " History of Creation," vol. i. p. 103. 28 (i. 83). Jean Lamarck, " Philosophic Zoologique, ou Exposition des Considerations relatives a 1'histoire naturelle dcs animaux," etc. 2 Tomes. Paris, 1809. Nouvelle edition, revue et precedee d'une introduction biographique par Charles Martins. Paris, 1873. 29 (i. 88). Wolfgang Goethe on Morphology (zur Morpho- logic). The formation and re-formation of organic bodies. On Goethe's morphological studies, cf. Oscar Schmidt (" Goethe's Verhaltniss zu den organischen Naturwissenschaften." Jena, 1853). Rudolph Yirchow, " Goethe as a Naturalist " (Berlin, 1861). Helmholtz, "On Goethe's Natural Scientific Works" (Brunswick, 1865). 30 (i. 96). Charles Darwin. His chief work is " On the Origin of Species by means of Natural Selection " (1859). 31 (i. 99). Darwin and Wallace. The general outlines of the theory of selection were discovered independently by Darwin and Wallace. It does not, however, follow that the services of the latter in furthering the science of evolution are at all comparable with those of the former. As many opponents of Darwin, especially the English Jesuit Mivart, have recently endeavoured to exalt Wallace at the expense of Darwin, and to depreciate the latter, I take this opportunity of expressly assert- ing that Darwin's services are very far the greater. 32 (i. 101). Thomas Huxley. In addition to the works mentioned in the text, the following popular works are especially to be recommended : " On Our Knowledge of the Causes of Phenomena in Organic Nature," and the " Elementary Phy- siology "(1871). 33 (i. 101). Gustav Jaeger, " Zoological Letters" ("Zoologische Briefe." Vienna, 1876), and the " Text-book of General Zoology*" (;1 Lehrbuch der Allgemeinen Zoologie." Stuttgart, 1875). NOTES. 465 34 (i. 101). Friedrich Rolle, "Man, his Descent and Morality represented in the light of the Darwinian Theory, and on the basis of Recent Geological Discoveries " (" Der Mensch, seine Abstammung und Gesittung im Lichte der Darwin'schen Lehre," etc.). Frankfort, 1866. 35 (i. 102). Ernst Haeckel, " Generelle Morphologie der Organismen." General outlines of the science of organic forms, mechanically shown in accordance with the theory of descent as reformed by Charles Darwin. Vol. i , " General Anatomy ; " vol. ii., " General History of Evolution." Berlin, 1866. 36 (i. 103). Charles Darwin, "The Descent of Man, and Selection in Relation to Sex." 2 vols. London, 1871. 37 (i. 108). Karl Gegenbaur, "Outlines of Comparative Anatomy "(" Grundzuge der vergleichenden Anatomic." Leipzig. 2nd ed., 1870). "Elements of Comparative Anatomy " (" Grundriss der vergleichenden Anatomie." 3rd (improved) edition, 1874). 38 (i. 114). Migration-theory. Moritz Wagner, " The Dar- winian Theory and the Law of Migration of Organisms " (" Die Darwin'sche Theorie und das Migrations-gesetz der Organ- ismen." Leipzig, 1868). August Weismann, " On the Influence of Isolation in the Formation of Species " (" Ueber den Einfluss der Isolirung auf die Artenbidung." Leipzig, 1871). 39 (i. 116). Carus Sterne, "Evolution and Dissolution" ("Wer- den Tind Vergehen "). A popular history of the evolution of nature as a whole. Berlin, 1876. Agassiz a "founder" of natural science. " Gegenwart." Berlin, 1876. 40 (i. 117). Ernst Haeckel, "The Chalk-sponges" ("Die Kalkschwamme ; Calcispongien oder Grantien." Berlin, 1872). A monograph and an attempted solution of the problem of the origin of species. Vol. i., "Biology of Chalk-sponges; " vol. ii., "Classification of Chalk-sponges;" vol. iii., "Atlas of Chalk- sponges " (with 60 plates). 41 (i. 124). On the Individuality of Cells and recent reforms* in the cell-theory, cf. my " Individualitatslehre," or " Tectologie " ("Generelle Morphologie," vol. i. pp. 239-274). Rudolf Virchow, " Cellular Pathologic." 4th edition. Berlin, 1871. 466 NOTES. 42 (i. 130). "The Plastid-theory and the Cell-theory."— "Jenaische Zcitschrift fur Naturwissenschaft," 1870, vol. v. p. 492. 43 (i. 138). Gegenbaur, " On the Structure and Evolution cf Vertebrate Eggs with Partial Yelk-cleavage." — " Archiv f . Anat. u. Phys." 1861, p. 491. 44 (i. 153). Ernst Haeckel, "On Division of Labour in Nature and Human Life," in the collection of Lectures by Virchow- Holtzendorf, 1 869. Sect. 78 2nd edition. 45 (i. 160). Monogony (Generatio neulralis). On the various forms of asexual reproduction (Schizogony, Sporogony, etc.), cf. " Generelle Morphologic," vol. ii. pp. 36-58. 46 (i. 160). Amphigony (Generatio sexualis). On the various forms of sexual reproduction (Hermaphroditism, Gonochorism, etc.), see " Generelle Morphologic, " vol. ii. pp. 58-69. 47 (i. 168). Fitful evolution and gradual evolution. The theory of fitful evolution has recently been developed especially by Kolliker, who, under the title of heterogeneous generation, opposes it to gradual evolution as maintained by us (" Zeitschr. f. Wissens. Zool.," vol. xiv. 1864, p. 181, and " Alcyonaria," 1872, pp. 384-415). This theory is distinguished by assuming entirely unknown causes for the "fitful evolution of species," a so-called " great law of evolution " (an empty word indeed !). On the contrary, we see, with Darwin, in the facts of Heredity and Adaptation sufficient known (partly inner, partly external) physiological causes, which explain the gradual evolution of species under the influence of the struggle for existence. 48 (i. 170). Immaculate Conception never occurs in the vertebrate tribe. On the other hand, parthenogenesis frequently occurs among Articulated Animals (Arthropoda) (note 16). 49 (i. 171). Fertilization of Flowers by insects. Charles Darwin on " The various contrivances by which British and Foreign Orchids are fertilized by Insects." Hermann Muller on. "The Fertilization of Flowers by Insects, and the correlative adaptations of both" ("Die Bsfruchtung der Blumen durch Insecten und die gcgonseitigen A-.passungen Beider ") . A con- NOTES. 467 tribution to our knowledge of causal connection in organic nature. Leipzig, 1873. 50 (i. 178). The Process of Fertilization has been very variously viewed, and was formerly often regarded as an entirely mysterious process, or even as a supernatural miracle. It now appears no more " wonderful or supernatural " than the process of digestion, of muscular movement, or of any other physiological function. For the earlier views, cf. Leuckart, Article " Zeugung " (generation) in R. Wagner's " Dictionary of Physiology," 1850. 51 (i. 179). Monerula. The simple, very transient, kernel- less condition, which we briefly call the "monerula," and, in accordance with the fundamental law of Biogeny, regard as a palingenetic reproduction of the phylogenetic Moneron parent- form, appears to vary to some extent in different organisms, especially in the matter of duration. In those cases in which it no longer occurs, and in which the kernel of the fertilized egg persists wholly or partially, we may regard this phenomenon as a later, kenogenetic curtailment of Ontogeny. 52 (i. 181). The Plasson of the monerula appears, mor- phologically, a homogeneous and structureless substance, like that of the Moneron. This is not contradicted by the fact that we ascribe a very complex molecular structure to the plastidules, or " plasson-molecules," of the monerula ; this latter will naturally be more complex in proportion as the organism which it ontogenetically constitutes is higher, and as the ancestral series of that organism is longer, in proportion as the preceding processes of Heredity and Adaptation are more numerous. 53 (i. 182). The Fundamental Significance of the Parent-cell, or cytula, as the foundation-stone of the young organism in the course of development, can only be rightly appreciated, if the part taken in its conetitution by the two generating cells is rightly appreciated, the part taken by the male sperm-cell and by the female egg-cell. 54 (i. 183). The One-celled Germ-organism, like the act of fertilization from which it results, has been very variously 468 NOTES. viewed. Cf. on this subject, in addition to the four important works, here quoted, by Auerbach, Biitschli, Hertvvig, and Stras- burger, the most recent annals of the progress of the history of evolution (Waldeyer in Virehow-Hirsch's " Jabresberichten," Berlin; Hertwig in Hofmann-Schwalbe's " Jahresberichten," Leipzig). 55 (i. 185). Protozoa and Metazoa. Cf. vol. i. p. 248; ii. 92. The Protozoa and Metazoa are genetically and anatomically so very distinct, that the former, as Protista, may even be excluded entirely from the animal kingdom, and may be regarded as a neutral intermediate kingdom between the plant and animal kingdoms. — " Generelle Morphologic," vol. i. pp. 191-230. Ac- cording to this view the Metazoa alone are really animals. 56 (i. 186). The Unity of the Zoogenetic Conception, result- ing from the Gastrasa-theory, has as yet not been destroyed by the numerous attacks directed against that theory : for none of these attacks have succeeded in substituting anything positive ; by pure negation no advance can be made in this dark ani difficult subject. 57 (i. 187). The Egg-cleavage and Gastrulation of Man, as represented diagrammatically in Figs. 12-17 of Plate II., is most probably in no essential way different from that of the Rabbit, which has as yet been most closely examined in this point. 58 (i. 188). Ernst Haeckel, " Arabian Corals " (" Arabischo Korallen"). "A Journey to the Coral Banks of the Red Sea, and a Glimpse into the Life of Coral Animals. A popular lecture, with scientific explanations," With 5 coloured elates, and 20 woodcuts. Berlin, 1876. 59 (i. 189). The Number of the Segmentella, or cleavage- cells, increases, in the original, pure forms of palingenetic egg- cleavage, in regular geometric progression. But the point to which this proceeds varies in the various archiblastic animals, so that the Morula, as the final result of the cleavage-process, consists sometimes of 32, sometimes of 64, sometimes of 128 cells, and so on. 60 (i. 189). The Mulberry-germ, or Morula. The seg« NOTES. 469 mentella, or cleavage- cells, which constitute the Morula at the close of palingenetic egg-cleavage, generally appear entirely similar, with morphological difference in size, form, or con- stitution. This does not, however, hinder the fact that these cells have separated, even during cleavage, into animal and vegetable cells, have differentiated physiologically, as is indicated in Figs. 2 and 3, Plate II., as probable. 61 (i. 189). The Bladder-germ of Archiblastic Animals (blastula, or blastosphcera), which is now commonly known as the germ-vesicle, or, more accurately, as the " germ-membrane vesicle," must not be confused with the essentially different " germ- vesicle " of amphiblastic mammals, which is better called the "intestinal-germ vesicle" (gastrocystis). The gastrocystis and the blastula are still often united under the name of " germ- vesicle, or vesicula blastodermica." Cf. vol. i. p. 290. 62 (i. 192). The Definition of the Gastrnla was first established by me in 1872, in my "Monograph of Chalk-sponges " (vol. i. pp. 333, 345, 466). There I already gave due weight to the " extremely great significance of the gastrula in reference to the general Phylogeny of the animal kingdom" (p. 333). " The fact that these larval forms re-occur in the most different animals, cannot, I think, be sufficiently estimated, and bears plain witness to the former common descent of all from the Gastrsea " (p. 345). 63 (i. 194). The Uniaxial Outline of the Gastrula is, on account of the two different poles of the axis, more accurately described as a diplopolic uniaxial form (a sternometric outline : conoid-form, or cone). Cf. my " Promorphology " (" Generelle Morphologic," vol. i. p. 426). 64 (i. 194). Primitive Intestine and Primitive Mouth. My distinction of the primitive intestine and primitive mouth (protogaster and protostoma) from the later, permanent intestine and mouth (inetagaster and metastoma) has been variously attacked ; it is, however, as much justified as the distinction of the primitive kidney from the permanent kidney, of the primitive vertebrae from the permanent vertebrae. The primitive intestine 4/O NOTES. forms but a part of the permanent intestine, and the primitive mouth (at least in the higher animals) does not become the permanent mouth. 65 (i. 196). Primitive germ-layers (blastnpTiylla) . As the two primary germ-layers (entoderma and exodenna) originally form the sole histogenetic rudiment of the whole body, and as the mesoderma, the nutritive yelk, and all other accessory parts of the germ have developed only secondarily from the former, 1 consider it very important to distinguish between the primary and secondary germ-layers. The latter, to distinguish them from fhe former, might be called "after germ-layers" (Wos- telasma). 66 (i. 201). Unequal Cleavage and Hood-gastrula (8eg- mentatio incequalis et Ampldgastrula). Next to Amphibia the most accessible examples for observation of unequal cleavage and the Amphigastrula are afforded by the indigenous Soft- bodied Animals (Mollusca) and Worms (Snails and Mussels, Earth Worms and Leeches). 67 (i. 202). The Colour of Amphibian-eggs is occasioned by the accumulation of dark colouring-matter at the animal pole of the egg. In consequence of this the animal-cells of the exoderm appear darker than the vegetative cells of the cntoderm. In most animals the reverse is the case; the protoplasm of the entoderm cells being usually darker and more coarsely granulated (vol. i. p. 197). 68 (i. 207). Hood-gastrula of Amphibia. Cf. Robert Remak, " On the Evolution of Batrachia " (" Uebtr die Entwickelung der Batrachier," p. 126 ; Plate XII. Figs. 3-7). Strieker's " Manual of Tissues " (" Handbuch der Gewebelehre," vol. ii. p. 1195- 1202; Figs. 399-402). Goette, "History of the Evolution of Bombinator" (" Entwickelungsgeschichte der Unke," p. 145; Plate II. Figs. 32-35). 69 (i. 214). Hood-gastrula of Mammals. Eduard van Beneden, " La maturation de 1'ceuf, la fecondation et les premieres phases du developpemcnt embryonnaire des Mammiferes, d'apres des recherches faitos chez le lapin." Brussels, 1875. No figures NOTES. 471 are given with these " Communication preliminaire ; " Van Beneden's description is, however, so clear, so thorough and care- ful, that they afford an entirely satisfactory insight into unequal egg-cleavage and the formation of the Hood-gastrula in Mammals. All other observers, who have studied the germination of Mam- malian eggs (among the most recent Kolliker, Rauber, and Hewson may be especially mentioned), have overlooked or failed to recognize the important features discovered by Van Beneden. 70 (i. 218). The Disc-gastrula (Disco-gastrula) of Osseous Fishes (Teleostei). Van Bambeke, " Recherches sur 1'embry- ologie des poissons osseux." Brussels, 1875. The transparent Fish-eggs, in which I observed discoid cleavage (Segmentatio discoidalis) and the formation of the Disc-gastrula by invagination, are accurately describad in my article on " The Gastrula and Egg-cleavage of Animals " (" Jen. Zeitschrift fur Naturwis- senschaft," 1875, vol. ix. p. 432-444; Plates IV., V). On the Disc-gastrula of Selachii, cf. Balfour, "The Development of Elasmo branch Fishes." — " Journ. of Anat. and Physiol.," vol. x. p. 517; Plates XX., XXIII. 71 (i. 221). Yelk-cells of Birds. The cell-like constituent parts, which occur in great number and variety in the nutritive yelk of Birds and Reptiles, as in most Fishes, are nothing less than true cells, as His and others have asserted. This does not mean that in this matter a distinct limit everywhere exists between the nutritive and the formative yelks, as in our oceanic Fish-eggs (Figs. 42, 43, note 70). On the contrary, originally (phylogenetically) the nutritive yelk originated from part of the onto derm. 72 (i. 223). Egg-cells of Birds. Notwithstanding the large nutritive yelk, the " after-egg " (metovum') of Birds and Reptiles is, in form- value, a single cell. The very small, active protoplasm of the " tread " does, however, indeed fall far short, in volume, of the huge mass of the yellow yelk-ball. The bird's eggs are absolutely the largest cells of the animal body. Cf. note 43, and Eduard van Beneden, " Recherches sur la composition efc la, 4/2 NOTES. signification de 1'oeuf." Brussels, 1870. Hubert Lmdwig, " On Egg-structure in the Animal Kingdom " (" Ueber die Eibildung in Thierreiche." Wiirzburg, 1874). 73 (i. 226). Discoidal cleavage (Segmentatio discoidalis) of Bird's eggs. Cf. Kolliker, " History of the Evolution of Man and the Higher Animals " (" Entvvickelungsgeschichte des Men- schen und der hoheren Thiere." 2nd edition, 1876, pp. 69-81 ; Figs. 16-22). 74 (i. 227). Disc-gastrula (Disco-gastrula) of Birds. Cf. Rauber, " On the Place of the Chick in the System of Evolu- tion " (" Ueber die Stellung des Hiihnchens im Entwickelungs- plan"). Leipzig, 1876. Foster and Balfour, " The Elements of Embryology." London, 1874. 75 (i. 231). Bladder-gastrula (Perigastrula) of Articulated Animals (Arthropoda) . Cf. Bobretzky, " Russian Essay on the Germ-history of Astacus and Palosmon." Kiew, 1873. Also my own article on the gastrula and egg-cleavage. — " Jen. Zeitschrift fur Naturwissenschaft." Vol. ix. pp. 444-452, Plate VI. 76 (i. 234). The Four-layer Theory, which was first clearly stated by Baer in 1837 (" Entwickelungsgeschichte der Thiere," vol. ii. pp. 46, 68), and which we have here carried out logically, yet appears the only form of the germ-layer theory, which, on comparative observation of all higher animals, supplies a universal law of germination for all and at the same time meets the inconsistent reputations of many observers. 77 (i. 239). 'Caspar Friedrich Wolff first indicated the Four- layer Theory (note 76). Cf. the remarkable sentence, quoted at vol. i. p. 45, from his pregnant work on the formation of the intestinal canal (note 19). 78 (i. 240). The Four Main Types of Gastrulation, which are diagrammatically distinguished in Plates II. and III., and in Tables III. and IV. (vol. i. pp. 241, 242), are of course connected by intermediate forms. These are transitions both between the primordial and the unequal forms, and between the primordial aud the superficial forms ; similarly, the unequal form "of egg-cleavage is connected by twixt- forms with the discoidal forms, which latter NOTES. 473 is again, perhaps, connected in the same way with the superficial form. 79 (i. 241). The Gastrulation of the various classes of animals has been far too little studied to enable us thoroughly to summarize the distribution of the various forms withir the separate classes. Yet it is already evident that primordial egg- cleavage and the formation of the Archigastrula occur in the lowest classes of each tribe. 80 (i. 243). The Rhythm of egg-cleavage is by no means as regular as might appear from the four first examples in the five tables. There are, on the contrary, many variations, and not infrequently an entirely irregular and very variable sequence of numbers occurs (especially in discoided cleavage). 81 (i. 246). Definition of the Type. Cf. Gcgenbaur, " Elements of Comparative Anatomy," 1874, p. 59. 82 (i. 246). Types and Phyla. According to the prevailing " Type-theory," the types of the animal kingdom are parallel, and entirely independent ; according to my " Gastnea-theory," on the contrary, they are divergent tribes, connected at the roots ; according to the view of Glaus and other opponents, the latter is no essential distinction. 83 (i. 248). The one-celled condition of Infusoria entirely forbids their morphological comparison with Metazoa. Cf. my article " On the Morphology of Infusoria " (" Jen. Zeitschrift fur Naturwissenschaft " 1873, vol. vii. p. 516-568). 84 (i. 257). The axes of the Vertebrate outline. Cf. my " Promorphology " (Stereometry of Organisms). — " Generello Morphologic," vol. i. pp. 374-574. " Singly double-outlines " (Dipleura), p. 519. "Bilateral-symmetrical" forms in the fourth signification of the word. 85 (i. 255). The Primitive Vertebrate Type, as it is repre- sented in Figs. 52-56, is a hypothetic diagram, which is principally founded on the outline of the Amphioxus, but in which the Comparative Anatomy of Ascidia and Appendicularia on the one side, of Cyclostorni and Selachii on the other, is regarded. This diagram is by no means meaut to be an " exact figure," but 474 NOTES. a provisional stage in the hypothetic reconstruction of the unknown, long extinct parent-form of Vertebrates, an "Archi- type." 86 (i. 258). Only very uncertain assumptions can be mado as to the sense-organs of the hypothetic parent-form, for these organs, more than any others, have been subject to adaptations, and in Ascidia, as in the Amphioxus, have probably been much atrophied. The earliest Vertebrates probably inherited a pair of eyes of very simple character and a pair of simple ear- vesicles from Worms. 87 (i. 267). The primitive kidneys were perhaps already metameric in the hypothetic parent-form of Vertebrates, so that in addition to the two longitudinal main canals (primitive kidney ducts) numerous transverse tubes (segmental canals) were connected with these main canals, a pair in each metameron of the middle part of the body. Perhaps these already opened through ciliated funnels into the body-cavity (coeloma), as is now the case in Annelids, and, according to Balfour, in the embryos of Selachii. Cf. Balfour, " Development of Elasmo- branch Fishes." — " Quarterly Journal of Microscopical Science." New Series, vol. xiv. p. 323 ; " Journal of Anat. and Physiol." vol. x. 88 (i. 273). The germination of Primitive Vertebrates. Cf. with Table VI., Table VII. (vol. i. p. 327), Table XI. (p. 467); also the diagrammatic figures in Plates IV. and V. with explana- tion (p. 321). 89 (i. 276). The Germ-forms of the earliest Vertebrates, as they are represented in diagrammatic cross sections in Figs. 62-69, can only, of course, be approximately guessed, and with the aid of Comparative Anatomy and Ontogeny. These hypothetic diagrams, therefore, by no means claim to be ac- cepted dogmatically, any more than do those in Figs. 52-56. (Cf. note 85.) 90 (i. 280). Main incidents in Vertebrate germination. Of the main palingenetic incidents here enumerated, perhaps the sixth, ainth, and tenth originally occurred in a very dif- NOTES. 475 ferent form. The other seven now appear to be pretty "well established. 91 (i. 285). The flat germ-disc of Birds, which even now, in the opinion of most embryologists, represents the first starting- point in the formation of the embryo, and to which all other germ-forms have been referred, is, on the contrary, a late and much modified germ-form, which has arisen in consequence of the extension of the gastrula over the greatly enlarging nutritive yelk. 92 (i. 289). Site of Fertilization. In Man, as in other Mammals, fertilization of the eggs probably usually takes place in the oviduct : here, the eggs which, at the rupture of the Graafian follicles, have emerged from the female ovary and passed into the outer opening of the oviduct, meet with the active sperm-cells of the male seed, which, during copulation, penetrated into the uterus, and from there passed into the inner opening of the oviduct. Rarely, fertilization occurs even on the ovary, or not till within the uterus. (Cf. Chapter XXV.) 93 (i. 293). The origin of the mesoderm in Mammals, as in other animals, is, at present, among the most obscure and con- tested points of Ontogeny. Remak, Balfour, and others derive it from the entoderm, Kolliker and others from the exoderm. Waldeyer, His, and others assert that both primary germ-layers take part in the formation of the mesoderm. The last assump- tion is, I believe, correct. (Cf. notes 76, 77.) 94 (i. 297). The Germ-shield (Notaspti). The ordinary view, that the germ-shield (= Remak's " Doppelschild ") is the earliest rudiment of the actual embryo, results in many erroneous conclusions. It is, therefore, necessary to point out especially that the germ-shield represents the first well-defined central dorsal part of the embryo. 95 (i. 317). Body Wall and Intestinal Wall. The morpho- logical distinction between the body wall and the intestinal wall, certainly primordial, is probably referable to the simple primary germ-layers of the Gastraea. If the skin-fibrous layer is derived from the exoderm, and the intestinal-fibrous layer from the 4/6 NOTES. entoderm, this most simply explains the progressive development of this distinction, which may be traced through the series of Worms, and up to Vertebrates. 96 (i. 320). Palingenetic and Kenogenetic germination. In the germ-history of Vertebrates no clear conception of the embryological process has yet been attained, because all authors have started from the higher Vertebrates (usually from the Chick) and have assumed that the form of evolution occurring in this case is original and typical. It is only since the germ- history of the Amphioxus has taught us the palingenetic, really original form of germination of Vertebrate organisms, that we have been enabled, by Comparative Ontogeny (and especially by the principles of the Gastraea theory), rightly to understand and to explain phylogenetically the kenogenetic forms of germination of higher Vertebrates. 97 (i. 321). The Diagrams in Plates IV. and V. are as simple and abstract as possible, in order to render the desired general explanation as easy as possible. 98 (i. 346). Primitive Vertebrae and Metamera. For the right conception of " primitive vertebral " structure it is espe- cially necessary to point out that the primitive vertebrae are much more than their name indicates. They must, in fact, be conceived as individual, consecutive sections of the trunk, which have arisen one after the other, as true " metamera," or consecutive pieces ("Generelle Morphologic," vol. i. p. 312). Each primitive vertebra of a Vertebrate, like each trunk-segment or metameron of an Annelid or Arthropod, contains all the essential, morphological constituent parts, characteristic of the corresponding animal-tribe. 99 (i. 349). Origin of the Primitive Vertebras. My concep- tion of these as individual, morphological "consecutive pieces," which, like the metamera of Cestods and Annelids, have arisen by terminal budding from a single unarticulated piece, has beeii much attacked. I therefore emphatically remark that I only understand this process in the widest sense. In both cases there is certainly a reproduction of individual, like parts, which have originated (in time and space) consecutively. NOTES. 477 100 (i. 361). The agreement among the germ-forms of various Mammals is instructive especially because it shows us how, by diversity in the mode of evolution, the most diverse structures can originate from one and the same form. As we actually see this in germ-forms, we may hypothetically assume the same to have occurred among tribe-forms. Moreover, this agreement is never absolute identity, but always only the very greatest similarity. Even the germs of the various individuals of a species are never actually identical. 101 (i. 366). The law of the ontogenetic connection of systematically allied animal-forms has many apparent exceptions. These are, however, fully explained by the adaptation of the germ to kenogenetic conditions of existence. Where the palin- genetic form of evolution of the germ has been accurately transmitted by heredity, that law is always in force. Cf. Fritz Miiller, "Fur Darwin" (note 111). 102 (i. 367). Earliest human germs. Cf. Kolliker, " History of the Evolution of Man " (" Entwickelungsgcschichte des Mer.- schen." 2nd edition, 1876, pp. 303-319). Also Eckcr, "Icones physiologies}." Leipzig, 1859. Plates XXV.-XXXI. The earliest human germs which have yet been certainly recognized, were from twelve to fourteen days old, and were observed by Prof. Allen Thomson, of Glasgow. No opportunity has ever occurred for the observation of earlier germs. 103 (i. 369). Human germs of three weeks (twenty to twenty- one days) exhibit in their whole structure thatphylogenetic stage of evolution which, among extant Vertebrates, is represented by the Cyclostomi (Lampreys and Hags, vol. ii. p. 103), and which must be referable to extinct Monorhine ancestors of similar structure. 104 (i. 370). Human germs of four weeks (twenty-five to thirty days), on the whole, exhibit in their whole structure that phylogenetic stage of evoliition, which is exhibited in Sharks and Rays, among extant Vertebrates, and which is referable to similar extinct Primitive Fish ancestors (Proselachii). Of course this comparison is affected by various kenogenetic modifications 478 NOTES. (both heterotopic and heterochronic), just as in the former. (Cf. note 108.) 105 (i. 374). The nose of Nosed-apes is much more different from that of other Apes than from that of Man. Moreover, even the extreme variety and variability in the external form of the human nose shows how small is the morphological yalue of this organ, so important to the physiognomy. 106 (i. 383). The bladder-like form of the human Allantois. Cf. W. Krause, " On the Allantois in Man " (" Ueber die Allan- tois des Meuschen."— "Archiv fur Anat. n. Physiol.," 1875, p. 215, Plate VI.). 107 (i. 400). The navel-cord (funiculus umbili calls), like the placenta, is an organ shared by Man exclusively with Pla- cental Animals. Cf. Chap. XIX. pp. 155-168, and Pigs. 200, 201. On the more minute structure of this organ, and on the special features of the embryonic blood-circulation, cf. Kolliker, " His- tory of the Evolution of Man." 2nd edition, 1876, pp. 319-363. 108 (i. 401). The Kenogeny of Man. In pointing out the phylogenetic significance of the separate incidents and periods of human germ-history, and in explaining them by reference to cor- responding processes and stages in the tribal history of our animal ancestors, we must always bear in mind that in Man, as in all higher animals, the original palingenetic cause of germination has nndergone much kenogenetic modification in consequence oi many adaptations to the very various conditions of embryonic life, that it has thus been much violated and contracted. The higher the organism develops, the more are especially these earliest stages of evolution abbreviated. 109 (i. 404). The sections of human germ-history, of which only four larger and ten smaller are mentioned here in reference to their phylogenetic significance, allow of much more division if their comparative Ontogeny is minutely examined. This phylogenetic significance may also be very well explained with fitting reference to kenogenetic displacements in place and time (vol. i. p. 13). 110 (i. 405). Figures of human embryos in all stages of NOTES. 479 germ-history were given iu very beautiful detail by M. P. Erdl thirty years ago : " The Evolution of Man, and of the Chick in the Egg" ("Die Entwickelung des Menschen, uiid des Hiihnchens im Ei." Leipzig, 1845). 111 (i. 409). Fritz Miiller, "Fur Darwin." Leipzig, 1864. A very excellent little book, in which the modification of the fundamental law of Biogeny (with reference to the Phylogeny of Crustacea) are explained for the first time. 112 (i. 413). The Method of Phylogeny is of the same morphological value as the well-known method of Geology, and may, therefore, claim exactly the same scientific acceptation. Cf. the excellent discourse by Eduard Strasburger, "On the Importance of Phylogenetic Methods in the Study of Living Beings." — "Jenaische Zeitschrift fur Naturwissenschaft," 1874, vol. viii. p. 56. 113 (i. 415). Johannes Miiller, " On the Structure and Vital Phenomena of Amphioxus lanceolatus." — Transactions of the Berlin Academy, 1844. 114 (i. 415). Eecent works on the Amphioxus. W. Rolph and E. Ray Lankester especially have recently added to our knowledge of the organology of the Amphioxus, Wilhelm Miiller and P. Langerhans to that of its histology. The literature of this subject is fully represented by W. Rolph, in his "Researches into the Structure of the Amphioxus " (" Untersuchungen iiber den Bau des Amphioxus." — " Morpholog. Jahrb.," vol. ii. p. 87, Plates V. and VII.), and in P. Langerhans, " On the Anatomy of the Amphioxus" ("Zur Anatomic des Amphioxus." — " Archiv. far Mikr. Anat.," vol. xii. p. 290, Plates XII.-XV.). 115 (i. 416). Acrania and Craniota. The separation of Vertebrates into Skull-less Animals (Acrania) and Skulled Animals (Craniota), which I first indicated in 1866 in my " Generelle Morphologic," appears to me absolutely essential for the phylogenetic explanation of the Vertebrate-tribe. 116 (i. 428). Max Schultze, "History of the Evolution of Petromyzon" ("Entwickelungsgeschichte von Petromyzon." Haar- lem, 1856). The Ontogeny of the Hags, which promises very important results, is yet, unfortunately, entirely unknown. 480 NOTES. 117 (i. 430). Savigny, "Memoires snr les Anintaux sans Vei-tebres." Vol. ii., Ascidies, 1816. Giard, "Recherches sur les Synascidies." — "Archives de Zoologie Experimentale," vol. i., 1872. 118 (i. 435). Syn-ascidia and Echinoderms. The Conn-theory of Echinoderms, which I explained in 1866 (" Generelle Mor- phologie," vol. ii. p. Ixiii), and which has been much attacked as " paradoxical," is as yet the sole theory attempting the genetic explanation of this remarkable group of animals. 119 (i. 442). Kowalevsky, "History of the Evolution of the Amphioxus and of Simple Ascidians" ("Memoires de 1'Acad. de S. Petersbourg." 7 Serie. Tom. x. and xi. 1867-8). 120 (i. 450). The metameric structure of the Amphioxus which is indicated in its nerve and muscle systems, undoubtedly shows that the notochord exists in Vertebrates previous to their metameric structure, and consequently that it is inherited from unarticulated Chorda Animals. 121 (i. 454). The Metamorphosis of the Amphioxus, through which the larva passes into the adult form, is not yet fullv known in all its details. This does not, however, affect the extraordinarily important bearing of the thoroughly known, earliest incidents in its germination on the palingenesis of Verte- brates. 122 (i. 455). Fertilization of Ascidia (Phallusia mammillata), Eduard Strasburger, " On Cell-structure and Cell-division, with Studies of Fertilization." 2nd edition. Jena, 1876, p. 306, Plate VIII. 123 (i. 462). Kupffer. The tribal relation of Ascidia to Vertebrates ("Archiv fur Mikros. Anat.," 1870, vol. vi. pp. 115-170). Oscar Hertwig, " Researches into the Structure and Evolution of the Cellulose Mantles of Tuniuata" (" Untersn- chnngen iiber den Bau und die Entwickelung des Cellulose- Mantels der Tunicaten "). Richard Hertwig, " Contribution to Knowledge of Ascidian Structure " (" Beitrage zur Kenntniss des Baues der Ascidien." — " Jenaische Zeitschrift fur Natnrwis- senschaft," 1873, vol. vii.). NOTES. 481 124 (i. 464). The Phylogenetic Importance of the Amphi- oxns cannot be too highly insisted on. Without knowledge of its Anatomy and Ontogeny, the origin of Vertebrates would be entirely dnbious, and their descent from Worms would appear incredible. 125 (i. 467). The Ontogenetic Cell-pedigree, as it is repre- sented, with reference to the Amphioxus, in Table XI., probably holds good, in its most important features, for all Vertebrates, and, therefore, also for Man. For, more than any other form, the Amphioxus by strict Heredity has accurately retained its Palingenesis. This histogenetic cell-pedigree is apparently well established as regards most and the chief features ; on the other hand, it yet appears doubtful with regard to the origin of the primitive kidneys, the testes, and ovaries. 126 (ii. 4). Milne-Edwards, "Lecons sur la Physiologic Comparee," vol. ix. 127 (ii. 6). Eternity of Organic Life. According to the monistic view, organic life is a further form of evolution of the inorganic word-processes, and had a beginning in time on our planet. In opposition to this, A. Fechner, among others, in his " Thoughts on the Creation and Evolution of Organisms," has stated certain opposed " kosmorganic pl^.ntasys " which appear entirely irreconcilable with the ontogenetic facts given here. 128 (ii. 18). Bernhard Cotta (" Geologie der G-egenwart," 1866; 4th edition, 1874) and Karl Zittel("Ans der Urzeit;" Miinchen, 1875, 2nd edition) have made some excellent remarks on the duration and the whole course of the organic history of the world. 129 (ii. 21). August Schleicher, "The Darwinian Theory and Philology" ("Die Darwin 'sche Theorie und die Sprach- wissenschaft." Weimar, 1863. 2nd edition, 1873). 130 (ii. 25). At first sight, most polyphyletic hypotheses appear more simple and easy than do monophyletic, but the former always present more difficulties the more they are considered. 131 (ii. 25). Those physiologists who desire an experi« 4^2 NOTES. mental proof of the theory of descent, merely thereby prove their extraordinary ignorance of the morphological scientific facts re- lating to this matter. 132 (ii. 30). Spontaneons generation. — " Generelle Mor- phologic," vol. i. pp. 167-190. "Monera and Spontaneous Gene- ration."— "Jenaische Zeitschrift fur Naturwissenschaft," 1871, vol. vi. pp. 37-42. 133 (ii. 33). The Absence of Organs in Monera. In saying that Monera are " organisms without organs," we understand the definition of organs in a morphological sense. In a physiological sense, on the other hand, we may call the variable plasson- processes of the body of the Moneron the " pseudopodia " organs. 134 (ii. 36). Induction and Deduction in Anthropogeny. "Generelle Morphologic," vol. i. pp. 79-88; vol. ii. p. 427. " History of Creation," vol. ii. p. 357. 135 (ii. 42). Animal Ancestors of Man. The number of species (or, more accurately, form-stages, which are distinguished as "species ") must, in the human ancestral line (in the course of many millions of years !), have amounted to many thousands ; the number of genera to many hundreds. 136 (ii. 47). Following Elsberg, we give the name of " plas- tidules" to the "molecules of plasson," to the smallest like parts of that albuminous substance which, according to the " plastid- theory," is the material substratum of all the active phenomena of life. Cf. my work on "The Perigenesis of Plastidules" (" Perigenesis der Plastidule oder Wellenzeugung der Lebens- theilchen." Berlin, 1876). This is an attempt to explain mechanically the elementary processes of evolution. 137 (ii. 49). Bathybius and the free protoplasm of ocean depths. Cf. my " Studies on Monera and other Protista." Leipzig, 1870, p. 86. The most recent observations on living Bathybius are those of Dr. Emil Bessel, who found this form on the coast of Greenland (in Smith's Sound), at a depth of about 550 ft. He noticed very active amoeboid movements in them, as well as the assumption of foreign particles (carmine, etc.). " It consists of nearly pure protoplasm, tinged most intensely bj NOTES. 483 a solution of carmine in ammonia. It contains fine gray granules of considerable refracting power, and besides the latter a great number of oleaginous drops, soluble in ether. It manifests very marked amoeboid motions, and takes up particles of carmine, etc." — Packard, " Life Histories of Animals, including Man." New York, 1876. 138 (ii. 50). The Philosophical Importance of Monera in explaining the most obscure biological questions cannot be sufficiently emphasized. Monograph of Monera. — " Jenaische Zeitschrift fur Naturwissenschaft," vol. iv., 1868, p. 64. 139 (ii. 54). The Nature and Significance of the Egg-cell can only be philosophically understood by means of phylogenetic examination. 140 (ii. 58). Synamoeba. Cieukowski, "On the Structure and Evolution of Labyrinthula " ("Uber den Ban und die Entwic- kelnng der Labyrinthuleen." — Arch, fiir Mikrosk. Anat., 1870, vol. iii. p. 274). Hertwig, "Microgromia Socialis." — Ibid. 141 (ii. 61). Catallacta, a new Protista-group (Mayosphcera planula). See " Jenaische Zeitschrift fiir Naturwissenschaft," vol. vi., 1871, p. 1. 142 (ii. 66). Haliphysema and Grastrophysema. Extant Gastraeads. See " Jeuaische Zeitschrift fiir Naturvvissenschaft," vol. xi., 1876, p. 1, Plates I.-VI. 143 (ii. 70). The five first stages in the evolution of the animal body, which are compared in Table XVII., and which are common to Man and all higher Animals, are established beyond all doubt as existing in the Ontogeny of most extant animals. As Comparative Anatomy shows that corresponding form-stages yet exist in the system of the lower animals, we may assume, in accordance with the fundamental law of Biogeny, that similar forms existed phylogenetically as most important ancestral forms. 144 (ii. 77). On the distinction of the axes, and on the geometric outline of the animal body, see " Promorphologie " (" Generelle Morphologic," vol. i. pp. 374-574). 145 (ii. 87). The hermaphrodite structure of our ancestral 4§4 NOTES. series was perhaps transmitted from the Chorda Animals even as far as the lower stages of Vertebrate ancestors. Cf . Chapter XXV . 146 (ii. 89). I am inclined to regard the Appendicularia as living Chorda Animals of the present day; they are the only Invertebrates permanently possessing a notochord, and thus, aa by many other peculiarities, distinguished from genuine Tnni- cates. 147 (ii. 105). Metamorphosis of Lampreys. That the blind Ammocoetes change into Petromyzon was known two hundred years ago (1666) to the fisherman Leonhard Baldner of Stras- burg ; but this observation remained unrecognized, and the modification was first discovered by August Miiller in 1854 (" Archiv fur Anat.," 1856, p. 325). Cf. Siebold, "The Fresh- water Fishes of Central Europe" ("Die Siisswasserfische von Mittel-Europa," 1863). 148 (ii. 114). Selachii as Primitive Fishes. The old disputes as to the systematic position and kindred of Selachii were first definitely settled by Gegenbaur, in the introduction to his classical work on " The Head-skeleton of Selachii." 149 (ii. 118). Gerard Krefft, " Description of a Gigantic Am- phib'an ; " and Albert Giinther, " Ceratodus, and its Systematic Position." — " Archiv fiir Naturgeschichte," 37, 1871, vol. i. p. 321 ; also "Phil. Trans.," 1871, Part II. p. 511, etc. 150 (ii. 129). The duration of metamorphosis of Amphibia varies much in the different forms of Frogs and Toads, the whole forming a complete phylogenetic series from the original, quite complete form, to the later, much shortened and vitiated heredity of modification. 151 (ii. 129). "All the histological features of the Land Salamander (Sdlamandra maculata) force the impression that it belongs to an entirely different epoch of terrestrial life than that of the Water Salamander (Tn'fom), externally so similar." — Robert Remak (" Entwickelung der Wirbelthiere," p. 117). 152 (ii. 130). Siredou and Amblystoma. Very various views have lately been expressed as to the phylogenetic significance to be attributed to the much-discussed modification of the Mexican NOTES. 485 Axolotl into an Amblystoma. Cf. on this subject especially August Weismann, in " Zeitsch. fiir wissensch. Zoologie," vol. xxv., Sup., pp. 297-334 153 (ii. 131). The Leaf- frog of Martinique (Hi/lodes mar- tinicensis) loses its gills on the seventh day, its tail and yelk-sac on the eighth day of egg-life. On the ninth or tenth day after fertilization the complete frog emerges from the egg. — Bavay, "Sur 1'Hylodes Martinicensis et ses Metamorphoses." "Journal, de Zool. par Grevais," vol. ii. 1873, p. 13. 154 (ii. 133). " Homo diluvii testis" = Andrias Scheuchzeri. " Sad bone of an ancient evil-doer ; Soften, stone, the heart of the new children of evil " (Diaconus Miller). Quenstedt. " Formerly and Now " (" Sonst und Jetzt," 1856, p. 239). 155 (ii. 133). The Amnion-structure of the three higher Vertebrate-classes, wanting in all lower Vertebrates, has no connection with the similar, but independently acquired Amnion- stracture (analogous, but not homologous) of higher Articu- lated Animals (Arthropod* i). 156 (ii. 138). The former existence of a Protamnion, the common parent-form of all Amniota, is undoubtedly shown by the Comparative Anatomy and Ontogeny of Reptiles, Birds, and Mammals. No fossil remains of such a Protamnion have, how- ever, yet been discovered. They must be sought in the Permian or Carboniferous formation. 157 (ii. 147). The former organisation of the Promanimalia may be hypothetically reconstructed from the Comparative Anatomy of the Salamander, Lizards, and Beaked Animals ( Ornitlwrliynclius). 158 (ii. 153). The Didelphic ancestors of Man may have been externally very different from all known Pouched Animals (Mar- supialia), but possessed all the essential internal characters of Marsupialia. 159 (ii. 163). The phylogenetic of the Semi-apes, as the primaeval placental parent-group, is not influenced by our ignor- ance of any fossil Prosimiae, for it is never safe to estimate palseontological facts as negative, but only as positive. 486 NOTES. 160 (ii. 168). On the structure of the Decidua very various theories have been given. Of. Kolliker, "History of the Evolution of Man " (" Entwickelungsgeschichte des Menschen." 2nd edition, 187] , pp. 319-376). Ercolani (Giambattista), " Sul pro- cesso formative della placenta." Bologna, 1870. " Le glandole otricolari del'utero." Bologna, 1868, 1873. Huxley, "Lectures on the Elements of Comparative Anatomy," 1864, pp. 101-112. 161 (ii. 172). Huxley, "Anatomy of Vertebrates," 1873, p. 382. Previously Huxley had separated the " Primates " into seven families of nearly equal systematic value. (See "Man's Place," etc., p. 119.) 162 (ii. 179). Darwin. Sexual selection in Apes and Man. — " Descent of Man," vol. ii. pp. 210-355. 163 (ii. 180). Man-like Holy Apes. Of all Apes, some Holy Apes (Semnopithecus') most resemble Man, in the form of their nose and the character of their hair (both that on the head and that on the beard). — Darwin, " Descent of Man," vol. i. p. 335 ; vol. ii. p. 172. 164 (ii. 182). Friedrich Miiller (" Allgemeine Ethnographic." Vienna, 1873, p. 29), on the supposed age of man. Families of languages (pp. 5, 15, etc.). 165 (ii. 183). The plate (XV.) representing the migrations, given in the " History of Creation," merely claims the value of a first attempt, is an hypothetic sketch, as I there expressly said, and as, in consequence of repeated attacks, I must here insist. 166 (ii. 201). The Leather-plate. The phylogenetic distinction of a special leather-plate, the outermost lamella separating from the skin-fibrous layer, is justified by Comparative Anatomy. 167 (ii. 204). Milk-glands. Huss, "Contributions to the History of the Evolution of the Milk-glands" ("Beitrage zur Entwickelungsgeschichte der Milch driisen "); and Gegenbaur, " On the Milk-gland Papillae " (" Jenaische Zeitschrift fur Naturwissenschaft," 1873, vol. vii. pp. 176, 204). 168 (ii. 208). On the hairy covering of Man and Apes, see Darwin, "Descent of Man," vol. i. pp. 20, 167, 180; vol. ii. pp. 280, 298, 335, etc. NOTES. 487 169 (ii. 217). Dorsal side and ventral sides are homologous In Vertebrates, Articulated Animals (Arthropod a), Soft-bodied Animals (Mollusca), and Worms, so that the dorsal marrow and the ventral marrow are not comparable. Of. Gegenbaur, "Morph. Jahrbuch," vol. i. pp. 5, 6. 170 (ii. 228). The unknown ontogenetic origin of the sym- pathetic nerve-system must probably, for phylogenetic reasons, be sought chiefly in the intestinal layer, not in the skin-layer. 171 (ii. 248). On the cavities connected with the nose, see Gegenbaur, " Elements of Comparative Anatomy," p. 580. 172 (ii. 260). The analogies in the germination of the higher sense organs were rightly grasped even by the earlier natural philosophers. The first more accurate sketches of the very obscure germ-history of the sense-organs, especially of the eye and ear, were given (1830) by Emil Huschke, of Jena (Isis, Meckel's Archiv, etc.). 173 (ii. 265). Hasse, "Anatomical Studies" (" Anatomische Studien "), chiefly of the organ of hearing. Leipzig, 1873. 174 (ii. 269). Johannes Rathke, "On the Gill-apparatus and the Tongue-bone " (" Ueber den Kiemen-apparat und des Zungenbein," 1832). Gegenbaur, " On the Head-skeleton of Selachii," 1872. (See note 124.) 175 (ii. 272). On the Rudimentary Ear-shell of Man, cf. Darwin, "Descent of Man," vol. i. pp. 17-19. 176 (ii. 276). Scarcely anywhere does Comparative Anatomy prove its high morphological value as with reference to the skeleton of Vertebrates : in this matter it accomplishes much more than Ontogeny. There is all the more reason to insist on this here, as Goette, in his gigantic history of the evolution of Bombinator, has recently denied all scientific value to Com- parative Anatomy, and asserted that Morphology is explained solely by Ontogeny. Cf. my " Aims and Methods of the Recent History of Evolution " (" Ziele und Wege der hentigen Ent- wickelungsgeschichte," 1875, p. 52, etc.). 177 (ii. 283). The Human Tail, like all other rudimentary organs, is very variable in point of size and development. In 488 NOTES. rare cases it remains permanently, projecting freely : usually ;.t disappears at an early period, as in Anthropoid Apes. 178 (ii. 284). On the Number of Vertebra in different Mam- mala, cf. Cuvier, "Le9ons d'Anatomie Comparee." 2nd edition, tome i., 1835, p. 177. 179 (ii. 293). On the earlier Skull-theory of Goethe and Oken, cf. Virchow, " Goethe as a Naturalist " (" Goethe als Natur- forscher," 1861, p. 103). 180 (ii 295.). Karl Gegenbaur, "The Head-skeleton of Selachii " (" Das Kopfskelet der Selachier"). As the foundation of a study of the head-skeleton of Vertebrates (1 872). 181 (ii. 301). Karl Gegenbaur, "On the Archipterygium." — "Jenaische Zeitschrift fur Naturwissenschaft," vol. vii. 1873, p. 131. 182 (ii. 304). Gegenbaur, " Researches into the Comparative Anatomy of Vertebrates " (" Untersuchungen zur Vergleichen- den Anatomie der Wirbelthiere "). Part I. Carpus and Tarsus (1864). Part II. The shoulder girdle of Vertebrates. Pectoral tins of Fishes (1866). 183 (ii. 305). Charles Martins, " Nouvelle comparaison des membres pelviens et thoraciques chez 1'homme et chez les mammiferes." — " Memoires de 1'Acad. de Montpellier," vol. iii. 1857. 184 (ii. 308). Ossification. Not all bones of the human body are first formed of cartilage. Cf. Gegenbaur, " On Primary and Secondary Bone-formation, with special reference to the Pri- mordial Skull Theory." — " Jenaisch. Zeitschrift fur Natur- wissenschaft," 1867, vol. iii. p. 54. 185 (ii. 308). Johannes Miiller, "Comparative Anatomy of Myxinoides." — " Transactions of the Berlin Academy," 1834-1842. 186 (ii. 314). The Homology of the Primitive Intestine and the two primary germ-layers is the postulate for morphological comparison of the various Metazoa-tribes. 187 (ii. 322). In the Evolution of the Intestine, Amphibia and Ganoids have, by heredity, retained the original Craniota-fonn more accurately than have Selachii and Osseous Fishes (Tcleoslei\ NOTES. 489 The palingenetic germination of Selachii has been much altered by kenogenetic adaptations. 188 (ii. 823). On the Homology of Scales and Teeth, cf. Gegenbaur, " Comparative Anatomy " (" Grundriss der vergl. Anatomie," 1874, pp/426, 582) ; also Oscar Hertwig, " Jenaische Zeitschrift fiir Naturwissenschaft," 1874, vol. viii. On the important distinction of homology (morphological resemblance) and Analogy (physiological resemblance), see Gegenbaur, as above, p. 63; also my " Generelle Morphologic," vol. i. p. 313. 189 (ii. 337). Wilhelm Miiller, "On the Hypobranchial Groove in Tunicates, and its Presence in the Amphioxus and Cyclostomi." — "Jenaische Zeitschrift fiir Naturwissenschaft," 1873, vol. viii. p. 327. 190 (ii. 358). The Nerve-muscular Cells of the Hydra throw the earliest light on the simultaneous, phylogenetic differentiation of nerve and muscle tissue. Cf. " Klemenberg, Hydra." Leipzig, 1872. 191 (ii. 383). The germ-history of the human heart accurately reproduces in all essential points its tribal history. This palin- genetic reproduction is, however, much contracted in particular points and vitiated by kenogenetic modifications of the original course of evolution, displacements partly in time, partly in place, which are the result of embryonic adaptations. 192 (ii. 383). On the Special Germ-history of the Human vascular system, cf. Kolliker, "History of the Evolution of Man" ("Entwickelungsgeschichte cbs Menschen." 2nd edition, 1876) ; also Rathke's excellent work on Ontogeny. 193 (ii. 387). The Homologies of the Primitive Organs, as they are here provisionally described in accordance with the Gash-sea- theory (note 24), can only be established by further co- operation between Comparative Anatomy and Ontogeny. Cf. Gegenbaur on Comparative Anatomy (" Grundriss der verglei- chenden Anatomie "). 194 (ii. 390). The Mechanism of Reproduction. As the functions of reproduction and of heredity, connected with re- production, are referable to growth, so the former as well as tho 490 NOTES. latter are finally explicable as the results of the attraction and rejection of homogeneous and heterogeneous particles. 195 (ii. 397). Eduard van Beneden, " De la Distinction origi- nelle du Testicule et de TO mire." Brussels, 1874. 196 (ii. 399). On the Original Hermaphrodite Structure of Vertebrates, cf. Waldeyer, "Ovary and Egg" ("Eierstock und Ei," 1872, p. 152) ; also Gegenbaur (" Grundriss der vergleichen- den Anatomie," 1874, p. 615). On the origin of the eggs from the ovary-epithelium, cf. Pfliiger, " On the Ovaries of Mammals and Man" ("Die Eierstocke der Saugethiere und des Menschen," 1863). 197 (ii. 423). On the special germ-history of the urinary and sexual organs, cf. Kolliker, " History of the Evolution of Man." On the homologies of these organs, see Gegenbaur (" Grundriss der vergleichenden Anatomie," 1874, pp. 610-628). 198 (ii. 443). Wilhelm Wundt, "Lectures on the Human and Animal Mind" (" Vorlesungen liber die Menschen- und Thier- seele." 1863). W. Wundt, "Outlines of Physiological Psy- chology" ("Grundziige der Physiologishen Psychologic," 1874). 199 (ii. 457). On Active (actual) and Latent (preteritial) forces, cf. Hermann Helmholtz, " Interoperation of Natural Forces " (" Wechselvvirkung der Naturkrafte," Part II., 1871). 200 (ii. 457). "Anthropology as Part of Zoology." — "Genercllo Morphologic," vol. ii. p. 43'2. " History of Creation," vol. i. 7 ; vol. ii. 347. INDEX. A.CALEPH.S, ii. 73, 92 Acoelomi, ii. 75, 92 Acorn-worms, ii. 86 Acrania, i. 116; ii. 97 Adam's apple, ii. 336 Adaptation, i. 158 After-birth, i. 400 Agassiz, thoughts on creation, i. 116 Air-tube (trachea), ii. 330, 333 Alali, ii. 182 Allantois, i. 380; ii. 135,411 Alluvial period, ii. 12 Amasta, ii. 146, 204 Aumion, i. 314, 386 animals, ii. 120, 133 sheaths of, i. 387 water, i. 314 Amniota, ii. 120, 133' Amreba, i. 142 ; ii. 152 false feet of, i. 142 Amoeboid egg-cells, i. 144; ii. 53 movements, i. 142; ii. c3 states, ii. 56 Amphibia, ii. 120, 122 Amphigastrula, i. 200, 241 Amphigonia, i. 160 Amphioxns, i. 413 ; ii. 98 blastula of, i. 443 body-form of, i. 417 cells of, their pedigree, i.467 chorda of. i. 417 distribution of, i. 415 gastrula of, i. 444 gorm-layers of, i. 447 Amphioxus, medullary tube of, i. 418 place of, in natural system, i. 416 sexual organs of, i. 425 side canals of, i. 423 significance of, i. 254. 427 Amphirhina, ii. 97, 101 Analogy, ii. 412 Anamnia, ii. 97, 120 Ancestral series of man, ii. 44, 184 Auimalcnlists, i. 37 Animal germ-layer, i. 194, 327 organs, ii. 192, 194 Anorgana, i. 156 ; ii. 30 Anthropocentric conception, ii. 457 Anthropoids, ii. 177, 189 Anthropolithic epoch, ii. 11, 16 Anthropozoic periods, ii. 12, 17 Antimera, i. 257 Anns, i. 339 ; :i. 323> 345 Anus-groove, i. 339 Anvil (Incus of ear), ii. 261, 268 Ape-men, ii. 44, 181 Apes, ii. 165, 189 eastern, ii. 172, 189 flat-nosed, ii. 172, 189 narrow-nosed, ii. 172, 189 question as to descent of, ii 165, 441 tailed, ii. 172, 189 western, ii. 172, 189 Aorta, i. 265 ; ii. 378 roots of, ii. 375 stem of, ii. 375 Aortal arches, ii. 375, 378 Appendicularia, i. 459-; ii. 90 492 INDEX. Archelminthes, ii. 76 Archigastrula, i. 198. 241 Archilithic epoch, ii. 9, 19 Archipterygium, ii. 301 Archizoic periods, ii. 9, 11 Area germinativa, i. 292 opaca, i. 296 pellucida, i. 296 Aristotle, i. 27 ; ii. 368 epigeneds, i. 29 heart formation, ii. 368 his history of evolution, i.27 Arm, lower, ii. 278, 304 upper, ii. 278, 304 Arteries omphalo-mescntericce, i. 395 umbilicales, i. 400 vertebrales, i. 395 vitellince, i. 395 Arteries, i. 393 Artery-arches, ii. 377 stalk, ii. 380 Arthropoda, ii. 92, 94 Articulated animals, ii. 92, 94 Articulation in man, i. 346 Ascidia, i. 429 ; ii. 90 blastula of, i. 455 chorda of, i. 456 communities of, i. 455 gastrnla of, i. 455 gill-sac of, i. 431 heart of, i. 433 homologies of, i. 465, 466 intestine of, i. 432 — mantle of, i. 430, 461 medullary tube of, i. 458 sexual organs of, i. 434 tail of, i. 456 Ascula, ii. 68 Atrium, ii. 374, 381 Auditory nerve, ii. 262 — organ, ii. 260 : vesicles, ii. 262 Auricular processes of heart, ii. 381 Axes of the body, i. 255 ; ii. 77 Axial cord, i. 301 rod (notochord), i. 302 skeleton, ii. 280, 299 Axis-plate, i. 299 Axuluil, ii. 126 B BAER, KARL ERNST, i. 50 his germ-layer theory, i. 51 his law, i. 58 life of, i. 52 on the bladder-like outline, ii. 62 on the human egg, i. 55 : ii. 424 on the notochord, i. 55 on type theory, i. 64 Balanoglossus, ii. 85 Bathybins, ii. 49 Batrachia, ii. 131 Bats, ii. 169, 187 Beaked animals, ii, 147, 187 Bell-sastrula, j. 198 Bilateral outline, i 257 ; ii. 74 Bimana, ii. 169 Biogeny, i. 24 ; ii. 434 fundamental law of, i. 6, 24 ; ii. 434 Birds, ii. 120, 138 gastrula of, i. 223 Bischoff, Wilhelm, i. 59 Bladder-gaetrula, i. 229, 241 Blastcea, ii. 31 Blastocceloma, i. 189 Blastoderma, i. 189 Blastodiscus, i. 227 Blastogeny, i. 24 Blastophylla, i. 196 Blastophyly, i. 24 Blastosphcera, i. 191 Blastula, i. 191, 242 Blind-intestine, ii. 330, 343 Blood-cells (corpuscles), i. 159 j ii 366 relationship, 5. 112 vessels, ii. 370 Bloodless worms, ii. 76 Bonnet, i. 40 Brain, i. 212, 232 bladders of, i. 343 } ii. 214 parts of, ii. 212 skull of, ii. 292 Breast-body, ii. 282 bone, ii. 282 cavity, i. 261 vertebrae, ii. 282 INDEX 493 Budding (gemmation), ii. 391 Bulbus arteriosus, ii. 374 Bulbus oculi, ii. 250 C.ENOL1THIC EPOCH, ii. 11, 15 Csenozoic period, ii. 15, 19 Calf-bone, ii. 278, 30 i Cambrian period, ii. 9, 19 Canalis auricularis, ii. 381 Carboniferous period, ii. 10, 19 Cardinal veins, i. 391 Carpus, ii. 278 Catarhinae, ii. 176, 189 Catastrophes, theory of, i. 76 Causal efficientes, i. 16, 80 ; ii. 455 finales, i. 16, 80 ; ii. 455 Cavum tympani, ii. 261, 270 Cell-division, i. 124 kernel (nucleus), i. 125 state, i. 12 1 substance, i. 125 Cells, i. 125 female, i. 171 ; ii. 392 male, i. 171 ; ii. 392 theory of, i. 60, 121 Central heart, ii. 120 medulla, ii. 210, 232 nerve-system, ii. 210 skeleton, ii. 280, 299 " Centre of sight," ii. 252 Ceratodus Fosteri, ii. 119 Cerebellum, ii. 212, 232 Cerebrum, ii. 212, 232 Cetacea, ii. 187 Cetomorpha, ii. 160, 187 Chalk period, ii. 14, 19 Chalk-sponges, i. 117 Chick, importance of, i. 31 Chimpanzee, ii. 178, 180 Chorda animals, ii. 84, 87 dorsalis, i. 255, 301 sheath, ii. 286 tissue of, ii. 286 vertebralis, i. 255, 301 Chordonia, i. 84, 87 Chorioidea, ii. 252, 258 Chorion, i. 387 ; ii. 158 frondosum, ii. 160 Iceve, ii. 160 Chorion, smooth, ii. 160 tufted, ii. 160 Chorology, i. 113 Chyle vessels, ii. 374 Cicatricula, i. 138 Circulation in Amphioxus, i. 423 Ascidia, i. 433 Fishes, ii. 375 germ-area, i. 397 Mammals, ii. 378 Clavicula, ii. 278, 304 Cleavage cells, i. 185 forms of, i. 242 of egg, i. 185, 241 partial, of bird's egg, i. 224 rhythm, i. 243 superficial, i. 200, 241 unequal, i. 200, 241 Clitoris, ii. 423, 431 Cloaca, ii. 145, 418 Cloacal animals, ii. 145, 187 Coalescence, i. 164 Coal period, ii. 11, 19 Coccyx, ii. 282 Cochlea, ii. 263, 268 Ccelenterata, ii. 73 Cceloma, i. 260 ; ii. 75 Ccelomati, ii. 75, 92 Columna vertebralis, i. 349 ; ii. 285 Comparative Anatomy, i. 107, 245 Concrescence, i. 164 Conjunctiva, ii. 259 Connective membrane of eye, ii. 251 tissue, ii. 363 ' Connectivum, ii. 361, 366 Convolutions of brain, ii. 226 Copulation organs, ii. 421 Copulativa, ii. 421 Coracoideum, ii. 278, 304 Cerium, ii. 200, 232 Cormogeny, i. 24 Cormophyly, i. 24 Cornea, ii. 251, 258 Costce, ii. 278, 282 Covering tissue, ii. 361 Craniota, ii. 100, 120 Cranium, ii. 291 Creation, i. 74, 79 ; ii. 183 Crooked intestine, ii. 319, 330 Cross-vertebrae, ii. 282 Crystalline lens, ii. 253, 258 Culture period, ii. 11 494 INDEX. Curves of embryo, i. 369 Cutis, ii. 200, 232 Cuvier, theory of catastrophes, i. 76 1 EAR, BOXFT-ETS o?, ii. 268 labyrinth of, ii. 262, 268 nmscles of, ii. 271 nerve of, ii. 266 pouch (utriculus) , ii. 262 sac (saccutus), ii. 262 shell of, ii. 269 snail of, ii. 263, 268 trmnpet, ii. 260 vehicles ii °65 Cyclostoma, ii. 101, 120 Cytods, i. 103 Cytula, i. 176 Cytococcus, i. 176 D DAI/TON, i. 51 Darwin, Charles, i. 96 net ccn or man, i. iiw Eclcction, theory o , i. Jo Echidna, ii. 147 Kchinoderma, i. 436 ; ii. 92 Egg-cell, i. 132 of Chick i 139 Darwinism, i. 95 Deoidua, ii. 161, 165 Bird i 139 Decidua-less animals, ii. 161 Deciduata, ii. 161, 187 Deduction, i. 101- ; ii. 37 Degree of development, i. 58 Derma, ii. 232 Descent, theory of, i. 84 Man i 137 • ii i95 cleavage, i. 185, 242 holoblastic, i. 215 human, i. 137 ; ii. 425 Devonian period, ii. 10, 19 Didelphia, ii. 149, 187 Differentiation, i. 152, 159 Digestive intestine, ii. 330 Digits, ii. 278 Diluvial period, ii. 12, 15 Dipnensta, ii. 115, 120 Discogastrula, i. 219, 241 Discoidal cleavage, i. 225, 242 Discoplacentalia, ii. 162, 187 Discus blastodermicus, i. 139, 226 Doellinger, i. 50 Dorsal farrow, i. 302 meroblastic, i. 216 Elbow, ii. 278, 301 Elementary organism, i. 124 Embryo of Vertebrates, i. 360 Embryology, i. 3 Empty intestine, ii. 319 Encephalon, ii. 232 Endoco&larimn, ii. 366, 400 Enteropneusta, ii. 86 Entoderma, i. 206, 236 Eocene period, ii. 11, 15 Epidermis, ii. 200, 232 Epididymis, ii. 417, 428 Epigenesis, i. 39, 41 Epigenesis, theory of, i. 39, 41 Epithelial tissue, ii. 361 Epithelium, ii. 361, 3G6 Epochs, duration of, ii. 3 Evolution of forms, i. 19 marroTT, n. --I Double-breathers, ii. 117, 120 " Donble-shield," i. 297 Dualism, i. 17 ; ii. 456 Dnalistic philosophy, i. 17 Duetus Gartneri, ii. 416, 431 - Mullcri ii 415 431 • history of i 1 "1 Rathkei, ii. 415, 431 _ Wolffii ii 415 431 Excretory organs, i. 267 ; ii. 403 Exoccelarium, ii. 369, 400 Exoderma, i. 195, 236 Dysteleology, i. 109 INDEX. 495 Extremities, ii. Ill, 306 Eye, ii. 250, 258 connective membrane of, ii. 251 lids, ii. 259 netted membrane of, ii. 252, 258 protective membrane of, ii 251 pupil of, ii. 230 rainbow membrane of, ii. 2Fi2 vascular membrane of, ii. 252 vesicles, i. 357 j ii. 253 FABRICTUS AB AQUAPENDENTE, i. 31 Face, development of, ii. 245, 316 skull of, ii. 278, 294 Fallopian canals, ii. 431 hydatids, ii. 431 Fatty layer of corium, ii. 232 Female breast, ii. 202 cells, i. 171, 392 copulatory organs, ii. 423 excretory ducts, ii. 415, 431 germ-glands, ii. 398 germ-layer, ii. 398 milk glands, ii. 202 phallus (Clitoris), ii. 428 sexual organs, ii. 423 sexual plate, ii. 401 uterus, ii. 417 Femur, ii. 278, 304 Fertilization, i, 169, 176 Fibula, ii. 278, 304 Fin, central /rod of, ii. 302 Fin, rays of, ii. 302 skeleton of, ii. 302 Final causes, i. 16 Fingers, ii. 278 Fishes, ii. 109, 120 fins of, ii. Ill — gastrula of, i. 219 • scales of, ii. 331 Five-digited foot, ii. 123 Flat-worms, ii. 76 Flesh, i. 259 Flesh-layer, i. 236 Foot, ii. 170 Force and matter, ii. 456 65 Forces, active, ii. 457 latent, ii. 457 . Formative functions, i. 15 yelk, i. 216 Forms, science of, i. 20 Frog-Batrachia, ii. 131 Frogs, ii. 131 egg cleavage of, i. 203 gastrula of, i. 207 — : larva of, ii. 127 metamorphosis of, ii. 126 Frontal process, ii. 244 Functions of evolution, ii. 155 science of, i. 19 Funiculus genitalis, ii. 418 unibilicalis, i. 383 ; ii. 168 O GALL-BLADDER, ii. 341 ducts, ii. 341 intestine, ii. 317, 330 Ganoid Fishes, ii. 112, 120 Gartnerian duct, ii. 416, 431 Gastraea, i. 232 ; ii. 66 theory of, i. 247 } ii. 195 Gastrseads, ii. 62, 70 Gastrocystis, i. 291 Qcuforodiseia, i. 292 Gastrula, i. 192; ii. 65 Bell-, i. 198 Bladder-, i. 200 Disc-, i. 200 Hood-, i. 200 Gegenbaur, i. 108; ii. 96 on Comparative Ana- tomy, ii. 96 Gegenbaur on head-skeleton, ii. 293 on theory of descent, i. 108 skull theory, ii. 293 theory of limbs, ii. 299 Generaiio spontanea, ii. 30 " Generelle Morphologic," i. 102 Geological hypotheses, i. 410 Germ, i. 3 Germ-area, i. 292 dark, i. 297 light, i. 297 • cavity, i. 189 disc, i. 139, 226 496 INDEX. Germ-epithelium, ii. 401 glands, ii. 398 history, i. 6, 24 layer, middle, i. 13 membrane, i. 189 membrane vesicle, i. 189 plate, ii. 401 point, i. 135 shield, i. 297 spot, i. 135 vesicle, i. 179, 291 Gibbon, ii. 178, 181 Glacial period, ii. 11 Glands of intestine, ii. 330 skin, i. 201 Giant phalli, ii. 422 Glomeruli renales, ii. 407 Gnathostomi, ii. 109 Goethe, Wolfgang, i. 88 his sknll theory, ii. 293 morphology, i. 88 on metamorphosis, i. 90 on reason, ii. 453 on specification, i. 90 Goette, Alexander, i. 65 Gonades, ii. 398 Gonochorismus, ii. 69, 395 Gonophori, ii. 402 Gorilla, i. 178, 180 Graafian follicles, ii. 424 Gubernaculum Hunteri, ii. 431 HAIB, ii. 205, 232 Hair-animals, ii. 205 Hairy covering, ii. 206 Halipliysema, ii. 66 Haller, Albrecht, i. 38 Hand, ii. 169 skeleton of, ii. 302 Hare-lip, ii. 246 Harrey, i. 31 Head-cap, i. 386 marrow, ii. 210 plate, i. 335 ribs, ii. 298 sheath, i. 387 Heart, auricle of, ii. 381 auricular processes of, ii. 381 Heart cavity, i. 394 development of, ii. 385 hnman, ii. 379, 382 mesentery, i. 394 ventricle of, ii. 381 Heopitheci, ii. 172 Heredity, i. 161 vitiated, i. 408 Hermaphrodites, ii. 395 Hermaphrodite gland, ii. 401 Vertebrates, ii. 408 Hermaphroditismus, ii. 69, 395 Hesperopitheci, ii. 172 Heteroehronism, i. 13 Heterotopism, i. 13 Hind-brain, ii. 221, 232 intestine, ii. 343 limbs, ii. Ill Hip-bone, ii. 278 His, Wilhelm, i. 64 Histogeny, i. 24 Histology, i. 24 Histophyly, i. 24 Hollow-worms, ii. 76 Holoblastic eggs, i. 215 Hologastrula, i. 241 Homology of primitive intestine, i. 247 ; ii. 321 of the animal tribes, ii. 387 germ-layers, i. 247 sexes, ii. 431 Hood-gastrnla, i. 200, 241 Hoofed animals, ii. 160, 188 Horn-plate, i. 307 Horn-stratnm of Epidermis, ii. 200 Humerus, ii. 278, 304 Hnxley, i. 101 ; ii. 294 germ-layer theory, i. 67 his Evidences, i. 101 Man and Ape, i. 101 primates, law of, ii. 177 skull theory, ii. 29 i Hylobates, ii. 181, 189 Hypospadia, ii. 423 IMMACULATE CONCEPTION, i. 170 IndeddvM ii. 159, 187 Individuality, i. 123 INDEX. 497 Individuality of cells, 5. 123 Lamina dermalis i. 273, 327 gastralis, i. 273, 327 Tndo- Germanic pedigree, ii. 23 Induction, i. 104 ; ii. 35 Inorganic history of earth, ii. 5 Insects, mental capacity of, ii. 448 Integumentum, ii. 199 Intestinal germ-disc, i. 291 vc -iclo i °91 inodcrma t.,, i. 3-7 tno T~tr -007 myxoga., ra 13, i. umbilicalis, i. 377 Vestibulum vaginae, ii. 431 Virginal generation, i. 170 VitMus, i. 135 WAGNER, MOIITTZ, i. 114 Wallace, Alfred, i. 98, 99 Water, amount of in body, ii. 7 Whale-like Animals, ii. 160, 1SS) Whales, ii. 1GO Wolff, Caspar Friedrich, i 40 his life, i. 41 his Natural Philosophy, i. 47 on formation of intestine, i. 4t on germ -lay era, i. 45 Theoria Generationis, i. 41 Wolffian bodies, ii. 411 duct, ii. 414, 431 Wolffs primitive kidneys, ii. 411 , 431 Woolly hair of embryo, ii. 200 Worms, i. 246 ; ii. 74 ancestors of, ii. 73 tribe of, ii. 73 Wrist, ii. 278 504 INDEX. YELK, i. 135 arteries, i. 395 cavity, i. 138 duct, i. 338; ii. 1G8 formative, i. 216 membrane, i. 138 ~ — nutritive, i. 216 sac, i. 337 Yelk veins, i. 395 vessels, i. 395 Zona, pelludda, i. 135 Zonaplacentalia, ii. 162, 187 Zoophyta, i. 246 ; ii. 73 University of California SOUTHERN REGIONAL LIBRARY FACILITY 405 Hilgard Avenue, Los Angeles, CA 90024-1388 Return this material to the library from which it was borrowed. A 000708108 6 »• •