"y tice, ta ~~ = te te! 4 oti a ean 3 7 LUTION OF MAN. International Science Library THE EVOLUTION OF MAN m@ POPULAR, EXPOSITION OF THE Principal Points of Human Ontogeny and Phylogeny FROM THE GERMAN OF ERNST. HAECKEL PROFESSOR IN THE UNIVERSITY OF JENA, AUTHOR OF ‘‘THE HISTORY OF CREATION,” ETC,’ IN TWO VOLUMES VOLE. I. H. LZ. fowle «= 2 Wew Pork ’ MADE BY THE WERNER COMPANY - *S _ AKRON, OHIO CONTENTS OF VOL. II. OE PAGE List of Plates — «.. the A ARE aee eo. Xill List of Woodcuts eee ece eee ese see aN List of Genetic Tables e ous eos soe AES Aion: CHAPTER XV. THE DURATION OF HUMAN TRIBAL HISTORY. Comparison of Ontogenetic and Phylogenetic Periods of Time.—Dura- tion 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 Rapid Ontogenetic Modification to the Slow Phylogenetic Metamorphosis.—Hstimate 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 Paleolithic Epoch. III. Second- ary, or Mesolithic Epoch. IV. Tertiary, or Cznolithic Epoch. V. Quaternary, or Anthropolithic Hpoch.—The Relative Duration of the Five EKpochs.—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 An- cestral Forms.—Monera originated by Spontaneous Generation. —Necessity of Spontaneous Generation Br se ese al vi CONTENTS. | CHAPTER XVI. THE ANCESTRY OF MAN. I. From THE MoNERA TO THE GASTRAA.. PAG? 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: Paleontology, Ontogeny, and Comparative Anatomy.—Unequal Certainty of the Various Special Hypotheses of Descent.—The Ancestral Line of Men in Twenty-two Stages: Hight Invertebrate and Fourteen Verte- brate Ancestors.—Distribution of these Twenty-two Parent-forms in the Five Main Divisions of the Organic History of the Harth.— First Ancestral Stage: Monera.—The Structureless and Homo- geneous Plasson of the Monera.—Differentiation of the Plasson into Nucleus, and the Vrotoplasm of the Cells——Cytods and Cells as Two Different Plastid-forms.—Vital Phenomena of Monera.— Organisms without Organs.—Second Ancestral Stage: Amecebez. —One-celled Primitive Animals of the Simplest and most Un- differentiated Nature.—The Amceboid Hgg-cells.—The Egg is Older than the Hen.—Third Ancestral Stage: Syn-Amoeba, Ontogeneti- cally reproduced in the Morula.—A Community of Homogeneous Ameeboid Cells.—Fourth Ancestral Stage: Planzea, Ontogenetically reproduced in the Blastula or Planula.—Fifth Ancestral Stage: Gastrza, Ontogenetically reproduced in the Gastrula and the Two- layered Germ-disc.—Origin of the Gastrzea by Inversion (invagt- natio) of the Planza.—Haliphysema and Gastrophysema.—Hxtant -Gastreeads coe eve es see eve oo. 34 CHAPTER XVIL THE ANCESTRAL SERIES OF MAN. TI. From tae 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, Acclomi and Coelomati: the former without, the latter with, a Body Cavity and Blood-vessel System.—Sixth Ancestral Stage: Archelminthes, most nearly allied to Turbellaria.— Descent of the CONTENTS. P Coolumati from the Acclomi.—Mantled Animals (Tumicata) and Chorda-Animals (Chordonia).—Seventh Stage: Soft-Worms (Scole- cida).—A Side Branch of the latter: the Acorn-Worm (Balano- glossus).—Differentiation of the Intestinal Tube into Gill-intes- tine and Stomach-intestine.—Highth 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 ChordasAnimals.—Separation of Vertebrates from the other Higher Animal Tribes (Articulated Animals, Star-Animals, Soft-bodied Animals).—Significance of the Metameric Formation.—Skull-less Animals (Acrania) and Skulled Animals (Cramiota).—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 (Myzi- noide and Petromyzonide) ... ove eee vee aw CHAPTER XVIII. THE PEDIGREE OF MAN. III. From tHz Primitive Fish to THE AMNIOTIC 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 Eartb.— 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 (Proteus and Avzolotl).—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 71 Vill CONTENTS, PAGE Lizard-like Parent-form (Protamnion).—First Formation of the Allantois and of the Amnion.—Branching of the Amnion Animale in Two Lines: on the one side, Reptiles (and Birds), on the other ‘ side, Mammals eee eee eee eee eee eee 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 Animals (Monotremata, or Ornithodelphia). —The Extinct Primitive Mammals (Promammalia) and the Extant Beaked Animals (Ornithostoma).—Seventeenth Ancestral Stage: Pouched Animals (Marsupialia, or Didelphia).—Extinct and Extant Pouched Animals.—Their Intermediate Position between Mono- tremes and Placental Animals.—Origin and Structure of Placental Animals (Placentalia, or Monodelphia).—Formation of the Pla- centa.—The Deciduous Embryonic Membrane (Decidwa).—Group of the Indecidua and of the Decidwata.—The Formation of the Decidua (vera, serotina, refleca) in Man and in Apes.—Highteenth Stage: Semi-apes (Prosimiev).—Nineteenth Stage: Tailed Apes (Menocerca).—Twentieth Stage : Man-like Apes (Anthropoides).— Speechless and Speaking Men (Mali. Homines) tee vee 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. 1x PAGE Touch).—Outer Skin (Epidermis) and Leather-skin (Cortwm).— 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 “ Peripheric Nervous System” .., jes eve eas eee eee 190 CHAPTER XXI. 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 Kye 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.—Hyelids. —Structure of the EHar.—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, Har 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 Har.—Rudimentary Muscles of the Kar-shell dis “45 - “ee eo ... 233 CONTENTS. CHAPTER XXII. “DEVELOPMENT OF THE ORGANS OF MOTION. The Motive Apparatus of Vertebrates.—These are constituted by the Th (a>) Passive and Active Organs of Motion (Skeleton and Musuwles).— The Significance of the Internal Skeleton of Vertebrates.—Struc- ture of the Vertebral Column.—Formation and Number of the Vertebrze.—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- bree.—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 (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.—Comparison 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 Ue see SAC CHAPTER XXIII. DEVELOPMENT OF THE INTESTINAL SYSTEM. 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 (esophagus).— 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.— PANE CONTENTS, x1 PAGE Differentiation of the Primitive Intestinal Tube into a Respiratory and a Digestive Intestine.—Gill-intestine and Stomach-intestine of the Amphioxus 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 .., Pee Jl 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 zach other.—New “Theories of Evolution” of His.—The “ En- velope Theory” and the “ Waste-rag Theory.”—Main Germ and Supplementary Germ.—Formative Yelk 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 Ascidia.—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) ... ... 348 CHAPTER XXV. DEVELOPMENT OF THE URINARY AND SEXUAL ORGANS. traportance 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 Kge-cell. —Fertilization.—Source of Love.—Original Hermaphroditism : B Later Separation of the Sexes (Gonochorism).—Original Develop. bi 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 KEpi- 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 Amphiorus.— 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 Fol- licle).—Origin of the External Sexual Organs.—Formation of the Cloaca.—Hermaphroditism in Man ... eee see o. 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 Monistie Philo- sophy and the Defeat of the Dualistic—Nature and Spirit.—Natural Science and Spiritual Science.—Conception of the World reformed by Anthropogeny ... Aas 204 se ase .. 432 Norres. Remarks and References to Literature a sos, 1459 INDEX re Hes bw ss ste cen 49] LIST OF PLATES. Plate XII. (between p. 130 and p. 131). The Australian Mud- - fish (Ceratodus Fosteri) ... ... Explanation _ Plate XIII. (between p. 130 and p. 131). The Mexican Axolotl q c _ (Siredon pisciformis) and the iat Land-salamander (Salamandra maculata)... ... Explanation Plate XIV. (between p. 180 and p. 181). Four Catarhines (Chimpanzee, Gorilla, Orang, Negro) ... Explanation ss XY. (between p. 188 and p. 189). Pedigree of Man eee. Explanation PAGE 118 129 181 184 LIST OF WOODCUTS. FIGURE 163. Moneron (Protameba) . 164. Bathybius, primitive slime 165. Monerula of Mammal 166. Cytula of Mammal . 167. Amceba . : . : _ 168. Ameeboid egg-cell . ° 169. Original egg-cleavage 170. Mulberry-germ (Morula) . 171. Germination of Monoxenia 172,173. Magosphera . 174-179. Gastrula of various animals . ° . 180, 181. Haliphysema . 182, 183. Ascula of a Sponge . 184,185. A Gliding-worm (Rhabdocelum) . 186. Acorn-worm (Balanoglose sus). . . . 187. Appendicularia ° ° 188. Ascidia . ° ° ° 189. Amphioxus . ° ° 190, Lamprey (Petromyzon) . 191, 192. Shark (Selachiz) 293. Larval Salamander . 194, Larvai Frog (Tadpole) 127 oes PAGE | FIGURE 46 | 195, 196. Beaked Animal (Orni- 49 thorhynchus) and _ its 51 skeleton : : 51 | 197. Pouched Animal (Marsu- 53 pial) with young . 53 | 198. Human ege-membranes . 55 | 199. Semi-ape (Lori) . 55 | 200. Human germ with its 57 membranes . ° A 60 | 201. Humanuterus, navel-cord, and embryo . 65 | 202. Head of Nose-ape . . 67 | 203. Tailed Ape (Sea-cat) 68 | 204. Skeleton of Gibbon ° 205. Skeleton of Orang-outang 80 | 206. Skeleton of Chimpanzee . 207. Skeleton of Gorilla .* 86 | 208. Skeleton of Man ., ° 90 | 209. Gastrula of Gastrophy- 90 sema . : : : 91 | 210. Germ-layers of Earth. 103 worms . . . 113 | 211. Nerve-system of Gliding- ey, worm . 5 - ° 212 Human skin-covering PAGS 148 142 158 164 166 167 175 175 178 178 178 178 178 198 198 198 200 LIST OF WOODCUTS. FIGURE ‘ 213. Epidermis cells . ° 214. Tear-glands . . ° 215, 216. Milk-glands . . 217, 218. Central marrow of humanembryo . . 219. Human brain . . ° 220. “A a. ae ° 4 221-223. Lyre-shaped embryo Chick . ° . ° 224-226. The five brain-blad- ders of the human germ 227. The five brain-bladders of Craniota ° ° ° 228. Brain of Shark ° = 229. Brain of Frog. ° ° 230. Brain of Rabbit . a 231. Nose of Shark : : 232-236. Development of the face in embryo Chick . 237. Nose and mouth cavities . 238-240. The face in the humanembryo . ° 241. Humaneye . ° . 242. Development of the eyes ae , 244. Human auditory passage 245. Humanauditory labyrinth 246-248. Development of the ear e es 3 e 249. Primitive skull with ear. vesicles : - = 250. Rudimentary ear-muscles 251, 252. Human skeleton . 253. Human vertebral column 254. Neck-vertebra - “ 255. Breast-vertebra . ° 256. Lumbar-vertebra . * PAGE ! FIGURE 201 | 257. Portion of notochord : 202 | 258-260. Growth of the primi- 203 tive vertebral series in ‘embryo Chick . . 210 | 261. Longitudinal section of 212 breast-vertebra . ° 213 | 262. Transverse section of same 263. Intervertebral disc . : 218 | 264. Human skull . : - 265. Head skeleton of Primi- 220 tive Fish J : 266. Primitive skull of Man . 222 | 267. Skeletonof fin of Ceratodus 222 | 268. Skeleton of fin of Acan- 222 thias . ° ° - 224 | 269. Skeleton of fin of Primi- 241 tive Fish : - é 270. Skeleton of hand of Frog 243 | 271. Skeletonof hand of Gorilla 246 | 272, Skeleton of human hand . 273. Skeleton of hand of Mam. 247 mal ; “ : 250 | 274. Gastrula of Olynthus 253 | 275. Human stomach . 2 256 | 276. Gastrula of Amphioxus . 260 | 277. Gastrula of Mammal 263 | 278,279. Human germ with yelk-sac and allantois . 264 | 280, Intestine of Turbellaria . 281. Intestine of Ascidia : 264 | 282. Intestine of Amphiorus . 270 | 283. ScalesofShark . 279 | 284, 285. Intestine of embryo 280 Dog with the intestinal 281 glands . . ° ° 281 | 286. Intestine with allantois . 281 | 287. Intestine of human germ xXV PAGE 286 288 290 291 291 292 296 297 302 302 302 302 302 302 306 313 317 321 321 324 327 327 328 332 334 338 339 XV1 LIST FIGURE : 288. Liver of human germ 289. Nail-tissue 5 - ° 290. Intestinal epithelium “ 291. Jelly-like tissue . 6 292. Cartilaginous tissue 293. Neuro-muscular cells - 294. Nerve-tissue . . - 295. Muscle-tissue . we ee , 296. Vascular tissue : : 297. Blood-cells . ‘ 298. Blood-vessels of a Worm. 299. Head with blood-vessels of Fish . H “ : 300-302. Arterial arches 303-306. “5 9 : 307-310. Development of the heart . : 3 : 311-314. Development of the heart . 315. Transverse through Haliphysema . section OF WOODCUTS. PAGE 342 362 362 3638 363 364 364 364 365 365 371 375 377 378 380 382 393 FIGURE 316. Rudiments of Urogenitalia 317. Primitive kidney of Bdello- stoma . . 318. Harliest primitive kidney rudiments . . . 319, 320. Primitive kidneys of Mammals _ ., . : 321. Development of urogeni- tal system . . . 322,323. ,, may: 324-326. 9 ” 327. Female sexual organs of Beaked Animal (Orni- thorhynchus). . : 328. Change of position of both kinds of sexual glands in human beings . : 329. Development of the human external sexual organs 330. Human egg-follicles ° PAGE 414 415 416 418 420 422 426 LIST OF GENETIC TABLES. Systematic Survey of palzeontological periods eee Systematic Survey of palzeontological formations Systematic Survey of the thickness of the forma- tions eee eee Pedigree of Indo- Satan pees Ke Systematic Survey of the most important ess in the animal ancestral line of Man Systematic Survey of the five first ee in the evolution of Man (phylogenetic, ontogenetic, sys- tematic) Systematic Survey of the Pieridae aon of ae animal kingdom ‘at aa Monophyletic pedigree of the acl Catan Systematic Survey of the phylogenetic system of Vertebrates .. eee cas Monophyletic neice OF es oss Systematic Survey of the periods of human riba history ‘ Systematic Survey of i pieloceneie ieee a Mammals, founded on the Gastrza Theory a Monophyletic pedigree of Mammals exe coe Pedigree of Apes wo a Systematic Survey of the organ- aioe of the pee body Systematic Biles of the Beano che of the human skin-covering Systematic Survey of the phylogenetic history of the human nerve-system veo eee ood 34 PAGE XXXIV. XXXYV. XXXVI. XXXVII. XXXVIII. XXXIX. XLITI. XLIV. LIST OF GENETIC TABLES. Systematic Survey of the ontogeny of the skin and nerve systems ee Systematic Survey of the plyigeeae of the tee nose ee “0 Systematic Survey of ie Snes of the aes eye Systematic Shee of the Gielen of the human ear Ro Systematic Somes of ‘the ontogeny of the human ear Systematic Sores of Hie eonstinion of ihe finan skeleton Systematic Survey of the pie leeean of the are skeleton Systematic Survey of he constihanie of ie human intestinal system ... Systematic Survey of the phylogeny of fe onan intestinal system Systematic Survey of the sedaenaes acomraige ia age, of the human tissue-groups (phylogenetic sequence of the tissues) : Systematic Survey of the sequence, accoudine i age, of the human organ-systems (phylogenetic sequence of the organs) cae ose Systematic Survey of the phylogeny of fis human vascular system Systematic Survey of the phploceny of the fisiaea heart a Be Systematic Survey of the homeless of Wore Articulated Animals (Arthropoda), Soft-bodied Animals (Mollusca), and Vertebrates ... Systematic Survey of the phylogeny of the Hime urinary and sexual organs Systematic Survey of the homologies of ‘the sexual organs in the two sexes of Mammals ,.. oo ® 367 387 43) THE EVOLUTION OF MAN. CHAPTER XV. THE DURATION OF HUMAN TRIBAL 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 Historv.—Relation of this Rapid Ontogenetic 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 Paleolithic Epoch. III. Secondary, or Mesolithic Epoch. IV. Tertiary, or Cenolithic 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. “Tn vain as yet hasit been attempted to draw an exact line of demarcation betyeen historic and prehistoric times; the origin of man and the period of his first appearance pass back into indefingble time; the so-called archaic age cannot be sharply distinguished from the present age. This is the fate of all geological, as of all historicai periods. The periods which we dis- tinguish are, therefore, more or less arbitrarily defined, and, like the divisions 2 THE EVOLUTION OF MAN. in systematic natural history, can only serve to bring the subject of our study better before us and to render it more manageable; but not to mark real distinctions between different things.’—BrRNHARD Corta (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 ege-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 93 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, ¢.g.,the Ox. In the Horse and the Ass it is somewhat longer, 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, 13 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.” 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 primzeval 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 do 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, 7.¢., 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, ete— 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 life27 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. 7 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 “ Alloemeine Natur. geschichte und Theorie des Himmels” and to the numerons Cosmogenies 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 Medusz, 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 Na as slg sa 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 inte 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 1s 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 XIIL, 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 Archilithie 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 Amoebze 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 Palzolithie, Palzeozoic, 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 ey 2 5 'pl 7a cate. So Systematic Survey of the Palzontological Periods, or the Greater Divisions in the History of the Organic Harth. I. First Epoch: The Archilithic, or Primordial Epoch. (Age of Skull-less Animals and Seaweed Forests.) 1, The Older Archilithic Epoch or Laurentian Period 2. The Middle Archilithic Epoch : Cambrian Period. 3. The Later Archilithic Epoch is Silurian Period. II. Second Epoch: The Paleolithic, or Primary Epoch. (Age of Fishes and Fern Forests.) 4. The Older Palzolithic Epoch or Devonian Period. 5. The Middle Paleolithic Epoch - Coal Period. 6. The Later Palzolithic Epoch - Permian Period. III. Third Epoch: The Mesolithic, or Secondary Epoch. (Age of Reptiles and Pine Forests, Conifere.) 7. The Older Mesolithic Epoch Or Triassic Period. 8. The Middle Mesolithic Epoch ee Jurassic Period. 9. The Later Mesolithic Epoch ~ Chalk Period. 10. th. 12. 13. 14, \5. IV. Fourth Epoch: The Cenolithic, or Tertiary Epoch. (Age of Mammals and Leaf Forests.) The Older Czenolithic Epoch or Eocene Period. The Middle Ceenolithic Epoch ~ Miocene Period. The Later Czenolithic Epoch fC Pliocene Period. V. Fifth Epoch: The Anthropolithic, or Quaternary Epoch. (Age of Man and Cultivated Forests.) Tks Older Anthropolithic Epoch or Ice Age, Glacial Period The Middle Anthropolithic Epoch 35 Post Glacial Period. The Later Anthropolithic Epoch i Period of Culture. (The Period of Culture is the Historic Period, or Period of Tradition.) ( 2 TABLE XUiage Systematic Survey of the Paleontological Formations, or the Fossiliferous Strata of the Harth’s Crust. ; Synonyms of Rock-Groups. Systems. Formations. Formate V. Quaternary ae XIV. Recent 36. Present Upper alluvial (Alluvium) 35. Recent .| Lower alluvial ipenodelititic XIII. Pleistocene § 34. Post glacial | Upper diluvial (Anthropozoic) (Diluvium) 33. Glacial Lower diluvial groups of strata : XII. Pliocene 32. Arvernian Upper pliocene IV. a (New tertiary) 31. Sub-Appenine | Lower pliocene coun XI. Miocene 30. Falunian Upper miocene C ae thi (Middle tertiary) 2 29. Limburgian | Lower miocene ase MEANS x Goes 28. Gypsum é Upper eocene (Czenozoic) 27. Nummulitic Middle eocene groups of strata \ (Old tertiary) i IX. Cretaceous. III. Secondary Group, or Mesolithic VIII. Jura (Mesozoic) groups of strata, VIL. Trias ‘VI. Permian (New red sand IL. Primary stone) Gee Palwoli hic V. Carboniferous (Palzeozvic) (Coal) Sea aac IV. Devonian (Old red sand- stone) III. Silurian or Archilithic Il. Cambrian (Archizoic) groups of strata . Laurentian ! | : | | | | | : | | | | | | I. Primordial / Growp, F — i=) . London clay - White chalk . Green sand - Neocomian . Wealden - Portlandian . Oxfordian . Bath . Lias . Keuper . Muschelkalk . Bunter sand . Mountain limestone (Zechstein) . Red sandstone . Carboniferous sandstone . Carboniferous limestone . Pilton . Ilfracombe Linton Ludlow Wenlock - Llandeilo . Potsdam . Longmynd . Labrador . Ottawa Lower eocene Upper cretaceous ‘Middle cretaceous Lower cretaceous The Kentish Weald Upper oolite Middle oolite Lower oolite Lias formation Upper trias Middle trias Lower trias Upper Permian Lower Permian Upper carbonifer- ous Lower carbonifer- ous Upper Devonian Middle Devonian Lower Devonian Upper Silurian Middle Silurian Lower Silurian Upper Cambrian Lower Cambrian Upper Laurentian Lower Laurentian GEOLOGICAL PERIODS. 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 Palzolithic 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 Paleolithic Period as the Age of Fishes. The Paleozoic 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 Hisenach is the best known. The appearance of the most ancient Amnion Animals (Amniota), to which the common parent-form of = {4 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 Paleolithic 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 inally by the oldest Amnion Animals, the Protamnia. After the Palzolithic 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 Paleolithic deposits. During this Epoch a very sreat 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 (Teleostev) 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 Archeopteryx). 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, Czenozoic, or Czenolithic 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, 85 16 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, %¢, 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 paleontological 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 Anthropolithie 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 Anthropolithie Epoch, while even the first half of this Epoch must be reckoned as a prehistoric period. Hence this last main period, reaching from the close of the Czenolithic 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 anthropccentric 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 18 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 5 - 53.6 II. Palzolithic, or palzeozoic (primary) Epoch : - 32.1 Ill. Mesolithic, or mesozoic (secondary) Epoch ° » 11.5 IV. Cznolithic, or cenozoic (tertiary) Epoch . 2.3 Ne SEP EDEEN or anthropozoic (quaternary) Mipoek 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 Czenozoic Epoch, the so-called Tertiary Epoch, during which the Placental 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.1% 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 palzeontological 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 that — PP to.ny TABLE XIV. Systematic Survey of the Neptunian fossiliferous strata of the earth with reference to their relative sectional thickness (130,000 feet circa). LL a ————————————_———— TV. Czenolithic Strata, circa 38000 feet. Pliocene, Miocene, Eocene. III. Mesolithic Strata. IX. Chalk System. POSTHCOH ees SOSSHS SHH LSS STTHHHTE ROT SEESSEEEE Deposits of the VIII. Jurassic System, Secondary Epoch, circa 15,000 feet. VII. Triassic System VI. Permian System. II. Palzolithic Strata. ee eee eet eee ses eetoonses ®etreene eece Deposits of the V. Coal System. Primary Epoch, circa 42,000 feet. IV. Devonian System. III. Silurian System, circa I. Archilithic Strata. 22.000 feet | COST OS Fea ee eee ess SSS FEF FCH FOS SEH TES SEES00 Deposits of the II. Cambrian System, circa 18,000 feet. Primordial Epoch, circa SPCC T ASHES ree sesresre sess oHsereeee @s:eeccee 70,000 feet. I. Laurentian System, circa, 80,000 feet. SL a I I TR a EE RE ns DE eg Ta BE PS Se ee 20 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 ot 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!” 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 Vertebrates, 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 intoa primitive German and a primitive Slavo-Lettic tongue. Similarly, the Ario- Romanic split up into a primitive Arian and a primitive Grzco-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 Friesian, 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 Grzeco-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. C 23 ) PARLE KV. Pedigree of the Indo-Germanic Languages. Anglo-Saxons High Germans Lithuanians Ancient Prussians Low Germans Letts Netherlanders | =—o” —,-——_—2 | | Ancient Saxons | | Baltic Races Saxons Friesians Sorbians | Polish L Ui Czechs Low Germans ‘ Scandinavians | West Sclaves Russians Goths Germans South Sclaves iy | | Primitive Germans . Ancient Brith South-east Sclaves Baicrcn) Seotck Trish a | Gauls Sclaves | \ Latins | Gaels Res SEAL | Brittanese ———_ Sclavo-Letts | Italians Kelts Sclavo-Germans Italo-Kelts Albanese Greeks | —,-—_-——— Primitive Thracians Indians Iranians Arians Grzco-Romars Ario-Romans Indo-Germans 24 THE EVOLUTION OF MAN. A close study of this pedigree of the Indo-Germanic 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 natwral 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 inore 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 Paleontology afford certain proof that all Vertebrates, MAN DESCENDED FROM EXTINCT FORMS. 25 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.1°? 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 experiment 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." 26 ‘THE EVOLUTION OF MAN. There is no doubt that Man is descended from an extinet 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 Seini-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 @ priori grounds, that all the earlier primitive languages, fundamental languages, and ancestral languages, from which the living dialects are i «i f I 4 : 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 Ario-Romanic and the Sclavo-Germanic tongues have leag been altogether extinct, as are also the primitive Arian and Greco-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 phylogenetic “Comparative Zoology.” The former can, however, adduce far more direct evidence than the latter, because the palzontological materials of Philology, the ancient monuments of extinct tongues, have been far better preserved than the palzeontological 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 higher 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 extinet 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 heredity. | 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 correct, 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 ANCESTORS. 29 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 Gastrza, that most important animal form in which we recognize the simplest conceivable prototype of an animal with two germ-layers. The Gastreea 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 Amceba, the peculiar significance of which depends on its resemblance to the human egeg-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 ameeboid 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 ameeboid 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 Amoebze ?” To this there is but one reply. Like all one-celled organ- isms, the Amcebee 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 wquivoca). We have neither time, nor indeed have we any occasion, to discuss at length the weighty question of spontaneous generation, On this subject 1 must refer you to my “History of Creation,” and, especially, to the second book of the Generelle Morphologie, and to the discussion on Monera and spontaneous generation in my “Studien tiber Moneren und andere Protista.’ 1? 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 cumplex 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.®’ 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 such 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 36 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, toexplain 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 assuining 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.” 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 he 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.™ CHAPTER XVI. THE ANCESTRY OF MAN. I. From tHe MoNnERA TO THE GASTRAA. Relation of the General Inductive Law of the Theory of Descent to the Special Deductive Laws of the Hypotheses of Descent.—-Incompleteness of the Three Great Records of Creation: Palzeontology, Ontogeny, and Comparative Anatomy.—Unequal Certainty of the Various Special Hypotheses of Descent.—The Ancestral Line of Men in Twenty-two Stages : Hight 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 : Amcebx.—One-celled Primitive Animals of the Simplest and most Un- differentiated Nature.— The Ameeboid Hgg-cells.—The Egg is Older than the Hen.—Third Ancestral Stage: Syn-Amoeba, Ontogenetically repro- duced in the Morula.—A Community of Homogeneous Ameceboid Cells.—- Fourth Ancestral Stage: Planzea, Ontogenetically reproduced in the Blastula or Planula.—Fifth Ancestral Stage: Gastrza, Ontogenetically reproduced in the Gastrula and the Two-layered Germ-disc.—Origin of the Gastrza by Inversion (invaginatio) of the Planzsa.—Hahphysema and Gastrophysema.—Extant Gastreeads. “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 sufliciently.”—Kar~ Ernst Barr (1828). GUIDED by the fundamental law of Biogeny and by the sure records of creation, we now turn to the interesting task of examining the animal parent-forms of the human 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, Paleontology, Comparative Anatomy, Dysteleology, Chorology, the Gikology 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 also a special deductive law of this kind, which is the logical conclusion from the general inductive law of the theory of descent.'*4 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 primeval parent- form. Between these two assumptions there is no third FAITH OR SCIENCE. 37 sourse. 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 OF 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 accord 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 relatwe 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, Palzeontology, 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 «1s in a petrified condition, there are at least a hundred, probably thousands » of extinct species which have left no trace of their existence. This extreme and most deplorable defectiveness of the palzeon- tological record of creation, upon which it :s 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 paleontological 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 record of creation, that of Ontogeny. For the Phylogeny 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 palin- 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 Phylogeny 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 40 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- spording 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 ege-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 primeval 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 pr-mitive 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 Gastrzea. Equally important in their bearing on the Phylogeny of Man, are the very important ontoge- uetieal form conditions which correspond to certain Worms A2 THE EVOLUTION OF MAN. Skull-less Animals (Acrania), Fishes, etc., ete. On the other hand, between these quite certain and most valuable phylo- genetic 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 Palzeon- tology, of Comparative Anatomy, and Ontogeny. In the first attempts to construct the human ancestral line, which I made in my Generelle Morphologie, 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. 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 most 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 Paleolithic Kpoch, three within the Mesolithic Epoch, and four within the Czenolithie 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- evver 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 oldest 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 »hilosophy. CAA) 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. Epochs of the Geological Periods Animal Nearest Organic of the Ancestral Stages Living Relatives History of the Organic History of of the Earth. of the Earth. Man. A ncestral Stages. 1. Monera Bathybius (Monera) Protamceba Simple Amcebre 2. Oldest Amcebe } (Automeba) 3. Ameeboid Socicties (Synamebia) Morula larvae 4, Ciliated planule (Planwad) Blastula larvee 5. Primitive Intes- I tinal animals { Gastrula larve Baptt aes 1. Laurentian Period , Gastreade) Archilithic 6. Trimitive Worms Gliding Worms or 2. Cambrian Period (Archelminthes) { (Turbellaria) i 5 e ? Between the glid- Primordial 3. Silurian Period ee ing worms and the Epoch Sea-squirts 8. Chorda animals Sea-squirts (Ascidia} (Chordonia) { (Appendicularia) IW (desc pececodedecaoceoe Cenoscedsbocccocce eontnoosécocotio ube N 9. Skull-less animals Lancelets (Acrania) Amphioxt) 10. Round-mouths ampreys (Cyclo: tomt) (Petromyzonta) 11. Primitive Fishes (Selachiz) Paleolithic BHMary Ep och ennui s IV. Cerenolithic or Tertiary Epoch 10 eg AF g Ge pe hey ake 0 tk en ne a ae EM Ne Epoch 4, Devonian Period . Coal Period . Permian Period . Triassic Period III. " Mesolithic or 8. Secondary > Jurassic Period . Chalk Period Eocene Period 1. Miocene Period 2. Pliocene Period 3. Diluvial Period 4. Alluvial Period 12. Salamander Fishes (Dipneusta) 13. Gilled Amphibia (Sozobranchia) 14. Tailed Amphibia —\__— (Sozura) ~, 15. Primitive Am- niota (Protamnia) 16. Primitive Mame mals (Promammalia) 17. Pouched Animals Gla Marsupialia) 18. Semi-Apes (Prosimie) 19. Tailed Narrow- nosed Apes 20. Men-like Apes or Tail-less Narrow- nosed Apes. 21. Speechless Men or Ape-like Men 22. Men speech | | | ; i - { ater-newt | ! | : capable of Sharks (Squalacet) Mud fish (Protoptera) Siren (Proteus) and Axolotl (Stredon) (Triton) ? Between Tailed Amphibians and Beaked animals Beaked animals (Monotrema) Pouched Rats (Drdelphyes) Lori (Stenops) Maki (Lemur) Nose Apes Holy Apes Gorilla, "Omnnate zee, Orang, Gibbon Cretins or Micro- cephali Australians aia 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 Gregarine, 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 body consists merely of soft, structureless plasson. However thoroughly we examine them with the help of the most delicate chemical reagents and the strongest optical mstruments, 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, a 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. Fic. 163.—A Moneron (Protameba) in the act of reproduction: 4, the whole Moneron, which moves, like the ordinary Amoeba, 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 (eg., Protomyxa, Myxastrum), very numerous, long, fine, and generally thread-like processes arise from the surface of the movable body, and these branch irregularly, inter- THE BATHYBIUS. 47 twining their free moving ends, so as to form anet. Huge masses of such slime-nets crawl upon the deepest bottom of the sea (Bathybws, 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,” 8° 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 consequence 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- fluent 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; a7 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 regenera- tion. The reproduction of Monera always occurs asexually. In the Protamceba (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 (B), and finally parts in the middle. Thus, in the simplest possible way, two new individuals proceed by self-division from one quite simple individual (C). Other 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 Jarge number of smaller globules (Protomonas, Protomyza ; 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. Studien,”-vol. i p. 182), The THE MONERON AND BATHYBIUS. 49 most remarkable of all Monera is the Bathybius, which was discovered by Huxley in 1868 (Fig. 164). This wonderful Moneren lives in the deepest parts of the sea, especially in Fie. 164.—Bathybius Heckelii (Huxley). A small piece of the formless and continually changing plasson-net of this Moneron from the Atlantic Ocean. the Atlantic Ocean, and in places covers the whole floor 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 amceboid 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.!* 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 first 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 by 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. 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 Amceba. 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 different sub- stances ; an inner firmer substance, the kernel, or nucleus, and an outer, softer substance, the cell-substance, or protoplasma. 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 wus manner, by the differentiation of the Monera, is 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 remembered that in the ay “y 4 i) ; THE MONERULA. 51 ege-cell of animals, cither before or after fertilization, the original kernel disappeared. We explained this phenomenon as a reversion or atavism, and assumed that the ege-cell, in accordance with the law of latent heredity, first fails back into the kernel-less, cytod stage (Fig. 165). It is only 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 cf Biogeny, it reproduces the original, oldest parent-form of the Moneron; we therefore _ callit the Monerula. (Cf. vol: 1. pp. 178-183.) } Fie. 165.—Monerula of Mammal (Rabbit). The fertilized egg-cell after ‘the loss of the nucleus is a simple ball of protoplasm (d). The outer covering of the latter is formed by the modified zona pellucida (z) together with a mucous layer (h) secreted on to the outside of the latter. In this a few sperm-cells are still visiblo (s). Fic. 166.—Parent-cell (Cytula) of a Mammal (Rabbit): i, parent- kernel ; n, nucleolus of the latter; , protoplasm of the parent-cell; 2, ¥ modified zona pellucida; s, sperm-cells ; h, outer albuminous covering. §2 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 ege-cell, as the reproduction of a one-celled parent-form, to which we ascribed the organization of an Amoeba (cf. Chap. VL). For the Amceba, as it yet lives widely distributed in the fresh and salt waters of the globe, must be regarded as the most undifferentiated and most original of the various one-celled Primitive Animals. As the immature primitive egg-cells (which as “primitive eggs” or Protova are found in the ovary of animals) are indistinguishable from ordinary Amcebe, we are justified in pointing to the Amoeba 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 ege-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 Amoebee. 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 Amcebee” (Fig. 168) 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 Amcebe (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 AMG@EBA. ee reference to the primeval ancestral form of the Amceba, directly enables us to give a definite answer to the old hu- morous riddle: Which was first, the egg or the hen?’ We can Fic. 167.—A crawling Amceba (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-voloured, roundish cell-kernel or nucleus. Fie. 168.—Egeg-cell of a Chalk-Sponge (Olynthus). The egg-cell creeps about in the body of the Sponge by extending variable processes, like those of the ordinary Amceba. 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 amceboid cell of the simplest form. The egg existed independently during thousands of years as a simplest one-celled organism, as the Amceba. 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 amce- 54 THE EVOLUTION OF MAN. boid cell. Even then, the egg was first a Gastreea-egg, then a Worm-egg, then an Acrania-ege, 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. The fact that this primitive ege-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 ego-cells of the most dissimilar animals. (Cf. Fig. 10, vol.i. p.134) It is not till later, when the primitive egg-cells, or the primitive egos (protova), 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 Ameceba, are, considered from a morphological point of view, simple organisms and individuals of the first order, Plastids, All subsequent stages in the ancestral chain are, on the me PRIMORDIAL EGG-CLEAVAGE. 55 other hand, compound organisms or individuals of higher order—social aggregations of a number of cells. The earliest of these, which, under the name of Synamcebe, we must rank as the third stage of our pedigree, are quite simple societies of all homogeneous undifferentiated 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 Fic. 169.—Original or primordial egg-cleavage. The parent-cell, or cytula, which resulted from the fertilization of the egg-cell, first breaks up, by a continuous and regular process of divisiou, into two cells (A), then into four (B), then into eight (C), and, lastly, into very numerous cleavage- cells (D). the Morula (Fig. 165), the parent-cell breaks up, by repeated civision, into numerous cells. We have already minutely examined this important process of ege-cleavage, and have found that all the various modes of the latter are modifications of a single mode, that of original or primordial cleavage. (Cf. Chap. VIIT., p. 188.) In the Ver- tebrate line this palingenetic form of Fic. 170.—Mulberry- germ, or morula. egg-cleavage has been accurately re- 56 THE EVOLUTION OF MAN. ts tained to the present time only by the Amphioxus, while all other Vertebrates have assumed a modified kenogenetic form of cleavage. (Cf. Table IIL, vol. i. p. 241.) The latte: 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, ete, ete. (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 Amcebz 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 GERMINATION OF A CORAL, a ee! A) rk (7 S ey ae 58 THE EVOLUTION OF MAN. Fic. 171.—Germination of a coral (Monozenia Darwinii): A, monerula s B, parent-cell (cytula); C, two cleavage-cells; D, four cleavage-cells; E, mulberry-germ (morula); F, vesicular germ (blastula); G, vesicular germ in section; H, infolded vesicular germ in section; J, gastrula in longitu. Ginal section; 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 socialis, and the Labyrinthule which were discovered by Cienkowski; formless masses of homogeneous and quite simple cells. In order to recognize the ancestors of the human races which developed first phylogenetically from the Syn- amoeba, 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 we called the germ-membrane (blastoderma), and the hollow globe the germ-membrane vesicle (blastula, or blasto- sphera). 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 (Zchinoderma) 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 ; and of other conditions of embryonic adaptation. We may therefore assume that the ontogenetic blastula is the repro- duction of a primeval 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 ege-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, #). 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 1847, been called the planula, or ciliated larva. This designation, is, however, used by different zoologists in different 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 true planula-form the blastula. 60 THE EVOLUTION OF MAN. Various kinds of Primitive Animals, which yet oxist both in the sea and in fresh water, are formed essentially liko 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 Planzeads, or Blastzeads, as they may be called, are formed in the very mixed society of the Flagellate, especially the Volvoces (for instance, Synura). I noticed in September, 1869, on the Island Gis-Oe, on the coast of Norway, another very interesting form, which I named Magosphera plunula (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. After Fic. 172.—The Norwegian Flimmer-ball (Magosphera planula), swim- ming by means of its vibratile fringes ; seen from the surface. Fic. 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. Hach cell contains both a kernel and a contractile vesicle. FLIMMER-LARV£. 61 having reached maturity the society dissolves. Each sepa- rate cell still lives a while independently, grows, and changes into a crawling Amceba. 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 Magosphera-form from which we started. This accomplishes the entire life-history of this remarkable Primitive Animal. If we compare these permanent blastula-forms with the freely swimming Flimmer-larve or planula-condition, of similar structure, of many other lower animals, we may with certainty infer therefrom the former existence of a primeval and long-extinct parent-form, the structure of which was essentially like that of the planula or blastula. We will call this the Planza, or Blastzea. 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. Planza-like Primitive Animals must certainly have existed and formed a distinct class of Protozoa, which we may call Flimmer-swimmers (Planewada). 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 THE EVOLUTION OF MAN. of the blastosphera, and, truly prophetically, insisted upon it in his classical “ Entwickelungsgeschichte der Thiere ” (vol. i. p. 223). The passage in question says: “ The further 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 primzeval ancestral form of the Planza, as the fifth stage in the human pedigree, is the Gastraea, a form which arises from the Planza. 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 germ-stage in the ontogeny of the most various animals (Fig. 171, I,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 mouth-opening. The intestinal wall consists THE GASTRA. 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 trustworthy intelligence as to the phylogenetic origin of the Gastrea from the Plana, 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, H). At last it is so great, that the outer, inverted part of the germ- membrane, or blastoderm, attaches itself closely to the imner, -uninverted portion (Fig. 171, 1). Now, if guided by this ontogenetic process, we wish to conceive the phylogenetic origin of the Gastreea in accordance with the fundamental law of Biogeny, we must imagine that the one-layered cell- society of the globular Planzea 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 Planza. 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, I), Medusee, Worms (Fig. 175, B) Star- animals (Echinoderma, C), Articulated Animals (Arthro- 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 GASTRAA. 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 6) 9% So es Fic. 174. Fic. 174,—(A) Gastrula of a Zoophyte (Gastrophysema), Haeckel. Fic. 175.—(B) Gastrula of a Worm (Arrow-worm, Sagitta). After Kowa- levsky. _ Fie. 176.—(C) Gastrula of an Hchinoderm (Star-fish, Uraster). After Alexander Agassiz. Fic. 177.—(D) Gastrula of an Arthropod (Primitive Crab, Nauplius). Fic. 178.—(Z) Gastrula of a Mollusc (Pond-snail, Limneus). After Karl Rabl. Fic. 179.—(F) Gastrula of a Vertebrate (Lancelet, Amphiozus) After Kowalevsky. ; \ A OF Tie + ; J 66 THE EVOLUTION OF MAN. lines of all these classes of animals have Jeveloped phylo- genetically from the same parent-form. This most signifi- cant primeval parent-form is the Gastreea. 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 gastrulz of this age. Pro- bably the primeval Gastreea, which has been extinet for 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 Gastrza 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 Plant-Animals (Zoophyta), especially in the Sponges. Two small animal forms are especially interesting in their bearing on this aspect of the Gastrea theory. They have as yet been little observed, but of all extant animals they are most nearly allied to the primeval Gastrzea, and may therefore be called “the Gastreads of the present day.”1 One of these animals, Haliphysema (Figs 180 and 181), has been described by Bowerbank as a Sponge; the other, Gastrophysema, by Carter as a Rhizopod (as “ Squa- mulina”’). The entire mature body of the developed person EXTANT GASTRAADS. 67 of Haliphysema forms a most simple, cylindrical or ege- shaped pouch, the wall of which consists of two cell-strata. The cavity of the pouch is the stomach-cavity, and the Fies. 180, 181.—Haliphysema primordiale, an extant Gastrza-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 upper end in the primitive mouth (m). Between the whip-cells (g) lie amceboid eggs (e). The skin-layer (h) below is encrusted with grains of sand, above with sponge-spicules. 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-grains, 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 Fies. 182, 183.—Ascula of a Sponge (Olynthus). Fig. 182, from the out- side; Fig. 183, in loncitudinal section: g, primitive intestine ; 0, primitive mouth ; i, intestinal layer; e, skin-layer. REPRODUCTION IN THE GASTRAADS. 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 Gastrea sexual repro- duction must have taken place in the same way. In the Gastrzads, just as in Plant-animals, both kinds of sexual cells—ege-cells and sperm-cells—must have formed in the same person; the oldest Gastreads 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. CT Te) TABLE X Vaid. Systematic Survey of the five earliest evolutionary stages of the Human An. cestral Line, compared with the five earliest stages of Individual and of Systematic Evolution. Form-Value Of the five earliest stages of the animal body. 1. Furst Stage. A quite simple cytod (a non-nucleated plas- tid). 2. Second Stage. A simple cell (a nucleated plastid). aM Third Stage. A quite simple ag- gregation of simple, similar cells. 4. Fourth Stage. A simple hollow globe, filled with liquid, the wall of which consists of a single stratum of homogeneous cells. Fifth Stage. A hollow body, with a single axis, the wall of which consists of different cell-strata 5 with an opening at one pole of the axis. Phylogeny. The five earliest stages in the evolu- | tion of the tribe. 1. Monera. The oldest animal Monera (originating by spontaneous gene- ration). ee Ameba. Oldest animal Ameeba. 3. Synameba. The oldest agerega- tion of animal Amcebe. A Plana. An animal hollow | globe, the wall of which consists of a single stratum of ciliated cells. (blastea.) 5. Gastrea. Parent-form of in- testinal animals, or Mctazoa. Simple pri- mitive intestine with primitive mouth. The ody-wall is formed by the exoderm and the entederm. Ontogeny, The five earliest stages in the evolu- tion of the germ. 1. Monerula, A non - nucleated animal-egg (after fer- tilization and after ; loss of the germ- vesicle). 2. Cytula. A nucleated, ferti- lized animal-egg (“first cleavage globule ’’). 3. Morula. “ Mulberry-germ.” A globular mass of cleavage-cells. 4. Blastula. A hollow globe, the wall of which consists of a single stratum of homogeneous cells (the Planula of lower animals). (blastosphera.) 5. Gastrula. Intestinal larva. A simple intestinal cavity with a mouth- opening. The body- wall is formed by the two primary germ- layers. The System, The five earliest stages in the animal system. ie Monera, Protameeba, Bathy- bius, and other extant Monera. 2. Ameba, Extant Ameba. 3. Labyrinthula. A mass of similar, one-celled primitive animals. 4. Magosphera. A hollow globe, the wall of which consists of a single stratum of homogeneous ciliated cells. 5. Haliphysema. A quite simple plant- animal. An unarticu- lated uniaxial person, the body-wallof which consists of the exodernt and the entoderm. CHAPTER XVII. THE ANCESTRAL SERIES OF MAN. I]. From THE PRIMITIVE WorRM TO THE SKULLED ANIMAL. The Four Higher Animal Tribes are descended from the Worm Tribe.—The Descendants of the Gastrza; in one direction the Parent Form of Plant- Animals (Sponges and SeaeNettles), 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 Coelomati: the former without, the latter with, a Body Cavity and Blood-vessel System.— Sixth Ancestral Stage : Archelminthes, most nearly allied to Turbellaria. —Descent of the Ccelomati from the Accelomi.—Mantled Animals (Tunicata) and Chorda-Animals (Chordonia).—Seventh Stage: Soft- Worms (Scolecida).—A Side Branch of the latter: the Acorn-Worm (Balanoglossus).—Differentiation of the Intestinal Tube into Gill-intes- tine and Stomach-intestine.—Highth 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 lribes (Articulated Animals, Star-Animals, Soft-bodied Animals).—Significance of the Metameric Formation.— Skull-less Animals (Acrania) and Skulled Animals (Craniota).—Ninth Ancestral Stage : Skull-less Animals.—Amphioxus and Primitive Verte. brate.—Development of Skulled Animals (Construction of the Head, Skull, and Brain).—Tenth Ancestral Stage: Skulled Animals, allied to the Cyclostomi (Myzinoide and Petromyzonide). “Not like the godsamI! Fall well I know; But like the worm which in the dust must go, And, finding in the dust his life and weal, Is crushed and buried by the traveller’s heel.— 72 THE EVOLUTION OF MAN. Why dost thou 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 dul), In search for truth, was sad and sore perplexed!” GOETHE. Boru in prose and in poetry man is very often compared to a worm. “A miserable worm,” “ common and almost compassionate phrases. If we cannot detect ary deep phylogenetic reference in this 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 muitiform 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 a poor worm,” are 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 Linnzeus. 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 primeval 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 Gastrza. Comparative Anatomy and Ontogeny clearly and sig- nificantly prove that the Gastrea 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 Gastrza, 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 Gastrea. On the one side, the common parent-form of the Worms developed from the Gastreea; as, on the other side, did the common parent-form of the Plant-animals. (Cf. Tables XVIII. and XIX.) The tribe of Plant-animals (Zoophytes, or Coelenterata) now comprehends, on the one side, the main class of Sponges (Spongie); on the other, the main class of the Sea-nettles (Acalephe}; to the former belong the Gastreads and Poriferee, to the latter the Hydroid-polyps, the Medusz, Ctenophore, and Corals. From the Comparative Anatomy and the Ontogeny of these we may infer, with great pro- 74 THE EVOLUTION OF MAN, | bability, that all these Plant-animals descend from a 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 (Acalephe) 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 front 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 Gastrea, the common paren 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 primeval 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 (Acelomi), 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 (Celomatz), 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 Accelomi, which are very nearly allied to the Gastreea and the Plant-animals, are to be regarded as an earlier and lower group, from which the more recent and higher division of the Ccelomati developed, perhaps towards the end of the Laurentian Period. We will first carefully examine the lower group of Worms, the Accelomi, among which we must look for the 76 THE EVOLUTION OF 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 cceloma,” and therefore without blood, or vascular system. The extant Accelomi are generally included in a single class, which, on account of their flattened bodies, are called Flat-worms (Plathelminthes). 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 Accelomi 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 Gastreads. The whole of these lowest Accelomi, 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 Accelomi, the Primitive Worms and the Flat-worms, represent in their externa) 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. (Cf. 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- — raxomia dipleura). 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 ts constantly conveyed 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 ‘loacal 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 (porus 146 THE EVOLUTION OF MAN. urogenitalis). The urinary bladder in the Monotremes also opens into the cloaca, and is separate from the two urinary ducts (Fig. 327, vo); in all other Mammals the ‘atter 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 (Amasta). 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 ife 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 (£dentata). 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.’ Some 148 THE EVOLUTION OF MAN. Fic. 195.—The Duck-billed Platy- pus (Ornithorhynchus paradowus). Fie. 196.—Skeleton of Platypus. Ty Zz a POUCHED ANIMALS, 149 small single molars, found in the uppermost strata of the Keuper formation in England and Wiirtemberg, and which are the oldest known vertebrate remains, probably belong to these primeval 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 (Dromatheriuwm 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- mammala. 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, tuis structure is alone sufficient to distinguish the Pouched Animals (Marswpialia). 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 [50 THE EVOLUTION OF MAN. lower jaws, no fragment of the rest of their bodies having been reserved. According to the logic usually applied to palzontology 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 paleontological 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 live 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 Czenolithic Epochs EXTANT POUCHED ANIMALS. I51 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 (0s coracoidewm) 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 wrogenitalis). 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 [52 THE EVOLUTION OF MAN born in a very imperfect condition, are carried by the mother fo a long time; until, in fact, they are completely developed (Fig. 197). In the large Giant Kangaroo, which iy yi it wh Ny ti ut cin ( , (7 : IM DRS NG : INNS ANY S ‘NY WOES ASS Ga 4. y 4 i SS SSS Ss I \ SS LRN ZEN } \ Sie NN ZN ZAHA ONY MMAR AY Witt tia WN ANY) ‘ VAN Ny) MTNA HN ead aly IAA ly Hy y} i SY \y \ \ NV “A MN Otis | ANN HEE NN BRCS f MAN Ait a oS ON NA Bh SS Fo = \ iM) \e SITS ~ QATAR SQ y NY \ NY N\ Ni WAN RY Pa SG SS \ AN AMA xu} uN Yes t,) NY x e ORES eee AA RSS WAN \\ WS RA - nh ‘ SSRN \\\." We < - FIN cl ; i. ih rn 4 Ka WHR XQ i ‘a \ AN A \\ \ Y ' | tayt ( \\W Fic. 197.—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 born 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.) 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 Secondary or Meso- lithic Epoch, can be referred with certainty to a Placental Animal, while we have plenty of placental fossils dating from every part of the Tertiary or Czenolithic Epoch. From this palzeontological 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 Czenolithie 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 Czenolithic 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 forms, amounted at most to about three per cent. of the entire duration of the organic history of the earth. (Cf p. 18.) | All Placental Animals are distinguished from the two lower Mammalian groups already considered, from the Cloacal Animals and Pouchéd 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 same 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 (ull 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 +e PLACENTAL ANIMALS. 155 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. 382). 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. 182-135, vol. i. p.877-380.) 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 intestina!-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 ege (chorion). In Cloacal Animals (Monotremata) and 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 grows 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 inereased 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 mammalianegg. 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 pellucida, and by the thick albuminous covering deposited externally on the latter (Fig. 19, Fig. 21, 2, h, vol. i. p. 178). We called these two outer coverings, which afterwards amaleamate, 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, «, 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 Fie. 198.—Egg-coverings of the human embryo (diagram- matic) : m, the thick fleshy wall of the uterus; plu, placenta, the inner stratum (plu') of which has extended processes between the chorion-tufts (chz) (chf, 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) ; dg, yelk-duct; ds, yelk-sac; dv, dr, decidua (dv, true, dr, false). The uterus-cavity (uh) opens below into. the vagina, above, on the right hand side, into an oviduct (t). (After Kolliker.) tufts arise in all directions from the scrous membrane, and thus before'long (in the human embryo in the third week) DEVELOPMENT OF THE PLACENTA. 159 the whole outer surface of the egg is covered with a dense forest of tufts (Fig. 134). These hollow tufts are now penetrated from within by the branching blood-vessels, which originate from the in- testinal fibrous layer of the allantois, and which contain the blood of the embryo, introduced through the navel vessels (Fig. 198, chz). On the other hand, dense networks of blood-vessels develop in the mucous membrane, which lines the inner surface of the uterus, particularly in the neighbourhood of the depressions into which the chorion- tufts penetrate (plu). These vascular networks receive the blood of the mother introduced through the uterus vessels. The whole mass of these two sets of vessels, which are here most intimately connected, together with the connecting and enveloping tissues, is called the placenta, or “ vascular cake.” Properly speaking, the placenta consists of two quite different, though closely connected, parts ; internally, of the embryonic placenta (placenta fetalis, Fig. 198, chz), and externally of the maternal placenta (placenta uterina, Fig. 198, plu). The latter is formed by the uterine mucous membrane and its blood vessels: the former by the secondary chorion and the navel vessels of the embryo. The mode in which these two “vascular cakes” com- bine to form the placenta, as well as the structure, form, and size of the latter, differs much in different Placental Animals and affords valuable data for natural classification, and hence also for the tribal history of the whole sub-class, The latter is primarily divisible into two main divisions, based on these differences: the lower Placental Animals, which are called IJndecidwa, and the Dee Placenta] Animals, or Deciduata. 44 160 THE EVOLUTION OF MAN. To the Indecidua, or lower Placental Animals, belong two very comprehensive and important vertebrate groups: (1) the Hoofed Animals (Ungulata)—the Tapirs, Horses, Swine, Ruminants, and others; (2) the Whale-like animals (Cetomorpha)—the Sea-cows, Porpoises, Dolphins, Whales, and others. In all these Indecidua the chorion tufts are distributed, singly or in bunches, over the entire surface of the chorion, or over the greater part of it. They are but very loosely attached to the mucous membrane of the uterus, so that the entire outer egg-membrane with its tufts might easily and without using force be drawn out of the depressions in the uterine mucous membrane, just as the hand is with- drawn from a glove. The two “vascular cakes” do not really coalesce at any point of their contact. Hence, at birth the “embryonic cake” (placenta fetalis) is alone removed; the “maternal cake” (placenta uterina) is not displaced. The entire mucous membrane of the gravid uterus is but little altered, and, at parturition, suffers no direct loss of substance. The structure of the placenta in the second and higher division of Placental Animals, the Deciduata, is very dif. ferent. To this comprehensive and very highly developed mammalian group belong all Beasts of Prey and all Insect- eaters, Gnawers (Rodentia), Elephants, Bats, Semi-apes, and, lastly, Apes and Man. In all these Deciduata the whole surface of the chorion is also at first thickly covered with tufts. These, however, afterwards disappear from part of the surface, while they develop all the more vigorously in the remainder. The smooth chorion (chorion leve, Fig. 198, chl) thus becomes distinct from the tufted chorion (chorion frondosum, Fig. 198, chf). On the former there are only INDECIDUA AND DECIDUA. 16] minute ‘and scattered tufts, or none at all; while the latter is thickly overgrown with highly developed and large tufts. In the Deciduata the tufted chorion alone forms the placenta. Yet more characteristic of the Deciduata is the very peculiar and intimate connection which is developed in these between the tufted chorion and the contiguous portion of the uterine mucous membrane, and which must be regarded as a true coalescence. The vascular tufts of the chorion push their branches into the sanguineous tissue of this mucous membrane in such a way, and the two sets of vessels are in such close contact and are so interlaced, that the embryonic placenta is no longer distinguishable from the maternal placenta; the two form one whole—a compact and apparently simple placenta. Owing to this intimate coalescence, a portion of the uterine mucous mem- brane of the mother comes away, at birth, with the firmly adherent egg-membrane. The portion of the mother’s body which is thus removed in parturition is called, on account of its separable nature, the deciduous membrane (decidua). All Placental Animals which possess this deciduous mem- brane are classed together as Deciduata. The removal of this membrane at parturition, of course, causes a greater or less loss of blood by the mother, which does not occur in the Indecidua. In the Deciduata, moreover, the lost portion of the uterine mucous membrane must be replaced, after parturition, by a renewal of the tissue. The structure of the placenta and deciduous membrane is, however, by no means identical throughout the compre- hensive group of Deciduata. On the contrary, there are many important differences in this respect, which are in 162 THE EVOLUTION OF MAN. some degree connected with other important structural characters (e.g., the structure of the brain, of the teeth, of the feet), and which may justly, therefore, be turned to account in the phylogenetic classification of Placentals. In the first place, two great groups of Deciduata may be distinguished according to the form of the placenta: in the one group it is ring-shaped or girdle-shaped; in the other it is discoid or cake-shaped. In Deciduata with girdle-shaped placenta (Zonoplacentalia) the poles of the oval egg take no part in the formation of the placenta. The “vascular cake” resembles a broad ring-like girdle, embracing the central zone of the egg. It is so in Beasts of Prey (Carnassia), both in the terrestrial forms (Carnivora) and in the marine forms (Pinnipedia). Te : : ' . ves . ty } } ' ¥ ‘ és { ’ 2 1 “ “ F \ . * i s i ‘ ( 189 ) Peeabin XXyY. Pedigree of Apes. Filan Homo Ape-like Sian Alalus Gorilla Orang-Outang Chimpanzee Gorilla Satyrus Gibbon a | Bis African ——_—_————4 filan-like Apes Asiatic fHlan-like Apes a SHlanzlike Apes Anthropoides Nose Apes Tall Apes Nasalis — Semnopithecus Sea Cat ————————— Cercopithecus Baboons Cynocephalus _ Silk Apes Hapalida Clutch-tails | Labidocerca Cailen Apes Menocerca a Flap-tails Aphyocerca Apes of Oly Worly Narrow-nosed Apes of Pew World Catarhinzy Flat-nosed Platyrhine ie Apes CHAPTER XxX. 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 Hxoderm, 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).—Onuter Skin (Hpidermis) and Leather-skin (Coriwm).—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 (Cerebellwm).—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), Twixt-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 “ Peripherio Nervous System.” “Hardly any part of the bodily frame, then, could be found better calculated 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, IQ! 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.’”—Mamn’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.”—Ibid, 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 {92 THE EVOLUTION OF MAN. organs; but I shall be compelled to explain the ontogenetic — 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. Hach 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 ORGAN-SYSTEMS. 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 (Cf. vol.i-pp. 58 and 196). Ofcourse, 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) kX ooaye Systematic Survey 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-fibrons layer; III. Intestinal-fibrous layer; IV. Intestinal-gland. ular layer.) - ANIMAL ORGAN-SYSTEM. VEGETATIVE ORGAN-SYSTEM. . Skin-covering { (Derma) 2. Central nerve- { system As Sensory . Peripheric nerve- Apparatus system Sensorium 4. Sense-organs (Organa sensuum) } . Muscle - system B. (active motive Motive organs) Apparatus 6. Skeleton-system Locomotorium oo motive organs) 7. Intestinal system { (Gaster) Cc. 8. Vascular system Wutritive (Organa circu- Apparatus lationis) Nutritorium . Renal system (Otiane urinaria) Bm etited | 10. Sexual organs | Apparatus (Organa sexualia)® ei ae Tac i Outer skin Leather skin Brain Spinal marrow Brain nerves Spinal nerves Intestinal nerves Organ of touch (skin) Organ of taste (tongue) Organ of smell (nose) Organ of sight (eye) Organ of hearing (ear) Skin muscles Skeleton muscles Vertebral column Skull Limb skeleton Digestive organ Respiratory organ Body cavity Lymph vessels Blood vessels Heart Kidneys Urinary ducts Urinary bladder Sexual glands ae Ovary) IT. Testes) Sexual ducts (1. Oviduct) (II. Seed duct) Copulatory organs . Sheath) or Penis) Epidermis, I. Corium, II. Encephalon . Medulla spinalis } I. Nervi cerebrales, I. + IJ Nervi spinales, IT. Sympatheticus, IT. + 111. Org. tactus Org. gustus Org. olfactus Org. visus Org. auditus L+il Musculi cutane M. skeleti Vv : Il. ertebrarium Cranium Sk. extremitatum O. digestiva - 0. reapitatoriayat +IV. Coeloma, IT. + ITI. Vasa lympha- tica II. +111 V. sanguifera Cor, III. Renes Ureteres } L(?) + IL Urocystis, III. + lV, Gonades (1. Ovaria) ITT.-+ IV. (?) (II. Testes) I. + II. (7) Gonophori (1. Oviduc- tus) (II. Sperma- ductus) Copulativa I. Vagina) II. Penis) LQ 40 tr + II. 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- form points unmistakably to a parent-form (the Gastrea) 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 Gastreea 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 Gastrzea 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, ete.; 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 externally 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 iowe1 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 Gastrea (Fig..209,e). This is the case in the lowest Plant Animals (Zoophyta), the Gas- treads, Sponges, and the lowest Hydroid Polyps, which are but little higher than the Gastreads. Just as all the vege- tative functions of these are performed by the 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 Gastreads. 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 SS TAS ( om| SSS Sea SS Fie. 209.—Gastrula of Gastrophysema (Gastrzead-class). Fie. 210.—Transverse section through an embryonic Earth-worm: hs, skin-sensory layer; hm, skin-fibrous layer; df, intestinal-fibrous layer; dd, intestinal-glandular layer ; a, intestinal cavity; c, body-cavity, or Celoma; nm, nerve-ganglia; u, primitive kidneys. : Fie. 211.—A Gliding Worm (Rhabdocelum). From the brain or upper throat ganglion (g) nerves (n) radiate towards the skin (f), the eyes (au), the organ of smell (na), and the mouth (7) : h, testes; e, ovaries. ‘THE SKIN. * 199 throat ganglion,” situated above the throat (Fig. 211, g; Plate V. Fig. 11, m). The complex central nervous system of all higher animals has developed from this simple rudiment. In the higher Worms, ¢.g., the Earth-worms, according to Kowalevsky, the earliest rudiment of the central nervous system (Fig 210, n) is a local thickening of the skin- sensory layer (hs), which afterwards becomes entirely detached from the horn-plate. Even the medullary tube of Vertebrates has the same origin. From the germ-history of Man, we already know that this medullary tube, the commencement of the central nervous system, originally develops from the outer skin-covering. Let us now turn aside from these very interesting features in evolution, and examine the development of the later human skin-covering, with its hairs, sweat-glands, ete. Physiologically, this outer covering (derma, or teyumentum) plays a double part. The skin, in the first place, forms the general protective covering (wntegumentwm commune) which covers the whole surface of the body, and protects all other parts. As such, it, at the same time, effects a certain ex- change of matter between the body and the surrounding atmospheric air (perspiration or skin-breathing). In the second place, the skin is the oldest and most primitive sense-organ, the organ of touch, which effects the sensation of the surrounding temperature and of the pressure or re- sistance of bodies with which it comes in contact. The human skin, like that of all higher animals, consists essentially of two distinct parts; of the outer-skin, and of the underlying leather-skin. The outer-skin (epidermis) consists only of simple cells, and contains no blood-vessels (Fig. 212, ab). It develops from the first of the secondary 200 THE EVOLUTION OF MAN. germ-layers from the skin-sensory layer, and, directly, from the horn-plate of the latter. The leather-skin (coriwm), on the contrary, consists principally of connective or fibrous Fic. 212.--Human skin in perpendicular section (after Hcker), much en- larged: a, horny stratum of outer-skin (epidermis); 6, mucous stratum of outer- skin; c, papilla of the leather-skin (corium); d, blood-vessels of the latter ; e, f, excretory ducts of the sweat-glands (g); h, fat- globules of the leather-skin ; i, nerve, passing above into a touch-body. tissue, contains numerous blood-vessels and nerves, and has a different origin. It develops from the outer stratum of the second secondary germ-layer, from the skin-fibrous layer. The leather-skin is much thicker than the outer-skm. In its deeper part, the “ swbcutis,” lie many masses of fat-cells (Fig. 212, h). Its upper part, the true “cutis,” or papillary layer, forms, over nearly the whole surface of the body, a number of microscopic cone-shaped warts, or papille, which fit into the overlying epidermis (c). These touch-warts, or sensory papillz, contain the most delicate of all the sensory ergans of the skin, the “corpuscula tactus.” Other papilla STRUCTURE OF THE SKIN. _ 201 contain merely the terminal loops of the nutritive blood- vessels of the skin (cd). All these different parts of the leather-skin originate, by differentiation, from the cells, origi- nally homogeneous, of the leather-plate, the outer lamella of the skin-fibrous layer (Fig. 112, hpr, vol. i. p. 352; Plates IV. and V.,1; Figs. 65-69, hf, p. 277). Analogously, all the constituent parts and appendages of the outer-skin (epidermis) originate, by differentiation, from the homogeneous cells of the horn-plate (Fig. 213). Ata Fig. 213.—Cells of the outer-skin (epidermis) of a human embryo of two months. (After Koelliker.) very early period, the simple cell-layer of this horn-plate splits into two dis- tinct strata. The imner, softer stratum (Hig. 212, b) is called the mucous layer; the outer, harder stratum (a), the horn-layer of the outer- skin, The surface of this horn-layer is continually worn out and thrown off; new cell-strata, produced by the erowth of the underlying mucous layer, take its place. Originally the outer-skin forms an entirely simple cover over the surface of the body. Afterwards, however, sundry appendages develop from this both internally and ex- ternally. The internal appendages are the skin-glands; the sweat-glands, the sebaceous glands, ete. The external appendages are hair, nails, ete. The glands of the skin-covering are at first merely solid plug-shaped growths of the outer-skin (epidermis), which penetrate into the underlying leather-skin (corvwm) (Fig. 214,). A canal afterwards forms inside these solid 202 THE EVOLUTION OF MAN. plugs (,, 3); eithor owing to the softening anc breaking up of the central cells, or as the result of a fluid internally secreted. Some of these skin-glands remain unbranched, as, for instance, the sweat-glands (e, f,g). These glands, which secrete the sweat, are of great length, their ends forming a coil; they never branch, however; and the same is to be said of the glands which secrete the fatty wax of the ears. | Fie. 214.—Rudiments of tear-glands from a human embryo of four months, (After Koelliker.) 1. Earliest rudiment the shape of a simple, solid plug. 2 and 3. Fur- ther developed rudiments, which branch and become hollow: a, a solid offshoot ; e, sell-covering of the hollow offshoot; f, rudiment of the fibrous covering, which afterwards forms the leather-skin round the glands. Most other skin-glands give out shoots and branches, as, for in- stance, the tear-giands, situated on the upper eyelid, which secrete the tears (Fig. 214), and also the sebaceous glands, which produce the fatty sebaceous matter, and generally opon into the hair-follicles. The sweat and sebaceous glands occur only in Mammals. The tear- glands, on the contrary, are found in all the three classes of Amnion Animals, in Reptiles, Birds, and Mammals. They are not represented in the lower Vertebrates. Very remarkable skin-glands, found in all Mammals, and in them exclusively, are the milk-glands (glandule mammales, Figs. 215, 216). They supply milk for the nourishment of the new-born Mammal. Notwithstanding SKIN-GLANDS. 203 their extraordinary size, these important organs are merely large -ehaceous skin-glands (Plate V. Fig. 16, md). The milk *; produced by liquefaction of the fatty milk-cells within the branched milk-gland pouch (Fig. 215, c), just as the sebaceous matter of the skin, and the fatty matter of the hair are produced by the breaking up of fatty sebaceous cells within the sebaceous skin-glands. The excretory passages of the milk-glands enlarge into sac-like milk-ducts (6), which again become narrower (a), and open, through from sixteen to twenty-four minute apertures, into the nipple of the breast. The first rudiment of this large and complex gland is a very simple conical plug in the le. 215.—-The breast of the female in section: c, grape-like glandular \obnles; 6b, enlarged milk-ducts; a, narrow excretory ducts, opening through “he breast-nipple. (After H. Meyer.) T'e. 216.—Milk-glands of a new-born child: a, original central gland b, smaller, and c, larger branches of the latter. (After Langer.) 204, THE EVOLUTION OF MAN. outer-skin (epidermis), which extends into the leather-skin (coriwm), and there branches. In the new-born child it consists merely of from twelve to eighteen radiating lobules (Fig. 216). These gradually branch, the excretory passages become hollow, and a large quantity of fatty matter collects between the lobules. Thus is developed the prominent breast of the female (mamma), on the summit of which rises the nipple (mammulla), adapted for being sucked.! The nipple does not appear until after the milk-gland is already formed; this ontogenetic phenomenon is very interesting, because the more ancient Mammals (the parent- forms of the entire class) had no nipples. In them, the milk simply emerged through a plane, sieve-like perforated spot in the abdominal skin, as is even now the case in the lowest extant Mammals, the Beaked Animals (Monotremata ; p.146). On account of this character these animals may be called Amasta (without nipple). In many of the lower mammals there are numerous milk-¢lands, situated at various points of the ventral side. In the human female there is usually only a pair of milk-glands, placed on the point of the breast, as in Apes, Bats, Elephants, and some other Mammals, Occasion- ally, however, even in the human female two pairs of breast, olands (or even more) appear, lying one behind the other ; this must be regarded as a reversion to an older parent- form. Sometimes these glands are well developed even in the male, and are capable of being sucked, though as a rule they exist in the male sex only as rudimentary organs with- out function. Just as the skin glands originate as local growths of the outer skin in an inward direction, so the appendages of the skin, called hair and nails, originate as local growths EXTERNAL APPENDAGES OF THE SKIN. 205 of the outer skin in an outward direction. The nails (wn- gues), which are important protective formations over the hind surface of the most sensitive parts of our limbs—the tips of the fingers and toes—are horny products of the epidermis, common to us with the Apes, In their place, the lower Mammals generally possess claws, and the Hoofed Animals (Ungulata) hoofs. The parent-form of Mammals undoubtedly had claws, such as appear in‘ a rudimentary state in the Salamander. The hoofs of the Hoofed Animals and the nails of Apes and of Man originated from the claws of more ancient Mammals. In the human embryo the first rudiment of the nails first appears (between the horn-layer and the mucous layer of this outer skin) in the fourth month. Their edges do not, however, project until the end of the sixth month. The most interesting and important appendages of the outer skin are the hairs, which, on account of their peculiar structure and mode of origin, must be regarded as very characteristic of the whole Mammalian class. Hairs, it is true, appear widely distributed in many lower animals, ¢.g., in Insects and Worms. But these hairs, like those of plants, are thread-like processes of the outer surface, and differ from Mammalian hairs in their characteristically finer structure and in their mode of development. Hence Oken rightly called Mammals “hairy animals.’ The hairs of Man, as of all other Mammals, consist simply of epidermic cells peculiarly differentiated and arranged. In their first state, they appear in the embryo as solid plug-shaped pro- cesses of the epidermis which penetrate into the underlying leather-skin (coriwm), as do the sebaceous and the sweat glands. As in the latter, the simple plug consists originally 206 THE EVOLUTION OF MAN. of the ordinary epidermic cells. Within this a firmer central cellular mass of conical shape soon forms. This increases considerably in length, detaches itself from the surrounding cellular mass, the “root-sheath,” and finally makes its way to the outside, appearing above the outer surface as a hair-stem. The deepest part, buried in the skin, the hair follicle, is the root of the hair, and is sur- rounded by the root-sheath. In the human embryo the first hairs make their appearance at the end of the fifth or in the beginning of the sixth month. During the last three or four months before birth the human embryo is usually covered by a thick coating of deli- cate woolly hairs. This embryonic wool-covering (lanugo) is often lost during the last weeks of embryonic life, and, at any rate, soon after birth, when it is replaced by the thinner permanent hair-covering. These later permanent hairs grow out of hair follicles which are developed from the root-sheaths of the deciduous woolly hair. In the human embryo, the embryonic woolly hair usually covers the entire body, with the exception of the palms of the hands and the soles of the feet. These parts remain bare, just as in all Apes and most other Mammals. Not un- frequently the woolly coat of the embryo differs considerably in colour from the later permanent hairy covering. Thus for instance, it sometimes happens in our own Indo-Ger- manic race that fair-haired parents are shocked to find their children, at their first appearance, covered by a dark brown, or even black, woolly covering. It is only after this has been shed, that the permanent fair hair, which the child inherits from its parents, makes its appearance. Occasionally the dark hair is retained for several weeks, THE HAIR AS A RUDIMENTARY ORGAN. 207 or even months, after birth. This remarkable woolly covering can only be explained as’ an inheritance from our primordial long-haired ancestors, the Apes. It is equally worthy of note that many of the higher Apes resemble Man in the thin coat of hair which coveis certain parts of their body. In most Apes, especially in the higher Catarhines, the face is nearly or even quite bare, or is covered with hairs as thin and as short as those of Man. In these Apes also, just as in Man, the hair on the back of the head is usually distinguished by its length, and the males often have much beard and whisker. (Cf. Fig. 202, p. 175). In both cases this masculine adornment has been acquired in consequence of sexual selection. In some Apes the breast and the inner sides of the joints are very thinly covered with hair—far less abundantly than is the back and the outer sides of the joints. On the other hand, we not unfrequently see the shoulders, the back, and the outer sides of the limbs thickly covered with hair in men of Indo-Germanic or Semitic race. It is a well-known fact that in some families abundant hair on the body is hereditary, as is the relative vigour and character of the hair-growth of the beard and head. These great differences in the total and partial hairiness of the body, which appear very striking not only when we compare different races of man, but even when we compare many families belonging ~ to the same race, are very simply explained by the fact that the entire hairy covering of Man is a rudimentary organ, an unused inheritance, which has been transmitted from the more hirsute Apes. In this matter, Man resembles the Elephant, Rhinoceros, Hippopotamus, Whale, and other Mammals of various orders which have also entirely or 47 208 THE EVOLUTION OF MAN. partially lost their original coat of hair in consequence of adaptation.168 _ The form of Adaptation which has degraded the growth of hair on most parts of the human body, while preserving it, or even greatly developing it, on certain parts, was, in ali probability, sexual selection. As Darwin has very clearly shown in his work on “The Descent of Man,” sexual selec- tion has had especially great influence in this respect. In consequence of the male Anthropoid Apes, in selecting a partner, preferring those females which were least hairy, and in consequence of the females preferring those suitors which were distinguished by peculiarly fine beard or head- hair, the general hirsuteness of the body was gradually degraded, while the beard and the hair of the head were advanced to a higher degree of perfection. Climatic con- ditions, and other circumstances unknown to us, may, however, also have promoted the loss of the hairy coat. In proof of the assertion that the hairy covering of Man is directly inherited from the Anthropoid Apes, we find, according to Darwin, a curious evidence in the direc- tion, otherwise inexplicable, in which the rudimentary hairs lie on our arms. Both on the upper and on the lower arm the hairs are directed towards the elbow, where they meet at an obtuse angle. Except in Man, this striking arrangement occurs only in the Anthropoid Apes, the Gorilla, Chimpanzee, Orang, and several species of Gibbons. In other Gibbons the hairs of both the lower and the upper arm are directed towards the hand, as in other Mammals. This remarkable peculiarity of Anthropoids and of Man can only be explained on the assumption that our common ape- like ancestors were accustomed, as they are even now, - THE NERVOUS SYSTEM. 209 during rain, to bring their hands together over their heads, or over a branch overhanging their heads. The reverse direction of the hairs, when the arms were in this position caused the rain to run off. Thus, even yet, the direction of the hairs on our lower arm testifies to this advantageous habit of our Ape-ancestors. If the skin and its appendages are minutely examined, Comparative Anatomy and Ontogeny supply many similar important “records of creation,’ showing that they are directly inherited from the skin-covering of the Ape. We obtained our skin and hair by inheritance, immediately from Anthropoid Apes, these from the lower Apes, which, in turn, inherited the same parts from lower Mammals. This is also true of the other great organ-system which is developed from the skin-sensory layer—of the nervous system and the sensory organs. This very highly developed organ system, which performs the highest vital functions— those of the mind—we have inherited immediately from the Apes, and mediately from Mammals of a lower order. The human nervous system, like that of all other Mammals, is, in its developed condition, a very complex apparatus, the anatomical arrangement and the physiological activity of which may, in general terms, be compared to a telegraph system. The central marrow (medulla), or cen- tral nervous system, represents the principal station, the in- numerable “ganglion cells” (Fig. 7, vol.i. p. 129) of which are connected with each other and with numerous very delicate conducting lines by their branched processes. The latter are the peripheric “ nerve fibres,” distributed over the whole surface of the body; these, together with their terminal apparatus, the sense-organs, etc., constitute the “conductive 210 THE EVOLUTION OF MAN. marrow, the peripheric nerve-system. Some, as sensory nerve-fibres, convey the sensations of the skin and of other sense-organs to the central medulla; others, as motor nerve- fibres, transmit the impulses from the central marrow to the muscles. Fic. 217.—Human embryo of three months, in natural size, seen from the dorsal side; the brain and dorsal marrow exposed (after Koelliker) : h, hemispheres of the cerebrum (fore-brain) ; m, “ four-bulbs” (mid-brain); c, small brain (hind-brain, or cerebellum). Below the latter is the three-cornered “neck-medulla” (after-brain). Fie. 218.—Central marrow of a human embryo of four months, in natural size, seen from the dorsal side (after Koelliker): h, large hemi- spheres; v, ‘‘four-bulbs;” c, small brain ; mo, neck-medulla. Below this the dorsal medulla marrow). The central nervous system, or central marrow (medulla centralis), is the actual organ of mental activities, in the stricter sense. Whatever view is taken of the intimate connection between this organ and its functions, it is, at least, certain that those of its special activities which we call sensation, volition, and thought, are in man, as in all the higher animals, inseparably connected with the normal development of this material organ. Hence we must neces- sarily take a deep interest in the history of the development THE CENTRAL MARROW. ‘ 211 of this organ. As it alone can give us the most important information as to the nature of our “mind,” it commands our most earnest attention. For if the central marrow develops in the human embryo exactly as in the embryos of all other Mammals, then the development of the human mental organ from the same central organ of other Mammals and, more remotely, from that of lower Vertebrates, cannot be questioned. It is, therefore, impossible to dispute the enormous significance of these phenomena of development. In order to appreciate these rightly, a few words must first be said as to the general form and anatomical construc- tion of the developed central marrow in Man. Like the central nervous system of all other Skulled Animals (Cra- niota), it consists of two distinct parts: firstly, of the brain or the medulla of the head (encephalon, or medulla ca- pitrs), and, secondly, of the spinal marrow (medulla spi- nalis). The former is enclosed in the bony skull, or “ brain cease,’ the latter in the bony vertebral canal, which is com- posed of a consecutive series of vertebree, shaped like signet rings. (Cf. Plate V. Fig. 16, m.). From the brain proceed twelve pairs of head nerves, from the spinal marrow thirty- one pairs of medullary or spinal nerves for the remainder of the body. The spinal marrow, when examined merely — anatomically, appears as a cylindrical cord with a spindle- shaped swelling in the region of the neck (at the last of the neck-vertebree) and another in the lumbar region (at the first lumbar vertebra, Figs. 217, 218). At the swelling at the throat the large nerves of the upper limbs pass off from the spinal marrow, and those of the lower limbs from the swelling in the lumbar region. The upper end of the spinal marrow passes through the neck-marrow (medulla oblon- 212 THE EVOLUTION OF MAN. gata) into the brain. The spinal marrow appears indeed to be a dense mass of nervous substance; but along its axis passes a very narrow canal, which is continued in front into the larger cavities of the brain, and which, like those cavities, is filled with a clear fluid. ’ The brain forms a considerable mass of nervous sub- stance, of very complex, minute structure, which occupies Fic. 219.—Human brain, seen from the lower side. (After H. Meyer.) Above (in front) is the large brain (cerebrum), with extensively branched furrows; below (be- hind) is the small brain (cere- bellum), with narrow parallel furrows. The Roman numbers indicate the roots of the twelve pairs of brain nerves in order from front to back. the greater part of the skull-cavity ; itis roughly distin- guishable into two main parts—the large and small brain (cerebrum and cerebellum). The former is situated in front and over the latter, and its surface exhibits the well- known characteristic convolutions and furrows (Figs. 219, 220). On its upper surface it is divided by a deep longi- tudinal slit into two lateral halves, the so-called “great hemispheres,” which are connected by means of a bridge, or “eross-piece” (corpus callosum). A deep transverse fissure separates the large brain (cerebrwm) from the smal] brain THE BRAIN. 213 ‘cerebellum). The latter is situated more posteriorly and inferiorly, and shows on its outer surface equally numerous furrows, which are, however, much finer and more regular, NS Gee Fic. 220.—Human brain, seen from the left side. (After H. Meyer.) The furrows of the large brain are indicated by large, thick lines, those of the 3mall brain by finer lines. Below the latter the neck-marrow is visible. f'—f?, frontal convolutions; Ce. a Ce. p, central convolutions; R, fissure of Rolan- dus; S, Sylvian fissure; T, temporal or parallel fissure; Pa, parietal lobe; An, the annectant convolutions ; PO, parieto-occipital fissure ; Su, supra-marginal convolution ; IP, intra-parietal fissure ; t, temporo-sphenoidal convolution. and between them are curved ridges (Fig. 219, lower part). The small bram is also divided into two lateral halves by a longitudinal furrow; these are the “small hemispheres,” _ which are connected at the top by a worm-like cross-piece, the “ brain-worm” (vermis), and at the bottom by a bridge (pons varolit; Fig. 219, VI.). Comparative Anatomy and Ontogeny show, however, that in Man, as in all other Skulled Animals, the brain originally consists not of two but of five distinct parts lying one behind another. These originally appear in the embryo of all 214 THE EVOLUTION OF MAN. Skulled Animals (Cramiota), from the Cyclostomiand Fishes up to Man, in exactly the same form, as five bladders placed one behind the other. Alike in their first rudiments, they, however, differ in their further development. In Man and all higher Mammals the first of these five bladders, the fore-brain, develops so excessively that, when mature, it forms, both in size and weight, by far the greater part of the whole brain. To it belong, not only the great hemispheres, but also the bridge (corpus calloswm), which connects these two, the olfactory lobes, from which proceed the nerves of smell, and most of the processes lying on the roof and floor of the great lateral cavities of the two hemispheres; such, for instance, as the large streaked bodies (corpora striata). On the other hand, the “centres of sight,’ which lie be- tween the streaked bodies, belong to the second main part, which develops from the twixt-brain ; and to the same part belong the third brain ventricle (which is single) and the processes known as the “funnel” (infundibulum), the gray mass, and the “cone” (conarvum). Behind these, and between the large brain and the small brain, we find a little mass, composed of two pairs of bosses, and called the “four bulbs,” on account of two superficial furrows which cross each other at right angles, thus quartering the whole mass (Figs. 217, m, 218, v). Though these “four bulbs” are very insignificant in Man and the higher Mammalia, they — constitute a distinct part of the brain, the third, or mid- brain, which is, on the contrary, especially well developed in the lower Vertebrates. The next or fourth part of the brain is the hind-brain, or small brain (cerebellum), in the strict sense of the term, with its single middle process, the “worm” (vermis), and its two lateral parts, the “small PARTS OF THE BRAIN. 215 hemispheres” (Figs. 217, c,218,c). Behind this comes, finally, the fifth and last part, the “neck-marrow ” (medulla oblon- gata, Fig. 218, mo), which includes the single fourth brain ventricle and the adjoining processes (pyramids, olives, and -restiform bodies). The neck medulla passes directly down into the spinal marrow. The narrow central canal of the spinal marrow extends into the wider “fourth ventricle” of the neck medulla, which is rhomboidal in shape, and the floor of which forms the “rhomboid groove.” From this proceeds a narrow duct, called the “aqueduct of Sylvius,” which leads through the “four-bulbs ” into the third ven- tricle, situated between the two “centres of sight;” and this cavity in turn is connected with the pair of lateral cavities which lie right and left in the large hemispheres. All the cavities of the central marrow are, therefore, directly connected together: Individually all these parts of the brain which we have enumerated have an infinitely complex, minute structure, which we cannot now study, and which hardly bears on our subject. This wonderful brain-struc- ture, as it occurs only in Man and the higher Vertebrates, is of the highest importance, simply because, in all Skulled Animals (Craniota), it develops from the same simple rudi- ments, from the five brain-bladders already enumerated. (Cf. Plates VI. and VII.) Before we direct our attention to the individual develop- ment of the complex brain from this series of simple bladders, we will, in order to understand the matter more clearly, glance fora moment at those lower animals which have no such brain. Even in the skull-less Vertebrates, in the Amphioxus, there is no real brain. Jn this case the whole central marrow is merely a simple cylindrical cord 216 THE EVOLUTION OF MAN. traversing the body longitudinally, and terminating in front almost as simply as at the other end: it is a simple medul- lary tube (Plate XI. Fig. 15, m). We found, however, that the rudiment of the same simple medullary tube occurs in the ascidian larva (Plate X. Fig. 5, m) and in the same cha- racteristic position, above the notochord. Moreover, when closely examined a small bladder-like swelling may be seen at the fore end of the medullary tube in these two closely allied animals ; this is the first indication of a separation of the medullary tube into brain (m,) and spinal marrow (m,). When, however, we consider the undeniable relationship of the Ascidia to the rest of the Worms, it is evident that the simple central marrow of the former exactly answers to the simple nerve-ganglion which, in the lower Worms lies above the throat (pharynx), and which has, therefore, long been called the “upper throat ganglion” (ganglion pharyngeum swperius). In the Gliding Worms (Turbellaria) the whole nerve system consists merely of this simple ganglion, which is — situated on the dorsal side of the body, and from which nerve- threads radiate to the different parts of the body (Fig. 211,97) This upper throat ganglion of the lower Worms is evidently the rudiment from which the more complex central marrow of the higher animals has developed. An elongation of the upper throat ganglion along the dorsal side gave rise to the medullary tube, which is characteristic of Vertebrates and the young forms of Ascidia alone. On the other hand, in all other animals, the central nerve system has de- veloped in a very different manner from the upper throat ganglion; in Articulated Animals (Arthropoda) especially, the latter has developed into a throat (pharyngeal) ring, with a ventral marrow ; this is the case, also, in the articu- THE NERVOUS SYSTEM IN THE LOWER ANIMALS. 217 lated Ringed Worms (Annelida) and the Star-animals (EZchi- noderma), which originated from Arthropods. The Soft- bodied Animals (Mollusca) also have a throat ring, which is quite unrepresented in Vertebrates. Only in Vertebrates the central marrow developed along the dorsal side, while in all other animals which have been named it developed aiong the ventral side of the body.” Descending below the Worms we find very many animals which are entirely without & nerve-system, and in which the functions of that system are performed simply by the outer skin-covering—by the cells of the skin-layer, or exoderm. This is the case in many low Plant Animals (Zoophyta), for instance, in all Sponges, and in the common fresh-water Polyp, the Hydra. It was also undoubtedly the case in all extinct Gastreads. In all Primitive Animals (Protozoa) the nerve-system is, of course, unrepresented, for these have not as yet attained to the development of germ- layers. In considering the individual development of the nerve- system in the human embryo, we must first of all start from the important fact already mentioned, that the first rudi- ment of the system is the simple medullary tube, which detaches itself from the outer germ-layer along the middle line of the lyre-shaped primitive germ. We found (Figs. _ 85-87, vol. i. p. 298) that the rectilineal primitive groove, or dorsal furrow, first arises in the centre of the lyre-shaped germ-dise. On each side of this rise the two parallel dorsal or medullary swellings. The free margins of these bend to- wards each other, coalesce, and form the closed medullary tube (Figs. 88—93,vol. i. pp.800-309). At first this tube lies directly under the horn-plate; it is, however, afterwards situate DUS): whe} THE EVOLUTION OF MAN. = = == S==S,y NN (M,—- z SS 5) f: => SSSS== —SS —<———SSSS ~ = —— —_—SSS=SS —— SS (H — Ez —>| ———S=> —_—- ——S SSS = ==> => E a: =. >- ji ll wil ll i! iy Ny SS il | Fies. 221-223.—Lyre-shaped (or sole-shaped) germ-shield of a Chick, in three consecutive stages of evolution, seea from the dorsal surface: about twenty times enlarged. Fig. 221, with six pairs of primitive vertebre. The brain a simple bladder (hb). The medullary furrow is wide open from the point.«, very wide at z. mp, Marrow (or medullary) plates; sp, side- plates; y, boundary between the throat cavity (sh) and the head-intestine (vd). Fig. 222, with ten pairs of primitive vertebra. The brain consists of three bladders: v, fore-brain; m, mid-brain; h, hind-brain. c, Heart; dv, yelk-veins. The medullary furrow is wide open behind (z). mp, Marrow- plates. Fig. 223, with sixteen pairs of primitive vertebre. The brain consists of five bladders: v, fore-brain; z, twixt-brain; m, mid-brain; h, hind-brain; ”, after-brain. a, Hye-vesicles; g, ear-vesicles; c, heart; dv, yelk-veins ; mp, marrow-plate. ww, primitive vertebre. DEVELOPMENT OF THE BRAIN. 219 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 (m,). 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, hb). 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 220 THE EVOLUTION OF MAN. parts. Two of these contractions first appear, and con- sequently the brain forms three consecutive bladders (Fig. Fies. 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 upper 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,h). 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 VIL., 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 (2); III., mid- brain (m); IV., hind-brain (h) ; and V., after-brain (7). 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, 2,) proceed primarily the “centres of sight” and the other parts which surround the so-called “third brain-ventricle,” also the “funnel” (infundibulum), the “cone” (conarvum), etc. The third bladder, the mid-brain (mesopsyche, m), furnishes the small eroup 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, n), forms the neck- marrow, or the “elongated marrow” (medulla oblongata), together with the rhomboid groove, the pyramids, olives, ete. 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.) Uz te. an =, t Fic. 227.—Brains of three embryonic Skulled Animals in vertical longi- tudinal sections: A, of a Shark (Heptanchus); B, of a Snake (Coluber); O, of a Goat (Capra); a, fore-brain ; b, twixt-brain ; c, mid-brain; d, hind-brain ; é, after-brain; s, primitive fissure of the brain. (After Gegenbaur.) Fig. 228.—Brain of a Shark (Scylliwm) 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 (0); d, twixt-brain; b, mid-brain (behind it, the insignificant rudiment of the hind-brain); a, after-brain. (After Busch.) Fic. 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 (6) ; 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; ¢, 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 (Myav- 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 MAN. 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 Fie. 230.—Brain of Rabbit: A, from the dorsal side; B, from the ventral side; lo, olfactory lobules; I., 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 (ventriculus 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-bladders. 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 Amnion Animals (Amniota), a noticeable curving of the rudimentary brain takes place, simultaneously BRAIN CURVATURE. 225 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. (Cf. Plates L, VI, and VIL) 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 C), 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 . go 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 thei 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. 229 are the inner membrane (pia mater), the central membrane (meninaz arachnoides), and the outer membrane (dura mater). All these parts are developed from the skin-fibrous layer. TABLE XXVIII. SysteMATIC SURVEY OF THE MOST IMPORTANT PERIODS IN THE PHYLOGENY OF THE HUMAN SKIN-COVERINGS. I. First Period: Skin of Gastreads. The entire skin-covering (including 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 Gastrzad 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 (coriwm), the muscle-plate and the skeleton-plate. The skin is potentially both covering and mind, III. Third Period: Skin of Chordonia. The skin-sensory 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 from the muscle and skeleton plates, 230 THE EVOLUTION OF MAN. V. Fifth Period: Skin of Cyclostoma. The outer-skin remains a simple, soft mucous layer of cells, but forms one-celled glands (cup-cells). The leather-skin (coriwm) differentiates into cutis and sub-cutis. : | VI. Siwth 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 an inner mucous layer. The ends of the toes are covered with horny sheaths (first rudiments of claws or nails). VIII. Highth Period: Skin of Mammals. The outer skin forms the appendages characteristic of Mammals only ; hair, and sebaceous, sweat, and milk glands, TABLE XXVIII. Systematic SURVEY OF THE MOST IMPORTANT PERIODS IN THE PHYLOGENY OF THE HumMAN NERVOUS SYSTEM. I. First Period: Medulla of Gastreads, The nerve system is not yet distinct from the skin, and, together with the latter, is represented by the simple cell-stratum of the exoderm, 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 Jayer, 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. ganglion. SURVEY OF HUMAN NERVOUS SYSTEM. 231 Ill. 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 ae 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 aorsal 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- secutive brain-bladders of simple structure. VI. Siath 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 Mammais. 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 Ma» ka Apes; & the brain of Ape-men ; and 7, the brain of Man. ( 232 ) TABLE XXIX. Systematic Survey of the Evolution of the Skin-covering and Nerve System. XXIX. A. Survey of the Evolution of the Skin-covering. Horn-layer of the outer / Hair Outer-skin skin Nails (Epidermis) (Stratum corneum) |} Sweat glands Product of the Skin-) Mucous layer of the ) Tear glands Ski sensory-layer outer skin Sebaceous glands in. (Stratum mucosum) \ Milk glands (Derma, : or Fibrous layer of the eonnece tissue Integumentum) Leather-skin leather skin , ite eas (Corium) (Cutis) Blo ue mi cay Product of the Skin-’) Fatty layer ofthe leather | p, Wee ee = d fibrous-layer skin pp ents pl cn (Subcutis) leather skin ——— ——— aa XXIX. B. Survey of the Evolution of the Central Marrow. (Protopsyche) _)| Streaked bodies Corpora striata Arch Forniz Cross piece Corpus callosum Centre of sight Thalamé opticr ae chamber of the Ventriculus tertius ain a rai Central Ws II. Twixt-brain bres hemispheres Pee cerebrt actory lobules Lobi olfactorit I. Fore-brain Lateral chambers Ventricult laterales (Deutopsyche) ) pineal body Conarium Central Nerve Ee Infundibulum stem (Ps a or Medulla { III. Mid-brai Four bulbs Corpus bigeminum y sy 7 Tam he Aqueduct of Sylvius Aqueductus Sylvit Centralis). (Mesopsyché) | Brain stalks ' Pedunculi cerebrt Product of the Small hemispheres Hemisphere cerebetli Skin-sensory IV. sna ie e) Brain worm Vermis cerebelli layer GOT Y ( Brain bridge Pons Varolit Params Cee py : ives orpora olivary V. ee an é Restiform bodies Corpora restiformia Cepipsyche’ Fourth chamber of the Ventriculus quartus brain VI. Dorsa: Marrow Notopsyche Medulla spinalis Enveloping mem- : p Medullar : 1. Soft medullary skin Pia mater y branes, with the 2. Centralmedullaryskin Arachnoidea trit blood coverings Teese GE th ee ~}\ 3. Hard medullary skin Dura mater (Meninges) Ns eye de te (Products of the skin-fibrous layer) and spinal cord ia el CHAPTER XXI. 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 Vascular Capsule and the Fibrous Capsule of the Eyeball.—Hye- lids.—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).— Conduct- ing Apparatus of Sound: Drum Cavity, Ear Bonelets, and Drom Mem- brane.—Origin of these from the First Gill-opening and the Parts immediately round it (the First and Second Gill-arch).—Rudimentary Outer Kar.—Rudimentary Muscles of the Ear-shell. “Systematic Physiology is based especially upon the history of develop- ment, and unless this is more complete, can never make rapid progress; for the history of development furnishes the philosopher with the materials necessary for the secure construction of a system of organic life. Hence apatomical and physiological researches should be prosecuted more from the 234 THE EVOLUTION OF MAN. point of view of development than is now the case; that is, we should study each organ, each tissue, and even each function simply with the view of determining whence they have arisen.”—Hmit HuscHkE (1832). THE sense-organs are undeniably among the most important and most interesting parts of the human body; through their activity alone we recognize the objects in the world around us. “Nihil est in intellectu, quod non prius fuerit in sensu.” They are the true springs of our mental life. Inno other part of the animal body can we point to such extremely delicate and complex anatomical contrivances, co-operating for a definite physiological aim; and in no other part of the body do these wonderful and very apt contrivances seem, at first, to indicate a premeditated creative design so conclu- sively. Hence it is that, in accordance with the received teleological view, it has been customary to admire the so- called “wisdom of the Creator” and the “purposive con- trivances of His Creation” especially in this matter. But on more mature consideration it will be observed that the Creator, according to this conception, does after all but play the part of an ingenious mechanic or of a skilful watch- maker; just, indeed, as all these cherished teleological conceptions of the Creator and His Creation are based on childish anthropomorphism. We admit that at first sight this teleological explana- tion seems to afford the simplest and fittest interpretation of these very apt contrivances. If the structure and funce- tions of the very highly developed sense-organs are alone regarded, it seems as though their origin is hardly explic- able except on the assumption of a supernatural creative act. But it is exactly on this point that the history of ORGANS OF SENSE. 235 evolution proves most clearly that this received conception is radically false. The history of evolution convinces us that the highly purposive and admirably constituted sense organs, like all other organs, have developed without premeditated aim ; that they originated by the same mechanical process of Natural Selection, by the same constant interaction of Adaptation and Heredity, by which all the other pur- posive contrivances of the animal organization have been slowly and gradually evolved during the “Struggle for Existence.” Like most other Vertebrates, Man possesses six distinct organs of sense, which accomplish seven distinct sensations. The external skin-covering accomplishes the sensation of pressure (resistance) and of temperature (warmth and cold). This is the earliest, the lowest, and the least differentiated organ of sense; it is distributed over the entire surface of the body. The other sensorial activities are localized. The sexual sense is limited to the skin-covering of the external sexual organs, just as the sense of taste is limited to the mucous membrane of the mouth-cavity (tongue and palate), and the sense of smell to the mucous membrane of the nose-cavity. Special mechanical contrivances of great com- plexity exist for the two highest and most differentiated organs of sense, the eye for the sense of sight, and the ear for that of hearing. Comparative Anatomy and Physiology show that in the low animals specialized sense-organs are entirely wanting, and that all sensations are transmitted through the outer surface of the skin-covering. The undifferentiated skin-layer, or exo- derm, of the Gastrzea is the simple cell-layer from which the 236 THE EVOLUTION OF MAN, differentiated sense-organs of all Intestinal Animals (Metazoa), and, therefore, of all Vertebrates, originally developed. Start- ing from the consideration that necessarily only the most superficial parts of the body, those immediately exposed to the outer world, could have accomplished sensations, we should be justified in conjecturing @ priori that the organs of sense also owe their origin tothe same part. This is, indeed, the fact. The most important part of all sense-organs develops from the outermost germ-layer, from the skin- sensory layer; in part, directly from the horn-plate, and, in part, from the brain, the foremost section of the medullary tube, after this has separated from the horn-plate. On comparing the individual development of the various organs of sense, we see that at first they make their appearance in the simplest conceivable form: only very gradually does that wonderful perfect structure develop by which the higher sense-organs eventually become the most remarkable and the most complex mechanisms of the entire organiza- tion. All organs of sense are, however, originally merely portions of the external skin-covering, in which sensorial nerves are distributed. Hven these nerves were originally homogeneous and undifferentiated in character. Gradually, by division of labour, the various functions or “ specific energies” of the different sensorial nerves developed. Simul- taneously the simple terminal expansions of these sense nerves in the skin-covering developed into extremely com- plex organs. The important bearings of these historic facts upon the ~ just appreciation of mental life will readily be perceived. The whole philosophy of the future will assume another NATURAL SYSTEM OF PSYCHOLOGY. 237 form as soon as Psychology has gained an accurate know- ledge of these genetic facts, and has made them the basis of its speculations. If the psychological teachings, published by the best known speculative philosophers, and still generally received, are impartially studied, the simplicity with which the authors bring forward their airy metaphysical speculations, regardless of all the significant ontogenetic facts by which their doctrines are clearly refuted, cannot fail to cause great sur- prise. And yet the history of evolution, in conjunction with the rapidly advancing Comparative Anatomy and Physiology of the sense-organs, affords the only safe founda- tion for the natural theory of the mind. With reference to the terminal expansions of the sensory nerves, the human organs of sense may be distri- buted into three groups, corresponding to three different stages of development. The first group includes those sense-organs, the nerves of which distribute themselves simply in the free surface of the skin-covering (organs of the sense of pressure, of heat, and of the sexual sense). In the second group, the nerves distribute themselves in the mucous membrane of cavities, which are originally grooves or inversions of the skin-covering (organs of taste and of smell). Finally, the third group is constituted by those very highly developed sense-organs, the nerves of which distribute themselves over an internal vesicle detached from ‘the skin-covering (organs of sight and hearing). This remarkable genetic relation is represented in the following table :— | 238 THE EVOLUTION OF MAN. 4 Three Groups. Sense-organs. Sense-nerves. Sense-fumctions. { I. Skin-covering I. Skin nerves , Sense of pressure A. Sense-organs in (outer skin, or (nervi cutanet) j Sense of warmth which the ter- epidermis, and minal expansions leather - skin, of the nerves are or coriwm) distributed inthe | Il. External II. Sexual nerves 3. Sexual sense outer skin-cover- sexual parts (nervi pudends) ing. (penis and chi- ) toris) which the ter- brane of the (nervus glosso- minal expansions mouth - cavity pharyngeus) of the nerves are (tongue and distributed over palate) inverted grooves|ITV.Mucousmem- IY. Olfactory 5. Sense of smell C B. Sense-organs in { Til. Mucousmem- III. Taste nerve 4. Sense of taste of the outer skin- brane of the nerve covering. nose-cavity (n. olfactorius) C. Sense-organs in which the ter- L minal expansions | y_ Hye V. Sight nerve 6. Sense of sight of the nerves are (n. opticus) vesicles _—_ sepa- , See ake ca (n. acousticus) ing external skin- covering. Of the developmental history of the lower organs of sense I have but little tosay. The development of the skin- covering, which is the organ of the sense of pressure (sense of touch) and of warmth, we have already traced (p. 209). I need only add that in the leather skin (corvwm) of Man, as of all higher Vertebrates, innumerable microscopic sense- organs develop, the direct relations of which to the sensa- tions of pressure or resistance, of warmth and of cold, are not yet ascertamed. These organs, in or upon which the sensitive skin-nerves terminate, are the so-called “touch hodies” and the “ Pacinian bodies,” named after their dis- MUCOUS MEMBRANE OF THE TONGUE AND NOSE. 236 coverer, Pacini. Similar bodies are also found in the organs of the sexual sense, in the penis of the male and in the _ clitoris of the female ; these are processes of the integument, and the development of which we shall consider presently, in connection with that of the other organs of generation. The development of the organ of taste, the tongue and the palate, we will also consider presently, in connection with that of the intestinal canal, to which these parts belong. To one point, however, [ will now call particular attention, viz, the mucous membrane of the tongue and palate, in which the taste-nerve terminates, is also in its origin a portion of the external skin-covering. For, as we found, the entire mouth-cavity originates, not as a part of the actual intes- tinal canal, but as a groove-like inversion of the external skin (vol. i. p. 338). Its mucous membrane, therefore, is formed, not from the intestinal layer, but from the skin- layer, and the taste-cells on the upper surface of the tongue and palate arise, not from the intestinal-glandular layer, but from the skin-sensory layer. This is equally true of the mucous membrane of the organ of smell, the nose. The history of the development of this sense-organ is, however, of far higher interest. Although the human nose, externally viewed, seems simple and single, yet in Man, as in all higher Vertebrates, it ‘consists of two perfectly distinct halves, of a right and a left nasal cavity. These two cavities are entirely separated by a vertical partition, so that the passage into the mght nasal cavity lies only through the right nostril, and into the left cavity only through the left nostril. Posteriorly the two nasal cavities open separately through the two posterior nasal ezhon into the head of the pharynx, so that the 9 240 THE EVOLUTION OF MAN. pharynx may be entered without touching the cavity of the mouth. This is the passage by which air is usually inhaled; the mouth being shut, it enters the pharynx, and thence . passes through the windpipe into the lungs. Both nasal eavities are separated from the mouth-cavity by the hori- zontal bony palate roof, to the back of which the soft palate and the uvula is attached, like a hanging curtain. In the upper and hinder portion of both nasal cavities the olfactory nerve extends over the mucous membrane, which lines these parts. This is the first pair of brain nerves, which issue from the skull-cavity through the sieve bone. Its branches extend partly over the partition wall, and partly over the inner side-walls of the nasal cavities, to which are attached the “shells,” or spongy bones of the nose—complex bony structures. These “shells” are much further developed in many of the higher Mammals than in Man. In all Mammals there are three of these “shells” — in each of the two nasal cavities. The sensation of smell is produced by a current of air, containing odoriferous matters, passing over the mucous membrane of the cavities, and there coming in contact with nerve-ends. The peculiar characters which distinguish the olfactory organ of Mammals from that of lower Vertebrates, are represented in Man. In all specific points the human nose exactly resembles that of the Catarhine Apes, some of which indeed possess an entirely human external nose (see face of the Nose-ape, Fig. 202, p.175). The first rudiment of the olfactory organ in the human embryo does not, however, show any signs of the fine form of the future catarhine nose. Indeed, it first appears in the same form which persists for life in Fishes, in the form of two. simple pits, THE NOSE. 241 or grooves in the skin of the upper surface of the head. In all Fishes two of these mere blind nose-pits are found in the upper surface of the head ; sometimes they are situated at the back, near the eyes, sometimes near the snout, or, again, near the mouth-opening (Fig. 191,n, p.113). They are lined by mucous membrane in folds, over which the end branches of the olfactory nerves spread. In this its original condition the double nose of all Amphirhina (p. 101) is entirely unconnected with the pri- mitive mouth-cavity. The connection, however, begins to Fie. 231.—Head of a Shark (Scyl- lium), from the ventral side: m, mouth opening; 0, nose grooves, or pits; 7, nasal furrow; mn, nose-flap in its natural position; n’, nose-flap turned up. (The dots are openings of mucous ducts.) (After Gegenbaur.) appear even in some Primitive Fishes (Selachiz); a super- ficial skin-furrow extends on each side from the nose-groove down to the adjacent corner of the mouth. This furrow, the nasal channel, or furrow (Fig. 231, r), is of great sig- nificance. In many Sharks (e.g., Scylliwm) a special process of the frontal skin, the nasal flap, or “inner nasal process,” overlaps the nasal furrow (n, 7’). Opposite to this the outer edge of the furrow rises and forms the “outer nasal process.” In Dipneusta and Amphibia these two nasal processes meet over the furrow and coalesce, thus forming a canal, the “nasal canal.” There is now a passage from the external nasal groove through this ganal directly into the mouth- 242 THE EVOLUTION OF MAN. cavity, which latter was developed independently of the groove. In the Dipneusta and the lower Amphibia the internal opening of the nasal canal lies well forward (behind the lips) ; in the higher Amphibia it lies further back. In the three highest vertebrate classes, the Amniota, the primary mouth-cavity is separated by the formation of the horizontal palate roof into two perfectly distinct cavities, the superior (or secondary) nasal cavity, and the inferior (or secondary) mouth-cavity. The nasal cavity is also separated by the vertical partition into two distinct halves, into a right and a left nasal cavity. Comparative Anatomy thus still shows us simultaneously, in the ascending series of the double-nostrilled Vertebrates, from Fishes up to Man, all the various stages of develop- ment of the nose which the very highly developed olfactory organ of the higher Mammals has passed through succes- sively in the different periods of its tribal history. The first rudiment of the organ of smell in the embryo of Man and in that of all the higher Mammals, makes its appearance in the same entirely simple form which is retained throughout life by the nose of Fishes. At a very early stage, and while no trace of the characteristic facial structure of Man is yet visible, a pair of small grooves appear on the front of the head, and before the primitive mouth-cavity; these were first discovered by Baer, and by him properly enough named “olfactory grooves” (“ Riechgruben,” Figs. 232, n, 233, n). These primitive nasal grooves are quite separate from the primitive mouth-cavity, or mouth indentation, which, as we found, likewise makes its appearance as a groove-like indentation of the external skin-covering, in front of the blind anterior extremity of the intestinal canal DEVELOPMENT OF THE NOSE. 243 This pair of nasal grooves, as well as the single mouth groove (Fig. 235, m), is lined by the horn-plate. The Per, bs 4 y zi i ul) i t x lin RIG. 235. Fic. 236. Fies. 232, 233.—Head of an embryonic Chick, on the third day of incubation: 232, from the front; 233, from the right side. n, Nose-rudi- ment (olfactory grooves); 1, eye-rudiment (sight-grooves); g, ear-rudiment (auditory grooves); v, fore-brain; gl, eye-slits; 0, upper jaw process; u, lower jaw process of the first gill arch. Fie. 234.—Head of an embryonic Chick, on the fourth day of incubation, from below: vn, nose-groove ; 0, upper jaw process of the first gill arch; wu, lower jaw process of the same; k”, second gill-arch; sp, choroidal fissure of the eye; s, throat (pharynx). Figs. 235, 236.—Two heads of embryonic Chicks : 235, at the end of the fourth day; 236, at the end of the fifth day of incubation. The letters as in Fig. 234. Additional letters are in, inner, and an, outer nasal process; nf, nasal furrow ; st, frontal process ; m, mouth-cavity. (After Koelliker.) All these figures are proportionately enlarged. 244 THE EVOLUTION OF MAN. original separation of the nasal groove from the mouth groove is, however, soon interrupted, for the frontal process (Fig. 235, st, Rathke’s “ Nasenfortsatz der Stirnwand”) is immediately formed above the mouth groove. Right and left the edges of this process project in the form of two lateral processes: these are the inner nasal processes, or nasal flaps (Fig. 235, 7m). On each side, opposite to these rises a parallel ridge between the eye and the nasal groove. These ridges are the outer nasal processes (Rathke’s “Nasen- dacher,” Fig. 235, an). Between the inner and outer nasal process a channel-like depression thus extends on each side from the nose groove toward the mouth groove (m), and this channel is, of course, the same nasal furrow or channel which we found in the Shark (Fig. 231, 7). As the two parallel edges of the inner and the euter nasal processes bend towards each other and coalesce above the nasal channel, the latter becomes a small tube—the primitive “nasal canal.” In this stage of its Ontogeny, therefore, the nose of Man and of all other Amnion Animals consists of two small narrow tubes—the “nasal canals ”—leading from the outer surface of the frontal skin into the simple pri- mitive mouth-cavity. This transient condition resembles the permanent condition of the nose in Dipneusta and Amphibia. (Cf. Plate I., Frontispiece, with explanation.) Specially significant in the modification of the open nasal channel into the closed nasal canal, is a plug-shaped forma- tion, which extends from below up to the lower extremities of both the nasal processes on each side, and unites with them. This is the upper jaw process (Figs. 232, 0, 236, 9, Plate I., 0). Below the mouth groove lie the gill arches, which are separated from one another by the gill openings UPPER JAW PROCESS. 245 (Plates I., VI, and VII.,&). The first of these gill arches, at present the most interesting to us, which we may call the jaw arch, develops the jaw-skeleton of the mouth (Plate I., x). A small process first grows out from the base of the front gill-arch: this is the upper jaw process. The first gill-arch itself develops a cartilage on its inner side, called after its discoverer, “Meckel’s cartilage,” on the outer surface of which the lower jaw forms (Figs. 232, wu, 236, uw). The upper jaw process forms the principal part of the entire framework of the upper jaw, viz., the palate bone and the wing bone, On its outer side the upper jaw bone, in the narrower sense, afterwards arises, while the middle portion of the upper jaw skeleton, the twixt jaw (intermaxillary bone) develops from the anterior portion of the frontal process. (See development of the face in Plate I.) In the further characteristic development of the face in the three higher vertebrate classes, the two upper jaw pro- cesses are of the highest importance. From them proceeds the palate roof, the important horizontal partition which grows into the simple primitive mouth-cavity, separating it into two quite distinct cavities. The upper cavity, into which the two nasal cavities open, now develops into the nasal cavity—a respiratory air passage and an olfactory organ. The lower cavity, on the other hand, forms, by itself, the permanent secondary mouth-cavity (Fig. 237, m)—the digestive food passage and the organ of taste. Both the upper smell-cavity and the lower taste-cavity open at the back into the throat (pharynx). The palate roof, separating these two cavities, is formed by the coalescence of two lateral portions —of the horizontal plates of the two upper jaw processes (palate-plates ; Fig. 237,). When these do not perfectly 246 THE EVOLUTION OF MAN. adhere in the middle line, the result is a permanent longi- tudinal cleft, through which there is an open passage from the mouth-cavity directly into the nasal cavity. The so- Fic. 237.— Diagrammatic transverse section a through the mouth and nose cavity. While the rf palate-plates () separate the original mouth-cavity p- into the lower secondary mouth-cavity (m) and the upper nasal cavity, the latter is parted by the ver- tical partition wall of the nose (e) into two distinct halves (n,n). (After Gegenbaur.) called “wolf's jaws” are thus caused. The “hare-lip” and “split lip”.is a slighter degree of this arrested develop- ment. | Simultaneously with the horizontal partition of the palate roof, a vertical wall by which the single nasal cavity is divided into two, a right and a left cavity, develops (Fig. 237, n, 7). This vertical partition of the nose (e) is formed by the middle part of the frontal process: above this gives rise by ossification to the vertical lamella of the sieve bone (cubiform plate), and below the great vertical bony partition wall—the “plough-share” (vomer), and in front to the twixt-jaw (os intermaaillare). Goethe was the first to show that in Man, just as in all the other Skulled Animals, the twixt-jaw appears as an independent bone between the two halves of the upper jaw. The vertical partition wall of the nose finally coalesces with the horizontal palate roof. The two nasal cavities are now as entirely separate from one another as from the secondary mouth- cavity. These three cavities open, however, at the back into the pharnyx, or jaw-cavity. THE HUMAN NOSE. 247 The double-nostrilled nose has now attained the structure characteristic of Man in common with all other Mammals. Its further development is very easily intelligible: it is limited to the formation of internal and external processes of the walls of both nasal cavities. Within the cavities develop the “ nose shells,” spongy bony structures, over which the olfactory mucous membrane spreads. The first brain. nerve, the olfactory nerve, with its delicate branches, passes eM a) , i (" sa HHA Colter aaa (PNA (VB) ‘Hah aor a 8 ” io | He Fies. 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, B= 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.) Fic. 240.—Face of a human embryo of eight weeks. (After Ecker.) Cf. Frontispiece, Plate I. Fig. Mr— M111, 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.!7 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 ‘Platyrhini, p. 175). (249) TABLE XXX. SysTEMATIC SURVEY OF THE CHIEF PHYLOGENETIC STAGES OF THE Human Noss. 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 mouth (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 (Sozobranchia). 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 Mammale. 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 proceeds 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. slusively 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 membrane (sclerotica) ; b, horn membrane (cornea); c, outer membrane (conjunctiva) ; d, circular veins of iris; e, vascular membrane (choroidea); f, ciliary muscle; g, corona ciliaris; h, rainbow membrane (iris); 1, optic nerve (n. opticus); k, anterior limit of the retina; l, crystalline lens (lens crystal- lina); m, inner cover of the horn membrane (water membrane, membrana Descemeti); n, pigment membrane (pigmentosa); o, retina; p, ““petits-canal ;” q, yellow spot of the retina. (After Helmholtz.) When fully developed, the human eye is a globular capsule (the eyeball, bulbus, Fig. 241). This les 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 (corpus vitrewm). The crystalline lens (Fig. 241, 2) 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 m, 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, b). 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 horny 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.110). 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 (0), which is a dilatation of the optic nerve (7). 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 pigment, n), 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, h), 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 iris 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 ciliaris, g), which surrounds the edge of the lens with about seventy large, and many smaller rays, DEVELOPMENT OF THE EYE. 253 In the embryo of Man, as in that of all other Amphi- rhina, two pear-shaped vesicles grow out laterally, at a very early period, from the foremost part of the first brain- bladder (Fig. 223, a, p.218). These bladder-like protuberances are the primary eye-vesicles. At first they are directed outward and forward, but they soon make their way further downward, so that after the specialization of the five brain- bladders, they lie at the base of the twixt-brain. The internal spaces within the two pear-shaped vesicles, which soon attain a considerable size, communicate through their hollow stalks with the cavity of the twixt-brain. Their outer covering is formed by the outer skin-covering (horn- plate and leather-plate). Where, on each side, the latter comes directly in contact with the most curved portion of the primary eye-vesicles, a thickening (/) arises, and at the same time a groove-like indentation (0) in the horn-plate (Pig. 242,1). This groove, which we will call the lens groove, changes into a closed sac, the thick-walled lens vesicle (2, 2), owing to the fact that the edges of the groove coalesce above Fie. 242.—Hye of an embryonic Chick in longitudinal section (1, of a germ after sixty-five hours of incubation; 2, of a somewhat older germ ; 3, of a germ four days old): h, horn-plate; 0, lens groove ; 1, lens (in 1, it still forms part of the epidermis, while in 2 and 3 it has separated); 2, thickening of the horn-plate at the point from which the lens separated ; gl, vitreous body; r, retina; u, pigment membrane. (After Remak.) 254 THE EVOLUTION OF MAN. it. Exactly as the medullary tube originally separates from the outer germ-layer does this lens-sac separate from the horn-plate, in which it originated. The space within this sac is afterwards entirely filled by the cells of its thick wall, and the solid crystalline lens is thus formed. The latter is, therefore, purely a formation of the epidermis. Together with the lens the small fragment of the leather-plate (coriwm) lying below the lens separates from the outer skin-covering. This small piece of the leather-skin very soon forms a highly vascular sac round the lens (capsula vasculosa lentis). Its anterior portion at first covers the pupillary orifice, and is then ‘known as the pupillary membrane (membrana pupillaris). Its back portion of the same membrane is called the “membrana capsulo-pupillaris.” This “vascular lens capsule, which merely serves to nourish the growing lens,” afterwards entirely disappears. The later, permanent lens capsule contains no vessels, and is a structureless secretion of the lens cells. As the lens thus separates from the horn-plate and grows inward, it must necessarily indent the adjoining primary eye-vesicles from without (Fig. 242, 1-3). This process may be compared to the inversion of the germ-mem- brane vesicle (blastula), which in the Amphioxus and in many low animals gives rise to the gastrula (vol. i.p.192). In both instances the inversion of one side of the closed vesicle proceeds until finally the inner, inverted portion touches the outer, uninverted portion of the wall of the vesicle, so that the cavity disappears. Just as in the gastrula the former ° part changes into the intestinal layer (entoderma), and the latter into the skin-layer (ewoderma), so in the inverted primary eye-vesicle the retina develops from the former ee DEVELOPMENT OF THE EYE. 255 (inner) part (Fig. 242, 7), and the black pigment membrane (w) from the latter (the outer, uninverted part). The hollow stalk of the primary eye-vesicle changes into the optic nerve, The lens (2) which enacts so important a part in this inverting process of the primary eye-vesicle, lies at first directly upon its inverted part, that is, on the retina (*). Very soon, however, the two separate, a new body, the vitreous body (corpus vitreum, gl), coming in between them. While the lens-sac is detaching itself, and the primary eye- vesicle is being inverted from without, another inversion simultaneously proceeds from beneath—from the superficial portion of the skin-fibrous layer, 2.e., from the leather-plate of the head. At the back of the lens and below it, a ledge-like process of the leather-plate arises (Fig. 243, g), which inverts the primary eye-vesicle (now shaped like a cup) from below, and presses in between the lens (/) and the retina (7) Thus the primary eye-vesicle assumes the form of a hood. The opening of this hood, answering to the face, is covered by the lens; but the opening, through which the neck would pass, answers to the indentation through which the leather-skin passes in between the lens and the retina (the inner wall of the hood). The space within this secondary eye-vesicle is almost filled by the vitreous body, which answers to the head wrapped in this hood. The hood itself is, properly speaking, double: the inner hood itself is the retina, and the outer one, directly surrounding the former, is the pigment membrane. The comparison with a hood renders this process of inversion, which is sometimes hard to explain, more clearly understood. The rudiment of the vitreous body (corpus vitrewm) is at first very incon- 50 256 THE EVOLUTION OF MAN. siderable (Fig. 243, g), and the retina disproportionally thick. As the former expands, the latter becomes much thinner, till at last the retina appears only as a very delicate Fie. 243.—Horizontal transverse section through the eye of a human embryo of four weeks; 100 times enlarged (after Koelliker): t, lens (the dark wall of which is equal to the diameter of the central cavity) ; g, vitreous body (connected with the leather-plate by a stalk, g'); v, vas- cular loop (penetrating through the stalk (g') into the vitreous body be- hind the lens); r, retina (inner, thicker, inverted lamella of the primary eye-vesicle); a, pigment membrane (outer, thinner, uninverted lamella of the same); h, intermediate space between the retina and the pigment membrane (remnant of the cavity of the primary eye-vesicle). coat of the thick, almost globular vitreous body, which fills the greater part of the secondary eye-vesicle. The outer layer of the vitreous body changes into a highly vascular capsule, the vessels of which afterwards disappear. The slit-like passage through which the rudiment of the vitreous body grows from below in between the lens and the retina, of course causes a break in the retina and the pigment-membrane. This break, which appears on the inner surface of the vascular membrane as a colourless streak, has been inaptly called the choroidal cleft, though the true vascular membrane is not cleft at all at this point (Fig. 234, sp, 235, sp, p. 243). A thin process of the vitreous body passes inward on the under surface of the optic nerve, which it inverts in the same way as the primary eye-vesicle was inverted. The hollow cylindrical optic nerve (the stalk of ae owe. - DEVELOPMENT OF THE EYE, 257 the primary eye-vesicle) is thus transformed into a channel, opening downward. The inverted lower surface attaches itself to the uninverted upper surface of the hollow stalk, so that the hollow space within the stalk, forming the com- munication between the cavity of the twixt-brain and of the primary eye-vesicle, now disappears. The two edges of the channel now grow downward toward each other, enclose the band-like process of the leather-plate, and coalesce beneath it. Thus this process now lies within the axis of the solid secondary optic nerve. It develops into a cord of connective tissue carrying the central blood-vessel of the retina (vusa centralia retine). An entirely fibrous covering, the fibrous capsule of the eye, now finally forms round the outside of the secondary eye-vesicle and its stalk (the secondary’optic nerve). It originates from the head-plate, from that part of the skin- fibrous layer which immediately encloses the eye-vesicle. This fibrous covering takes the form of a completely-closed globular sac, which surrounds the whole ball of the eye, and on the outer side of this, grows in between the lens and the horn-plate. The globular wall of the capsule soon separates, by fission of the surface, into two distinct membranes. The inner membrane becomes the choroidea, or vascular layer; in front it forms the ciliated crown (corona ciliaris) and the iris. The outer membrane, on the other hand, becomes the white enveloping, or protective membrane (sclerotica), and, in front, forms the transparent horny membrane (cornea). The rudiments of all the essential parts of the eye are now formed, and its further development is only in details, in the complex differentiation and combination of the several parts. ( 258 ) TABLE XX RT, Systematic Survey of the Development of the Human Eye. — I. Systematic Survey of those parts of the Human Eye which develop from the first of the Secondary Germ-layers, the Skin-sensory Layer. 1. Stem of the primary 1. Optic nerve Nervus opticus eye-vesicle iM 2. Inner (inverted) part of 2. Retina Retina Products of the dine Nay Ge: lat vesicle Marrow-plate 3. Outer (uninverted) part 3. Screen, or pig- Pigmentosa (lamina of the primary eye- ment-coat pigmentr) vesicle 4. Vesicle separated from 4. Crystalline lens Lens crystallina the horny plate uN Dig of the | 5. Outer epidermic skin 5. Connective mem- Conjunctiva brane ; Horn-plate 6. Inverted portionsofthe 6. Tear-glands Glandule lacrymales epidermic skin (I. Systematic Survey of those parts of the Human Eye which develop from the second of the Secondary Germ-layers, the Skin-fibrous Layer. 4,8. Ledge-like process of 7%, Vitreous body Corpus vitreum the corium on the 8. Vascular mem- Capsula vasculosa lower side of the pri- brane of the corporis vitret mary eye-vesicle vitreous body C. 9. Continuation of the 9. Central vessels of Vasa centralia Products of the corium process the retina retine Leather-plate | 10. Pupillary membrane, 10. Vascular mem- Capsula vasculosa P with its capsule brane of the lentis crystalline lens 11. Folds of the leather 11. Eyelids Palpebrs \ skin (cortwm) 12,13. Vascular mem- 12, Vascular mem- Choroidea . brane of the eye- brane A ball (capsula vas- 13. Rainbow mem- Iris Products of the culosa bulbt)) brane Skull-plate 14, 15. Fibrousmembrane 14. Protective mem- sclerotica of the eyeball (cap- brane sula fibrosa bulbi) 15. Horny membrane Cornea \ 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 ut, R 1, 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 (Selachit) 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- mentary 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. 260 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.% 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 (externai and middle ear) and an apparatus for producing the sensation of sound (internal ear). The outer ear opens in the ear-shell (concha Fie. 244,—Auditory organ of man (left ear, seen from the front; natural size) : a, ear-shell: b, external ear-canal; c, drum, or tympanic membrane ; d, cavity of drum; e, ear-trumpet; f,g, h, the three ear bonelets (f, hammer ; _g, anvil; h, stirrup); %, ear-pouch (utriculus); k, the three semi-circular canals; l, ear-sac (sacculus) ; m, snail (cochlea) ; n, auditory nerve. auras), 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 (b). The inner end of this THE EAR. 261 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 aspecial 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 (pharynz). This canal is called the Eustachian tube (tuba Hustachit). 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 fg, h). 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 (A) 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 directly 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 (endolymph), 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, ' ealled 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, b). 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 twe 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 Fie. 245.—The bony labyrinth of the human ear (left side): a, vestibule; 6, cochlea; c, upper semi- circular canal; d, posterior semi-circular canal; e, outer semi-circular canal; f, fenestra 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. 246, 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, 0). 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, f, vol.1. p. 382.) This is called the appendage of the labyrinth recessus labyrvntha, Fig. 246, lr). Fic. 246.—Development of the ear-labyrinth of a Chick, in five con- secutive stages (A-—H) (cross-sections through the rudimentary skull): fl, ear-groove; lv, ear-vesicle; lr, labyrinth appendage; c, rudiment of the cochlea; csp, hind semi-circular canal; cse, outer semi-circular canal; jr, jugular vein. (After Reissner.) Fies. 247, 248.—Head of an embryonic Chick, on the third day of incuba- tion: 247 in front, 248 from the right; », rudimentary nose (olfactory groove); 1, rudimentary eye (ocular groove) ; g, rudimentary ear (auditory groove) ; v, fore-brain; gl, eye-slit; 0, process of the upper jaw; wu, 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, n, the five grooves of the skull cavity in which the five brain bladders are situated (fore, twixt, mid, hind, and after brains); 0, primary, pear-shaped auditory vesicle (showing through); a, eye (showing through); no, optic nerve; p, canal of the hypophysis; ¢, 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 (Selachw) remains permanently open, and opens above on .the skull (ductus endolymphaticus). In Mam. mals, on the contrary, the appendage of the labyrinth _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 peirous bone in the form of a narrow canal, and is called the aque- duct of the vestibule (aqgueductus vestibult). 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, L, 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 Cyclostomi 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 LS a ae PLA 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. 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, (207 "9 TABLE XXXEIE.. Systematic SuRVEY OF THE CHIEF STAGES IN THE DEVELOPMENT OF THE HumMAN Har, 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. II. Second Stage. The differentiated place of the horn-plate, at which the auditory nerve appeared, forms a small special auditory groove in the skin, which has an outer orifice in the appendage called the “labyrinth.” Ill. Third Stage. The auditory groove has detached itself from the horn-lamina, and forms a small closed auditory vesicle filled with fluid. The “labyrinth-appendage” becomes rudimentary (Aqueductus vestibuli). 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- rhina). 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 Amniote. VII. Seventh Stage. The first gill-opening (the blow-hole of Selachians) changes into the tympanic cavity and the Hustachian tube; the former is externally closed by the tympanic membrane (Amphibia). VIII. Fighth 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 Survey of the Development of the Human Kar. I. Survey of the parts of the Internal Ear. (Apparatus perceptive of sound.) 1. Stalk of the primary ear-vesicle 1, Aqueduct of the vestibule (Duc- Aqueductus vestibulé s. Recessus labyrinthi tus _endolym- phaticus) A. : ; 2, 3. Upper part of the 2. Ear-pouch Utriculus Erodneisiot the primary ear-vesicle 3. Three semi-circu- Canales semi-circu- Horn-plate lar, or curved ares canals 4,5. Lower part of the 4. Ear-sac Sacculus primary ear-vesicle 5. “The snail” Cochlea 6. Auditory nerve 6. Auditory nerve Nervus acusticus B 7. Bony covering of the 7. Osseous labyrinth Labyrinthws osseus Products atone membranous laby- : rinth Head-plate 8. Bony covering of the 8. “The stony bone” Os petrosum whole internal ear II. Survey of the parts of the Intermediate and External Ear. (Apparatus for the conveyance of sound.) 9. Inner part of the first 9. Eustachiantube Tuba Hustachit gill-opening C. 10. Central part of the first Products of the gill-opening first Gill-opening | 11. Closed part of the first gill-opening 12. Upper part of the second gill-arch D. oducts of the 13. Upper part of the first Pr Sage aes gill-arch Gill-arches 14, Central part of the first gill-arch E. Product of the 15. eich circle ) nnulus tympanicus Head-plate 16. Circular membranous fold at the closed part ae of the of the first gill- open Skin-covering yout 10. Tympanic cavity Cnterior of the drum) 11. Tympanic mem- brane (Head of the drum) 12. Stirrup (First bonelet of the ear) 13. Anvil (Second bonelet of the ear) 14. Hammer (Third bonelet of the ear) 15. Bony outer audi- tory passage 16. Ear-shell 17. Rudimentary ear- muscles Cavum tympanr UMembrana tympani Stapes Incug Malleus Meatus cuditortus Osseus Concha auris Husculi conch DEVELOPMENT OF THE 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 (Selachiz) 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.!™ _ Finally, as regards the external ear, the ear-shell (concha auris), 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, ete., 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 AY SLelt = A\ ) \ ( iy tg LAE SUE % Ly LE___ ECE ———— H 4, Y) f y f Uy ly) YW, Frc. 250.—Rudimentary ear-muscles on the human skull: a, upward muscle (m. attollens) ; b, forward muscle (m. attrahens) ; c, backward muscle (m. vetrahens) ; d, larger muscle of the helix (m. helicis major) ; e, smaller muscle of the helix (m. helicis minor); f, muscle of the tragus (m. tragicus) ; g, 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. 27! 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 (b) 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. (Cf. the similarly folded ear in the embryo of the Pig and Cow, Plate VII. Fig. H mt.and C m1) 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 the 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 51 272 THE EVOLUTION OF MAN. ‘a 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 or 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,145 CHAPTER (2 XT DEVELOPMENT OF THE ORGANS OF MOTION. The Motive Apparatus of Vertebrates.—These are constituted by the Passive and Active Organs of Motion (Skeleton and Muscles).—The Significance of the Internal Skeleton of Vertebrates.—Structure of the Vertebral Column.—Formation and Number of the Vertebrz.—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 Vertebrz.—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 (Ribs 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- parison 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. “Tn 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 remam 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.”—KarL G&GENBAUR (1872). Amon 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 arrauyement 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 (Mollusca). In the great group of the Articulated Animals (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. 275 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 reversed 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 Anatomy 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 276 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 ad 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. . 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 sind ahnlich, doch keine gleichet der andern; Und so deutet der Chor auf ein geheimes Gesetz.” IMPORTANCE OF THE SKELETON. 2/7 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 ( 278 ) TABLE XXXIV. Systematic Survey of the Arrangement of the Human Skeleton. A. Central Skeleton, or Axial Skeleton. a Spine. A.a. Vertebral Bodies and Upper Arches. 1. Skull la. Pre-vertebral skull (Cranium) ) 2b. Vertebral skull 7 Neck vertebra 2. Vertebral ee Chest)’ > column < 5 Hip (Columna | 5 Vertebre of the sacrum vertebralis) | 4 a v9» bail (coccyzx) { 1. Products of the gill- A.b. Lower Vertebral Arches. Producta arcuum arches branchialium {f Ribs and breast- Coste et sternum bone B. Bones connecting the Extremities, B.a. Bones connecting the Anterior Limbs: Bones of the Shoulder. 1. Shoulder-blade Scapula (2. Primitive key-bone Procoracoides +) (3. Raven bone Coracoides +) 4. Collar-bone,or key-bone Clavicula B.b. Bones connecting the Lower Limbs: Bones of the Pelvis. 1, Intestinal bone Os tlium 2. Pubic bone Os pubis 3. Hip-bone Os ischi C. Jointed Skeleton of the Limbs. ‘C.a. Skeleton of the Fore Limbs. I. First Division: UPPER ARM. 1. Upper arm bone Humerus II. Szconp Division: LowER ARM. 2. Spoke-bone Radius 3. Ell-bone Ulna III. Turrp Drvision: Hann. III. A. Wrist Carpus Original parts. Modified parts. a. Radical = Scaphoideum b. Intermedium = Innatum c. Ulnar = Triquetrum [d. Central = Intermedium F] e. Carpal I. = Trapezium Ba ee II. = Trapezoides ieee UE yy) OU = Capitatum { Bis sys VS ate Vi = Hamatum Ill. B. Palm ofthe Hand Metacarpus (5) IIL. C, Five Fingers Digiti (14 bones Phalanges) C.b. Skeleton of the Hind Limbs. I. First Division: THIGH. 1. Thigh-bone Femur II. Szeconp Division: Lza, 2. Shin-bone Tibia 3. Calf-bone Fibula III. Turrp Division: Foot, Ill. Ankle Tarsus Original parts. Modified parts. a. Tibial = s b. Intermedium } = Astragalus c. Fibular = Calcaneus d. Central = Naviculare re. Tarsal I. = Cuneiform L Saieiae II. = aa he G5 ¢ ele: = 5 Bee EV Sct Wi = Cuboides III. B. Sole of the Foot Metatarsus (5) Ill. C. Five Toes Digitt (14 bones Phalanges) 279 SKELETON. HUMAN Doce sr ww estoresreesaseeeer eres eeeas Gs CNA ee = 3 OEE. = ae (ie eee ee we ert 7) Pes = seid aa ase SukoeneaRaee Ss a= S== —SS ——=>= SSS = = = =f = =e = = SSS SS SS SS eSeseesSe55555—— = —— — . _ a es eS eT = i Ut Fies. 258-260.—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 yertebrz. The brain is a sim- ple bladder (hb). The spinal furrow from w remains wide open ; behind, at z, it is much enlarged. mp, Marrow-plates; sp, side-plates; y, limit be- tween the pharynx cavity (sh) and the head-intestine (vd). Fig. 259, with ten pairs of primitive vertebre. The brain has separated into three bladders: v, fore-brain; m, mid-brain; h, hind-brain; c, heart; dv, yelk- veins. The spinal furrow is still wide open (2). mp, Marrow-plates. Fig. 260, with sixteen pairs of primitive vertebra. The brain has separated PRIMITIVE VERTEBR&. 289 into five bladders: v, fore-brain; 2, 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 ; ww, 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 vertebre. We have already seen how these true vertebree develop from the skeleton-plate of the primitive vertebre 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 cells, which secrete a firm, elastic “intercellular sub- stance,’ and thus produce cartilaginous tissue. Like most other parts of the skeleton, the rudimentary vertebrz 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 vi radiate bone-cells (Fig. 5, vol. i. p. 126). Fie. 261.—Three She ; breast-vertebre of a Lhe original axis of the vertebral column, humanembryoofeight +he notochord, is more or less compressed SL enna ra by the cartilaginous tissue which grows gitudinal section: v, ; cartilaginous vertebral vigorously round it. In lower Verte- ee li, interverte- brates (2.¢., in Primitive Fishes) a more ral discs ; ch, noto- chord. (After Koel or less considerable portion of the noto- otker) 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 through the axis of the thick cartilaginous ver- tebra] column (Fig. 261, ch). In the cartilaginous vertebral bodies themselves, which afterwards ossify, the thin remnant of the notochord (Fig. 262, ch) 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, i). In a new-born child, a large, pear-shaped cavity, filled with a gelatinous cell-mass, is visible in each intervertebral dis¢ » EVOLUTION OF THE NOTOCHORD: 291 (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. Fic. 262.—A breast-vertebra of the same embryo in lateral cross-section : ev, cartilaginous vertebral bodies; ch, notochord; pr, square process; a, vertebral arch (upper); ¢, upper end of rib (lower arch). (After Koelliker.) Fic. 263.—Intervertebral disc of new-born child in cross-section : a, remnant of the notochord. (After Koelliker.) When the cartilaginous vertebra 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 292 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- tects 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 @ priorz 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 skuil (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 cranw) below, and the boldly arched roof of the skull Fie. 264.—Human skull, (forme cranw) 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 jJaw- 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. 293 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 vertebre, yet the old comparative anatomists at the close of the eighteenth century correctly believed that the skull is originally merely a series of modified vertebre. 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 vertebre of the back of the skull) are also derivable from vertebre.” 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!” 1% 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 Larlier 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 vertebre; they supposed that in this way they could prove that the developed mammalian skull consists of from three to six original vertebre. The hindmost of these skull-vertebree 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 vertebrze 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 (Cramiota), the Cyclostomi and the Selachii. In these the skull retains throughout life the form of a simple cartilaginous capsule—of an imarticulate “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-vertebree ” 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, 4.¢., 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. He was the first te employ the phylogenetic method, which, in this as in all morphological questions, leads most surely and quickly to the result. He showed that the Primitive Fishes (Selachw, 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 vertebree, 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- fied 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. vertebre,” in the selachian skull (Fig. 265), 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 (bc) 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 (0s palato-quadratum, o) and the Fia. 265.—Head skeleton of a Primitive Fish: n, nose-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 ; bc, hind lip- cartilage ; 0, primitive upper jaw (palato quadratum); u, primitive lower jaw; II., tongue-arch; III.—VIII., first to sixth gill-arches. (After Gegen- baur.) primitive lower jaw (wz); IV.,the tongue arch (I1.), and V. to X., six true gill arches, in the stricter sense of that. term (IIT.-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 vertebrz. 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 4 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. 266), 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 Fie. 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); 0, 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 vertebrz ; 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 metamera. The fact that the primitive verte- brate skull is a much modified and profoundly transformed organ, and by no means a primitive structure, is also evl1- 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 the 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 (corvwm), and only secondarily, come into closer relations with the skull. How, in Man, this most simple and primordial rudiment of the primitive skull develops, onto- genetically, from the head-plates, and how, in the mean- time, the anterior extremity of the notochord is enclosed in the base of the skull, has already been explained. (Cf. vol. i. p. 878; Figs. 145 and 146, p. 393.) | The main features in the history of the development of — the gill-arches, which must now be regarded as skull-ribs, has been told. Of the four original rudimentary gill-arches of Mammals (Plates I. and VIL, Figs. 232-236, p. 243), the first lies between the primitive mouth-opening and the first gili- opening. From the base of this gill-arch the “upper jaw process” develops, and this unites, in the manner already _ described, with the internal and the external nasal provesses on each side, and forms the chief parts of the upper jaw skele- ton palate-bones, wing-bones, ete. (Cf p. 245 and 268.) The rest of the first gill-arch, now distinguished as the “ lower-jaw process,” forms out of its base two ear bonelets—the hammer (malleus) and the anvil (vncus); the rest of its mass becomes a long strip of cartilage, called, after its discoverer, “ Meckel’s cartilage.” On the external surface of this cartilage origin- ates, as a surface-bone (formed of cellular matter from the leather-plate), the permanent bony lower jaw. From the base of the second gill-arch in Mammalia originate the third ear bonelet, the stirrup (stapes), and from the subse- quent parts, in order, the stirrup-muscle, the styloid process of the temporal bone, the styloid band, and the small horn of the. tongue-bone. Finally, the third gill-arch becomes cartilagir- ous only at its anterior portion, and here, by the union of its two halves, is formed the body of the tongue-bone (copula EVOLUTION OF THE SKULL. 299 hyoidea) and its great horn on each side. The fourth gill- arch appears in the mammalian embryo only as a transient, rudimentary embryonic organ, and does not develop inte special parts. Of the posterior gill-arches (the fifth and sixth pairs), which are permanent in the Primitive Fishes, no trace is visible in the embryo of higher Vertebrates. The latter have long been lost. The four gill-openings in the human embryo are also only interesting as transient rudi- mentary organs, which soon disappear entirely by concre- scence. The first gill-opening (between the first and second gill-arches) alone is of permanent importance; from it develops the drum, or tympanic cavity of the ear, and the Eustachian tube. (Cf. p. 269, and Plate 1, with explan- ation.) | Not only did Gegenbaur, in his model “ Researches into the Comparative Anatomy of Vertebrates,” first correctly explain the skull and its relation to the vertebral column, but he also first performed the no less weighty and interest- ing task of showing the phylogenetic derivation of the skeleton of the limbs in all Vertebrates from one primordial form. Few parts of the body in the different Vertebrates are subjected, by adaptation to various circumstances, to such an infinite variety of modifications as the limbs, in point of size, form, and special fitness for certain purposes, and yet we are now able to refer them all to one common here- ditary form. Vertebrates are distinguishable as regards the structure of their limbs into three large main groups. The lowest and most ancient Vertebrates, the skull-less and jaw- less classes, like all their invertebrate ancestors, had no paired limbs; this condition is yet represented in the Am- phioxus and in the Cyclostomi (Figs. 189,190). The second 300 THE EVOLUTION OF MAN. main group consists of the two classes of true Fishes, and of the Dipneusta ; in these, two pairs of lateral limbs, in the shape of many-fingered swimming-fins—one pair of pectoral fins (the fore legs) and one pair of abdominal fins (hind legs)— are originally always present (Figs. 191, 192, Plate XII). Finally, the third main group embraces the four higher vertebrate classes: Amphibia, Reptiles, Birds, and Mammals; in these the same two pairs of legs exist originally, but in the form of five-fingered feet. The digits or fingers are often fewer than five; sometimes, also, the feet are quite : aborted. But the original parent-form of the entire sroup had anteriorly and poserorly five digits (Pentadactylism, _p. 123). SoA As regards the Phylogeny of the limbs, from their Comparative Anatomy it appears, therefore, that the extre- mities originated in the’ Fishes, in the Primitive Fishes (Selachw), and werd “transmitted from these to all higher Vertebrates (all the Amphirhina), first in the form of many-fingered fins, and afterwards as five-fingered feet (Figs. 267-272). The anterior extremity—the pectoral fin (or the fore leg)—is originally shaped precisely like the posterior extremity—the ventral fin (or the hind leg). In the one, as in the other, the true limb, externally promi- nent, is distinguishable from the internal, concealed girdle, by which the limb is attached to the spinal column—the shoulder-girdle above, the pelvic girdle below. The genuine primitive form of the paired limbs, as it existed in the most ancient of the Primitive Fishes during the Silurian Period, occurs to this day in perfect preserva- ~ tion in the ancient Ceratodus, and very curious Mud-fish of Australia (p.119, Plate XII). In this, both the pectoral and THE LIMBS. 301 the ventral fin is a flat, oval paddle, in which we find a feathered or biserial cartilaginous skeleton (Fig. 267). This skeleton consists firstly of a strong, articulated fin-rod or “stem” (Fig. 267, A B), which extends from the 2ase te the tip of the fin, and secondly, of a double row of thin, _ feathered rays (rr), which are attached to both sides of the central rod, like the pinnze of a pinnate leaf. This primi- tive fin, first recognized by Gegenbaur, and by him called the Archipterygium, is attached to the spinal column by means of a simple girdle in the shape of a cartilaginous arch.'* In some Sharks and Rays, especially when very young, this same primitive fin also occurs in a more or less modified form. But in most Primitive Fishes the fin is already essentially modified, in that the rays on one side of the stem are partly or altogether lost, and are retained only on the other side (Fig. 268). Hence arises the half-feathered, or uniserial fish-fin, inherited by the other fishes from the Selachii (Fig. 269). ; Gegenbaur first showed how the five-fingered leg of Amphibia is developed from this uniserial fin (Fig. 270) and is inherited by three classes of Amniota. In those Dip- neusta which were the ancestors of the Amphibia, the fin rays on the other side of the stem also were gradually degraded in development, and were in a great measure lost (the light- coloured cartilages in Fig. 269). Only the four lowest rays (shaded in Fig. 269) were retained ; and these are the four outer digits of the foot (second to fifth digits). The first, or great digit (toe), on the contrary, originated from the lower part of the fin-rod. From the middle and upper parts of this fin-rod developed the long main stem of the limbs THE EVOLUTION OF MAN. Yi Y Cer i LU. a Uy Wf My UV. Y D, Ou: We \ \\\ ~N Ni NS N N We; YYy Wh, yp Wd, EVOLUTION OF THE LIMBS. 303 Fie. 267.—Bones of pectoral fins of Ceratodus (Archipterygium, or bilateral pinnate skeleton) : 4 B, series of cartilaginous pieces forming the | ventral stem of the fin; rr, rays of the fin. (After Giinther.) Fig. 268.—Bones of pectoral fin of an earlier Primitive Fish (Acanthias). Most of the rays of the medial edge of the fin (B) have disappeared; only a few (R’) remain. RR, rays of the lateral edge of fin; mt, Metap. terygium ; ms, Mezopterygium; p, Propterygium. (After Gegenbaur.) Fic. 269.—Bones of pectoral fin of a more recent Primitive Fish, or Selachian. The rays of the medial edge of the fin have entirely dis- appeared. The shaded part on the right is that portion which develops into the five-fingered hand of higher Vertebrates (b, the three basal pieces of the fin; mt, Metapterygium; rudiment of the humerus; ms, Mezoptery- gium; p, Propterygium). (After Gegenbaur.) Fie. 270.—Bones of the fore-limb of an Amphibian: h, upper arm (humerus); 1, wu, lower arm (r, radius; u, ulna); 1, c,i, c, %, root-bones of the hand, first row (r, radial; 7, intermediate; c, central; %, ulnary); 1, 2, 3, 4, 5, root-bones of the hand, second row. (After Gegenbaur.) Fic. 271.—Bones of hand of Gorilla. (After Huxley.) Fie. 272.—Bones of human hand, seen from the back. (After H. Meyer.) which is so prominent in the higher Vertebrata as the upper arm (or leg) (Fig. 270, r and uw) and the lower arm (or leg, h). The many-fingered fish-fins thus gave rise, by a process of gradual reversion and differentiation, to the five-fingered amphibian foot, which occurs first in the Sozobranchia, and which, from them, has been transmitted on the one hand to Reptiles, and to Mammals, up to Man, on the other (Fig. 272). Simultaneously with the reduction of the number of the fin- rays to four, a further differentiation affected the fin-stem or rod; it became transversely divided into the upper and lower arms (or legs), and a modification took place in the girdle, which in the higher Mammals originally consists, both anteriorly and posteriorly, of three bones. The simple arch of the original shoulder-girdle separates, on each side into an upper (dorsal) piece—the shoulder-blade (scapula) 53 == 304 THE EVOLUTION OF MAN. and a lower (ventral) piece; the anterior portion of the latter constitutes the pro-key (or collar) bone (procoracot- dewm) and its posterior part the raven-bone (coracoideum). The simple arch of the pelvic girdle breaks up, correspond- ingly, into an upper (dorsal) piece—the intestinal bone (os iliwm), and a lower (ventral) piece; the anterior portion of the latter becomes the pubic bone (0s pubis) and the posterior portion the hip-bone (0s ischw). Table XXXTV., p. 278, shows the correspondence of these three parts of the pelvic girdle with those of the shoulder-girdle. The latter, however, in the key-bone or collar-bone (clavicula), possesses a, fourth, wanting in the former. (Cf, Gegenbaur.®) As in the girdle, so in the trunk of the limbs there is originally an absolute agreement between the anterior and posterior limbs. The first section of the trunk is supported by a single strong bone—in the anterior limbs, the upper arm (hwmerus) ; in the posterior, the upper leg (femur). The second section, on the other hand, contains two bones— on the anterior extremity the spoke-bone (radius, Fig. 270, r), and the ell-bone (ulna, Fig. 270, w); in the posterior the two corresponding bones, the shin-bone (tibia) and calf-bone (fibula). (Cf. skeletons in Fig. 196 and Figs. 204-208). Moreover, the subsequent small and numerous bones of the wrist (carpus) and of the ankle (tarsus) cor- respond; so do the five bones of the middle of the hand (metacarpus) and of the middle of the foot (metatarsus). Finally, the same is true of the five digits attached to these parts, which in their characteristic structure of a series of bone-pieces correspond in the anterior and posterior limbs. Charles Martins, of Montpellier, an excellent morphologist has shown that, in detail, the anterior and posterior limbs correspond.!® ' a shes HOMOLOGY OF THE LIMBS. 305 As Comparative Anatomy thus shows that the skeleton of the limbs in Man is composed of the same bones, and in the same manner as the skeleton in the four higher verte- brate classes, we may justly infer their common descent from a single parent-form. This parent-form was the most ancient Amphibian possessing five digits both on the fore and on the hind limbs. The outermost part of the limbs has, indeed, been very much modified by adaptation to various conditions of life. The diversities in this point within the mammalian class are enormous. The slender limbs of the swift Deer and the strong; springy legs of the Kangaroo, the climbing feet of the Sloth and the digging paws of the Mole, the fins of the Whale and the wings of the Bat, are all instances. It will, of course, be admitted by all that these organs of locomotion are as diverse as possible in point of size, form, and special function. And yet the internal bony skeleton is substantially the same in them all. In all these different forms of limbs the same characteristic bones are always represented in essentially the same strongly inherited combination ; and here we have a weighty confirm- ation of the theory of descent, such as is hardly afforded by the Comparative Anatomy of any other organ. (Cf. Plate IV. p. 34, vol. u1. of “ History of Creation.”) True, in the limbs of the different Mammals, the skeleton is subject to various arrests of development and reversions, in addition to those due to special adaptation (Fig. 273). Thus, in the fore foot (or hand) of the Dog the first digit, or thumb, is aborted (Fig. 273 II.). In the Pig (III.) and the Tapir (V.) this digit has entirely disappeared. So, too, in the Rumi- nants (e.g., the Ox, Fig. IV.) the second and fifth digits are | also aborted, and only the third and fourth are well deve 306 THE EVOLUTION OF MAN. toped. Finally, in the Horse, only one digit, the third, is perfectly developed (Fig. VL, 3). And yet all these diverse fore-feet, as also the hand of the Ape (Fig. 271) and the human hand (Fig. 272), have originated from the same common five-fingered parent-form. This is proved, not only by the rudiments of the aborted digits, but also by the homologous disposition of the wrist-bones (Fig. 273, a—p). (Vide supra, p. 124.) The same story is also told by the germ-history of the limbs, which is originally identical, not only in all Mammals, but in all Vertebrates. However different the limbs of the various Skulled Animals (Cramniota) afterwards appear in their fully developed state, they nevertheless all originate from the same simple rudiment. (Cf. Plates VI. and VIL, Fic. 273.—Skeleton of hand or fore-foot of six Mammals. I. Man; Il — Dog; III. Pig; IV. Ox; V. Tapir; VI. Horse. r, Radius; wu, ulna; a, scaphoid; 6, semi-lunar; c, triquetrum (cuneiform); d, trapezium; & yg trapezoid ; f, capitatum. (unciform process) ; g, hamatum (unciform bone); , pisiform; 1, thumb; 2, digit; 3, middle finger; 4, ring finger; 5, little finger. (After Gegenbaur.) ORIGIN OF THE LIMBS, 307 vol. i. p. 362; f, fore-leg, b, hind-leg.) In all, the first rudi- ment of each limb in the embryo is a simple wart, or small knob, which grows from the side of the body between the dorsal and ventral sides (Figs. 119 and 120, vol. i. pp. 357, 359 ; 136 and 137, pp. 381, 382). The cells composing these knobs belong to the skin-fibrous layer. The outer surface is coated by the horn-plate, which is rather thicker at the apex of the protuberance (Plate IV. Fig. 5,x). The two anterior protuberances appear at a rather earlier period than the two posterior. By differentiation of the cells, these simple rudiments develop immediately, in Fishes and in the Dipneusta, into fins. In the higher vertebrate classes, on the contrary, each of the four protuberances, in the course uf its development, assumes the form of a stalked plate, the inner portion of which being narrower and thicker, the outer broader and thinner. The inner portion, or the handle of the plate, then divides into two sections: the upper and lower legs (or arms). Four notches then appear in the free edge of the plate, and these gradually become deeper; these are the divisions between the five digits (Plate VIII. Fig. 1). The latter soon become more prominent. At first, however, all the five digits, both on the fore and on the hind limbs, are joined by a thin connecting web-like membrane; this recalls the original adaptation of the foot as a swimming-fin. The further development of the limbs from this most simple rudiment takes place in the same way in all Vertebrates ; that is, by the modification of certain groups of the cells of the skin-fibrous layer into cartilage, of other groups into muscles, yet others into blood-vessels, nerves, ete. Probably the differentiation of all these various tissues occurs actually in the limbs. Like the vertebral column and the skull, the 308 THE EVOLUTION OF MAN. bony parts of the limbs are also formed at first from soft undifferentiated cell-groups of the skin-fibrous layer. These afterwards change into cartilage, and from these the per- manent bones originate by a tertiary process.) The development of the muscles, or the active organs of locomotion, is, as yet, of much less interest than that of the skeleton, or the passive instruments of motion. The Com- parative Anatomy of these is, indeed, of much higher im- portance than their Embryology. But as very little attention has, as yet, been paid to the Comparative Anatomy and Ontogeny of the muscular system, we have only very general ideas of its Phylogeny also. The muscular system -as a whole has developed in the most intimate reciprocal correlation with the bone system.™ ( 309 ) TABLE XXXV. Systematic SURVEY OF THE MOST IMPORTANT PERIODS IN THE PHYLOGENY OF THE HUMAN SKELETON. I. First Period: Skeleton of the Chordonia (Fig. 187, p. 90). The entire skeleton is formed by the notochord. II. Second Period: Skeleton of the Acrania (Fig. 189, p. 91). A notochord-membrane, the dorsal continuation of which forms a cover: ing round the medullary tube, is formed round the notochord. IIl. Third Period : Skeleton of the Cyclostoms (Fig. 190, p. 108). A cartilaginous primordial skull develops round the anterior extremity of the notochord, from the notochord-membrane. An outer cartilaginous gill-skeleton forms round the gills. IV. Fourth Period : Skeleton of the older Selachis (Fig. 268, p. 302). A primitive vertebral column, with upper and lower arches (the gill- arches and ribs) forms round the notochord. The remnant of the outer gill- skeleton remains with the inner. Two pairs of limbs, with pinnate (biserial) skeletons, appear. V. Fifth Period: Skeleton of the more recent Selachii (Fig. 269, p. 302). The anterior gill-arches change into lip-cartilage and jaw-arches. The external gill-skeleton is lost. The skeleton of the two pairs of fins becomes uniserial (semi-pinnate). VI. Sixth Period: Skeleton of the Dipneusta (Fig. 2, Plate XII.). The skull becomes partially ossified ; as does the shoulder-girdle. VII. Seventh Period: Skeleton of the Amphibia (Fig. 270, p. 302). The gill-arches are modified into parts of the tongue-bone, and of the jaw- apparatus. On the semi-pinnate skeletons of the fins the rays diminish in number to four, thus giving rise to the five-toed foot. The vertebral column ossifies. 310 THE EVOLUTION OF MAN. VIII. Eighth Period: Skeleton of the Monotremata (Fig. 196, p. 148). The vertebral column, skull, jaws, and limbs, acquire the definite characteristics of Mammals. IX. Ninth Period: Skeleton of the Marsupialia (Fig. 197, p. 152). The coracoid bone of the shoulder-girdle becomes atrophied, and the remnant of it amalgamates with the shoulder-blade. _X. Tenth Period : Skeleton of the Semi-apes (Fig. 199, p. 164). The pouch-bones, which distinguish Monotremes and Marsupials, disappear. XI. Eleventh Period: Skeleton of the Anthropoid Apes (Figs. 204-208, p. 179). The skeleton acquires the peculiar development shared by Man ex: clusively with the Anthropoid Apes. a CHAPTER XXIIL DEVELOPMENT OF THE INTESTINAL SYSTEM. The Primitive Intestine of the Gastrula.—Its Homolagy, 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 (ewsophagus).—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 (Metagaster).—Secondary Formation of the Mouth and Anus from the 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 a Digestive Intestine.—Gill-intestine and Stomach- Intestine of the Amphioxus 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. ‘Cautious people require us 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 unreasonableness of this demand is apparent enough from the fact that 312 THE 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 demand, to reap any harvest from this field. But, further than this, history teaches us clearly, that no age in which scientific inquiry has been active, has been able so to deny itself, as, setting the goal of its researches in the future, to refrain from drawing conclusions for itself from its larger or smaller treasury of observations, and from trying to fill the gaps with hypotheses. It would, indeed, be a hopeless proceeding, if, in order to avoid losing any part of our possessions, we should refuse tc acquire any possessions whatever.”—KarL Ernst BAER (1819). 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 primeval collection of homogeneous cells (Synamebium), of the phylogenetic existence of which we yet have evidence in the ontogenetic developmental form of the mulberry-germ (J/orula), 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 Fic. 274.—Gastrula of a Chalk-sponge (Olynthus): A, from outside; B, in longitudinal section through the axis; g, primitive intestine ; 0, primi: tive mouth; 7, intestinal layer, or entoderm ; e, skin-layer, or exoderm. series: in the Sponges, Sea-nettles (Acalephw), Worms, Soft-bodied Animals (Mollusca), Articulated Animals (Arthro- poda), 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, g); its simple opening, the primitive mouth (protostoma, 0), is at once mouth and anus; and the two cell-strata which compose its wall, are the two primary germ-layers: the inner, the nutritive, or vegetative germ- layer, is the intestinal layer (entoderma, 1%); and the outer, covering layer, which, by means of its cilia, is also the agent of motion, is the animal layer, or skin-layer (exoderma, e). This highly important fact, that the gastrula appears as an early larval condition in the individual development of the most varied animals, and that this gastrula always exhibits the same structure, and that the very differently developed intestinal canals of the most varied animals, arises, onto- genetically, from the same extremely simple gastrula- intestine, this very important fact justifies, in accordance with the fundamental law of Biogeny, two conclusions, which involve important results, and of which one is general and one special. The general conclusion is an inductive one, and may be stated thus: The very variously formed intestinal canal of all the different Intestinal Animals has developed, phylogenetically, from one common and extremely simple primitive intestine, from the intestinal - cavity of the Gastrea, that primeeval common parent-form which is at the present reproduced, in accordance with the fundamental law of Biogeny, in the gastrula. The second, the special conclusion, which is connected with the former, is deductive, and may be stated thus: The intestinal canal in Man as a whole is homologous with the intestinal canal in all other animals; it has the same original significance, and has developed from the same rudimentary form. Before proceeding to trace the history of the develop- ment of the human intestinal canal in detail, it will be THE HUMAN INTESTINAL CANAL 315 necessary briefly to get a correct idea of the more general conditions of the formation of the intestinal canai in the developed Man. Not until this is known can the development of the several parts be correctly understood. (Cf. Plates IV. and V., vol. i. p. 321.) The intestinal canal in the developed Man is, in all essential points, exactly similar in form to those of all other higher Mammals, and, especially, to that of the Catarhines, the Nazrow-nosed Apes of the Old World. The entrance to the intestinal canal is the mouth-opening (Plate V. Fig. 16,0). Food and drink pass first through this into the mouth-cavity, in the lower part of which is the tongue. The human mouth-cavity is hedged with thirty-two teeth, attached in two rows to the two jaws, the upper and lower. It has already been stated that the series of teeth is formed in Man exactly as in the Catarhine Apes, but differs from the corresponding part in all other animals (p. 173). Above the mouth-cavity is the double nose-cavity ; the two parts of this are separated by the par- tition-wall of the palate. But, as we have seen, the nasal cavity is not originally separated at all from the mouth- cavity, a common nasal and mouth cavity being primarily formed in the embryo, and this separates at a later period into two separate stories by the hard palate-roof: the upper is the nasal cavity, the lower isthe mouth-cavity. The nasal cavity is connected with certain air-filled bony cavities; the jaw-cavities in the upper jaw, the frontal cavities in the frontal bone, and the sphenoid cavities in the sphenoid bone. Numerous glands of various kinds open into the mouth-cavity, particularly many small mucous glands and three pairs of large salivary glands. The human mouth-cavity is half closed at the back by 316 THE EVOLUTION OF MAN. the vertical curtain which we call the soft palate, and in the centre of the lower part of which is situated the _uvula, A glance with the mouth open into a mirror is sufficient to show the form. The uvula is of importance, because it occurs only in Men and in Apes. On both sides of the soft palate are the tonsils (tonsillw). Through the gate-like arched opening situated beneath the soft palate, we pass into the throat-cavity (pharyna; Plate V. Fig. 16, sh), which lies behind the mouth-cavity. This is only partly visible in the open mouth when reflected in the mirror. Into the throat-cavity a narrow passage opens on each side (the Eustachian tube of the ear), which leads directly into the tympanic cavity of the ear (Fig. 244, e, p. 260). The throat-cavity is continued into a long narrow tube, the gullet (wsophagus, sr). Through this the masticated and swallowed food passes down into the stomach. The wind-pipe (trachea, lr) also opens into the upper part of the throat, and leads thence to the lungs. The opening of this is protected by the epiglottis, over which the food passes. The respiratory organs, the two lungs (Plate IV. Fig. 8, Ju), are situated, in Man, as in all Mammals, in the right and left sides of the breast-cavity (thorax), and midway between them is the heart (Fig. 8, hr, hl). At the upper end of the wind-pipe (¢rachea), below the epiglottis just spoken of, is a peculiarly differ- entiated section, the larynx, which is protected by a carti- laginous frame. The larynx is the most important organ of the human voice and speech, and also develops from a part of the intestinal canal. In front of the larynx lies the thyroid gland (thyreoidea), which occasionally enlarges to the so-called “ goitre.” THE HUMAN INTESTINAL CANAL. 317 The gullet (wsophagus) passes downward through the thorax, along the vertebral column, behind the lungs and the heart, and enters the ventral cavity, after penetrating the diaphragm. The latter (Fig. 16, z) is a membranous, muscular, transverse partition, which in all Mammals (and only in these) completely separates the chest-cavity (thorax, c,) from the ventral cavity (c,). As has been said, this division does not originally exist; at first a common chest and ventral cavity, the cceloma, or the pleuro- peritoneal cavity, is formed in the embryo. It is only afterwards that the diaphragm forms a muscular, horizontal partition between the chest and the ventral cavities. This partition then completely separates the two cavities, and is penetrated only by separate organs, passing through the } i} mi) Dy My h Fic. 275.—Human stomach and gall-intestine in longitudinal section: a, cardia (limit of the cesophagus); b, fundus (blind sac of the left side) ; c, pylorus fold; d, pylorus valve; e, pylorus-cavity; fg h, gall-intestine ; t, mouth of the gall-duct and of the pancreas duct. (After H. Meyer.) chest-cavity into the ventral cavity. One of the most important of these organs is the gullet (esophagus). After this has passed through the diaphragm into the ventral cavity it enlarges into the stomach in which digestion 318 THE EVOLUTION OF MAN. especially takes place. The stomach of an adult man (Fig. 275, Plate V. Fig. 16, mg) is an oblong sac, placed somewhat obliquely, the left side of which widens into a blind-sae, the base of the stomach or fundus (6), while the right side narrows, and passes at the right end, called the pylorus (¢), into the small intestine. Between these two parts of the intestine is a valve, the pyloric valve (d), which only opens when the food-pulp (chyme) passes from the stomach into the small intestine. The stomach itself is the most important digestive organ, and serves especially to dissolve the food. The muscular wall of the stomach is comparatively thick, and, on the outside, has strong muscle- layers, which effect the digestive movements of the stomach ;—on the inside, it has a great number of small glands, the gastric glands, which secrete the gastric juice. — Next to the stomach follows the longest part of the whole intestinal canal, the central, or small intestine (chylogaster). Its principal function is to effect the absorp- tion of the fluid mass of digested food, or the food-pulp (chyme), and it is again divided into several sections, the first of which, the one immediately following the stomach, is called the gall-intestine, or “twelve-finger intestine” (duodenum, Fig. 275, fgh). The gall-intestine forms a short loop curved like a horse-shoe. The largest glands of the intestinal canal open into it: the liver, the most important digestive gland, which furnishes the bile, or gall, and a very large salivary gland, the ventral salivary gland, or pancreas, which secretes the digestive saliva. Both of these glands pour the juices they secrete, the bile and pancreatic juice, into the duodenum (7) near each other. In adults the liver is a very large gland, well supplied with blood, lying on the THE HUMAN INTESTINAL CANAL. 319 right side immediately below the diaphragm, and separated by the latter from the lungs (Plate V. Fig. 16, /b). The pancreas lies somewhat further back and more to the left (Fig. 16, »). 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 (lium). 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 (cwcwm), 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 8, 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 secrete 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 54 320 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. 5,9). The curving-point was distinguished as the middle-plate (Fig. 99, mp). The mesentery is, at first, very short (Plate V. Fig. 14,9); 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 gastrzad ancestors, and which the extant gastrula now exhibits. We have already shown (in Chapter VIII.) that the peculiar Hood-gastrula (Ampho- gastrula) of Mammals (Fig. 277) may be referred back to the original Bell-gastrula (Archigastrula) 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 Gastreea, and in which the whole body of the animal is intestine. ! The peculiar form and mode in which the complex ——— ee 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 Fic. 276.—Archigastrula of Amphioxus (in longitudinal section): d, primitive intestine ; 0, primitive mouth; i, intestinal layer; e, skin-laycr. Fie. 277.—Amphigastrula of Mammal (in longitudinal section). The primitive intestine (d) and primitive mouth (0) are filled up by the cells of the intestinal layer (7) ; 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 secondary intestine, which alone “a 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 (Selachiz), Bony Fishes (Teleostez), 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).384 We have already seen in what a peculiar way the development of the intestine takes place ontogenetically m 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 cavity, 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.}& 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, ,, vol.1. 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, ——— SSS SS | | ( ( aD Fe c oe ¢ ‘way jp iy mi Ze» OD Von p \ 2 {}) y a AWM ES OS PAP SD af Zo DA 5. ps a Baas Ope _ Oe =n UA a< Fic. 278.—Human embryo of the third week, with the aninion 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). Fic. 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 (f) and legs (0) are already commenced: v, fore-brain; z, twixt-brain; m, mid-brain; h, hind- brain; n, after-brain; a, eye; k, three gill-arches; c, heart; s, tail. ee 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 from 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 3206 _ 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, ete. (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 (Rhabdocelum, 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 Lancelet is connected, as may be conjectured with some probability, with the formation of EARLY FORMS OF THE INTESTINAL CANAL. 327 the gill-openings, which appear directly behind it on the intestine. The front portion of the intestine has thus a 8 ; ATTN Ss L, \\ o. Wee 3 & S a ET 8/5 rm ~ SS Ss EMCCCEUEL SEELEY gy HM) OTT SE Trtprit dost LEE OO TMA Hin Q KS SSS LUT ry JAY _FA ANN AW iS or ow ) (wim 4 S DOT tg S297 TTT Tiida

JOO OreR | [o| ban Z (chr a olelol ojo) GS) “il ic) Mane Zs ZZ () IO {ig) foo] SSS Q“yy POVEUIULELE LLL By sx 2D a) Sees a — —= Fic. 280.—A simple Gliding Worm (Rhabdoce!um) m, mouth; sd, throat- epithelium ; sm, throat muscle-mass; d, stomach-intestime; nc, renal ducts; f, ciliated outer-skin ; nm, openings of the latter; au, eye; na, nose-pit. Fie. 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 (0) is above; at the opposite, tail end, the ascidian has become adherent. The gill-intestine (br), 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 (a’); m, mantle. (After Gegenbaur.) 328 become a respiratory organ. THE EVOLUTION OF MAN. I have already pointed out how characteristic this adaptation is of Vertebrates and oy Csophagus = Cardia a Stomachus oe) Pylorus 2 (=| Duodenum S Hepar be: Pancreas b Jejunum 5 (Vesicula sack ® lis) E Tleum om) = 3 5 3 & ~ Q 2 a = Allantots) Orethra Grocystis (excepting the cavity of the Anus, which is formed by the skin-layer). THE MOUTH-SKELETON. 331 We have 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. (Cf. 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."*8 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 Fic. 283. — Scales of a i Shark (Centrephorus calceus). take a wholly different form; but On each rhomboid bone-tablet, notwithstanding that the anterior lying in the leather-skin, rises oe : : Bail tec ce nenered aes 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- ~~ 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 (érachea) 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 iminediately behind the gills, and soon separates into two lateral halves (Figs., 284, c, 285, c; Plate V. Figs. 13, 15, 16, dw). 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 (celoma), 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 swimmine-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 (pharynax), and is expelled through 334 THE EVOLUTION OF MAN. 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- Fic. 284.—Intestine of an embryonic Dog (which is representex 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; f, liver; 9g, walls of the opened yelk-sac, into which the central intestine opens by a wide aperture ; h, rectum. Fic. 285.—The same intestine, seen from the right side: a, lungs; 6, 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 q | 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 (pharynx), 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 ; 55 336 | 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, 7 We must say a few words about an interesting rudi- mentary organ of the respiratory intestine, the thyroid gland (thyreoidea), 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 esthetically 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, y).1°9 The second main section of the intestinal canal, the stomach or digestive intestine, undergoes modifications no less 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 (jejwnwm), 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 very various structures. We have already discussed two of these appendages—the yelk-sac, which protrudes from the middle of the intestinal tube (Fig. 286, ¢), and the allantois, which grows out of the last portion of the pelvic intestine as a large sac-like protuberance (uw). The protuberances from the - middle of the intestine are the two great glands which open into the duodenum, the liver (2) and the ventral salivary gland. | Immediately behind the bladder-like rudiment of the lungs (Fig. 286,/) 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 MAN. 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 Fic. 286.—Longitudinal section through an embryonic Chick on thie fifth day of incubation: d, intestine; 0, mouth; a,anus; l, lungs; h, liver; g, mesentery; v, auricle of heart; k, ventricle of heart; b, arterial arches ; Il, aorta; c, yelk-sac; m, yelk-duct; wu, allantois; 7, stalk of allantois; n, amnion; w, 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-sae 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- Hic. 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; 2, tongue; v, right, v’, left ventricle of heart; o’, left auricle of heart; b, origin of aorta; b’ b” b'”, 1st, 2nd, 3rd aorta-arches; cc’ c”, hollow vein; ae, lungs (y, lung-arteries); e, stomach; m, primitive kidneys (j, left yelk-vein; s, pylorus; a, right yelk- artery; n, navel-artery ; uw, navel-vein) ; x, yelk-duct; 7%, 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 340 THE EVOLUTION OF MAN. the inner stratum, on the contrary, innumerable minor 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, m) 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, 9). The most prominent part of the loop into which the yelk-sae 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 (ileum). 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 we THE SMALL INTESTINE. 341 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 (yeyunum) which succeeds, and the last section of the small intestine, the crooked intestine (ilewm). | 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, 7, 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-clandular 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 Fic. 288.—Chest and abdominal viscera of a human embryo of twelve weeks, in natural size. (After Koelliker.) The head is omitted; the chest and abdominal walls removed. The greater part of the abdominal cavity is occupied by the liver, from an opening in the centre of which the blind- intestine (cecum, v), with the worm appendage, protrudes. Above the diaphragm the heart is visible in the centre, with the small lungs on the right and left. halves are equally well developed; afterwards the left half hes considerably behind the right. In consequence of the asymmetrical development 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 1:36; 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. Five 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 Bauhinz) forms, which divides the large intestine from the small intestine; behind appears a pouch-like protuberance, which grows larger and becomes the blind- intestine (cwcwm) (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 (wrethra), 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, w). In the Dipneusta and Amphibia, in which this blind-sac first appears, it remains within the body-cavity (celoma), 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 (7) remains, its upper portion forming the central navel THE URINARY BLADDER. 345 band of the urinary vesicle (ligamentum vesico-wmbilicale 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 “wrachus”) 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 XXYV.) ( 346 ) EXPLANATION OF PLATE I.—(FRrontispisce.) DEVELOPMENT OF THE FACE. The twelve figures in Plate I. represent the faces of four different Mammals in three distinct stages of individual evolution: M1—Mi111 that of Man, Bi—Br1t of the Bat, C1-Ci of the Cat, S1-Sm1 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 ; k, nose-roof ; 0, 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; 7, fourth gill-arch; g, ear-fissure (remains of the front gill-opening); z, tongue. (Cf. Plates VI. and VIL, Figs. 282-236, p. 243; also Figs. 123, 124, vol. i. p. 370.) TABLE XXXVII. SysTEMATIC SURVEY OF THE MOST IMPORTANT PERIODS IN THE PHYLOGENY OF THE HUMAN INTESTINAL SYSTEM. I. First Period: Intestine of Gastrea (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 acquires, 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 Ascidia. SURVEY OF HUMAN INTESTINAL SYSTEM. 347 IV. Fourth Period: Intestine of Skull-less Antmals (Acramta) (Fig. 282; Plate XI. Fig. 15). The gill-streaks appear between the gill-openings of the respiratory intestine ; a liver blind-sac grows from the stomach-pouch of the digestive intestine ; as in the Amphioxus. V. Fifth Pertod: 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 blind-sac. VI. Sixth Period: Intestine of Primitive Fishes (p. 114). Cartilaginons gill-arches appear between the gill-openings. The fore- most of these form the lip-cartilages and the jaw-skeleton (upper and lower jaw). The swimming-bladder grows from the pharynx. The ventral-salivary gland appears near the liver, as in Selachii. | VII. Seventh 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 Lepidosiren. 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 of Monotremes (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 Amnion Animals. X. Tenth Period: Intestine of Marsupials (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: Intestine of Catarhine Apes (p. 176). All parts of the intestine, and especially the teeth-apparatus, acquire the characteristic development common to Man and Catarhine 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 Ontogenetic 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 Selachii.—Double Auricle in Dipneusta and Amphibia.—Double Ven. tricle in Birds and Mammals.—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 precede 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. Treats ment of the history of development without preparatory study is only too APPLICATION OF THE LAW OF BIOGENY. 349 likely to lead to groping in the dark; and it not anfrequently leads to the most unfortunate results—far inferior to those which might be established beyond question without any study of the history of development,”’— ALEXANDER Braun (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 tenaciously to the inherited germ-history ; and, owing to heredity, accurately retain the mode of evolution inherited from primzeval 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-forms, the primzeval 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, 350 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 (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. 35! 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 has 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 a 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 diflicult field. Positive proof of this assertion can be gained by studying the complex vascular system as explained in the classical works of Johannes Miller, 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 productions 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 56 352 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. (Cf. vol. i. pp. 65, 66.) 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 ot: 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 zrotesque 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 THE 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. (Cf. 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 His 354 THE EVOLUTION OF MAN, caims to have caused by his new conception of germ- 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-disec 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 larvee 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 ee oa eee oe 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 ccelom (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 cceloma. 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- mately to infer the phylogenetic sequence, in which these organs gradually developed, one after the other, in the ancestral line of animals. In the “ Gastrea 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 (D), III. The kidney system (#). IV. The vascular system (F)). V. The skeleton system (G@). VI. The sexual system (H). (Cf. Table XX XIX., 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 Gastrea. 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 (Chordonia). In them, after the skin- system and the intestinal system, two other organ-systems simultaneously arise; these are the nervous and the mus- cular systems. T’he way in which these two organ-systemg 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.% 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 eell (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. 1. 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 informs. Even the lowest classes of Worms, which have as yet neither body-cavity nor vascular system, are furnished with these primitive kidneys (Fig. 280, ne, 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 (Aca@lomi) 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 (Plathelmvinthes), the Gliding Worms (Turbellaria), the Sucking Worms (Jrematoda), and the Tape Worms. In the higher Worms, which are therefore called Coelomati, a body-cavity (celoma), filled with blood, first begins to form; 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 (Ifollusca), and the Star Animals (Zchinoderma), and we may, therefore, infer that they have all acquired these, as a common inheritance from the Coelomati; 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 heterochronism, to displace- ment in time in the germ-history, that is, a gradual dis- arrangement in the original phylogenetic: 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 Gastrea; 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, which compose these systems, are of different ages and of varying morphological value. As we were justified in drawing 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 XX XIX., 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 (epithelvwm) ; 2, connective tissue (connectivum); 3, nerve and muscular tissue (newro-musculum); and 4, vas- cular tissue (vasaliwm). Of these, in accordance with the Gastrea theory, we must regard the covering-tissue as the oldest and most original form, as the actual primary or 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 (%). 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), Fic. 289.—Tissue of the nails (flattened epithelium): a-e, cells of the upper strata; f, g, cells of the lower strata. Fie. 290.—Tissue of the covering of the small intestine (columnar epithelium) : a, side view of three cells (with thicker, porous borders) ; }, surface view of four cells. (After Frey.) hairs, skin-glands, ete., 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. 363 Connective tissue (connectivwm) must be regarded as forming, in order of phylogenetic age, the second main eroup of tissues. This is morphologically characterized by the intercellular substance, which develops between the Fie. 291.—Jelly-like tissue from the vitreous body of an embryo of four months (round cells as jelly-like intercellular substance). Fic. 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. 1. p. 126), the fatty tissue, and the chorda tissue as the earlier; the fibrous, - cartilaginous (Fig. 292), and bone-tissue (Fig. 5, vol. 1. 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 (newro-musculwm) is of much more recent origin than the connective tissue. If epithelial tissue represents a primary period in tribal history, and 264. THE EVOLUTION OF MAN. connective tissue a secondary period, then we may cha- racterize a third, much later period, by nerve-muscle tissue. = ==} Mmmm (nm LICR Tl Fic. 295. Fig. 294. Fic. 293.—Nerve-muscle tissue. Three cells from Hydra: n, outer, nervous; ™, inner, muscular part of the cells. (After Kleinenberg.} Fic. 294.—Nerve-tissue (from a spinal nerve knot): a, anterior, b, posterior root of the spinal nerve; d,e, fibrous nerve-stem; f, g, h,1, nerve cells in ganglion (f, unipolar, g,h, bipolar cells); k, 1, nerve fibres. (After Frey.) Fie. 295.—Muscle-tissue. Three pieces of striped muscle fibre (a). In- terfibrous fat-cells (6). (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. 293; 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 (vasaliwm) must be regarded as forming Fic, 296.—Vascular tissue (vasaliuwm). A hair-vessel from the mesentery: a, vascular cells; b, the kernels of these (“ endothelium’”’), Fic. 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 Eeamuaeny: ; a, surface view; b, edge view. (After Wagner.) C366) { X TABLE XX XY LET, Systematic Survey of the Sequence, according to Age, of the Human Tissue-groups. (Phylogenetic Classification of Vertebrate Tissues. ) FIRST GROUP: PRIMARY TISSUES (Epithelium). 1. First Histotocicat Stace or Evouvrion. I. Covering-tissue (Hpithelium). 1, Outer skin (Epidermis) I. A. Skin-covering tissue (Epithelium dermale).\ 5" Gianas of outer skin Skin-layer, or Exoderm, of Gastrula (after-{ 2 - ; : aes wards ékin-sensory layer) 3. eich site of origin cf the sperm I. B. Intestinal covering tissue (Zptthel. gastrale). (1. Real intestinal epithelium Intestinal layer, or Entoderm, of Gastrula{ 2. Epithelium of the intestinal glands (afterwards the intestinal-glandular layer) 3. (Earliest site of origin of egg-cell ?) SECOND GROUP: SECONDARY TISSUES. (All derived from the Covering-tissue, or Epithelium.) 2. Seconp HisToLoGicaL STAGE OF EVoLUTION. II. Connective-tissue (Connectivum). II. ¢. Filling-up tissue (Tela conjunctiva) Coe Det yas uisaue ; A A 2. tty ti [surrounding] connective tissue) Fatty tissue 3. Fibrous tissue 4. Chorda tissue 5. Cartilaginous tissue 6. Bone-tissue II. D. Supporting tissue (Tela skeletaris) (firmer (supporting) connective tissue) = 3. Tuirp HistroLtocicaL STAGE OF EVOLUTION. III. Nerve-muscle Tissue (Neuro-musculum). 1. Nerve-cells " a. Peripheric nerve-cells (Rod-cells of (Ganglion-cells) the sense-organs) Ill. FE. Nerve-tissue (Tela 1,6. Central nerve-cells (mind-cells) mervea). Original outer portion of the nerve- 2,a. Sheath less nerve-fibres (pale, or muscle cells of the | 2. Nerve-fibres medulla-less fibres) Exoderm . (Nerve-tubes) 2.6, Sheathed nerve-fibres (dark fibres with medulla) IIL. F. Se rete Ce 1. One-celled muscle- (1. a. Smooth contractile fibre-cells ee Hen Pee fibres ‘11 b. Striped contractile fibre-cells nner portion of tie’) 2. Many-celledmuscle- (2. a. Smooth muscle-masses } nerve-muscle cells of pee + ae Gee fibres 2. b. Striped muscle-masses 4. FourtH HisroLtocicat STaGEe or EVoLurion. IV. Vascular Tissue (Vasalium). 1, a, Exoccelarinm (Parietal Coelom-epl- thelium) (and secondary site of we CHS Cw origin of the sperm-cells /) YG ela vasalis). Taner} lium) ) 2B. Endocoslarinum (Visceral eoelom-ept- wall-covering of the thelium) (and secondary site of Coelom system f origin of the egg-cells 7) 2. NCgeean ithe-) 2-4. Endothelium of the lymph-vessels lium) P 2. 6b. Endothelium of the blood-vessels lymphatica). Liquid contents of the Ceelo system 1. Lymph (Colourless blood-cells and fluid intercellular substance) [V. 4. Lympbh-tissue (Tela {: Blood (Red blood-cells and fluid intercellular substance) SE AE C2307") . TABLE: XX XDRk Systematic Survey of the Sequence, according to Age, of the Human 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 StTaGE IN THE EVOLUTION oF ORGANS. I. Skin and Intestinal Systems. The two Systems appear first, and simultaneously, in the Gastread ancestors. Al. I. 4. Skin-system (Systema dermale) Bil. I. B. Intestinal system (Systema gastrale) B3. Gill-intestine and stomach-intestine Simple exoderm A2. Outer skin (Skin-sensory layer) and leather skin (Skin-fibrous layer) A 3. Outer skin, with hairs, glands, ete. Simple entoderm B 2. Intestinal epithelium (Intestinal-glan- Gastrzeads Worms Mammals Gastrzads dular layer) and intestinal muscular } Worms skin (Intestinal-fibrous layer) Chorda-animals 2. SeconD STAGE IN THE EVOLUTION oF ORGANS. II. Nerve and Muscle Systems. The two Systems appear first, and simultaneously, in the Primitive Worm ancestors. C1. II. C. Nerve-system C2. los. (Systema nerveum) II. D. Muscle-system en (Systema muscalare) D3. Upper throat ganglia Simple medullary tube Brain and spinal marrow Skin-muscle pouch Side muscles of the trunk Trunk and limb muscles Primitive Worms Chorda-animals Monorhina Primitive Worms Acrania Fishes 8. Tarrp STAGE IN THE EVoLUTION OF ORGAXS. III. Kidney and Vascular Systems. The two Systems first appear, one after the other, in the Soft-worm ancestors (Scolecida). Ill. #. Kidney-system (Systema renale) ITI. F. Vascular system (Systema vasculare) . Primitive kidney canals . Segmental canals . Primitive kidneys . Permanent kidneys . Simple ccelom . Dorsal and ventral vessels . Heart (part of the ventral vessel) . Heart, with auricle and ventricle Scolecida Acrania? Monorhina Protamnia Scolecida Worms Chorda-animals Monorhina 4. FourtH STaGE IN THE EVOLUTION OF ORGANS, IV. Skeleton and Sexual Systems. The two Systems first appear, one after the other, in the Chordonia-ancestors. G1. IV. G. Skeleton-system G2. (Systema skeletare) i 3. G4 73 Ne IV. H. Sexual system H2. (Systema sexuale) H3. 4. 57 Simple notochord Cartilaginous primitive skull Gill-arches, ribs, limbs . Limbs, with five digits Simple hermaphrodite glands Distinct testes and ovaries Seed-duct and oviduct Phallus (penis, clitoris) Chorda-animals Monorhina Selachii Amphibia Chorda-animals Acrania Selachii 368 THE 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 ecelom, chest-cavity, ventral cavity, heart-cavity, blood-vessels, ete. (Fig. 296). In addition to this vascular carpet (endo- thelium), 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, “narablastic” origin (from the nutritive yelk); they are, however, products of the intestinal-fibrous layer (and partly, perhaps, of the skin-fibrous layer). As the cceloma 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 Gastrea 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- vessels, 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, Medusze), but also all lower Worms (Acelomi), 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 (ceeloma), or of a system of connected spaces, round the intestinal tube, in which cavities the nutritive fiuid (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 series of intermediate stages between the lowest bloodless Primitive Worms (Archelmanthes) and the Chorda-worms (Chordonia), 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 layer 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- celar), the outer, parietal, part by the skin-fibrous layer (exocelar), The ccelom fluid, collected between the two, 370 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 cceloma, 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 trans- 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 . , b, 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 the 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 ends into a main vessel (Fig. 188, ec p. 90; Plate XI. Fig. 14, hz). The original position Fic. 298.—Blood-vessel system of a Ringed Worm (Saenuris); 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 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 372 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 earbonic 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- _ vessels which 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 amceboid cells (Fig. 9, vol. 1. 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 hzemo- 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, hv, 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, hk). From here it is propelled by the gill-artery stem (the foremost section of the ventral vessel) into the gills. In Primitive Fishes (Selachit), an arterial stalk (bulbus 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 ealled 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. Fre. 299.—Head of an embryonic Fish, with the rudiment and the blood-vessel system; seen from the left side: dc, Cuverian duct (point of anion of the front and hind main veins); sv, venous sinus (enlarged terminal portion of the Cuverian duct); a, auricle; v, main chamber ; abr, gill-artery stem; s, gill-openings (between the arterial arches); ad, aorta; c’, head-artery (carotis); , 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 developement of the arterial 376 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 arteriai 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 haif 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 (Lys N PIAA = LIMO ZS, Cr” b EZ FY, na Fic. 316.—Transverse section through the pelvic region and the hind limbs of an embryo Chick in the fourth day of incubation, enlarged about 40 times: h, horn-plate; w, medullary tube; n, canal of the medullary tube; wu, primitive kidneys; w, notochord; e, hind limbs; }, allantois canal in ventral wall; ¢t, aorta; v, cardinal veins; a, intestine; d, intestinal. glandular layer; f, intestinal-fibrous layer; g, germ-epithelium : 7, dorsal muscles; c, body-cavity, or Coelom. (After Waldeyer.) intestinal-fibrous layer (in Fig. 5, Plate IV., coloured. red) ; those which line the inner surface of the external wall of the abdomen (“ exocelar”) are, on the contrary, the product of the skin-fibrous layer (coloured blue in Fig. 5, Plate IV.) ; but the sexual cells which make their appearance at the boundary line between the two forms of ccelom-cells, and . DIFFERENTIATION OF THE SEXEN. 401 which insert themselves, to a certain extent, between the endoccelar and the exoccelar, there forming the germ- plate, cannot be referred either to the intestinal-fibrous layer or to the skin-fibrous layer, but directly to the two primary germ-layers; for there are important grounds for supposing that even the first rudiment of the sexual plate is, probably, hermaphroditic, and that this “sexual epithelium” (visible, in Man and all other Vertebrates, between the exo- cceelar and the endoccelar) represents a primeval and simple hermaphrodite gland. (Cf. vol. i. p. 256, Figs. 52-56, e, h.) The inner half of this, in contact with the intestinal-fibrous ayer, which is derived from the intestinal-glandular layer, would be the rudiment of the ovary; its outer half, in contact with the skin-fibrous layer, which originates from the intestinal-glandular layer, would be the rudiment of the testes. This is, of course, only conjectural. We ought, accordingly, to distinguish two different sexual plates or germ-epithelia; the female sexual plate, a product of the intestinal layer, which gives rise to the ovary-epithelium—the mother cells of the ova (“ovary- plate”); and the male sexual plate, lying externally over the former, and which is a product of the skin-layer, from which originates the testes-epithelium—the mother cells of the sperm-threads (“testes-plate ”); but even the first recog- nizable rudiments of the two sexual plates appear, indeed, 80 intimately associated in the human embryo and in those of the higher Vertebrates, that hitherto they have been re- garded as a single, undifferentiated, common rudiment of an organ ; and it is still possible that the two kinds of sexual glands arise by secondary differentiation from a common rudiment. 402 THE EVOLUTION OF MAN. Though we must recognize the formation of the two kinds of sexual cells, and in their union at fertilization as the one essential act of sexual reproduction, yet, in the great majority of animals, other organs exist which also take part in the act of fertilization. The most important of these secondary sexual organs are the exit-ducts which serve to conduct the mature sexual cells out of the body, and, next to these, the copulative organs, which transmit the fertilizing sperm from the male person to the female, in which the eggs are situated. These latter organs exist only in the higher animals of various tribes, and are far less widely distributed than the exit-ducts. Even these latter, however, are only of secondary formation, and are wanting in many animals of the lower groups. In these, as a rule, the mature sexual cells are simply ejected from the body. In some cases they pass out directly through the outer skin-covering (as in the Hydra and many of the Hy- droidea); in other cases, they enter the stomach-cavity, and are ejected through the mouth-opening (in Gastreeads, Sponges, and other Hydroid Polypes and Coral Animals) ; in yet other cases, they enter the body-cavity and pass out through a special aperture in the ventral wall (porus genitalis). The latter is the casein many Worms and even in a few lower Vertebrates (Cyclostoma and a few Fishes). These indicate the earliest condition of this matter as it was in our ancestors. On the other hand, in all higher, and most lower Vertebrates (as also in most higher Invertebrates) special tube-shaped exit- ducts from the sexual cells, or sexual ducts (gonophori), are present in both sexes. In the female these convey the egg-cells out from the ovaries, and hence they have been EGG-DUCTS AND SPERM-DUCTS. 403 ealled ego-ducts (oviductus, or tube fallopie). In the male sex these tubes convey the sperm-cells from the testes, and hence they are called sperm-ducts (spermaductus, or vasa deferentia). The original, genetic condition of these two outlets is exactly the same in Man as in all higher Vertebrates, while in most Invertebrates it is entirely different; for while in the latter the sexual ducts develop directly from the sexual glands, or from the external skin, or from the in- testinal canal, in Vertebrates an organ-system is employed for the conveyance of the sexual products; one which origin- ally had a very different significance and function—the kidney system, or urinary organs. The original, primary func- tion of these organs is simply to eliminate useless matter from the body in a liquid form. The liquid product of this secretion is called the urine, and is discharged either directly through the external skin, or through the last section of the intestine. The tube-shaped “urinary ducts” only second- arily absorb the sexual products also and convey them out; they thus become “urogenital ducts” (ductus wrogenitales). This remarkable secondary combination of the urinary and the sexual organs into a common “ urogenital apparatus,” or “urogenital system,” is highly characteristic of the higher Vertebrates. In the lowest of these it is, however, wanting, while, on the other hand, it is found in the higher Ringed Worms (Annelida). .To estimate this rightly, we must first glance at the comparative economy of the urinary organs as a whole. The kidney system or urinary system (systema uro- poeticum) is one of the earliest and most important organ- systems in the differentiated animal body, as has already 404 THE EVOLUTION OF MAN. been incidentally mentioned. (Cf. Chapter XVII.) It is found almost universally distributed, not only in the higher animal tribes, but even in the more primitive Worm tribe. Among the latter it even occurs in the lowest and most imperfect known Worms—the Flat Worms (Plathelminthes) (Fig. 184, nc, p. 80). Although these accelomatous Worms have no body-cavity, no blood, no vascular system, they always have a kidney system. It consists of a pair of simple or of branched canals, lined by a layer of cells, which absorb useless juices from the tissues and discharge them through an external skin-opening (Fig. 184, mm). Not only the free-living Gliding Worms (Turbellaria), but also the parasitic Sucking Worms (Trematoda), and even the still more degraded Tape Worms, which, in consequence of their parasitic habit of life, have lost their intestinal canal, are all provided with these “ kidney canals” or primi- tive kidneys. Usually these canals in the Worms are called excretory organs, and in former times they used to be called water-vessels. Phylogenetically they must be regarded as highly-developed pouch-like skin-glands resembling the sweat-glands of Mammals, and, like these, developed from the skin-sensory layer. (Cf. Fig. 210, , p. 198, and Fig. 214, p. 202.) | While in these lowest unsegmented Worms only a single pair of kidney ducts is present, in the higher segmented Worms these ducts exist in greater numbers. In Ringed Worms (Annelida), in which the body is composed of a creat number of segments, or metamera, a pair of these primitive kidneys (hence known as segmenta] organs, or canals) exists in each separate segment.. In this case, also, the canals are very simple tubes, which, on account of theiz i | —. — THE PRIMITIVE KIDNEYS. 405 coiled or looped form, are called “coiled canals.” To the primary, external aperture in the outer skin, originally alone present, a secondary, internal aperture into the body- cavity (celoma) is now added, This opening is provided with vibratory cilia, and is thus enabled to- absorb the secretional juices from the body-cavity and to discharge them from the body. Now in these Worms also the sexual cells, which develop inthe simplest form upon the inner surface of the abdominal wall, pass, when mature, into the ccelom, are drawn into the internal, funnel-shaped ciliated openings of the kidney canals, and are carried out of the body with the urine. Thus the urine-forming “coiled canals,” or “primitive kidneys,” serve, in the female Ringed Worms, as “ oviducts,” and, in the male, as “sperm-ducts.” It would of course be most interesting to know the condition, on this point, of the Amphioxus, which, standing midway between Worms and Vertebrates, affords us so much valuable information. Unfortunately this animal, for the present, affords no solution of this matter. At present we know nothing certainly as to the relation between the urinary and the sexual organs of the Amphi- oxus. Some zoologists assert that this animal has no kidneys; others regard the two long “side canals” as atrophied primitive kidney ducts (Fig. 152, 8, vol. i. p. 428) ; yet others consider certain glandular epidermis-swellings on the inner surface of the gill-cavity to be rudimentary kidneys. Most probably, a great reversion has affected the original primitive kidney canals in the Amphioxus, amounting per- haps to their entire phylogenetic loss. Very interesting inferences may be drawn from the Vertebrates of the next stage—the Monorhina, or Cyclos- 406 THE EVOLUTION OF MAN. toma. Although both orders of this class—the Myxinoides as well as the Petromyzontes—possess developed, urine- secreting kidneys, these organs do not in this case serve te carry away the sexual cells. These cells pass directly from the germ-glands into the ccelom, and are discharged through a, posterior aperture in the abdomen. The condition of the primitive kidneys in these is, however, very interesting, and a A throws light on the complex kidney structure of the higher Vertebrates. In the first place, in the Myxi- noides (Bdellostoma) we find a long tube, the primitive kidney duct (protureter, Fig. 317, a), on each side. This opens internally into the ccelom through a ciliated funnel- shaped aperture (as in Ringed Worms); it opens externally through an opening in the outer skins A great number of small horizontal tubes (“ segmental canals,” or primi- Fie. 317.—A. Portion of kidney of Bdel- lostoma: a, primitive kianey duct (protu- reter); 0b, segmental canals, or primitive urine canals (tubuli wrinifert); ¢, kidney- vesicles (capsule Malphigiane).—B. Por- tion of the same, much enlarged : ¢, kidney- vesicle, with the glomerulus ; d, approaching artery ; ¢, retreating artery. (After Johannes Miller.) tive urine tubes) open on its inner side. Each of these terminates in a blind, vesicular capsule (c) enclosing a THE PRIMITIVE KIDNEY OF SKULLED ANIMALS. 407 knot of blood-vessels (glomerulus, an arterial net, Fig. 317, B,c). Afferent arterial branches (vasa afferentia) con- vey arterial blood into the coiled branches of the “glome- rulus” (d), and efferent arterial branches (vasa efferentia) again carry it out of the glomerulus (e). _ In Primitive Fishes (Selachiz) also there is a longitudi- nal series of segmental canals, which open outwardly in the primitive kidney ducts. The segmental canals (a pair in each metameron of the central part of the body) open, in this case, freely into the body-cavity, through a ciliated funnel (as in Ringed Worms, or Annelids).