THE EVOLUTION OF MAN.

Ilntcrnational Science Xibrar\>

THE

Evolution of Man

A 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

VOL. M.

Ube Merner Company

1&oo\^ /IDanufacturerg

Bftrout ©bio

Authorized Edition

MADE BY

THE WERNER COMPANY

AKRON, OHIO

CONTENTS OF VOL. II.

List of Platei ••• ••• •^ — ••• •*• ^^

List of Woodcuts ... ••• ••• ••• ••• ^^^

List of Genetic Tables ... .m ^m m* ••• xvii

CHAPTER X\.

THE DURATION OF HUMAN TRIBAL HISTORY.

C!omparison of Ontogenetic and Phylogenetio Perrodg of Time. — Dora-
tion of Germ -history in Man and in Different Animals. — Extreme
Brevity of the Latter in Comparison with the Immeasorable Long
Periods of Tribal History. — Relation of this Rapid Ontogenetic
Modification to the Slow Phylogenetio Metamorphosis. — Estimate
of the Past Duration of the Organic World, founded on the Belatire
Thickness of Sedimentary Rock-strata, or Neptanian Formationi.
— The Five Main Divisions in the Latter : I. Primordial, or
Archilithic Epoch. 11. Primary, or Palaeolithic Epoch. III. Second-
ary, or Mesolithic Epoch. IV. Tertiary, or Cssnolithio Epoch.
V. Qaatemary, or Anthropolithic Epoch. — The Relative Dnration
of the Five Epochs.— The Results of Comparative Philology as
Explaining the Phylogeny of Species. — The Inter-relations of the
Main Stems and Branches of the Indo-Germanio Languages are
Analogous to the Inter-relations of the Main Stems and Branches
of the Vertebrate Tribe. — The Parent Forms in both Cases are
Extinct. — The Most Important Stages among the Human An-
eestral Forms. — Monera originated by Spontaneous Generation.
— Neeesniy of Bpontaneovs Qeneration

▼1 OONTENTB.

CHAPTER XVL

THK ANCESTEY OF MAN.
L Fbom thb Moneba to thb Gastbaa.

FAOl

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 Eecords of Creation : Palaeontology,
Ontogeny, and Comparative Anatomy. — Unequal Certainty of the
Various Special Hypotheses of Descent. — The Ancestral Line of
Men in Twenty-two Stages : Eight Invertebrate and Fourteen Verte-
brate Ancettors. — Distribution of these Twenty-tw© Parent-forms
in the Five Main Divisions of the Organic History of the Earth. —
First Ancestral Stage : Monera. — The Structureless and Homo«
geneons Plasson of the Monera. — Differentiation of the Flasson
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 : Amoebse.
— One-celled Primitive Animals of the Simplest and most Un-
differentiated Nature. — The Amoeboid Egg-cells. — The Egg is Older
than the Hen. — Third Ancestral Stage : Syn-Amoeba, Ontogeneti-
cally reproduced in the Morula. — A Community of Homogeneous
Amoeboid Cells. — Fourth Ancestral Stage : Planaea, Ontogeneti cally
reproduced in the Blastula or Planula. — Fifth Ancestral Stage :
Gastraea, Ontogenetically reproduced in the Gastrula and the Two-
layered Germ-disc. — Origin of the Gastraea by Inversion (^invagi-
natio) of the Planssa. — Haliphysema and Gastrophysema. — Extant
Gastrssada ... ... ... ... .^ ... 94

CHAPTER XVIL

THE ANCESTRAL SERIES OF MAN.

n. Fbom the Pbimitiye Wobm to the Skulled Animal.

The Four Higher Animal Tribes are descended from the Worm Tribe.
— The Descendants of the Gastraea; in one direction the Parent
Form of Plant- Animals (Sponges and Sea-Nettles), in the other
the Parent Form of Worms. — Radiate form of the former, Bilateral
form of the latter. — The Two Main Divisions of the Worms,
Acoelomi and Coelomati : the former without, the latter with, a
Body Cavity and Blood-vessel System. — Sixth Ancestral Stage :
Arohelmiuthea, most nearly allied to Tnrbellaria. — Descent of the

C0NTEKT8. tH

rAo«

Coelomati from the Accelomi. — Mantled Animals {Ttmicata) and

Chorda- Animals (Chordonia). — Seventh Stage: Soft- Worms (Scole.
cida). — A Side Branch of the latter : the Acom-Worm (Balano-
glossus). — Differentiation of the Intestinal Tnbe into Gill-intes-
tine and Stomach-intestine. — Eighth Stage : Chorda- Animals (Chor-
donia).— Ascidian Larva exhibits the Outline of a Choi-da- Animal. —
Construction of the Notochord. — Mantled Animals and Verte-
brates as Diverging Branches of Chorda-Animals. — Separation of
Vertebrates from the other Higher Animal Tribes (Articulated
Animals, Star-Animals, Soft-bodied Animals). — Significance of the
Metamerio Formation. — Skull-less Animals (Acrania) and Skulled
Animals (Cra/niota) . — 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 (Mywi-
noidcs and Petromyzonidoe) ... m« ^m m* ••• 71

CHAPTER XVIII.

THE PEDIGREE OF MAN.

m. Fbom the Pbimitivb 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 Granoids
(Qanoides), the Osseous Fishes (Teleostei). — Dawn of Terrestrial
Life on the Earth. — Modification of the Swimming-bladder into
the Lungs. — Intermediate Position of the Dipneusta between the
Primitive Fishes and Amphibia. — The Three Extant Dipneusta
(Protopterus, Lepidosiren, Geratodus) . — 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. — DiSerent Stages in Amphibian Metamorphosis. — The
Gilled Batrachians (Proteus and Axolotl). — The Tailed Batrachians
(Salamanders and Mud-fish). — Frog Batrachians (Frogs and
Toads). — Chief Group of the Amnion Animals, or Amniota (Reptiles,
Birds, and Mammals). — Descent of all the Amniota from a Common

• ••

nn CONTENTa

PAOR

Lizard .like Pttrent-form (Protanmion) . — First Formation of the
Allantois and of the Amnion. — Branching of the Amnion Animab
in Two Lines : on the one side, Beptiles (and Birds), on the other
side, MammalB ... ... ... m* m* ••• 107

CHAPTER XIX.

THE PEDIGREE OF MAN.

lY. From the Fbimxtiyb Mammal to thi Apb.

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 : Gloacal Animals (IToTto^r&mata, or Omithodelphia),
—The Extinct Primitive Mammals (Promo/mmaUa) and the Extant
Beaked AnimitlH (Omithostoma) . — Seventeenth Ancestral Stage:
Pouched Animals {McurtupiaUa, or Didelphia). — Extinct and Extant
Pouched Animals. — Their Intermediate Position between Mono-
tremes and Placental Animals. — Origin and Structure of Placental
Animals (PlacentaUoj or MonodeVphia). — Formation of the Pla-
centa.— The Deciduous Embryonic Membrane (Decidua). — Group
of the Indecidtia and of the Decid/uata. — The Formation of the
Decidua (vera, serotina, reflexa) in Man and in Apes. — Eighteenth
Stage: Semi-apes (ProBimice). — Nineteenth Stage : Tailed Apes
{Menocerea). — Twentieth Stage : Man-like Apes (Anthropoides), —
Bpeeohleu and Speaking Men (Mali. Hominet) ,„ .^ 140

CHAPTER XX.

SUB raSTOBT 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 ;
afterwardfl, the Skin.oovering specialized from the Nerve-system.
— ^Doable Function of the Skin (as a Covering and as Organ of

CX)NTENTa Ix

rAOB

Touch). — Outer Skin (Epidermis) and Leather-skin (^Corium). —

Appendages of the Epidermis : Skin-glands (Sweat-glands, Tear-
glands, Sebaceous Glands, Milk-glands); Nails and Hair. — The
Embryonic Wool-covering. — Hair of the Head and of the Beard. —
Influence of Sexual Selection. — JSLrrangement of the Nerve-system.
— Motor and Sensory Nerves. — Central Marrow : Brain and Dorsal
Marrow. — Constitution of the Human Brain : Large Brain ((7«f>»-
hrum) fjid Small Brain {Cerebellum). — Comparative Anatomy of
the Central Marrow. — Germ-history of the Medullary-tube. — Sepau-
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). — Yarious
Formation of the Five Brain-bladders in the various Vertebrate
Classes. — Development of the Conductive Marrow, or "Peripheric
Nervous System " ... ,., „, . ,^ »^ ,,, 190

CHAPTER XXL

DEVELOPMENT OF THE SENSB-ORGANa

Qrijfin 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 B«of. — 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 Vaa-
oular Capsule and the Fibrous Capsule of the Eyeball. — Eyelids.
— Structure of the Ear. — The Apparatus for Perception of Sound :
Labyrinth and Auditory Nerve. — Origin of the Labyrinth from
the Primitive Ear Vesicles (by Separation from the Horn-plate). —
Conducting Apparatus of Sound : Drum Cavity, Ear Bonelets, and
Drum Membrane. — Origin of these from the First Gill-opening
and the Parts immediately round it (the First and Second Gill-
arch). — Eudimentary Outer Ear, — Eudimentary Muscles of the
Ear.shell ... ... ... ... ... ... SM

X CONTENTS.

CHAPTER XXII.

DEVELOPMENT OP THE ORGANS OF MOTION.

The Motive Apparatus of Vertebrates. — These are constituted by the
Passive and Active Organs of Motion (Skeleton and Muaoles). —
The Significance of the Internal Skeleton of Vertebrates. — Struo
ture of the Vertebral Column. — Formation and Number of the
Vertebrae. — 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-
bwe. — Intervertebral Discs. — Head-skeleton (Skull and Gill-arches).
— ^Vertebral Theory of the Skull (Goethe and Oken, Huxley and
Gegenbaur). — Primitive Skull, or Primordial Craniam. — Its Forma-
tion from Nine or Ten Coalescent Metamera. — The Gill-arohes
(Ribs of -the Head). — Bones of the Two Pairs of Limbs. — Deyelop-
ment of the Five-toed Foot, adapted for Walking, from the Many-
toed Fin of the Fish.— The Primitive Fin of the Selachians
(Archipterygiwn 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 ... .„ ... 278

CHAPTER XXIII.

DEVELOPMENT OF THE INTESTINAL SYSTEM.

The Primitive Intestine of the Gastrula. — Its Homology, or Morpho-
logical Identity in all Animals (excepting the Protozoa). — Survey
of the Structure of the Developed Intestinal Canal in Man. — The
Mouth-cavity. — The Throat (pharynx). — The Gullet (oesophagiis). —
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-glandnlar
Layer, and of all other parts of the Intestine from the Intestinal-
fibiofis Layer. — Simple Intestinal Pouch of the Lower Worms. —

CONTENTS. n

PASS

Differeiitiatkm of the Primitive Intestinal Tube into a Eespiratory
and a Digestive Intestine. — Gill-intestine and Stomach-intestine of
the Amphioxna 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 Lxmgs
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 ... ... 811

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
2ach 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 (Phy logeny) ... ... 848

CHAPTER XXV.

DEVELOPMENT OF THE URINARY AND SEXUAL ORGANS.

Importance of Reproduction. — Growth. — Simplest Forms of Asemal

Reproduction; Division and the Formation of Buds (Gemmation). —

Simplest Forms of Sexual Reproduction : Amalgamation of Two

Differentiated Cells ; the Male Sperm-cell and the Female Egg-cell.

-Fertilization. — Source of Love. — Original Hermaphroditism ;

Sft CXniTENTB.

rA«B

Later 8«pu»tioii of the Sezea (Gonoohorinn). — Origfxml Deyelop.
ment of the TVo Kinds of Sexual Cells from the Two Primary
Qerm-layers. — The Male Exoderm and Female Entoderm. — Develop,
ment of the Testes and Ovaries. — Passage of the Sexual Cells into
the Coelom. — Hermaphrodite Eudiment of the Embryonic Bpi-
thelium, or Sexual Plate. — Channels of Exit, or Sexual Dnots. —
Egg-duct and Seed-duct.— Development of these from the Primitive
Kidney Ducts. — Excretory Organs of worms. — " Coiled Canals " of
Binged Worms (Annelida). — Side Canals of the Arryphioxui. —
Primitive Kidneys of the Myxinoides. — Primitive Badneys of Skulled
Animals (Craniota). — Development of the Permanent Secondary
Eadneys 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
Gloaoa. — Hermaphroditism in Man ... ... .•• ... 388

CHAPTER XXVI

RESULTS OF ANTHEOPOQBNT.

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. — Liheritances 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 (Cocciis). — 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.
— InJBuence of Anthropogeny on the Victory of the Monistic Philo-
sophy and the Defeat of the Dualistic. — Nature and Spirit. — Natural
Science and Spiritual Science. — Conception of the World reformed
by Anthropogeny ... ... ... ... ... ... 432

N^OTSS. Remarks and References to Literature ••• ... 459

U^DaX -.. ... ••• (•• ••• ••• 491

LIST OF PLATES.

Plate XII. (between p. 130 and p. 131). The Australian Mud-
fish ((7(?rf/?ofZ?/« ^t's^eri) ... ... ... Exjilanation

Plate XIII. (between p. 130 and p. 131). The Mexican Axolotl
{Siredon pisciformis) and the European Land-salamander
{Sdlamandra maculat(i) ... ... ... Explanation

Plate Xiy. (between p. 180 and p. 181). Four Catarhines
(Chimpanzee, Gorillas, Orang, Negro) ... Explanation

Plate XV. (between p. 188 and p. 189). Pedigree of Man

Explanation

PAGE

118

129

181

184

LIST OF WOODCUTS.

FIQTTRE

163. Moneron (Prof anuBba)
164(. Bathybius, primitive slime

165. Monemla of Mammal

166. Cytula of Mammal .

167. Amoeba ....

168. Amoeboid egg-cell . •

169. Original egg-cleavage

170. Mulberry-germ (Morula) .

171. Germination of Monoxenia
172, 173. Magospbaera .
174-179. Gastrula of various

animals . .

180, 181. Haliphysema .
182, 183. Ascula of a Sponge .
184, 185. A Gliding-worm

(Rhabdocoelum)

186. Acorn- worm {Balanoglos'

8USj > a • •

187. Appendicularia • •

188. Asoidia . . • •

189. AmphiozuB • . •

190. Lamprey {Petromyxon) .
191,192. Shark (Selachii) .

193. Larval Salamander .

194. Lanral Frog (Tadpole) .

46

wtmnm

195, 196. Beaked AniiBal(Omt.

PMOM

49
51

thorJiAfnchus) and ita
skeleton . ' ,

148

61

197.

Pouched Animal (Marsu-

53
53

198.

pial) with young .
Human egg-membranes .

152

158

55

199.

Semi-ape (Lori)

164

65

200.

Human gei-m with its

57

membranes ,

166

60

201.

Human uterus, navel-cord,
and embryo . ,

167

65

202.

Head of Nose-ape .

175

67
68

203.
204.

Tailed Ape (Sea-cat)'
Skeleton of Gibbon

175

178

80

205.
206.

Skeleton of Orang-outang
Skeleton of Chimpanzee .

178
178

207.

Skeleton of Gorilla .

178

86

208.

Skeleton of Man

178

90

209.

Gastrula of Gastrophy-

90

soma ....

198

91

210.

Germ-layers of Earth.

103

worms

198

113

127

211.

Nerve-systein of Glidiag-
worm ....

198

127

212

Human skin-ooTwrixig

»0

LIST OF WOODCUTS.

nOCKB

rA«B

213. Epidermis oelli •

201

214. Tear-glands . • •

202

215, 216. Milk-glands .

203

217, 218. Central marrow of

human embryo

210

219. Hnman brain • • •

212

220. t) »» • • •

213

221-223. Lyre-shaped embryo

Chick ....

218

224-226. The five brain-blad-

ders of the hnman germ

220

827. The fiye brain-bladders of

Orcmiota • • .

222

228. Brain of Shark

222

229. Brain of Frog .

222

230. Brain of Eabbit

224

231. Nose of Shark

241

232-236. Derelopment of the

face in embryo Chick .

243

237. Nose and mouth cavities .

246

238-240. The face in the

human embryo • •

247

241. Human eye . .

250

242^ Development of the eyes

253

243.

256

244. Human auditory passage

260

246. Human auditory labyrinth

263

246-248. Development of the

ecur . . . •

264

240. Primitive skull with ear-

vesicles

264

260. Eudimentary ear-muscles

270

251, 252. Human skeleton

279

263. Human vertebral oolnmn

280

254. Neok-vertebra • •

281

265. Braast-vertebr» • •

281

26& LoBbAr-vertebm • •

181

navMM WAmm

267. Portion of noioohOTd . 286

258-260. Growth of the primi.

tive vertebral series in

embryo Chick • . 288

261. Longitudinal aeotion of

breast-vertebra . . 290

262. Transverse section of same 291

263. Intervertebral disc . . 291

264. Human skull ... 292

265. Head skeleton of Primi.

tive Fish . . .296

266. Primitive skull of Man . 297

267. Skeleton of fin of Cera*o<iu« 302

268. Skeleton of fin of AecM.

thiaa . . • .302

269. Skeleton of fin of Primi-

tive Fish . . .302

270. Skeleton of hand of Frog 302

271. Skeletonof hand of Gorilla 302

272. Skeleton of human hand . 302

273. Skeleton of hand of Mam-

mal .... 306

274. Oastrula of OVynthw . 313

275. Human stomach . • 817

276. Gastrula of Amphioxas . 321

277. Gastrula of Mammal . 821
278, 279. Human germ with

yelk-sac and allantois . 324

280. Intestine of Tv/rhellaria . 827

281. Intestine of Ascidia . 327

282. Intestine of Amphioxva . 328

283. Scales of Shark • . 832

284. 286. Intestine of embryo

D<^ with the intestinal

glands .... 834

286. Intestine with allantois . 388-

287. latMtiJM of hamaa g«na 888

XVI

LIST OF WOODCUXa

nevn

288. Liver of humAn germ

289. Nail-tissae . • •

290. Intestinal epithelium

291. Jelly-like tissue

292. Cartilaginous tis^e

293. Neuro-musonlar o«Ils .
294). NerT«>tiMae •

295. MuBole-tissne . •

296. Yasonlar tiMua • •

297. Blood-oelk

298. Blood.T«««l8 of » Worm .

299. HeiMl with blood-resselB

of Fiflk . . • •
300-302. Axteriai arobei
303-306. „ w

307-310. Developmen* of the

hesft . . • •
311-314. Derelopment of the

heart • * • •
315. Transverse section

through Haliphjrsema .

rAOB

FIOUKK

MA*

342

316. Rn^TUBnit at UrogmUkilia

400

862

317. Primitive kidney of JBdeWo.

862

stoma . . . ■

406

S63

318. Earliest primitive kidMj

863

rudiments

408

864

319, 320. Primitive kidneyt of

864

Mammals

409

864

821. Development of nrogoni*

865

tal system .

414

865

822, 823. » »

416

371

324-326. „ n
827. Female sexual organs of

416

875

Beaked Animal (OrrU-

877

thorhynehm)

418

878

828. Change of position of both
kinds of sexual glands

380

in human beings .
829. Development of the human

420

882

external sexual organs

422

380. Human egg-folliclefi

426

393

LIST OF GENETIC TABLES.

■«o*-

TABU rAtt

XTT. Systematic Sturey of pal»ontological periods ••• 11
Xni. Systematic Survey of palaeontological formations ... 12
XTV. Systematic Survey of the thickness of the forma-
tions ... ... ... •«• ... XSr

XV. Pedigree of Indo-Germanic languages .- ... 23

XVI. Systematic Survey of the most important stages in

the animal ancestral line of Man ... ... 44

XVII. Systematic Survey of the five first stages in the
evolution of Man (phylogenetic, ontogenetic, sys-
tematic^ ... ... ,., ... ... 7U

XVIII. Systematic Survey of the phylogenetic system of the

animal kingdom ... ... ... ... 92

XTX. Monophyletic pedigree of the a-nima.! kingdom ... 93
XX. Systematic Survey of the phylogenetic system of

Vertebrates... ... ... ... ... 120

XXI. Monophyletic pedigree of Vertebrates ... ... 121

XXn. Systematic Survey of the periods of human tribal

history ... ... ... „. ... 184

"y^TTI. Systematic Survey of the phylogenetic system of

Mammals, founded on the Gastraea Theory ... 187

XXTV. Monophyletic pedigree of Mammals ,,, m. 188

XXV. Pedigree of Apes ... ... ... ... 189

XXVI. Systematic Survey of the organ-systems <rf the human

UOiXjr ••• ««• «9, ««« ••• XifV

XXVIL Systematic Survey of the phylogenetic history of the

human skin-covering ... ... ... 229

XXVUL ^ Systematic Survey of the phylogenetic history of the

human nerve-system ... ••• ••• 2S0

34

* • •

Xvm LIST OF GENETIC TABLKa .

TABLC '

XXIX. Systematic Survey of the ontogeny of the Bkin and

nerve systems ... ... ... ... 232

XXX. Systematic Survey of the phylogeny of the human

nose ... tts ... ... ... JnSr

XXXI. Systematic Survey of the ontogeny of the human

eye ••• ... ... ... ... iSoo

XXXII. Systematic Survey of the phylogeny of the humui

ear .«• ••• ... •*• ... ^07

XXXni. Systematic Survey of the ontogeny of the human

t^cl*X ••« ••• ■•• ••• ••! ^00

XXXIV. Systematic Survey of the constitution of the human

SKeieron ... ... ... ... ... Jtio

XXXV. Systematic Survey of the phylogeny of the human

skeleton ... ... ... ... 309

XXXVI. Systematic Survey of the constitution of the human

intestinal system ... ... ... ... 330

XXXVII. Systematic Survey of the phylogeny of the human

intestinal system ... „. ..•. ... 346

XXXVm. Systematic Survey of the sequence, according to
age, of the human tissue-groups (phylogenetic
sequence of the tissues) ... ... ... 366

XXXTX. Systematic Survey of the sequence, according to
age, of the human organ-systems (phylogenetic
sequence of the organs) ... ... ,., 367

XL. Systematic Survey of the phylogeny of the human

vascular system ... ... ... ... 384

XLI. Systematic Survey of the phylogeny of the human

nearc ... ... ... ... ... ooO

XLn. Systematic Survey of the homologies of Wormis,
Articulated Animals {Arthropoda), Soft-bodied
Animals (Mollusca), and Vertebrates ... ... 387

XLm. Systematic Survey of the phylogeny of the human

urinary and sexual organs ... ... ... 428

XLIY. Systematic Survey of the homologies of the sexual

organs in the two sexes of TVIammals ... •* 431

THE EYOLUTION OF MAN

-••»•

CHAPTER XV.

THE DURATION OP 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 History. — 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
Palaeolithic Epoch. III. Secondary, or Mesolithic Epoch. IV. Tertiary,
or Caenolithic Epoch. V. Quaternary, or Anthropolithic Epoch. — The
Relative Duration of the Five Epochs. — The Results of Comparative
Philology as Explaining the Phylogeny of Species. — The Inter-relations
of the Main Stems and Branches of the Indo-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 Greneration.

"In vain as yet hat it been attempted to draw an exact line of demar(»tion
between historic and prehistoric times ; the origin of man and the period of
his first appearance pass back into indefinable time ; the so-called archaic
age cannot be sharply distinguished from the present age. This is the fate
of all geological, as of all historical periods. The periods which we dis-
iinguish are, therefore, more or less arbitrarily defined, and, like the div isious

3 THE EVOLUTION OF MAN.

in systematio natnral history, can only serve to bring the subject of onr
study better before ns and to render it more manageable ; bnt not to mark
real distinctions between different things." — Bernhard Cotta (1866).

OuE 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
these 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, befort

TntB EEQUIRED FOR THE DEVELOPMENT OF MAN. 3

beginning this task, it -stlU 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 difi'erence between the duration
of the germ-history of Man on the one hand, and of his
tribal history on the other. The brief period in which the
Ontogeny of the human individual takes place, bears no
proportion to the infinitely long period required for the
Phylogeny of the human tribe. The human individual
requires nine months for its perfect development from the
fertilized egg- cell to the moment at which it is born and
quits the mother's body. The human embryo, therefore,
passes through the whole course of its development in the
brief space of 40 weeks (usually in exactly 280 days).
Each man is really older by this period than is usually
assumed. When, for example, a child is said to be 9 J years
old, he is in reality 10 years old. For individual existence
does not begin at the moment of birth, but -at the
moment of fertilization. In many other Mammals the
duration of the embryonic development is the same as in
Man, e.g., the Ox. In the Horse and the Ass it is somewhat
longer, viz., from 43 to 45 weeks; in the Camel it is lo
months. In the largest Mammals the embryo requires a
much longer time for its complete formation within the
maternal body; in the Rhinoceros, for instance, 1|- year,
in the Elephant 90 weeks. In the latter case, therefore
{cestation 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 ; Kabbits 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 ftiU 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 Qgg}^^

In all cases, however, the duration of the Ontogeny
appears infinitely brief when compared with the enormous,
the infinitely long period during which the Phylogeny, or
gradual development of the ancestral series, took place.
This period is not to be measured by years and centuries,
but by thousands and millions of years. Many millions of
years must indeed have elapsed while the most perfect
vertebrate organism, Man, gradually developed from the
primaeval one-celled ancestral organism. The opponents of
the development theory, who regard this gradual develop-
ment of Man from lower animal forms, and his original
descent from a one-celled primitive animal as incredible,

DURATION OF HUMAN GERM-HISTOKY. 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 fi^ approximately their length in hundreds or in thousands
of years, or to establish any absolute measure of their,
duration, liut 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 Nej)tunian strata or sedimentary rock, i.e., all those
strata which have been deposited, as mud, at the bottom
of the ocean, or under fresh water. These stratified sedi-
mentary rocks — limestone, clay, marl, sandstone, slate, etc. —
which constitute the great mass of mountain-chains, and
which are often several thousand feet in thickness, afibrd
us data for estimating the relative lengths of the various
periods of the earth's history.

For the sake of completeness, I must say a few words as
to the development of the earth as a whole, briefly indicating
a few of the more prominent facts relating to this matter.
At the very outset we are confronted with the weighty
fact, that life originated on our planet at a certain definite
period. This is a proposition that is no longer gainsaid by
any competent geologist. We now know for certain that
organic life upon our planet actually began at a certain
time, and that it did not exist there from eternity, as
some have supposed. The indisputable proofs of this are
furnished, on the one hand, by physico-astronomical cos-
mogeny ; on the other, by the Ontogeny of organisms. Species
and tribes, like individuals, do not enjoy a perpetual
life.^^"^ 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 tliia
point I must refer the reader to Kant's " Allgemeine Natur*
geschicbte und Theorie des Himmels " and to the numerous
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 Medusae, the body contains even more than
99 per cent, of water, and not even one per cent, of solid
matter. No organism can exist and perform its vital
functions without water. Without water there is no life.

Water in the fluid state, which is, therefore, in-
dispensable for the existence of life, could not, however,
appear upon the earth until after the temperature of the
surface of the fiery globe had sunk to a certain point.
Before this it existed only in the form of steam. As
soon, however, as the first drop of water in a fluid state was
precipitated by cooling from the envelope of steam, it began
its geological action, and from that time to this it has
efiected continual changes in the modification of the hard

C7

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 ol
Organic Nature," the most important fact in the past history
of our earth is ooze, and the question a^ 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 a.^ ooze at the
bottom of the waters, and only afterwards hardened into
solid stone.

As has already been said, it is possible, ^y bringing
together and comparing the various rock-strata from many
places on the surface of the earth, to obtain an approximate
conception of the relative ages of these various strata.
Geologists have long agreed that there is an entirely definite
historical sequence of the various formations. The various
gi'oups of strata which lie one over another correspond to
successive periods in the earth's organic history, during
which they were deposited in the shape of mud at the
bottom of the sea. Gradually this mud was hardened into
solid rock. The latter, by alternate upheaval and depres-
sion of the surface of the earth, was lifted above the water,
and assumed the form of mountains. Four or five main
periods in the earth's organic history, answering to the
larger and smaller groups of these sedimentary rock-strata,
are usually distinguished. These main periods are sub-
divided into numerous subordinate or lesser periods. From
twelve to fifteen of the latter are usually assumed. (Cf.
Tables XII. and XIII, pp. H, 12.) The relative thick-

GEOLOGICAL TIMK 9

ness of the various groups of strata affords the means of
approximately estimating the relative length of these various
divisions of time. Of course we cannot say, "In a hun-
dred years on the average a stratum of a certain thick-
ness (say two inches) is deposited, and therefore a rock-
stratum of a thousand feet in thickness is 600,000
years old." For different rock-formations of equal thick-
ness may have occupied periods of very various length
in their deposition and consolidation. From the thickness
of the formation we may, however, approximately judge of
the relative length of the period during which it was
formed.

Of the four or five main periods of the earth's organic
history, our acquaintance with which is indispensable for
our Phylogeny of the human race, the first and oldest is
known as the Primordial, Archizoic, or Archilithic Epoch.
If we estimate the total thickness of all the sedimentary
strata as averaging about 130,000 feet, then 70,000 feet belong
to this first epoch — more than one half From this and other
circumstances we may conclude that the corresponding
Primordial or Archilithic Epoch must alone have been con-
siderably longer than the whole long period between the
close of the Archilithic and the present time. Probably the
Primordial Epoch was much longer than might appear from
the ratio of 7 : 6, which -we have given. This Epoch is divided
into three sub-periods, known as the Laurentian, Cambrian,
and Silurian, corresponding to the three principal groups of
sedimentary rock-strata which constitute the Archilithic
rocks. The enormous length of time required for the forma-
tion at the bottom of the primordial sea of these gigantic
strata, of over 70,000 feet in thickness, must, at all events.

10 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 oui
planet — viz., the Monera. From these, one-celled plants and
animals first developed — the Amoebae and many kinds of
Protista. During this same Archilithic Epoch, also, all the
invertebrate ancestors of the human race developed from
these one-celled organisms. We draw this conclusion from
the fact that towards the close of the Silurian period
a few remains of fossil Fishes are already to be found, viz.,
Selachians and Ganoids. These are, however, much more
highly organized and of later origin than the lowest
Vertebrates (the Amphioxus), or than the various skull-less
Vertebrates allied to Amphioxus, which must have lived
during this time. The latter must necessarily have been
preceded by all the invertebrate ancestors of man. Hence
we may characterize this entire epoch as the " age of man's
invertebrate ancestors;" or, with special reference to the
oldest representatives of the Vei'tebrate 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 PalaeoHthic,
Palaeozoic, or Primary Epoch, which is also separable into
three sub-periods : the Devonian, the Carboniferous, and the
Permian. During the Devonian Period the Old Red Sand-
stone, or Devonian system was formed ; during the Car-
boniferous, those great beds of coal were deposited which

( " )

TABLE XII.

Syetematio Surrey of the Palaeontological Periods, or the Gh?eat«r DiTiincaQ&

in the History of the Organic Earth.

I. First Epoch : The Archilithic, or Primordial Epoch,
(Age of Stall-less Animals and Seaweed Forestg.)

1. The Older Archilithic Epoch or Lanrentian Period

2. The Middle Archilithic Epoch • „ Cambrian Period.

3. The Later Archilithic Epoch „ Silurian Period.

n. Second Epoch : The Palaeolithic, or Primary Epoch,
(Age of Fishes and Fern Forests.)

4. The Older Palseolithio Epoch or Devonian Period.

5. The Middle Palaeolithic Epooh ,, Coal Period,

^. The Later Palaeolithic Epoch ,9 Permian Period.

m. Third Epoch : The Mesolithic, or Secondary Epoch.
(Age of Eeptiles and Pine Forests, Coniferce.)

7. The Older Mesolithic Epoch or Triassic Period.

8. The Middle Mesolithic Epoch i^ Jnrassic Period.

9. The Later Mesolithic Epoch „ Chalk Period.

rV. Fourth Epoch : The Csenolithic, or Tertiamf Epoch,
(Age of Mammals and Leaf Forests.)

10. The Older Caenolithic Epoch op Eocene Period.

11. The Middle Caenolithic Epoch ^ Miocene Period.

12. The Later Caenolithic Epoch „ Pliocene Period.

V. Fifth Epoch : The Anthropolithic, or Quatema/ry Epoch.
(Age of Man and Cultivated Forests.)

15. Tte Older Anthropolithic Epoch or loe Age, Glacial Period
14. The Middle Anthropolithic Epoch „ Post Glacial Period.

16. The Later Anthropolithic Epoch „ Period of Culture.

(The Period of Culture is the Historic Period, or Period of Tradition.)

( 12 )

TABLE XIII.

Systematic Survey of the Palaeontological Formations, or the FossiliferoiiB

Strata of the Earth's Crust.

Boeh-Qrou^8,

Systems.

Formations.

Synoruyms of
Formutions,

V. Quatemcmf

Qrov/pt

or

Anthropolithic

(Anthropozoic)

groups of strata '

IV. Tertiary
Qrowp,^
or

Csenolithic

(CsBnozoic)

groups of strata

XIV. Recent

(Anuvinm)
XIII. Pleistocene
(Diluvium)

{

36. Present
35. Recent
34. Post glacial
33. Glacial

XII. Pliocene ^ 32. Arvemian

5 32.
I 31.

(New tertiary) \ 31. Sub-Appenlne
XI. Miocene i 30. Falunian

(Middle tertiary) ( 29. Limburgian
„ -c, (28. Gypsum

,^' J^°^,^^® , ) 27. NummuUtic
(Old tertiary) | 26. London clay

/iX. Cretaceous.

III. Secondary
' Growp,

or

Mesolithic

(Mesozoic)

groups of strata

V

VIII. Jura

VII. Trias

25. White chalk

24. Green sand

23. Neocomian

22. Wealden

21. Portlandian

20. Oxfordian

19. Bath

18. Lias

17. Keuper

16. Muschelkalk

15. Bunter sand

II. Primary

Grou'p,

or

Palaeoli ihic

(Palaeozoic)

groups of strataf

VI. Permian f ^^ ^°^^tain
(New red sand- J (z^'Sf^)

' (_ 13. Red sandstone

12. Carboniferous

I. Primordial

Qrowp,

or

Archilithic

(Ai'chizoic)

groups of strata

V. Carboniferous
(Coal)

IV. Devonian
(Old red sand-
stone)

III. Silurian

II. Cambrian
I. Laarentiaa

sandstone
11. Carboniferous

limestone
10. Pilton
9. Ilfracombe
8. Linton

7. Indlow
6. Wenlock
5. Llandeilo
4. Potsdam
3. Longmynd
2. Labrador
1. Ottawa

Upper alluvial
Lower alluvial
Upper diluvial
Lower diluvial

Upper pliocene
Lower pliocene
Upper miocene
Lower miocene
Upper eocene
Middle eocene
Lower eocene

Upper cretaceous
Middle ci'etaceous
Lower cretaceous
The Kentish Weald
Upper oolite
Middle ool'te
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 Lanrentian
Lower

GEOLOGICAL PERIODS. 1 3

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
Bomewhat more, others considerably less. In any case,
this Palaeolithic Epoch, taken as a whole, is consider-
ably shorter than the Archilithic, but yet is considerably
longer than all the following Epochs taken together. The
strata deposited during this Primary Epoch supply fossil
animal remains in great abundance ; besides numerous
species of Invertebrates we find also very many Verte-
brates— Fishes preponderating. As early as the Devonian,
and even during the Carboniferous and the Peimian
Periods, there existed so great a number of Fishes, espe-
cially Primitive Fishes (Sharks) and Ganoids, that we may
designate the entire Palaeolithic Period as the Age of Fishes.
The Palaeozoic Ganoids especially are represented by a
large number of forms.

But even during this period some Fishes began to
accustom themselves to living upon the land, and thus gave
rise to the Amphibian class. Even in the carboniferous
system we find fossil remains of Amphibia — the earliest
terrestrial and air-breathing animals. In the Permian
Period the variety of these Amphibia becomes greater. To-
wards its close the first Amnion-animals, the tribal ancestors
of the true higher Vertebrate classes, seem first to appear.
These are a few lizard-like animals, of which the Protero-
saurus from the Cupriferous Slate at Eisenach is the best
known. The appearance of the most ancient Amnion
Animals (Amniota), to which the common parent-form of

14 THE EVOLUTION OF MAN.

Reptiles, Birds, and Mammals must have belonged, seems
in fact to be referred by these oldest reptilian remains back
to the ctose of the Palaeolithic Epoch. During this Epoch
the ancestors of the human race must accordingly have
been represented, first by true Fishes, then by Mud-Fishes
(Bipneusta) and Amphibia, and finally by the oldest
Amnion Animals, the Protamnia.

After the Palaeolithic Epoch comes a third main division
of the earth's organic history, known as the Mesolithic, or
Secondary Epoch. This is again distinguished into three
subdivisions — the Triassic, the Jurassic, and the Cretaceous
Periods. The approximate thickness of the strata-groups,
formed during these three Periods from the beginning of
the Triassic dowti to the end of the Cretaceous Period,
amounts in all to about 15,000 feet, not one half the thick-
ness of the Palaeolithic deposits. During this Epoch a very
great and varied development took place in all divisions of
the animal kingdom. In the vei*tebrate tribe especially a
number of new and interesting forms developed. Among
Fishes the Osseous Fishes (Teleostei) now first appear. But
the Reptiles surpass all others both in numbers and in
diversity of species — the most remarkable and the most
familiar forms being the gigantic extinct Dragons (Dino-
saurians), the Sea-Dragons (Halisaurians), and the Flying
Lizards (Pterosaurians). In reference to this predomiuance 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-Hke
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. 1 5

and with lizard's tail, belonging to this period (Odonr-
tomia Archoeopteryx). Finally, it was during this period
that there appeared upon the scene that most perfect and,
for us, most important vertebrate class, the mammalian
class. The oldest fossil remains of these have been found
in the most recent Triassic strata, viz., molar teeth of
a small insectivorous Pouched Animal (Marsupial). Numer-
ous remains occur somewhat later in the Jura system, and
a few in the chalk. All the remains of Mammals from this
Mesolithic Epoch with which we are acquainted belong to
the low Pouched Animal division ; and among these were
undoubtedly the ancestors of Man. On the other hand,
not a single undisputed relic has yet been discovered
throughout all this period of one of the higher Mammals
(Placentalia). This last division, of which Man is a member,
did not develop till later, in the immediately subsequent
Tertiary Epoch.

The fourth main division of the history of the organic
earth, the Tertiary, Csenozoic, or Csenolithic Epoch, was of
much shorter duration than the preceding. For the strata
deposited during this period are in all only about 8000 feet
in thickness. This Epoch, also, is divided into three sub-
divisions, known as the Eocene, Miocene, and Pliocene
Periods. During these periods the most diverse develop-
ment of the higher classes of plants and animals took place
and the fauna and flora of our globe now approached
nearer and nearer to their present character. The most
highly developed class of animals, that of Mammals, now
attained pre-eminence. This Tertiary Epoch may, therefore,
be called the age of Mammals. The most perfect section

of this class, the Placental Animals, among which is Man,

35

1 6 THE EVOLUTION OF MAN.

now first appeared. The first appearance of Man — or to
speak more correctly — the development of man from the
most nearly allied ape-form, dates probably either from fhe
Miocene or the Pliocene Period, — from the middle or the
latest section of the Tertiary Epoch. Perhaps, as is assumed
by others, Man strictly so-called, i.e., Man gifted with
language, first developed from the speechless man-like Apes,
in the subsequent Anthropolithic Age.

At all events, the perfect development and distribution
of the various races of Man dates from the fifth and last
main division of the organic history of the earth, and hence
this Epoch has been called the Anthropolithic, or Anthro-
pozoic, and also the Quaternary Epoch. It is true that, in
the present imperfect state of our palseontological and
prehistoric knowledge, we cannot solve the problem as to
whether the development of Man from the nearest allied
Ape-forms took place in the beginning of this Anthropolithic
Epoch, or as early as the middle or towards the close of the
preceding Tertiary Epoch. This much, however, is certain,
that the true development of human culture dates only
from the Anthropolithic Epoch, and that this latter con-
stitutes only an insignificantly small section of the entire
enormous period of time occupied in the development of
the organic earth. When we reflect upon this, it appears
absurd to speak of the brief span of man's period of cu:
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." 1 7

History, or more correctly, History of People, is itself
only the latter half of the Anthropolithic Epoch, while
oven the first half of this Epoch must be reckoned as a
prehistoric period. Hence this last main period, reaching
from the close of the Csenolithic 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 aU the
rest of the organic population of the globe, took place
during' its course.

Since the awakening of the human consciousness,
human vanity and human arrogance have delighted in
regarding Man as the real main-purpose and end of all
earthly life, and as the centre of terrestrial Nature, for whose
use and service all the activities of the rest of creation were
from the first defined or predestined by a "wise providence."
How utterly baseless these presumptuous anthropocentric
conceptions are, nothing could evince more strikingly than
a comparison of the duration of the Anthropozoic or Quater-
nary Epoch with that of the preceding Epochs. For even
though the Anthropolithic Epoch may embrace several hun-
dreds of thousands of years, how small is this time when
compared with the millions of years that have elapsed since
the beginning of the world's organic history down to the
first appearance of the human race !

K the entire duration of the organic history of the earth,
from the generation of the first Monera down to the present
day, is divided into a hundred equal parts, and if then,
corresponding with the relative average thickness of the
intervening strata-systems, the respective percentages are

1 8 THE EVOLUTION OF MAK.

assigned to the relative durations of the five main divisions
or Epochs, the latter will be found to be about as follows : —

I. Arohilithio, or archizoio (primordial) Epoch • • 68.6

IL Palaeolithic, or palaeozoic (primary) Epoch , • 82^

HI. Mesolithic, or mesozoio (secondary) Epoch • • 11.6

IT. Oasnolithic, or cenozoio (tertiary) Epoch . . • L8

T. Attthropolithio, or anthropczoio (quaternary) Epoch . OJi

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 (XIY.), in which the relative thicknesses of -
the strata systems deposited within these Epochs is repre- 1
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 I
with the immeasurable duration of those earlier epochs during
which Man did not exist upon this planet. Even the great
Csenozoic Epoch, the so-called Tertiary Epoch, during which
the 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.^^

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 palseontological 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 belbf« na verj m«ch easier. Tke science k thai

( 19 )

TABLE Xiy.

Syitematic Snrvey of the Neptunian fossiliferous strata of the earth with
reference to their relative sectional thickness (130,000 feet circa).

IV. CaBnolithic Strata,
circa 3000 feet.

Pliocene, >^ocene, Eooen*.

TTT. Mesolithio Strata.

TX. Chalk System.

Deposits of tht

Seofmdary Epooh, cixoa

YIIL Jnrassic System.

U»000 feet.

VIL Triassio System

n. FalsBolithic Strata.

VL Permian System.

Deposits of the

V. Coal System.

Primary Epoch, oirom

42,000 feet.

IV. Devonian System.

III. Silurian System, circa

L Archilithic Strata.

22,000 feet.

Deposits of the

II. Cambrian System, circa

Anmordial Epoch, oiroa

18,000 feet.

70,000 feet

I. Lam>entian System, circa

«0,000 feet.

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 lanixuages
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 whc have no conception of natural evolution. Such
brilliant results have been attained in Comparative Philology
that only one who is wilfully blind can fail to recognize the
natural evolution of language. The latter is necessarily
evident to the student of nature. For speech is a physio-
logical function of the human organism, developing simul-
taneously with its special organs, the larynx and the tongue,
and simultaneously with the functions of the brain. It is,
therefore, quite natural that in the history of the evolution
of languages, and in their whole system, we should find the
same correlations as in the history of the evolution of
organic species and in their whole system. The various

METHOD OF COMPARATIVE PHILOLOGY. 21

larger and smaller groups of speech-forms, which are distin-
guished in Comparative Philology as primitive languages,
fundamental languages, parent languages, derived languages,
dialects, patois, etc., correspond perfectly in their mode of
development with the various larger and smaller groups of
organisms classed in sj^stems 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, aie
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-
[)arative Anatomy and evolutionary history of languages is
treated by the same phylogenetic method which we employ
:n 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 MAX.

form of the primitive Vertebrates. The primitive Indo-
Germanic root-tongue first separated into two chief stems ;
the Slavo-Germanic and the Ario-Romanic main-trunks.
The Slavo-Germanic then branched into a primitive German
and a primitive Slavo-Lettic tongue. Similarly, the Ario-
Romanic split up into a primitive Arian and a primitive
Grseco-Romanic language (p. 23). If we continue our
examination of this pedigree of the four primitive Indo-
Germanic languages, we find that the primitive Germanic
tongue divided into three chief branches — a Scandinavian,
a Gothic, and a Teutonic branch. From the Teutonic branch
proceeded, on the one hand. High German, and, on the
other hand. Low German, to which latter belong the various
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 Graeco-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
^m the primitive Arian language.

( »3 )

TABLE XT.

Pedigree of the Indo-Germamc Langtiagwi.

Lithnanians Ancient Prussians
Letts

Anglo-Saxons

Baltic BaoM

Low Germans

Netherlandere

High Germaai

Ancient Saxonfl

Serbians
Polish
Czechs

West Sclaves

Bassians
South S Clares

South-east SolaveB

Sclaves

Bazong

FriesioDB

Low Germaos

Boandinariana

Goths Oemuuui

PrimitlTe German!

Sclavo-Letts

Bomans

Ancient Britinli

Ancient Scotch

Irish

GhMiis

Latins ^

Gktels

Brittanese

I

itm MM*

Italians

Eelti

Solavo-Germani

Italo-Eetei

Albanese

Greeks

Primitive Thracittns
Indians Iranians

Ario-Bomaaa

I

I»do-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 natural science. It, indeed, long ago anticipated in its
own province the ph^dogenetic 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 more vivid would
be the results if even the first elements of Comparative
Philology were taught instead of the distasteful composi-
tion of Latin exercises in Ciceronian style !

I have entered with this detail into the " Comparative
Anatomy" and the history of the evolution of languages,
because it is unsurpassed as a means of illustrating the
Phylogeny of organic species. We find that in structure
and in development these primitive languages, parent
languages, derived languages, and dialects, correspond
exactly like the classes, orders, genera, and species of the
animal kingdom. The " natural system " is in both cases
phylogenetic. Just as Comparative Anatomy, Ontogeny, and
Palaeontology afibrd certain proof that all Vertebrates,

MAN DESCENDED FKOM EXTINCT FORMS. 2$

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.^^
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 museuma 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 theii
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.^^^

EVOLUTION OF MAJC.

There is no doubt that Man is descended from an extinct
mamTnalian form, which, if we could see it, we should
certainly class with the Apes. It is equally certain that
this primitive Ape in turn descended from an unknown
Semi-ape, and the latter from an extinct Pouched Animal
But then it is beyond a doubt that it is only in respect of
essential internal structure, and on account of their similarity
in the distinctive anatomical characters of the order, that all
these extinct ancestral forms can be spoken of as members
of the yet extant mammalian orders. In external form, in
generic and specific characters, they must have been more or
less — perhaps even greatly — different from all living repre-
sentatives of the orders to which they belonged. For it
must be accepted as a quite universal and natural fact in
phylogenetic evolution that the parent-forms themselves,
with their specific characters, became extinct at a more or
less distant period. Those extant forms which come nearest
to them, yet differ from them more or less, perhaps even
very essentially. Hence in our phylogenetic researches
and in our comparative view of the still living divergent
descendants all we can undertake to do is to determine
how far , the latter depart from the parent-form. We may
quite confidently assume that no single older parent-form
has reproduced itself without modification down to our time.

We find this same state of things on comparing various
extinct and living languages, which have sprung from one
common primitive tongue. If, from this point of view, we
examine the genealogical tree of the Indo-Germanic
languages, we may conclude, on d priori grounds, that all
the earlier primitive languages, fundamental languages, and
ancestral languages, from which the living dialects are

COMPARATIYE METHODS OF PHILOLOGY AND ZOOLOGY. 2^

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 long
been altogether extinct, ^ are also the primitive Arian and
Graeco-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-
Gtermanic 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 palaeontological materials of Philology, the
ancient monuments of extinct tongues, have been far better
preserved than the palseontological 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
AmphioxuA, or the Shark of our day, or any extant species

2$ THE EVOLUTION OF MAN.

of Amphibian is an actual parent-form of the highei Vei^
tebrates and of Man. On the contrary, this important
assertion must be clearly understood to mean, that the living
forms, which have been mentioned, are side branches, which
are much more nearly allied, and similar to the extinct
common parent-forms, .than any other known animal forms.
In their internal characteristic structure they remain so
similar to the unknown parent-forms, that we should class
them both in one order, if we had the latter before us in
a living state. But the direct descendants of the primitive
forms have never remained unmodified. Hence it is
quite impossible that among the living species of animals
we should find the actual ancestors of the human race in
their characteristic specific forms. The essential and charac-
teristic features, which more or less closely connect living
forms with the extinct common parent-forms, are to be
found in the internal structure of the body, not in the
external specific form. The latter has been much modified
by adaptation. The former has been more or less retained
by 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 smgle
primitive V^ertebrate species. The genealogical line of the
Vertebrates, therefore, is also that of Man.

AMCEBOID ANCESTOBS. 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 Gastrsea, that most important animal form in which
we recognize the simplest conceivable prototype of an animal
with two germ-layers. The Gastrsea itself must have ori-
ginated from among those lowest of all simple animal forms,
which are now included by the name of Primitive Animals
[Protozoa). Among these we have already considered that
primitive form which possesses most interest for us — the
one-celled Amoeba, the peculiar significance of which depends
on its resemblance to the human egg-cell. Here we have
reached the lowest of those impregnable points, at which
the value of our fundamental law of Biogeny is directly
found, and at which, from the embryonic evolutionary stage,
we can directly infer the extinct parent-form. The amoeboid
nature of the young egg-cell, and the one-celled condition
in which each Man begins his existence as a simple parent-
cell or cytula-cell, justify us in affirming that the oldest
ancestors of the human race (as of the whole animal
kingdom) were simple amoeboid cells.

30 THE EVOLUTION OF MAN.

Here arises another question : " Whence, in the begin-
ning of the organic history of the earth, at the commence-
ment of the Laurentian period, came the earliest Amoebae ? "
To this there is but one reply. Like all one-celled organ-
isms, the Amoebae 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 cequivoca).

We have neither time, nor indeed have we any occasion,
to discuss at length the weighty question of spontaneous
generation. On this subject I must refer you to my
" History of Creation," and, especially, to the second book
of the Gen^relle Morphologie^ and to the discussion on
Monera and spontaneous generation in my " Studien tiber
Moneren imd andere Protista." ^^ 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 BO 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-'
r(xUo spontanea) and assume it as a necessary h3rpo-
thesis in explanation of the first beginning of life upon

SPONTANEOUS GENERATIOlf. 3 J

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, liave been formed, by a process purely
chemical, from purely inorganic carbon combinations, that
very complex nitrogenized carbon compound which we call
plasson, or " primitive slime," and which is the oldest material
substance in which all vital activities are embodied. In
the lowest depths of the sea such homogeneous amorphous
protoplasm probably still lives, in its simplest character, under
the name of Bathybius.^^'^ 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 imder the term "spontaneous generation," and
r.6

32 • THE EVOLUTION OF MAN.

because some have supposed that the problem could be
solved by means of the crudest experiments. The doctrine
of spontaneous generation cannot be experimentally refuted.
For each experiment with a negative result merely proves
that under the conditions (always very artificial) supplied by
us, no organism has been produced from inorganic combina-
tions. Neither can the theory of spontaneous generation
be experimentally proved unless great difficulties are over*
come ; and even if in our own time Monera were produced
daily by spontaneous generation — as is very possible — yet
the absolute empiric proof of this fact would be extremely
difficult — indeed, in most cases impossible. He, however,
who does not assume a spontaneous generation of Monera,
in the sense here indicated, to explain the first origin of life
upon our earth, has no other resource but to believe in a
supernatural miracle ; and this, in fact, is the questionable
standpoint still taken by many so-called " exact naturalists,"
who thus renounce their own reason.

Sir William Thomson has indeed tried to avoid the
necessary hypothesis of spontaneous generation by assuming
that the organic inhabitants of our earth originally de-
scended from germs which proceeded from the inhabitants
of other planets, and which, with fragments of the latter,
with meteorites, accidentally fell on to the earth. This
hypothesis has met with much applause, and was even
supported by Helmholtz. Friederich Zoellner, an acute
physicist, has, however, refuted it in his excellent natural-
philosophical work " Ueber die Natur der Cometen," a
critical book containing most valuable contributions to the
history and theory of knowledge.^^'' 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 difierentiated 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 Moneea to the Gaste^a.

Relation of the General Inductive Law of the Theory of Descent to tbo

Special Deductive Laws of the Hypotheses of Descent. — Incompleteness
of the Three Great Records of Creation : Palaeontology, Ontogeny, and
Comparative Anatomy. — Unequal Certainty of the Various Special
Hypotheses of Descent. — The Ancestral Line of Men in Twenty-two
Stages : Eight Invertebrate and Fourteen Vertebrate Ancestors. — Distri-
bution of .these Twenty-two Parent-forms in the Five Main Divisions of
the Organic History of the Earth. — First Ancestral Stage : Monera. —
The Structureless and Homogeneous Plasson of the Monera. — DiflFeren-
tiation of the Plasson into Nucleus, and the Protoplasm of the Cells. —
Cytods and Cells as Two Different Plastid-forms. — Vital Phenomena
of Monera. — Organisms without Organs. — Second Ancestral Stage :
Amoebae. — One-celled Primitive Animals of the Simplest and most Un-
differentiated Nature. — The Amoeboid Egg-cells. — The Egg is Older than
the Hen. — Third Ancestral Stage : Syn- Amoeba, Ontogenetically repro-
duced in the Morula. — A Community of Homogeneous Amoeboid Cells. —
Fourth Ancestral Stage : Planaea, Ontogenetically reproduced in the
Blastula or Planula. — Fifth Ancestral Stage : Gastraea, Ontogenetically
reproduced in the Gastrula and the Two-layered Germ-disc. — Origin of
the Gastraea by Inversion (^invagination of the Planaea. — Haliphysema
and Gastrophysema. — Extant Gastraeads.

" 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

lor one's self. How slowly progress is made in the knowledge eren of self-
evident matters, especially when respectable authorities disagree, I myself
nave experienced sufficiently." — Kael Ernst Baer (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, Palaeontology, Comparative Anatomy,
Dysteleology, Chorology, the (Ekology of organisms, all the
important general laws, which we infer from multitudinous
phenomena of these difierent 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 betwe^i all

36 THE EVOLUTION OF MAN.

fchese 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 oi 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.^^

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 liuman race,
lies between two radically different assumptions : We must
either accustom ourselves to the idea that all the various
species of animals and plants, Man also included, ori-
ginated independently of each other by the supernatural
process of a divine "creation," which as such is entirely
removed from the sphere of scientific observation — or we
are compelled to accept the theory of descent in its entirety,
and trace the human race, equally with the various animal
and plant species, from an entirely simple primaeval parent-
form. Between these two assumptions there is no third

FAITH OR SCIENCE. 3/

course. 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 amono^ these we can aofain
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 hyi^otheses of descent, wliich are
founded entirely on special deductive inferences, is very
unequal Several r>^ ^hese conclusions are akeady fully

38 THE EVOLUTION OF MAN.

established ; others, on the contrary, are most dou]:>tful ; 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 relative certainty of
the special (deductive) hypothesis of descent. We can
never in any case prove the whole ancestral line of an-
cestors of an organism with the same certainty with which
we regard the theory of descent as the only scientific expla-
nation of the organic forms. On the contrary, the special
proof of all separate parent-forms must always remain
more or less incomplete and hypothetical. That is quite
natural. For all the records of creation upon which we
tely are in a great measure incomplete, and will always
lemain incomplete; just as in the case of Comparative
[^hUology.

Above all, Palaeontology, the most ancient of all records
of creation, is in the highest degree incomplete. We know
that all the petrifications with which we are acquainted
form but an insignificantly small fragment of the whole
number of animal forms and plant forms which have ever
existed. For each extinct species obtained by us in a petrified
condition, there are at least a hundred, probably thousands
of extinct species which have left no trace of their existence.
This extreme and most deplorable defectiveness of the palseon-
tological record of creation, upon which it is impossible to
insist too strongly, is very easily accounted for. The very
conditions under which organic remains become petrified
neoeesitaie it. It is also partly explicable as the result of

INCOMPLETENESS OF THE BIOLOGICAL RECORDa 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 coimt
less petrifications which are hidden in the huge moun-
tain chains of Asia and Africa, we knov/ 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 palseontological 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 OP 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." 4I

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-
sp&nding phylogenetic forms, it will have been understood
that some germ-forms may with certainty be regarded as
reproductions of corresponding parent-forms. We recog-
nized the human egg-cell and the parent-cell which results
from the impregnation of the latter as the first and most
important form of this kind.

From the weighty fact that the egg of the human being,
like the egg of all other animals, is a simple cell, it may be
quite certainl}^ inferred that a one-celled parent-form once
existed, from which all the many-celled animals, Man in-
cluded, developed.

A second very significant germ-form, which evidently
reproduces a primaeval parent-form, is the germ-vesicle
(Blastula), a simple hollow sphere, the wall of which con-
sists of a single cell-stratum. A third extremely import-
ant form in germ-history, which may be quite safely and
directly referred back to the tribal history, is the true Gas-
trula. This most interesting larval form already exhibits
the animal body composed of two germ-layers, and fur-
nished with the fundamental primitive organ, the intestinal
canal Now, as the same two-layered germ-condition, with
the primitive rudiment of the intestinal canal, is common to
all the other animal tiibes (with the single exception of the
Primitive Animals, Protozoa), we may certainly from this
mfer a common parent-fonn of similar construction to the
Gastnila, the Gastrijea. Equally important in their bearing
on the Phylogeny of Man, are the very important ontoge-
netical form conditions which correspond to certain Worms,

4^ THB KTOLTJTION OF MAN.

Skull-less Animals (Acrania), Fishes, etc., etc. 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 whiioh
are satisfactorily explained by reasons which have already
been named, especially by the incompleteness of Palaeon-
tology, of Comparative Anatomy, and Ontogeny.

In the first attempts to construct the human ancestral
line, which I made in my Generelle 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 Palaeolithic Epoch, three
within the Mesolithic Epoch, and four within the Caenolithic
Epoch. In the last, the Anthropolithic Age, Man already
•ziBted.

THE TWENTY-TWO ANCESTRAL STAGES. 43

K we would now undertake the diflBcult attempt to dis-
covei 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
pthilMophy.

( 44 )
TABLE XVI.

Systematic Survey of the most Important Stages in the Animal

Ancestral Line of Man.

M N = Boundary between the Invertebrate and the Vertebrate Ancestors.

Epochs of the
Organic

History of the
Earth.

Geological Periods

of the

Organic History

of the Earth.

Animal
Ancestral Stages

«^
Man.

Nearest
Living Relative*
of the
A ncestral Stages.

L

Archilithic

or

Primordial

Epoch

1. Lanrentian Period
^ 2. Cambrian Period
3. SUorian Period

■

1. Monera
{Monera)

2. Oldest Amoebae

3. Amoeboid Societies
{Synamcebia')

4. Ciliated planulae

{^Plani^adrp^

5. Primitive Intes-
tinal animals
'yOastraadf^)

6. iVimitive Worms
(^Archelminthes')

7. Soft- worms
(Scolecida)

8. Chorda animals
(jChordonia)

M-

9. Skull-less animals

(^Acrania)
10. Round-mouths

(^Cycloytonii)

11. Primitive Fishes

{^Selachii)

I

Bathy})iu8

Protamoeba

Simple Amoebae

(Automoeba)

Morula larvae
Blastula larvtt

Gastrula larvae

Gliding Worms
(^7\irbellaria)
? Between the glid-
ing worms and the
Sea-squirts
Sea-squirts (Ascidice]
(Appendicularia)

N

Lancelets

(Amphioxi')

Lampreys

{Petromyzonta')

Sharks

{Squalacei)

n.

Palaeolithic

or

Primary

Epoch

4. Devonian Period

5. Coal Period

6. Permian Period

12,

Salamander Fishes
(^Dipneusta)

13. Gilled Amphibia
i^Sozobranchia)

14. Tailed Amphibia
jSozura)

I

Mud fish

{Protoptera)

Siren {Proteus)

and AxolotI

(Siredon)
Water-newt

(Triton)

m.

Mesolithic

or

Secondary

Epoch

1. Triassic Period

8. Jurassic Period

9. Chalk Period

{

15. Primitive Am-

niota

{Protamnia)

16. Primitive Mam

mals J

(Promammalia) (

17. Pouched Animals j
(Marsupialia) \

? Between Tailed
Amphibians and
Beaked animals

Beaked animals
{Monotrema)

Pouched Rate
(Didelphyes)

TV.

Osnolithic

or

Tertiary

Epoch

l(V, Eocene Period

11. Miocene Period

12. Pliocene Period

18. Semi-Apes (

(Prosimios) \

19. Tailed Narrow- j

nosed Apes )

20. Men-like Apes or j"
Tail-less Narrow- <

nosed Apes. (

21. Speechless Men or i
Ape-like Men (

Lori {Stenopa)

Maki ( Lemur)

Nose Apes

Holy Apes

Gorilla, Chimpao*

zee, Orang,

Gibbon

Cretins or Micfo*

cepbali .

▼.

Quaternary
Xpooh

u

Diluvial Period
AUuvlAl Period

32. Men capaUe
Bpeech

"I

Australians and
Papoans

MONERON AND MORULA. 45

The soft slimeKke plasson-substance of the body of the
Moneron is commonly called " pi^otoplasma," and identified
with the cell-substance of ordinary animal and plant cells.
As, however, Eduard van Beneden, in his excellent work
upon the Gregarinae, first clearly pointed out, we must,
strictly speaking, distinguish thoroughly between the
plasson of cytods and the protoplasm of cells. This dis-
tinction is of special importance in its bearing on the history
of evolution. As was before incidentally mentioned, we
must assume two different stages of evolution in those ele-
mentary organisms, which, as formative particles or plastids,
represent organic individuality of the first order. The older
and lower stage is that of the cytods, in which the w^hole 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 diff'erent
kinds of albuminous substances, into the inner cell-kerne]
(nucleus), and the outer cell-substance (profoplasma), 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
instruments, we yet find that all the parts are completely
homogeneous. These Monera are, therefore, in the strictest
sense of the word, '' organisms without organs ; " or even, in
a strictly philosophical sense, they might not even be called
" organisms," since they possess no organs, since they are
not composed of various particles. They can only be called
organisms, in so far as they are capable of exercising the

46 THE EVOLUTION OF MAN.

organic phenomena of life, of nutrition, reproduction, sensa-
tion, and movement. If we tried to construct, 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.

Fig. 163. — A Moneron (Protamoeha) in the act of reprodnction : A, 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 Protamoeba (Fig. 163),
the formless body, during its movements, invariably de-
velops only a few, short, and blunt processes, which project
like fingers, slowly altering their form and size^ but never
branching. In other Monera, on the other hand {e.g.,
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 BATHTBIUS. ^y

twining their free moving ends, so as to form a net. Huge
masses of such slime-nets crawl upon the deepest bottom
of the sea {Bathyhius, Fig. 164). Within these soft sHme-
like plasaon-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 smaUest par-
ticles, or molecules, of the Moneron-body, called "plas-
tidules,"i86 displace each other, change their relative
positions, and thus efiect 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, reaUy homogeneous and structureless;

45 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 Protamoeba (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 ((7). 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 (VaTripyreUa), or into a
large number of smaller globules {Protomonas, Protomyxa ;
cf Plate I. in the " Natural History of Creation "). After
a time, these globules begin to move, split the integument
by their movement, and emerge; after which they float
about by means of a long, thin, thread-shaped process.
Each again passes by simple growth into the mature form.
Thus, it is possible to distinguish different genera and
species of Monera, on one hand, by the form of the different
processes of the body, and, on the other hand, by the
different kind and manner of reproduction. In the appendix
to my monograph of the Monera I enumerated eight genera
and sixteen species ("BioL Studien," voL i p. 182). The

THE MONERON AND BATHYBIUS.

49

most remarkable of all Monera is the Bathybius, which was
di^co veered by Huxley in 1868 (Fig. 164). This wonderful
Muneron lives in tlie dee])est parts of the sea, especially in

Pio. 164. — Bathybius Haeckelii (Huxley^. A Boiall piece of the formlesB
ind continually changing plasson-net of this Moueron from the Atlantic
f )cean .

tlie 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 amoeboid, movements of these formless pieces of
plasson, which were first observ^ed by the English zoologists
Carpenter and Wyville Thomson, have recently been again
observed by the German Arctic voyager, Emil Besaels, in
the Bathybius of the coast of Greenland ^^

The origin and importance of these huge w^ww^ of
living, formless plasson-bodies in the lowest depths of the

50 THE EVOLUTION OF MAK.

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 cy tod-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 undifieren-
tiated cell-form, which, at the present time, still leads an
independent solitary life, as the Amoeba. For the first and
oldest process of organic difierentiation, which aflfected the
homogeneous and structureless plasson-body of the Monera,
caused the separation of the latter into two difierent sub-
stances ; an inner firmer substance, the kernel, or nucleus,
and an outer, softer substance, the cell-substance, or
protoplaama. By this extremely important separative pro-
cess, by the difierentiation 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 tnis manner, by the difierentiation 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 difier-
entiation in Ontogeny. It will be remembered that in the

THE MONERULA.

51

egg-cell of animals, either before or after fertilization, the
original kernel disappeared. We explained this phenomenon
as a reversion or atavism, and assumed that the egg-cell,
in accordance "with the law of latent heredity, first fails
back into the kernel-less, cytod stage (Fig. 165). It is 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 kemel-less cytod-con-
dition, intermediate between the egg-celi and the parent-
cell, is an interesting germ-form, because, in accordance
with the fundamental law of Biogeny, it reproduces the
origina], oldest parent-form of the Moneron; we therefore
call it the Monerula, (Cf vol. i. pp. 178-183.)

f Fig. 165.— Monerula of Mammal (Rabbit). The fertilized egg-cell after
the loss or the nucleus is a simple ball of protoplasri (d). The outer covering
of the latter is formed bj the modified zona pellucida (z) together with
a rnucous layer (h) secreted on to the outside of the latter. In this a few
sperm. cells are still visible ^s).

Fig. 166.-Parent-cell (CyUda) of a Ma'_>imal (Eabbit) : k, parent,
kernel ; n, nucleolus of the latter ; p, protoplasm of the parent-cell ; z,
modified zona pellucida ; s, sperm-cells ; h, outer albuminous covering.

53 THE EVOLUTION OF MAN.

We have already explained the one-celled germ-form,
which we see in the original egg-cell and the parent-cell
which is originated by the fertilization of the egg-cell, aa
the reproduction of a one-celled parent-form, to which we
ascribed the organization of an Amoeba (cf Chap. VI.). Fcmt
the Amoeba, 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
Amoebae, 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 egg-cell," As evidence of the striking cor-
respondence of the two cells, it was incidentally men-
tioned that in the case of some Sponges the real eggs of
these animals were formerly described as parasitic Amoebse.
Large one-celled Amoeba-like organisms were seen creeping
about in the interior of the Sponge, and were mistaken for
parasites. It was only afterwards that it was discovered
that these "parasitic Amoebae " (Fig. 168) are really the eggs
of the Sponge, from which the young Sponges develop.
These egg-ceUs of the Sponge are, however, so like the
true common Amoebae (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

A^KEB^.

53

reference to the primseval ancestral form of the Amoeba,
directly enables us to give a definite answer to the old hu-
morous riddle : Which was first, the egg or the hen ? We can

Fig. 167. — A crawling Amoeba (mucli enlarged). The whole organism has
vthe form -value of a simple naked cell and moves about by means of change-
able proceSses, which are extended from the protoplasmic body and again
drawn in. In the inside is the bright-coloured, roundish cell-kernel or
nucleus.

Fig. 168. — Egg-cell of a Chalk-Sponge {OVynthus). The egg-cell creeps
about in the body of the Sponge by extending variable processes, like those
of the ordinary Amoeba.

now very simply answer this Sphinx-question, with which
our opponents try to shake or even to refute the Theory of
Evolution. The egg existed much earlier than the hen. Of
course it did not exist in the form of a bird's egg, but as an
undifferentiated amoeboid cell of the simplest form. The
egg existed independently during thousands of years as a
simplest one-celled organism, as the Amoeba. It was only
after the descendants of these one-celled Primitive Animals
had developed into many-celled animal forms, and after
theso had sexually difierentiated, that the egg, in the present
physiological sense of the word, originated from the amoe-

54 THE EVOLUTION OF MAN.

boid cell. Even then, the egg was first a Gastrgea-egg, then
a Worm-egg, then an Acrania-egg, then a Fish-egg, an Am-
phibian-egg, a Reptile-egg, and lastly, a Bird-egg. The egg
of the Bird, as it now is, is a most complex historical pro-
duct, the result of countless processes of heredity, which
have occurred in the course of many millions of years.^^

The fact that this primitive egg-form, as it first appears
in the ovary of the most dissimilar animals, is always of
one form, an undifferentiated cell, of the simplest amoeboid
character, has already been pointed out as an especially
important phenomenon. In this earliest young condition,
immediately after the individual egg-cell has originated in
consequence of a separation of the cells of the maternal
ovary, no essential difference is recognizable in the egg-cells
of the most dissimilar animals. (Cf. Fig. 10, voL i. p. 134.) It
is not till later, when the primitive egg-cells, or the *primitive
eggs (jprotova), have absorbed different kinds of nutritive
yelk, and have surrounded themselves with variously fornled
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 diff^erent con-
ditions of existence both of the egg itself and of the animal
which forms the egg.

The two first and oldest ancestral forms of the human
race, which we have now considered, the Moneron and the
Amoeba, are, considered from a morphological point of view,
simple organisms and individuals of the first order, Plastids.
All subsequent stages in the ancestral chain are, on the

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 Synamoebse,
we must rank as the third stag^ 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

Fig. 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 dirisioii, into two cells (A), then into
four (B), then into eight (C), and, lastly^ iato very numerous cleavage-
cells (D).

the Morula (Fig. 165), the parent-cell breaks up, by repeated
division, into numerous cells. We have already minutely
examined this important process of
egg-cleavage, and have found that all
the various modes of the latter are
modifications of a single mode, that
of original or primordial cleavage.
(Cf. Chap. YIIL, p. 188.) In the Ver-
tebrate line this palingenetic form of
egg-cleavage has been accurately re- Fig. 170.-Muibeny.

°° ° •^ germ, or morula.

56 THE EVOLUTION OF MAN.

fcained to the present time only by the Amphioxus, while
all other Vertebrates have assumed a modified kenogenetic
form of cleavage. (Cf. Table III., voL i. p. 241.) The latter
certainly originated at a later period than the former, and
the egg-cleavage of the Amphioxus is, therefore, extremely
interesting (vol. i. p. 442). In this the parent-ceU first
parts into two similar ceUs, the two first cleavage-cells
(Fig. 169, A). From these, by continuous division, arise
4, 8, 16, 32, 64 ceUs, etc., etc. (Fig. 169). The final result of
this primordial cleavage was, we found, the formation of a
globular mass of cells, which was entirely composed of homo-
geneous, undifierentiated cells of the simplest characfter
(Figs. 170, and 171, E), 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-ceUed animal body in the same entirely simple
primitive condition in which, in the earlier Laurentian
primitive epoch, it first originated from the one-ceUed
amoeboid primitive animal form. The morula reproduces,
in accordance with the fundamental law of Biogeny, the
ancestral form of the Synamoeba. For the first cell-com-
munities, which then formed, and which laid the first
foundation of the higher many-ceUed animal body, must
have consisted entirely of homogeneous and quite simple
amoeboid ceUs. The earliest Amoebae lived isolated hermit
lives, and the amoeboid cells, which originated &om 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 conununities arose, owing to the fact that the

GERMINATION OF A CORAL.

$t THE EVOLUTION OF MAN.

Fia. 171. — Germination of a coral (Monoxenia Darwinii) : A, monerals i
B, parent-cell (cytula) ; C, two cleavage-cells ; D, four cleavage -cells ; E,
mulberry-germ (morula); F, vesicular germ {hlastula) ; G, vesicular germ
in section ; H, infolded vesicular germ in section ; J, gastrola in los^tn-
dinal section; K, gastrulay 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 sodalis, and the Lahyrinthulce 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 fi^om the Syn-
amoeba, we need only contiuue to trace the ontogenetic
modification of th^ 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 soHd 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 (bldstoderTna), and the hollow
globe the germ-membrane vesicle Q>lastula, or hlaato-
sphcera).

THE BLASTULA. 59

The interesting blastula germ-form is also of great sig-
nificance, for the modification of the mulberry-germ into the
germ-membrane vesicle takes place in the same way in a
great many animals of very dissimilar tribes ; for instance, in
many Plant-animals and Worms, in the Ascidians, in many
Star-animals (Echinoderma) and Soft-bodied Animals
{Mollusca)y 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 primaeval phylogenetic ancestral form, and that
all animals (with the exception of the lower Primitive
Animals) have originated from a common parent-form, the
structure of which was essentially that of a germ-mem-
brane vesicle. In many lower animals, the evolution of the
blastula takes place not within the egg-coverings, but out-
side this, free in water. Very soon after this, each cell of
the germ-membrane begins to extend one or more movable,
hair-like protoplasmic processes; owing to the fact that
these cilia or whips vibrate in the water the whole body
swims about (Fig. 171, F). This vesicular larva, the body-
wall of which forms a cell-stratum, and which rotates and
swims by means of the united vibrations of the cilia, has,
ever since the year 1847, been called the planula, or ciliated
larva. This designation, is, however, used by difierent
zoologists in difierent 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 exist
both in the sea and in fresh water, are formed essentially
like the blastula, and which, in a certain sense, may be con-
sidered as permanent or persistent blastula-forms, hoUov/
vesibles, the wall of which is formed of a single stratum of
ciliated homogeneous cells. These Planseads, or Blastseads,
as they may be called, are formed in the very mixed society
of the Flagellatae, 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- il/a^osp^OBra planula (Figs. 172, 173). The fully
developed body of this forms a globular vesicle, the wall of
which is composed of from thirty to forty vibratory homo*
geneous cells, and which swims about freely in the sea. After

Fig. 172. — The Norwegian Flimmer-ljall (MagospJicera planula), swim,
ming by means of its vibratile fringes ; seen from tte surface.

Fig. 173. — The same, in section. The pear-shaped cells are seen bound
together in the centre of the gelatinous sphere by a thread-like process.
Bech. cell contains both a kernel and a contractile vesicle.

FLDOfEB-LABYJB. 6l

having reached maturity the society dissolves. Each sepa-
rate cell still lives a while independently, grows, and changes
into a crawling Amoeba. This afterwards assumes a globu-
lar form, and encases itself by exuding a structureless
integument. The cell now has just the appearance of a
common animal egg. After it has remained for a 'time in
this quiescent state, the cell breaks up, by means of con-
tinued division, first into 2, then into 4, 8, 16, 82 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 Magosphsera-form froo
which we started. This accomplishes the entire life-history
of this remarkable Primitive Animal.-^*^

K we compare these permanent blastida-forms with the
freely swimming Flimmer-larvse or planula-condition, of
similar structure, of many other lower animals, we may
with certainty infer therefrom the former existence of a
primaeval and long-extinct parent-form, the structure of
which was essentially like that of the planula or blastula.
We will call this the Plansea, or Blastsea. 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 ceUs,
covered with cilia. Many difierent kinds and species of
Plansea-like Primitive Animals must certainly have existed
and formed a distinct class of Protozoa, which we may call
Flimmer-swimmers (Planceada). A remarkable proof of the
natural philosophical genius with which Karl Ernst Baer
penetrated into the deepest secrets of the history of animal
evolution, is that, as early as the year 1828 (ten years before
the cell-theory was established), he guessed the significance

62 THE EVOLUTION OF MAK.

of the blastosphgera, and, truly prophetically, insisted upon
it in his classical " Entwickelungsgeschichte der Thierc"
(vol. L 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 ceU-stratum, was
not then known. And yet Baer, in spite of the extreme
deficiency of his empiric grounds, ventured the bold state-
ment ; " At their first appearance all animals are perhaps
alike, and are merely hollow globes."

Next to the primaeval ancestral form of the Plansea, as
the fifth stage in the human pedigree, is the Gastrgea, a form
which arises from the Plansea. 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, liK). 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 GASTRSA. 65

of two cell-sti-ata, which are, in fact, the two primary germ-
layers, the animal skin-layer, and the vegetative intestinal
layer.

The ontogenetic origin of the gastmla from the blastula
at the present day affords us trustworthy intelligence as to
the phylogenetic origin of the Gastrsea from the Plansea.
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 inner,
uninverted portion (Fig. 171, /). Now, if guided by this
ontogenetic process, we wish to conceive the phylogenetic
origin of the Gastrsea in accordance with the fundamental
law of Biogeny, we must imagine that the one-layered cell-
society of the globular Plansea 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 ceUs undei*took the
function of movement and covering. Thus originated the
first division of labour among the originally homogeneous
cells of the Planaea.

The first result of this earliest histological difierentia-
tion was the distinction of two difierent 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

04 THE EyoLunoN or man.

of the hollow formed the inner or vegetative layer, aooom-
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. Thia
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, /), Medusa, Worms (Fig. 175, B) Star-
animals {Echinodermay (7), Articulated Animals {Arthro-
poda, D), Soft-bodied Animals (Molluaca, E), and Verte-
brates {¥), 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
leferable to that original palingenetic form (voL i. p. 231). The
gastrula proved to be a germ-form common to all cIhhhoi of |

DEVELOPMENT OF THE GASTR^A.

65

animals, with the exception of the Protozoa. This highly
important fact justifies the inference in accordance with the
fundamental law of Biogeny, that the various ancestral

Fig. 176.

Fig. 177.

Pig. 178.

Fig. 174

Fig. 179,

Fig. 174. — (A) Gastrnla of a Zoophyte (GastropTiysema), Haeckel.
Fig. 175. — {B) Gustrula of a Worm (Arrow-worm, Sagitta). After Kowa-
levsky.

Fig. 176. — (C) GabLiuia jf au Echiuoderm (Star-fish, TJraster). After
Alexander Agassiz.

Fig. 177. — (D) Gustrula of an Arthropod (Primitive Crab, Nauplius).

Fig. 178.— (E) Gastrnla of a Mollusc (Pond-snail, Limnoeus). After ,
Karl Rabl.

Pig. I79,_(2r') Gastrnla of a Vertebrate (Lancelot, Amphioxus) After
Kowalevsky.

66 THE EVOLUTION OF MAN.

lines of all these classes of animals have developed phylo-
genetically from the same parent-form. This most signifi-
cant primaeval parent-form is the Gastraea.

The Gastrea was at any rate already present in the
sea during the Laurentian period, and by means of its
vibratory fringe hurried about in the water, just like the
yet extant free-moving ciliated gastrulae of this age. Pro-
bably the primaeval Gastraea, which has been extinct 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 Gastraea had already acquired sexual re-
production, and did not only propagate its species asexually
(by division — ^bud-formation or spore-formation), as was
probably the case with the four preceding ancestral stages.
Presumably, single cells of the primary germ-layers as-
sumed the character of egg-cells, others that of fertilizing
seed-cells. (Cf Chapter XXV.) This hypothesis is founded
on the fact that sexual reproduction is yet met with in the
same simple forms in the lowest Plant- Animals (Zoophyta),
especially in the Sponges.

Two small animal forms are especially interesting in
their bearing on this aspect of the Gastraea theory. They
have as yet been little observed, but of all extant animals
they are most nearly allied to the primaeval .Gastraea, and
may therefore be called "the Gastraeads of the present
day." ^^ 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 GASTRJilADS.

67

of Haliphysema forms a most simple, cylindrical or egg-
shaped pouch, the wall of which consists of two cell-strata.
The cavity of the pouch is the stomach-cavity, and the

Pigs. 180, 181. — Haliphijsema primordiale, an extant Gastrsea-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 amoeboid eggs (e). The skin-layer Qi) below
is encrusted with grains of sand, above with sponge-spicnles.

opening at the top is the mouth-opening (Fig. 181, m).
The two cell-strata forming the wall of the pouch are the

68 THE EVOLUTION OF MAN.

two primary germ-layers. These most simple Plant- Animals
differ from the gastriila 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

Figs. 182, 183.— Ascnla of a Sponge (Olynthus). Fig. 182, from the ont-
side ; Fig. 18.S, in longitudinal section : g, primitive intestine ; o, primitive
mouth ; i, intestinal layer ; e, Bkin-layer.

BSFBODXTCnON IN THE OASTR^ADS. 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 Halipkysema.

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 Gastraea sexual repror
duction must have taken place in the same way. In the
Grastrseads, just as in Plant-animals, both kinds of sexual
cells — egg-cells and sperm-cells — must have formed in the
same person; the oldest Gastrseads must, therefore, have
been hermaphrodite. For Comparative Anatomy shows
that hermaphroditism, that is, the union of both kinds of
sexual ceUs 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.

( 70 )

TABLE XTII.

Systomatio Snryey of the five earliest evolutionary stages of the Haman Aiii
oestral Line, compared with the five earliest stages of Individual and
of Systematic Evolution.

Form-Value

Of the five earliest
Rtagea of the animal
bo47.

1.

Firtt Stage.

A quite simple cjtod
(a non*nacleated plas-
tid).

8«eond Sttige,

A. simple cell (•
Ducleated plastid).

Third Stage.

A qnite simple ag-
gregation of simple,
similar oella..

1

Fourth Stage.

A simple boUow globe,
filled with liquid, the
wall of which consists
of a single stratum of
bomogeneeuB cells.

Fifth Stage.

A hollow body, with
a single axis, the wall
»f which consists of
different cell-strata ;
with an opening at one
pole of Um

Phylogeny.

The five earliest
stages in the evolu-
tion of the tribe.

1.

Monera.

The oldest animal
Monera (originating
by spontaneous gene-
ration).

2.

Amoeba.
Oldest animal Amoeba.

3.

Synamoeba.

The oldest aggrega-
tion of animal AmoelMe.

4.

Flansea.

An animal hollow
globe, the wall of
which consists of a
single stratum of
ciliated cells.
(piaitcBO.')

5.

Gastrsea.

Parent-form of In-
testinal animals, or
Mftazoa. Simple pri-
mitive intestine with
primitive mouth. The
body-wall is formed
by the exoderm and
the entoderm.

Ontogeny.

The five earliest
stages in the evolu-
tion of the germ.

1.

Monerala.

A non - nucleated

animal-egg (after fer-

and after

the

tilization
loss of
vesiele).

germ-

I

2.

Cytnla.

A nucleated, ferti-
lized animal-egg (" first
cleavage globule ").

3.

Morula.

•• Mulberry-germ."
A globular mass of
cleavage-cells.

4.

Blastnla.

A hollow globe, the
wall of which consists
of a single stratum
of homogeneous cells
(the Planula of lower
animals).

(blastosptuera.)

5.

Oastmla.

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.

1.

Monera.

Protamoeba, Bathy-
bius, and other extant
Monera.

2.

Amoeba.

Extant Amoiba.

L^byrinthnla.

A mass of similar,
one - celled primitive
animals.

4.

Magosphaera.

A hollow globe, the
wall of which consists
of a single stratum of
homogeneous ciliated
cells.

5.

Ealiphysema.

A quite simple plant-
animal. An unarticu-
lated uniaxial person,
the body-wall of which
consists of the exoderm
and the entoderm.

, CHAPTER XVIL
THE ANCESTRAL SERIES OF MAN.

II. Feom the Primitive Woem to the Skulled Animal.

rhe Foar Higher Animal 'Jribes are descended from the Worm Tribe. — Tlu
Descendants of the Gastraea ; in one direction the Parent Form of Plant-
Animals (Sponges and Sea-Nettles), in the other the Parent Form of
Worms. — Radiate form of the former, Bilateral form of the latter. — The
Two Main Divisions of the Worms, Accelomi and 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 Coelomati from the Accelomi. — Mantled Animals
{Tunicata) and Chorda- Animals (Chordoma). — 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. — Eighth Stage : Chorda- Animals (Chor-
donia). — Ascidian Larva exhibits the Outline of a Chorda- Animal. —
Construction of the Notochord. — Mantled Animals and Vertebrates as
Diverging Branches of Chorda- Animals. — Separation of Vertebrates from
the other Higher Animal Tribes (Articulated Animals, Star-Animals,
Soft-bodied Animals). — Significance of the Metameric Formation. —
Skull-less Animals (Acrania) and Skulled Animals (Cro/niota). — Ninth
Ancestral fetage : 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 (Myzinoidoe and PetromyzonidcB).

" Not like the gods am I ! Full well I know |
But like the worm which in the dust must go,
And, finding in the dust his life and weal,
Is crashed and buried by the traveller's heel.—

79 THB EYOLUnON OF MAN.

Wlij do«t tlion grin at me, thou hollow sknll f

As though of old thy brain, like mine, was Tezed,

Had looked to find bright day, but in the twilight dvll.
In Maroh for truth, was sad and sore perplexed ! "

Both in prose and in poetry man is very often compared
to a worm. " A miserable worm," " a poor worm," are
common and almost compassionate phrases. If we cannot
detect any deep phylogenetic reference in this zoological
metaphor, we might at least safely assert that it contains
an unconscious comparison with a low condition of animal
developmetit which is interesting in its bearing on the
pedigree of the human race. For there is no doubt that
the vertebrate tribe, in common with those of the other
higher classes of animals, have developed phylogenetically
from that multiform group of lower invertebrate animals
which are now called Worms. However closely we limit
the zoological significance of the word " Worm," it yet
remains indubitable that a large number of extinct Worms
must be reckoned among the direct ancestors of the human
race.

The group of Worms (VerTnes) 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 Linnaeus.
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 oft*. On the one side, among the
widely divergent classes of Worms, are found the parent-
formB of the four higher tribes of animals, the Molluscs,
Star-animals, Articulates, and Vertebrates ; on the other side,

DEYILOPMENT OF WORMS AND PLANT-ANIMAIA f$

several comprehensive groups and also single isolated genera
of Worms are to be regarded as root-suckers which have
sprouted directly from the rest of the primaeval family-tree
of the Worms. Some of these suckers have evidently
changed but little from the long-extinct parent-form, the
Primitive Worm (Prothelmis), which is immediately con-
nected with the Gastrsea.

Comparative Anatomy and Ontogeny clearly and sig-
nificantly prove that the Gastra3a must be regarded a^
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 Gastraea, and which also yet
develop directly from the gastrula. K 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 Gastraea. On the one side, the common parent-form of
the Worms developed from the Gastraea ; as, on the other
side, did the common parent-form of the Plant-animals.
(Of Tables XVIII. and XIX.)

The tribe of Plant-animals (Zoophytes, or Ccdenterata)
now comprehends, on the one side, the main class of Sponges
(Spongice) ; on the other, the main class of the Sea-nettles
(Acalephce); to the former belong the Gastraeads and
Poriferae, to the latter the Hydroid^polyps, the Medusae,
Ctenophorae, and Corals. From the Comparative Anatomy
and the Ontogeny of these we may infer, with great pro-

74 THE EVOLUTION OF MAK.

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 (Accdephce) transverse axes have been
differentiated in the course of further evolution, thus giving
rise to a characteristic radiate structure with a pyramidal
general outline.

In distinction from this predominant radiate outline of
Plant-animals, a marked bilateral general outline is de-
veloped from the first in the second offshoot from the
gastrula, in the Worms. As the radiate form is marked by
adaptation to an adherent mode of life, so is the bilateral
form by adaptation to certain definite acts of free loco-
motion. The constant direction and carriage of the body
which would be maintained in this mode of free locomotion,
conditioned the two-sided, or bilateral outline of the
symmetrical Worms. Even the parent-form of the latteu,
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 outUne of the
body. The former, the bilateral outline, has been inherited
by the human race from the Worms.

Except through the Gastrsea, the common parent-form
of Plant-animals and Worms, the human race is, therefore,

THE WORMS AS ANCESTORS OF MAH. 75

not related to the Plant-animals. It will be our next task
to consider more closely the pedigree of Man in so far as
it coincides with that of the Worms. Let us examine how
far the Comparative Anatomy and Ontogeny of Worms
justify us in looking among the latter for primaeval ancestors
of Vertebrates, and therefore of Man. For this end we must
first consider the zoological system of Worms. In accord-
ance with the most recent investigations of the Comparative
Anatomy and Ontogeny of Worms, we divide (without
reference to the many and various peculiarities of the
numerous separate classes, which in this place do not
interest us) the whole mass of forms within this tribe
into two large main groups. The first main group, which
we call Bloodless Worms (Acoelomi), 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 gi'oup, on the contrary, called
Blood- worms (Coelomati), are distinguished from the former
by the possession of a true body-cavity, and also by the
presence of a blood -like fluid, which fills this cavity ;
most of them also develop special blood-vessels, which
again cause further correlated advances in structure. The
relation of these two main groups of Worms is very evi-
dently phylogenetic. The Acoelomi, which are very nearly
allied to the Gastrsea and the Plant-animals, are to be
regarded as an earlier and lower group, from which the
more recent and higher division of the Coelomati developed,
perhaps towards the end of the Laurentian Period.

We will first carefully examine the lower group otf
Worms, the Acoelomi, among which we must look for tlM

76 THE EVOLUTION OF MAN.

sixth ancestral stage of the human race, the stage imme
diately following the gastrula. The name "Acoelomi '
signifies " Worms without a body-cavity, or coeloma," and
therefore without blood, or vascular system. The extant
Acoelomi are generally included in a single class, which, on
account of their flattened bodies, are called Flat-worms
{Plathelminthes). To this class belong the Gliding- worms
(TurbeUaria), which live independently in the water; also
the parasitic intestinal Sucking-worms (Trematoda), and
the Tape- worms (Cestoda), which have become yet more
degraded *by parasitism. The phylogenetic relations of the
three forms of Flat- worms are very evident ; the Sucking-
worms originated from the free Gliding- worms by adaptation
to a parasitic mode of life ; and, by a yet more completely
parasitic life, the Tape-worms originated from the Sucking-
worms. These are striking examples of the gradually
increasing degeneration of the most important organs.

In addition to these well-known extant Flat- worms,
great numbers of other Acoelomi must have lived during
the Archilithic ^poch, 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 Gastraeads. The whole of these lowest Acoelomi, among
which the common parent-form of the whole Worm tribe
(the Prothelmis) must have been, may be classed as " Primi-
tive Worms " (Archelminthes).

The two classes of the Acoelomi, the Primitive Wo^m^
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 WOBMa 77

oufc 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.
(C£ 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
ajscula, has but one axis with imequal poles {Monaxonia
diplopola). The typical outline of Worms, as of Vertebrates,
is, on the contrary, bilateral, with tranverse axes (Stau-
raxonia dipleura)}^

The whole outer surface of the Gliding-worms (Turhel-
laria) is covered, as in the gastrula, with a thick, fine
ciliated coat ; that is, with a fur-like covering of extremely
fine and close microscopic hairs, which are direct processes
of the uppermost cells of the epidermis, and maintain an
uninterrupted whirling or vibratory motion (Fig. 184,/).
The constant vibrations of these cilia cause a continued
current of water over the surface of the body. Fresh water
is constantly 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 Archelminthes. They inherited this
ciliated dress directly from the Gastrsea.

If we now make various vertical sections (longitudinal
and transverse) through the simple body of the Gliding- worms
(and that of the Archelminthes which are certainly very
closely allied to the former), we soon discover that their
internal structure is considerably higher than that of the
Gastrseads. We first observe that the two primary germ-
layers (inherited from the Gastriea) have differentiated into
several cell-strata. The skin-layer and the intestinal layer
have each split into two strata. The four secondary germ-
layers, which are thus produced, are the same that we found
resulted from the first differentiation of the two primary
germ-layers in the embryo of the Vertebrate also. (Cf the
transverse sections through the larval Amphioxus and
Earth-worm, Figs. 50 and 51, p. 236, and Plate IV. Fig. 2;
Plate V. Fig. 10.)

The highly important histological differentiation of these
four secondary germ-layers led directly to further organolo-
gical processes of differentiation, by which the organism of
the Primitive Worms was soon considerably raised above
that of the Gastrseads. In the latter there was really, in
a morphological sense, but a single organ, the primitive intes-
tine, with its mouth-opening. The whole body was nothing
but an intestinal canal ; the intestinal wall was at the
same time the wall of the body. Of the two cell-layers^
forming this intestinal wall, the inner accomplished the
functions of nutrition, the outer those of motion and
covering. As some of the cells of the primary germ-layers
developed into egg-cells, and others into sperm-cells, these
layers also performed the function of reproduction. In the

GLIDING-WOKMa 79

Primitive Worms, however, simultaneously with the forma-
tion of the secondary germ-layers, these various functions
also began to be distributed to various organs, which detached
themselves from the original main organ, the primitive in-
testine. Special organs originated for reproduction (sexual
glands), for secretion (kidneys), for motion (muscles), and
for sensation (nerves and sense-organs).

In order to obtain an approximate picture of the sim-
plest form in which all these various organs first appeared
in the Primitive Worms, it is only necessary to examine
the most imperfect forms of Gliding- worms (Turhellaria), as
they exist at the present time in salt and fresh water. They
are mostly very small and insignificant Worms of the simplest
form, many being scarcely a millimetre or a few millimetres in
length. In the simplest species of Gliding- worms the greater
part of the oval body is occupied by the intestinal canal.
This is a very regularly shaped pouch with an opening, re-
presenting both mouth and anus (Fig. 184, m). At the
anterior section of the intestinal tube, which is separated
as a throat (pharynx, sd), the fibrous layer is very thick,
a thick muscular layer. Immediately outside the intestinal-
fibrous layer lies the skin-fibrous layer, which in most
worms appears as a large skin-muscle sac. Above the
throat in Gliding-worms a nerve system of the simplest
form is already visible in front, a pair of small nerve-
knots, or ganglia, which from their position are called the
"upper throat ganglia," or " brain " (Fig. 185, g). Delicate
nerve-threads {n) pass from this to the muscles and to the
ciliated skin-sensory layer. A pair of quite simple eyes
(au) and nose-pits (no) are to be found in a few Gliding-

worma The Flat-worms are also universally provided with
30

So

THE EVOLUTIOX OF MAN

a pair of simple kidney-canals ( " excretory organs " ), in
the form of two long, thin, glandular tubes, which traverse
the right and left sides of the intestine and open at the
hinder end of the body (Fig. 184, nm). We found that the

■w>

Fig. 184. — A simple Gliding-worm (Rhabdocoslum): w, mouth; sd, throat-
epithelium ; S7n, throat-muscles ; d, stomach-intestine ; 7ic, kidney ducts ;
nm, opening of the kidneys ; an, eye ; 7ia, nose-pit.

Fig. 185. — The same Gliding-worm, showing the remaining organs : g,
brain; au, eye; na, nose-pit; n, nerves; h, testes; ^, male opening j
?> female opening j e, ovary ; /, ciliated outer-skin.

STRUCTURE OF THE GLIDINO-WORMS. 8l

two primitive kidney canals in the vertebrate embryo
also appeared at a very early period, shortly after the first
differentiation of the middle germ-layer {mesoderTna). The
appearance of these at so early a period shows that the
kidneys are very important primordial organs. It also
shows their universal existence in all Flat- worms ; for even
the Tape-worms, which, in consequence of the adoption of a
parasitic mode of life, have lost the intestine, yet have the
two secreting primitive kidneys, or " excretory ducts." The
latter seem, therefore, to be older and of greater physiologi-
cal importance than the blood-vessel system, which is wholly
wanting in the Flat- worms. The sexual organs appear
in many of the Gliding- worms in a very complex form ,'
while in others their form is very simple. Most of them
are hermaphrodites ; that is, each individual worm has
both male and female sexual organs. In the simplest
forms we find a testis in the anterior part (Fig. 185, h),
a single or double ovary behind (a). One of these simplest
existing Acoelomi, such as we find among the lowest Rhab-
docoela, may give us an approximate idea of the structure
of the Primitive Worm, which forms the sixth stage in
the human pedigree.

These ancestors of the human race, which, on account
of their general organization, must be placed among the
Bloodless Worms {Acodomi), must have been represented
during the Archilithic Epoch by a large number of various
Worm forms. The lowest of these must have been directly
connected with the Gastrseads (the fifth ancestral stage); the
most highly developed must, on the other hand, have been
directly connected with the Coelomati (the seventh stage),
Afly however, our present knowledge of the Comparativt

8a THE EVOLUTION OF MAN.

Anatomy and Ontogeny of the Acoelomi is very fragmen
tary, and much too imperfect to enable us to point with
certainty to the series of the various stages, we will not
attempt a detailed arrangement of them. "We will turn
instead to the seventh stage in the human pedigree, which
belonged to the multiform group of the Blood-bearing
Worms (Coelomati).

The great organic advance in structure by which the
Blood-bearing worms, or Coelomati, developed from the
older Bloodless Worms, or Acoelomi, consisted in the for-
mation of a body-cavity (coeloma), and of a nutritive juice
filling the latter, the first blood. All the lower animals
with which we have yet occupied ourselves in our Phy-
logeny, all the Primitive Animals and Plant-animals, are,
like the Acoelomi, bloodless and without a body-cavity. In
the formation of a special vascular system, the earliest
Coelomati made a very great advance. Much of the com-
plexity in the organic structure in the four higher tribes of
animals is based on the differentiation of the vascular
system, which they have inherited from the Blood-bearing
Worms.

The first development of a true body-cavity (axlomd)
is referable to the separation of the two fibrous layers ; to
the formation of a spacious cavity between the outer skin-
fibrous layer and the inner intestinal-fibrous layer. In the
fissure-like gaps, which formed between the two germ-layers,
a juice collected, which penetrated through the intestinal
wall. This juice was the first blood, and the gaps between
the two germ-layers formed the first rudiment of the body-
cavity. The union of these gaps formed the simple coelom,
the large cavity, containing blood or lymph, which plays m

BLOOD-BEARING WORMS. 83

important a part in all the higher animals as the receptacle
of the very extensive intestines. The formation of this
coelom, and of the blood-vessels developed in connection with
it, exercised a very great influence on the further evolution
of the animal organization. The most important result was,
that it allowed the conveyance of rich nutritive juices to
those parts of the body lying near the circumference, and
developing at a considerable distance from the intes-
tinal canal. The intimate correlation, or reciprocity of the
parts, necessarily occasioned, in direct connection with the
progressive development of the blood-vessel system, many
other important advances in the structure of the body of
the Blood-bearing Worms.

Just as among the Acoelomi, so also among the Coelomati,
the pedigree of our race must have passed through a large
number of diverse ancestral stages. But among extant
Coelomati (which form but a very small fraction of this once
multiform group), there are but very few Worms which can
with certainty be regarded as nearly allied to the long-
extinct ancestors of Man. In this respect, but a single
class of Coelomati is really of prominent importance ; these
are the Mantled Animals {Tunicata), to which belong the
Ascidia already known to us. Our careful examination of
the structure and germ-history of the Ascidian and the
Amphioxus have shown the extreme importance of these
very interesting animal forms. (Cf. Chapters XIII. and
XIV.) That examination fully justifies us in asserting
that among the ancestors of the Vertebrates (and therefore
of Man) there was an unknown extinct coelomate species,
to which the nearest allied form among extant animals is
the Appendicularia (Fig. 187), of which we have already

84 THE EVOLUTION OF MAK.

spoken, and the tailed Ascidian larva. We will for the
present call this kind of Worm, which was primarily dis-
tinguished by the possession of a notochord, the Chorda-
animal (Chordonium). The Ascidians on the one hand, and
the Vertebrates on the other, developed, as two diverging
branches, from these Chorda-animals. The common parent-
form of the Chorda-animals themselves was a coelomate form,
which finally must have descended from the Acoelcmi, and
from the Archelminthes.

Many connecting intermediate forms must, of course, have
existed between these two groups of Worms, between the
Primitive Worms and the Chorda-animals. Unfortunately,
however, zoological knowledge is at present especially im-
perfect with regard to these important intermediate forma
of the multiform Worm tribe. For very evident reasons,
none of these Worms could leave fossil remains. For, like
the great majority of other Worms, they had no hard parts
in their bodies. Most even of the known fossil Worms
are worthless, for they tell us little or nothing of the most im-
portant structural features of the soft body. Fortunately,
however, we can in great measure satisfactorily fill the con-
siderable palseontological gap in this part of our pedigree,
with the help of the Comparative Anatomy and Ontogeny of
Worms. If, on the one hand, we examine the structure and
mode of development of the lower Worms from the Gliding-
Worms (Turbellaria), and, on the other hand, the Anatomy
and Ontogeny of the Ascidians, it is not difiicult, step by
step, to re-construct in imagination the connecting inter-
mediate forms, and to insert a series of extinct ancestral
forms between the Acoelomi and the Chordonia. This
series of forms under the name of Soft-worms {Scolecida)

DEVELOPMENT OF CHORDA- ANIMALS. 85

we will consider as the seventh stage in the human
pedigree.

An examination of the Comparative Anatomy of the
various Scolecid forms, which we might perhaps distinguish
here, would lead us much too far into the difficult details
of the Comparative Anatomy and Ontogeny of the Worms.
For our purpose it seems more important to call attention
to those phylogenetic advances, by means of which the
organization of the earliest Blood-bearing Worms was in
the end elevated to that of the Chorda-animals. The Com-
parative Anatomy and Ontogeny of the Gliding-worms
and of the Ascidians justify us in giving special weight to
the sio:nificant differentiation of the intestinal canal into two
distinct divisions ; into an anterior division (the gill-intes-
tine), which accomplishes respiration, and a posterior divi-
sion (the stomach-intestine), which accomplishes digestion.
As in Gastrseads and Primitive Worms, so also in the Ascidian
larva, the intestinal canal is at first a simple pouch-like
body, provided merely with a mouth-opening. A second
opening, the anus, does not develop till a later period. Gill-
openings afterwards appear in the anterior section of the
intestinal canal, by which the whole anterior intestine is
transformed into a gill-body. This remarkable arrange-
ment is, as we found, quite peculiar to Vertebrates, and,
except in the Ascidians, occurs nowhere else. Among extant
Worms there is, however, a single isolated and very remark-
able Worm form, which in this respect may be regarded
as distantly allied to the Ascidia and to Vertebrates, and
perhaps as an off-shoot from the Soft-worms (Scolecida).
This is the so-called "Acorn-worm" {BalanoglossiLS, Fig.
186), which Kves in the sand of the sea-shore. The in-

96

THE EVOLUTION OP MAN.

leresting points connecting this with Ascidians and the

Skull-less Animals (Acrania)
were first accurately observed
and explained by Gegenbaur. Al-
though this singular Balanoglossus
is in many other respects peculiar
in its organization, so that Gegen-
baur rightly ranked it as the re-
presentative of a special class
(Enter opneusta), yet the structure
of the anterior section of the in-
testinal tube is exactly similar to
that of Ascidians and SkuU-less
Animals (k), a gill body, the walls
of which are pierced on either side
by gill-openings and are supported
by gill-arches. Now, although the
Acorn-worm in other points of its
structure may differ very con-
siderably from those extinct Soft-
worms (Scolecidce), which we must
regard as direct ancestors of our
race, and as intermediate links
between the Primitive Worms

Pig. 186. — A young Acorn -worm (Bal-
anoglossus). (After Alexander Agassiz.)
r, acorn-like proboscis ; h, collar ; k, gill
openings and gill-arches of the anterior in-
testine, in a long row one behind another
on each side ; d, digestive posterior intefr-
tine, filling the greater part of the body-
cavity ; V. intestinal, vessel, lying: beti
two peirailel loiOs ot sKm.; a, tosua.

sorr-woBMB. 9f

and the Chorda-animals, yet, in virtue of this characteristic
structure of the gill-intestine, it may be considered a re-
motely allied collateral line of the Soft-worms. The
development of an anus (Fig. 186, a) at the end opposite
to the mouth, is also a considerable advance in the struc-
ture of the intestine. The further development of the
blood-vessel system in the Acorn-worm also indicates a
marked advance. In the ciliary surface of the skin, on
the contrary, it recalls the Gliding- worms. The sexes are
separated, while our scolecid ancestors were probably
hermaphrodite.^^

From a branch of the Soft- worms, the group of Chorda-
animals (Ghordonia), the common parent-group of the
Mantle-animals and Vertebrates also developed. The process
which primarily led to the development of this important
group of the coelomati, was the formation of the inner
axial skeleton (the notochord, or chorda dor sails), wliich
at the present day we find permanently retained in its
simplest form in the lowest Vertebrate, the Amphioxus.
We saw that this notochord is already found in the tailed
and free-swimming larva of the Ascidian (Plate X. Fig. 5).
The chorda does, indeed, serve specially as a support for
the rudder-like tail of the larval Ascidian, but its anterior
extremity passes in between the intestinal and medullary
tubes within the actual body of the larva. A transverse
section of this larva therefore shows that arrangement of
the most important organs which is characteristic of the
vertebrate type : in the centre is the firm notochord, which
supports the other organs and serves especially as a base
and point of attachment for the motive trunk muscles ;
above this notochord, on the dorsal side, is the central

SS THE EVOLUTION OF MAN.

nervous system in the form of a medullary tube ; below, on
the ventral side, is the intestinal tube, the anterior half of
which is a respiratory gill-intestine, its posterior half a
digestive stomach-intestine. It is true that the free-
swimming larva of the extant Ascidian possesses this typical
vertebrate character only for a short time ; it soon relin-
quishes its free roving mode of life, puts off its oar-like tail
with the notochord, adheres to the bottom of the sea, and
then undergoes that very great retrogression, the surprising
final result of which we have already observed (Chapters
XIII. and XIV,). Nevertheless, the Ascidian larva, in its
very transitory evolution (for a brief space), affords us a
picture of the long extinct Chordona-form, which must
be regarded as the common parent-form of Mantle-animals
and Vertebrates. There is even yet extant a small and
insignificant form of Mantle-animal which throughout life
retains the structure of the Ascidian larva with its oar-
like tail and its free-swimming mode of life, and which
reproduces itself in this form. This is the remark-
able Appendicularia (Fig. 187), which we have already
examined.

If we ask ourselves what conditions of adaptation could
possibly have had so remarkable a result as the develop-
ment of the notochord, and the modification of a branch
of the Soft- worms into the parent-form of the Chorda-
animals, we may with great probability answer, that this
result was effected by the habituation of the creeping
Soft-worm to a swimming mode of life. By energetic and
continued swimming movements, the muscles of the trunk
would be greatly developed, and a strong internal point oi
attachment would be very favourable to this musculai

THE ASCIDIANS. 89

activity. A support of this kind might arise by enlarge-
ment and concrescence of the germ-layers along the longi-
tudinal axis of the body; and the differentiation of an
independent bony cord from this axial cord gave rise to the
notochord. (Cf Fig. 88, 89, vol. i. pp. 300, 301.) In corre-
lation to the formation of this central notochord, the simple
nerve-ganglia, lying over the throat in the Soft-worms,
lengthened into a long nerve-cord, reaching from front to
rear, above the notochord ; in this way, the medullary tube
originated from the " upper throat ganglia."

As we have already minutely considered the great
lignificance of the Ascidians (Fig. 188) in this respect, as
well as their close relations to the Amphioxus (Fig. 189).
we will not tarry longer over this point now. I will
repeat, that we must by no means regard the Ascidian
OS the direct parent-form of the Amphioxus and of the
other Vertebrates. On the contrary, we assert that, on
the one hand the Ascidians, and on the other the Ver-
tebrates, have both descended from one unknown
Worm form, which has long been extinct ; the nearest
relatives of this among existing animals are the Ascidiai-
larvae and the Appendicularia (Fig. 187). This unknown
common parent-form must have belonged to the group of
Chorda-animals, which we pointed out as the eighth
ancestral stage in the human pedigree. ^^ Although we
cannot form an entirely satisfactory idea as to all points
of external and internal structure of this Chorda-animal,
there is no doubt that, like its near relatives, the
Mantle-animals, and like the preceding ancestral stage
represented by the Soft- worms and Primitive Worms, it
must be classified in the natural system of the animal

90

THE EVOLUTION OF MAN.

kingdom as a genuine Worm. The difi'erence between it
and other genuine Worms cannot have been greater than is

v.d;

^-(y

'm.

Fio. 187. — Append icularia, seen from the left side : m, montli ; k, giU-
intestine ; o, oesophagus ; v, stomach ; a, anus ; n, nerve-ganglia (nppex
throat-knots) ; g, ear-vesicle ; /, ciliated groove under the gill j fc, heart f
(, testes ; «, ovary ; c, notochord ; «, tail.

Fig. 188. — Structure of an Ascidian (seen from the left, z» in Fig. 153
and Fig. 14, Plate XI.) : sb, giU-sao j v, stomach ; t, large intestine j c,
heart ; t, testes ; vd, seed-duct ; o, ovary j o\ matured eggs in the body.
oavitj. (After Milne Edwards.)

THE AMPHIOXU&

91

11

111

fch* difference between the extant Tape- worms and Ringed
Worms (Annelida). Moreover, in a certain sense we may
regard the extant Appendicularia as a last remnant of the
Chordonia class.

We have now studied the most import-
ant animal forms which occur in the pedigree
of the human race, and which, in the zoo-
logical system, must be classed among the
Worms. In leaving this lower class, and
tracing our ancestry henceforth exclusively
within the vertebrate tribe, we at once ^
leave behind the great majority of animal
(brms, which branched off from the worm
Liibe in entirely different directions. When,
ill a previous chapter (IX.), the vertebrate /
ature of man was proved, it was incidentally
entioned that the very great majority of «
animals are in no way directly allied to our ,
tribe. The parent-forms of the three other '
higher Animal tribes,the Articulated Animals
{Arihro'poda), Star-animals (Echinoderma),
and Soft-bodied Animals (MoUusca), like
the vertebrate tribe, originated from the

Fig. 189. — Lancelet (^Amphiomus Icmceolatus), twice
the actual size, seen from the left (the longitudinal
axis is represented vertically, the mouth turned up-
ward, the tail downward, as in Plate XI. Fig. 15) :
a, mouth-opening, surrounded by cilia ; h, anal open,
iug ; c, ventral opening {Porus ahdominalia) ; d, gill-
body I 0, qtomach j /, liver-coecum ; g^ large intes-
tine ; h, coelom : », notochord (under it the aorta) ;
k, trches of the aorta ; I, main gill-artery ; tn, ewellingi
on its branches; n, hollow Tttiii} 0, intoitinal Tttin.

( 92 )

TABLE XVIII.

Bj^tematio Survey of the Phylogenetio System of the Animal Kingdom,
founded on the Gastraea Theory and the Homology of the Germ-layers.

Tribes «r Phyla

<tfthe

tMimaX Kingdom.

Main Classes or

Branches of the

Animal Kinydom.

Classet

of the

AnifMil Kingdmn.

Systematic Namtt

ofthd

Classes.

First Sub-kinqdom : Primitive Animals (Protozoa).
Animals without germ-layers, intestine, or true tissues.

A.

^rttnittbe
Animals
Protozoa

Egg-animals
Ovularia

1. Monera

2. AmcebsB

3. Gregarinae

IL Infusorial animalt t 4. Sucking InfuMri*

Infusoria

It;

Ciliated Infusoria

1. Monera

% Lobosa
S. Gregarina

4. AcineUs

5. Ciliata

Second Sub-kingdom : Intestinal Animals (Metazoa).
Animals with two primary germ-layers, intestines and tissues.

B.

plant*
Animals
Zoophytes

C.

SKorms
VermeB

D.

Soft^bDtiteti
Animals
Mollusoa

E.
Star==<ammals
Echinoderma

F.

^rticulatrt

Animals

Arthropoda

G.

Ftrtebrate

'Enimals

y«rtebrata

III. Sponges
Sponffia

TV. Sea-nettles
Acalephoe

T. Bloodless worms
AcaUnna

{:

VI.

Blood-worms
CcBlomati

\

18.

19.

V20.

/ VU. Headless shell-fish r 21.
Acephala \ 22.

Vni. Head-bearing
shell-lish
Eucephala

IX. Ringed -arms
Colobrachia
X. Armless
Lipobrachia

XI. GiU-breathers
Carides

Primitive
animals
Sponges

{8. Corals
9. Hood-jeUiet
10. Comb-jellia

(11. Primitive worms
12. Flat-worms
/13. Round- worms
1 14. Arrow-worms
I 15. Wheel-animalcules
J 16. Moss-polyps
17. Mantle-animals
Acorn-worms
Star-worms
Ringed-wormi

Lamp-shells
Mussels

intestiasl t. Gastr»adi

23.
24.

Snails
Cuttles

t. Porifera
•. Coralla
f . Hydromednsa
!•. Ctenophora

11. Archelminthes

12. Plathelmlnthes

13. Nemathelminthes

14. Choetognathi

15. Rotatoria

16. Bryozoa

17. Tunicata

18. Enteropneusts

19. Gephyrea

30. Annelida

31. Spirobranchla

32. LamellibranchiR

23. Cochlides

24. Cephalopoda

Xn. Tube-breathers
Tracheata

\ 26. Sea-lilies

f 27. Sea-urchins

1 28. Sea-cncumbefs

i 29. Crabs

I 30. Spiders

< 31. Centipedes

32. Flies

26.
26.
37.
38.

Asterida
Crinoida
Ecblnida
Holotharice

39. Cmstaoea

30.

31.
32.

Arachnida
Myriopoda
Insect*

Xni. Skull-less
Acrania
XIV

/ 33. Tube-hearts (Lsnos-
l lets)

Single -nostrilled f 34. Round-moutbs
Monorrhina I (Lampreys)

35. Fishes

36. Mud-fish

37. Amphibians

38. Reptiles

39. Birds

40. Mammals

XV. Amnion-lesB
Ancumnia

xn. AmnioD-animals ^
Ammiota

33. Leptocariis
M. Cjclostoma

35. Pisces

36. Dipneusta

37. Amphibia

38. Reptilia

39. Aves

40. MsmmsHs

( 93 )

TABLE XIX.

tfonophyletio Pedigree of the Animal Kingdom, founded on the GastrtM
Theory and the Homology of the Germ-layers."

It

I

§

II

IS
■<^

•S'2 >

^r

5 ♦* 5

N ►:; OJ -
•<&^ >< Oi

(, v-^ a> ri

eg gO

H

I

I.

1 ai ^ 05

o

^ to 4

^5* -

S a** M
S C S o3

15

fi)

Articalates
Arthropoda

Vertebrates
Vertebrata

St&r-anfmals
Echinoderma

v^

Soft-bodied Aninuds
Mollusca

Coelomati.
( Wormt with body-cavity)

Plant-animals

Zoophyta
(^CoBlenUratd)

SpongM

Spongiee

Sea-nettles
(^Acalephce)

Flat Womn
PlathelmlQtbe

Protascus

GaBtrjea radialis
{stationary)

Acoelomi
iWorwu without body-eavUjf)

Prothelmii

Gastrsea bilateraUs
{crawling)

Gastraea
^Ontogeny: Gattridd)

Primitiye Anim^t*
Protozoa

PlanaeadA
{OtUoffeny: BUutuia')

CUiAU

Adseto

OKgarlnse

InfusoiiA

SynamoebA
{Ontogeny: Morula')

AmoeUaa

\

AmoebsB

(Ontogeny: CytuU)

Monera
{Ontogeny: MtntnltQ

94 THE EVOLUnOW OF MAN.

worm tribe ; but the parent-forms of the three formei
belong to worm-groups quite distinct from that of the
Chordonia. It is only far down at the common root of the
group of Coelomati, that we assume a common source for
these various tribal forms. (Cf. Tables XVIII. and XIX.)
It is especially necessary to remember that there is no
direct blood-relationship between Vertebrates and Articu-
lated Animals.

The Articulated Animals (Arthropoda), to which the
most comprehensive of all classes of animals, that of Insects,
and also the Spiders, Centipedes, as weU as the Crabs, or
Crustaceans, belong, are descendants of articulated Worms,
the nearest allies of which are the extant Ringed Worms
(Annelida). The tribe of Star-animals (Echinoderma),
which includes the Star-fishes, Sea-lilies, Sea-urchins, and
Sea-cucumbers, must also have descended from similar articu-
lated Worms."^ The parent-form of the Soft-bodied Animals
(Mollwsca), which include the Cuttles, Snails, Mussels, and
Lamp-shells, must also be sought among the Worms. But
the Coelomati, from which these three higher animal tribes
originated, differed entirely in character from the Chorda-
animals. Unlike the latter, they never developed a noto-
ohord. In them, the anterior section of the intestinal tube
was never modified into a gill-body with gill-openings; nor
were the upper throat-ganglia developed into a medullary

tube. In a word, in Articulated Animals, Star-animaJs, and

if

Soft-bodied Animals, as well as in their ancestors among
the Blood-bearing Worms, the typical structural peculiari-
ties which are exclusively characteristic of the vertebrate
tribe and of their immediate invertebrate progenitors, were
never present Thus the great majority of all Miimalfl are

DEVELOPMENT OF VERTEBRATES FROM INVERTEBRATES. 95

in no way the subject of our further investigations, which
are only concerned with the Vertebrates.

The development of the Vertebrates from the Inverte-
brates most nearly related to them, the Chorda- Animals,
occurred millions of years ago, during the Archilithic Epoch,
(See Table XII., p. 11.) This is unmistakably shown by
the fact that the most recent sedimentary rock-strata
which were deposited during that immense period of time,
the higher layers of the Upper Silurian formation, contain
remains of fossil Fishes (Primitive Fishes, Selachii). As
these Fishes, although they belong to the lowest stage of
the Skulled Animals (Craniota), yet possess a compara-
tively high organization, and as they must necessarily have
been preceded by a long progressive series of lower Skull-
less Vertebrates, we must attribute the origin of the oldest
Skull-less Animals (A crania) from the Chorda-animals to
a much earlier part of the Archilithic Epoch. Therefore,
not only all the invertebrate ancestors of our race, but also
the earliest form of our vertebrate progenitors must have
developed in that primordial time, which includes the
Laurentian, Cambrian, and Silurian Periods. (C£ Tables
XIIL, XIV., and XVI., pp. 12, 19, 44.)

Unfortunately, Palaeontology can give us absolutely no

information with regard either to the structure of our oldest

vertebrate ancestors, or to the time of their appearance;

for their bodies were as soft and as destitute of hard

parts capable of fossilization, as were the bodies of all

our preceding invertebrate ancestors. It is, therefore, not

surprising, but quite natural, that we find no fossil

remains of the former in the Archilithic formations. The

Fishes in which the soft cartilaginous skeleton was partly
40

96 THE EVOLUTION OF MAN.

modified into hard bone, are the earliest Vertebrates capable
v./ leaving petrified records of their existence and structure.
Fortunately, this want is more than counterbalanced
by the much more important testimony of Comparative
Anatomy and Ontogeny, which henceforth form our
safest guides within the Vertebrate pedigree. Thanks to
the classic researches of Cuvier, Johannes Miiller, Huxley,
and especially of Gegenbaur, we are in possession of such
extensive and instructive records of creation in this most
important branch of tribal history, that we can prove at
least the more significant features in the development of our
Vertebrate ancestors, with the most gi^atifying certainty.

The characteristic peculiarities by which Vertebrates
in general are distinguished from all Invertebrates, engaged
our attention some time ago, when we examined the structure
of the ideal Primitive Vertebrate (Figs. 52-56, p. 256). The
most prominent characters were as follows: (1) the formation
of the notochord between the medullary and intestinal tubes;
(2) the differentiation of the intestinal tube into an anterior
gill-intestine and a posterior stomach-intestine ; (3) the
inner articulation, or formation of metamera. The Verte-
brates share the first two qualities with the larval Ascidians
and with the Chorda-animals ; the third quality is entirely
peculiar to them. Accordingly, the most important struc-
tural advance, by which the earliest vertebrate forms origin-
ated from the most nearly allied Chorda-Animals, consisted
in an internal metameric structure. This showed itself
first most distinctly in the articulation of the muscular
system, which broke up on the right and left into a series
of consecutive muscular plates. At a later period the
articulation declared itself prominently in the skeleton, and

CLASSIFICATION OF VERTEBRATB8L 97

nervous and blood-vessel systems. As we have already
seen, this process of articulation, or metameric formation,
must essentially be regarded as terminal germination.
Each distinct trunk-segment, or metameron, represents an
individual. Thus the Vertebrates with their internal
segmentation stand in a similar relation to their inarticulate
Invertebrate ancestors, the Chorda Animals, as do the out-
wardly segmented Ringed Worms (Annelida) and Articu-
lated Animals (Arthropoda) to the simple inarticulate
Worms from which they originated.

The tribal history of Vertebrates is rendered much more
intelligible by the natural classification of the tribe which
I proposed first in my Generelle Morphologie (1866), and
afterwards improved in many ways in " The Natural History
of Creation" (Chap. XX., p. 192, etc.). In accordance with
that, existing Vertebrates must be divided into at least
eight classes, as follows : —

STSTEMATIO SURVEY OF THE EIGHT CLASSES OF

VERTEBRATES.

A. Sknll-lfiM (^Aerania) 1. Tnbe-hearted 1. Leptocardia

ia. Single-nostiilled {Monorhina) 2. Round-moaths 2. C7ck)*toina

/ L [3. Fishes 3. Pisces

1 Amnion-less J. 4. Mud-fishes 4. Dlpneusta

b. Doable-nostrilldd j Anamnia [ 5. Amphibians 6. Amphibia

Awiphirhina { II ^6. Rpptiles 6. Reptilia

f With Amnion < 7. Birds Y. Aves

V Amniota { 8. Mammals 8. Mammalia

The whole Vertebrate tribe may primarily be divided
into the two main sections of the Skull-less and the
Skulled Vertebrates. Of the earlier and lower section, that
of the Skull-less (Acrania), the Amphioxus is alone extant.
To the more recent and higher section, the Skulled (Cra-
niota), belong all other existing Vertebrates up to Man. The

98 THE EVOLUTION OF MAN.

Craniota branched off from the Acrania, as these did from
the Chorda Animals. Our exhaustive study of the Compara-
tive Anatomy and Ontogeny of the Ascidian and the
Amphioxus have already afforded proof of this relation. (Cf
Chapters XIII. and XIV., and Plates X. and XI. with the
explanations.) I will only repeat, as the most important
fact, that the Amphioxus develops from the egg in exactly
the same way as the Ascidian. In both, the original Bell-
gastrula (Figs. 4 and 10) originates in an exactly similar
manner, by primordial cleavage from the simple parent-cell
(Figs. 1 and 7). From this originates that remarkable larva,
which develops a medullary tube on the dorsal side of the
intestinal tube, and between the two a notochord. At a
later period, both in the Ascidian and in the Amphioxus, the
intestinal tube differentiates into an anterior gill-intestine
and a posterior stomach-intestine. In accordance with the
fundamental principle of Biogeny, from these very important
facts we may deduce the following statement of great phylo-
genetic importance : the Amphioxus, the lowest Vertebrate
form, and the Ascidian, the most nearly allied Invertebrate
form, have both descended from one single extinct Worm
form, which must have possessed the essential structure of
the Chorda Animals.

The Amphioxus, as has already been often shown, is
of extreme importance ; not only because it thus fiUs the
great gap between the Invertebrates and the Vertebrates,
but also because it represents, at the present time, the
typical Vertebrate in its simplest form; and because it
directly affords the best standpoint from which to examine
the gradual historic evolution of the whole tribe. If the
Btructure and germ-history of the Amphioxus were un^

THB AMPHIOXUS AS THE ANCESTOR OF MAN. 99

known to us, the whole subject of the development of
the Vertebrate tribe, and thus of our own race, would Ije
enveloped in an impenetrable veil. The accurate anatomical
and ontogenetic knowledge of the Amphioxus, attain'.''
during the last few years, has alone pierced that heavy veil
formerly supposed to be impenetrable. If the Amphioxus i
compared with the developed Man or any other of tl.
higher Vertebrates, a great number of striking dissimilarities
will be seen. The Amphioxus has no specialized head, nu
brain, no skull, no jaws, no limbs ; it is without a central-
ized heart, a developed liver and kidneys, a jointed vertebral
column ; every organ appears in a much simpler and more
primitive form than lq the higher Vertebrates and in Man
(Cf. Table X., vol. L p. 466.) And yet, in spite of all thest
various deviations from the structure of other Vertebra te^
the Amphioxus is a genuine, unmistakable Vertebra it-
and if, instead of the developed Man, the human embr\ >
at an early period of its Ontogeny is compared witi
the Amphioxus, we shall find perfect parallelism betweei
the two in all essential points. (Cf Table IX., vol. L p. 465.;
This highly important parallelism justifies the conclusion
that all the Skulled Animals (Craniota) have descended
from a common primaeval parent-form, the structure of
which was essentially that of the Amphioxus. This parent-
form, the earliest Primitive Vertebrate, possessed the
peculiar characters of the Vertebrates, and yet was without
all those important peculiarities that distinguish the Skulled
Animals from the SkuU-less. Although the Amphioxus ap-
pears peculiarly organized in many respects, and although
it may not be regarded as an unmodified descendant of the
Primitive Vertebrate, yet it must have inherited from the

100 THE EVOLUTION OF MAN.

latter the distinguishing characteristic features already
mentioned. We cannot therefore say that the Amphioxus ia
the progenitor of the Vertebrates ; but we may certainly say
that the Amphioxus of all known animals is nearest allied
to this progenitor ; both belong to the same limited family
group, to the lowest Vertebrate class, that of the Skull-less
Animals (Acrania). In the human pedigree, this group
forms the ninth stage of the ancestral chain, the first among
Vertebrate ancestors. From this Skull-less group was
ieveloped the Amphioxus on the one side, and on the other
fche parelit-form of the Skulled Animals (Craniota).

The comprehensive group of the Skulled Animals
includes aU known Vertebrates, with the single excep-
tion of the Amphioxus. All these Skulled Animals
possess a distinct head, inwardly specialized from the
trunk, and this contains a skull, enclosing a brain. This
head also carries three of the higher sense-organs, which are
partially wanting in the Skull-less Animals (nose, ears, and
eyes). At first, the brain appears in a very simple form, as
an anterior bladder-like extension of the medullary tube
(Plate XI. Fig. 16, mj). This, however, is soon distributed by
several tranverse grooves — first into three, and afterwards
into a series of five consecutive brain-bladders. In the
formation of the head, skull, and brain, together with the
higher sense-organs, lies the most essential advance made
by the skulled parent-form beyond its skull-less ancestors.
Other organs, however, also soon rose to a higher grade
of development; a compact centralized heart appeared, a
more perfect liver and kidneys; and in other directions
also important advance was made.

The Skull-less Animals may be primarily subdivided

SKULL-LESS ANIMALS. lOI

into two differing main sections, that of the Single-nostrils
(Monorhina), and that of the Double-nostrils (Amphirhina).
Of the former there are but very few extant forms, which
are called Round-mouths {Cyclostcmia). These are, however,
of great interest, because in their whole structure they are
intermediate between the Skull-less Animals and the Double-
nostrils {Amphirhina). Their organization is much higher
than that of the Skull-less Animals, much lower than that
of the Double-nostrils ; they thus form a very welcome
phylogenetic link between those two divisions. We may
therefore represent them as a special, tenth stage in the
human ancestral seriea

The few existing species of the class of Round-mouths are
distributed into two different orders, which are distinguished
as the Hags and the Lampreys. The Hags {Myxinoides)
have long, cylindrical, worm-like bodies. Linnaeus classed
them among Worms, but later zoologists have placed them,
sometimes among the Fishes, sometimes Amphibians, and
again with Molluscs. The Hags live in the sea and are
usually parasitic on Fishes, into the skin of which they
penetrate by means of their round sucking mouths and
their toothed tongues. They are occasionally found in the
body-cavity of Fishes — for example, of the Cod and Stur-
geon— having penetrated to the interior in their passage
through the skin. The second order, that of the Lampreys
(Petromyzontea), includes those well-known " Nine eyes,"
common at the seaside; the little river Lamprey {Petro-
myzon Jluviatilis) and the large sea Lamprey {Fetromyzon
marinvs, Fig. 190).

The animals included in the two groups of the Myxi'
noides and the PetroTnyzontes, are called Round-mouths

102 THE EVOLUTION OF MAN.

[Cycloetoma), from the fact that their mouth forms a circular
or semi-circular opening. The upper and under jaws,
which appear in all the higher Vertebrates, are completely
wanting in the Round-mouths, as in the Amphioxus. All
other Vertebrates are therefore distinguishable from them
;ls "Jaw-mouthed" (Gnathostomi). The Round-mouths may
rJso be called " Single-nostrils " (Monorhina), because they
have but a single nasal tube, while the Gnathostomi are all
furnished with a pair of nasal cavities, a right and a left
nose-cavity (" Double-nostrilled," Amphirhina). But in
addition to these peculiarities, the Jaw-mouths are also
distinguished by many other remarkable structural arrange-
ments, and are further removed from the Fishes than the
latter are from Man. They must, therefore, evidently be
regarded as the last remnant of a very old and very low
class of Vertebrates, which are far below the structural
stage of a genuine Fish. To mention here briefly only
the most important, the Round-mouths are entirely with-
out any trace of limbs. Their slimy skin is quite
naked and smooth, without scales. They are wholly
destitute of a bony skeleton. The inner skeleton axis is
a very simple inarticulate notochord, like that of the
Amphioxus. In the Lampreys alone a rudimentary articu-
lation is indicated by the fact that upper arches appear in
the vertebral tube proceeding from the notochord sheath.
At the anterior end of the chorda a skull is developed in
its very simplest form. From the notochord sheath pro-
ceeds a small soft-membraneous skull capsule, which
becomes partly cartilaginous: this capsule encloses the
brain. The important apparatus of the gill-arches, the
tongue-bone, etc., which is inherited by all Vertebrates

LAMFBET&

I03

from Fishes to Man, is wholly wanting in
the Round-mouths. They have, indeed, a
superficial, cartilaginous gill-skeleton, but this
is of quite different morphological significance.
On the other hand, in them we meet, for the
first time, with a brain, that important
mental organ, which has been transmitted
from the Single-nostrils up to Man. It is true
that in the Round-mouths the brain appears
merely as a very small and comparatively
insignificant swelling of the spinal chord ; at
first a simple bladder (Plate XL Fig. 16, mj),
which afterwards separates into five consecu-
tive brain-bladders, as in the brains of all
Double-breathers. These five simple primitive
brain-bladders, which reappear in a similar
form in the embryos of all higher Vertebrates,
from Fishes up to Man, and which undergo
a very complex modification, remain in the
Round-mouths, in a very low and undifferen-
tiated stage of development. The histological
elementary structure of the nervous system is
also much more imperfect than in other Verte-
brates. While in the latter the oro:an ot
hearing ahvays has three semi-circular canals,
in the Lampreys it has but two, and in the
Hags but one. In most other points also,
the organization of the Round-mouths is

Fio. 190. — The large Sea-lamprey (Petromyzon marim
wus), much reduced in size. A series of seven gill-opeo-
ings are visible below the eje.

r^

104 THE EVOLUTION OF KAN.

much simpler and more imperfect, as, for instance, in the
structure of the heart, the circulatory system, and the
kidneys. In them, as in the Amphioxus, the anterior
portion of the intestinal canal does, indeed, form respiratory
gills ; but these respiratory organs are developed in a very
peculiar way : in the form of six or seven little pouches, or
sacs, which lie on both sides of the anterior intestine and
communicate with the throat (pharynx) by inner openings,
and by outer ones with the external skin. This is a very
peculiar formation of the respiratory organs, quite cha-
racteristic of this class of animals. They have therefore
been called the " Pouch-gills " {Marsupohranchii). The
absence of one very important organ found in the Fishes,
the swimming-bladder, from which the lungs of the higher
Vertebrates have developed, should be especially noticed.

In their germ-history, as in their whole anatomical struc-
ture, the Round-mouths present many peculiarities. They
are even peculiar in the unequal cleavage of the egg, which
most nearly approaches that of the Amphibians (Fig. 31,
vol. L p. 203). This results in the formation of a Hood-
gastfula, like that of Amphibians (Plate II. Fig. 11). From
this develops a very simple organized larval form, which is
closely allied to the Amphioxus, and which, for that reason,
we examined and compared with the latter (voL i. p. 428,
and Plate VIII. Fig. 16). The gradual germ-evolution of
these larvae of the Round-mouths explains very clearly and
unmistakably the gradual evolution of the Skulled from the
Skull-less class of Vertebrates. At a later period, from
chia simple Lamprey larva is developed a blind and tooth-
less larval form, which is so very ditierent from the mature
Lamprey that, until twenty years ago, it was generally

ROUND-MOUTHa IO5

described as a peculiar form of fish under the name of
Ammocoetes. By a further metamorphosis this blind and
toothless Ammocoetes is transformed into the Lamprey with
eyes and teeth {Petromyzon)}^'^

Summing up all these peculiarities in the structure and
embryology of the Round-mouths, we may assert that the
oldest Skulled Animals, or Craniota, diverged in two lines ;
one of these lines has continued up to the present time
but little modified; it is represented by the Cyclostoma,
or Monorhina, forming a collateral line which has made
but little progress, but has remained at a very low stage of
development. The other line, the direct line in the pedigi^ee
of the Vertebrates, advanced in a straight line to the Fishes,
and by new adaptations attained many important improve-
menta

In order rightly to appreciate the phylogenetic signi-
ticance of interesting remnants of primaeval groups of
animals, such as the Round-mouths, it is necessary to study
minutely their various peculiar characters philosophically
and with the aid of Comparative Anatomy. A careful
distinction must be drawn between the hereditary cha-
racters which have been accurately transmitted to the
present day by heredity from common, primaeval ancestors,
now extinct, on the one hand ; and, on the other, those
special adaptive peculiarities which the existing remnant
of that primaeval group have, in the course of time, gained
secondarily by adaptation. To the latter class belong,
for example, in the Round-mouths, the peculiar formation
of the sino'le nostril and the round suckinor mouth: as
well as special structural arrangements of the epidermis
and the pouch-shaped gills. But, on the other hand, to the

I06 THE EVOLUTION OF MAN.

former class of characteristics, which alone have any phylo-
genetic significance, belong the primitive formation of the
vertebral column and the brain, the absence of the swim-
ming-bladder, of jaws, limbs, etc.

In the animal system, the Round-mouths are usually
classed among Fishes; but that this is quite incorrect is
apparent from the simple fact that, in all important and
prominent structural peculiarities, they are further removed
from the Fishes than the Fishes are from the Mammals and
from Man.

CHAPTER XVIII.
THE PEDIGREE OF ]VIAN.

III. Feom the Primitive Fish to the Amniotic Animal.

Comparative Anatomy of the Vertebrates. — The Characteristic Qualities of
the Donble-nostrilled and Jaw-mouthed : the Double-Nostrils, the Gill-
arch Apparatus, with the Jaw-arches, the Swimming-bladder, the Two
Pairs of Limbs. — Relationship of the Three Groups of Fishes : the Pri-
mitive Fishes (^Selachii), the Ganoids {Ganoides), the Osseous Fishes
(Teleostei). — Dawn of Terrestial Life on the Earth. — Modification of
the Swimming-bladder into the Lungs. — Intermediate Position of the
Dipneusta between the Primitive Fishes and Amphibia. — The Three
Extant Dipneusta (Protopterus, LepidosireUy Ceratodua). — Modification of
the Many-toed Fin of the Fish into the Five-toed Foot. — Causes and
Effects of the latter. — Descent of all Higher Vertebrates from a Five-toed
Amphibian. — Intermediate Position of the Amphibians between the
Lower and Higher Vertebrates. — Modification or Metamorphosis of
Frogs. — Different Stages in Amphibian Metamorphosis. — The Gilled
Batrachians (Proteus and Axolotl) . — The Tailed Batrachians (Salaman.
ders and Mud-fish). — Frog Batrachians (Frogs and Toads). — Chief
Ghroup of the Amnion Animals, or Amniota (Reptiles, Birds, and
Mammals). — Descent of all the Amniota from a Common Lizard.like
Parent-form (Protamnion) . — First Formation of the Allantois and of the
Amnion. — Branching of the Amnion Animals in Two Lines : on the one
side, Reptiles (and Birds), on the other side. Mammals.

** The imagination is an indispensable faculty ; for it ifl that which, by
forming new combinations, occasions important discoveries. The naturalist
needs both the discriminating power of abstract reason, and the generalizing
power q{ tb0 imagination, and that the two should be harmoniously inter.

I08 THE EVOLUTION OF MAK.

related. If the proper balance of these faculties is deatroyed, the natnralist
is hurried into chimerical fancies by his imagination ; while the same gift
leads the gifted naturalist of sufficient strength of reason to the most
important discoveries." — Johannes Mullee (1834).

The further we proceed in human tribal history, the nar-
rower does that part of the animal kingdom become within
which we must look for extinct ancestors of the human
race. At the same time, the evidence as to the history of
the evolution of our race given by what we have called the
records of creation, the evidence of Ontogeny, of Compara-
tive Anatomy, and of Palaeontology, grows constantly more
extensive, complete, and trustworthy. It is therefore
natural that Phylogeny should assume a more definite form
the nearer we approach the higher and the highest stages
of the animal kingdom.

Comparative Anatomy especially has done far more for
our knowledge of these higher stages of evolution in the
animal kingdom than for the lower. This important
science, which aims at a true philosophy of organic forms,
has made greater progress in the Vertebrate tribe than in any
section of the Invertebrate. Cuvier, Meckel, and Johannes
Miiller had already laid a deep and extensive foundation ;
and now the Comparative Anatomy of Vertebrates has
recently been powerfully advanced by the admirable inves-
tigations of Owen and Huxley, and, especially, has been
perfected to such a degree by the unsurpassed labours of
Gegenbaur, that it now forms one of the strongest supports
o£ the Theory of Descent. Relying on the evidence thus
fiimished, we can now, with a great degree of certainty,
recognize the most important outlines of the series of stages
and the r&mi£cationa of the Vertebrate pedigree^

PRIMTnVE FISHESL lOQ

TLat part of the animal kingdom with which we are
now concerned has become so narrow, even before we have
left the Archilithic Epoch, that but a single one of the
seven tribes of the animal kingdom forms the object of our
study. Even within this tribe we have passed the lowest
steps, and have risen above the Skull-less (Acrania) and
Double-nostrilled Vertebrates (Alonorhina), to the class of
i'ishes. The latter are the first of the great main division
of Vertebrates distinguished by mouths with jaws and by
double nostrils (AmphirhiTia, or Onathostcmia). From Fishes
we start again, as from that class of Vertebrates which are
indubitably shown by Comparative Anatomy and Ontogeny
to be the ancestral class of all higher Vertebrates, all Am-
phirhina. Of course no existing Fish can be regarded as
the direct parent-form of the higher Vertebrates. But it is
equally certain that from a common extinct Fish-like
parent-form we may trace all those Vertebrates from Fishes
up to Man, which are included under the name of Am-
phirhina. If this primaeval parent-form were extant, we
should undoubtedly describe it as a genuine Fish and class
it among Fishes. Fortunately, the Comparative Anatomy
and Classification of the Fishes has been so far advanced
(thanks to the labours of Johannes Miiller and Gegenbaur)
that we can very clearly distinguish these most important
and interesting genealogical relations.

In order correctly to understand the human pedigree
within the Vertebrate tribe, it is very important to bear in
mind the distinguishing characteristics, separating Fishes
and all the other Double-nostrils (AmphirhiTia) from
Single-nostrilled and Skull-less Animals (Monorhina and
Acrania). These very distinguishing characteristic marks

no THE EVOLUTION OF MAN.

Fishes have in common with all other Double-nostrils up
to Man, and it is on this parallelism that we found our
claim of relationship to Fishes. (Cf. Table X., vol. i.
p. 466.) The following characters of the Double-nostrils
must be especially indicated as the systematic anatomical
features of the highest importance : (1) the double structure
of the nose ; (2) the internal gill-arch apparatus, together
with the jaw-arches ; (3) the swimming-bladder, or lungs ;
and (4) the two pairs of limbs.

As to the nasal structure, on which is based the distinc-
tion of the Single-nostrils (Monorhina) from the Double-
nostrils {Amphirhina), it is certainly significant that even in
Fishes the earliest rudiment of the nose consists of two en-
tirely distinct lateral grooves or pits in the outer surface of
the head, just as is the case in the embryo of Man and of all
higher Vertebrates. On the other hand, in Single-nostrils
and Skull-less Vertebrates the first rudiment of the nose is,
from the first, a single pit in the centre of the forehead
region. No less important is the higher development of the
skeleton of the gill-arch and of the jaw apparatus connected
with it, as it occurs in all Double-nostrils from Fishes to
Man. It is true that the primitive modification of the
anterior intestine into the gill-intestine, which occurs even
in Ascidians, is developed in all Vertebrates from one simple
rudiment; and in this respect the gill-openings, which in
all Vertebrates and also in Ascidians pierce the wall of the
gill-intestine, are quite characteristic. But the external
framework of the gills, which in aU Skull-less and Single-
nostrilled Animals (Acraniota and Monorhina) supports
the gill-body, is displaced in all Double-nostrils {Amphi-
rhi/na) by an internal gill-skeleton which replaces the former

DOUBLE-NOSTRILS AND SINGLE-NOSTRILS. Ill

This internal gill-support consists of a consecutive series of
cartilaginous arches, which are situated between the gill-
openings within the wall of the throat (pharynx), and
extend round the throat. The foremost of these pairs of
gill-arches changes into the jaw-arch (maxillary arch),
which gives rise to the upper and lower jaws.

A third essential character by which all Double-nostrils
are well distinguished from all those lower Vertebrates
which we have already considered, is the formation of a
blind sac which protrudes from the anterior portion of the
intestinal canal, and which in the Fishes becomes the air-
filled swimming-bladder (Plate V. Fig. 13, lit). As this
organ, in proportion as it contains a greater or less quantity
of air, or in proportion as this air is more or less compressed,
imparts a higher or lower specific gravity to the Fish, it
acts as a hydrostatic apparatus. By this means the Fish
can rise or sink in the water. This swimming-bladder is
the organ from which the lung of higher Vertebrates has
developed. The fourth and last main character of Double-
nostrils is the presence of two pairs of extremities or
members in the primitive arrangement of the embryo ; a
pair of fore limbs, which in Fishes are called pectoral fins
(Fig. 191, v), and a pair of hind limbs, which in Fishes are
called ventral fins (Fig. 191, A). The Comparative Anatomy
of these fins is of supreme interest, because they contain
the rudiments of all those parts of the skeleton which, in
all the higher Vertebrates up to Man, form the skeleton or
support of the extremities of the fore and hind limbs. In
Skull-less and Singie-nostrilled Animals there is, on the
contrary, no trace of these extremities. In addition to

these four most important main characters of the Amphi-

41

112 THE EVOLUTION OF MAN.

rhina, we might further mention the presence of a sym
pathetic nerve-system, a spleen, a ventral salivary gland ;
organs which are not represented in the lower Vertebrates
already considered. All these important parts have trans-
mitted themselves from Fishes up to Man, and from this
circumstance alone it is evident how wide a chasm sepa-
rates the Fishes from the Skull-less and Single-nostrilled
Animals (Acraniota and Monorhina). Fishes and Man
possess all these characters in common (Table X.).

Turning now to consider the Fish class in greater detail,
we may divide it primarily into three main groups, or sub-
classes, the genealogies of which are evident. The first
and most ancient group is that of the Primitive Fishes
(Selachii), the best-known extant representatives of which
are the members of the much-varied orders of Sharks and
Rays (Figs. 191, 192). These are followed by a series of
further developed Fish forms, < by the sub-class of Mucous
Fishes (Ganoides). The greater number of these have long
been extinct, and only very few living representatives are
known ; these are the Sturgeon and Huso of European seas,
the Polypterus of African, and the Lepidosteus and Amia
of American rivers. The earlier abundance of forms belono^-
ing to this interesting group is, however, proved by the
abundance of their fossil remains. From these Mucous
Fishes originated the third sub-class, that of the Osseous
Fishes (Teleostei), to which belong most extant Fishes, espe-
cially nearly all our river fish. Comparative Anatomy
and Ontogeny very clearly show that the Ganoids sprang
from the Selachii, just as the Teleostei sprang from the
Ganoids. But, on the other hand, a second siJe-line, or
rather the main ascending line of the Vertebrate tribe,

EMBRYOS OF SHARKS

113

Fig. 191.

Fig. 192,

114 THE EVOLUTION OF MAN.

Fig. 191. — Embryo of a Shark (Scymnus lichia), seen from ventral side ;
V, pectoral fins (in front of these five pairs of gill-openinsfs) ; h, ventral fins ;
a, anal opening ; s, tail fin ; k, external gill-tufts ; d, yelk-sac (the greater
part of this has been removed) ; g, eye ; n, nose ; m, mouth fissure.

Fig. 192. — Developed Man-shark (Carcharias melanopterus], Been from
the left side : r^ first, r^ second dorsal fin; «, tail fin; a, anal fin; v, pectoral
fins ; h, ventral fins.

developed in another direction from the Primitive Fishes;
this line leads upward through the Dipneusta group to the
important class of Amphibia.

This significant relationship between the three groups
of Fishes has been placed beyond all doubt by the re-
searches of Gegenbaur on the subject. The lucid discussion
on the " systematic position of the Selachii " which that
author inserted in the introduction to his classic study of
the "head skeleton of the Selachii," must be regarded as
definitely proving this important relation.^^^ In Primitive
Fishes (Selachii), however, the scales (skin appendages)
and the teeth (jaw appendages) are identical in formation
and structure, while in the other two groups of Fishes
(Mucous and Osseous Fishes) these organs have already
become distinct and differentiated. Moreover, in Primitive
Fishes, the cartilaginous skeleton (the vertebral column
and the skull, as well as the members) is of the simplest
and most primitive nature, of which the bony skeletons
of Mucous and Osseous Fishes must be regarded as a
modification. It is true that in certain respects (in the
structure of the heart and of the intestinal canal) Mucous
Fishes fully coincide with Primitive Fishes, and differ from
Osseous Fishes. But a comparative review of all the
anatomical relations plainly shows that the Mucous Fishes
constitute a connecting group between Primitive and

MUD-FISHES. 115

Osseous Fishes. The Primitive Fishes (Selachii) form the
most ancient and original group of Fishes. From these,
in one direction, all other Fishes have developed ; the
Mucous Fishes first, which, at a much later period (in the
Jurassic, or the Chalk Period), gave rise to the Osseous
Fishes. In another direction, the Primitive Fishes gave
rise to the parent-forms of the higher Vertebrates, directly
to the Dipneusta, and thus to Amphibians. Regarding the
Selachii as forming the eleventh stage in our pedigree, these
would be followed by the Dipneusta group as the twelfth
stage, and by the Amphibian group as the thirteenth stage.
The advance efiected in the development of the Mud-
fishes (Dipneusta) from the Primitive Fishes is of great mo-
ment, and is connected with a very noticeable cl^ange, which
took place in the beginning of the Palseozoic, or Primary
Period in organic life as a whole. For the very numerous
fossil remains of plants and animals which are now known to
belong to the first three epochs of the history of the earth —
to the Laurentian, the Cambrian, and the Silurian Periods,
are exclusively those of aquatic plants and animals. From
this palseontological fact, taken in connection with certain
weighty geological and biological considerations, we may
infer, with tolerable certainty, that at that time no land-
animals yet existed. During the whole of the enormous
Archizoic Period — during many millions of years — the living
population of our globe were all water-dwellers : a very
remarkable fact, when it is remembered that this period
embraces the larger half of the entire organic history of the
earth. The lower animal tribes are even now exclusively,
or with very few exceptions, aquatic. But during the
Archizoic, or Primordial Epoch, the higher animal tribes

Il6 THE EVOLUTION OF MAN.

continued exclusively adapted to aquatic habits of life. It
was not till later that they adopted a land life. The earliest
fossils of terrestrial animals occur in the Devonian strata,
which were deposited in the beginning of the second great
division of the earth's history (the Palaeozoic Epoch). They
increase greatly in number in the deposits of the Coal and
Permian Periods. Even in these early formations many
terrestrial and air-breathing species, both of the Arthro-
pod and of the Vertebrate tribe, occur ; while their aquatic
ancestors of the Silurian Period breathed nothing but
water. * This physiologically significant modification of the
mode of respiration is the most influential change that
affected the animal organism in the transition from water
to dry land. In the first place it caused the development
of an air-breathing organ, the lung, the water-breathing
gills having previously acted as respiratoiy organs. Simul-
taneously, however, it efi'ected a remarkable change in the
circulation of the blood and in the organs connected with
this ; for these are always most closely correlated with the
respiratory organs. In addition to these, other organs also,
either in consequence of more remote correlation with the
respiratory organs, or in consequence of new adaptations,
were more or less modified.

Within the Vertebrate tribe it was undoubtedly a branch
of the Primitive Fishes (Selachii) which, during the De-
vonian Period, made the first successful effort to accustom
itself to terrestrial life and to breathe atmospheric air. In
this the swimming-bladder was especially of service, for it
succeeded in adapting itself to respiration of air, and so
became a lung. The immediate consequence of this was
fche modification of the heart and nose. While true Fishes

EVOLUTION OF MUD-FISHES. II 7

have only two blind nose-pits on the surface of the head,
these now became connected with the mouth-cavity by an
open passage. A canal formed on each side, leading directly
from the nose-pit into the mouth-cavity, and thus even
while the mouth-opening was closed the necessary atmo-
spheric air could be introduced into the lungs. While,
moreover, in all true Fishes the heart consists simply of two
compartments, an auricle, which receives the venous blood
from the veins of the body, and a ventricle, which forces
this blood through an arterial expansion into the gills, the
auricle, owing to the formation of an incomplete partition
wall, is now divided into a right and a left half
The riD:ht auricle alone now received the venous blood of
the body, while the left auricle received the pulmonic
venous blood passing from the lungs and the gills to the
heart. The simple blood-circulation of the true Fishes thus
became the so-called double circulation of the higher Ver-
tebrates ; and this development resulted, in accordance with
the laws of correlation, in further progress in the structure
of other organs.

The vertebrate class, which thus first adapted itself to
the habit of breathing air, and which originated from a
branch of the Selachii, are called Mud-fishes {Dipneusta),
or Double-breathers, because, like the lowest Amphibia,
they retain the earlier mode of breathing through the gills,
in addition to the newly acquired lung-respiration. This
class must have been represented by numerous and diverse
genera during the Palaeolithic Epoch (during the Devonian,
Carboniferous, and Permian Periods). As, however, the
skeleton is soft and cartilaginous, like that of the Selachii,
they naturally left no fossil remaina The hard teeth of

Il8 THE EVOLUTION OF MAN.

single genera (Ceratodus) could alone endure ; these occur,
for instance, in the Trias. At the present time there are
only three extant genera of this whole class : Protopterua
annectens, in the rivers of tropical Africa (White Nile,
Niger, Quillimane, etc.) ; Lepidosiren paraddxa, in tropical
South America (in the tributaries of the Amazon) ; and
Ceratodus Fosteri, in the swamps of Southern Australia
(Plate XII.).^^^ This wide distribution of the three isolated
descendants of the class is alone sufficient to prove that
they are the last remnants of a group which was formerly
very widely developed. The whole structure of their
bodies shows that the group to which they belong forms the
transition between Fishes and Amphibia, The direct tran-
sitional structure between the two classes is so clearly
expressed in the whole organization of these curious animals,
that zoologists yet dispute whether the Dipneusta are
Fishes or Amphibia. Some well-known zoologists still class
them among Amphibia, while they are usually placed
among Fishes. In fact, the characters of both the classes
are so united in the Dipneusta that the answer to the
question as to their nature depends entirely upon the mean-
ing attached to the terms " Fish " and " Amphibian." In
their mode of life they are true Amphibia. During the
tropical winter, in the rainy season, they swim in the water
like Fishes and inhale water through the gills. During the
dry season they burrow in the mud as it dries up, and
during that period breathe air through lungs, like Am-
phibians and higher Vertebrates. In this two-fold respira-
tion they do, it is true, coincide with the lower Amphibia,
and stand far above Fishes. Yet, in most other characters
they more nearly resemble the latter, and stand below the

EXTANT MUD-FISHES. 1 19

former. Their external appearance is entirely like that of
Fishes.

The head of the Dipneusta is not distinct from the
trunk. The skin is covered with large fish-scales. The
skeleton is soft, cartilaginous; its development has been
arrested at a very low stage, just as in the lower Primitive
Fishes. The notochord is retained entire. The two pairs of
limbs are very simple fins of primitive structure, like those
of the lowest Primitive Fishes. The structure of the braia,
of the intestinal tube, and the sexual organs, is also as in
Primitive Fishes. The Dipneusta, or Mud-fishes, have, there-
fore, by heredity, accurately retained many features of a
lower organization derived from our primseval Fish ancestors,
while their adoption of the habit of breathing air through
lungs introduced a great advance in the vertebrate organi-
zation.

Moreover, the three extant Mud-fishes differ a good deal
from one another in important points of structure. The
Australian Mud-fish (Ceratodus), which was first described
at Sidney in 1870 by Gerard Kreffl, and which attains a
length of six feet, appears in an especial degree to represent
a primaeval and very conservative animal form (Plate XII.).
This is especially true of the structure of its simple lung,
and of its fins, which contain a pinnate skeleton. In the
African Mud-fish (Protopterus), on the contrary, and in the
American form {Lepidosiren) the double lung is present, as
in all higher Vertebrates ; nor is the fin-skeleton pinnate.
In addition to the internal gills, Protopterus has also ex-
ternal gills, which are wanting in Lepidosiren. Those
unknown Dipneusta, which were among our direct ancestors
and which formed the connecting link between the Selachii

( I20 )

TABLE XX.

SysteiiuUic Survey of the Phyhgenetic Classification of

Vertebrates.

I. SkuII4ess (Acrania), or ^ubc-f)eartet( (Leptocardia).
Vertebrates without a specialized head, skull, brain, or centralized heart.

1. &ku\Ultss
Aorania

I. Tube-hearted
Leptocardia

1. Lancelets

1. Amphioxida

II. Animals hittfj skulls (Craniota) and with centraltjeti fjearts (Pachycardia).
Vertebrates with specialized head, with skull and brain, and with a

centralized heart.

Main-clasiet

of the

Skulled Animals.

Classes

of the

Skulled Aninals.

Sub-classes

of the

Skulled Animal*.

Systematic NaToe

of the

Sub-class€t.

2. 5mglc=
0ostrilktJ
Monorhina

3. yon«
atnnionatt
Anaimiia

II. Round mouths
Cyclostoma

III. Fishes
Pisces

IV. Mud-fishes
Dipneusta

V. Batrachians
Aviphihia

k *atnnton5
"Knimals
Amniota

VI. Reptiles
Beptilia

VII. Birds

Aves

VIII. Mammals
Marrnnalia

2. Hags, or

Mucous Fish

3. Lampreys

4. Primitive Fish

5. Ganoid Fish

6. Osseous Fish

7. Single-lunged

8. Double -lunged

9. Mailed Batra-

chians
10. Naked Batra-
chians

^IL Lizards

12. Snakes

13. Crocodiles

14. Tortoises

15. Sea -dragons

16. Dragons

17- FlyiugKeptiles

18. BeakedAninials

19. Long-tailed

20. Fan.tailed

21. Bush-tailed

/22. Cloacal Animals

23. Pouched Ani-
mals

24. Placental Ani-
mals

1

2. Hyperotreta
(Myxinoida)

3. Hyperoartia
(Petromyzontia)

4. Selachii

5. Ganoides

6. Teleostei

T.lMonopneumonee

8. Dipneumones

9. Phractamphibia

10. Lissamphibia

11. Lacertilia

12. Ophidia
18. Crocodilia

14. Chelonia

15. Halisauria

16. Dinosauria

17. Pterosauria

18. Anomodonta

19. SaururaB

20. Carinatae

21. Ratitas

22. Monotrema

23. Marsupialia

24. Placent&lia

Oweons fisk
TeleoaUi

Ganoid fish
OanoidsM

( 131 )

TABLE XXL

Pedigree of Vertebrates. {Of. Plate XV.)

8. fEammals
Mammalia

Single-lnnged
Monop nen mones

Double-Inn ged
Dypneumones

7. Birds
Aves

I
6. Reptiles

Repti/ia

Mud -fish
Protoptm'i

^mnionMnimBls
Amniota

I
5. ISattarfjiafts
Ampliibia

4. fHutJ^fisfj
Dipneusta

Primitive fishes Selaohii
3. Fishes Pisces

©oufaU^nostrillrt
Amphirhina

■ — Lampreys Hags

Petromytontes Myxinoid"-

2. Round-mouths

Cylostoina

Sea-equirts

Ascidico

Sea-barrels

Thaliacea

IHantlcTJ ^nimali
Tuuicata

5>ingIf'n0striIIctJ Monorhina
,S)kuiIrti'llutmal;a Craniota

1. Tube-heaited
Leptoco/rdia

Acrania
Vtxithxatts
Vertebrata

C!)ortia^^nimate
Chordouia

Worms
Vermes

122 THE EVOLUTION OF MAN.

and the Amphibians, were doubtless in many respects
different from their three direct descendants of the present
time, but in the most essential characters they must have
coincided with the latter. Unfortunately, the germ-history
of the three surviving Mud -fishes is as yet entirely un-
known; probably at some future time it will afford us
further important information as to the tribal history of the
lower Vertebrates and so of our ancestors.

Very important information of this kind has been
supplied by the next Vertebrate class, that of the Batra-
chians (Amphihia), which is directly connected with the
Dipneusta, from which it originated. To this class belong
the Axolotl, Salamanders (Plate XIII.), Toads, and Frogs.
Formerly, after the example of Linnaeus, all Reptiles (Lizards,
Snakes, Crocodiles, and Tortoises) were also classed among
Amphibia. But these animals are of a far higher organiza-
tion, and in the most important characters of their ana-
tomical structure are more nearly allied to Birds than to
Amphibians. The true Amphibia, on the other hand, are
more nearly allied to the Double-breathers and to Primitive
Fishes : they are also much older than Reptiles. Even as
earl}'' as the Carboniferous Period numerous very highly
developed Amphibia (some of large size) were extant, whereas
the earliest Reptiles first appear only towards the close of
the Permian Period. In all probability the Amphibia were
developed from Double-breathers at an even earlier period —
during the Devonian Period. The extinct Amphibia, of
which fossil remains have been preserved from that most
ancient Primaeval Epoch — and these are especially numerous
in the Trias — were distinguished by a large bony coat of mail
overlying the skin (like that of the Crocodile), while most

BATRACHIANS. I23

of the yet extant Amphibians have a smooth and slippery
skin. The latter, also, are on an average smoother than the
former, and must be regarded as their stunted posterity.

Among the Amphibia of the present time we are
therefore, unable to find any forms that are directly referable
tx) the pedigree of the human race, or that are to be re-
garded as ancestors of the three higher Vertebrate classes ;
yet, in important points of their internal anatomical struc-
ture, and especially in their germ-development, they cor-
respond so closely with us, that we are justified in affirming
that between the Double-breathers (Dipneusta) on the one
hand, and the three higher Vertebrate classes (grouped
together as Amniota) on the other, there existed a series of
extinct intermediate forms which, if we had them before us,
we should class among Amphibia. The whole organization
of the extant Amphibia represents a transitional group of
this kind. In the important matters of respiration and
circulation of the blood, they are still closely allied to the
Double-breathers, although in other respects they rise above
the latter This is especially true with respect to the ad-
vanced structure of their limbs or extremities. The latter
here for the first time appear as feet with five digits. The
thorough researches of Gegenbaur have shown that the fins
of Fishes, concerning which very en^oneous views were pre-
viously held, are feet with numerous digits ; that is to say,
the several cartilaginous or osseous rays, many of which occur
in every Fish-fin, correspond to the fingers or digits on the
limbs of higher Vertebrates. The several joints of each ray
correspond to the several joints of each digit. In the Double-
breathers the fin yet retains the same structure as in Fishes,
and it was only gradually that the five-toed form of foot,

124 THE EVOLUTION OF MAN.

w^hich occurs for the first time in Amphibians, was developed
from this multi -digitate form. This reduction in the numbei
of the digits from ten to five occurred in those Dipneusta
which must be regarded as the parent-forms of the Amphibia,
probably as early as the latter half of the Devonian Period —
or, at latest, in the immediately subsequent Carboniferous
Period. Several fossil Amphibia with five digits have already
been found in the strata of the latter period. Fossil foot-
prints of the same animals are very numerous in the
Trias {Cher other iun).

The great significance of the five digits depends on the
fact that this number has been transmitted from the
Amphibia to all higher Vertebrates. It would be impossible
to discover any reason why in the lowest Amphibia, as well
as in Reptiles and in higher Vertebrates up to Man,
there should always originally be five digits on each of the
anterior and posterior limbs, if we denied that heredity
from a common five-fingered parent-form is the efiicient
cause of this phenomenon : heredity can alone account foi
it. In many Amphibia, certainly, as well as in many higher
Vertebrates, we find less than five digits. But in all these
cases it can be shown that separate digits have retrograded,
and have finally been completely lost.

The causes which effected the development of the five-
fingered foot of the higher Vertebrates in this Amphibian
parent-form from the many-fingered foot, must certainly be
found in the adaptation to the totally altered functions
which the limbs had to discharge during the transition from
an exclusively aquatic life to one which was partially
terrestrial. While the many-fingered fins of the Fish had
previously served almost exclusively to propel the body

PINS AND FINGERS. 12$

through the water, they had now also to afford support to
the animal while creeping upon land. This effected a
modification both of the skeleton and of the muscles of
the limbs. The number of fin rays was gradually lessened,
and was finally reduced to five. These five remaining
rays now, however, developed more vigorously. The soft
cartilaginous rays became hard bones. The rest of the
skeleton also became considerably more firm. The move-
ments of the body became not only more vigorous, but
also more varied. The separate portions of the skeleton
system, and consequently those of the muscular system also,
became more and more differentiated. Owing to the intimate
correlation of the muscular to the nervous system, the latter
also naturally made marked progress in point of function
and structure. We therefore find that the brain is very
much more developed in the higher Amphibia than in
Fishes, in Mud-fishes, and in the lower Amphibia.

The organs which are most modified in consequence of
an amphibious mode of life are, as we have already seen in
the Double-breathers (Dipneusta), those of respiration and
of the circulation of the blood. The first advance in
organization necessitated by the transition from aquatic to
terrestrial habits of life was, of course, the formation of an
air-breathing organ, a lung. This developed directly from
the swimming-bladder which these animals had inherited
from the Fishes. At first the function of this organ would
be quite subordinate to the more ancient organ, used for the
respiration of water, the gills. Hence we find that the
lowest Amphibia, the Gilled Amphibia, like the Dipneusta,
spend the greater part of their lives in the water, and that
accordingly they breathe water through gills. It is only

126 THE EVOLUTION OF MAN.

Tor brief intervals that they rise to the surface of the water
or creep out of the water on to the land ; and at these times
they breathe air through lungs. Some, however, of the
Tailed Amphibians, the Axolotl and the Salamander, live
exclusively in the water only when young, and afterwards
usually remain on land. In the adult state they breathe
only air through lungs. This is also the case with the most
highly developed Amphibians, the Frog-amphibia (Frogs and
Toads) ; some of the latter have even entirely lost the
gilled larval form.^^^ The same is true of a few small
snake-like Amphibia, the Csecilise, which, like earth-worms,
live in the ground.

The high degree of interest attached to the natural
history of the Amphibian class is especially due to the fact
that they hold a position exactly intermediate between the
higher and the lower Vertebrates. While the lower Am-
phibia are in their whole organization directly allied to the
Dipneusta and the Fishes, living mostly in the water and
respiring water through gills, the higher Amphibia are no
less directly related to the Amnion Animals, for, like the
latter, they live mostly on land, and breathe air through
lungs. But when young the higher forms resemble the lower,
and only attain their own higher degree of development
after undergoing complete modification. The individual
germ-history of most higher Amphibians still accurately
reproduces the tribal history of the whole class ; and the
various stages of modification which were necessitated in
certain low Vertebrates by the transition from aquatic to
terrestrial habits during the Devonian or Carboniferous
Period, are still to be seen every spring in each Frog as it
develops from the egg in our ditches and pools.

I

IMPORTANCE OF AMPHIBIA.

127

Like the Tailed Salamanders (Fig. 193), each common
Frog emerges from the egg in a larval form, totally different
jccm that of the full-grown Frog (Fig. 194). The short

I

Fig, 193. — Larva of Spotted Land-Newt {Salamandra macuJata), from
the ventral- Side. Li tiie centre a yelk-sac yet protrudes from the intestine.
The external gills are prettily branched and tree-like. The two pairs of
limbs are yet very small.

Fig. 194. — Larva of the Common Grass-Frog (Rana temforaria) , a so-
called tadpole : m, mouth; n, a pair of suction cups used in clinging to stones;
d, skin-fold, which gives rise to the gill-roof ; behind are the gill-openings,
from which the gill branches protrude ; s, tail-muscles ; /, skin-fold of the
tail, forming a float.

■s

irunk is produced into a long tail, which in form and struc-
42

125 THE EVOLUTION OF MAM.

ture reseml:)les the tail of a Fish (s). At first it has no
limbs. Respiration is accomplished solely by gills, which
are at first external (k) and afterwards internal. Corre-
spo'ndingly, the heart is also of the same form as in the
Fishes, and consists of only two compartments — an auricle,
which receives the venous blood of the body, and a ven-
tricle, which drives it through the arterial bulb into the
gills.

Numbers of these fish-like Frog larvae, or " tadpoles," as
they are called, swim about every spring in all ponds and
pools, using their muscular tails for propulsion, just as is
done by Fishes and larval Ascidians. The remarkable
transformation of the fish-like form into that of the Froji
does not take place till after the tadpole has grown to n
certain size. From the throat grows a closed sac which
develops into a pair of large sacs; these are the lungs.
The simple chamber of the heart is divided into two auricles,
owing to the formation of a partition wall, and simul-
taneously considerable changes of structure occur in the
main arterial trunks. Previously all the blood passed from
the heart-chamber through the aorta arches into the gills ;
but only part of it now passes to the gills, while another
part passes thiough the newly formed lung arteries into the
lungs. From the lungs arterial blood returns into the left
auricle of the heart, while the venous blood of the body
collects in the right auricle. As both of the auricles open
into the simple ventricle, the latter contains mixed blood.
The fish-like form has now passed into the Dipneusta form.
During the further course of modification the gills, with
the gill- vessels, are entirely lost, and respiration is now pei-
formed by the lungs alone. Yet later, the long tail is also

GILLED BATRACHIANS. 1 29

rejected, and the Frog now leaps about on the land on legs
which have sprouted in the mean time.^^

This remarkable metamorphosis of the higher Amphibia
ia very instructive in its bearing on Man's ancestral history,
and is especially interesting owing to the fact that the
various groups of extant Amphibia have remained stationary
at various stages of their tribal history, which, in accord-
ance with the fundamental law of Biogeny, are reproduced
in this germ-history. First, there is a very low order of
Amphibia, the Gilled Batrachians {Sozohranchia), which, like
Fishes, retain their gills throughout life. To this order
belong, among others, the well-known blind " 01m " of the
Adelsberg Cave {Proteus anguineus), the Mud-eel of South
Carolina {Siren lacertina), and the Axolotl of Mexico (Sire-
don pisciformiis ; Plate XIII. Fig. 1). All these Gilled
Batrachians are fish-like animals with long tails, and in
point of respiratory organs and of circulation of the blood
they remain throughout life stationary at the Dipneusta
stage. They possess both gills and lungs, and can either
respire water through the gills or air through the lungs, as
occasion requires. In another order, the Salamanders, the
gills are lost during metamorphosis, and in the adult state air
only is breathed through lungs. This order bears the name
of Tailed Batrachians {Sozura) because they retain the tail
throughout life. To this order belong the common Water-
Newts \Triton) which swarm in all ponds during the
summer, and the black, yeUow-speckled Land-Salamanders
(Saiamandra) foiuad in damp woods (Plate XIII. Fig. 2).
The latter are among the moat remarkable of our indigenous
animals, sundry anatomical characters proving them to bo
very ancient and highly conservative Vertebrates.^^ A

130 THE EVOLUTION OF MAN.

few Tailed Batrachians retain the gill-opening in the side
of the neck, though the gills themselves are lost {Meno-
poma). If the larv?e of the Salamanders (Fig. 193) and
Tritons are compelled to remain in water, and not allowed
to get on land, they may, under favourable conditions, be
made to retain their gills. In this fish-like condition they
become sexually mature, and will throughout life remain
compulsorily in the lower stage of development of the
Gilled Batrachians. The opposite experiment was made
some years ago in the case of the Mexican Gilled Batra-
chian, the fish-like AxolotJ (Siredon pisciformis; Plate
XIII. Fig. 1). This animal had previously been regarded
as a permanent Gilled Batrachian, remaining throughout
life in this fish-like condition. But of the hundreds of
these animals kept in the Jardin des Plantes at Paris, a few
individuals, for some unknown reason, crept to land, lost
their gills, and changed into a form closely allied to that of
the Salamander (Amhlystoma, Fig. 2). In this state they
became sexually mature.^^^ This phenomenon, which at
first excited a lively interest, has since been repeatedly
observed with care. Zoologists regarded the fact as some-
thing peculiarly wonderful, though each spring every
common Frog and Salamander passes through the same
modification. In these animals we can in the same way
follow each step in the significant metamorphosis of the
aquatic and gill-respiring animal into the terrestrial and
lung-respiring animal. That which thus takes place in the
individual during germ-evolution, took place in the same
way in the whole class during the course of its tribal
history.

The metamorphosis which takes place in the third order

HAECKRL S EVOLUTION OF MAN.

PLATE XII.

'm

t^>ii

i

m

Tigl

^m^^

liS^

/"

X

If

A/

x.\'

J/,

-^\

(>«

j([i

>'-i

S^

Cera^odus
Torsteri

^j

'^»^

vS*s

HA.ECKEL S EVOLUTION OF MAN

PLATE XIII.

f^2

\i\

■•j^-^^'
:?"%

m^

Fig. I. Siredon pisciformis.

Fig. 2. Salamandra maculata.

TAILED BATRACHIANS AND FROG BATRACHIANS. I31

of Amphibia, the Frog Batrachians (Batrachia, or Anura),
is yet more complete than in the Salamanders. To these
belong all the various kinds of Toads, Water-frogs, Tree-
frogs, etc. In the course of transformation these lose not
only the gills, but also the tail, which drops off in some
cases earlier, in others later. In this respect the various
species differ somewhat from one another. In most Frog
Batrachians the larvae drop the tail very early, and the
tail-less frog-like form subsequently grows considerably
larger. Other species, on the contrary, as, for instance, the
Pseudes paradoxus of Brazil, as also an European Toad (Pelo-
hatesfuscus) remain for a very long time in the fish fonn,
and retain a lengthy tail till they have almost attained
their full size; hence, after their metamorphosis is com-
pleted, they appear much smaller than before. The opposite
extreme is seen in some Frogs but recently brought under
notice, which have lost the whole of their historic meta-
morphoses, and in which no tailed and gilled larva emerges
from the egg, but the perfect Frog, without tail or gills.
These Frogs inhabit isolated oceanic islands, the climate
of which is very dry, and which are often for a con-
siderable leno-th of time without fresh water. As fresh
water is indispensable for gill-respiring tadpoles, these Frog-
have adapted themselves to this local deficiency and have
entirely relinquished their original metamorphosis, e.g..
Hylodes martinicensis}^

The ontooenetic loss of o'ills and tail in Frosts and Toads
can of course only be phylogenetically explained as owing
to the fact that these animals have descended from long-
tailed salamander-like Amphibians. This is also proved
beyond doubt by the Comparative Anatomy of the two

132 THE EVOLUTION OF MAN.

^oups. This remarkable transformation is, in other res\ ecta
also, of general interest, as throwing a flood of light upon the
Phylogeny of the Tail-less Apes and of Man. Man's ances-
tors were also long-tailed gill- breathing animals, resembling
Gilled Batrachians, as is irrefutably demonstrated by the
tail and the gill arches in the human embryo.

During the Palgeozoic Epoch, and probably in the Car-
boniferous Period, there is no doubt that the Amphibian
class embraced a series of forms which must be regarded as
direct ancestors of Mammals, and so of Man. On grounds
derived from Comparative Anatomy and Ontogeny, we must
not, however, look for these Amphibian ancestors of ours —
as might perhaps be supposed — among the Tail-less Frog
Batrachians, but only among the lower Tailed Amphibians.
We can with certainty point to at least two extinct Batra-
chian forms as direct ancestors of Man, as the thirteenth
and fourteenth stages in our pedigree. The thirteenth
ancestral form must have been closely allied to the Double-
breathers (Dipneusta), must, like these, have possessed per-
manent gills, but must have been already characterized by
having five digits on each foot ; and were they still living we
should place them in the group of Gilled Batrachians, with
the Proteus and the Axolotl (Plate XIII. Fig. 1). The
fourteenth ancestral form, on the other hand, must indeed
have retained the long tail, but must have lost the gills, and
hence the nearest aUied forms among extant Tailed Batra-
chians would be the Water-Newts and Salamanders
(Plate XIII. Fig. 2). Indeed, in the year 1725 the fossil
skeleton of one of these extinct Salamanders (closely aUieJ
to the present giant Salamander of Japan) was described
by the Swiss naturalist, Scheuchzer, as the skeleton of

PRIMITIVE AMNION ANIMALS. 1 33

a fossil Man dating from the Deluge ! (" Homo diluvii
testis." i«^)

As the vertebrate form occurring in our pedigree imme-
diately after these Batrachian ancestors — and, therefore, as
the fifteenth stage — let us now examine a lizard-like animal,
^ which no fossil remains have been obtained, and which
k not even proximately represented in any extant animal
form, but the former existence of which we may infer with
the utmost certainty from certain comparative anatomical
and ontogenetica] facts. This important animal form we
will call the Protamnion, or Primitive Amniotic animal.
All Vertebrates higher than the Amphibia — that is, the three
classes of Reptiles, Birds, and Mammals — are so essentially
distinct in their whole structure from all the lower Verte-
brates which we have as yet considered, and, on the other
hand, have so much in common, that we may class tlienj
together in one group as Amnion Animals {Arriniota). It it
only in these three classes of animals that we find that
remarkable envelope of the embryo known as the amnion.
(Cf. vol. i. p. 386.) The latter must probably be regarded as
a kenogenetic adaptation, as caused by the sinking of the
embryo into the yelk-sac.^^^

All known Amnion Animals, all Reptiles, Birds, and
Mammals (Man included), coincide in so many important
points of organization and development that we are fully
justified in asserting their common descent from a single
parent form. If the testimony of Comparative Anatomy
and Ontogeny is entirely unquestionable in any point, it is
certainly so here. For all the special peculiarities and
characters, which appear accompanying and following the
formation of the amnion; and which we found in the

134 THE EVOLUTION OF MAH.

development of the human embryo; all the many peculiari-
ties in the development of the organs which we shall
presently notice in detail ; and, finally, the chief special
arrangements of the internal structure of the body in all
fully developed Amnion Animals; all these so clearly demon-
strate the common origin of all Amnion Animals from a
single extinct parent-form, that it is impossible to conceive
their origin as polyphyletic, and that they originated from
several independent parent- forms. This unknown common
parent-form is the Primitive Amnion Animal {Protam-
nion). ' In external appearance the Protamnion was most
probably an intermediate form between the Salamanders
and the Lizards.

It was probably during the Permian Period that the
Protamnion originated ; perhaps at the beginning, perhaps
at the close of that period. This we know from the fact
that the Amphibia did not attain their full development till
the Carboniferous Period, and that toward the close of the
Permian the first fossil Reptiles make their appearance —
or, at least, fossils (Proterosaurus, Phopalodon) which must
in all probability be referred to lizard-like Reptiles. Among
the great and pregnant modifications of the vertebrate
organization determined during this period by the develop-
ment of the first Amnion Animals from salamander-like
Amphibians, the three following are especially important :
the total loss of water-breathing gills and modification of
the gill-arches into other organs ; the formation of the
allantois, or primitive urinary sac ; and, finally, the develop-
ment of the amnion.

The total loss of the respiratory gills must be regarded
as one of the most prominent characters of all Amnion

BYOLUTION OF AMNION ANIMALS. I35

1

Animals. All these, even such as live in the water, e.g.
whales, respire only air through lungs, never water through
gills. While all Amphibians, with very few exceptions, in
the young state retain their gills for a longer or shorter
period, and breathe through gills for some time (if not
always), from this point gill-respiration entirely ceases.
Even the Protamnion must have entirely ceased to breathe
water. The gill-arches, however, remain, and develoji
into very different organs (partly rudimentary) ; into the
various parts of the tongue-bone, into certain portions of the
jaw apparatus, the organ of hearing, etc. But no trace oj
gill-leaves, of real respiratory organs on the gill-arches, are
ever found in the embrvo of Amnion Animals.

With this total loss of the gills is probably connected
the formation of another organ, which we have akead}
described as occurring in human Ontogeny ; this is the
allantois, or primitive urinary sac. (See vol. i. p. 379.) In
all probability the urinary bladder of the Dipneusta is to be
regarded as the first beginning of the allantois. Even in
the American Mud-fish {Lepidosiven) we find an urinary
bladder, which grows from the lower wall of the posterior
extremity of the intestine, and serves as a receptacle for
the renal secretions. This organ has been inherited by the
Amphibia, as may be seen in any Frog. But it is only in
the three higher Vertebrate classes that the allantois attains
a special development ; in these it protrudes at an early
period from the body of the embryo, forming a large sac
filled with liquid, and traversed by a considerable number
of large blood-vessels. This sac also discharges a portion of
the nutritive functions. In the higher Mammals and in
Man the allantois afterwards forms the placenta.

1^6 THE EVOLUTION OF MAN.

The formation of the amnion and the allantois, together
with the total loss of the gills and the exclusive adoption
of lung-respiration, are the most important characters by
which all Amnion Animals are distinguished from the lower
Vertebrates which we have been considerinor. In addition
to these there are a few subordinate characters which are
constantly inherited by Amnion Animals, and are altogether
wanting in animals without an amnion. One striking em-
bryonic character of the Amnion Animals is the great curva-
ture of the head and neck of the embryo. In the Anamnia
the embryo is from the first either nearly straight, or else
the whole body is bent in a sickle-shaped curve corre-
sponding to the curvature of the yelk sac, to which the
embryo is attached by its ventral surface ; but there are
no marked angles in the longitudinal axis (Plate VI.
Fig. F). In all Amnion Animals, on the contrary, the
body is very noticeably bent at an early age, so that the
back of the embryo is much arched outwards, the head
pressed almost at right angles against the breast, and the
tail inclined on to the abdomen. The tail extremity, as it
bends inwards, approaches so near to the frontal side of
the head, that the two often nearly touch (Plates VI, and
VII.). This striking triple curvature of the embryonic
body, which has already been considered when we studied
the Ontogeny of ^lan, and in which we distinguished the
skull-curve, neck-curve, and tail-curve (vol. i. p. 871), is a
characteristic peculiarity common to the embryos of all
Reptiles, Birds, and Mammals. But in the formation of man)»
internal organs also, an advance is observable in all the
Amnion Animals which ranks them above the highest of
the non-aronionate forms. Above all, a partition wall foims

MAN AS AN AMNION ANIMAL. 1 37

within the simple ventricle of the heart, dividing it into a
right and a left ventricle. In connection with the complete
metamoi^hosis of the gill-arches, a further development of
the organ of hearing takes place. A considerable advance
is also noticeable in the development of the brain, the skele-
ton, the muscular system, and other parts. Finally, the
reconstruction of the kidneys must be regarded as a mdst
important modification. In all the lower Vertebrates as yet
considered, we have found the primitive kidneys, which
appear very early in the embryos of aU higher Vertebrates
up to Man, acting as a secretory or urinary apparatus. In
Amnion Animals, however, these early primitive kidneys
lose their function at an early period of embryonic life, and
it is assumed by the permanent " secondary kidneys," which
grow out of the terminal portion of the primitive kidney
ducts.

Looking: back at the whole of these characters of Amnion
Animals, it is impossible to doubt that all animals of this
group, all Reptiles, Birds, and Mammals, had a common
origin, and constitute a single main division of kindred
forms. To this division belongs our own race. In his
whole organization and germ-history Man is a true Amnion
Animal, and, in common with all other Amniota, has
descended from the Protamnion. Although this whole
group originated at the end, or perhaps even in the middle,
of the Palseozoic Epoch, it did not attain its full de-
velopment and its full perfection till the Mesozoic Epoch.
The two classes of Birds and Mammals then first appeared.
Nor did the Reptilian class develop in its fuU variety
until the Mesozoic Epoch, which is, therefore, called the "Age
of Reptiles." The unknown and extinct Protamnion, the

138 THE EV0LT7TI0N OF MAN.

parent-form of iue entire group, must have been very nearly
allied to the Reptiles in its whole organization, even though
it cannot be regarded as a true Reptile in the present
meaning of the term. Of all known Reptiles, certain Lizards
are most nearly allied to the Protamnion ; and in the
outward form of its body we may imagine the latter as
an intermediate form between the Salamander and the
Lizard.^^

The Comparative Anatomy and Ontogeny of the Am-
nionate group clearly explains its genealogy. The gi*oup
which directly descended from Protamnion gave rise to two
divergent branches. The first of these, which will in future
receive our whole attention, forms the Mammalian group.
The other branch, which assumed an entirely different course
of progressive development, and which is connected with
the mammalian branch only as the root, is the compre-
hensive group constituted by Reptiles and Birds. The two
latter forms may be classed together as Monocondylia, or
SauTopsides. The common parent-form of these is an
extinct lizard-like Reptile. From this, the Serpents, Croco-
diles, Tortoises, Dragons, etc. — in short, all the various forms
of the Reptilian gi'oup — developed in different directions.
The remarkable group formed by the Birds also developed
directly from an offshoot of the Reptilian group, as is now
definitely proved. Down to a late time the embryos of
Reptiles and of Birds are yet identical, and even later they
are in some respects surprisingly similar. (See Plate VI. Fig.
T and C.) In their entire organization the resemblance
between the two is so great that ^o anatomist now denies
that the Birds originated from Reptiles. The Mammalian
line is connected at its roots with the Reptilian line but

GENEALOGY OF AMNION ANIMALS. 1 39

afterwards diverged entirely from the latter, and developed
in an entirely peculiar direction. The highest result of the
development of the Mammalian line is Man, the so-called
" Crown of Creation.**

CHAPTER XIX.

THE PEDIGREE OF MAN.

IV. Feom the Primitive Mammal to the Ape.

The Mammalian Character of Man. — Common Descent of all Mainmalb
from a Single Parent-form (Promammalian). — Bifurcation of the Am
nion Animals into Two Main Lines : on the one side, Reptiles and Birds,
on the other. Mammals. — Date of the Origin of Mammals : the Trias
Period. — The Three Main Groups or Sub-classes of Mammals : their
Genealogical Relations. — Sixteenth Ancestral -Stage: Cloacal Animals
(Monotremata, or Ornithodelphia) . — 'Ihe Extinct Primitive Mammals
(^Promammalia) and the Extant Beaked Animals {Ornitho stoma) . —
Seventeenth Ancestral Stage : Pouched Animals {Marsupialia, or Didel-
phia). — Extinct and Extant Pouched Animals. — Their Intermediate
Position between Monotremes and Placental Animals. — Origin and
Structtire of Placental Animals {Placentalia, or Monodelphia) . — Forma-
tion of the Placenta. — The Deciduous Embryonic Membrane (Decidua).
— Group of the Indecidua and of the BecidMata. — The Formation of the
Decidua {vera, serotina, rejlexa) in Man and in Apes. — Eighteenth
Stage: Semi-apes {Prosimice) . — Nineteenth Stage : Tailed Apes {Meno'
eerca). — Twentieth Stage : Man-like Apes (Anthropoides). — Speechless
and Speaking Men {Mali. Homines).

** A century of anatomical research brings ns back to the conolnsion of
Linnaeus, the great lawgiver of systematic zoology, that man is a member
of the same order as the apes and lemurs. Perhaps no order of mammals
presents us with so extraordinary a series of gradations as this, leading us
insensibly from the crown and summit of the animal creation down to
creatures from which there is but a step, as it seems, to the lowest, smallest,
»ixd least intelligent of the placental mammalia. It is as if nature iierself

"man's place in nature." 141

had foreseen the axrogance of man, and with Roman Beverity had provided
that his intellect, by its very tnumphs, should call iuto {n'ominence the
slaves, admonishing the conqueror that he is but dust." — Thomas Huxlev
(1863).

Among those zoological facts which afford us points of
support in researches into the pedigree of the human race,
the position of Man in the Mammalian class is one of the
most important and fundamental. Much as zoologists havr
long disagreed in their opinions as to Man's particular place
■ in this class, and especially in their ideas of his relation to
the most nearly related group, that of the Apes, yet no
naturalist has ever doubted that Man is a genuine Mammal
in the whole structure and development of his body. Every
anatomical museum, every manual of Comparative Anatomy,
affords proof that the structure of the human body shares
all those peculiarities which are common to all ^Lammals,
and by which the latter are definitely distinguished from all
other animals.

Now, if we examine this established anatomical fact
phylogenetically, and in the light of the Theory of Descent,
we arrive immediately at the conclusion that Man is of a
common stock with all the other Mammals, and springs
from a root common to them. The various characteristics
in which all Mammals coincide, and in which they diffei
from all other animals, are, moreover, of such a kind, that a
polyphyletic hypothesis appears in a special degree inad-
missible in their case. It is inconceivable that all existing
and extinct Mammals have sprung from several ditlerent
and originally separate root-forms. We are compelled, if
we in any way acknowledge the Theory of Evolution, to
assume the monophyletic hypothesis, that all Mammals,

142 THE EVOLUTION OF MAN.

including Man, must be traced from a single common mam-
malian parent-form. This long extinct primaeval root-form
and its immediate descendants — which differ from each
other hardly more than do several species of one genus — we
will call Primitive Mammals (Promarrinialia). As we have
already seen, this root-form developed from the ancient
parent-form of the Primitive Amnion Animals in a direction
wholly different from that followed by the Reptile group,
which afterwards gave rise to the more highly developed
class of Birds. The differences which distinguish Mammals
on the one side, from Reptiles and Birds on the other, are so
important and characteristic, that we may quite safely as-
sume a bifurcation of this kind in the vertebrate family tree.
Reptiles and Birds — which we classed together as Monocon-
dylia, or Sauropsida — coincide entirely, for instance, in the
characteristic structure of the skull and brain, which is
strikingly dissimilar from that of the same parts in Mam-
mals. In Reptiles and Birds, the skull is connected with the
first cervical vertebra (the atlas) by a single joint-process
(condyle) of the occipital bone; in Mammals, on the con-
trary (as in Amphibians), the condyle is double. In the
former, the under jaw is composed of many parts, and is
connected with the skull by a peculiar bone of the jaw
(the square bone) so as to be movable ; in the latter, on the
contrary, the lower jaw consists of but two bone-pieces,
which are directly attached to the temporal bone. Again,
the skin of the Sauropsida (Reptiles and Birds) is covered
with scales or feathers, that of the Mammals with hair.
The red blood-cells of the former are nucleated, those of the
latter non-nucleated. The eggs of the former are very
large, are provided with a large nutritive yelk, and undergo

REPTILES AND MAMMALS. 143

discoidal cleavage resulting in a Disc-gastrula ; the eggs of
the latter are very small, and their unequal cleavage results
in the formation of a Hood -gas trula. Finally, two charac-
ters entirely peculiar to Mammals, and by which these
are distinguished both from Birds and Reptiles and from all
other animals, are the presence of a complete diaphragm,
and of the milk-glands (manionce), by means of which the
new-bom young are nourished by the milk of the mother.
It is only in Mammals that the diaphragm forms a transverse
partition- wall across the body-cavity (coeloma), completely
separating the chest from the ventral cavity. (Cf. Plate V.
Fig. 16 z.) It is only among Mammals that the mother
nourishes the young with her milk ; and the whole class are
well named from this.

These important facts in Comparative Anatomy and
Ontogeny clearly show that the tribe of Amnion Animals
(Amniota) bifurcated from the very first into two main
diverging lines ; on the one side, the Reptilian line, from
which the Birds afterwards developed ; on the other side,
the Mammalian line. The same facts also prove as indu-
bitably that Man originated from the latter line. For Man,
in common with Mammals, shares all the characteristics we
have mentioned, and is distinguished by them from aF
other animals. And, finally, these facts indicate as ceii^inly
those advances in vertebrate structure by which one branch
of the Primitive Amnion Animals developed into the parent-
form of Mammals. The most prominent of these advances
were (1) the characteristic modification of the skull and
brain ; (2) the formation of a covering of hair ; (3) the com-
plete development of the diaphragm ; and (4) the formation

of the milk-ijlands and the adaptation to the suckling of
43

144 THE EVOLUTION OF MAN.

the young. Intimately connected with these, other im-
portant structural modifications gradually occurred.

The period at which these important advances, which
laid the first foundation of the Mammalian class, took place,
may most probably be placed in the first part of the
Mesolithic, or Secondary Epoch, in the Triassic Period.
For the oldest known fossil remains of Mammals occur in
sedimentary rock -strata of the most recent deposits of the
Triassic Period, in the upper Keuper. It is possible,
indeed, that the parent-forms of Mammals may have
appeared earlier (perhaps even at the close of the Palajo-
lithic Epoch, in the Permian Period). But no fossil remains
of Mammals belonging to that period are as yet known.
Throughout the Mesolithic Epoch, throughout the Triassic,
Jurassic, and Calcareous Periods, fossil remains of Mammals
are very scarce, and indicate a very limited development
of the whole class. During this Mesolithic Epoch, Reptiles
play the chief part, and Mammals are of quite secondary
importance. It is, however, especially significant and
interesting, that all mammalian fossil remains of the
Mesozoic Epoch belong to the older and inferior division
of Pouched Animals {Marsupialia), a few probably even
to the yet older division of the Gloacal Animals (Mono-
trema). Among them, no traces of the third and most
highly developed division of the Mammals, the Placental
Animals, have as yet boen found. The last, to which Man
belongs are much more recent, and their fossil remains do
not occur till much later — in the succeeding Csenolithic
Epoch ; in the Tertiary Period. This palseontological fact
is very significant, because it harmonizes perfectly with
that order of the development of Mammals which is un-

THE THREE MAMMALIAN GROUPa 1 45

rnistakably indicated by Comparative Anatomy and Onto-
geny.

These show that the whole Mammalian class is divisible
into three main groups, or sub-classes, corresponding to
three successive stages of phylogenetic evolution. These
three stages which consequently represent three important
ancestral stages in the human pedigree, were first dis-
tinguished in the year 1816 by the celebrated French
zoologist, Blainville, who named them, according to the
difierent structure of the female organs of re {production,
Orniiliodelphia, Didelphia, and Monodelphia {^eX<pvg,
which, being interpreted, is uterus). It is not, however,
only in the varied structure of the sexual organs that these
three classes differ from one another, but in many other
respects also, so that we can safely maintain the important
phylogenetic statement : The Monodxlphia, or Placental
Animals, have descended from the Dldelphia, or Pouched
Animals; and the latter, again, have descended from the
Cloacal Animals, or Oriiithodelphia.^

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. 827). 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 Monotreines 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 (coracoideuTri), 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 should er-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-
[)hibian characters must have been present in the parent

EXTANT CLOACAL ANIMALS. 1 47

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 gi'ouped together as the Beaked Animal family (Orni-
thostoma). Both of these are confined to Australia and the
neighbouring island of Van Diemen's Land, or Tasmania ;
both are becoming less numerous year by year, and will
soon be classed, with all their blood relations, among the
extinct animals of our globe. One of these forms passes
its life swimming about in rivers, and builds subterranean
dwellings on the banks : this is the well-known Duck-
billed Platypus {0r7iithorJiynchus 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 itseK
up into a ball like a hedgehog. Neither of these extant
Beaked Animals possesses true bony teeth, and, in this
point, they resemble the Toothless Mammals (Edentata).
The absence of teeth, together with other peculiarities of
the Ornithostomata, is probably the result of comparatively
recent adaptation. Those extinct Cloacal Anima's 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.

Fig. 195.— The Duck-billed Platy-
pus (Ornitliorhynchus paradoxus).
Fig. 193.— Skeleton of Platypus.

POUCHED ANBIALS. I49

Bmall 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 primseval Promammalia. These teeth, by
their form, indicate species that lived on insects ; the species
lias been called Microlestes antiqitus. Teeth belonging to
another closely allied Primitive Mammal (Dromatheriuni
sUvestre) have recently been discovered in the Nortli
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 represent! n<:
two distinct and divergent lines of descent from the Pro
mammalia. This second Mammalian sub-class is ver}
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,
tiiis structure is alone sufficient to distinguish the Pouched
Animals (Marsupialia). 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

ISO THE EVOLUTION OF MAN.

lower jaws, no fragment of the rest of their bodies having
been reserved. According to the logic usually applied to
palaeontology by the " exact " opponents of the theory of
evolution, the inference drawn from this fact would be
that these Mammals had no bones except lower jaws. The
remarkable circumstance is, after all, very easily accounted
for. The lower jaw of Mammals being a solid and excep-
tionally hard bone, but very loosely attached to the skull, it
is easily detached from the carcase as the latter is carried
down by some river, and, falling to the bottom, is retained
in the mud. The rest of the carcase is carried on further,
and is gradually destroyed. As all the mammalian lower
jaws found, in England, in the Jurassic strata of Stonesfield
and Purbeck, exhibit this peculiar process characteristic of
the Pouched Animals {Marsupialia), we may infer, from
this palaeontological 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 Csenolithic Epochs.

EXTANT POUCHED ANIMALS. 15 1

The Neptunian deposits of these epochs in all quarters of
the globe, and even ui Europe, contain abundant marsupial
remains in great variety, some of them being of very large
size. From this we ma} infer that the extant Pouched
Animal? 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 sur\dvors 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 (o8 coracoideum) attached to the shoulder girdle. A
more important step consists in the fact that a cloaca is no
longer formed ; the cavity of the rectum, together with the
anal opening, is separated by a partition wall from the urinary
and sexual opening (sinus urogenitalis). Moreover, all
Pouched Animals develop special nipples on the milk-glands,
which are sucked by the young after birth. These nipples
project into the cavity of a pouch, or marsupium, in the
ventral side of the mother. This pouch is supported by
a couple of marsupial bones. In it the young, which are

152

THE EVOLUTION OF MAN

bom in a very imperfect condition, are carried by the
mother foj a long time ; until, in fact, they are completely
develoj^'^d (Fig. 197). In the large Giant Kangaroo, which

Fig. 197. — The Crab-eating l^ouched Kat {Philander cancrivorus), A
female with two yoang 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 fui^ther 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 (perh§>ps from several
blanches) 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 oi
Placental Animals (Placentalia). This very highly de-
veloped group of Mammals, the third and last, came into
the world at a considerably later period. No single known
fossil, belonging to any portion of the Secondaiy or Meso-
lithic Epoch, can be referred with certainty to a Placental
Animal, while we have plenty of placental fossils dating
from ever} part of the Tertiary or Ca^nolithic Epoch. From
this pal^eontological 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
Csenolithic 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 Caenolithic
Epoch was. Judging from the relative thicknesses of the
various strata-formations we were able to sav that this

ft/

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. (C£ p. 18.)

All Placental Animals are distinguished from the two
lower Mammalian groups already considered, from the
Cloacal Animals and Pouched Animals, by many prominent
peculiarities. All these characters are present in Man ; a
most significant fact. For on the most accurate comparative
anatomical and ontogenetical researches, we may base the
irrefutable proposition that Man is in every respect a true
Placental Animal; in l^imare 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
full development only in Placental Animals ; in the Pouched
Animals and Cloacal Animals it exists merely as an insigni-
icant rudiment. It is true that in their brain structure
the lowest of the Placental Animals yet resemble Pouched
Animals very nearly; but within the Placental gi'oup we
can trace a continuous series of progressive stages in the
development of the brain, ascending quite gradually from

PLACENTAL ANIMALS. 153

the lowest stage to the very highly developed mind-orgarj
of the Aloiikey and of Man. (C£ Chapter XX.) Th.
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 Tnarsupialia) 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 irj
Placental Animals.

The character, however, which especially distinguishes
Placental Animals, and which has justly given its name to
the entire sub-class, is the development of the placenta, or
vascular cake. We have already spoken of this organ, in
describing the development of the allantois in the human
embryo (vol i. p. 882). The urinary sac or allantois, that
peculiar bladder which grows out of the posterior portion of
the intestinal canal, is, we found, formed at an early stage in
the Imm^n embryo just as in the germs of all other Amnion
Animals. (Cf Figs. 132-135, vol. i. p. 877-880.) The thin wall
of this sac consists of the same two layers, or skins, as the
wall of the intestine itself ; internally of the intestinal-glan-
dular layer, and externally of the intestinal-fibrous layer.
The cavity of the allantois is filled with fluid ; this primi-

156 THE EVOLUTION OF MAN.

tive Tirine must be chiefly the product of the primitive
kidneys. The intestinal fibrous layer of the allantois is
traversed by large blood-vessels which accomplish the nutri-
ment and, especially, the respiration of the embryo ; these
are the navel-vessels, or umbilical vessels (vol. i. p. 400). In
all Reptiles and Birds the allantois becomes an immense
sac, which encloses the embryo with the amnion, and which
does not coalesce with the outer covering of the egg
(chorion). In Cloacal Animals (Alonotremata) and Pouched
Animals {Marsupi(dia) the allantois is also of this natuie.
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 coiTesponding 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 ol
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. 1 5/

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 increased
consumption of food. In this point there is a very striking
difference between the lower and the higher Mammals. In
Cloacal Animals and Pouched Animals, in which the embryo
remains for a comparatively brief time in the uterus, and is
bom in a very immature condition, the circulation as it exists
in the yelk-sac and in the allantois suffices for nutrition, as
in birds and reptiles. In Placental Animals, on the contrary,
in which gestation is very protracted, and the embryo
remains much longer in the uterus, there attaining its full
development within its investing membranes, a new ap-
paratus is required to convey a direct supply of richer
nutritive matter ; and this is admirably effected by the
development of the placenta.

In order rightly to understand and appreciate the for-
mation of this placenta and its important modifications in
different Placental Animals, we must once more glance at the
external coverings of the mammalian egg. The outermost oi
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 j)(^^lucida, and by the thick
albuminous covering deposited externally on the latter
(Fig. 19, Fig. 21, z, h, vol. i. p. 178).

We called these two outer coverings, which afterwards
amalgamate, the j^'^^ochorion. This prochorioii 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, .-;, sh, p. 385.) This
is at first a very smooth, thin membrane, surrounding the
entire egg, as a closed spherical vesicle, and consisting of c.
single layer of exoderm cells. The chorion, however, be-
comes very soon studded with a number of little protuber-
ances or tufts (Fig. 139, 5, chi^). 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, howevei, not solid, but hollow, like the fino-ers 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, growino-
and branching rapidly. In the spaces between them, new

Fig. 198. — Egg-coverings of
the human embryo (cliagrain-
matic) : m, the thick fleshy wall
of the uterus ; plu, placenta,
the inner stratum {j^lu) of
which has extended processes
between the chorion-tufts {cJiz)
(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). (.'Jter
Kolliker.)

tufts arise in all directions from the serous membrane, and
thus before long (in the human embryo in the third week)

DEVELOPMENT OF THE PLACENTA, 1 59

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). Th6se 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 {jjlacenta foetalis, 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
Ajiimals 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 difterences : the lower Placental Animals,
which are called Indecidua, and the higher PlacentaJ

Animals, or Deciduata.
44

l6o THE EVOLUTION OF MULN.

To tlie Iiidecidua, or lower Placental Animals, belong
two very comprehensive and important vertebrate groups :
(1) the Hoofed Animals ( Ungulata) — the Tapirs, Ilorses,
Swine, Ruminants, and others ; (2) the Whale-like animals
(Cetomorpha) — the Sea-cows, Porpoises, Dolphins, Whales,
and others. In all these Tndecidua 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 dt*awn 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 foetalis) 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 Pre}^ 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 leave, Fig. 198,
ohl) thus becomes distinct from the tufted chorion (chorion
(rondosum. Fig. 198, chf). On the former there are only

INDECIDUA AND DECIDUA. l6l

minute and scattered tufts, or none at all ; while the lattei
is thickly overgrown with highly developed and large tufts,
[n the Deciduata the tufted chorion alone formg 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. Ow'ng 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).
Ail 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-
licnsive group of Deciduata. On the contrary, there are
many important difierences in this respect, which are in

1 62 THE EVOLUTION OF MAN.

gome degree connected with other important structural
characters {e.g., the structure of the brain, of the teeth, ot 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). A similar girdle-shaped placenta is
found in the False-hoofed Animals (Chelophora) : the
elephants, and Klip Das (Hyrax) with its allies, which were
formerly classed as Hoofed Animals. All these Zonoplacen-
talia belong to one or more side-branches of the Deciduata,
which are not nearly allied to Man.

The second and most highly developed group is formed
by the Deciduata with discoidal placenta {Discoplacentalia).
The formation of the placenta is here most localized and
its structure most fully developed. The placenta forms a
thick, spongy cake, usually in the form of a circular or
oval disc, and attached only to one side of the uterine
waU. The greater part of the embryonic egg-membrane is,
therefore, smooth, without developed tufts. To the Disco-
placentalia belong the Semi-apes and Insect-eaters, the
Diggers (Effbdienta) and the Sloths, Rodents and Bats,
Apes and Man. Comparative Anatomy enables us to infer
that of these various orders the Semi-apes are the parent-

SEMI-APES. 163

-Toup from which all other Discoplacentals, and perhaps
even all Deciduous Animals, have developed as divergent
branches. (Cf. Tables XXIII. and XXIV.)

The Semi-apes (Prosimice) are now represented only by
very few forms. These, however, are very interesting, and
must be regarded as the last remnants of a group once rich
in forms. This group is certainly very ancient, and was
probably very prominent during the Eocene Epoch. Their
present degraded descendants are scattered widely over the
southern portion of the Old World. Most of the species
inhabit Madagascar ; a few the Sunda Islands ; a few others
the continents of Asia and Africa. No living or fossil Semi-
apes have, as yet, been found in Europe, America, or Aus-
tralia.-^^^ The widely scattered posterity of the Semi-apes
is considerably diversified. Some forms seem nearly allied
to the Marsupials, especially to the Pouched-rats. Others
(MacTotarsi) are very near akin to the Insect-eaters, and
yet others (Cheiromys) to the Gnawers (Rodentia). One
genus (Galeopithecus) forms a direct transition to the Bats.
Finally, some of the Semi-apes (Brachytarsi) approach very
near to true Apes. Among the latter are some tail-less forms
{e.g., the Lori, Stenops, Fig. 199), From these highly in-
teresting and important relations of the Semi-apes to tht
various Discoplacental orders, we may fairly infer thai
of the extant representatives of this group, they are tht
nearest to the common primitive parent-form. Among the
direct common ancestors of Apes and Men, there must have
been some Deciduata which we should class among the
Semi-apes, were we to see them alive. We may therefore
consider this order as a special stage, following the Pouched
Animals, as the eighteenth stage in the human pedigree.

164

THE EVOLUTION OF MAN.

Probably our ancestors among the Semi-apes closely re-
sembled the extant Brachytarsi or Lemurs {Lertiur, LichaTir-

J'- "

Fig. 199. — The Slender Lori of Ceylon (Stenops gracilis).

otus, Stenops), and, like these, led a quiet life, climbing on
trees. The extant Semi-apes are mostly nocturnal animals
of, gentle and melancholy disposition, subsisting on fruits.

APES. 165

The Semi-apes are immediately followed by the true
A-pes (Simice), as the nineteenth stage in the human pedi-
oree. It has long been beyond doubt that of all animals
the Apes are in all respects the most nearly allied to Man.
Just as, on the one side, the lowest Apes approach very near
to the Semi-apes, so, on the other side, do the highest Apes
most closely resemble Man. By carefully studying the Com-
parative Anatomy of Apes and i\Ian, it is possible to trace a
gradual, uninterrupted advance in the Ape-organization up to
the purely human structure; and on impartially testing this
" Ape-question," which has lately been agitated with such
passionate interest, we shall infallibly have to acknowledge
the important fact, which was first explicitly laid down by
Huxley, that "whatever system of organs be studied, the
comparison of their modifications in the ape series leads to
one and the same result — that the structural differences
which separate Man from the Gorilla and Chimpanzee are
not so great as those which separate the Gorilla from the
lower Apes." In phylogenetic language this pregnant law
established in so masterly a manner by Huxley, is equiva-
lent to the popular phrase : Man is descended from the Ape.
In order to become convinced of the truth of this law,
let us now once more consider the placenta and deciduous
membrane, on the varied structure of which we justly laid
special stress. Men and Apes, in the structure of theii* disc-
shaped placenta and m their decidua, do, indeed, coincide
on the whole with all other Discoplaeental Animals. But
in the more delicate structure of these parts Man is dis-
tinguished by peculiarities which he shares only with Apes,
and which are absent in other Deciduata. Thus in Man
and in the Apes three distinct parts are recognized in the

1 66

I'HE EVOLUTION OF MAN

deciduous membrane ; these parts may be called the outer,
the inner, and the placental deciduous membrane. The
outer or true membrane {d. externa or vera, Fig. 198, dv,
Fig. 200, g)y is that portion of the uterine mucous membrane
which coats the internal surface of the uterus wherever the

Fig. 200. — Human embryo, twelve weeks old, with its coverings ; natural
size. The navel cord passes from the navel to the placenta : 6, amnion ;
c, chorion; </, placenta; d\ remains of tufts on the smooth chorion;/, de-
cidtia reflexa (inner) ; </, decidtia vera (outer). (After Bernhard Schultze.)

latter is not attached to the placenta. The placental or
spongy deciduous membrane {d. jplacentalis or serotina^
Fig. 198j_^Zt^5 Fig. 200, d) is simply the maternal placenta

THE DECIDUOUS MEMBRANE IN MAN AND APES. 1 6/

itself, or the maternal part of the " vascular cake " {pla^
centa uterina), i.e., that part of the uterine mucous mem-
brane which coalesces intimately with the chorion- tufts of

Fig. 201. — Mature human embryo (at the end of pregnancy), in its natural
jwsition, taken out of the uterus. On the inner surface of the latter (on
the left) is the placenta, which is attached to the navel of the child by the
narel cord. (After Bernhard Schultze.)

the embryonic placenta {^^lacenta foetalis). Lastly, the
inner or false deciduous membrane (d. interna or rejiexa.
Fig. 198, dr, Fig. 200, /) is that portion of the uterine mucous
membrane which, as a peculiar thin envelope, covers all the
rest of the egg-surface, lying immediately over the tuftless
smooth chorion {chorion Iceve). The origin of these three
distinct deciduous membranes, concerning which erroneous
notions have been entertained (still retained in the nomen-
clature), is plain enough ; the external or true deciduous

1 68 THE EVOLUTION OF MAN.

membrane is a peculiar modification, afterwards lost, of the
superficial layer of the original mucous membrane of the
utoi-us. The placental membrane is that portion of the
preceding which is completely modified by the intrusion of
the chorion-tufts and is employed in forming the placenta.
Lastly, the inner deciduous membrane is formed by a
ring-shaped fold of the mucous membrane (at the point
of union of the d. vera and the d. serotina) which rises,
•grows round the egg, and closes in the same way as the
amnion.^^^

The peculiar anatomical characters which- mark the human
L'gg-membrane re-occur, in the same form, only in Apes. Al]
other Discoplacental Animals present greater or less differ-
ences in these points, the conditions being generally more
simple. This is the case, for instance, in the structure of
the placenta itself, in the coalescence of the chorion tufts
with the deddua serotina. The matured human placenta
is a circular (rarely oval) disc of a soft, spongy character,
6 to 8 inches in diameter, about 1 inch thick, and weiohino
from 1 to l^^ lb. Its convex, external surface (that which
coalesces with the uterus) is very uneven, and tufted. Its
internal, concave surface (that which is turned towards the
cavity of the egg) is quite smooth, and clothed by the amnion
(Fig. 198, a). From near the centre of the placenta springs
the navel cord (funiculus v/mhilicalis), the development
of which we have already observed (vol. L p. 883). It also is
coated by the amnion as with a sheath, which at the navel
end passes directly into the abdominal skin (Fig. 200, 201).
The mature navel cord is a cylindrical cord, coiled spirally
around its axis, and usually about 20 inches long and 5- inch
thick. It consists of gelatinous connective tissues (" Whar-

THE HUMAN PLACENTA. 1 69

ton's jelly "), in which are contained the remnants of the
yelk- vessels and of the great navel vessels ; the two navel
arteries which convey the blood of the embryo to the pla-
centa, and the great navel vein which brings back the blood
from the latter to the heart. The numerous fine branches
of these embr^^onic navel vessels pass into the branched
chorion tufts of the foetal placenta, and with these, finally,
grow, in a very peculiar way, into large blood-filled cavities,
which spread themselves in the uterine placenta and con-
tain blood from the mother. The anatomical relations, very
complex and difiicult to comprehend, which are developed
between the embryonic and the maternal placenta, exist in
this form only in Man and in the higher Apes, while in aU
other Deciduous Animals their form is more or less different.
The navel cord, also, is proportionately longer in Man and
in Apes than in other Mammals.

As in these important characters, so also in every other
morphological respect, Man appears as a member of the
order of Apes, and cannot be separated from the latter. The
great originator of systematic description of nature, Karl
Linnaeus, with prophetic penetration, united Men, Apes,
Semi-apes, and Bats in a single natural division, under the
name of Primates, that is, the first, the lords of the animal
kingdom. Later naturalists dissolved this order of Primates.
The GSttingen anatoixdst, Blumenbach, first placed Man in
a special order, which he called that of Two-handed Animals
{BiinaTia) ; in a second order, he united Apes and Semi-
apes under the name of Four-handed Animals (Quad-
ruma,na), while a third order included the distantly related
Bats (Chiroptera). The separation of the Bimana and
Quadrumana was retained by Cuvier and most succeeding

I/O THE EVOLUTION OF MAN.

/-oologists. It seems very important, but is really wholly
unjustifiable. This was first shown in the year 1863 by
Huxley. Supported by very accurate Comparative Anato-
mical researches, he proved that Apes are as " two-handed "
as Men, or, conversely, that Men are as " four-handed " as
Apes. Huxley showed, with convincing clearness, that the
ideas previously held of the hand and the foot were false,
and were incorrectly founded on physiological instead of on
morphological distinctions. The circumstance that in the
hand, the thumb may be opposed to the other four fingers,
thus permitting the act of grasping, appeared especially to
distinguish the hand from the foot, in which the correspond-
ing great toe cannot be thus opposed to the four remaining
toes. Apes, on the contrary, can grasp in this way with the
hind-foot as well as with the fore-foot, and were therefore
regarded as four-handed. Many tribes, however, among the
lower races of men, especially many negro tribes, use the foot
in the same way as the hand. In consequence of early habit
and continued practice, they are able to grasp as well with
the foot as with the hand (for example, in climbing, they
grasp the branches of trees). Even new-bom children of our
own race have a very strong grasping power in the great toe,
with which they can hold a spoon as fast as with the hand.
The physiological distinction between hand and foot can,
therefore, neither be strictly carried out, nor scientifically
established. Morphological characters must be used for this
purpose.

A sharp morphological distinction of this kind — that is,
»ne founded on anatomical structure — between hand and
foot, between the anterior and the posterior limbs, is actually
possible. There are essential and permanent difierences

MAN AND APK I/I

both in the structure of the bony skeleton and in that of
the muscles which are attached to the hand and the foot ;
and these are exactly the same in Man and in the Ape.
There is, for instance, an essential difference in the arrange-
ment and number of the wrist-bones of the hand {carpus)
and the ankle-bones of the foot {tarsus). The muscle-masses
present equally constant differences. The posterior ex-
tremity, the foot, has always three muscles (a short flexor
muscle, a short extensor muscle, and a long muscle attached
to the muscles of the tibia) which are never present in
the anterior extremity, the hand. The disposition of the
muscles is also very different in the two sets of limbs.
These characteristic differences between the anterior and
the posterior extremities occur in Man just as in Apes.
There can, therefore, be no doubt, that the foot of the
Ape deserves the name as truly as that of the Man ; and
that all true Apes are as genuinely two-handed animals
{Bimana) as Man. Thus the usual distinction of the Apes
as Quadrumana is wholly unjustifiable.

It might now be asked whether, quite apart from these,
there are not other marks by which Man is more widely
separated from the Apes than are the difierent species of
Apes from each other. Huxley has given a final negative
to this question so convincingly, that the opposition now
raised against him in many quarters must be regarded as
completely unfounded and ineffective. Based on an accurate
study of the Comparative Anatomy of all parts of the body,
Huxley brought forward very significant proof that, in
every anatomical respect, the difierences between the highest
and the lowest Apes are greater than the corresponding
diflferences between the highest Apes and Man. He ther©-

1/2 THE EVOLUTION OF MAN.

fore restored Linnseus's order of Primates (excluding the
Bats), and divided it into three different sub-orders, the
first of which is formed by the Semi-apes (Lemurida), the
second by the true Apes (Simiadce), and the third by Men
(Anthropidce)}^^

Yet, if we proceed logically and without prejudice, in
accordance with the principles of scientific reasoning, we
find, on the basis of Huxley's own law, this division in-
adequate, and must go considerably further. As I first
showed in 1866, in treating this question in my Generelle
Morphologie, we are fully justified in taking at least one
impoii:ant step further, in assigning to Man his natural
place in one of the divisions of the Ape-order. All the
characters distinctive of this one division of the Apes are
present in Man, while they are absent in other Apes. We
are, therefore, not justified in forming a distinct order for
Man apart from the true Apes.

The order of the true Apes (Simice), the Semi-apes being
excluded, has long been divided into two natural main
groups, which, among other points, are distinguished by
their geographical distribution. Those of one division
(Hesperopitheci, or Western Apes) live in the New World,
in America. The other division, to which Man belongs, is
that of the Heopitheci, or Eastern Apes ; these live in the
Old World, in Asia, Africa, and, formerly, in Europe. All
the Apes of the Old World, all Heopitheci, share, in common
with Man, all those characteristics to which special promin-
ence is justly given, in distinguishing these two groups of
^pes, in zoological classification ; among these characteristics
the structure of the teeth is most prominent. The objec-
tion is at once evident that the teeth are, in a physiological

THE TEETH. 173

sense, much too subordinate a part of the body to justify so
great a weight being attached to their structure in so im-
portant a question. There are, however, good reasons for
this prominent consideration of the structure of the teeth ;
and it is with perfect correctness and propriety that sys-
tematic zoologists have, for more than a century, given
special weight to this character in systematically dis-
tinguishing and arranging the mammaHan orders. The
number, form, and disposition of the teeth are transmitted
much more accurately within the respective orders of the
mammals than are most other zoological characteristics.
The structure of the human teeth is well known. In matu-
rity there are 32 teeth in our jaws, and of these 32 teeth,
8 are front-teeth, 4 cannie-teeth, and 20 molar-teeth. The
eight front-teeth or incisors {denies incisivi), which are
situated in the centre of the jaws, exhibit characteristic
differences in the upper and lower jaw. In the upper the
inner incisors are larger than the outer; in the lower jaw,
on the contrary, the inner incisors are smaller than the
outer. Next to these, on each side, both in the upper and
lower jaw, is a corner- tooth, which is larger than the in-
cisors, the so-called eye-tooth, or canine (dens caninus).
Sometimes this tooth becomes very prominent in Men, as in
most Apes and many other Mammals, and forms a sort of
tusk. Finally, next to this, on each side, and in each jaw,
ire situated five back-teeth, or molar- teeth (denies molar es\
of which the two foremost (the bicuspid teeth) are small,
have but a single fang, and are subject to the change of
teeth, while the three hinder molars are much larger, have
two fangs, and do not appear till after the temporary teetn
have been shed (so-called "grinders"). The Apes of the

1/4 THE EVOLUTION OF MAN.

Old World have exactly this human structure of the teeth, —
all Apes which have as yet been found, either living or as
fossils, in Africa, Asia, and Europe. All Apes of the New
World, on the contrary, all American Apes, have an extra
tooth on both sides of each jaw ; this is a biscupid tooth.
Thus they have six back-teeth on both sides of each jaw, —
in all, thirty-six teeth. This characteristic difference be-
tween the Eastern and Western Apes has been so constantly
transmitted within the two groups, that it is of the greatest
value to us. A small family of South American Apes does,
indeed, appear to form an exception in this respect. The
pretty little Silk Apes, or Marmosets (Ilapalida), namely, to
which the Brush-monkey (Midas) and the tufted Marmoset
(Jacchus) belong, have but five back-teeth in each half of
the jaw, instead of six, and, accordingly, seem to approach
nearer the Eastern Apes. But on closer observation it
is found that, like all the Western Apes, they have the
three biscupids, and that the hindmost grinder has been
lost. Thus this apparent exception confirms the value of
the distinction.

Among the other marks by which the two main groups
of the Apes are distinguished, the structure of the nose is
specially important and prominent. In all Old World Apes
the structure of the nose is the same as in Man ; namely, a
comparatively narrow partition of the two halves, so that
the nostrils are directed downwards. In a few Eastern Apes,
the nose projects as prominently and is as characteristically
formed as in Man. We have already called attention,
in this respect, to the remarkable Nose-ape (Semno-
pitheciis nasicus), which has a well-curved and long nose
(Fig. 202). Most (»f the Eastern Apes have, it is true, a

THE NOSE.

175

somewhat flatter nose, as, for instance, has the white-nosed
Sea-cat (Cercopithecus petaurista, Fig. 203) ; yet in all the
partition of the nose is narrow and thin. On the contrary,
all American Apes have a different nasal structure. In
them, the partition is peculiarly broadened and thickened
below, and the wings of the nose are not developed, in con-
sequence of which the nostiils are not below, but are
turned outwards. This characteristic difference in the
structure of the nose has also been so accurately trans-

FiG. 202. — Head of Nose. ape (Semnopithecus nasicus).

Fig. 203. — The white-nosed Sea-cat (Cercopithecus petauristd).

mittod in both groups, that, on account of it, the Apes of
the New World have been called Flat-nosed (Platyrhince\
and those of the Old World Narrow-nosed (Gatarhince).
The former are, on the average, inferior in organization.

45

1/6 THE EVOLUTION OP MAK.

The division of the order of Apes into two sub-orders,
the PlatyrhincB and the Caiarhince, is, on account of the
constant hereditary characters, now generally accepted by
zoologists, and receives much support from the geographical
distribution of the two groups between the New and Old
Worlds. From this follows the direct inference, very im-
portant in its bearing on the Phylogeny of Apes, that, from
the primaeval common parent-form of the Ape-order, two
diverging lines branched out at a very early period, one of
which spread over the New World, the other over the Old.
It is certain that all the Flat-nosed Apes, on the one hand,
are descendants of a common parent-form, and, on the other
hand, all the Narrow-nosed Apes from another

An inference concerning our own pedigree may be drawn
from this. Man has exactly the same characters, the same
peculiar formation o£ the teeth and nose, as all the
Catarhinae, and is as thoroughly distinguished by these
charateristics from the Platyrhinae. We are therefore com-
pelled, in classifying the Primates, to assign to Man a place
in the Narrow-nosed group. The bearing of this on our
tribal history is, that Man is immediately related in blood
to the apes of the Old World, and may be traced from a
parent-form common to all other Catarhinae also. Man is
a genuine Narrow-nosed Ape in his whole structure and
in origin, and has descended from some unknown, extinct
Catarhine form in the Old World. On the other hand, the
Apes of the New World, the Flat-nosed group, constitute a
diverging branch of our family tree, and stand in no near
genealogical relation to the human race.

We have now reduced the circle of our nearest aUies
to the small group, containing comparatively few forraii^

MAN'S RELATION TO APES. 1/7

which is represented by the sub-order of the Narrow-nosed^
or Eastern Apes. Finally, the question which now re-
mains to be answered is — what position in this sub-order
must be assigned to Man, and whether other inferences as
to the structure of our immediate ancestors may be drawn
from this position. The comprehensive and acute researches
into the Comparative Anatomy of Man and the various
Catarhinae, which Huxley has recorded in his work on the
" Evidence as to Man's Place in Nature," are of the greatest
value in furnishing the answer to these important questions.
The inevitable conclusion is, that the diiFerence between
Man and the highest Narrow-nosed Apes (the Gorilla, Chim-
panzee, Orang) is slighter in every respect than the corre-
sponding differences between the highest and the lowest
Catarhines (the Sea-cat, Macaque, Baboon). Even within
the small group of the Tail-less man-like Apes {Anihro-
poides) the several genera do not differ less from each other
than they do from Men. This is seen by a glance at the
skeletons represented here, as arranged by Huxley (Figs.
204j-208). If the skull, or the vertebral column, together
with the rib-system, or the anterior or posterior members,
are compared; or if the comparison is extended to the
muscular system, the circulatory system, the brain, etc.,
a candid and unprejudiced examination always results in
the same conclusion, that Man does not differ more from
the higher Catarhines than the extreme forms of the latter
(for example, the Gorilla and Baboon) differ from each
other. "We can, therefore, complete the important propo-
sition already quoted^ from Huxley : We may take what-
ever system of organs we will, — ^the comparison of their
modifications within the ranks of the Catarhinse leads us

178

THE EVOLUTION OF MAN;

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■VOLUTION OF MAN FROM APES. 1/9

to one and the same conclusion : that the anatomical dif-
ferences that distinguish Man from the most highly developed
Catarhinse (the Orang, Gorilla, Chimpanzee), are not so great
as those which separate the latter from the lowest Catarhinae
(Sea-cat, Macaque, Baboon).

We must, therefore, consider the proof complete, that Man
is descended from other Narrow-nosed Apes (Catarhi/noB),
Although future researches into the Comparative Anatomy
and Ontogeny of the existing Catarhines, as well as of their
fossil relatives, promise us various new details, yet no
future discovery can ever overthrow that important pro-
position. Our Catarhine ancestors must, of course, have
passed through a long series of varied forms, before Man
finally developed as the most perfect form. The following
must be considered as the most important advances by
which this " Creation of Man," his differentiation from the
most nearly allied Catarhine Apes, was efiected : Habituation
fco upright carriage and, in connection with this, the greater
differentiation of the anterior and posterior limbs ; also, the
development of articulate speech and its organ, the larynx ;
and lastly, and especially, the more perfect development of
the brain and its function, the soul ; sexual selection must
have exerted an extraordinarily important influence, as
Darwin has conclusively proved in his celebrated work on
sexual selection.^®

With reference to these advances, we may, among our
Catarhine ancestors, distinguish at least four important
ancestral stages, marking prominent epochs in the great
historical process of the origin of Man. As the nineteenth
stage in the human pedigree, next to the Semi-apes, we may
place the oldest and lowest Catarhine Apes, which developed

l80 THE EVOLUTION OF MAN.

from the former by the formation of the chaiacteristic
catarhine head, and by the peculiar modification of the
teeth, the nose, and the brain. This oldest parent-form of
the whole Catarhine group must, certainly, have been
thickly covered with hair, and must have had a long tail ;
was, in fact, a Tailed Ape (Menocerca, Fig. 203). They
were already in existence during the earlier part of the
Tertiary Epoch (during the Eocene Period), as is shown by
fossil remains of Eocene Catarhines. Among extant Tailed
Apes, the Slender Apes (Semnopitheci) are perhaps most
nearly related to this parent-form.^^

As the twentieth stage in the human pedigree, next to
these Tailed Apes, we must rank the Tail-less man-like Apes
(Anthropoides), under which name the most highly de-
veloped Catarhines, those most nearly related to Man, have
been grouped. They originated from the Tailed Catarhines,
by the loss of the tail, the partial loss of their hairy cover-
ing, and the further development of the brain, the latter
being indicated in the preponderating development of the
brain-skull over the facial skull. At the present time but
few forms of this remarkable family are in existence ; they
are distributed into two different groups, an African and an
Asiatic group. The African Man-like Apes are limited to
the western part of tropical Africs., but are probably dis-
tributed over Central Africa in several species. Only two
species are well known : the Goiilla {Pongo gorilla, or
Gorilla engina), the largest of all Apes (Fig. 207) ; and the
smaller Chimpanzee (Fongo troglodytes, or Engeco troglo-
dytes), which may be seen in several zoological gardens
(Figs. 206, Plate XIV. Figs. 1, 2). Both the African Man-
like Apes are black in colour, and like their countrymen,

HAECKELS EVOLUTION OF MA>f.

PLATE XIV,

1 Gorilla.

Pirn- ^Ai \y~^Z'^At^

'll^7i>h-r il.-t .

3. Orang.

4. Negro.

ICAN-LIEE APEa l8l

the Negroes, have the head long from back to front (doli-
chocephalic). The Asiatic Man-like Apes are, on the con-
trary mostly of a brown, or yellowish brown colour, and
have the head short from back to front (brachycephalic),
like their countrymen, the Malays and Mongols. The
largest Asiatic Man-like Ape is the well-known Orang, or
Orang-outang (Fig. 128), which is indigenous in the Sunda
Islands (Borneo, Sumatra), and is brown in colour. Two
species have recently been distinguished : the great Orang
(Satyrus Orang ; Fig. 205, Plate XIV. Fig. 3), and the small
Orang (Satyrus morio). A genus of smaller Anthropoids
(Fig. 204), the Gibbons (Hylobatea), live on the main-land
of Southern Asia and on the Sunda Islands ; from four to
eight different species of these have been distinguished.
Neither of these living Anthropoids can be indicated as the
Ape absolutely most like Man. The Gorilla approaches
nearest to Man in the structure of the hand and foot, the
Chimpanzee in important structural details in the skull,
the Orang in the development of the brain, and the Gibbon
in that of the thorax. It is evident that no single one of
these existing Man-like Apes is among the direct ancestors
of the human race ; they are all the last scattered remnants
of an old, catarhine branch, once numerous, from which the
human race has developed as a special branch and in a
special direction.

Although Man (Homo) ranks immediately next to this
anthropoid family, from which he doubtless directly origin-
ated, yet the Ape-men (Pithecanthropi) may be inserted
here, as an important intermediate form between the two,
and as the twenty-first stage in our ancestral series. In the
"Natural History of Creation*' (voL ii p. 293), I have

1 82 THE EVOLUTION OF MAW.

applied this name to the speechless Primitive Men (Alali\
who made their appearance in what is usually called the
human form, that is, having the general structure of Men
(especially in the differentiation of the limbs) — but yet
being destitute of one of the most important qualities of
Man, namely, articulate speech, as well as of the higher
mental development connected with speech. The higher
differentiation of the larynx and of the brain occasioned by
the latter, first gave rise to the true " Man."

Comparative Philology has recently shown that the
present, human language is polyphyletic in origin, that
several, and probably many, different original languages
must be recognized, as having developed independently from
each other. The history of the development of languages
also teaches us (its Ontogeny in every child, as well as its
Phylogeny in every race), that the actual rational lan-
guage of men developed gradually, only after the body
had developed into the specific human form. It is even
probable that the formation of language did not begin till
after the differentiation of the various species, or races of
men, and this presumably occurred in the beginning of the
Quaternary Epoch, or the Diluvial Period. The Ape-men,
or Alaliy were therefore probably already in existence
toward the close of the Tertiary Epoch, during the Pliocene
Period, perhaps even as early as the Miocene Period.^^

Lastly, the genuine or speaking human being (Homo)
must be considered as the twenty-second and final stage
in our animal pedigree. Man originated from the pre-
ceding stage in consequence of the gradual improvement
of inarticulate animal sounds into true human articulate
speech. Only very uncertain conjectures can be formed at

PRIMEVAL MAN. 1 83

to the time and place of this true "Creation of Man.**
It is probable that Primgeval Man originated during the
Diluvial Epoch, in the torrid zone of the Old World, either
on the continent of tropical Asia or Africa, or on an earlier
continent which has now sunk below the surface of the Indian
Ocean, and which extended from Eastern Africa (Madagas-
car and Abyssinia) to Eastern Asia (the Sunda Islands
and Eastern India). In my " Natural History of Creation "
(Chapter XXIII. and Table XV), I have already fuUy
discussed the important evidence as to the former existence
of this large continent, called Lemuria, and how the distribu-
tion of the various species and races of men probably took
place from this " Paradise " over the surface of the earth
In the same place, I have also fully discussed the inter-
relations of the various races and species of the human
race.^^

TABLE XXII.

Systematic Survey of the Periods in the Tribal History of the

Human Race.
(Compare Table VIII., vol. i. p. 402.)

FIRST MAIN PERIOD IN TRIBAL HISTORY.

The Flastid Anoeston of Man.

The form of the ancestors of man is equal to the simple indiyidiuJ of ikt
flirit order, • single plastid.

First Stage : Moneron Series (Fig. 163, p. 46).
The ancestors of man are single, living, simple cytods.

Second Stage : Amoeba Series (Fig. 167 p. 68).
The ancestoi's of man are single, living, nniple cells.

SECOND MAIN PERIOD IN TRIBAL HISTORY.

The many-celled Primitive Animal Ancestors of Man.

The ancestors of man consist of a closely -united society of many homo-
geneous cells ; hence their form-value is that of individuals of the second
order, of Idorgana.

Third Stage : Synamoeha Series (Fig. 170, p. 56).

The ancestors of man are many-celled primitive animals of the simplest
kind : solid masses of simple, homogeneous cells.

Fourth Stage : Flanssa Series (Figs. 172, 173, p. 60).

The ancest« )rs of man are many-celled primitive animals of a charactei
like that of the Magosphcera and certain planula-larvsa, of equal rank with
the ontogenetic Blastula or BlastosphoBra ; hollow spheres, the wall of which
cunsists of a singlt stratom of ciliated cells.

SYSTEMATIC SURVEY OF THE HUilAN RACl, I83

THIRD MAIN PERIOD IN TRIBAL HISTORY,

The Inyertebrate Intestinal Animftl Ancestors of Uao.

The iJicestors of man have the form-ralne of individnals of the third
order, of inartiOTilate individuals. The body encloses an intestinal cavity
with a month, and consists at first of two primary germ-lftjers, afterwards
of four secondary germ-layera.

Fifth Stage : Gartrsea Series (Figs. 174-179, p. 66).

The ancestors of man have the form-value and stmoture of a Gastmla.
The body consists merely of a simple primitive intestine, the wall of which
is formed of the two primary germ-layers.

Sixth Stage : Chordonium Series (Figs. 184-188, p. 80-90),

The ancestors of man are worms : at first, primitive worms, allied to the
Turhellaria; afterwards worms of higher rank, Scoimda; finally, notochord*
animals with the organization of the ascidian larvae. The body is composed
of four secondary germ-layers.

FOURTH MAIN PERIOD IN TRIBAL HISTORY.

The Vertebrate Ancestors of Man.

The ancestors of man are vertebrates, and their form-value is, therefore, -
that of an articulated individual, or a chain of metamera. The skin-sensory
layer is specialized into the horn-plate, medullary tube, and primitive
kidneys. The skin-fibrous layer has divided into the leather.plate, primitive
vertebrae (muscular plate and skeleton-plate), and the notochord. From
the intestinal.fibrous layer originates the heart with the main blood-vessels
and the fleshy intestinal wall. From the intestinal-glandular layer, the
epithelium of the intestinal tube is formed. The formation of metamera is
constant.

Seventh Stage : Aorania Series (Fig. 189 ; PL XL Fig. 15).

The ancestors of man are skull-less vertebrates, like the extant Amphi-
oxos. The body already forms a chain of metamera, several primitive
vertebrae having separated off. The head is not yet entirely distinct from
the trurk. The medullary tube has not separated into brain-bladders. The
heart is very simple, without chambers. The skull is still wanting ; as are
also the jaws and limbs.

1 86 THE EVOLUTION OF MAN.

Eighth Stage: Monorhina Series (Fig. 190; PI. XI. Fig. 1(J).

The ancestors of man are jaw-less skulled animals (resembling the
developed Myxinoides and Petromyzontes) . The number of the metamera is
increasing. The head is becoming more distinctly differentiated from the
Srnnk. The anterior end of the medullary tube swells into a bladder-like
itructure and forms the brain, which is soon differentiated into five brain-
bladders. At the sides of these appear the three higher organs of sense.
The heart is divided into auricle and ventricle. The jaws, limbs, and
iwimming-bladder are still wanting.

Ninth Stage : lohthyoda Series (Figs. 191, 192; PI. XII. and XIH.).

The ancestors of man are fish-like skulled animals : first, Primitive
Fishes {Selachii), then mnd-fishes (Bipneusta), then gilled Batrachians (Sozura).
The ancestors belonging to this Ichthyoda stage develop two pairs of limbs:
a pair of anterior limbs (pectoral fins) and a pair of posterior limbs (ventral
fins). The gill-arches are formed between the gill-openings, and from them
are formed the first pair of jaw-arches (upper and lower jaws). The
swimming-bladder (lungs), liver, and pancreas grow from the intestinal
canal.

Tenth Stage : Anmiota Series (Figs. 195-208 ; PL XTV.).

The ancestors of man are amnion-animals or gill-less vertebrates : first,
Primitive amniota (Protamnia), then Primitive mammals (Monotrema) ; next,
Pouched animals (Marsupialta) j then Semi-apes (Prosimiae), and, lastly.
Apes {Sitnioe), The ape-ancestors of man are first tailed Catarhini, theo
tail-less Catarhini (Anthropoides), then speechless Ape-men (Alali), and at
last genuine, speaking men. The ancestors belonging to this amnionata
series develop an amnion and allantois, and graduallj acquire the mam.
malian stroctare, and at lost the specific homaa form.

( 187 )

TABLE XXIII.

SjBtematio Survey of the Phylogenetic Classification of MammAls.

L

Virst
Sab-class of

/ Cloacal
Animals
{Monotrema, or
Ornithodelphia)

1. Primitive Mammata

2. Beaked Animals

Promammalia
OmithosUma

IL

Second
Sub-class of

' Pouched
Animals

{^Marsupialia, or
Diddphia)

3. Herbivorous Pouched Animals

4. Carnivorous i'ouched Animals

Botanophaga
Zoophaga

in. (a)

, Placental
Mammals with-

' 6. Hoofed Animals r Single-hoofed
Ungulata \ Double-hoofed

Perissodactyla

Artiodactyla

out Decidua, with
Tufted Placenta

Indecidua
yilliplacentalia

6. Whale-like
Animals
Cetomorpha

■

Sea-cows
Whales

Sirenia
Cetaoea

/

III. (6)

Placental
Mammals with

7. Pseudo-hoofed

Animals

Chdophora

Rock Conies
Elephants

lAimnungia
Prohoscidta

m.

Third
Snb-class of

Decidua, with ^
Girdle Placenta
Becidimta
Zonoplacen talia

8. Beasts of Prey
Carmuaia
\

' Land Beasts of
J prey

^ Marine Beasts of
( prey

Camivora
Piimipedia

Placental
Mammals
{Placentalia,

9. Semi-apes
Prosimict

' Fingered animals
J Long-footed
"i Flying Lemur
( Lemurs

Leptodactyla
Macrotarsi
Ptenopleura
Brachytarti

or Mono-
delphia)

ni. (e)

Placental
Manoials with /
Decidua, with (
Discoid Placenta

10. Gnawing Anl

mals
Rodentia

11. Toothless
Edentata

_ / Squirrel species
" 1 Mouse species

J Porcupine species

' Hare species

( Digging an'wiftls
I Sloths

Seiuromorpha
Myomorpha
Hystrichomorpha
Lagomorpha

Effodientia
Bradypoda

Deciduata
DiKoplacentalia

12. Insect-eaters
Insectiwra

I With CoBP.um
1 Without Coecnm

Mmotyphta
Lipotyphla

\

13. Flying Animals ) Flying Foxes
Chiroptera ( Bats

Pterocynes
Nycteridu

14. Apes
[ SitMm

( Flat-nosed
\liarTOw-D08ed Apes

Platyrhina
Catarhtna

C i88 )

TABLE XXIV.

Pedigree of MammaU,

Elephants
Prohoscidta

Rook Conies

TjOk'n'mingia

False-hoofed

Chelophora Flat-nosed Apes
PlatyrhincB

IHan
Homiues

I Bats

Man-like Apes Ny derides
Anthropoidea \

I Flying Foxes

Narrow-nosed Apes Pterocynes
CatarhincB jFIstng Animals
Chiroptera

Sea beasts of ptvy
Pirmipedia

®ttafoino[*^nitnaIs

Eodentia Spcs

Fingered animals Simiae

Whales Leptodactyla

Cetacea |

I ^" « -"'

Sea-Cows I

Sirenia \

B2Ef)aIc=fnmil2

Cetomorpha

Land beasts of prey
Carnwora
Flying Lemurs |

Ptenopleura Bcasts of ^ttg
I Camassia

Lemnrs

Brachyta/rsi

Toothless
Edentata

Insect-eaters

Long-footed Insectivora

Macrotarn

^aoifa Animal!
TTngulata

SBaitfjDut BectHtui
Indeciduata

Semi -apes
ProsimicB

©tciliuous Animals
Deoiduata

^lacnttal Inimals
Placentalia

Herbivorous Pouched Animals
Marsvpialia hota/nophaga

Carnivorous Pouched Animals
Marsupialia zoophaga

I

Beaked Animals

Omithostoma

!

^oucf}eti "Unttnalf
Marsupialia

Primitive Mammals

Promammalia

Cloaca I Animals

hasckel's evolution of man.

PLATE XV,

PEDIGREE OF MAN.

( i89 )

TABLE XXV.

Pedigree of Apes.

Homo

AlaluB

Gorilla

Ohimpansee Oorilla

Engeco

African

Or&ng-Ontsng
Satyrut

Gibbon

Hylohatm

•Hsiattc

fHan4t^e "apes
Anthropoides

Silk Apes
Hapalida

Clntcli. tails
Lahidocerca

Nose Apes
Tall Apes NaaaM*

Sermuypithecus

Sea Cat

Cercopithecus

Baboons
Cynoeephahts

I

Flap.tails
Aphyocerea

aipes of j^dD WLoxln

Flat-nosed
PlatyrliinaB

Catlcti "apes
Menoceroa

Slpes of ^Vn WioxtU

Narrow-nosed
Catarhina

Riniifla

Pzonmui

CHAPTER XX.

THE HISTORY OF THE EVOLUTION OF THE EPIDERMIS
AND THE NERVOUS SYSTEM.

A.uiraal and Vegetative Organ. systema. — Original Eelations of these to the
Two Primary Germ-layers. — Sensory Apparatus. — Constituentg of
Sensory Apparatus : originally only the Exoderm, or Skin-layer; after-
wards, the Skin-covering specialized from the Nerve-system. — Double
Function of the Skin (as a Covering and as Organ of Touch). — Outer
Skin (Epidermis) and Leather-skin (^Corium). — Appendages of the Epi-
dermis : Skin-glands (Sweat-glands, Tear-glands, Sebaceous Glands,
Milk-glands); Nails and Hair. — The Embryoaw 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 {Cerehrtim) and Small Brain {Cerebellum). — Comparative
Anatomy of the Central Marrow. — Germ-history of the Medullary- tube.
— Separation of the Medullary-tube into Brain and Dorsal Marrow,
— Modification of the Simple Brain-bladder into Five Consecutive Brain-
bladders : Fore-brain (Large Brain, or Oerehrum), Twixt-brain ("Centre
of Sight"), Mid-brain ('« Four Bulbs "), Hind-brain (Small Brain, or Qere.
helium), 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."

"Hardly any part of the bodily frame, then, could be found better
oalculated to illustrate the truth that the structural diSerences between
Mau and the highest Ape are of less value than those between the highest

THE DEVELOPMENT OF THE OEGANS. I9I

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." — Ma/n*s Place in Nature, p. 94 (1863).

"As if to demonstrate, by a striking example, the impoasibilitj 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 braim 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

19a THE EVOLUTION OF MAX.

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 hour
inseparably the evolution of the germ is connected with
that of the tribe. The Ontogeny of the organs is intelligible
and explicable only through their Phylogeny ; just as the
germ-history of the entire body (the "person ") is rendered
intelligible only by the history of the tribe. Each germ-
form is determined by a con-esponding 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 oigans,
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

AKIMAL AND VEGETATIVE ORGAN-SYSTEMS. 1 93

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
{^erm-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 skiu-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
(C£ vol. i. pp. 53 and 196). Of course, in making this important
assumption, we pre-suppose the correctness of Baer's view,
according to which the skin-fibrous layer (the "flesh
stratum " of Baer) must have originated (phylogenetically)
from the exoderm, and, on the other hand, the iutestinal-
fibrous layer (Baer's "vascular layer") from the entoderm.
This influential view, which is yet much disputed, is, we

( 194 )

TABLE XXVI.

Sjitematio Survey of the Organ- Systems of the Hnman 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 j
II. Skin.fibrous layer ; III. Intestinal-fibrous layer ; IV. Intestinal-gland,
alar layer.)

H
I

o

;z5

Sensory

Apparatus

Sen$9ri%im

'"1. Skin-covering
(^Derma)

( Outer skin
I Leather skin

■L

Motive
Apparatus
Loeomoleritt

a. Central nerve- ( Brain

Bystem I Spihal marrow

3. Periphericnerve- Jgf;y^^g7;^g"g
system | Intestinal nervee

4. Sense-orguu
(^Orgcma semuwn)

Mnscle • system
(active motive-
organs)

/^ Organ of tonch (skin)
Organ of taste (tongue)
Organ of smell (nose)
Organ of sight (eye)

. Organ of hearing (ear)

Skin muscles
Skeleton mosclei

Skeleton-system j Vertebral colamn
(passive motive < Skull
organs) ( Limb skeleton

Epidermis, L
Corium, II.

Encephalon ^ j

Medulla spinalis j

Nervi cerebrales, I. + II
Nervi spinales, II.
Sympatheticus.II. + ILL

Org. tactus

Org. gustus

Org. olfactus [• I. + 11

Org. visus

Org. audituB

Mnsculi cutane
M. skeleti

Vertebrarium

Cranium

Sk. extremitatum

n.

H

CO

CO
I

<

O
»

5

c.

Nutritive
Apparatus
Nutritorium

''t. Intestinal system f Digestive organ
(ftwter) \ Respiratory organ

8. Vascular system
(^Organa circv^
lationis)

< Body cavity
Lymph vessels

Blood vessels
Heart

Reproductive
Apparatus ^

9. Renal system J nH°f7v*H„^t.

'Sexual glands
(I. Ovary)
(II. Testes)
Sexual ducts
(I. Oviduct)

10. Sexual organs

(n. Seed dBct)

Copulatory oi
^Sheath)
(n. Penis)

0. digestiva \„, ,y
O.respiratoria/"^'*'^'

Coeloma, II. + III.
Vasa lympha- \

tica llL+IIL

V. sanguifera I
Cor, III.

Urocystis, III. + lY.

\

Gonades

LOvaria)in, + IV.(?)
II. Testes) L + U. (?)

Gonophori »

(I. Ovidac- I
tus) U.(?)4'n

(II. Spenua- 1
dnctos) /

Copulativa \

a.Tagina)W ♦ n.

(II. Penis) [

tHE SENSORY APPARATtTS. I$$

think, securely fomided 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 Gasti*aea)
common to all animals, the Protozoa alone excepted ; in
this long extinct parent-form the entire body of the animal
consisted throughout life of the two primary germ-layers, as
is yet the case, for a short time, in the Gastrula. In the
Gastrsea the simple skin-layer did actually represent all the
animal organs and functions, and the simple intestinal layer,
on the other hand, all the vegetative organs and functions ;
potentially, this is even yet the case in the Gastrula.

In studying the development of the first important
part of the animal sphere, the sensory apparatus, or sen-
sorium, we shall now find how well adapted this Gastrsea
Theory is to explain, not only in a morphological but
in a physiological sense, the most important facts in the
history of evolution. This sensory apparatus consists of two
very distinct parts, having, apparently, nothing in common :
in the first place, the external skin-coveiing {Derma),
together with its appendages, the hair, nails, sweat-glands,
etc. ; and, secondly, the nervous system, situated internally.
The latter includes the central nervous system (brain and
spinal chord), the peripheric brain-nerves and medullary
nerves, and finally, the organs of sense. In the fully
developed vertebrate body these two main constituents of
the sensorium are entirely separate ; the skin lying entirely
extemallv 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-
losfical oT3servations 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
nervoa.

DERMIC ORIGIN OF THE SENSORY ORGANS. 1 97

This view is fully confirmed by the results of Comparative
Anatomy. Comparative Anatomy shows that many lowei
animals possess no nervous system, although, in common
with higher animals, they exercise the functions of sensation,
volition, and thought. In the Primitive Animals (Protozoa),
which do not even form germ-layers, of course the nervous
system, like the skin-covering, is wanting. Even in the
second main division of the animal kingdom — in the Metazoa
or Intestinal Animals — there is at first no nervous system.
The functions of these are performed by the simple cell-
layer of the exoderm, which the lower Intestinal Animals
have inherited directly from the Gastraea (Fig. 209, e). This
is the case in the lowest Plant Animals (Zoophyta), the Gas-
trseads. Sponges, and the lowest Hydroid Polyps, which are
but little higher than the Gastrseads. 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 ceU 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 (Archelmintkes)
which were developed directly from the Gastrseads. Even
those Primitive Worms in which the two primary germ-layers
had already split into the four secondary germ-layers (Plate
V. Fig. 10), seem not to have possessed a nervous system
distinct from the skin. The skin-sensory layer must, even in
these long-extinct Worms, have been at once skin-covering
and nerve-system. But already in the Flat Worms {Platel-
minthes), and especially in the Gliding Worms (TurbeUaria)
which of all existing forms approach nearest to the Primitive

198

THE EVOLUTION OF MAN.

Worms, we find an independent nerve-system, distinct and
separate from the outer skin-covering. This is the " upper

Fig. 209. — Gastrnla of Gastrophysema (Gastraead-class).

Fio. 210. — Transverse section through an embryonic Earth-worm : hs,
skin-sensory layer; ^m, skin-fibrons layer; (?/, intestinal-fibrous layer; dd,
intestinal-glandular layer ; a, intestinal cavity ; c, body-cavity, or Coeloma ;
^^, nerve-ganglia; u, primitive kidneys.

Fig. 211. — A Gliding Worm (Rhahdoccelum). From the brain or upper
throat ganglion (g) nerves (n) radiate towards the skin (/), the eyes (au),
the organ of smell {no), and the mouth (in) : h, testes; e, ovaries.

THE SKIN. 199

throat ganglion," situated above the throat (Fig. 211, gr ; Plate
V. Fig. 11, m). The complex central nervous system of all
higher animals has developed from this simple rudiment.
In the higher Worms, e.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 (Jis), 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, etc.
Physiologically, this outer covering (derma, or tegumentwm)
plays a double part. The skin, in the first place, forms the
general protective covering (integumentum commuTie) 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, ah). 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 (corium), on
the contrary, consists principally of connective or fibrous

Ftq. 212.— Human skin

in [.erpendicular section
(aft CI Kcker), much en-
lar'_re<.i : a, horuy stratum of
uuLrt rkin (epidermiti) ; 6,
tuucuus stratum of outer-
skin ; c, papillae of the
leather-skin (corium) ; d,
biuud- vessels of the latter;
e, /, excretory ducts of the
sweat-ijlands (p) ; 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 " aubcutis," 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 papillae, which
fit into the overlying epidermis (c). These touch-warts, or
sensory papillge, contain the most delicate of all the sensory
organs of the skin, the " corpwscula tactius" Other papillae

STRUCTURE OF THE SKIN. lOI

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 skm-fibrous layer (Fig. 112,Aj>r, vol. i. p. 352; Plates lY.
and v., I', Figs. 65-69, hf, p. 277).^^^

Analogously, all the constituent parts and appendages ot
the outer-skin (epidermis) origmate, by differentiation, from
the homogeneous cells of the horn-plate (Fig. 213). At a

Fig. 213. — Cells of the outer-skin {epidermis) of
a human embryo of two months. (After KoelUker.)

very early period, the simple cell-layer
of this horn-plate splits into two dis-
tinct strata. The inner, softer stratum
(Fig. 212, h) 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
growth 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
appendaojes develop from this both internally and ex-
temall3\ The internal appendages are the skin-glands;
the sweat-glands, the sebaceous glands, etc. The external
appendages are hair, nails, etc.

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 (corium)
(Fig. 214 1 ). A canal afterwards forms inside these solid

202

THE EVOLUTION OF MAN.

plugs (2, 3), either owing to the softening an 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, / g). These glands, which
secrete the sweat, are of great length, their ends forming a
coil ; they :evor branch, however ; and the same is to be
said of the glands which secrete the fatty wax of the ears.

Fig. 214. — Kudiments of tcar-glands
from a human embryo of four monthco
(After Koelliker.) 1. Earliest I'udiment the
shape of a simple, solid ping. 2 and 3. Fur-
ther developed rudiments, which branch
and become hollow : a, a solid offshoot ;
e, cell- covering of the hollow offshoot ; /,
rudiment of the fibrons 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-glands, situated
on the upper eyelid, which secrete
the tears (Fig. 214), and also the
sebaceous glands, .. aich produce the fatty sebaceous matter,
and generally open 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 (glandulcB
mammales, Figs. 215, 216). They supply milk for z. r
nourishment of the new-born Mammal. Notwithstandino;

SKIN-GLANDS.

203

their extra^ordinary size, these important organs are merely
large sebaceous skin-glands (Plate V. Fig. 16, md). The
milk is produced by liquefaction of the fatty milk-cells
within the branched milk-giand 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 (b), 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

FiQ. 215. — The breast of the female in section : c, grape-like glandular
lobules ; h, enlarged milk-ducts ; a, narrow excretory ducts, opening through
the breast-nipple. (After H. Meyer.)

Fig. 216. — Milk-glands of a new-bom child : a, original central gland
2?j ema-Uer, 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-bom 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 (onam.m.illa), 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
Am/ista (without nipple). In many of the lower mammals
there are numerous milk-glands, 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
A.pes, Bats, Elephants, and some other Mammals. Occasion-
ally, however, even in the human female two pairs of breast
glands (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

KXTERKAL APPENDAGES OF THE SKIK. 20$

of the outer skin in an outward direction. The nails (un-
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, e.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 (corium), as do the sebaceous and the sweat
glands. As in the latter, the simple plug consists oiiginally

206 THE KVOLUTION OF MAK.

of th« 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 parmanent fair hair, which the
child inherits from its parents, makes its appearance.
Occasionally the dark hair is retained for several weeksj

THE HAIB Ifl A BUDBIENTARY ORGAH. 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 highei
Apes resemble Man in the thin coat of hair which coveii
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 ihe 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 thoir origmal coat of hair in consequence of
adaptation.^^

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 all
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 eyen 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
surfeice of the body; these, together with their terminal
apparatus, the sense-organs, etc., constitute the " conductive

2IO

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.

Fig. 217. — Huinan embryo
of three months, in natural
size, seen from the dorsal side ;
the brain and dorsal marrow
exposed (after Koelliker) : }i,
hemispheres of the cerebrum
(fore-brain) ; m, "four-bulbs"
(mid-brain) ; c, small brain
(hind-brain, or cerehellum).
Below the latter is the
three -cornered "neck-medulla"
(after-brain) .

'Fig. 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

TBI: CENTRAL MARROW. til

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 meduUa of the head (encephalon, or medulla ca-
pitis), and, secondly, of the spinal marrow (medulla spi-
nalis). The former is enclosed in the bony skuU, or " brain
case," the latter in the bony vertebral canal, which is com-
posed of a consecutive series of vertebrse, 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-vertebrse) and another in the Imnbar region (at the
first lumbar vertebra, Figs. 217, 218). At the swelling at
the throat the large nerves of the upper limbs pass ofi" 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 EVOLUtlON OV MA^.

gata) into the brain. The spinal marrow a])pears 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

Fig. 219. — Hnman brain,
Been 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-
helium), with narrow parallel
furrows. The Roman numbers
indicate the roots of the twelve
pairs of brain nerves in order
from front to back.

the gi-eater part of the skull-ca\aty ; it is roughly distin-
guishable into two main parts — the large and small brain
{cerehrum and cerebellum). The former is situated in
front and over the latter, and its surface exhibits the well-
known characteristic convolutions and furrows (Figa. 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
" cross-piece " {corpus callosum). A deep transverse fissure
separates the large brain {cer^hrwm) from the small 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,

I'O

Fig. 220. — Hnman brain, seen fiom the left side. (After H. Meyer.) The
furrows of the large brain are indicated by large, thick lines, those of the
imall brain by finer lines. Below the latter the neck-marrow is visible, f^-p,
frontal convolutions ; Ce. a Ce. p, central convolutions ; B, fissui'e of Eolan-
ins; S, Sylvian fissure; T, temporal or pai-allel fissure; Pa, parietal lobe; An,
the annectant convolutions^; PO, parieto-occipital fissure ; Su, supra-marginal
csonvolution ; IP, intra-parietal fissure ; t, temporo-sphenoidal convolution.

and between them are curved ridges (Fig. 219, lower part).
The small brain 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
{jponfis varolii ; 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 (Craniota), from the Cyclostomi and Fishes
up to Man, in exactly the same form, as five bladders
placed one behind the other. Alike in their first rudiments,
they, however, difier 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 callosum), 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" (infundihulum), the
gray mass, and the " cone " (conarium). 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 " wonn " (vermis), and its two lateral parts, the " small

PAKTS OF THE BRAIN. 21$

hemispheres ^ (Figs. 217, c, 21 8, c) . Behind this comes, finally,
the fifth and last part, the " neck-marrow " (medulla oblon-
gata, Fig. 218, mo), which includes the single fourth Inain
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 for a moment at those lower animals which
have no such brain. Even in the skull-less Vertebrates, in
the Amphioxus, there is no real brain. In this case the
whole central marrow is merely a simple cylindrical cord

2l6 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 (mj) 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 (jpharynx), and which has, therefore, long been
■called the "upper throat ganglion " (ganglion pharyngeum
miperius). In the Gliding Worms (Turhellaria) 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 difierent parts of the body (Fig. 211,gn)
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 maiTow ; this is the case, also, in the articU'

NERVOUS SYSTEM IK THE LOWER AinHAUL ^I^

*

lated Ringed Worms {Annelida) and the Star-animals (Echi-
noderraa), 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
along the ventral side of the body.-^^

Descending below the Worms we find very many
animals which are entirely without a 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 Gastrseads. In all Primitive Animals
{Protozoa) the nerve-system is, of course, unrepresented, for
these have not as yet attained to the development of germ-
layera

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
hne 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- disc. 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 Hes directly
under the horn-plate; it is, however, afterwards situate

2lS

THE EVOLUTION OF MAN.

mff-~

EiGS. 221-223. — Lyre-shaped (or sole-shaped) germ-shield of a Chick, in
three consecutive stages of evolution, seen from the dorsal surface : about
twenty times enlarged. Fig. 221, with six pairs of primitive vertebrae.
The brain a simple bladder Qib). The medullary furrow is wide open from
the point x, 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 vertebrae. The brain consists of
three bladders : r, fore-brain; m, mid-brain ; /i, hind-brain, c, Heart ; dv,
yelk-veins. The medullary furrow is wide open behind (z). wp, Marrow-
plates. Fig. 223, with sixteen pairs of primitive vertebrae. The brain
consists of five bladders : v, fore-brain ; z, twixt-brain ; m, niid-brain ; h,
hind-brain ; n, after-brain, a, Eye-vesicles ; g, ear- vesicles ; c, heart ; dv,
yelk- veins ; wp, marrow-plate, uw, primitive vertebrae.

DEVELOPMENT OF THE BRAIN. 319

quite internally, the upper edges of the primiti v^e 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-
iiiota), commences in the same way and in exactly the same
simple form m 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
iiieduUary 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, rrii). For the central
medulla of Vertebrates thus first distinctly differentiates
into its two main sections, the brain (rrii) and the spinal
marrow (mj). 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, hh). 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.

5 ^- * Figs. 224-226.— Central mar.

row of human embryo in the
seventh week, two cm. long.
(After Koelliker.) Fig. 226,
view of the whole embryo from
the dorsal side ; the brain and
dorsal marrow laid bare. Fig.
225, the brain and upper part
of the dorsal marrow from the

left side. Fig. 224, the brain from above : v, fore-brain ; «, 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
di^dsions are formed (Fig. 223 : cf. also Plate V. Figs.
13-16 ; Plates VI. and VII., second cross-line). These five
fundamental brain-bladders, which re-occur in the same form
in the embryos of all the Skulled Animals (Craniota), were
first clearly recognized by Baer, who understood their true
importance and distinguished them, according to their rela-
tive positions, by very appropriate names, which are still in
general use : I., fore-brain (y) ; II., twixt-brain (0); IIL, mid-
brain (m) ; IV., hind-brain (h) , and V., after-brain (n).

In all Skulled Animals, from the Cyclostomi to Man,
the same parts, although in very various forms, develop
from these five original brain-bladders. The first bladder,
the fore-brain (protopsyche, v), forms by far the largest part
of the so-called " great brain " (cerebrum) ; it forms the two
^eat 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 ANIMAL& 221

bladder, the twixt-brain (deutopsyche, z,) proceed primarily
the " centres of sight " and the other parts which surround
the so-called "third brain- ventricle," also the "funnel"
(infuTidibulum), the " cone " (conarium), etc. The third
bladder, the mid-brain (mesopsyche, m), furnishes the small
group of the " four bulbs," together with the " aqueduct of
Sylvius." From the fourth bladder, the hind-brain (meta-
psyche, h), the greater part of the so-called " little brain "
(cerehellurn) develops ; the central " worm " (vermU), and
the two lateral "small hemispheres." The fifth bladder,
finally, the after-brain (epipsyche, n), forms the neck-.
marrow, or the "elongated marrow" (medulla ohloTigata),
together with the rhomboid groove, the pyramids, olives, etc.
The very highest importance must certainly be ascribed
to the fact, seen in Comparative Anatomy and Ontogeny,
that the brain is originally formed in exactly the same way
in the embryos of all Skulled Animals {Graniota)y 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 meduUary 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 difierent, both internally and externally, may be
traced back to one common condition. And yet, aU 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. (C£ Plates VI. and VII., second cross-
line.)

'Ai

\IV

s...

in,'-

A

Fig. 227. — Brains of three embryonic Skulled Animals in vertical longi-
tudinal sections: A, of a Shark (Heptanchus) ; B, of a Snake (Coluber); C, of
a Goat (Ca^ra) ; a, fore -brain ; fe, twixt-brain ; c, mid-brain ; d, hind-brain ;
e, after-brain; s, px'imitive fissure of the brain. (After Gegenbaur.)

Fig. 228. — Brain of a Shark (ScylUum) from the dorsal side : g, fore-
brain ; h, olfactory bulbs of the fore -brain, which send the large olfactory
nerves to the large nose capsules (o) ; d, twixt-brain ; h, mid-brain (behind
it, the insignificant rudiment of the hind-brain) ; a, after-brain. (After
Busch.)

Fig. 229. — Brain and dorsal maiTow of a Frog : A, from the dorsal side ;
B, from the ventral side ; a, olfactory bulbs, in front of the fore-brain (&) ;
i, funnel at the base of the twixt-brain ; c, mid-brain ; d, hind-brain ; s,
rhomboid groove in the after-brain ; m, dorsal marrow (very short in the
frog) ; m', root-processes of the spinal nerves ; t, fibre at the end of the
dorsal marrow. (After Gegenbaur.)

CXnfPA&jLTIYE VIEW OF BRAIN DEVELOPMENT. 223

A thorough comparison of the corresponding stages of
development in the brain in the various Skulled Animals
(Craniota) is very instructive. If it is applied to the whole
series of skulled classes, the following extremely interest-
ing facts soon become evident : in the Cyclostomi {Myxi •
noidea 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 ^ve 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 smaU 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

226 THE EVOLUTION OF MAN.

the animal, thus proving that the higher mental activities,
consciousness and thought, conscious volition and sensation,
may be destroyed one by one, and finally entirely anni-
hilated. If the animal thus treated is artificially fed, it
may be kept alive for a long time ; for the nourishment of
the entire body, digestion, respiration, the circulation of the
blood, secretion, in short, the vegetative functions, are in
no way destroyed by this destruction of the most important
mental organs. Conscious sensation and voluntary motion,
the capacity for thought and the combination of the various
higher mental activities, have alone been lost.

This fore-brain, the source of all these most wonderful
nervous activities, reaches that high degree of perfection only
in the higher Placental Animals (Placentalia) ; a fact which
explains very clearly why the higher Mammals so far excel
the lower in intellectual capacity. While the "mind" of the
lower Placental Animals does not exceed that of Birds and
Reptiles, we find among the higher Placentalia an uninter-
rupted gradation up to Apes and Man. Accordingly, their
anterior brains show surprising differences in the degree of
perfection. In the lower Mammals, the surface of the great
hemispheres (the most important part) is entirely smooth
and even. The fore-brain, too, remains so small that it
does not even cover the mid-brain above (Fig. 230). One
stage higher, and this latter is indeed entirely covered by
the excessive growth of the fore-brain ; but the hind-brain
remains free and uncovered. At last, in Apes and in Man,
the fore-brain covers the hind-brain also. A similar gradual
advance may also be traced in the development of the
peculiar furrows and protuberances which are so charac-
teristically prominent on the surface of the large brain

CONVOLUTIONS OF BRAIN. 227

{cerebrum) of higher Mammals (Figs. 219, 220). If the
brains of the various mammalian groups are compared with
reference to these convolutions and furrows, it appears that
theii gradual development is entirely proportionate with
the development of the higher intellectual activities. Much
attention has recently been devoted to this particular
branch of the Anatomy of the brain, and very striking
individual differences have been found even within the
human race. In all human individuals distinguished by
peculiar ability and great intellect, these swellings and
furrows on the surface of the great hemispheres exhibit a
much greater development than in common average men;
while in the latter, again, they are more developed than in
Cretins and others of unusually feeble intellect. There are
also similar gradations in the internal structure of the fore-
brain in Mammals. The great cross-piece {corpus callosunt),
especiaJly, the bridge between the two great hemispheres,
is developed only in Placental Animals. Other arrange-
merts, 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

2J8 the EVOLtmON 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-
structura* The extremely complex and perfect active
phenomena within the nerve-ceUs, 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 difierentiation 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

OBIQIN OF THE FUNCTIONS OF THE BRAIN. 229

are the inner membrane (pia mater), the central membrane
(meniTix arachnoides), and the outer membrane {dura
mater). All these parts are developed from the skin-fibrous
layer.

TABLE XXVII.

Ststkmatic Susvet of the most important Periods ih th« PHTLOonrr

OP THE Human Skin-coverings.

I. First Period : Skin of Gastrceads.

The entire skin-oovering (including the nervons 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
Amphiozns.

n. Second Period : Shin of Primitive Worma,

The simple exoderm of the Gastraead has thickened and split into two
distinct layers, or secondary germ-layers : the skin-sensory layer (rudiment
of the horn-plate and nerve-system) and the skin-fibrous layer (rudiment of
the leather skin (corium), the muscle-plate and the skeleton-plate. The
skin is potentially both covering and nund.

m. Third Period : Skin of Chordonia.

The skin-sensory layer has differentiated into the horn-plate (epidermis) j
and the central marrow (upper throat ganglia) separated from it j the latter
elongates into a meduUary tube. The skin-fibrous layer has differentiated
into the leather plate (corium) and, below this, the skin-muscular pouch (as
in all Worms).

rV. Fowrth Period : Skin of Acranick.

The horn-plate yet forms a simple epidermis. The leather-plate ii fUly
differentiated from the mnsole and skeleton plates.

230 THE EVOLUTION OF MAJNT.

V. Fifth Period : Skin of Cyelostoma.

The onter-skin remains a simple, soft mucous layer of cells, but forma
one-celled glands (cup-cells). The leather-skin (corivm) differentiates into
cutis and suh-cutis.

VI. Sixth Period : Skin of Prtmitive Fishes.

The outer skin is still simple. The leather skin forms placoid soalei or
small bony tablets, as in the Selachii.

VII. Seventh Period : Skin of Amphibia,

The outer skin differentiates into an outer hom-layer, and an inner
mucous layer. The ends of the toes are covered with homy sheaths (first
rudiments of^lslaws or nails).

VIII. Eighth Period : Skin of Mammals.

The outer skin forms the appendages characteristic of Mammals cmly ;
hair, and sebaceous, sweat, and milk glands.

TABLE XXVIII.

Systematic Suevei of the most important Periods in the PHTLOoiirr

OF THE Human Nervous System.

I. First Period ; Medulla of QastroBods.

The nOTve 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
AmphiozQS.

n. Second Period : Medulla of Primitive Worms,

The central nerve system is yet, at first, a part of the skin-sensory layer,
and afterwards consists of a throat medulla, a simple nerve-ganglion lying
above the throat ; as it is now in the lower Worms : the upper throat-
ganglion.

SXTBYEY OF HUKAN NEBYOUS SYSTEM. 33 1

m. Third Period : Medulla of Qhordonia.

The central nerve system consists of a simple mednllarj tnbe, an
elongation of the upper throat ganglion, which ia separated from thn inteo-
tine by a notoohord (chorda dorsaUs).

lY. Fourth Period : Medulla ofAerania.

The simple medullary tube differentiates into two parts : a head, and a
cLsfTsal part. The head medulla resembles a small, pear-shaped, simple
swelling (the primitive brain, or first rudiment of the brain) on the anterior
oztremity of the long cylindrical spinal marrow.

Y. Fifth Period : Medulla of CyclostonM,

The simple, bladder-like radiment of the brain divides into flre oon-
■eontiTe brain-bladders of simple straotnre.

YI. SitBth Period : Medulla of Primitive Fishes,

The five brain-bladders differentiate into a form similar to that now
permanently retained by the Selachii.

YII. Seventh Period : Medulla of Amphibia,

The differentiation of the five brain-bladders progresses to that stmotnre
which is now characteristic of the brain in Amphibia.

^ Yin. Eighth Period : Medulla of Mammals.

The brain attains the characteristic peculiarities distinctive of Mammals.
The following may be distinguished as subordinate stages of development ;
1, the brain of Monotremes ; 2, the brain of Marsupials j 3, the brain of
Semi-apes; 4, the brain of Apes; 5, the brain of Mav^Ufca ApM| 4 the
brain of Ape-men j and 7| the brain of Man.

( «32 )

TABLE XXIX.

Bjitematio Sxxrrej of the Evolution of the Skiii.ooTeriQg aad

Nerve System.

XXIX. A. Survey of the Evolntloii of the SMn-oovering.

fUm

(Derma,

or

nUegununitm)

i Horn-layer of the outer
skin
(Stratum comeum)
Mucous layer of the
outer skin
{Stratum mueoium)

Leather-ekln

{Corium)

Product of the Skln-

fibrous-Uyer

Fibrous layer of the
leather ekin
{Cutis)
Fatty layer of tne leather
skin

{SubeuHi)

/Hair
NailB
Sweat glands

ITear glands
Sebaceous glands
Milk glands

/Connective tlsMN
Fatty tissue
Musciilar tissue'
Blood-vessels
Papillffi of taste
nerves of
leather skin

azkl
the

XXIX. B. Sorvej of the Evolution of the Central Marrow.

Central Marrow,

or

Central Nerve
System

(Psyche, or Medulla
Centralit),

Product of the

Skin-sensory

layer

I. Fore-brain
(FrotoptycAe)

n. Twixt-braln

(^Deutopsyche')

{Great hemispheres
Olfactory lobules
Lateral chambers
Streaked bodies
Arch
Ooee piece

r Centre of sight
I Third chamber of the
< brain
1 Pineal body
I. Funnel

rFour bulbe

m. Mld-braln

IV. Hind-brain

{Metapsyche)

After-brain
{£^p8yche'^y

r Small hemispheres
A Brain worm
(^ Brain bridge

{Pjrramids
Olives
Restiform bodies
Fourth chamber of the
brain

\TI. Dorsal Marrow Notoptyehe

Eemifph(Brce eertbrt

Lobi olf actor ii

Ventriculi lateraXm

Corpora striata

F^nix

Corpus ecMotum

Tkalami optiei
Ventriculus tertUu

Conarium
InfundibtUvm

Corpus bigeminum
Aqun'ductus Sylvii
Pedunculi cerebri

Eemisphara cerebeUi
Vermis cerebelli
Pom Varolii

Corpora pyramiddlia
Corpora oliraria
Corpora resti/ormia
Ventriculus quartus

Mediulla spinaiiM

Kednllary
eoverings
{Menifigm)

^Enreloprng mem- (^ g^^ medullary eWn
branes, with the Central medull.ry skin
nutn ive blood- J 3 ^^^^ medullary skin

I^'splill^ ^^ I <^^«^ ^ "^« ddn-fibro«8 U^)

Pia mater

Arachnoidea
Dura mater

CHAPTER XXI.

DEVELOPMENT OF THE SENSE-ORGANS.

Origin of the most highly Purposive Sense-organs by no Preoonoeivo<3
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. — Eye
lids. — Structure of the Ear. — The Apparatus for Perception of Sound :
Labyrinth and Auditory Nerve. — Origin of the Labyrinth from thf
Primitive Ear Vesicles (by Separation from the Horn-plate). — Conduct
ing Apparatus of Sound : Drum Cavity, Ear Bonelets, and Drum Meuj -
brane. — Origin of these from the First GiU-opening and the Parte
immediately round it (the First and Second Gill-aroh). — Eudimentai ;
Outer Ear. — Eudimentary Muscles of the Ear-sheU.

" Systematic Physiology is based especially upon the history of develop-
ment, and unless this is more complete, can never make r%pid progress j for
the history of development furnishes the philosopher with the materials
necessary for the secure construction of a system of organic life. Hence
anatoniical and physiological researches should be prosecuted more from th«

234 THE EVOLUTION OF MAN.

point of view of development than is now the case ; that is, we should study
(»ach organ, each tissue, and even each function simply with the view of
determining whence they have arisen." — Emil 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. In no
other part of the animal body can we point to such extremely
delicate' aiid 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
tirst, 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-
caUed " 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
bhildish 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 func-
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. 23$

evolution proves most clearty 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 diuing 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 Gastrsea is the simple cell-layer from which the

236 THE EVOLUTION OF MAN.

differentiated sense-organs of all Intestinal Animals (Jl/e^azoa),
and, therefore, of aU. 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 d priori that the organs of
sense also owe their origin to the 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 hom-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. Even 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.
Thfd whole philosophy of the future will assume another

1

NATUBAL 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 stUl generally received,
are impartially studied, the simplicity with which the authors
bring foi-ward their airy metaphysical speculations, regardless
of all the significant ontogenetic facts by which theii
doctrines are clearly refuted, cannot fail to causie great sur-
prise. And yet the history of evolution, in conjunction
with the rapidly advancing Comparative Anatomy and
Physiology of the sense-organs, afibrds 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
Btages 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 : —

a3»

THE EVOLUTION OF MAN.

Senur^ngant.

o6flM-fKr0M.

Sem^/uneticma.

A* Sense-organs in
which the ter-
minal expansions
of the nerves are*
distributed in the
outer akin-oorer.
ing.

B. Sense-organs in
which . the ter-
minal expansions
of the nerves are
distributed over
inverted grooves
of the outer skin.
covering.

0. Sense-organs in
which the ter-
minal expansions
of the nerves are
distributed over-'
vesicles sepa-
rated from the
external skin.
ooTering.

I. Skin-covering
(outer skin, or
epidermis, and
leather - skin,
or corivm)

n. External
sexual parts
(penis and eli-
toria)

m. Muoousmem-
brane of the
mouth - cavity
(tongue and
palate)

IV. Mucous mem-
brane of the
nose-cavity

V. Bye
VI. Ear

I. Skin nerves ) Sense of pressure
(nervi cutcmei) i Sense of warmth

II. Sexual nerves 3. Sexual senM
(nervi pudendi)

m. Taste nerve
(nervus gloaso-
pharyngev>8)

TV. Olfactory
nerve
(n. olfactorius)

4. SeoBO of taste

6. Sense of smeU

V. Sight nerve 6. Sense of sight
{n. opticus)

VI. Ear-nerve 7. Sense of hear-
(n. acousticus) ing

Of the developmental history of the lower organs of
sense I have but little to say. 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 (corium) of Man,
as of aU 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 ascertained. These organs, in or upon which the
sensitive skin-nerves terminate, are the so-caUed "touch
bodies " and the " Pacinian bodies/' named after their dis-

MUCOUS MEMBRANE OF THE TONGUE AND NOSE. 239

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, I 'vill now call particular attention,
viz., the mucous membrane of the tongue and palate, in
which the taste-nerv^ 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 oimple
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 right 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 apertures into the head of the pharynx, so that the
49

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
cavities 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 waU, 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 smeU
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 thfl
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 NOSK

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, 71, p. 113). They are
lined by mucous membrane in folds, over vrhich 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

7n

Fig. 231.— Head of a Shark {Scyl-
Hum), from the ventral side : m, mouth
opening ; o, nose grooves, or pits ; r,
nasal furrow; w, 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 (Selachii) ; 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., Scyllium) a special process
of the frontal skin, the nasal flap, or " inner nasal process,"
overlaps the nasal furrow (n, n'). Opposite to this the outer
edge of the furrow rises and forms the "outer nasal process."
In Dipncusta 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 tli rough this canal directly into the mouth-

343 "^BX BVOLUnON 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 Ues 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 svxices-
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 f imple 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 " (" Kiechgruben," 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
ftimi oi the l^iind anterior extremity of the intestinal caiiaL

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

Fig. 232.

a n

Fig. 233.

X.

an

-rp

m

inf

■n f

Fig. 235.

Fig. 236.

Figs. 232, 233.— Head of an embryonic Chick, on the third day of
incubation : 232, from the front ; 233, from the right side, w, Nose-rudi-
ment (olfactory grooves) ; I, eye-rudiment (sight-grooves) ; g, ear-rudiment
(auditory grooves); -u, fore-brain ; gl, eye-slits ; o, upper jaw process; w,
lower jaw process of the first gill arch.

Fig. 234. — Head of an embryonic Chick, on the fourth day of incubation,
from below: n, nose -groove ; o, upper jaw process of the first gill arch;
y, lower jaw process of the same ; /c", 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 ; 7j/,
nasal furrow ; st, frontal process ; m, mouth-cavity. (After Koelliker.)

All these figures are proportionately enlarged.

244 '™^ EVOLtJTIOK OF MAIT.

original separation of the nasal groove from the mouth
groove is, however, soon interrupted, for the frontal procesa
(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, in). 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, r). As
the two parallel edges of the inner and the outer 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, o, 236, o,
Plate I., o). Below the mouth groove lie the gill arches,
which are separated from one another by the gill openings

UPPER JAW PROCESS. 345

(Plates I., YI., and YII., k). 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., u).
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, "Meckels cartilage," on the outer surface of
which the lower jaw forms (Figs. 232, u, 236, u). 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, w) — 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, p). 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-

FiG. 237. — Diagrammatic transverse section
through the mouth and nose cavity. While the
palate-plates (2^) separate the original mouth-cavity
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, v). (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, n). 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 tlie great vertical
bony partition wall — the " plough -share " {fomei'), and in
front to the twixt-jaw (os interTnaxillare). Goethe was
the first to show that in Man, just as in all the other Skulled
Animals, the twixt-jaw a^^pears 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 lias 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

Figs. 238, 239. — Upper part of the body of a human embryo (16 mm. in
length) dm:ing the sixth week : Fig. 238, from the left side ; Fig. 239, from
the front. The origin of the nose in two lateral halves,
originally separate, is still plainly visible. The nose and
upper lip are disproportionately great in comparison with
the rest of the face, especially with the lower lip.
(After Kollman.)

Fig. 240. — Face of a human embryo of eight weeks.
(After Ecker.) Cf. Frontispiece, Plate I. Fig. Mi—
Mm.

248 THE EVOLUTION OF MAN.

from the large brain through the roof of both nasal cavities
into the cavities, and extends over the olfactory mucous
membrane. At the same time, by inversion of the nasal
mucous membrane, the minor cavities of the nose, which are
afterwards filled with air, and which communicate directly
with the two nasal cavities, arise (frontal cavities, cavities of
the sphenoid bone, jaw cavities, etc.). In this special stage
of development they occur only in Mammals.^^

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.

STtTEMATIC SUBVKT 07 THE ChIEF FhYLOOENETIC StAGH OV TBB

Human Nose.

First Stage : Nose of the earlier rrimitive Fishes.

'Hie nose is formed by a pair of simple skin-groores (nose-pits) in ike
Mtter 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
khe higher Selachians).

Third Stage : Nose of the Dipnmtsta.

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
{Sozohranchia).

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 Frotamnia.

The primitive mouth-cavity, into which both nasal canals open, separates,
in consequence of the fonnation 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 Mammal».

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 Mamtnals.

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 e»»
oiusively oharacteiistic of Catarhine ADes 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.

t

Fig. 241. — The human eye in transverse section: a, protective membrane
(sclerotica); &, horn membrane {cin-nea); c, outer membrane (conjunctiva);
d, circular veins of iris ; e, vascular membrane (clioroidea) ; /, ciliary
muscle ; g, corona ciliaris ; h, rainbow membrane {iris) ; i, optic nerve
{n. opticus) ; k, anterior limit of the retina ; I, crystalline lens {lens crystal-
Una)', m, inner cover of the horn membrane (water membrane, membrana
Descemeti); n, pigment membrane {pigmentosa); o, retina; p, ^'petits-csinal;"
q, yellow spot of the retina. (After Helmholtz.)

When fully developed, the human eye is a globular
capsule (the eyeball, hidbus, Fig. 241). This lies in the

THE EYE. 251

bodxj orbit of th« skuU, 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 vitreum). The crystalline lens
(Fig. 241, l) 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 peUucid,
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 (corTiea, h). On
its outer surface the homy membrane is covered by a very
thin coating of outer skin (epidermis) ; this coating is
called the connecting membrane (conjunctiva); it extends
from the homy membrane over the inner surface of both
eyelids — the upper and lower folds of skin which on closing
the eyes are drawn together over them. At the inner
comer 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
darkrred, highly vascular membrane, the vascular mem-
brane (choroidea, e), and within this the retina (0), which
is a dilatation of the optic nerve (i). This latter is the
second brain nerve. It extends from the " centre of sight "
(the second brain-bladder) to the eye, penetrates the outer
coats of this, and then extends, as the retina, between the
vascular membrane (choroidea) and the vitreous body
(corpus vitreum). Between the retina and the vascular
membrane lies another very delicate membrane, which
is commonly, but wrongly, considered as part of the latter.
This is the black pigment membrane (pigmentosa, lamina
pigTuenti, 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
gi'anules. 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, /i), 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
pupU, 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,
afid mamj smaller rays.

DEVELOPMENT OF THE EYE. 253

In the embryo of jMan, 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 {I) arises, and at the
same time a groove-like indentation (0) in the horn-plate
(Fig. 242, 1). This groove, which we will call the lens groove,
changes into a closed sac, the thick- walled lens vesicle (2, Q,
owing to the fact that the. edges of the groove coalesce above

y

a-

I-

i

IS

w

\

IV

Fig. 242.— Eye 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) : ?i, hom-plate ; 0, lens groove ; Z, lens (in 1,
it still forms part of the epidermis, while in 2 and 3 it has separated); »,
thickening of the hom-plate at the point from which the lens separated j
;Z, vitreous body ; r, retina; u, pigment membrane. (After Remak.)

2 54 THE EVOLUTION OF MAJI.

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 (corivm)
lying below the lens separates from the outer skin-covering.
This siiiall 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 " memhrana 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 (hlastula), 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 (eTitoderTna), and
the latter into the skin-layer (exoderTna), so in the inverted
primary eye-vesicle the retina develops from the former

DEVELOPMENT OF THE EYE. 255

(inner) part (Fig. 242, r), 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 (I) 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 (r).
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, i.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 (I) and the retina (r)
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 liood 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 vitreum) 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

Fig. 243, — Horizontal traijaverBe
Bection through the eye of a human
embryo of four weeks ; 100 times
enlarged (after Koelliker) : i, 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, tfiinner, uninverted
lamella of the same) ; K, 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, s'p, 235, sp, p. 243). A thin process of the vitreous body
passes inward on the under surfiice of the optic nerve, which
it inverts in the same way as tlie primary eye-vesicle was
inverted. The hollow cylindi-ical optic nerve (tlie stalk of

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-
n^unication 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 {vasa centralia retince).

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
hom-plate. The globular waU 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 homy membrane
(cornea). The rudiments of all the essential parts of the
eye are now formed, and its farther development is only in
details, in the complex differentiation and combination of
the several parts.

( 258 )

TABLE XXXI.

B/rtemstic Surrey of the Development of the Human Eye.

L Qjstcinatlo Surrey of those parts of the Hnman Eye which develop from the first of tkn
Secondary Germ-layers, the Skin-sensory Layer.

A.
Products of the

Marrow-plat«

B.

ProdnctB of the

Horn-plate

^1. stem of the primary
eye-vesicle

Inner (inverted) part of
the primary eye-
vesicle

Outer (uuinverted) part
of the primary eye-
vesicle

Vesicle separated from
the horny plate

6. Outer epidermic skin

6. Inverted portions of the
epidermic sldn

1. Optic nerve
a. Betliu

3. Screen, or pig-
ment-coat

4. Crystalline lens

6. Connective mem-
brane

6. Te&r-glands

Iftrvut optieu*
Bttina

Pigmentosa (tamina
pigmentiy

Lens cryiidllina
Conjunctiva
Cflandulaa lacrymalet

11. Systemailo Survey of those parts of the Human Eye which develop from the second of th<

Secondary Germ-layers, the Skin-fibrous Layer.

'"7, 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 vitrei
mary eye-vesicle vitreous body

Prod«eU«f the
LeathMT-plate

Products of the

sioOHplftt*

9. Central vessels of
the retina

VasaceniraUa
retince

9. Continuation of the
corium process

10. Pupillary membrane, 10. Vascular mem- Capsula vatculotek
with its capsule brane of the lentis cryst4MinA

lens

11. Folds of the leather 11. Eyelids
^ skin (corium)

Palpti)f\m

flit 13. Vascular mem-
brane of the eye-
ball (capsula vas-
culosa bulbij)

14, 16. Fibrous membrane
of the eyeball (cap-
tviajibrosa bulbi)

mem- Cfktroide^
mem- M$

12. Vascular

brane

13. Rainbow

brane

14. Protective mem- Sclerotiea

brane
18. Homymembraos Oomea

THE NICTirATING 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 (epiderTnis). From
the outer skin — the homy 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 homy membrane (cornea) from the head-plates.

The outer protective organs for the eye, the eyelids, are
merely simple folds of skin, which, in the human embryo,
appear in the third month. In the fourth month the upper
eyelid adheres to the lower, and the eye then remains
covered by them till birth. (Plate VII. Fig. M ill., R iii.,
etc.) The two eyelids usually again separate shortly before
birth, but sometimes not till after. Our skuUed ancestors
had, in addition to these, a third eyelid, the nictitating
membrane, which was drawn over the eye from the inner
comer. Many Primitive Fishes (SelacJiii) 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 comer 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.

26o

THB EVOLUTION OF MAN.

The ear of Vertebrates develops in many important
points similarly to the eye and nose, but yet in other
lespects 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 laiter, it consists of two principal parts, an
apparatus for the conveyance of sound (external and middle
ear) and an apparatus for producing the sensation of sound
(internaJ ear). The outer ear opens in the ear-shell (concha

Fie. S44. — Anditory organ of man (left ear, seen from the front; natnra]
aiae) : a, ear-shell : h, external ear-caual ; c, drum, or tympanic membrane j
d, cavity of drum ; e, ear-trumpet; /, g, 7i, the three ear honelets (/, hammer ;
g, anvil ; h, stirnip) ; i, ear-pouch (utriculus) ; k, the three semi-circula*
canals ; I, ear-sac (sacculus) ; m, snail (cochlea) ; n, auditory nerve.

auris), situated at the side of the head (Fig. 224, a). From
this the outer ear-canal, which is usually about an inch long^
leads to the inside of the head (h). The inner end of this

THE EAU. 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 a special tube with
the moutli-cavity. This tube is somewhat longer, but much
narrower than the outer ear-canal ; it leads inward and
forward in an oblique direction from the inside wall of the
tympanum and opens behind the inner nostrils (or Choana)
into the upper part of the cavity of the throat (pharynx).
This canal is called the Eustachian tube (tuha Eustachii).
It equalizes the pressure of the air in the tympanic
cavity, and the outer atmospheric air which enters by
the ear canal. Both the Eustachian tube and the tympanic
cavity are lined by a thin, mucous membrane, which
is a direct continuation of the mucous membrane of the
throat. Within the tympanic cavity are the three bonelets
of the ear, which, from their characteristic shape, are called
the hammer, the anvil, and the stirrup (Fig. 244 /, g, 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 (h) lies next to the anvil
toward the inside, and touches with its base the outer waD
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 ol
sound from without throu!ih 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 smaU stones, composed
of a mass of microscopic calcareous crystals (otoliths). The
organs of hearing of most Invertebrates have essentially
the bame 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 aU Amphirhina
is divided into two sacs. The larger sac is called the
auditory pouch (utriculus), and has three curved appendages,
called the semi-circular canals (c, d, e) ; the smaller sac is
called the auditory sac (saccuhts), and is connected with a
peculiar appendage, which in Man and the higher Mammals
is distinguiBhed by a spiral form, like the shell of a snails and

DEVELOPMENT OF THE EAB. 263

henoe is called the " snail " {cochlea, h). On the thin wall
of this delicate membranous labyrinth, the auditory nerve,
which passes from the after-brain to the labyrinth, is dis-
tributed in a very complex manner. It divides into two
main branches, the nerve of the cochlea, and the nerve of
vestibule, for the remaining part of the labyrinth. The
former seems specially to determine the quality of the sound
heard, the latter its quantity. The nerve of the cochlea

Fio. 245. — The bony labyrinth of the human ear
(left side) : o, vestibnle ; h, cochlea ; c, upper semi-
circular canal; d, posterior semi-circular canal; «,
outer semi-circular canal ; f, fenestra ovalis ; g, fenestra
rotwnda. (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
lu^aring is very simple in the human embryo, as in those
uf 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
gilJ -opening, a little wart-like thickening of the horn-plate
arises on each side (Figs. 24;6, 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 ita
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, Iv ; 249, 0). The outer part has

264 THE EVOLUTION OF MAN.

elongated into a thin stalk, which at first opens outward in
a narroAv canal. (Cf. Fig. 137,/, vol. i. p. 382.) This is called
the appendage of the labyrinth recessus labyrinthi,Fig. 246, Ir).

Fig. 246. — Development of the ear-labyrinth of a Chick, in five con-
secutive stages (A-E) (cross-sections through the rudimentary skull) : fl,
ear-groove ; Iv, ear-vesicle ; Ir, labyrinth appendage ; r., rudiment of the
cochlea; csp, hind semi-circular canal; cse, outer semi-circular canal;
jr, jugular vein. (After Eeissner.)

Figs. 247, 248. — Head of an embryonic Chick, on the third day of incuba-
tion : 247 in front, 248 from the right ; n, rudimentary nose (olfactory
groove) ; I, rudimentary eye (ocular groove) ; g, rudimentary ear (auditory
groove) ; V, fore-brain ; gl, eye-slit ; o, process of the upper jaw ; u, process
of the lower jaw of the first gill-arch. (After Koelliker.)

Fig. 249. — Primitive brain of human embryo of four weeks, in vertical
section, and the left half observed from within : v, z, m, h, n, the five grooves of
the skull cavity in which the five brain bladders are situated (fore, twixt,
mid, hind, and after brains) ; o, primary, pear-shaped auditory vesicle
(showing through) ; a, eye (showing through) ; Tio, optic nerve ; p, canal of
the hypophysis ; t, central skull-pieces. (From Koelliker,)

DEVELOPMENT OF THE EAR. ' 265

In lower Vertebrates, this develops into a peculiar cavity
filled with calcareous crystals, which in some Primitive
Fishes (Selachii) remains permanently open, and open^
above on the skull (ductus endolymphaticus). In Mam-*
mala, 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 petrous
bone in the form of a narrow canal, and is called the aque-
duct of the vestibule {aquceductus vestihuli).

Only the inner and lower part (extended like a bladder)
of the detached ear-vesicle develops into the differentiated
and extremely complex structure which is afterwards known
as the " secondary labyrinth." This vesicle separates at a
very early stage into an upper, larger section, and a lower,
smaller section. The former gives rise to the ear-pouch
(utriculus) with the three semi-circular canals; from the
latter proceeds the ear-sac (sacculus) with the "snail"
(cochlea, Fig. 246, c). The three semi-circular canals
originate as simple pocket-like processes from the ear-
pouch (Fig. 246, E, cse and csp). In the centre of each of
these processes, the two walls coalesce, and separate them-
selves from the utricle, while their extremities still commn
nicate with its cavity. In all Double-nostrils (Amjpliirhina)
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

266 THE EVOLUTION OF MAN.

various stages in its ontogenetic development still exist
permanently side by side in the ranks of the lower Vert.e-
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 facia]
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, secotidary formation,
which only afterwards connects itself with tlie 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.

( 267 )
TABLE XXXII.

SnmCATIG SURYEY OF THE GhIEF StAOES IN THK DbVSLOPMSIIT

OF THE Human Ear.

I. First Stage.

The auditory nerre is an ordinary sensitive skin-nerve, which, daring the

differentiation of the horn-plate, appears at a certain point on the skin of

the head.

n. 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."

III. 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"
becomM mdimentaiy {Aquaductv^ vestihuli) .

rV. Fov/rth Stage.
The auditory vesicle differentiates into two connected parts, the ear-
pouch (utricuhia) and the ear.sac (^sacculus). Each of the two vesicles
receives a special main branch of the auditory nerve.

V. Fifth Stage.

Three semi-oironlar canals grow from the ear -pouch (as in all Amphi-

rhind),

VL Sixth Stage.

The ** snail '* (cochlea) grows from the ear-sac in Fishes and Amphibia ;
it is very insignificant, and is only developed as an independent part in the

Amniota.

VII. Seventh Stage.

The first gill-opening (the blow-hole of Selachians) changes into the
tympanic cavity and the Eustachian tube ; the former is externally closed
by the tympanic membrane (Amphibia).

VIII. Eighth Stage.

The small bones of the ear (ossicula cuuditus) (the hammer (malleiU) and
anvil (incus) from the first gill-arch, the stirrup (atapes) &om 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 onrved rim wMh
a small ear-flap (as in Anthropoid Apes and Men).

( 2^^ )

TABLE XXXIII.

SjEitematio Survey of the Development of the Hainan Ear.

I. Surrey of the parts of the Internal Ear. (Apparatus perceptlre of souaA.)

1. Stalk of the primary 1. 4<luP<^uct of the Aqu(Eductu$ vesttbuli

ear- vesicle vestibule (Due- s.Recettutldhyrintki

tus endolym-
phaticiis)

riiLjii-f^jif thf J 2, 3. Upper part of the 2. Ear-pouch Utriculut

mancis oi me < primary ear-vesicle 3. Three seml-circu- Canale* ttmi-cireu-

Eonx-plate

lar, or curved
canals

4, 5. Lower part of the 4. Ear-sac

primary ear- vesicle 5. "The snail"

lart*

Sacculut
Cochlea

B.

Products of the

Head-plate

6. Auditory nerve 6. Auditory nerve Ntrvus aeuttiau

1. Bony covering of the 7. Osseous labyrinth Labyrinthiu otaexu
membranous laby-
rinth

8. Bony covering of the 8. " The stony bone " 0$ petrotum

\ whole internal ear

n. Survey of the parts of the Intermediate and External Ear. (Apparatus for the

conveyance of sound.)

C
Products of the

first

OiU-opeuing

/- 9. Inner part of the first

gill-opening

10. Central p irt of the first

gill-opening

11. Closed part of the first
gill-opening

/12. Upper part of the
second gill-arch

D.

Products of the
first two

Gill-arohei

Prodack of the

Head-plate

Product of the
Skm-oov«ring

<

13. Upper part of the first

gill-arch

14. Central part of the first

gill-arch

16. Tympanic circle

(Annulus tympanicut)

'16. Circular membranous
fold at the closed part
of the first gill-
oponing

9. Eustachian tube Thiba Eustachii

10. Tympanic cavity Cavwm tympani

(Interior of the
drum)

11. Tympanic mem- Mcmbrana tympani

brane (Head of
the drum)

12. Stirrup (First Sta/pt»

bonelet of the
ear)

13. Anvil (Second IneuB

bonelet of the
ear)

14. Hammer (Third MaOeuM

bonelet of the
ear)

15. Bony outer audi- Maahu oudUorim

tory passage otteut

16. Ear-shell

Oonchaauria

17. Rudimentary ear- Muaouli oonchat

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 {Selachii) 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 giU-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 difierent stages of development and partly also of
atrophy. The ear-shell has atrophied in most a(^uatic

270

THE EVOLUTION OF MAN.

Mammals. Most of these have even lost it entirely ; this
is so, for example, in the Sea-cows and Whales, and most
Seals. On the other hand, in the great majority of Pouched
Animals (Marsupialia) and Placental Animals (Placentalia),
the ear-shell is well developed, receives and concentrates the
waves of sound, and is provided with a highly-developed
muscular apparatus, by means of which it can be turned
freely to all sides, and at the same time can be changed in
form„ Every one must have noticed how strongly and freely
our domestic Mammals, Horses, Cows, Dogs, Rabbits, etc.,
can " prick " their ears, erect them and turn them in different
directions. Most Apes yet retain the power of doing this,
and our ancient Ape progenitors could also do it. The more

Fig. 250. — Rudimentary ear.muscles on the human skull : a, upward
muscle {m. attollens) ; l, forward muscle (m. attrahens) ; c, backward muscle
(m. retrahens) ; d, larger muscle of the helix (m. helicis major) ; e, smaller
muscle of the helix (m. helicis minor); f, muscle of the tragus (m. tragicus);
(/ 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 gi'adually

THE EAR IN MAN AND APES. 271

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 gi'adually increased. On the
other hand, no man is able to erect the ear-sheU by the
upward muscle (a), or to change its form by the little inner
muscles of the ear (d, 6, /, 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 iii. and G in.) On carefully
comparing the ear-sheUs 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 comer

appears, the last trace of the free prominent point of the ear
51

2/- THE EVOLUTION OF MAN.

a our older Ape ancestors. Thus it is possible even here,
vsrith 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
orgaiis.iT^

CHAPTER XXII.
DEVELOPMENT OF THE ORGANS OF MOTION.

The Motire Apparatus of Vertebrates. — These jure constitnted 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 Vertebrae. — The Ribs
and Breast-bone. — Germ-histoiy of the Vertebral Column. — The Noto-
chord. — The Primitive Vertebral Plates. — The Foniiation of the Meta-
mera. — Cartilaginous and Bony Vertebrae. — Intervertebral Discs. —
Head-skeleton (Skull and Gill-arches). — Vertebral Theory of the Skull
(Goethe and Oken, Huxley and Gegenbaur). — Primitive Skull, or
Primordial Cranium. — Its Formation from Nine or Ten Cofilescent
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 Toea
of the Fins. — Many-fingered and Five-fingered Vertebrates. — Com-
paiTson of the Anterior Limbs (Pectoral Fins) and the Posterior Limbfl
(Ventral Fins). — Shoulder Girdle and Pelvis Girdle. — Germ-history of
the Limbs. — Development of the Muscles.

• In forming his estimate of my entire theory, the reader may begin with
the details and examine the fundamental facts on which I base my con-
elnsions. But it is equally necessary to connect the detached facts, and
estimate their bearing on the whole. He who in the world of organisms seea
only disconnected existencesj in which some organic similarities appear M

2/4 THE EVOLUTION OF MAN.

accidental ooinoidenoes, will remain a stranger to the reanlts of this
inyestigation j not merely because he does not comprehend the concln-
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 Gcoenbavk
(1878).

Among those features of the organization which are specially
characteristic of the vertebrate tribe as such, the peculiar
arrangement of the motive apparatus, or " locomotorium,"
undoubtedly occupies a principal place. As in all the
higher animals, the active organs of motion, the muscles,
form the most important part of this apparatus ; these are
the fleshy bands which, by means of their peculiar contrac-
tibility, of their power of contracting and shortening, move
the various parts of the body, and thus change the position
of the entire body. The arrangement of these muscles is,
however, entirely peculiar in Vertebrates, and differs from
the arrangement common to aU 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 th© 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 difierent 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 recoo:nized as lon^ acjo as the be^inninfr
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 Gk)ethe

2y6 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 choruB 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
d 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 afibrds 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
afibrd conviction of the true tribal relationship of all
Vertebrates. All the separate parts of which this bony
&ame is composed appear in other Mammals, in a great

• •* AUe Gestalten sind ahnlioh, doch keine gleichet der andem;
Und so deatet der Cbor auf ein geheimes Gesetz."

IMPORTANCE OF THE SKELETON. 27;

yariety 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 ]\Ian, (Cf
Table XXXIV. and Fig. 251, the human skeleton from the

( 2/8 )

TABLE XXXIV.

Bjetem&tio Snrvey of the Arrangement of the Hnman SkeletooL
A. Central Skeleton, or Axial Skeleton. Spine.

A.». Vertebral Bodies and Upper Arches,

1. Skall i 1 a. Pre- vertebral skull
{Cranium) \ 2 b. Vertebral skull

r 1 Neck vertebra

2. Vertebral ] 12 Chest „
column -^ 5 Hip „

( Columna • 6 Vertebrae of the eacmm
vertebralit) (.4 „ » „ tail (coccyx)

A.b. Lower Vertebral Arches.

1. Products of the gill-
arches

Producta arcuum
branchialium

2. Ribfl and bmct- Costee et itemtm
bone

B. Bones connecting the Extremities.

B.a. Bones connecting the Anterior Limbs:
Bones of the Shoulder.

1. Shoulder-blade Scapula
1. Primitive key-bone Procoracoidesf)
[3. Raven Lone Coracoides f)
4. Collar-bone, or key-bone Clavicula

B.b. Bones connecting the Lower Limbs :
Bones of the Pelvis.

1. Intestinal bone Os ilium

2. Pubic bone Os pubis

3. Hip-bone Os ischi

0. Jointed Skeleton of the Limbt.

C.ft. Skekton of the Ftrre Limbs.

I. F1B8T Division: Upper A km.
1. Uppar arm bone Humerus

II. Second Divimon ;

3. Spoke-bone
3. EU-bone

Lower Arm.

Radius
Ulna

in. Thibd Division : Hand.

m. A. Wrirt

Original parta
/ a. Radical
I b. TDtermedima
\ c. Ulnar
'Ld. Central
i e. Carpal I.

I/. „ n.
] g. .. III.
' *. „ IV.

+ v.

Carpus
Modified parti.
Scaphoideum

■■ Liinatum
■■ Triquetium
■■ Intermedium f ]
; Trapezium
■■ Trapezoides
■■ Capitatum
Hamatum

III. B. Palm of the Hand
III. C. Five Fingem
(14 booee

Metacarpus (5)

Digiti

Phalanges)

C.b. Skeleton of the Hind Limit.

I. First Division: Thigh.

1. Thigh-bone Femur

II. Sboovd DrviBioir : Lso.

2. Shin-bone Tibia

3. Calf-bone Fibula

III. TraBD Division: Foot.

III. Ankle
Original parts.

a. Tibial

b. Intormedium

c. FibuLir
. d. Central

Tarsal I.
., II.
. .. III.
. „ IV. + V.

Tarsus
Modified parte.

'\ — Astragalus

= Calcaneus
= Navicular*
- Cuneiform

= Cuboides

L
IL
UL

1

III. B. Sole of the Foot

III. C. Five Toes
(14 bones

Metatarsits (5)

Digiti

Phalanges)

HUMAN SKELETON.

279

^?i^'^

Fig. 251.

Fig. 252.

28o

THE EVOLUTION OF MAN.

I,

Fig. 253.—
Human vertebral
colnmn (in an up-
right position ;
from the right
side). (After H.
Meyer.)

right side (without arms); Fig. 252, the
entire skeleton from the front.) In Man,
as in all other Mammals, the skeleton is
primarily distinguishable into the axial
skeleton, or spine, and the skeleton of the
appendages, or the bony frame of the limbs.
The spine consists of the vertebral column
and of the skull ; the latter being the pecu'
liarly modified anterior part of the former.
The ribs are the appendages of the vertebral
column ; the tongue-bone (os Ungues), the
lower jaw, and the other products of the
gill-arches, are those of the skull. The
skeletons of the two pairs of limbs, or ex-
tremities, are composed of two different
parts : of the bony frame of the actual, pro-
minent extremities, and of the inner girdle
skeleton, by which the limbs are attached
to the vertebral column. The girdle skele-
ton of the arms (or fore limbs) is the
shoulder girdle ; the girdle skeleton of the
legs (or the hind limbs) is the pelvic
girdle.

The bony vertebral column in human
beings (coluriina vertehralis, or vertehra-
rium. Fig. 253) is composed of thirty-three
or thirty-four circular pieces of bone, which
lie one behind the other (one above the
other in the usual upright position of
man). These bones {vertehroe) are sepa-
rated from each other by elastic cushions.

DEVELOPMENT OF THE SKELETON.

281

tlie intervertebral discs (iigamenta intervertebralia), and
at the same time, are connected by joints, so that the
entire vertebral column forms a firm and solid axis, which
is, however, flexible and elastic, capable of moving freely
in all directions. In the various regions of the trunk,
the vertebrge differ in form and connection, so that the
following are distinguished in the human vertebral column,
beginning from above : seven neck- vertebrae, twelve breast-
vertebrse, five lumbar-vertebrse, five cross-vertebrae, and
four to five tail-vei*tebr8e. The uppermost, those directly in

Fig. 254. — Third neck-vertebra of man.
Fig. 255. — Sixth breast. vertebra of man.
Fig. 256. — Second lumbar-vertebra of man.

contact with the skull, are the neck-vertebrae (Fig. 254),
and are distingTiished by a hole found in each of the
two lateral processes. There are seven neck-vertebrae in
Man, as in nearly all other Mammals, whether the neck
is long, as in the Camel and the Girafie, or short, as in the
Mole and the Hedo'ehog;. The fact that the number of these
neck-vertebrae is always seven, — and there are but few
exceptions (explicable by adaptation), — is a strong argu-
ment for the common descent of all Mammals ; it can only
be accounted for as a strict transmission from a common

2SS THE EVOLUTION OF MAN.

parent-form, from some Promammal which had seven neek>
vertebrae. If each animal species had been a distinct crea-
tion, it would have been far more to the purpose to have
furnished the long-necked Mammalia with a larger, and the
short-necked with a smaller number of neck- vertebrae. The
neck-vertebrae are immediately followed by those of the breast
or thorax, which, in Man and most other Mammals, number
twelve or thirteen (usually twelve). Attached to the sides
of each breast-vertebra (Fig. 255) is a pair of ribs — ^long
curved processes of bone lying in and supporting the wall of
the thorax. The twelve pairs of ribs, with the connecting
intercostal muscles and the breast-bone {stemvm) constitute
the breast-body {thorax, Fig. 252, p. 279). In this elastic
and yet firm thorax lie the double lung, and between the
two halves of this, the heart. The chest-vertebrae are
followed by a short but massive section of the vertebral
column, formed by five large vertebrae. These are the
lumbar-vertebrae (Fig. 256), which bear no ribs and have
no perforations in their lateral processes. Next conies the
cross-bone (sacrum), which is inserted between the two
halves of the pelvic girdle. This cross-bone consists of five
fixed and amalgamated cross-vertebrae. Last comes a small
rudimentary tail- vertebral column, the rump-bone (coccyx).
This bone consists of a varying number (usually four, more
rarely three or five) of small aborted vertebrae ; it is a
useless rudimentary organ, retaining no physiological sig-
nificance either in Man or in the Tail-less Apes or Anthro-
poids. (Cf Figs. 204-208.) Morphologically it is, however,
very interesting, as afibrding incontrovertible evidence oi
the descent of Man and of Anthropoids from Long-tailed
Apes. For this assumption afiords the only possible

THE VEBTEBRJL

283

explanation of this rudimentary tail. In the human

embryo, indeed, during the earlier stages of germ-history,
the tail projects considerably. (Cf. Plate VII. Fig. M il.,
and Figs. 123, s, 124, s, vol. i. p. 370.) It afterwards becomes
adherent, and is no longer CKternally visible. Yet traces
of the aborted tail- vertebrae, as well as of the rudimentary
muscles, which formerly moved them, persist throughout life.
According to the earlier anatomists the tail in the female
human being has one vertebra more than that of the male
(four in the latter, five in the former).^''^

Ifuniber of VtrUbrce im variout Oatarhini.

Neck
Verte-
bra.

Tail-
less

(Man (Fig. 208)

Orang (Fig. 205)

Gibbon (Fig. 204)

Gorilla (Fig. 207)

Chimpanzee (Fig. 206)

i Mandril (Mormon choras)
Drill {Mormon leuco'phceus) ...
Rhesus {Imius rhesus)
Sphinx (Papio sphinx)
Simpai (Semnopithecus melus)

7
7
7
7

7

7
7
7
7
7

Chest
or tho-
racic
Verte-
bra.

Lum-
bar

VerU-
bra.

12
12
13
13
14

13
12
12
13
12

5
5
5
4
4

6

7
7
6
7

Cfrott
or

tacral
Verte-
bra.

mil

Verte-
bra.

5
4

4

4

4

3
3
2
3
3

4
5
3
6
5

Ittal.

33
33
32
33
34

5

8

18

24

31

34

37
46
53
60

The number of vertebrae in the human vertebral column
is usually thirty-three in aU ; but it is an interesting fact
that this number frequently varies, one or another vertebra
failing, or a new, supernumerary vertebra inserting itself.
Not unfrequently, also, a rib, capable of free motion, forms
on the last neck-vertebra or on the first lumbar-vertebra, so
that thus there are thirteen breast, and six neck, or four
lumbar vertebrae. In this way contiguous vertebrae in the
different sections of the vei-tebral column may replace each

284 THE EVOLUTION OF MAN.

»

other. On the other hand, the above comparison of the
number of vertebrae in different tail-less and tailed Catarhines
shows considerable fluctuations in these numbers even 'n
this one family.^'®

To understand the history of the development of the
human vertebral column, we must now study the form and
combination of the vertebrae in somewhat greater detail
The main outline of each vertebra is that of a signet ring
(Figs. 254?-256). The thicker part, which faces the ventral
side, is called the body of the vertebra, and it forms a short
disc of bone ; the thinner forms a semi-circular arch — the
vertebral arch, which is turned toward the dorsal side of the
body. The arches of all the consecutive vertebrae are so con-
nected by thin ligaments {ligamenta intercruralia) that the
space enclosed by them all in common forms a long canal.
In this spinal, vertebral canal lies, as we have seen, the hind
portion of the central nervous system, the spinal marrow.
The front part of this, the brain, is enclosed in the skull-
cavity, and hence the skull itself is merely the anterior
section of the vertebral column, modified in a peculiar way.
rhe base or ventral side of the bladder-shaped brain-capsule
was originally formed by a number of coalescent vertebral
bodies, the amalgamated upper vertebral arches of which
formed the arched or ventral side of the skull.

While the firm, massive vertebral bodies constitute the
true central axis of the skeleton, the dorsal arches serve to
enclose and protect the central marrow. Analogous arches
also develop on the ventral side as a protection for the
thoracic and abdominal viscera. These inferior or ventral
vertebral arches, proceeding from the ventral side of the
vertebral bodies, form a canal in many low Vertebrates in,

NATUBB OF THE VERTEBRfi. 385

which are enclosed the large blood-vessels on the under
surface of the vertebral column — the aorta and the tail vein.
In higher Vertebrates most of these inferior vertebral arches
are lost or become merely rudimentary. But in the breast
section of the vertebral column they develop into strong,
independent bony arches, the ribs (costce). The ribs are, in
fact, merely large vertebral arches which have become
independent, and have broken their original connection
with the vertebral bodies. The gill arches, of which we
have spoken so often, are of similar origin ; they are actual
head-ribs in the strictest sense — processes which have
actually originated from the lower arches of the skull-
vertebrae, and which correspond with the ribs. Even the
mode of connection of the right and left halves of the arches
on the ventral side is the same in both instances. The
chest is closed in front by the intervention, between the
upper ribs, of the breast-bone (sternum) — a single bone
originating from two corresponding side-halves. The gill-
body is also closed in front by the intervention of a single
piece of bone — the copula lingualis.

In now turning from this anatomical examination of the
constitution of the vertebral column to the question of its
development, I may, as regards the first and most important
features in the evolution, refer the reader to the explanation
already given of the germ-history of the vertebral column
(Chap. XIL, vol L pp. 369-378). In the first place, it is ne-
cessary to recollect the important fact that in Man, as in all
other Vertebrates, a simple, unarticulated cartilaginous rod
at first occupies the place of the articulated vertebral column.
This firm but flexible and elastic cartilaginous rod is the
well-known notochord (chorda dorsalis). In the lowest Ver-

286

THE EVOLUTION OF MAN.

tebrate, the Amphioxus, this persists throughout life in this
very simple form, and penrianently constitutes the whole
internal skeleton (Fig. 151, i,voL i. p. 420 ; Plate XI. Fig. 15).
But even in the Mantle Animals (Tunicata), the nearest
invertebrate allies of Vertebrata, we find this same noto
chord; transitorily in the transient larval tail of Ascidia
(Plate X. Fig. 5, ch) ; permanently in the Appendicularia
(Fig. 162). The Mantle Animals, as weU as the Acrania,
have undoubtedly inherited the notochord from a common
worm-like parent-form, and these primaeval worm ancestors
are the Chorda Animals (Chordonia, p. 91).

Long before any trace of a skull, limbs, etc., appears in the
human embryo or in that of any of the higher Vertebrates —
in that early stage when the whole body is represented only
y6 by the lyre-shaped germ-disc — in the cen-
tral line of this latter, directly under the
primitive groove or medullary furrow, ap-
pears the simple chorda dorsalis. (C£ Figs.
84-87, vol. i. pp. 297, 298, surface view;
Figs. 66-70, 89-93, transverse section ; also
Plates IV., v., ch.) As a cylindrical chord it
traverses the longitudinal axis of the body,
and is equally pointed at both ends. The
cells which compose the chord (Fig. 257, b)
Fig. 267.— PbT- come, in common with all the other ceUs of
tion of notochord ^^^ skeleton, from the skin-fibrous layer

(zhorda iAyrsaUs) of _ *'

an embryo sheep : They most resemble certain cartilage cells ;
a, sheath; b, cells, g^ special " chordaL tissuc " is often said to

exist; but this must not be regarded as
more than a special form of cartilaginous tissue. At an
early period the notochord envelopes itself in a structureless
sheath (a) as clear as glass, which is secreted by its cells.

DEVELOPMENT OF THE VERTEBRAL COLUMN. 287

This perfectly simple, inarticulate, primary axial
skeleton is soon replaced by an axticulated, secondary
axial skeleton, called the " vertebral column." On each side
of the notochord the primitive vertebral bands or primitive
vertebral plates (vol. L p. 306, Fig. 92, ww) differentiate from
the inner portion of the skin-fibrous layer. The inner part
of these primitive vertebral bands, which immediately sur-
rounds the notochord, is the skeleton-plate, or skeleton
stratum (i.e., the cell-layer forming the skeleton), which
furnishes the tissue for the rudiments of the permanent
vertebral column and of the skull. In the anterior half
of the body the primitive vertebral plate remains a simple,
continuous, unbroken layer of tissue, and soon expands into
a thin- walled vesicle, which surrounds the brain ; this is the
primordial skulL In the posterior half, on the contrar}^,
the primitive vertebral plate breaks up into a number of
homologous cube-shaped pieces, lying one behind the other ,
these are the several primitive vertebrae. The number
of these is at first very small, but soon increases, as the
germ grows in the posterior direction (Figs. 258-260, uw).
The first and earliest primitive vertebrae are the foremost
neck-vertebrae ; the posterior neck- vertebrae then originate ;
then the anterior breast-vertebrae, etc. The lowest of the
tail- vertebrae arise last. This successive ontogenetic growth
of the vertebral column in a direction from front to rear
may be explained phylogenetically by regarding the many-
membered vertebrate body as a secondary product, which
has originated from an originally inarticulate parent-form
by progressive metameric development, or articulation.
Just as the many-membered Worms (Earth-worm, Leech)

and the closely allied Arthropods (Crabs, Insects) originally
52

288

THE EVOLUTION OF MAN.

*»'.<»•

Figs. 258-260. — Lyre-sliaped germ-shield of a Chick, in three consecutive
stages of development ; seen from the dorsal side ; enlarged about twenty
times. Fig. 258, with six pairs of primitive vertebrae. The brain is a sim-
ple bladder (hh). The spinal furrow from x remains wide open ; behind, at
z, it is much enlarged, mp, Marrow-plates ; sp, side-plates ; y, limit be-
tween the pharynx cavity (s/i) and the head-intestine (vd). Fig. 259, with
ten pairs of primitive vertebrae. The brain has separated into three
bladders: v, fore-brain; w, mid-brain; h, hind-brain; c, heart; dv, yelk-
veins. The spinal furrow is still wide open (z). m/p, Marrow-plates.
Fig. 260, with sixteen pairs of primitive vertebrge. The brain has separated

PKIMITIVE VEllTEBR^. 289

into ftre bladders : v, fore-brain ; z, twixt-brain j m, mid-bi-ain ; \, hind,
brain ; n, after-brain ; o^ eye-vesicles ; g, ear-veeioles j c, heart j d«, yelk-
reias ; mp, marrow-plate } uto, prinutive vertebra.

developed from an inaxticulate 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 vertebrae.
We have already seen how these true vertebrae develop from
the skeleton-plate of the primitive vertebrae or metamera.
The right and left lateral halves of each primitive vertebra,
originally separate, unite. The ventral edges, meeting below
the meduUary tube, surround the chord and thus form the
rudiments of the vertebral bodies ; the dorsal edges, meeting
above the medullary tube, form the first rudinicnts of the
vertebral arches. (Cf Figs, 95-98, and Plate IV. Figs. 3-8.)

290

THE EVOLUTION OF MAN.

Fig. 261.— Three
breast-vertebn© of a

In all Skulled Animals (Graniota), most of the soft,
undifferentiated cells which originally constitute the
skeleton-plate, afterwards change into cartilage ceUs, which

secrete a firm, elastic "intercellular sub-
stance," and thus produce cartilaginous
tissue. Like most other parts of the
skeleton, the rudimentary vertebrae soon
pass into a cartilaginous condition, and,
in the higher Vertebrates, the cartila-
ginous tissue is afterwards replaced by
the rigid bony tissue with its peculiar
radiate bone-ceils (Fig. 5, vol. i. p. 126).
The original axis of the vertebral column,
human embryo of eight the notochord, is more or less compressed

weeks, in lateral Ion- 1,1 l•^ • x* i-i

gitudinal section : v, ^y ^^^^ cartilagmous tissuc which grows
cartilaginous vertebral vigorously round it. In lower Verte-
brates {i.e., in Primitive Fishes) a more
or less considerable portion of the noto-
chord remains within the vertebral
bodies. In Mammals, on the contrary, it disappears almost
entirely. In the human embryo, even at the end of the
second month, the notochord is seen only as a thin thread
which passes through the axis of the thick cartilaginous ver-
tebral cu]umi\ (Fig. 261, ch). In the cartilaginous vertebral
bodies themsjlves, which afterwards ossify, the thin remnant
of the notochord (Fig. 262, clt) 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, li).
In a new-born child, a large, pear-shaped cavity, filled with
a gelatinous cell-mass, is visible in each intervertebral disc

bodies ; li, intervertO'
bral discs ; ch, note
chord. (After Koel
liker.)

EVOLUTION OF THE NOTOCHOKD.

291

(Fig. 263, a). This " gelatinous nucleus " of the elastic ver-
tebral disc becomes less sharply defined, but persists
throudiout life in all MamMals, while in Birds and Rep-
tiles, even the last remnant of the notochord vanishes.

Ftg. 262. — A breast-vertebra of the same embryo in lateral cross-section :
cv, cartilaginous vertebral bodies ; cli, notochord ; pr, square process ;
a, vertebral arch (upper) ; c, upper end of rib (lower arch). (After
Koelliker.)

Fig. 263. — Intervertebral disc of new-born child in cross-section :
a, remnant of the notochord. (After Koelliker.)

When the cartilaginous vertebrae afterwards ossify, the first
deposit of bone-substance (the first "bone-nucleus") in
the vertebral bodies is formed immediately round the rem-
nant of the notochord, and soon completely displaces the
latter. A special bone kernel or nucleus is then formed in
each half of the cartilaginous vertebral arch. It is not till
after birth that the ossification progresses so far that the
three bone -nuclei approach each other. The two bony
halves of the arch unite during the first year, but it is not
till much later, till between the eighth and the twelfth
year, that they unite with the bony vertebral body.

The bony skull (craniu7ii\ which must be regarded as

292

TITE 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 a priori grounds, that the bony envelope ^
of the brain is a peculiar modification of that of the dorsal
marrow. It is true, that if the developed human skull
(Fig. 264) is considered by itself, it is impossible to under-
stand how it can be merely the modified anterior portion of
the vertebral column. It is a complex, capacious bony
structure, consisting of no less than twenty bones, diflfering

widely in form and size. Seven of
these skull-bones constitute the
spacious case which encloses the
brain, and in which we distinguish
the strong, massive floor of the skull
(basis cranii) below, and the
boldly arched roof of the skull
(fornix cranii) above. The other
thirteen bones form the "facia]
skull," which especially provides the bony envelopes of
the higher sense-organs, and at the same time as the jaw-
skeleton, encircles the entrance to the intestinal canal.
The lower jaw (usually regarded as the twenty-first
skull-bone) is jointed to the skull-floor, and behind this,
embedded in the roots of the tongue, we find the tongue-
bone, which, like the lower jaw, has originated from the
gill-arches, togetlijer with a portion of the lower arch, which
originally developed as " skull-ribs " from the ventral side
of the skuU-floor.

Pig. 264. — Htunan skull,
from the right side.

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 vertebrae, yet the old comparative
anatomists at the close of the eighteenth century correctly
believed that the skull is originally merely a series of
modified vertebrae. In 1790, Goethe " picked up out of the
sand of the Jews' burying-ground among the downs near
Venice, a dismembered skull of a sheep; he at once per-
ceived that the face bones (like the three vertebrae of the
back of the skull) are also derivable from vertebrae." And,
in 1806, Oken (without knowing of Goethe's discovery), at
Ilsenstein, on the way to the Brocken, " found a beautifully
bleached skull of a hind ; the thought flashed through him,
It is a vertebral column ! " ^'^^

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 skuU skeleton of the Selachii
is the only record which affords definite proof of the verte-
bral theory of the skuIL Earlier comparative anatomists
erred in starting from the developed mammalian skull, and

294 '"^E EVOLUTION OF MAN.

in comparing the several component bones with the separate
parts of vertebrae ; they supposed that in this way they
could prove that the developed mammalian skull consists
of from three to six original vertebrae. The hindmost of
these skull-vertebrae was, according to them, the occipital
bone. A second and a third vertebra were represented by
the sphenoid bone, with the parietal bones, and by the
frontal bone, etc. The elements of anterior skull vertebrae
were even supposed to exist in the face bones. In opposi-
tion to this view, Huxley first called attention to the fact
that in the embryo this bony skull originally develops
from a simple cartilaginous vesicle, and that in this simple
cartilaginous " primitive skull " not the slightest trace of a
constitution of vertebrate parts is visible. This is equally
true of the skulls of the lowest and most ancient Skulled
Animals (Graniota), the Cyclostomi and the Selachii. In
these the skull retains throughout life the form of a simple
cartilaginous capsule — of an inarticulate "primitive or
primordial skull." K the older skull-theory, as it was
accepted from Goethe and Oken by most comparative
anatomists, were correct, then in these lowest Skulled
Animals especially, and in the embryos of the higher Skulled
Animals, the constitution of the " primitive skull " by a
series of " skull-vertebrae " would be very clearly evident.

This simple and obvious consideration, first duly em-
phasized by Huxley, indeed overturns the famous " Verte-
brate Theory of the Skull," as held by the older comparative
anatomiflts. Yet the entirely correct fundamental idea
holds good, i.«., 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 THEOllY. 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 to 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 (Selachii, 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-
geneticaUy. He also pointed out that the gill-arches of the
Selachii show that their primordial skull was originally
formed of a considerable number — at least nine or ten —
primitive vertebrae, and that the brain-nerves, which branch
from the base of the brain, entirely confirm this. These
brain-nerves — with the exception of the first and the second
pairs (the olfactory and the optic nerves) — are merely modi-
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 giveji 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 "skuU-ribs," or "lower arches of skull-

296 THE EVOLUTION OF MAN.

vertebrae,** 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
(be) of an upper and a lower piece ; III., the jaw-arch,
which also consists of two pieces on each side, — viz., the
primitive upper jaw (os palato-qitadratum, o) and the

Fig. 266. — Head skeleton of a I'limitivt^ Fish: n, nose. groove; eth,, regioL
of the sieve-bone ; orh, eye-cavity ; la, wall of eai-labyriuth ; occ, occipital
region of the primitive skull ; cu, vertebral column ; a, front ; be, hind lip-
cartilage ; 0, primitive upper jaw (palato quadrcttum) ; u, primitive lower
jaw ; II., tongue-arch ; III.-VIII., first to sixth gill-arohes. (After Gegen-
baur.)

primitive lower jaw (u); IV., the tongue arch (II.), and V. to
X., six true gill arches, in the stricter sense of that term
(III.-VIII.). 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 skuU " of the Primitive
Fishes originally develops from an equal number (nine at
the least) of primitive vertebrse. The base of the skull is
formed by the vertebral bodies ; the roof of the skidl by the
upper vertebral arches. The coalescence an<l {una Igamation
of these into a single capsule is, however, so ancient, that

EVOLUTION OF THE SKULL.

297

their primordial separate condition now appears effaced by
the action of the "law of abridged heredity," and is no
longer demonstrable in the Ontogeny.

In the human primitive skull (Fig. 26G), and in that of
all higher Vertebrates, which has been moditied, phylog^
netically, from the primitive skull of the Sclachii, five con-
secutive divisions are visible at a certain early period of
development; these one might be tempted to refer to five

Fig. 2GG. — Primitive skull of human
embryo of four weeks; vertical S'-c-iion,
the left half seen from the inside : v, z,
m, h, n, the five grooves in the skull
cavity, in which 1 in 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 vertebrae ; they are, however, merely the
result of adaptation to the five primitive brain -bladders,
arid, like the latter, they rather correspond to a larger
number of metamera. TIi ; fact that the primitive verte-
brate skull is a much moditied and profoundly transformed
organ, and by no means a primitive structure, is also evi-
dent in the circumstance that its rudiment, originally a soft
membrane, commonly assumes the cartilaginous state only
at its base and on the sides, while it remains membranous
at 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

29S THE EVOLUTION OF MAN.

leather-skin (corium), and only secondarily, come into closer
relations with tlie 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. 378; Figs. 145 and 146, p. 393.)

The main features in the history of the development of
tlie gill-arches, which must now be regarded as skull-ribs,
lias been told. Of the four original rudimentary gill-arches or
Mammals (Plates I. and VII., 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 processes
on each side, and forms the chief parts of the upper jaw skele-
ton palate-bones, wing-bones, etc. (Cf p. 245 and 208.) The
rest of the first gill-aich, now distinguished as the " lower-jaw
process," forms out of its base two ear bonelets — the hammer
(malleus) and the anvil (incus) ; 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 eai" 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 cartilagin-
ous only at its anterior portion, and here, by the union of iUi
two halves, is formed the body of the tongue-bone {cojmla

BVOLUnON 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 into
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 embiyo 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 I., with explan-
ation.)

Not only did Gegenbaur, in his model " Researches into
the Comparative Anatomy of Vertebrates," fii'st 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 Hmbs in aU Vertebrates from one primordial
form. Few parts of the body in the difierent 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
(he DIpneusta ; in these, two pairs of lateral limbs, in the
fsliape of many-fingered swimming-fins — one pair of pectoral
tins (the fore legs) and one pair of abdominal fins (hind legs) —
aio 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 group
had anteriorly and posteriorly five digits (Pentadactylism,
p. 123).

As regards the Phylogeny of the limbs, from their
Comparative Anatomy it appears, therefore, that the extre-
mities ori<^inated in the Fishes, in the Primitive Fishes
(Selachii), and were transmitted from these to all higher
Vertebrates (all the ATnphirhina), 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 Umbs, 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 XIL). 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 3ase to
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 pinnae 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 tluee classes of Amniota. In those Dip-
Qeusta 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

302

THE EVOLUTION OF MAN.

Fig. 267.

Fig. 269.

Fig. 270.

Fifi. 272.

IVOLUnON OF THE LIMBS. 303

Fio. 267. — Bones of pectoral fins of Ceratodus (Archipterygium, or
bilateral pinnate skeleton) : A 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 ; onlj
a few (E*) remain. R R, rays of the lateral edge of fin ; mt, Metap.
terygiam ; ms, Mezopterygiura ; p, Propteryginm. (After Gegenbaur.)

Fig. 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 vrhich develops into
the five-fingered hand of higher Vertebrates (h, the three basal pieces of
the fin ; mtj Metapteryginm ; rudiment of the humerus ; ms, Mezoptery-
gimn; p, Propteryginm). (After Gegenbaur.)

Fig. 270. — Bones of the fore-limb of an Amphibian : h, upper arm
(humerv^) ; r, u, lower arm (r, radius ; u, ulna) ; r, c, i, c, H,, root-bones of the
hand, first row (r, radial ; i, intermediate ; c, central ; u, ulnary) ; 1, 2, 3, 4, 6,
root-bones of the hand, second row. (After Gegenbaur.)

Fig. 271. — Bones of hand of Gorilla. (After Huxley.)

Fig. 272. — Bones of human hand, seen from the back. (After H. Meyer.)

which is SO prominent in the higher Yertebrata as the upper
arm (or leg) (Fig. 270, r and u) 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 afiected 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 shuuldei-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 (procbrcLcoi-
deuTTh) 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 ilium), and a lower (ventral) piece ; the anterior portion
of the latter becomes the pubic bone (os pubis) and the
posterior portion the hip-bone (os ischii). Table XXXIY.,
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 (hu/merus) ; in the posterior, the upper leg (feviur).
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, u) ; 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 (r)ietatar&iis).
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
conrospond.^^

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
th(^ f^ame 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 o^vift Deer and the strong, springy legs of the
Kano^aroo, the climbing feet of the Sloth and the diffodng
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. Plat^
IV. p. 34, voL ii. 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 XL). 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-

3o6

THE EVOLUTION OF MAN.

loped. Finally, in the Horse, only one digit, the third, is
perfectly developed (Fig. VI., 3). And yet all these diverse
fore-feet, as also the hand of the Ape (Fig. 271) and the
human hand (Fig. 272j, 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 difierent the limbs of the
various Skulled Animals (Graniota) afterwards appear in
their fully developed state, they nevertheless all originate
from the same simple rudiment. (Cf. Plates VI. and VII.,

Fig. 273. — Skeleton of hand or fore-foot of six Mammals. I. Man; II.
Dog; III. Pig; IV. Ox; V. Tapir; VI. Horse, r, Eadius; u, ulna;
a, scaphoid ; h, semi-lunar ; c, triquetrum (cuneiform) ; d, trapezium ; ev
trapezoid ; /, capitatum (unciform process) ; g, hamatum (unciform bone)>
'P, pisiform ; 1, thumb ; 2, digit ; 3, middle finger ; 4, ring finger ; 5, little
finger. (After Gegenbanr.)

OSIQIN OF THE LIMBS. 30;

voL L p. 362 ; / fore-leg, h, 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 ratlier earlier period than the
two posterior. By difierentiation 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
jf its development, assumes the form of a stalked plate, the
'nner 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; thi-
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 Vertebratets ,
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, etc. Probably
the difierentiation of aU these various tissues occurs actuall}'
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 Phjdogeny also. Tlie mii.sciilar systeu)
as a whole has developed in the most intimate reciprocal
correlation with the bone svstem.^^^

( 309 )

TABLE XXXV.

Systematic Survey of the most Important Periods in the Phylggkny

OF THE Human Skeleton.

L First Period : Skeleton of the Chordonia (Fig. 187, p. 90).
The entire skeleton is foiined by the notochord.

II. Second Period : Skeleton of the Acrania (Fig. 189, p. 91).

A notocliord-niembrane, tlie dorsal continuation of which fomui a cover
ing roand the medullary tube, is formed round the notochord.

in. Third Period : Skeleton of the CyclostonU (Fig. 190, p. 103).

A cartilaginous primordial skull develops round the anterior extremity
of the notochord, from the notochord-membrane. An outer cartila^nous
gill-skeleton forms round the gills.

IV. Fourth Period : Skeleton of the older Selachii (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 (biaerial)
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 XH.).
The skull becomes partially ossified j 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
oolumn ossifies.

3IO THE EVOLUTION OF MAN.

Vin. Eighth Period : Skeleton of the Monotremata (Fig. 196, p. 148).

The vertebral colnmn, skull, jaws, and limbs, acquire the definite

oharaoteristics of Mammals.

IX. Ninth Period : Skeleton of the MarsupiaUa (Fig. 197, p. 152).

The coracoid bone of the shoulder.girdle becomes atrophied, and tba
remnant of it amalgamates with the shoulder-blade.

X. Tenth Period : Skeleton of the Semi-apes (Fig. 199, p. 164).

The poaoh-bones, which distinguish Monotremes and Marsupials,
disappear.

.XI. Eleventh Period : Skeleton of the Anthropoid Apm
(Figs. 204^208, p. 179).

The skeleton acquires the peculiar development shared by Man ex.
clusively with the Anthropoid Apes.

CHAPTER XXIIL

DEVELOPMENT OF THE INTESTINAL SYSTEM.

Tix Primitive Intestine of the Gastmla. — Its Homology, or Morphological
Identity in all Animals (excepting the Protoaoa). — Survey of the
Structure of the Developed Intestinal Canal in Man. — The Moatb>
cavity. — The Throat (pharynx). — The Gullet (oesophagus). — The "Wind-
pipe (trachea) and Lungs. — The Larynx. — The Stomach. — The Small
Intestine. — The Lirer 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 Amphioxos 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
Eespiratory 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 ourselres to gathering material/ii
and to leave it to posterity to luise a scientific structure from those
materials ; because only in that way can we escape the ignjDminy of having
the theories we believed in overthrown by the advance of knowledge. The
unreasonableness of this demand is apparent enough from the faei that

312 THE EVOLUTION OF MAN.

Comparatire Anatomy, like every other science, is endless ; and therefore
the endlessness of the accumulation of materials would nerer allow men, if
they complied with this demand, to reap any harvest from this field. But,
farther than this, history teaches us clearly, that no age in which scientific
inquiry has been active, has been able so to deiiy 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 to
acquire any possessions whatever." — Kakl 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 akeadv seen, the result of the first di^dsion of
labour in the homogeneous cells of the earliest many-celled
animal body must have been the formation of a nutritive
intestinal canal. The first duty and the first need of every
organism is self-support. This task is accomplished by the
two functions of nutrition and of the covering of the body.
When, therefore, in the primaeval collection of homogeneous
cells (Synamoebium), of the phylogenetic existence of which
we yet have evidence in the ontogenetic developmental
form of the mulberry-germ {M uvula), 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 v:ry remarkable
germ-form of the gastrula, which we have shown to exist
in wonderful uniformity throughout the whole animal

Fig. 274.— Gastrnla of a Chalk-spong^ (Olynthus) : Ay from outside;
5, in longitudinal section through the axis ; g, primitive intestine ; 0, primi-
tive mouth ; i, intestinal layer, or entoderm ; e, skin-layer, or exoderm.

series : in the Sponges, Sea-nettles {Acalephoi), Worms,
Soft-bodied Animals (i/o^Zusca), Aii:iculated Animals {Art/iro-
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, ii:. the primitive intestine

314 ^tHE EVOLUTION OF MAX.

(protogctder, g) ; its simple opening, tlie primitive mouth
(protostoma, o), 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, i) ; 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 Gastraea, that primaeval 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

aeceflsaiy briefly to get a correct idea of the more
general conditions of the formation of the intestinal canal
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 Narrow-nosed Apes of the
Old World. The entrance to the intestinal canal is the
mouth-opening (Plate V. Fig. 16, o). 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 is the 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

3l6 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 (tonsiUce). Through the
gate-like arched opening situated beneath the soft palate,
we pass into the throat-cavity (pharynx; Plate V. Fig.
IG, sh), which lies behind the mouth-cavity. This is only
partly 'visible in the open mouth when leflected in the
mirror. Into the throat-cavity a narrow passage opens on
each side (the Eustachian tube of tlie ear), Avhich 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 (oesojjJtagas, s/'). Through this
the masticated and swallowed food passes down into the
stomach. The wind-pipe {trachea, Ir) 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, lu), 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, hi). At the upper end of the wind-pipe {trachea),
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-caUed " goitre."

i

THE HUMAN INTESTINAL CANAL.

317

The gullet (oesophagus) passes do^Ynward 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, 2;) 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^J. 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

Fig. 275. — Human stomach and gall-intestine in longitudinal section :
a, cardia (limit of the oesophagus) ; b, fundus (blind sac of the left side) ;
c, pylorus fold; d, pylorus valve; e, pylorus-cavity ; /g ^, gall-intestine ;
i, 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 [oesophagus). After
this has passed through the diaphragm into the ventral,
cavity it enlarges into the stomach in which dioestion

3l8 THE EVOLUTION OF MAX.

especially takes place. The stomach of an adult man
(Fig. 275, Plate Y. Fig. 16, mg) is an oblong sac, placed
somewhat obliquely, the left side of which widens into
a blind-sac, the base of the stomach or fundus (6), while the
right side narrows, and passes at the right end, called
the pylorus (e), 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 itseh" 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, fg h). 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 saKvary 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 (i) near each other. In adults the liver
UB 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, Ih), The
pancreas lies somewhat further back and more to the
left (Fig. 16, p). The small intestine is so long that it
has to lie in many folds in order to find room in the limited
space of the ventral cavity ; these coils are the bowels.
They are divided into an upper intestine, called the empty
intestine (jejunum), and a lower, the crooked intestine
(ilii4m^). 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 (coecum), the atrophied
extremity of which is a well-known rudimentary organ, the
vermiform process (processus vermiforTnis). The large
intestine (colon) consists of three parts , an ascending part
on the right, a transverse central part, and a descending
part on the left. The latter finally curves like an S, called
the "sigmoid flexure,*' into the last part of the intestinal
canal, above the rectum, which opens at the back by the
anus (Plate V. Fig. 16, a). Both the large intestine and
the small intestine are furnished with numerous glands,
most of them very small, and which 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 MAH.

from the intestinal-fibrous layer, at the point where this
curves into the outer lamina of the side-layer, into the
skin-fibrous layer (Plate IV. Fig. 5, g). The curving-point
was distinguished as the middle-plate (Fig. 99, mp). The
mesentery is, at first, very short (Plate V. Fig. 14,^) ; but it
soon lengthens considerably at the central part of the intes-
tinal canal, and takes the form of a thin, transparent,
membranous plate, which has to be the more extended the
further the folds of the intestine diverge from the place
where they are first attached to the vertebral column. The
blood-vessels, lymphatic vessels, and nerves which enter
the intestinal canal traverse this mesentery.

Although, therefore, the intestinal canal; in the adult
human being forms an extremely complex organ, and
though it shows in its details so many intricate and delicate
structural arrangements, — into which we cannot enter
here, — this entire structure has developed, historically,
from that simplest form of primitive intestine which
was possessed by our gastrsead ancestors, and which the
extant gastrula now exhibits. We have already shown (in
Chapter YIII.) that the peculiar Hood-gastrula {Arajphi-
gastrula) of Mammals (Fig. 277) may be referred back
to the original Bell-gastrula (Archigastrtda) 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
Gastrsea, and in which the whole body of the animal is
intestine.

The peculiar form and mode in which the complex

DEVELOPMENT OF THE INTESTINAL CANAL.

321

human intestinal canal develops from the simple gastiaila
and which is similar to that in other Mammals, can there-
fore be only correctly understood when it is considered in
the light of Phylogeny. We must, accordingly, distinguish

Fig. 276. — Archigastrula of Amphioxns (in longitudinal secti(n): d,
I'rimitive intestine; 0, primitive month; i, intestinal layer; e, skin-layc^o

Fig. 277. — Amphigastrula of Mammal (in longitudinal section). Th.c
primitive intestine (d) and primitive mouth (0) are filled up by 1\\2 cells of
the intestinal layer (i) ; e, skin-layer. ,

«

between the original p;:imary intestine ("the primitive
intestine, or 'protogastev ") 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

322 THE EVOLUTION OF MAN.

gives rise to the various parts of the differentiated intestinal
system, and the transient yelk-sac, which serves only as a
storehouse of materials for the building of the embryo.
The yelk-sac attains its greatest development in Primitive
Fishes {Selachii), Bony Fishes (Teleostei), Reptiles, and
Birds. In Mammals, and especially in Placental Animals,
it is atrophied. The peculiar intestinal development of the
Cyclostomi, Ganoids, and Amphibia must be regarded as
an intermediate form, between the palingenetic intestinal
development of the Skull-less animals, and the kenoge-
netic intestinal development of the Amnion Animals (Am-
niota)}^

We have already seen in what a peculiar way the
development of the intestine takes place ontogenetically in
the human embryo and in that of other Mammals. Imme-
diately from the gastrula of these originates a globular
intestinal germ-vesicle (gastrocystis), filled with fluid (Figs.
72, 73, vol L p. 289). In the waU 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 waU 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 waU,
by which it is connected with the intestinal germ-vesicle
(Plate V. Fig. l^). The latter, in the course of development,
becomes continually smaller, as the intestinal canal continuei

MmELOPMENT OF THE INTESTINAL CAKiJL 3^3

fco 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 aU 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 ie
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, / ;
Plate V. Fig. 14, at). In this more developed form — repK -
gented in the diagram (Fig. 94, 4, voL i. p. 312) — the intestina '

3^4

THE EVOLUTION OF MAN.

canal of Man, like that of all other Mammals, now forms a
slightly-curved, cylindrical tube, which has an opening at
both ends, and from the lower wall of which depend two
sacs; the anterior navel-bladder, or yelk-sac, and the pos-
terior allantois, or primitive urinary sac.

Microscopic observation shows that the thin wall of this
simple intestinal tube and of its two bladder-like append-
ages is composed of two distinct cell-strata. The inner,
which coats the entire cavity, consists of larger, darker cells.

Fig. 278.— Human embryo of the third week, with the amnion and
allantois. The great globular yelk-sac is below, the bladder-like allantois
on the right ; there are as yet no limbs. The germ, with its appendages, is
enclosed in the tufted membrane {chorion).

Fig. 279. — Human embryo, with amnion and allantois, in the fourth
week. (After Krause.) The amnion (w) lies pretty close to the body. The
greater part of the yelk-sac (d) has been torn away. Behind this the allan-
tois appears as a small pear-shaped bladder. Arms (/) and legs (h) are
already commenced : v, fore-brain ; ;::, twixt-brain ; m, mid-brain ; h, hind-
brain ; ?i, after-brain ; a, eye ; k, three gill-arches ; c, heart ; s, tail.

mUDIMENT OF THE INTESTINAL CANAL. 325

and ifl 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 famished, 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 difierentiation 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, aU 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 j the fibrous connective tissue and the
smooth muscles which compose its fieshy 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 shorty everything belonging to the intestine, with ih«

326 THE EVOLUTION OF MAH.

exception of tlie intestinal epithelium. From the intestinal*
fibrous layer originates also the entire mesentery with all
the adjacent parts, the heart, the large blood-vessels of the
body, etc. (Plate V. Fig. 16).

Let us now turn aside for a moment from this original
rudimentary intestine of Mammals, in order to institute a
comparison between it and the intestinal canal of those
lower Vertebrates and Worms, which we have learned to
recognize as the ancestors of Man. In the simplest Gliding-

* ^^^

worm, or Turbellaria (Rhabdocoelum, 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 (wi). 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 giU-
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 aU Skulled Animals (Graniota). The secondary forma-
tion of the mouth of the Lancelet is connected, as may be
conjectured with some probability, with the formation oi

EARLY FORMS OF THE INTESTINAL CANAL.

.27

the gill-openings, which appear directly behind it on the
intestine. The front portion of the intestine has thus

nnv

Fig. 280. — A simple Gliding Worm (Rhahdoccelum) m, mouth; sd, throat-
epithelium; sm, throat muscle-mass; d, stomiach-iiitestine ; nc, renal ducts;
/, ciliated outer-skin ; nr)%, openings of the latter ; au, eye ; na, nose-pit.

Fig. 281. — Structure of an Ascidian (seen from the left side, as in Plate
XI. Fig. 14). The dorsal side is turned toward the right, the ventral side to
the left ; the mouth-opening (0) is above ; at the opposite, tail end, the
ascidian has become adherent. The gill-intestine {hr), perforated by many
openings, extends into the stomach-intestine. The terminal intestine
opens through the anus (a) into the gill-cavity (ci), 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

THE EVOLUTION OF MAN.

Ji

mt'

become a respiratory organ. I have already pointed out
how characteristic this adaptation is of Vertebrates and

Mantle Animals {Tunicata, p. 87). The
phjlogenetic origin of the gill-openings in-
dicates the beginning of a new epoch in
the tribal history of Vertebrates.

The most important process we meet
with in the further ontogenetic development
of the intestinal canal in the human embryo,
is the origin of the gill-openings. At the
head of the human embryo, the wall of the
throat very early unites with the outer wall
of the body, and four openings then form
on the right and left sides of the neck,
behind the mouth, and these lead directly
from without into the throat-cavity. These
openings are the gill-openings, and the par-
titions separating them are the gill-arches
(Figs. 116-118, vol. i. p. 356; Plates I.
and v., Fig. 15, ks). These embryonic for-
mations are very interesting ; for they show

Fig. 282. — Lancelet (Amphioxus lanceolatus), double
the natural size, seen from the left side (Lhe longi-
tudinal axis is perpendicular, the mouth end above,
the tail end below (as in Plate XI. Fig. 15) : a, mouth-
opening, surrounded by bristles ; h, anal opening ; c,
gill-pore ( porus hranchialis) ; d, gill-body ; e, stomach ;
/, liver ; g, small intestine ; li, gill-cavity ; i, notochord,
below which is the aorta; A-, aortal arch; /, main stem
of the gill-artery ; m, swellings on the branches of
the latter ; n, hollow vein ; o, intestinal vein.

that all the higher Vertebrates when in a very young state,

ORIGIN OF THE GILL-OPENINOS. 329

reproduce, in accordance with the fundamental principle of
Biogeny, the same process which was originally of the
greatest importance to the development of the whole verte-
brate tribe. This process was the differentiation of the
intestinal canal into two sections : an anterior, respiratory
part, the gill-intestine, which serves only for breathing,
and a posterior, digestive part, the stomach -intestine, which
serves only for digestion. As we meet with this very
characteristic differentiation of the intestinal tube into two,
physiologically, very distinct main sections, not only in
the Amphioxus, but also in the Ascidian and the Appen -
dicularia, we can safely conclude that it also existed in
our common ancestors, the Chorda Animals (Ghordonia),
especially as even the Acorn Worm (Balanoglossus) has
it (Fig. 186, p. 86). All other Invertebrate Animals are
entirely without this peculiar arrangement.

The number of the gill-openings is stiU very large in the
Amphioxus, as in Ascidians and in the Acorn Worm. In
the Skulled Animals it is, on the contrary, very much
lessened. Fishes mostly have from four to six pairs of gill-
openings. In the embryos of Man and the higher Verte-
brates also, only three or four pairs are developed, and these
appear at a very early period. The gill-openings are perma-
nent in Fishes, and afford a passage to the water which has
been breathed in through the mouth (Figs. 191, 192, p. 113;
Plate V. Fig. 13, ks). On the other hand, the Amphibians
lose them partially, and aU the higher Vertebrates entirely.
In the latter, only a single vestige of the giU-openings remains,
the remnant of the first gill-opening. This changes into a
part of the organ of hearing ; from it originates the outer
ear-canal, the tympanic cavity, and the Eustachian tub >

( 330 )

TABLE XXXVI.

BjiiMinHn Sorrey of the Development of the Human InteBtinal Sjstem,
VJi. — The parts nuirked thoB f are processes from the intestinal tnbe.

flMt buUh seotion
of the

Intestinal

System :

Mm Respiratory

Intestine
(Gill Intestine).

PnooASTEB.
MqMTdUortus.)

/

1.

Month-oavlly

Nose-cavity
{Cavumnaat)

a

Seoend main
Motion of the

Intestinal

System:

Digestive

Intestine

(Stomach Intev*

tine).

PSFTOOASTBX.

5.

Mentk-^eaiag

Lipa
Jaws
Teeth

Tongue

Tongue bone
f Salivary glan^

Soft palate
' Uvula

Noee canal
•f Jaw cavities
+ Frontal cavities
,j Ethmoid cavity

IIsthmoB of the thiMt
Tonsils
PharjTix
+ Eustachian tube
f Tympanic cavity
+ Brain-appendage
•j- Thyroid gland

Lnng-cavity
( CavumpKimoni*)

(i

Anterior Intestine j
(^Protogaster) 1

f Larynx
■ Windpipe
Longs

Gullet

Stomach-openliig
Stomach
Stomach exit

6.

Central Intestine
{Muogast0r)

I Gal
fLh

fPai

Posterior Intestlae

{EpigatUr)

Urinary Intestiae
(I^<ui«r)

Gall-intestine
■ Liver
Pancreas
Empty intestine
|(f Yelk-sac, or n*Tel-
bladder)
Crooked intestine

Large intestine
■f- Blind intestine
f Vermiform prooeis al
the ccBcuoi

Rectum

Anal opening

( ("f Primitive urinary mo
\ T Urinary tube
UOdaeiyUadte

JStmaerii

MaxiXUz

DenUs

Lingua

Os hyoidea

GlandulcB talivaUs

Velum -palatinum

Umda

Meatus narium
Sinus maxUiara
Sinus frontdUs
Sinus ethwutidaUs ,

Isthmus faueivm

TonsilloR

Pharynx

ThiiHJL Bustachii

Cavum tympani

Hypophysis

Thyrecid/ea

Larynx
Trachea
Pulmonu

(EsophaguB
Cardia
Stomachut
Pylorus

Duodcnvm

Bepar

Pancreas

Jejunum

( Vesicula umbiliei^-

lU)
Hewn

Golan
Coecum

Processus
formis
Rectum,
Anus

AOantoia)
Urethra

vermt'

i

4J K

■2 a

^ to

a>

"« 4)
I-

fe'S

Coo
•I—

_a ID

I®

1^

I

THE MOUTH-SKELETON. 331

We have already considered this remarkable formation, and
will only caU 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 giU-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 aU the three
higher vertebrate classes, thus also in Man, the tongue-bone
(os hyoides) and the bonelets of the ear originate from th«
giU-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
thk respect, exactly similar to their teeth (Fig. 288). Thus
the human teeth, in their earliest origin, are modified fish-

v^^

THE EVOLUTION OF MAN.

scales. ^^ 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-
,,|v tions, an anterior gill-intestine, and
Jj a posterior stomach-intestine, like
the intestinal canal of the Lancelet
and the Ascidian. By the develop-
ment of the jaws and gill-arches
it is modified into a true Fish-
intestine. Afterwards, however, the
gill-intestine, which is a memorial
of the Fish-ancestors, as such, is
entirely lost. The parts that remain
Shark (Centrcphorus calceus). take a wholly different form ; but

Oneachrhomboid bone-tablet, j^Q^^|^}jS^a^jj(^ijig that the anterior
lying in the leather-skin, rises . • , , • i i j.i

a small, three-cornered tooth, section of our intestinal canal thus
(After Gegenbaur.). surrenders entirely its original

form of gill-intestine, it yet retains its physiological func-
tion as a respiratory intestine; for the extremelj in-

FiG. 283. — Scales of a

THE BREATHING APPARATUS. 333

teresting and remarkable discovery is now made that even
the permanent respiratory organ of the higher Vertebrates,
the air-breathing lungs, has also developed from this anterior
section of the intestinal canal. Our lungs, together with
the wind-pipe (trachea) and the larynx, develop from the
ventral waU of the anterior intestine. This entire great
breathing-apparatus, which occupies the greater part of
the chest (thorax) in the developed Man, is at first merely
a very small and simple vesicle or sac, which grows out
from the intestinal canal immediately behind the gills, and
soon separates into two lateral halves (Figs., 284, c, 285, c ;
Plate V. Figs. 13, 15, 16, lu). This vesicle occurs in all
Vertebrates except in the two lowest classes, the Acrania and
CyclostomL In the lower Vertebrates, however, it develops,
not into lungs, but into an air-filled bladder of considerable
size, occupying a great part of the body-cavity (coeloma),
and which is of quite a difierent significance from the
lungs. It serves, not for breathing, but as an hydrostatic
apparatus: for vertical swimming movements it is the
swimming-bladder of Fish ; but the lungs of Man and of
all other air-breathing Vertebrates develop from the same
simple bladder-like appendage of the anterior intestine,
which, in Fishes, becomes the swimming-bladder.

Originally this sac also has no respiratory function, but
serves only as an hydrostatic apparatus, augmenting or
diminishing the specific gravity of the body. Fishes, in
which the swimming-bladder is fully developed, are able to
compress it, and thus to condense the air contained in il
The air sometimes also escapes from the intestinal canal
through an air-passage which connects the swimming-
bladder with the throat (pharynx), and is expelled through

334

THE EVOLUTION OF MAK.

the mouth ; in this way the circumference of the swim-
ming-bladder is diminished, and the fish becomes heavier
and sinks. When the animal is again about to ascend,
the swimming-bladder is distended by remitting the com-

FiG. 284. — Intestine of an embryonic Dog (which is representev^ in Fig.
137, vol. i. p. 382; after Bischoff), from the ventral side : a, gill-arches (four
pairs) ; 6, rudimentary throat and larynx ; c, lungs ; d, stomach ; /, liver ; g,
walls of the opened yelk-sac, into which the central intestine opens by a
wide aperture ; /i, rectum.

Fig. 285. — The same intestine, seen from the I'ight side : a, lungs ; t,
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

ITOLUTION OF THE LUNQflL 335

long, 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
limg 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 lind 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 lunga 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 o£

55

33^ THE EVOLUTION OF MAM.

»

Comparative Anatomywe can trace the progressive develop
ment of this important organ from its very simple rudiment
in the lower Amphibia up to the complex and vocal appara-
tus represented by the larynx of Birds and Mammals.

Though these organs of voice, speech, and air-respiration
develop so differently in the various higher Mammals, they
yet all arise from the same simple original rudiment —
from a vesicle which grows out of the wall of the anterior
intestine. We have thus satisfied ourselves of the interest-
ing fact that both the respiratory apparatus of Vertebrates
develop from the fore part of the intestinal canal ; first, the
primary and more primitive water- respiring apparatus, the
gill-body, which is altogether lost in the three higher
vertebrate classes ; and, afterwards, the secondary and more
recent air-breathing apparatus, which acts in Fishes only
as a swimming-bladder, but as a lung from the Dipneusta
upwards.

We must say a few words about an interesting rudi-
mentary organ of the respiratory intestine, the thyroid
gland (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 aesthetically interesting, |
because in certain mountainous dis^xicts 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 " hypobranchlal groove," which we hiiv©

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)}^

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 (jejunv/m), and
jrooked intestine (ileus); and the hind intestme (large
aatestine 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, c), and the allantois, which grows out of the
last portion of the pelvic intestine as a large sac-Hke
piotuberance (w). The protuberances from the middle
of the intestine are the two great glands which open
;nto the duodenum, the liver (h) and the ventral salivary

gland.

Immediately behind the bladder-like rudiment of the
lungs (Fig. 286, i) comes that portion of the intestinal' tube
which forms the most important part of the digestive
apparatus, viz., the stomach (Figs. 284, d, 285, h). 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

Fig. 286. — Longitudinal section througli an embryonic Chick on the
fifth day of incubation : d, intestine ; o, mouth ; a, anus ; I, lungs ; h, liver ;
g, mesentery ; v, auricle of heart ; k, ventricle of heart ; h, arterial arches ;
J, aorta; c, yelk-sac; w, yelk-duct; u, allantois ; r, 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-sac begin to develop unequally.
As the left side of the spindle-shaped pouch grows much
more vigorously than the right, and as, at the same time,

DEVELOPMENT OF THE STOMACH.

339

there occurs a considerable obliquity of its axis, it soon

assumes an oblique position. The upper end lies more to

the left and the lower end more

to the right. The anterior end

extends so as to form the long

narrow canal of the gullet

{oesophagus) ; below the latter,

the blind-sac of the stomach

{fundus) bulges out to tlie left,

and thus the later form of the

stomach is gradually developed

(Fig. 287, e ; Fig. 275, p. 317). The

axis, which was originally verti-

FiG. 287. — Human embryo of five
weeks, from the ventral side ; opened
(enlarged). The breast wall, abdominal
wall, and liverj have been removed. 3,
external nasal process ; 4, upper jaw ; 6,
lower jaw ; z, tongue ; v, right, v', left
ventricle of heart ; o', left auricle of
heart ; b, origin of aorta ; h' h" h'", Ist,
2nd, 3rd aorta-arches ; c c' 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 ; n, navel-vein) ;
X, yelk-duct ; i, terminal intestine ; 8,
tail; 9, fore-limb ; 9', hind-limb. (After
Coste.)

cal, now inclines from a higher point on the left to a lowei'
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 MAIC.

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, ^r, and Fig. 186, vol. i. p. 381.)
Before the abdominal wall closes, a horseshoe-shaped loop of
intestine (Fig. 136,771) protrudes from the ventral opening of
the embryo, and into the curve of this the yelk-sac or navel -
bladder opens (n). The thin, delicate membrane which
secures this intestinal loop to the ventral side of the vertebral
column, and occupies the inside of this horseshoe curve, is
the first rudiment of the mesentery (Fig. 286, g). The most
prominent part of the loop into which the yelk-sac opens
(Fig. 287, x), and which is afterwards closed by the intestinal
navel, represents that part of the small intestine which is
afterwards called the crooked intestine (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 distinguisli differentiate later in a very simple
way; these are the gall-intestine (duodenwin), which is
next to the stomach, the long empty intestine (jejunum)
which succeeds, and the last section of the small intestine,
the crooked intestine (ileuvi).

The two large glands which we have already named, the
liver and the ventral salivary gland, grow out, as protuber-
ances, from the gall-intestine, or duodenum. The liver first
appears in the form of two small sacs, situated right and left
just behind the stomach (Figs. 284,/, 285, c). In many low
Vertebrates the two livers remain quite separate for a long
time (in the Myxinoides for life), and coalesce only imper-
fectly. In higher Vertebrates, on the other hand, the two
livers coalesce more or less completely at an early period,
and constitute one large organ. The intestinal-glandular
layer, which lines the hollow, pouch-like rudiment of the
liver, sends a number of branched processes into the investing
intestinal-fibrous layer ; as these solid processes (rows of
gland-cells) again branch out, and as their branches coalesce,
the peculiar netted structure of the developed liver is
produced. The liver-cells, as the secreting organs which
form the bile, all originate from the intestinal-glandular
layer. The fibrous mass of connective tissue, which joins
this great cellular network into a large compact organ, and
which invests the whole, comes, on the other hand, from the
intestinal-fibrous layer. From the latter originate also the
great blood-vessels which traverse the entire liver, and
the innumerable netted branches of which are interlaced
with the network of the liver-cells. The gall-ducts, which
traverse the entire liver, collecting the bile and discharging
it into the intestine, originate as intercellular passages along

342

THE EVOLUTION OF MAN.

the axis of the solid cell-cords ; they all discharge into the
two primitive main gall or biliary ducts, which originate
from the base of the two original protuberances of the
intestine. In Man, and in many other Vertebrates, these
two ducts afterwards unite, and form one simple gall-duct,
which discharges into the ascending portion of the gall-
intestine. The gall bladder originates as a hollow pro-
tuberance of the right primitive liver duct. The gi^owth
of the liver is at first exceedingly rapid ; in the human
embryo, even in the second month, it attains such dimen-
sions that during the third month it occupies by far the
largest part of the body-cavity (Fig. 288). At first, both

Fig. 288. — Chest and abdominal viscera of a
human embryo of twelve weeks, in natural size.
( After Koelliker. ) The head 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 centime of which the blind-
intestine {coecum, 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
lies considerably behind the right. In consequence of the
asymmetrical developm-ent and alteration in the position of
the stomach and other abdominal viscera, the whole of the
liver is eventually forced over on to the right side. Although
the growth of the liver is, afterwards, not so excessive, even
at the end of gestation, it is comparatively much larger in
the embryo than in the adult. In the latter, its weight

THE LARGE INTESTINS. 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.

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
difierentiates into different sections, of which the foremost
and longest is called the colon, the shorter and hinder the
rectimi. At the commencement of the former a valve
(vcdvula Bauhini) forms, which divides the large intestine
from the small intestine ; behind appears a pouch-like
protuberance, which grows larger and becomes the blind-
intestine (coecumi) (Fig. 288, v). In plant-eating Mammals

344 TH^ 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 individuab other-
wise perfectly healthy. In our plant-eating ancestors this
rudimentary organ was larger, and was of physiological
value.

Finally, w© must mention another important appendage
of the intestinal tube; this is the urinary bladder (uro-
eystis) with the urinary tube (urethra), which in develop-
ment and in morphological character belong to the intestinal
system. These urinary organs, which act as receptacles and
excretory passages for the urine secreted by the kidneys,
originate from the inner part of the allantois-stalk. The
allantois develops, as a sac-like protuberance, from the
anterior wall of the last section of the intestine (Fig. 286, u).
In the Dipneusta and Amphibia, in which this blind-sac
first appears, it remains within the body-cavity (coeloma),
and acts entirely as a urinary bladder. In aU Amniota, on
the other hand, it protrudes considerably out of the body-
cavity of i^e embryo, and forms the large embryonic
" primitive urinary sac," which, in higher Mammals, forms
the placenta. At birth this is lost ; but the long allantois-
stalk (r) remains, its upper portion forming the central navel

•raE URINABY BLADDER. 34$

band of the urinary vesicle (ligamentum vesico-^mhilicale
medium), a rudimentary organ which extends as a solid
cord from the top of the urinary bladder to the navel. The
lower part of the allantois-pedicle (the " urachus ") remains
hollow, and forms the urinary bladder. At first, in Man,
as in the lower Vertebrates, this organ discharges into the
last section of the posterior intestine, and there is, there-
fore, a true " cloaca," receiving both urine and excrement ;
but, among the Mammals, this cloaca is permanent only in
the Cloacal Animals, or Monotremes, as in Birds, E-eptiles,
and Amphibia. In all other Mammals (Marsupialia and
Placentalia) a transverse partition forms at a later period,
and separates the urinary-sexual aperture in front from the
anal aperture behind, (Cf Chapter XXV.)

( 346 )

EXPLANATION OF PLATE I.— (Frontispiece.)

Development or the Face.

The twelve figures in Plate I. represent the faces of four different
Mammals in three distinct stages of individual evolution : Mi-Miii that of
Man, Bi-Biii of the Bat, Ci-Ciii of the Cat, Si-Siii of the Sheep. The
three different stages of evolution have been chosen to correspond as far as
possible ; they have been reduced to about the same size, and are seen from
in front. In all the figures the letters indicate the same : a, eye ; v, fore-
brain ; m, mid-brain ; s, frontal process ; fe, nose-roof ; o, upper jaw process
(of the first gill-arch) ; u, lower jaw process (of the first gill-arch) ; h,
second gill-arch ; d, third gill-arch ; r, fourth gill-arch ; g, ear-fissun'
(remains of the front gill-opening) ; z, tongue. (Cf . Plates VI. and VII.,
Pigs. 232-236, p. 243 ; also Figs. 123, 124, vol. i. p. 370.)

TABLE XXXVII. *

Systematic Survey op the most Important Periods in thi
Phylogeny of the Human Intestinal System.

I. First Period : Intestine of Gastrcea (Figs. 274-277 ; Plate V. Figs. 9, 10).
The whole intestinal system is a simple pouch (primitive intestine), the
eixnple cavity of which has one orifice (the primitive mouth).

n. 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.

[IT. 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.

i

SURVEY OF HUMAN INTESTINAL SYSTEM.

347

IV. Fourth Period : Intestine of Skull-less Animala {Acra/nta)
(Fig. 282 ; Plate XI. Fig. 15).
The gill-streaks appear between the gill-openiDgs of the respiratory
intestine ; a liver blind.sao grows from the stomach-pouoh of the digestire
intestine ; as in the Amphioxru.

V. Fifth Period : Intestine of CyelosUma (Plate XI. Fig. 16).
The thyroid gland develops from the ciliated grooTe on the base of
the gills (hypobranchial groove). A compact liver-gland develops from
the liver blind-sac.

VI. 8i«Bth Period : Intestine of Primitive Fishes (p. 114).
Cartilaginous gill-arches appear between the gill-openings. The fore-
most of these form the lip-cartilages and the 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.

Vni. 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).
nie existing cloaca is separated by a partition wall into an anterior
urinary-sexual aperture and a posterior anal aperture.

XI. Eleventh Period : Intestine of Cata/rhine Apes (p. 176).
All parts of the intestine, and especially the teeth-apparatus, acquire the
ohaxaoteristio development common to Man and Gatarhine 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. — Ontogen}/
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. — Former,
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 Siguificanoe
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 Conmiencemeni
of the Latter ; Coeloma. — Dorsal Vessel and Ventral Vessel of Worms:
— Simple Heart of Asoidia. — 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.; — FftraUelism of the
Tribal-history (Phylogeny).

"Morphological oomparison of the adult conditions ahonld 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 j
it is thus provided, as it were, to see each step in the formative process in
its true relation with the condition which is finally to be reached. Treat-
ment of the histoxy of derelopment without preparatory study is (mlj too

APPLICATION OP THE LAW OF BIOGENY. 349

likely to lead to groping in the dark ; and it not unfrequently leadg to the
most unfortnnate results — far inferior to those which might be established
beyond question without any study of the history of development," —
Albxandkb Brauk (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
ifhe 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 primaeval animal ancestors j 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 primaeval ^letamorphosis 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 MAK.

vitiated, and abbreviated, in the course of time, that w©
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 haif 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 wiU now examine. If we attempted to infer the
original structural features of our older animal ancestors
solely from the phenomena which the individual develop-
ment of these organ-systems, in the embryo of Man and of
other high Vertebrates, exhibit, we should obtain wholly

HIS ON THE VASCULAR SYSTEM. 351

erroneous views. By many influential embryonic adap-
tations, among which the development of an extensive
nutritive yelk must be regarded as the most important, the
original course of development of the vascular system 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
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 difiicult field.
Positive proof of this assertion can be gained by studying
the complex vascular system as explained in the classical
works of Johannes Miiller, Heinrich Eathke, 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 Phylcgeny. 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

353 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
fortL (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 otl er hand, it soon
becomes evident that he conceives form-constructing
" Mother Nature " merely as a kind of clever dressmaker ;
by cutting out the germ-layers in various ways, by bend-
ing, folding, pulling, and splitting them, this clever semp-
stress easily brings into existence the various forms of
animal species, by " development " (!). The bendings and
foldings especially play the most important part. Not only
the differentiation of head and trunk, of right and left, of
central stem and periphery, but also the rudiment of the
limbs, as also the articulation of the brain, the sense-organs,
the primitive vertebral column, the heart, and the earliest
intestines, can be shown, with convincing necessity (!) to be
mechanical results of the first development of folds. Most
grotesque is the mode in which the dressmaker proceeds in
forming the two pairs of limbs. Their first form is deter-
mined by the crossing of four folds bordering the body,

HIS OM THE VASCULAR SYSTEM. 353

"like the four comers of a letter." Yet this wonderful
" envelope theory " of the vertebrate limbs is surpassed by
the " waste-rag theory " (Hollen-lappen Theorie) which His
gives of the origin of the rudimentary organs : " Organs
(like the hypophysis and the thyroid gland) to which no
physiological part has yet been assigned, are embryonic
remnants, comparable to the clippings, which in the cutting
of a dress cannot be entirely avoided, even by the most
economical use of the material " (!). Nature, therefore, in
cutting out, throws the superfluous rags of tissue into the
waste heap. Had our skull-less ancestors of the Silurian
age had any presentiment of such aberrations of intellect
of their too speculative human descendants, they would
certainly have preferred relinquishing possession of the
hypobranchial groove on the gill-body, instead of trans-
mitting it to the extant Amphioxus, and of leaving a
remnant of it to us, in the equally unsightly as useless
thyroid gland. (Of p. 836).

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 ^eek 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 afibrds especial occasion for
this ; for among the most important advances which Hit

354 THE EVOLUTION OF MAN.

claims 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 arcbiblast" (the
germ-disc composed of the two primary germ-layers).

The whole of this artificial development theory of His,
and above all the unnatural distinction between the supple-
mentary and the main germ, collapses like a card house
when the Anatomy and Ontogeny of the Amphioxus, that
invaluable lowest Vertebrate, is contemplated, which alone
can elucidate the most difficult and darkest features in the
development of the higher Vertebrates, and thus also of
Man. The gastrula of the Amphioxus alone overthrows
the whole artificial theory; for this gastrula teaches us
that all the various organs and tissues of complete Verte-
brates originally developed entirely from the two primary
germ-layers. The developed Amphioxus, like all other
Vertebrates, has a differentiated vascular system and
a skeleton of " connective substance tissues " extending
throughout its body, and yet there is in this case no " sup-
plementary germ" from which these tissues can originate
thus, contrasting with the other tissues.

The larvae of the Amphioxus, arising from the original
bell-gastrula {archigastrula), in its further development,
throws the most important rays of light also upon the diffi-
cult history of development of the vascular system. In the
first place, it answers the very important question, which

THE VASCULAB SYSTEM. 355

we have already frequently indicated, as to the origin of
the four secondary germ-layers ; it clearly shows that the
skin-fibrous layer originates from the exoderm, the intes-
tinal-fibrous layer, on the contrary, in an analogous manner,
from the entoderm of the gastrula ; the cavity thus caused
between the two fibrous layers is the first rudiment of the
body-cavity, or the coelom (Figs. 50, 51, vol. i. p. 236). As the
Amphioxus larva thus shows that the fission of the layers
is the same in the lowest Vertebrates as in the Worms, it at
the same time represents the phylogenetic connection be-
tween the Worms and the higher Vertebrates. As, more-
over, the primitive vascular stems in the Amphioxus
originate in the intestinal wall, and in this, as in the em-
bryos of all other Vertebrates, proceed from the intestinal-
fibrous layer, proof is afforded us that the earlier embryolo-
gists were right in calling the latter the vascular layer.
Finally, the Comparative Ontogeny of the different verte-
brate classes further convinces us that the vascular layer
is originally everywhere the same. The vascular system in
Man, as in all Skulled Animals, forms a complex apparatus
of cavities, which are filled with juices, or fiuids, 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 eonstant 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 diflferentiated 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 wideljy
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 pliylogenetic 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

▲GE 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 " Gastrsea 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 (^EJ). IV. The vascular system (F).
V. The skeleton system (G). VI. The sexual system (H).
(Cf Table XXXIX., p. 367.)

In the first place," the gastrula proves that in all animals
with the exception of the, Primitive Animals (Protozoa), —
therefore, in all Intestinal Animals (Metazoa), — two primary
organ-systems originally arose simultaneously and first;
these were the skin-system (skin-covering) and the intes-
tinal system (stomach-pouch). The first is represented, in
its earliest and simplest form, by the skin-layer or exoderm,
the latter by the intestinal layer or entoderm of the Gastraea.
As we can ascribe the same origin, and, therefore, also the
same morphological significance, to these two primary germ-
layers in all Intestinal Animals, from the simplest Sponge
to Man, the homology of these two layers seems sufficient
proof of the above assumption.

Immediately after the differentiation of the two primary
germ-layers, an inner or outer skeleton develops in many
lower animals (e.g., in Sponges, Corals, and other Plant

358 THE EVOLUTION OF MAN.

Animals). In the ancestors of Vertebrates, the development
of the skeleton did not take place till much later, in the
Chorda Animals (Ghordonia), In them, after the skin-
system and the intestinal system, two other organ-systems
simultaneously arise ; these are the nervous and the mus-
cular S3^s terns. The way in which these two organ-systems
which mutually condition each other, developed simulta-
neously and independently, in reciprocal action and yet in
opposition to each other, was first explained by Nicholaus
Kleinenberg in his excellent monograph on the Hydra, the
common fresh- water Polyp. ^^^ In this interesting little
animal, single ceUs of the skin-layer send fibre-shaped pro-
cesses inward, which acquire the power of contraction, the
capacity, characteristic of the muscles, of contracting in a
constant direction. The outer, roundish part of the exo-
derm cell remains sensitive and acts as the nervous element,
the inner, fibre-shaped part of the same cell becomes con-
tractile, and, incited to contraction by the former part, acts
as the muscular element (Fig. 293). These remarkable
neuro-muscular cells thus still unite in a single individual
of the first order the functions of two organ-systems. One
step further; the inner, muscular half of the neuro-muscular
cell (Fig. 293, m) acquires its own nucleus, and separates
from the outer, nervous half (n), and both organ-systems
have their independent element of form. The fission of the
muscular skin-fibrous layer from the nervous skin-sensory
layer in embryonic Worms confirms this important phylo-
genetic process (Figs. 50, 51, vol. i. p. 236).

These four organ-systems, which have been mentioned
were already in existence, when an apparatus developed,
tertiarily, in the human ancestral line, which, at first

THE KIDNEYS. 359

sight, seems of subordinate significance, but which proves,
by its early appearance in the animal series and in the
embryo, that it must be very ancient and, consequently, of
great physiological and morphological value. This is the
urinary apparatus, or kidney system, the organ-system
which secretes and removes the useless fluids from the body
We have already seen how soon the primitive kidneys
appear in the embryo of all Vertebrates, long before any
trace of the heart is discoverable. Correspondingly, we also
find a pair of simple primitive kidney ducts (the so-called
excretory ducts or lymphatic vessels) almost universally
difiused in the Worm tribe, which is so rich in forms. Even
the lowest classes of Worms, which have as yet neither
body-cavity nor vascular system, are furnished with these
primitive kidneys (Fig. 280, nCy 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 {Acoslomi) 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 (Plathelniintkes), the Gliding Worms
(Turhellaria), the Sucking Worms {Trematoda), and the
Tape Worms. In the higher Worms, which are therefore
called Coelomati, a body-cavity {coeloma), 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 Coelomati to the four higher animal
tribes.

These organ-systems are common to Vertebrates and to
the three higher animal tribes, the Articulated Animals

$6o TETE EVOLUTION OF MAN.

(Arthropodd), the Soft-bodied Animals -(il/o^Zitsca), and the
Star Animals {Echinoderma), 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 oflf
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 contiary.

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 Gastraea;
but in all these low animals, the reproductive cells do not

AGE OF THE TISSUEa 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
vaiying 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 XXXIX., p. 367).

The tissues of the human body, arising by division of
labour, the separation and the connection of the component
cells, may be distributed, with reference to their develop-
ment, in the four following distinct gi'oups : — 1, covering-
tissue {epitheliutn) ; 2, connective tissue (connectivum); 3,
nerve and muscular tissue (neuro-musculum) ; and 4, vas-
cular tissue (vasalium). Of' these, in accordance with the
Gastrsea theory, we must regard the covering-tissue as the
oldest and most original form, as the actual primary o)
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 ceUs of

362

THE EVOLUTION OF MAN.

which are arranged in a simple strata-like way, and extend
over the outer and inner surface of the body as a protective
and secreting cover. This is proved by the simple fact that
the formation of the tissues of the animal body begins
with the formation of the gastrula, and that the latter
itself consists solely of two simple epithelial strata, of the
skin-layer (Fig. 274, e), and of the intestinal-layer (i).
Histologically, the two primary germ-layers are simple
epithelia. When these, afterwards, separate into the four
secondary germ -layers, the skin-sensory layer becomes the
outermost of the external coverings (dermal-epithelium) ;
the intestinal-glandular layer becomes the innermost of the
internal coverings (gastral-epithelium). The tissue of the
outer skin and of all its appendages, such as nails (Fig. 289),

Fig. 289. — Tissue of the nails (flattened epithelium) : a-e, cells of tha
npper strata ; /, g, cells of the lower strata.

Fig. 290. — Tissue of the covering of the small intestine (columnai
epithelium) : a, side view of thi-ee cells (with thicker, porous borders) ; &,
surface view of four cells. (After Frey.)

hairs, skin-glands, etc., arise from the skin-sensory layer.
(Cf. Table XXIX., p. 232.) The inner covering of the intes-
tinal tube and of its intestinal glands originates, on the
other hand, from the intestinal-glandular layer (Fig. 290).

TISSUES. 3^3

Connective tissue (connectivum) must be regarded as
forming, in order of phylogenetic age, the second main
group of tissues. This is morphologically characterized by
the intercellular substance, which develops between the

Fig. 291. — Jelly-like tissue from the vitreons body of an embryo of four
months (round cells as jelly-like intercellular substance).

Fig. 292. — Cartilage-tissue of the fibrous or netted cartilage of the ear-
shell : a, cells ; h, intercellular mass ; c, fibres in the latter. (After Frey. )

cells, physiologically, by the double part which it plays,
as connecting substance and as complementary substance
between the other tissues, as an inner supporting substance
and as a protective covering for the inner organs. Of the
numerous forms and varieties of connective tissue, we regard
the jelly-like tissue (Fig. 291 ; Fig. 6, vol. i. p. 126), the fatty
tissue, and the chorda tissue as the earlier; the fibrous,
cartilaginous (Fig. 292), and bone- tissue (Fig. 5, vol. i. p. 126)
as the more recent formations. All these various forms of
connective tissue are products of the middle germ-layer,
or mesoderm ; or, more accurately, of the two fibrous layers,
of which the skin-fibrous layer is originally derived from
the exoderm, the intestinal-fibrous layer from the entoderm.
The nerve-muscular tissue (neuro-musculum) is of much
more recent origin than the connective tissue. If epithelial
tissue represents a primary period in tribal history, and

3^4

THE EVOLUTION OF MAN.

connective tissue a secondary period, then we may cha-
racterize a third, much later period, by nerve-muscle tissue.

Fig. 293.

Fig. 294.

Fig. 293. — Nerve-muscle tissue. Three cells from Hydra : w, outer,
nervous ; m, inner, muscular part of the cells. (After Kleinenberg.\

Fig. 294. — Nerve-tissue (from a spinal nerve knot) : a, anterior, h^
posterior root of the spinal nerve ; d, e, fibrous nerve- stem ; /, g, h, i, nerve
cells in ganglion (/, unipolar, g, h, bipolar cells) ; k, I, nerve fibres. (After
Frey.)

Fig. 295. — Muscle-tissue. Three pieces of striped muscle fibre (a). In=
terfibroui: f a,t.cells (&). (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 gi^eater part of the nerve-tissue is
derived from the skin-sensory layer, the greater part of the
muscle-tissue from the skin-fibrous layer.

Vascular tissue (vasalium) must be regarded as forming

Fig. 296. — Ta:^cnlar tissne {vasalium). A hair- vessel from the
mesentery : a, vascular cells ; h, the kernels of these ("endothelium").

Fig. 297. — Eed 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 {Cohitis) ; 8, Lamprey (Petromyzon) ;
a, surface view ; h, edge view. (After Wagner, j

( 36e )

TABLE XXXYIII.

fljlfetmatio Stirvey of the Sequence, according to Age, of the

Tissue-groTips.

(Phylogenetic Classification of Vertebrate Tissues.)

FIRST GROUP: PRIMARY TISSUES (Epithelium).

1. FntsT Histological Stage of Evolutioh.
L Covering-tissue (Epithelium).

I. A. Skl»^»T«ring tissue ^Epithelium cU^-male). (^ gfSs oAufrSlT"^

SkiB-layer. or Exodenn of Gastrula (after- ^ f^^^f s'ite^Vorigin of the f^^
wMdB skin-sensory layer) I cells?) » -f«»"-

I. B. Intestinal covering tiBBue (Epithet. gastraleX (\. Real intestinal epithelium

Intestinal layer, or Entoderm, of Gastrula •< 2. Epithelium of the intestinal glandi
(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. Second Histological Stage of Evolution.
II. Connective-tissue (Connectivum).

Jelly-like tissu*
Fatty tissue
Fibrous tis'^na
Chorda tissue

II. C. FilHng-Tip tissue (TeJa conjuiKti/va) (softer jg*
[surrounding] connective tissue) "i ^

II. D. Supporting tissue {Tela skeletaris) (firmer/ J; CaitU^inous tissue
[flupportmg] connective tissue) | g Bone-ttssue

8. Third Histological Stage of Evolution.

III. Nerve-muscle Tissue

in..

m.

I Kerre-tiesue (Tela
mervea). Original outer
portion of the nerre-
muBcle cells of the
Exodenn

P. MuBcle-tissne {Tela
mtacularis). Original
Inner portion of the
nerve-muscle cells of
the Exodenn

Nerve-cells
(Ganglion-cells)

Nerve-fibres
(Nerve-tubes)

One-celled muscle-
fibres

Many-celled muscle-
fibres

(Neuro-miksculum).

[ 1. a. Peripheric nerve-cells (Rod-cells o!

< the sense-organs)

( 1. 6. Central nerve-cells (mind -cells)

(2. a. Sheath less nerve-fibres (pale, or
medulla-less fibres)
2. 6. Slieathed nerve-fibres (dark fibres
with medulla)

1. o. Smooth contractile fibre-cella
1 b. Striped contractile fibre-cell*

2. a. Smooth muscle-masses
2. h. Striped muscle-masses

4. Fourth Histological Stage of EvoLunov.

IV.

Vascular Tissue (Vasalium).

I. a.

IT.0. Vascular lining tissue
{Tela vasalis). Inner
wall-covering of the
OoBlom system

fW.K. I^rmph-tissue {Tela (
lymphatiea'). Liquid 1 1.
eentents of tlM C<elom 1 2.

Ccelarium

(Ccelom

lium)

epithe-^j b.

Endothelium f

(Vascular epitbe- <

Uum) (

Exocoelarium (Parietal Ocelom-epl-
thelium) (and secondary site oi
origin of the sperm-cells H)

Endocoelarium (Visceral coelom-eirf-
thelium) (and secondary site of
origin of the egg-cells })*

Endothelium of the lymph-vessels
Endothelium of the blood-vessels

')

Lymph (Colourless blood-cells and fiuid intercellular
Blood (Red blood-cells and fluid intercellular substeaee)

( 367 )
TABLE XXXIX.

8yBtematio Snrvey of the Sequence, according to Age, of the Hi

Organ-systems.

(Phylogenetic Classification of Vertebrate Organs.)
(On the right are given the Ancestral Stages, 'n which the respective
organs probably first appeared.)

1. FiBST Stage in the ETOLrrroy of Ohgajti.

I. Skin and Intestinal Systems.

Tbe two Systems appear first, and simultaneously, in the Oaatread aaeetton.

I A 1. Simple exoderm (Hstreafdi

A 2. Outer skin (Skin-sensory layer) and ) Worms
leather skin (Skin-fibrous layer) j
A 3. Outer skin, with hairs, glands, etc. Mammals

!£ 1. Simple entoderm Gastneads

B 2. Intestinal epithelium (Intestlnal-glan- '\
dular layer) and intestinal muscular \ WorBis
skin (Intestinal-fibrous layer) )

B 3. Gill-intestine and stomach -intestine Ghorda-anim*lB

S. Second Stage is the Evolution op Obqamb.
II. Nerve and Muscle Systems.
Tke two Sjitems appear first, and simultaneously, in the PrimitiT* Warm aooestors.

n r Nprve svstem I ^ ^- ^PP^"" ^^^^^^ S-nglia Primitive Worms

11. U. jNerve-sysiem ^2. Simple medullary tube Chorda-animals

iSystema nervtam) ( ^ 3 3,.^/^ ^^^ spinal marrow Monorhina

TT n M«o,,i» Q,To*a»« lD\. Skin-muscle pouch Primitive Worms

"^w™ SvJ«r-^ \ ^ 2- Side muscles of the trunk Acrania

iSyitaaa mtocotore) | ^ 3 ^^^^j^ ^^ ^^^ muscles Fishes

8. Third Stage in the Evolution of Oboajm^

III. Kidney and Vascular Systems.

TW tw« Systems first appear, one after the other. In the Soft-wona ABoeskon (JSKUcida).

1^1. Primitive kidney canAli Scolecida

^2. Segmental canals Acrania?

^3. Primitive kidneys Monorhina

E4,. Permanent kidneys Ptotamnia

ri^l. Simple coelom Scolecida

OL F. Vascular system \ F2. Dorsal and ventral vessel* Worms

{Syitema vatculare) j /«' 3. Heart (part of the ventral vewel) Chorda-animals

I J? 4. Heart, with auricle and ventricle Monorhina

4. FouBTH Stage in the Evolution of Osgani.

rV. Skeleton and Sexual Systems.

Tbe two Systems first appear, one after tbe other, in the Cliordonia-anceston.

/-(? 1. Simple notochord Chorda-animals

IV. O. ^eleton-system J G 2. Cartilaginous primitive ikoU Monorhina

{SysteiMk $ktUtare) J (? 3. Gill-arches, ribs, limbs Selachii

V. 6^ 4. Limbs, with five digits Amphibia

{^1. Simple hermaphrodite glands Chorda-animala

H2. Distinct testes and ovaries Acrania

JI3. Seed-duct and oviduct Selachii

^4. Phallus (penis, clitoris) Piotamnia

57

36S THB EYOLUnON OF MAX.

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 coelom,
chest-cavity, ventral cavity, heart-cavity, blood-vessels, etc.
(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,
" parablastic " origin (from the nutritive yelk) ; they are,
however, products of the intestinal-fibrous layer (and partly,
perhaps, of the skin-fibrous layer). As the coeloma and the
whole vascular system is of more recent phylogenetic origin,
its peculiar tissues must also be more recent.

This phylogenetic explanation of the ontogenetic suc-
cession of the tissues and of the organ systems arising from
them, appears to me to be satisfactorily proved by Com-
parative Anatomy, and by the Gastrsea theory. If it is
correct, it discloses an interesting glimpse into the entirely
various age of the most important constituent parts of our
body. The human skin and intestine are, according to this,
many thousands of years older than the muscles and nerves;
these again are much more ancient than kidneys and blood-
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

BUDIMENTABY VASCULAR SYSTEM. 369

begin till a comparatively late period. Not only ail Plant
Animals (Sponges, Corals, Hydropolyps, Medusae), but also
all lower Worms {Aco&lorai), are entirely destitute of
vascular system. In both groups, the fluid acquired by
digestion Ie 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
(c€eloina)f or of a system of connected spaces, round the
intestinal tube, in which cavities the nutritive fluid (blood)
exuded through the intestinal wall, collects.

In the human ancestral line we meet with this first
rudiment of the vascular system in that group of Worms
which we spoke of as Soft Worms (Scolecida; p. 86).
The Soft Worms, as we said, formed a series of intermediate
stages between the lowest bloodless Primitive Worms
(Archelminthes) and the Chorda- worms (C%ordonia), 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 coelom, 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-poljrps (Bryozoa)
m the Wheel-animalcule {Rotatoria), and in other lower
Worms. The inner, visceral, part of the wall of the coelom
is, naturally, formed by the intestinal-fibrous layer (endo-
oodar), the outer, parietal, part by the skin-fibrous layer
(eococwlar), "Jlje po^lom fluid, collected between the two,

370 THE ICVOLUTION OP MAN.

may contain detached cells (lymph-cells) from either fibrous
layer.

A first advance in the development of this most primi-
tive vascular system was accomplished by the formation of
canals or blood-conducting tubes, which developed, inde-
pendently of the coeloma, in the intestinal wall, that is, in
the intestinal-fibrous layer of the wall. These real blood-
vessels, in the stricter sense, appear in very different
form in Worms of the intermediate and higher groups ;
sometimes they are very simple, sometimes very complex.
Two primordial " primitive vessels " must be regarded as
representing that form, which probably formed the first of
the more complex vascular system of Vertebrates ; these are
a dorsal vessel, which passes from front to back along the
middle line of the dorsal wall of the intestine, and a ventral
vessel which passes, in the same direction, along the middle
line of the ventral wall Both at the front and at the
back these two vessels are linked together by a loop sur-
rounding the intestines. The blood enclosed in the two
tubes is driven forward by the peristaltic contraction of
this.

The further development of this simplest rudimentary
blood-vessel system is evident in the class of the Ringed
Worms (Annelida), in which we find it in very variorw
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-waU and
branched, so as to conduct blood to this part. As in those
ancestral Worms, which we have called Chordoma, the
front section of the intestine changed into a gill-body, theM

THE VASCULAR SYSTEM.

371

t

\

asculai- 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
(Balanoglossiis) 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, c. p. 90;
Plate XI. Fig. 14, hz). The original position

Fig. 298. — Blood-vessel system of a Einged 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 ot

li-
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 alternatinof 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.

Ah in the more recent Chorda Animals, which gave
rise to the Vertebrate tribe, the newer direction became
permanent, the two vessels which proceeded from the
two ends of the heart-pouch, acquired a constant signifi-
cance. The front section of the ventral vessel, since then,
has steadily conducted the blood from the heart, acting,
consequently, as an artery; the hinder section of the
ventral vessel, on the contrary, leads the blood, circulating
in the body, back into the heart, and must, therefore, be
called a vein. In reference to their relation to the two
sections of the intestine, we may speak of the latter, more
accurately, as the intestinal vein, and of the former as the
gill-artery. The blood contained in both vessels, which
alone fills the heart also, is venous blood ; that is, containing
much carbonic acid. On the other hand, the blood which
flows from the gills into the dorsal vessel is there re-
furnished with oxygen ; is arterial blood. The most delicate
branches of the arteries and veins pass into each other,
within the tissue, through a network of extremely fine
neutral hair-vessels or capillaries (Fig. 296).

If we now turn from the Ascidia to the nearest allied
form, the Amphioxus, we are immediately surprised by an
apparent retrogi'ession in the development of the vascular

DEVELOPMENT OF THE VASCULAR SYSTEM. 373

gystem. The Amphioxus, as has been stated, haa 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, (Of. 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 giU-arches,
which accomplish respiration, and absorb oxygen fix)m 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 SkuUed
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 Chordoma and
transmitted it directly to the older SkuUed 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 amoeboid cells (Fig. 9, vol. i. p. 132), which are also
distributed in the blood as colourless blood-cells (corpuscles) ;
the latter contains, in addition, the much greater quantity
of red blood-cells (corpuscles), which gives the blood of
Skulled Animals its red colour. The distinction, common to
aU 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 difterent portions of an
original unitary, primitive blood-vessel system (or haemo-
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
(atriv/m, 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 gilJ -artery stem (the foremost section of
the ventral vessel) into the gills.

In Primitive Fishes {Selachii), an arterial stalk (huUms
arteriosus), sepaiated by valves, originates, as a distinct
section, from the foremost end of the ventricle. It forma
the enlarged, hindmost end of the gill-artery stem (Fi^

DEVELOPMENT OF THE VASCULAR SYSTEM.

375

299, ahr). From each side of this, from five to seven gill-
arteries proceed; these rise between the gill-openings (s)
to the gill-arches, encircle the throat, and combine above
into a common aorta-stem, the continuation of which,
passing backward above the intestine, corresponds to the
dorsal vessel of Worms. As the arched arteries distribute
themselves in a respiratory capillary net over the gill-
arches, they thus contain venous blood in their lower part
(as arterial gill-arches), and arterial blood in their upper
part (as aorta-arches). The points at which separate aorta-
arches unite, which occur on the right and left sides, are /
called aorta-roots. Of an originally greater number of
aorta-arches, only five pairs are retained, and from these
five (Fig. 300), in all higher Vertebrates, the most im-
portant parts of the arterial system develop.

Fig. 299. — Head of an. embryonic Fish, with the mdiment and the
blood-vessel system ; seen from the left side : dc, Cuverian duct (point of
anion of the front and hind main veins) ; av, venons Binns (enlarged
terminal portion of the Cuverian duct) j a, auricle ; v, main chamber ;
ahr, gill-artery stem; s, gill-openings (between the arterial arches); ad,
aorta; e', head-artery (carotis) ; n, nose-groove. (After Gegenbaur.)

The appearance of the lungs, connected with the respi-
ration of air, which first occurs in the Dipneusta, is most
important in the further developement of the arterial

3/6 THE EVOLUTION OF MA«.

system. In Dipneusta, the auricle of the heart separatefl

into two halves by the formation of an incomplete partition.
Only the right auricle now absorbs the venous blood of the
body-veins. The left auricle, on the other hand, absorbs
the arterial blood of the lung-veins ; both auricles dis-
charge in common into the simple ventricle, in which the
two kinds of blood mingle, and are then propelled through
the arterial stalk into the arterial arches. From the last of
these latter spring the lung-arteries (Fig. 301, p)) these
convey a part of the mixed blood into the lungs, while the
remainder is driven through the aorta into the body.

From the Dipneusta upward, we trace a progressive
development of the vascular system, which finally leads,
with the loss of gill respiration, to a complete separation of
the two parts of the double circulatory system. In Am-
phibia, the partition between the two auricles becomes
complete. In their young form, these yet retain gill-
respiration and the circulatory system as in Fishes, and the
heart contains only venous blood; at a later period, the
lungs, with their vessels, are developed also, and the main
chamber of the heart then contains mixed blood In Pro-
tamnia and Reptiles, the main chamber and the arterial
stalk belonging to it begin to separate, by the formation of
a longitudinal partition, into two halves,, and this partition
becomes complete in the higher reptiles on the one side, in
the parent-form of Mammals on the other. The right half
of the heart alone now contains venous blood, the left half
only arterial, as in all Birds and Mammals. The right
auricle receives venous blood from the body-veins, and the
right ventricle propels this through the lung-arteries into
the lungs ; from there it returns as arterial blood through

DOUBLE CIRCULATORY SYSTEM.

377

the lung- veins to the left auricle, and is driven through the
left ventricle into the body-arteries. Between t!.o 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,
ui- body-circulation. Only in the two highest Vertebrate

Fig. 300.— The five arterial arcbes of Skulled Animals (1-5) in their
original form : a, arterial stalk ; a", main stem of the aorta ; c, head-
artery (carotis, anterior continuation of the aorta-roots). (After Rathke.)

Fig. 301 — The five arterial arches of Birds; the light portions of the
rudiment disappear ; only the dark parts are permanent. Letters as in
Fig. 300 : s, arteries of the clavicula (sub-clavian) ; p, lung-artery ; p',
branches of the same. (After Eathke.)

Fig. 302. — The five arterial arches of Mammals. Letters as in Fig. 301 :
V, vertebra] artery ; h, Botalli's duct (open in the embryo, afterwards
closed). (After Rathke.)

classes, in Birds and Mammals, is this complete separation
of the two courses of the circulation perfect. Moreover, this
separation has taken place in the two classes independently
of each other, as is shown by the unequal development of
the aortas. In Birds, which are descended from Reptiles,

17^

THE ^VOLUTION OF MAJN.

the right half of the fourth arterial arch has become the
periuanent arterial arch {ar<M8 aortcB, Fig. 301). On the
other hand, the latter has developed from the left half of
the same arch (Fig. 302) in Mammals, which are directly
descended from the Protamnia.

On comparing the arterial system in the various classes
of the Skulled Animals {Craniota) in its matured condition,
it appears in very various forms, and yet it develops, in
all, from the same primitive form. This development takes
place in man exactly as in other Mammals ; especially is the
modification of the five arterial arches precisely the same in
both cases (Figs. 303-306). At first, only a single pair of

Figs. 303-306. — Metamorphosis of the five arterial arches in the human
embryo (diagi-am after Rathke) : ta, axterial stalk ; 1, 2, 3, 4, 5, the arterial
arches from the first to the fifth pair; ad, main stem of the aorta; ate,
roots of the aorta. In Fig. 303, three of the arterial arches are given ; in Fig.
304, the whole five (those indicated by dots are not yet developed) ; iii Fig. 305,
the first two have again disappeared ; in Fig. 306, the permanent arterial
stems axe represented. The dotted parts disappear, s, Sub.clavian artery ;
r, vertebral artery ; aa?, axillary artery ; c, carotid artery (c', outer, e",
tnner carotis) ; j9, pulmonary artery (lung-artery) .

arches develop, and these lie on the inner surface of the
first pair of gill-arches (Figs. 147-150, vol. i. pp. 895-398;
Fig. 303). A second and a third pair of arches then develop

DEVELOPMENT OF THE HEABT. 379

behind the first, and these are situated on the inner surface
of the second and third gill-arches. At length, a fourth and
a fifth pair appear behind the third (Fig. 304) ; but while
the latter are developing, the first two are again disappear-
ing t)y growing together (Fig. 305). The permanent main
arteries develop only fi'om the three posterior arterial
arches (3, 4, 5, in Fig. 304), the lung-arteries from the last
(p ; Fig. 306). (Of. with this Fig. 802.)

The human heart also (Fig. 314) develops exactly like that
3f other Mammals. We have already considered the first prin-
ciples of its germ-history (vol. i. pp. 392-395, Figs. 143-147),
which essentially corresponds with its Phylogeny.^®^ We saw
that the very first rudiment of the heart is a spindle-shaped
thickening of the intestinal-fibrous layer in the ventral wall
of the head-intestine (Fig. 143, df). This spindle-shaped
formation then becomes hollow, forms a simple pouch, and
separates from the place at which it originated, so that it
then lies freely in the cardiac cavity (Figs. 145, 146). This
pouch bends into the form of an S (Fig. 144, c), and, at the
same time, turns spirally on an imaginary axis, so that the
posterior part Kes on the dorsal surface of the anterior
part. The combined yelk-veins open into its posterior
extremity ; from the anterior extremity proceed the arterial
arches (Fig. 150, voL i. p. 398).

This first rudiment of the human heart, which encloses
a very simple cavity, corresponds to the heart of the As-
cidians, and must be regarded as a reproduction of the heart
of the Chordoma ; it now, however, separates into two, and
then three parts, thus exhibiting for a very brief period the
heart-structure of the Cyclostoma and of Fishes. The spiral
turn and curve of the heart increases, and, simultaneouslyi

38o

THE EVOLUTION OF MAN,

two shallow transverse indentations of the circumference
appear, which externally mark the three sections (Figs. 307,
308). The anterior section, which is turned toward the

—e r

th'-

Pig. 307. — Heart of an embryonic Rabbit, from behind : a, yelt-veins ;
b, auriculae ; c, auricle (atrium) ; d, ventricle ; e, artery-stalk ; /, base of the
three pairs of arterial arches. (After Bischoff.)

Fig. 308. — Heart of the same embryo (Fig. 307), from the front: v,
yelk-veins ; a, auricle ; ca, auricular canal ; I, left ventricle ; r, right
ventricle ; ta, artery-stalk. (After Bischoff.)

Fig. 309. — Heart and head of an embryonic Dog, from the front :
a, fore-brain ; h, eyes ; c, mid-brain ; d, primitive lower jaw ; e, primitive
upper Jaw ; /, gill-arches ; g, right auricle ; h, left auricle ; i, left ventricle ;
k, right ventricle. (After Bischoff.)

Fig. 310. — Heart of the same embryo, from behind : a, entrance of the
yelk-veins ; h, left auricular process ; c, right auricular process ; d, auricle ;
e, auricular canal ; /, left ventricle ; g, right ventricle ; h, artery-stalk.
(After Bischoff.) ^

ventral side, and from which the aortal arches spring,
reproduces the arterial stalk (bulbvs arteriosus) of the
Selachii. The central section is the rudiment of a simple
chamber, or ventricle (ventrimdus) ; and the posterior
section, the one turned toward the dorsal side, into which
the yelk-veins open, is the rudiment of a simple auricle

DEVELOPMENT OF THE HEABT. 38 1

(atrium). The latter, like the simple auricle of the heart
of the Fish, forms a pair of lateral protuberances, the heart
ears, or auricular appendages {av/riculcB, Fig. 307,6); and
hence the indentation between the auricle and ventricle is
called the auricular canal (canalia auricularis, Fig. 308, ca).
The heart of the human embryo is now a complete Fisl
heart

Corresponding exactly with the Phylogeny of the human
heart (Table XLI), its Ontogeny exhibits a gradual tran-
sition from the Fish heart through the Amphibian heart to
the Mammalian heart. The most important step in this
advance is the formation of a longitudinal partition, im-
perfect at first, afterwards complete, by which aU the three
sections of the heart are separated into a right (venous^ and
a left (arterial) half (Cf Figs. 309-314.) The auricle
{atrium) is thus divided into a right and a left auricle, each
of which acquires its respective auricular process ; the body-
veins discharge into the right auricle (ascending and de-
scending vena cavce, Fig. 311, c, Fig. 313, c) ; the left auricle
receives the lung- veins. Similarly, a superficial "inter-
ventricular furrow" (sulcus irUerventricularis, Fig. 312,8)
appears at an early period on the main chamber of the
heart, the external expression of the internal partition, by
the formation of which the ventricle is divided into two
chambers, a right (venous) and a left (arterial) ventricle.
Finally, a longitudinal partition forms, in a similar way,
in the third section of the primitive heart, which so much
resembles that of a Fish, in the arterial stalk, which is also
externally indicated by a longitudinal furrow (Fig. 312, a/).
This separates the cavity of the artery-stalk into two
lateral halves ; the main lung artery, which opens into the

382

THE EVOLUTION OF MAN.

Fig. 311.

Fig. 313.

Fig. 314.

Fig. 311. — Heart of a human embryo of four weeks ; 1, from the front ;
2, from the back ; 3, open, and with the upper half of the amricle reuioved ;
a', left auricular process ; a", right auricular process ; v', left ventricle ;
v", right ventricle ; ao, artery-stalk ; c, upper hollow vein {vena cava) {cd,
right, cs, left) ; s, rudiment of the partition, between the chambers. (After
Koelliker.)

Fig. 312. — Heart of a human embryo of six weeks, from the front:
r, right ventricle ; t, left ventricle ; s, furrow between the two ventricles ;
ta, artery-stalk ; af, furrow on its surface ; at the right and left are the
two large auricular processes of the heart. (After Ecker.)

Fig. 313. — Heart of a human embryo of eight weeks, from behind:
a', left auricular process ; a", right auricular process ; v', left ventricle ;
v", right ventricle ; cd', right upper vena cava ; cs, left upper vena cava ;
ci, lower vena cava. (After Koelliker.)

Fig. 314. — Heart of human adult, perfectly developed, from the front, in
its natural position : a, right auricular process (below it, the right ventricle) ;
b, left auricular process (below it, the left ventricle) ; C, upper vena cava ;
V, lung -veins ; P, lung-artery ; d, Botalli's duct ; A, aorta. (After Meyer.)

right ventricle, and the aorta-trunk, which opens into the
left ventricle. Not until all these partitions are complete,
is the lesser, or lung-circulation, entirely distinct from the

POSITION OF THE RUDIMENTARY HEABT. 583

greater, or body-circulation ; the right half of the heart is
the centre of motion for the former, the left half for the
latter. (Cf Table XLI.)

In the human embryo, and in all other Amniota, the
heart originally lies far forward on the lower side of the
head, as in Fishes it remains permanently near the throat.
Afterwards, with the advancing development of the neck
and chest, the heart continually moves further back, until
at last it is situated in the lower part of the breast between
the lungs. At first its position is symmetrical, in the central
plane of the body, so that its longitudinal axis corre-
sponds with that of the body (Plate IV. Fig. 8). In most
Mammals it retains this symmetrical position permanently ;
but in the Apes the axis begins to incline obliquely, and to
move the apex of the heart to the left side. This inclination
is carried furthest in the Man-like Apes; in the Chim-
panzee, Gorilla, and Orang, which also resemble Man in
this oblique position of the heart.

The germ-history of aU other parts of the vascular system,
like that of the heart, point out many and valuable facts re-
garding the history of our descent. But as an accurate know-
ledge of the complex arrangement of the entire vascular system
of Man and other Vertebrates is required, in order to follow the
matter sufficiently far to make it intelligible, we cannot here
enter into any further detaiL^^^ Moreover, many important
features in the Ontogeny of the vascular system, especially
in regard to the derivation of its various parts from the
secondary germ-layers, are as yet very obscure and doubtful
This is true, for example, of the question as to the origin of
the ccelom-epithelium — that is, of the cell-layer coating the

body-cavity. Probably there is an important phylogenetio

58

3^4 THE EYOLUnON OF MAK.

distinctioii between the exocoelar, or the parietal corfom-
epithelium, which originates from the skin-fibrous layer, and
the endocoelar, or the visceral ccelom-epithelium, which
ie derived fnym the intestinal-fibrous layer. The former
is, perhaps, connected with the male germ-epithelium (the
rudiment of the testes), the latter with the female germ-
epithelium (the rudiment of the ovary). (Cf. Chapter XXV.)

TABLE XL.

ST*nM4nC SUSTIT OV THE MOST IMPOKTANT PesIODS IN THS PhTLOOSJIT

07 THE Human Yasculae System.

I. Fint Period : Vascular System of the earlier Seolecida.
Between the skin-covering and the intestinal wall is formed a simple
bodj-oavity (cesloma), or a perienteric cavitj (as in the extant Bryo»oa and
otber Coslomati).

IL Second Period : Veueular System of the wtore recent Seolecida.

The first real blood-ressels form in the intestinal wall (in the intestinal*
Abroos layer), a dorsal vessel in the central line of the dorsal side of the
intestinal tnbe, and a ventral vessel in the central line of its ventral side.
The two vessels are oonnected bj several oiroolar vessels, enoiroling the
inteatixM.

nL Third Period : VmeeuUtr System cf ike ea/rUer Chordoma.

Bj the modification of the anterior half of the intestine into a gill-
intestine, the anterior section of the ventral vessel becomes a gill-arterj,
and the anterior section of the dorsal vessel a gill- vein | between the two
a gill capillary network develops.

rf. FovHh Period : VaseuJair System of the mare reeemt Chordoma,

l%e portion of the ventral vessel, Ijing immediately behind the gfB>-
te a simple beart-poook (Aaddian).

PHTLOGENT OF THE HUMAN HEART. 385

V. Fifth Period : Vascular System of the Acraitiim,

The rentral vessel (intestinal vein) forms, round the deTeloping Krer-
MO, the first rudiment of a vena portee system.

YI. Siafth Period : Vascular System of the Cyclostomi.

The aingle-ctambered heart divides into two chambers ; ft po8t«rior
T©ntricle, and an anterior auricle. The lymph-ressel system derelops side
hj side with the blood-vessel system.

Vn. Seventh Period : Vascular System of the Prim/itive Fishes, or Selachii.

From the anterior section of the main chamber of the heart arises an
•riery-stalk or trunk, from which five (?) pairs of arterial arches proceed.

Vni. Eighth Period : Vascular System of the Mud-fishes.
From the last (fifth) pair of arterial arches the Inng-exteries develop,
as in the Dipneusta.

IX. Ninth Period : Vascular System of Amphihui.

The gill-arches gradually disappear with the gills. The right and left
aortal arches remain.

X. Tenth Period ; Vascular System of MammmLs,

The separation of the greater from the lesser oiroolation is complete.
The right aortal arch unites with Botalli's duct.

TABLE ^LL

STimfATIC SUBVBT OF THK MOST IMPOKTANT FeEIODS IK THI PhTLOCKWT

o? THE Human Heart.

L First Period : Heart of Chordonia,

The heaH forms a simple spindle-shaped enlargement of the ventral
Teaeel, with an alternating blood-ourrent (as in Ascidia).

II. Second Period : Heart of Acrania.

The heart is like that of Chordonia, but the blood-current acquires
a constant directicm, petssing only from book to front. (Betrt^raded in
Amphioxns.)

386 THE EVOLUTION OF MAN.

m. Third Period : Hea/rt of Cyclostoma.

The heart dirides into two chambers, a posterior auricle (airwm) and
an anterior rentricle (yentricuUis).

IV. Fowrih Period : Heart of Primitive Fishes.
From the anterior section of the ventricle is diflferentiated an arterial
stalk (hullm$ a/rteriosus) , as in all Selachii.

V. Fifth Period : HeaH of the Mud-fishes.

The auricle divides, bj an imperfect and interrupted partition, into
a right and a left half, as in Dipneasta.

VI. Sixth Period : Heart of Atnvhihia.

The 'partition between the right and left auricles becomes complete, a> in
the higher Amphibia.

Vn. Seventh Period : Heart of Protamnia.

The main chamber of the heart divides, by an incomplete partition, into
a right and a left half, as in Reptiles.

VIII. Eighth Period : Heart oj Monotrema.

The partition between the right and left ventricles becomes completOi as
in all Mammals.

IX. Ninth Period : Hea/rt of Marsupials.

The valves between the auricles and ventricles (atrio.ventricular valves),
together with the connecting filaments and papillary muscles belonging to
them, are differentiated from the muscular masses of Monotremes.

X. Tenth Period : Heart of Apes.

The main axis of the heart, lying in the central line of the bodj
ueoomes oblique, so th&t the' apex ia turned to the left, aa in Apeft vad

( 387 )

TABLE XLII.

Sjitematic Survey of those Primitive Organs which most probably be
regarded as homologous in Worms, Articulated Animals, Soft-bodied
Animals, and Vertebrates.***

Wdms
(Fermet).

Articulated

Animals
{Arthropoda).

Soft-bodied
Animals
{Molluscdy

Vertebrates
{rerUbrmtm).

L Producti

J of the Differentiation of the Skin-sensory La^er,

1. Ooter skin

1. Chitinous skin

1. Outer skin

1. Outer skin

{Epidermis')
1. Brain (upper throat-

(ffypodermis^

{Epidermis)
2. Brain (upper throat-

{Epidermis)

2. Brain (upper throat-

3. Medullary tube (an-

ganglia)

ganglia)

ganglia)

terior part)
S. Primitive kidney-

3. Ebccretory organB

3. Shell-glands of the

3. Rudimentary kid-

(water - vessels,

Crustacean

neys (Primitive

ducts {Proture-

segmental organs)

(trachea of the

kidneys)

teres) and seg-

Tracheata?)

mental orgaos

IL Products of the Differentiation of the Skin-fibrous Layer.

1, Leather-skin
{Corium)

(together with the
circular muscle-
pouch ?)

S. Longitudinal
muscle-pouch

i. Exocoelar innermost
cell- layer of the
body-wall (also
Bale germ-plate?)

4. Leather-skin
(Rudiment)

6. Trunk-musclea

6. Exocoelar innermost
cell-layer of the
body- wall (also
male germ-plate?) I

4. Leather-skin

(Corium)

(together with the

muscles of the

skin ?)
6. Inner trunk-muscles

6. Exocoelar parietal
epithelium of the
coelom (also male
germ-plate?)

4. Leather-skln

(Cbrium)
(together with the
muscular layer of
the skin ?)
6. Side tmnk-muscles

6. Exoccelar parietal
epithelium of the
coelom (also male
germ-plate ?)

m. Products of the Differentiation of the Intestinal-fibrous Laymr.

T. Body-cavity

7. Body-cavity

1. Body-cavity

7. Pleuro-perttoDeal

{Coeloma)

{Cceloma)

{Coelomd)

cavity

•l Endocoelar outer-

8. Eiidoccelar outer-

8. lidocflelar visceral

8. Kndocoelar visceral

most cell-layer

most cell-layer

epithelium of the

epithelium of the

of the intestinal

of the intestinal

coelom (together

coelom (together

wall (together

wall (together

with the female

with the female

with the female

with the female

germ-plate ?)

germ-plate ?)

germ-plate ?)

germ-plate ?)

•. Dorsal vessel

9. Heart

9. Chamber of the
heart (and main
artery)

!«. Ventral vesael

10.

10.

10. Heart (and gUl-

artery)

11. Intestinal wall (ex-

1L Intestinal wall (ex-

11. Intestinal wall (ex-
cept the epithe-

11. Intestinal wall (ex-

cept the epithe-

cept the epithe-

cept the epith^

lium)

Uum)

Uum)

lium)

IV. Products of the Differentiation of the Intestinal-glam^dular Layer.

12. Intestinal epithe
lioffi

12. Intestinal epithe-
liom

12. Intestinal epithe-
lium

12. Intestinal
Uun

CHAPTER XXV.

DEVELOPMENT OF THE URINARY AND SEXUAL ORGANS.

Importance of Reproduction. — Growth. — Simplest Forms of Asexnal Repro-
duction : Division and the Formation of Buds (Gemmation) . — Simplest
Forms of Sexual Reproduction : Amalgamation of Two Differentiated
Cells ; the Male Sperm-cell and the Female Egg-cell. — Fertilization. —
Source of Love. — Original Hermaphroditism ; Later Separation of the
Sexes (Gonoohorism). — Original Development of the Two Elinds of
Sexual Cells from the Two Primary Germ-layers. — The Male Exoderm
and Female Entoderm. — Development of the Testes and Ovaries. —
Passage of the Sexual Cells into the Ccelom. — Hermaphrodite Rudiment
of the Embryonic Epithelium, or Sexual Plate. — Channels of Exit, or
Sexual Ducts. — Egg-duct and Seed-duct. — Development of these irom
the Primitive Kidney Ducts. — Excretory Organs of Worms. — *' Coiled
Canals " of Ringed Worms (Annelida). — Side Canals of the AmpKiowus.
— Primitive Kidneys of the Myxinoides. — Primitive Kidneys of Skulled
Animals (Craniotd), — Development of the Permanent Secondary
Kidneys in Amniota. — Development of the Urinary Bladder from the
AUantois. — Differentiation of the Primary and Secondary Primitive
Kidney Ducts. — The Miillerian Duct (Egg-duct) and the Wolflian Dnot
(Seed-duct). — Change of Position of the Germ-glands in Mammals. —
Formation of the Egg in Mammals (Graafian Follicle). — Origin of the
External Sexual Organs. — Formation of the Cloaca. — Hermaphroditism
in Mjuu

** The most important truths in Natural Science are discovered, neither
by the mere analysis of philosophical ideas, nor by simple experience, bal
by rffiectiw€ •mperitn^ which distinguishes the essential from the accidental

IMPORTANCE OF THE RKPRODTJCIIVE STSTKIC 389

in the phenomea* obeerred, and thiu finds principles from wUch mtaij
experiences can be derived. This is more than mere experience ; it is,
■o to speak, philosophical experience." — Johajjnes Mullbr (1840).

If we judge of the importance of the organ-systems of the
animal body according to the number and variety of
phenomena which they present, and according to the
physiological interest connected with them, we must recog-
nize as one of the most important and interesting organic
systems, the one to the development of which we now,
finally, turn ; the system of the reproductive organs. Just
as nutrition is the first and most important condition of
self-preservation of the organic individual, so by repro-
duction alone is the preservation of the kind or species
effected, or, rather, the preservation of the long series of
generations, which in their genealogical connection form the
sum of the organic tribe, or phylum. No organic individual
enjoys an eternal life. To each is granted but a short
span of time for his individual evolution, a brief, fleeting
moment in the long millions of years of the earth's organic
history.

Reproduction in connection with Heredity has, there-
fore, long been regarded as, after nutrition, the most
important fundamental function of the organism, and it is
customary to make this a primary distinction between
living bodies and lifeless or inorganic bodies. But this
distinction is in reality not so deep and thorough as it at
first appears, and as is generally assumed. For, if the
nature of the phenomena of reproduction is closely con-
sidered, it is soon seen that it may be reduced to a more
general quality, that of growth, which belongs to inorganic,
sm well as to organic bodies. Reproduction is a nutrition

390 THE EVOLUTION OF MAN.

and a growth of the organism beyond the individual size,
which, therefore, raises a part of the organism to the rank
of a whole (vol. i. p. 159). This is most clearly seen by
observing the reproduction of the simplest and lowest
organisms, especially of the Monera (p. 46) and of the one-
celled Amoeba (p. 48). In these, the simple individual pos-
sesses only the form-value of a single plastid. As soon as,
by continued nutrition and simple growth, this has reached
a certain size, it does not exceed that size, but falls, by
simple. division, into two similar halves. Each of these
two halves thenceforth leads an independent life, and again
grows, till, having reached the same limit of growth, it once
more divides. At each of these simple self-divisions, two
new central points of attraction for the particles of the
body are formed, as foundations of the two new indi-
viduals.^

In many other Primitive Animals {Protozoa)^ the simple
reproduction is accomplished, not by division, but by the
formation of buds (gemmation). In this case, the growth,
which prepares the way for reproduction, is not total (as in
the case of division), but partial. Hence in the case of
gemmation, the product of local growth, which, as a bud,
forms a new individual, can be distinguished, as a young
individual, from the parent-organism from which it
originates. The latter is older and larger than the former.
In the case of division, on the contrary, the two products
are of equal age and of equal form-value. Further
differentiated forms of asexual reproduction, connected
with gemmation, are, thirdly, the formation of germ-buds,
and, fourthly, the formation of germ-cells. The latter,
however, brings us directly to sexual reproduction, for which

BUDIMENTARY REPRODUCTIVE SYSTEM. 39 1

the opposed differentiation of the two sexes is the condition.
In my Generelle Morphologic (vol. ii. pp. 32-71), and in
my " Natural History of Creation " (vol. i p. 183), I
have fully discussed the connection of these various forms
of reproduction.

None of the earliest ancestors of Man and of the higher
animals were capable of the higher function of sexual
reproduction, but multiplied only in an asexual manner, by
division or gemmation, by the formation of germ-buds, or of
germ-cells, as is still the case with most Primseval Animals
or Protozoa, It was not until a later period in the organic
history of the earth, that sexual difference of the two
sexes could arise ; and this took place at first in the
simplest manner by the severance of two cells which
amalgamated from the community of the many-celled
organism. We may say that, in this case, growth, which is
the condition necessary to reproduction, was attained by
the union of two full-grown cells into a single cell which
then exceeded its proper size (" copulation " or conjuga-
tion"). At first, the two united cells may have been
entirely alike. Soon, however, by natural selection, a con-
trast must have arisen between them. For it must have
been very advantageous to the newly-created individual in
the struggle for existence, to have inherited various quali-
ties from the two parent-cells. The complete development
of this progressive contrast between the two producing
cells, led to sexual differentiation. One ceU became a
female egg-ceU, the other, a male seed or sperm celL

The simplest form of sexual reproduction among existing
animals, is exhibited in Gastrgeads and the lower Sponges,
especially the Chalk Sponges, and, also, in the simplest

39^ 1VE STOUmON OF MAX,

Hydroid Polyps. In the H&liphyBema (Fig. 315) and in
the Olynthus the whole body is a simple intestinal pouch,
which ifl only essentially distinguished from the gastnila by
the fact that it is adherent by the end opposite the mouih.
The thin wall of the pouch consists only of the two
primary germ -layers. As soon as it is sexually mature,
single cells of the wall become female egg-cells, others
become male sperm-cells, or seed-cells; the former grow
very large, as they form a considerable number of yelk-
granules in their protoplasm (Fig. 181, e); the latter, on the
contrary, by continued division, become very small, and
modify into movable "pin-shaped" spermatozoa (Fig. 17,
vol. i. p. 173). Both kinds of cells sever themselves from their
birthplace, the primary germ-layers, faU either into the
surrounding water or into the intestinal cavity, and there
unite by amalgamation. This is the very important process
of the fertilization of the egg-ceU by the sperm-cell. (Cf,
Fig. 18, vol. i. p. 175.)

These simplest processes of sexual reproduction, as
exhibited at the present time in the lowest Plant Animals,
especially in the Chalk Sponges and Hydroid Polyps, inform
us of several extremely important and significant facts ;
in the first plawje, we learn, that for sexual reproduction in
its simplest form, nothing more is required than the
blending or amalgamation of two differing cells, a female
egg-cell and a male sperm-cell, or seed-ceU. AH other
circumstances, and aU the other extremely complex pheno-
mena, accompanying the act of sexual reproduction in the
nigher animals, are of a subordinate and secondary charac-
l;er, and have only attached themselves secondarily to that
simplest primary process of copulation or fertilization, or

RELATION OF THE SEXES.

393

have arisen by differentiation. But, now, if we consider
what an extraordinarily important part is ever3rwhere
played by the relation of the two sexes in organic nature,
in the vegetable kingdom, as in animal
and human life ; how the reciprocal
inclination and attraction of the sexes,
love, giv.es the impetus of the most
varied and remarkable processes, is,
even, one of the most important
mechanical causes of the hio^hest
differentiation in life ; — if we consider
this, we cannot over-estimate this re-
tracing of " love " to its primitive '^it
source, to the power of attraction be-
tween two differing cells. Ever}-
where throughout animated nature ^/J^^

Fig. 315. — Longitudinal section through a
Haliphysema (Gastrceada) The egg-cells (e) are
enlarged epithelial cells of the entoderm (g),
and lie freely in the pi^imitiye intestinal cavity
{(l): w, mouth-opening ; /j,exoderm.

the greatest results proceed from this most insignificant
cause. It is /only necessary to think of the part played in
nature by the flowers, the reproductive organ of flowering
plants ; or of the multitude of wonderful phenomena
caused by sexual selection in animal life ; or, finally, of the
important influence exerted by love on human life : the coa-
lescence of two cells is everywhere the single, original
impelling motive ; everywhere this apparently trivial pro-

394 THE EVOLUTION OF MAK.

cess exerts the greatest influence on the development of the
most varied circumstances. We may, indeed, assert, that
no other organic process can be, even remotely, compared to
this in extent and intensity of differentiating effect. For
is not the Semitic myth of Eve, who seduced Adam to
knowledge, and is not the old Greek legend of Paris and
Helen, and are not very many other famous fictions, merely
the poetical expression of the immeasurable influence, which
love, in connection with " sexual selection," ^ has exerted,
ever since the differentiation of the two sexes, on the pro-
gress of the world's history ? All other passions that agitate
the human breast are in their combined efiects far less
powerful than love, which inflames the senses and fools the
understanding. On the one hand, we gratefully glorify love
as the source of the most splendid creations of art ; of the
noblest productions of poetry, of plastic art and of music :
we reverence in it the most powerful factor in human
civilization, the basis of family life, and, consequently, of
the development of the state. On the other hand, we fear
in it the devouring flame which drives the unfortunate to
ruin, and which has caused more misery, vice, and crime,
than all the other evils of the human race taken together.
So wonderful is love, and so immeasurably important is its
influence on mental life, on the most varied functions of the
medullary tube, that in this point, more than in any other,
" supernatural " causation seems to mock every natural
explanation. And yet, notwithstanding all this, the com-
parative history of evolution leads us back very clearly and
indubitably to the oldest and simplest source of love, to
the elective aj9&nity of two differing cells : the sperm-cell
and the egg-cell.

HERMAPHRODITISM. 395

Just as the lowest Plant Animals exhibit this most
simple origin of the complex phenomena of reproduction,
so, in the second place, they reveal the highly important
fact, that the earliest and most primitive sexual relation
was hermaphroditism, and that the separation of the sexes
originated from this only secondarily (by division of labour).
Hermaphroditism is prevalent in lower animals of the most
different groups; in these, each single individual, when
sexually mature, each person, contains male and female
sexual cells, and is, therefore, capable of self-fertilization
and self-reproduction. Thus, not only in the lowest Plant
Animals just mentioned (the Gastrseads, Chalk-sponges,
and many Hydroid Polyps) do we find egg-cells and
sperm-cells united in one and the same person ; but
many Worms (for example, the Ascidians, Eaiih Worms
and Leeches), many Snails (the common garden Snail), and
many other invertebrate animals are also hermaphrodite.
All the earlier invertebrate ancestors of man, from the
Gastrseada up to the Chordoma, must also have been her-
maphrodite. So, probably, were also the earliest Skulled
Animals (Figs. 52-56, e, h, vol. i. p. 256). One extremely
weighty piece of evidence of this is afforded by the remark-
able fact, that even in Vertebrates, in Man as well as other
Vertebrates, the original rudiment of the sexual organs is
hermaphrodite. The separation of the sexes (Oonocho-
rism). the assignment of the two kinds of sexual cells
to ditierent individuals, originated from hermaphroditism
only in the farther course of tribal history. At first, male
and female individuals differed only in the possession of the
two kinds of cells, but in other respects were exactly alike,

ia now the case in the Amphioxus and the Cyclostoma.

396 THE EVOLUTION OF MAN.

Not until a later period, by the law of sexual selection, so
briUiantly elucidated by Darwin, were developed the so-
called " secondary sexual characters," that is, those dif-
ferences in the male and female sexes which are exhibited,
not in the sexual organs themselves, but in other parte of
the body (for example, the beard of the man, the breast of
the woman).*

The third important fact, taught us by the lower Plant
Animals, refers to the earliest origin of the two kinds of
sexual cells. For, as in Gastrseads, and in many Sponges and
Hydroids, in which we meet with the simplest rudiments
of sexual differentiation, the whole body consists throughout
life only of the two primary germ-layers, the two kinds of
sexual cells can, therefore, only have originated from cells
of the two primary germ-layers. This simple discovery is
of extreme importance, because the question of the first
origin of the egg-cells as well as of the sperm-cells in the
higher animals — and especially in Vertebrates — presents
unusual difficulties. In these animals it usually appears
as if the sexual cells developed, not from one of the two
primary, but from one of the four secondary germ-layers.
If, as most authors assume, they do originate from the
middle-layer, or mesoderm, the fact is due to an ontogenetic
heterotopism, to a displacement in position. (Cf vol. i. p. 13.)
Unless the unjustifiable and paradoxical assumption, that
the sexual cells are of entirely different origin in the higher
and in the lower animals, is accepted, we are compelled to
derive them originally (phylogenetically), in the former as in
the latter, from one of the two primary germ-layers. It must
then be assumed that these cells of the skin-layer or of
the intestinal layer, which must be regiuxled as the earliest

OBIQIN OF THE SEXUAL CELLS. |97

progenitors of the sperm-cells and of the egg-cella, with-
drew, during the separation of the skin-fibrous layer from
the skin-sensory layer, or of the intestinal-fibrous layer
fi'om the intestinal-glandular layer, into the body-cavity
oceloTna), which was in process of formation; and that
they thus acquired the internal position between the two
fibrous layers, which appears as their original position,
when the sexual cells first become distinct in the vertebrate
embryo. Otherwise, we should be obliged to accept the
improbable polyphyletic hypothesis, that the origin of the
egg-cells and sperm-cells is different in the higher and in
the lower animals, that their origin in the former ia inde-
pendent of that in the latter.

K we, accordingly, derive the two kinds of sexual cells
from the two primary germ-layers in man as in all other
animals, the farther question arises : Did the female egg-
cells and the male sperm-cells develop from both primary
germ-layers, or fit)m one only ? and, in the latter case, from
which of the two ? This important and interesting question
is one of the most difficult and obscure problems in the
history of evolution, and, up to the present moment, no full
and clear solution has been attained. On the contrary,
the most opposite answers are given to it even yet by
naturalists of note. Among the various possible solutions
only two have been generally considered. It has been
supposed that both kinds of sexual cells originally de-
veloped from the same primary germ-layer, either from the
skin-layer or the intestinal layer ; but almost as many and
as able observers have accepted the one as the origin as
the other. Quite recently the Belgian naturalist, Eduard
▼an Beueden, has asserted, on the contrary, that the ^g-ce^

39^ THE EVOLUTION OF MAN.

originate from the intestinal layer, the sperm-colls from the
skin-layer.^^^ In Gastraeads, Sponges, and Hydro-medusae
this appears really to be the case. The development of the
sexual differences, which is so rich in results, must, ac-
cordingly, have commenced even during the differentiation
of the two primary germ-layers in the simplest and lowest
^ant Animals ; the exoderm would be the male germ-layer,
the entoderm, the female. If this discovery of Van Beneden
is established and proves to be a universal law, Biology wiU
gain a most pregnant advance ; for not only would all the
contradictory empiric explanations be answered, but a new
path would be opened for philosophic reflection on one of
the most important of biogenetic processes.

K we now trace the Phylogeny of the sexual organs
in our earliest Metazoic ancestors further, as it is indicated,
at the present time, in the Comparative Anatomy and
Ontogeny of the lowest Worms and Plant Animals, we
note, as the first advance, the accumulation of the cells of
both sexes into definite groups. While in Sponges and
the lowest Hydra-Polyps single scattered cells separate from
the cell-layers of the two primary germ-layers, and become
isolated and free sexual cells, in the higher Plant Animals
and Worms we find these same cells associated and col-
lected into groups of aggregate cells, which are, hence-
forward, called " sexual glands," or " germ-glands " (gonades).
It is only now that we can speak of sexual organs in the
morphological sense. The female germ-glands which, as
such, in their simplest form constitute a mass of homo-
genous egg-cells, are the ovaries (ovaria, or oophora; Fig.
211, e, p. 198). The male germ-glands, which in their
joimitive form also consist merely of a mass of sperm-cellsj

DXYELOPMENT OF THE SEXUAL OmaJJKM. 599

the testes (testiculi, or orchides; Fig. 211, ^). We find
the ovaries and testes in this earliest and simplest shape
not only in many Worms (Annelida) and Plant Animals,
but also in the lowest Vertebrates, in the Skull-less Animals
(Acranid). In the anatomy of the Amphioxus we found the
ovaries of the female and the testes of the male consisting
of twenty to thirty elliptic or roundly four-cornered simple
sacs, of small size, attached to the inside of the gill-cavity
on each side of the intestine. (Cf. voL i. p. 425.)

Only a single pair of germ-glands, lying far down in the
floor of the body-cavity (Fig. 316, g), exist in all Skulled
Animals (Graniota). The first traces of these appear in the
coelom-epithelium. Probably, in this case also, the male
sperm-cells originate from the skin-layer, the female egg-
cells, on the contrary, from the intestinal layer. The earliest
traces are visible in the embryo at the point where the
skin-fibrous layer and the intestinal-fibrous layer meet in
the middle plate (mesentery-plate) (Fig. 318, mp, p. 408).
At this very important point in the ccelom-wall, where the
endocoelar (or visceral coelom-epithelium) merges into the
exocoelar (or parietal coelom-epithelium), in the embryo of
Man and the other Skulled Animals a small aggregation ol
cells becomes visible, at a very early period, and this, accord-
ing to Waldeyer,^^^ we may call the " germ-epithelium," or
(corresponding with the other plate-shaped rudiments of
organs) the sexual plate (Fig. 316, y ; Plate IV. Fig. o,k). The
cells of this germ-plate, or sexual plate (la/mella sexualis) are
essentially distinguished by their cylindrical form and by
their chemical constitution from the other cells of the
coelom ; they are of quite difierent significance from the flat
o«lk oi the ''seroiu coelom-epithelium" which line the

59

400

THE EVOLUTION OF MAN.

remainder of the body-cavity (coeloma). Of tliese latter —
the true ccjelom -cells — those which invest the intestinal
tube and the mesentery (" endocoelar ") originate from the

Fig. 316, — Transverse section . through the pelvic region and the hind
limbs of an embryo Chick in the fourth day of incubation, enlarged about
40 tirjies : h, horn-plate ; w, medullary tube ; n, canal of the medullary
tube ; u, primitive kidneys ; x, notochord ; e, hind limbs ; b, allantois canal
in ventral wall; t, aorta; v, cardinal veins'; a, intestine; d, intestinal-
glandular layer; /, intestinal-fibrous layer; g, germ-epithelium • r, 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 (" exocoelar ") are, on the contrary, the product
of the skin-fibrous layer (coloured blue in Fig. 5, Plate I\\) ;
but the sexual cells which make their appearance at the
boundary line between the two forms of coelom-cells, and

DIFFEBENTIATION OF THE SEXES. 4OI

which insert themselves, to a certain extent, between the
endocoelar and the exocoelar, 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-
coelar and the endocoelar) represents a primaeval and simple
hermaphrodite gland. (Cf. voL L 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 distingiiish 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,
Bo 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 conimon
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 Gastrseads,
Sponges, and other Hydro^ Polypes and Coral Animals) ;
in yet other cases, they enter the body-cavity and
pass out through a special aperture in the ventral wall
(poru8 genitalia). The latter is the case in many Worms
and even in a few lower Vertebrates (Cyclostoma and
a few Fishes). These indicate the earliest condition of
lids matter as it was in our ancestors. On the other
hand, in aU 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-cella out from the ovaries, and hence they have bees

SGO-BUGTS AND SPfiRM-DUCm. 403

called egg-ducts {oviductus, or tuhoe faZlopice). In the
male sex these tubes convey the sperm-cells from the testes,
and hence they are called sperm-ducts (spermddiLctua, 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 dticts" (ductus urogenitates).
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 foimd 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 (aystema uro-
poeticum) is one of the earliest and most important organ-
systems in the differentiated animal body, as has already

404 THE EVOLUTION OF MAK.

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 acoelomatous 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 ceUs, which
absorb useless juices from the tissues and discharge them
through an external skin-opening (Fig. 184, n/m). Not
only the free-living Gliding Worms (Turhellaria), 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 caUed
water-vessels. PhylogeneticaUy they must be regarded as
highly-developed pouch-like skin-glands resembling the
sweat-glands of Mammals, and, Mke these, developed from
the skin-sensory layer. (Cf. Fig. 210, n, 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
great number of segments, or metamera, a pair of these
primitive kidneys (hence known as segmental organs, or
canals) exists in each separate segment. In this case, also,
the canals are very simple tubes, which, on account of theii

THE PRIMITIVE KIDNKTH. 4O5

ooiled 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 {coeloma) 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 in the simplest form upon the inner
surface of the abdominal wall, pass, when mature, into the
coelom, 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. 162, S, vol. i. p. 423) ;
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 Cyclod-

40«

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 to
carry away the sexual cells. These cells pass directly from
the germ-glands into the coelom, and are discharged through
a posterior aperture in the abdomen. The condition of the
primitive kidneys in these is, however, very interesting, and

throws light on the complex kidney
structure of the higher Vertebrates.
In the first place, in the My^xi-
noides (Bdellostoma) we find a long
tube, the primitive kidney duct
(protureter, Fig. 317, a), on each
side. This opens internally into
the coelom through a ciliated funnel-
shaped aperture (as in Ringed
Worms) ; it opens externally through
an opening in the outer skin. A
oreat number of small horizontal
tubes (" segmental canals," or primi-

FiG. 317.—^. Portion of kidney of Bdel-
lostoma : a, primitive kioney duct (jprotu-
reter); h, segmental canals, or primitive
urine canals (tuhuli uriniferi) ; c, kidney,
vesicles (capsulae Malphigianoe'), — B. Por-
tion of the same, much enlarged : c, kidney.
vesicle, with the glomeruhia ; d, approaching
artery ; «, retreating artery. (After Johannes
Miiller.)

fcive urine tubes) open on its inner side. Each of these
terminates in a blind, vesicular capsule (c) enclosing a

THE PRBHTIYE KEDNET OF SKULLED ANIMALS. 4O7

knot of blood-vessels (glomervZua, an arterial net. Tig.
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 effererUia)
again carry it out of the glomerulus (e).

In Primitive Fishes (Selachii) 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). A part of this
organ forms a compact primitive kidney, while the rest is
employed in the formation of the sexual organs.

The primitive kidney in the embryo of Man and in that
of all other Skulled Animals (Craniota) is first formed in
the same simple shape which persists throughout life in
Myxinoides, and partly in Selachii. We found this primi-
tive organ in the human embryo at that early period just
succeeding the separation in the skin-sensory layer, of the
medullary tube from the horn-plate, and the differentiation,
in the skin-fibrous layer, of the notochord, the primitive
vertebral plate, and the skin-muscle plate. As the first
rudiment of the primordial kidneys, a long thin, thread-like
string of cells, which is soon hollowed out into a canal,
appears in this case, on each side, immediately below the
horn-plate ; this extends in a straight line from front to
back, and is plainly seen in the cross section of the embryo
(Fig. 318) in its original position in the space between the
horn-plate (A), the primitive vertebrae (uw), and the skin-
muscle plate (hpl). The first origin of this primitive
kidney duct is still a matter of dispute, some ontogenista

408 - THE EVOLCTTION OF MAN.

referring it to the horn-plate, others to the primitive ver-
tebral plate, and yet others to the skin-muscle plate. Pro-
bably its earliest (phylogenetic) origin is to be found in the
skin-sensory layer ; but it very soon quits its superficial

ch u>r ao sp <^<^ '^f

Fig. 318. — Transverse section through the embryo of a Chick, on the
second day of incubation : h, horn-plate ; m, medullary tube ; ung, primitive
kidney duct ; ch, notochord ; mv, primitive vertebral cord ; h'pl, skin-
fibrous layer ; df, intestinal-fibrous layer ; m/p, mesentery-plate, or middle
plate (point of attachment of the two fibrous layers) ; sp, body-cavity
(cosloma) ; ao, primitive aorta ; dd, intestinal-glandular layer. (After
Kolliker.)

position, passes inward, between the primitive vertebral
plates and the side plates, and finally lies upon the inner
surface of the body-cavity. (Cf Figs. 66-69, u, vol. i. p. 277,
^ and Figs. 95-98, p. 319; also Plate IV. Figs. 3-6, u.) While
the primitive kidney duct is thus making its way inward,
on its inner and under side appear a large number of small
horizontal tubes (Fig. 319, a), exactly corresponding to the
segmental canals of the Myxinoides (Fig. 317, h). Like the
latter, these are, probably, originally protuberances of the
primitive kidney ducts (Fig. 316, t^). At the blind, inner
end of each of the primitive urinary tubes an arterial
glomerulus is formed, which grows into this blind end
from within, forming a " vascular coil." The glomerulus
t(j a certain extent expands the bladder-like blind end
of the small urinary tubes. As the primitive urinary tubes,

RUDIMENTARY PRIMITIVE KIDNEYS.

409

which are, at first, very short, grow longer and broader,
each of the two primitive kidneys assumes the form of a
semi-pinnate leaf (Fig. 320)ii fThe urinary tubes (u) repre-

FiG. 319. — Rudimentary primitive kidney of embryonic Dbg. The pos-
terior portion of the body of the embryo is seen from the ventral side,
covered by the intestinal layer of the yelk-sac, which has been torn away,
and thrown back in front in order to show the primitive kidney ducts with
the primitive kidney tubes (a) : h, primitive vertebrae ; c, dorsal medulla ;
d, passage into the pelvic intestinal cavity. (After Bischoff.)

Fig. 320. — -Primitive kidney of a human embryo : n, the urine-tubes of
the primitive kidney ; iv, Wolffian duct ; iv', upper end of the latter (Mor-
gagni's hydatid) ; m, Miillenan duct ; m', upper end of the latter (Fallopian
hydatid) ; g, hermaphrodite gland. (After Kobelt.) ,

sent the tissue and the primitive kidney duct (tu) the
mid-rib. On the inner margin of the primitive kidney the
rudiment of the hermaphrodite sexual gland already

410 THE EVOLUTION OF MAK.

appears as a body of considerable size. The posterior end
of the primitive kidney duct opens into the lower extremity
of the last section of the rectum, so that this organ becomes
a cloaca But this opening of the primitive kidney duct
into the intestinal canal must be regarded, phylogenetically,
as a secondary condition. Originally, as is indicated clearly
in the Cyclostoma, they issued through the external abdo-
minal skin, quite independently of the intestinal canal, thus
proving their early phylogenetic origin from the horn-plate,
as outer skin glands.

While in the Myxinoides the primitive kidneys per-
manently retain this simple form, as they do partially in
Primitive Fishes (Selachii), in all other Craniota it appears
only temporally in the embryo, as the ontogenetic repro-
duction of the primordial phylogenetic condition. In these
Skulled Animals the primitive kidney, by vigorous growth,
increases in length, and by the increase in number and the
coiling of the urinary tubes, very soon assumes the form of
a large compact gland, of oblong, oval, or spindle-shaped
form, which extends longitudinally through the greater
part of the body-cavity {coeloraa) of the embryo (Figs. 123,m,
124,m, vol. i. p. 370). In this case, it lies near the middle line,
directly under the primitive vertebral column, and extends
from the region of the heart to the cloaca. The right and
left primitive kidneys lie parallel and close together, being
separated only by the mesentery, that narrow, thin lamella
which connects the central intestine with the lower surface
of the primitive vertebral column. The excretory duct of
each primitive kidney, the protureter, traverses the lower
and outer side of the gland in a posterior direction, and
opens into the cloaca, dose to the root of the aUantois ; at

WOLFFIAN BODIES. 4 II

a later period, it opens into the allantois itself (Fig. 136, o.
voL i. p. 381). '

The primitive kidney (primordial kidney) in the embr}''o
of Amniota was formerly called the " Wolffian body," also
the " Okenian body." In all cases it acts for a time as a
true kidney, draining and secreting the useless fluids of the
embryonic body, and discharging them into the cloaca and
then into the allantois. The " primitive urine " collects in
the latter organ, and hence the allantois in the embryo of
man and of the other Amniota acts as a real urinary bladder,
or " primitive urinary sac ; " yet it is in no way geneti-
cally connected with the primitive kidneys, but is rather,
as we have already seen, a pouch-like protuberance of the an-
terior wall of the terminal intestine (Fig. 135, u, vol. i. p. 380),
The allantois is, therefore, a product of the intestinal layer,
while the primitive kidneys are a product of the skin-
layer. Phylogenetically we must conceive that the allan-
tois originated as a pouch-shaped protuberance of the
cloacal wall resulting from the distension caused by the
collection in the cloaca of the primitive urine secreted
by the primordial kidneys. It is, originally, a blind sac
belonging to the rectum (Plate V. Fig. 15, Kb). The true
urinary bladder of Vertebrates, evidently, first appeared in
Dipneusta (in the Lepidosiren), and was thence transmitted,
first to the Amphibia, and then to the Amniota. In the
embryo of the latter it protrudes far out of the yet unclose<l
abdominal wall. Many Fishes, indeed, also possess a so-
called urinary bladder. But this is merely a local disten-
sion in the lower section of the primitive kidney ducts.
and hence, both in origin and in constitution, is essentially
distinct from the true urinary bladder. The two structures

413 THE ETOLUTION OF MAN.

are only physiologically comparable; they are, therefore,
analogous, as having the same function; morphologically,
however, they are not to be compared, or are not homo-
logous.^® The false urinary bladder in Fishes is a pro-
duct of the primitive kidney duct, therefore of the skin-
layer; the true urinary bladder in Dipneusta, Amphi-
bia, and Amniota is, on the contrary, a blind-sac of the
terminal intestine, and hence a product of the intestinal
layer.

In all low Skulled Animals {Graniota), without amnion
(in Cyclostoma, Fishes, Dipneusta, and Amphibia), the
urinary organs remain in an inferior stage of development,
in so far as the primitive kidneys (protonepkra), though
much modified, here act permanently as urine-secreting
glands. In the three higher vertebrate classes, included in
the term Amnion Animals, on the contrary, this is the case
only for a short period during early embryonic life. The
permanent, or secondary kidneys (renes, or metanephra),
which are peculiar to these three classes, are very early
developed. These originate, not (as was long believed, on
the authority of Remak) as entirely new, independent
glands of the intestinal tube, but from the posterior section
of the primitive kidney duct (protureter). From the latter,
near where it opens into the cloaca, a simple pouch — the
secondary kidney duct — grows out, and this increases con-
siderably in length forwards; from the blind, upper, or
anterior portion of this the permanent kidney originates,
precisely as the primitive kidney originates from the pri-
mitive kidney duct. The secondary kidney duct gives rise
to a number of small blind tubes — the secondary urinary
tabes — and the blind capsule-shaped ends of these

THK SECONDARY KIDNETSw 413

are occupied by vascular coils {glomervli). The further
growth of these tubes results in the compact secondary
kidney, which, in Man and most higher Mammals, acquires
the well-known bean-like form ; in the lower Mammalia,
in Birds and in Reptiles, on the other hand, it is separated
into several lobes. The lower, or posterior part of the
permanent kidney duct retains the form of a simple canal,
widens, and thus forms the permanent urine duct {ureter).
At first this canal, yet united with the last section of the
primitive kidney duct, discharges into the cloaca; at a
later period, it separates from the primitive kidney duct,
and yet later from the rectum, and then it discharges into
the permanent urinary bladder {vesica urinaria). The
latter originates from the posterior, or lower part of the
stalk of the allantois (urachus), which widens and becomes
spindle-shaped before opening into the cloaca. The anterior,
or upper part of the allantois-stalk, which passes in the
abdominal wall of the embryo to the navel, afterwards
disappears, a useless cord-shaped remnant alone remaining
as a rudimentary organ : this is the single urinary-bladder
navel-cord (ligamentwfn vesico-v/mbilicale mediwm). On
the right and left of this, in the adult Man, there are two
other rudimentary organs : the lateral urinary-bladder navel-
cords (ligamenta vesico-umhilicalia lateralia). These are
the obsolete cord-Kke remnant of the former navel-arteries
{arterioB vmibilicales, vol. i. p. 400 ; Fig. 326, a).

Although in Man, as in all other Amnion Animals, the
primitive kidneys are thus very early displaced by the
secondary kidneys, and although the latter alone afterwards
act as urinary organs, the former are not, however, alto-
gether discarded. Indeed, the primitive kidney ducts acquii e

414

THE EVOLUTION OF MAN.

a high physiological significance, as they modify into
cretory ducts of the sexual glands. In all Amphirhina or
Gnathostomi — therefore in all Vertebrates from Fishes up
to Man — at a very early period, a second similar canal
appears in the embryo at the side of each primitive kidney
duct. This canal is commonly called, after its discoverer,
Johannes Muller, " Miiller's duct " (ductus Mulleri), while
the earlier, primitive kidney duct is distinguished as the
"Wolffian duct" {ductus Wolfii). The actual origin of
Miiller's duct is still undetermined ; Comparative Anatomy
and Ontogeny seem, however, to indicate that it proceeds
by differentiation from the Wolffian duct. It is, probably,
most correct to say, that the original (primary) primitive
V kidney duct breaks up by difierentiation (or fission) into
two secondary, similar ducts; these are the Wolffian and

Jhtk. 321. — Primitive kidneya and rudiments of the sexnal orgam. A and
B, of Amphibia (Frog larvae) ; A, earlier, B, later condition. 0, of a Mam-
mal (embryo of Ox) : u, primitive kidneys ; k, sexual glands (rudiments of
testes and ovaries). The primary primitive kidney duct (ug in Fig. A)
separates (in B and C) into the two secondary primitive kidney ducts ; the
Miillerian duct (») and the Wolffian duct (u9')> which unite behind into a
genital oord (gr) ; I, groin-oord of the primitive kidneys. (Aftar Qegenbaor.)

DEVELOPMENT OF THE WOLFFIAN DUCTa

415

the Mullerian ducts. The latter (Fig. 320, w) lies imme-
diately inside the former (Fig. 320 m). Both open pos-
teriorly into the cloaca.

Obscure and uncertain as
is the origin of the Mullerian
and Wolffian ducts, their later
history is clear and definite.
In all Double-nostrilled (^m-
phirhina) and Jaw-moutbed
(Ghuithostoini) animals, from
Primitive Fishes up to Man,
the Wolffian duct becomes the
seed-duct, and the Miillerian
duct, the oviduct. In each
sex only one of these is per-

Fios. 322, 323. — Urinary and sexnal
organs of an Amphibian (Water- Newt,
or Triton). Fig. 322 (A), female;
Fig. 323 (B), male : r, primitire kid-
ney ; ov, ovary ; od, egg -dnct and
Rathke'e duct, both formed from the
Mullerian duct ; u, primitive urinary
duct — acting, in man, also as seed-
duot (v«) — opening below into WolfFs
duct (tt') ; ms, ovary-mesentery {ma-
ova/rwm). (After Gegenbaur.)

sistent; the other entirely disappears, or leaves only a
remnant as a rudimentary organ. In the male sex, in
which the two Wolffian ducts become sperm-ducts, certain
rudiments of the Mullerian duct are often found, which
we will caU " Rathke's canals " (Fig. 323, c). In the female
«ex, where, on the contrary, the two Miillerian ducts
60

4i6

THE EVOLUTION OF MAN.

become oviducts, traces of the Wolffian ducts remain, and
are known as " Gartner's canals."

'iiri"Ti'"«r,«i,'^)^''^"iv

3.

FlOS. 324-326. — Urinary and sexnal organs of an embryonic Ox. Fig.
324, of female embryo of 1^ inch in length ; Pig. 325, of male embryo
of 24 inches in length ; Fig. 326, of female embryo of 2^ inches in length :
w, primitive kidney ; wg, Wolff's duct ; n», Miiller's duct ; m', upper end of
he latter (opened at <)> *> lower thickened end of the same (rudiment
of otenu) ; g, genital cord j h, testes {h', lower, h", upper testis-oord) ;
0, orary ; 0', lower ovary-cord ; t, groin-cord of the primitive kidney ;
d, diaphragm>oord of the primitive kidney ; n, permanent kidneys (below
these the S-shaped urine-duct ; between the two the reotmn) ; v, urine-
bladd«r ; a, navel-artery. (After KSlliker.)

The most interesting facts in reference to this remark-
able development of the primitive kidney ducts and their
anion with the sexual glands are exhibited in Amphibia
(Figs. 321-323). The first rudiment of the primitive kidney
ducts and their differentiation into the Miillerian and

DEVELOPMENT OF THE HUKAN KIDNKT. 417

Wolffian ducts is identical in both sexes, as is the case in
the embryos of Mammals (Fig. 321, G, Fig. 324). In the
female Amphibia the Miillerian duct on each side develops
into a large ovary (Fig. 322, od), while the Wolffian duct acts
permanently as a urinaiy duct {n). In the male, on the
contrary, the Miillerian duct persists only as a rudimentary
organ, without functional significance, as Rathke's canal
(Fig. 323, c) ; the Wolffian duct serves, in this case also, as a
orinary duct, but also as a sperm or seed duct, the seminal
tubes (ye) from the testes (t) entering the upper part of the
primitive kidneys, and there uniting with the urinary canals.

In Mammals these conditions, persistent in Amphibia, are
rapidly traversed by the embryo in an early period of its
development (Fig. 321, G). The primitive kidneys, which
in non-amnionate Vertebrates persist throughout life as the
urine-secretory organ, are superseded by the secondary
kidneys. The actual primitive kidneys disappear almost
entirely in the embryo at an early period, leaving but small
traces. In the male Mammal the supplementary testis
(epididymis) develops from the upper part of the primitive
kidney ; in the female the same part gives rise to a useless
rudimentary organ, the supplementary ovary (parovarvu/m)

In the female Mammal the Miillerian ducts undergo very
considerable changes. The actual ovaries develop only from
its upper part ; the lower part widens out into a spindle-
shaped pouch, with a thick, fleshy wall, within which the
fertilized egg develops into the embryo. This pouch is the
womb (uterus). At first the two uteri are perfectly
separate, and open on each side of the urine-bladder (vu)
into the cloaca, as is yet permanently the case in the
lowest living Mammals, the Beaked Anima.1s (Omithosftcma) ;

4i8

THE EVOLUTION OF MAN.

but even in Pouched Animals (Marsupialia) sl connection
forms between the two Mullerian ducts, and in Placental
Animals they coalesce below with the rudimentary Wolffian

ducts, forming with them a single
"sexual cord" (funiculus geni'
talis).- But the original indepen-
dence of the two parts of the
uterus, and of the two vagina
canals which proceed out of their
lower extremities, persists in many
lower Placental Animals, while in
the higher members of the same
group, these organs gradually
coalesce to form one single organ.

Fig. 327.— Female sexual The process of COaleSCence ad-
organs of a Beaked Animal vances steadily from below (or

(Omithorhynchus, Figs. 195, i i • i\ i / n

196): 0, ovaries; t, oviduct; ^om behmd) upwards (or for-
u, uterus ; sug, urinary sexual wards). While in many Gnawing
cavity {sinus urogenitalis); the j^^;^^^^^^ (Rodeutia, e.g., Hares and

two parts of the uterus open ^ ^ ' u ^

into this at u' : ci, cloaca. Squirrels) two separate uteri open
(After Gegenbaur.) jj^^^ |]-^g vagina canal which has

already become simple, in other Gnawing Animals, as also
in Beasts of Prey, Whales, and Hoofed Animals {Ungulata),
the lower halves of the two uteri are already coalescent,
their upper halves (the so-called horns, "cornua") remaining
distinct (" uterus hicornis "). In Bats and Semi-apes these
upper horns are very short, while the unified lower part
becomes longer. Finally, in Apes, as in Man, th^ cohesion
of the two parts is complete, one simple pear-shaped uterus-
pouch alone remaining, and into this the oviducts open on
^ach side,

POSITION OF THE HUMAN SEXUAL onOANft. 4t^

In the male Mammal also, a similar coalescence of the
lower portion of the Mullerian and Wolffian ducts takes
place. In this case also, these ducts form a single " sexual
cord " (Fig. 325, g), which likewise opens into the original
urinary sexual cavity (sinus urogenitalis), which develops
from the lower part of the urinary bladder (v). While,
however, in the male Mammal the Wolffian ducts develop
into the permanent sperm-ducts, only very slight traces of
the Miillerian ducts remain as rudimentary organs. The
most remarkable of these is the " male uterus " (tUerus
masculinus), which originates from the lowest, coalescent
portion of the Miillerian ducts, and which is homologous
with the female uterus. It forms a small flask-shaped
vesicle, entirely without physiological significance, which
opens into the urinary tubes between the two sperm-ducts
and the prostatic lobes (yesicula prostatica).

The internal sexual organs in Mammals undergo very
peculiar modifications in point of position. At first the
germ-glands, in both sexes, lie deep down in the ventral
cavity, on the inner side of the primitive kidneys (Figs.
320, g, 321, k), attached to the vertebral column by a short
mesentery (in the male, the mesorchium ; in the female,
viesovarium). It is only, however, in Monotremes that this
original position of the germ-glands is (as in lower Verte-
tebrates) permanent. In all other Mammals (Marsupials as
well as Placentals) these glands quit their place of origin
and make their way more or less downward (or towards the
posterior extremity), following the course of a cord which
extends from the primitive kidney to the groin region of
the abdominal waU. This is the groin-cord of the primitive
kidney; in the male, the "Hunterian guiding-cord" (^vher-

420

THE EVOLUTION OF MAN,

naculum testis) (Fig. 328, M, gh) ; in the female, the round
uterus-cord (Fig. 328, F,t). In the latter the ovaries
migrate more or less in the direction of the small pelvis, or

^A..

. 9h

n- A

Fig. 328, M.

Fig. 328, F.

Fig. 328. — Original position of the sexual glands in the abdominal cavity
of the human embryo (of three months). Fig. 328, M, male (natural size) :
h, testis ; gh, the conducting-cord of the testis ; wg, seed-duct ; h, urinary
bladder ; uh, lower hollow vein (vena ca/od) ; nn, supplementary kidneys ;
n, kidneys. Fig. 328 F, female (somewhat enlarged) : r, I'ound uterus-cord
(below this the urine-bladder, above it the ovary) ; r', kidney ; s, sup-
plementary kidney ; c, blind-intestine {caecum) ; o, small net ; om, large
net (between the two is the stomach) ; Z, spleen. (After Kolliker.)

even enter this. In the male the testis quits the abdominaj
cavity altogether, passing through the groin-canal, and
enters a sac-shaped, distended fold of the external skin-
covering. The coalescence of the right and left folds
(" sexual folds ") gives rise to the testis-sac {scrotum).. The
various Mammals exhibit the various stages of this migra-
tion. In the Elephant and in Whales the testes descend
very little, and lie below the kidneys. In many Gnawing-
Animals (Rodentia) and Beasts of Prey (Carnaria) they
enter the groin-canal. In most higher Mammals they pass
down through this into the testis-sac ; usually the walls of

EXTERNAL SEXUAL OROAWfik 421

the groin-canal coalesce. When, however, this remains
open, the testes are able to descend periodically (in the rutting
season) into the testis-sac, returning again into the abdo-
minal cavity {e.g., in Pouched Animals or Marsupialia,
Gnawing Animals, Bats, etc.).

Another peculiarity of Mammals is the formation of the
external sexual organs which, as copulative organs, serve
to carry the fertilizing sperm from the male into the
female organism in the act of copulation. Organs of this
sort are altogether wanting in most lower Vertebrates. In
those which are aquatic (e.g., Acrania, Cyclostoma, and most
Fishes) the eggs and sperm are simply discharged into the
water, and their coming together is the result of some lucky
accident which in this way brings about impregnation. On
the other hand, in many Fishes and Amphibia which bring
forth their young alive, there is a direct transfer of the
sperm from the male to the female orgauism ; and this is
the case in all Amniota (Reptiles, Birds, and Mammals). In
these animals the urinary and genital organs always open
originally into the lower part of the rectum, which thus
forms a " cloaca " (p. 345) ; but among Mammals the cloaca
is permanent only in the Beaked Animals (Ornithostoma) ,
which have, on this account, been called Cloacal Animals
{Monotrema, Fig. 327, cl). In all other Mammals a lateral
partition wall develops in the cloaca (in the human embryo
about the middle of the third month), by which the latter
is separated into two cavities The urinary sexual canal
passes into the anterior cavity (sinus urogenitalis), and it
is through this cavity alone that the urinary and sexual
products are discharged, while the *' anal cavity," which lies
behind it, serves merely to eject the excrement through the

422

THE EVOLUTION OF MAN.

anus. Even before the appearance of this partition, in the
Pouched Animals (Marsupialia) and Placental Animals, a
conical papilla — the sexual protuberance (phallus, Fig. 329,
A,e, JB, e) — rises on the anterior part of the circumference of

^- Ji

i.

1^1

B.

Fig. 329. — External sexual organs of the human embryo : A, neutral
germ (in the eighth week ; twice the natural size ; with cloaca) ; B, neutral
germ (in the ninth week ; twice the natural size ; anus distinct from the
urogenital opening) ; C, female germ in the eleventh week ; D, male germ
in the fourteenth week ; e, sexual protuberance (phallus') ; /, sexual furrow ;
hi, sexual folds ; r, Raphe (point of union of the penis and scrotum) ;
a, anus; ug, urinary sexual opening; n, navel-cord; s, tail. (After Ecker.)
Cf. Table XLIV., p. 431.

the cloaca-opening. The apex of this is swollen into a knob
(the " acorn," glans). On the under side appears a furrow
(sulcus genitalis, f), and on each side of the latter a skin-
fold, or sexual fold (hi). The phallus is especially the organ
of the " sexual sense," and over it are distributed the sexual

FALSE HERMAPHRODITISM. 423

nerves (nervi pudendi) which are especially concerned in
producing the sexual sensations (p. 238). In the male the
phallus develops into the masculine "penis" (Fig. 329, D, «);
in the female it becomes the much smaller " clitoris " (Fig.
329, 0, e) ; only in some Apes {Ateles) does this become im-
usually large. The " fore-skin " (prcBputiv/m), in both
sexes, also develops as a skin-fold from the anterior part of
the circumference of the phallus. In the male sexual furrow
the lower side of the phaUus receives the urogenital
canal, and, as a continuation of the latter, modifies, by the
coalescence of its two parallel edges, into a closed canal —
the male urinary tube (urethra). In the female this occurs
only in a few instances (in some Semi-apes, Gnawing Animals
or Rodentia, and Moles) ; as a rule the sexual furrow remains
open and its edges are developed into the labia minora.
The labia majora of the female develops from the two
parallel skin-folds which appear on each side of the sexual
furrow. In the male these last folds coalesce, forming a
closed sac, the testis-sac (scrotum). Occasionally this
coalescence does not take place, and the sexual furrow also
sometimes remains open (hypospadia). In these cases the
external male genitalia resemble the female, and this phe-
nomenon has often been mistaken for hermaphroditism
(pseudo-hermaphroditism).^^'

From this and other cases of false "hermaphroditism,"
the much less frequent cases of "true hermaphroditism" are
very distinct. This exists only when the essential organs of
reproduction, both kinds of germ-glands, are united in one
individual. Either an ovary is then developed on the right,
and a testis on the left (or vice versa) ; or testes and ovaries
are developed on both sides, one more, the other less

424 THE EVOLUTION OF MAN.

perfectly. As we have already seen that the original
rudiment of the sexual organs is really hermaphroditic in
all Vertebrates, and that the separation of the sexes is only
due to a one-sided development of this hermaphroditic
rudiment, these remarkable cases offer no theoretic diffi-
culties. They very seldom, however, occur in Man and the
higher Vertebrates. On the other hand, we find original
hermaphroditism constant in some lower Vertebrates, as in
some Fishes of the Perch kind (Serranus), and in some
Amphibia {Bombinator and in Toads). In these cases, the
male has usually a rudimentary ovary at the upper ex-
tremity of the testis ; on the other hand, the female has
sometimes a rudimentary testis, without function. This
also occurs occasionally in Carp and some other Fishes.
We have already seen how the original hermaphroditism
is maintained in the excretory ducts, in Amphibia.

In the germ-history of the human urinary and sexual
organs, the outlines of the history of human descent have
been faithfully maintained up to the present time. We can
trace their development in the human embryo step by step
in the same gradations as are exhibited, one after another,
in the comparison of the urogenitals in Acrania, Cyclostomi,
Fishes, Amphibians, and then further, in the series of
Mammals, in Cloacal Animals {Monotr ernes). Pouched
Animals (Marsupialia), and the various Placental Animals.
(Cf Table XLIII.) Ail the structural peculiarities of the
urogenitals, distinguishing Mammals from other Verte-
brates, are also present in Man ; and in all special charac-
teristics the latter resembles the Apes, and especially the
Anthropoid Apes. As eviaence that the special peculiarities
of Mammals have been transmitted to Man, I will finally

FORMATION OF ♦THE EOOa 42$

briefly notice the similar manner in which the eggs are
formed in the ovary. In all Mammals, the mature eggs are
contained in peculiar vesicles, which, after their discoverer,
Regner De Graaf (1677), are called the Graafian follicles.
These were formerly regarded as the actual eggs, which
were, however, discovered by Baer within the Graafian
foUicles (vol. i. p. 55). Each foUicle (Fig. 330, G) consists of a
round, fibrous capsule, which contains fluid and is coated by
several layers of cells. At one point this cellular layer has
a knob-like enlargement (C, 6), and, there, surrounds the
real Qgg {C, a). The mammalian ovary is, originally, a very
simple oblong Kttle body (Fig. 320, g), formed only of
connective tissue, and blood-vessels, and surrounded by a
cell-layer (the epithelium of the ovary, or the female germ-
epithelium). From this epithelium, cords of cells grow
inward, into the connective tissue or " stroma " of the
ovary (Fig. 330, A, h). "Single cells of these cords increase
in size and become egg-cells (primitive eggs. A, c); but the
Laeater number of the cells remain small and form an
enveloping and nutritive cellular layer (the follicle-epi-
thelium) round each egg.

In Mammals the follicle-epithelium is at first one-
layered (Fig. 330, B, 1), afterwards many-layered (5, 2). In
all other Vertebrates, the egg-ceU is, indeed, enclosed in a
permanent covering of small cells, an egg-follicle ; but only
in Mammals does fluid accumulate between the growing
follicle-cells, and thus extends the follicle into a round
bladder of considerable size, on the inner wall of which
the egg lies excentrically. In this point, as in his whole
Morphology, Man unmistakably indicates his descen+j firom
Mammal&

426

THE EVOLUTION OF ]\LA.N.

Fig. 330, B.

Fig. 330, C.

HISTORICAL IMPORTANCE OF THE SEXUAL ORGANS. 427

Fig. 880. — Derelopment of human ovules within the female orary. — A.
Vertical section through the ovary of a new-bom female : a, epithelium of
the ovary ; h, rudiment of an egg-cord ; c, young eggs in the epithelium ;
d, longer egg-cord with the follicles ; e, group of young follicles ; /, single
young follicle ; g, blood-vessels in the connective tissue (stroma) of the
ovary. In the cords the young primitive eggs can be distinguished from
the Burronnding cells of the follicle by their relatively large size. (After
Waldeyer). — 330, B. Two young follicles isolated;* in 1, the cells of the
follicle form but a single layer around the young primitive egg ; in 2, they
torai a double layer j in 2, they begin to form the primary chorion (a), or
the «ona pelUtcida (vol. i. p. 135) . — 330, C. A mature human Graafian follicle :
a, the mature egg ; h, the surrounding follicle-cells ; c, the epithelial cells of
the follicle ; d, the fibrous membrane of the follicle ; «, its outer surface.

The entire natural history of the human sexual organs
is one of the branches of Anthropology which affords the
strongest proofs of the origin of the human race from the
animal kingdom. Each man, on knowing the pertinent
facts, and without prejudice, judging these comparatively,
can but be convinced that he is descended from lower
Vertebrates. The general, and the more minute structure,
the activity and the individual evolution of the sexual *
organs, is exactly the same in Man as in Apes. This is as
true of the male as of the female, of the internal as of the
external genitalia. The differences in this matter between
Man and the most man-like Apes are far less than the
differences between the various forms of Apes. As, how-
ever, all Apes are undoubtedly from a common origin, this
fact alone proves, with absolute certainty, the descent of
Man from Apes.

( 428 )

TABLE XLIII.

Ststkmatio BtmvBT of the most Important Periods ih thb Prtloobn)
OF THE Urinary and Sexual Organs of Man.^*'

XLin. A. First main diTision : the BexuiU organs (G) and the arinary
organs (U) are distinct. (The seznal or genital system (G) and th«
excretory or urinary system act independently of each other.)

I. First Period : OenitaU a/ad Kidneys of OastrcBads.

Q. Single, scattered cells of the entoderm change into egg-oeHs ; single,
scattered cells of the exoderm into sperm-oells.

U. Special arinary organs are as yet wholly waiting. Seoretrai i»
performed by the cells of the exoderm.

IL Second Period : Genitals and Kidmeys of Primitive Worms,

G. The egg-cells of the entoderm gather into groups (ovary.plates) ; as
do the sperm-cells of the exoderm (testis. plates).

U. A pair of simple pouch-like skin-glands (products of the skin-sensory
layer) develop into extremely simple kidney-canals (excretory organs of the
Plat-worms, Platelminthes).

III. Third Period : Genitals and Kidneys of Scotedda, *"

Q. After the differentiation of the four secondary germ-layers is com.
plete, the egg-cells pass from the skin-sensory layer into the skin-fibrous
layer ; the sperm-cells also pass from the intestinal-glandular layer into the
intestinal -fibrous layer.

U. After the formation of the coelom is completed, the blind inner ends
of the two kidney •canals (or *' primitive kidney ducts ") open into the body-
cavity (eoslotna).

rV. Fourth Period : Genitals amd Kidm.eys of Chordonia.

0. The groups of egg-cells (ovarial plates) and the groups of sperm-ceHi
(testes-plates) meet at the boundary between the endocoelar (the visceral
intostinal-fibrous layer of the coelom-epithelium) and the exocoelar (the
parietal skin-fibrous layer of the coelom-epithelium), so as to form the
hermaphrodite glands.

T7. The primitive kidney ducts differentiate into an excretory and •
glandular pari.

V. Fifth Period : Genitals a/nd Kidneys of Aerania.

O. The sexes become distinct. In the female, only the ovary m de-
raloped ; in the male, only the testes.

XJ. The primitive kidney ducts remaan simple (atrophied in Atnphioausy

PHYLOGENY OF URINARY AND SEXUAL OROANa 429

TI. Sixth Period : Oenitals amd Kidney $ of Cyclostom:

G.. The seraal glands (nomerons in Acrania) coalesce into a p&ir.

U. The primitive kidney ducts send out lateral branches which acquire
rascular ooils (glomeruli) (the semi-pinnate primitiye kidneys of BdeUo-
stomtk).

XLIII. B. Second main diyision : the genital organs (G) and tiie vrixuurj
organs (U) become united. (The sexual system and the urinary system
are united in the " urogenital system.")

VII. Seventh Period : Urogenitals of Primitive Pishes (SeUiehii).

The primary primitive kidney duct differentiates on each side, f(wrming
two secondary canals ; the Wolffian duct, which develops into the seed-duct,
and the Miillerian duct, which develops into the oviduct. Both genital
doots originally open behind the anus (JProselachii).

VIII. Eighth Period : Urogenitals of Dipneusta,

A cloaca is formed by the union of the urogenital opening and the cavity
of the anus. The single urinary bladder grows out from the anterior wall of
the rectum (Lepidosiren).

IX. Ninth Period : Urogenitals of Amphibia,

Prom the uppermost part of the primitive kidney which is in process of
atrophy, proceeds, in the male sex, the supplementary testis ; in the female
sex, the supplementary ovary. The Wolffian duct yet acts, in both sexes, an
a urinary canal, and, in the male, also as the seed-duct. The MiilleriaD
duct acts in the female sex as oviduct ; in the male it is a radimentar^
organ (Bathke's duct).

X. Tenth Period : Urogenitals of ProtamiUm.

Hie atrophied primitive kidney is replaced by the permanent seoondary
kidney as the urinary organ. The urinary bladder grows out from the
ventral orifice of the embryo and fuM-ms the allantois. From the anterior
wall of the cloaca grows the sexual protuberance (phaUiks), whlok, in the
male, develops to the penis, in the female, to the clitoriB.

I

XI. Eleventh Period : Urogenitals of MonoWemse*

Tbe lower end of the oviduct enlarges on each side to % mafcnlar
atema.

XII. Twelfth Period : Urogenitals of lfar«i(piaZ««.

The cloaca is separated by a partition into an anteri(»: urogenitftl opening
%pertura Mrogenitalis) and a posterior anal opening (onta). Fron the

430 THE EVOLUTION OP MAW.

low«r part of the nteras the ragina. canal passes ont on each tide. Ae
oyarieB and testes begin to move downward from their place of formation.

XIII. Thirteenth Period : Urogenitals of 8em%.a/pe»,

The lower parts of the Miillerian and the Wolffian dncts ooalesee into
a sexual cord. The coalescence of the two uteri at the lower part gives
riie to the uterus hicomia. A part of the allantois becomes the plaoenta.

XIV. Fourteenth Period : Urogenitals of Apes.

The two uteri coalesce throughout their entire length, forming a single
pear-shaped uterus, as in Man.

A

( 431 )

TABLE XLIV.

Sjatematic Sorrej of the Homologies of the Sexual OrgaoB in the tyro 8«zea

of Mammals.

XLIV. A. Homologies op the Internal Sexual OKOAKt.

9. CiNMmon Rtidimentt of tk»
iHUmai StxiMl Organt.

M. JnUmdl Male Parta.

F. Interttcu Ikatait PmrU.

1.

Male germ -gland (testes-plate

1. Testis

1. Rndimentary testlB dis-

in the •mbryo, product of the

(TettU, or OrchU)

appears,— remaina in

•kin-layer ?)

some AmphlbiA

i.

Female germ-gland (ovary-plate,

2. (Rudimentary ovary, dis-

2. Ovary

product of the intestinal

appears, — remains in

(Ovarium, or Oopkoron)

layer ?)

some Amphibia)

5.

WolfBan duct (lateral primitive

3. Seed-duct

3. Oartnerlan dxict (rndi-

kidney duct)

( S[>e rmadiLctuM)

mentary eaoal)

* a. Miillerian duct

4*. Kathke'8 duct (rodi-

4 a. Oviduct

(Ductus MiilUri, central pri-

mentary canal io

(Oviductut, n M>a

mitive kidney duct)

Amphibia)

Fallopice)

4 6. Upper part of the Mailerlan

4 h. Hydatit Morgagni

4 b. Hydatit FaM«f%a

4 c. Lower part •f the Miillerian

4 e. rterut maicuXxnut

4 e. Utenu, sheath (vo^tna)

duct

( Vesicula proitatica)

K.

Remnant of the primitive kidney

5. Supplementary teste*

6. Supplementary ovary

(Protonephron, corput Wolf-

(EpididymU)

(Parovarium)

6.

JIX)

Groin ligament of the primitive

%. Honterian guiding-cord

9. Bound utenu-oord

kidney

CGubemacuium S*m-

(Ligamentum uteri

{Ligamentymk protonephroiW'

Uri)

rotundum)

guinale)

7.

Sexual mesentery

7. Testis-mesentery

7. Ovary -mesoitery

(JiesenUrium sexudU)

(^Mesorchium)

(Jtet9varivm)

XLIV. B. Homologies oi the External Sexual Oboans.

9. Otmmon Rudiments of the
WKtemal Sexual Organs.

M. External Male Parts.

F. External Female FturU.

t. Sexual protaberaooe
(PhaUus)

t. Fore-skin

8. Penis

t. Male fore-skin

t. OitOTii

9. Female fore-sUn

(Prceputiutm)

10. Sexual folds

(Plica genitaUs')

11. FlMure between tne two sexual

(Praputium penit)

10. Testes-sac

(Scrotum)

11. Seam of the testis-sae

(Prceputium ditoridis)
19. I^iapudendiwu^joret

11. FeaiAle FkIm

folds
12. Sexual edges (edges «f the
sexual furrow)

(Raphe scroti)
12. Edges of the sexual
furrow coalesce

12. Labia pudendi m imer%

la. Urogenital canal

(Sinus urogenitalis)
14. Qlandular appendages of Um

13. Ureters

(Urethra)

14. Cowper's glands

13.Antechamber of the vac&u

( \est\bul%im vagimm)
U. Bartboli's gl«B^

61

CHAPTER XXVI.

RESULTS OF ANTHROPOGENY.

Review of the Germ-history as given. — Its Explanation by the Fundamental
Law of Biogeny. — Its Cansed Rel&tion to the History of the Tribe. —
Rudimentary Organs of Man. — Dysteleology, or the Doctrine of Pur-
poselessness. — 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 (Cocctw). — Mind in Man and in Ape. — The Organ of Mentui
Activity : the Central Nervous System. — The Ontogeny and Phy.
logenj of the Mind. — The Monistic and Dualistic Theories of the
Mind. — Heredity of the Mind. — Bearing of the Fundamental Law of
Biogeny on Psychology. — Influence of Anthropogeny on the Victory of
the Monistic Philosophy and the Defeat of the Dualistic. — Nature and
Spirit. — Natural Science and Spiritual Science. — Conception of the
World reformed by Anthropogeny.

" The Theory of Descent is a general inductive law which results with
absolute necessity from the comparative synthesis of all the phenomena of
organio nature, and especially from the threefold parallel of phylogenetio,
ontogenetic, and systematic evolution. The doctrine that man has de-
veloped from lower Vertebrates, and immedately from genuine Apes, is
a special deductive conclusion, which results with absolute necessity from
the general inductive law of the Theory of Descent. This view of ' man's

SUMMARY. 433

place in natnre,* oftnnot, we believe, be made too prominent. If the Theorv
of Descent is correct as a whole, then the theory that man has developed
from lower Vertebrates is simply an unavoidable deductive conclusion fruui
that general inductive law. Hence, all farther discoveries which may ii)
future enrich our knowledge of the phyletic development of man, can only
be confirmative of special points of that deduction, which resta on the
broadest inductive basis." — Qenerelle Morphologie (1866).

As we have now traversed the wonderful territory of

the history of human development, and learned its mo^f
important parts, it seems appropriate that, at the close of
our travels, we should look back on the road behind us,
and, on the other hand, glance forward along the further
path of knowledge into which our road will lead in future.
We started from the simplest facts of the history of man's
individual development; ontogenetic facts which can, at
any moment, be shown and established by microscopic or
anatomic research. The first and most important of these
ontogenetic facts is, that every man, like every other
animal, is at the commencement of his individual existence,
a simple cell. This egg-cell exhibits precisely the same
structure and mode of origin as that of any other Mammal.
From this cell proceeds, by repeated division, a many-celled
body, the mulberry-germ (morula); this changes into a
cup-germ (gastrula), and this, again, into an intestinal
germ-vesicle (yastrocystis). The two distinct cell-strata
which compose its wall are the two primary germ-
layers ; the skin-layer (cxoderrrui) and the intestinal layer
(entoderma). This double-layered germ-form is the onto-
genetic reproduction of that extremely important phylo-
genetic parent-form of all Intestinal Animals, to which we
have given the name Gastrsea.

As the human germ, like that of other Intestinal Animals,

4.34 THE EVOLUTION OF MAN.

passes through this gastrula-form, we are enabled to trace
its phylogenetic origin back to the Gastrsea. By tracing the
germ-history of the two-layered germ still farther, we found
that, by fission, four secondary layers are produced from
the two original germ-layers. These have exactly the same
constitution and genetic significance in Man as in all other
Vertebrates. From the skin-sensory layer develops the
outer skin (epidermis) and the central nervous system, and,
probably, the kidney system. The skin-fibrous layer forms
the leather-skin (corium) and the organs of motion (the
skeleton and muscle systems). From the intestinal-fibrous
layer originates the vascular system and the fleshy wall of
the intestine. The intestinal-glandular layer, finally, forms
only the epithelium, or the inner cellular layer of the
intestinal-mucous membrane and of the intestinal glands.

The manner in which these various organic systems
develop from the four secondary germ-layers, is, from the
very first, exactly the same in Man as in all other Verte-
brates. The germ-history of each separate organ afibrded
proof that the human embryo takes exactly the same special
direction in its difierentiation and formation, which, except
in Man, occurs only in the other Vertebrates. Within this
great animal tribe we then traced, step by step, and stage
ofber stage, the farther development which takes place in the
entire body as well as in all its several parts. This higher
development takes place in the human embryo in the form
peculiar to Mammals. Finally, we saw, that even within
this class the various stages of phylogenetic development,
which determine the natural classification of Mammals,
correspond throughout to the various stages of ontogenetic
formation through which the human embryo passes in the

rUNDAMENTiX LAW OF BIOQENt. 435

further course of its development. We were thus enabled
to determine the place of Man more definitely in the system
of this class, and accordingly to establish the nature of his
relation to the various mammalian orders.

The course of reasoning which we adopted in explaining
these ontogenetic facts, was simply the logical carrying out
of the fundamental law of Biogeny. In so doing we have
constantly tried to carry out the significant distinction
between palingenetic and kenogenetic phenomena. Palin-
genesis, or "the history of inheritance," alone enabled us to
draw direct conclusions from observed germ-forms as to the
tribal forms transmitted by heredity. On the other hand,
these conclusions were more or less endangered, wherever
Kenogenesis, or "vitiated evolution," was introduced by new
adaptations. The whole understanding of the history of in-
dividual evolution depends on the recognition of this most
important relation. We stand here on the border-line which
sharply divides the new from the old method of scientific
investigation, the new from the old conception of the world.
All the results of recent morphological research drive us
with irresistible force to the recognition of this fundamental
principle of Biogeny, and of its far-reaching consequences.
These are, it is true, irreconcilable with the customary
mythological ideas of the world, and with the powerful
prejudices engrafted into us in early youth by theosophie
instruction ; but, without this fundamental law of Biogeny,
without the distinction between Palingenesis and Keno-
genesis, and without the Theory of Descent, upon which
these are based, we are entirely unable to understand the
facts of organic development ; without these, we cannot
afford the faintest explanation of any part of this great and

436 THE EVOLUTION OF MAH.

wonderful world of phenomena. But, if we recognize the
causal relation between the development of the germ and
that of the tribe, if we recognize the true causal connection
of Ontogeny and Phylogeny, which is expressed in that law,
then the wonderful phenomena of Ontogeny explain them-
selves most simply; then the facts of germ-development
appear but the necessary mechanical effects of the develop-
ment of the tribe, conditioned by the laws of Heredity and
Adaptation. The inter-operation of these laws among the
everywhere-active influences of the struggle for existence,
— or, as we may simply say with Darwin, Natural Selection,
— is amply sufficient to explain to us the entire process of
germ-history by the history of the tribe. Darwin's chief
merit lies in the fact, thait by the discovery of the inter-
action of the phenomena of Heredity and Adaptation, he
prepared the way for a correct, logical understanding of the
history of Evolution.

Among the numerous and important evidences that we
have found for the truth of this view of our development
history, I will only call attention here once more to the
peculiarly valuable records of creation afforded by Dystele-
ology, or the doctrine of purposelessness, the science dealing
with rudimentary organs. It is impossible to emphasize
too often and too strongly the high morphological import-
ance of those remarkable parts of the body, which are,
physiologically, completely worthless and useless. In every
system of organs we find, in Man and in all higher Verte-
brates, some of these worthless primaeval heirlooms, which
have been inherited from our lower vertebrate ancestors.
Thus, first, we find on the outer surface of the body a scanty
rudimentary covering of hair, which is thicker only on the

1

BITDIMENTAIIT OEGANS. 43/

head, in the armpits, and on some other parts of the body. The
short hairs on the greater part of the surface of our bodies are
entirely useless, are without any physiological significance ;
they are the last scanty remains of the much more fully
developed haiiy covering of our Ape ancestors (p. 208). The
sense-organs exhibit a series of the most remarkable rudimen-
tary parts. As we have seen, the whole external shell of the
ear, with its cartilages, muscles, and membranes, is, in Man,
a useless appendage, destitute of the physiological importance
that was formerly, erroneously, attributed to it It is the
atrophied remnant of the pointed, freely-moving, and much
more highly developed mammalian ear, the muscles of which
we retain, although we can no longer use them (p. 271).
Again, we found, at the inner comer of the human eye, the
remarkable little crescent-shaped fold, which is of no use to
us, and is of interest only as being the last vestige of the
nictitating membrane ; of that third inner eyelid which is
still of great physiological importance in Sharks and many
Amnion Animals (p. 259). Numerous and interesting
dysteleological proofs are also afforded by the apparatus of
motion, both by the bony and the muscular systems. I
will only cite the free, projecting tail of the human embryo,
and the rudimentary caudal vertebrae developed in the
latter, together with the pertinent muscles ; this whole
organ is entirely useless to Man, but is of great interest as
the atrophied remnant of the long tail of our earlier Ape
ancestors, which was composed of numerous vertebrae and
muscles (p. 283), From these same ancestors we have also
inhtrited various bone-processes and muscles, which were of
great use to them in their climbing life among the trees, but
with us have fallen out of use. At various points under the

433 THE EVOLUTION OF MAN.

skill we also have entirely unused skin-muscles ; vestiges of
the largely developed skin-muscles of our lower mam-
malian ancestors. It was the function of this " panniculus
camosus " to contract and wrinkle the skin, as we may see
any day done by horses to drive away flies. We still
possess an active remnant of this great skin-muscle in the
muscle of the forehead, by means of which we wrinkle the
forehead and draw up the eyebrows ; but we are no longer
able to move at will another considerable remnant of it, the
great skin-muscle of the neck (platysma myoides).

As in these animal organ-systems of our body, so also in
the vegetative apparatus, we meet with many rudimentary
organs, most of which we have incidentally noticed. I will
only cite the remarkable thyroid gland (thyreoidea), the
rudiment of the crop and the remnant of the ciliated groove
(hypobranchial groove) present in Chordonia, Ascidia, and
Arcrania, on the lower part of the gill-body (pp. 336, 353) ;
also the vermiform process of the blind-intestine (ccscwm)
(p. 344)). In the vascular system we find many useless
ducts, the vestiges of disused vessels which were formerly
active blood-channels ; such, for instance, are the " ductus
BotaUi" between the lung-artery and the aorta, and
the ** ductus venosua Arantii*' between the vena portce
and vena cava, and many others. The numerous rudi-
mentary organs of the urinary and sexual systems (p. 415)
ar© especially interesting. Most of these are developed in
on© sex and rudimentary in the other. Thus, in the male,
the seed-ducts form from the Wolffian ducts, of which the
only traces remaining in the female are the Gartnerian
canals. On the other hand, from the Miillerian ducts in the
female are developed the oviducts and the uterus ; while io

HISTORICAL IMPORTANCE OF THE RUDI^HENTARY ORGANS. 439

the male, only the lower extremities of these ducts remain,
forming the useless male uterus (vesicula prostatica). In
the nipples and mammary glands, the male possesses other
rudimentary organs which, as a rule, are functional only in
the female (p. 204).

A closer anatomical examination of the human body
would bring to our notice a number of other rudimentary
organs, aU of which can be explained only by the Theory of
Descent. They are among the most important evidences foi'
the truth of the mechanical theory of nature, and among the
most overwhelming proofs against the prevailing teleologica]
ideas of creation. If, in accordance with this latter view, Man
and every other organism had been designed for his life-
purpose from the beginning, and had been called into existence
by an act of creation, the existence of these rudimentary
organs would be an incomprehensible enigma ; it would be
impossible to understand why the Creator should have laid
this useless burden on his creatures in their life-journey, so
arduous at the best. On the other hand, by means of the
Theory of Descent we can explain their existence in the
most simple way, and say : The rudimentary organs arc
parts of the body, which, in the course of centuries,
have gradually fallen out of use ; organs which perfoimed
definite functions in our animal ancestors, but which, in
lis, have lost their physiological importance. They have
become useless in consequence of our adaptation to new
circumstances, but yet are transmitted from generation to
generation by heredity, and have only slowly atrophied.

Like these rudimentary organs, so also all the other
organs of oui body have been transmitted to us from
Mammals, and, immediately, from our Ape ancestors. The

440 THE EVOLUTION OF MA2f.

iiiiman body includes no single organ which is not in-
licrited from Apes ; but, by means of our fundamental law
of Biogeny, we can trace the origin of our several systems
of organs yet further down to various lower ancestral
grades. Thus, for instance, we can say that we have
inherited the earliest organs of our body, the outer-skin
(epidermis) and the intestinal canal, from the Gastraeads,
the nervous and muscular systems from the lower Worms
{Archelminthes), the vascular system, body-cavity (cceloma),
and blood from Soft Worms (Scolecida), the notochord and
the gill-intestine from Chorda Animals, the differentiated
organs of sense from the Cyclostoma, the limbs and the
Miillerian ducts from Primitive Fishes (Selachii), and the ex-
ternal reproductive organs from Primitive Mammals (Pro-
niammalia). When we stated the " law of the ontogenetic
connection of systematically allied forms," and determined
the relative age of the organs, we saw how we could draw
such phylogenetic conclusions as these from the ontogenetic
succession of the organ-systems (vol. i. p. 390 ; ii, 357).

By the help of this important law and of Comparative
Anatomy, we were also enabled to determine definitely
"man's place in nature," or, as we may say, to assign to
man his position in the system of the animal kingdom. It
is now usual, in the more recent zoological systems, to
distribute the whole animal kingdom into the seven tribeS;
or phyla, which are again sub-divided, in round numbers^
into about forty classes ; and these classes into about
two hundred orders. According to his whole organization,
Man is undoubtedly, primarily, a member of but a single
tribe, that of Vertebrates ; secondly, he is a member of but
a single class, that of Mammals ; and, thirdly, a membei

"man's place in nature.*' 441

of but a single order, that of the Apes. All the character
istic peculiarities, distinguishing Vertebrates from the other
six tribes, distinguishing Mammals from the other forty
classes, and distinguishing Apes from the remaining twro
hundred orders of the animal kingdom, are also present
in Man. Turn and twist as we may, we cannot escape thii3
anatomical and systematic fact. Quite recently this very
fact has led to the liveliest discussion, and has occasioned,
especially, many disputes about the specific anatomical
relationship of Man to Apes. The most astounding views
on this "ape question," or "pithecoid theory," have been
uttered. It will therefore be well to examine it closely
once more at this point, and to separate the essential from
the non-essential in it.

We will start from the undisputed fact, that Man, at all
events, — whether his special blood-relationship to Apes is
acknowledged or denied, — is a genuine Mammal, is a Pla-
cental Mammal. This fundamental truth can be so easily
proved at any moment by investigations in Comparative
Anatomy, that it has been unanimously acknowledged since
the separation of the Placental from the lower Mammals
fPouched Animals, or Marsupialia, and Beaked Animals, or
Ornithostoma). But, from this, every logical adherent of
the doctrine of development at once draws the conclusion,
that man is descended from one and the same common
parent-form, together with all other Placental Animals, from
the progenitor of the Placentalia, just as, further, we must
necessarily suppose a common mammalian ancestral form
of all the various Mammals (Placentalia), Pouched Animals,
and Cloacal Animals (Monotremata) ; but by this the grea/.,
all-agitatiog main question of man's place in nature is

442 THE EVOLUTION OP MAN.

conclusively settled, whether we ascribe to Man a nearer ot
a more remote relationship to Apes. No matter whether
Man is, in a phylogenetic sense, a member of the Ape order
(or, if it is preferred, of the Primate order) or not, — in
any case, his direct blood-relationship to all other Mammals,
and especially to the Placental Mammals, is established. It
may be that the inter-relations of the various Mammals
are quite different from those now hypothetically assumed ;
but, in any case, the common descent of Man and all
other Mammals from a common parent-forin is indis-
putable. This primaeval, long since extinct parent-form,
which probably developed during the Triassic Period, was
the monotreme ancestral form of all Mammals.

If this fundamental and extremely significant principle
is borne in mind, the " ape question " will appear to us
in a wholly different light from that in which it is usually
presented. A little reflection will bring conviction that
this question has not the importance that has of late been
attributed to it ; for the origin of the human race from
a series of various mammalian ancestors, and the historical
development of the latter from an earlier series of lower
vertebrate ancestors, remains indubitably established, no
matter whether the genuine " Apes " are regarded as the
nearest animal ancestors of the human race or not. But,
it having become habitual to lay the principal weight of
the entire question of the origin of man on this very
*• descent from Apes," I find myself compelled to return
once more to it here, and to recall those facts in Com-
parative Anatomy and Ontogeny, which conclusively setth
this " ape question."

The shortest way to the goal is the one taken by

t€

HUXLEY'S LAW/ 443

Huxley in his celebrated work, which we have eo often
quoted, on the " Evidences as to Man's Place in Nature/' —
the way afforded by Comparative Anatomy and Ontogeny.
We have to compare objectively all the several organs of
Man with the same organs in the higher Apes, and then to
ascertain whether the differences between the former and
the latter are greater than the corresponding differences
between the higher and lower Apes. The indubitable and
indisputable result of this comparative anatomical investi-
gation which was conducted with the greatest candour and
accuracy, was the important law, which, in honour of its
discoverer, we have named Huodey's Law ; namely, that the
physical differences between the organization of Man and
that of the most highly developed Apes known to us, are
much smaller than the corresponding differences between
the higher and lower Apes. We might even define this law
yet more exactly by excluding entirely the Platyrhina or
American Apes as being more remote relatives, and limiting
our comparison to the narrower circle of relatives, the
Catarhina, or Apes of the Old World. Even within this
small group of Mammals, we found the differences of struc-
ture between the higher and lower Narrow-nosed Apes, for
example between the Gorilla and the Baboon, much greater
than the differences between these Man-like Apes and Maa
When, in addition, we now turn to Ontogeny, and when we
find there, according to our " law of the ontogenetic con-
nection of systematically related forms, that the embryos of
Man and of the Man-like Apes, are identical for a longer
period than the embryos of the highest and of the lowest
Apes, we are certainly obliged to bring ouisi.'lvcs, whether
with a good or a bad grace, to acknowledge our origin from

444 THE EVOLUTION OF MAN.

the Ape order. From the facts exliibited by Comparative
Ajiatomy, we can undoubtedly form in imagination an
approximate image of the structure of our ancestors during
the older Tertiary Period ; we may fill out the details as we
will, yet this image will be a genuine Ape, and a true
Catarhine. For Man has all the physical characters dis-
tinguishing the Catyrhina from the Platyrhina. Accord-
ingly, in the mammalian pedigree, we must derive the
human race directly from the Catarhine group, and refer
the origin of Man to the Old World For the entire group
of the Catarhine Apes has, as yet, been confined to the Old
World, just as the group of the Platyrhine Apes has been
limited to the New. Only the earliest root-form, that from
which both groups sprang, was common to them; probably
it originated from the Semi-apes of the Old World.

Therefore, although it is thus indubitably established as
the result of our objective scientific inv^uiry, that the human
race is directly descended from the Apes of the Old World,
yet we will once more state emphatically that this signifi-
cant fact is not of as great importance to the main question
of the origin of Man, as is generally supposed. For, even
if we entirely ignore the fact or thrust it aside, this will
not affect all that the zoological facts of Comparative
Anatomy and the history of development have taught us
concerning the placental character of Man. These clearly
prove the common descent of Man and the other Mammals.
It is evident also, that the main question cannot be in the
least evaded or set aside by the statement : " Man is, indeed,
a Mammal; but he branched off from the others quite at
the root of the class, and has no nearer relationship with
any other extant Mammal." At all events, the relationship

EVOLUTION AND SENTIMENT. 445

is evidently more or less close if we comparatively examine
the relation of the Mammalian class to the remaining forty
classes of the animal kingdom. All Mammals, including
Man, are, at least, of common origin, and it is equally
certain that their common parent-forms gradually developed
fix)m a long series of lower Vertebrates.

Feeling, evidently, rather than understanding, induces
most people to combat the theory of their "descent from
Apes." It is simply because the organism of the Ape appears
a caricature of Man, a distorted likeness of ourselves in a
not very attractive form, because the customary aesthetic
ideas and self-glorification of Man are touched by this in so
sensitive a point, that most men shrink from recognizing
their descent from Apes. It seems much pleasanter to be
descended from a more highly developed, divine being,
and hence, as is well knoTVTi, human vanity has, from the
earliest times, flattered itself by assuming the original
descent of the race from gods or demi-gods. The churcli
with that sophistical distortion of ideas of which she i-
so great an adept, has managed to extol this ridiculous
pride as Christian humility ; and those people who
reject with haughty horror every suggestion of descent
from lower animals, and consider themselves children of
God, those very people are exceedingly fond of boasting
about their childlike humility of spirit. In most of the
sermons delivered against the progress of the doctrine
of evolution, human "vanity and conceit play throughout
a prominent part ; and, although we have inherited this
characteristic weakness from Apes, yet we must confess to
having developed it to a degree of perfection which
completely overthrows the unprejudiced judgment of the

446 THE EVOLUTION OF MAX.

" sound understanding of man." We ridicule th© childiBh
follies occasioned by the pride of ancestry among the
nobility, from the splendid Middle Ages down to our own
time, and yet no small portion of this groundless prida
of nobility lurks in a great majority of men. Just as most
people prefer to trace their pedigree from a decayed baron
or, if possible, from a celebrated prince, rather than from
an unknown, humble peasant, so they prefer seeing the pro-
genitor of the human race in an Adam degraded by the Fall,
rather than in an Ape capable of higher development and
progress. It is a matter of taste, and such genealogical
preferences do not, therefore, admit of discussion. StiU I
must confess that, personally, I am as proud of my paternal
grandfather, who was simply a Silesian peasant, as of my
maternal grandfather, who raised himself from the position
of a Rhenish lawyer to the highest posts in the council
of state. And it is also much more to my individual taste
to be the more highly developed descendant of a primaeval
Ape ancestor, who, in the struggle for existence, had de-
veloped progressively from lower Mammals, as they from
stUl lower Vertebrates, than the degraded descendant of
an Adam, god-like, but debased by the Fall, who was formed
from a dod of earth, and of an Eve, created from a rib of
Adam. As regards this celebrated " rib," I must here ex-
pressly add as a supplement to the history of the develop-
ment of the skeleton, that the number of ribs is the same in
man and in woman. In the latter as well as in tiie former,
the ribs originate from the skin-fibrous layer, and are to be re-
garded phy logenetically as lower or ventral vertebrae (p. 285).
Now I certainly hear some one say : " That may all be
right and correct as far as the human body is concerned, and,

EVOLUTION OF THE MIND. 447

from the facts presented, it is certainly no longer to be
doubted that tliis has actually developed gradually, step by
step, from the long ancestral series of Vertebrates ; but it is
quite otherwise with the ' spirit of man,* with the human
mind, which cannot possibly have developed in a similar
way from the mind of lower Vertebrates." Let us see if the
known facts of Comparative Anatomy, Physiology, and
Evolution can meet this grave objection. We shall best
gain firm ground from which to start in this matter by
comparatively examining the minds of the difierent Verte-
brates. Side by side within the various classes, orders,
genera, and species of Vertebrates, we find so great a variety
of vertebral intellects, that, at first sight, one can scarcely
deem it possible that they can all be derived from the mind
of a common " Primitive Vertebrate." First, there is the
little Lancelet, which has no brain at all, but only a simple
medullary tube, the entire mental capacity remaining at
the very lowest grade occuning among Vertebrates. The
Cyclostomi, also, standing just above, exhibit a hardly
higher mental life, though they have a brain. Passing on to
Fishes, we find then- intelhgence, as is well known, also
at a very low point. Not until from these we ascend to the
Amphibia, is any essential progress in mental de^^elopment
observable. This is much greater in Mammals, although,
even here, in the Beaked Animals {Ornithostoma), and the
next higher class, the stupid Pouched Animals {Marrj/pials),
the entire mental activity is still of a very low order ; but
if we pass on from these to Placental Animals, within this
multiform group we find such numerous and important
steps in difierentiation and improvement, that the mental
differences between the most stupid Placental Animals (for
62

448 THE EVOLCTTIOlf OF MAN.

instance, Sloths and Armadillos) and the most intelligent
animals of the same group (for instance, Dogs and Apes),
seem much more considerable than the intellectual dif-
ferences between those lowest Placentals and the Pouched
Animals, or even the lower Vertebrates. Those differences
are, at any rate, much more considerable than the dif-
ferences in the intellectual life of dogs, apes, and men. And
yet all these animals are allied members of a single class.^**
This fact is shown to a yet more surprising degree in
the Comparative Psychology of another class of animals,
which is specially interesting for many reasons, that of
Insects. It is well known that many Insects exhibit a
mental capacity approximately as highly developed as is
possessed by Man only of the vertebrate group. It is needless
to speak of the celebrated organized communities and states
of Bees and Ants ; every one knows that very remarkable
social arrangements occur among these, such as occur in an
equal degree of development only in the higher races of
men, and nowhere else in the animal kingdom. I will only
aUude to the civil organization and government among
Monarchical bees and Republican ants, to their division
into various orders : the queen, the drone nobility, the
workers, the nurses, soldiers, and so on. Among the most
remarkable phenomena in this extremely interesting field of
life, is certainly the cattle-keeping of certain Ants, which
tend plant-lice for the sake of their milk and regularly
coUect their honey-juice. Even more remarkable is the
slave-holding of the large red Ants, which steal the young
of the small black species and rear them to slave-labour.
It has long been known that all these civil and social
arrangements of the Ants were originated by the systematic

COMPARATIVE MENTAL CAPACITISa 449

oo-operation of numerous citizens, understanding each other.
Numerous observations have placed the astoundingly high
intellectual development of these little Articulated Animab
beyond all doubt. With this let us compare, as Darwin
has done, the intellectual capacity of many lower, and,
especially, of many parasitic. Insects. There, for example,
are the Scale Insects (Coccus) which, when mature, consist
of an entirely immovable shield-shaped body attached to
the leaves of .plants. Their feet are atrophied. Their
mouths are embedded into the tissue of the plant, the
juices of which they suck. The whole mental activity of
this motionless female parasite consists in the enjoyment it
derives from sucking these juices and from sexual inter-
course with the unattached male. The same is true of the
maggot-like female of the Twisted-wings (Strepsiptera),
which spends its whole life, wingless and footless, as a
motionless parasite in the body of the wasp. There can be
no suspicion of any higher mental activity there. If these
brutish parasites are compared with the mentally active
and sensible ants, it will certainly be admitted, that the
psychical diflferences between the two are much greater
than those between the highest and lowest Mammals,
between Beaked Animals (Ornithostoma), Pouched Animals
{Marsupialia), and Armadillos on the one hand, and Dogs,
Apes, and Men on the other. And yet aU those insects
belong, without question, to the single class of Arthropoda,
just as all these Mammals undoubtedly belong to the single
class of Vertebrates ; and just as every logical adherent of
the doctrine of evolution must assume a common parent-
form for aU those Insects, so also he must necessarily assert
a common descent for all these Mammala.

450 THE EVOLUTION OF MAN.

Turning now from observing the comparative mental
capacity of the various animals to the question as to the
organs of these functions, we receive the answer, that in all
higher animals they are invariably connected with certain
groups of cells, those cells which compose the central
nervous system. All naturalists, without exception, agree
that the central nervous system is the organ of the mental
life of animals, and this assertion is at any time capable
of experimental proof If the central nervous system is
wholly or partially destroyed, the " mind," or the psychical
activity of the animal, is wholly or partially annihilated at
the same time. We must, therefore, next inquire what is
the character of the mental organ in man. The undeniable
answer to this question has already been given. Man's
mental organ is, in its whole structure and origin, the same
as that of all other Vertebrates. It originates as a simple
medullary tube from the outer skin of the embryo, from
the skin-sensory layer, or the first of the secondary germ-
layers. In the course of its gradual development it passes
through the same stages of progression in the human
embryo as in that of all other Vertebrates, and as these
latter have undoubtedly a common origin, so must also the
brain and spinal cord be of the same origin in all.

Physiological obsei'vation and experiment teaches, more-
over, that the relation of the " mind " to its organ, the brain
and spinal marrow, is exactly the same in Man as in all
other Mammals. The former can in no case act without
the latter; the one is connected with the other, as is
muscular movement with muscle. Therefore, the mind can
develop only in connection with its organ. Adherents of
Uie Theory of Descent, who concede the causal connection

DEVELOPMENT OF THE HUMAN MIND. 45 1

between Ontogeny and Phylogeny, are now compelled to
recognize the following propositions : The mind, or " psyche,"
of man has developed together with, and as the function of
the medullary tube, and just as even now the brain and
spinal marrow develop in each human individual from the
simple medullary tube, so the human " mind," or the mental
capacity of the entire human race, has developed gradually,
step by step, from the mind of lower Vertebrates. Just as
even now in every individual of the human race the
wonderful and complex structure of the brain develops
step by step from exactly the same rudiment, from the
same five simple brain-bladders, as in all other Skulled
Animals (Craniota), so the human mind has gradually
developed in the course of millions of years from the mind
of lower Skulled Animals ; and as now the brain of every
human embryo difierentiates according to the special type
of the Ape-brain, so also the human psyche has historicallj
difierentiated from the Ape-mind.

This monistic idea will, of course, be indignantly re-
jected by most people, who accept the contrary dualistic
view, which denies the inseparable connection of the brain
and the mind, and regards " body and mind " as entirely
separate and distinct ; but how shall we reconcile this
commonly accepted view with the facts taught by the
history of evolution ? The dualistic view is, at least, as
irreconcilably opposed to Ontogeny as to Phylogeny. If
we agree with the majority of men, that the mind is a self-
existent, independent being, which has originally nothing
to do with the body, but only dwells in it for a time, and
which gives expression to its emotions through the brain,
as the piano-player through his instrument, then we must

45a TKEi EVOLUTION OF MAN.

suppose a period in the human germ-history, at which the
mind enters the body, enters the brain ; and we must also
suppose a moment at death, at which it leaves the body ;
and further, as every man inherits certain individual
mental qualities from each parent, we must suppose that
portions of the mind of each were transferred to the germ
at the time of its procreation. A little piece of the father's
mind accompanied the sperm-cell, a little piece of the
mother's mind remained with the egg-cell. This dualistic
view entirely fails to explain the phenomena of evolution.
We all know that the new-bom child has no consciousness,
no knowledge of itself and of the objective world. Who-
ever has children of his own, and follows their mental
development candidly, cannot possibly deny that processes
of biological evolution are at work there. Just as all other
functions of the body develop in connection with their
organs, so does the mind develop in connection with the
brain. And this gradual development of the child's mind
is such a wonderful and beautiful phenomenon, that every
mother and every father with eyes to see takes unwearied
delight in observing it. The text-books of Psychology
alone are ignorant of any such development, and we are
almost forced to the conclusion that their authors them-
Belves never had any children. The human mind, as it is
represented in the great majority of psychological works,
is only the one-sided mind of a learned philosopher, who,
indeed, knows many books, but nothing of the process of
evolution, and does not suspect that even his own mind has
developed.

These same dualistic philosophers must, of course, it
they are consistent, also assume that there was a moment

K£Aii02(. 453

in the Phylogeny of the human mind at which this mind
first entered the vertebrate body of man. Accordingly, at
the time when the human body developed from the body
of the Anthropoid Ape (thus, probably, in the latter part of
the Tertiary Period), a specific human mind-element — or, as it
is usually expressed, a "divine spark " — must have suddenly
entered or been breathed into the brain of the Anthropoid
Ape, and there have associated itself with the already
existing Ape-mind. I need not point out the theoretic
difficulties involved in this conception. I will only remark
that even this " divine spark," by which the mind of Man
is said to be distinguished from that of all other animals,
must itself be a thing capable of evolution, and has actually
developed progressively in the course of human history.
This "divine spark "is usually understood to be "reason,'
and is ascribed to man as a mental function distinguishing
him from all "irrational animals." Comparative Psycho-
logy, however, teaches that this frontier-post between man
and beast is altogether untenable.^^^ We must either take
the idea of reason in its broader sense, in which case it
belongs to the higher Mammals (the Ape, Dog, Elephant,
Horse), as much as to the majority of men ; or we must
conceive it in its narrower sense, and then it is lacking in
the majority of men, as well as in most animals. On the
whole, that which Goethe's Mephistopheles said of his time,
11 true of Man's reason to-day :

*• He might have kept himself more right
Hadst Thon ne'er shewn to him a glimpse of heaven'i ligki.
He calls it Reason, but Thou seest
Its use but makes him beastlier than the beast."

It therefore, we must abandon this generally preferred.

454 THE EVOLUTION OP HO.

and, in many respects, very pleasant dualistic theory of the
mind, as being wholly untenable, because irreconcilable
with genetic facts, then the opposite monistic view alone
remains to us, according to which the human mind, like
that of any other animal, is a function of the central nervous
system, with which it has developed in inseparable con-
nection. Ontogenetically, we see this in every child ;
phylogenetically, we must assert it in accordance with the
fundamental law of Biogeny. In every human embryo
the medullary tube develops from the skin-sensory layer,
and from the anterior part of that tube the five brain-
bladders of Skulled Animals (Graniota), and from these
the mammalian brain (at first with the characteristics of
the lower, then with those of the higher Mammals).
Just as this entire ontogenetic process is but a short repro
duction, occasioned by Heredity, of the same process in the
Phylogeny of Vertebrates, so also the wonderful mental
activity of the human race has gradually developed, step
by step, in the course of many thousands of years, from tho
less perfect mental activity of the lower Vertebrates. And
the evolution of the mind in each child is only a brief
reproduction of that long phylogenetic process.

The extraordinary and important bearing of Anthro-
pogeny on Philosophy, in the light of the fundamental prin-
ciple of Biogeny, now becomes apparent. The speculative
philosophers who will take possession of the facts of On-
togeny and explain them phylogenetically (according to that
law), will introduce a greater advance in the history of
Philosophy than has been made by the greatest thinkers of
all previous centuries. Undoubtedly every clear and logical
thinker must draw from the facts of Comparative Anatomy

PHILOSOPHICAL ASPECT OF EVOLUTION. 455

and Ontogeny which have been brought forward, a mass
of suggestive thoughts and reflections which cannot fail
of their effect on the further development of the philo-
sophical study of the universe. Neither can it be doubted
that these facts, if properly weighed, and judged without pre-
judice, will lead to the decisive victory of that philosophical
tendency, which we distinguish, briefly, as monistic or
mechanical, in distinction from the dualistic or teleological,
on which most philosophical systems of ancient, mediaeval,
and modem times are based. This mechanical, or monistic
philosophy, asserts that everywhere the phenomena of
human life, as well as those of external nature, are undei
the control of fixed and unalterable laws, that there is
everywhere a necessary causal connection between pheno-
mena, and that, accordingly, the whole knowable universe
forms one undivided whole, a " raonon" It further asserts,
that all phenomena are produced by mechanical causes
(causce ejfficientes), not by pre-arranged, purposive causes
{causae finales). Hence there is no such thing as "free-
will " in the usual sense. On the contrary, in the light of
this monistic conception of nature, even those phenomena
which we have been accustomed to regard as most free and
independent, the expressions of the human will, appear as
subject to fixed laws as any other natural phenomenon
Indeed, each unprejudiced and searching test appKed to the
action of our " free-will " shows that the latter is never
reaUy free, but is always determined by previous causal
conditions, which are eventually referable either to Heredity
or to Adaptation. Accordingly, we cannot assent to the
popular distinction between nature and spirit. Spirit
exists everywhere in nature, and we know of no spirit out-

456 THE EVOLUTION OF MJlN.

side of nature. Hence, also, the usual distinction between
natural science and mental science is entirely untenable.
Every real science is at the same time both a natural and a
mental science. Man is not above nature, but in nature.

The opponents of the doctrine of evolution are very fond
of branding the monistic philosophy grounded upon it as
" materialism," by confusing philosophical materialism with
the wholly different and censurable moral materialism.
Strictly, however, our " monism " might, as accurately or as
inaccurately, be called spiritualism as materialism. The
real materialistic philosophy asserts, that the vital pheno-
mena of motion, like all other phenomena of motion, are
effects or products of matter. The other, opposite extreme,
spiritualistic philosophy, asserts, on the contrary, that
matter is the product of motive force, and that all material
forms are produced by free forces entirely independent of
the matter itself. Thus, according to the materialistic
conception of the universe, matter, or substance, precedes
motion, or active force. According to the spiritualistic con-
ception of the universe, on the contrary, active force or
motion precedes matter. Both views are dualistic, and we
hold them both to be equally false. A contrast to both
views is presented in the monistic philosophy, which can as
little believe in force without matter, as in matter without
force. It is only necessary to reflect on this for a time,
from a strictly scientific standpoint, to find that on close,
examination it is impossible clearly to represent the one
without the other. As Goethe says, "Matter can never
exist and act without spirit; neither can spirit without
matter." "

The " spirit " and " mind " of man are but forces which

BKASON A FUNCTION OF MIND. 45/

are inseparably connected with the material substance of
our bodies. Just as the motive force of our flesh is involved
in the muscular form-elepaent, so is the thinking force of
our spirit involved in the form-element of the brain. Our
spiritual forces are as much functions of this part of the
body, as every force is a function of a material body. W©
know of no matter which does not possess force, and, con-
versely, of no forces that are not connected with matter.
When the forces manifest themselves in the phenomena of
motion, they are called active forces ; if, on the other hand,
the forces are in a state of rest, or of equilibrium, they are
called latent forces.^^^ This is as true of inorganic natural
substances as of organic. The magnet attracting iron-
filings, powder exploding, steam driving the locomotive, are
active inorganic substances ; they work by active force just
as does the sensitive mimosa, when it folds its leaves at a
touch, — as does the Amphioxus, when it buries itself in the
sand, — as does man, when he thinks. Only in these latter
cases the combination of the different forces, appearing as
phenomena of motion, are much more complex and much
less easily recognized than in the former cases.

Anthropogeny has led us to the conclusion that even in
the entire history of the evolution of man, in the history of
the germ, as well as in that of the tribe, no other active
forces have been at work, than in the rest of organic and
inorsanic nature; All the forces at work there can be
reduced at last to growth — to that fundamental function of
evolution by which the forms of inorganic, as well as of
organic bodies, originate. Growth, again, itself rests on the
attraction and repulsion of like and unlike particles.^^^ It
boa given rise to Man and to Ape, to Palm and Alga, to

453 THE EVOLUTION OF MAN.

crystal and water. Hence the evolution of man has taken
place according to the same " eternal, immutable laws,"
as has the evolution of any other natural body.

It is true that the prejudices that stand in the way of
the general recognition of this " Natural Anthropogeny "
are even yet intensely powerful ; otherwise the ancient
strife between the various philosophical systems would
already have been decided in favour of "Monism.*' But
it can be foreseen with certainty that a more general
acquaintance with genetic facts, will gradually destroy
those prejudices and bring about the victory of the
natural idea of "Man's Place in Nature." The fear is
often expressed in opposition to this view that it will cause
a retrogression in the intellectual and moral development
of man ; but, on the contrary, I cannot withhold my convic-
tion, that the very reverse will be true, that by it the pro-
gressive development of the human spirit will be advanced
in an unusual degree. At all events, I hope and trust tliat
I have, in these chapters, afforded convincing proof that
the only way to attain a true scientific knowledge of the
human organism, is by employing the method which we
must acknowledge to be alone correct and successful in the
study of organic nature, — by following the course of the
ffistory of Evolution.200

NOTES.

UEMARKS AND REFERENCES TO LITBRATURB.

1 (vol. i. p. 2). Anthropogeny (Greek) = History of the Bvoln-
fcion of Man; from Anthropos (av^pwTros) = man, and genea (yevea)
= Evolution history. There is no especial Greek word for " the
history of evolution ; " in its place is used either yeved ( = de-
scent), or yoveia (= generation). If goneia is preferred to
genea, the word must be written Anthropogeny. The word
" Anthropogony,'^ used first by Josephus, means, however, only
" the generation of man." Genesis (yei/eo-ts) means " origination,
or evolution ; " therefore Anthrojpogenesis = " the evolution of

man."

2 (i. 3). Embryo (Greek) = germ {tfjiPpvov). Really to cvtos
r^s ya(TTpo'i ppvov (Eust.), i.e. the unborn germ in the mother's
body (Latin foetus, or, better, fetus). In accordance with this
original sense, the term embryo should only be applied to those
young organisms which are still enclosed in the egg- coverings.
(Cf. " Generelle Morphologie," vol. ii. p. 20.) Inaccurately, how-
ever, various free-moving young forms of low animals (larvae)
are often spoken of as embryos. Embryonic life ends at birth.

3 (i. 5). Embryology (Greek) = Germ-science, from embryon
{tfxppvov) = germ, and logos (Aoyos) = science. Even now the
whole history of the evolution of the individual is erroneously
called "embryology." For corresponding with the term
"embryo" (see note 2), by "embryology," or " embryogony,"
should only be understood " the history of the evolution of ihe

460 NOTES.

individual within the egg-coverings." As soon as the organism

has left there, it is no longer a real " embryo." The later changes
of this form the subject of the science of Metamorphoses, or
MetainoTrphology.

4 (i. 5). Ontogeny (Greek) =" germ-history," or "the
history of the evolution of the individual;" from oKra = indi-
viduals, and genea (ycvea) = history of evolution. (Cf. note 1.)
Ontogeny, as the "history of the evolution of the individual,"
embraces both Embryology and Metamo^phology (note 3). —
"Grenerelle Morphologic," vol. ii. p. 30.

5 (i. 5). Phylogeny (Greek) = tribal history, or " the pa-
IsBontological history of evolution ; " from phylon (^uAoi/) = tribe,
fcnd genea (yci/ea) = history of evolution. The phylon includes
all organisms connected by blood, which are descended from a
common typical parent-form. Phylogeny includes Pala3ontology
and Genealogy. — " Generelle Morphologic," vol. ii. p. 305.

6 (i. 6). Biogeny (Greek) = the history of the evolution of
organisms or of living natural bodies in the widest sense
(Genea tu biu.) )Stos = life.

7 (i. 6). The fundamental law of Biogeny. Cf. my "General
History of the Evolution of Organisms" (" Generelle Morphologic,"
1866, vol. ii.), p. 300 (Essays on the causal connection of biogenetic
and phyletic evolution) ; also the " Monograph of Chalk
Sponges " (" Monographic der Kalkschwamme," 1872, vol. i. 471);
also my " Natural History of Creation."

8 (i 10). Palingenesis (Greek) = original evolution, from
palingenesia (TroAtvyevco-ia) = new-birth, renewal of the former
course of evolution. Therefore, Palingeny = inherited history
(from TToXtv = reproduced, and y€vca= history of evolution).

9 (i. 10). Kenogenesis (Greek) = modified evolution, from
kenos (/c€i/os) = strange, meaningless; and genea (yevea) = history
of evolution. The modifications introduced into Palingenesis
by Kenogenesis are vitiations, strange, meaningless additions to
the original, true course of evolution. Kenogeny — vitiated
history.

10 (i. 12). Latin dafinitioB ol the fundamental law 9k

NOTES. 461

Biogen J : " Ontogenesis snmmarinm vel recapitnlatio est phy-
logeneseoa, tanto integrins, quanto hereditate palingenesis con-
serratnr, tanto minns integrum, quanto adaptatione kenogenesis
introdncitnr." Cf . my " Aims and Methods of Recent History
of Evolution " (" Ziele und Wage der Heutigen Entwickelunga-
geschichte," p. 11. Jena, 1876).

11 (i. 17). Mechanical and purposive causes. Meclianical
natural philosophy assumes that throughout nature, in organic
as well as in inorganic processes, only non-purposive, mechanical,
necessarily-working causes exist (causce ejfficienteSy mecho/nism,
causality). On the other hand, vitalistic natural philosophy
asserts that the latter are at work only in inorganic processes,
which in certain other, purposive, special causes are at work,
conscious or purposive causes, working for a definite end (causce
UnaleSy Vitalism^ Teleology). (Cf. " Generelle Morphologic,"
vol. i. p. 94.)

12 (i. 17). Monism and Dualism. Unitary philosophy, or
Monism, is neither extremely materialistic nor extremely spirit-
ualistic, but resembles rather a union and combination of these
opposed principles, in that it conceives all nature as one whole
and nowhere recognizes any but mechanical causes. Binarv
philosophy, on the other hand, or Dualism, regards nature and
spirit, matter and force, inorganic and organic nature as distinct
and independent existences. (Cf. vol. ii. p. 456.)

18 (i. 20). Morphology and Physiplogy. Morphology (as
the science of forms) and Physiology (as the science of the
functions of organisms) are indeed connected, but co-ordinate
sciences, independent of each other. The two together constitute
Biology, or the " science of organisms." Each has its peculiar
methods and aids. (Cf . ** Generelle Morphologic," toL i, pp.
17-21.)

14 (i. 24). Morphogeny and Physiogeny. Biogeny, or the
'* history of the evolution of organisms," up to the present time
has been almost exclusively Morphogeny. Just as this first
opens the way to a true knowledge of organic forms, so will
Physiogeny afterwarda make a tme recognition of functions

462 NOTES.

possible, by discovering tbeir historic evolntion. Its future
promises to be most fruitful. Cf . " Aims and Methods of the
Recent History of Evolution" (" Ziele und Wege der Heutigen
Entwickelungsgeschichte," pp. 92-98. Jena, 1876).

15 (i. 27). Aristotle. Five books on the generation and
evolution of animals (Trcpt ^cowv yei/ccreos).

16 (i. 28). Parthenogenesis. On " virginal generation,"
or the ''immaculate conception" of Invertebrates, especially of
Articulated Animals {Crustacea, Insecta, etc.), see Siebold.
"Remarks on Parthenogenesis among Arthropoda " (" Beitrage
zur Parthenogenesis der Arthropoden." Leipzig, 1871). Georg
Seidlitz, " Parthenogenesis and its Relation to other Forms of
Generation in the Animal Kingdom " (" Die Parthenogenesis und
ihr Verbal tniss zu den iibrigen Zeugungs-Arten im Thierreich."
Leipzig, 1872).

17 (i. 34). The Preformation-theory. This theory is, in
Germany, usually called " E volutions- theorie,"iu distinction from
the " Epigenesis-theorie." As, however, in England, France, and
Italy, the latter is, on the contrary, usually called the theory of
evolution, evolution and epigenesis being used as synonymous
terms, it appears better to call the former " the theory of pre-
formation." Recently Kolliker has called his " theory of hetero-
genous generation " " Evolutionism " (note 47). Cf. preface,

p. XXX.

18 (L 37). Alfred KirchhofE, "Caspar Friedrich Wolff, his
Life and Teaching in the Science of Organic Evolution." —
" Jenaische Zeitschrift fiir Naturwissenschaft," 1868, vol. iv
p. 193.

19 (i. 43). Part of the writings left by Wolff have not yet
been published. His most important works are the dissertation
for the degree of doctor, Theoria generationis (1759), and hii-
model treatise " de formatione intestinorum " (on the formation
of the intestinal canal). — "Nov. Comment. Acad. Sc. PetropoL,"
xii. 1768; xiii. 1769. Translated into German by Meckel.
Halle, 1812.

20 (i. 51). Christian Pander, ^^Sistoria metamorphoseos, qnam

NOTES. 463

ovmn incubattun prioribns qninque diebns snbit." Vicebergi,
1817. (Dissertatio inauguralis.) " Contribntions toward the
history of the evolntion of the chick within the egg.^* (" Beitrage
znr Entwickelungsgeschichte des Huhnchens im Eie." Wurz-
burg, 1817.)

21 (i. 52). Karl Ernst Baer, "On the Evolution of Animals.
Observations and Reflections " (" Ueber Entwickelungsgeschichte
der Thiere. Beobachtung und Reflexion." 2 vols. Konigsberg,
1827-1837). In addition to this chief work, cf. " Story of the
Life and Writings of Dr. Karl Ernst Baer, told by himself "
(" Nachrichten iiber Leben und Schriften des Dr. Karl Ernst
Baer, mitgetheilt von ihm selbst." Petersburg, 1865).

22 (i. 60). Albert Kolliker. His "History of the Evolution of
Man and the Higher Animals " (" Entwickelungsgeschichte des
Menschen und der hoheren Thiere *') . The 2nd (corrected) edition,
1876, contains (pp. 28-40) a catalogue of ontogenetic literature.
On the newer contributions to this, cf. the " Jahresberichte
iiber die Leistungen und Fortschritte der Medicin " (Berlin), by
Virchow and Hirsch (the " History of Evolution,** by Waldeyer) ;
also the " Jahresberichte iiber die Fortschritte der Anatomie und
Physiologic,'* by Hofmann and Schwalbe (Leipzig) ; the "History
of Evolution," by R. Hertwig and Nitsche. Most of Kowalev-
sky's researches are contained in the " Memoires de 1* Academic
imperiale de St. Petersburg " (from the year 1866). Others are
published in Max Schultze's "Archiv fur mikroskopische
Anatomie," and in other periodicals.

23 (i. 60). Theodor Schwann, " Microscopic Researches into
the Identity in Structure and Growth of Plants and Animalg "
(" Mikroskopische Untersuchungen iiber die Uebereinstimmung
in der Structur und Wachsthum der Thiere und Pflanzen."
Berlin, 1839).

24 (i. 69). Ernst Haeckel, the Gastreea Theory, phylogenetic
classification of the animal kingdom and homology of the germ-
layers. — " Jenaische Zeitschrift fiir Naturwissenschaft," voL yiii.
1874, pp. 1-56.

25 (i. 75). Ernst Haeckel, "The History of Creation."
London, 1876.

464 NOTES.

26 (i. 81). Fritz Schultzo, " Kant and Darwin." A con-
tribntion to the history of tho science of evolution. Jena, 1875.

27 (i. 81). Immanuel Kant, "Critique of Teleological Rea-
son" ("Kritik der teleologischen Urtheilskraft "). 1790. § 74
and § 79. Cf. also my " History of Creation," vol. i. p. 103.

28 (i. 83). Jean Lamarck, " Philosophie Zoologiqne, ou
Exposition des Considerations relatives a I'histoire natureUe des
animaux," etc. 2 Tomes. Paris, 1809. Nouvelle edition, revue
et precedee d*une introduction biographique par Charles Martins.
Paris, 1873.

29* (i. 88). Wolfgang Goethe on Morphology (zur Morpho-
logie). The formation and re-formation of organic bodies. On
Goethe's morphological studies, cf. Oscar Schmidt (" Goethe's
Yerhaltniss zu den organischen Naturwissenschaften." Jena,
1853). Rudolph Yirchow, "Goethe as a Naturalist" (Berlin,
1861). Helmholtz, " On Goethe's Natural Scientific Works "
(Brunswick, 1865).

30 (i. 96). Charles Darwin. His chief work is " On the
Origin of Species by means of Natural Selection " (1859).

31 (i. 99). Darwin and Wallace. The general outlines of
the theory of selection were discovered independently by Darwin
and Wallace. It does not, however, follow that the services
or the latter in furthering the science of evolution are at ail
comparable with those of the former. As many opponents of
Darwin, especially the English Jesuit Mivart, have recently
endeavoured to exalt Wallace at the expense of Darwin, and to
depreciate the latter, I take this opportunity of expressly assert-
ing that Darwin's services are very far the greater.

32 (i. 101). Thomas Huxley. In addition to the works
mentioned in the text, the following popular works are especially
to be recommended : " On Our Knowledge of the Causes of
Phenomena in Organic Nature," and the "Elementary Phy-
siology " (1871).

33 (i. 101). Gustav Jaeger, " Zoological Letters " ("Zoologische
Briefe." Vienna, 1876), and the " Text-book of General Zoology"
(" Lehrbuch der Allgemeinen Zoologie." Stuttgart, 1876).

NOTES. 465

34 (1. 101). Friedricli Rolle, "Man, his Descent and Morality
represented in the light of the Darwinian Theory, and on the
basis of Recent Greological Discoveries " (" Der Mensch, seine
Abstammung und Gesittung im Lichte der Darwin'schen Lehre,"
etc.). Frankfort, 1866.

35 (i. 102). Ernst Haeckel, " Generelle Morphologie der
Organismen." General outlines of the science of organic forms,
mechanically shown in accordance with the theory of descent as
reformed by Charles Darwin. Vol. i., " General Anatomy ; "
vol. ii., " General History of Evolution." Berlin, 1866.

36 (i. 103). Charles Darwin, "The Descent of Man, and
Selection in Relation to Sex." 2 vols. London, 1871.

37 (i. 108). Karl Gegenbaur, "Outlines of Comparative
Anatomy" ("Grundziige der vergleichenden Anatomie." Leipzig.
2nd ed., 1870). "Elements of Comparative Anatomy " (" Grundriss
der vergleichenden Anatomie." 3rd (improved) edition, 1874).

38 (i. 114). Migration-theory. Moritz Wagner, " The Dar-
winian Theory and the Law of Migration of Organisms " (" Die
Darwin'sche Theorie und das Migrations-gesetz der Organ-
ismen." Leipzig, 1868). August Weismann, "On the Influence
oP Isolation in the Formation of Species " (" Ueber den Einfluss
der Isolirang auf die Artenbidung." Leipzig, 1871).

39 (i. 116). Carus Sterne, "Evolution and Dissolution" (" Wer-
den und Vergehen"). A popular history of the evolution of
nature as a whole. Berlin, 1876. Agassiz a "founder" of
natural science. " Gegenwart." Berlin, 1876.

40 (i. 117). Ernst Haeckel, "The Chalk-sponges" ("Die
Kalkschwamme ; Calcispongien oder Grantien." Berlin, 1872).
A monograph and an attempted solution of the problem of the
origin of species. Vol. i., " Biology of Chalk-sponges ; " vol. ii.,
" Classification of Chalk-sponges ; " vol. iii., " Atlas of Chalk-
sponges " (with 60 plates).

41 (i. 124). On the Individuality of Cells and recent reforms
in the cell-theory, cf. my " Individuahtatslehre," or " Tectologie "
("Generelle Morphologie," vol. i. pp. 239-274). Rudolf
Virchow, "(/ellular Pathologic." 4th edition. Berlin, 1871.

4-66 MOTEa

42 (i. 130). "The Plastid-tlieory and the Cell-theorf.**—
" Jenaische Zeits'chrift fiir Naturwissenschaft," 1870, vol. v. p.
492.

43 (i. 138). Gegenbaur, " On the Stmcture and Evolution of
Vertebrate Eggs with Partial Yelk-cleavage." — " Archiv f . Anat.
u. Phys." 1861, p. 491.

44 (i. 163). Ernst Haeckel, "On Division of Labour in Nature
and Human Life," in the collection of Lectures by Virchow-
Holtzendorf, 1869. Sect. 78 2nd edition.

45 (i. 160). Monogony {Generatio neutralis). On the various
forms of asexual reproduction (Schizogony, Sporogony, etc.), cf.
" Generelle Morphologic," vol. ii. pp. 36-58.

46 (i. 160). Amphigony (^Generatio sexualis). On the various
forms of sexual reproduction (Hermaphroditism, Gonochorism,
etc.), see " Generelle Morphologic," vol. ii. pp. 58-69.

47 (i. 168). Fitful evolution and gradual evolution. The
theory of fitful evolution has recently been developed especially
by Kolliker, who, under the title of heterogeneous generation,
opposes it to gradual evolution as maintained by us (" Zeitschr.
f. Wissens. Zool.," vol. xiv. 1864, p. 181, and " Alcyonaria, " 1872,
pp. 384-415). This theory is distinguished by assuming entirely
unknown causes for the "fitful evolution of species," a so-called
"great law of evolution" (an empty word indeed!). On the
contrary, we see, with Darwin, in the facts of Heredity and
Adaptation sufficient known (partly inner, partly external)
physiological causes, which explain the gradual evolution of
species under the influence of the struggle for existence.

48 (i. 170). Immaculate Conception never occurs in the
vertebrate tribe. On the other hand, parthenogenesis frequently
occurs among Articulated Animals (Artliropoda) (note 16).

49 (i. 171). Fertilization of Flowers by insects. Charles
Darwin on " The various contrivances by which British and
Foreign Orchids are fertilized by Insects." Hermann Miiller on
" The Fertilization of Flowers by Insects, and the correlative
adaptations of both " (" Die B«fruchtung der Blumen durch
Insecten nnd die gegonseitigen Ajipassungen Beider "). A con*

NOTES. 467

fcribntion to our knowledge of causal connection in organic
nature. Leipzig, 1873.

60 (i. 178). The Process of Fertilization has been very
varionsly viewed, and was formerly often regarded as an
entirely mysterions process, or even as a snpernatural miracle.
It now appears no more " wonderful or supernatural " than the
process of digestion, of muscular movement, or of any other
physiological function. For the earlier views, cf. Leuckart,
Article " Zeugung " (generation) in R. "Wagner's " Dictionary
of Physiology," 1850.

61 (i. 179). Monerula. The simple, very transient, kernel-
less condition, which we briefly call the "monerula," and, in
accordance with the fundamental law of Biogeny, regard as a
palingenetic reproduction of the phylogenetic Moneron parent-
form, appears to vary to some extent in different organisms,
especially in the matter of duration. In those cases in which
it no longer occurs, and in which the kernel of the fertilized
egg persists wholly or partially, we may regard this phenomenon
as a later, kenogenetic curtailment of Ontogeny.

52 (i. 181). The Plasson of the monerula appears, mor-
phologically, a homogeneous and structureless substance, like
that of the Moneron. This is not contradicted by the fact that
we ascribe a very complex molecular structure to the plastidules,
OP " plasson-molecules," of the monerula ; this latter will
natnrally be more complex in proportion as the organism which
it ontogenetically constitutes is higher, and as the ancestral
series of that organism is longer, in proportion as the preceding
processes of Heredity and Adaptation are more numerous.

63 (i. 182). The Fundamental Significance of the Parent-cell,
or cytnla, as the foundation-stone of the young organism in the
course of development, can only be rightly appreciated, if the
part taken in its constitution by the two generating cells is
rightly appreciated, the part taken by the male sperm-cell and
by the female egg-cell.

64 (i. 183). The One-celled Germ-organism, like the act of
fertilization from which it results, has been very variously

46S NOTES.

▼lewed. Cf. on this stibiect, in addition to the fonr important
works, here qnoted, by Anerbach, Biitschli, Hertwig, and Stras-
bnrger, the most recent annals of the progress of the history of
evolution (Waldeyer in Virchow-Hirsch's " Jahresberichten,"
Berlin ; Hertwig in Hofmann-Schwalbe's " Jahresberichten., *
Leipzig).

55 (i. 185). Protozoa and Metazoa. Cf. vol. i. p. 248; ii. 92.
The Protozoa and Metazoa are genetically and anatomically so
very distinct, that the former, as Protista, may even be excluded
entirely from the animal kingdom, and may be regarded as a
neutral intermediate kingdom between the plant and animal
kingdoms. — "Generelle Morphologic," vol. i. pp. 191-230. Ac-
cording to this view the Metazoa alone are really animals.

56 (i. 186). The Unity of the Zoogenetic Conception, result-
ing from the Gastraea-theory, has as yet not been destroyed by
the numerous attacks directed against that theory : for none of
these attacks have succeeded in substituting anything positive ;
by pure negation no advance can be made in this dark ani
difficult subject.

57 (i. 187). The Egg-cleavage and Gastrulation of Man, as
represented diagrammatically in Figs. 12-17 of Plate II., is most
probably in no essential way different from that of the Rabbit,
which has as yet been most closely examined in this point.

58 (i. 188). Ernst Haeckel, " Arabian Corals " (" Arabische
Korallen"). "A Journey to the Coral Banks of the Red Sea, and
a Glimpse into the Life of Coral Animals. A popular lecture,
with scientific explanations." With 5 coloured plates, and 20
woodcuts. Berlin, 1876.

59 (i. 189). The Number of the Segmentella, or cleavage-
cells, increases, in the original, pure forms of palingenetic egg-
cleavage, in regular geometric progression. But the point to
which this proceeds varies in the various archiblastic animals,
BO that the Morula, as the final result of the cleavage-process,

' consists sometimes of 32, sometimes of 64, sometimes of 128
cells, and so on.

60 (L 189). The Mulberry-germ, or Morula. The seg-

KOTSS. 469

menteOa, or cleaTage- cells, which constitute the Morula at the
close of palingenetic egg-cleavage, generally appear entirely
similar, with morphological difference in size, form, or con-
gtitntion. This does not, however, hinder the fact that these
cells have separated, even during cleavage, into animal and
vegetable cells, have differentiated physiologically, as is indicated
in Figs. 2 and 3, Plate II., as probable.

61 (i. 189). The Bladder-germ of Archiblastic Animals
(hlastulaf or hlastosphcera)', which is now commonly known as
the germ-vesicle, or, more accurately, as the " germ-membrane
vesicle," must not be confused with the essentially different
" germ-vesicle " of amphiblastic mammals, which is better called
the *' intestinal-germ vesicle" (gastrocystia) . The gastrocystis
and the blastula are still often united under the name of " germ-
v^esicle, or vesicula hlastodermica." Cf. vol. i. p. 290.

62 (i. 192). The Definition of the Gastrula was first
established by me in 1872, in my "Monograph of Chalk-sponges "
(vol. i. pp. 383, 345, 4i66). There I already gave due weight to
the " extremely great significance of the gastrula in reference
to the general Phylogeny of the animal kingdom" (p. 333).
" The fact that these larval forms re-occur in the most different
animals, cannot, I think, be sufficiently estimated, and bears
plain witness to the former common descent of all from the
Gastrsea " (p. 345).

63 (i. 194). The Uniaxial Outline of the Gastrula is, on
account of the two different poles of the axis, more accurately
described as a diplopolic uniaxial form (a sternometric outline :
conoid-form, or cone). Cf. my " Promorphology " (" Generelle
Morphologic," vol. i. p. 426).

64 (i. 194). Primitive Intestine and Primitive Mouth. My
distinction of the primitive intestine and primitive moutli
(protogaster and protostoma) from the later, permanent intestine
and mouth (metagaster and metastoma) has been variously
attacked ; it is, however, as much justified as the distinction of
the primitive kidney from the permanent kidney, of the primitive
vertebrae from the permanent vertebrae. The primitive intestiiif

470 Nona

forms but a part of the permanent intestine, and the primitiTe
mouth (at least in the higher animals) does not become the
permanent month.

65 (i. 196). Primitive germ-layers (hlastopkyUa) . As the
two primary germ-layers (^entoderma and exodermd) originally
form the sole histogenetic rudiment of the whole body, and as
the mesodemia, the nutritive yelk, and all other accessory parts
of the germ have developed only secondarily from the former,
1 consider it very important to distinguish between the primary
and secondary germ-layers. The latter, to distinguish them
from the former, might be called "after germ-layers" (Jblcu-
telasmd).

66 (i. 201). Unequal Cleavage and Hood-gastrula (Seg-
mentatio inoBqualis et Amphigastruld) . Next to Amphibia the
most accessible examples for observation of unequal cleavage
and the Amphigastrula are afforded by the indigenous Soft-
bodied Animals (Mollusca) and Worms (Snails and Mussels,
Earth Worms and Leeches).

67 (i. 202). The Colour of Amphibian- eggs is occasioned by
the accumulation of dark colouring- matter at the auimal pole of
the egg. In consequence of this the animal-cells of the exoderm
appear darker than the vegetative cells of the entoderm. In
most animals the reverse is the case; the protoplasm of the
entoderm cells being usually darker and more coarsely granulated
(vol. i. p. 197).

68 (i. 207). Hood-gastrula of Amphibia. Cf. Robert Remak,
" On the Evolution of Batrachia " (" Ueber die Entwickelung der
Batrachier," p. 126 ; Plate XII. Figs. 3-7). Strieker's " Manual
of Tissues" ("Handbuch der Gewebelehre," vol. ii. p. 1195-
1202; Figs. 399-402). Goette, "History of the Evolution of
Bomhinator" (" Entwickelungsgeschichte der Unke," p. 145;
Plate II. Figs. 32-35).

69 (i. 214). Hood-gastrula of Mammals. Eduard van
Beneden, " La maturation de Toeuf, la fecondation et les premieres
phases du developpement embryonnaire des Mammiferes, d'aprea
des recherches faites chez le lapin." Brussels, 1875 . No figures

NOTE& 471

are given with these *' Coranninication pr^liminaire ; " Van
Bcneden's description is, however, so clear, so thoroagh and care-
ful, that thev afford an entirely satisfactory insight into unequal
egg-cleavage and the formation of the Hood-gastrula in Mammals.
All other observers, who have studied the germination of Mam-
malian eggs (among the most recent Kolliker, Rauber, and
Hewsou may be especially mentioned), have overlooked or
failed to recognize the important features discovered by Van
Beneden.

70 (i. 218). The Disc-gastrula (Disco-gastrvla) of Osseous
Fishes (Teleostei). Van Bambeke, " Recherches sur I'embry-
ologie des poissons osseux." Brussels, 1875. The transparent
Fish-eggs, in which I observed discoid cleavage (Segmentatio
discoidalis) and the formation of the Disc-gastrula by invagination,
are accurately described in my article on " The Gastrula and
Egg-cleavage of Animals " (" Jen. Zeitschrift fiir Naturwis-
senschaft," 1875, vol. ix. p. 432-444; Plates IV., V.). On the
Disc-gastrula of Selachiiy cf. Balfour, " The Development of
Elasmo branch Fishes." — " Joum. of Anat. and Physiol.," vol. x.
p. 517; Plates XX., XXIII.

71 (i. 221). Yelk-cells of Birds. The cell-like constituent
parts, which occur in great number and variety in the nutritive
yelk of Birds and Reptiles, as in most Fishes, are nothing less
than true cells, as His and others have asserted. This does not
mean that in this matter a distinct limit everywhere exists
between the nutritive and the formative yelks, as in our oceanic
Fish-eggs (Figs. 42, 43, note 70). On the contrary, originally
(phylogenetically) the nutritive yelk originated from part of the
entoderm.

72 (i, 223) Egg-cells of Birds. Notwithstanding the large
nntritive yelk; the " after-egg " (metovum) of Birds and Reptiles
10, in form- value, a single cell. The very small, active protoplasm
of the " tread " does, however, indeed fall far short, in volume,
of the huge mass of the yellow yelk-ball. The bird's eggs are
absolutely the largest cells of the animal body. Cf. note 43, and
Eduard van Beneden, " Recherches sur la composition et kb

4/2 NOTEa.

4

signification de Tceuf." Brussels, 1870. Hubert Lndwig, "On
Bgg-stnictnre in tbe Animal Kingdom " (" Ueber die Eibildung
in Thierreicbe." Wiirzburg, 1874).

73 (i. 226). Discoidal cleavage {Segmentatio discoidalis) ol
Bird's eggs. Cf . Kolliker, " History of tbe Evolution of Man
and tbe Higber Animals " (" Entwickelungsgescbicbte des Men-
scben und der boberen Tbiere." 2nd edition, 1876, pp. 69-81 ;
Figs. 16-22).

74 (i. 227), Disc-gastrula (Disco-gastrula) of Birds. Cf.
Rauber, "On tbe Place of tbe Cbick in tbe System of Evolu-
tion " (" XJeber die Stellung des Hiibncbens im Entwickelungs-
plan "). Leipzig, 1876. Foster and Balfour, " Tbe Elements of
Embryology." London, 1874.

75 (i. 231). Bladder-gastrula (Perigastrula) of Articulated
Animals (^Arthrojpoda) . Cf. Bobretzky, "Russian Essay on tbe
Gerra-bistory of Astacus and Pal?emon." Kiew, 1873. Also my
own article on tbe gastrula and egg-cleavage. — " Jen. Zeitscbrift
fiir Naturwissenscbaft." Vol. ix. pp. 444-452, Plate VI.

^Q (i. 234). Tbe Four-layer Tbeory, wbicb was first clearly
stated by Baer in 1837 (" Entwickelungsgescbicbte der Tbiere,"
vol. ii. pp. 46, 68), and wbicb we bave bere carried out logically,
yet appears tbe only form of tbe germ-layer tbeory, wbicb,
on comparative observation of all bigber animals, supplies a
universal law of germination for all and at tbe same time meets
tbe inconsistent reputations of many observers.

77 (i. 239). Caspar Friedricb Wolff first indicated tbe Four-
layer Tbeory (note 7Q^. Cf. tbe remarkable sentence, quoted at
vol. i. p. 45, from bis pregnant work on tbe formation of the
intestinal canal (note 19).

78 (i. 24Q). Tbe Four Main Types of Gastmlation, wbicb
are diagrammatical ly distinguisbed in Plates II. and III., and in
Tables III. and IV. (vol. i. pp. 241, 242), are of course connected
by intermediate forms. Tbese are transitions botb between tbe
primordial and tbe unequal forms, and between tbe primordial and
tbe superficial forms ; similarly, tbe unequal form of egg-cleavage
is connected by twixt-f orms with tbe discoidal forms, which latter

NOTES. 473

is again, perhaps, connected in tlie same way with the enperficial
form.

79 (i. 241). The Gustmlation of the varions classes of
animals has been far too little studied to enable ns thoroughly
to summarize the distribution of the various forms withir the
separate classes. Yet it is already evident that primordial egg-
cleavage and the formation of the Archigastrula occur in the
lowest classes of each tribe.

80 (i. 243). The Rhythm of egg-cleavage is by no means as
regular as might appear from the four first examples in the five
tables. There are, on the contrary, many variations, and not
infrequently an entirely irregular and very variable sequence of
numbers occurs (especially in discoided cleavage).

81 (i. 246). Definition of the Type. Cf. Gegenbanr,
'' Elements of Comparative Anatomy," 1874, p. 59.

82 (i. 246). Types and Phyla. AccordiDg to the prevailing
" Type- theory," the types of the animal kingdom are parallel,
and entirely independent ; according to my " GastraBa- theory,"
on the contrary, they are divergent tribes, connected at the
roots ; according to the view of Glaus and other opponents, the
latter is no essential distinction.

83 (i. 248). The one-ceUed condition of Infusoria entirely
forbids their morphological comparison with Metazoa. Cf. my
article " On the Morphology of Infusoria " (" Jen. Zeitschrift
fiir Naturwissenschaft " 1873, vol. vii. p. 516-568).

84 (i. 257). The axes of the Vertebrate outline. Cf. my
" Promorphology " (Stereometry of Organisms). — " Generelle
Morphologic," vol. i. pp. 374-574. " Singly double-outlines "
(Dipleura), p. 519. "Bilateral-symmetrical" forms in the fourth
signification of the word.

85 (i. 255). The Primitive Vertebrate Type, as it is repre-
sented in Figs. 52-56, is a hypothetic diagram, which is principally
founded on the outline of the Amphioxus, but^ in which the
Comparative Anatomy of Ascidia and Appendicularia on the
one side, of Cyclostomi and Selachii on the other, is regarded
This diagram is by no means meant to be an " exact figure," but

474 NOTES.

a provisional stage in the hypothetic reconstruction of the
unknown, long extinct parent-form of Vertebrates, an "Archi-
tjpe."

86 (i. 268). Only very uncertain assumptions can be made
as to the sense-organs of the hypothetic parent-form, for these
organs, more than any others, have been subject to adaptation!,
and in Ascidia, as in the Amphioius, have probably been much
atrophied. The earliest Vertebrates probably inherited a pair
of eyes of very simple character and a pair of simple ear- vesicles
from Worms.

87 (i.. 267). The primitive kidneys were perhaps already
metameric in the hypothetic parent-form of Vertebrates, so that
in addition to the two longitudinal main canals (primitive
kidney ducts) numerous transverse tubes (segmental canals)
were connected with these main canals, a pair in each metameron
of the middle part of the body. Perhaps these already opened
through ciliated funnels into the body-cavity {cosloma)^ as is
now the case in Annelids, and, according to Balfour, in the
embryos of Selachii. Cf. Balfour, " Development of Elasmo-
branch Fishes." — " Quarterly Journal of Microscopical Science."
New Series, vol. liv. p. 323 ; " Journal of Anat. and Physiol."
vol. X.

88 (i. 273). The germination of Primitive Vertebrates. Cf.
<vith Table VI., Table VII. (vol. i. p. 327), Table XI. (p. 467);
also the diagrammatic figures in Plates IV. and V. with explana-
tion (p. 321).

89 (i. 276). The Germ-forms of the earliest Vertebrates, as
they are represented in diagrammatic cross sections in Figs.
62-69, can only, of course, be approximately guessed, and with
the aid of Comparative Anatomy and Ontogeny. These
hypothetic diagrams, therefore, by no means claim to be ac-
cepted dogmatically, any more than do those in Figs. 52-56.
(Cf. note 85.)

90 (i. 280). Main incidents in Vertebrate germination. Of
the main palingenetic incidents here enumerated, perhaps the
sixth, ninth, and tenth originally occurred in a very dif«

K0TB8. 475

ferent form. The other seven now appear to be prettj well

established.

91 (i. 285). The flat germ-disc of Birds, which even now, in
the opinion of most embryologists, represents the first starting-
point in the formation of the embryo, and to which all other
germ-forms have been referred, is, on the contrary, a late and
much modified germ-form, which has arisen in consequence of
the extension of the gastmla over the greatly enlarging nutritive
yelk.

92 (i. 289). Site of Fertilization. In Man, as in other
Mammals, fertilization of the eggs probably usually takes place
in the oviduct : here, the eggs which, at the rupture of the
Graafian follicles, have emerged from the female ovary and
passed into the outer opening of the oviduct, meet with the
active sperm-cells of the male seed, which, during copulation,
penetrated into the uterus, and from there passed into the inner
opening of the. oviduct. Rarely, fertilization occurs even on the
ovary, or not till within the uterus. (Cf. Chapter XXV.)

93 (i. 293). The origin of the mesoderm in Mammals, as in
other animals, is, at present, among the most obscure and con-
tested points of Ontogeny. Remak, Balfour, and others derive
it from the entoderm, Kolliker and others from the exoderm.
Waldeyer, His, and others assert that both primary germ-layers
take part in the formation of the mesoderm. The last assump-
tion is, I believe, correct. (Cf. notes 76^ 77.^

94 (i. 297). The Germ-shield (Notaspis). The ordinary
view, that the germ-shield (= Remak's " Doppelschild ") is the
earliest rudiment of the actual embryo, results in many erroneous
conclusions. It is, therefore, necessary to point out especially
that the germ- shield represents the first well-defined central
dorsal part of the embryo.

95 (i. 317). Body Wall and Intestinal Wall. The morpho-
logical distinction between' the body wall and the intestinal wall,
certainly primordial, is probably referable to the simple primary
germ-layers of the Grastrasa. If the skin-fibrous layer is derived
from the exoderm, and the intestinal-fibrous layer from the

4/6 NOTES.

entoderm, this most simply explains the progressive development
of this distinction, which may be traced through the series of
Worms, and np to Vertebrates.

96 (i. 320). Palingenetic and Kenogenetic germination. In
the germ-history of Vertebrates no clear conception of the
embryological process has yet been attained, because all authors
have started from the higher Vertebrates (usually from the
Chick) and have assumed that the form of evolution occurring
in this case is original and typical. It is only since the germ-
history of the Amphioxus has taught us the palingenetic, really
original form of germination of Vertebrate organisms, that we
have been enabled, by Comparative Ontogeny (and especially by
the principles of the Gastraea theory), rightly to understand and
to explain phylogenetically the kenogenetic forms of germination
of higher Vertebrates.

97 (i. 321). The Diagrams in Plates IV. and V. are as simple
and abstract as possible, in order to render the desired general
explanation as easy as possible.

98 (i. 346). Primitive Vertebrae and Metamera. For the
right conception of " primitive vertebral " structure it is espe-
cially necessary to point out that the primitive vertebras are
much more than their name indicates. They must, in fact, be
conceived as individual, consecutive sections of the trunk,
which have arisen one after the other, as true " metamera," or
consecutive pieces (" Generelle Morphologic," vol. i. p. 312).
Each primitive vertebra of a Vertebrate, like each trunk-segment
or metameron of an Annelid or Arthropod, contains all the
essential, morphological constituent parts, characteristic of the
corresponding animal-tribe.

99 (i, 349). Origin of the Primitive Vertebras. My concep-
tion of these as individual, morphological " consecutive pieces,"
which, like the metamera of Cestods and Annelids, have arisen
by terminal budding from a single unarticulated piece, has beeii
much attacked. I therefore emphatically remark that I only
understand tliis process in the widest sense. In both cases there
Is certainly a reproduction of individual, like parts, which ]»t«
originated (in time and space) consecutively.

KOTsa 477

100 (i. 361). The agreement among the germ-formfl of
various Mammals is instructive especially because it shows us
how, by diversity in the mode of evolution, the most diverse
gtmctures can originate from one and the same form. As we
actually see this in germ-forms, we may hypothetically assume
the same to have occurred among tribe-forms. Moreover,
this agreement is never absolute identity, but always only
the very greatest similarity. Even the germs of the yarious
individuals of a species are never actually identical.

101 (i. 366). The law of the ontogenetic connection of
systematically allied animal-forms has many apparent exceptions.
These are, however, fully explained by the adaptation of the
germ to kenogenetic conditions of existence. Where the palin-
genetic form of evolution of the germ has been accurately
transmitted by heredity, that law is always in force. Cf . Fritz
Miiller, " Fiir Darwin " (note 111).

102 (i. 367). Earliest human germs. Cf. Kolliker, "History
of the Evolution of Man " ('* Entwickelungsgeschichte des Men-
schen." 2nd edition, 1876, pp. 303-319). Also Ecker, "Icones
physiologicse." Leipzig, 1859. Plates XXV.-XXXI. The
earliest human germs which have yet been certainly recognized,
were from twelve to fourteen days old, and were observed by
Prof. Allen Thomson, of Glasgow. No opportunity has ever
occurred for the observation of earlier germs.

103 (i. 369). Human germs of three weeks (twenty to twenty-
one days) exhibit in their whole structure that phylogenetic stage
of evolution which, among extant V^ertebrates, is represented by
the Cyclostomi (Lampreys and Hags, vol. ii. p. 103), and which
must be referable to extinct Monorhine ancestors of similar
etructure.

104 (i. 370). Human germs of four weeks (twenty-five to
thirty days), on the whole, exhibit in their whole structure that
phylogenetic stage of evolution, which is exhibited in Sharks
and Rays, among extant Vertebrates, and which is referable to
similar extinct Primitive Fish ancestors (Proselachii). Of course
thi« comparison is afEected by various kenogenetic modifications

47^ vomi

(both heterotopic and heterochronic), just as in the foniMK.

(Cf. note 108.)

105 (i. 374). The nose of Nosed-apes is much more difFererit
from that of other Apes than from that of Man. Moreover,
even the extreme variety and variability in the external form
of the hnman nose shows how small is the morphological value
of this organ, so important to the physiognomy.

106 (i. 383). The bladder-like form of the hnman Allantois.
Cf . W. Kranse, " On the Allantois in Man " (*' Ueber die ADan-
tois des Menschen.**— "Archiv fiir Anat. n. Physiol.," 1875, p. 215,
Plate VI.).

107 (i. 400). The navel-cord (ftmicuhu umbilicalis), like
the placenta, is an organ shared by Man exclusively with Pla-
cental Animals. Cf . Chap. XIX. pp. 155-168, and Figs. 200, 201.
On the more minnte structure of this organ, and on the special
features of the embryonic blood-circulation, cf. Kolliker, " His-
tory of the Evolution of Man.*' 2nd edition, 1876, pp. 319-363.

108 (i. 401). The Kenogeny of Man. In pointing out the
phylogenetic significance of the separate incidents and periods of
human germ-history, and in explaining them by reference to cor-
responding processes and stages in the tribal history of our animal
ancestors, we must always bear in mind that in Man, as in all
higher animals, the original palingenetic cause of germination
has undergone much kenogenetic modification in consequence oi
many adaptations to the very various conditions of embryonic
life, that it has thus been much violated and contracted. The
higher the organism develops, the more are especially these
earliest stages of evolution abbreviated.

109 (L 404). The sections of human germ-history, of which
only four larger and ten smaller are mentioned here in reference
to their phylogenetic significance, allow of much more division
if their comparative Ontogeny is minutely examined. This
phylogenetic significance may also be very well explained with
fitting reference to kenogenetic displacements in place and time
(voL L p. 13).

110 (i. 405). Figures of human embryos in all stages of

s
HOTE& 479

^rm-hif?tory were given in very beantifnl detail by M. P. Erdl
thirty years ago : " The Evolution of Man, and of the Chick in
the Egg" ("Die Entwickelung des Menschen, nnd des Hiihnchens
imEi." Leipzig, 1845).

111 (i. 409). Fritz Miiller, "Fiir Darwin." Leipzig, 1864.
A very excellent little book, in which the modification of the
fundamental law of Biogeny (with reference to the Phylogeny of
Crustacea) are explained for the first time.

112 (i. 413). The Method of Phylogeny is of the same
morphological value as the well-known method of Geology, and
may, therefore, claim exactly the same scientific acceptation.
Cf. the excellent discourse by Eduard Strasburger, " On the
Importance of Phylogenetic Methods in the Study of Living
Beings." — '* Jenaiache Zeitschrift fiir Naturwissenschaft," 1874,
rol. viii. p. 56.

113 (i. 415). Johannes Miiller, " On the Structure and Yital
Phenomena of Arwpliioxus ZanceoZaf^^s."— Transactions of tlir
Berlin Academy, 1844.

114 (i. 415). Recent works on the Amphioxus. W. Rolpl)
and B. Ray Lankester especially have recently added to our
knowledge of the organology of the Amphioxus, Wilhelm Miiller
and P. Langerhans to that of its histology. The literature of
this enbject is fully represented by W. Rolph, in his "Researches
into the Structnre of the Amphioxns " (" Untersuchungen iiber
den Ban des Amphioxns." — "Morpholog. Jahrb.," vol. ii. p. 87,
Plates V. and YII.), and in P. Langerhans, " On the Anatomy of
the Amphioxus" ("Zur Anatomie des Amphioxus." — *'Archiv.
fur Mikr. Anat.," vol. xii. p. 290, Plates XII.-XV.).

115 (i. 416). Acrania and Craniota. The separation of
Vertebrates into Skull-less Animals {Acrania) and Sknllod
Animals (Craniota), which I first indicated in 1866 in my
" Generelle Morphologic," appears to me absolutely essential for
the phylogenetic explanation of the Vertebrate-tribe.

116 (i. 428). Max Schultze, "History of the Evolution of
Petromyzon" ("EntwickelungsgeschichtevonPe^omi/2(w." Haar-
lem, 1856), The Ontogeny of the Hags, which promises very

anportant results, is yet, nnfortnnately, entirely nnknovni.
64

49o Nona

117 (i. 450). Savignj, ** M^moiros but les Animanr lanfl
Vert^bree/* Vol. ii, Ascidics, 1816. Giard, " Recherchea enr
les Synascidies." — "Archives do Zoologie Experimentale/'Tol. i.,
1872.

118 (i. 436). Syn-aacidia and Echinoderms. The Corm-theoiy
of Echinoderms, which I explained in 1866 (" Generelle Mor-
phologie/* vol. ii. p. liiii), and which has been much attacked as
" paradoxical," is as yet the sole theory attempting the genetic
explanation of this remarkable gronp of animals.

119 (i. 442). Kowalevsky, "History of the Evolution of the
Amphioxus and of Simple Ascidians" (" Memoires de I'Acad. de
S. Petersbourg." 7 Serie. Tom. x. and xi. 1867-8).

120 (i. 450). The metameric structure of the Amphioxus
\v'hich is indicated in its nerve and muscle systems, undoubtedly
sliows that the notochord exists in Vertebrates previous to their
metameric structure, and consequently that it is inherited from
nnarticulated Chorda Animals.

121 (i. 454). The Metamorphosis of the Amphioxus, through
which the larva passes into the adult form, is not yet fully
known in all its details. This does not, however, affect the
extraordinarily important bearing of the thoroughly known,
earliest incidents in its germination on the palingenesis of Verte-
brate*.

122 (i. 465). Fertilization of Ascidia (Phallusia mammillata),
Eduard Strasburger, " On Cell-structure and Cell-dirision, with
Studies of Fertilization." 2nd edition. Jena, 1876, p. 306,
Plate VIII.

123 (i. 462). Kupffer. The tribal relation of Ascidia to
Vertebrates ("Archiv fur Mikros. Anat.," 1870, vol. vi. pp.
115-170). Oscar Hertwig, " Researches into the Structure and
Evolution of the Cellulose Mantles of Tunicata " (" Untersu-
chungen uber den Bau und die Entwickelung des Cellulose-
Mantels der Tnnicaten"). Richard Hertwig, "Contribution to
Knowledge of Ascidian Structure " (" Beitrage zur Kenntniss
des Baues der Ascidien." — " JenaiAche Zeitschrift fur Naturwis-
•enschaft," 1873, vol. vii.).

NOTES. 4S1

124 (i. 464). The Pliylogenetic Importance of the Amphi-
oxoB cannot be too highly insisted on. Without knowledge of
its Anatomy and Ontogeny, the origin of Vertebrates would be
entirely dubious, and their descent from Worms would appear
incredible.

125 (i. 467). The Ontogenetic Cell-pedigree, as it is repre-
sented, with reference to the Amphioius, in Table XI., probablj
holds good, in its most important features, for all Vertebrates,
and, therefore, also for Man. For, more than any other form,
the Amphioxus by strict Heredity has accurately retained its
Palingenesis. This histogenetic cell-pedigree is apparently well
established as regards most and the chief features ; on the other
hand, it yet appears doubtful with regard to the origin of the
primitive kidneys, the testes, and ovaries.

126 (ii. 4). Milne-Edwards, "Le9on8 sur la Physiologic
Comparee," vol. ix.

127 (ii. 6). Eternity of Organic Life. According to the
monistic view, organic life is a further form of evolution of the
inorganic word-processes, and had a beginning in time on our
planet. In opposition to this, A. Fechner, among others, in his
" Thoughts on the Creation and Evolution of Organisms," has
stated certain opposed " kosmorganic pl^-ntasys " which appear
entirely irreconcilable with the ontogenetic facts given here.

128 (ii. 18). Bemhard Cotta (" Geologic der Gegenwart,"
1866; 4th edition, 1874) and Karl Zittel("Aus der Urzeit;"
Miinchen, 1875, 2nd edition) have made some excellent remarks
on the duration and the whole course of the organic history of
the world.

129 (ii. 21). August Schleicher, "The Darwinian Theory
and Philology " ( " Die Darwin'sche Theorie und die Sprach-
wissenschaft." Weimar, 1863. 2nd edition, 1873).

130 (ii. 25). At first sight, most polyphyletic hypotheses
appear more simple and easy than do monophyletic, but the
former always present more difficulties the more they are
oonsidered.

X^X (ii. 25)« Those physiologists who desire an experi>

4^2 NOTEa

mental proof of tlie theory of descent, merely thereby prove theif
extraordinary ignorance of the morphological scientific facts re-
lating to this matter.

132 (ii. 30). Spontaneous generation. — " Generelle Mor-
phologic," vol. i. pp. 167-190. "Monera and Spontaneous Gene-
ration."— "Jenaische Zeitschrift fur Naturwissenschaft," 1871,
vol. vi. pp. 37-42.

133 (ii. 33). The Absence of Organs in Monera. In saying
that Monera are " organisms without organs," we understand the
definition of organs in a morphological sense. In a physiological
sense, on the other hand, we may call the variable plasson-
processes of the body of the Moneron the " pseudojjodia " organs.

134 (ii. 36). Induction and Deduction in Anthropogeny.
"Generelle Morphologic," vol i. pp. 79-88; vol. ii. p. 427.
" History of Creation," vol. ii. p. 357.

135 (ii. 42). Animal Ancestors of Man. The number of
species (or, more accurately, form-stages, which are distinguished
as " species ") must, in the human ancestral line (in the course of
many millions of years !), have amounted to many thousands ;
the number of genera to many hundreds.

136 (ii. 47). Following Elsberg, we give the name of "plas-
tidules" to the "molecules of plasson," to the smallest like parts
of that albuminous substance which, according to the " plastid-
theory," is the material substratum of all the active phenomena
of life. Cf. my work on " The Perigenesis of Plastidules "
(" Perigenesis der Plastidule oder Wellenzeugung der Lebens-
theilchen." Berlin, 1876). This is an attempt to explain
mechanically the elementary processes of evolution.

137 (ii. 49). Batliybius and the free protoplasm of ocean
depths. Cf. my " Studies on Monera and other Protista."
Leipzig, 1870, p. 86. The most recent observations on living
Bathybius are those of Dr. Emil Bessel, who found this form on
the coast of Greenland (in Smith's Sound), at a depth of about
550 ft. He noticed very active amoeboid movements in them,
as well as the assumption of foreign particles (carmine, etc.).
" It consists of nearly pure protoplasm, tinged most intensely by

NOTES. 4S3

a solution of (jarmine in ammonia. It contains fine gray gi*anulea
of considerable refracting power, and besides the latter a great
number of oleaginous drops, soluble in ether. It manifests very
marked amoeboid motions, and takes up particles of carmine, etc."
— Packard, " Life Histories of Animals, including Man." New
York, 1876.

138 (ii. 50). The Philosophical Importance of Monera in
explaining th.e most obscure biological questions cannot be
sujEciently emphasized. Monograph of Monera. — " Jenaische
Zeitschrift fur Naturwissenschaft," vol. iv., 1868, p. 64.

139 (ii. 54). The Nature and Significance of the Egg- cell can
only be philosophically understood by means of phylogenetic
examination.

14j0 (ii. 58). Synamoeba. Cienkowski, "On the Structnre
and Evolution of Labvrinthula " ("Uber den Ban und die Entwic-
kelung der Labyrinthuleen."^Arch. fiir Mikrosk. Anat., 1870,
vol. iii. p. 274). Hertwig, " Microgromia Socialis." — Thid.

141 (ii. 61). Catallacta, a new Protista-group (Magosphcera
planula). See " Jenaische Zeitschrift fiir Naturwissenschaft,"
vol. vi., 1871, p. 1.

142 (ii. 66). Haliphysema and Gastrophysema. Extant
Gastraeads. See " Jenaische Zeitschrift fiir Naturwissenschaft,"
vol. xi., 1876, p. 1, Plates I.-YI.

143 (ii. 70). The five first stages in the evolution of the
animal body, which are compared in Table XVII., and which
are common to Man and all higher Animals, are established
beyond all doubt as existing in the Ontogeny of most extant
animals. As Comparative Anatomy shows that corresponding
form-stages yet exist in the system of the lower animals, we
may assume, in accordance with the fundamental law of Biogeny,
that similar forms existed phylogenetically as most important
ancestral forms.

144 (ii. 77). On the distinction of the axes, and on the
geometric outline of the animal body, see " Promorphologie "
(" Generelle Morphologic," vol. i. pp. 374-574).

145 (ii. 87). The hermaphrodite structure of our ancestral

4^4 NOTES.

series was perhaps transmitted from the Chorda Animals even as
far as the lower stages of Vertebrate ancestors. Cf . Chapter XXV.

146 (ii. 89). I am inclined to regard the Appendicularia as
living Chorda Animals of the present day; they are the only
Invertebrates permanently possessing a notochord, and thus, as
by many other peculiarities, distingnished from genuine Tuni-
cates.

147 (ii. 105). Metamorphosis of Lampreys. That the blind
Ammocoetes change into Petromyzon was known two hundred
years ago (1666) to the fisherman Leonhard Baldner of Stras-
burg ; -but this observation remained unrecognized, and the
modification was first discovered by August Miiller in 1854
(" Archiv fur Anat.,'* 1856, p. 325). Cf. Siebold, " The Fresh-
water Fishes of Central Europe** ("Die Siisswasserfische von
Mittel-Europa," 1863).

148 (ii. 114). Selachii as Primitive Fishes. The old disputes
as to the systematic position and kindred of Selachii were first
definitely settled by Gegenbaur, in the introduction to his classical
work on " The Head-skeleton of Selachii."

149 (ii. 118). Gerard Krefft, "Description of a Gigantic Am-
phibian ; ** and Albert Giinther, " Ceratodus, and its Systematic
Position." — "Archiv fiir Naturgeschichte," 37, 1871, vol. i. p.
321 ; also "Phil. Trans.," 1871, Part II. p. 511, etc.

150 (ii. 129). The duration of metamorphosis of Amphibia
varies much in the different forms of Frogs and Toads, the whole
forming a complete phylogenetic series from the original, quite
'complete form, to the later, much shortened and vitiated heredity
of modification.

161 (ii. 129). " AH the histological features of the Land
Salamander (Salamandra maculata) force the impression that it
belongs to an entirely different epoch of terrestrial life than that
of the Water Salamander {Triton), externally so similar." — Robert
Remak (" Entwickelung der Wirbelthiere," p. 117).

152 (ii. 130). Siredon and Amblystoma. Very various views
have lately been expressed as to the phylogenetic significance to
be attributed to the much- discussed modification of the Mexican

NOTES. 485

Axolotl into an Amblystoma. Of. on this Bnbject especially
August Weismann, in " Zeitsch. fur wissenscli. Zoologie," rol.
XXV., Sup., pp. 297-334.

153 (ii. 131). The Leaf-frog of Martinique (Hylodes rruur-
Hnicensis) loses its gills on the seventh day, its tail and yelk-sao
on the eighth day of egg-life. On the ninth or tenth day after
fertilization the complete frog emerges from the egg. — Bavay,
**Sur I'Hylodes Martinicensis et ses Metamorphoses." "Journal,
de Zool. par Q-revais," vol. ii. 1873, p. 13.

154 (ii. 133). " Homo diluvii testis " = Andrias Scheuchzeri.
•* Sad bone of an ancient evil-doer ; Soften, stone, the heart of
the new children of evil " (Diaconus Miller). Quenstedt.
"Formerly and Now" (" Sonst und Jetzt," 1856, p. 239).

155 (ii. 133). The Amnion-structure of the three higher
Vertebrate-classes, wanting in all lower Vertebrates, has no
connection with the similar, but independently acquired Amnion-
structure (analogous, but not homologous) of higher Articu-
lated Animals (Arthropoda).

156 (ii. 138). The former existence of a Protamnion, the
common parent-form of all Amniota, is undoubtedly shown by
the Comparative Anatomy and Ontogeny of Reptiles, Birds, and
Mammals. No fossil remains of such a Protamnion have, how-
ever, yet been discovered. They must be sought in the Permian
or Carboniferous formation.

167 (ii. 147). The former organisation of the Promammalia
may be hypothetically reconstructed from the Comparative
Anatomy of the Salamander, Lizards, and Beaked Animals
(OmithorhyncJiics).

158 (ii. 153). The Didelphic ancestors of Man may have been
externally very different from all known Pouched Animals (Jfar-
awpialia)^ but possessed all the essential internal characters of
Marsupialia.

159 (ii. 163). The phylogenetic of the Semi-apes, as the
primaeval placental parent-group, is not influenced by our ignor-
ance of any fossil Prosimise, for it is never safe to estimate
palaBontological facts as negative, but only as 'positive.

4^6 NOTES.

160 (ii. 168). On the stnicture of the Decidna very variotis
theories have been given. Cf . Kolliker, " History of the Evolution
of Man " (" Entwickelungsgeschichte des Menschen." 2nd
edition, 187] , pp. 319-376). Ercolani (Giambattista), " Snl pro-
cesso formativo della placenta." Bologna, 1870. "Le glandole
otricolari derntero." Bologna, 1868, 1873. Huxley, "Lectures
on the Elements of Comparative Anatomy," 1864, pp. 101-112.

161 (ii. 172). Huxley, " Anatomy of Vertebrates," 1873,
p. 382. Previously Huxley had separated the " Primates " into
seven families of nearly equal systematic value. (See "Man's
Place," etc., p. 119.)

162 (ii. 179). Darwin. Sexual selection in Apes and Msa. —
" Descent of Man," vol. ii. pp. 210-355.

163 (ii. 180). Man-like Holy Apes. Of all Apes, some Holy
Apes (^Semnojpithecms) most resemble Man, in the form of their
nose and the character of their hair (both that on the head and
that on the beard). — Darwin, " Descent of Man," vol. i. p. 335 ;
vol. ii. p. 172.

164 (ii. 182). Friedrich Muller (" AUgemeine Ethnographie."
Vienna, 1873, p. 29), on the supposed age of man. Families of
languages (pp. 5, 15, etc.).

165 (ii. 183). The plate (XV.) representing the migrations,
given in the " History of Creation," merely claims the value of
a first attempt, is an hypothetic sketch, as I there expressly said,
and as, in consequence of repeated attacks, I must here insist.

166 (ii. 201). The Leather-plate. The phylogenetic distinction
of a special leather-plate, the outermost lamella separating from
the skin-fibrous layer, is justified by Comparative Anatomy.

167 (ii. 204). Milk-glands. Huss, "Contributions to the
History of the Evolution of the Milk- glands" ("Beitrage zur
Entwickelungsgeschichte der Milchdriisen ") ; and Gegenbaur,
" On the Milk-gland Papillae " (" Jenaische Zeitschrift fiir
Natorwissenschaft," 1873, vol. vii. pp. 176, 204).

168 (ii. 208). On the hairy covering of Man and Apes, see
Darwin, "Descent of Man," vol. i. pp. 20, 167, 180; vol. ii.
pp. 280, 298, 335, etc.

NOTES. 487

169 (li. 217). Dorsal side and ventral sides are homologous
in Vertebrates, Articulated Animals (Arthropoda), Soft-bodied
Animals {Mollusca), and Worms, so that the dorsal marrow and
the ventral marrow are not comparable. Cf. Gegenbaur, "Morph.
Jahrbuch," vol. i. pp. 5, 6.

170 (ii. 228). The unknown ontogenetic origin of the sym-
pathetic nerve-system mnst probably, for phylogenetic reasons,
be songht chiefly in the intestinal layer, not in the skin-layer.

171 (ii. 248). On the cavities connected with the nose, see
Gegenbaur, " Elements of Comparative Anatomy," p. 580.

172 (ii. 260). The analogies in the germination of the higher
sense organs were rightly grasped even by the earlier natural
philosophers. The first more accurate sketches of the very
obscure germ-history of the sense-organs, especially of the eye
and ear, were given (1830) by Emil Huschke, of Jena (Isis,
Meckel's Archiv, etc.).

173 (ii. 265). Hasse, "Anatomical Studies" (** Anatomische
Studien "), chiefly of the organ of hearing. Leipzig, 1873.

174 (ii. 269). Johannes Rathke, " On the Gill- apparatus and
the Tongue-bone " (" Ueber den Kiemen-apparat und des
Zungenbein," 1832). Gegenbaur, " On the Head-skeleton of
Selachii," 1872. (See note 124.)

175 (ii. 272). On the Rudimentary Ear-shell of Man, cf.
Darwin, "Descent of Man," vol. i. pp. 17-19.

176 (ii. 276). Scarcely anywhere does Comparative Anatomy
prove its high morphological value as with reference to the
skeleton of Vertebrates : in this matter it accomplishes much
more than Ontogeny. There is all the more reason to insist on
tliis here, as Goette, in his gigantic history of the evolution of
Bombinator, has recently denied all scientific value to Com-
parative Anatomy, and asserted that Morphology is explained
solely by Ontogeny. Cf . my " Aims and Methods of the Recent
History of Evolution " (" Ziele und Wege der heutigen Ent-
wickelungsgeschichte," 1875, p. 52, etc.).

177 (ii. 283). The Human Tail, like all other rudimentary
organa, is very variable in point of size and development. In

488 NOTES.

rare cases it remains permanently, projecting freely : nsnally it
disappears at an early period, as in Anthropoid Apes.

178 (ii. 284). On the Number of Yertebrss in different Mam-
mals, cf. Cuvier, "Lemons d* Anatomic Comparee." 2nd edition,
tome i., 1835, p. 177.

179 (iL 293). On the earHer Sknll-theory of Goethe and Oken,
cf . Virchow, " Goethe as a Naturalist " (" Goethe als Natur-
forscher," 1861, p. 103).

180 (ii 295.). Karl Gegenbaur, "The Head-skeleton of
Selachii" ("Das Kopfskelet der Selachier"). As the foundation
of a study of the head-skeleton of Vertebrates (1872).

181 (ii. 301). Karl Gegenbaur, " On the Archipterygium."
— " Jenaische Zeitschrift fiir Naturwissenschaft," vol. viL 1873,
p. 131.

182 (ii. 304). Gegenbaur, " Researches into the Comparative
Anatomy of Vertebrates" (" Untersuchungen zur Vergleichen-
den Anatomic der Wirbelthiere "). Part I. Carpus and Tarsus
(1864). Part II. The shoulder girdle of Vertebrates. Pectoral
fins of Fishes (1866).

183 (ii. 305). Charles Martins, "Nouvelle comparaison des
membres pelviens et thoraciques chez I'hommc et chez les
mammiferes." — "Memoircs dc TAcad. de Montpellier," vol. iii
1857.

184 (ii. 308). Ossification. Not all bones of the human bod}
arc first formed of cartilage. Cf. Gegenbaur, " On Primary and
Secondary Bone-formation, with special reference to the Pri-
mordial Skull Theory." — " Jenaisch. Zeitschrift fiir Natur-
wissenschaft," 1867, vol. iii. p. 54.

185 (ii 308). Johannes Miiller, "Comparative Anatomy of
Myxinoides." — " Transactions of the Berlin Academy," 1834-1842.

186 (ii. 314). The Homology of the Primitive Intestine and
the two primary germ-layers is the postulate for morphological
comparison of the various Metazoa-tribes.

187 (ii. 322). In the Evolution of the Intestine, Amphibia and
Gunoids have, by heredity, retained the original Craniota-form
more accurately than have Selachii and Osseous Fishes (Teleotlei).

NOTES. 489

The palir genetic germination of Selachii has been nmch altered
by kenogenetic adaptations.

188 (ii. 323). On the Homology of Scales and Teeth, cf.
Gegenbanr, " Comparative Anatomy " (" Grundriss der vergl.
Anatomie,'* 1874, pp. 426, 582) ; also Oscar Hertwig, " Jenaische
Zeitschrift fiir Naturwissenschaft," 1874, vol. viii. On the
important distinction of homology (morphological resemblance)
and Analogy (physiological resemblance), see Gegenbaur, as
above, p. 63; also my " Generelle Morphologic, '* vol. i. p. 313.

189 (ii. 337). Wilhelm Miiller, "On the Hypobranchial
Groove in Tunicates, and its Presence in the Amphioxus and
Oyclostomi." — "Jenaische Zeitschrift fur Naturwissenschaft,"
1873, vol. viii. p. 327.

190 (ii. 358). The Nerve- mnscnlar Cells of the Hydra throw
the earliest light on the simultaneous, phylogenotic diSerentiation
of nerve and muscle tissue. Cf. "Klemenberg, Hjdra." Leipzig,
1872.

191 (ii. 383). The germ-history of the human heart accurately
reproduces in all essential points its tribal history. This paliu-
genetic reproduction is, however, much contracted in particular
points and vitiated by kenogenetic modifications of the origiual
course of evolution, displacements partly in time, partly in place,
which are the result of embryonic adaptations.

192 (ii. 383). On the Special Germ-history of the Hnman
vascular system, cf. Kolliker, "History of the Evolution of Man"
("Bntwickelungsgeschichte des Menschen." 2nd edition, 1876) ;
also Rathke's excellent work on Ontogeny.

193 (ii. 387). The Homologies of the Primitive Organs, as
they are here provisionally described in accordance with the
Qtistreea- theory (note 24), can only be established by further co-
operation between Comparative Anatomy and Ontogeny. Cf.
Gegenbaur on Comparative Anatomy (" Grundriss der verglei-
chenden Anatomic ").

194 (ii. 390). The Mechanism of Reproduction. As the
functions of reproduction and of heredity, connected with re-
production, are referable to growth, so the former as well as tho

490 NOTsa

latter are finally explicable as the results of the attraction and
rejection of homogeneous and heterogeneous particles.

195 (ii. 397). Eduard van Beneden, " De la Distinction origi-
nelle du Testicule et de I'Ovaire." Brussels, 1874.

196 (ii. 399). On the Original Hermaphrodite Structure of
Vertebrates, cf. Waldeyer, " Ovary and Egg " (" Eierstock und
Ei," 1872, p. 152) ; also Gegenbaur (*' Grundriss der vergleichen-
den Anatomie," 1874, p. 615). On the origin of the eggs from the
ovary-epithelium, cf. Pfliiger, " On the Ovaries of Mammals and
Man" ("Die Eierstocke der Saugethiere und des Menschen,*'
1863). •

197 (ii. 423). On the special germ-history of the urinary and
sexual organs, cf. Kolliker, " History of the Evolution of Man."
On the homologies of these organs, see Gegenbaur (" Grundriss
der vergleichenden Anatomie," 1874, pp. 610-628).

198 (ii. 448). Wilhelm Wiindt, "Lectures on the Human and
Animal Mind" (" Vorlesungen iiber die Menschen- und Thier-
seele." 1863). W. Wundt, "Outlines of Physiological Psy-
chology" ("Grundziige der Physiologishen Psychologie," 1874).

199 (ii. 457). On Active (actual) and Latent (preteritial)
forces, cf. Hermann Helmholtz, " Interoperation of Natural
Forces " (" Wechselwirkung der Naturkrafte," Part II., 1871).

200 (ii. 457). "Anthropology as Part of Zoology." — " Generello
Morphologie," voL iL p. 4!32. ** History of Creation,*' voL L 7 j
vol. ii d47.

INDEX.

AcAiEPH^, u. 73, 92

Acoelomi, ii. 75, 92

Acorn- worms, ii. 86

Acrania, i. 116 ; ii. 97

Adam's apple, ii. 336

Adaptation, i. 158

After-birth, i. 400

Agassiz, thoughts on creation, i. 116

Air-tube (trachea), ii. 330, 333

Alali, ii. 182

Allantois, i. 380 ; ii. 135, 411

Alluvial period, ii. 12

Amasta, ii. 146, 204

Amnion, i. 314, 386

animals, ii. 120, 133

sheaths of, i. 387

water, i. 314

Amniota, ii. 120, 133
Amoeba, i. 142 ; ii. 152

false feet of, i. 142

Amoeboid egg-cells, i. 144 ; ii. 53

movements, i. 142 ; ii. 53

states, ii. 56

Amphibia, ii. 120, 122
Amphigastrula, i. 200, 241
Amphigonia, i. 160
Amphioxus, i. 413 ; ii. 98

blastula of, i. 443

body-form of, i. 417

cells of, their pedigree,

i. 467

chorda of. i. 417

distribution of, i. 416

— — gastrula of, i. 444

"■ — germ-layers of, i. 447

Amphiozns, mednllarj t«be of, L 418
place of, in natural

system, i. 416

sexual organs of, i. 425

— side canals of, i. 423

significance o^ L 254,

427
Attvphirhina, ii. 97, 101
Analogy, ii. 412
Anaraniay ii. 97, 120
Ancestral series of man, ii. 44, 184
Animalcnlists, i. 37
Animal germ-layer, i. 194, 327

organs, ii. 192, 194

Anorgana, i. 156 ; ii. 30

Anthropocentric conception, ii. 457

Anthropoids, ii. 177, 189

Anthropolithic epoch, ii. 11, 16

Anthropozoic periods, ii. 12, 17

Antimera, i. 257

Anus, i. 339 j ?i. 323, 345

Anus-groove, i. 339

Anvil (Incus of ear), il 261, 268

Ape-men, ii. 44, 181

Apes, ii. 165, 189

eastern, ii. 172, 189

flat-nosed, ii. 172, 189

narrow- nosed, ii. 172, 189

question as to descent of, ii

165, 441

tailed, ii. 172, 189

western, ii. 172, 189

Aorta, i. 265 ; ii. 378

roots of, ii. 375

— — ^ stem of, ii. 375
Aortal arches, ii. 375, 378
Appendiculariot i. 459 ; ii. 90

492

INDEX.

Arehelnwnthes, ii. 76
Archigastrula, i. 198, 241
Archilithic epoch, ii. 9, 19
Arehipterygiv/m, ii. 301
Arohizoio periods, ii. 9, 11
Area gertninativa, i. 292

opaca, i. 296

pellucida, i, 296

Arietotle, 1. 27 ; ii. 368

epigenesis, i. 29

heart formation, ii. 368

■ his history of eyolution,
i. 27
Ann, lower, ii. 278, 804

upper, ii. 278, 304

ArtericB omphalo-mesenteriecOf i. 895

vmbilicales, i. 400

vertehrales, i. 396

vitelUncB, 1. 395

Arteries, i. 393
Artery-arohes, ii. 377

stalk, ii. 380

Arthropoda, ii. 92, 94
Articulated animalB, ii. 92, 94
Articulation in man, i. 346
Ascidia, i. 429 ; ii. 90

ilastula of, L 455

chorda of, i. 456

— — communities of, i. 455

gastrula of, i. 455

gill-sac of, i. 431

heart of, i. 433

homologies of, i. 465, 466

intestine of, i. 432

mantle of, i. 430, 461

medullary tube of, i. 458

sexual organs of, L 434

tail of, i. 456

AsoulOf ii. 68
Atrium, ii. 874, 881
Auditory nerve, ii. 262
— — — organ, ii. 260

passage, ii. 269

vesiclei, ii. 262

Auricular processes of heart, iL 881
Axes of the body, i. 255 ; ii. 77
Axial cord, L 301

rod (notochord), i. SOS

skeleton, u. 280, 899

Axis-plate, i. 299

iL im

B

Baeb, Karl Ernst, i. 60

his germ-layer theory, 1. 61

his law, i. 58

' life of, i. 52

on the bladder.like outline,

ii. 62
■■ on the human egg, L 55 ; ii,
424

on the notochord, i. 65

on type theory, L 64

Balanoglo88U8, ii. 85

Bathybius, ii. 49

Batrachia, ii. 131

Bats, ii. 169, 187

Beaked animals, ii, 147, 187

Bell-gastrula, i. 198

Bilateral outline, i. 257 ; ii. 74

Bimana, ii. 169

Biogeny, L 24 ; ii. 434

fundamental law of, i. 6,

24 ; ii. 434
Birds, ii. 120, 138

gastrula of, i. 223

Bischoflf, Wilhebn, i. 59
Bladder-gaetrula, i. 229, 241
Blastcea, ii. 31
BlastocoBloma, i. 189
Blastoderma, i. 189
Blastodiscus, i. 227
Blastogeny, i. 24
Blastophylla, i. 196
Blastophyly, L 24
Blastosphcera, i. 191
Blastula, i. 191, 242
Blind-intestine, ii. 330, 348
Blood-cells (corpuscles), i. 169} M.
366
■ ■■ relationship, i. 112

vessels, ii. 870

Bloodless worins, ii. 76
Bonnet, i. 40
Brain, i. 212, 232

bladders of, i. 348 f fi. 114

parts of, ii. 212

skull of, ii. 29S

Breast>body, ii. 288

bone, ii. 282

■ i o»vity, i. 261

ii. Ml

INDEX

493

Bndding (gemmation), ii. 891
Bulhus arteriosv^, ii. 374
BuUms oculif ii. 250

Cjeicolithic epoch, ii. 11, 16
Csenozoic period, ii. 15, 19
Calf -bone, ii. 278, 304
Cambrian period, ii. 9, 19
Canalis auricularis, ii. 381
Carboniferous period, ii. 10, 19
Cardinal veins, i. 391
Carpus, ii. 278
CatarhincB, ii. 176, 189
Catastrophes, theory of, i. 76
Causae eficientes, i. 16, SO ; ii. 455

finales, i. 16, 80 ; ii. 455

Cavum tywpani, ii. 261, 270
Cell-division, i. 124

kernel (nucleus), L 126

state, i. 124

substance, i. 125

Cells, i. 125

female, i. 171 ; ii. 392

male, i. 171 ; ii. 392

theory of, L 60, 121

Central heart, ii. 120

medulla, ii. 210, 232

nerve-system, ii. 210

skeleton, ii. 280, 299

« Centre of sight," ii. 252
Ceratod'us Fosteri, ii. 119
Cerehellwm, ii. 212, 232
Cerehrvm, ii. 212, 232
Cetacea, ii. 187
Cetomorpha, ii. 160, 187
Chalk period, ii. 14, 19
Chalk-sponges, i. 117
Chick, importance of, L 31
Chimpanzee, ii. 178, 180
Chorda animals, ii. 84, 87

dwsaUa, i. 255, 301

sheath, ii. 286

tissue of, ii. 286

vertehralis, i. 255, 301

Chordoma, i. 84, 87
Chorioidea, ii. 252, 258
Chorion, i. 387 ; ii. 158

frondosu/m, ii. 160

I«v«, ii, 160

Chorion, smooth, ii. 160

tufted, ii. 160

Chorology, i- 113
Chyle vessels, ii. 374
Cicatricula, i. 138
Circulation in Amphioxus, i.

Ascidia, i. 433

Fishes, ii. 375

germ-area, i. 397

Mammals, ii, 378

Clavicula, ii. 278, 304
Cleavage cells, L 185

forms of, i. 242

of e^^, i. 185, 241

partial, of bird's egg, L 224

rhythm, i. 243

1 superficial, i. 200, 241

unequal, i. 200, 241

Clitoris, ii. 423, 431
Cloaca, ii. 145, 418
Cloacal animals, ii. 145, 187
Coalescence, i. 164

Coal period, ii. 11, 19

Coccyx, n. 282

Cochlea, iL 263, 268

Coel&nterata, ii. 73

Cceloma, i. 260 ; ii. 76

Ccelomati, ii. 75, 92

Columna vertehralis, i. 349 ; ii. 286

Comparative Anatomy, i. 107, 245

Concrescence, i. 164

Conjunctiva, ii. 259

Connective membrane of eye, ii, 261

tissue, ii. 363

Connectivum, ii. 361, 366

Convolutions of brain, ii. 228

Copulation organs, ii. 421

Copulativa, ii. 421

Coracoideum, ii. 278, 304

Corium, ii. 200, 232

Cormogeny, i. 24

Cormophyly, i. 24

Cornea, ii. 251, 258

CostoB, ii. 278, 282

Covering tissue, ii. 361

Craniota, ii. 100, 120

Crarwu/m, ii. 291

Creation, i. 74, 79 ; ii, 188

Crooked intestine, ii. 319, 880

Cross-vertebrae, ii. 282

Crystalline lens, ii. 253, 268

Culture period, ii. 11

494

INDEX.

Cnrres of embryo, i. 369

Cutis, ii. 200, 232

Cuvier, theory of cjatastropheg, i. 76

theory of types, i. 57

Cycloetoma, ii. 101, 120
Cytods, i. 103
Cytula, i. 176
CytooocciiB, L 176

Daxtov, i. 51

Darwin, Charles, L 96

descent of man, i. 103

^ selection, theory of, i. 95

Bemal selection, i. 103

Erasmus, L 96

Darwinism, i. 95
Deoidua, ii. 161, 165

animals, ii. 161

Decidna^less animals, ii. 161
Decidniata, ii. 161, 187
Deduction, i. 104 ; ii. 37
Degree of development, i. 58
Derma, ii. 232
Descent, theory of, i. 84

of man, i. 104

Devonian period, ii. 10, 19
IHdeVphia, ii. 149, 187
Differentiation, i. 152, 159
Digestive intestine, ii. 330
Digits, ii. 278
Dilarial period, iu 12, 15
Dipneosta, ii. 115, 120
DuBOOgastmla, i. 219, 241
Discoidal cleayage, i. 225, 242
D»«eopIac««aaZ»a, ii. 162, 187
Biscu* hla^odermicvis, i. 139, 226
Doellinger, i. 50
Dorsal fnirow, i. 302

marrow, ii. 221

— — — swellings, i. 303
Donbla-breathers, ii. 117, 120

nostrils, ii. 97, 101

« Double-shield," i. 297
Dualism, i. 17 ; ii. 456
Dualistic philosophy, i. 17
Ductut Qartneri, ii. 416, 431

Mullen, ii. 415, 431

Bathkei, ii. 415, 431

WoljSHi, ii. 415, 431

DysfcelMlogy, i. 109

Eae, bonelets of, ii. 268

labyrinth of, ii. 262, 268

muscles of, ii. 271

nerve of, ii. 266

pouch {viriculus) , ii. 262

sac (sacculus), ii. 262

shell of, ii. 269

snail of, ii. 263, 268

trumpet, ii. 260

vesicles, ii. 265

wax, glands of, ii. 262

Echidna, ii. 147
Echinoderma, L 435 ; ii. 92
Egg-cell, i. 132

of Chick, i. 139

Bird, i. 139

Mammal, i. 137

Man, i. 137 ; ii. 425

sponge, i. 144

cleavage, i. 185, 242

holoblastic, i. 215

human, i. 137 ; ii. 425

membranes, i, 375 ; ii. 158

meroblastic, i. 216

Elbow, ii. 278, 304
Elementary organism, i. 124
Embryo of Vertebrates, i. 360
Embryology, i. 3
Empty intestine, ii. 319
Encephalon, ii. 232
Endoccelarium, ii. 366, 400
Enteropneusta, ii. 86
Entoderms, i. 206, 236
Eocene period, ii. 11, 16
Epidermis, ii. 200, 232
Epididymis, ii. 417, 428
Epigenesis, i. 39, 41
Epigenesis, theory of, i. 39, 41
Epithelial tissue, ii. 361
Epithelium,, ii. 361, 366
Epochs, duration of, ii. 3
Evolution of forms, i. 19

of functions, i. 19

history of, i. 1, 24

theory of, i. 34

Excretory organs, i. 267 ; ii. 408
Exoccelarium, ii. 369, 400
EwedermQ, i. 195, 236

iHDJUL

495

Extremitlee, ii. Ill, 806
Eye, ii. 250, 258

connective membrane of, ii.

251

lids, ii. 259

netted membrane of, ii. 252,

268
— - ]»t>teotive membrane of, ii 251
— — pupil of, ii. 250

rainbow membrane of, ii. 252

I rascular membrane of, ii. 252
Twiclea, L S67 1 ii 263

Fabbicitts ab Aquapindente, i. 81
Face, development of, ii. 245, 846

skull of, ii. 278, 294

Fallopian canals, ii. 431

hydatids, ii. 431

Fatty layer of oorium, ii. 232
Female breast, ii. 202

cells, i. 171, 392

Oqpnlatory organs, ii. 423

excretory dncts, ii. 415, 431

— — — — germ-glands, ii. 398

germ-layer, ii. 398

— — — milk glands, ii. 202
— -^ phallus (Clitoris), ii. 428
— — sexual organs, ii. 423

sexual plate, iL 401

uterus, ii. 417

Femur, ii. 278, 304
Fertilization, i, 169, 176
Fibula, ii. 278, 304
Fia, central ffod of, ii. 802
Fin, rays of, ii. 302

■ skeleton of, ii. 808
Final causes, i. 16
Fingers, ii. 278
Fishes, ii. 109, 120

fins of, ii. Ill

gastrula of, i. 219

scales of, ii. 331

Fire-digited foot, ii, 128
Flat-worms, ii. 76
Flesh, i. 259
Flesh-kyer, i. 286
Foot, ii. 170
Flovoe and matter, ii. 466

65

Forces, active, 8. 487

latent, ii. 457

Formative functions, i. 16

yelk, i. 216

Forms, science of, i. 20
Frog-Batrachia, ii. 181
Frogs, ii. 131

egg-cleavage of, i. 208

' gastrula of, i. 207

- ■ larva of, ii. 127

metamorphosis of, iL 126

Frontal process, ii. 244
Functions of evolution, ii. 156

science of, i. 19

Funiculus genitalis, ii. 418
umbiUcaUs, i. 883 ; ii. 16b

e

Gall-bladder, ii. 841

ducts, ii. 341

intestine, ii. 317, 380

Ganoid Fishes, ii 112, 120

Gartnerian duct, ii. 416, 431

GastrcBa, i. 232 ; ii. 66

theory of, i. 247 ; ii. 195

Gastraeads, ii. 62, 70

Gastrocystis, i. 291

Qastrodiscus, i. 292

Gastrula, i. 192 ; ii. 65

Bell-, i. 198

Bladder-, i. 200

Disc-, i. 200

Hood-, i. 200

Gegenbaur, i. 108; ii. 96

on Comparative Ana-
tomy, ii. 96

Gegenbaur on head-skeleton, ii. 29.3

on theory of descent,

i.108

skull theory, ii. 293

theory of limbs, ii. 299

Oeneratio spontanea, ii. 30

" Generelle Morphologie," i. 102

Geological hypotheses, i. 410

Germ, i. 3

Germ-area, i. 292

- dark, i. 297

light, i. 297

cavity, i. 189
diao, i 189»

496

nmEx.

Germ-epithellBm, ii. 401

glands, ii. 398

history, i. 6, 24

layer, middle, i. 13

membrane, i. 189

membrane vesicle, i. 189

plate, ii. 401

point, i. 135

shield, i. 297

spot, i. 135

yesicle, i. 179, 291

Gibbon, ii. 178, 181
Glacial period, ii. 11
Glands of intestine, ii. 330

skin, i. 201

Giant phalli, ii. 422
Qhymeruli renales, ii. 407
Onathostomi, ii. 109
Goethe, Wolfgang, i. 88

his skull theory, ii. 293

morphology, i. 88

on metamorphosis, i. 90

on reason, ii. 453

on specification, i. 90

Goette, Alexander, i. 65
Gonades, ii. 398
Gonochorisrmis, ii. 69, 396
Gonophori, ii. 402
Gorilla, i. 178, 180
Graafian follicles, ii. 424
GvhemaeuluMi Hv/nterif ii. 4i31

Hai«, ii. 205, 232
Hair-animals, ii. 205
Hairy covering, ii. 206
Haliph/ysema, ii. 66
Haller, Albrecht, i. 38
Hand, ii. 169

skeleton of, ii. 302

Hare-lip, ii. 246
Harvey, i. 31
Head-cap, i. 386

marrow, ii. 210

plate, i. 335

ribs, ii. 298

sheath, i. 387

Heart, auricle of, ii. 381

- ■ ftorioalar procesgei of, ii.

381

Heart oarity, i. 894

development of, ii. 88S

human, ii. 379, 382

— ^— — mesentery, i. 394

ventricle of, iL 381

Heopitheciy Li. 172

Heredity, i. 161

vitiated, i. 408

Hermaphrodites, ii. 395

Hermaphi-odite gland, ii. 401

Vertebrates, ii. 4Ui

Hermaphroditisrmis, ii. 69, 396

Hesperopitheci, ii. 172

Heterochronism, i. 13

Heterotopism, i. 13

Hind-brain, ii. 221, 232

intestine, ii. 343

limbs, ii. Ill

Hip-bone, ii. 278

His, Wilhelm, i. 64

Histogeny, i. 24

Histology, i. 24

Histophyly, i. 24

Hollow-worms, ii. 76
Holoblastic eggs, i. 216

Hologastrula, i. 241

Homology of primitive intestin©, i

247 ; ii. 321
I of the animal tribee, iL

387
- germ-layers, i. 24t

sexes, ii. 431

Hood-gastrula, i. 200, 241
Hoofed animals, ii. 160, 188
Horn-plate, i. 307
Horn-stratum of Epidermitf ii. 200
Humerus, ii. 278, 304
Huxley, i. 101 ; ii. 294

germ -layer theory, i. 67

— — — his Evidences, i. 101
— — — Man and Ape, i. 101

primates, law of, ii. 177

skull theory, ii. 294

Hylohates, ii. 181, 189
Hypospadia, ii. 423

Immacvlati conception, i. 170
luiiMvhM ii. 159, 187
Ixtdividaality, i. 123

INDEX.

497

ladiTidaality of oells, i. 123

of metamera, i. 348

Indo. Germanic pedigree, ii. 23
Induction, L 104 ; ii. 35
Inorganic history of earth, ii. 5
Insects, mental capacity of, ii. 4-^8
Iniegume^Uxtm, ii. 199
Intestinal germ-disc, i. 291
• vesicle, i. 291

head-caviiy, i. 336

Intestine, after, ii. 321

'■ blind, ii. 330

crooked, ii. 319, 8S0

dijjestive, ii. 330

empty, ii. 330

middle, ii. 330

stomach, \L 330

Inrertebrates, i. 414
Iri», ii. 252, 258

Jasgek, Qustav, i. 101
Jaw arches, ii. 102

lower, ii. 102

upper, ii. 102

JxiracBio period, ii. 14, 19

Kaitt, Imm AircKL, i. 79
KidnejB, ii. 403, 412
Kidney system, ii. 403

primitive, iL 306, 410

Kenogenesis, i. 12
Kftnogenetic cleavage, i. 231
Kernel of oell (nncleus), i. 127
Kleinenberg, Nioolaus, ii. 358
KOUiker, Albert, i. 69, 62
KowaloTsky, Aogost, L 59, 441

Labtkiifth of kar, ii. 260

Labyrinthulae, ii. 58

Lamarck, Jean, i. 82

his life, i. 82

■ Man and Ape, i. 85

" '" *' Philosophie Zoologiqne,
i. 83

»

La/mina dermdUa i. 273, 327

gastralis, i. 273, 327

inodermalis, i. 327

inojastralis, i. 327

myxogastralis, i. 327

neurodermalia, i, 827

Lampreys, ii. 101, 121
Lancelet, i. 253, 413
Lankester, Ray, i. GO
Lanugo, ii. 206
Larynx, ii. 330

Latebra (of bird's egg), i. 138
Laurentian period, ii. 9, 19
Layers (Lamince), i. 273, 327
Leather.plate, i. 327

skin, ii. 200, 232

Leenwenhoek, i. 37
Leg, lower, ii. 278

upper, ii. 278

Leibnitz, i. 39
Lemuria, ii. 183
Lemurs, ii. 164
Lens, ii, 251, 254
Lepidosiren parcuioxa, ii. 119
Leptocardia, ii. 120
Limbs, ii. Ill, 306

fore, ii. 302

hind, ii. 306

skeleton of, ii. 305

theory of, ii. 305

Linnaeus, Karl, i. 7S
Lip-cartilage, ii. 245

fissure, ii. 246

Liver, ii. 330, 341
Lizards, iL 120, 129
LocomotoriMmf ii. 194, 274
Lori, ii. 163
Lyell, Charles, i. 77
Lymph-cells, ii. 366
T«ss«ls, ii. 278

Macula ftrmmativa, i. 133
Magosphwra pUvnula,, iL 60
Male breast, ii. 204
cells, i. 171 ; ii. 392

copulatory organs, ii. 42.'^

excretory ducts, ii. 414, A^

germ-glands, ii. 398

germ-layer, ii. 898

498

INDEX.

Male milk-glandB, ii. 204

phallus (Penis), ii. 423

sexual organs, ii. 431

sexual plate, ii. 401

uterus, ii. 419

Malpighi, i. 31

Malthus, i. 98

Mamilla, ii. 204

Mammaf Ii. 204 -

Mammalia, ii. 141, 187

Mammals, egg-cleavage of, i. 210

gastrula of, i. 213

— ■ mental capacities of, ii.

448
Man.4kpes, ii. 178
Mantle animals, ii. 83
Marsupobranchii, ii. 104
Mwrsupialia, ii. 149, 187
Martins, Charles, ii. 304
Materialism, ii. 456
Maternal placenta, ii. 160
Matter, ii. 457
Mechanism in nature, i. 80
Meckel's cartilaa^e, ii. 298
Medulla, ii. 211,^^232

capitis, ii. 211

centralis, ii. 211

— oblongata, ii. 211

— — — — spinalis, ii. 211
MedoUary furrow, i. 302

membranes, ii. 228

plate, i. 327

swellings, i. 308

tube, i. 305

Mmingea, ii. 228, 232

Meroblastic eggs, 216

Merogastrula, i. 241

Mesentery, ii. 320

Mesoderma, i. 236, 278

Meaolithio epoch, ii. 14, 19

Mesozoic periods, ii. 12, 14

Meta,carj>us, ii. 278, 304

Metagaster, ii. 321

Metagastrula, i. 199

Metameray i. 346

Metamerio structure, i. 347

Metanephra, ii. 412

Metatargus, ii. 278, 304

MetoMa (intestinal animals), i. 248 {

n.92
ldicrol4*t4»t ii. 149
Mid.brain, ii. 221, 282

Middle germ-layer, i. 278

intestine, ii. 330

layers, i. 278

'■ part of foot, ii. 278, 304

hand, ii. 278, 304

Migration, theory of, i. 114
Milk, ii. 202

glands, ii. 143, 202

Mind, ii. 226, 447

activity of, ii. 210

cells of, i. 129

development of, ii. 450

heredity of, ii. 452

Molliisca, ii. 92, 94
Monads, i. 39
Monera, i. 180 ; ii. 43
Monerula, i. 179

Monistic philosophy, i. 16 ; ii, 466
Monocondyles, ii. 138
Monodelphia, ii. 151, 187
Monogeny, i. 160
Monophyletio origin, ii. 277
MoTwrhina, ii. 101, 120
Monotrema, ii. 145, 187
Monstrons evolution, i. 168
Morphogeny, i. 21, 24
Morphology, i. 21
Morphophyly, i. 24
Morula, i. 189
Motor apparatus, ii. 194, 274

germinative layer, i. 330

Mouth, i. 338; ii. 315, 830

cavity, ii. 315, 330

groove, i. 338

Mud.fishes, ii. 115, 120
Mulberry -germ, i. 189
Miiller, Fritz, i. 59, 408

Hermann, i. 170

Johannes, i. 59 ; ii. 96, 414

Miillerian duct, ii. 414, 431
Muscles, i. 259 ; ii. 364
Muscle-plate, i. 353

system, ii. 308

Mymvnoideg, ii. 101, 12Q

N

Nails, ii. 204, 232

Natural history of creation, L lOt

philosophy, i. 82

Navel, i. 815. 335

INDEX.

499

t^ATel arteries, i. 400

cord, i. 384 ; ii. 168

mesentery arteries, i. 395

veins, i. 399

reins, i. 399

vesicle, i. 337, 377

Neok curvature, i. 369

marrow, ii 211, 222

rertebrae, ii. 281

Nerre-oells, i. 126

system, ii. 211, 232

Neuro-muscular cells, ii. 232, 236
Nictitating membrane, ii. 259
Nipples of milk-glands, ii. 202
Nipple-less animals, ii. 146, 204
Nose, i. 374; iL 247

of Ape, ii. 175

cavities, ii. 245

flaps, ii. 243

furrow, ii. 244

grooves, ii. 244

processes, ii. 242

roofs, ii. 242

Notaspis, i. 297
Nucleohis, i. 133
Nucleus^ i. 125
Nutrition, i. 158
Nutritive yelk, i. 216

CKkoloot, L 114
Oken, Lorenz, i. 49
Olfactory grooves, ii. 240

nerve, ii. 239

organ, ii. 239

Ontogenesis (evolution of the germ),

i. 12
Ontogenetic fission of the layers,
i. 238

unity, i. 366

Ontogeny, L 6
Oophora, ii. 898, 429
Optic nerve, ii. 250
Orang-outang, ii. 178, 181
Orchides, ii. 399, 429
Organic history of the earth, ii. 7
Organisms without organs, ii. 46
Organogeny, i. 24
Organology, ii. 192
Oiganophjly, i. S4

Organ-Bygtemt, age of, ii. 357, 367

human, ii. 194

Original cleavage, i. 198, 241
Ornithodelphia, ii. 145, 187
Omithorhynchus, ii. 147
Omithostoma, ii. 147, 187
Os ilmm, ii. 278, 304
Os ischii, ii. 278, 304
Os pubis, ii. 278, 304
Outer skin, ii. 200, 232
Ovary, ii. 398, 430

plate, ii. 401

Oviduct, ii. 403, 429
OvococcuSf i. 183
Ovojtlasma, i. 183
Ovtda holohlasta, i. 215, 241

merohlasta, i. 216, 241

Ovulists, i. 37
Ovulvm, L 171, 183

Pachycardia, i. 120
Palate, ii. 330

roof, ii. 330

soft, ii. 330

Palaeolithic epoch, ii. 10, 19
Palaeontology, i. 106
Palaeozic periods, u. 12, 13
Palingenesis, i. 10
Palingenetic cleavage, L 211
Palingeny, i. 11
Pcmcreas, ii. 330, 343
Pander, Christian, i. 51
Paradise, ii. 183
Parallelism in evolution, ii. 70
Parent-cell, i. 176

kernel, i. 176

Parov(vrivMty ii. 417, 4.31
Parthenogenesi*, i. iO, 170
Partial clearage, i. 316, 240

of bird's egg, I 22 1

Pastrana, Julia, i. 374
Pedigree, i. 112

of animals, ii. 98

Apes, ii. 189

cells, i. 467

Indo- Germanic Ian.

guages, ii. 23
-^— — ^— Mammals, ii. 188
— ^— — — mas, ii. 23

500

INDEX.

Pedigree of Vertebrates, ii. 121
Pelvic girdle, ii. 278, 304

intestinal cavity, i. 335

Prnvit, ii. 423, 431
Pentadaetylia, ii. 123, 802
Perigcutrula, i. 230, 241
Peripheric nerve- system, ii. 228
Permian period, ii. 10
P»tromyM<mte$, ii. 101, 120
PhallM, ii. 422, 431
Phalhuia, i. 454
Pharyna, ii. 316, 830
Philology, ii 20

comparative, ii. 21

Philoiophy, i. 17 ; ii. 456
Phylogene»i$ (evolution of the tribe),
i. 12

Phjlogenetio fission of the layers,
i. 238

hypotheses, i. 413

Phylogeny, i. 5, 72

Physiogeny, i. 21, 24

Physiology, i. 20

comparative, i. 20

Physiophyly, L 24

Pig, i. 362

Pigment-membrane, ii. 252

Pithecanthropi, ii. 181

Pithecoid theory, ii. 441

Placenta, i. 883 ; ii. 155, 168

disc-shaped, ii. 162, 187

©mbryonic, ii. 160

fcetalis, ii. 160

— — — — girdle-shaped, ii. 162, 187

maternal, ii. 160

uterina, ii. 160

Placental animals, ii. 153, 187

Placentalia, ii. 153, 187

FUkfUM, ii. 61

PbauMds, ii. 61

Pbknt-animals,

Planula, ii. 59

Plasson, i. 130 ; ii. 43

FlMtids, L 130 ; ii. 46

Plastid ancestors, ii. 184

FlMtid theory, i. 130

Plastidoles, ii. 47

Plates {lamella), i. 803

PUOkehninthea, ii. 76
PIntyrMno, ii. 175, 189
Plonro-peritoneal cavity, L 26Q
poriod, iL 11, 16

Polydaetylia, ii. 123
Poms genitalis, ii. 402
Post-glacial period, ii. 11
Pouch-bones, ii. 151
Pouched animals, ii. 149, 187
Praedelineation theory, i. 37
PrBBformation, i. 34

theory, i. 34

Prmputium, ii. 423, 431
Pressure, sense of, ii. 238
Primary axial skeleton, iL 285

age, ii. 10, 11

germ -layers, i. 196

Primates, ii. 169

Primitive amnion animals, ii. 13t

animal ancestors, ii. 184

animals, or Protozoa, i. M#

clavicula, ii. 278

egg {Protovum), i. 134

fins, ii. 303

Fishes, ii. 112, 120

furrow, i. 226

germ-layers, i. 195

groove, i. 335

intestine, i. 444 ; ii. 318

kidney, i. 306 ; ii. 410

ducts, ii. 406

Mammal, ii. 142, 187 ,

Man, ii. 182

• mouth, i. 444 ; ii. 313

skull, ii. 296

slime, ii. 43

streak, i. 299

urine-bladder, ii. 411

urine-sac, i. 379 ; ii. 41 1

vertebrae, i. 346

— '■ vertebral cords, i. 306

vertebral plates, i. 346

■ — Vertebrate (ideal), L 256

(real), iL 98

Worm, ii. 74, 80

Primordial cleavage, i. 198, 241

kidneys, i. 307 ; ii. 410

skull, ii. 297

times, ii. 9, 11

Prochorion, ii. 157
Procoracoidevm,, ii. 278, 304
Promammalia, ii. 142, 187
ProsimicB, ii. 163, 187
Protamosha, ii, 46
Protamnionf ii. '83
Prothelmitt ii. 76

OTDEX.

501

FT<4oga$t€r, L 444 ; iL 314
Frotorrvyza, ii. 46
ProtoTiephra, ii. 410
Protoplasma, i. 131
Protopterua armecten*, ii. 119
Protoioa, ii. 248
Protureter, ii. 406
Peeadopodia of Amoeb», i. 142
Ptyche, ii. 225, 446
Piychology, ii. 448
Pttbio bone, ii. 278
Pwnctum germinativumf L 186
Pupil, ii. 252, 258
'—— membrane, ii. 254

SaeUiM, ii. 278, 304

Rathke, Heinrich, i. 59

Rathke's duct, ii. 415, 431

Reason, ii. 453

Reicbert, Bogulaus, i. 61

Kemak, Robert, i. 62

Re7ie$, ii. 412

Reprodaction, i. 159

Reproductive organs, ii. 392, 418

Reptiles, ii. 120, 138

Eespiratorj intestine, i. 262 ; ii. 330

organs, i. 262 ; ii. 333

Retina, ii 252, 258
Bibs, ii. 285
Rolle, Priedrich, i. 101
Bound -month 8, ii. 101, 120
Budimentary organs, i. 109
Bmnp bone, ii. 282

vertebrae, ii. 282

SuBCOni, anus of, i. 206
BwKKmi'a nutritive oavitj, L 907

Salamakdei, ii. 127
Salivary glands,
Bwuropsida, ii. 138
Scapula, ii. 278, 303
Schleidon, M. J., i. 60, 128
8<diwaim, Theodor, i, 60
aeUrvtiem, i Ul, 268

Seoleeidoi ii 88
Scrotum, ii. 423, 431
Sea-Lettlea, ii. 73, 92
Secondary age, ii. 11, 14

axial skeleton, ii 291

germ -layers, i 235, 3fTh

kidneys, ii. 412

; sexual chanicter, ii. 3i^

— strata of earth, ii. 12

Seed (male), i. 36

animalcules, i. 173

cells, i. 173

duct, ii. 403, 429

Segmental canals, ii 406
Segmentation, i. 186
Segmentella, i. 186
Selachii, ii 112, 121
Selection, theory of, i 96
Semi-apes, ii. 164, 187
Semicircular canals -of ear, ii. 26r?

268
Semper, Karl, i 91, 426
Sense oiP pressure, ii. 238

of warmth, ii. 238

Sense-organs, ii. 238
Sensorium, ii. 194
Sensory a]'paratus, ii. 194

functions, ii. 238

layer, i. 236, 229

nerves, ii. 238

Sexes, separation of, ii. 69, 896
Sexual cells, origin of,

cord, ii. 418

ducts, ii. 402, 429

folds, ii. 422, 431

furrow, ii. 422 431

glands, ii. 398

■■ nerves, ii. 238

organs, i 266

plates, ii. 399

selection, i 103 ; ii. 894

sense, ii. 238

Sheath {Va<fina), ii. 417, 481

of amnion, i 387

Shin-bone, ii. 278, 304
Shoulder-blade, ii. 278, 804

■ girdle, ii. 278, 304

Side-layers, i. 303

plates, i. 303

sheath, i. 308

Silurian period, ii. 9, 19
Shuniatt ii. 165

50J

INDEX.

Sing^*-noetnl8, ii. 101, 120
SmtLS urogenitalis, ii. 419, 421
Siredon, ii. 129
Skeleton, ii. 278

Skeleton-forming cell-layer, ii. 287
Skeleton, muscles of, ii. 194

plate, ii. 287

Skin, ii. 195, 229

corering, ii. 195, 232

fibrous layer, i. 236

■- — glands, ii. 201, 232

layer, i. 236

muscle layer, i. 236

moBcles, ii. 194

— .— narel, i. 317

iMrres, ii 238

sensory layer, i. 236

stratum, i. 236

SkHll, ii. 292

floor, ii. 292

roof, ii. 292

vertebrae, ii. 294

theory of, iL 295 *

Skulled Animals, ii. 100
Skull-less Animals, i. 416 ; ii. 99
Small brain, ii. 213, 232
Soft-bodied Animals, ii. 92, 94
Soft worms (Scolecida), ii. 86
Sozohranchia, ii. 129
Sozura, ii. 129
Species (idea of), i. 73, 116
Sperma, i. 171
8permaduetu$, ii. 403, 429
Spermalists, i. 37
Spermatasoa, i. 172
Sp^rm-cells, i. 172
Sperm^coccxis, i. 7 S3
3p«rmopla*ma, i. 188
3p«rmviwnf L 183
Spine, ii. 280
SpiritualijBii, ii. 456
Spoke-bone (Radiiut), ii. 278, 304
Sponges, ii. 73, 92
Spontaneous generation, ii. 80
Star.animals, i. 435
Stenops, iL 164
SUrnun^ ii. 278
Stomach, ii. 330

intestine, ii. 830

8trT!ggle for existence, i. 96
S^theuHs, iL 232

ii*7V

Superficial clerwage, i. 229
Sweat-glands, ii. 202
Swimming-bladder, ii. Ill, 336
Sylvian aqueduct, ii. 221
Synairuebinm, ii. 56
System of animals, ii. 92
germ-layerH, i. 273, 327

mammals, ii. 188

organs, ii. 194

tissues, ii. 366

vertebrates, ii. 120

Tadpolks, ii. 128

Tail, human, i. 372 ; iL 28S

Tail-cap, i. 387

curvature, L 369

sheath, i. 387

vertebrae, ii. 283

Tailed apes, ii. 180

Batrachia, ii. 129

Tarsus, ii. 278

Taste, nerve of, ii. 238
— sense of, ii. 238
Teeth, ii. 173, 331
Tegumentum, ii. 199
Teleology, i. 16, 109
Teleostei, ii. 115, 120
Terminal budding, i. 349
Tertiary age, ii. 11, 15
Testes, ii. 399, 429
— — ^^ change of place of, iL 419

sac, ii. 423, 431

TesticuU, ii. 399, 429
Theoria Oenerationis, L 41
Thorax, ii. 282
Thyroid gland, ii. 336
Tibia, ii. 278, 304
Tissues, ii. 362, 366

age of, ii. 361, 366

connective, ii. 363

— — — covering, ii. 361, 368

vascular, ii. 361, 366

Total cleavage, i. 217, 242
Tongue, ii. 331

arch, ii. 296

bone, ii. 298

Tortoise, ii. 120
ToMk bodies, ii. 220

INDEX.

S03

Toneh, organ of, ii. 199, 238
Transition forms, i. 117
Tread, i. 138
Triasaio period, ii. 14, 19
Tribal history, i. 7, 24
Trophic germ-la jer, i. 239
TubcB FalUypicB, iL 431
Tube-hearta, ii. 120
TvinietUa, ii. 83, 92
Twhellaria, ii. 79
Twixt-brain, ii. 220, 282

jaw, ii. 246

Tjmpasie oarity, ii. 261, 268

membrane, ii. 261, 2fJ8

Tympanum, ii. 261, 268
Types in animal kingdom, i. 56, 246
of , L 66, 246

Ulna, ii. 278, 304

UnguUita, ii. 160, 188

CTnitary conception of the world, i.

17 ; ii. 456
Urachus, ii. 413
Ureter, ii. 413
Urethra, ii. 423, 431
Urinary bladder, ii. 418

ducts, ii. 406

organs, ii. 403

sac, i. 379

sexnal cavity, ii. 419

sexnal duct, ii. 403

system, ii. 403

Ulenis, ii. 417, 431

bicornis, ii. 418 '

masculirvus, ii. 419

{Timlo, ii. 430

Vchgina, ii. 417, 481

Vamjyyrella, ii. 48

Van Beneden, Eduard, i. 60, 209;

ii. 398
7m« deferentia, ii. 403, 429

wnbiUcalia i. 399

Vawrnlar syitem, ii. 384

Vegetatire germ-layer, i, 196, 327

organs, ii. 193, 194

Veins, i. 393
Ventral cavity, i. 316

plates, i. 316

vessel, i. 423

— wall, i. 316

Ventricle of heart, ii. 374

Vermal appendage of coeoum, ii. 34

Vertebraa, ii. 280

number of, ii. 288

Vertebral arches, ii. 284

bodies, ii. 284

canal, ii. 284

column, ii. 280

Vertehrarium, ii. 278

Vertebrat-es, ii. 92

Vertebrates, ancestoni o^ ii. 186

mental capacities o!

ii. 446

pedigree of, ii. 93

system of, ii. 97

Vesicula hlastodermica, i. 290

germinativa, i. 133

prostatica, n. 419

umhilicalii, i. 377

VesUhulum vagirux, ii. 431
Virginal generation, i. 170
Vitellus, i. 135

Wagwer, Moritz, i. 114
Wallace, Alfred, i. 98, 99
Water, amount of in body, ii. 7
Whale.like Animals, ii. 160, 189
Whales, ii. 160
Wolff, Caspar Friedrioh, I 40

his life, i. 41

his Natural Philosophy, i. 47

on formation of intestine, i. 44

on germ-layers, i. 45

Theoria QeneratiorUg, L 41

Wolffian bodies, ii. 411

duct, ii. 414, 431

Wolff's primitive kidneys, ii. 411, 4;'« '
Woolly hair of embryo, ii. 206
Worms, i. 246 ; ii. 74

ancestors of, ii. 78

- tribe of, ii. 73

Wrist, ii. 278

^04

iKDBt.

Vklk, i. ISft

arteries, L 396

cavity, i. 138

duct, i. 388; ii. 168

fonnativa, i. 216

membrane, i. 138

— mxtritire, i. 216
— no, L 887

T«lk T«ma, L 895

TeseelB, i, 396

Zona pMueida, i. 135
ZofUiplaeentaUa, iL 162, 187
Zcophyta, i. 2^ ; iL 78

(15)

THE END.