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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/6710.txt b/6710.txt new file mode 100644 index 0000000..4c04fc5 --- /dev/null +++ b/6710.txt @@ -0,0 +1,12884 @@ +The Project Gutenberg EBook of The Evolution of Man, V.2, by Ernst Haeckel +#2 in our series by Ernst Haeckel + +Copyright laws are changing all over the world. Be sure to check the +copyright laws for your country before downloading or redistributing +this or any other Project Gutenberg eBook. + +This header should be the first thing seen when viewing this Project +Gutenberg file. Please do not remove it. Do not change or edit the +header without written permission. + +Please read the "legal small print," and other information about the +eBook and Project Gutenberg at the bottom of this file. Included is +important information about your specific rights and restrictions in +how the file may be used. You can also find out about how to make a +donation to Project Gutenberg, and how to get involved. + + +**Welcome To The World of Free Plain Vanilla Electronic Texts** + +**eBooks Readable By Both Humans and By Computers, Since 1971** + +*****These eBooks Were Prepared By Thousands of Volunteers!***** + + +Title: The Evolution of Man, V.2 + +Author: Ernst Haeckel + +Release Date: October, 2004 [EBook #6710] +[Yes, we are more than one year ahead of schedule] +[This file was first posted on January 17, 2003] + +Edition: 10 + +Language: English + +Character set encoding: ASCII + +*** START OF THE PROJECT GUTENBERG EBOOK THE EVOLUTION OF MAN, V.2 *** + + + + +Produced by Sue Asscher <asschers@bigpond.com> + + + + +THE EVOLUTION OF MAN + +A POPULAR SCIENTIFIC STUDY + + +BY + +ERNST HAECKEL + +VOLUME 2. + +HUMAN STEM-HISTORY, OR PHYLOGENY. + + + +TRANSLATED FROM THE FIFTH (ENLARGED) EDITION BY JOSEPH MCCABE. + + + +[ISSUED FOR THE RATIONALIST PRESS ASSOCIATION, LIMITED.] + + + + +WATTS & CO., +17, JOHNSON'S COURT, FLEET STREET, LONDON, E.C. +1911. + + + + +CONTENTS OF VOLUME 2. + + +LIST OF ILLUSTRATIONS. + + +INDEX. + + +CHAPTER 2.16. STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT. + + +CHAPTER 2.17. EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT. + + +CHAPTER 2.18. DURATION OF THE HISTORY OF OUR STEM. + + +CHAPTER 2.19. OUR PROTIST ANCESTORS. + + +CHAPTER 2.20. OUR WORM-LIKE ANCESTORS. + + +CHAPTER 2.21. OUR FISH-LIKE ANCESTORS. + + +CHAPTER 2.22. OUR FIVE-TOED ANCESTORS. + + +CHAPTER 2.23. OUR APE ANCESTORS. + + +CHAPTER 2.24. EVOLUTION OF THE NERVOUS SYSTEM. + + +CHAPTER 2.25. EVOLUTION OF THE SENSE-ORGANS. + + +CHAPTER 2.26. EVOLUTION OF THE ORGANS OF MOVEMENT. + + +CHAPTER 2.27. EVOLUTION OF THE ALIMENTARY SYSTEM. + + +CHAPTER 2.28. EVOLUTION OF THE VASCULAR SYSTEM. + + +CHAPTER 2.29. EVOLUTION OF THE SEXUAL ORGANS. + + +CHAPTER 2.30. RESULTS OF ANTHROPOGENY. + + +LIST OF ILLUSTRATIONS. + +FIGURE 2.210. THE LANCELET. + +FIGURE 2.211. SECTION OF THE HEAD OF THE LANCELET. + +FIGURE 2.212. SECTION OF AN AMPHIOXUS-LARVA. + +FIGURE 2.213. DIAGRAM OF PRECEDING. + +FIGURE 2.214. SECTION OF A YOUNG AMPHIOXUS. + +FIGURE 2.215. DIAGRAM OF A YOUNG AMPHIOXUS. + +FIGURE 2.216. TRANSVERSE SECTION OF LANCELET. + +FIGURE 2.217. SECTION THROUGH THE MIDDLE OF THE LANCELET. + +FIGURE 2.218. SECTION OF A PRIMITIVE-FISH EMBRYO. + +FIGURE 2.219. SECTION OF THE HEAD OF THE LANCELET. + +FIGURES 2.220 AND 2.221. ORGANISATION OF AN ASCIDIA. + +FIGURES 2.222 TO 2.224. SECTIONS OF YOUNG AMPHIOXUS-LARVAE. + +FIGURE 2.225. AN APPENDICARIA. + +FIGURE 2.226. Chroococcus minor. + +FIGURE 2.227. Aphanocapsa primordialis. + +FIGURE 2.228. PROTAMOEBA. + +FIGURE 2.229. ORIGINAL OVUM-CLEAVAGE. + +FIGURE 2.230. MORULA. + +FIGURES 2.231 AND 2.232. Magosphaera planula. + +FIGURE 2.233. MODERN GASTRAEADS. + +FIGURES 2.234 AND 2.235. Prophysema primordiale. + +FIGURES 2.236 AND 2.237. Ascula of Gastrophysema. + +FIGURE 2.238. Olynthus. + +FIGURE 2.239. Aphanostomum Langii. + +FIGURES 2.240 AND 2.241. A TURBELLARIAN. + +FIGURES 2.242 AND 2.243. Chaetonotus. + +FIGURE 2.244. A NEMERTINE WORM. + +FIGURE 2.245. AN ENTEROPNEUST. + +FIGURE 2.246. SECTION OF THE BRANCHIAL GUT. + +FIGURE 2.247. THE MARINE LAMPREY. + +FIGURE 2.248. FOSSIL PRIMITIVE FISH. + +FIGURE 2.249. EMBRYO OF A SHARK. + +FIGURE 2.250. MAN-EATING SHARK. + +FIGURE 2.251. FOSSIL ANGEL-SHARK. + +FIGURE 2.252. TOOTH OF A GIGANTIC SHARK. + +FIGURES 2.253 TO 2.255. CROSSOPTERYGII. + +FIGURE 2.256. FOSSIL DIPNEUST. + +FIGURE 2.257. THE AUSTRALIAN DIPNEUST. + +FIGURES 2.258 AND 2.259. YOUNG CERATODUS. + +FIGURE 2.260. FOSSIL AMPHIBIAN. + +FIGURE 2.261. LARVA OF THE SPOTTED SALAMANDER. + +FIGURE 2.262. LARVA OF COMMON FROG. + +FIGURE 2.263. FOSSIL MAILED AMPHIBIAN. + +FIGURE 2.264. THE NEW ZEALAND LIZARD. + +FIGURE 2.265. Homoeosaurus pulchellus. + +FIGURE 2.266. SKULL OF A PERMIAN LIZARD. + +FIGURE 2.267. SKULL OF A THEROMORPHUM. + +FIGURE 2.268. LOWER JAW OF A PRIMITIVE MAMMAL. + +FIGURES 2.269 AND 2.270. THE ORNITHORHYNCUS. + +FIGURE 2.271. LOWER JAW OF A PROMAMMAL. + +FIGURE 2.272. THE CRAB-EATING OPOSSUM. + +FIGURE 2.273. FOETAL MEMBRANES OF THE HUMAN EMBRYO. + +FIGURE 2.274. SKULL OF A FOSSIL LEMUR. + +FIGURE 2.275. THE SLENDER LORI. + +FIGURE 2.276. THE WHITE-NOSED APE. + +FIGURE 2.277. THE DRILL-BABOON. + +FIGURES 2.278 TO 2.282. SKELETONS OF MAN AND THE ANTHROPOID APES. + +FIGURE 2.283. SKULL OF THE JAVA APE-MAN. + +FIGURE 2.284. SECTION OF THE HUMAN SKIN. + +FIGURE 2.285. EPIDERMIC CELLS. + +FIGURE 2.286. RUDIMENTARY LACHRYMAL GLANDS. + +FIGURE 2.287. THE FEMALE BREAST. + +FIGURE 2.288. MAMMARY GLAND OF A NEW-BORN INFANT. + +FIGURE 2.289. EMBRYO OF A BEAR. + +FIGURE 2.290. HUMAN EMBRYO. + +FIGURE 2.291. CENTRAL MARROW OF A HUMAN EMBRYO. + +FIGURES 2.292 AND 2.293. THE HUMAN BRAIN. + +FIGURES 2.294 TO 2.296. CENTRAL MARROW OF HUMAN EMBRYO. + +FIGURE 2.297. HEAD OF A CHICK EMBRYO. + +FIGURE 2.298. BRAIN OF THREE CRANIOTE EMBRYOS. + +FIGURE 2.299. BRAIN OF A SHARK. + +FIGURE 2.300. BRAIN AND SPINAL CORD OF A FROG. + +FIGURE 2.301. BRAIN OF AN OX-EMBRYO. + +FIGURES 2.302 AND 2.303. BRAIN OF A HUMAN EMBRYO. + +FIGURE 2.304. BRAIN OF THE RABBIT. + +FIGURE 2.305. HEAD OF A SHARK. + +FIGURES 2.306 TO 2.310. HEADS OF CHICK-EMBRYOS. + +FIGURE 2.311. SECTION OF MOUTH OF HUMAN EMBRYO. + +FIGURE 2.312. DIAGRAM OF MOUTH-NOSE CAVITY. + +FIGURES 2.313 AND 2.314. HEADS OF HUMAN EMBRYOS. + +FIGURES 2.315 AND 2.316. FACE OF HUMAN EMBRYO. + +FIGURE 2.317. THE HUMAN EYE. + +FIGURE 2.318. EYE OF THE CHICK EMBRYO. + +FIGURE 2.319. SECTION OF EYE OF A HUMAN EMBRYO. + +FIGURE 2.320. THE HUMAN EAR. + +FIGURE 2.321. THE BONY LABYRINTH. + +FIGURE 2.322. DEVELOPMENT OF THE LABYRINTH. + +FIGURE 2.323. PRIMITIVE SKULL OF HUMAN EMBRYO. + +FIGURE 2.324. RUDIMENTARY MUSCLES OF THE EAR. + +FIGURES 2.325 AND 2.326. THE HUMAN SKELETON. + +FIGURE 2.327. THE HUMAN VERTEBRAL COLUMN. + +FIGURE 2.328. PIECE OF THE DORSAL CORD. + +FIGURES 2.329 AND 2.330. DORSAL VERTEBRAE. + +FIGURE 2.331. INTERVERTEBRAL DISK. + +FIGURE 2.332. HUMAN SKULL. + +FIGURE 2.333. SKULL OF NEW-BORN CHILD. + +FIGURE 2.334. HEAD-SKELETON OF A PRIMITIVE FISH. + +FIGURE 2.335. SKULLS OF NINE PRIMATES. + +FIGURES 2.336 TO 2.338. EVOLUTION OF THE FIN. + +FIGURE 2.339. SKELETON OF THE FORE-LEG OF AN AMPHIBIAN. + +FIGURE 2.340. SKELETON OF GORILLA'S HAND. + +FIGURE 2.341. SKELETON OF HUMAN HAND. + +FIGURE 2.342. SKELETON OF HAND OF SIX MAMMALS. + +FIGURES 2.343 TO 2.345. ARM AND HAND OF THREE ANTHROPOIDS. + +FIGURE 2.346. SECTION OF FISH'S TAIL. + +FIGURE 2.347. HUMAN SKELETON. + +FIGURE 2.348. SKELETON OF THE GIANT GORILLA. + +FIGURE 2.349. THE HUMAN STOMACH. + +FIGURE 2.350. SECTION OF THE HEAD OF A RABBIT-EMBRYO. + +FIGURE 2.351. SHARK'S TEETH. + +FIGURE 2.352. GUT OF A HUMAN EMBRYO. + +FIGURES 2.353 AND 2.354. GUT OF A DOG EMBRYO. + +FIGURES 2.355 AND 2.356. SECTIONS OF HEAD OF LAMPREY. + +FIGURE 2.357. VISCERA OF A HUMAN EMBRYO. + +FIGURE 2.358. RED BLOOD-CELLS. + +FIGURE 2.359. VASCULAR TISSUE. + +FIGURE 2.360. SECTION OF TRUNK OF A CHICK-EMBRYO. + +FIGURE 2.361. MEROCYTES. + +FIGURE 2.362. VASCULAR SYSTEM OF AN ANNELID. + +FIGURE 2.363. HEAD OF A FISH-EMBRYO. + +FIGURES 2.364 TO 2.370. THE FIVE ARTERIAL ARCHES. + +FIGURES 2.371 AND 2.372. HEART OF A RABBIT-EMBRYO. + +FIGURES 2.373 AND 2.374. HEART OF A DOG-EMBRYO. + +FIGURES 2.375 TO 2.377. HEART OF A HUMAN EMBRYO. + +FIGURE 2.378. HEART OF ADULT MAN. + +FIGURE 2.379. SECTION OF HEAD OF A CHICK-EMBRYO. + +FIGURE 2.380. SECTION OF A HUMAN EMBRYO. + +FIGURES 2.381 AND 2.382. SECTIONS OF A CHICK-EMBRYO. + +FIGURE 2.383. EMBRYOS OF SAGITTA. + +FIGURE 2.384. KIDNEYS OF BDELLOSTOMA. + +FIGURE 2.385. SECTION OF EMBRYONIC SHIELD. + +FIGURES 2.386 AND 2.387. PRIMITIVE KIDNEYS. + +FIGURE 2.388. PIG-EMBRYO. + +FIGURE 2.389. HUMAN EMBRYO. + +FIGURES 2.390 TO 2.392. RUDIMENTARY KIDNEYS AND SEXUAL ORGANS. + +FIGURES 2.393 AND 2.394. URINARY AND SEXUAL ORGANS OF SALAMANDER. + +FIGURE 2.395. PRIMITIVE KIDNEYS OF HUMAN EMBRYO. + +FIGURES 2.396 TO 2.398. URINARY ORGANS OF OX-EMBRYOS. + +FIGURE 2.399. SEXUAL ORGANS OF WATER-MOLE. + +FIGURES 2.400 AND 2.401. ORIGINAL POSITION OF SEXUAL GLANDS. + +FIGURE 2.402. UROGENITAL SYSTEM OF HUMAN EMBRYO. + +FIGURE 2.403. SECTION OF OVARY. + +FIGURES 2.404 TO 2.406. GRAAFIAN FOLLICLES. + +FIGURE 2.407. A RIPE GRAAFIAN FOLLICLE. + +FIGURE 2.408. THE HUMAN OVUM. + + + + +CHAPTER 2.16. STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT. + +In turning from the embryology to the phylogeny of man--from the +development of the individual to that of the species--we must bear in +mind the direct causal connection that exists between these two main +branches of the science of human evolution. This important causal +nexus finds its simplest expression in "the fundamental law of organic +development," the content and purport of which we have fully +considered in the first chapter. According to this biogenetic law, +ontogeny is a brief and condensed recapitulation of phylogeny. If this +compendious reproduction were complete in all cases, it would be very +easy to construct the whole story of evolution on an embryonic basis. +When we wanted to know the ancestors of any higher organism, and, +therefore, of man--to know from what forms the race as a whole has +been evolved we should merely have to follow the series of forms in +the development of the individual from the ovum; we could then regard +each of the successive forms as the representative of an extinct +ancestral form. However, this direct application of ontogenetic facts +to phylogenetic ideas is possible, without limitations, only in a very +small section of the animal kingdom. There are, it is true, still a +number of lower invertebrates (for instance, some of the Zoophyta and +Vermalia) in which we are justified in recognising at once each +embryonic form as the historical reproduction, or silhouette, as it +were, of an extinct ancestor. But in the great majority of the +animals, and in the case of man, this is impossible, because the +embryonic forms themselves have been modified through the change of +the conditions of existence, and have lost their original character to +some extent. During the immeasurable course of organic history, the +many millions of years during which life was developing on our planet, +secondary changes of the embryonic forms have taken place in most +animals. The young of animals (not only detached larvae, but also the +embryos enclosed in the womb) may be modified by the influence of the +environment, just as well as the mature organisms are by adaptation to +the conditions of life; even species are altered during the embryonic +development. Moreover, it is an advantage for all higher organisms +(and the advantage is greater the more advanced they are) to curtail +and simplify the original course of development, and thus to +obliterate the traces of their ancestors. The higher the individual +organism is in the animal kingdom, the less completely does it +reproduce in its embryonic development the series of its ancestors, +for reasons that are as yet only partly known to us. The fact is +easily proved by comparing the different developments of higher and +lower animals in any single stem. + +In order to appreciate this important feature, we have distributed the +embryological phenomena in two groups, palingenetic and cenogenetic. +Under palingenesis we count those facts of embryology that we can +directly regard as a faithful synopsis of the corresponding +stem-history. By cenogenesis we understand those embryonic processes +which we cannot directly correlate with corresponding evolutionary +processes, but must regard as modifications or falsifications of them. +With this careful discrimination between palingenetic and cenogenetic +phenomena, our biogenetic law assumes the following more precise +shape:--The rapid and brief development of the individual (ontogeny) +is a condensed synopsis of the long and slow history of the stem +(phylogeny): this synopsis is the more faithful and complete in +proportion as the original features have been preserved by heredity, +and modifications have not been introduced by adaptation. + +In order to distinguish correctly between palingenetic and cenogenetic +phenomena in embryology, and deduce sound conclusions in connection +with stem-history, we must especially make a comparative study of the +former. In doing this it is best to employ the methods that have long +been used by geologists for the purpose of establishing the succession +of the sedimentary rocks in the crust of the earth. This solid crust, +which encloses the glowing central mass like a thin shell, is composed +of different kinds of rocks: there are, firstly, the volcanic rocks +which were formed directly by the cooling at the surface of the molten +mass of the earth; secondly, there are the sedimentary rocks, that +have been made out of the former by the action of water, and have been +laid in successive strata at the bottom of the sea. Each of these +sedimentary strata was at first a soft layer of mud; but in the course +of thousands of years it condensed into a solid, hard mass of stone +(sandstone, limestone, marl, etc.), and at the same time permanently +preserved the solid and imperishable bodies that had chanced to fall +into the soft mud. Among these bodies, which were either fossilised or +left characteristic impressions of their forms in the soft slime, we +have especially the more solid parts of the animals and plants that +lived and died during the deposit of the slimy strata. + +Hence each of the sedimentary strata has its characteristic fossils, +the remains of the animals and plants that lived during that +particular period of the earth's history. When we make a comparative +study of these strata, we can survey the whole series of such periods. +All geologists are now agreed that we can demonstrate a definite +historical succession in the strata, and that the lowest of them were +deposited in very remote, and the uppermost in comparatively recent, +times. However, there is no part of the earth where we find the series +of strata in its entirety, or even approximately complete. The +succession of strata and of corresponding historical periods generally +given in geology is an ideal construction, formed by piecing together +the various partial discoveries of the succession of strata that have +been made at different points of the earth's surface (cf. Chapter +2.18). + +We must act in this way in constructing the phylogeny of man. We must +try to piece together a fairly complete picture of the series of our +ancestors from the various phylogenetic fragments that we find in the +different groups of the animal kingdom. We shall see that we are +really in a position to form an approximate picture of the evolution +of man and the mammals by a proper comparison of the embryology of +very different animals--a picture that we could never have framed from +the ontogeny of the mammals alone. As a result of the above-mentioned +cenogenetic processes--those of disturbed and curtailed +heredity--whole series of lower stages have dropped out in the +embryonic development of man and the other mammals especially from the +earliest periods, or been falsified by modification. But we find these +lower stages in their original purity in the lower vertebrates and +their invertebrate ancestors. Especially in the lowest of all the +vertebrates, the lancelet or Amphioxus, we have the oldest stem-forms +completely preserved in the embryonic development. We also find +important evidence in the fishes, which stand between the lower and +higher vertebrates, and throw further light on the course of evolution +in certain periods. Next to the fishes come the amphibia, from the +embryology of which we can also draw instructive conclusions. They +represent the transition to the higher vertebrates, in which the +middle and older stages of ancestral development have been either +distorted or curtailed, but in which we find the more recent stages of +the phylogenetic process well preserved in ontogeny. We are thus in a +position to form a fairly complete idea of the past development of +man's ancestors within the vertebrate stem by putting together and +comparing the embryological developments of the various groups of +vertebrates. And when we go below the lowest vertebrates and compare +their embryology with that of their invertebrate relatives, we can +follow the genealogical tree of our animal ancestors much farther, +down to the very lowest groups of animals. + +In entering the obscure paths of this phylogenetic labyrinth, clinging +to the Ariadne-thread of the biogenetic law and guided by the light of +comparative anatomy, we will first, in accordance with the methods we +have adopted, discover and arrange those fragments from the manifold +embryonic developments of very different animals from which the +stem-history of man can be composed. I would call attention +particularly to the fact that we can employ this method with the same +confidence and right as the geologist. No geologist has ever had +ocular proof that the vast rocks that compose our Carboniferous or +Jurassic or Cretaceous strata were really deposited in water. Yet no +one doubts the fact. Further, no geologist has ever learned by direct +observation that these various sedimentary formations were deposited +in a certain order; yet all are agreed as to this order. This is +because the nature and origin of these rocks cannot be rationally +understood unless we assume that they were so deposited. These +hypotheses are universally received as safe and indispensable +"geological theories," because they alone give a rational explanation +of the strata. + +Our evolutionary hypotheses can claim the same value, for the same +reasons. In formulating them we are acting on the same inductive and +deductive methods, and with almost equal confidence, as the geologist. +We hold them to be correct, and claim the status of "biological +theories" for them, because we cannot understand the nature and origin +of man and the other organisms without them, and because they alone +satisfy our demand for a knowledge of causes. And just as the +geological hypotheses that were ridiculed as dreams at the beginning +of the nineteenth century are now universally admitted, so our +phylogenetic hypotheses, which are still regarded as fantastic in +certain quarters, will sooner or later be generally received. It is +true that, as will soon appear, our task is not so simple as that of +the geologist. It is just as much more difficult and complex as man's +organisation is more elaborate than the structure of the rocks. + +When we approach this task, we find an auxiliary of the utmost +importance in the comparative anatomy and embryology of two lower +animal-forms. One of these animals is the lancelet (Amphioxus), the +other the sea-squirt (Ascidia). Both of these animals are very +instructive. Both are at the border between the two chief divisions of +the animal kingdom--the vertebrates and invertebrates. The vertebrates +comprise the already mentioned classes, from the Amphioxus to man +(acrania, lampreys, fishes, dipneusts, amphibia, reptiles, birds, and +mammals). Following the example of Lamarck, it is usual to put all the +other animals together under the head of invertebrates. But, as I have +often mentioned already, the group is composed of a number of very +different stems. Of these we have no interest just now in the +echinoderms, molluscs, and articulates, as they are independent +branches of the animal-tree, and have nothing to do with the +vertebrates. On the other hand, we are greatly concerned with a very +interesting group that has only recently been carefully studied, and +that has a most important relation to the ancestral tree of the +vertebrates. This is the stem of the Tunicates. One member of this +group, the sea-squirt, very closely approaches the lowest vertebrate, +the Amphioxus, in its essential internal structure and embryonic +development. Until 1866 no one had any idea of the close connection of +these apparently very different animals; it was a very fortunate +accident that the embryology of these related forms was discovered +just at the time when the question of the descent of the vertebrates +from the invertebrates came to the front. In order to understand it +properly, we must first consider these remarkable animals in their +fully-developed forms and compare their anatomy. + +We begin with the lancelet--after man the most important and +interesting of all animals. Man is at the highest summit, the lancelet +at the lowest root, of the vertebrate stem. + +It lives on the flat, sandy parts of the Mediterranean coast, partly +buried in the sand, and is apparently found in a number of seas.* (* +See the ample monograph by Arthur Willey, Amphioxus and the Ancestry +of the Vertebrates; Boston, 1894.) It has been found in the North Sea +(on the British and Scandinavian coasts and in Heligoland), and at +various places on the Mediterranean (for instance, at Nice, Naples, +and Messina). It is also found on the coast of Brazil and in the most +distant parts of the Pacific Ocean (the coast of Peru, Borneo, China, +Australia, etc.). Recently eight to ten species of the amphioxus have +been determined, distributed in two or three genera. + +(FIGURE 2.210. The lancelet (Amphioxus lanceolatus), twice natural +size, left view. The long axis is vertical; the mouth-end is above, +the tail-end below; a mouth, surrounded by threads of beard; b anus, c +gill-opening (porus branchialis), d gill-crate, e stomach, f liver, g +small intestine, h branchial cavity, i chorda (axial rod), underneath +it the aorta; k aortic arches, l trunk of the branchial artery, m +swellings on its branches, n vena cava, o visceral vein. + +FIGURE 2.211. Transverse section of the head of the Amphioxus. (From +Boveri.) Above the branchial gut (kd) is the chorda, above this the +neural tube (in which we can distinguish the inner grey and the outer +white matter); above again is the dorsal fin (fh). To the right and +left above (in the episoma) are the thick muscular plates (m); below +(in the hyposoma) the gonads (g). ao aorta (here double), c corium, ec +endostyl, f fascie, gl glomerulus of the kidneys, k branchial vessel, +ld partition between the coeloma (sc) and atrium (p), mt transverse +ventral muscle, n renal canals, of upper and uf lower canals in the +mantle-folds, p peribranchial cavity, (atrium), sc coeloma (subchordal +body-cavity), si principal (or subintestinal) vein, sk perichorda +(skeletal layer).) + +Johannes Muller classed the lancelet with the fishes, although he +pointed out that the differences between this simple vertebrate and +the lowest fishes are much greater than between the fishes and the +amphibia. But this was far from expressing the real significance of +the animal. We may confidently lay down the following principle: The +Amphioxus differs more from the fishes than the fishes do from man and +the other vertebrates. As a matter of fact, it is so different from +all the other vertebrates in its whole organisation that the laws of +logical classification compel us to distinguish two divisions of this +stem: 1, the Acrania (Amphioxus and its extinct relatives); and 2, the +Craniota (man and the other vertebrates). The first and lower division +comprises the vertebrates that have no vertebrae or skull (cranium). +Of these the only living representatives are the Amphioxus and +Paramphioxus, though there must have been a number of different +species at an early period of the earth's history. + +Opposed to the Acrania is the second division of the vertebrates, +which comprises all the other members of the stem, from the fishes up +to man. All these vertebrates have a head quite distinct from the +trunk, with a skull (cranium) and brain; all have a centralised heart, +fully-formed kidneys, etc. Hence they are called the Craniota. These +Craniotes are, however, without a skull in their earlier period. As we +already know from embryology, even man, like every other mammal, +passes in the earlier course of his development through the important +stage which we call the chordula; at this lower stage the animal has +neither vertebrae nor skull nor limbs (Figures 1.83 to 1.86). And even +after the formation of the primitive vertebrae has begun, the +segmented foetus of the amniotes still has for a long time the simple +form of a lyre-shaped disk or a sandal, without limbs or extremities. +When we compare this embryonic condition, the sandal-shaped foetus, +with the developed lancelet, we may say that the amphioxus is, in a +certain sense, a permanent sandal-embryo, or a permanent embryonic +form of the Acrania; it never rises above a low grade of development +which we have long since passed. + +The fully-developed lancelet (Figure 2.210) is about two inches long, +is colourless or of a light red tint, and has the shape of a narrow +lancet-formed leaf. The body is pointed at both ends, but much +compressed at the sides. There is no trace of limbs. The outer skin is +very thin and delicate, naked, transparent, and composed of two +different layers, a simple external stratum of cells, the epidermis, +and a thin underlying cutis-layer. Along the middle line of the back +runs a narrow fin-fringe which expands behind into an oval tail-fin, +and is continued below in a short anus-fin. The fin-fringe is +supported by a number of square elastic fin-plates. + +In the middle of the body we find a thin string of cartilage, which +goes the whole length of the body from front to back, and is pointed +at both ends (Figure 2.210 i). This straight, cylindrical rod +(somewhat compressed for a time) is the axial rod or the chorda +dorsalis; in the lancelet this is the only trace of a vertebral +column. The chorda develops no further, but retains its original +simplicity throughout life. It is enclosed by a firm membrane, the +chorda-sheath or perichorda. The real features of this and of its +dependent formations are best seen in the transverse section of the +Amphioxus (Figure 2.211). The perichorda forms a cylindrical tube +immediately over the chorda, and the central nervous system, the +medullary tube, is enclosed in it. This important psychic organ also +remains in its simplest shape throughout life, as a cylindrical tube, +terminating with almost equal plainness at either end, and enclosing a +narrow canal in its thick wall. However, the fore end is a little +rounder, and contains a small, almost imperceptible bulbous swelling +of the canal. This must be regarded as the beginning of a rudimentary +brain. At the foremost end of it there is a small black pigment-spot, +a rudimentary eye; and a narrow canal leads to a superficial +sense-organ. In the vicinity of this optic spot we find at the left +side a small ciliated depression, the single olfactory organ. There is +no organ of hearing. This defective development of the higher +sense-organs is probably, in the main, not an original feature, but a +result of degeneration. + +Underneath the axial rod or chorda runs a very simple alimentary +canal, a tube that opens on the ventral side of the animal by a mouth +in front and anus behind. The oval mouth is surrounded by a ring of +cartilage, on which there are twenty to thirty cartilaginous threads +(organs of touch, Figure 2.210 a). The alimentary canal divides into +sections of about equal length by a constriction in the middle. The +fore section, or head-gut, serves for respiration; the hind section, +or trunk-gut, for digestion. The limit of the two alimentary regions +is also the limit of the two parts of the body, the head and the +trunk. The head-gut or branchial gut forms a broad gill-crate, the +grilled wall of which is pierced by numbers of gill-clefts (Figure +2.210 d). The fine bars of the gill-crate between the clefts are +strengthened with firm parallel rods, and these are connected in pairs +by cross-rods. The water that enters the mouth of the Amphioxus passes +through these clefts into the large surrounding branchial cavity or +atrium, and then pours out behind through a hole in it, the +respiratory pore (porus branchialis, Figure 2.210 c). Below, on the +ventral side of the gill-crate, there is in the middle line a ciliated +groove with a glandular wall (the hypobranchial groove), which is also +found in the Ascidia and the larvae of the Cyclostoma. It is +interesting because the thyroid gland in the larynx of the higher +vertebrates (underneath the "Adam's apple") has been developed from +it. + +(FIGURE 2.212. Transverse section of an Amphioxus-larva, with five +gill-clefts, through the middle of the body. + +FIGURE 2.213. Diagram of the preceding. (From Hatschek.) A epidermis, +B medullary tube, C chorda, C1 inner chorda-sheath, D visceral +epithelium, E sub-intestinal vein. 1 cutis, 2 muscle-plate (myotome), +3 skeletal plate (sclerotome), 4 coeloseptum (partition between dorsal +and ventral coeloma), 5 skin-fibre layer, 6 gut-fibre layer, I myocoel +(dorsal body-cavity), II splanchnocoel (ventral body-cavity).) + +Behind the respiratory part of the gut we have the digestive section, +the trunk or liver (hepatic) gut. The small particles that the +Amphioxus takes in with the water--infusoria, diatoms, particles of +decomposed plants and animals, etc.--pass from the gill-crate into the +digestive part of the canal, and are used up as food. From a somewhat +enlarged portion, that corresponds to the stomach (Figure 2.210 e), a +long, pouch-like blind sac proceeds straight forward (f); it lies +underneath on the left side of the gill-crate, and ends blindly about +the middle of it. This is the liver of the Amphioxus, the simplest +kind of liver that we meet in any vertebrate. In man also the liver +develops, as we shall see, in the shape of a pouch-like blind sac, +that forms out of the alimentary canal behind the stomach. + +The formation of the circulatory system in this animal is not less +interesting. All the other vertebrates have a compressed, thick, +pouch-shaped heart, which develops from the wall of the gut at the +throat, and from which the blood-vessels proceed; in the Amphioxus +there is no special centralised heart, driving the blood by its +pulsations. This movement is effected, as in the annelids, by the thin +blood-vessels themselves, which discharge the function of the heart, +contracting and pulsating in their whole length, and thus driving the +colourless blood through the entire body. On the under-side of the +gill-crate, in the middle line, there is the trunk of a large vessel +that corresponds to the heart of the other vertebrates and the trunk +of the branchial artery that proceeds from it; this drives the blood +into the gills (Figure 2.210 l). A number of small vascular arches +arise on each side from this branchial artery, and form little +heart-shaped swellings or bulbilla (m) at their points of departure; +they advance along the branchial arches, between the gill-clefts and +the fore-gut, and unite, as branchial veins, above the gill-crate in a +large trunk blood-vessel that runs under the chorda dorsalis. This is +the principal artery or primitive aorta (Figure 2.214 D). The branches +which it gives off to all parts of the body unite again in a larger +venous vessel at the underside of the gut, called the subintestinal +vein (Figures 1.210 o and 2.212 E). This single main vessel of the +Amphioxus goes like a closed circular water-conduit along the +alimentary canal through the whole body, and pulsates in its whole +length above and below. When the upper tube contracts the lower one is +filled with blood, and vice versa. In the upper tube the blood flows +from front to rear, then back from rear to front in the lower vessel. +The whole of the long tube that runs along the ventral side of the +alimentary canal and contains venous blood may be called the +"principal vein," and may be compared to the ventral vessel in the +worms. On the other hand, the long straight vessel that runs along the +dorsal line of the gut above, between it and the chorda, and contains +arterial blood, is clearly identical with the aorta or principal +artery of the other vertebrates; and on the other side it may be +compared to the dorsal vessel in the worms. + +(FIGURE 2.214. Transverse section of a young Amphioxus, immediately +after metamorphosis, through the hindermost third (between the +atrium-cavity and the anus). + +FIGURE 2.215. Diagram of preceding. (From Hatschek.) A epidermis, B +medullary tube, C chorda, D aorta, E visceral epithelium, F +subintestinal vein. 1 corium-plate, 2 muscle-plate, 3 fascie-plate, 4 +outer chorda-sheath, 5 myoseptum, 6 skin-fibre plate, 7 gut-fibre +plate, I myocoel, II splanchnocoel, I1 dorsal fin, I2 anus-fin.) + +The coeloma or body-cavity has some very important and distinctive +features in the Amphioxus. The embryology of it is most instructive in +connection with the stem-history of the body-cavity in man and the +other vertebrates. As we have already seen (Chapter 1.10), in these +the two coelom-pouches are divided at an early stage by transverse +constrictions into a double row of primitive segments (Figure 1.124), +and each of these subdivides, by a frontal or lateral constriction, +into an upper (dorsal) and lower (ventral) pouch. + +These important structures are seen very clearly in the trunk of the +amphioxus (the latter third, Figures 2.212 to 2.215), but it is +otherwise in the head, the foremost third (Figure 2.216). Here we find +a number of complicated structures that cannot be understood until we +have studied them on the embryological side in the next chapter (cf. +Figure 1.81). The branchial gut lies free in a spacious cavity filled +with water, which was wrongly thought formerly to be the body-cavity +(Figure 2.216 A). As a matter of fact, this atrium (commonly called +the peribranchial cavity) is a secondary structure formed by the +development of a couple of lateral mantle-folds or gill-covers (M1, +U). The real body-cavity (Lh) is very narrow and entirely closed, +lined with epithelium. The peribranchial cavity (A) is full of water, +and its walls are lined with the skin-sense layer; it opens outwards +in the rear through the respiratory pore (Figure 2.210 c). + +On the inner surface of these mantle-folds (M1), in the ventral half +of the wide mantle cavity (atrium), we find the sex-organs of the +Amphioxus. At each side of the branchial gut there are between twenty +and thirty roundish four-cornered sacs, which can clearly be seen from +without with the naked eye, as they shine through the thin transparent +body-wall. These sacs are the sexual glands they are the same size and +shape in both sexes, only differing in contents. In the female they +contain a quantity of simple ova (Figure 2.219 g); in the male a +number of much smaller cells that change into mobile ciliated cells +(sperm-cells). Both sacs lie on the inner wall of the atrium, and have +no special outlets. When the ova of the female and the sperm of the +male are ripe, they fall into the atrium, pass through the gill-clefts +into the fore-gut, and are ejected through the mouth. + +(FIGURE 2.216. Transverse section of the lancelet, in the fore half. +(From Ralph.) The outer covering is the simple cell-layer of the +epidermis (E). Under this is the thin corium, the subcutaneous tissue +of which is thickened; it sends connective-tissue partitions between +the muscles (M1) and to the chorda-sheath. N medullary tube, Ch +chorda, Lh body-cavity, A atrium, L upper wall of same, E1 inner wall, +E2 outer wall, Lh1 ventral remnant of same, Kst gill-reds, M ventral +muscles, R seam of the joining of the ventral folds (gill-covers), G +sexual glands.) + +Above the sexual glands, at the dorsal angle of the atrium, we find +the kidneys. These important excretory organs could not be found in +the Amphioxus for a long time, on account of their remote position and +their smallness; they were discovered in 1890 by Theodor Boveri +(Figure 2.217 x). They are short segmented canals; corresponding to +the primitive kidneys of the other vertebrates (Figure 2.218 B). Their +internal aperture (Figure 2.217 B) opens into the body-cavity; their +outer aperture into the atrium (C). The prorenal canals lie in the +middle of the line of the head, outwards from the uppermost section of +the gill-arches, and have important relations to the branchial vessels +(H). For this reason, and in their whole arrangement, the primitive +kidneys of the Amphioxus show clearly that they are equivalent to the +prorenal canals of the Craniotes (Figure 2.218 B). The prorenal duct +of the latter (Figure 2.218 C) corresponds to the branchial cavity or +atrium of the former (Figure 2.217 C). + +(FIGURE 2.217. Transverse section through the middle of the Amphioxus. +(From Boveri.) On the left a gill-rod has been struck, and on the +right a gill-cleft; consequently on the left we see the whole of a +prorenal canal (x), on the right only the section of its fore-leg. A +genital chamber (ventral section of the gonocoel), x pronephridium, B +its coelom-aperture, C atrium, D body-cavity, E visceral cavity, F +subintestinal vein, G aorta (the left branch connected by a branchial +vessel with the subintestinal vein), H renal vessel. + +FIGURE 2.218. Transverse section of a primitive fish embryo +(Selachii-embryo, from Boveri.). To the left pronephridia (B), the +right primitive kidneys (A). The dotted lines on the right indicate +the later opening of the primitive kidney canals (A) into the prorenal +duct (C). D body-cavity, E visceral cavity, F subintestinal vein, G +aorta, H renal vessel.) + +If we sum up the results of our anatomic study of the Amphioxus, and +compare them with the familiar organisation of man, we shall find an +immense distance between the two. As a fact, the highest summit of the +vertebrate organisation which man represents is in every respect so +far above the lowest stage, at which the lancelet remains, that one +would at first scarcely believe it possible to class both animals in +the same division of the animal kingdom. Nevertheless, this +classification is indisputably just. Man is only a more advanced stage +of the vertebral type that we find unmistakably in the Amphioxus in +its characteristic features. We need only recall the picture of the +ideal Primitive Vertebrate given in a former chapter, and compare it +with the lower stages of human embryonic development, to convince +ourselves of our close relationship to the lancelet. (Cf. Chapter +1.11.) + +It is true that the Amphioxus is far below all other living +vertebrates. It is true that it has no separate head, no developed +brain or skull, the characteristic feature of the other vertebrates. +It is (probably as a result of degeneration) without the auscultory +organ and the centralised heart that all the others have; and it has +no fully-formed kidneys. Every single organ in it is simpler and less +advanced than in any of the others. Yet the characteristic connection +and arrangement of all the organs is just the same as in the other +vertebrates. All these, moreover, pass, during their embryonic +development, through a stage in which their whole organisation is no +higher than that of the Amphioxus, but is substantially identical with +it. + +(FIGURE 2.219. Transverse section of the head of the Amphioxus (at the +limit of the first and second third of the body). (From Boveri) a +aorta (here double), b atrium, c chorda, co umlaut coeloma +(body-cavity), e endostyl (hypobranchial groove), g gonads (ovaries), +kb gill-arches, kd branchial gut, l liver-tube (on the right, +one-sided), m muscles, n renal canals, r spinal cord, sn spinal +nerves, sp gill-clefts.) + +In order to see this quite clearly, it is particularly useful to +compare the Amphioxus with the youthful forms of those vertebrates +that are classified next to it. This is the class of the Cyclostoma. +There are to-day only a few species of this once extensive class, and +these may be distributed in two groups. One group comprises the +hag-fishes or Myxinoides. The other group are the Petromyzontes, or +lampreys, which are a familiar delicacy in their marine form. These +Cyclostoma are usually classified with the fishes. But they are far +below the true fishes, and form a very interesting connecting-group +between them and the lancelet. One can see how closely they approach +the latter by comparing a young lamprey with the Amphioxus. The chorda +is of the same simple character in both; also the medullary tube, that +lies above the chorda, and the alimentary canal below it. However, in +the lamprey the spinal cord swells in front into a simple pear-shaped +cerebral vesicle, and at each side of it there are a very simple eye +and a rudimentary auditory vesicle. The nose is a single pit, as in +the Amphioxus. The two sections of the gut are also just the same and +very rudimentary in the lamprey. On the other hand, we see a great +advance in the structure of the heart, which is found underneath the +gills in the shape of a centralised muscular tube, and is divided into +an auricle and a ventricle. Later on the lamprey advances still +further, and gets a skull, five cerebral vesicles, a series of +independent gill-pouches, etc. This makes all the more interesting the +striking resemblance of its immature larva to the developed and +sexually mature Amphioxus. + +While the Amphioxus is thus connected through the Cyclostoma with the +fishes, and so with the series of the higher vertebrates, it is, on +the other hand, very closely related to a lowly invertebrate marine +animal, from which it seems to be entirely remote at first glance. +This remarkable animal is the sea-squirt or Ascidia, which was +formerly thought to be closely related to the mussel, and so classed +in the molluscs. But since the remarkable embryology of these animals +was discovered in 1866, there can be no question that they have +nothing to do with the molluscs. To the great astonishment of +zoologists, they were found, in their whole individual development, to +be closely related to the vertebrates. When fully developed the +Ascidiae are shapeless lumps that would not, at first sight, be taken +for animals at all. The oval body, frequently studded with knobs or +uneven and lumpy, in which we can discover no special external organs, +is attached at one end to marine plants, rocks, or the floor of the +sea. Many species look like potatoes, others like melon-cacti, others +like prunes. Many of the Ascidiae form transparent crusts or deposits +on stones and marine plants. Some of the larger species are eaten like +oysters. Fishermen, who know them very well, think they are not +animals, but plants. They are sold in the fish markets of many of the +Italian coast-towns with other lower marine animals under the name of +"sea-fruit" (frutti di mare). There is nothing about them to show that +they are animals. When they are taken out of the water with the net +the most one can perceive is a slight contraction of the body that +causes water to spout out in two places. The bulk of the Ascidiae are +very small, at the most a few inches long. A few species are a foot or +more in length. There are many species of them, and they are found in +every sea. As in the case of the Acrania, we have no fossilised +remains of the class, because they have no hard and fossilisable +parts. However, they must be of great antiquity, and must go back to +the primordial epoch. + +The name of "Tunicates" is given to the whole class to which the +Ascidiae belong, because the body is enclosed in a thick and stiff +covering like a mantle (tunica). This mantle--sometimes soft like +jelly, sometimes as tough as leather, and sometimes as stiff as +cartilage--has a number of peculiarities. The most remarkable of them +is that it consists of a woody matter, cellulose--the same vegetal +substance that forms the stiff envelopes of the plant-cells, the +substance of the wood. The tunicates are the only class of animals +that have a real cellulose or woody coat. Sometimes the cellulose +mantle is brightly coloured, at other times colourless. Not +infrequently it is set with needles or hairs, like a cactus. Often we +find a mass of foreign bodies--stone, sand, fragments of +mussel-shells, etc.--worked into the mantle. This has earned for the +Ascidia the name of "the microcosm." + +(FIGURE 2.220. Organisation of an Ascidia (left view); the dorsal side +is turned to the right and the ventral side to the left, the mouth (o) +above; the ascidia is attached at the tail end. The branchial gut +(br), which is pierced by a number of clefts, continues below in the +visceral gut. The rectum opens through the anus (a) into the atrium +(cl), from which the excrements are ejected with the respiratory water +through the mantle-hole or cloaca (a); m mantle. (From Gegenbaur.) + +FIGURE 2.221. Organisation of an Ascidia (as in Figure 2.220, seen +from the left). sb branchial sac, v stomach, i small intestine, c +heart, t testicle, vd sperm-duct, o ovary, o apostrophe ripe ova in +the branchial cavity. The two small arrows indicate the entrance and +exit of the water through the openings of the mantle. (From +Milne-Edwards.)) + +The hind end, which corresponds to the tail of the Amphioxus, is +usually attached, often by means of regular roots. The dorsal and +ventral sides differ a good deal internally, but frequently cannot be +distinguished externally. If we open the thick tunic or mantle in +order to examine the internal organisation, we first find a spacious +cavity filled with water--the mantle-cavity or respiratory cavity +(Figure 2.220 cl). It is also called the branchial cavity and the +cloaca, because it receives the excrements and sexual products as well +as the respiratory water. The greater part of the respiratory cavity +is occupied by the large grated branchial sac (br). This is so like +the gill-crate of the Amphioxus in its whole arrangement that the +resemblance was pointed out by the English naturalist Goodsir, years +ago, before anything was known of the relationship of the two animals. +As a fact, even in the Ascidia the mouth (o) opens first into this +wide branchial sac. The respiratory water passes through the +lattice-work of the branchial sac into the branchial cavity, and is +ejected from this by the respiratory pore (a apostrophe). Along the +ventral side of the branchial sac runs a ciliated groove--the +hypobranchial groove which we have previously found at the same spot +in the Amphioxus. The food of the Ascidia also consists of tiny +organisms, infusoria, diatoms, parts of decomposed marine plants and +animals; etc. These pass with the water into the gill-crate and the +digestive part of the gut at the end of it, at first into an +enlargement of it that represents the stomach. The adjoining small +intestine usually forms a loop, bends forward, and opens by an anus +(Figure 2.220 a), not directly outwards, but first into the mantle +cavity; from this the excrements are ejected by a common outlet (a +apostrophe) together with the used-up water and the sexual products. +The outlet is sometimes called the branchial pore, and sometimes the +cloaca or ejection-aperture. In many of the Ascidiae a glandular mass +opens into the gut, and this represents the liver. In some there is +another gland besides the liver, and this is taken to represent the +kidneys. The body-cavity proper, or coeloma, which is filled with +blood and encloses the hepatic gut, is very narrow in the Ascidia, as +in the Amphioxus, and is here also usually confounded with the wide +atrium, or peribranchial cavity, full of water. + +There is no trace in the fully-developed Ascidia of a chorda dorsalis, +or internal axial skeleton. It is the more interesting that the young +animal that emerges from the ovum HAS a chorda, and that there is a +rudimentary medullary tube above it. The latter is wholly atrophied in +the developed Ascidia, and looks like a small nerve-ganglion in front +above the gill-crate. It corresponds to the upper "gullet-ganglion" or +"primitive brain" in other vermalia. Special sense-organs are either +wanting altogether or are only found in a very rudimentary form, as +simple optic spots and touch-corpuscles or tentacles that surround the +mouth. The muscular system is very slightly and irregularly developed. +Immediately under the thin corium, and closely connected with it, we +find a thin muscle tube, as in the worms. On the other hand, the +Ascidia has a centralised heart, and in this respect it seems to be +more advanced than the Amphioxus. On the ventral side of the gut, some +distance behind the gill-crate, there is a spindle-shaped heart. It +retains permanently the simple tubular form that we find temporarily +as the first structure of the heart in the vertebrates. This simple +heart of the Ascidia has, however, a remarkable peculiarity. It +contracts in alternate directions. In all other animals the beat of +the heart is always in the same direction (generally from rear to +front); it changes in the Ascidia to the reverse direction. The heart +contracts first from the rear to the front, stands still for a minute, +and then begins to beat the opposite way, now driving the blood from +front to rear; the two large vessels that start from either end of the +heart act alternately as arteries and veins. This feature is found in +the Tunicates alone. + +Of the other chief organs we have still to mention the sexual glands, +which lie right behind in the body-cavity. All the Ascidiae are +hermaphrodites. Each individual has a male and a female gland, and so +is able to fertilise itself. The ripe ova (Figure 2.221 o apostrophe) +fall directly from the ovary (o) into the mantle-cavity. The male +sperm is conducted into this cavity from the testicle (t) by a special +duct (vd). Fertilisation is accomplished here, and in many of the +Ascidiae developed embryos are found. These are then ejected with the +breathing-water through the cloaca (q), and so "born alive." + +If we now glance at the entire structure of the simple Ascidia +(especially Phallusia, Cynthia, etc.) and compare it with that of the +Amphioxus, we shall find that the two have few points of contact. It +is true that the fully-developed Ascidia resembles the Amphioxus in +several important features of its internal structure, and especially +in the peculiar character of the gill-crate and gut. But in most other +features of organisation it is so far removed from it, and is so +unlike it in external appearance, that the really close relationship +of the two was not discovered until their embryology was studied. We +will now compare the embryonic development of the two animals, and +find to our great astonishment that the same embryonic form develops +from the ovum of the Amphioxus as from that of the Ascidia--a typical +chordula. + + +CHAPTER 2.17. EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT. + +The structural features that distinguish the vertebrates from the +invertebrates are so prominent that there was the greatest difficulty +in the earlier stages of classification in determining the affinity of +these two great groups. When scientists began to speak of the affinity +of the various animal groups in more than a figurative--in a +genealogical--sense, this question came at once to the front, and +seemed to constitute one of the chief obstacles to the carrying-out of +the evolutionary theory. Even earlier, when they had studied the +relations of the chief groups, without any idea of real genealogical +connection, they believed they had found here and there among the +invertebrates points of contact with the vertebrates: some of the +worms, especially, seemed to approach the vertebrates in structure, +such as the marine arrow-worm (Sagitta). But on closer study the +analogies proved untenable. When Darwin gave an impulse to the +construction of a real stem-history of the animal kingdom by his +reform of the theory of evolution, the solution of this problem was +found to be particularly difficult. When I made the first attempt in +my General Morphology (1866) to work out the theory and apply it to +classification, I found no problem of phylogeny that gave me so much +trouble as the linking of the vertebrates with the invertebrates. + +But just at this time the true link was discovered, and at a point +where it was least expected. Towards the end of 1866 two works of the +Russian zoologist, Kowalevsky, who had lived for some time at Naples, +and studied the embryology of the lower animals, were issued in the +publications of the St. Petersburg Academy. A fortunate accident had +directed the attention of this able observer almost simultaneously to +the embryology of the lowest vertebrate, the Amphioxus, and that of an +invertebrate, the close affinity of which to the Amphioxus had been +least suspected, the Ascidia. To the extreme astonishment of all +zoologists who were interested in this important question, there +turned out to be the utmost resemblance in structure from the +commencement of development between these two very different +animals--the lowest vertebrate and the mis-shaped, sessile +invertebrate. With this undeniable identity of ontogenesis, which can +be demonstrated to an astounding extent, we had, in virtue of the +biogenetic law, discovered the long-sought genealogical link, and +definitely identified the invertebrate group that represents the +nearest blood-relatives of the vertebrates. The discovery was +confirmed by other zoologists, and there can no longer be any doubt +that of all the classes of invertebrates that of the Tunicates is most +closely related to the vertebrates, and of the Tunicates the nearest +are the Ascidiae. We cannot say that the vertebrates are descended +from the Ascidiae--and still less the reverse--but we can say that of +all the invertebrates it is the Tunicates, and, within this group, the +Ascidiae, that are the nearest blood-relatives of the ancient +stem-form of the vertebrates. We must assume as the common ancestral +group of both stems an extinct family of the extensive vermalia-stem, +the Prochordonia or Prochordata ("primitive chorda-animals"). + +In order to appreciate fully this remarkable fact, and especially to +secure the sound basis we seek for the genealogical tree of the +vertebrates, it is necessary to study thoroughly the embryology of +both these animals, and compare the individual development of the +Amphioxus step by step with that of the Ascidia. We begin with the +ontogeny of the Amphioxus. + +From the concordant observations of Kowalevsky at Naples and Hatschek +at Messina, it follows, firstly, that the ovum-segmentation and +gastrulation of the Amphioxus are of the simplest character. They take +place in the same way as we find them in many of the lower animals of +different invertebrate stems, which we have already described as +original or primordial; the development of the Ascidia is of the same +type. Sexually mature specimens of the Amphioxus, which are found in +great quantities at Messina from April or May onwards, begin as a rule +to eject their sexual products in the evening; if you catch them about +the middle of a warm night and put them in a glass vessel with +seawater, they immediately eject through the mouth their accumulated +sexual products, in consequence of the disturbance. The males give out +masses of sperm, and the females discharge ova in such quantity that +many of them stick to the fibrils about their mouths. Both kinds of +cells pass first into the mantle-cavity after the opening of the +gonads, proceed through the gill-clefts into the branchial gut, and +are discharged from this through the mouth. + +The ova are simply round cells. They are only 1/250 of an inch in +diameter, and thus are only half the size of the mammal ova, and have +no distinctive features. The clear protoplasm of the mature ovum is +made so turbid by the numbers of dark granules of food-yelk or +deutoplasm scattered in it that it is difficult to follow the process +of fecundation and the behaviour of the two nuclei during it (Chapter +1.7). The active elements of the male sperm, the cone-shaped +spermatozoa, are similar to those of most other animals (cf. Figure +1.20). Fecundation takes place when these lively ciliated cells of the +sperm approach the ovum, and seek to penetrate into the yelk-matter or +the cellular substance of the ovum with their head-part--the thicker +part of the cell that encloses the nucleus. Only one spermatozoon can +bore its way into the yelk at one pole of the ovum-axis; its head or +nucleus coalesces with the female nucleus, which remains after the +extrusion of the directive bodies from the germinal vesicle. Thus is +formed the "stem-nucleus," or the nucleus of the "stem-cell" (cytula, +Figure 1.2). This now undergoes total segmentation, dividing into two, +four, eight, sixteen, thirty-two cells, and so on. In this way we get +the spherical, mulberry-shaped body, which we call the morula. + +The segmentation of the Amphioxus is not entirely regular, as was +supposed after the first observations of Kowalevsky (1866). It is not +completely equal, but a little unequal. As Hatschek afterwards found +(1879), the segmentation-cells only remain equal up to the +morula-stage, the spherical body of which consists of thirty-two +cells. Then, as always happens in unequal segmentation, the more +sluggish vegetal cells are outstripped in the cleavage. At the lower +or vegetal pole of the ovum a crown of eight large entodermic cells +remains for a long time unchanged, while the other cells divide, owing +to the formation of a series of horizontal circles, into an increasing +number of crowns of sixteen cells each. Afterwards the +segmentation-cells get more or less irregularly displaced, while the +segmentation-cavity enlarges in the centre of the morula; in the end +the former all lie on the surface of the latter, so that the foetus +attains the familiar blastula shape and forms a hollow ball, the wall +of which consists of a single stratum of cells (Figure 1.38 A to C). +This layer is the blastoderm, the simple epithelium from the cells of +which all the tissues of the body proceed. + +These important early embryonic processes take place so quickly in the +Amphioxus that four or five hours after fecundation, or about +midnight, the spherical blastula is completed. A pit-like depression +is then formed at the vegetal pole of it, and in consequence of this +the hollow sphere doubles on itself (Figure 1.38 D). This pit becomes +deeper and deeper (Figure 1.38 E and F); at last the invagination (or +doubling) is complete, and the inner or folded part of the +blastula-wall lies on the inside of the outer wall. We thus get a +hollow hemisphere, the thin wall of which is made up of two layers of +cells (Figure 1.38 E). From hemispherical the body soon becomes almost +spherical once more, and then oval, the internal cavity enlarging +considerably and its mouth growing narrower (Figure 2.213). The form +which the Amphioxus-embryo has thus reached is a real "cup-larva" or +gastrula, of the original simple type that we have previously +described as the "bell-gastrula" or archigastrula (Figures 1.29 to +1.35). + +As in all the other animals that form an archigastrula, the whole body +is nothing but a simple gastric sac or stomach; its internal cavity is +the primitive gut (progaster or archenteron, Figure 1.38 g, 1.35 d), +and its aperture the primitive mouth (prostoma or blastoporus, o). The +wall is at once gut-wall and body-wall. It is composed of two simple +cell-layers, the familiar primary germinal layers. The inner layer or +the invaginated part of the blastoderm, which immediately encloses the +gut-cavity is the entoderm, the inner or vegetal germ-layer, from +which develop the wall of the alimentary canal and all its appendages, +the coelom-pouches, etc. (Figures 1.35 and 1.36 i). The outer stratum +of cells, or the non-invaginated part of the blastoderm, is the +ectoderm, the outer or animal germ-layer, which provides the outer +skin (epidermis) and the nervous system (e). The cells of the entoderm +are much larger, darker, and more fatty than those of the ectoderm, +which are clearer and less rich in fatty particles. Hence before and +during invagination there is an increasing differentiation of the +inner from the outer layer. The animal cells of the outer layer soon +develop vibratory hairs; the vegetal cells of the inner layer do so +much later. A thread-like process grows out of each cell, and effects +continuous vibratory movements. By the vibrations of these slender +hairs the gastrula of the Amphioxus swims about in the sea, when it +has pierced the thin ovolemma, like the gastrula of many other animals +(Figure 1.36). As in many other lower animals, the cells have only one +whip-like hair each, and so are called flagellate (whip) cells (in +contrast with the ciliated cells, which have a number of short lashes +or cilia). + +In the further course of its rapid development the roundish +bell-gastrula becomes elongated, and begins to flatten on one side, +parallel to the long axis. The flattened side is the subsequent dorsal +side; the opposite or ventral side remains curved. The latter grows +more quickly than the former, with the result that the primitive mouth +is forced to the dorsal side (Figure 1.39). In the middle of the +dorsal surface a shallow longitudinal groove or furrow is formed +(Figure 1.79), and the edges of the body rise up on each side of this +groove in the shape of two parallel swellings. This groove is, of +course, the dorsal furrow, and the swellings are the dorsal or +medullary swellings; they form the first structure of the central +nervous system, the medullary tube. The medullary swellings now rise +higher; the groove between them becomes deeper and deeper. The edges +of the parallel swellings curve towards each other, and at last unite, +and the medullary tube is formed (Figures 1.83 m and 1.84 m). Hence +the formation of a medullary tube out of the outer skin takes place in +the naked dorsal surface of the free-swimming larva of the Amphioxus +in just the same way as we have found in the embryo of man and the +higher animals within the foetal membranes. + +Simultaneously with the construction of the medullary tube we have in +the Amphioxus-embryo the formation of the chorda, the coelom-pouches, +and the mesoderm proceeding from their wall. These processes also take +place with characteristic simplicity and clearness, so that they are +very instructive to compare with the vermalia on the one hand and with +the higher vertebrates on the other. While the medullary groove is +sinking in the middle line of the flat dorsal side of the oval embryo, +and its parallel edges unite to form the ectodermic neural tube, the +single chorda is formed directly underneath them, and on each side of +this a parallel longitudinal fold, from the dorsal wall of the +primitive gut. These longitudinal folds of the entoderm proceed from +the primitive mouth, or from its lower and hinder edge. Here we see at +an early stage a couple of large entodermic cells, which are +distinguished from all the others by their great size, round form, and +fine-grained protoplasm; they are the two promesoblasts, or polar +cells of the mesoderm (Figure 1.83 p). They indicate the original +starting-point of the two coelom-pouches, which grow from this spot +between the inner and outer germinal layers, sever themselves from the +primitive gut, and provide the cellular material for the middle layer. + +Immediately after their formation the two coelom-pouches of the +Amphioxus are divided into several parts by longitudinal and +transverse folds. Each of the primary pouches is divided into an upper +dorsal and a lower ventral section by a couple of lateral longitudinal +folds (Figure 1.82). But these are again divided by several parallel +transverse folds into a number of successive sacs, the primitive +segments or somites (formerly called by the unsuitable name of +"primitive vertebrae"). They have a different future above and below. +The upper or dorsal segments, the episomites, lose their cavity later +on, and form with their cells the muscular plates of the trunk. The +lower or ventral segments, the hyposomites, corresponding to the +lateral plates of the craniote-embryo, fuse together in the upper part +owing to the disappearance of their lateral walls, and thus form the +later body-cavity (metacoel); in the lower part they remain separate, +and afterwards form the segmental gonads. + +In the middle, between the two lateral coelom-folds of the primitive +gut, a single central organ detaches from this at an early stage in +the middle line of its dorsal wall. This is the dorsal chorda (Figures +1.83 and 1.84 ch). This axial rod, which is the first foundation of +the later vertebral column in all the vertebrates, and is the only +representative of it in the Amphioxus, originates from the entoderm. + +In consequence of these important folding-processes in the primitive +gut, the simple entodermic tube divides into four different +sections:-- + +1. underneath, at the ventral side, the permanent alimentary canal or +permanent gut; + +2. above, at the dorsal side, the axial rod or chorda; and + +3. the two coelom-sacs, which immediately sub-divide into two +structures:-- + +3A. above, on the dorsal side, the episomites, the double row of +primitive or muscular segments; and + +3B. below, on each side of the gut, the hyposomites, the two lateral +plates that give rise to the sex-glands, and the cavities of which +partly unite to form the body-cavity. At the same time, the neural or +medullary tube is formed above the chorda, on the dorsal surface, by +the closing of the parallel medullary swellings. + +All these processes, which outline the typical structure of the +vertebrate, take place with astonishing rapidity in the embryo of the +Amphioxus; in the afternoon of the first day, or twenty-four hours +after fertilisation, the young vertebrate, the typical embryo, is +formed; it then has, as a rule, six to eight somites. + +The chief occurrence on the second day of development is the +construction of the two permanent openings of the gut--the mouth and +anus. In the earlier stages the alimentary tube is found to be +entirely closed, after the closing of the primitive mouth; it only +communicates behind by the neurenteric canal with the medullary tube. +The permanent mouth is a secondary formation, at the opposite end. +Here, at the end of the second day, we find a pit-like depression in +the outer skin, which penetrates inwards into the closed gut. The anus +is formed behind in the same way a few hours later (in the vicinity of +the additional gastrula-mouth). In man and the higher vertebrates also +the mouth and anus are formed, as we have seen, as flat pits in the +outer skin; they then penetrate inwards, gradually becoming connected +with the blind ends of the closed gut-tube. During the second day the +Amphioxus-embryo undergoes few other changes. The number of primitive +segments increases, and generally amounts to fourteen, some +forty-eight to fifty hours after impregnation. + +Almost simultaneously with the formation of the mouth the first +gill-cleft breaks through in the fore section of the Amphioxus-embryo +(generally forty hours after the commencement of development). It now +begins to nourish itself independently, as the food material stored up +in the ovum is completely used up. The further development of the free +larvae takes place very slowly, and extends over several months. The +body becomes much longer, and is compressed at the sides, the head-end +being broadened in a sort of triangle. Two rudimentary sense-organs +are developed in it. Inside we find the first blood-vessels, an upper +or dorsal vessel, corresponding to the aorta, between the gut and the +dorsal cord, and a lower or ventral vessel, corresponding to the +subintestinal vein, at the lower border of the gut. Now, the gills or +respiratory organs also are formed at the fore-end of the alimentary +canal. The whole of the anterior or respiratory section of the gut is +converted into a gill-crate, which is pierced trellis-wise by numbers +of branchial-holes, as in the ascidia. This is done by the foremost +part of the gut-wall joining star-wise with the outer skin, and the +formation of clefts at the point of connection, piercing the wall and +leading into the gut from without. At first there are very few of +these branchial clefts; but there are soon a number of them--first in +one, then in two, rows. The foremost gill-cleft is the oldest. In the +end we have a sort of lattice work of fine gill-clefts, supported on a +number of stiff branchial rods; these are connected in pairs by +transverse rods. + +(FIGURES 2.222 TO 2.224. Transverse sections of young Amphioxus-larvae +(diagrammatic, from Ralph.) (Cf. also Figure 2.216.) In Figure 2.222 +there is free communication from without with the gut-cavity (D) +through the gill-clefts (K). In Figure 2.223 the lateral folds of the +body-wall, or the gill-covers, which grow downwards, are formed. In +Figure 2.224 these lateral folds have united underneath and joined +their edges in the middle line of the ventral side (R seam). The +respiratory water now passes from the gut-cavity (D) into the +mantle-cavity (A). The letters have the same meaning throughout: N +medullary tube, Ch chorda, M lateral muscles, Lh body-cavity, G part +of the body-cavity in which the sexual organs are subsequently formed. +D gut-cavity, clothed with the gut-gland layer (a). A mantle-cavity, K +gill-clefts, b = E epidermis, E1 the same as visceral epithelium of +the mantle-cavity, E2 as parietal epithelium of the mantle-cavity.) + +At an early stage of embryonic development the structure of the +Amphioxus-larva is substantially the same as the ideal picture we have +previously formed of the "Primitive Vertebrate" (Figures 1.98 to +1.102). But the body afterwards undergoes various modifications, +especially in the fore-part. These modifications do not concern us, as +they depend on special adaptations, and do not affect the hereditary +vertebrate type. When the free-swimming Amphioxus-larva is three +months old, it abandons its pelagic habits and changes into the young +animal that lives in the sand. In spite of its smallness (one-eighth +of an inch), it has substantially the same structure as the adult. As +regards the remaining organs of the Amphioxus, we need only mention +that the gonads or sexual glands are developed very late, immediately +out of the inner cell-layer of the body-cavity. Although we can find +afterwards no continuation of the body-cavity (Figure 2.216 U) in the +lateral walls of the mantle-cavity, in the gill-covers or mantle-folds +(Figure 2.224 U), there is one present in the beginning (Figure 2.224 +Lh). The sexual cells are formed below, at the bottom of this +continuation (Figure 2.224 S). For the rest, the subsequent +development into the adult Amphioxus of the larva we have followed is +so simple that we need not go further into it here. + +We may now turn to the embryology of the Ascidia, an animal that seems +to stand so much lower and to be so much more simply organised, +remaining for the greater part of its life attached to the bottom of +the sea like a shapeless lump. It was a fortunate accident that +Kowalevsky first examined just those larger specimens of the Ascidiae +that show most clearly the relationship of the vertebrates to the +invertebrates, and the larvae of which behave exactly like those of +the Amphioxus in the first stages of development. This resemblance is +so close in the main features that we have only to repeat what we have +already said of the ontogenesis of the Amphioxus. + +The ovum of the larger Ascidia (Phallusia, Cynthia, etc.) is a simple +round cell of 1/250 to 1/125 of an inch in diameter. In the thick +fine-grained yelk we find a clear round germinal vesicle of about +1/750 of an inch in diameter, and this encloses a small embryonic spot +or nucleolus. Inside the membrane that surrounds the ovum, the +stem-cell of the Ascidia, after fecundation, passes through just the +same metamorphoses as the stem-cell of the Amphioxus. It undergoes +total segmentation; it divides into two, four, eight, sixteen, +thirty-two cells, and so on. By continued total cleavage the morula, +or mulberry-shaped cluster of cells, is formed. Fluid gathers inside +it, and thus we get once more a globular vesicle (the blastula); the +wall of this is a single stratum of cells, the blastoderm. A real +gastrula (a simple bell-gastrula) is formed from the blastula by +invagination, in the same way as in the amphioxus. + +Up to this there is no definite ground in the embryology of the +Ascidiae for bringing them into close relationship with the +Vertebrates; the same gastrula is formed in the same way in many other +animals of different stems. But we now find an embryonic process that +is peculiar to the Vertebrates, and that proves irrefragably the +affinity of the Ascidiae to the Vertebrates. From the epidermis of the +gastrula a medullary tube is formed on the dorsal side, and, between +this and the primitive gut, a chorda; these are the organs that are +otherwise only found in Vertebrates. The formation of these very +important organs takes place in the Ascidia-gastrula in precisely the +same way as in that of the Amphioxus. In the Ascidia (as in the other +case) the oval gastrula is first flattened on one side--the subsequent +dorsal side. A groove or furrow (the medullary groove) is sunk in the +middle line of the flat surface, and two parallel longitudinal +swellings arise on either side from the skin layer. These medullary +swellings join together over the furrow, and form a tube; in this +case, again, the neural or medullary tube is at first open in front, +and connected with the primitive gut behind by the neurenteric canal. +Further, in the Ascidia-larva also the two permanent apertures of the +alimentary canal only appear later, as independent and new formations. +The permanent mouth does not develop from the primitive mouth of the +gastrula; this primitive mouth closes up, and the later anus is formed +near it by invagination from without, on the hinder end of the body, +opposite to the aperture of the medullary tube. + +During these important processes, that take place in just the same way +in the Amphioxus, a tail-like projection grows out of the posterior +end of the larva-body, and the larva folds itself up within the round +ovolemma in such a way that the dorsal side is curved and the tail is +forced on to the ventral side. In this tail is developed--starting +from the primitive gut--a cylindrical string of cells, the fore end of +which pushes into the body of the larva, between the alimentary canal +and the neural canal, and is no other than the chorda dorsalis. This +important organ had hitherto been found only in the Vertebrates, not a +single trace of it being discoverable in the Invertebrates. At first +the chorda only consists of a single row of large entodermic cells. It +is afterwards composed of several rows of cells. In the Ascidia-larva, +also, the chorda develops from the dorsal middle part of the primitive +gut, while the two coelom-pouches detach themselves from it on both +sides. The simple body-cavity is formed by the coalescence of the two. + +When the Ascidia-larva has attained this stage of development it +begins to move about in the ovolemma. This causes the membrane to +burst. The larva emerges from it, and swims about in the sea by means +of its oar-like tail. These free-swimming larvae of the Ascidia have +been known for a long time. They were first observed by Darwin during +his voyage round the world in 1833. They resemble tadpoles in outward +appearance, and use their tails as oars, as the tadpoles do. However, +this lively and highly-developed condition does not last long. At +first there is a progressive development; the foremost part of the +medullary tube enlarges into a brain, and inside this two single +sense-organs are developed, a dorsal auditory vesicle and a ventral +eye. Then a heart is formed on the ventral side of the animal, or the +lower wall of the gut, in the same simple form and at the same spot at +which the heart is developed in man and all the other vertebrates. In +the lower muscular wall of the gut we find a weal-like thickening, a +solid, spindle-shaped string of cells, which becomes hollow in the +centre; it begins to contract in different directions, now forward and +now backward, as is the case with the adult Ascidia. In this way the +sanguineous fluid accumulated in the hollow muscular tube is driven in +alternate directions into the blood-vessels, which develop at both +ends of the cardiac tube. One principal vessel runs along the dorsal +side of the gut, another along its ventral side. The former +corresponds to the aorta and the dorsal vessel in the worms. The other +corresponds to the subintestinal vein and the ventral vessel of the +worms. + +With the formation of these organs the progressive development of the +Ascidia comes to an end, and degeneration sets in. The free-swimming +larva sinks to the floor of the sea, abandons its locomotive habits, +and attaches itself to stones, marine plants, mussel-shells, corals, +and other objects; this is done with the part of the body that was +foremost in movement. The attachment is effected by a number of +out-growths, usually three, which can be seen even in the +free-swimming larva. The tail is lost, as there is no further use for +it. It undergoes a fatty degeneration, and disappears with the chorda +dorsalis. The tailless body changes into an unshapely tube, and, by +the atrophy of some parts and the modification of others, gradually +assumes the appearance we have already described. + +(FIGURE 2.225. An Appendicaria (Copelata), seen from the left. m +mouth, k branchial gut, o gullet, v stomach, a anus, n brain (ganglion +above the gullet), g auditory vesicle, f ciliated groove under the +gills, h heart, t testicles, e ovary, c chorda, s tail.) + +Among the living Tunicates there is a very interesting group of small +animals that remain throughout life at the stage of development of the +tailed, free Ascidia-larva, and swim about briskly in the sea by means +of their broad oar-tail. These are the remarkable Copelata +(Appendicaria and Vexillaria, Figure 2.225). They are the only living +Vertebrates that have throughout life a chorda dorsalis and a neural +string above it; the latter must be regarded as the prolongation of +the cerebral ganglion and the equivalent of the medullary tube. Their +branchial gut also opens directly outwards by a pair of branchial +clefts. These instructive Copelata, comparable to permanent +Ascidia-larvae, come next to the extinct Prochordonia, those ancient +worms which we must regard as the common ancestors of the Tunicates +and Vertebrates. The chorda of the Appendicaria is a long, cylindrical +string (Figure 2.225 c), and serves as an attachment for the muscles +that work the flat oar-tail. + +Among the various modifications which the Ascidia-larva undergoes +after its establishment at the sea-floor, the most interesting (after +the loss of the axial rod) is the atrophy of one of its chief organs, +the medullary tube. In the Amphioxus the spinal marrow continues to +develop, but in the Ascidia the tube soon shrinks into a small and +insignificant nervous ganglion that lies above the mouth and the +gill-crate, and is in accord with the extremely slight mental power of +the animal. This insignificant relic of the medullary tube seems to be +quite beyond comparison with the nervous centre of the vertebrate, yet +it started from the same structure as the spinal cord of the +Amphioxus. The sense-organs that had been developed in the fore part +of the neural tube are also lost; no trace of which can be found in +the adult Ascidia. On the other hand, the alimentary canal becomes a +most extensive organ. It divides presently into two sections--a wide +fore or branchial gut that serves for respiration, and a narrower hind +or hepatic gut that accomplishes digestion. The branchial or head-gut +of the Ascidia is small at first, and opens directly outwards only by +a couple of lateral ducts or gill-clefts--a permanent arrangement in +the Copelata. The gill-clefts are developed in the same way as in the +Amphioxus. As their number greatly increases we get a large +gill-crate, pierced like lattice work. In the middle line of its +ventral side we find the hypobranchial groove. The mantle or +cloaca-cavity (the atrium) that surrounds the gill-crate is also +formed in the same way in the Ascidia as in the Amphioxus. The +ejection-opening of this peribranchial cavity corresponds to the +branchial pore of the Amphioxus. In the adult Ascidia the branchial +gut and the heart on its ventral side are almost the only organs that +recall the original affinity with the vertebrates. + +The further development of the Ascidia in detail has no particular +interest for us, and we will not go into it. The chief result that we +obtain from its embryology is the complete agreement with that of the +Amphioxus in the earliest and most important embryonic stages. They do +not begin to diverge until after the medullary tube and alimentary +canal, and the axial rod with the muscles between the two, have been +formed. The Amphioxus continues to advance, and resembles the +embryonic forms of the higher vertebrates; the Ascidia degenerates +more and more, and at last, in its adult condition, has the appearance +of a very imperfect invertebrate. + +If we now look back on all the remarkable features we have encountered +in the structure and the embryonic development of the Amphioxus and +the Ascidia, and compare them with the features of man's embryonic +development which we have previously studied, it will be clear that I +have not exaggerated the importance of these very interesting animals. +It is evident that the Amphioxus from the vertebrate side and the +Ascidia from the invertebrate form the bridge by which we can span the +deep gulf that separates the two great divisions of the animal +kingdom. The radical agreement of the lancelet and the sea-squirt in +the first and most important stages of development shows something +more than their close anatomic affinity and their proximity in +classification; it shows also their real blood-relationship and their +common origin from one and the same stem-form. In this way, it throws +considerable light on the oldest roots of man's genealogical tree. + + +CHAPTER 2.18. DURATION OF THE HISTORY OF OUR STEM. + +Our comparative investigation of the anatomy and ontogeny of the +Amphioxus and Ascidia has given us invaluable assistance. We have, in +the first place, bridged the wide gulf that has existed up to the +present between the Vertebrates and Invertebrates; and, in the second +place, we have discovered in the embryology of the Amphioxus a number +of ancient evolutionary stages that have long since disappeared from +human embryology, and have been lost, in virtue of the law of +curtailed heredity. The chief of these stages are the spherical +blastula (in its simplest primary form), and the succeeding +archigastrula, the pure, original form of the gastrula which the +Amphioxus has preserved to this day, and which we find in the same +form in a number of Invertebrates of various classes. Not less +important are the later embryonic forms of the coelomula, the +chordula, etc. + +Thus the embryology of the Amphioxus and the Ascidia has so much +increased our knowledge of man's stem-history that, although our +empirical information is still very incomplete, there is now no defect +of any great consequence in it. We may now, therefore, approach our +proper task, and reconstruct the phylogeny of man in its chief lines +with the aid of this evidence of comparative anatomy and ontogeny. In +this the reader will soon see the immense importance of the direct +application of the biogenetic law. But before we enter upon the work +it will be useful to make a few general observations that are +necessary to understand the processes aright. + +We must say a few words with regard to the period in which the human +race was evolved from the animal kingdom. The first thought that +occurs to one in this connection is the vast difference between the +duration of man's ontogeny and phylogeny. The individual man needs +only nine months for his complete development, from the fecundation of +the ovum to the moment when he leaves the maternal womb. The human +embryo runs its whole course in the brief space of forty weeks (as a +rule, 280 days). In many other mammals the time of the embryonic +development is much the same as in man--for instance, in the cow. In +the horse and ass it takes a little longer, forty-three to forty-five +weeks; in the camel, thirteen months. In the largest mammals, the +embryo needs a much longer period for its development in the womb--a +year and a half in the rhinoceros, and ninety weeks in the elephant. +In these cases pregnancy lasts twice as long as in the case of man, or +one and three-quarter years. In the smaller mammals the embryonic +period is much shorter. The smallest mammals, the dwarf-mice, develop +in three weeks; hares in four weeks, rats and marmots in five weeks, +the dog in nine, the pig in seventeen, the sheep in twenty-one and the +goat in thirty-six. Birds develop still more quickly. The chick only +needs, in normal circumstances, three weeks for its full development. +The duck needs twenty-five days, the turkey twenty-seven, the peacock +thirty-one, the swan forty-two, and the cassowary sixty-five. The +smallest bird, the humming-bird, leaves the egg after twelve days. +Hence the duration of individual development within the foetal +membranes is, in the mammals and birds, clearly related to the +absolute size of the body of the animal in question. But this is not +the only determining feature. There are a number of other +circumstances that have an influence on the period of embryonic +development. In the Amphioxus the earliest and most important +embryonic processes take place so rapidly that the blastula is formed +in four hours, the gastrula in six, and the typical vertebrate form in +twenty-four. + +In every case the duration of ontogeny shrinks into insignificance +when we compare it with the enormous period that has been necessary +for phylogeny, or the gradual development of the ancestral series. +This period is not measured by years or centuries, but by thousands +and millions of years. Many millions of years had to pass before the +most advanced vertebrate, man, was evolved, step by step, from his +ancient unicellular ancestors. The opponents of evolution, who declare +that this gradual development of the human form from lower animal +forms, and ultimately from a unicellular organism, is an incredible +miracle, forget that the same miracle takes place within the space of +mine months in the embryonic development of every human being. Each of +us has, in the forty weeks--properly speaking, in the first four +weeks--of his development in the womb, passed through the same series +of transformations that our animal ancestors underwent in the course +of millions of years. + +It is impossible to determine even approximately, in hundreds or even +thousands of years, the real and absolute duration of the phylogenetic +period. But for some time now we have, through the research of +geologists, been in a position to assign the relative length of the +various sections of the organic history of the earth. The immediate +data for determining this relative length of the geological periods +are found in the thickness of the sedimentary strata--the strata that +have been formed at the bottom of the sea or in fresh water from the +mud or slime deposited there. These successive layers of limestone, +sandstone, slate, marl, etc., which make up the greater part of the +rocks, and are often several thousand feet thick, give us a standard +for computing the relative length of the various periods. + +To make the point quite clear, I must say a word about the evolution +of the earth in general, and point out briefly the chief features of +the story. In the first place, we encounter the principle that on our +planet organic life began to exist at a definite period. That +statement is no longer disputed by any competent geologist or +biologist. The organic history of the earth could not commence until +it was possible for water to settle on our planet in fluid condition. +Every organism, without exception, needs fluid water as a condition of +existence, and contains a considerable quantity of it. Our own body, +when fully formed, contains sixty to seventy per cent of water in its +tissues, and only thirty to forty per cent of solid matter. There is +even more water in the body of the child, and still more in the +embryo. In the earlier stages of development the human foetus contains +more than ninety per cent of water, and not ten per cent of solids. In +the lower marine animals, especially certain medusae, the body +consists to the extent of more than ninety-nine per cent of sea-water, +and has not one per cent of solid matter. No organism can exist or +discharge its functions without water. No water, no life! + +But fluid water, on which the existence of life primarily depends, +could not exist on our planet until the temperature of the surface of +the incandescent sphere had sunk to a certain point. Up to that time +it remained in the form of steam. But as soon as the first fluid water +could be condensed from the envelope of steam, it began its geological +action, and has continued down to the present day to modify the solid +crust of the earth. The final outcome of this incessant action of the +water--wearing down and dissolving the rocks in the form of rain, +hail, snow, and ice, as running stream or boiling surge--is the +formation of mud. As Huxley says in his admirable Lectures on the +Causes of Phenomena in Organic Nature, the chief document as to the +past history of our earth is mud; the question of the history of past +ages resolves itself into a question about the formation of mud. + +As I have said, it is possible to form an approximate idea of the +relative age of the various strata by comparing them at different +parts of the earth's surface. Geologists have long been agreed that +there is a definite historical succession of the different strata. The +various superimposed layers correspond to successive periods in the +organic history of the earth, in which they were deposited in the form +of mud at the bottom of the sea. The mud was gradually converted into +stone. This was lifted out of the water owing to variations in the +earth's surface, and formed the mountains. As a rule, four or five +great divisions are distinguished in the organic history of the earth, +corresponding to the larger and smaller groups of the sedimentary +strata. The larger periods are then sub-divided into a series of +smaller ones, which usually number from twelve to fifteen. The +comparative thickness of the groups of strata enables us to make an +approximate calculation of the relative length of these various +periods of time. We cannot say, it is true, "In a century a stratum of +a certain thickness (about two feet) is formed on the average; +therefore, a layer 1000 feet thick must be 500,000 years old." +Different strata of the same thickness may need very different periods +for their formation. But from the thickness or size of the stratum we +can draw some conclusion as to the RELATIVE length of the period. + +The first and oldest of the four or five chief divisions of the +organic history of the earth is called the primordial, archaic, or +archeozoic period. If we compute the total average thickness of the +sedimentary strata at about 130,000 feet, this first period comprises +70,000 feet, or the greater part of the whole. For this and other +reasons we may at once conclude that the corresponding primordial or +archeolithic period must have been in itself much longer than the +whole of the remaining periods together, from its close to the present +day. It was probably much longer than the figures I have quoted (7 : +6) indicate--possibly 9 : 6. Of late years the thickness of the +archaic rocks has been put at 90,000 feet. + +SYNOPSIS OF THE PALEONTOLOGICAL FORMATIONS, OR THE FOSSILIFEROUS +STRATA OF THE CRUST. + +COLUMN 1 : Groups (V. down to I.). + +COLUMN 2 : Systems (XIV. down to I.). + +COLUMN 3 : Formations (38 down to 1). + +COLUMN 4 : Synonyms of Formations. + +V. Anthropolithic group, or anthropozoic (quaternary) group of strata +: XIV. Recent (alluvium) : 38. Present : Upper alluvial. + +V. Anthropolithic group, or anthropozoic (quaternary) group of strata +: XIV. Recent (alluvium) : 37. Recent : Lower alluvial. + +V. Anthropolithic group, or anthropozoic (quaternary) group of strata +: XIII. Pleistocene (diluvium) : 36. Post-glacial : Upper diluvial. + +V. Anthropolithic group, or anthropozoic (quaternary) group of strata +: XIII. Pleistocene (diluvium) : 35. Glacial : Lower diluvial. + +IV. Cenolithic group, or cenozoic (tertiary) group of strata : XII. +Pliocene (neo-tertiary) : 34. Arverne : Upper pliocene. + +IV. Cenolithic group, or cenozoic (tertiary) group of strata : XII. +Pliocene (neo-tertiary) : 33. Subapennine : Lower pliocene. + +IV. Cenolithic group, or cenozoic (tertiary) group of strata : XI. +Miocene (middle tertiary) : 32. Falun : Upper miocene. + +IV. Cenolithic group, or cenozoic (tertiary) group of strata : XI. +Miocene (middle tertiary) : 31. Limbourg : Lower miocene. + +IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xb. +Oligocene (old tertiary) : 30. Aquitaine : Upper oligocene. + +IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xb. +Oligocene (old tertiary) : 29. Ligurium : Lower oligocene. + +IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xa. +Eocene (primitive tertiary) : 28. Gypsum : Upper eocene. + +IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xa. +Eocene (primitive tertiary) : 27. Coarse chalk : Middle eocene. + +IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xa. +Eocene (primitive tertiary) : 26. London clay : Lower eocene. + +III. Mesolithic group, or mesozoic (secondary) group of strata : IX. +Chalk (cretaceous) : 25. White chalk. : Upper cretaceous. + +III. Mesolithic group, or mesozoic (secondary) group of strata : IX. +Chalk (cretaceous) : 24. Green Sand : Middle cretaceous. + +III. Mesolithic group, or mesozoic (secondary) group of strata : IX. +Chalk (cretaceous) : 23. Neocomian : Lower cretaceous. + +III. Mesolithic group, or mesozoic (secondary) group of strata : IX. +Chalk (cretaceous) : 22. Wealden : Weald-formation. + +III. Mesolithic group, or mesozoic (secondary) group of strata : VIII. +Jurassic : 21. Portland : Upper oolithic. + +III. Mesolithic group, or mesozoic (secondary) group of strata : VIII. +Jurassic : 20. Oxford : Middle oolithic. + +III. Mesolithic group, or mesozoic (secondary) group of strata : VIII. +Jurassic : 19. Bath : Lower oolithic. + +III. Mesolithic group, or mesozoic (secondary) group of strata : VIII. +Jurassic : 18. Lias : Liassic. + +III. Mesolithic group, or mesozoic (secondary) group of strata : VII. +Triassic : 17. Keuper : Upper triassic. + +III. Mesolithic group, or mesozoic (secondary) group of strata : VII. +Triassic : 16. Muschelkalk : Middle triassic. + +III. Mesolithic group, or mesozoic (secondary) group of strata : VII. +Triassic : 15. Bunter : Lower triassic. + +II. Paleolithic group, or paleozoic (primary) group of strata : VIb. +Permian : 14. Zechstein : Upper permian. + +II. Paleolithic group, or paleozoic (primary) group of strata : VIb. +Permian : 13. Neurot sand : Lower permian. + +II. Paleolithic group, or paleozoic (primary) group of strata : VIa. +Carboniferous (coal-measures) : 12. Carboniferous sandstone : Upper +carboniferous. + +II. Paleolithic group, or paleozoic (primary) group of strata : VIa. +Carboniferous (coal-measures) : 11. Carboniferous limestone : Lower +carboniferous. + +II. Paleolithic group, or paleozoic (primary) group of strata : V. +Devonian : 10. Pilton : Upper devonian. + +II. Paleolithic group, or paleozoic (primary) group of strata : V. +Devonian : 9. Ilfracombe : Middle devonian. + +II. Paleolithic group, or paleozoic (primary) group of strata : V. +Devonian : 8. Linton : Lower devonian. + +II. Paleolithic group, or paleozoic (primary) group of strata : IV. +Silurian : 7. Ludlow : Upper silurian. + +II. Paleolithic group, or paleozoic (primary) group of strata : IV. +Silurian : 6. Wenlock : Middle silurian. + +II. Paleolithic group, or paleozoic (primary) group of strata : IV. +Silurian : 5. Llandeilo : Lower silurian. + +I. Archeolithic group, or archeozoic (primordial) group of strata : +III. Cambrian : 4. Potsdam : Upper cambrian. + +I. Archeolithic group, or archeozoic (primordial) group of strata : +III. Cambrian : 3. Longmynd : Lower cambrian. + +I. Archeolithic group, or archeozoic (primordial) group of strata : +II. Huronian : 2. Labrador : Upper laurentian. + +I. Archeolithic group, or archeozoic (primordial) group of strata : I. +Laurentian : 1. Ottawa : Lower laurentian. + +The primordial period falls into three subordinate sections--the +Laurentian, Huronian, and Cambrian, corresponding to the three chief +groups of rocks that comprise the archaic formation. The immense +period during which these rocks were forming in the primitive ocean +probably comprises more than 50,000,000 years. At the commencement of +it the oldest and simplest organisms were formed by spontaneous +generation--the Monera, with which the history of life on our planet +opened. From these were first developed unicellular organisms of the +simplest character, the Protophyta and Protozoa (paulotomea, amoebae, +rhizopods, infusoria, and other Protists). During this period the +whole of the invertebrate ancestors of the human race were evolved +from the unicellular organisms. We can deduce this from the fact that +we already find remains of fossilised fishes (Selachii and Ganoids) +towards the close of the following Silurian period. These are much +more advanced and much younger than the lowest vertebrate, the +Amphioxus, and the numerous skull-less vertebrates, related to the +Amphioxus, that must have lived at that time. The whole of the +invertebrate ancestors of the human race must have preceded these. + +The primordial age is followed by a much shorter division, the +paleozoic or Primary age. It is divided into four long periods, the +Silurian, Devonian, Carboniferous, and Permian. The Silurian strata +are particularly interesting because they contain the first fossil +traces of vertebrates--teeth and scales of Selachii (Palaeodus) in the +lower, and Ganoids (Pteraspis) in the upper Silurian. During the +Devonian period the "old red sandstone" was formed; during the +Carboniferous period were deposited the vast coal-measures that yield +us our chief combustive material; in the Permian (or the Dyas), in +fine, the new red sandstone, the Zechstein (magnesian limestone), and +the Kupferschiefer (marl-slate) were formed. The collective depth of +these strata is put at 40,000 to 45,000 feet. In any case, the +paleozoic age, taken as a whole, was much shorter than the preceding +and much longer than the subsequent periods. The strata that were +deposited during this primary epoch contain a large number of fossils; +besides the invertebrate species there are a good many vertebrates, +and the fishes preponderate. There were so many fishes, especially +primitive fishes (of the shark type) and plated fishes, during the +Devonian, and also during the Carboniferous and Permian periods, that +we may describe the whole paleozoic period as "the age of fishes." +Among the paleozoic plated fishes or Ganoids the Crossopterygii and +the Ctenodipterina (dipneusts) are of great importance. + +During this period some of the fishes began to adapt themselves to +living on land, and so gave rise to the class of the amphibia. We find +in the Carboniferous period fossilised remains of five-toed amphibia, +the oldest terrestrial, air-breathing vertebrates. These amphibia +increase in variety in the Permian epoch. Towards the close of it we +find the first Amniotes, the ancestors of the three higher classes of +Vertebrates. These are lizard-like animals; the first to be discovered +was the Proterosaurus, from the marl at Eisenach. The rise of the +earliest Amniotes, among which must have been the common ancestor of +the reptiles, birds, and mammals, is put back towards the close of the +paleozoic age by the discovery of these reptile remains. The ancestors +of our race during this period were at first represented by true +fishes, then by dipneusts and amphibia, and finally by the earliest +Amniotes, or the Protamniotes. + +The third chief section of the organic history of the earth is the +Mesozoic or Secondary period. This again is subdivided into three +divisions Triassic, Jurassic, and Cretaceous. The thickness of the +strata that were deposited in this period, from the beginning of the +Triassic to the end of the Cretaceous period, is altogether about +15,000 feet, or not half as much as the paleozoic deposits. During +this period there was a very brisk and manifold development in all +branches of the animal kingdom. There were especially a number of new +and interesting forms evolved in the vertebrate stem. Bony fishes +(Teleostei) make their first appearance. Reptiles are found in +extraordinary variety and number; the extinct giant-serpents +(dinosauria), the sea-serpents (halisauria), and the flying lizards +(pterosauria) are the most remarkable and best known of these. On +account of this predominance of the reptile-class, the period is +called "the age of reptiles." But the bird-class was also evolved +during this period; they certainly originated from some division of +the lizard-like reptiles. This is proved by the embryological identity +of the birds and reptiles and their comparative anatomy, and, among +other features, from the circumstance that in this period there were +birds with teeth in their jaws and with tails like lizards +(Archeopteryx, Odontornis). + +Finally, the most advanced and (for us) the most important class of +the vertebrates, the mammals, made their appearance during the +mesozoic period. The earliest fossil remains of them were found in the +latest Triassic strata--lower jaws of small ungulates and marsupials. +More numerous remains are found a little later in the Jurassic, and +some in the Cretaceous. All the mammal remains that we have from this +section belong to the lower promammals and marsupials; among these +were most certainly the ancestors of the human race. On the other +hand, we have not found a single indisputable fossil of any higher +mammal (a placental) in the whole of this period. This division of the +mammals, which includes man, was not developed until later, towards +the close of this or in the following period. + +The fourth section of the organic history of the earth, the Tertiary +or Cenozoic age, was much shorter than the preceding. The strata that +were deposited during this period have a collective thickness of only +about 3,000 feet. It is subdivided into four sections--the Eocene, +Oligocene, Miocene, and Pliocene. During these periods there was a +very varied development of higher plant and animal forms; the fauna +and flora of our planet approached nearer and nearer to the character +that they bear to-day. In particular, the most advanced class, the +mammals, began to preponderate. Hence the Tertiary period may be +called "the age of mammals." The highest section of this class, the +placentals, now made their appearance; to this group the human race +belongs. The first appearance of man, or, to be more precise, the +development of man from some closely-related group of apes, probably +falls in either the miocene or the pliocene period, the middle or the +last section of the Tertiary period. Others believe that man properly +so-called--man endowed with speech--was not evolved from the +non-speaking ape-man (Pithecanthropus) until the following, the +anthropozoic, age. + +In this fifth and last section of the organic history of the earth we +have the full development and dispersion of the various races of men, +and so it is called the Anthropozoic as well as the Quaternary period. +In the imperfect condition of paleontological and ethnographical +science we cannot as yet give a confident answer to the question +whether the evolution of the human race from some extinct ape or lemur +took place at the beginning of this or towards the middle or the end +of the Tertiary period. However, this much is certain: the development +of civilisation falls in the anthropozoic age, and this is merely an +insignificant fraction of the vast period of the whole history of +life. When we remember this, it seems ridiculous to restrict the word +"history" to the civilised period. If we divide into a hundred equal +parts the whole period of the history of life, from the spontaneous +generation of the first Monera to the present day, and if we then +represent the relative duration of the five chief sections or ages, as +calculated from the average thickness of the strata they contain, as +percentages of this, we get something like the following relation:-- + +I. Archeolithic or archeozoic (primordial) age : 53 : 6. + +II. Paleolithic or paleozoic (primary) age : 32 : 1. + +III. Mesolithic or mesozoic (secondary) age : 11 : 5. + +IV. Cenolithic or cenozoic (tertiary) age : 2 : 3 + +V. Anthropolithic or anthropozoic (quaternary) age : 0 : 5. + +Total : 100 : 0. + +In any case, the "historical period" is an insignificant quantity +compared with the vast length of the preceding ages, in which there +was no question of human existence on our planet. Even the important +Cenozoic or Tertiary period, in which the first placentals or higher +mammals appear, probably amounts to little over two per cent of the +whole organic age. + +Before we approach our proper task, and, with the aid of our +ontogenetic acquirements and the biogenetic law, follow step by step +the paleontological development of our animal ancestors, let us glance +for a moment at another, and apparently quite remote, branch of +science, a general consideration of which will help us in the solving +of a difficult problem. I mean the science of comparative philology. +Since Darwin gave new life to biology by his theory of selection, and +raised the question of evolution on all sides, it has often been +pointed out that there is a remarkable analogy between the development +of languages and the evolution of species. The comparison is perfectly +just and very instructive. We could hardly find a better analogy when +we are dealing with some of the difficult and obscure features of the +evolution of species. In both cases we find the action of the same +natural laws. + +All philologists of any competence in their science now agree that all +human languages have been gradually evolved from very rudimentary +beginnings. The idea that speech is a gift of the gods--an idea held +by distinguished authorities only fifty years ago--is now generally +abandoned, and only supported by theologians and others who admit no +natural development whatever. Speech has been developed simultaneously +with its organs, the larynx and tongue, and with the functions of the +brain. Hence it will be quite natural to find in the evolution and +classification of languages the same features as in the evolution and +classification of organic species. The various groups of languages +that are distinguished in philology as primitive, fundamental, parent, +and daughter languages, dialects, etc., correspond entirely in their +development to the different categories which we classify in zoology +and botany as stems, classes, orders, families, genera, species, and +varieties. The relation of these groups, partly co-ordinate and partly +subordinate, in the general scheme is just the same in both cases; and +the evolution follows the same lines in both. + +When, with the assistance of this tree, we follow the formation of the +various languages that have been developed from the common root of the +ancient Indo-Germanic tongue, we get a very clear idea of their +phylogeny. We shall see at the same time how analogous this is to the +development of the various groups of vertebrates that have arisen from +the common stem-form of the primitive vertebrate. The ancient +Indo-Germanic root-language divided first into two principal +stems--the Slavo-Germanic and the Aryo-Romanic. The Slavo-Germanic +stem then branches into the ancient Germanic and the ancient +Slavo-Lettic tongues; the Aryo-Romanic into the ancient Aryan and the +ancient Greco-Roman. If we still follow the genealogical tree of these +four Indo-Germanic tongues, we find that the ancient Germanic divides +into three branches--the Scandinavian, the Gothic, and the German. +From the ancient German came the High German and Low German; to the +latter belong the Frisian, Saxon, and modern Low-German dialects. The +ancient Slavo-Lettic divided first into a Baltic and a Slav language. +The Baltic gave rise to the Lett, Lithuanian, and old-Prussian +varieties; the Slav to the Russian and South-Slav in the south-east, +and to the Polish and Czech in the west. + +We find an equally prolific branching of its two chief stems when we +turn to the other division of the Indo-Germanic languages. The +Greco-Roman divided into the Thracian (Albano-Greek) and the +Italo-Celtic. From the latter came the divergent branches of the +Italic (Roman and Latin) in the south, and the Celtic in the north: +from the latter have been developed all the British (ancient British, +ancient Scotch, and Irish) and Gallic varieties. The ancient Aryan +gave rise to the numerous Iranian and Indian languages. + +This "comparative anatomy" and evolution of languages admirably +illustrates the phylogeny of species. It is clear that in structure +and development the primitive languages, mother and daughter +languages, and varieties, correspond exactly to the classes, orders, +genera, and species of the animal world. In both cases the "natural" +system is phylogenetic. As we have been convinced from comparative +anatomy and ontogeny, and from paleontology, that all past and living +vertebrates descend from a common ancestor, so the comparative study +of dead and living Indo-Germanic tongues proves beyond question that +they are all modifications of one primitive language. This view of +their origin is now accepted by all the chief philologists who have +worked in this branch and are unprejudiced. + +But the point to which I desire particularly to draw the reader's +attention in this comparison of the Indo-Germanic languages with the +branches of the vertebrate stem is, that one must never confuse direct +descendants with collateral branches, nor extinct forms with living. +This confusion is very common, and our opponents often make use of the +erroneous ideas it gives rise to for the purpose of attacking +evolution generally. When, for instance, we say that man descends from +the ape, this from the lemur, and the lemur from the marsupial, many +people imagine that we are speaking of the living species of these +orders of mammals that they find stuffed in our museums. Our opponents +then foist this idea on us, and say, with more astuteness than +intelligence, that it is quite impossible; or they ask us, by way of +physiological experiment, to turn a kangaroo into a lemur, a lemur +into a gorilla, and a gorilla into a man! The demand is childish, and +the idea it rests on erroneous. All these living forms have diverged +more or less from the ancestral form; none of them could engender the +same posterity that the stem-form really produced thousands of years +ago. + +It is certain that man has descended from some extinct mammal; and we +should just as certainly class this in the order of apes if we had it +before us. It is equally certain that this primitive ape descended in +turn from an unknown lemur, and this from an extinct marsupial. But it +is just as clear that all these extinct ancestral forms can only be +claimed as belonging to the living order of mammals in virtue of their +essential internal structure and their resemblance in the decisive +anatomic characteristics of each ORDER. In external appearance, in the +characteristics of the GENUS or SPECIES, they would differ more or +less, perhaps very considerably, from all living representatives of +those orders. It is a universal and natural procedure in phylogenetic +development that the stem-forms themselves, with their specific +peculiarities, have been extinct for some time. The forms that +approach nearest to them among the living species are more or +less--perhaps very substantially--different from them. Hence in our +phylogenetic inquiry and in the comparative study of the living, +divergent descendants, there can only be a question of determining the +greater or less remoteness of the latter from the ancestral form. Not +a single one of the older stem-forms has continued unchanged down to +our time. + +We find just the same thing in comparing the various dead and living +languages that have developed from a common primitive tongue. If we +examine our genealogical tree of the Indo-Germanic languages in this +light, we see at once that all the older or parent tongues, of which +we regard the living varieties of the stem as divergent daughter or +grand-daughter languages, have been extinct for some time. The +Aryo-Romanic and the Slavo-Germanic tongues have completely +disappeared; so also the Aryan, the Greco-Roman, the Slavo-Lettic, and +the ancient Germanic. Even their daughters and grand-daughters have +been lost; all the living Indo-Germanic languages are only related in +the sense that they are divergent descendants of common stem-forms. +Some forms have diverged more, and some less, from the original +stem-form. + +This easily demonstrable fact illustrates very well the analogous case +of the origin of the vertebrate species. Phylogenetic comparative +philology here yields a strong support to phylogenetic comparative +zoology. But the one can adduce more direct evidence than the other, +as the paleontological material of philology--the old monuments of the +extinct tongue--have been preserved much better than the +paleontological material of zoology, the fossilised bones and imprints +of vertebrates. + +We may, however, trace man's genealogical tree not only as far as the +lower mammals, but much further--to the amphibia, to the shark-like +primitive fishes, and, in fine, to the skull-less vertebrates that +closely resembled the Amphioxus. But this must not be understood in +the sense that the existing Amphioxus, or the sharks or amphibia of +to-day, can give us any idea of the external appearance of these +remote stem-forms. Still less must it be thought that the Amphioxus or +any actual shark, or any living species of amphibia, is a real +ancestral form of the higher vertebrates and man. The statement can +only rationally mean that the living forms I have referred to are +COLLATERAL LINES that are much more closely related to the extinct +stem-forms, and have retained the resemblance much better, than any +other animals we know. They are still so like them in regard to their +distinctive internal structure that we should put them in the same +class with the extinct forms if we had these before us. But no direct +descendants of these earlier forms have remained unchanged. Hence we +must entirely abandon the idea of finding direct ancestors of the +human race in their characteristic EXTERNAL FORM among the living +species of animals. The essential and distinctive features that still +connect living forms more or less closely with the extinct common +stem-forms lie in the internal structure, not the external appearance. +The latter has been much modified by adaptation. The former has been +more or less preserved by heredity. + +Comparative anatomy and ontogeny prove beyond question that man is a +true vertebrate, and, therefore, man's special genealogical tree must +be connected with that of the other Vertebrates, which spring from a +common root with him. But we have also many important grounds in +comparative anatomy and ontogeny for assuming a common origin for all +the Vertebrates. If the general theory of evolution is correct, all +the Vertebrates, including man, come from a single common ancestor, a +long-extinct "Primitive Vertebrate." Hence the genealogical tree of +the Vertebrates is at the same time that of the human race. + +Our task, therefore, of constructing man's genealogy becomes the +larger aim of discovering the genealogy of the entire vertebrate stem. +As we now know from the comparative anatomy and ontogeny of the +Amphioxus and the Ascidia, this is in turn connected with the +genealogical tree of the Invertebrates (directly with that of the +Vermalia), but has no direct connection with the independent stems of +the Articulates, Molluscs, and Echinoderms. If we do thus follow our +ancestral tree through various stages down to the lowest worms, we +come inevitably to the Gastraea, that most instructive form that gives +the clearest possible picture of an animal with two germinal layers. +The Gastraea itself has originated from the simple multicellular +vesicle, the Blastaea, and this in turn must have been evolved from +the lowest circle of unicellular animals, to which we give the name of +Protozoa. We have already considered the most important primitive type +of these, the unicellular Amoeba, which is extremely instructive when +compared with the human ovum. With this we reach the lowest of the +solid data to which we are to apply our biogenetic law, and by which +we may deduce the extinct ancestor from the embryonic form. The +amoeboid nature of the young ovum and the unicellular condition in +which (as stem-cell or cytula) every human being begins its existence +justify us in affirming that the earliest ancestors of the human race +were simple amoeboid coils. + +But the further question now arises: "Whence came these first amoebae +with which the history of life began at the commencement of the +Laurentian epoch?" There is only one answer to this. The earliest +unicellular organisms can only have been evolved from the simplest +organisms we know, the Monera. These are the simplest living things +that we can conceive. Their whole body is nothing but a particle of +plasm, a granule of living albuminous matter, discharging of itself +all the essential vital functions that form the material basis of +life. Thus we come to the last, or, if you prefer, the first, question +in connection with evolution--the question of the origin of the +Monera. This is the real question of the origin of life, or of +spontaneous generation. + +We have neither space nor occasion to go further in this Chapter into +the question of spontaneous generation. For this I must refer the +reader to the fifteenth chapter of the History of Creation, and +especially to the second book of the General Morphology, or to the +essay on "The Monera and Spontaneous Generation" in my Studies of the +Monera and other Protists.* (* The English reader will find a luminous +and up-to-date chapter on the subject in Haeckel's recently written +and translated Wonders of Life.--Translator.) I have given there fully +my own view of this important question. The famous botanist Nageli +afterwards (1884) developed the same ideas. I will only say a few +words here about this obscure question of the origin of life, in so +far as our main subject, organic evolution in general, is affected by +it. Spontaneous generation, in the definite and restricted sense in +which I maintain it, and claim that it is a necessary hypothesis in +explaining the origin of life, refers solely to the evolution of the +Monera from inorganic carbon-compounds. When living things made their +first appearance on our planet, the very complex nitrogenous compound +of carbon that we call plasson, which is the earliest material +embodiment of vital action, must have been formed in a purely chemical +way from inorganic carbon-compounds. The first Monera were formed in +the sea by spontaneous generation, as crystals are formed in the +mother-water. Our demand for a knowledge of causes compels us to +assume this. If we believe that the whole inorganic history of the +earth has proceeded on mechanical principles without any intervention +of a Creator, and that the history of life also has been determined by +the same mechanical laws; if we see that there is no need to admit +creative action to explain the origin of the various groups of +organisms; it is utterly irrational to assume such creative action in +dealing with the first appearance of organic life on the earth. + +This much-disputed question of "spontaneous generation" seems so +obscure, because people have associated with the term a mass of very +different, and often very absurd, ideas, and have attempted to solve +the difficulty by the crudest experiments. The real doctrine of the +spontaneous generation of life cannot possibly be refuted by +experiments. Every experiment that has a negative result only proves +that no organism has been formed out of inorganic matter in the +conditions--highly artificial conditions--we have established. On the +other hand, it would be exceedingly difficult to prove the theory by +way of experiment; and even if Monera were still formed daily by +spontaneous generation (which is quite possible), it would be very +difficult, if not impossible, to find a solid proof of it. Those who +will not admit the spontaneous generation of the first living things +in our sense must have recourse to a supernatural miracle; and this +is, as a matter of fact, the desperate resource to which our "exact" +scientists are driven, to the complete abdication of reason. + +A famous English physicist, Lord Kelvin (then Sir W. Thomson), +attempted to dispense with the hypothesis of spontaneous generation by +assuming that the organic inhabitants of the earth were developed from +germs that came from the inhabitants of other planets, and that +chanced to fall on our planet on fragments of their original home, or +meteorites. This hypothesis found many supporters, among others the +distinguished German physicist, Helmholtz. However, it was refuted in +1872 by the able physicist, Friedrich Zollner, of Leipzig, in his +work, On the Nature of Comets. He showed clearly how unscientific this +hypothesis is; firstly in point of logic, and secondly in point of +scientific content. At the same time he pointed out that our +hypothesis of spontaneous generation is "a necessary condition for +understanding nature according to the law of causality." + +I repeat that we must call in the aid of the hypothesis only as +regards the Monera, the structureless "organisms without organs." +Every complex organism must have been evolved from some lower +organism. We must not assume the spontaneous generation of even the +simplest cell, for this itself consists of at least two parts--the +internal, firm nuclear substance, and the external, softer cellular +substance or the protoplasm of the cell-body. These two parts must +have been formed by differentiation from the indifferent plasson of a +moneron, or a cytode. For this reason the natural history of the +Monera is of great interest; here alone can we find the means to +overcome the chief difficulties of the problem of spontaneous +generation. The actual living Monera are specimens of such organless +or structureless organisms, as they must have boon formed by +spontaneous generation at the commencement of the history of life. + + +CHAPTER 2.19. OUR PROTIST ANCESTORS. + +Under the guidance of the biogenetic law, and on the basis of the +evidence we have obtained, we now turn to the interesting task of +determining the series of man's animal ancestors. Phylogeny us a whole +is an inductive science. From the totality of the biological processes +in the life of plants, animals, and man we have gathered a confident +inductive idea that the whole organic population of our planet has +been moulded on a harmonious law of evolution. All the interesting +phenomena that we meet in ontogeny and paleontology, comparative +anatomy and dysteleology, the distribution and habits of +organisms--all the important general laws that we abstract from the +phenomena of these sciences, and combine in harmonious unity--are the +broad bases of our great biological induction. + +But when we come to the application of this law, and seek to determine +with its aid the origin of the various species of organisms, we are +compelled to frame hypotheses that have essentially a DEDUCTIVE +character, and are inferences from the general law to particular +cases. But these special deductions are just as much justified and +necessitated by the rigorous laws of logic as the inductive +conclusions on which the whole theory of evolution is built. The +doctrine of the animal ancestry of the human race is a special +deduction of this kind, and follows with logical necessity from the +general inductive law of evolution. + +I must point out at once, however, that the certainty of these +evolutionary hypotheses, which rest on clear special deductions, is +not always equally strong. Some of these inferences are now beyond +question; in the case of others it depends on the knowledge and the +competence of the inquirer what degree of certainty he attributes to +them. In any case, we must distinguish between the ABSOLUTE certainty +of the general (inductive) theory of descent and the RELATIVE +certainty of special (deductive) evolutionary hypotheses. We can never +determine the whole ancestral series of an organism with the same +confidence with which we hold the general theory of evolution as the +sole scientific explanation of organic modifications. The special +indication of stem-forms in detail will always be more or less +incomplete and hypothetical. This is quite natural. The evidence on +which we build is imperfect, and always will be imperfect; just as in +comparative philology. + +The first of our documents, paleontology, is exceedingly incomplete. +We know that all the fossils yet discovered are only an insignificant +fraction of the plants and animals that have lived on our planet. For +every single species that has been preserved for us in the rocks there +are probably hundreds, perhaps thousands, of extinct species that have +left no trace behind them. This extreme and very unfortunate +incompleteness of the paleontological evidence, which cannot be +pointed out too often, is easily explained. It is absolutely +inevitable in the circumstances of the fossilisation of organisms. It +is also due in part to the incompleteness of our knowledge in this +branch. It must be borne in mind that the great majority of the +stratified rocks that compose the crust of the earth have not yet been +opened. We have only a few specimens of the innumerable fossils that +are buried in the vast mountain ranges of Asia and Africa. Only a part +of Europe and North America has been investigated carefully. The whole +of the fossils known to us certainly do not amount to a hundredth part +of the remains that are really buried in the crust of the earth. We +may, therefore, look forward to a rich harvest in the future as +regards this science. However, our paleontological evidence will (for +reasons that I have fully explained in the sixteenth chapter of the +History of Creation) always be defective. + +The second chief source of evidence, ontogeny, is not less incomplete. +It is the most important source of all for special phylogeny; but it +has great defects, and often fails us. We must, above all, clearly +distinguish between palingenetic and cenogenetic phenomena. We must +never forget that the laws of curtailed and disturbed heredity often +make the original course of development almost unrecognisable. The +recapitulation of phylogeny by ontogeny is only fairly complete in a +few cases, and is never wholly complete. As a rule, it is precisely +the earliest and most important embryonic stages that suffer most from +alteration and condensation. The earlier embryonic forms have had to +adapt themselves to new circumstances, and so have been modified. The +struggle for existence has had just as profound an influence on the +freely moving and still immature young forms as on the adult forms. +Hence in the embryology of the higher animals, especially, +palingenesis is much restricted by cenogenesis; it is to-day, as a +rule, only a faded and much altered picture of the original evolution +of the animal's ancestors. We can only draw conclusions from the +embryonic forms to the stem-history with the greatest caution and +discrimination. Moreover, the embryonic development itself has only +been fully studied in a few species. + +Finally, the third and most valuable source of evidence, comparative +anatomy, is also, unfortunately, very imperfect; for the simple reason +that the whole of the living species of animals are a mere fraction of +the vast population that has dwelt on our planet since the beginning +of life. We may confidently put the total number of these at more than +a million species. The number of animals whose organisation has been +studied up to the present in comparative anatomy is proportionately +very small. Here, again, future research will yield incalculable +treasures. But, for the present, in view of this patent incompleteness +of our chief sources of evidence, we must naturally be careful not to +lay too much stress in human phylogeny on the particular animals we +have studied, or regard all the various stages of development with +equal confidence as stem-forms. + +In my first efforts to construct the series of man's ancestors I drew +up a list of, at first ten, afterwards twenty to thirty, forms that +may be regarded more or less certainly as animal ancestors of the +human race, or as stages that in a sense mark off the chief sections +in the long story of evolution from the unicellular organism to man. +Of these twenty to thirty stages, ten to twelve belong to the older +group of the Invertebrates and eighteen to twenty to the younger +division of the Vertebrates. + +In approaching, now, the difficult task of establishing the +evolutionary succession of these thirty ancestors of humanity since +the beginning of life, and in venturing to lift the veil that covers +the earliest secrets of the earth's history, we must undoubtedly look +for the first living things among the wonderful organisms that we call +the Monera; they are the simplest organisms known to us--in fact, the +simplest we can conceive. Their whole body consists merely of a simple +particle or globule of structureless plasm or plasson. The discoveries +of the last four decades have led us to believe with increasing +certainty that wherever a natural body exhibits the vital processes of +nutrition, reproduction, voluntary movement, and sensation, we have +the action of a nitrogenous carbon-compound of the chemical group of +the albuminoids; this plasm (or protoplasm) is the material basis of +all vital functions. Whether we regarded the function, in the monistic +sense, as the direct action of the material substratum, or whether we +take matter and force to be distinct things in the dualistic sense, it +is certain that we have not as yet found any living organism in which +the exercise of the vital functions is not inseparably bound up with +plasm. + +The soft slimy plasson of the body of the moneron is generally called +"protoplasm," and identified with the cellular matter of the ordinary +plant and animal cells. But we must, to be accurate, distinguish +between the plasson of the cytodes and the protoplasm of the cells. +This distinction is of the utmost importance for the purposes of +evolution. As I have often said, we must recognise two different +stages of development in these "elementary organisms," or plastids +("builders"), that represent the ultimate units of organic +individuality. The earlier and lower stage are the unnucleated +cytodes, the body of which consists of only one kind of albuminous +matter--the homogeneous plasson or "formative matter." The later and +higher stage are the nucleated cells, in which we find a +differentiation of the original plasson into two different formative +substances--the caryoplasm of the nucleus and the cytoplasm of the +body of the cell (cf. Chapter 1.6.) + +(FIGURE 2.226. Chroococcus minor (Nageli), magnified 1500 times. A +phytomoneron, the globular plastids of which secrete a gelatinous +structureless membrane. The unnucleated globule of plasm (bluish-green +in colour) increases by simple cleavage (a to d). + +The Monera are permanent cytodes. Their whole body consists of soft, +structureless plasson. However carefully we examine it with our finest +chemical reagents and most powerful microscopes, we can find no +definite parts or no anatomic structure in it. Hence, the Monera are +literally organisms without organs; in fact, from the philosophic +point of view they are not organisms at all, since they have no +organs. They can only be called organisms in the sense that they are +capable of the vital functions of nutrition, reproduction, sensation, +and movement. If we were to try to imagine the simplest possible +organism, we should frame something like the moneron. + +The Monera that we find to-day in various forms fall into two groups +according to the nature of their nutrition--the Phytomonera and the +Zoomonera; from the physiological point of view, the former are the +simplest specimens of the plant (phyton) kingdom, and the latter of +the animal (zoon) world. The Phytomonera, especially in their simplest +form, the Chromacea (Phycochromacea or Cyanophycea), are the most +primitive and the oldest of living organisms. The typical genus +Chroococcus (Figure 2.226) is represented by several fresh-water +species, and often forms a very delicate bluish-green deposit on +stones and wood in ponds and ditches. It consists of round, light +green particles, from 1/7000 to 1/2500 of an inch in diameter. + +(FIGURE 2.227. Aphanocapsa primordialis (Nageli), magnified 1000 +times. A phytomoneron, the round plastids of which (bluish-green in +colour) secrete a shapeless gelatinous mass; in this the unnucleated +cytodes increase continually by simple cleavage.) + +The whole life of these homogeneous globules of plasm consists of +simple growth and reproduction by cleavage. When the tiny particle has +reached a certain size by the continuous assimilation of inorganic +matter, it divides into two equal halves, by a constriction in the +middle. The two daughter-monera that are thus formed immediately begin +a similar vital process. It is the same with the brown Procytella +primordialis (formerly called the Protococcus marinus); it forms large +masses of floating matter in the arctic seas. The tiny plasma-globules +of this species are of a greenish-brown colour, and have a diameter of +1/10,000 to 1/5000 of an inch. There is no membrane discoverable in +the simplest Chroococcacea, but we find one in other members of the +same family; in Aphanocapsa (Figure 2.227) the enveloping membranes of +the social plastids combine; in Gloecapsa they are retained through +several generations, so that the little plasma-globules are enfolded +in many layers of membrane. + +Next to the Chromacea come the Bacteria, which have been evolved from +them by the remarkable change in nutrition which gives us the simple +explanation of the differentiation of plant and animal in the protist +kingdom. The Chromacea build up their plasm directly from inorganic +matter; the Bacteria feed on organic matter. Hence, if we logically +divide the protist kingdom into plasma-forming Protophyta and +plasma-consuming Protozoa, we must class the Bacteria with the latter; +it is quite illogical to describe them--as is still often done--as +Schizomycetes, and class them with the true fungi. The Bacteria, like +the Chromacea, have no nucleus. As is well-known, they play an +important part in modern biology as the causes of fermentation and +putrefaction, and of tuberculosis, typhus, cholera, and other +infectious diseases, and as parasites, etc. But we cannot linger now +to deal with these very interesting features; the Bacteria have no +relation to man's genealogical tree. + +We may now turn to consider the remarkable Protamoeba, or unnucleated +Amoeba. I have, in the first volume, pointed out the great importance +of the ordinary Amoeba in connection with several weighty questions of +general biology. The tiny Protamoebae, which are found both in fresh +and salt water, have the same unshapely form and irregular movements +of their simple naked body as the real Amoebae; but they differ from +them very materially in having no nucleus in their cell-body. The +short, blunt, finger-like processes that are thrust out at the surface +of the creeping Protamoeba serve for getting food as well as for +locomotion. They multiply by simple cleavage (Figure 2.228). + +(FIGURE 2.228. A moneron (Protamoeba) in the act of reproduction. A +The whole moneron, moving like an ordinary amoeba by thrusting out +changeable processes. B It divides into two halves by a constriction +in the middle. C The two halves separate, and each becomes an +independent individual. (Highly magnified.)) + +The next stage to the simple cytode-forms of the Monera in the +genealogy of mankind (and all other animals) is the simple cell, or +the most rudimentary form of the cell which we find living +independently to-day as the Amoeba. The earliest process of inorganic +differentiation in the structureless body of the Monera led to its +division into two different substances--the caryoplasm and the +cytoplasm. The caryoplasm is the inner and firmer part of the cell, +the substance of the nucleus. The cytoplasm is the outer and softer +part, the substance of the body of the cell. By this important +differentiation of the plasson into nucleus and cell-body, the +organised cell was evolved from the structureless cytode, the +nucleated from the unnucleated plastid. That the first cells to appear +on the earth were formed from the Monera by such a differentiation +seems to us the only possible view in the present condition of +science. We have a direct instance of this earliest process of +differentiation to-day in the ontogeny of many of the lower Protists +(such as the Gregarinae). + +The unicellular form that we have in the ovum has already been +described as the reproduction of a corresponding unicellular +stem-form, and to this we have ascribed the organisation of an Amoeba +(cf. Chapter 1.6). The irregular-shaped Amoeba, which we find living +independently to-day in our fresh and salt water, is the least +definite and the most primitive of all the unicellular Protozoa +(Figure 1.16). As the unripe ova (the protova that we find in the +ovaries of animals) cannot be distinguished from the common Amoebae, +we must regard the Amoeba as the primitive form that is reproduced in +the embryonic stage of the amoeboid ovum to-day, in accordance with +the biogenetic law. I have already pointed out, in proof of the +striking resemblance of the two cells, that the ova of many of the +sponges were formerly regarded as parasitic Amoebae (Figure 1.18). +Large unicellular organisms like the Amoebae were found creeping about +inside the body of the sponge, and were thought to be parasites. It +was afterwards discovered that they were really the ova of the sponge +from which the embryos were developed. As a matter of fact, these +sponge-ova are so much like many of the Amoebae in size, shape, the +character of their nucleus, and movement of the pseudopodia, that it +is impossible to distinguish them without knowing their subsequent +development. + +Our phylogenetic interpretation of the ovum, and the reduction of it +to some ancient amoeboid ancestral form, supply the answer to the old +problem: "Which was first, the egg or the chick?" We can now give a +very plain answer to this riddle, with which our opponents have often +tried to drive us into a corner. The egg came a long time before the +chick. We do not mean, of course, that the egg existed from the first +as a bird's egg, but as an indifferent amoeboid cell of the simplest +character. The egg lived for thousands of years as an independent +unicellular organism, the Amoeba. The egg, in the modern physiological +sense of the word, did not make its appearance until the descendants +of the unicellular Protozoon had developed into multicellular animals, +and these had undergone sexual differentiation. Even then the egg was +first a gastraea-egg, then a platode-egg, then a vermalia-egg, and +chordonia-egg; later still acrania-egg, then fish-egg, amphibia-egg, +reptile-egg, and finally bird's egg. The bird's egg we have experience +of daily is a highly complicated historical product, the result of +countless hereditary processes that have taken place in the course of +millions of years. + +The earliest ancestors of our race were simple Protophyta, and from +these our protozoic ancestors were developed afterwards. From the +morphological point of view both the vegetal and the animal Protists +were simple organisms, individualities of the first order, or +plastids. All our later ancestors are complex organisms, or +individualities of a higher order--social aggregations of a plurality +of cells. The earliest of these, the Moraeada, which represent the +third stage in our genealogy, are very simple associations of +homogeneous, indifferent cells--undifferentiated colonies of social +Amoebae or Infusoria. To understand the nature and origin of these +protozoa-colonies we need only follow step by step the first embryonic +products of the stem-cell. In all the Metazoa the first embryonic +process is the repeated cleavage of the stem-cell, or first +segmentation-cell (Figure 2.229). We have already fully considered +this process, and found that all the different forms of it may be +reduced to one type, the original equal or primordial segmentation +(cf. Chapter 1.8). In the genealogical tree of the Vertebrates this +palingenetic form of segmentation has been preserved in the Amphioxus +alone, all the other Vertebrates having cenogenetically modified forms +of cleavage. In any case, the latter were developed from the former, +and so the segmentation of the ovum in the Amphioxus has a great +interest for us (cf. Figure 1.38). The outcome of this repeated +cleavage is the formation of a round cluster of cells, composed of +homogeneous, indifferent cells of the simplest character (Figure +2.230). This is called the morula (= mulberry-embryo) on account of +its resemblance to a mulberry or blackberry. + +(FIGURE 2.229. Original or primordial ovum-cleavage. The stem-cell or +cytula, formed by fecundation of the ovum, divides by repeated regular +cleavage first into two (A), then four (B), then eight (C), and +finally a large number of segmentation-cells (D). + +FIGURE 2.230. Morula, or mulberry-shaped embryo.) + +It is clear that this morula reproduces for us to-day the simple +structure of the multicellular animal that succeeded the unicellular +amoeboid form in the early Laurentian period. In accordance with the +biogenetic law, the morula recalls the ancestral form of the Moraea, +or simple colony of Protozoa. The first cell-communities to be formed, +which laid the early foundation of the higher multicellular body, must +have consisted of homogeneous and simple amoeboid cells. The oldest +Amoebae lived isolated lives, and even the amoeboid cells that were +formed by the segmentation of these unicellular organisms must have +continued to live independently for a long time. But gradually small +communities of Amoebae arose by the side of these eremitical Protozoa, +the sister-cells produced by cleavage remaining joined together. The +advantages in the struggle for life which these communities had over +the isolated cells favoured their formation and their further +development. We find plenty of these cell-colonies or communities +to-day in both fresh and salt water. They belong to various groups +both of the Protophyta and Protozoa. + +To have some idea of those ancestors of our race that succeeded +phylogenetically to the Moraeada, we have only to follow the further +embryonic development of the morula. We then see that the social cells +of the round cluster secrete a sort of jelly or a watery fluid inside +their globular body, and they themselves rise to the surface of it +(Figure 1.29 F, G). In this way the solid mulberry-embryo becomes a +hollow sphere, the wall of which is composed of a single layer of +cells. We call this layer the blastoderm, and the sphere itself the +blastula, or embryonic vesicle. + +This interesting blastula is very important. The conversion of the +morula into a hollow ball proceeds on the same lines originally in the +most diverse stems--as, for instance, in many of the zoophytes and +worms, the ascidia, many of the echinoderms and molluscs, and in the +amphioxus. Moreover, in the animals in which we do not find a real +palingenetic blastula the defect is clearly due to cenogenetic causes, +such as the formation of food-yelk and other embryonic adaptations. We +may, therefore, conclude that the ontogenetic blastula is the +reproduction of a very early phylogenetic ancestral form, and that all +the Metazoa are descended from a common stem-form, which was in the +main constructed like the blastula. In many of the lower animals the +blastula is not developed within the foetal membranes, but in the open +water. In those cases each blastodermic cell begins at an early stage +to thrust out one or more mobile hair-like processes; the body swims +about by the vibratory movement of these lashes or whips (Figure 1.29 +F). + +We still find, both in the sea and in fresh water, various kinds of +primitive multicellular organisms that substantially resemble the +blastula in structure, and may be regarded in a sense as permanent +blastula-forms--hollow vesicles or gelatinous balls, with a wall +composed of a single layer of ciliated homogeneous cells. There are +"blastaeads" of this kind even among the Protophyta--the familiar +Volvocina, formerly classed with the infusoria. The common Volvox +globator is found in the ponds in the spring--a small, green, +gelatinous globule, swimming about by means of the stroke of its +lashes, which rise in pairs from the cells on its surface. In the +similar Halosphaera viridis also, which we find in the marine plancton +(floating matter), a number of green cells form a simple layer at the +surface of the gelatinous ball; but in this case there are no cilia. + +Some of the infusoria of the flagellata-class (Signura, Magosphaera, +etc.) are similar in structure to these vegetal clusters, but differ +in their animal nutrition; they form the special group of the +Catallacta. In September, 1869, I studied the development of one of +these graceful animals on the island of Gis-Oe, off the coast of +Norway (Magosphaera planula), Figures 2.231 and 2.232). The +fully-formed body is a gelatinous ball, with its wall composed of +thirty-two to sixty-four ciliated cells; it swims about freely in the +sea. After reaching maturity the community is dissolved. Each cell +then lives independently for some time, grows, and changes into a +creeping amoeba. This afterwards contracts, and clothes itself with a +structureless membrane. The cell then looks just like an ordinary +animal ovum. When it has been in this condition for some time the cell +divides into two, four, eight, sixteen, thirty-two, and sixty-four +cells. These arrange themselves in a round vesicle, thrust out +vibratory lashes, burst the capsule, and swim about in the same +magosphaera-form with which we started. This completes the life-circle +of the remarkable and instructive animal. + +If we compare these permanent blastulae with the free-swimming +ciliated larvae or blastulae, with similar construction, of many of +the lower animals, we can confidently deduce from them that there was +a very early and long-extinct common stem-form of substantially the +same structure as the blastula. We may call it the Blastaea. Its body +consisted, when fully formed, of a simple hollow ball, filled with +fluid or structureless jelly, with a wall composed of a single stratum +of ciliated cells. There were probably many genera and species of +these blastaeads in the Laurentian period, forming a special class of +marine protists. + +It is an interesting fact that in the plant kingdom also the simple +hollow sphere is found to be an elementary form of the multicellular +organism. At the surface and below the surface (down to a depth of +2000 yards) of the sea there are green globules swimming about, with a +wall composed of a single layer of chlorophyll-bearing cells. The +botanist Schmitz gave them the name of Halosphaera viridis in 1879. + +The next stage to the Blastaea, and the sixth in our genealogical +tree, is the Gastraea that is developed from it. As we have already +seen, this ancestral form is particularly important. That it once +existed is proved with certainty by the gastrula, which we find +temporarily in the ontogenesis of all the Metazoa (Figure 1.29 J, K). +As we saw, the original, palingenetic form of the gastrula is a round +or oval uni-axial body, the simple cavity of which (the primitive gut) +has an aperture at one pole of its axis (the primitive mouth). The +wall of the gut consists of two strata of cells, and these are the +primary germinal layers, the animal skin-layer (ectoderm) and vegetal +gut-layer (entoderm). + +The actual ontogenetic development of the gastrula from the blastula +furnishes sound evidence as to the phylogenetic origin of the Gastraea +from the Blastaea. A pit-shaped depression appears at one side of the +spherical blastula (Figure 1.29 H). In the end this invagination goes +so far that the outer or invaginated part of the blastoderm lies close +on the inner or non-invaginated part (Figure 1.29 J). In explaining +the phylogenetic origin of the gastraea in the light of this +ontogenetic process, we may assume that the one-layered cell-community +of the blastaea began to take in food more largely at one particular +part of its surface. Natural selection would gradually lead to the +formation of a depression or pit at this alimentary spot on the +surface of the ball. The depression would grow deeper and deeper. In +time the vegetal function of taking in and digesting food would be +confined to the cells that lined this hole; the other cells would see +to the animal functions of locomotion, sensation, and protection. This +was the first division of labour among the originally homogeneous +cells of the blastaea. + +(FIGURE 2.231. The Norwegian Magosphaera planula, swimming about by +means of the lashes or cilia at its surface. + +FIGURE 2.232. Section of Magosphaera planula, showing how the +pear-shaped cells in the centre of the gelatinous ball are connected +by a fibrous process. Each cell has a contractile vacuole as well as a +nucleus.) + +The effect, then, of this earliest histological differentiation was to +produce two different kinds of cells--nutritive cells in the +depression and locomotive cells on the surface outside. But this +involved the severance of the two primary germinal layers--a most +important process. When we remember that even man's body, with all its +various parts, and the body of all the other higher animals, are built +up originally out of these two simple layers, we cannot lay too much +stress on the phylogenetic significance of this gastrulation. In the +simple primitive gut or gastric cavity of the gastrula and its +rudimentary mouth we have the first real organ of the animal frame in +the morphological sense; all the other organs were developed +afterwards from these. In reality, the whole body of the gastrula is +merely a "primitive gut." I have shown already (Chapters 1.8 and 1.9) +that the two-layered embryos of all the Metazoa can be reduced to this +typical gastrula. This important fact justifies us in concluding, in +accordance with the biogenetic law, that their ancestors also were +phylogenetically developed from a similar stem-form. This ancient +stem-form is the gastraea. + +The gastraea probably lived in the sea during the Laurentian period, +swimming about in the water by means of its ciliary coat much as free +ciliated gastrulae do to-day. Probably it differed from the existing +gastrula only in one essential point, though extinct millions of years +ago. We have reason, from comparative anatomy and ontogeny, to believe +that it multiplied by sexual generation, not merely asexually (by +cleavage, gemmation, and spores), as was no doubt the case with the +earlier ancestors. Some of the cells of the primary germ-layers +probably became ova and others fertilising sperm. We base these +hypotheses on the fact that we do to-day find the simplest form of +sexual reproduction in some of the living gastraeads and other lower +animals, especially the sponges. + +The fact that there are still in existence various kinds of +gastraeads, or lower Metazoa with an organisation little higher than +that of the hypothetical gastraea, is a strong point in favour of our +theory. There are not very many species of these living gastraeads; +but their morphological and phylogenetic interest is so great, and +their intermediate position between the Protozoa and Metazoa so +instructive, that I proposed long ago (1876) to make a special class +of them. I distinguished three orders in this class--the Gastremaria, +Physemaria, and Cyemaria (or Dicyemida). But we might also regard +these three orders as so many independent classes in a primitive +gastraead stem. + +The Gastremaria and Cyemaria, the chief of these living gastraeads, +are small Metazoa that live parasitically inside other Metazoa, and +are, as a rule, 1/50 to 1/25 of an inch long, often much less (Figure +2.233, 1 to 15). Their soft body, devoid of skeleton, consists of two +simple strata of cells, the primary germinal layers; the outer of +these is thickly clothed with long hair-like lashes, by which the +parasites swim about in the various cavities of their host. The inner +germinal layer furnishes the sexual products. The pure type of the +original gastrula (or archigastrula, Figure 1.29 I) is seen in the +Pemmatodiscus gastrulaceus, which Monticelli discovered in the +umbrella of a large medusa (Pilema pulmo) in 1895; the convex surface +of this gelatinous umbrella was covered with numbers of clear +vesicles, of 1/25 to 1/8 inch in diameter, in the fluid contents of +which the little parasites were swimming. The cup-shaped body of the +Pemmatodiscus (Figure 2.233, 1) is sometimes rather flat, and shaped +like a hat or cone, at other times almost curved into a semi-circle. +The simple hollow of the cup, the primitive gut (g), has a narrow +opening (o). The skin layer (e) consists of long slender cylindrical +cells, which bear long vibratory hairs; it is separated by a thin +structureless, gelatinous plate (f) from the visceral or gut layer +(i), the prismatic cells of which are much smaller and have no cilia. +Pemmatodiscus propagates asexually, by simple longitudinal cleavage; +on this account it has recently been regarded as the representative of +a special order of gastraeads (Mesogastria). + +Probably a near relative of the Pemmatodiscus is the Kunstleria +Gruveli (Figure 2.233, 2). It lives in the body-cavity of Vermalia +(Sipunculida), and differs from the former in having no lashes either +on the large ectodermic cells (e) or the small entodermic (i); the +germinal layers are separated by a thick, cup-shaped, gelatinous mass, +which has been called the "clear vesicle" (f). The primitive mouth is +surrounded by a dark ring that bears very strong and long vibratory +lashes, and effects the swimming movements. + +Pemmatodiscus and Kunstleria may be included in the family of the +Gastremaria. To these gastraeads with open gut are closely related the +Orthonectida (Rhopalura, Figure 2.233, 3 to 5). They live +parasitically in the body-cavity of echinoderms (Ophiura) and +vermalia; they are distinguished by the fact that their primitive +gut-cavity is not empty, but filled with entodermic cells, from which +the sexual cells are developed. These gastraeads are of both sexes, +the male (Figure 1.3) being smaller and of a somewhat different shape +from the oval female (Figure 1.4). + +The somewhat similar Dicyemida (Figure 1.6) are distinguished from the +preceding by the fact that their primitive gut-cavity is occupied by a +single large entodermic cell instead of a crowded group of sexual +cells. This cell does not yield sexual products, but afterwards +divides into a number of cells (spores), each of which, without being +impregnated, grows into a small embryo. The Dicyemida live +parasitically in the body-cavity, especially the renal cavities, of +the cuttle-fishes. They fall in several genera, some of which are +characterised by the possession of special polar cells; the body is +sometimes roundish, oval, or club-shaped, at other times long and +cylindrical. The genus Conocyema (Figures 1.7 to 1.15) differs from +the ordinary Dicyema in having four polar pimples in the form of a +cross, which may be incipient tentacles. + +The classification of the Cyemaria is much disputed; sometimes they +are held to be parasitic infusoria (like the Opalina), sometimes +platodes or vermalia, related to the suctorial worms or rotifers, but +having degenerated through parasitism. I adhere to the +phylogenetically important theory that I advanced in 1876, that we +have here real gastraeads, primitive survivors of the common +stem-group of all the Metazoa. In the struggle for life they have +found shelter in the body-cavity of other animals. + +(FIGURE 2.233. Modern gastraeads. +Figure 1. Pemmatodiscus gastrulaceus (Monticelli), in longitudinal +section. +Figure 2. Kunstleria gruveli (Delage), in longitudinal section. (From +Kunstler and Gruvel.) +Figures 3 to 5. Rhopalura Giardi (Julin): Figure 3 male, Figure 4 +female, Figure 5 planula. +Figure 6. Dicyema macrocephala (Van Beneden). +Figures 7 to 15. Conocyema polymorpha (Van Beneden): Figure 7 the +mature gastraead, Figures 8 to 15 its gastrulation. d primitive gut, o +primitive mouth, e ectoderm, i entoderm, f gelatinous plate between e +and i (supporting plate, blastocoel).) + +The small Coelenteria attached to the floor of the sea that I have +called the Physemaria (Haliphysema and Gastrophysema) probably form a +third order (or class) of the living gastraeads. The genus Haliphysema +(Figures 2.234 and 2.235) is externally very similar to a large +rhizopod (described by the same name in 1862) of the family of the +Rhabdamminida, which was at first taken for a sponge. In order to +avoid confusion with these, I afterwards gave them the name of +Prophysema. The whole mature body of the Prophysema is a simple +cylindrical or oval tube, with a two-layered wall. The hollow of the +tube is the gastric cavity, and the upper opening of it the mouth +(Figure 2.235 m). The two strata of cells that form the wall of the +tube are the primary germinal layers. These rudimentary zoophytes +differ from the swimming gastraeads chiefly in being attached at one +end (the end opposite to the mouth) to the floor of the sea. + +In Prophysema the primitive gut is a simple oval cavity, but in the +closely related Gastrophysema it is divided into two chambers by a +transverse constriction; the hind and smaller chamber above furnishes +the sexual products, the anterior one being for digestion. + +The simplest sponges (Olynthus, Figure 2.238) have the same +organisation as the Physemaria. The only material difference between +them is that in the sponge the thin two-layered body-wall is pierced +by numbers of pores. When these are closed they resemble the +Physemaria. Possibly the gastraeads that we call Physemaria are only +olynthi with the pores closed. The Ammoconida, or the simple tubular +sand-sponges of the deep-sea (Ammolynthus, etc.), do not differ from +the gastraeads in any important point when the pores are closed. In my +Monograph on the Sponges (with sixty plates) I endeavoured to prove +analytically that all the species of this class can be traced +phylogenetically to a common stem-form (Calcolynthus). + +(FIGURES 2.234 AND 2.235. Prophysema primordiale, a living gastraead. + +FIGURE 2.234. The whole of the spindle-shaped animal (attached below +to the floor of the sea). + +FIGURE 2.235. The same in longitudinal section. The primitive gut (d) +opens above at the primitive mouth (m). Between the ciliated cells (g) +are the amoeboid ova (e). The skin-layer (h) is encrusted with grains +of sand below and sponge-spicules above. + +FIGURES 2.236 TO 2.237. Ascula of gastrophysema, attached to the floor +of the sea. Figure 2.236 external view, 2.237 longitudinal section. g +primitive gut, o primitive mouth, i visceral layer, e cutaneous layer. +(Diagram.) + +FIGURE 2.238. Olynthus, a very rudimentary sponge. A piece cut away in +front.) + +The lowest form of the Cnidaria is also not far removed from the +gastraeads. In the interesting common fresh-water polyp (Hydra) the +whole body is simply an oval tube with a double wall; only in this +case the mouth has a crown of tentacles. Before these develop the +hydra resembles an ascula (Figures 2.236 and 2.237). Afterwards there +are slight histological differentiations in its ectoderm, though the +entoderm remains a single stratum of cells. We find the first +differentiation of epithelial and stinging cells, or of muscular and +neural cells, in the thick ectoderm of the hydra. + +In all these rudimentary living coelenteria the sexual cells of both +kinds--ova and sperm cells--are formed by the same individual; it is +possible that the oldest gastraeads were hermaphroditic. It is clear +from comparative anatomy that hermaphrodism--the combination of both +kinds of sexual cells in one individual--is the earliest form of +sexual differentiation; the separation of the sexes (gonochorism) was +a much later phenomenon. The sexual cells originally proceeded from +the edge of the primitive mouth of the gastraead. + + +CHAPTER 2.20. OUR WORM-LIKE ANCESTORS. + +The gastraea theory has now convinced us that all the Metazoa or +multicellular animals can be traced to a common stem-form, the +Gastraea. In accordance with the biogenetic law, we find solid proof +of this in the fact that the two-layered embryos of all the Metazoa +can be reduced to a primitive common type, the gastrula. Just as the +countless species of the Metazoa do actually develop in the individual +from the simple embryonic form of the gastrula, so they have all +descended in past time from the common stem-form of the Gastraea. In +this fact, and the fact we have already established that the Gastraea +has been evolved from the hollow vesicle of the one-layered Blastaea, +and this again from the original unicellular stem-form, we have +obtained a solid basis for our study of evolution. The clear path from +the stem-cell to the gastrula represents the first section of our +human stem-history (Chapters 1.8, 1.9, and 2.19). + +The second section, that leads from the Gastraea to the Prochordonia, +is much more difficult and obscure. By the Prochordonia we mean the +ancient and long-extinct animals which the important embryonic form of +the chordula proves to have once existed (cf. Figures 1.83 to 1.86). +The nearest of living animals to this embryonic structure are the +lowest Tunicates, the Copelata (Appendicaria) and the larvae of the +Ascidia. As both the Tunicates and the Vertebrates develop from the +same chordula, we may infer that there was a corresponding common +ancestor of both stems. We may call this the Chordaea, and the +corresponding stem-group the Prochordonia or Prochordata. + +From this important stem-group of the unarticulated Prochordonia (or +"primitive chorda-animals") the stems of the Tunicates and Vertebrates +have been divergently evolved. We shall see presently how this +conclusion is justified in the present condition of morphological +science. + +We have first to answer the difficult and much-discussed question of +the development of the Chordaea from the Gastraea; in other words, +"How and by what transformations were the characteristic animals, +resembling the embryonic chordula, which we regard as the common +stem-forms of all the Chordonia, both Tunicates and Vertebrates, +evolved from the simplest two-layered Metazoa?" + +The descent of the Vertebrates from the Articulates has been +maintained by a number of zoologists during the last thirty years with +more zeal than discernment; and, as a vast amount has been written on +the subject, we must deal with it to some extent. All three classes of +Articulates in succession have been awarded the honour of being +considered the "real ancestors" of the Vertebrates: first, the +Annelids (earth-worms, leeches, and the like), then the Crustacea +(crabs, etc.), and, finally, the Tracheata (spiders, insects, etc.). +The most popular of these hypotheses was the annelid theory, which +derived the Vertebrates from the Worms. It was almost simultaneously +(1875) formulated by Carl Semper, of Wurtzburg, and Anton Dohrn, of +Naples. The latter advanced this theory originally in favour of the +failing degeneration theory, with which I dealt in my work, Aims and +Methods of Modern Embryology. + +This interesting degeneration theory--much discussed at that time, but +almost forgotten now--was formed in 1875 with the aim of harmonising +the results of evolution and ever-advancing Darwinism with religious +belief. The spirited struggle that Darwin had occasioned by the +reformation of the theory of descent in 1859, and that lasted for a +decade with varying fortunes in every branch of biology, was drawing +to a close in 1870-1872, and soon ended in the complete victory of +transformism. To most of the disputants the chief point was not the +general question of evolution, but the particular one of "man's place +in nature"--"the question of questions," as Huxley rightly called it. +It was soon evident to every clear-headed thinker that this question +could only be answered in the sense of our anthropogeny, by admitting +that man had descended from a long series of Vertebrates by gradual +modification and improvement. + +In this way the real affinity of man and the Vertebrates came to be +admitted on all hands. Comparative anatomy and ontogeny spoke too +clearly for their testimony to be ignored any longer. But in order +still to save man's unique position, and especially the dogma of +personal immortality, a number of natural philosophers and theologians +discovered an admirable way of escape in the "theory of degeneration." +Granting the affinity, they turned the whole evolutionary theory +upside down, and boldly contended that "man is not the most highly +developed animal, but the animals are degenerate men." It is true that +man is closely related to the ape, and belongs to the vertebrate stem; +but the chain of his ancestry goes upward instead of downward. In the +beginning "God created man in his own image," as the prototype of the +perfect vertebrate; but, in consequence of original sin, the human +race sank so low that the apes branched off from it, and afterwards +the lower Vertebrates. When this theory of degeneration was +consistently developed, its supporters were bound to hold that the +entire animal kingdom was descended from the debased children of men. + +This theory was most strenuously defended by the Catholic priest and +natural philosopher, Michelis, in his Haeckelogony: An Academic +Protest against Haeckel's Anthropogeny (1875). In still more +"academic" and somewhat mystic form the theory was advanced by a +natural philosopher of the older Jena school--the mathematician and +physicist, Carl Snell. But it received its chief support on the +zoological side from Anton Dohrn, who maintained the anthropocentric +ideas of Snell with particular ability. The Amphioxus, which modern +science now almost unanimously regards as the real Primitive +Vertebrate, the ancient model of the original vertebrate structure, +is, according to Dohrn, a late, degenerate descendant of the stem, the +"prodigal son" of the vertebrate family. It has descended from the +Cyclostoma by a profound degeneration, and these in turn from the +fishes; even the Ascidia and the whole of the Tunicates are merely +degenerate fishes! Following out this curious theory, Dohrn came to +contest the general belief that the Coelenterata and Worms are "lower +animals"; he even declared that the unicellular Protozoa were +degenerate Coelenterata. In his opinion "degeneration is the great +principle that explains the existence of all the lower forms." + +If this Michelis-Dohrn theory were true, and all animals were really +degenerate descendants of an originally perfect humanity, man would +assuredly be the true centre and goal of all terrestrial life; his +anthropocentric position and his immortality would be saved. +Unfortunately, this trustful theory is in such flagrant contradiction +to all the known facts of paleontology and embryology that it is no +longer worth serious scientific consideration. + +But the case is no better for the much-discussed descent of the +Vertebrates from the Annelids, which Dohrn afterwards maintained with +great zeal. Of late years this hypothesis, which raised so much dust +and controversy, has been entirely abandoned by most competent +zoologists, even those who once supported it. Its chief supporter, +Dohrn, admitted in 1890 that it is "dead and buried," and made a +blushing retraction at the end of his Studies of the Early History of +the Vertebrate. + +Now that the annelid-hypothesis is "dead and buried," and other +attempts to derive the Vertebrates from Medusae, Echinoderms, or +Molluscs, have been equally unsuccessful, there is only one hypothesis +left to answer the question of the origin of the Vertebrates--the +hypothesis that I advanced thirty-six years ago and called the +"chordonia-hypothesis." In view of its sound establishment and its +profound significance, it may very well claim to be a THEORY, and so +should be described as the chordonia or chordaea theory. + +I first advanced this theory in a series of university lectures in +1867, from which the History of Creation was composed. In the first +edition of this work (1868) I endeavoured to prove, on the strength of +Kowalevsky's epoch-making discoveries, that "of all the animals known +to us the Tunicates are undoubtedly the nearest blood-relatives of the +Vertebrates; they are the most closely related to the Vermalia, from +which the Vertebrates have been evolved. Naturally, I do not mean that +the Vertebrates have descended from the Tunicates, but that the two +groups have sprung from a common root. It is clear that the real +Vertebrates (primarily the Acrania) were evolved in very early times +from a group of Worms, from which the degenerate Tunicates also +descended in another and retrogressive direction." This common extinct +stem-group are the Prochordonia; we still have a silhouette of them in +the chordula-embryo of the Vertebrates and Tunicates; and they still +exist independently, in very modified form, in the class of the +Copelata (Appendicaria, Figure 2.225). + +The chordaea-theory received the most valuable and competent support +from Carl Gegenbaur. This able comparative morphologist defended it in +1870, in the second edition of his Elements of Comparative Anatomy; at +the same time he drew attention to the important relations of the +Tunicates to a curious worm, Balanoglossus: he rightly regards this as +the representative of a special class of worms, which he called +"gut-breathers" (Enteropneusta). Gegenbaur referred on many other +occasions to the close blood-relationship of the Tunicates and +Vertebrates, and luminously explained the reasons that justify us in +framing the hypothesis of the descent of the two stems from a common +ancestor, an unsegmented worm-like animal with an axial chorda between +the dorsal nerve-tube and the ventral gut-tube. + +The theory afterwards received a good deal of support from the +research made by a number of distinguished zoologists and anatomists, +especially C. Kupffer, B. Hatschek, F. Balfour, E. Van Beneden, and +Julin. Since Hatschek's Studies of the Development of the Amphioxus +gave us full information as to the embryology of this lowest +vertebrate, it has become so important for our purpose that we must +consider it a document of the first rank for answering the question we +are dealing with. + +The ontogenetic facts that we gather from this sole survivor of the +Acrania are the more valuable for phylogenetic purposes, as +paleontology, unfortunately, throws no light whatever on the origin of +the Vertebrates. Their invertebrate ancestors were soft organisms +without skeleton, and thus incapable of fossilisation, as is still the +case with the lowest vertebrates--the Acrania and Cyclostoma. The same +applies to the greater part of the Vermalia or worm-like animals, the +various classes and orders of which differ so much in structure. The +isolated groups of this rich stem are living branches of a huge tree, +the greater part of which has long been dead, and we have no fossil +evidence as to its earlier form. Nevertheless, some of the surviving +groups are very instructive, and give us clear indications of the way +in which the Chordonia were developed from the Vermalia, and these +from the Coelenteria. + +While we seek the most important of these palingenetic forms among the +groups of Coelenteria and Vermalia, it is understood that not a single +one of them must be regarded as an unchanged, or even little changed, +copy of the extinct stem-form. One group has retained one feature, +another a different feature, of the original organisation, and other +organs have been further developed and characteristically modified. +Hence here, more than in any other part of our genealogical tree, we +have to keep before our mind the FULL PICTURE of development, and +separate the unessential secondary phenomena from the essential and +primary. It will be useful first to point out the chief advances in +organisation by which the simple Gastraea gradually became the more +developed Chordaea. + +We find our first solid datum in the gastrula of the Amphioxus (Figure +1.38). Its bilateral and tri-axial type indicates that the +Gastraeads--the common ancestors of all the Metazoa--divided at an +early stage into two divergent groups. The uni-axial Gastraea became +sessile, and gave rise to two stems, the Sponges and the Cnidaria (the +latter all reducible to simple polyps like the hydra). But the +tri-axial Gastraea assumed a certain pose or direction of the body on +account of its swimming or creeping movement, and in order to sustain +this it was a great advantage to share the burden equally between the +two halves of the body (right and left). Thus arose the typical +bilateral form, which has three axes. The same bilateral type is found +in all our artificial means of locomotion--carts, ships, etc.; it is +by far the best for the movement of the body in a certain direction +and steady position. Hence natural selection early developed this +bilateral type in a section of the Gastraeads, and thus produced the +stem-forms of all the bilateral animals. + +The Gastraea bilateralis, of which we may conceive the bilateral +gastrula of the amphioxus to be a palingenetic reproduction, +represented the two-sided organism of the earliest Metazoa in its +simplest form. The vegetal entoderm that lined their simple gut-cavity +served for nutrition; the ciliated ectoderm that formed the external +skin attended to locomotion and sensation; finally, the two primitive +mesodermic cells, that lay to the right and left at the ventral border +of the primitive mouth, were sexual cells, and effected reproduction. +In order to understand the further development of the gastraea, we +must pay particular attention to: (1) the careful study of the +embryonic stages of the amphioxus that lie between the gastrula and +the chordula; (2) the morphological study of the simplest Platodes +(Platodaria and Turbellaria) and several groups of unarticulated +Vermalia (Gastrotricha, Nemertina, Enteropneusta). + +We have to consider the Platodes first, because they are on the border +between the two principal groups of the Metazoa, the Coelenteria and +the Coelomaria. With the former they share the lack of body-cavity, +anus, and vascular system; with the latter they have in common the +bilateral type, the possession of a pair of nephridia or renal canals, +and the formation of a vertical brain or cerebral ganglion. It is now +usual to distinguish four classes of Platodes: the two free-living +classes of the primitive worms (Platodaria) and the coiled-worms +(Turbellaria), and the two parasitic classes of the suctorial worms +(Trematoda) and the tape-worms (Cestoda). We have only to consider the +first two of these classes; the other two are parasites, and have +descended from the former by adaptation to parasitic habits and +consequent degeneration. + +(FIGURE 2.239. Aphanostomum Langii (Haeckel), a primitive worm of the +platodaria class, of the order of Cryptocoela or Acoela. This new +species of the genus Aphanostomum, named after Professor Arnold Lang +of Zurich, was found in September, 1899, at Ajaccio in Corsica +(creeping between fucoidea). It is one-twelfth of an inch long, +one-twenty-fifth of an inch broad, and violet in colour. a mouth, g +auditory vesicle, e ectoderm, i entoderm, o ovaries, a spermaries, f +female aperture, m male aperture.) + +The primitive worms (Platodaria) are very small flat worms of simple +construction, but of great morphological and phylogenetic interest. +They have been hitherto, as a rule, regarded as a special order of the +Turbellaria, and associated with the Rhabdocoela; but they differ +considerably from these and all the other Platodes (flat worms) in the +absence of renal canals and a special central nervous system; the +structure of their tissue is also simpler than in the other Platodes. +Most of the Platodes of this group (Aphanostomum, Amphichoerus, +Convoluta, Schizoprora, etc.) are very soft and delicate animals, +swimming about in the sea by means of a ciliary coat, and very small +(1/10 to 1/20 inch long). Their oval body, without appendages, is +sometimes spindle-shaped or cylindrical, sometimes flat and +leaf-shaped. Their skin is merely a layer of ciliated ectodermic +cells. Under this is a soft medullary substance, which consists of +entodermic cells with vacuoles. The food passes through the mouth +directly into this digestive medullary substance, in which we do not +generally see any permanent gut-cavity (it may have entirely +collapsed); hence these primitive Platodes have been called Acoela +(without gut-cavity or coelom), or, more correctly, Cryptocoela, or +Pseudocoela. The sexual organs of these hermaphroditic Platodaria are +very simple--two pairs of strings of cells, the inner of which (the +ovaries, Figure 2.239 o) produce ova, and the outer (the spermaria, s) +sperm-cells. These gonads are not yet independent sexual glands, but +sexually differentiated cell-groups in the medullary substance, or, in +other words, parts of the gut-wall. Their products, the sex-cells, are +conveyed out behind by two pairs of short canals; the male opening (m) +lies just behind the female (f). Most of the Platodaria have not the +muscular pharynx, which is very advanced in the Turbellaria and +Trematoda. On the other hand, they have, as a rule, before or behind +the mouth, a bulbous sense-organ (auditory vesicle or organ of +equilibrium, g), and many of them have also a couple of simple optic +spots. The cell-pit of the ectoderm that lies underneath is rather +thick, and represents the first rudiment of a neural ganglion +(vertical brain or acroganglion). + +The Turbellaria, with which the similar Platodaria were formerly +classed, differ materially from them in the more advanced structure of +their organs, and especially in having a central nervous system +(vertical brain) and excretory renal canals (nephridia); both +originate from the ectoderm. But between the two germinal layers a +mesoderm is developed, a soft mass of connective tissue, in which the +organs are embedded. The Turbellaria are still represented by a number +of different forms, in both fresh and sea-water. The oldest of these +are the very rudimentary and tiny forms that are known as Rhabdocoela +on account of the simple construction of their gut; they are, as a +rule, less than a quarter of an inch long and of a simple oval or +lancet shape (Figure 2.240). The surface is covered with ciliated +epithelium, a stratum of ectodermic cells. The digestive gut is still +the simple primitive gut of the gastraea (d), with a single aperture +that is both mouth and anus (m). There is, however, an invagination of +the ectoderm at the mouth, which has given rise to a muscular pharynx +(sd). It is noteworthy that the mouth of the Turbellaria (like the +primitive mouth of the Gastraea) may, in this class, change its +position considerably in the middle line of the ventral surface; +sometimes it lies behind (Opisthostomum), sometimes in the middle +(Mesostomum), sometimes in front (Prosostomum). This displacement of +the mouth from front to rear is very interesting, because it +corresponds to a phylogenetic displacement of the mouth. This probably +occurred in the Platode ancestors of most (or all?) of the Coelomaria; +in these the permanent mouth (metastoma) lies at the fore end (oral +pole), whereas the primitive mouth (prostoma) lay at the hind end of +the bilateral body. + +In most of the Turbellaria there is a narrow cavity, containing a +number of secondary organs, between the two primary germinal layers, +the outer or animal layer of which forms the epidermis and the inner +vegetal layer the visceral epithelium. The earliest of these organs +are the sexual organs; they are very variously constructed in the +Platode-class; in the simplest case there are merely two pairs of +gonads or sexual glands--a pair of testicles (Figure 2.241 h) and a +pair of ovaries (e). They open externally, sometimes by a common +aperture (Monogonopora), sometimes by separate ones, the female behind +the male (Digonopora, Figure 2.241). The sexual glands develop +originally from the two promesoblasts or primitive mesodermic cells +(Figure 1.83 p). As these earliest mesodermic structures extended, and +became spacious sexual pouches in the later descendants of the +Platodes, probably the two coelom-pouches were formed from them, the +first trace of the real body-cavity of the higher Metazoa +(Enterocoela). + +The gonads are among the oldest organs, the few other organs that we +find in the Platodes between the gut-wall and body-wall being later +evolutionary products. One of the oldest and most important of these +are the kidneys or nephridia, which remove unusable matter from the +body (Figure 2.240 nc). These urinary or excretory organs were +originally enlarged skin-glands--a couple of canals that run the +length of the body, and have a separate or common external aperture +(nm). They often have a number of branches. These special excretory +organs are not found in the other Coelenteria (Gastraeads, Sponges, +Cnidaria) or the Cryptocoela. They are first met in the Turbellaria, +and have been transmitted direct from these to the Vermalia, and from +these to the higher stems. + +Finally, there is a very important new organ in the Turbellaria, which +we do not find in the Cryptocoela (Figure 2.239) and their gastraead +ancestors--the rudimentary nervous system. It consists of a couple of +simple cerebral ganglia (Figure 2.241 g) and fine nervous fibres that +radiate from them; these are partly voluntary nerves (or motor fibres) +that go to the thin muscular layer developing under the skin; and +partly sensory nerves that proceed to the sense-cells of the ciliated +epiderm (f). Many of the Turbellaria have also special sense-organs; a +couple of ciliated smell pits (na), rudimentary eyes (au), and, less +frequently, auditory vesicles. + +On these principles I assume that the oldest and simplest Turbellaria +arose from Platodaria, and these directly from bilateral Gastraeads. +The chief advances were the formation of gonads and nephridia, and of +the rudimentary brain. On this hypothesis, which I advanced in 1872 in +the first sketch of the gastraea-theory (Monograph on the Sponges), +there is no direct affinity between the Platodes and the Cnidaria. + +(FIGURE 2.240. A simple turbellarian (Rhabdocoelum). m mouth, sd +gullet epithelium, sm gullet muscles, d gastric gut, nc renal canals, +nm renal aperture, au eye, na olfactory pit. (Diagram.) + +FIGURE 2.241. The same, showing the other organs. g brain, au eye, na +olfactory pit, n nerves, h testicles, male symbol male aperture, +female symbol female aperture, e ovary, f ciliated epiderm. (Diagram.) + +(FIGURES 242 AND 243. Chaetonotus, a rudimentary vermalian, of the +group of Gastrotricha. m mouth, s gullet, d gut, a anus, g brain, n +nerves, ss sensory hairs, au eye, ms muscular cells, h skin, f +ciliated bands of the ventral surface, nc nephridia, nm their +aperture, e ovaries.)) + +Next to the ancient stem-group of the Turbellaria come a number of +more recent chordonia ancestors, which we class with the Vermalia or +Helminthes, the unarticulated worms. These true worms (Vermes, lately +also called Scolecida) are the difficulty or the lumber-room of the +zoological classifier, because the various classes have very +complicated relations to the lower Platodes on the one hand and the +more advanced animals on the other. But if we exclude the Platodes and +the Annelids from this stem, we find a fairly satisfactory unity of +organisation in the remaining classes. Among these worms we find some +important forms that show considerable advance in organisation from +the platode to the chordonia stage. Three of these phenomena are +particularly instructive: (1) The formation of a true (secondary) +body-cavity (coeloma); (2) the formation of a second aperture of the +gut, the anus; and (3) the formation of a vascular system. The great +majority of the Vermalia have these three features, and they are all +wanting in the Platodes; in the rest of the worms at least one or two +of them are developed. + +Next and very close to the Platodes we have the Ichthydina +(Gastrotricha), little marine and fresh-water worms, about 1/250 to +1/1000 inch long. Zoologists differ as to their position in +classification. In my opinion, they approach very close to the +Rhabdocoela (Figures 2.240 and 2.241), and differ from them chiefly in +the possession of an anus at the posterior end (Figure 2.242 a). +Further, the cilia that cover the whole surface of the Turbellaria are +confined in the Gastrotricha to two ciliated bands (f) on the ventral +surface of the oval body, the dorsal surface having bristles. +Otherwise the organisation of the two classes is the same. In both the +gut consists of a muscular gullet (s) and a glandular primitive gut +(d). Over the gullet is a double brain (acroganglion, g). At the side +of the gut are two serpentine prorenal canals (water-vessels or +pronephridia, nc), which open on the ventral side (nm). Behind are a +pair of simple sexual glands or gonads (Figure 2.243 e). + +While the Ichthydina are thus closely related to the Platodes, we have +to go farther away for the two classes of Vermalia which we unite in +the group of the "snout-worms" (Frontonia). These are the Nemertina +and the Enteropneusta. Both classes have a complete ciliary coat on +the epidermis, a heritage from the Turbellaria and the Gastraeads; +also, both have two openings of the gut, the mouth and anus, like the +Gastrotricha. But we find also an important organ that is wanting in +the preceding forms--the vascular system. In their more advanced +mesoderm we find a few contractile longitudinal canals which force the +blood through the body by their contractions; these are the first +blood-vessels. + +(FIGURE 2.244. A simple Nemertine. m mouth, d gut, a anus, g brain, n +nerves, h ciliary coat, ss sensory pits (head-clefts), au eyes, r +dorsal vessel, l lateral vessels. (Diagram.) + +FIGURE 2.245. A young Enteropneust (Balanaglossus). (From Alexander +Agassiz.) r acorn-shaped snout, h neck, k gill-clefts and gill-arches +of the fore-gut, in long rows on each side, d digestive hind-gut, +filling the greater part of the body-cavity, v intestinal vein or +ventral vessel, lying between the parallel folds of the skin, a anus. + +Figure 2.246. Transverse section of the branchial gut. A of +Balanoglossus, B of Ascidia. r branchial gut, n pharyngeal groove, +asterisk ventral folds between the two. Diagrammatic illustration from +Gegenbaur, to show the relation of the dorsal branchial-gut cavity (r) +to the pharyngeal or hypobranchial groove (n).) + +The Nemertina were formerly classed with the much less advanced +Turbellaria. But they differ essentially from them in having an anus +and blood-vessels, and several other marks of higher organisation. +They have generally long and narrow bodies, like a more or less +flattened cord; there are, besides several small species, giant-forms +with a width of 1/5 to 2/5 inch and a length of several yards (even +ten to fifteen). Most of them live in the sea, but some in fresh water +and moist earth. In their internal structure they approach the +Turbellaria on the one hand and the higher Vermalia (especially the +Enteropneusta) on the other. They have a good deal of interest as the +lowest and oldest of all animals with blood. In them we find +blood-vessels for the first time, distributing real blood through the +body. The blood is red, and the red colouring-matter is haemoglobin, +connected with elliptic discoid blood-cells, as in the Vertebrates. +Most of them have two or three parallel blood-canals, which run the +whole length of the body, and are connected in front and behind by +loops, and often by a number of ring-shaped pieces. The chief of these +primitive blood-vessels is the one that lies above the gut in the +middle line of the back (Figure 2.244 r); it may be compared to either +the dorsal vessel of the Articulates or the aorta of the Vertebrates. +To the right and left are the two serpentine lateral vessels (Figure +2.244 l). + +After the Nemertina, I take (as distant relatives) the Enteropneusta; +they may be classed together with them as Frontonia or Rhyncocoela +(snout-worms). There is now only one genus of this class, with several +species (Balanoglossus); but it is very remarkable, and may be +regarded as the last survivor of an ancient and long-extinct class of +Vermalia. They are related, on the one hand, to the Nemertina and +their immediate ancestors, the Platodes, and to the lowest and oldest +forms of the Chordonia on the other. + +The Enteropneusta (Figure 2.245) live in the sea sand, and are long +worms of very simple shape, like the Nemertina. From the latter they +have inherited: (1) The bilateral type, with incomplete segmentation; +(2) the ciliary coat of the soft epidermis; (3) the double rows of +gastric pouches, alternating with a single or double row of gonads; +(4) separation of the sexes (the Platode ancestors were +hermaphroditic); (5) the ventral mouth, underneath a protruding snout; +(6) the anus terminating the simple gut-tube; and (7) several parallel +blood-canals, running the length of the body, a dorsal and a ventral +principal stem. + +On the other hand, the Enteropneusta differ from their Nemertine +ancestors in several features, some of which are important, that we +may attribute to adaptation. The chief of these is the branchial gut +(Figure 2.245 k). The anterior section of the gut is converted into a +respiratory organ, and pierced by two rows of gill-clefts; between +these there is a branchial (gill) skeleton, formed of rods and plates +of chitine. The water that enters at the mouth makes its exit by these +clefts. They lie in the dorsal half of the fore-gut, and this is +completely separated from the ventral half by two longitudinal folds +(Figure 2.246 A*). This ventral half, the glandular walls of which are +clothed with ciliary epithelium and secrete mucus, corresponds to the +pharyngeal or hypo-branchial groove of the Chordonia (Bn), the +important organ from which the later thyroid gland is developed in the +Craniota (cf. Chapter 2.16). The agreement in the structure of the +branchial gut of the Enteropneusts, Tunicates, and Vertebrates was +first recognised by Gegenbaur (1878); it is the more significant as at +first we find only a couple of gill-clefts in the young animals of all +three groups; the number gradually increases. We can infer from this +the common descent of the three groups with all the more confidence +when we find the Balanoglossus approaching the Chordonia in other +respects. Thus, for instance, the chief part of the central nervous +system is a long dorsal neural string that runs above the gut and +corresponds to the medullary tube of the Chordonia. Bateson believes +he has detected a rudimentary chorda between the two. + +Of all extant invertebrate animals the Enteropneusts come nearest to +the Chordonia in virtue of these peculiar characters; hence we may +regard them as the survivors of the ancient gut-breathing Vermalia +from which the Chordonia also have descended. Again, of all the +chorda-animals the Copelata (Figure 2.225) and the tailed larvae of +the ascidia approach nearest to the young Balanoglossus. Both are, on +the other hand, very closely related to the Amphioxus, the Primitive +Vertebrate of which we have considered the importance (Chapters 2.16 +and 2.17). As we saw there, the unarticulated Tunicates and the +articulated Vertebrates must be regarded as two independent stems, +that have developed in divergent directions. But the common root of +the two stems, the extinct group of the Prochordonia, must be sought +in the vermalia stem; and of all the living Vermalia those we have +considered give us the safest clue to their origin. It is true that +the actual representatives of the important groups of the Copelata, +Balanoglossi, Nemertina, Icthydina, etc., have more or less departed +from the primitive model owing to adaptation to special environment. +But we may just as confidently affirm that the main features of their +organisation have been preserved by heredity. + +We must grant, however, that in the whole stem-history of the +Vertebrates the long stretch from the Gastraeads and Platodes up to +the oldest Chordonia remains by far the most obscure section. We might +frame another hypothesis to raise the difficulty--namely, that there +was a long series of very different and totally extinct forms between +the Gastraea and the Chordaea. Even in this modified chordaea-theory +the six fundamental organs of the chordula would retain their great +value. The medullary tube would be originally a chemical sensory +organ, a dorsal olfactory tube, taking in respiratory-water and food +by the neuroporus in front and conveying them by the neurenteric canal +into the primitive gut. This olfactory tube would afterwards become +the nervous centre, while the expanding gonads (lying to right and +left of the primitive mouth) would form the coeloma. The chorda may +have been originally a digestive glandular groove in the dorsal middle +line of the primitive gut. The two secondary gut-openings, mouth and +anus, may have arisen in various ways by change of functions. In any +case, we should ascribe the same high value to the chordula as we did +before to the gastrula. + +In order to explain more fully the chief stages in the advance of our +race, I add the hypothetical sketch of man's ancestry that I published +in my Last Link [a translation by Dr. Gadow of the paper read at the +International Zoological Congress at Cambridge in 1898]:-- + + A. MAN'S GENEALOGICAL TREE, FIRST HALF: EARLIER SERIES OF ANCESTORS, + WITHOUT FOSSIL EVIDENCE. + +COLUMN 1 : CHIEF STAGES. +COLUMN 2 : ANCESTRAL STEM-GROUPS. +COLUMN 3 : LIVING RELATIVES OF ANCESTORS. + + +STAGES 1 TO 5. PROTIST ANCESTORS. UNICELLULAR ORGANISMS. + +1 to 2. Protophytes. : 1. Monera. Without nucleus. : Chromacea. +(Chroococcus.) Phycochromacea. + +1 to 2. Protophytes. : 2. Algaria. Unicellular algae. : 2. Paulotomea. +Palmellacea. Eremosphaera. + +3 to 5. Protozoa. : 3. Lobosa. Unicellular (amoebina) rhizopods. : 3. +Amoebina. Amoeba Leucocyta. + +3 to 5. Protozoa. : 4. Infusoria. Unicellular. : 4. Flagellata. +Euflagellata. Zoomonades. + +3 to 5. Protozoa. : 5. Blastaeades. Multicellular hollow spheres. : 5. +Catallacta. Magosphaera, Volvocina, Blastula. + +STAGES 6 TO 11. INVERTEBRATE METAZOA ANCESTORS. + +6 to 8. Coelenteria, without anus and body-cavity. : 6. Gastraeades. +With two germ-layers. : 6. Gastrula. Hydra, Olynthus, Gastremaria. + +6 to 8. Coelenteria, without anus and body-cavity. : 7. Platodes I. +Platodaria (without nephridia). : 7. Cryptocoela. Convoluta, Proporus. + +6 to 8. Coelenteria, without anus and body-cavity. : 8. Platodes II. +Platodinia (with nephridia). : 8. Rhabdocoela. Vortex, Monotus. + +9 to 11. Vermalia, with anus and body-cavity. : 9. Provermalia. +(Primitive Worms.) Rotatoria. : 9. Gastrotricha. Trochozoa, +Trochophora. + +9 to 11. Vermalia, with anus and body-cavity. : 10. Frontonia. +(Rhynchelminthes.) Snout-worms. : 10. Enteropneusta. Balanoglossus, +Cephalodiscus. + +9 to 11. Vermalia, with anus and body-cavity. : 11. Prochordonia. +Chorda-worms. : 11. Copelata. Appendicaria. Chordula-larvae. + +STAGES 12 TO 15. MONORHINA ANCESTORS. + +Oldest vertebrates without jaws or pairs of limbs, single nose. : 12. +Acrania I. (Prospondylia.) : 12. Amphioxus larva. + +Oldest vertebrates without jaws or pairs of limbs, single nose. : 13. +Acrania II. More recent. : 13. Leptocardia. Amphioxus. + +Oldest vertebrates without jaws or pairs of limbs, single nose. : 14. +Cyclostoma I. (Archicrania.) : 14. Petromyzonta larvae. + +Oldest vertebrates without jaws or pairs of limbs, single nose. : 15. +Cyclostoma II. More recent. : 15. Marsipobranchia. Petromyzonta. + +B. MAN'S GENEALOGICAL TREE, SECOND HALF: LATER ANCESTORS, WITH FOSSIL +EVIDENCE. + +COLUMN 1 : GEOLOGICAL PERIODS. +COLUMN 2 : ANCESTRAL STEM-GROUPS. +COLUMN 3 : LIVING RELATIVES OF ANCESTORS. + +Silurian. : 16. Selachii. Primitive fishes. Proselachii. : 16. +Natidanides. Chlamydoselachius. Heptanchus. + +Silurian. 17. Ganoides. Plated-fishes. Proganoides. : 17. +Accipenserides. (Sturgeons.) Polypterus. + +Devonian. : 18. Dipneusta. Paladipneusta. : 18. Neodipneusta. +Ceratodus. Protopterus. + +Carboniferous. : 19. Amphibia. Stegocephala. : 19. Phanerobranchia. +Salamandrina. (Proteus, triton.) + +Permian. : 20. Reptilia. Proreptilia. : 20. Rhynchocephalia. Primitive +lizards. Hatteria. + +Triassic. : 21. Monotrema. Promammalia. : 21. Ornithodelphia. Echidna. +Ornithorhyncus. + +Jurassic. : 22. Marsupalia. Prodidelphia. : 22. Didelphia. Didelphys. +Perameles. + +Cretaceous. : 23. Mallotheria. Prochoriata. : 23. Insectivora. +Erinaceida. (Ictopsida +.) + +Older Eocene. : 24. Lemuravida. Older lemurs. Dentition. 3. 1. 4. 3. : +24. Pachylemures. (Hyopsodus +), (Adapis +). + +Neo-Eocene. : 25. Lemurogona. Later lemurs. Dentition. 2. 1. 4. 3. : +25. Autolemures. Eulemur. Stenops. + +Oligocene. : 26. Dysmopitheca. Western apes. Dentition. 2. 1. 3. 3. : +26. Platyrrhinae. (Anthropops +), (Homunculus +). + +Older Miocene. : 27. Cynopitheca. Dog-faced apes (tailed). : 27. +Papiomorpha. Cynocephalus. + +Neo-Miocene. : 28. Anthropoides. Man-like apes (tail-less). : 28. +Hylobatida. Hylobates. Satyrus. + +Pliocene. : 29. Pithecanthropi. Ape-men (alali, speechless). : 29. +Anthropitheca. Chimpanzee. Gorilla. + +Pleistocene. : 30. Homines. Men, with speech. : 30. Weddahs. +Australian negroes. + + +CHAPTER 2.21. OUR FISH-LIKE ANCESTORS. + +Our task of detecting the extinct ancestors of our race among the vast +numbers of animals known to us encounters very different difficulties +in the various sections of man's stem-history. These were very great +in the series of our invertebrate ancestors; they are much slighter in +the subsequent series of our vertebrate ancestors. Within the +vertebrate stem there is, as we have already seen, so complete an +agreement in structure and embryology that it is impossible to doubt +their phylogenetic unity. In this case the evidence is much clearer +and more abundant. + +The characteristics that distinguish the Vertebrates as a whole from +the Invertebrates have already been discussed in our description of +the hypothetical Primitive Vertebrate (Chapter 1.11, Figure 1.98 to +1.102). The chief of these are: (1) The evolution of the primitive +brain into a dorsal medullary tube; (2) the formation of the chorda +between the medullary tube and the gut; (3) the division of the gut +into branchial (gill) and hepatic (liver) gut; and (4) the internal +articulation or metamerism. The first three features are shared by the +Vertebrates with the ascidia-larvae and the Prochordonia; the fourth +is peculiar to them. Thus the chief advantage in organisation by which +the earliest Vertebrates took precedence of the unsegmented Chordonia +consisted in the development of internal segmentation. + +The whole vertebrate stem divides first into the two chief sections of +Acrania and Craniota. The Amphioxus is the only surviving +representative of the older and lower section, the Acrania +("skull-less"). All the other vertebrates belong to the second +division, the Craniota ("skull-animals"). The Craniota descend +directly from the Acrania, and these from the primitive Chordonia. The +exhaustive study that we made of the comparative anatomy and ontogeny +of the Ascidia and the Amphioxus has proved these relations for us. +(See Chapters 2.16 and 2.17.) The Amphioxus, the lowest Vertebrate, +and the Ascidia, the nearest related Invertebrate, descend from a +common extinct stem-form, the Chordaea; and this must have had, +substantially, the organisation of the chordula. + +However, the Amphioxus is important not merely because it fills the +deep gulf between the Invertebrates and Vertebrates, but also because +it shows us to-day the typical vertebrate in all its simplicity. We +owe to it the most important data that we proceed on in reconstructing +the gradual historical development of the whole stem. All the Craniota +descend from a common stem-form, and this was substantially identical +in structure with the Amphioxus. This stem-form, the Primitive +Vertebrate (Prospondylus, Figures 1.98 to 1.102), had the +characteristics of the vertebrate as such, but not the important +features that distinguish the Craniota from the Acrania. Though the +Amphioxus has many peculiarities of structure and has much +degenerated, and though it cannot be regarded as an unchanged +descendant of the Primitive Vertebrate, it must have inherited from it +the specific characters we enumerated above. We may not say that +"Amphioxus is the ancestor of the Vertebrates"; but we can say: +"Amphioxus is the nearest relation to the ancestor of all the animals +we know." Both belong to the same small family, or lowest class of the +Vertebrates, that we call the Acrania. In our genealogical tree this +group forms the twelfth stage, or the first stage among the vertebrate +ancestors (Chapter 2.20). From this group of Acrania both the +Amphioxus and the Craniota were evolved. + +The vast division of the Craniota embraces all the Vertebrates known +to us, with the exception of the Amphioxus. All of them have a head +clearly differentiated from the trunk, and a skull enclosing a brain. +The head has also three pairs of higher sense-organs (nose, eyes, and +ears). The brain is very rudimentary at first, a mere bulbous +enlargement of the fore end of the medullary tube. But it is soon +divided by a number of transverse constrictions into, first three, +then five successive cerebral vesicles. In this formation of the head, +skull, and brain, with further development of the higher sense-organs, +we have the advance that the Craniota made beyond their skull-less +ancestors. Other organs also attained a higher development; they +acquired a compact centralised heart with valves and a more advanced +liver and kidneys, and made progress in other important respects. + +We may divide the Craniota generally into Cyclostoma ("round-mouthed") +and Gnathostoma ("jaw-mouthed"). There are only a few groups of the +former in existence now, but they are very interesting, because in +their whole structure they stand midway between the Acrania and the +Gnathostoma. They are much more advanced than the Acrania, much less +so than the fishes, and thus form a very welcome connecting-link +between the two groups. We may therefore consider them a special +intermediate group, the fourteenth and fifteenth stages in the series +of our ancestors. + +(FIGURE 2.247. The large marine lamprey (Petromyzon marinus), much +reduced. Behind the eye there is a row of seven gill-clefts visible on +the left, in front the round suctorial mouth.) + +The few surviving species of the Cyclostoma are divided into two +orders--the Myxinoides and the Petromyzontes. The former, the +hag-fishes, have a long, cylindrical, worm-like body. They were +classed by Linne with the worms, and by later zoologists, with the +fishes, or the amphibia, or the molluscs. They live in the sea, +usually as parasites of fishes, into the skin of which they bore with +their round suctorial mouths and their tongues, armed with horny +teeth. They are sometimes found alive in the body cavity of fishes +(such as the torsk or sturgeon); in these cases they have passed +through the skin into the interior. The second order consists of the +Petromyzontes or lampreys; the small river lamprey (Petromyzon +fluviatilis) and the large marine lamprey (Petromyzon marinus, Figure +2.247). They also have a round suctorial mouth, with horny teeth +inside it; by means of this they attach themselves by sucking to +fishes, stones, and other objects (hence the name Petromyzon = +stone-sucker). It seems that this habit was very widespread among the +earlier Vertebrates; the larvae of many of the Ganoids and frogs have +suctorial disks near the mouth. + +The class that is formed of the Myxinoides and Petromyzontes is called +the Cyclostoma (round-mouthed), because their mouth has a circular or +semi-circular aperture. The jaws (upper and lower) that we find in all +the higher Vertebrates are completely wanting in the Cyclostoma, as in +the Amphioxus. Hence the other Vertebrates are collectively opposed to +them as Gnathostoma (jaw-mouthed). The Cyclostoma might also be called +Monorhina (single-nosed), because they have only a single nasal +passage, while all the Gnathostoma have two nostrils (Amphirhina = +double-nosed). But apart from these peculiarities the Cyclostoma +differ more widely from the fishes in other special features of their +structure than the fishes do from man. Hence they are obviously the +last survivors of a very ancient class of Vertebrates, that was far +from attaining the advanced organisation of the true fish. To mention +only the chief points, the Cyclostoma show no trace of pairs of limbs. +Their mucous skin is quite naked and smooth and devoid of scales. +There is no bony skeleton. A very rudimentary skull is developed at +the foremost end of their chorda. At this point a soft membranous +(partly turning into cartilage), small skull-capsule is formed, and +encloses the brain. + +The brain of the Cyclostoma is merely a very small and comparatively +insignificant swelling of the spinal marrow, a simple vesicle at +first. It afterwards divides into five successive cerebral vesicles, +like the brain of the Gnathostoma. These five primitive cerebral +vesicles, that are found in the embryos of all the higher vertebrates +from the fishes to man, and grow into very complex structures, remain +at a very rudimentary stage in the Cyclostoma. The histological +structure of the nerves is also less advanced than in the rest of the +vertebrates. In these the auscultory organ always contains three +circular canals, but in the lampreys there are only two, and in the +hag-fishes only one. In most other respects the organisation of the +Cyclostoma is much simpler--for instance, in the structure of the +heart, circulation, and kidneys. We must especially note the absence +of a very important organ that we find in the fishes, the +floating-bladder, from which the lungs of the higher Vertebrates have +been developed. + +When we consider all these peculiarities in the structure of the +Cyclostoma, we may formulate the following thesis: Two divergent lines +proceeded from the earliest Craniota, or the primitive Craniota +(Archicrania). One of these lines is preserved in a greatly modified +condition: these are the Cyclostoma, a very backward and partly +degenerate side-line. The other, the chief line of the Vertebrate +stem, advanced straight to the fishes, and by fresh adaptations +acquired a number of important improvements. + +(FIGURE 2.248. Fossil Permian primitive fish (Pleuracanthus Dechenii), +from the red sandstone of Saarbrucken. (From Doderlein.) I Skull and +branchial skeleton: o eye-region, pq palatoquadratum, nd lower jaw, hm +hyomandibular, hy tongue-bone, k gill-radii, kb gill-arches, z +jaw-teeth, sz gullet-teeth, st neck-spine. II Vertebral column: ob +upper arches, ub lower arches, hc intercentra, r ribs. III Single +fins: d dorsal fin, c tail-fin (tail-end wanting), an anus-fin, ft +supporter of fin-rays. IV Breast-fin: sg shoulder-zone, ax fin-axis, +ss double lines of fin-rays, bs additional rays, sch plates. V Ventral +fin: p pelvis, ax fin-axis, ss single row of fin-rays, bs additional +rays, sch scales, cop penis. + +FIGURE 2.249. Embryo of a shark (Scymnus lichia), seen from the +ventral side, v breast-fins (in front five pairs of gill-clefts), h +belly-fins, a anus, s tail-fin, k external gill-tuft, d yelk-sac +(removed for most part), g eye, n nose, m mouth-cleft.) + +The Cyclostoma are almost always classified by zoologists among the +fishes; but the incorrectness of this may be judged from the fact that +in all the chief and distinctive features of organisation they are +further removed from the fishes than the fishes are from the Mammals, +and even man. With the fishes we enter upon the vast division of the +jaw-mouthed or double-nosed Vertebrates (Gnathostoma or Amphirhina). +We have to consider the fishes carefully as the class which, on the +evidence of palaeontology, comparative anatomy, and ontogeny, may be +regarded with absolute certainty as the stem-class of all the higher +Vertebrates or Gnathostomes. Naturally, none of the actual fishes can +be considered the direct ancestor of the higher Vertebrates. But it is +certain that all the Vertebrates or Gnathostomes, from the fishes to +man, descend from a common, extinct, fish-like ancestor. If we had +this ancient stem-form before us, we would undoubtedly class it as a +true fish. Fortunately the comparative anatomy and classification of +the fishes are now so far advanced that we can get a very clear idea +of these interesting and instructive features. + +In order to understand properly the genealogical tree of our race +within the vertebrate stem, it is important to bear in mind the +characteristics that separate the whole of the Gnathostomes from the +Cyclostomes and Craniota. In these respects the fishes agree entirely +with all the other Gnathostomes up to man, and it is on this that we +base our claim of relationship to the fishes. The following +characteristics of the Gnathostomes are anatomic features of this +kind: (1) The internal gill-arch apparatus with the jaw arches; (2) +the pair of nostrils; (3) the floating bladder or lungs; and (4) the +two pairs of limbs. + +The peculiar formation of the frame work of the branchial (gill) +arches and the connected maxillary (jaw) apparatus is of importance in +the whole group of the Gnathostomes. It is inherited in rudimentary +form by all of them, from the earliest fishes to man. It is true that +the primitive transformation (which we find even in the Ascidia) of +the fore gut into the branchial gut can be traced in all the +Vertebrates to the same simple type; in this respect the gill-clefts, +which pierce the walls of the branchial gut in all the Vertebrates and +in the Ascidia, are very characteristic. But the EXTERNAL, superficial +branchial skeleton that supports the gill-crate in the Cyclostoma is +replaced in the Gnathostomes by an INTERNAL branchial skeleton. It +consists of a number of successive cartilaginous arches, which lie in +the wall of the gullet between the gill-clefts, and run round the +gullet from both sides. The foremost pair of gill-arches become the +maxillary arches, from which we get our upper and lower jaws. + +The olfactory organs are at first found in the same form in all the +Gnathostomes, as a pair of depressions in the fore part of the skin of +the head, above the mouth; hence, they are also called the Amphirhina +("double-nosed"). The Cyclostoma are "one-nosed" (Monorhina); their +nose is a single passage in the middle of the frontal surface. But as +the olfactory nerve is double in both cases, it is possible that the +peculiar form of the nose in the actual Cyclostomes is a secondary +acquisition (by adaptation to suctorial habits). + +A third essential character of the Gnathostomes, that distinguishes +them very conspicuously from the lower vertebrates we have dealt with, +is the formation of a blind sac by invagination from the fore part of +the gut, which becomes in the fishes the air-filled floating-bladder. +This organ acts as a hydrostatic apparatus, increasing or reducing the +specific gravity of the fish by compressing or altering the quantity +of air in it. The fish can rise or sink in the water by means of it. +This is the organ from which the lungs of the higher vertebrates are +developed. + +(FIGURE 2.250. Fully developed man-eating shark (Carcharias +melanopterus), left view. r1 first, r2 second dorsal fin, s tail-fin, +a anus-fin, v breast-fins, h belly-fins.) + +Finally, the fourth character of the Gnathostomes in their simple +embryonic form is the two pairs of extremities or limbs--a pair of +fore legs (breast-fins in the fish, Figure 2.250 v) and a pair of hind +legs (ventral fins in the fish, Figure 2.250 h). The comparative +anatomy of these fins is very interesting, because they contain the +rudiments of all the skeletal parts that form the framework of the +fore and hind legs in all the higher vertebrates right up to man. +There is no trace of these pairs of limbs in the Acrania and +Cyclostomes. + +Turning, now, to a closer inspection of the fish class, we may first +divide it into three groups or sub-classes, the genealogy of which is +well known to us. The first and oldest group is the sub-class of the +Selachii or primitive fishes; the best-known representatives of which +to-day are the orders of the sharks and rays (Figures 2.248 to 2.252). +Next to this is the more advanced sub-class of the plated fishes or +Ganoids (Figures 2.253 to 2.255). It has been long extinct for the +most part, and has very few living representatives, such as the +sturgeon and the bony pike; but we can form some idea of the earlier +extent of this interesting group from the large numbers of fossils. +From these plated fishes the sub-class of the bony fishes or Teleostei +was developed, to which the great majority of living fishes belong +(especially nearly all our river fishes). Comparative anatomy and +ontogeny show clearly that the Ganoids descended from the Selachii, +and the Teleostei from the Ganoids. On the other hand, a collateral +line, or rather the advancing chief line of the vertebrate stem, was +developed from the earlier Ganoids, and this leads us through the +group of the Dipneusta to the important division of the Amphibia. + +(FIGURE 2.251. Fossil angel-shark (Squatina alifera), from the upper +Jurassic at Eichstatt. (From Zittel.) The cartilaginous skull is +clearly seen in the broad head, and the gill-arches behind. The wide +breast-fin and the narrower belly-fin have a number of radii; between +these and the vertebral column are a number of ribs.) + +The earliest fossil remains of Vertebrates that we know were found in +the Upper Silurian (Chapter 2.18), and belong to two groups--the +Selachii and the Ganoids. The most primitive of all known +representatives of the earliest fishes are probably the remarkable +Pleuracanthida, the genera Pleuracanthus, Xenacanthus, Orthocanthus, +etc. (Figure 2.248). These ancient cartilaginous fishes agree in most +points of structure with the real sharks (Figures 2.249 and 2.250); +but in other respects they seem to be so much simpler in organisation +that many palaeontologists separate them altogether, and regard them +as Proselachii; they are probably closely related to the extinct +ancestors of the Gnathostomes. We find well-preserved remains of them +in the Permian period. Well-preserved impressions of other sharks are +found in the Jurassic schist, such as of the angel-fish (Squatina, +Figure 2.251). Among the extinct earlier sharks of the Tertiary period +there were some twice as large as the biggest living fishes; +Carcharodon was more than 100 feet long. The sole surviving species of +this genus (C. Rondeleti) is eleven yards long, and has teeth two +inches long; but among the fossil species we find teeth six inches +long (Figure 2.252). + +From the primitive fishes or Selachii, the earliest Gnathostomes, was +developed the legion of the Ganoids. There are very few genera now of +this interesting and varied group--the ancient sturgeons (Accipenser), +the eggs of which are eaten as caviare, and the stratified pikes +(Polypterus, Figure 2.255) in African rivers, and bony pikes +(Lepidosteus) in the rivers of North America. On the other hand, we +have a great variety of specimens of this group in the fossil state, +from the Upper Silurian onward. Some of these fossil Ganoids approach +closely to the Selachii; others are nearer to the Dipneusts; others +again represent a transition to the Teleostei. For our genealogical +purposes the most interesting are the intermediate forms between the +Selachii and the Dipneusts. Huxley, to whom we owe particularly +important works on the fossil Ganoids, classed them in the order of +the Crossopterygii. Many genera and species of this order are found in +the Devonian and Carboniferous strata (Figure 2.253); a single, +greatly modified survivor of the group is still found in the large +rivers of Africa (Polypterus, Figure 2.255, and the closely related +Calamichthys). In many impressions of the Crossopterygii the floating +bladder seems to be ossified, and therefore well preserved--for +instance, in the Undina (Figure 2.254, immediately behind the head). + +Part of these Crossopterygii approach very closely in their chief +anatomic features to the Dipneusts, and thus represent +phylogenetically the transition from the Devonian Ganoids to the +earliest air-breathing vertebrates. This important advance was made in +the Devonian period. The numerous fossils that we have from the first +two geological sections, the Laurentian and Cambrian periods, belong +exclusively to aquatic plants and animals. From this paleontological +fact, in conjunction with important geological and biological +indications, we may infer with some confidence that there were no +terrestrial animals at that time. During the whole of the vast +archeozoic period--many millions of years--the living population of +our planet consisted almost exclusively of aquatic organisms; this is +a very remarkable fact, when we remember that this period embraces the +larger half of the whole history of life. The lower animal-stems are +wholly (or with very few exceptions) aquatic. But the higher stems +also remained in the water during the primordial epoch. It was only +towards its close that some of them came to live on land. We find +isolated fossil remains of terrestrial animals first in the Upper +Silurian, and in larger numbers in the Devonian strata, which were +deposited at the beginning of the second chief section of geology (the +paleozoic age). The number increases considerably in the Carboniferous +and Permian deposits. We find many species both of the articulate and +the vertebrate stem that lived on land and breathed the atmosphere; +their aquatic ancestors of the Silurian period only breathed water. +This important change in respiration is the chief modification that +the animal organism underwent in passing from the water to the solid +land. The first consequence was the formation of lungs for breathing +air; up to that time the gills alone had served for respiration. But +there was at the same time a great change in the circulation and its +organs; these are always very closely correlated to the respiratory +organs. Moreover, the limbs and other organs were also more or less +modified, either in consequence of remote correlation to the preceding +or owing to new adaptations. + +(FIGURE 2.252. Tooth of a gigantic shark (Carcharodon megalodon), from +the Pliocene at Malta. Half natural size. (From Zittel.)) + +In the vertebrate stem it was unquestionably a branch of the +fishes--in fact, of the Ganoids--that made the first fortunate +experiment during the Devonian period of adapting themselves to +terrestrial life and breathing the atmosphere. This led to a +modification of the heart and the nose. The true fishes have merely a +pair of blind olfactory pits on the surface of the head; but a +connection of these with the cavity of the mouth was now formed. A +canal made its appearance on each side, and led directly from the +nasal depression into the mouth-cavity, thus conveying atmospheric air +to the lungs even when the mouth was closed. Further, in all true +fishes the heart has only two sections--an atrium that receives the +venous blood from the veins, and a ventricle that propels it through a +conical artery to the gills; the atrium was now divided into two +halves, or right and left auricles, by an incomplete partition. The +right auricle alone now received the venous blood from the body, while +the left auricle received the venous blood that flowed from the lungs +and gills to the heart. Thus the double circulation of the higher +vertebrates was evolved from the simple circulation of the true +fishes, and, in accordance with the laws of correlation, this advance +led to others in the structure of other organs. + +(FIGURE 2.253. A Devonian Crossopterygius (Holoptychius nobilissimus, +from the Scotch old red sandstone. (From Huxley.) + +FIGURE 2.254. A Jurassic Crossopterygius (Undina penicillata), from +the upper Jurassic at Eichstatt. (From Zittel.) j jugular plates, b +three ribbed scales. + +FIGURE 2.255. A living Crossopterygius, from the Upper Nile +(Polypterus bichir). + +FIGURE 2.256. Fossil Dipneust (Dipterus Valenciennesi), from the old +red sandstone (Devon). (From Pander.) + +FIGURE 2.257. The Australian Dipneust (Ceratodus Forsteri). B view +from the right, A lower side of the skull, C lower jaw. (From +Gunther.) Qu quadrate bone, Psph parasphenoid, PtP pterygopalatinum, +Vo vomer, d teeth, na nostrils, Br branchial cavity, C first rib. D +lower-jaw teeth of the fossil Ceratodus Kaupi (from the Triassic).) + +The vertebrate class, that thus adapted itself to breathing the +atmosphere, and was developed from a branch of the Ganoids, takes the +name of the Dipneusts or Dipnoa ("double-breathers"), because they +retained the earlier gill-respiration along with the new pulmonary +(lung) respiration, like the lowest amphibia. This class was +represented during the paleozoic age (or the Devonian, Carboniferous, +and Permian periods) by a number of different genera. There are only +three genera of the class living to-day: Protopterus annectens in the +rivers of tropical Africa (the White Nile, the Niger, Quelliman, +etc.), Lepidosiren paradoxa in tropical South America (in the +tributaries of the Amazon), and Ceratodus Forsteri in the rivers of +East Australia. This wide distribution of the three isolated survivors +proves that they represent a group that was formerly very large. In +their whole structure they form a transition from the fishes to the +amphibia. The transitional formation between the two classes is so +pronounced in the whole organisation of these remarkable animals that +zoologists had a lively controversy over the question whether they +were really fishes or amphibia. Several distinguished zoologists +classed them with the amphibia, though most now associate them with +the fishes. As a matter of fact, the characters of the two classes are +so far united in the Dipneusts that the answer to the question depends +entirely on the definition we give of "fish" and "amphibian." In +habits they are true amphibia. During the tropical winter, in the +rainy season, they swim in the water like the fishes, and +breathe water by gills. During the dry season they bury themselves in +the dry mud, and breathe the atmosphere through lungs, like the +amphibia and the higher vertebrates. In this double respiration they +resemble the lower amphibia, and have the same characteristic +formation of the heart; in this they are much superior to the fishes. +But in most other features they approach nearer to the fishes, and are +inferior to the amphibia. Externally they are entirely fish-like. + +(FIGURE 2.258. Young ceratodus, shortly after issuing from the egg, +magnified ten times. k gill-cover, l liver. (From Richard Semon.) + +FIGURE 2.259. Young ceratodus six weeks after issuing from the egg. s +spiral fold of gut, b rudimentary belly-fin. (From Richard Semon.)) + +In the Dipneusts the head is not marked off from the trunk. The skin +is covered with large scales. The skeleton is soft, cartilaginous, and +at a low stage of development, as in the lower Selachii and the +earliest Ganoids. The chorda is completely retained, and surrounded by +an unsegmented sheath. The two pairs of limbs are very simple fins of +a primitive type, like those of the lowest Selachii. The formation of +the brain, the gut, and the sexual organs is also the same as in the +Selachii. Thus the Dipneusts have preserved by heredity many of the +less advanced features of our primitive fish-like ancestors, and at +the same time have made a great step forward in adaptation to +air-breathing by means of lungs and the correlative improvement of the +heart. + +Ceratodus is particularly interesting on account of the primitive +build of its skeleton; the cartilaginous skeleton of its two pairs of +fins, for instance, has still the original form of a bi-serial or +feathered leaf, and was on that account described by Gegenbaur as a +"primitive fin-skeleton." On the other hand, the skeleton of the pairs +of fins is greatly reduced in the African dipneust (Protopterus) and +the American (Lepidosiren). Further, the lungs are double in these +modern dipneusts, as in all the other air-breathing vertebrates; they +have on that account been called "double-lunged" (Dipneumones) in +contrast to the Ceratodus; the latter has only a single lung +(Monopneumones). At the same time the gills also are developed as +water-breathing organs in all these lung-fishes. Protopterus has +external as well as internal gills. + +The paleozoic Dipneusts that are in the direct line of our ancestry, +and form the connecting-bridge between the Ganoids and the Amphibia, +differ in many respects from their living descendants, but agree with +them in the above essential features. This is confirmed by a number of +interesting facts that have lately come to our knowledge in connection +with the embryonic development of the Ceratodus and Lepidosiren; they +give us important information as to the stem-history of the lower +Vertebrates, and therefore of our early ancestors of the paleozoic +age. + + +CHAPTER 2.22. OUR FIVE-TOED ANCESTORS. + +With the phylogenetic study of the four higher classes of Vertebrates, +which must now engage our attention, we reach much firmer ground and +more light in the construction of our genealogy than we have, perhaps, +enjoyed up to the present. In the first place, we owe a number of very +valuable data to the very interesting class of Vertebrates that come +next to the Dipneusts and have been developed from them--the Amphibia. +To this group belong the salamander, the frog, and the toad. In +earlier days all the reptiles were, on the example of Linne, classed +with the Amphibia (lizards, serpents, crocodiles, and tortoises). But +the reptiles are much more advanced than the Amphibia, and are nearer +to the birds in the chief points of their structure. The true Amphibia +are nearer to the Dipneusta and the fishes; they are also much older +than the reptiles. There were plenty of highly-developed (and +sometimes large) Amphibia during the Carboniferous period; but the +earliest reptiles are only found in the Permian period. It is probable +that the Amphibia were evolved even earlier--during the Devonian +period--from the Dipneusta. The extinct Amphibia of which we have +fossil remains from that remote period (very numerous especially in +the Triassic strata) were distinguished for a graceful scaly coat or a +powerful bony armour on the skin (like the crocodile), whereas the +living amphibia have usually a smooth and slippery skin. + +The earliest of these armoured Amphibia (Phractamphibia) form the +order of Stegocephala ("roof-headed") (Figure 2.260). It is among +these, and not among the actual Amphibia, that we must look for the +forms that are directly related to the genealogy of our race, and are +the ancestors of the three higher classes of Vertebrates. But even the +existing Amphibia have such important relations to us in their +anatomic structure, and especially their embryonic development, that +we may say: Between the Dipneusts and the Amniotes there was a series +of extinct intermediate forms which we should certainly class with the +Amphibia if we had them before us. In their whole organisation even +the actual Amphibia seem to be an instructive transitional group. In +the important respects of respiration and circulation they approach +very closely to the Dipneusta, though in other respects they are far +superior to them. + +This is particularly true of the development of their limbs or +extremities. In them we find these for the first time as five-toed +feet. The thorough investigations of Gegenbaur have shown that the +fish's fins, of which very erroneous opinions were formerly held, are +many-toed feet. The various cartilaginous or bony radii that are found +in large numbers in each fin correspond to the fingers or toes of the +higher Vertebrates. The several joints of each fin-radius correspond +to the various parts of the toe. Even in the Dipneusta the fin is of +the same construction as in the fishes; it was afterwards gradually +evolved into the five-toed form, which we first encounter in the +Amphibia. This reduction of the number of the toes to six, and then to +five, probably took place in the second half of the Devonian +period--at the latest, in the subsequent Carboniferous period--in +those Dipneusta which we regard as the ancestors of the Amphibia. We +have several fossil remains of five-toed Amphibia from this period. +There are numbers of fossil impressions of them in the Triassic of +Thuringia (Chirotherium). + +(FIGURE 2.260. Fossil amphibian from the Permian, found in the Plauen +terrain near Dresden (Branchiosaurus amblystomus). (From Credner.) A +skeleton of a young larva. B larva, restored, with gills. C the adult +form, natural size.) + +The fact that the toes number five is of great importance, because +they have clearly been transmitted from the Amphibia to all the higher +Vertebrates. Man entirely resembles his amphibian ancestors in this +respect, and indeed in the whole structure of the bony skeleton of his +five-toed extremities. A careful comparison of the skeleton of the +frog with our own is enough to show this. It is well known that this +hereditary number of the toes has assumed a very great practical +importance from remote times; on it our whole system of enumeration +(the decimal system applied to measurement of time, mass, weight, +etc.) is based. There is absolutely no reason why there should be five +toes in the fore and hind feet in the lowest Amphibia, the reptiles, +and the higher Vertebrates, unless we ascribe it to inheritance from a +common stem-form. Heredity alone can explain it. It is true that we +find less than five toes in many of the Amphibia and of the higher +Vertebrates. But in all these cases we can prove that some of the toes +atrophied, and were in time lost altogether. + +The causes of this evolution of the five-toed foot from the many-toed +fin in the amphibian ancestor must be sought in adaptation to the +entire change of function that the limbs experienced in passing from +an exclusively aquatic to a partly terrestrial life. The many-toed fin +had been used almost solely for motion in the water; it had now also +to support the body in creeping on the solid ground. This led to a +modification both of the skeleton and the muscles of the limbs. The +number of the fin-radii was gradually reduced, and sank finally to +five. But these five remaining radii became much stronger. The soft +cartilaginous radii became bony rods. The rest of the skeleton was +similarly strengthened. Thus from the one-armed lever of the many-toed +fish-fin arose the improved many-armed lever system of the five-toed +amphibian limbs. The movements of the body gained in variety as well +as in strength. The various parts of the skeletal system and +correlated muscular system began to differentiate more and more. In +view of the close correlation of the muscular and nervous systems, +this also made great advance in structure and function. Hence we find, +as a matter of fact, that the brain is much more developed in the +higher Amphibia than in the fishes, the Dipneusta, and the lower +Amphibia. + +The first advance in organisation that was occasioned by the adoption +of life on land was naturally the construction of an organ for +breathing air--a lung. This was formed directly from the +floating-bladder inherited from the fishes. At first its function was +insignificant beside that of the gills, the older organ for +water-respiration. Hence we find in the lowest Amphibia, the gilled +Amphibia, that, like the Dipneusta, they pass the greater part of +their life in the water, and breathe water through gills. They only +come to the surface at brief intervals, or creep on to the land, and +then breathe air by their lungs. But some of the tailed Amphibia--the +salamanders--remain entirely in the water when they are young, and +afterwards spend most of their time on land. In the adult state they +only breathe air through lungs. The same applies to the most advanced +of the Amphibia, the Batrachia (frogs and toads); some of them have +entirely lost the gill-bearing larva form.* (* The tree-frog of +Martinique (Hylades martinicensis) loses the gills on the seventh, and +the tail and yelk-sac on the eighth, day of foetal life. On the ninth +or tenth day after fecundation the frog emerges from the egg.) This is +also the case with certain small, serpentine Amphibia, the Caecilia +(which live in the ground like earth-worms). + +(FIGURE 2.261. Larva of the Spotted Salamander (Salamandra maculata), +seen from the ventral side. In the centre a yelk-sac still hangs from +the gut. The external gills are gracefully ramified. The two pairs of +legs are still very small.) + +The great interest of the natural history of the Amphibia consists +especially in their intermediate position between the lower and higher +Vertebrates. The lower Amphibia approach very closely to the Dipneusta +in their whole organisation, live mainly in the water, and breathe by +gills; but the higher Amphibia are just as close to the Amniotes, live +mainly on land, and breathe by lungs. But in their younger state the +latter resemble the former, and only reach the higher stage by a +complete metamorphosis. The embryonic development of most of the +higher Amphibia still faithfully reproduces the stem-history of the +whole class, and the various stages of the advance that was made by +the lower Vertebrates in passing from aquatic to terrestrial life +during the Devonian or the Carboniferous period are repeated in the +spring by every frog that develops from an egg in our ponds. + +(FIGURE 2.262. Larva of the common grass-frog (Rana temporaria), or +"tadpole." m mouth, n a pair of suckers for fastening on to stones, d +skin-fold from which the gill-cover develops; behind it the +gill-clefts, from which the branching gills (k) protrude, s +tail-muscles, f cutaneous fin-fringe of the tail.) + +The common frog leaves the egg in the shape of a larva, like the +tailed salamander (Figure 2.261), and this is altogether different +from the mature frog (Figure 2.262). The short trunk ends in a long +tail, with the form and structure of a fish's tail (s). There are no +limbs at first. The respiration is exclusively branchial, first +through external (k) and then internal gills. In harmony with this the +heart has the same structure as in the fish, and consists of two +sections--an atrium that receives the venous blood from the body, and +a ventricle that forces it through the arteries into the gills. + +We find the larvae of the frog (or tadpoles, Gyrini) in great numbers +in our ponds every spring in this fish-form, using their muscular +tails in swimming, just like the fishes and young Ascidia. When they +have reached a certain size, the remarkable metamorphosis from the +fish-form to the frog begins. A blind sac grows out of the gullet, and +expands into a couple of spacious sacs: these are the lungs. The +simple chamber of the heart is divided into two sections by the +development of a partition, and there are at the same time +considerable changes in the structure of the chief arteries. +Previously all the blood went from the auricle through the aortic +arches into the gills, but now only part of it goes to the gills, the +other part passing to the lungs through the new-formed pulmonary +artery. From this point arterial blood returns to the left auricle of +the heart, while the venous blood gathers in the right auricle. As +both auricles open into a single ventricle, this contains mixed blood. +The dipneust form has now succeeded to the fish-form. In the further +course of the metamorphosis the gills and the branchial vessels +entirely disappear, and the respiration becomes exclusively pulmonary. +Later, the long swimming tail is lost, and the frog now hops to the +land with the legs that have grown meantime. + +This remarkable metamorphosis of the Amphibia is very instructive in +connection with our human genealogy, and is particularly interesting +from the fact that the various groups of actual Amphibia have remained +at different stages of their stem-history, in harmony with the +biogenetic law. We have first of all a very low order of Amphibia--the +Sozobranchia ("gilled-amphibia"), which retain their gills throughout +life, like the fishes. In a second order of the salamanders the gills +are lost in the metamorphosis, and when fully grown they have only +pulmonary respiration. Some of the tailed Amphibia still retain the +gill-clefts in the side of the neck, though they have lost the gills +themselves (Menopoma). If we force the larvae of our salamanders +(Figure 2.261) and tritons to remain in the water, and prevent them +from reaching the land, we can in favourable circumstances make them +retain their gills. In this fish-like condition they reach sexual +maturity, and remain throughout life at the lower stage of the gilled +Amphibia. + +(FIGURE 2.263. Fossil mailed amphibian, from the Bohemian +Carboniferous (Seeleya). (From Fritsch.) The scaly coat is retained on +the left.) + +We have the reverse of this experiment in a Mexican gilled salamander, +the fish-like axolotl (Siredon pisciformis). It was formerly regarded +as a permanent gilled amphibian persisting throughout life at the +fish-stage. But some of the hundreds of these animals that are kept in +the Botanical Garden at Paris got on to the land for some reason or +other, lost their gills, and changed into a form closely resembling +the salamander (Amblystoma). Other species of the genus became +sexually mature for the first time in this condition. This has been +regarded as an astounding phenomenon, although every common frog and +salamander repeats the metamorphosis in the spring. The whole change +from the aquatic and gill-breathing animal to the terrestrial +lung-breathing form may be followed step by step in this case. But +what we see here in the development of the individual has happened to +the whole class in the course of its stem-history. + +The metamorphosis goes farther in a third order of Amphibia, the +Batrachia or Anura, than in the salamander. To this belong the various +kinds of toads, ringed snakes, water-frogs, tree-frogs, etc. These +lose, not only the gills, but also (sooner or later) the tail, during +metamorphosis. + +The ontogenetic loss of the gills and the tail in the frog and toad +can only be explained on the assumption that they are descended from +long-tailed Amphibia of the salamander type. This is also clear from +the comparative anatomy of the two groups. This remarkable +metamorphosis is, however, also interesting because it throws a +certain light on the phylogeny of the tail-less apes and man. Their +ancestors also had long tails and gills like the gilled Amphibia, as +the tail and the gill-arches of the human embryo clearly show. + +For comparative anatomical and ontogenetic reasons, we must not seek +these amphibian ancestors of ours--as one would be inclined to do, +perhaps--among the tail-less Batrachia, but among the tailed lower +Amphibia. + +The vertebrate form that comes next to the Amphibia in the series of +our ancestors is a lizard-like animal, the earlier existence of which +can be confidently deduced from the facts of comparative anatomy and +ontogeny. The living Hatteria of New Zealand (Figure 2.264) and the +extinct Rhyncocephala of the Permian period (Figure 2.265) are closely +related to this important stem-form; we may call them the +Protamniotes, or Primitive Amniotes. All the Vertebrates above the +Amphibia--or the three classes of reptiles, birds, and mammals--differ +so much in their whole organisation from all the lower Vertebrates we +have yet considered, and have so great a resemblance to each other, +that we put them all together in a single group with the title of +Amniotes. In these three classes alone we find the remarkable +embryonic membrane, already mentioned, which we called the amnion; a +cenogenetic adaptation that we may regard as a result of the sinking +of the growing embryo into the yelk-sac. + +All the Amniotes known to us--all reptiles, birds, and mammals +(including man)--agree in so many important points of internal +structure and development that their descent from a common ancestor +can be affirmed with tolerable certainty. If the evidence of +comparative anatomy and ontogeny is ever entirely beyond suspicion, it +is certainly the case here. All the peculiarities that accompany and +follow the formation of the amnion, and that we have learned in our +consideration of human embryology; all the peculiarities in the +development of the organs which we will presently follow in detail; +finally, all the principal special features of the internal structure +of the full-grown Amniotes--prove so clearly the common origin of all +the Amniotes from single extinct stem-form that it is difficult to +entertain the idea of their evolution from several independent stems. +This unknown common stem-form is our primitive Amniote (Protamnion). +In outward appearance it was probably something between the salamander +and the lizard. + +It is very probable that some part of the Permian period was the age +of the origin of the Protamniotes. This follows from the fact that the +Amphibia are not fully developed until the Carboniferous period, and +that the first fossil reptiles (Palaehatteria, Homoeosaurus, +Proterosaurus) are found towards the close of the Permian period. +Among the important changes of the vertebrate organisation that marked +the rise of the first Amniotes from salamandrine Amphibia during this +period the following three are especially noteworthy: the entire +disappearance of the water-breathing gills and the conversion of the +gill-arches into other organs, the formation of the allantois or +primitive urinary sac, and the development of the amnion. + +One of the most salient characteristics of the Amniotes is the +complete loss of the gills. All Amniotes, even if living in water +(such as sea-serpents and whales), breathe air through lungs, never +water through gills. All the Amphibia (with very rare exceptions) +retain their gills for some time when young, and have for a time (if +not permanently) branchial respiration; but after these there is no +question of branchial respiration. The Protamniote itself must have +entirely abandoned water-breathing. Nevertheless, the gill-arches are +preserved by heredity, and develop into totally different (in part +rudimentary) organs--various parts of the bone of the tongue, the +frame of the jaws, the organ of hearing, etc. But we do not find in +the embryos of the Amniotes any trace of gill-leaves, or of real +respiratory organs on the gill-arches. + +With this complete abandonment of the gills is probably connected the +formation of another organ, to which we have already referred in +embryology--namely, the allantois or primitive urinary sac (cf. +Chapter 1.15). It is very probable that the urinary bladder of the +Dipneusts is the first structure of the allantois. We find in these a +urinary bladder that proceeds from the lower wall of the hind end of +the gut, and serves as receptacle for the renal secretions. This organ +has been transmitted to the Amphibia, as we can see in the frog. + +The formation of the amnion and the allantois and the complete +disappearance of the gills are the chief characteristics that +distinguish the Amniotes from the lower Vertebrates we have hitherto +considered. To these we may add several subordinate features that are +transmitted to all the Amniotes, and are found in these only. One +striking embryonic character of the Amniotes is the great curve of the +head and neck in the embryo. We also find an advance in the structure +of several of the internal organs of the Amniotes which raises them +above the highest of the anamnia. In particular, a partition is formed +in the simple ventricle of the heart, dividing into right and left +chambers. In connection with the complete metamorphosis of the +gill-arches we find a further development of the auscultory organs. +Also, there is a great advance in the structure of the brain, +skeleton, muscular system, and other parts. Finally, one of the most +important changes is the reconstruction of the kidneys. In all the +earlier Vertebrates we have found the primitive kidneys as excretory +organs, and these appear at an early stage in the embryos of all the +higher Vertebrates up to man. But in the Amniotes these primitive +kidneys cease to act at an early stage of embryonic life, and their +function is taken up by the permanent or secondary kidneys, which +develop from the terminal section of the prorenal ducts. + +(FIGURE 2.264. The lizard (Hatteria punctata = Sphenodon punctatus) of +New Zealand. The sole surviving proreptile. (From Brehm.)) + +Taking all these peculiarities of the Amniotes together, it is +impossible to doubt that all the animals of this group--all reptiles, +birds, and mammals--have a common origin, and form a single +blood-related stem. Our own race belongs to this stem. Man is, in +every feature of his organisation and embryonic development, a true +Amniote, and has descended from the Protamniote with all the other +Amniotes. Though they appeared at the end (possibly even in the +middle) of the Paleozoic age, the Amniotes only reached their full +development during the Mesozoic age. The birds and mammals made their +first appearance during this period. Even the reptiles show their +greatest growth at this time, so that it is called "the reptile age." +The extinct Protamniote, the ancestor of the whole group, belongs in +its whole organisation to the reptile class. + +The genealogical tree of the amniote group is clearly indicated in its +chief lines by their paleontology, comparative anatomy, and ontogeny. +The group succeeding the Protamniote divided into two branches. The +branch that will claim our whole interest is the class of the Mammals. +The other branch, which developed in a totally different direction, +and only comes in contact with the Mammals at its root, is the +combined group of the reptiles and birds; these two classes may, with +Huxley, be conveniently grouped together as the Sauropsida. Their +common stem-form is an extinct lizard-like reptile of the order of the +Rhyncocephalia. From this have been developed in various directions +the serpents, crocodiles, tortoises, etc.--in a word, all the members +of the reptile class. But the remarkable class of the birds has also +been evolved directly from a branch of the reptile group, as is now +established beyond question. The embryos of the reptiles and birds are +identical until a very late stage, and have an astonishing resemblance +even later. Their whole structure agrees so much that no anatomist now +questions the descent of the birds from the reptiles. On the other +hand, the mammal line has descended from the group of the +Sauromammalia, a different branch of the Proreptilia. It is connected +at its deepest roots with the reptile line, but it then diverges +completely from it and follows a distinctive development. Man is the +highest outcome of this class, the "crown of creation." The hypothesis +that the three higher Vertebrate classes represent a single +Amniote-stem, and that the common root of this stem is to be found in +the amphibian class, is now generally admitted. + +(FIGURE 2.265. Homoeosaurus pulchellus, a Jurassic proreptile from +Kehlheim. (From Zittel.)) + +The instructive group of the Permian Tocosauria, the common root from +which the divergent stems of the Sauropsids and mammals have issued, +merits our particular attention as the stem-group of all the Amniotes. +Fortunately a living representative of this extinct ancestral group +has been preserved to our day; this is the remarkable lizard of New +Zealand, Hatteria punctata (Figure 2.264). Externally it differs +little from the ordinary lizard; but in many important points of +internal structure, especially in the primitive construction of the +vertebral column, the skull, and the limbs, it occupies a much lower +position, and approaches its amphibian ancestors, the Stegocephala. +Hence Hatteria is the phylogenetically oldest of all living reptiles, +an isolated survivor from the Permian period, closely resembling the +common ancestor of the Amniotes. It must differ so little from this +extinct form, our hypothetical Protamniote, that we put it next to the +Proreptilia. The remarkable Permian Palaehatteria, that Credner +discovered in the Plauen terrain at Dresden in 1888, belongs to the +same group (Figure 2.266). The Jurassic genus Homoeosaurus (Figure +2.265), of which well-preserved skeletons are found in the Solenhofen +schists, is perhaps still more closely related to them. + +Unfortunately, the numerous fossil remains of Permian and Triassic +Tocosauria that we have found in the last two decades are, for the +most part, very imperfectly preserved. Very often we can make only +precarious inferences from these skeletal fragments as to the anatomic +characters of the soft parts that went with the bony skeleton of the +extinct Tocosauria. Hence it has not yet been possible to arrange +these important fossils with any confidence in the ancestral series +that descend from the Protamniotes to the Sauropsids on the one side +and the Mammals on the other. Opinions are particularly divided as to +the place in classification and the phylogenetic significance of the +remarkable Theromorpha. Cope gives this name to a very interesting and +extensive group of extinct terrestrial reptiles, of which we have only +fossil remains from the Permian and Triassic strata. Forty years ago +some of these Therosauria (fresh-water animals) were described by Owen +as Anomodontia. But during the last twenty years the distinguished +American paleontologists, Cope and Osborn, have greatly increased our +knowledge of them, and have claimed that the stem-forms of the Mammals +must be sought in this order. As a matter of fact, the Theromorpha are +nearer to the Mammals in the chief points of structure than any other +reptiles. This is especially true of the Thereodontia, to which the +Pureosauria and Pelycosauria belong (Figure 2.267). The whole +structure of their pelvis and hind-feet has attained the same form as +in the Monotremes, the lowest Mammals. The formation of the scapula +and the quadrate bone shows an approach to the Mammals such as we find +in no other group of reptiles. The teeth also are already divided into +incisors, canines, and molars. Nevertheless, it is very doubtful +whether the Theromorpha really are in the ancestral line of the +Sauromammals, or lead direct from the Tocosauria to the earliest +Mammals. Other experts on this group believe that it is an independent +legion of the reptiles, connected, perhaps, at its lowest root, with +the Sauromammals, but developed quite independently of the +Mammals--though parallel to them in many ways. + +One of the most important of the zoological facts that we rely on in +our investigation of the genealogy of the human race is the position +of man in the Mammal class. However different the views of zoologists +may have been as to this position in detail, and as to his relations +to the apes, no scientist has ever doubted that man is a true mammal +in his whole organisation and development. Linne drew attention to +this fact in the first edition of his famous Systema Naturae (1735). +As will be seen in any museum of anatomy or any manual of comparative +anatomy; the human frame has all the characteristics that are common +to the Mammals and distinguish them conspicuously from all other +animals. + +(FIGURE 2.266. Skull of a Permian lizard (Palaehatteria longicaudata). +(From Credner.) n nasal bone, pf frontal bone, l lachrymal bone, po +postorbital bone, sq covering bone, i cheek-bone, vo vomer, im +inter-maxillary.) + +If we examine this undoubted fact from the point of view of phylogeny, +in the light of the theory of descent, it follows at once that man is +of a common stem with all the other Mammals, and comes from the same +root as they. But the various features in which the Mammals agree and +by which they are distinguished are of such a character as to make a +polyphyletic hypothesis quite inadmissible. It is impossible to +entertain the idea that all the living and extinct Mammals come from a +number of separate roots. If we accept the general theory of +evolution, we are bound to admit the monophyletic hypothesis of the +descent of all the Mammals (including man) from a single mammalian +stem-form. We may call this long-extinct root-form and its earliest +descendants (a few genera of one family) "primitive mammals" or +"stem-mammals" (Promammalia). As we have already seen, this root-form +developed from the primitive Proreptile stem in a totally different +direction from the birds, and soon separated from the main stem of the +reptiles. The differences between the Mammals and the reptiles and +birds are so important and characteristic that we can assume with +complete confidence this division of the vertebrate stem at the +commencement of the development of the Amniotes. The reptiles and +birds, which we group together as the Sauropsids, generally agree in +the characteristic structure of the skull and brain, and this is +notably different from that of the Mammals. In most of the reptiles +and birds the skull is connected with the first cervical vertebra (the +atlas) by a single, and in the Mammals (and Amphibia) by a double, +condyle at the back of the head. In the former the lower jaw is +composed of several pieces, and connected with the skull so that it +can move by a special maxillary bone (the quadratum); in the Mammals +the lower jaw consists of one pair of bony pieces, which articulate +directly with the temporal bone. Further, in the Sauropsids the skin +is clothed with scales or feathers; in the Mammals with hair. The red +blood-cells of the former have a nucleus; those of the latter have +not. In fine, two quite characteristic features of the Mammals, which +distinguish them not only from the birds and reptiles, but from all +other animals, are the possession of a complete diaphragm and of +mammary glands that produce the milk for the nutrition of the young. +It is only in the Mammals that the diaphragm forms a transverse +partition of the body-cavity, completely separating the pectoral from +the abdominal cavity. It is only in the mammals that the mother +suckles its young, and this rightly gives the name to the whole class +(mamma = breast). + +(FIGURE 2.267. Skull of a Triassic theromorphum (Galesaurus +planiceps), from the Karoo formation in South Africa. (From Owen.) a +from the right, b from below, c from above, d tricuspid tooth. N +nostrils, NA nasal bone, Mx upper jaw, Prf prefrontal, Fr frontal +bone, A eye-pits, S temple-pits. Pa Parietal eye, Bo joint at back of +head, Pt pterygoid-bone, Md lower jaw.) + +From these pregnant facts of comparative anatomy and ontogeny it +follows absolutely that the whole of the Mammals belong to a single +natural stem, which branched off at an early date from the +reptile-root. It follows further with the same absolute certainty that +the human race is also a branch of this stem. Man shares all the +characteristics I have described with all the Mammals, and differs in +them from all other animals. Finally, from these facts we deduce with +the same confidence those advances in the vertebrate organisation by +which one branch of the Sauromammals was converted into the stem-form +of the Mammals. Of these advances the chief were: (1) The +characteristic modification of the skull and the brain; (2) the +development of a hairy coat; (3) the complete formation of the +diaphragm; and (4) the construction of the mammary glands and +adaptation to suckling. Other important changes of structure proceeded +step by step with these. + +The epoch at which these important advances were made, and the +foundation of the Mammal class was laid, may be put with great +probability in the first section of the Mesozoic or secondary age--the +Triassic period. The oldest fossil remains of mammals that we know +were found in strata that belong to the earliest Triassic period--the +upper Kueper. One of the earliest forms is the genus Dromatherium, +from the North American Triassic (Figure 2.268). Their teeth still +strikingly recall those of the Pelycosauria. Hence we may assume that +this small and probably insectivorous mammal belonged to the +stem-group of the Promammals. We do not find any positive trace of the +third and most advanced division of the Mammals--the Placentals. These +(including man) are much younger, and we do not find indisputable +fossil remains of them until the Cenozoic age, or the Tertiary period. +This paleontological fact is very important, because it fully +harmonises with the evolutionary succession of the Mammal orders that +is deduced from their comparative anatomy and ontogeny. + +The latter science teaches us that the whole Mammal class divides into +three main groups or sub-classes, which correspond to three successive +phylogenetic stages. These three stages, which also represent three +important stages in our human genealogy, were first distinguished in +1816 by the eminent French zoologist, Blainville, and received the +names of Ornithodelphia, Didelphia, and Monodelphia, according to the +construction of the female organs (delphys = uterus or womb). Huxley +afterwards gave them the names of Prototheria, Metatheria, and +Epitheria. But the three sub-classes differ so widely from each other, +not only in the construction of the sexual organs, but in many other +respects also, that we may confidently draw up the following important +phylogenetic thesis: The Monodelphia or Placentals descend from the +Didelphia or Marsupials; and the latter, in turn, are descended from +the Monotremes or Ornithodelphia. + +Thus we must regard as the twenty-first stage in our genealogical tree +the earliest and lowest chief group of the Mammals--the sub-class of +the Monotremes ("cloaca-animals," Ornithodelphia, or Prototheria, +Figures 2.269 and 2.270). They take their name from the cloaca which +they share with all the lower Vertebrates. This cloaca is the common +outlet for the passage of the excrements, the urine, and the sexual +products. The urinary ducts and sexual canals open into the hindmost +part of the gut, while in all the other Mammals they are separated +from the rectum and anus. The latter have a special uro-genital outlet +(porus urogenitalis). The bladder also opens into the cloaca in the +Monotremes, and, indeed, apart from the two urinary ducts; in all the +other Mammals the latter open directly into the bladder. It was proved +by Haacke and Caldwell in 1884 that the Monotremes lay large eggs like +the reptiles, while all the other Mammals are viviparous. In 1894 +Richard Semon further proved that these large eggs, rich in food-yelk, +have a partial segmentation and discoid gastrulation, as I had +hypothetically assumed in 1879; here again they resemble their +reptilian ancestors. The construction of the mammary gland is also +peculiar in the Monotremes. In them the glands have no teats for the +young animal to suck, but there is a special part of the breast +pierced with holes like a sieve, from which the milk issues, and the +young Monotreme must lick it off. Further, the brain of the Monotremes +is very little advanced. It is feebler than that of any of the other +Mammals. The fore-brain or cerebrum, in particular, is so small that +it does not cover the cerebellum. In the skeleton (Figure 2.270) the +formation of the scapula among other parts is curious; it is quite +different from that of the other Mammals, and rather agrees with that +of the reptiles and Amphibia. Like these, the Monotremes have a +strongly developed caracoideum. From these and other less prominent +characteristics it follows absolutely that the Monotremes occupy the +lowest place among the Mammals, and represent a transitional group +between the Tocosauria and the rest of the Mammals. All these +remarkable reptilian characters must have been possessed by the +stem-form of the whole mammal class, the Promammal of the Triassic +period, and have been inherited from the Proreptiles. + +(FIGURE 2.268. Lower jaw of a Primitive Mammal or Promammal +(Dromatherium silvestre) from the North American Triassic. i incisors, +c canine, p premolars, m molars. (From Doderlein.)) + +During the Triassic and Jurassic periods the sub-class of the +Monotremes was represented by a number of different stem-mammals. +Numerous fossil remains of them have lately been discovered in the +Mesozoic strata of Europe, Africa, and America. To-day there are only +two surviving specimens of the group, which we place together in the +family of the duck-bills, Ornithostoma. They are confined to Australia +and the neighbouring island of Van Diemen's Land (or Tasmania); they +become scarcer every year, and will soon, like their blood-relatives, +be counted among the extinct animals. One form lives in the rivers, +and builds subterraneous dwellings on the banks; this is the +Ornithorhyncus paradoxus, with webbed feet, a thick soft fur, and +broad flat jaws, which look very much like the bill of a duck (Figures +2.269 and 2.270). The other form, the land duck-bill, or spiny +ant-eater (Echidna hystrix), is very much like the anteaters in its +habits and the peculiar construction of its thin snout and very long +tongue; it is covered with needles, and can roll itself up like a +hedgehog. A cognate form (Parechidna Bruyni) has lately been found in +New Guinea. + +These modern Ornithostoma are the scattered survivors of the vast +Mesozoic group of Monotremes; hence they have the same interest in +connection with the stem history of the Mammals as the living +stem-reptiles (Hatteria) for that of the reptiles, and the isolated +Acrania (Amphioxus) for the phylogeny of the Vertebrate stem. + +The Australian duck-bills are distinguished externally by a toothless +bird-like beak or snout. This absence of real bony teeth is a late +result of adaptation, as in the toothless Placentals (Edentata, +armadillos and ant-eaters). The extinct Monotremes, to which the +Promammalia belonged, must have had developed teeth, inherited from +the reptiles. Lately small rudiments of real molars have been +discovered in the young of the Ornithorhyncus, which has horny plates +in the jaws instead of real teeth. + +(FIGURE 2.269. The Ornithorhyncus or Duck-mole. (Ornithorhyncus +paradoxus). + +FIGURE 2.270. Skeleton of the Ornithorhyncus.) + +The living Ornithostoma and the stem-forms of the Marsupials (or +Didelphia) must be regarded as two widely diverging lines from the +Promammals. This second sub-class of the Mammals is very interesting +as a perfect intermediate stage between the other two. While the +Marsupials retain a great part of the characteristics of the +Monotremes, they have also acquired some of the chief features of the +Placentals. Some features are also peculiar to the Marsupials, such as +the construction of the male and female sexual organs and the form of +the lower jaw. The Marsupials are distinguished by a peculiar +hook-like bony process that bends from the corner of the lower jaw and +points inwards. As most of the Placentals have not this process, we +can, with some probability, recognise the Marsupial from this feature +alone. Most of the mammal remains that we have from the Jurassic and +Cretaceous deposits are merely lower jaws, and most of the jaws found +in the Jurassic deposits at Stonesfield and Purbeck have the peculiar +hook-like process that characterises the lower jaw of the Marsupial. +On the strength of this paleontological fact, we may suppose that they +belonged to Marsupials. Placentals do not seem to have existed at the +middle of the Mesozoic age--not until towards its close (in the +Cretaceous period). At all events, we have no fossil remains of +indubitable Placentals from that period. + +The existing Marsupials, of which the plant-eating kangaroo and the +carnivorous opossum (Figure 2.272) are the best known, differ a good +deal in structure, shape, and size, and correspond in many respects to +the various orders of Placentals. Most of them live in Australia, and +a small part of the Australian and East Malayan islands. There is now +not a single living Marsupial on the mainland of Europe, Asia, or +Africa. It was very different during the Mesozoic and even during the +Cenozoic age. The sedimentary deposits of these periods contain a +great number and variety of marsupial remains, sometimes of a colossal +size, in various parts of the earth, and even in Europe. We may infer +from this that the existing Marsupials are the remnant of an extensive +earlier group that was distributed all over the earth. It had to give +way in the struggle for life to the more powerful Placentals during +the Tertiary period. The survivors of the group were able to keep +alive in Australia and South America because the one was completely +separated from the other parts of the earth during the whole of the +Tertiary period, and the other during the greater part of it. + +(FIGURE 2.271. Lower jaw of a Promammal (Dryolestes priscus), from the +Jurassic of the Felsen strata. (From Marsh.)) + +From the comparative anatomy and ontogeny of the existing Marsupials +we may draw very interesting conclusions as to their intermediate +position between the earlier Monotremes and the later Placentals. The +defective development of the brain (especially the cerebrum), the +possession of marsupial bones, and the simple construction of the +allantois (without any placenta as yet) were inherited by the +Marsupials, with many other features, from the Monotremes, and +preserved. On the other hand, they have lost the independent bone +(caracoideum) at the shoulder-blade. But we have a more important +advance in the disappearance of the cloaca; the rectum and anus are +separated by a partition from the uro-genital opening (sinus +urogenitalis). Moreover, all the Marsupials have teats on the mammary +glands, at which the new-born animal sucks. The teats pass into the +cavity of a pouch or pocket on the ventral side of the mother, and +this is supported by a couple of marsupial bones. The young are born +in a very imperfect condition, and carried by the mother for some time +longer in her pouch, until they are fully developed (Figure 2.272). In +the giant kangaroo, which is as tall as a man, the embryo only +develops for a month in the uterus, is then born in a very imperfect +state, and finishes its growth in the mother's pouch (marsupium); it +remains in this about nine months, and at first hangs continually on +to the teat of the mammary gland. + +(FIGURE 2.272. The crab-eating Opossum (Philander cancrivorus). The +female has three young in the pouch. (From Brehm.) + +From these and other characteristics (especially the peculiar +construction of the internal and external sexual organs in male and +female) it is clear that we must conceive the whole sub-class of the +Marsupials as one stem group, which has been developed from the +Promammalia. From one branch of these Marsupials (possibly from more +than one) the stem-forms of the higher Mammals, the Placentals, were +afterwards evolved. Of the existing forms of the Marsupials, which +have undergone various modifications through adaptation to different +environments, the family of the opossums (Didelphida or Pedimana) +seems to be the oldest and nearest to the common stem-form of the +whole class. To this family belong the crab-eating opossum of Brazil +(Figure 2.272) and the opossum of Virginia, on the embryology of which +Selenka has given us a valuable work (cf. Figures 1.63 to 1.67 and +1.131 to 1.135). These Didelphida climb trees like the apes, grasping +the branches with their hand-shaped hind feet. We may conclude from +this that the stem-forms of the Primates, which we must regard as the +earliest Lemurs, were evolved directly from the opossum. We must not +forget, however, that the conversion of the five-toed foot into a +prehensile hand is polyphyletic. By the same adaptation to climbing +trees the habit of grasping their branches with the feet has in many +different cases brought about that opposition of the thumb or great +toe to the other toes which makes the hand prehensile. We see this in +the climbing lizards (chameleon), the birds, and the tree-dwelling +mammals of various orders. + +Some zoologists have lately advanced the opposite opinion, that the +Marsupials represent a completely independent sub-class of the +Mammals, with no direct relation to the Placentals, and developing +independently of them from the Monotremes. But this opinion is +untenable if we examine carefully the whole organisation of the three +sub-classes, and do not lay the chief stress on incidental features +and secondary adaptations (such as the formation of the marsupium). It +is then clear that the Marsupials--viviparous Mammals without +placenta--are a necessary transition from the oviparous Monotremes to +the higher Placentals with chorion-villi. In this sense the Marsupial +class certainly contains some of man's ancestors. + + +CHAPTER 2.23. OUR APE ANCESTORS. + +The long series of animal forms which we must regard as the ancestors +of our race has been confined within narrower and narrower circles as +our phylogenetic inquiry has progressed. The great majority of known +animals do not fall in the line of our ancestry, and even within the +vertebrate stem only a small number are found to do so. In the most +advanced class of the stem, the mammals, there are only a few families +that belong directly to our genealogical tree. The most important of +these are the apes and their predecessors, the half-apes, and the +earliest Placentals (Prochoriata). + +The Placentals (also called Choriata, Monodelphia, Eutheria or +Epitheria) are distinguished from the lower mammals we have just +considered, the Monotremes and Marsupials, by a number of striking +peculiarities. Man has all these distinctive features; that is a very +significant fact. We may, on the ground of the most careful +comparative-anatomical and ontogenetic research, formulate the thesis: +"Man is in every respect a true Placental." He has all the +characteristics of structure and development that distinguish the +Placentals from the two lower divisions of the mammals, and, in fact, +from all other animals. Among these characteristics we must especially +notice the more advanced development of the brain. The fore-brain or +cerebrum especially is much more developed in them than in the lower +animals. The corpus callosum, which forms a sort of wide bridge +connecting the two hemispheres of the cerebrum, is only fully formed +in the Placentals; it is very rudimentary in the Marsupials and +Monotremes. It is true that the lowest Placentals are not far removed +from the Marsupials in cerebral development; but within the placental +group we can trace an unbroken gradation of progressive development of +the brain, rising gradually from this lowest stage up to the elaborate +psychic organ of the apes and man. The human soul--a physiological +function of the brain--is in reality only a more advanced ape-soul. + +The mammary glands of the Placentals are provided with teats like +those of the Marsupials; but we never find in the Placentals the pouch +in which the latter carry and suckle their young. Nor have they the +marsupial bones in the ventral wall at the anterior border of the +pelvis, which the Marsupials have in common with the Monotremes, and +which are formed by a partial ossification of the sinews of the inner +oblique abdominal muscle. There are merely a few insignificant +remnants of them in some of the Carnivora. The Placentals are also +generally without the hook-shaped process at the angle of the lower +jaw which is found in the Marsupials. + +(FIGURE 2.273. Foetal membranes of the human embryo (diagrammatic). m +the thick muscular wall of the womb. plu placenta [the inner layer +(plu apostrophe) of which penetrates into the chorion-villi (chz) with +its processes]. chf tufted, chl smooth chorion. a amnion, ah amniotic +cavity, as amniotic sheath of the umbilical cord (which passes under +into the navel of the embryo--not given here), dg vitelline duct, ds +yelk sac, dv, dr decidua (vera and reflexa). The uterine cavity (uh) +opens below into the vagina and above on the right into an oviduct +(t). (From Kolliker.)) + +However, the feature that characterises the Placentals above all +others, and that has given its name to the whole sub-class, is the +formation of the placenta. We have already considered the formation +and significance of this remarkable embryonic organ when we traced the +development of the chorion and the allantois in the human embryo +(Chapter 1.15). The urinary sac or the allantois, the curious vesicle +that grows out of the hind part of the gut, has essentially the same +structure and function in the human embryo as in that of all the other +Amniotes (cf. Figures 1.194 to 1.196). There is a quite secondary +difference, on which great stress has wrongly been laid, in the fact +that in man and the higher apes the original cavity of the allantois +quickly degenerates, and the rudiment of it sticks out as a solid +projection from the primitive gut. The thin wall of the allantois +consists of the same two layers or membranes as the wall of the +gut--the gut-gland layer within and the gut-fibre layer without. In +the gut-fibre layer of the allantois there are large blood-vessels, +which serve for the nutrition, and especially the respiration, of the +embryo--the umbilical vessels (Chapter 1.15). In the reptiles and +birds the allantois enlarges into a spacious sac, which encloses the +embryo with the amnion, and does not combine with the outer foetal +membrane (the chorion). This is the case also with the lowest mammals, +the oviparous Monotremes and most of the Marsupials. It is only in +some of the later Marsupials (Peramelida) and all the Placentals that +the allantois develops into the distinctive and remarkable structure +that we call the placenta. + +The placenta is formed by the branches of the blood-vessels in the +wall of the allantois growing into the hollow ectodermic tufts (villi) +of the chorion, which run into corresponding depressions in the mucous +membrane of the womb. The latter also is richly permeated with +blood-vessels which bring the mother's blood to the embryo. As the +partition in the villi between the maternal blood-vessels and those of +the foetus is extremely thin, there is a direct exchange of fluid +between the two, and this is of the greatest importance in the +nutrition of the young mammal. It is true that the maternal vessels do +not entirely pass into the foetal vessels, so that the two kinds of +blood are simply mixed. But the partition between them is so thin that +the nutritive fluid easily transudes through it. By means of this +transudation or diosmosis the exchange of fluids takes place without +difficulty. The larger the embryo is in the placentals, and the longer +it remains in the womb, the more necessary it is to have special +structures to meet its great consumption of food. + +In this respect there is a very conspicuous difference between the +lower and higher mammals. In the Marsupials, in which the embryo is +only a comparatively short time in the womb and is born in a very +immature condition, the vascular arrangements in the yelk-sac and the +allantois suffice for its nutrition, as we find them in the +Monotremes, birds, and reptiles. But in the Placentals, where +gestation lasts a long time, and the embryo reaches its full +development under the protection of its enveloping membranes, there +has to be a new mechanism for the direct supply of a large quantity of +food, and this is admirably met by the formation of the placenta. + +Branches of the blood-vessels penetrate into the chorion-villi from +within, starting from the gut-fibre layer of the allantois, and +bringing the blood of the foetus through the umbilical vessels (Figure +2.273 chz). On the other hand, a thick network of blood-vessels +develops in the mucous membrane that clothes the inner surface of the +womb, especially in the region of the depressions into which the +chorion-villi penetrate (plu). This network of arteries contains +maternal blood, brought by the uterine vessels. As the connective +tissue between the enlarged capillaries of the uterus disappears, wide +cavities filled with maternal blood appear, and into these the +chorion-villi of the embryo penetrate. The sum of these vessels of +both kinds, that are so intimately correlated at this point, together +with the connective and enveloping tissue, is the placenta. The +placenta consists, therefore, properly speaking, of two different +though intimately connected parts--the foetal placenta (Figure 2.273 +chz) within and the maternal or uterine placenta (plu) without. The +latter is made up of the mucous coat of the uterus and its +blood-vessels, the former of the tufted chorion and the umbilical +vessels of the embryo (cf. Figure 1.196). + +(FIGURE 2.274. Skull of a fossil lemur (Adapis parisiensis,), from the +Miocene at Quercy. A lateral view from the right, half natural size. B +lower jaw, C lower molar, i incisors, c canines, p premolars, m +molars.) + +The manner in which these two kinds of vessels combine in the +placenta, and the structure, form, and size of it, differ a good deal +in the various Placentals; to some extent they give us valuable data +for the natural classification, and therefore the phylogeny, of the +whole of this sub-class. On the ground of these differences we divide +it into two principal sections; the lower Placentals or Indecidua, and +the higher Placentals or Deciduata. + +To the Indecidua belong three important groups of mammals: the Lemurs +(Prosimiae), the Ungulates (tapirs, horses, pigs, ruminants, etc.), +and the Cetacea (dolphins and whales). In these Indecidua the villi +are distributed over the whole surface of the chorion (or its greater +part) either singly or in groups. They are only loosely connected with +the mucous coat of the uterus, so that the whole foetal membrane with +its villi can be easily withdrawn from the uterine depressions like a +hand from a glove. There is no real coalescence of the two placentas +at any part of the surface of contact. Hence at birth the foetal +placenta alone comes away; the uterine placenta is not torn away with +it. + +The formation of the placenta is very different in the second and +higher section of the Placentals, the Deciduata. Here again the whole +surface of the chorion is thickly covered with the villi in the +beginning. But they afterwards disappear from one part of the surface, +and grow proportionately thicker on the other part. We thus get a +differentiation between the smooth chorion (chorion laeve, Figure +2.273 chl) and the thickly-tufted chorion (chorion frondosum, Figure +2.273 chf). The former has only a few small villi or none at all; the +latter is thickly covered with large and well-developed villi; this +alone now constitutes the placenta. In the great majority of the +Deciduata the placenta has the same shape as in man (Figures 1.197 and +1.200)--namely a thick, circular disk like a cake; so we find in the +Insectivora, Chiroptera, Rodents, and Apes. This discoplacenta lies on +one side of the chorion. But in the Sarcotheria (both the Carnivora +and the seals, Pinnipedia) and in the elephant and several other +Deciduates we find a zonoplacenta; in these the rich mass of villi +runs like a girdle round the middle of the ellipsoid chorion, the two +poles of it being free from them. + +(FIGURE 2.275. The Slender Lori (Stenops gracilis) of Ceylon, a +tail-less lemur.) + +Still more characteristic of the Deciduates is the peculiar and very +intimate connection between the chorion frondosum and the +corresponding part of the mucous coat of the womb, which we must +regard as a real coalescence of the two. The villi of the chorion push +their branches into the blood-filled tissues of the coat of the +uterus, and the vessels of each loop together so intimately that it is +no longer possible to separate the foetal from the maternal placenta; +they form henceforth a compact and apparently simple placenta. In +consequence of this coalescence, a whole piece of the lining of the +womb comes away at birth with the foetal membrane that is interlaced +with it. This piece is called the "falling-away" membrane (decidua). +It is also called the serous (spongy) membrane, because it is pierced +like a sieve or sponge. All the higher Placentals that have this +decidua are classed together as the "Deciduates." The tearing away of +the decidua at birth naturally causes the mother to lose a quantity of +blood, which does not happen in the Indecidua. The last part of the +uterine coat has to be repaired by a new growth after birth in the +Deciduates. (Cf. Figures 1.199 and 1.200.) + +In the various orders of the Deciduates, the placenta differs +considerably both in outer form and internal structure. The extensive +investigations of the last ten years have shown that there is more +variation in these respects among the higher mammals than was formerly +supposed. The physiological work of this important embryonic organ, +the nutrition of the foetus during its long sojourn in the womb, is +accomplished in the various groups of the Placentals by very different +and sometimes very elaborate structures. They have lately been fully +described by Hans Strahl. + +The phylogeny of the placenta has become more intelligible from the +fact that we have found a number of transitional forms of it. Some of +the Marsupials (Perameles) have the beginning of a placenta. In some +of the Lemurs (Tarsius) a discoid placenta with decidua is developed. + +While these important results of comparative embryology have been +throwing further light on the close blood-relationship of man and the +anthropoid apes in the last few years (Chapter 1.15), the great +advance of paleontology has at the same time been affording us a +deeper insight into the stem-history of the Placental group. In the +seventh chapter of my Systematic Phylogeny of the Vertebrates I +advanced the hypothesis that the Placentals form a single stem with +many branches, which has been evolved from an older group of the +Marsupials (Prodidelphia). The four great legions of the +Placentals--Rodents, Ungulates, Carnassia, and Primates--are sharply +separated to-day by important features of organisation. But if we +consider their extinct ancestors of the Tertiary period, the +differences gradually disappear, the deeper we go in the Cenozoic +deposits; in the end we find that they vanish altogether. The +primitive stem-forms of the Rodents (Esthonychida), the Ungulates +(Chondylarthra), the Carnassia (Ictopsida), and the Primates +(Lemuravida) are so closely related at the beginning of the Tertiary +period that we might group them together as different families of one +order, the Proplacentals (Mallotheria or Prochoriata). + +Hence the great majority of the Placentals have no direct and close +relationship to man, but only the legion of the Primates. This is now +generally divided into three orders--the half-apes (Prosimiae), apes +(Simiae), and man (Anthropi). The lemurs or half-apes are the +stem-group, descending from the older Mallotheria of the Cretaceous +period. From them the apes were evolved in the Tertiary period, and +man was formed from these towards its close. + +The Lemurs (Prosimiae) have few living representatives. But they are +very interesting, and are the last survivors of a once extensive +group. We find many fossil remains of them in the older Tertiary +deposits of Europe and North America, in the Eocene and Miocene. We +distinguish two sub-orders, the fossil Lemuravida and the modern +Lemurogona. The earliest and most primitive forms of the Lemuravida +are the Pachylemurs (Hypopsodina); they come next to the earliest +Placentals (Prochoriata), and have the typical full dentition, with +forty-four teeth (3.1.4.3. over 3.1.4.3.). The Necrolemurs (Adapida, +Figure 2.274) have only forty teeth, and have lost an incisor in each +jaw (2.1.4.3. over 2.1.4.3.). The dentition is still further reduced +in the Lemurogona (Autolemures), which usually have only thirty-six +teeth (2.1.3.3. over 2.1.3.3.). These living survivors are scattered +far over the southern part of the Old World. Most of the species live +in Madagascar, some in the Sunda Islands, others on the mainland of +Asia and Africa. They are gloomy and melancholic animals; they live a +quiet life, climbing trees, and eating fruit and insects. They are of +different kinds. Some are closely related to the Marsupials +(especially the opossum). Others (Macrotarsi) are nearer to the +Insectivora, others again (Chiromys) to the Rodents. Some of the +lemurs (Brachytarsi) approach closely to the true apes. The numerous +fossil remains of half-apes and apes that have been recently found in +the Tertiary deposits justify us in thinking that man's ancestors were +represented by several different species during this long period. Some +of these were almost as big as men, such as the diluvial lemurogonon +Megaladapis of Madagascar. + +(FIGURE 2.276. The white-nosed ape (Cercopithecus petaurista).) + +Next to the lemurs come the true apes (Simiae), the twenty-sixth stage +in our ancestry. It has been beyond question for some time now that +the apes approach nearest to man in every respect of all the animals. +Just as the lowest apes come close to the lemurs, so the highest come +next to man. When we carefully study the comparative anatomy of the +apes and man, we can trace a gradual and uninterrupted advance in the +organisation of the ape up to the purely human frame, and, after +impartial examination of the "ape problem" that has been discussed of +late years with such passionate interest, we come infallibly to the +important conclusion, first formulated by Huxley in 1863: "Whatever +systems of organs we take, the comparison of their modifications in +the series of apes leads to the same result: that the anatomic +differences that separate man from the gorilla and chimpanzee are not +as great as those that separate the gorilla from the lower apes." +Translated into phylogenetic language, this "pithecometra-law," +formulated in such masterly fashion by Huxley, is quite equivalent to +the popular saying: "Man is descended from the apes." + +(FIGURE 2.277. The drill-baboon (Cynocephalus leucophaeus) (From +Brehm.)) + +In the very first exposition of his profound natural classification +(1735) Linne placed the anthropoid mammals at the head of the animal +kingdom, with three genera: man, the ape, and the sloth. He afterwards +called them the "Primates"--the "lords" of the animal world; he then +also separated the lemur from the true ape, and rejected the sloth. +Later zoologists divided the order of Primates. First the Gottingen +anatomist, Blumenbach, founded a special order for man, which he +called Bimana ("two-handed"); in a second order he united the apes and +lemurs under the name of Quadrumana ("four-handed"); and a third order +was formed of the distantly-related Chiroptera (bats, etc.). The +separation of the Bimana and Quadrumana was retained by Cuvier and +most of the subsequent zoologists. It seems to be extremely important, +but, as a matter of fact, it is totally wrong. This was first shown in +1863 by Huxley, in his famous Man's Place in Nature. On the strength +of careful comparative anatomical research he proved that the apes are +just as truly "two-handed" as man; or, if we prefer to reverse it, +that man is as truly four-handed as the ape. He showed convincingly +that the ideas of hand and foot had been wrongly defined, and had been +improperly based on physiological instead of morphological grounds. +The circumstance that we oppose the thumb to the other four fingers in +our hand, and so can grasp things, seemed to be a special distinction +of the hand in contrast to the foot, in which the corresponding great +toe cannot be opposed in this way to the others. But the apes can +grasp with the hind-foot as well as the fore, and so were regarded as +quadrumanous. However, the inability to grasp that we find in the foot +of civilised man is a consequence of the habit of clothing it with +tight coverings for thousands of years. Many of the bare-footed lower +races of men, especially among the negroes, use the foot very freely +in the same way as the hand. As a result of early habit and continued +practice, they can grasp with the foot (in climbing trees, for +instance) just as well as with the hand. Even new-born infants of our +own race can grasp very strongly with the great toe, and hold a spoon +with it as firmly as with the hand. Hence the physiological +distinction between hand and foot can neither be pressed very far, nor +has it a scientific basis. We must look to morphological characters. + +As a matter of fact, it is possible to draw such a sharp morphological +distinction--a distinction based on anatomic structure--between the +fore and hind extremity. In the formation both of the bony skeleton +and of the muscles that are connected with the hand and foot before +and behind there are material and constant differences; and these are +found both in man and the ape. For instance, the number and +arrangement of the smaller bones of the hand and foot are quite +different. There are similar constant differences in the muscles. The +hind extremity always has three muscles (a short flexor muscle, a +short extensor muscle, and a long calf-muscle) that are not found in +the fore extremity. The arrangement of the muscles also is different +before and behind. These characteristic differences between the fore +and hind extremities are found in man as well as the ape. There can be +no doubt, therefore, that the ape's foot deserves that name just as +much as the human foot does, and that all true apes are just as +"bimanous" as man. The common distinction of the apes as +"quadrumanous" is altogether wrong morphologically. + +But it may be asked whether, quite apart from this, we can find any +other features that distinguish man more sharply from the ape than the +various species of apes are distinguished from each other. Huxley gave +so complete and demonstrative a reply to this question that the +opposition still raised on many sides is absolutely without +foundation. On the ground of careful comparative anatomical research, +Huxley proved that in all morphological respects the differences +between the highest and lowest apes are greater than the corresponding +differences between the highest apes and man. He thus restored Linne's +order of the Primates (excluding the bats), and divided it into three +sub-orders, the first composed of the half-apes (Lemuridae), the +second of the true apes (Simiadae), the third of men (Anthropidae). + +But, as we wish to proceed quite consistently and impartially on the +laws of systematic logic, we may, on the strength of Huxley's own law, +go a good deal farther in this division. We are justified in going at +least one important step farther, and assigning man his natural place +within one of the sections of the order of apes. All the features that +characterise this group of apes are found in man, and not found in the +other apes. We do not seem to be justified, therefore, in founding for +man a special order distinct from the apes. + +The order of the true apes (Simiae or Pitheca)--excluding the +lemurs--has long been divided into two principal groups, which also +differ in their geographical distribution. One group (Hesperopitheca, +or western apes) live in America. The other group, to which man +belongs, are the Eopitheca or eastern apes; they are found in Asia and +Africa, and were formerly in Europe. All the eastern apes agree with +man in the features that are chiefly used in zoological classification +to distinguish between the two simian groups, especially in the +dentition. The objection might be raised that the teeth are too +subordinate an organ physiologically for us to lay stress on them in +so important a question. But there is a good reason for it; it is with +perfect justice that zoologists have for more than a century paid +particular attention to the teeth in the systematic division and +arrangement of the orders of mammals. The number, form, and +arrangement of the teeth are much more faithfully inherited in the +various orders than most other characters. + +Hence the form of dentition in man is very important. In the fully +developed condition we have thirty-two teeth; of these eight are +incisors, four canine, and twenty molars. The eight incisors, in the +middle of the jaws, have certain characteristic differences above and +below. In the upper jaw the inner incisors are larger than the outer; +in the lower jaw the inner are the smaller. Next to these, at each +side of both jaws, is a canine (or "eye tooth"), which is larger than +the incisors. Sometimes it is very prominent in man, as it is in most +apes and many of the other mammals, and forms a sort of tusk. Next to +this there are five molars above and below on each side, the first two +of which (the "pre-molars") are small, have only one root, and are +included in the change of teeth; the three back ones are much larger, +have two roots, and only come with the second teeth. The apes of the +Old World, or all the living or fossil apes of Asia, Africa, and +Europe, have the same dentition as man. + +(FIGURES 2.278 TO 2.282. Skeletons of man and the four anthropoid +apes. (From Huxley.) Cf. Figures 1.203 to 1.209. + +FIGURE 2.278. Gibbon (Hylobates). + +FIGURE 2.279. Orang (Satyrus). + +FIGURE 2.280. Chimpanzee (Anthropithecus). + +FIGURE 2.281. Gorilla (Gorilla). + +FIGURE 2.282. Man (Homo).) + +On the other hand, all the American apes have an additional pre-molar +in each half of the jaw. They have six molars above and below on each +side, or thirty-six teeth altogether. This characteristic difference +between the eastern and western apes has been so faithfully inherited +that it is very instructive for us. It is true that there seems to be +an exception in the case of a small family of South American apes. The +small silky apes (Arctopitheca or Hapalidae), which include the +tamarin (Midas) and the brush-monkey (Jacchus), have only five molars +in each half of the jaw (instead of six), and so seem to be nearer to +the eastern apes. But it is found, on closer examination, that they +have three premolars, like all the western apes, and that only the +last molar has been lost. Hence the apparent exception really confirms +the above distinction. + +Of the other features in which the two groups of apes differ, the +structure of the nose is particularly instructive and conspicuous. All +the eastern apes have the same type of nose as man--a comparatively +narrow partition between the two halves, so that the nostrils run +downwards. In some of them the nose protrudes as far as in man, and +has the same characteristic structure. We have already alluded to the +curious long-nosed apes, which have a long, finely-curved nose. Most +of the eastern apes have, it is true, rather flat noses, like, for +instance, the white-nosed monkey (Figure 2.276); but the nasal +partition is thin and narrow in them all. The American apes have a +different type of nose. The partition is very broad and thick at the +bottom, and the wings of the nostrils are not developed, so that they +point outwards instead of downwards. This difference in the form of +the nose is so constantly inherited in both groups that the apes of +the New World are called "flat-nosed" (Platyrrhinae), and those of the +Old World "narrow-nosed" (Catarrhinae). The bony passage of the ear +(at the bottom of which is the tympanum) is short and wide in all the +Platyrrhines, but long and narrow in all the Catarrhines; and in man +this difference also is significant. + +This division of the apes into Platyrrhines and Catarrhines, on the +ground of the above hereditary features, is now generally admitted in +zoology, and receives strong support from the geographical +distribution of the two groups in the east and west. It follows at +once, as regards the phylogeny of the apes, that two divergent lines +proceeded from the common stem-form of the ape-order in the early +Tertiary period, one of which spread over the Old, the other over the +New, World. It is certain that all the Platyrrhines come of one stock, +and also all the Catarrhines; but the former are phylogenetically +older, and must be regarded as the stem-group of the latter. + +What can we deduce from this with regard to our own genealogy? Man has +just the same characters, the same form of dentition, auditory +passage, and nose, as all the Catarrhines; in this he radically +differs from the Platyrrhines. We are thus forced to assign him a +position among the eastern apes in the order of Primates, or at least +place him alongside of them. But it follows that man is a direct blood +relative of the apes of the Old World, and can be traced to a common +stem-form together with all the Catarrhines. In his whole organisation +and in his origin man is a true Catarrhine; he originated in the Old +World from an unknown, extinct group of the eastern apes. The apes of +the New World, or the Platyrrhines, form a divergent branch of our +genealogical tree, and this is only distantly related at its root to +the human race. We must assume, of course, that the earliest Eocene +apes had the full dentition of the Platyrrhines; hence we may regard +this stem-group as a special stage (the twenty-sixth) in our ancestry, +and deduce from it (as the twenty-seventh stage) the earliest +Catarrhines. + +We have now reduced the circle of our nearest relatives to the small +and comparatively scanty group that is represented by the sub-order of +the Catarrhines; and we are in a position to answer the question of +man's place in this sub-order, and say whether we can deduce anything +further from this position as to our immediate ancestors. In answering +this question the comprehensive and able studies that Huxley gives of +the comparative anatomy of man and the various Catarrhines in his +Man's Place in Nature are of great assistance to us. It is quite clear +from these that the differences between man and the highest +Catarrhines (gorilla, chimpanzee, and orang) are in every respect +slighter than the corresponding differences between the highest and +the lowest Catarrhines (white-nosed monkey, macaco, baboon, etc.). In +fact, within the small group of the tail-less anthropoid apes the +differences between the various genera are not less than the +differences between them and man. This is seen by a glance at the +skeletons that Huxley has put together (Figures 2.278 to 2.282). +Whether we take the skull or the vertebral column or the ribs or the +fore or hind limbs, or whether we extend the comparison to the +muscles, blood-vessels, brain, placenta, etc., we always reach the +same result on impartial examination--that man is not more different +from the other Catarrhines than the extreme forms of them (for +instance, the gorilla and baboon) differ from each other. We may now, +therefore, complete the Huxleian law we have already quoted with the +following thesis: "Whatever system of organs we take, a comparison of +their modifications in the series of Catarrhines always leads to the +same conclusion; the anatomic differences that separate man from the +most advanced Catarrhines (orang, gorilla, chimpanzee) are not as +great as those that separate the latter from the lowest Catarrhines +(white-nosed monkey, macaco, baboon)." + +We must, therefore, consider the descent of man from other Catarrhines +to be fully proved. Whatever further information on the comparative +anatomy and ontogeny of the living Catarrhines we may obtain in the +future, it cannot possibly disturb this conclusion. Naturally, our +Catarrhine ancestors must have passed through a long series of +different forms before the human type was produced. The chief advances +that effected this "creation of man," or his differentiation from the +nearest related Catarrhines, were: the adoption of the erect posture +and the consequent greater differentiation of the fore and hind limbs, +the evolution of articulate speech and its organ, the larynx, and the +further development of the brain and its function, the soul; sexual +selection had a great influence in this, as Darwin showed in his +famous work. + +With an eye to these advances we can distinguish at least four +important stages in our simian ancestry, which represent prominent +points in the historical process of the making of man. We may take, +after the Lemurs, the earliest and lowest Platyrrhines of South +America, with thirty-six teeth, as the twenty-sixth stage of our +genealogy; they were developed from the Lemurs by a peculiar +modification of the brain, teeth, nose, and fingers. From these Eocene +stem-apes were formed the earliest Catarrhines or eastern apes, with +the human dentition (thirty-two teeth), by modification of the nose, +lengthening of the bony channel of the ear, and the loss of four +pre-molars. These oldest stem-forms of the whole Catarrhine group were +still thickly coated with hair, and had long tails--baboons +(Cynopitheca) or tailed apes (Menocerca, Figure 2.276). They lived +during the Tertiary period, and are found fossilised in the Miocene. +Of the actual tailed apes perhaps the nearest to them are the +Semnopitheci. + +If we take these Semnopitheci as the twenty-seventh stage in our +ancestry, we may put next to them, as the twenty-eighth, the tail-less +anthropoid apes. This name is given to the most advanced and man-like +of the existing Catarrhines. They were developed from the other +Catarrhines by losing the tail and part of the hair, and by a higher +development of the brain, which found expression in the enormous +growth of the skull. Of this remarkable family there are only a few +genera to-day, and we have already dealt with them (Chapter 1.15)--the +gibbon (Hylobates, Figure 1.203) and orang (Satyrus, Figures 1.204 and +1.205) in South-Eastern Asia and the Archipelago; and the chimpanzee +(Anthropithecus, Figures 1.206 and 1.207) and gorilla (Gorilla, Figure +1.208) in Equatorial Africa. + +The great interest that every thoughtful man takes in these nearest +relatives of ours has found expression recently in a fairly large +literature. The most distinguished of these works for impartial +treatment of the question of affinity is Robert Hartmann's little work +on The Anthropoid Apes. Hartmann divides the primate order into two +families: (1) Primarii (man and the anthropoid apes); and (2) Simianae +(true apes, Catarrhines and Platyrrhines). Professor Klaatsch, of +Heidelberg, has advanced a different view in his interesting and +richly illustrated work on The Origin and Development of the Human +Race. This is a substantial supplement to my Anthropogeny, in so far +as it gives the chief results of modern research on the early history +of man and civilisation. But when Klaatsch declares the descent of man +from the apes to be "irrational, narrow-minded, and false," in the +belief that we are thinking of some living species of ape, we must +remind him that no competent scientist has ever held so narrow a view. +All of us look merely--in the sense of Lamarck and Darwin--to the +original unity (admitted by Klaatsch) of the primate stem. This common +descent of all the Primates (men, apes, and lemurs) from one primitive +stem-form, from which the most far-reaching conclusions follow for the +whole of anthropology and philosophy, is admitted by Klaatsch as well +as by myself and all other competent zoologists who accept the theory +of evolution in general. He says explicitly (page 172): "The three +anthropoid apes--gorilla, chimpanzee, and orang--seem to be branches +from a common root, and this was not far from that of the gibbon and +man." That is in the main the opinion that I have maintained +(especially against Virchow) in a number of works ever since 1866. The +hypothetical common ancestor of all the Primates, which must have +lived in the earliest Tertiary period (more probably in the +Cretaceous), was called by me Archiprimus, Klaatsch now calls it +Primatoid. Dubois has proposed the appropriate name of Prothylobates +for the common and much younger stem-form of the anthropomorpha (man +and the anthropoid apes). The actual Hylobates is nearer to it than +the other three existing anthropoids. None of these can be said to be +absolutely the most man-like. The gorilla comes next to man in the +structure of the hand and foot, the chimpanzee in the chief features +of the skull, the orang in brain development, and the gibbon in the +formation of the chest. None of these existing anthropoid apes is +among the direct ancestors of our race; they are scattered survivors +of an ancient branch of the Catarrhines, from which the human race +developed in a particular direction. + +(FIGURE 2.283. Skull of the fossil ape-man of Java (Pithecanthropus +erectus), restored by Eugen Dubois.) + +Although man is directly connected with this anthropoid family and +originates from it, we may assign an important intermediate form +between the Prothylobates and him (the twenty-ninth stage in our +ancestry), the ape-men (Pithecanthropi). I gave this name in the +History of Creation to the "speechless primitive men" (Alali), which +were men in the ordinary sense as far as the general structure is +concerned (especially in the differentiation of the limbs), but lacked +one of the chief human characteristics, articulate speech and the +higher intelligence that goes with it, and so had a less developed +brain. The phylogenetic hypothesis of the organisation of this +"ape-man" which I then advanced was brilliantly confirmed twenty-four +years afterwards by the famous discovery of the fossil Pithecanthropus +erectus by Eugen Dubois (then military surgeon in Java, afterwards +professor at Amsterdam). In 1892 he found at Trinil, in the residency +of Madiun in Java, in Pliocene deposits, certain remains of a large +and very man-like ape (roof of the skull, femur, and teeth), which he +described as "an erect ape-man" and a survivor of a "stem-form of man" +(Figure 2.283). Naturally, the Pithecanthropus excited the liveliest +interest, as the long-sought transitional form between man and the +ape: we seemed to have found "the missing link." There were very +interesting scientific discussions of it at the last three +International Congresses of Zoology (Leyden, 1895, Cambridge, 1898, +and Berlin, 1901). I took an active part in the discussion at +Cambridge, and may refer the reader to the paper I read there on "The +Present Position of Our Knowledge of the Origin of Man" (translated by +Dr. Gadow with the title of The Last Link). + +An extensive and valuable literature has grown up in the last ten +years on the Pithecanthropus and the pithecoid theory connected with +it. A number of distinguished anthropologists, anatomists, +paleontologists, and phylogenists have taken part in the controversy, +and made use of the important data furnished by the new science of +pre-historic research. Hermann Klaatsch has given a good summary of +them, with many fine illustrations, in the above-mentioned work. I +refer the reader to it as a valuable supplement to the present work, +especially as I cannot go any further here into these anthropological +and pre-historic questions. I will only repeat that I think he is +wrong in the attitude of hostility that he affects to take up with +regard to my own views on the descent of man from the apes. + +The most powerful opponent of the pithecoid theory--and the theory of +evolution in general--during the last thirty years (until his death in +September, 1902) was the famous Berlin anatomist, Rudolf Virchow. In +the speeches which he delivered every year at various congresses and +meetings on this question, he was never tired of attacking the hated +"ape theory." His constant categorical position was: "It is quite +certain that man does not descend from the ape or any other animal." +This has been repeated incessantly by opponents of the theory, +especially theologians and philosophers. In the inaugural speech that +he delivered in 1894 at the Anthropological Congress at Vienna, he +said that "man might just as well have descended from a sheep or an +elephant as from an ape." Absurd expressions like this only show that +the famous pathological anatomist, who did so much for medicine in the +establishment of cellular pathology, had not the requisite attainments +in comparative anatomy and ontogeny, systematic zoology and +paleontology, for sound judgment in the province of anthropology. The +Strassburg anatomist, Gustav Schwalbe, deserved great praise for +having the moral courage to oppose this dogmatic and ungrounded +teaching of Virchow, and showing its untenability. The recent +admirable works of Schwalbe on the Pithecanthropus, the earliest races +of men, and the Neanderthal skull (1897 to 1901) will supply any +candid and judicious reader with the empirical material with which he +can convince himself of the baselessness of the erroneous dogmas of +Virchow and his clerical friends (J. Ranke, J. Bumuller, etc.). + +As the Pithecanthropus walked erect, and his brain (judging from the +capacity of his skull, Figure 2.283) was midway between the lowest men +and the anthropoid apes, we must assume that the next great step in +the advance from the Pithecanthropus to man was the further +development of human speech and reason. + +Comparative philology has recently shown that human speech is +polyphyletic in origin; that we must distinguish several (probably +many) different primitive tongues that were developed independently. +The evolution of language also teaches us (both from its ontogeny in +the child and its phylogeny in the race) that human speech proper was +only gradually developed after the rest of the body had attained its +characteristic form. It is probable that language was not evolved +until after the dispersal of the various species and races of men, and +this probably took place at the commencement of the Quaternary or +Diluvial period. The speechless ape-men or Alali certainly existed +towards the end of the Tertiary period, during the Pliocene, possibly +even the Miocene, period. + +The third, and last, stage of our animal ancestry is the true or +speaking man (Homo), who was gradually evolved from the preceding +stage by the advance of animal language into articulate human speech. +As to the time and place of this real "creation of man" we can only +express tentative opinions. It was probably during the Diluvial period +in the hotter zone of the Old World, either on the mainland in +tropical Africa or Asia or on an earlier continent (Lemuria--now sunk +below the waves of the Indian Ocean), which stretched from East Africa +(Madagascar, Abyssinia) to East Asia (Sunda Islands, Further India). I +have given fully in my History of Creation, (chapter 28) the weighty +reasons for claiming this descent of man from the anthropoid eastern +apes, and shown how we may conceive the spread of the various races +from this "Paradise" over the whole earth. I have also dealt fully +with the relations of the various races and species of men to each +other. + +SYNOPSIS OF THE CHIEF SECTIONS OF OUR STEM-HISTORY. + +FIRST STAGE: THE PROTISTS. + +Man's ancestors are unicellular protozoa, originally unnucleated +Monera like the Chromacea, structureless green particles of plasm; +afterwards real nucleated cells (first plasmodomous Protophyta, like +the Palmella; then plasmophagous Protozoa, like the Amoeba). + +SECOND STAGE: THE BLASTAEADS. + +Man's ancestors are round coenobia or colonies of Protozoa; they +consist of a close association of many homogeneous cells, and thus are +individuals of the second order. They resemble the round +cell-communities of the Magospherae and Volvocina, equivalent to the +ontogenetic blastula: hollow globules, the wall of which consists of a +single layer of ciliated cells (blastoderm). + +THIRD STAGE: THE GASTRAEADS. + +Man's ancestors are Gastraeads, like the simplest of the actual +Metazoa (Prophysema, Olynthus, Hydra, Pemmatodiscus). Their body +consists merely of a primitive gut, the wall of which is made up of +the two primary germinal layers. + +FOURTH STAGE: THE PLATODES. + +Man's ancestors have substantially the organisation of simple Platodes +(at first like the cryptocoelic Platodaria, later like the +rhabdocoelic Turbellaria). The leaf-shaped bilateral-symmetrical body +has only one gut-opening, and develops the first trace of a nervous +centre from the ectoderm in the middle line of the back (Figures 2.239 +and 2.240). + +FIFTH STAGE: THE VERMALIA. + +Man's ancestors have substantially the organisation of unarticulated +Vermalia, at first Gastrotricha (Ichthydina), afterwards Frontonia +(Nemertina, Enteropneusta). Four secondary germinal layers develop, +two middle layers arising between the limiting layers (coeloma). The +dorsal ectoderm forms the vertical plate, acroganglion (Figure 2.243). + +SIXTH STAGE: THE PROCHORDONIA. + +Man's ancestors have substantially the organisation of a simple +unarticulated Chordonium (Copelata and Ascidia-larvae). The +unsegmented chorda develops between the dorsal medullary tube and the +ventral gut-tube. The simple coelom-pouches divide by a frontal septum +into two on each side; the dorsal pouch (episomite) forms a +muscle-plate; the ventral pouch (hyposomite) forms a gonad. Head-gut +with gill-clefts. + +SEVENTH STAGE: THE ACRANIA. + +Man's ancestors are skull-less Vertebrates, like the Amphioxus. The +body is a series of metamera, as several of the primitive segments are +developed. The head contains in the ventral half the branchial gut, +the trunk the hepatic gut. The medullary tube is still simple. No +skull, jaws, or limbs. + +EIGHTH STAGE: THE CYCLOSTOMA. + +Man's ancestors are jaw-less Craniotes (like the Myxinoida and +Petromyzonta). The number of metamera increases. The fore-end of the +medullary tube expands into a vesicle and forms the brain, which soon +divides into five cerebral vesicles. In the sides of it appear the +three higher sense-organs: nose, eyes, and auditory vesicles. No jaws, +limbs, or floating bladder. + +NINTH STAGE: THE ICHTHYODA. + +Man's ancestors are fish-like Craniotes: (1) Primitive fishes +(Selachii); (2) plated fishes (Ganoida); (3) amphibian fishes +(Dipneusta); (4) mailed amphibia (Stegocephala). The ancestors of this +series develop two pairs of limbs: a pair of fore (breast-fins) and of +hind (belly-fins) legs. The gill-arches are formed between the +gill-clefts: the first pair form the maxillary arches (the upper and +lower jaws). The floating bladder (lung) and pancreas grow out of the +gut. + +TENTH STAGE: THE AMNIOTES. + +Man's ancestors are Amniotes or gill-less Vertebrates: (1) Primitive +Amniotes (Proreptilia); (2) Sauromammals; (3) Primitive Mammals +(Monotremes); (4) Marsupials; (5) Lemurs (Prosimiae); (6) Western apes +(Platyrrhinae); (7) Eastern apes (Catarrhinae): at first tailed +Cynopitheca; then tail-less anthropoids; later speechless ape-men +(Alali); finally speaking man. The ancestors of these Amniotes develop +an amnion and allantois, and gradually assume the mammal, and finally +the specifically human, form. + + +CHAPTER 2.24. EVOLUTION OF THE NERVOUS SYSTEM. + +The previous chapters have taught us how the human body as a whole +develops from the first simple rudiment, a single layer of cells. The +whole human race owes its origin, like the individual man, to a simple +cell. The unicellular stem-form of the race is reproduced daily in the +unicellular embryonic stage of the individual. We have now to consider +in detail the evolution of the various parts that make up the human +frame. I must, naturally, confine myself to the most general and +principal outlines; to make a special study of the evolution of each +organ and tissue is both beyond the scope of this work, and probably +beyond the anatomic capacity of most of my readers to appreciate. In +tracing the evolution of the various organs we shall follow the method +that has hitherto guided us, except that we shall now have to consider +the ontogeny and phylogeny of the organs together. We have seen, in +studying the evolution of the body as a whole, that phylogeny casts a +light over the darker paths of ontogeny, and that we should be almost +unable to find our way in it without the aid of the former. We shall +have the same experience in the study of the organs in detail, and I +shall be compelled to give simultaneously their ontogenetic and +phylogenetic origin. The more we go into the details of organic +development, and the more closely we follow the rise of the various +parts, the more we see the inseparable connection of embryology and +stem-history. The ontogeny of the organs can only be understood in the +light of their phylogeny, just as we found of the embryology of the +whole body. Each embryonic form is determined by a corresponding +stem-form. This is true of details as well as of the whole. + +We will consider first the animal and then the vegetal systems of +organs of the body. The first group consists of the psychic and the +motor apparatus. To the former belong the skin, the nervous system, +and the sense-organs. The motor apparatus is composed of the passive +and the active organs of movement (the skeleton and the muscles). The +second or vegetal group consists of the nutritive and the reproductive +apparatus. To the nutritive apparatus belong the alimentary canal with +all its appendages, the vascular system, and the renal (kidney) +system. The reproductive apparatus comprises the different organs of +sex (embryonic glands, sexual ducts, and copulative organs). + +As we know from previous chapters (1.11 to 1.13), the animal systems +of organs (the organs of sensation and presentation) develop for the +most part out of the OUTER primary germ-layer, or the cutaneous (skin) +layer. On the other hand, the vegetal systems of organs arise for the +most part from the INNER primary germ-layer, the visceral layer. It is +true that this antithesis of the animal and vegetal spheres of the +body in man and all the higher animals is by no means rigid; several +parts of the animal apparatus (for instance, the greater part of the +muscles) are formed from cells that come originally from the entoderm; +and a great part of the vegetative apparatus (for instance, the +mouth-cavity and the gonoducts) are composed of cells that come from +the ectoderm. + +In the more advanced animal body there is so much interlacing and +displacement of the various parts that it is often very difficult to +indicate the sources of them. But, broadly speaking, we may take it as +a positive and important fact that in man and the higher animals the +chief part of the animal organs comes from the ectoderm, and the +greater part of the vegetative organs from the entoderm. It was for +this reason that Carl Ernst von Baer called the one the animal and the +other the vegetative layer (see Chapter 1.3). + +The solid foundation of this important thesis is the gastrula, the +most instructive embryonic form in the animal world, which we still +find in the same shape in the most diverse classes of animals. This +form points demonstrably to a common stem-form of all the Metazoa, the +Gastraea; in this long-extinct stem-form the whole body consisted +throughout life of the two primary germinal layers, as is now the case +temporarily in the gastrula; in the Gastraea the simple cutaneous +(skin) layer ACTUALLY represented all the animal organs and functions, +and the simple visceral (gut) layer all the vegetal organs and +functions. This is the case with the modern Gastraeads (Figure 2.233); +and it is also the case potentially with the gastrula. + +We shall easily see that the gastraea theory is thus able to throw a +good deal of light, both morphologically and physiologically, on some +of the chief features of embryonic development, if we take up first +the consideration of the chief element in the animal sphere, the +psychic apparatus or sensorium and its evolution. This apparatus +consists of two very different parts, which seem at first to have very +little connection with each other--the outer skin, with all its hairs, +nails, sweat-glands, etc., and the nervous system. The latter +comprises the central nervous system (brain and spinal cord), the +peripheral, cerebral, and spinal nerves, and the sense-organs. In the +fully-formed vertebrate body these two chief elements of the sensorium +lie far apart, the skin being external to, and the central nervous +system in the very centre of, the body. The one is only connected with +the other by a section of the peripheral nervous system and the +sense-organs. Nevertheless, as we know from human embryology, the +medullary tube is formed from the cutaneous layer. The organs that +discharge the most advanced functions of the animal body--the organs +of the soul, or of psychic life--develop from the external skin. This +is a perfectly natural and necessary process. If we reflect on the +historical evolution of the psychic and sensory functions, we are +forced to conclude that the cells which accomplish them must +originally have been located on the outer surface of the body. Only +elementary organs in this superficial position could directly receive +the influences of the environment. Afterwards, under the influence of +natural selection, the cellular group in the skin which was +specifically "sensitive" withdrew into the inner and more protected +part of the body, and formed there the foundation of a central nervous +organ. As a result of increased differentiation, the skin and the +central nervous system became further and further separated, and in +the end the two were only permanently connected by the afferent +peripheral sensory nerves. + +(FIGURE 2.284. The human skin in vertical section (from Ecker), highly +magnified, a horny layer of the epidermis, b mucous layer of the +epidermis, c papillae of the corium, d blood-vessels of same, ef ducts +of the sweat-glands (g), h fat-glands in the corium, i nerve, passing +into a tactile corpuscle above.) + +The observations of the comparative anatomist are in complete accord +with this view. He tells us that large numbers of the lower animals +have no nervous system, though they exercise the functions of +sensation and will like the higher animals. In the unicellular +Protozoa, which do not form germinal layers, there is, of course, +neither nervous system nor skin. But in the second division of the +animal kingdom also, the Metazoa, there is at first no nervous system. +Its functions are represented by the simple cell-layer of the +ectoderm, which the lower Metazoa have inherited from the Gastraea +(Figure 1.30 e). We find this in the lowest Zoophytes--the Gastraeads, +Physemaria, and Sponges (Figures 2.233 to 2.238). The lowest Cnidaria +(the hydroid polyps) also are little superior to the Gastraeads in +structure. Their vegetative functions are accomplished by the simple +visceral layer, and their animal functions by the simple cutaneous +layer. In these cases the simple cell-layer of the ectoderm is at once +skin, locomotive apparatus, and nervous system. + +(FIGURE 2.285. Epidermic cells of a human embryo of two months. (From +Kolliker.)) + +When we come to the higher Metazoa, in which the sensory functions and +their organs are more advanced, we find a division of labour among the +ectodermic cells. Groups of sensitive nerve cells separate from the +ordinary epidermic cells; they retire into the more protected tissue +of the mesodermic under-skin, and form special neural ganglia there. +Even in the Platodes, especially the Turbellaria, we find an +independent nervous system, which has separated from the outer skin. +This is the "upper pharyngeal ganglion," or acroganglion, situated +above the gullet (Figure 2.241 g). From this rudimentary structure has +been developed the elaborate central nervous system of the higher +animals. In some of the higher worms, such as the earth-worm, the +first rudiment of the central nervous system (Figure 1.74 n) is a +local thickening of the skin-sense layer (hs), which afterwards +separates altogether from the horny plate. In the earliest Platodes +(Cryptocoela) and Vermalia (Gastrotricha) the acroganglion remains in +the epidermis. But the medullary tube of the Vertebrates originates in +the same way. Our embryology has taught us that this first structure +of the central nervous system also develops originally from the outer +germinal layer. + +Let us now examine more closely the evolution of the human skin, with +its various appendages, the hairs and glands. This external covering +has, physiologically, a double and important part to play. It is, in +the first place, the common integument that covers the whole surface +of the body, and forms a protective envelope for the other organs. As +such it also effects a certain exchange of matter between the body and +the surrounding atmosphere (exhalation, perspiration). In the second +place, it is the earliest and original sense organ, the common organ +of feeling that experiences the sensation of the temperature of the +environment and the pressure or resistance of bodies that come into +contact. + +The human skin (like that of all the higher animals) is composed of +two layers, the outer and the inner or underlying skin. The outer skin +or epidermis, consists of simple ectodermic cells, and contains no +blood-vessels (Figure 2.284 a, b). It develops from the outer germinal +layer, or skin-sense layer. The underlying skin (corium or hypodermis) +consists chiefly of connective tissue, contains numerous blood-vessels +and nerves, and has a totally different origin. It comes from the +outermost parietal stratum of the middle germinal layer, or the +skin-fibre layer. The corium is much thicker than the epidermis. In +its deeper strata (the subcutis) there are clusters of fat-cells +(Figure 2.284 h). Its uppermost stratum (the cutis proper, or the +papillary stratum) forms, over almost the whole surface of the body, a +number of conical microscopic papillae (something like warts), which +push into the overlying epidermis (c). These tactile or sensory +particles contain the finest sensory organs of the skin, the touch +corpuscles. Others contain merely end-loops of the blood-vessels that +nourish the skin (c, d). The various parts of the corium arise by +division of labour from the originally homogeneous cells of the +cutis-plate, the outermost lamina of the mesodermic skin-fibre layer +(Figure 1.145 hpr, and Figures 1.161 and 1.162 cp). + +In the same way, all the parts and appendages of the epidermis develop +by differentiation from the homogeneous cells of this horny plate +(Figure 2.285). At an early stage the simple cellular layer of this +horny plate divides into two. The inner and softer stratum (Figure +2.284 b) is known as the mucous stratum, the outer and harder (a) as +the horny (corneous) stratum. This horny layer is being constantly +used up and rubbed away at the surface; new layers of cells grow up in +their place out of the underlying mucous stratum. At first the +epidermis is a simple covering of the surface of the body. Afterwards +various appendages develop from it, some internally, others +externally. The internal appendages are the cutaneous glands--sweat, +fat, etc. The external appendages are the hairs and nails. + +The cutaneous glands are originally merely solid cone-shaped growths +of the epidermis, which sink into the underlying corium (Figure 2.286 +1). Afterwards a canal (2, 3) is formed inside them, either by the +softening and dissolution of the central cells or by the secretion of +fluid internally. Some of the glands, such as the sudoriferous, do not +ramify (Figure 2.284 efg). These glands, which secrete the +perspiration, are very long, and have a spiral coil at the end, but +they never ramify; so also the wax-glands of the ears. Most of the +other cutaneous glands give out buds and ramify; thus, for instance, +the lachrymal glands of the upper eye-lid that secrete tears (Figure +2.286), and the sebaceous glands which secrete the fat in the skin and +generally open into the hair-follicles. Sudoriferous and sebaceous +glands are found only in mammals. But we find lachrymal glands in all +the three classes of Amniotes--reptiles, birds, and mammals. They are +wanting in the lower aquatic vertebrates. + +(FIGURE 2.286. Rudimentary lachrymal glands from a human embryo of +four months. (From Kolliker.) 1 earliest structure, in the shape of a +simple solid cone, 2 and 3 more advanced structures, ramifying and +hollowing out. a solid buds, e cellular coat of the hollow buds, f +structure of the fibrous envelope, which afterwards forms the corium +about the glands.) + +The mammary glands (Figures 2.287 and 2.288) are very remarkable; they +are found in all mammals, and in these alone. They secrete the milk +for the feeding of the new-born mammal. In spite of their unusual size +these structures are nothing more than large sebaceous glands in the +skin. The milk is formed by the liquefaction of the fatty milk-cells +inside the branching mammary-gland tubes (Figure 2.287 c), in the same +way as the skin-grease or hair-fat, by the solution of fatty cells +inside the sebaceous glands. The outlets of the mammary glands enlarge +and form sac-like mammary ducts (b); these narrow again (a), and open +in the teats or nipples of the breast by sixteen to twenty-four fine +apertures. The first structure of this large and elaborate gland is a +very simple cone in the epidermis, which penetrates into the corium +and ramifies. In the new-born infant it consists of twelve to eighteen +radiating lobes (Figure 2.288). These gradually ramify, their ducts +become hollow and larger, and rich masses of fat accumulate between +the lobes. Thus is formed the prominent female breast (mamma), on the +top of which rises the teat or nipple (mammilla). The latter is only +developed later on, when the mammary gland is fully-formed; and this +ontogenetic phenomenon is extremely interesting, because the earlier +mammals (the stem-forms of the whole class) have no teats. In them the +milk comes out through a flat portion of the ventral skin that is +pierced like a sieve, as we still find in the lowest living mammals, +the oviparous Monotremes of Australia. The young animal licks the milk +from the mother instead of sucking it. In many of the lower mammals we +find a number of milk-glands at different parts of the ventral +surface. In the human female there is usually only one pair of glands, +at the breast; and it is the same with the apes, bats, elephants, and +several other mammals. Sometimes, however, we find two successive +pairs of glands (or even more) in the human female. Some women have +four or five pairs of breasts, like pigs and hedgehogs (Figure 1.103). +This polymastism points back to an older stem-form. We often find +these accessory breasts in the male also (Figure 1.103 D). Sometimes, +moreover, the normal mammary glands are fully developed and can suckle +in the male; but as a rule they are merely rudimentary organs without +functions in the male. We have already (Chapter 1.11) dealt with this +remarkable and interesting instance of atavism. + +(FIGURE 2.287. The female breast (mamma) in vertical section. c +racemose glandular lobes, b enlarged milk-ducts, a narrower outlets, +which open into the nipple. (From H. Meyer.)) + +While the cutaneous glands are inner growths of the epidermis, the +appendages which we call hairs and nails are external local growths in +it. The nails (Ungues) which form important protective structures on +the back of the most sensitive parts of our limbs, the tips of the +fingers and toes, are horny growths of the epidermis, which we share +with the apes. The lower mammals usually have claws instead of them; +the ungulates, hoofs. The stem-form of the mammals certainly had +claws; we find them in a rudimentary form even in the salamander. The +horny claws are highly developed in most of the reptiles (Figure +2.264), and the mammals have inherited them from the earliest +representatives of this class, the stem-reptiles (Tocosauria). Like +the hoofs (ungulae) of the Ungulates, the nails of apes and men have +been evolved from the claws of the older mammals. In the human embryo +the first rudiment of the nails is found (between the horny and the +mucous stratum of the epidermis) in the fourth month. But their edges +do not penetrate through until the end of the sixth month. + +The most interesting and important appendages of the epidermis are the +hairs; on account of their peculiar composition and origin we must +regard them as highly characteristic of the whole mammalian class. It +is true that we also find hairs in many of the lower animals, such as +insects and worms. But these hairs, like the hairs of plants, are +thread-like appendages of the surface, and differ entirely from the +hairs of the mammals in the details of their structure and +development. + +The embryology of the hairs is known in all its details, but there are +two different views as to their phylogeny. On the older view the hairs +of the mammals are equivalent or homologous to the feathers of the +bird or the horny scales of the reptile. As we deduce all three +classes of Amniotes from a common stem-group, we must assume that +these Permian stem-reptiles had a complete scaly coat, inherited from +their Carboniferous ancestors, the mailed amphibia (Stegocephala); the +bony scales of their corium were covered with horny scales. In passing +from aquatic to terrestrial life the horny scales were further +developed, and the bony scales degenerated in most of the reptiles. As +regards the bird's feathers, it is certain that they are modifications +of the horny scales of their reptilian ancestors. But it is otherwise +with the hairs of the mammals. In their case the hypothesis has lately +been advanced on the strength of very extensive research, especially +by Friedrich Maurer, that they have been evolved from the cutaneous +sense-organs of amphibian ancestors by modification of functions; the +epidermic structure is very similar in both in its embryonic +rudiments. This modern view, which had the support of the greatest +expert on the vertebrates, Carl Gegenbaur, can be harmonised with the +older theory to an extent, in the sense that both formations, scales +and hairs, were very closely connected originally. Probably the +conical budding of the skin-sense layer grew up UNDER THE PROTECTION +OF THE HORNY SCALE, and became an organ of touch subsequently by the +cornification of the hairs; many hairs are still sensory organs +(tactile hairs on the muzzle and cheeks of many mammals: pubic hairs). + +This middle position of the genetic connection of scales and hairs was +advanced in my Systematic Phylogeny of the Vertebrates (page 433). It +is confirmed by the similar arrangement of the two cutaneous +formations. As Maurer pointed out, the hairs, as well as the cutaneous +sense-organs and the scales, are at first arranged in regular +longitudinal series, and they afterwards break into alternate groups. +In the embryo of a bear two inches long, which I owe to the kindness +of Herr von Schmertzing (of Arva Varallia, Hungary), the back is +covered with sixteen to twenty alternating longitudinal rows of scaly +protuberances (Figure 2.289). They are at the same time arranged in +regular transverse rows, which converge at an acute angle from both +sides towards the middle of the back. The tip of the scale-like wart +is turned inwards. Between these larger hard scales (or groups of +hairs) we find numbers of rudimentary smaller hairs. + +The human embryo is, as a rule, entirely clothed with a thick coat of +fine wool during the last three or four weeks of gestation. This +embryonic woollen coat (Lanugo) generally disappears in part during +the last weeks of foetal life but in any case, as a rule, it is lost +immediately after birth, and is replaced by the thinner coat of the +permanent hair. These permanent hairs grow out of hair-follicles, +which are formed from the root-sheaths of the disappearing +wool-fibres. The embryonic wool-coat usually, in the case of the human +embryo, covers the whole body, with the exception of the palms of the +hands and soles of the feet. These parts are always bare, as in the +case of apes and of most other mammals. Sometimes the wool-coat of the +embryo has a striking effect, by its colour, on the later permanent +hair-coat. Hence it happens occasionally, for instance, among our +Indo-Germanic races, that children of blond parents seem--to the +dismay of the latter--to be covered at birth with a dark brown or even +a black woolly coat. Not until this has disappeared do we see the +permanent blond hair which the child has inherited. Sometimes the +darker coat remains for weeks, and even months, after birth. This +remarkable woolly coat of the human embryo is a legacy from the apes, +our ancient long-haired ancestors. + +(FIGURE 2.288. Mammary gland of a new-born infant, a original central +gland, b small and c large buds of same. (From Langer.)) + +It is not less noteworthy that many of the higher apes approach man in +the thinness of the hair on various parts of the body. With most of +the apes, especially the higher Catarrhines (or narrow-nosed apes), +the face is mostly, or entirely, bare, or at least it has hair no +longer or thicker than that of man. In their case, too, the back of +the head is usually provided with a thicker growth of hair; this is +lacking, however, in the case of the bald-headed chimpanzee +(Anthropithecus calvus). The males of many species of apes have a +considerable beard on the cheeks and chin; this sign of the masculine +sex has been acquired by sexual selection. Many species of apes have a +very thin covering of hair on the breast and the upper side of the +limbs--much thinner than on the back or the under side of the limbs. +On the other hand, we are often astonished to find tufts of hair on +the shoulders, back, and extremities of members of our Indo-Germanic +and of the Semitic races. Exceptional hair on the face, as on the +whole body, is hereditary in certain families of hairy men. The +quantity and the quality of the hair on head and chin are also +conspicuously transmitted in families. These extraordinary variations +in the total and partial hairy coat of the body, which are so +noticeable, not only in comparing different races of men, but also in +comparing different families of the same race, can only be explained +on the assumption that in man the hairy coat is, on the whole, a +rudimentary organ, a useless inheritance from the more thickly-coated +apes. In this man resembles the elephant, rhinoceros, hippopotamus, +whale, and other mammals of various orders, which have also, almost +entirely or for the most part, lost their hairy coats by adaptation. + +(FIGURE 2.289. Embryo of a bear (Ursus arctos), twice natural size. A +seen from ventral side, B from the left.) + +The particular process of adaptation by which man lost the growth of +hair on most parts of his body, and retained or augmented it at some +points, was most probably sexual selection. As Darwin luminously +showed in his Descent of Man, sexual selection has been very active in +this respect. As the male anthropoid apes chose the females with the +least hair, and the females favoured the males with the finest growths +on chin and head, the general coating of the body gradually +degenerated, and the hair of the beard and head was more strongly +developed. The growth of hair at other parts of the body (arm-pit, +pubic region) was also probably due to sexual selection. Moreover, +changes of climate, or habits, and other adaptations unknown to us, +may have assisted the disappearance of the hairy coat. + +The fact that our coat of hair is inherited directly from the +anthropoid apes is proved in an interesting way, according to Darwin, +by the direction of the rudimentary hairs on our arms, which cannot be +explained in any other way. Both on the upper and the lower part of +the arm they point towards the elbow. Here they meet at an obtuse +angle. This curious arrangement is found only in the anthropoid +apes--gorilla, chimpanzee, orang, and several species of +gibbons--besides man (Figures 1.203 and 1.207). In other species of +gibbon the hairs are pointed towards the hand both in the upper and +lower arm, as in the rest of the mammals. We can easily explain this +remarkable peculiarity of the anthropoids and man on the theory that +our common ancestors were accustomed (as the anthropoid apes are +to-day) to place their hands over their heads, or across a branch +above their heads, during rain. In this position, the fact that the +hairs point downwards helps the rain to run off. Thus the direction of +the hair on the lower part of our arm reminds us to-day of that useful +custom of our anthropoid ancestors. + +The nervous system in man and all the other Vertebrates is, when fully +formed, an extremely complex apparatus, that we may compare, in +anatomic structure and physiological function, with an extensive +telegraphic system. The chief station of the system is the central +marrow or central nervous system, the innumerable ganglionic cells or +neurona (Figure 1.9) of which are connected by branching processes +with each other and with numbers of very fine conducting wires. The +latter are the peripheral and ubiquitous nerve-fibres; with their +terminal apparatus, the sense-organs, etc., they constitute the +conducting marrow or peripheral nervous system. Some of them--the +sensory nerve-fibres--conduct the impressions from the skin and other +sense-organs to the central marrow; others--the motor +nerve-fibres--convey the commands of the will to the muscles. + +The central nervous system or central marrow (medulla centralis) is +the real organ of psychic action in the narrower sense. However we +conceive the intimate connection of this organ and its functions, it +is certain that its characteristic actions, which we call sensation, +will, and thought, are inseparably dependent on the normal development +of the material organ in man and all the higher animals. We must, +therefore, pay particular attention to the evolution of the latter. As +it can give us most important information regarding the nature of the +"soul," it should be full of interest. If the central marrow develops +in just the same way in the human embryo as in the embryo of the other +mammals, the evolution of the human psychic organ from the central +organ of the other mammals, and through them from the lower +vertebrates, must be beyond question. No one can doubt the momentous +bearing of these embryonic phenomena. + +(FIGURE 2.290. Human embryo, three months old, natural size, from the +dorsal side: brain and spinal cord exposed. (From Kolliker.) h +cerebral hemispheres (fore brain), m corpora quadrigemina (middle +brain), c cerebellum (hind brain): under the latter is the triangular +medulla oblongata (after brain). + +FIGURE 2.291. Central marrow of a human embryo, four months old, +natural size, from the back. (From Kolliker.) h large hemispheres, v +quadrigemina, c cerebellum, mo medulla oblongata: underneath it the +spinal cord.) + +In order to understand them fully we must first say a word or two of +the general form and the anatomic composition of the mature human +central marrow. Like the central nervous system of all the other +Craniotes, it consists of two parts, the head-marrow or brain (medulla +capitis or encephalon) and the spinal-marrow (medulla spinalis or +notomyelon). The one is enclosed in the bony skull, the other in the +bony vertebral column. Twelve pairs of cerebral nerves proceed from +the brain, and thirty-one pairs of spinal nerves from the spinal cord, +to the rest of the body (Figure 1.171). On general anatomic +investigation the spinal marrow is found to be a cylindrical cord, +with a spindle-shaped bulb both in the region of the neck above (at +the last cervical vertebra) and the region of the loins (at the first +lumbar vertebra) below (Figure 2.291). At the cervical bulb the strong +nerves of the upper limbs, and at the lumbar bulb those of the lower +limbs, proceed from the spinal cord. Above, the latter passes into the +brain through the medulla oblongata (Figure 2.291 mo). The spinal cord +seems to be a thick mass of nervous matter, but it has a narrow canal +at its axis, which passes into the further cerebral ventricles above, +and is filled, like these, with a clear fluid. + +The brain is a large nerve-mass, occupying the greater part of the +skull, of most elaborate structure. On general examination it divides +into two parts, the cerebrum and cerebellum. The cerebrum lies in +front and above, and has the familiar characteristic convolutions and +furrows on its surface (Figures 2.292 and 2.293). On the upper side it +is divided by a deep longitudinal fissure into two halves, the +cerebral hemispheres; these are connected by the corpus callosum. The +large cerebrum is separated from the small cerebellum by a deep +transverse furrow. The latter lies behind and below, and has also +numbers of furrows, but much finer and more regular, with convolutions +between, at its surface. The cerebellum also is divided by a +longitudinal fissure into two halves, the "small hemispheres"; these +are connected by a worm-shaped piece, the vermis cerebelli, above, and +by the broad pons Varolii below (Figure 2.292 VI). + +(FIGURE 2.292. The human brain, seen from below. (From H. Meyer.) +Above (in front) is the cerebrum with its extensive branching furrows; +below (behind) the cerebellum with its narrow parallel furrows. The +Roman numbers I to XII indicate the roots of the twelve pairs of +cerebral nerves in a series towards the rear.) + +But comparative anatomy and ontogeny teach us that in man and all the +other Craniotes the brain is at first composed, not of these two, but +of three, and afterwards five, consecutive parts. These are found in +just the same form--as five consecutive vesicles--in the embryo of all +the Craniotes, from the Cyclostoma and fishes to man. But, however +much they agree in their rudimentary condition, they differ +considerably afterwards. In man and the higher mammals the first of +these ventricles, the cerebrum, grows so much that in its mature +condition it is by far the largest and heaviest part of the brain. To +it belong not only the large hemispheres, but also the corpus callosum +that unites them, the olfactory lobes, from which the olfactory nerves +start, and most of the structures that are found at the roof and +bottom of the large lateral ventricles inside the two hemispheres, +such as the corpora striata. On the other hand, the optic thalami, +which lie between the latter, belong to the second division, which +develops from the "intermediate brain "; to the same section belong +the single third cerebral ventricle and the structures that are known +as the corpora geniculata, the infundibulum, and the pineal gland. +Behind these parts we find, between the cerebrum and cerebellum, a +small ganglion composed of two prominences, which is called the corpus +quadrigeminum on account of a superficial transverse fissure cutting +across (Figures 2.290 m and 2.291 v). Although this quadrigeminum is +very insignificant in man and the higher mammals, it forms a special +third section, greatly developed in the lower vertebrates, the "middle +brain." The fourth section is the "hind-brain" or little brain +(cerebellum) in the narrower sense, with the single median part, the +vermis, and the pair of lateral parts, the "small hemispheres" (Figure +2.291 c). Finally, we have the fifth and last section, the medulla +oblongata (Figure 2.291 mo), which contains the single fourth cerebral +cavity and the contiguous parts (pyramids, olivary bodies, corpora +restiformia). The medulla oblongata passes straight into the medulla +spinalis (spinal cord). The narrow central canal of the spinal cord +continues above into the quadrangular fourth cerebral cavity of the +medulla oblongata, the floor of which is the quadrangular depression. +From here a narrow duct, called "the aqueduct of Sylvius," passes +through the corpus quadrigeminum to the third cerebral ventricle, +which lies between the two optic thalami; and this in turn is +connected with the pairs of lateral ventricles which lie to the right +and left in the large hemispheres. Thus all the cavities of the +central marrow are directly interconnected. All these parts of the +brain have an infinitely complex structure in detail, but we cannot go +into this. Although it is much more elaborate in man and the higher +Vertebrates than in the lower classes, it develops in them all from +the same rudimentary structure, the five simple cerebral vesicles of +the embryonic brain. + +But before we consider the development of the complicated structure of +the brain from this simple series of vesicles, let us glance for a +moment at the lower animals, which have no brain. Even in the +skull-less vertebrate, the Amphioxus, we find no independent brain, as +we have seen. The whole central marrow is merely a simple cylindrical +cord which runs the length of the body, and ends equally simply at +both extremities--a plain medullary tube. All that we can discover is +a small vesicular bulb at the foremost part of the tube, a degenerate +rudiment of a primitive brain. We meet the same simple medullary tube +in the first structure of the ascidia larva, in the same +characteristic position, above the chorda. On closer examination we +find here also a small vesicular swelling at the fore end of the tube, +the first trace of a differentiation of it into brain and spinal cord. +It is probable that this differentiation was more advanced in the +extinct Provertebrates, and the brain-bulb more pronounced (Figures +1.98 to 1.102). The brain is phylogenetically older than the spinal +cord, as the trunk was not developed until after the head. If we +consider the undeniable affinity of the Ascidiae to the Vermalia, and +remember that we can trace all the Chordonia to lower Vermalia, it +seems probable that the simple central marrow of the former is +equivalent to the simple nervous ganglion, which lies above the gullet +in the lower worms, and has long been known as the "upper pharyngeal +ganglion" (ganglion pharyngeum superius); it would be better to call +it the primitive or vertical brain (acroganglion). + +Probably this upper pharyngeal ganglion of the lower worms is the +structure from which the complex central marrow of the higher animals +has been evolved. The medullary tube of the Chordonia has been formed +by the lengthening of the vertical brain on the dorsal side. In all +the other animals the central nervous system has been developed in a +totally different way from the upper pharyngeal ganglion; in the +Articulates, especially, a pharyngeal ring, with ventral marrow, has +been added. The Molluscs also have a pharyngeal ring, but it is not +found in the Vertebrates. In these the central marrow has been +prolonged down the dorsal side; in the Articulates down the ventral +side. This fact proves of itself that there is no direct relationship +between the Vertebrates and the Articulates. The unfortunate attempts +to derive the dorsal marrow of the former from the ventral marrow of +the latter have totally failed (cf. Chapter 2.20). + +(FIGURE 2.293. The human brain, seen from the left. (From H. Meyer.) +The furrows of the cerebrum are indicated by thick, and those of the +cerebellum by finer lines. Under the latter we can see the medulla +oblongata. f1 to f2 frontal convolutions, C central convolutions, S +fissure of Sylvius, T temporal furrow, Pa parietal lobes, An angular +gyrus, Po parieto-occipital fissure.) + +When we examine the embryology of the human nervous system, we must +start from the important fact, which we have already seen, that the +first structure of it in man and all the higher Vertebrates is the +simple medullary tube, and that this separates from the outer germinal +layer in the middle line of the sole-shaped embryonic shield. As the +reader will remember, the straight medullary furrow first appears in +the middle of the sandal-shaped embryonic shield. At each side of it +the parallel borders curve over in the form of dorsal or medullary +swellings. These bend together with their free borders, and thus form +the closed medullary tube (Figures 1.133 to 1.137). At first this tube +lies directly underneath the horny plate; but it afterwards travels +inwards, the upper edges of the provertebral plates growing together +between the horny plate and the tube, joining above the latter, and +forming a completely closed canal. As Gegenbaur very properly +observes, "this gradual imbedding in the inner part of the body is a +process acquired with the progressive differentiation and the higher +potentiality that this secures; by this process the organ of greater +value to the organism is buried within the frame." (Cf. Figures 1.143 +to 1.146). + +(FIGURES 2.294 TO 2.296. Central marrow of the human embryo from the +seventh week, 4/5 inch long. (From Kolliker.) + +FIGURE 2.294. The brain from above, v fore brain, z intermediate +brain, m middle brain, h hind brain, n after brain. + +FIGURE 2.295. The brain with the uppermost part of the cord, from the +left. + +FIGURE 2.296. Back view of the whole embryo: brain and spinal cord +exposed.) + +In the Cyclostoma--a stage above the Acrania--the fore end of the +cylindrical medullary tube begins early to expand into a pear-shaped +vesicle; this is the first outline of an independent brain. In this +way the central marrow of the Vertebrates divides clearly into its two +chief sections, brain and spinal cord. The simple vesicular form of +the brain, which persists for some time in the Cyclostoma, is found +also at first in all the higher Vertebrates (Figure 1.153 hb). But in +these it soon passes away, the one vesicle being divided into several +successive parts by transverse constrictions. There are first two of +these constrictions, dividing the brain into three consecutive +vesicles (fore brain, middle brain, and hind brain, Figure 1.154 v, m, +h). Then the first and third are sub-divided by fresh constrictions, +and thus we get five successive sections (Figure 1.155). + +In all the Craniotes, from the Cyclostoma up to man, the same parts +develop from these five original cerebral vesicles, though in very +different ways. The first vesicle, the fore brain (Figure 1.155 v), +forms by far the largest part of the cerebrum--namely, the large +hemispheres, the olfactory lobes, the corpora striata, the callosum, +and the fornix. From the second vesicle, the intermediate brain (z), +originate especially the optic thalami, the other parts that surround +the third cerebral ventricle, and the infundibulum and pineal gland. +The third vesicle, the middle brain (m), produces the corpora +quadrigemina and the aqueduct of Sylvius. From the fourth vesicle, the +hind brain (h), develops the greater part of the cerebellum--namely, +the vermis and the two small hemispheres. Finally, the fifth vesicle, +the after brain (n), forms the medulla oblongata, with the +quadrangular pit (the floor of the fourth ventricle), the pyramids, +olivary bodies, etc. + +We must certainly regard it as a comparative-anatomical and +ontogenetic fact of the greatest significance that in all the +Craniotes, from the lowest Cyclostomes and fishes up to the apes and +man, the brain develops in just the same way in the embryo. The first +rudiment of it is always a simple vesicular enlargement of the fore +end of the medullary tube. In every case, first three, then five, +vesicles develop from this bulb, and the permanent brain with all its +complex anatomic structures, of so great a variety in the various +classes of Vertebrates, is formed from the five primitive vesicles. +When we compare the mature brain of a fish, an amphibian, a reptile, a +bird, and a mammal, it seems incredible that we can trace the various +parts of these organs, that differ so much internally and externally, +to common types. Yet all these different Craniote brains have started +with the same rudimentary structure. To convince ourselves of this we +have only to compare the corresponding stages of development of the +embryos of these different animals. + +(FIGURE 2.297. Head of a chick embryo (hatched fifty-eight hours), +from the back, magnified forty times. (From Mihalkovics.) vw anterior +wall of the fore brain. vh its ventricle. au optic vesicles, mh middle +brain, kh hind brain, nh after brain, hz heart (seen from below), vw +vitelline veins, us primitive segment, rm spinal cord.) + +This comparison is extremely instructive. If we extend it through the +whole series of the Craniotes, we soon discover this interesting fact: +In the Cyclostomes (the Myxinoida and Petromyzonta), which we have +recognised as the lowest and earliest Craniotes, the whole brain +remains throughout life at a very low stage, which is very brief and +passing in the embryos of the higher Craniotes; they retain the five +original sections of the brain unchanged. In the fishes we find an +essential and considerable modification of the five vesicles; it is +clearly the brain of the Selachii in the first place, and subsequently +the brain of the Ganoids, from which the brain of the rest of the +fishes on the one hand and of the Dipneusts and Amphibia, and through +these of the higher Vertebrates, on the other hand, must be derived. +In the fishes and Amphibia (Figure 2.300) there is a preponderant +development of the middle brain, and also the after brain, the first, +second, and fourth sections remaining very primitive. It is just the +reverse in the higher Vertebrates, in which the first and third +sections, the cerebrum and cerebellum, are exceptionally developed; +while the middle brain and after brain remain small. The corpora +quadrigemina are mostly covered by the cerebrum, and the oblongata by +the cerebellum. But we find a number of stages of development within +the higher Vertebrates themselves. From the Amphibia upwards the brain +(and with it the psychic life) develops in two different directions; +one of these is followed by the reptiles and birds, and the other by +the mammals. The development of the first section, the fore brain, is +particularly characteristic of the mammals. It is only in them that +the cerebrum becomes so large as to cover all the other parts of the +brain (Figures 2.293 and 2.301 to 2.304). + +There are also notable variations in the relative position of the +cerebral vesicles. In the lower Craniotes they lie originally almost +in the same plane. When we examine the brain laterally, we can cut +through all five vesicles with a straight line. But in the Amniotes +there is a considerable curve in the brain along with the bending of +the head and neck; the whole of the upper dorsal surface of the brain +develops much more than the under ventral surface. This causes a +curve, so that the parts come to lie as follows: The fore brain is +right in front and below, the intermediate brain a little higher, and +the middle brain highest of all; the hind brain lies a little lower, +and the after brain lower still. We find this only in the +Amniotes--the reptiles, birds, and mammals. + +(FIGURE 2.298. Brain of three craniote embryos in vertical section. A +of a shark (Heptarchus), B of a serpent (Coluber), C of a goat +(Capra). a fore brain, b intermediate brain, c middle brain, d hind +brain, e after brain, s primitive cleft. (From Gegenbaur.) + +FIGURE 2.299. Brain of a shark (Scyllium), back view. g fore-brain, h +olfactory lobes, which send the large olfactory nerves to the nasal +capsule (o), d intermediate brain, b middle brain; behind this the +insignificant structure of the hind brain, a after brain. (From +Gegenbaur.) + +FIGURE 2.300. Brain and spinal cord of the frog. A from the dorsal, B +from the ventral side. a olfactory lobes before the (b) fore brain, i +infundibulum at the base of the intermediate brain, c middle brain, d +hind brain, s quadrangular pit in the after brain, m spinal cord (very +short in the frog), m apostrophe roots of the spinal nerves, t +terminal fibres of the spinal cord. (From Gegenbaur.) + +FIGURE 2.301. Brain of an ox-embryo, two inches in length. (From +Mihalkovics, magnified three times.) Left view; the lateral wall of +the left hemisphere has been removed, st corpora striata, ml +Monro-foramen, ag arterial plexus, ah Ammon's horn, mh middle brain, +kh cerebellum. dv roof of the fourth ventricle, bb pons Varolii, na +medulla oblongata.) + +Thus, while the brain of the mammals agrees a good deal in general +growth with that of the birds and reptiles, there are some striking +differences between the two. In the Sauropsids (birds and reptiles) +the middle brain and the middle part of the hind brain are well +developed. In the mammals these parts do not grow, and the fore-brain +develops so much that it overlies the other vesicles. As it continues +to grow towards the rear, it at last covers the whole of the rest of +the brain, and also encloses the middle parts from the sides (Figures +2.301 to 2.303). This process is of great importance, because the fore +brain is the organ of the higher psychic life, and in it those +functions of the nerve-cells are discharged which we sum up in the +word "soul." The highest achievements of the animal body--the +wonderful manifestations of consciousness and the complex molecular +processes of thought--have their seat in the fore brain. We can remove +the large hemispheres, piece by piece, from the mammal without killing +it, and we then see how the higher functions of consciousness, +thought, will, and sensation, are gradually destroyed, and in the end +completely extinguished. If the animal is fed artificially, it may be +kept alive for a long time, as the destruction of the psychic organs +by no means involves the extinction of the faculties of digestion, +respiration, circulation, urination--in a word, the vegetative +functions. It is only conscious sensation, voluntary movement, +thought, and the combination of various higher psychic functions that +are affected. + +(FIGURE 2.302. Brain of a human embryo, twelve weeks old. (From +Mihalkovics, natural size.) Seen from behind and above. ms +mantle-furrow, mh corpora quadrigemina (middle brain), vs anterior +medullary ala, kh cerebellum, vv fourth ventricle, na medulla +oblongata.) + +The fore brain, the organ of these functions, only attains this high +level of development in the more advanced Placentals, and thus we have +the simple explanation of the intellectual superiority of the higher +mammals. The soul of most of the lower Placentals is not much above +that of the reptiles, but among the higher Placentals we find an +uninterrupted gradation of mental power up to the apes and man. In +harmony with this we find an astonishing variation in the degree of +development of their fore brain, not only qualitatively, but also +quantitatively. The mass and weight of the brain are much greater in +modern mammals, and the differentiation of its various parts more +important, than in their extinct Tertiary ancestors. This can be shown +paleontologically in any particular order. The brains of the living +ungulates are (relatively to the size of the body) four to six times +(in the highest groups even eight times) as large as those of their +earlier Tertiary ancestors, the well-preserved skulls of which enable +us to determine the size and weight of the brain. + +(FIGURE 2.303. Brain of a human embryo, twenty-four weeks old, halved +in the median plane: right hemisphere seen from inside. (From +Mihalkovics, natural size.) rn olfactory nerve. tr funnel of the +intermediate brain, vc anterior commissure, ml Monro-foramen, gw +fornix, ds transparent sheath, bl corpus callosum, br fissure at its +border, hs occipital fissure, zh cuneus, sf occipital transverse +fissure, zb pineal gland, mh corpora quadrigemina, kh cerebellum. + +In the lower mammals the surface of the cerebral hemispheres is quite +smooth and level, as in the rabbit (Figure 2.304). Moreover, the fore +brain remains so small that it does not cover the middle brain. At a +stage higher the middle brain is covered, but the hind brain remains +free. Finally, in the apes and man, the latter also is covered by the +fore brain. We can trace a similar gradual development in the fissures +and convolutions that are found on the surface of the cerebrum of the +higher mammals (Figures 2.292 and 2.293). If we compare different +groups of mammals in regard to these fissures and convolutions, we +find that their development proceeds step by step with the advance of +mental life. + +Of late years great attention has been paid to this special branch of +cerebral anatomy, and very striking individual differences have been +detected within the limits of the human race. In all human beings of +special gifts and high intelligence the convolutions and fissures are +much more developed than in the average man; and they are more +developed in the latter than in idiots and others of low mental +capacity. There is a similar gradation among the mammals in the +internal structure of the fore brain. In particular the corpus +callosum, that unites the two cerebral hemispheres, is only developed +in the Placentals. Other structures--for instance, in the lateral +ventricles--that seem at first to be peculiar to man, are also found +in the higher apes, and these alone. It was long thought that man had +certain distinctive organs in his cerebrum which were not found in any +other animal. But careful examination has discovered that this is not +the case, but that the characteristic features of the human brain are +found in a rudimentary form in the lower apes, and are more or less +fully developed in the higher apes. Huxley has convincingly shown, in +his Man's Place in Nature (1863), that the differences in the +formation of the brain within the ape-group constitute a deeper gulf +between the lower and higher apes than between the higher apes and +man. + +The comparative anatomy and physiology of the brain of the higher and +lower mammals are very instructive, and give important information in +connection with the chief questions of psychology. + +(FIGURE 2.304. Brain of the rabbit. A from the dorsal, B from the +ventral side, lo olfactory lobes, I fore brain, h hypophysis at the +base of the intermediate brain, III middle brain, IV hind brain, V +after brain, 2 optic nerve, 3 oculo-motor nerve, 5 to 8 cerebral +nerves. In A the roof of the right hemisphere (I) is removed, so that +we can see the corpora striata in the lateral ventricle. (From +Gegenbaur.)) + +The central marrow (brain and spinal cord) develops from the medullary +tube in man just as in all the other mammals, and the same applies to +the conducting marrow or "peripheral nervous system." It consists of +the SENSORY nerves, which conduct centripetally the impressions from +the skin and the sense-organs to the central marrow, and of the MOTOR +nerves, which convey centrifugally the movements of the will from the +central marrow to the muscles. All these peripheral nerves grow out of +the medullary tube (Figure 1.171), and are, like it, products of the +skin-sense layer. + +The complete agreement in the structure and development of the psychic +organs which we find between man and the highest mammals, and which +can only be explained by their common origin, is of profound +importance in the monistic psychology. This is only seen in its full +light when we compare these morphological facts with the corresponding +physiological phenomena, and remember that every psychic action +requires the complete and normal condition of the correlative brain +structure for its full and normal exercise. The very complex molecular +movements inside the neural cells, which we describe comprehensively +as "the life of the soul," can no more exist in the vertebrate, and +therefore in man, without their organs than the circulation without +the heart and blood. And as the central marrow develops in man from +the same medullary tube as that of the other vertebrates, and as man +shares the characteristic structure of his cerebrum (the organ of +thought) with the anthropoid apes, his psychic life also must have the +same origin as theirs. + +If we appreciate the full weight of these morphological and +physiological facts, and put a proper phylogenetic interpretation on +the observations of embryology, we see that the older idea of the +personal immortality of the human soul is scientifically untenable. +Death puts an end, in man as in any other vertebrate, to the +physiological function of the cerebral neurona, the countless +microscopic ganglionic cells, the collective activity of which is +known as "the soul." I have shown this fully in the eleventh chapter +of my Riddle of the Universe. + + +CHAPTER 2.25. EVOLUTION OF THE SENSE-ORGANS. + +The sense-organs are indubitably among the most important and +interesting parts of the human body; they are the organs by means of +which we obtain our knowledge of objects in the surrounding world. +Nihil est in intellectu quod non prius fuerit in sensu. They are the +first sources of the life of the soul. There is no other part of the +body in which we discover such elaborate anatomical structures, +co-operating with a definite purpose; and there is no other organ in +which the wonderful and purposive structure seems so clearly to compel +us to admit a Creator and a preconceived plan. Hence we find special +efforts made by dualists to draw our attention here to the "wisdom of +the Creator" and the design visible in his works. As a matter of fact, +you will discover, on mature reflection, that on this theory the +Creator is at bottom only playing the part of a clever mechanic or +watch-maker; all these familiar teleological ideas of Creator and +creation are based, in the long run, on a similar childlike +anthropomorphism. + +However, we must grant that at the first glance the teleological +theory seems to give the simplest and most satisfactory explanation of +these purposive structures. If we merely examine the structure and +functions of the most advanced sense-organs, it seems impossible to +explain them without postulating a creative act. Yet evolution shows +us quite clearly that this popular idea is totally wrong. With its +assistance we discover that the purposive and remarkable sense-organs +were developed, like all other organs, without any preconceived +design--developed by the same mechanical process of natural selection, +the same constant correlation of adaptation and heredity, by which the +other purposive structures in the animal frame were slowly and +gradually brought forth in the struggle for life. + +Like most other Vertebrates, man has six sensory organs, which serve +for eight different classes of sensations. The skin serves for +sensations of pressure and temperature. This is the oldest, lowest, +and vaguest of the sense-organs; it is distributed over the surface of +the body. The other sensory activities are localised. The sexual sense +is bound up with the skin of the external sexual organs, the sense of +taste with the mucous lining of the mouth (tongue and palate), and the +sense of smell with the mucous lining of the nasal cavity. For the two +most advanced and most highly differentiated sensory functions there +are special and very elaborate mechanical structures--the eye for the +sense of sight, and the ear for the sense of hearing and space +(equilibrium). + +Comparative anatomy and physiology teach us that there are no +differentiated sense-organs in the lower animals; all their sensations +are received by the surface of the skin. The undifferentiated +skin-layer or ectoderm of the Gastraea is the simple stratum of cells +from which the differentiated sense-organs of all the Metazoa +(including the Vertebrates) have been evolved. Starting from the +assumption that necessarily only the superficial parts of the body, +which are in direct touch with the outer world, could be concerned in +the origin of sensations, we can see at once that the sense-organs +also must have arisen there. This is really the case. The chief part +of all the sense-organs originates from the skin-sense layer, partly +directly from the horny plate, partly from the brain, the foremost +part, of the medullary tube, after it has separated from the horny +plate. If we compare the embryonic development of the various +sense-organs, we see that they all make their appearance in the +simplest conceivable form; the wonderful contrivances that make the +higher sense-organs among the most remarkable and elaborate structures +in the body develop only gradually. In the phylogenetic explanation of +them comparative anatomy and ontogeny achieve their greatest triumphs. +But at first all the sense-organs are merely parts of the skin in +which sensory nerves expand. These nerves themselves were originally +of a homogeneous character. The different functions or specific +energies of the differentiated sense-nerves were only gradually +developed by division of labour. At the same time, their simple +terminal expansions in the skin were converted into extremely complex +organs. + +The great instructiveness of these historical facts in connection with +the life of the soul is not difficult to see. The whole philosophy of +the future will be transformed as soon as psychology takes cognisance +of these genetic phenomena and makes them the basis of its +speculations. When we examine impartially the manuals of psychology +that have been published by the most distinguished speculative +philosophers and are still widely distributed, we are astonished at +the naivete with which the authors raise their airy metaphysical +speculations, regardless of the momentous embryological facts that +completely refute them. Yet the science of evolution, in conjunction +with the great advance of the comparative anatomy and physiology of +the sense-organs, provides the one sound empirical basis of a natural +psychology. + +(FIGURE 2.305. Head of a shark (Scyllium), from the ventral side. m +mouth, o olfactory pits, r nasal groove, n nasal fold in natural +position, n apostrophe nasal fold drawn up. (The dots are openings of +the mucous canals.) (From Gegenbaur.)) + +In respect of the terminal expansions of the sensory nerves, we can +distribute the human sense-organs in three groups, which correspond to +three stages of development. The first group comprises those organs +the nerves of which spread out quite simply in the free surface of the +skin itself (organs of the sense of pressure, warmth, and sex). In the +second group the nerves spread out in the mucous coat of cavities +which are at first depressions in or invaginations of the skin (organs +of the sense of smell and taste). The third group is formed of the +very elaborate organs, the nerves of which spread out in an internal +vesicle, separated from the skin (organs of the sense of sight, +hearing, and space). + +(FIGURES 2.306 AND 2.307. Head of a chick embryo, three days old: +2.306 front view, 2.307 from the right. n rudimentary nose (olfactory +pits), l rudimentary eyes (optic pits), g rudimentary ear (auscultory +pit), v fore brain, gl eye-cleft, o process of upper jaw, u process of +lower jaw of the first gill-arch. + +FIGURE 2.308. Head of a chick embryo, four days old, from below. n +nasal pit, o upper-jaw process of the first gill-arch, u lower-jaw +process of same, k double apostrophe second gill-arch, sp choroid +fissure of eye, s gullet. + +FIGURES 2.309 AND 2.310. Heads of chick embryos: 2.309 from the end of +the fourth, 2.310 from the beginning of the fifth week. Letters as in +Figure 2.308, except: in inner, an outer, nasal process, nf nasal +furrow, st frontal process, m mouth. (From Kolliker.) Figures 2.306 to +2.310 are magnified to the same extent.) + +There is little to be said of the development of the lower +sense-organs. We have already considered (Chapter 2.24) the organ of +touch and temperature in the skin. I need only add that in the corium +of man and all the higher Vertebrates countless microscopic +sense-organs develop, but the precise relation of these to the +sensations of pressure or resistance, of warmth and cold, has not yet +been explained. Organs of this kind, in or on which sensory cutaneous +nerves terminate, are the "tactile corpuscles" (or the Pacinian +corpuscles) and end-bulbs. We find similar corpuscles in the organs of +the sexual sense, the male penis and the female clitoris; they are +processes of the skin, the development of which we will consider later +(together with the rest of the sexual parts, Chapter 2.29). The +evolution of the organ of taste, the tongue and palate, will also be +treated later, together with that of the alimentary canal to which +these parts belong (Chapter 2.27). I will only point out for the +present that the mucous coat of the tongue and palate, in which the +gustatory nerve ends, originates from a part of the outer skin. As we +have seen, the whole of the mouth-cavity is formed, not as a part of +the gut-tube proper, but as a pit-like fold in the outer skin (Chapter +1.13). Its mucous lining is therefore formed, not from the visceral, +but from the cutaneous layer, and the taste-cells at the surface of +the tongue and palate are not products of the gut-fibre layer, but of +the skin-sense layer. + +This applies also to the mucous lining of the olfactory organ, the +nose. However, the development of this organ is much more interesting. +Although the nose seems superficially to be simple and single, it +really consists, in man and all other Gnathostomes, of two completely +separated halves, the right and left cavities. They are divided by a +vertical partition, so that the right nostril leads into the right +cavity alone and the left nostril into the left cavity. They open +internally (and separately) by the posterior nasal apertures into the +pharynx, so that we can get direct into the gullet through the nasal +passages without touching the mouth. This is the way the air usually +passes in respiration; the mouth being closed, it goes through the +nose into the gullet, and through the larynx and bronchial tubes into +the lungs. The nasal cavities are separated from the mouth by the +horizontal bony palate, to which is attached behind (as a dependent +process) the soft palate with the uvula. In the upper and hinder parts +of the nasal cavities the olfactory nerve, the first pair of cerebral +nerves, expands in the mucous coat which clothes them. The terminal +branches of it spread partly over the septum (partition), partly on +the side walls of the internal cavities, to which are attached the +turbinated bones. These bones are much more developed in many of the +higher mammals than in man, but there are three of them in all +mammals. The sensation of smell arises by the passage of a current of +air containing odorous matter over the mucous lining of the cavities, +and stimulating the olfactory cells of the nerve-endings. + +Man has all the features which distinguish the olfactory organ of the +mammals from that of the lower Vertebrates. In all essential points +the human nose entirely resembles that of the Catarrhine apes, some of +which have quite a human external nose (compare the face of the +long-nosed apes). However, the first structure of the olfactory organ +in the human embryo gives no indication of the future ample +proportions of our catarrhine nose. It has the form in which we find +it permanently in the fishes--a couple of simple depressions in the +skin at the outer surface of the head. We find these blind olfactory +pits in all the fishes; sometimes they lie near the eyes, sometimes +more forward at the point of the muzzle, sometimes lower down, near +the mouth (Figure 2.249). + +(FIGURE 2.311. Frontal section of the mouth and throat of a human +embryo, neck half-inch long. "Invented" by Wilhelm His. The vertical +section (in the frontal plane, from left to right) is so constructed +that we see the nasal pits in the upper third of the figure and the +eyes at the sides: in the middle third the primitive gullet with the +gill-clefts (gill-arches in section); in the lower third the pectoral +cavity with the bronchial tubes and the rudimentary lungs.) + +This first rudimentary structure of the double nose is the same in all +the Gnathostomes; it has no connection with the primitive mouth. But +even in a section of the fishes a connection of this kind begins to +make its appearance, a furrow in the surface of the skin running from +each side of the nasal pit to the nearest corner of the mouth. This +furrow, the nasal groove or furrow (Figure 2.305 r), is very +important. In many of the sharks, such as the Scyllium, a special +process of the frontal skin, the nasal fold or internal nasal process, +is formed internally over the groove (n, n apostrophe). In contrast to +this the outer edge of the furrow rises in an "external nasal +process." As the two processes meet and coalesce over the nasal groove +in the Dipneusts and Amphibia, it is converted into a canal, the nasal +canal. Henceforth we can penetrate from the external pits through the +nasal canals direct into the mouth, which has been formed quite +independently. In the Dipneusts and the lower Amphibia the internal +aperture of the nasal canals lies in front (behind the lips); in the +higher Amphibia it is right behind. Finally, in the three higher +classes of Vertebrates the primary mouth-cavity is divided by the +formation of the horizontal palate-roof into two distinct +cavities--the upper (secondary) nasal cavity and the lower (secondary) +mouth-cavity. The nasal cavity in turn is divided by the construction +of the vertical septum into two halves--right and left. + +(FIGURE 2.312. Diagrammatic section of the mouth-nose cavity. While +the palate-plates (p) divide the original mouth-cavity into the lower +secondary mouth (m) and the upper nasal cavity, the latter in turn is +divided by the vertical partition (e) into two halves (n, n). (From +Gegenbaur.)) + +Comparative anatomy shows us to-day, in the series of the double-nosed +Vertebrates, from the fishes up to man, all the different stages in +the development of the nose, which the advanced olfactory organ of the +higher mammals has passed through at various periods in the course of +its phylogeny. It first appears in the embryo of man and the higher +Vertebrates, in which the double fish-nose persists throughout life. +At an early stage, before there is any trace of the characteristic +human face, a pair of small pits are formed in the head over the +original mouth-cavity; these were first discovered by Baer, and +rightly called the "olfactory pits" (Figures 2.306 n and 2.307 n). +These primitive nasal pits are quite separate from the rudimentary +mouth, which also originates as a pit-like depression in the skin, in +front of the blind fore end of the gut. Both the pair of nasal pits +and the single mouth-pit (Figure 2.310 m) are clothed with the horny +plate. The original separation of the former from the latter is, +however, presently abolished, a process forming above the +mouth-pit--the "frontal process" (Figure 2.309 st). Its outer edge +rises to the right and left in the shape of two lateral processes; +these are the inner nasal processes or folds (in). Opposite to these a +parallel ridge is formed on either side between the eye and the nasal +pit; these are the outer nasal processes (an). Thus between the inner +and outer nasal processes a groove-like depression is formed on either +side, which leads from the nasal pit towards the mouth-pit (m); this +groove is, as the reader will guess, the same nasal furrow or groove +that we have already seen in the shark (Figure 2.305 r). As the +parallel edges of the inner and outer nasal processes bend towards +each other and join above the nasal groove, this is converted into a +tube, the primitive nasal canal. Hence the nose of man and all the +other Amniotes consists at this embryonic stage of a couple of narrow +tubes, the nasal canals, which lead from the outer surface of the +forehead into the rudimentary mouth. This transitory condition +resembles that in which we find the nose permanently in the Dipneusts +and Amphibia. + +A cone-shaped structure, which grows from below towards the lower ends +of the two nasal processes and joins with them, plays an important +part in the conversion of the open nasal groove into the closed canal. +This is the upper-jaw process (Figures 2.306 to 2.310 o). Below the +mouth-pit are the gill-arches, which are separated by the gill-clefts. +The first of these gill-arches, and the most important for our +purpose, which we may call the maxillary (jaw) arch, forms the +skeleton of the jaws. Above at the basis a small process grows out of +this first gill-arch; this is the upper-jaw process. The first +gill-arch itself develops a cartilage at one of its inner sides, the +"Meckel cartilage" (named after its discoverer), on the outer surface +of which the lower jaw is formed (Figures 2.306 to 2.310 u). The +upper-jaw process forms the chief part of the skeleton of that jaw, +the palate bone, and the pterygoid bone. On its outer side is +afterwards formed the upper-jaw bone, in the narrower sense, while the +middle part of the skeleton of the upper jaw, the intermaxillary, +develops from the foremost part of the frontal process. + +The two upper-jaw processes are of great importance in the further +development of the face. From them is formed, growing into the +primitive mouth-cavity, the important horizontal partition (the +palate) that divides the former into two distinct cavities. The upper +cavity, into which the nasal canals open, now develops into the nasal +cavity, the air-passage and the organ of smell. The lower cavity forms +the permanent secondary mouth (Figure 2.312 m), the food-passage and +the organ of taste. Both the upper and lower cavities open behind into +the gullet (pharynx). The hard palate that separates them is formed by +the joining of two lateral halves, the horizontal plates of the two +upper-jaw processes, or the palate-plates (p). When these do not, +sometimes, completely join in the middle, a longitudinal cleft +remains, through which we can penetrate from the mouth straight into +the nasal cavity. This is the malformation known as "wolf's throat." +"Hare-lip" is the lesser form of the same defect. At the same time as +the horizontal partition of the hard palate a vertical partition is +formed by which the single nasal cavity is divided into two +sections--a right and left half (Figure 2.312 n, n). + +(FIGURES 2.313 AND 2.314. Upper part of the body of a human embryo, +two-thirds of an inch long, of the sixth week; Figure 2.313 from the +left, Figure 2.314 from the front. The origin of the nose and the +upper lip from two lateral and originally separate halves can be +clearly seen. Nose and upper lip are large in proportion to the rest +of the face, and especially to the lower lip. (From Kollmann.)) + +The double nose has now acquired the characteristic form that man +shares with the other mammals. Its further development is easy to +follow; it consists of the formation of the inner and outer processes +of the walls of the two cavities. The external nose is not formed +until long after all these essential parts of the internal organ of +smell. The first traces of it in the human embryo are found about the +middle of the second month (Figures 2.313 to 2.316). As can be seen in +any human embryo during the first month, there is at first no trace of +the external nose. It only develops afterwards from the foremost nasal +part of the primitive skull, growing forwards from behind. The +characteristic human nose is formed very late. Much stress is at times +laid on this organ as an exclusive privilege of man. But there are +apes that have similar noses, such as the long-nosed ape. + +(FIGURE 2.315. Face of a human embryo, seven weeks old, (From +Kollmann.) Joining of the nasal processes (e outer, i inner) with the +upper-jaw process (o), n nasal wall, a ear-opening.) + +The evolution of the eye is not less interesting and instructive than +that of the nose. Although this noblest of the sensory organs is one +of the most elaborate and purposive on account of its optic perfection +and remarkable structure, it nevertheless develops, without +preconceived design, from a simple process of the outer germinal +layer. The fully-formed human eye is a round capsule, the eye-ball +(Figure 2.317). This lies in the bony cavity of the skull, surrounded +by protective fat and motor muscles. The greater part of it is taken +up with a semi-fluid, transparent gelatinous substance, the corpus +vitreum. The crystalline lens is fitted into the anterior surface of +the ball (Figure 2.317 l). It is a lenticular, bi-convex, transparent +body, the most important of the refractive media in the eye. Of this +group we have, besides the corpus vitreum and the lens, the watery +fluid (humor aqueus) that is found in front of the lens (at the letter +m in Figure 2.317). These three transparent refractive media, by which +the rays of light that enter the eye are broken up and re-focussed, +are enclosed in a solid round capsule, composed of several different +coats, something like the concentric layers of an onion. The outermost +and thickest of these envelopes is the white sclerotic coat of the +eye. It consists of tough white connective tissue. In front of the +lens a circular, strongly-curved, transparent plate is fitted into the +sclerotic, like the glass of a watch--the cornea (b). At its outer +surface the cornea is covered with a very thin layer of the epidermis; +this is known as the conjunctiva. It goes from the cornea over the +inner surface of the eye-lids, the upper and lower folds which we draw +over the eye in closing it. At the inner corner of the eye we have a +rudimentary organ in the shape of the relic of a third (inner) +eye-lid, which is greatly developed, as "nictitating (winking) +membrane," in the lower Vertebrates (Chapter 1.5). Underneath the +upper eye-lid are the lachrymal glands, the product of which, the +lachrymal fluid, keeps the outer surface of the eye smooth and clean. + +Immediately under the sclerotic we find a very delicate, dark-red +membrane, very rich in blood-vessels--the choroid coat--and inside +this the retina (o), the expansion of the optic nerve (i). The latter +is the second cerebral nerve. It proceeds from the optic thalami (the +second cerebral vesicle) to the eye; penetrates its outer envelopes, +and then spreads out like a net between the choroid and the corpus +vitreum. Between the retina and the choroid there is a very delicate +membrane, which is usually (but wrongly) associated with the latter. +This is the black pigment-membrane (n). It consists of a single +stratum of graceful, hexagonal, regularly-joined cells, full of +granules of black colouring matter. This pigment membrane clothes, not +only the inner surface of the choroid proper, but also the hind +surface of its anterior muscular continuation, which covers the edge +of the lens in front as a circular membrane, and arrests the rays of +light at the sides. This is the well-known iris of the eye (h), +coloured differently in different individuals (blue, grey, brown, +etc.); it forms the anterior border of the choroid. The circular +opening that is left in the middle is the pupil, through which the +rays of light penetrate into the eye. At the point where the iris +leaves the anterior border of the choroid proper the latter is very +thick, and forms a delicate crown of folds (g), which surrounds the +edge of the lens with about seventy large and many smaller rays +(corona ciliaris.) + +At a very early stage a couple of pear-shaped vesicles develop from +the foremost part of the first cerebral vesicle in the embryo of man +and the other Craniotes (Figures 1.155 a and 2.297 au). These growths +are the primary optic vesicles. They are at first directed outwards +and forwards, but presently grow downward, so that, after the complete +separation of the five cerebral vesicles, they lie at the base of the +intermediate brain. The inner cavities of these pear-shaped vesicles, +which soon attain a considerable size, are openly connected with the +ventricle of the intermediate brain by their hollow stems. They are +covered externally by the epidermis. + +(FIGURE 2.316. Face of a human embryo, eight weeks old (From Ecker.)) + +At the point where this comes into direct contact with the most curved +part of the primary optic vesicle there is a thickening (l) and also a +depression (o) of the horny plate (Figure 2.318, I). This pit, which +we may call the lens-pit, is converted into a closed sac, the +thick-walled lens-vesicle (2, l), the thick edges of the pit joining +together above it. In the same way in which the medullary tube +separates from the outer germinal layer, we now see this lens-sac +sever itself entirely from the horny plate (h), its source of origin. +The hollow of the sac is afterwards filled with the cells of its thick +walls, and thus we get the solid crystalline lens. This is, therefore, +a purely epidermic structure. Together with the lens the small +underlying piece of corium-plate also separates from the skin. + +As the lens separates from the corneous plate and grows inwards, it +necessarily hollows out the contiguous primary optic vesicle (Figure +2.318, 1 to 3). This is done in just the same way as the invagination +of the blastula, which gives rise to the gastrula in the amphioxus +(Figure 2.38 C to F). In both cases the hollowing of the closed +vesicle on one side goes so far that at last the inner, folded part +touches the outer, not folded part, and the cavity disappears. As in +the gastrula the first part is converted into the entoderm and the +latter into the ectoderm, so in the invagination of the primary optic +vesicle the retina (r) is formed from the first (inner) part, and the +black pigment membrane (u) from the latter (outer, non-invaginated) +part. The hollow stem of the primary optic vesicle is converted into +the optic nerve. The lens (l), which has so important a part in this +process, lies at first directly on the invaginated part, or the retina +(r). But they soon separate, a new structure, the corpus vitreum (gl), +growing between them. While the lenticular sac is being detached and +is causing the invagination of the primary optic vesicle, another +invagination is taking place from below; this proceeds from the +superficial part of the skin-fibre layer--the corium of the head. +Behind and under the lens a last-shaped process rises from the +cutis-plate (Figure 2.319 g), hollows out the cup-shaped optic vesicle +from below, and presses between the lens (l) and the retina (i). In +this way the optic vesicle acquires the form of a hood. + +(FIGURE 2.317. The human eye in section. a sclerotic coat, b cornea, c +conjunctiva, d circular veins of the iris, e choroid coat, f ciliary +muscle, g corona ciliaris, h iris, i optic nerve, k anterior border of +the retina, l crystalline lens, m inner covering of the cornea +(aqueous membrane), n pigment membrane, o retina, p Petit's canal, q +yellow spot of the retina. (From Helmholtz.)) + +Finally, a complete fibrous envelope, the fibrous capsule of the +eye-ball, is formed about the secondary optic vesicle and its stem +(the secondary optic nerve). It originates from the part of the +head-plates which immediately encloses the eye. This fibrous envelope +takes the form of a closed round vesicle, surrounding the whole of the +ball and pushing between the lens and the horny plate at its outer +side. The round wall of the capsule soon divides into two different +membranes by surface-cleavage. The inner membrane becomes the choroid +or vascular coat, and in front the ciliary corona and iris. The outer +membrane is converted into the white protective or sclerotic coat--in +front, the transparent cornea. The eye is now formed in all its +essential parts. The further development--the complicated +differentiation and composition of the various parts--is a matter of +detail. + +(FIGURE 2.318. Eye of the chick embryo in longitudinal section (1. +from an embryo sixty-five hours old; 2. from a somewhat older embryo; +3. from an embryo four days old). h horny plate, o lens-pit, l lens +(in 1. still part of the epidermis, in 2. and 3. separated from it), x +thickening of the horny plate at the point where the lens has severed +itself, gl corpus vitreum, r retina, u pigment membrane. (From +Remak.)) + +The chief point in this remarkable evolution of the eye is the +circumstance that the optic nerve, the retina, and the pigment +membrane originate really from a part of the brain--an outgrowth of +the intermediate brain--while the lens, the chief refractive body, +develops from the outer skin. From the skin--the horny plate--also +arises the delicate conjunctiva, which afterwards covers the outer +surface of the eyeball. The lachrymal glands are ramified growths from +the conjunctiva (Figure 2.286). All these important parts of the eye +are products of the outer germinal layer. The remaining parts--the +corpus vitreum (with the vascular capsule of the lens), the choroid +(with the iris), and the sclerotic (with the cornea)--are formed from +the middle germinal layer. + +The outer protection of the eye, the eye-lids, are merely folds of the +skin, which are formed in the third month of human embryonic life. In +the fourth month the upper eye-lid reaches the lower, and the eye +remains covered with them until birth. As a rule, they open wide +shortly before birth (sometimes only after birth). Our craniote +ancestors had a third eye-lid, the nictitating membrane, which was +drawn over the eye from its inner angle. It is still found in many of +the Selachii and Amniotes. In the apes and man it has degenerated, and +there is now only a small relic of it at the inner corner of the eye, +the semi-lunar fold, a useless rudimentary organ (Chapter 1.5). The +apes and man have also lost the Harderian gland that opened under the +nictitating membrane; we find this in the rest of the mammals, and the +birds, reptiles, and amphibia. + +The peculiar embryonic development of the vertebrate eye does not +enable us to draw any definite conclusions as to its obscure +phylogeny; it is clearly cenogenetic to a great extent, or obscured by +the reduction and curtailment of its original features. It is probable +that many of the earlier stages of its phylogeny have disappeared +without leaving a trace. It can only be said positively that the +peculiar ontogeny of the complicated optic apparatus in man follows +just the same laws as in all the other Vertebrates. Their eye is a +part of the fore brain, which has grown forward towards the skin, not +an original cutaneous sense-organ, as in the Invertebrates. + +(FIGURE 2.319. Horizontal transverse section of the eye of a human +embryo, four weeks old (magnified one hundred times). (From Kolliker.) +t lens (the dark wall of which is as thick as the diameter of the +central cavity), g corpus vitreum (connected by a stem, g, with the +corium), v vascular loop (pressing behind the lens inside the corpus +vitreum by means of this stem g), i retina (inner thicker, invaginated +layer of the primary optic vesicle), a pigment membrane (outer, thin, +non-invaginated layer of same), h space between retina and pigment +membrane (remainder of the cavity of the primary optic vesicle). + +FIGURE 2.320. The human ear (left ear, seen from the front, natural +size), a shell of ear, b external passage, c tympanum, d tympanic +cavity, e Eustachian tube, f, g, h the three bones of the ear (f +hammer, g anvil, h stirrup), i utricle, k the three semi-circular +canals, l the sacculus, m cochlea, n auscultory nerve.) + +The vertebrate ear resembles the eye and nose in many important +respects, but is different in others, in its development. The +auscultory organ in the fully-developed man is like that of the other +mammals, and especially the apes, in the main features. As in them, it +consists of two chief parts--an apparatus for conducting sound +(external and middle ear) and an apparatus for the sensation of sound +(internal ear). The external ear opens in the shell at the side of the +head (Figure 2.320 a). From this point the external passage (b), about +an inch in length, leads into the head. The inner end of it is closed +by the tympanum, a vertical, but not quite upright, thin membrane of +an oval shape (c). This tympanum separates the external passage from +the tympanic cavity (d). This is a small cavity, filled with air, in +the temporal bone; it is connected with the mouth by a special tube. +This tube is rather longer, but much narrower, than the outer passage, +leads inwards obliquely from the anterior wall of the tympanic cavity, +and opens in the throat below, behind the nasal openings. It is called +the Eustachian tube (e); it serves to equalise the pressure of the air +within the tympanic cavity and the outer atmosphere that enters by the +external passage. Both the Eustachian tube and the tympanic cavity are +lined with a thin mucous coat, which is a direct continuation of the +mucous lining of the throat. Inside the tympanic cavity there are +three small bones which are known (from their shape) as the hammer, +anvil, and stirrup (Figure 2.320, f, g, h). The hammer (f) is the +outermost, next to the tympanum. The anvil (g) fits between the other +two, above and inside the hammer. The stirrup (h) lies inside the +anvil, and touches with its base the outer wall of the internal ear, +or auscultory vesicle. All these parts of the external and middle ear +belong to the apparatus for conducting sound. Their chief task is to +convey the waves of sound through the thick wall of the head to the +inner-lying auscultory vesicle. They are not found at all in the +fishes. In these the waves of sound are conveyed directly by the wall +of the head to the auscultory vesicle. + +The internal apparatus for the sensation of sound, which receives the +waves of sound from the conducting apparatus, consists in man and all +other mammals of a closed auscultory vesicle filled with fluid and an +auditory nerve, the ends of which expand over the wall of this +vesicle. The vibrations of the sound-waves are conveyed by these media +to the nerve-endings. In the labyrinthic water that fills the +auscultory vesicle there are small stones at the points of entry of +the acoustic nerves, which are composed of groups of microscopic +calcareous crystals (otoliths). The auscultory organ of most of the +Invertebrates has substantially the same composition. It usually +consists of a closed vesicle, filled with fluid, and containing +otoliths, with the acoustic nerve expanding on its wall. But, while +the auditory vesicle is usually of a simple round or oval shape in the +Invertebrates, it has in the Vertebrates a special and curious +structure, the labyrinth. This thin-membraned labyrinth is enclosed in +a bony capsule of the same shape, the osseous labyrinth (Figure +2.321), and this lies in the middle of the petrous bone of the skull. +The labyrinth is divided into two vesicles in all the Gnathostomes. +The larger one is called the utriculus, and has three arched +appendages, called the "semi-circular canals" (c, d, e). The smaller +vesicle is called the sacculus, and is connected with a peculiar +appendage, with (in man and the higher mammals) a spiral form +something like a snail's shell, and therefore called the cochlea (= +snail, b). On the thin wall of this delicate labyrinth the acoustic +nerve, which comes from the after-brain, spreads out in most elaborate +fashion. It divides into two main branches--a cochlear nerve (for the +cochlea) and a vestibular nerve (for the rest of the labyrinth). The +former seems to have more to do with the quality, the latter with the +quantity, of the acoustic sensations. Through the cochlear nerves we +learn the height and timbre, through the vestibular nerves the +intensity, of tones. + +(FIGURE 2.321. The bony labyrinth of the human ear (left side). a +vestibulum, b cochlea, c upper canal, d posterior canal, e outer +canal, f oval fenestra, g round fenestra. (From Meyer.) + +FIGURE 2.322. Development of the auscultory labyrinth of the chick, in +five successive stages (A to E). (Vertical transverse sections of the +skull.) fl auscultory pits, lv auscultory vesicles, lr labyrinthic +appendage, c rudimentary cochlea, csp posterior canal, cse external +canal, jv jugular vein. (From Reissner.)) + +The first structure of this highly elaborate organ is very simple in +the embryo of man and all the other Craniotes; it is a pit-like +depression in the skin. At the back part of the head at both sides, +near the after brain, a small thickening of the horny plate is formed +at the upper end of the second gill-cleft (Figure 2.322 A fl). This +sinks into a sort of pit, and severs from the epidermis, just as the +lens of the eye does. In this way is formed at each side, directly +under the horny plate of the back part of the head, a small vesicle +filled with fluid, the primitive auscultory vesicle, or the primary +labyrinth. As it separates from its source, the horny plate, and +presses inwards and backwards into the skull, it changes from round to +pear-shaped (Figures 2.322 B lv and 2.323 o). The outer part of it is +lengthened into a thin stem, which at first still opens outwards by a +narrow canal. This is the labyrinthic appendage (Figure 2.322 lr). In +the lower Vertebrates it develops into a special cavity filled with +calcareous crystals, which remains open permanently in some of the +primitive fishes, and opens outwards in the upper part of the skull. +But in the mammals the labyrinthic appendage degenerates. In these it +has only a phylogenetic interest as a rudimentary organ, with no +actual physiological significance. The useless relic of it passes +through the wall of the petrous bone in the shape of a narrow canal, +and is called the vestibular aqueduct. + +It is only the inner and lower bulbous part of the separated +auscultory vesicle that develops into the highly complex and +differentiated structure that is afterwards known as the secondary +labyrinth. This vesicle divides at an early stage into an upper and +larger and a lower and smaller section. From the one we get the +utriculus with the semi-circular canals; from the other the sacculus +and the cochlea (Figure 2.320 c). The canals are formed in the shape +of simple pouch-like involutions of the utricle (cse and csp). The +edges join together in the middle part of each fold, and separate from +the utricle, the two ends remaining in open connection with its +cavity. All the Gnathostomes have these three canals like man, whereas +among the Cyclostomes the lampreys have only two and the hag-fishes +only one. The very complex structure of the cochlea, one of the most +elaborate and wonderful outcomes of adaptation in the mammal body, +develops originally in very simple fashion as a flask-like projection +from the sacculus. As Hasse and Retzius have pointed out, we find the +successive ontogenetic stages of its growth represented permanently in +the series of the higher Vertebrates. The cochlea is wanting even in +the Monotremes, and is restricted to the rest of the mammals and man. + +The auditory nerve, or eighth cerebral nerve, expands with one branch +in the cochlea, and with the other in the remaining parts of the +labyrinth. This nerve is, as Gegenbaur has shown, the sensory dorsal +branch of a cerebro-spinal nerve, the motor ventral branch of which +acts for the muscles of the face (nervus facialis). It has therefore +originated phylogenetically from an ordinary cutaneous nerve, and so +is of quite different origin from the optic and olfactory nerves, +which both represent direct outgrowths of the brain. In this respect +the auscultory organ is essentially different from the organs of sight +and smell. The acoustic nerve is formed from ectodermic cells of the +hind brain, and develops from the nervous structure that appears at +its dorsal limit. On the other hand, all the membranous, +cartilaginous, and osseous coverings of the labyrinth are formed from +the mesodermic head-plates. + +(FIGURE 2.323. Primitive skull of the human embryo, four weeks old, +vertical section, left half seen internally. v, z, m, h, n the five +pits of the cranial cavity, in which the five cerebral vesicles lie +(fore, intermediate, middle, hind, and after brains), o pear-shaped +primary auscultory vesicle (appearing through), a eye (appearing +through), no optic nerve, p canal of the hypophysis, t central +prominence of the skull. (From Kolliker.)) + +The apparatus for conducting sound which we find in the external and +middle ear of mammals develops quite separately from the apparatus for +the sensation of sound. It is both phylogenetically and +ontogenetically an independent secondary formation, a later accession +to the primary internal ear. Nevertheless, its development is not less +interesting, and is explained with the same ease by comparative +anatomy. In all the fishes and in the lowest Vertebrates there is no +special apparatus for conducting sound, no external or middle ear; +they have only a labyrinth, an internal ear, which lies within the +skull. They are without the tympanum and tympanic cavity, and all its +appendages. From many observations made in the last few decades it +seems that many of the fishes (if not all) cannot distinguish tones; +their labyrinth seems to be chiefly (if not exclusively) an organ for +the sense of space (or equilibrium). If it is destroyed, the fishes +lose their balance and fall. In the opinion of recent physiologists +this applies also to many of the Invertebrates (including the nearer +ancestors of the Vertebrates). The round vesicles which are considered +to be their auscultory vesicles, and which contain an otolith, are +supposed to be merely organs of the sense of space ("static vesicles +or statocysts"). + +The middle ear makes its first appearance in the amphibian class, +where we find a tympanum, tympanic cavity, and Eustachian tube; these +animals, and all terrestrial Vertebrates, certainly have the faculty +of hearing. All these essential parts of the middle ear originate from +the first gill-cleft and its surrounding part; in the Selachii this +remains throughout life an open squirting-hole, and lies between the +first and second gill-arch. In the embryo of the higher Vertebrates it +closes up in the centre, and thus forms the tympanic membrane. The +outlying remainder of the first gill-cleft is the rudiment of the +external meatus. From its inner part we get the tympanic cavity, and, +further inward still, the Eustachian tube. Connected with this is the +development of the three bones of the mammal ear from the first two +gill-arches; the hammer and anvil are formed from the first, the +stirrup from the upper end of the second, gill-arch. + +(FIGURE 2.324. The rudimentary muscles of the ear in the human skull. +a raising muscle (M. attollens), b drawing muscle (M. attrahens), c +withdrawing muscle (M. retrahens), d large muscle of the helix (M. +helicis major), e small muscle of the helix (M. helicis minor), f +muscle of the angle of the ear (M. tragicus), g anti-angular muscle +(M. antitragicus). (From H. Meyer.)) + +Finally, the shell (pinna or concha) and external meatus (passage to +the tympanum) of the outer ear are developed in a very simple fashion +from the skin that borders the external aperture of the first +gill-cleft. The shell rises in the shape of a circular fold of the +skin, in which cartilage and muscles are afterwards formed (Figures +2.313 and 2.315). This organ is only found in the mammalian class. It +is very rudimentary in the lowest section, the Monotremes. In the +others it is found at very different stages of development, and +sometimes of degeneration. It is degenerate in most of the aquatic +mammals. The majority of them have lost it altogether--for instance, +the walruses and whales and most of the seals. On the other hand, the +pinna is well developed in the great majority of the Marsupials and +Placentals; it receives and collects the waves of sound, and is +equipped with a very elaborate muscular apparatus, by means of which +the pinna can be turned freely in any direction and its shape be +altered. It is well known how readily domestic animals--horses, cows, +dogs, hares, etc.--point their ears and move them in different +directions. Most of the apes do the same, and our earlier ape +ancestors were also able to do it. But our later simian ancestors, +which we have in common with the anthropoid apes, abandoned the use of +these muscles, and they gradually became rudimentary and useless. +However, we possess them still (Figure 2.324). In fact, some men can +still move their ears a little backward and forward by means of the +drawing and withdrawing muscles (b and c); with practice this faculty +can be much improved. But no man can now lift up his ears by the +raising muscle (a), or change the shape of them by the small inner +muscles (d, e, f, g). These muscles were very useful to our ancestors, +but are of no consequence to us. This applies to most of the +anthropoid apes as well. + +We also share with the higher anthropoid apes (gorilla, chimpanzee, +and orang) the characteristic form of the human outer ear, especially +the folded border, the helix and the lobe. The lower apes have pointed +ears, without folded border or lobe, like the other mammals. But +Darwin has shown that at the upper part of the folded border there is +in many men a small pointed process, which most of us do not possess. +In some individuals this process is well developed. It can only be +explained as the relic of the original point of the ear, which has +been turned inwards in consequence of the curving of the edge. If we +compare the pinna of man and the various apes in this respect, we find +that they present a connected series of degenerate structures. In the +common catarrhine ancestors of the anthropoids and man the +degeneration set in with the folding together of the pinna. This +brought about the helix of the ear, in which we find the significant +angle which represents the relic of the salient point of the ear in +our earlier simian ancestors. Here again, therefore, comparative +anatomy enables us to trace with certainty the human ear to the +similar, but more developed, organ of the lower mammals. At the same +time, comparative physiology shows that it was a more or less useful +implement in the latter, but it is quite useless in the anthropoids +and man. The conducting of the sound has scarcely been affected by the +loss of the pinna. We have also in this the explanation of the +extraordinary variety in the shape and size of the shell of the ear in +different men; in this it resembles other rudimentary organs. + + +CHAPTER 2.26. EVOLUTION OF THE ORGANS OF MOVEMENT. + +The peculiar structure of the locomotive apparatus is one of the +features that are most distinctive of the vertebrate stem. The chief +part of this apparatus is formed, as in all the higher animals, by the +active organs of movement, the muscles; in consequence of their +contractility they have the power to draw up and shorten themselves. +This effects the movement of the various parts of the body, and thus +the whole body is conveyed from place to place. But the arrangement of +these muscles and their relation to the solid skeleton are different +in the Vertebrates from the Invertebrates. + +(FIGURE 2.325. The human skeleton. From the right. + +FIGURE 2.326. The human skeleton. Front.) + +In most of the lower animals, especially the Platodes and Vermalia, we +find that the muscles form a simple, thin layer of flesh immediately +underneath the skin. This muscular layer is very closely connected +with the skin itself; it is the same in the Mollusc stem. Even in the +large division of the Articulates, the classes of crabs, spiders, +myriapods, and insects, we find a similar feature, with the difference +that in this case the skin forms a solid armour--a rigid cutaneous +skeleton made of chitine (and often also of carbonate of lime). This +external chitine coat undergoes a very elaborate articulation both on +the trunk and the limbs of the Articulates, and in consequence the +muscular system also, the contractile fibres of which are attached +inside the chitine tubes, is highly articulated. The Vertebrates form +a direct contrast to this. In these alone a solid internal skeleton is +developed, of cartilage or bone, to which the muscles are attached. +This bony skeleton is a complex lever apparatus, or PASSIVE apparatus +of movement. Its rigid parts, the arms of the levers, or the bones, +are brought together by the actively mobile muscles, as if by +drawing-ropes. This admirable locomotorium, especially its solid +central axis, the vertebral column, is a special feature of the +Vertebrates, and has given the name to the group. + +(FIGURE 2.327. The human vertebral column (standing upright, from the +right side). (From H. Meyer.)) + +In order to get a clear idea of the chief features of the development +of the human skeleton, we must first examine its composition in the +adult frame (Figure 2.325, the human skeleton seen from the right; +Figure 2.326, front view of the whole skeleton). As in other mammals, +we distinguish first between the axial or dorsal skeleton and the +skeleton of the limbs. The axial skeleton consists of the vertebral +column (the skeleton of the trunk) and the skull (skeleton of the +head); the latter is a peculiarly modified part of the former. As +appendages of the vertebral column we have the ribs, and of the skull +we have the hyoid bone, the lower jaw, and the other products of the +gill-arches. + +The skeleton of the limbs or extremities is composed of two groups of +parts--the skeleton of the extremities proper and the zone-skeleton, +which connects these with the vertebral column. The zone-skeleton of +the arms (or fore legs) is the shoulder-zone; the zone-skeleton of the +legs (or hind legs) is the pelvic zone. + +(FIGURE 2.328. A piece of the axial rod (chorda dorsalis), from a +sheep embryo. a cuticular sheath, b cells. (From Kolliker.)) + +The vertebral column (Figure 2.327) in man is composed of thirty-three +to thirty-five ring-shaped bones in a continuous series (above each +other, in man's upright position). These vertebrae are separated from +each other by elastic ligaments, and at the same time connected by +joints, so that the whole column forms a firm and solid, but flexible +and elastic, axial skeleton, moving freely in all directions. The +vertebrae differ in shape and connection at the various parts of the +trunk, and we distinguish the following groups in the series, +beginning at the top: Seven cervical vertebrae, twelve dorsal +vertebrae, five lumbar vertebrae, five sacral vertebrae, and four to +six caudal vertebrae. The uppermost, or those next to the skull, are +the cervical vertebrae (Figure 2.327); they have a hole in each of the +lateral processes. There are seven of these vertebrae in man and +almost all the other mammals, even if the neck is as long as that of +the camel or giraffe, or as short as that of the mole or hedgehog. +This constant number, which has few exceptions (due to adaptation), is +a strong proof of the common descent of the mammals; it can only be +explained by faithful heredity from a common stem-form, a primitive +mammal with seven cervical vertebrae. If each species had been created +separately, it would have been better to have given the long-necked +mammals more, and the short-necked animals less, cervical vertebrae. +Next to these come the dorsal (or pectoral) vertebrae, which number +twelve to thirteen (usually twelve) in man and most of the other +mammals. Each dorsal vertebra (Figure 1.165) has at the side, +connected by joints, a couple of ribs, long bony arches that lie in +and protect the wall of the chest. The twelve pairs of ribs, together +with the connecting intercostal muscles and the sternum, which joins +the ends of the right and left ribs in front, form the chest (thorax). +In this strong and elastic frame are the lungs, and between them the +heart. Next to the dorsal vertebrae comes a short but stronger section +of the column, formed of five large vertebrae. These are the lumbar +vertebrae (Figure 1.166); they have no ribs and no holes in the +transverse processes. To these succeeds the sacral bone, which is +fitted between the two halves of the pelvic zone. The sacrum is formed +of five vertebrae, completely blended together. Finally, we have at +the end a small rudimentary caudal column, the coccyx. This consists +of a varying number (usually four, more rarely three, or five or six) +of small degenerated vertebrae, and is a useless rudimentary organ +with no actual physiological significance. Morphologically, however, +it is of great interest as an irrefragable proof of the descent of man +and the anthropoids from long-tailed apes. On no other theory can we +explain the existence of this rudimentary tail. In the earlier stages +of development the tail of the human embryo protrudes considerably. It +afterwards atrophies; but the relic of the atrophied caudal vertebrae +and of the rudimentary muscles that once moved it remains permanently. +Sometimes, in fact, the external tail is preserved. The older +anatomists say that the tail is usually one vertebra longer in the +human female than in the male (or four against five); Steinbach says +it is the reverse. + +(FIGURE 2.329. Three dorsal vertebrae, from a human embryo, eight +weeks old, in lateral longitudinal section. v cartilaginous vertebral +body, li inter-vertebral disks, ch chorda. (From Kolliker.) + +(FIGURE 2.330. A dorsal vertebra of the same embryo, in lateral +transverse section. cv cartilaginous vertebral body, ch chorda, pr +transverse process, a vertebral arch (upper arch), c upper end of the +rib (lower arch). (From Kolliker.)) + +In the human vertebral column there are usually thirty-three +vertebrae. It is interesting to find, however, that the number often +changes, one or two vertebrae dropping out or an additional one +appearing. Often, also, a mobile rib is formed at the last cervical or +the first lumbar vertebra, so that there are then thirteen dorsal +vertebrae, besides six cervical and four lumbar. In this way the +contiguous vertebrae of the various sections of the column may take +each other's places. + +In order to understand the embryology of the human vertebral column we +must first carefully consider the shape and connection of the +vertebrae. Each vertebra has, in general, the shape of a seal-ring +(Figures 1.164 to 1.166). The thicker portion, which is turned towards +the ventral side, is called the body of the vertebra, and forms a +short osseous disk; the thinner part forms a semi-circular arch, the +vertebral arch, and is turned towards the back. The arches of the +successive vertebrae are connected by thin intercrural ligaments in +such a way that the cavity they collectively enclose represents a long +canal. In this vertebral canal we find the trunk part of the central +nervous system, the spinal cord. Its head part, the brain, is enclosed +by the skull, and the skull itself is merely the uppermost part of the +vertebral column, distinctively modified. The base or ventral side of +the vesicular cranial capsule corresponds originally to a number of +developed vertebral bodies; its vault or dorsal side to their combined +upper vertebral arches. + +(FIGURE 2.331. Intervertebral disk of a new-born infant, transverse +section. a rest of the chorda. (From Kolliker.)) + +While the solid, massive bodies of the vertebrae represent the real +central axis of the skeleton, the dorsal arches serve to protect the +central marrow they enclose. But similar arches develop on the ventral +side for the protection of the viscera in the breast and belly. These +lower or ventral vertebral arches, proceeding from the ventral side of +the vertebral bodies, form, in many of the lower Vertebrates, a canal +in which the large blood-vessels are enclosed on the lower surface of +the vertebral column (aorta and caudal vein). In the higher +Vertebrates the majority of these vertebral arches are lost or become +rudimentary. But at the thoracic section of the column they develop +into independent strong osseous arches, the ribs (costae). In reality +the ribs are merely large and independent lower vertebral arches, +which have lost their original connection with the vertebral bodies. + +If we turn from this anatomic survey of the composition of the column +to the question of its development, I may refer the reader to earlier +pages with regard to the first and most important points (Chapter +1.14). It will be remembered that in the human embryo and that of the +other vertebrates we find at first, instead of the segmented column, +only a simple unarticulated cartilaginous rod. This solid but flexible +and elastic rod is the axial rod (or the chorda dorsalis). In the +lowest Vertebrate, the Amphioxus, it retains this simple form +throughout life, and permanently represents the whole internal +skeleton (Figure 2.210 i). In the Tunicates, also, the nearest +Invertebrate relatives of the Vertebrates, we meet the same +chorda--transitorily in the passing larva tail of the Ascidia, +permanently in the Copelata (Figure 2.225 c). Undoubtedly both the +Tunicates and Acrania have inherited the chorda from a common +unsegmented stem-form; and these ancient, long-extinct ancestors of +all the chordonia are our hypothetical Prochordonia. + +Long before there is any trace of the skull, limbs, etc., in the +embryo of man or any of the higher Vertebrates--at the early stage in +which the whole body is merely a sole-shaped embryonic shield--there +appears in the middle line of the shield, directly under the medullary +furrow, the simple chorda. (Cf. Figures 1.131 to 1.135 ch). It follows +the long axis of the body in the shape of a cylindrical axial rod of +elastic but firm composition, equally pointed at both ends. In every +case the chorda originates from the dorsal wall of the primitive gut; +the cells that compose it (Figure 2.328 b) belong to the entoderm +(Figures 2.216 to 2.221). At an early stage the chorda develops a +transparent structureless sheath, which is secreted from its cells +(Figure 2.328 a). This chordalemma is often called the "inner +chorda-sheath," and must not be confused with the real external +sheath, the mesoblastic perichorda. + +(FIGURE 2.332. Human skull. + +FIGURE 2.333. Skull of a new-born child. (From Kollmann.) Above, in +the three bones of the roof of the skull, we see the lines that +radiate from the central points of ossification; in front, the frontal +bone; behind, the occipital bone; between the two the large parietal +bone, p. s the scurf bone, w mastoid fontanelle, f petrous bone, t +tympanic bone, l lateral part, b bulla, j cheek-bone, a large wing of +cuneiform bone, k fontanelle of cuneiform bone.) + +But this unsegmented primary axial skeleton is soon replaced by the +segmented secondary axial skeleton, which we know as the vertebral +column. The provertebral plates (Figure 1.124 s) differentiate from +the innermost, median part of the visceral layer of the coelom-pouches +at each side of the chorda. As they grow round the chorda and enclose +it they form the skeleton plate or skeletogenetic layer--that is to +say, the skeleton-forming stratum of cells, which provides the mobile +foundation of the permanent vertebral column and skull (scleroblast). +In the head-half of the embryo the skeletal plate remains a +continuous, simple, undivided layer of tissue, and presently enlarges +into a thin-walled capsule enclosing the brain, the primordial skull. +In the trunk-half the provertebral plate divides into a number of +homogeneous, cubical, successive pieces; these are the several +primitive vertebrae. They are not numerous at first, but soon increase +as the embryo grows longer (Figures 1.153 to 1.155). + +(FIGURE 2.334. Head-skeleton of a primitive fish, n nasal pit, eth +cribriform bone region, orb orbit of eye, la wall of auscultory +labyrinth, occ occipital region of primitive skull, cv vertebral +column, a fore, bc hind-lip cartilage, o primitive upper jaw +(palato-quadratum), u primitive lower jaw, II hyaloid bone, III to +VIII first to sixth branchial arches. (From Gegenbaur.) + +FIGURE 2.335. Roofs of the skulls of nine Primates (Cattarrhines), +seen from above and reduced to a common size. 1 European, 2 Brazilian, +3 Pithecanthropus, 4 Gorilla, 5 Chimpanzee, 6 Orang, 7 Gibbon, 8 +Tailed ape, 9 Baboon.) + +In all the Craniotes the soft, indifferent cells of the mesoderm, +which originally compose the skeletal plate, are afterwards converted +for the most part into cartilaginous cells, and these secrete a firm +and elastic intercellular substance between them, and form +cartilaginous tissue. Like most of the other parts of the skeleton, +the membranous rudiments of the vertebrae soon pass into a +cartilaginous state, and in the higher Vertebrates this is afterwards +replaced by the hard osseous tissue with its characteristic stellate +cells (Figure 1.6). The primary axial skeleton remains a simple chorda +throughout life in the Acrania, the Cyclostomes, and the lowest +fishes. In most of the other Vertebrates the chorda is more or less +replaced by the cartilaginous tissue of the secondary perichorda that +grows round it. In the lower Craniotes (especially the fishes) a more +or less considerable part of the chorda is preserved in the bodies of +the vertebrae. In the mammals it disappears for the most part. By the +end of the second month in the human embryo the chorda is merely a +slender thread, running through the axis of the thick, cartilaginous +vertebral column (Figures 1.182 ch and 2.329 ch). In the cartilaginous +vertebral bodies themselves, which afterwards ossify, the slender +remnant of the chorda presently disappears (Figure 2.330 ch). But in +the elastic inter-vertebral disks, which develop from the skeletal +plate between each pair of vertebral bodies (Figure 2.329 li), a relic +of the chorda remains permanently. In the new-born child there is a +large pear-shaped cavity in each intervertebral disk, filled with a +gelatinous mass of cells (Figure 2.331 a). Though less sharply +defined, this gelatinous nucleus of the elastic cartilaginous disks +persists throughout life in the mammals, but in the birds and most +reptiles the last trace of the chorda disappears. In the subsequent +ossification of the cartilaginous vertebra the first deposit of bony +matter ("first osseous nucleus") takes place in the vertebral body +immediately round the remainder of the chorda, and soon displaces it +altogether. Then there is a special osseous nucleus formed in each +half of the vertebral arch. The ossification does not reach the point +at which the three nuclei are joined until after birth. In the first +year the two osseous halves of the arches unite; but it is much +later--in the second to the eighth year--that they connect with the +osseous vertebral bodies. + +(FIGURE 2.336. Skeleton of the breast-fin of Ceratodus (biserial +feathered skeleton). A, B, cartilaginous series of the fin-stem. rr +cartilaginous fin-radii. (From Gunther.) + +FIGURE 2.337. Skeleton of the breast-fin of an early Selachius +(Acanthias). The radii of the median fin-border (B) have disappeared +for the most part; a few only (R) are left. R, R, radii of the lateral +fin-border, mt metapterygium, ms mesopterygium, p propterygium. (From +Gegenbaur.) + +FIGURE 2.338. Skeleton of the breast-fin of a young Selachius. The +radii of the median fin-border have wholly disappeared. The shaded +part on the right is the section that persists in the five-fingered +hand of the higher Vertebrates. (b the three basal pieces of the fin: +mt metapterygium, rudiment of the humerus, ms mesopterygium, p +propterygium.) (From Gegenbaur.)) + +The bony skull (cranium), the head-part of the secondary axial +skeleton, develops in just the same way as the vertebral column. The +skull forms a bony envelope for the brain, just as the vertebral canal +does for the spinal cord; and as the brain is only a peculiarly +differentiated part of the head, while the spinal cord represents the +longer trunk-section of the originally homogeneous medullary tube, we +shall expect to find that the osseous coat of the one is a special +modification of the osseous envelope of the other. When we examine the +adult human skull in itself (Figure 2.332), it is difficult to +conceive how it can be merely the modified fore part of the vertebral +column. It is an elaborate and extensive bony structure, composed of +no less than twenty bones of different shapes and sizes. Seven of them +form the spacious shell that surrounds the brain, in which we +distinguish the solid ventral base below and the curved dorsal vault +above. The other thirteen bones form the facial skull, which is +especially the bony envelope of the higher sense-organs, and at the +same time encloses the entrance of the alimentary canal. The lower jaw +is articulated at the base of the skull (usually regarded as the XXI +cranial bone). Behind the lower jaw we find the hyoid bone at the root +of the tongue, also formed from the gill-arches, and a part of the +lower arches that have developed as "head-ribs" from the ventral side +of the base of the cranium. + +Although the fully-developed skull of the higher Vertebrates, with its +peculiar shape, its enormous size, and its complex composition, seems +to have nothing in common with the ordinary vertebrae, nevertheless +even the older comparative anatomists came to recognise at the end of +the eighteenth century that it is really nothing else originally than +a series of modified vertebrae. When Goethe in 1790 "picked up the +skull of a slain victim from the sand of the Jewish cemetery at +Venice, he noticed at once that the bones of the face also could be +traced to vertebrae (like the three hind-most cranial vertebrae)." And +when Oken (without knowing anything of Goethe's discovery) found at +Ilenstein, "a fine bleached skull of a hind, the thought flashed +across him like lightning: 'It is a vertebral column.'" + +(FIGURE 2.339. Skeleton of the fore leg of an amphibian. h upper-arm +(humerus), ru lower arm (r radius, u ulna), rcicu apostrophe, +wrist-bones of first series (r radiale, i intermedium, c centrale, u +apostrophe ulnare). 1, 2, 3, 4, 5 wrist-bones of the second series. +(From Gegenbaur.) + +FIGURE 2.340. Skeleton of gorilla's hand. (From Huxley.) + +FIGURE 2.341. Skeleton of human hand, back. (From Meyer.)) + +This famous vertebral theory of the skull has interested the most +distinguished zoologists for more than a century: the chief +representatives of comparative anatomy have devoted their highest +powers to the solution of the problem, and the interest has spread far +beyond their circle. But it was not until 1872 that it was happily +solved, after seven years' labour, by the comparative anatomist who +surpassed all other experts of this science in the second half of the +nineteenth century by the richness of his empirical knowledge and the +acuteness and depth of his philosophic speculations. Carl Gegenbaur +has shown, in his classic Studies of the Comparative Anatomy of the +Vertebrates (third section), that we find the most solid foundation +for the vertebral theory of the skull in the head-skeleton of the +Selachii. Earlier anatomists had wrongly started from the mammal +skull, and had compared the several bones that compose it with the +several parts of the vertebra (Figure 2.333) they thought they could +prove in this way that the fully-formed mammal skull was made of from +three to six vertebrae. + +The older theory was refuted by simple and obvious facts, which were +first pointed out by Huxley. Nevertheless, the fundamental idea of +it--the belief that the skull is formed from the head-part of the +perichordal axial skeleton, just as the brain is from the simple +medullary tube, by differentiation and modification--remained. The +work now was to discover the proper way of supplying this philosophic +theory with an empirical foundation, and it was reserved for Gegenbaur +to achieve this. He first opened out the phylogenetic path which here, +as in all morphological questions, leads most confidently to the goal. +He showed that the primitive fishes (Figures 2.249 to 2.251), the +ancestors of all the Gnathostomes, still preserve permanently in the +form of their skull the structure out of which the transformed skull +of the higher Vertebrates, including man, has been evolved. He further +showed that the branchial arches of the Selachii prove that their +skull originally consisted of a large number of (at least nine or ten) +provertebrae, and that the cerebral nerves that proceed from the base +of the brain entirely confirm this. These cerebral nerves are (with +the exception of the first and second pair, the olfactory and optic +nerves) merely modifications of spinal nerves, and are essentially +similar to them in their peripheral expansion. The comparative anatomy +of these cerebral nerves, their origin and their expansion, furnishes +one of the strongest arguments for the new vertebral theory of the +skull. + +(FIGURE 2.342. Skeleton of the hand or fore foot of six mammals. I +man, II dog, III pig, IV ox, V tapir, VI horse. r radius, u ulna, a +scaphoideum, b lunare, a triquetrum, d trapezium, e trapezoid, f +capitatum, g hamatum, p pisiforme. 1 thumb, 2 index finger, 3 middle +finger, 4 ring finger, 5 little finger. (From Gegenbaur.)) + +We have not space here to go into the details of Gegenbaur's theory of +the skull. I must be content to refer the reader to the great work I +have mentioned, in which it is thoroughly established from the +empirico-philosophical point of view. He has also given a +comprehensive and up-to-date treatment of the subject in his +Comparative Anatomy of the Vertebrates (1898). Gegenbaur indicates as +original "cranial ribs," or "lower arches of the cranial vertebrae," +at each side of the head of the Selachii (Figure 2.334), the following +pairs of arches: I and II, two lip-cartilages, the anterior (a) of +which is composed of an upper piece only, the posterior (bc) from an +upper and lower piece; III, the maxillary arches, also consisting of +two pieces on each side--the primitive upper jaw (os palato-quadratum, +o) and the primitive lower jaw (u); IV, the hyaloid bone (II); +finally, V to X, six branchial arches in the narrower sense (III to +VIII). From the anatomic features of these nine to ten cranial ribs or +"lower vertebral arches" and the cranial nerves that spread over them, +it is clear that the apparently simple cartilaginous primitive skull +of the Selachii was originally formed from so many (at least nine) +somites or provertebrae. The blending of these primitive segments into +a single capsule is, however, so ancient that, in virtue of the law of +curtailed heredity, the original division seems to have disappeared; +in the embryonic development it is very difficult to detect it in +isolated traces, and in some respects quite impossible. It is claimed +that several (three to six) traces of provertebrae have been +discovered in the anterior (pre-chordal) part of the Selachii-skull; +this would bring up the number of cranial somites to twelve or +sixteen, or even more. + +(FIGURES 2.343 TO 2.345. Arm and hand of three anthropoids. + +FIGURE 2.343. Chimpanzee (Anthropithecus niger). + +FIGURE 2.344. Veddah of Ceylon (Homo veddalis). + +FIGURE 2.345. European (Homo mediterraneus). (From Paul and Fritz +Sarasin.)) + +In the primitive skull of man (Figure 2.323) and the higher +Vertebrates, which has been evolved from that of the Selachii, five +consecutive sections are discoverable at a certain early period of +development, and one might be induced to trace these to five primitive +vertebrae; but these sections are due entirely to adaptation to the +five primitive cerebral vesicles, and correspond, like these, to a +large number of metamera. That we have in the primitive skull of the +mammals a greatly modified and transformed organ, and not at all a +primitive formation, is clear from the circumstance that its original +soft membranous form only assumes the cartilaginous character for the +most part at the base and the sides, and remains membranous at the +roof. At this part the bones of the subsequent osseous skull develop +as external coverings over the membranous structure, without an +intermediate cartilaginous stage, as there is at the base of the +skull. Thus a large part of the cranial bones develop originally as +covering bones from the corium, and only secondarily come into close +touch with the primitive skull (Figure 2.333). We have previously seen +how this very rudimentary beginning of the skull in man is formed +ontogenetically from the "head-plates," and thus the fore end of the +chorda is enclosed in the base of the skull. (Cf. Figs 1.145 and +Chapters 1.13 and 1.14.) + +The phylogeny of the skull has made great progress during the last +three decades through the joint attainments of comparative anatomy, +ontogeny, and paleontology. By the judicious and comprehensive +application of the phylogenetic method (in the sense of Gegenbaur) we +have found the key to the great and important problems that arise from +the thorough comparative study of the skull. Another school of +research, the school of what is called "exact craniology" (in the +sense of Virchow), has, meantime, made fruitless efforts to obtain +this result. We may gratefully acknowledge all that this descriptive +school has done in the way of accurately describing the various forms +and measurements of the human skull, as compared with those of other +mammals. But the vast empirical material that it has accumulated in +its extensive literature is mere dead and sterile erudition until it +is vivified and illumined by phylogenetic speculation. + +Virchow confined himself to the most careful analysis of large numbers +of human skulls and those of anthropoid mammals. He saw only the +differences between them, and sought to express these in figures. + +Without adducing a single solid reason, or offering any alternative +explanation, he rejected evolution as an unproved hypothesis. He +played a most unfortunate part in the controversy as to the +significance of the fossil human skulls of Spy and Neanderthal, and +the comparison of them with the skull of the Pithecanthropus (Figure +2.283). All the interesting features of these skulls that clearly +indicated the transition from the anthropoid to the man were declared +by Virchow to be chance pathological variations. He said that the roof +of the skull of Pithecanthropus (Figure 2.335, 3) must have belonged +to an ape, because so pronounced an orbital stricture (the horizontal +constriction between the outer edge of the eye-orbit and the temples) +is not found in any human being. Immediately afterwards Nehring showed +in the skull of a Brazilian Indian (Figure 2.335, 2), found in the +Sambaquis of Santos, that this stricture can be even deeper in man +than in many of the apes. It is very instructive in this connection to +compare the roofs of the skulls (seen from above) of different +primates. I have, therefore, arranged nine such skulls in Figure +2.335, and reduced them to a common size. + +(FIGURE 2.346. Transverse section of a fish's tail (from the tunny). +(From Johannes Muller.) a upper (dorsal) lateral muscles, a +apostrophe, b apostrophe lower (ventral) lateral muscles, d vertebral +bodies, b sections of incomplete conical mantle, B attachment lines of +the inter-muscular ligaments (from the side).) + +We turn now to the branchial arches, which were regarded even by the +earlier natural philosophers as "head-ribs." (Cf. Figures 1.167 to +1.170). Of the four original gill-arches of the mammals the first lies +between the primitive mouth and the first gill-cleft. From the base of +this arch is formed the upper-jaw process, which joins with the inner +and outer nasal processes on each side, in the manner we have +previously explained, and forms the chief parts of the skeleton of the +upper jaw (palate bone, pterygoid bone, etc.) (Cf. Chapter 2.25.) The +remainder of the first branchial arch, which is now called, by way of +contrast, the "upper-jaw process," forms from its base two of the +ear-ossicles (hammer and anvil), and as to the rest is converted into +a long strip of cartilage that is known, after its discoverer, as +"Meckel's cartilage," or the promandibula. At the outer surface of the +latter is formed from the cellular matter of the corium, as covering +or accessory bone, the permanent bony lower jaw. From the first part +or base of the second branchial arch we get, in the mammals, the third +ossicle of the ear, the stirrup; and from the succeeding parts we get +(in this order) the muscle of the stirrup, the styloid process of the +temporal bone, the styloid-hyoid ligament, and the little horn of the +hyoid bone. The third branchial arch is only cartilaginous at the +foremost part, and here the body of the hyoid bone and its larger horn +are formed at each side by the junction of its two halves. The fourth +branchial arch is only found transitorily in the mammal embryo as a +rudimentary organ, and does not develop special parts; and there is no +trace in the embryo of the higher Vertebrates of the posterior +branchial arches (fifth and sixth pair), which are permanent in the +Selachii. They have been lost long ago. Moreover, the four gill-clefts +of the human embryo are only interesting as rudimentary organs, and +they soon close up and disappear. The first alone (between the first +and second branchial arches) has any permanent significance; from it +are developed the tympanic cavity and the Eustachian tube. (Cf. +Figures 1.169 and 2.320.) + +It was Carl Gegenbaur again who solved the difficult problem of +tracing the skeleton of the limbs of the Vertebrates to a common type. +Few parts of the vertebrate body have undergone such infinitely varied +modifications in regard to size, shape, and adaptation of structure as +the limbs or extremities; yet we are in a position to reduce them all +to the same hereditary standard. We may generally distinguish three +groups among the Vertebrates in relation to the formation of their +limbs. The lowest and earliest Vertebrates, the Acrania and +Cyclostomes, had, like their invertebrate ancestors, no pairs of +limbs, as we see in the Amphioxus and the Cyclostomes to-day (Figures +2.210 and 2.247). The second group is formed of the two classes of the +true fishes and the Dipneusts; here there are always two pairs of +limbs at first, in the shape of many-toed fins--one pair of +breast-fins or fore legs, and one pair of belly-fins or hind legs +(Figures 2.248 to 2.259). The third group comprises the four higher +classes of Vertebrates--the amphibia, reptiles, birds, and mammals; in +these quadrupeds there are at first the same two pairs of limbs, but +in the shape of five-toed feet. Frequently we find less than five +toes, and sometimes the feet are wholly atrophied (as in the +serpents). But the original stem-form of the group had five toes or +fingers before and behind (Figures 2.263 to 2.265). + +The true primitive form of the pairs of limbs, such as they were found +in the primitive fishes of the Silurian period, is preserved for us in +the Australian dipneust, the remarkable Ceratodus (Figure 2.257). Both +the breast-fin and the belly-fin are flat oval paddles, in which we +find a biserial cartilaginous skeleton (Figure 2.336). This consists, +firstly, of a much segmented fin-rod or "stem" (A, B), which runs +through the fin from base to tip; and secondly of a double row of thin +articulated fin-radii (r, r), which are attached to both sides of the +fin-rod, like the feathers of a feathered leaf. This primitive fin, +which Gegenbaur first recognised, is attached to the vertebral column +by a simple zone in the shape of a cartilaginous arch. It has probably +originated from the branchial arches.* (* While Gegenbaur derives the +fins from two pairs of posterior separated branchial arches, Balfour +holds that they have been developed from segments of a pair of +originally continuous lateral fins or folds of the skin.) + +We find the same biserial primitive fin more or less preserved in the +fossilised remains of the earliest Selachii (Figure 2.248), Ganoids +(Figure 2.253), and Dipneusts (Figure 2.256). It is also found in +modified form in some of the actual sharks and pikes. But in the +majority of the Selachii it has already degenerated to the extent that +the radii on one side of the fin-rod have been partly or entirely +lost, and are retained only on the other (Figure 2.337). We thus get +the uniserial fin, which has been transmitted from the Selachii to the +rest of the fishes (Figure 2.338). + +(FIGURE 2.347. Human skeleton. (Cf. Figure 2.326.) + +FIGURE 2.348. Skeleton of the giant gorilla. (Cf. Figure 1.209.)) + +Gegenbaur has shown how the five-toed leg of the Amphibia, that has +been inherited by the three classes of Amniotes, was evolved from the +uniserial fish-fin.* (* The limb of the four higher classes of +Vertebrates is now explained in the sense that the original fin-rod +passes along its outer (ulnar or fibular) side, and ends in the fifth +toe. It was formerly believed to go along the inner (radial or tibial) +side, and end in the first toe, as Figure 2.339 shows.) In the +dipneust ancestors of the Amphibia the radii gradually atrophy, and +are lost, for the most part, on the other side of the fin-rod as well +(the lighter cartilages in Figure 2.338). Only the four lowest radii +(shaded in the illustration) are preserved; and these are the four +inner toes of the foot (first to fourth). The little or fifth toe is +developed from the lower end of the fin-rod. From the middle and upper +part of the fin-rod was developed the long stem of the limb--the +important radius and ulna (Figure 2.339 r and u) and humerus (h) of +the higher Vertebrates. + +In this way the five-toed foot of the Amphibia, which we first meet in +the Carboniferous Stegocephala (Figure 2.260), and which was inherited +from them by the reptiles on one side and the mammals on the other, +was formed by gradual degeneration and differentiation from the +many-toed fish-fin (Figure 2.341). The reduction of the radii to four +was accompanied by a further differentiation of the fin-rod, its +transverse segmentation into upper and lower halves, and the formation +of the zone of the limb, which is composed originally of three limbs +before and behind in the higher Vertebrates. The simple arch of the +original shoulder-zone divides on each side into an upper (dorsal) +piece, the shoulder-blade (scapula), and a lower (ventral) piece; the +anterior part of the latter forms the primitive clavicle +(procoracoideum), and the posterior part the coracoideum. In the same +way the simple arch of the pelvic zone breaks up into an upper +(dorsal) piece, the iliac-bone (os ilium), and a lower (ventral) +piece; the anterior part of the latter forms the pubic bone (os +pubis), and the posterior the ischial bone (os ischii). + +There is also a complete agreement between the fore and hind limb in +the stem or shaft. The first section of the stem is supported by a +single strong bone--the humerus in the fore, the femur in the hind +limb. The second section contains two bones: in front the radius (r) +and ulna (u), behind the tibia and fibula. (Cf. the skeletons in +Figures 2.260, 2.265, 2.270, 2.278 to 2.282, and 2.348.) The +succeeding numerous small bones of the wrist (carpus) and ankle +(tarsus) are also similarly arranged in the fore and hind extremities, +and so are the five bones of the middle-hand (metacarpus) and +middle-foot (metatarsus). Finally, it is the same with the toes +themselves, which have a similar characteristic composition from a +series of bony pieces before and behind. We find a complete parallel +in all the parts of the fore leg and the hind leg. + +When we thus learn from comparative anatomy that the skeleton of the +human limbs is composed of just the same bones, put together in the +same way, as the skeleton in the four higher classes of Vertebrates, +we may at once infer a common descent of them from a single stem-form. +This stem-form was the earliest amphibian that had five toes on each +foot. It is particularly the outer parts of the limbs that have been +modified by adaptation to different conditions. We need only recall +the immense variations they offer within the mammal class. We have the +slender legs of the deer and the strong springing legs of the +kangaroo, the climbing feet of the sloth and the digging feet of the +mole, the fins of the whale and the wings of the bat. It will readily +be granted that these organs of locomotion differ as much in regard to +size, shape, and special function as can be conceived. Nevertheless, +the bony skeleton is substantially the same in every case. In the +different limbs we always find the same characteristic bones in +essentially the same rigidly hereditary connection; this is as +splendid a proof of the theory of evolution as comparative anatomy can +discover in any organ of the body. It is true that the skeleton of the +limbs of the various mammals undergoes many distortions and +degenerations besides the special adaptations (Figure 2.342). Thus we +find the first finger or the thumb atrophied in the fore-foot (or +hand) of the dog (II). It has entirely disappeared in the pig (III) +and tapir (V). In the ruminants (such as the ox, IV) the second and +fifth toes are also atrophied, and only the third and fourth are well +developed (VI, 3). Nevertheless, all these different fore-feet, as +well as the hand of the ape (Figure 2.340) and of man (Figure 2.341), +were originally developed from a common pentadactyle stem-form. This +is proved by the rudiments of the degenerated toes, and by the +similarity of the arrangement of the wrist-bones in all the pentanomes +(Figure 2.342 a to p). + +If we candidly compare the bony skeleton of the human arm and hand +with that of the nearest anthropoid apes, we find an almost perfect +identity. This is especially true of the chimpanzee. In regard to the +proportions of the various parts, the lowest living races of men (the +Veddahs of Ceylon, Figure 2.344) are midway between the chimpanzee +(Figure 2.343) and the European (Figure 2.345). More considerable are +the differences in structure and the proportions of the various parts +between the different genera of anthropoid apes (Figures 2.278 to +2.282); and still greater is the morphological distance between these +and the lowest apes (the Cynopitheca). Here, again, impartial and +thorough anatomic comparison confirms the accuracy of Huxley's +pithecometra principle (Chapter 1.15). + +The complete unity of structure which is thus revealed by the +comparative anatomy of the limbs is fully confirmed by their +embryology. However different the extremities of the four-footed +Craniotes may be in their adult state, they all develop from the same +rudimentary structure. In every case the first trace of the limb in +the embryo is a very simple protuberance that grows out of the side of +the hyposoma. These simple structures develop directly into fins in +the fishes and Dipneusts by differentiation of their cells. In the +higher classes of Vertebrates each of the four takes the shape in its +further growth of a leaf with a stalk, the inner half becoming +narrower and thicker and the outer half broader and thinner. The inner +half (the stalk of the leaf) then divides into two sections--the upper +and lower parts of the limb. Afterwards four shallow indentations are +formed at the free edge of the leaf, and gradually deepen; these are +the intervals between the five toes (Figure 1.174). The toes soon make +their appearance. But at first all five toes, both of fore and hind +feet, are connected by a thin membrane like a swimming-web; they +remind us of the original shaping of the foot as a paddling fin. The +further development of the limbs from this rudimentary structure takes +place in the same way in all the Vertebrates according to the laws of +heredity. + +The embryonic development of the muscles, or ACTIVE organs of +locomotion, is not less interesting than that of the skeleton, or +PASSIVE organs. But the comparative anatomy and ontogeny of the +muscular system are much more difficult and inaccessible, and +consequently have hitherto been less studied. We can therefore only +draw some general phylogenetic conclusions therefrom. + +It is incontestable that the musculature of the Vertebrates has been +evolved from that of lower Invertebrates; and among these we have to +consider especially the unarticulated Vermalia. They have a simple +cutaneous muscular layer, developing from the mesoderm. This was +afterwards replaced by a pair of internal lateral muscles, that +developed from the middle wall of the coelom-pouches; we still find +the first rudiments of the muscles arising from the muscle-plate of +these in the embryos of all the Vertebrates (cf. Figures 1.124, 1.158 +to 1.160, 2.222 to 2.224 mp). In the unarticulated stem-forms of the +Chordonia, which we have called the Prochordonia, the two +coelom-pouches, and therefore also the muscle-plates of their walls, +were not yet segmented. A great advance was made in the articulation +of them, as we have followed it step by step in the Amphioxus (Figures +1.124 and 1.158). This segmentation of the muscles was the momentous +historical process with which vertebration, and the development of the +vertebrate stem, began. The articulation of the skeleton came after +this segmentation of the muscular system, and the two entered into +very close correlation. + +The episomites or dorsal coelom-pouches of the Acrania, Cyclostomes, +and Selachii (Figure 1.161 h) first develop from their inner or median +wall (from the cell-layer that lies directly on the skeletal plate +[sk] and the medullary tube [nr]) a strong muscle-plate (mp). By +dorsal growth (w) it also reaches the external wall of the +coelom-pouches, and proceeds from the dorsal to the ventral wall. From +these segmental muscle-plates, which are chiefly concerned in the +segmentation of the Vertebrates, proceed the lateral muscles of the +stem, as we find in the simplest form in the Amphioxus (Figure 2.210). +By the formation of a horizontal frontal septum they divide on each +side into an upper and lower series of myotomes, dorsal and ventral +lateral muscles. This is seen with typical regularity in the +transverse section of the tail of a fish (Figure 2.346). From these +earlier lateral muscles of the trunk develop the greater part of the +subsequent muscles of the trunk, and also the much later "muscular +buds" of the limbs.* (* The ontogeny of the muscles is mostly +cenogenetic. The greater part of the muscles of the head (or the +visceral muscles) belong originally to the hyposoma of the vertebrate +organism, and develop from the wall of the hyposomites or ventral +coelom-pouches. This also applies originally to the primary muscles of +the limbs, as these too belong phylogenetically to the hyposoma. (Cf. +Chapter 1.14)) + + +CHAPTER 2.27. THE EVOLUTION OF THE ALIMENTARY SYSTEM. + +The chief of the vegetal organs of the human frame, to the evolution +of which we now turn our attention, is the alimentary canal. The gut +is the oldest of all the organs of the metazoic body, and it leads us +back to the earliest age of the formation of organs--to the first +section of the Laurentian period. As we have already seen, the result +of the first division of labour among the homogeneous cells of the +earliest multicellular animal body was the formation of an alimentary +cavity. The first duty and first need of every organism is +self-preservation. This is met by the functions of the nutrition and +the covering of the body. When, therefore, in the primitive globular +Blastaea the homogeneous cells began to effect a division of labour, +they had first to meet this twofold need. One half were converted into +alimentary cells and enclosed a digestive cavity, the gut. The other +half became covering cells, and formed an envelope round the +alimentary tube and the whole body. Thus arose the primary germinal +layers--the inner, alimentary, or vegetal layer, and the outer, +covering, or animal layer. (Cf. Chapter 2.19.) + +When we try to construct an animal frame of the simplest conceivable +type, that has some such primitive alimentary canal and the two +primary layers constituting its wall, we inevitably come to the very +remarkable embryonic form of the gastrula, which we have found with +extraordinary persistence throughout the whole range of animals, with +the exception of the unicellulars--in the Sponges, Cnidaria, Platodes, +Vermalia, Molluscs, Articulates, Echinoderms, Tunicates, and +Vertebrates. In all these stems the gastrula recurs in the same very +simple form. It is certainly a remarkable fact that the gastrula is +found in various animals as a larva-stage in their individual +development, and that this gastrula, though much disguised by +cenogenetic modifications, has everywhere essentially the same +palingenetic structure (Figures 1.30 to 1.35). The elaborate +alimentary canal of the higher animals develops ontogenetically from +the same simple primitive gut of the gastrula. + +This gastraea theory is now accepted by nearly all zoologists. It was +first supported and partly modified by Professor Ray-Lankester; he +proposed three years afterwards (in his essay on the development of +the Molluscs, 1875) to give the name of archenteron to the primitive +gut and blastoporus to the primitive mouth. + +Before we follow the development of the human alimentary canal in +detail, it is necessary to say a word about the general features of +its composition in the fully-developed man. The mature alimentary +canal in man is constructed in all its main features like that of all +the higher mammals, and particularly resembles that of the +Catarrhines, the narrow-nosed apes of the Old World. The entrance into +it, the mouth, is armed with thirty-two teeth, fixed in rows in the +upper and lower jaws. As we have seen, our dentition is exactly the +same as that of the Catarrhines, and differs from that of all other +animals (Chapter 2.23). Above the mouth-cavity is the double nasal +cavity; they are separated by the palate-wall. But we saw that this +separation is not there from the first, and that originally there is a +common mouth-nasal cavity in the embryo; and this is only divided +afterwards by the hard palate into two--the nasal cavity above and +that of the mouth below (Figure 2.311). + +At the back the cavity of the mouth is half closed by the vertical +curtain that we call the soft palate, in the middle of which is the +uvula. A glance into a mirror with the mouth wide open will show its +shape. The uvula is interesting because, besides man, it is only found +in the ape. At each side of the soft palate are the tonsils. Through +the curved opening that we find underneath the soft palate we +penetrate into the gullet or pharynx behind the mouth-cavity. Into +this opens on either side a narrow canal (the Eustachian tube), +through which there is direct communication with the tympanic cavity +of the ear (Figure 2.320 e). The pharynx is continued in a long, +narrow tube, the oesophagus (sr). By this the food passes into the +stomach when masticated and swallowed. Into the gullet also opens, +right above, the trachea (lr), that leads to the lungs. The entrance +to it is covered by the epiglottis, over which the food slides. The +cartilaginous epiglottis is found only in the mammals, and has +developed from the fourth branchial arch of the fishes and amphibia. +The lungs are found, in man and all the mammals, to the right and left +in the pectoral cavity, with the heart between them. At the upper end +of the trachea there is, under the epiglottis, a specially +differentiated part, strengthened by a cartilaginous skeleton, the +larynx. This important organ of human speech also develops from a part +of the alimentary canal. In front of the larynx is the thyroid gland, +which sometimes enlarges and forms goitre. + +The oesophagus descends into the pectoral cavity along the vertebral +column, behind the lungs and the heart, pierces the diaphragm, and +enters the visceral cavity. The diaphragm is a membrano-muscular +partition that completely separates the thoracic from the abdominal +cavity in all the mammals (and these alone). This separation is not +found in the beginning; there is at first a common breast-belly +cavity, the coeloma or pleuro-peritoneal cavity. The diaphragm is +formed later on as a muscular horizontal partition between the +thoracic and abdominal cavities. It then completely separates the two +cavities, and is only pierced by several organs that pass from the one +to the other. One of the chief of these organs is the oesophagus. +After this has passed through the diaphragm, it expands into the +gastric sac in which digestion chiefly takes place. The stomach of the +adult man (Figure 2.349) is a long, somewhat oblique sac, expanding on +the left into a blind sac, the fundus of the stomach (b apostrophe), +but narrowing on the right, and passing at the pylorus (e) into the +small intestine. At this point there is a valve, the pyloric valve +(d), between the two sections of the canal; it opens only when the +pulpy food passes from the stomach into the intestine. In man and the +higher Vertebrates the stomach itself is the chief organ of digestion, +and is especially occupied with the solution of the food; this is not +the case in many of the lower Vertebrates, which have no stomach, and +discharge its function by a part of the gut farther on. The muscular +wall of the stomach is comparatively thick; it has externally strong +muscles that accomplish the digestive movements, and internally a +large quantity of small glands, the peptic glands, which secrete the +gastric juice. + +(FIGURE 2.349. Human stomach and duodenum, longitudinal section. a +cardiac (end of oesophagus), b fundus (blind sac of the left side), c +pylorus-fold, d pylorus-valves, e pylorus-cavity, fgh duodenum, i +entrance of the gall-duct and the pancreatic duct. (From Meyer.) + +FIGURE 2.350. Median section of the head of a hare-embryo, one-fourth +of an inch in length. (From Mihalcovics.) The deep mouth-cleft (hp) is +separated by the membrane of the throat (rh) from the blind cavity of +the head-gut (kd). hz heart, ch chorda, hp the point at which the +hypophysis develops from the mouth-cleft, vh ventricle of the +cerebrum, v3, third ventricle (intermediate brain), v4 fourth +ventricle (hind brain), ck spinal canal.) + +Next to the stomach comes the longest section of the alimentary canal, +the middle gut or small intestine. Its chief function is to absorb the +peptonised fluid mass of food, or the chyle, and it is subdivided into +several sections, of which the first (next to the stomach) is called +the duodenum (Figure 2.349 fgh). It is a short, horseshoe-shaped loop +of the gut. The largest glands of the alimentary canal open into +it--the liver, the chief digestive gland, that secretes the gall, and +the pancreas, which secretes the pancreatic juice. The two glands pour +their secretions, the bile and pancreatic juice, close together into +the duodenum (i). The opening of the gall-duct is of particular +phylogenetic importance, as it is the same in all the Vertebrates, and +indicates the principal point of the hepatic or trunk-gut (Gegenbaur). +The liver, phylogenetically older than the stomach, is a large gland, +rich in blood, in the adult man, immediately under the diaphragm on +the left side, and separated by it from the lungs. The pancreas lies a +little further back and more to the left. The remaining part of the +small intestine is so long that it has to coil itself in many folds in +order to find room in the narrow space of the abdominal cavity. It is +divided into the jejunum above and the ileum below. In the last +section of it is the part of the small intestine at which in the +embryo the yelk-sac opens into the gut. This long and thin intestine +then passes into the large intestine, from which it is cut off by a +special valve. Immediately behind this "Bauhin-valve" the first part +of the large intestine forms a wide, pouch-like structure, the caecum. +The atrophied end of the caecum is the famous rudimentary organ, the +vermiform appendix. The large intestine (colon) consists of three +parts--an ascending part on the right, a transverse middle part, and a +descending part on the left. The latter finally passes through an +S-shaped bend into the last section of the alimentary canal, the +rectum, which opens behind by the anus. Both the large and small +intestines are equipped with numbers of small glands, which secrete +mucous and other fluids. + +For the greater part of its length the alimentary canal is attached to +the inner dorsal surface of the abdominal cavity, or to the lower +surface of the vertebral column. The fixing is accomplished by means +of the thin membranous plate that we call the mesentery. + +Although the fully-formed alimentary canal is thus a very elaborate +organ, and although in detail it has a quantity of complex structural +features into which we cannot enter here, nevertheless the whole +complicated structure has been historically evolved from the very +simple form of the primitive gut that we find in our +gastraead-ancestors, and that every gastrula brings before us to-day. +We have already pointed out (Chapter 1.9) how the epigastrula of the +mammals (Figure 1.67) can be reduced to the original type of the +bell-gastrula, which is now preserved by the amphioxus alone (Figure +1.35). Like the latter, the human gastrula and that of all other +mammals must be regarded as the ontogenetic reproduction of the +phylogenetic form that we call the Gastraea, in which the whole body +is nothing but a double-walled gastric sac. + +We already know from embryology the manner in which the gut develops +in the embryo of man and the other mammals. From the gastrula is first +formed the spherical embryonic vesicle filled with fluid +(gastrocystis, Figure 1.106). In the dorsal wall of this the +sole-shaped embryonic shield is developed, and on the under-side of +this a shallow groove appears in the middle line, the first trace of +the later, secondary alimentary tube. The gut-groove becomes deeper +and deeper, and its edges bend towards each other, and finally form a +tube. + +As we have seen, this simple cylindrical gut-tube is at first +completely closed before and behind in man and in the Vertebrates +generally (Figure 1.148); the permanent openings of the alimentary +canal, the mouth and anus, are only formed later on, and from the +outer skin. A mouth-pit appears in the skin in front (Figure 2.350 +hp), and this grows towards the blind fore-end of the cavity of the +head-gut (kd), and at length breaks into it. In the same way a shallow +anus-pit is formed in the skin behind, which grows deeper and deeper, +advances towards the blind hinder end of the pelvic gut, and at last +connects with it. There is at first, both before and behind, a thin +partition between the external cutaneous pit and the blind end of the +gut--the throat-membrane in front and the anus-membrane behind; these +disappear when the connection takes place. + +Directly in front of the anus-opening the allantois develops from the +hind gut; this is the important embryonic structure that forms into +the placenta in the Placentals (including man). In this more advanced +form the human alimentary canal (and that of all the other mammals) is +a slightly bent, cylindrical tube, with an opening at each end, and +two appendages growing from its lower wall: the anterior one is the +umbilical vesicle or yelk-sac, and the posterior the allantois or +urinary sac (Figure 1.195). + +The thin wall of this simple alimentary tube and its ventral +appendages is found, on microscopic examination, to consist of two +strata of cells. The inner stratum, lining the entire cavity, consists +of larger and darker cells, and is the gut-gland layer. The outer +stratum consists of smaller and lighter cells, and is the gut-fibre +layer. The only exception is in the cavities of the mouth and anus, +because these originate from the skin. The inner coat of the +mouth-cavity is not provided by the gut-gland layer, but by the +skin-sense layer; and its muscular substratum is provided, not by the +gut-fibre, but the skin-fibre, layer. It is the same with the wall of +the small anus-cavity. + +If it is asked how these constituent layers of the primitive gut-wall +are related to the various tissues and organs that we find afterwards +in the fully-developed system, the answer is very simple. It can be +put in a single sentence. The epithelium of the gut--that is to say, +the internal soft stratum of cells that lines the cavity of the +alimentary canal and all its appendages, and is immediately occupied +with the processes of nutrition--is formed solely from the gut-gland +layer; all other tissues and organs that belong to the alimentary +canal and its appendages originate from the gut-fibre layer. From the +latter is also developed the whole of the outer envelope of the gut +and its appendages; the fibrous connective tissue and the smooth +muscles that compose its muscular layer, the cartilages that support +it (such as the cartilages of the larynx and the trachea), the +blood-vessels and lymph-vessels that absorb the nutritive fluid from +the intestines--in a word, all that there is in the alimentary system +besides the epithelium of the gut. From the same layer we also get the +whole of the mesentery, with all the organs embedded in it--the heart, +the large blood-vessels of the body, etc. + +(FIGURE 2.351. Scales or cutaneous teeth of a shark (Centrophorus +calceus). A three-pointed tooth rises obliquely on each of the +quadrangular bony plates that lie in the corium. (From Gegenbaur.)) + +Let us now leave this original structure of the mammal gut for a +moment, in order to compare it with the alimentary canal of the lower +Vertebrates, and of those Invertebrates that we have recognised as +man's ancestors. We find, first of all, in the lowest Metazoa, the +Gastraeads, that the gut remains permanently in the very simple form +in which we find it transitorily in the palingenetic gastrula of the +other animals; it is thus in the Gastremaria (Pemmatodiscus), the +Physemaria (Prophysema), the simplest Sponges (Olynthus), the +freshwater Polyps (Hydra), and the ascula-embryos of many other +Coelenteria (Figures 2.233 to 2.238). Even in the simplest forms of +the Platodes, the Rhabdocoela (Figure 2.240), the gut is still a +simple straight tube, lined with the entoderm; but with the important +difference that in this case its single opening, the primitive mouth +(m), has formed a muscular gullet (sd) by invagination of the skin. + +(FIGURE 2.352. Gut of a human embryo, one-sixth of an inch long, +magnified fifteen times. (From His. Showing: Epiglottis, Tongue, +Hypophysis, Hepatic duct, Tail, Allantoic duct, Tail-gut, Umbilical +cord, Larynx, Rudimentary lungs, Stomach, Pancreas, Bladder, Wolffian +duct, Rudimentary kidneys.)) + +We have the same simple form in the gut of the lowest Vermalia +(Gastrotricha, Figure 2.242, Nematodes, Sagitta, etc.). But in these a +second important opening of the gut has been formed at the opposite +end to the mouth, the anus (Figure 2.242 a). + +We see a great advance in the structure of the vermalian gut in the +remarkable Balanoglossus (Figure 2.245), the sole survivor of the +Enteropneust class. Here we have the first appearance of the division +of the alimentary tube into two sections that characterises the +Chordonia. The fore half, the head-gut (cephalogaster), becomes the +organ of respiration (branchial gut, Figure 2.245 k); the hind half, +the trunk-gut (truncogaster), alone acts as digestive organ (hepatic +gut, d). The differentiation of these two parts of the gut in the +Enteropneust is just the same as in all the Tunicates and Vertebrates. + +It is particularly interesting and instructive in this connection to +compare the Enteropneusts with the Ascidia and the Amphioxus (Figures +2.220 and 2.210)--the remarkable animals that form the connecting link +between the Invertebrates and the Vertebrates. In both forms the gut +is of substantially the same construction; the anterior section forms +the respiratory branchial gut, the posterior the digestive hepatic +gut. In both it develops palingenetically from the primitive gut of +the gastrula, and in both the hinder end of the medullary tube covers +the primitive mouth to such an extent that the remarkable medullary +intestinal duct is formed, the passing communication between the +neural and intestinal tubes (canalis neurentericus, Figures 1.83 and +1.85 ne). In the vicinity of the closed primitive mouth, possibly in +its place, the later anus is developed. In the same way the mouth is a +fresh formation in the Amphioxus and the Ascidia. It is the same with +the human mouth and that of the Craniotes generally. The secondary +formation of the mouth in the Chordonia is probably connected with the +development of the gill-clefts which are formed in the gut-wall +immediately behind the mouth. In this way the anterior section of the +gut is converted into a respiratory organ. I have already pointed out +that this modification is distinctive of the Vertebrates and +Tunicates. The phylogenetic appearance of the gill-clefts indicates +the commencement of a new epoch in the stem-history of the +Vertebrates. + +In the further ontogenetic development of the alimentary canal in the +human embryo the appearance of the gill-clefts is the most important +process. At a very early stage the gullet-wall joins with the external +body-wall in the head of the human embryo, and this is followed by the +formation of four clefts, which lead directly into the gullet from +without, on the right and left sides of the neck, behind the mouth. +These are the gill or gullet clefts, and the partitions that separate +them are the gill or gullet-arches (Figure 1.171). These are most +interesting embryonic structures. They show us that all the higher +Vertebrates reproduce in their earlier stages, in harmony with the +biogenetic law, the process that had so important a part in the rise +of the whole Chordonia-stem. This process was the differentiation of +the gut into two sections--an anterior respiratory section, the +branchial gut, that was restricted to breathing, and a posterior +digestive section, the hepatic gut. As we find this highly +characteristic differentiation of the gut into two different sections +in all the Vertebrates and all the Tunicates, we may conclude that it +was also found in their common ancestors, the Prochordonia--especially +as even the Enteropneusts have it. (Cf. Chapters 1.12, 1.14 and 2.20, +and Figures 2.210, 2.220, 2.245.) It is entirely wanting in all the +other Invertebrates. + +(FIGURE 2.353. Gut of a dog-embryo (shown in Figure 1.202, from +Bischoff), seen from the ventral side, a gill-arches (four pairs), b +rudiments of pharynx and larynx, c lungs, d stomach, f liver, g walls +of the open yelk-sac (into which the middle gut opens with a wide +aperture), h rectum. + +FIGURE 2.354. The same gut seen from the right. a lungs, b stomach, c +liver, d yelk-sac, e rectum.) + +There is at first only one pair of gill-clefts in the Amphioxus, as in +the Ascidia and Enteropneusts; and the Copelata (Figure 2.225) have +only one pair throughout life. But the number presently increases in +the former. In the Craniotes, however, it decreases still further. The +Cyclostomes have six to eight pairs (Figure 2.247); some of the +Selachii six or seven pairs, most of the fishes only four or five +pairs. In the embryo of man, and the higher Vertebrates generally, +where they make an appearance at an early stage, only three or four +pairs are developed. In the fishes they remain throughout life, and +form an exit for the water taken in at the mouth (Figures 2.249 to +2.251). But they are partly lost in the amphibia, and entirely in the +higher Vertebrates. In these nothing is left but a relic of the first +gill-cleft. This is formed into a part of the organ of hearing; from +it are developed the external meatus, the tympanic cavity, and the +Eustachian tube. We have already considered these remarkable +structures, and need only point here to the interesting fact that our +middle and external ear is a modified inheritance from the fishes. The +branchial arches also, which separate the clefts, develop into very +different parts. In the fishes they remain gill-arches, supporting the +respiratory gill-leaves. It is the same with the lowest amphibia, but +in the higher amphibia they undergo various modifications; and in the +three higher classes of Vertebrates (including man) the hyoid bone and +the ossicles of the ear develop from them. (Cf. Chapter 2.25.) + +(FIGURE 2.355. Median section of the head of a Petromyzon-larva. (From +Gegenbaur,) h hypobranchial groove (above it in the gullet we see the +internal openings of the seven gill-clefts), v velum, o mouth, c +heart, a auditory vesicle, n neural tube, ch chorda.) + +From the first gill-arch, from the inner surface of which the muscular +tongue proceeds, we get the first structure of the maxillary +skeleton--the upper and lower jaws, which surround the mouth and +support the teeth. These important parts are wholly wanting in the two +lowest classes of Vertebrates, the Acrania and Cyclostoma. They appear +first in the earliest Selachii (Figures 2.248 to 2.251), and have been +transmitted from this stem-group of the Gnathostomes to the higher +Vertebrates. Hence the original formation of the skeleton of the mouth +can be traced to these primitive fishes, from which we have inherited +it. The teeth are developed from the skin that clothes the jaws. As +the whole mouth cavity originates from the outer integument (Figure +2.350), the teeth also must come from it. As a fact, this is found to +be the case on microscopic examination of the development and finer +structure of the teeth. The scales of the fishes, especially of the +shark type (Figure 2.351), are in the same position as their teeth in +this respect (Figure 2.252). The osseous matter of the tooth (dentine) +develops from the corium; its enamel covering is a secretion of the +epidermis that covers the corium. It is the same with the cutaneous +teeth or placoid scales of the Selachii. At first the whole of the +mouth was armed with these cutaneous teeth in the Selachii and in the +earliest amphibia. Afterwards the formation of them was restricted to +the edges of the jaws. + +Hence our human teeth are, in relation to their original source, +modified fish-scales. For the same reason we must regard the salivary +glands, which open into the mouth, as epidermic glands, as they are +formed, not from the glandular layer of the gut like the rest of the +alimentary glands, but from the epidermis, from the horny plate of the +outer germinal layer. Naturally, in harmony with this evolution of the +mouth, the salivary glands belong genetically to one series with the +sudoriferous, sebaceous, and mammary glands. + +Thus the human alimentary canal is as simple as the primitive gut of +the gastrula in its original structure. Later it resembles the gut of +the earliest Vermalia (Gastrotricha). It then divides into two +sections, a fore or branchial gut and a hind or hepatic gut, like the +alimentary canal of the Balanoglossus, the Ascidia, and the Amphioxus. +The formation of the jaws and the branchial arches changes it into a +real fish-gut (Selachii). But the branchial gut, the one reminiscence +of our fish-ancestors, is afterwards atrophied as such. The parts of +it that remain are converted into entirely different structures. + +(FIGURE 2.356. Transverse section of the head of a Petromyzon-larva. +(From Gegenbaur.) Beneath the pharynx (d) we see the hypobranchial +groove; above it the chorda and neural tube. A, B, C stages of +constriction.) + +But, although the anterior section of our alimentary canal thus +entirely loses its original character of branchial gut, it retains the +physiological character of respiratory gut. We are now astonished to +find that the permanent respiratory organ of the higher Vertebrates, +the air-breathing lung, is developed from this first part of the +alimentary canal. Our lungs, trachea, and larynx are formed from the +ventral wall of the branchial gut. The whole of the respiratory +apparatus, which occupies the greater part of the pectoral cavity in +the adult man, is at first merely a small pair of vesicles or sacs, +which grow out of the floor of the head-gut immediately behind the +gills (Figures 2.354 C, 1.147 l). These vesicles are found in all the +Vertebrates except the two lowest classes, the Acrania and +Cyclostomes. In the lower Vertebrates they do not develop into lungs, +but into a large air-filled bladder, which occupies a good deal of the +body-cavity and has a quite different purport. It serves, not for +breathing, but to effect swimming movements up and down, and so is a +sort of hydrostatic apparatus--the floating bladder of the fishes +(nectocystis, Chapter 2.21). However, the human lungs, and those of +all air-breathing Vertebrates, develop from the same simple vesicular +appendage of the head-gut that becomes the floating bladder in the +fishes. + +At first this bladder has no respiratory function, but merely acts as +hydrostatic apparatus for the purpose of increasing or lessening the +specific gravity of the body. The fishes, which have a fully-developed +floating bladder, can press it together, and thus condense the air it +contains. The air also escapes sometimes from the alimentary canal, +through an air-duct that connects the floating bladder with the +pharynx, and is ejected by the mouth. This lessens the size of the +bladder, and so the fish becomes heavier and sinks. When it wishes to +rise again, the bladder is expanded by relaxing the pressure. In many +of the Crossopterygii the wall of the bladder is covered with bony +plates, as in the Triassic Undina (Figure 2.254). + +This hydrostatic apparatus begins in the Dipneusts to change into a +respiratory organ; the blood-vessels in the wall of the bladder now no +longer merely secrete air themselves, but also take in fresh air +through the air-duct. This process reaches its full development in the +Amphibia. In these the floating bladder has turned into lungs, and the +air-passage into a trachea. The lungs of the Amphibia have been +transmitted to the three higher classes of Vertebrates. In the lowest +Amphibia the lungs on either side are still very simple transparent +sacs with thin walls, as in the common water-salamander, the Triton. +It still entirely resembles the floating bladder of the fishes. It is +true that the Amphibia have two lungs, right and left. But the +floating bladder is also double in many of the fishes (such as the +early Ganoids), and divides into right and left halves. On the other +hand, the lung is single in Ceratodus (Figure 2.257). + +(FIGURE 2.357. Thoracic and abdominal viscera of a human embryo of +twelve weeks, natural size, (From Kolliker.) The head is omitted. +Ventral and pectoral walls are removed. The greater part of the +body-cavity is taken up with the liver, from the middle part of which +the caecum and the vermiform appendix protrude. Above the diaphragm, +in the middle, is the conical heart; to the right and left of it are +the two small lungs.) + +In the human embryo and that of all the other Amniotes the lungs +develop from the hind part of the ventral wall of the head-gut (Figure +1.149). Immediately behind the single structure of the thyroid gland a +median groove, the rudiment of the trachea, is detached from the +gullet. From its hinder end a couple of vesicles develop--the simple +tubular rudiments of the right and left lungs. They afterwards +increase considerably in size, fill the greater part of the thoracic +cavity, and take the heart between them. Even in the frogs we find +that the simple sac has developed into a spongy body of peculiar +froth-like tissue. The originally short connection of the pulmonary +sacs with the head-gut extends into a long, thin tube. This is the +wind-pipe (trachea); it opens into the gullet above, and divides below +into two branches which go to the two lungs. In the wall of the +trachea circular cartilages develop, and these keep it open. At its +upper end, underneath its pharyngeal opening, the larynx is +formed--the organ of voice and speech. The larynx is found at various +stages of development in the Amphibia, and comparative anatomists are +in a position to trace the progressive growth of this important organ +from the rudimentary structure of the lower Amphibia up to the +elaborate and delicate vocal apparatus that we have in the larynx of +man and of the birds. + +We must refer here to an interesting rudimentary organ of the +respiratory gut, the thyroid gland, the large gland in front of the +larynx, that lies below the "Adam's apple," and is often especially +developed in the male sex. It has a certain function--not yet fully +understood--in the nutrition of the body, and arises in the embryo by +constriction from the lower wall of the pharynx. In many mining +districts the thyroid gland is peculiarly liable to morbid +enlargement, and then forms goitre, a growth that hangs at the front +of the neck. But it is much more interesting phylogenetically. As +Wilhelm Muller, of Jena, has shown, this rudimentary organ is the last +relic of the hypobranchial groove, which we considered in a previous +chapter, and which runs in the middle line of the gill-crate in the +Ascidia and Amphioxus, and conveys food to the stomach. (Cf. Chapter +2.16, Figure 2.246). We still find it in its original character in the +larvae of the Cyclostomes (Figures 2.355 and 2.356). + +The second section of the alimentary canal, the trunk or hepatic gut, +undergoes not less important modifications among our vertebrate +ancestors than the first section. In tracing the further development +of this digestive part of the gut, we find that most complex and +elaborate organs originate from a very rudimentary original structure. +For clearness we may divide the digestive gut into three sections: the +fore gut (with oesophagus and stomach), the middle gut (duodenum, with +liver, pancreas, jejunum, and ileum, and the hind gut (colon and +rectum). Here again we find vesicular growths or appendages of the +originally simple gut developing into a variety of organs. Two of +these embryonic structures, the yelk-sac and allantois, are already +known to us. The two large glands that open into the duodenum, the +liver and pancreas, are growths from the middle and most important +part of the trunk-gut. + +Immediately behind the vesicular rudiments of the lungs comes the +section of the alimentary canal that forms the stomach (Figures 2.353 +d and 2.354 b). This sac-shaped organ, which is chiefly responsible +for the solution and digestion of the food, has not in the lower +Vertebrates the great physiological importance and the complex +character that it has in the higher. In the Acrania and Cyclostomes +and the earlier fishes we can scarcely distinguish a real stomach; it +is represented merely by the short piece from the branchial to the +hepatic gut. In some of the other fishes also the stomach is only a +very simple spindle-shaped enlargement at the beginning of the +digestive section of the gut, running straight from front to back in +the median plane of the body, underneath the vertebral column. In the +mammals its first structure is just as rudimentary as it is +permanently in the preceding. But its various parts soon begin to +develop. As the left side of the spindle-shaped sac grows much more +quickly than the right, and as it turns considerably on its axis at +the same time, it soon comes to lie obliquely. The upper end is more +to the left, and the lower end more to the right. The foremost end +draws up into the longer and narrower canal of the oesophagus. +Underneath this on the left the blind sac (fundus) of the stomach +bulges out, and thus the later form gradually develops (Figures 2.349 +and 1.184 e). The original longitudinal axis becomes oblique, sinking +below to the left and rising to the right, and approaches nearer and +nearer to a transverse position. In the outer layer of the +stomach-wall the powerful muscles that accomplish the digestive +movements develop from the gut-fibre layer. In the inner layer a +number of small glandular tubes are formed from the gut-gland layer; +these are the peptic glands that secrete the gastric juice. At the +lower end of the gastric sac is developed the valve that separates it +from the duodenum (the pylorus, Figure 2.349 d). + +Underneath the stomach there now develops the disproportionately long +stretch of the small intestine. The development of this section is +very simple, and consists essentially in an extremely rapid and +considerable growth lengthways. It is at first very short, quite +straight, and simple. But immediately behind the stomach we find at an +early stage a horseshoe-shaped bend and loop of the gut, in connection +with the severance of the alimentary canal from the yelk-sac and the +development of the first mesentery. The thin delicate membrane that +fastens this loop to the ventral side of the vertebral column, and +fills the inner bend of the horseshoe formation, is the first rudiment +of the mesentery (Figure 1.147 g). We find at an early stage a +considerable growth of the small intestine; it is thus forced to coil +itself in a number of loops. The various sections that we have to +distinguish in it are differentiated in a very simple way--the +duodenum (next to the stomach), the succeeding long jejunum, and the +last section of the small intestine, the ileum. + +From the duodenum are developed the two large glands that we have +already mentioned--the liver and pancreas. The liver appears first in +the shape of two small sacs, that are found to the right and left +immediately behind the stomach (Figures 2.353 f, and 2.354 c). In many +of the lower Vertebrates they remain separate for a long time (in the +Myxinoides throughout life), or are only imperfectly joined. In the +higher Vertebrates they soon blend more or less completely to form a +single large organ. The growth of the liver is very brisk at first. In +the human embryo it grows so much in the second month of development +that in the third it occupies by far the greater part of the +body-cavity (Figure 2.357). At first the two halves develop equally; +afterwards the left falls far behind the right. In consequence of the +unsymmetrical development and turning of the stomach and other +abdominal viscera, the whole liver is now pushed to the right side. +Although the liver does not afterwards grow so disproportionately, it +is comparatively larger in the embryo at the end of pregnancy than in +the adult. Its weight relatively to that of the whole body is 1 : 36 +in the adult, and 1 : 18 in the embryo. Hence it is very important +physiologically during embryonic life; it is chiefly concerned in the +formation of blood, not so much in the secretion of bile. + +Immediately behind the liver a second large visceral gland develops +from the duodenum, the pancreas or sweetbread. It is wanting in most +of the lowest classes of Vertebrates, and is first found in the +fishes. This organ is also an outgrowth from the gut. + +The last section of the alimentary canal, the large intestine, is at +first in the embryo a very simple, short, and straight tube, which +opens behind by the anus. It remains thus throughout life in the lower +Vertebrates. But it grows considerably in the mammals, coils into +various folds, and divides into two sections, the first and longer of +which is the colon, and the second the rectum. At the beginning of the +colon there is a valve (valvula Bauhini) that separates it from the +small intestine. Immediately behind this there is a sac-like growth, +which enlarges into the caecum (Figure 2.357 v). In the plant-eating +mammals this is very large, but it is very small or completely +atrophied in the flesh-eaters. In man, and most of the apes, only the +first portion of the caecum is wide; the blind end-part of it is very +narrow, and seems later to be merely a useless appendage of the +former. This "vermiform appendage" is very interesting as a +rudimentary organ. The only significance of it in man is that not +infrequently a cherry-stone or some other hard and indigestible matter +penetrates into its narrow cavity, and by setting up inflammation and +suppuration causes the death of otherwise sound men. Teleology has +great difficulty in giving a rational explanation of, and attributing +to a beneficent Providence, this dreaded appendicitis. In our +plant-eating ancestors this rudimentary organ was much larger and had +a useful function. + +Finally, we have important appendages of the alimentary tube in the +bladder and urethra, which belong to the alimentary system. These +urinary organs, acting as reservoir and duct for the urine excreted by +the kidneys, originate from the innermost part of the allantoic +pedicle. In the Dipneusts and Amphibia, in which the allantoic sac +first makes its appearance, it remains within the body-cavity, and +functions entirely as bladder. But in all the Amniotes it grows far +outside of the body-cavity of the embryo, and forms the large +embryonic "primitive bladder," from which the placenta develops in the +higher mammals. This is lost at birth. But the long stalk or pedicle +of the allantois remains, and forms with its upper part the middle +vesico-umbilical ligament, a rudimentary organ that goes in the shape +of a solid string from the vertex of the bladder to the navel. The +lowest part of the allantoic pedicle (or the "urachus") remains +hollow, and forms the bladder. At first this opens into the last +section of the gut in man as in the lower Vertebrates; thus there is a +real cloaca, which takes off both urine and excrements. But among the +mammals this cloaca is only permanent in the Monotremes, as it is in +all the birds, reptiles, and amphibia. In all the other mammals +(marsupials and placentals) a transverse partition is afterwards +formed, and this separates the urogenital aperture in front from the +anus-opening behind. (Cf. Chapters 2.22 and 2.29.) + + +CHAPTER 2.28. EVOLUTION OF THE VASCULAR SYSTEM. + +The use that we have hitherto made of our biogenetic law will give the +reader an idea how far we may trust its guidance in phylogenetic +investigation. This differs considerably in the various systems of +organs; the reason is that heredity and variability have a very +different range in these systems. While some of them faithfully +preserve the original palingenetic development inherited from earlier +animal ancestors, others show little trace of this rigid heredity; +they are rather disposed to follow new and divergent CENOGENETIC lines +of development in consequence of adaptation. The organs of the first +kind represent the CONSERVATIVE element in the multicellular state of +the human frame, while the latter represent the PROGRESSIVE element. +The course of historic development is a result of the correlation of +the two tendencies, and they must be carefully distinguished. + +There is perhaps no other system of organs in the human body in which +this is more necessary than in that of which we are now going to +consider the obscure development--the vascular system, or apparatus of +circulation. If we were to draw our conclusions as to the original +features in our earlier animal ancestors solely from the phenomena of +the development of this system in the embryo of man and the other +higher Vertebrates, we should be wholly misled. By a number of +important embryonic adaptations, the chief of which is the formation +of an extensive food-yelk, the original course of the development of +the vascular system has been so much falsified and curtailed in the +higher Vertebrates that little or nothing now remains in their +embryology of some of the principal phylogenetic features. We should +be quite unable to explain these if comparative anatomy and ontogeny +did not come to our assistance. + +The vascular system in man and all the Craniotes is an elaborate +apparatus of cavities filled with juices or cell-containing fluids. +These "vessels" (vascula) play an important part in the nutrition of +the body. They partly conduct the nutritive red blood to the various +parts of the body (blood-vessels); partly absorb from the gut the +white chyle formed in digestion (chyle-vessels); and partly collect +the used-up juices and convey them away from the tissues (lymphatic +vessels). With the latter are connected the large cavities of the +body, especially the body-cavity, or coeloma. The lymphatic vessels +conduct both the colourless lymph and the white chyle into the venous +part of the circulation. The lymphatic glands act as producers of new +blood-cells, and with them is associated the spleen. The centre of +movement for the circulation of the fluids is the heart, a strong +muscular sac, which contracts regularly and is equipped with valves +like a pump. This constant and steady circulation of the blood makes +possible the complex metabolism of the higher animals. + +But, however important the vascular system may be to the more advanced +and larger and highly-differentiated animals, it is not at all so +indispensable an element of animal life as is commonly supposed. The +older science of medicine regarded the blood as the real source of +life. Even in the still prevalent confused notions of heredity the +blood plays the chief part. People speak generally of full blood, half +blood, etc., and imagine that the hereditary transmission of certain +characters "lies in the blood." The incorrectness of these ideas is +clearly seen from the fact that in the act of generation the blood of +the parents is not directly transmitted to the offspring, nor does the +embryo possess blood in its early stages. We have already seen that +not only the differentiation of the four secondary germinal layers, +but also the first structures of the principal organs in the embryo of +all the Vertebrates, take place long before there is any trace of the +vascular system--the heart and the blood. In accordance with this +ontogenetic fact, we must regard the vascular system as one of the +latest organs from the phylogenetic point of view; just as we have +found the alimentary canal to be one of the earliest. In any case, the +vascular system is much later than the alimentary. + +(FIGURE 2.358. Red blood-cells of various Vertebrates (equally +magnified). 1. of man, 2. camel, 3. dove, 4. proteus, 5. +water-salamander (Triton), 6. frog, 7. merlin (Cobitis), 8. lamprey +(Petromyzon). a surface-view, b edge-view. (From Wagner.) + +FIGURE 2.359. Vascular tissues or endothelium (vasalium). A capillary +from the mesentery. a vascular cells, b their nuclei.) + +The important nutritive fluid that circulates as blood and lymph in +the elaborate canals of our vascular system is not a clear, simple +fluid, but a very complex chemical juice with millions of cells +floating in it. These blood-cells are just as important in the +complicated life of the higher animal body as the circulation of money +is to the commerce of a civilised community. Just as the citizens meet +their needs most conveniently by means of a financial circulation, so +the various tissue-cells, the microscopic citizens of the +multicellular human body, have their food conveyed to them best by the +circulating cells in the blood. These blood cells (haemocytes) are of +two kinds in man and all the other Craniotes--red cells (rhodocytes or +erythrocytes) and colourless or lymph cells (leucocytes). The red +colour of the blood is caused by the great accumulation of the former, +the others circulate among them in much smaller quantity. When the +colourless cells increase at the expense of the red we get anaemia (or +chlorosis). + +(FIGURE 2.360. Transverse section of the trunk of a chick-embryo, +forty-five hours old. (From Balfour.) A ectoderm (horny-plate), Mc +medullary tube, ch chorda, C entoderm (gut-gland layer), Pv primitive +segment (episomite), Wd prorenal duct, pp coeloma (secondary +body-cavity). So skin-fibre layer, Sp gut-fibre layer, v blood-vessels +in latter, ao primitive aortas, containing red blood-cells.) + +The lymph-cells (leucocytes), commonly called the "white corpuscles" +of the blood, are phylogenetically older and more widely distributed +in the animal world than the red. The great majority of the +Invertebrates that have acquired an independent vascular system have +only colourless lymph-cells in the circulating fluid. There is an +exception in the Nemertines (Figure 2.358) and some groups of +Annelids. When we examine the colourless blood of a cray-fish or a +snail (Figure 2.358) under a high power of the microscope, we find in +each drop numbers of mobile leucocytes, which behave just like +independent Amoebae (Figure 1.17). Like these unicellular Protozoa, +the colourless blood-cells creep slowly about, their unshapely +plasma-body constantly changing its form, and stretching out +finger-like processes first in one direction, then another. Like the +Amoebae, they take particles into their cell-body. On account of this +feature these amoeboid plastids are called "eating cells" +(phagocytes), and on account of their motions "travelling cells" +(planocytes). It has been shown by the discoveries of the last few +decades that these leucocytes are of the greatest physiological and +pathological consequence to the organism. They can absorb either solid +or dissolved particles from the wall of the gut, and convey them to +the blood in the chyle; they can absorb and remove unusable matter +from the tissues. When they pass in large quantities through the fine +pores of the capillaries and accumulate at irritated spots, they cause +inflammation. They can consume and destroy bacteria, the dreaded +vehicles of infectious diseases; but they can also transport these +injurious Monera to fresh regions, and so extend the sphere of +infection. It is probable that the sensitive and travelling leucocytes +of our invertebrate ancestors have powerfully co-operated for millions +of years in the phylogenesis of the advancing animal organisation. + +The red blood-cells have a much more restricted sphere of distribution +and activity. But they also are very important in connection with +certain functions of the craniote-organism, especially the exchange of +gases or respiration. The cells of the dark red, carbonised or venous, +blood, which have absorbed carbonic acid from the animal tissues, give +this off in the respiratory organs; they receive instead of it fresh +oxygen, and thus bring about the bright red colour that distinguishes +oxydised or arterial blood. The red colouring matter of the blood +(haemoglobin) is regularly distributed in the pores of their +protoplasm. The red cells of most of the Vertebrates are elliptical +flat disks, and enclose a nucleus of the same shape; they differ a +good deal in size (Figure 2.358). The mammals are distinguished from +the other Vertebrates by the circular form of their biconcave red +cells and by the absence of a nucleus (Figure 1.1); only a few genera +still have the elliptic form inherited from the reptiles (Figure 1.2). +In the embryos of the mammals the red cells have a nucleus and the +power of increasing by cleavage (Figure 1.10). + +The origin of the blood-cells and vessels in the embryo, and their +relation to the germinal layers and tissues, are among the most +difficult problems of ontogeny--those obscure questions on which the +most divergent opinions are still advanced by the most competent +scientists. In general, it is certain that the greater part of the +cells that compose the vessels and their contents come from the +mesoderm--in fact, from the gut-fibre layer; it was on this account +that Baer gave the name of "vascular layer" to this visceral layer of +the coeloma. But other important observers say that a part of these +cells come from other germinal layers, especially from the gut-gland +layer. It seems to be true that blood-cells may be formed from the +cells of the entoderm before the development of the mesoderm. If we +examine sections of chickens, the earliest and most familiar subjects +of embryology, we find at an early stage the "primitive-aortas" we +have already described (Figure 2.360 ao) in the ventral angle between +the episoma (Pv) and hyposoma (Sp). The thin wall of these first +vessels of the amniote embryo consists of flat cells (endothelia or +vascular epithelia); the fluid within already contains numbers of red +blood-cells; both have been developed from the gut-fibre layer. It is +the same with the vessels of the germinative area (Figure 2.361 v), +which lie on the entodermic membrane of the yelk-sac (c). These +features are seen still more clearly in the transverse section of the +duck-embryo in Figure 1.152. In this we see clearly how a number of +stellate cells proceed from the "vascular layer" and spread in all +directions in the "primary body-cavity"--i.e. in the spaces between +the germinal layers. A part of these travelling cells come together +and line the wall of the larger spaces, and thus form the first +vessels; others enter into the cavity, live in the fluid that fills +it, and multiply by cleavage--the first blood-cells. + +But, besides these mesodermic cells of the "vascular layer" proper, +other travelling cells, of which the origin and purport are still +obscure, take part in the formation of blood in the meroblastic +Vertebrates (especially fishes). The chief of these are those that +Ruckert has most aptly denominated "merocytes." These "eating +yelk-cells" are found in large numbers in the food-yelk of the +Selachii, especially in the yelk-wall--the border zone of the germinal +disk in which the embryonic vascular net is first developed. The +nuclei of the merocytes become ten times as large as the ordinary +cell-nucleus, and are distinguished by their strong capacity for +taking colour, or their special richness in chromatin. Their +protoplasmic body resembles the stellate cells of osseous tissue +(astrocytes), and behaves just like a rhizopod (such as Gromia); it +sends out numbers of stellate processes all round, which ramify and +stretch into the surrounding food-yelk. These variable and very mobile +processes, the pseudopodia of the merocytes, serve both for locomotion +and for getting food; as in the real rhizopods, they surround the +solid particles of food (granules and plates of yelk), and accumulate +round their nucleus the food they have received and digested. Hence we +may regard them both as eating-cells (phagocytes) and travelling-cells +(planocytes). Their lively nucleus divides quickly and often +repeatedly, so that a number of new nuclei are formed in a short time; +as each fresh nucleus surrounds itself with a mantle of protoplasm, it +provides a new cell for the construction of the embryo. Their origin +is still much disputed. + +(FIGURE 2.361. Merocytes of a shark-embryo, rhizopod-like yelk-cells +underneath the embryonic cavity (B). (From Ruckert.) z two embryonic +cells, k nuclei of the merocytes, which wander about in the yelk and +eat small yelk-plates (d), k smaller, more superficial, lighter +nuclei, k apostrophe a deeper nucleus, in the act of cleavage, k +asterisk chromatin-filled border-nucleus, freed from the surrounding +yelk in order to show the numerous pseudopodia of the protoplasmic +cell-body.) + +Half of the twelve stems of the animal world have no blood-vessels. +They make their first appearance in the Vermalia. Their earliest +source is the primary body-cavity, the simple space between the two +primary germinal layers, which is either a relic of the +segmentation-cavity, or is a subsequent formation. Amoeboid +planocytes, which migrate from the entoderm and reach this +fluid-filled primary cavity, live and multiply there, and form the +first colourless blood-cells. We find the vascular system in this very +simple form to-day in the Bryozoa, Rotatoria, Nematoda, and other +lower Vermalia. + +The first step in the improvement of this primitive vascular system is +the formation of larger canals or blood-conducting tubes. The spaces +filled with blood, the relics of the primary body-cavity, receive a +special wall. "Blood-vessels" of this kind (in the narrower sense) are +found among the higher worms in various forms, sometimes very simple, +at other times very complex. The form that was probably the incipient +structure of the elaborate vascular system of the Vertebrates (and of +the Articulates) is found in two primordial principal vessels--a +dorsal vessel in the middle line of the dorsal wall of the gut, and a +ventral vessel that runs from front to rear in the middle line of its +ventral wall. From the dorsal vessel is evolved the aorta (or +principal artery), from the ventral vessel the principal or +subintestinal vein. The two vessels are connected in front and behind +by a loop that runs round the gut. The blood contained in the two +tubes is propelled by their peristaltic contractions. + +(FIGURE 2.362. Vascular system of an Annelid (Saenuris), foremost +section. d dorsal vessel, v ventral vessel, c transverse connection of +two (enlarged in shape of heart). The arrows indicate the direction of +the flow of blood. (From Gegenbaur.) + +The earliest Vermalia in which we first find this independent vascular +system are the Nemertina (Figure 2.244). As a rule, they have three +parallel longitudinal vessels connected by loops, a single dorsal +vessel above the gut and a pair of lateral vessels to the right and +left. In some of the Nemertina the blood is already coloured, and the +red colouring matter is real haemoglobin, connected with elliptical +discoid cells, as in the Vertebrates. The further evolution of this +rudimentary vascular system can be gathered from the class of the +Annelids in which we find it at various stages of development. First, +a number of transverse connections are formed between the dorsal and +ventral vessels, which pass round the gut ring-wise (Figure 2.362). +Other vessels grow into the body-wall and ramify in order to convey +blood to it. In addition to the two large vessels of the middle plane +there are often two lateral vessels, one to the right and one to the +left; as, for instance, in the leech. There are four of these parallel +longitudinal vessels in the Enteropneusts (Balanoglossus, Figure +2.245). In these important Vermalia the foremost section of the gut +has already been converted into a gill-crate, and the vascular arches +that rise in the wall of this from the ventral to the dorsal vessel +have become branchial vessels. + +We have a further important advance in the Tunicates, which we have +recognised as the nearest blood-relatives of our early vertebrate +ancestors. Here we find for the first time a real heart--i.e. a +central organ of circulation, driving the blood into the vessels by +the regular contractions of its muscular wall, it is of a very +rudimentary character, a spindle-shaped tube, passing at both ends +into a principal vessel (Figure 2.221). By its original position +behind the gill-crate, on ventral side of the Tunicates (sometimes +more, sometimes less, forward), the head shows clearly that it has +been formed by the local enlargement of a section of the ventral +vessel. We have already noticed the remarkable alternation of the +direction of the blood stream, the heart driving it first from one +end, then from the other (Chapter 2.16). This is very instructive, +because in most of the worms (even the Enteropneust) the blood in the +dorsal vessel travels from back to front, but in the Vertebrates in +the opposite direction. As the Ascidia-heart alternates steadily from +one direction to the other, it shows us permanently, in a sense, the +phylogenetic transition from the earlier forward direction of the +dorsal current (in the worms) to the new backward direction (in the +Vertebrates). + +(FIGURE 2.363. Head of a fish-embryo, with rudimentary vascular +system, from the left. dc Cuvier's duct (juncture of the anterior and +posterior principal veins), sv venous sinus (enlarged end of Cuvier's +duct), a auricle, v ventricle, abr trunk of branchial artery, s +gill-clefts (arterial arches between), ad aorta, c carotid artery, n +nasal pit. (From Gegenbaur.) + +FIGURE 2.364. The five arterial arches of the Craniotes (1 to 5) in +their original disposition, a arterial cone or bulb, a double +apostrophe aorta-trunk, c carotid artery (foremost continuation of the +roots of the aorta). (From Rathke.) + +FIGURE 2.365. The five arterial arches of the birds; the lighter parts +of the structure disappear; only the shaded parts remain. Letters as +in Figure 2.364. s subclavian arteries, p pulmonary artery, p +apostrophe branches of same, c apostrophe outer carotid, c double +apostrophe inner carotid. (From Rathke.) + +FIGURE 2.366. The five arterial arches of mammals; letters as in +Figure 2.365. v vertebral artery, b Botall's duct (open in the embryo, +closed afterwards). (From Rathke.)) + +As the new direction became permanent in the earlier Prochordonia, +which gave rise to the Vertebrate stem, the two vessels that proceed +from either end of the tubular heart acquired a fixed function. The +foremost section of the ventral vessel henceforth always conveys blood +from the heart, and so acts as an artery; the hind section of the same +vessel brings the blood from the body to the heart, and so becomes a +vein. In view of their relation to the two sections of the gut, we may +call the latter the intestinal vein and the former the branchial +artery. The blood contained in both vessels, and also in the heart, is +venous or carbonised blood--i.e. rich in carbonic acid; on the other +hand, the blood that passes from the gills into the dorsal vessel is +provided with fresh oxygen--arterial or oxydised blood. The finest +branches of the arteries and veins pass into each other in the tissues +by means of a network of very fine, ventral, hair-like vessels, or +capillaries (Figure 2.359). + +When we turn from the Tunicates to the closely-related Amphioxus we +are astonished at first to find an apparent retrogression in the +formation of the vascular system. As we have seen, the Amphioxus has +no real heart; its colourless blood is driven along in its vascular +system by the principal vessel itself, which contracts regularly in +its whole length (cf. Figure 2.210). A dorsal vessel that lies above +the gut (aorta) receives the arterial blood from the gills and drives +it into the body. Returning from here, the venous blood gathers in a +ventral vessel under the gut (intestinal vein), and goes back to the +gills. A number of branchial vascular arches, which effect respiration +and rise in the wall of the branchial gut from belly to back, absorb +oxygen from the water and give off carbonic acid; they connect the +ventral with the dorsal vessel. As the same section of the ventral +vessel, which also forms the heart in the Craniotes, has developed in +the Ascidia into a simple tubular heart, we may regard the absence of +this in the Amphioxus as a result of degeneration, a return in this +case to the earlier form of the vascular system, as we find it in many +of the worms. We may assume that the Acrania that really belong to our +ancestral series did not share this retrogression, but inherited the +one-chambered heart of the Prochordonia, and transmitted it directly +to the earliest Craniotes (cf. the ideal Primitive Vertebrate, +Prospondylus, Figures 1.98 to 1.102). + +(FIGURES 2.367 TO 2.370. Metamorphosis of the five arterial arches in +the human embryo (diagram from Rathke). la arterial cone, 1, 2, 3, 4, +5 first to fifth pair of arteries, ad trunk of aorta, aw roots of +aorta. In Figure 2.367 only three, in Figure 2.368 all five, of the +aortic arches are given (the dotted ones only are developed). In +Figure 2.369 the first two pairs have disappeared again. In Figure +2.370 the permanent trunks of the artery are shown; the dotted parts +disappear, s subclavian artery, v vertebral, ax axillary, c carotid (c +apostrophe outer, c double apostrophe inner carotid), p pulmonary.) + +The further phylogenetic evolution of the vascular system is revealed +to us by the comparative anatomy of the Craniotes. At the lowest stage +of this group, in the Cyclostomes, we find for the first time the +differentiation of the vasorium into two sections: a system of +blood-vessels proper, which convey the RED blood about the body, and a +system of lymphatic vessels, which absorb the colourless lymph from +the tissues and convey it to the blood. The lymphatics that absorb +from the gut and pour into the blood-stream the milky food-fluid +formed by digestion are distinguished by the special name of +"chyle-vessels." While the chyle is white on account of its high +proportion of fatty particles, the lymph proper is colourless. Both +chyle and lymph contain the colourless amoeboid cells (leucocytes, +Figure 1.12) that we also find distributed in the blood as colourless +blood-cells (or "white corpuscles"); but the blood also contains a +much larger quantity of red cells, and these give its characteristic +colour to the blood of the Craniotes (rhodocytes, Figure 2.358). The +distinction between lymph, chyle, and blood-vessels which is found in +all the Craniotes may be regarded as an outcome of division of labour +between various sections of our originally simple vascular system. In +the Gnathostomes the spleen makes its first appearance, an organ rich +in blood, the chief function of which is the extensive formation of +new colourless and red cells. It is not found in the Acrania and +Cyclostomes, or any of the Invertebrates. It has been transmitted from +the earliest fishes to all the Craniotes. + +The heart also, the central organ of circulation in all the Craniotes, +shows an advance in structure in the Cyclostomes. The simple, +spindle-shaped heart-tube, found in the same form in the embryo of all +the Craniotes, is divided into two sections or chambers in the +Cyclostomes, and these are separated by a pair of valves. The hind +section, the auricle, receives the venous blood from the body and +passes it on to the anterior section, the ventricle. From this it is +driven through the trunk of the branchial artery (the foremost section +of the ventral vessel or principal vein) into the gills. + +In the Selachii an arterial cone is developed from the foremost end of +the ventricle, as a special division, cut off by valves. It passes +into the enlarged base of the trunk of the branchial artery (Figure +2.363 abr). On each side 5 to 7 arteries proceed from it. These rise +between the gill-clefts (s) on the gill-arches, surround the gullet, +and unite above into a common trunk-aorta, the continuation of which +over the gut corresponds to the dorsal vessel of the worms. As the +curved arteries on the gill-arches spread into a network of +respiratory capillaries, they contain venous blood in their lower part +(as arches of the branchial artery) and arterial blood in the upper +part (as arches of the aorta). The junctures of the various aortic +arches on the right and left are called the roots of the aorta. Of an +originally large number of aortic arches there remain at first six, +then (owing to degeneration of the fifth arch) only five, pairs; and +from these five pairs (Figure 2.364) the chief parts of the arterial +system develop in all the higher Vertebrates. + +(FIGURE 2.371. Heart of a rabbit-embryo, from behind, a vitelline +veins, b auricles of the heart, c atrium, d ventricle, e arterial +bulb, f base of the three pairs of arterial arches. (From Bischoff.) + +FIGURE 2.372. Heart of the same embryo (Figure 2.371), from the front. +v vitelline veins, a auricle, ca auricular canal, l left ventricle, r +right ventricle, ta arterial bulb. (From Bischoff.)) + +The appearance of the lungs and the atmospheric respiration connected +therewith, which we first meet in the Dipneusts, is the next important +step in vascular evolution. In the Dipneusts the auricle of the heart +is divided by an incomplete partition into two halves. Only the right +auricle now receives the venous blood from the veins of the body. The +left auricle receives the arterial blood from the pulmonary veins. The +two auricles have a common opening into the simple ventricle, where +the two kinds of blood mix, and are driven through the arterial cone +or bulb into the arterial arches. From the last arterial arches the +pulmonary arteries arise (Figure 2.365 p). These force a part of the +mixed blood into the lungs, the other part of it going through the +aorta into the body. + +From the Dipneusts upwards we now trace a progressive development of +the vascular system, which ends finally with the loss of branchial +respiration and a complete separation of the two halves of the +circulation. In the Amphibia the partition between the two auricles is +complete. In their earlier stages, as tadpoles (Figure 2.262), they +have still the branchial respiration and the circulation of the +fishes, and their heart contains venous blood alone. Afterwards the +lungs and pulmonary vessels are developed, and henceforth the +ventricle of the heart contains mixed blood. In the reptiles the +ventricle and its arterial cone begin to divide into two halves by a +longitudinal partition, and this partition becomes complete in the +higher reptiles and birds on the one hand, and the stem-forms of the +mammals on the other. Henceforth, the right half of the heart contains +only venous, and the left half only arterial, blood, as we find in all +birds and mammals. The right auricle receives its carbonised or venous +blood from the veins of the body, and the right ventricle drives it +through the pulmonary arteries into the lungs. From here the blood +returns, as oxydised or arterial blood, through the pulmonary veins to +the left auricle, and is forced by the left ventricle into the +arteries of the body. Between the pulmonary arteries and veins is the +capillary system of the small or pulmonary circulation. Between the +body-arteries and veins is the capillary system of the large or +body-circulation. It is only in the two highest classes of +Vertebrates--the birds and mammals--that we find a complete division +of the circulations. Moreover, this complete separation has been +developed quite independently in the two classes, as the dissimilar +formation of the aortas shows of itself. In the birds the RIGHT half +of the fourth arterial arch has become the permanent arch (Figure +2.365). In the mammals this has been developed from the LEFT half of +the same fourth arch (Figure 2.366). + +(FIGURE 2.373. Heart and head of a dog-embryo, from the front, a fore +brain, b eyes, c middle brain, d primitive lower jaw, e primitive +upper jaw, f gill-arches, g right auricle, h left auricle, i left +ventricle, k right ventricle. (From Bischoff.) + +FIGURE 2.374. Heart of the same dog-embryo, from behind. a +inosculation of the vitelline veins, b left auricle, c right auricle, +d auricle, e auricular canal, f left ventricle, g right ventricle, h +arterial bulb, (From Bischoff) + +FIGURE 2.375. Heart of a human embryo, four weeks old; 1. front view, +2. back view, 3. opened, and upper half of the atrium removed. a +apostrophe left auricle, a double apostrophe right auricle, v +apostrophe left ventricle, v double apostrophe right ventricle, ao +arterial bulb, c superior vena cava (cd right, cs left), s rudiment of +the interventricular wall. (From Kolliker.) + +FIGURE 2.376. Heart of a human embryo, six weeks old, front view. r +right ventricle, t left ventricle, s furrow between ventricles, ta +arterial bulb, af furrow on its surface; to right and left are the two +large auricles. (From Ecker.) + +FIGURE 2.377. Heart of a human embryo, eight weeks old, back view. a +apostrophe left auricle, a double apostrophe right auricle, v +apostrophe left ventricle, v double apostrophe right ventricle, cd +apostrophe right superior vena cava, ci inferior vena cava. (From +Kolliker.)) + +If we compare the fully-developed arterial system of the various +classes of Craniotes, it shows a good deal of variety, yet it always +proceeds from the same fundamental type. Its development is just the +same in man as in the other mammals; in particular, the modification +of the six pairs of arterial arches is the same in both (Figures 2.367 +to 2.370). At first there is only a single pair of arches, which lie +on the inner surface of the first pair of gill-arches. Behind this +there then develop a second and third pair of arches (lying on the +inner side of the second and third gill-arches, Figure 2.367). +Finally, we get a fourth, fifth, and sixth pair. Of the six primitive +arterial arches of the Amniotes three soon pass away (the first, +second, and fifth); of the remaining three, the third gives the +carotids, the fourth the aortas, and the sixth (number 5 in Figures +2.364 and 2.368) the pulmonary arteries. + +The human heart also develops in just the same way as that of the +other mammals (Figure 2.378). We have already seen the first rudiments +of its embryology, which in the main corresponds to its phylogeny +(Figures 1.201 and 1.202). We saw that the palingenetic form of the +heart is a spindle-shaped thickening of the gut-fibre layer in the +ventral wall of the head-gut. The structure is then hollowed out, +forms a simple tube, detaches from its place of origin, and henceforth +lies freely in the cardiac cavity. Presently the tube bends into the +shape of an S, and turns spirally on an imaginary axis in such a way +that the hind part comes to lie on the dorsal surface of the fore +part. The united vitelline veins open into the posterior end. From the +anterior end spring the aortic arches. + +(FIGURE 2.378. Heart of the adult man, fully developed, front view, +natural position. a right auricle (underneath it the right ventricle), +b left auricle (under it the left ventricle), C superior vena cava, V +pulmonary veins, P pulmonary artery, d Botalli's duct, A aorta. (From +Meyer.)) + +This first structure of the human heart, enclosing a very simple +cavity, corresponds to the tunicate-heart, and is a reproduction of +that of the Prochordonia, but it now divides into two, and +subsequently into three, compartments; this reminds us for a time of +the heart of the Cyclostomes and fishes. The spiral turning and +bending of the heart increases, and at the same time two transverse +constrictions appear, dividing it externally into three sections +(Figures 2.371 and 2.372). The foremost section, which is turned +towards the ventral side, and from which the aortic arches rise, +reproduces the arterial bulb of the Selachii. The middle section is a +simple ventricle, and the hindmost, the section turned towards the +dorsal side, into which the vitelline veins inosculate, is a simple +auricle (or atrium). The latter forms, like the simple atrium of the +fish-heart, a pair of lateral dilatations, the auricles (Figure 2.371 +b); and the constriction between the atrium and ventricle is called +the auricular canal (Figure 2.372 ca). The heart of the human embryo +is now a complete fish-heart. + +(FIGURE 2.379. Transverse section of the back of the head of a +chick-embryo, forty hours old. (From Kolliker.) m medulla oblongata, +ph pharyngeal cavity (head-gut), h horny plate, h apostrophe thicker +part of it, from which the auscultory pits afterwards develop, hp +skin-fibre plate, hh cervical cavity (head-coelom or cardiocoel), hzp +cardiac plate (the outermost mesodermic wall of the heart), connected +by the ventral mesocardium (uhg) with the gut-fibre layer or visceral +coelom-layer (dfp apostrophe), Ent entoderm, ihh inner (entodermic?) +wall of the heart; the two endothelial cardiac tubes are still +separated by the cenogenetic septum (s) of the Amniotes, g vessels.) + +In perfect harmony with its phylogeny, the embryonic development of +the human heart shows a gradual transition from the fish-heart, +through the amphibian and reptile, to the mammal form, The most +important point in the transition is the formation of a longitudinal +partition--incomplete at first, but afterwards complete--which +separates all three divisions of the heart into right (venous) and +left (arterial) halves (cf. Figures 2.373 to 2.378). The atrium is +separated into a right and left half, each of which absorbs the +corresponding auricle; into the right auricle open the body-veins +(upper and lower vena cava, Figures 2.375 c and 2.377 c); the left +auricle receives the pulmonary veins. In the same way a superficial +interventricular furrow is soon seen in the ventricle (Figure 2.376 +s). This is the external sign of the internal partition by which the +ventricle is divided into two--a right venous and left arterial +ventricle. Finally a longitudinal partition is formed in the third +section of the primitive fish-like heart, the arterial bulb, +externally indicated by a longitudinal furrow (Figure 2.376 af). The +cavity of the bulb is divided into two lateral halves, the +pulmonary-artery bulb, that opens into the right ventricle, and the +aorta-bulb, that opens into the left ventricle. When all the +partitions are complete, the small (pulmonary) circulation is +distinguished from the large (body) circulation; the motive centre of +the former is the right half, and that of the latter the left half, of +the heart. + +The heart of all the Vertebrates belongs originally to the hyposoma of +the head, and we accordingly find it in the embryo of man and all the +other Amniotes right in front on the under-side of the head; just as +in the fishes it remains permanently in front of the gullet. It +afterwards descends into the trunk, with the advance in the +development of the neck and breast, and at last reaches the breast, +between the two lungs. At first it lies symmetrically in the middle +plane of the body, so that its long axis corresponds with that of the +body. In most of the mammals it remains permanently in this position. +But in the apes the axis begins to be oblique, and the apex of the +heart to move towards the left side. The displacement is greatest in +the anthropoid apes--chimpanzee, gorilla, and orang--which resemble +man in this. + +As the heart of all Vertebrates is originally, in the light of +phylogeny, only a local enlargement of the middle principal vein, it +is in perfect accord with the biogenetic law that its first structure +in the embryo is a simple spindle-shaped tube in the ventral wall of +the head-gut. A thin membrane, standing vertically in the middle +plane, the mesocardium, connects the ventral wall of the head-gut with +the lower head-wall. As the cardiac tube extends and detaches from the +gut-wall, it divides the mesocardium into an upper (dorsal) and lower +(ventral) plate (usually called the mesocardium anterius and posterius +in man, Figure 2.379 uhg). The mesocardium divides two lateral +cavities, Remak's "neck-cavities" (Figure 2.379 hh). These cavities +afterwards join and form the simple pericardial cavity, and are +therefore called by Kolliker the "primitive pericardial cavities." + +(FIGURE 2.380. Frontal section of a human embryo, one-twelfth of an +inch long in the neck, magnified forty times; "invented" by Wilhelm +His. Seen from ventral side. mb mouth-fissure, surrounded by the +branchial processes, ab bulbus of aorta, hm middle part of ventricle, +hl left lateral part of same, ho auricle, d diaphragm, vc superior +vena cava, vu umbilical vein, vo vitelline space, lb liver, lg hepatic +duct.) + +The double cervical cavity of the Amniotes is very interesting, both +from the anatomical and the evolutionary point of view; it corresponds +to a part of the hyposomites of the head of the lower +Vertebrates--that part of the ventral coelom-pouches which comes next +to Van Wijhe's "visceral cavities" below. Each of the cavities still +communicates freely behind with the two coelom-pouches of the trunk; +and, just as these afterwards coalesce into a simple body-cavity (the +ventral mesentery disappearing), we find the same thing happening in +the head. This simple primary pericardial cavity has been well called +by Gegenbaur the "head-coeloma," and by Hertwig the "pericardial +breast-cavity." As it now encloses the heart, it may also be called +cardiocoel. + +The cardiocoel, or head-coelom, is often disproportionately large in +the Amniotes, the simple cardiac tube growing considerably and lying +in several folds. This causes the ventral wall of the amniote embryo, +between the head and the navel, to be pushed outwards as in rupture +(cf. Figure 1.180 h). A transverse fold of the ventral wall, which +receives all the vein-trunks that open into the heart, grows up from +below between the pericardium and the stomach, and forms a transverse +partition, which is the first structure of the primary diaphragm +(Figure 2.380 d). This important muscular partition, which completely +separates the thoracic and abdominal cavities in the mammals alone, is +still very imperfect here; the two cavities still communicate for a +time by two narrow canals. These canals, which belong to the dorsal +part of the head-coelom, and which we may call briefly pleural ducts, +receive the two pulmonary sacs, which develop from the hind end of the +ventral wall of the head-gut; they thus become the two pleural +cavities. + +The diaphragm makes its first appearance in the class of the Amphibia +(in the salamanders) as an insignificant muscular transverse fold of +the ventral wall, which rises from the fore end of the transverse +abdominal muscle, and grows between the pericardium and the liver. In +the reptiles (tortoises and crocodiles) a later dorsal part is joined +to this earlier ventral part of the rudimentary diaphragm, a pair of +subvertebral muscles rising from the vertebral column and being added +as "columns" to the transverse partition. But it was probably in the +Permian sauro-mammals that the two originally separate parts were +united, and the diaphragm became a complete partition between the +thoracic and abdominal cavities in the mammals; as it considerably +enlarges the chest-cavity when it contracts, it becomes an important +respiratory muscle. The ontogeny of the diaphragm in man and the other +mammals reproduces this phylogenetic process to-day, in accordance +with the biogenetic law; in all the mammals the diaphragm is formed by +the secondary conjunction of the two originally separate structures, +the earlier ventral part and the later dorsal part. + +Sometimes the blending of the two diaphragmatic structures, and +consequently the severance of the one pleural duct from the abdominal +cavity, is not completed in man. This leads to a diaphragmatic rupture +(hernia diaphragmatica). The two cavities then remain in communication +by an open pleural duct, and loops of the intestine may penetrate by +this "rupture opening" into the chest-cavity. This is one of those +fatal mis-growths that show the great part that blind chance has in +organic development. + +(FIGURE 2.381. Transverse section of the head of a chick-embryo, +thirty-six hours old. Underneath the medullary tube the two primitive +aortas (pa) can be seen in the head-plates (s) at each side of the +chorda. Underneath the gullet (d) we see the aorta-end of the heart +(ae), hh cervical cavity or head coelom, hk top of heart, ks +head-sheath, amniotic fold, h horny plate. (From Remak.) + +(FIGURE 2.382. Transverse section of the cardiac region of the same +chick-embryo (behind the preceding). In the cervical cavity (hh) the +heart (h) is still connected by a mesocard (hg) with the gut-fibre +layer (pf). d gut-gland layer, up provertebral plates, jb rudimentary +auditory vesicle in the horny plate, hp first rise of the amniotic +fold. (From Remak.)) + +Thus the thoracic cavity of the mammals, with its important contents, +the heart and lungs, belongs originally to the HEAD-PART of the +vertebrate body, and its inclusion in the trunk is secondary. This +instructive and very interesting fact is entirely proved by the +concordant evidence of comparative anatomy and ontogeny. The lungs are +outgrowths of the head-gut; the heart develops from its inner wall. +The pleural sacs that enclose the lungs are dorsal parts of the +head-coelom, originating from the pleuroducts; the pericardium in +which the heart afterwards lies is also double originally, being +formed from ventral halves of the head-coelom, which only combine at a +later stage. When the lung of the air-breathing Vertebrates issues +from the head-cavity and enters the trunk-cavity, it follows the +example of the floating bladder of the fishes, which also originates +from the pharyngeal wall in the shape of a small pouch-like +out-growth, but soon grows so large that, in order to find room, it +has to pass far behind into the trunk-cavity. To put it more +precisely, the lung of the quadrupeds retains this hereditary +growth-process of the fishes; for the hydrostatic floating bladder of +the latter is the air-filled organ from which the air-breathing organ +of the former has been evolved. + +There is an interesting cenogenetic phenomenon in the formation of the +heart of the higher Vertebrates that deserves special notice. In its +earliest form the heart is DOUBLE, as recent observation has shown, in +all the Amniotes, and the simple spindle-shaped cardiac tube, which we +took as our starting-point, is only formed at a later stage, when the +two lateral tubes move backwards, touch each other, and at last +combine in the middle line. In man, as in the rabbit, the two +embryonic hearts are still far apart at the stage when there are +already eight primitive segments (Figure 1.134 h). So also the two +coelom-pouches of the head in which they lie are still separated by a +broad space. It is not until the permanent body of the embryo develops +and detaches from the embryonic vesicle that the separate lateral +structures join together, and finally combine in the middle line. As +the median partition between the right and left cardiocoel disappears, +the two cervical cavities freely communicate (Figure 2.381), and form, +on the ventral side of the amniote head, a horseshoe-shaped arch, the +points of which advance backwards into the pleuro-ducts or pleural +cavities, and from there into the two peritoneal sacs of the trunk. +But even after the conjunction of the cervical cavities (Figure 2.381) +the two cardiac tubes remain separate at first; and even after they +have united a delicate partition in the middle of the simple +endothelial tube (Figures 2.379 s and 2.382 h) indicates the original +separation. This CENOGENETIC "primary cardiac septum" presently +disappears, and has no relation to the subsequent permanent partition +between the halves of the heart, which, as a heritage from the +reptiles, has a great PALINGENETIC importance. + +Thorough opponents of the biogenetic law have laid great stress on +these and similar cenogenetic phenomena, and endeavoured to urge them +as striking disproofs of the law. As in every other instance, careful, +discriminating, comparative-morphological examination converts these +supposed disproofs of evolution into strong arguments in its favour. +In his excellent work, On the structure of the Heart in the Amphibia +(1886), Carl Rabl has shown how easily these curious cenogenetic facts +can be explained by the secondary adaptation of the embryonic +structure to the great extension of the food-yelk. + +The embryology of all the other parts of the vascular system also +gives us abundant and valuable data for the purposes of phylogeny. But +as one needs a thorough knowledge of the intricate structure of the +whole vascular system in man and the other Vertebrates in order to +follow this with profit, we cannot go into it further here. Moreover, +many important features in the ontogeny of the vascular system are +still very obscure and controverted. The characters of the embryonic +circulation of the Amniotes, which we have previously considered +(Chapter 1.15), are late acquisitions and entirely cenogenetic. (Cf. +Chapter 1.15 and Figures 1.198 to 1.202.) + + +In the Selachii also we find a longitudinal row of segmental canals on +each side, which open outwards into the primitive renal ducts +(nephrotomes, Chapter 1.14). The segmental canals (a pair in each +segment of the middle part of the body) open internally by a ciliated +funnel into the body-cavity. From the posterior group of these organs +a compact primitive kidney is formed, the anterior group taking part +in the construction of the sexual organs. + +In the same simple form that remains throughout life in the Myxinoides +and partly in the Selachii we find the primitive kidney first +developing in the embryo of man and the higher Craniotes (Figures +2.386 and 2.387). Of the two parts that compose the comb-shaped +primitive kidney the longitudinal channel, or nephroduct, is always +the first to appear; afterwards the transverse "canals," the excreting +nephridia, are formed in the mesoderm; and after this again the +Malpighian capsules with their arterial coils are associated with +these as coelous outgrowths. The primitive renal duct, which appears +first, is found in all craniote embryos at the early stage in which +the differentiation of the medullary tube takes place in the ectoderm, +the severance of the chorda from the visceral layer in the entoderm, +and the first trace of the coelom-pouches arises between the limiting +layers (Figure 2.385). The nephroduct (ung) is seen on each side, +directly under the horny plate, in the shape of a long, thin, +thread-like string of cells. It presently hollows out and becomes a +canal, running straight from front to back, and clearly showing in the +transverse section of the embryo its original position in the space +between horny plate (h), primitive segments (uw), and lateral plates +(hpl). As the originally very short urinary canals lengthen and +multiply, each of the two primitive kidneys assumes the form of a +half-feathered leaf (Figure 2.387). The lines of the leaf are +represented by the urinary canals (u), and the rib by the outlying +nephroduct (w). At the inner edge of the primitive kidneys the +rudiment of the ventral sexual gland (g) can now be seen as a body of +some size. The hindermost end of the nephroduct opens right behind +into the last section of the rectum, thus making a cloaca of it. +However, this opening of the nephroducts into the intestine must be +regarded as a secondary formation. Originally they open, as the +Cyclostomes clearly show, quite independently of the gut, in the +external skin of the abdomen. + +(FIGURE 2.395. Primitive kidneys and germinal glands of a human +embryo, three inches in length (beginning of the sixth week), +magnified fifteen times. k germinal gland, u primitive kidney, z +diaphragmatic ligament of same, w Wolffian duct (opened on the right), +g directing ligament (gubernaculum), a allantoic duct. (From +Kollmann.)) + +In the Myxinoides the primitive kidneys retain this simple comb-shaped +structure, and a part of it is preserved in the Selachii; but in all +the other Craniotes it is only found for a short time in the embryo, +as an ontogenetic reproduction of the earlier phylogenetic structure. +In these the primitive kidney soon assumes the form (by the rapid +growth, lengthening, increase, and serpentining of the urinary canals) +of a large compact gland, of a long, oval or spindle-shaped character, +which passes through the greater part of the embryonic body-cavity +(Figures 1.183 m, 1.184 m, 2.388 n). It lies near the middle line, +directly under the primitive vertebral column, and reaches from the +cardiac region to the cloaca. The right and left kidneys are parallel +to each other, quite close together, and only separated by the +mesentery--the thin narrow layer that attaches the middle gut to the +under surface of the vertebral column. The passage of each primitive +kidney, the nephroduct, runs towards the back on the lower and outer +side of the gland, and opens in the cloaca, close to the +starting-point of the allantois; it afterwards opens into the +allantois itself. + +(FIGURES 2.396 TO 2.398. Urinary and sexual organs of ox-embryos. +Figure 2.396, female embryo one and a half inches long; Figure 2.397, +male embryo, one and a half inches long. Figure 2.398 female embryo +two and a half inches long. w primitive kidney, wg Wolffian duct, m +Mullerian duct, m apostrophe upper end of same (opened at t), i lower +and thicker part of same (rudiment of uterus), g genital cord, h +testicle, (h apostrophe, lower and h double apostrophe, upper +testicular ligament), o ovary, o apostrophe lower ovarian ligament, i +inguinal ligament of primitive kidney, d diaphragmatic ligament of +primitive kidney, nn accessory kidneys, n permanent kidneys, under +them the S-shaped ureters, between these the rectum, v bladder, a +umbilical artery. (From Kolliker.)) + +The primitive or primordial kidneys of the amniote embryo were +formerly called the "Wolffian bodies," and sometimes "Oken's bodies." +They act for a time as kidneys, absorbing unusable juices from the +embryonic body and conducting them to the cloaca--afterwards to the +allantois. There the primitive urine accumulates, and thus the +allantois acts as bladder or urinary sac in the embryos of man and the +other Amniotes. It has, however, no genetic connection with the +primitive kidneys, but is a pouch-like growth from the anterior wall +of the rectum (Figure 1.147 u). Thus it is a product of the visceral +layer, whereas the primitive kidneys are a product of the middle +layer. Phylogenetically we must suppose that the allantois originated +as a pouch-like growth from the cloaca-wall in consequence of the +expansion caused by the urine accumulated in it and excreted by the +kidneys. It is originally a blind sac of the rectum. The real bladder +of the vertebrate certainly made its first appearance among the +Dipneusts (in Lepidosiren), and has been transmitted from them to the +Amphibia, and from these to the Amniotes. In the embryo of the latter +it protrudes far out of the not yet closed ventral wall. It is true +that many of the fishes also have a "bladder." But this is merely a +local enlargement of the lower section of the nephroducts, and so +totally different in origin and composition from the real bladder. The +two structures can be compared from the physiological point of view, +and so are ANALOGOUS, as they have the same function; but not from the +morphological point of view, and are therefore not HOMOLOGOUS. The +false bladder of the fishes is a mesodermic product of the +nephroducts; the true bladder of the Dipneusts, Amphibia, and Amniotes +is an entodermic blind sac of the rectum. + +In all the Anamnia (the lower amnionless Craniotes, Cyclostomes, +Fishes, Dipneusts, and Amphibia) the urinary organs remain at a lower +stage of development to this extent, that the primitive kidneys +(protonephri) act permanently as urinary glands. This is only so as a +passing phase of the early embryonic life in the three higher classes +of Vertebrates, the Amniotes. In these the permanent or after or +secondary (really tertiary) kidneys (renes or metanephri) that are +distinctive of these three classes soon make their appearance. They +represent the third and last generation of the vertebrate kidneys. The +permanent kidneys do not arise (as was long supposed) as independent +glands from the alimentary tube, but from the last section of the +primitive kidneys and the nephroduct. Here a simple tube, the +secondary renal duct, develops, near the point of its entry into the +cloaca; and this tube grows considerably forward. With its blind upper +or anterior end is connected a glandular renal growth, that owes its +origin to a differentiation of the last part of the primitive kidneys. +This rudiment of the permanent kidneys consists of coiled urinary +canals with Malpighian capsules and vascular coils (without ciliated +funnels), of the same structure as the segmental mesonephridia of the +primitive kidneys. The further growth of these metanephridia gives +rise to the compact permanent kidneys, which have the familiar +bean-shape in man and most of the higher mammals, but consist of a +number of separate folds in the lower mammals, birds, and reptiles. As +the permanent kidneys grow rapidly and advance forward, their passage, +the ureter, detaches altogether from its birth-place, the posterior +end of the nephroduct; it passes to the posterior surface of the +allantois. At first in the oldest Amniotes this ureter opens into the +cloaca together with the last section of the nephroduct, but +afterwards separately from this, and finally into the permanent +bladder apart from the rectum altogether. The bladder originates from +the hindmost and lowest part of the allantoic pedicle (urachus), which +enlarges in spindle shape before the entry into the cloaca. The +anterior or upper part of the pedicle, which runs to the navel in the +ventral wall of the embryo, atrophies subsequently, and only a useless +string-like relic of it is left as a rudimentary organ; that is the +single vesico-umbilical ligament. To the right and left of it in the +adult male are a couple of other rudimentary organs, the lateral +vesico-umbilical ligaments. These are the degenerate string-like +relics of the earlier umbilical arteries. + +Though in man and all the other Amniotes the primitive kidneys are +thus early replaced by the permanent kidneys, and these alone then act +as urinary organs, all the parts of the former are by no means lost. +The nephroducts become very important physiologically by being +converted into the passages of the sexual glands. In all the +Gnathostomes--or all the Vertebrates from the fishes up to man--a +second similar canal develops beside the nephroduct at an early stage +of embryonic evolution. The latter is usually called the Mullerian +duct, after its discoverer, Johannes Muller, while the former is +called the Wolffian duct. The origin of the Mullerian duct is still +obscure; comparative anatomy and ontogeny seem to indicate that it +originates by differentiation from the Wolffian duct. Perhaps it would +be best to say: "The original primary nephroduct divides by +differentiation (or longitudinal cleavage) into two secondary +nephroducts, the Wolffian and the Mullerian ducts." The latter (Figure +2.387 m) lies just on the inner side of the former (Figure 2.387 w). +Both open behind into the cloaca. + +However uncertain the origin of the nephroduct and its two products, +the Mullerian and the Wolffian ducts, may be, its later development is +clear enough. In all the Gnathostomes the Wolffian duct is converted +into the spermaduct, and the Mullerian duct into the oviduct. Only one +of them is retained in each sex; the other either disappears +altogether, or only leaves relics in the shape of rudimentary organs. +In the male sex, in which the two Wolffian ducts become the +spermaducts, we often find traces of the Mullerian ducts, which I have +called "Rathke's canals" (Figure 2.394 c). In the female sex, in which +the two Mullerian ducts form the oviducts, there are relics of the +Wolffian ducts, which are called "the ducts of Gaertner." + +(FIGURE 2.399. Female sexual organs of a Monotreme (Ornithorhynchus, +Figure 2.269). o ovaries, t oviducts, u womb, sug urogenital sinus; at +u apostrophe is the outlet of the two wombs, and between them the +bladder (vu). cl cloaca. (From Gegenbaur.) + +FIGURES 2.400 AND 2.401. Original position of the sexual glands in the +ventral cavity of the human embryo (three months old). + +FIGURE 2.400 male (natural size). h testicles, gh conducting ligament +of the testicles, wg spermaduct, h bladder, uh inferior vena cava, nn +accessory kidneys, n kidneys. + +FIGURE 2.401 female, slightly magnified. r round maternal ligament +(underneath it the bladder, over it the ovaries). r apostrophe +kidneys, s accessory kidneys, c caecum, o small reticle, om large +reticle (stomach between the two), l spleen. (From Kolliker.)) + +We obtain the most interesting information with regard to this +remarkable evolution of the nephroducts and their association with the +sexual glands from the Amphibia (Figures 2.390 to 2.395). The first +structure of the nephroduct and its differentiation into Mullerian and +Wolffian ducts are just the same in both sexes in the Amphibia, as in +the mammal embryos (Figures 2.392 and 2.396). In the female Amphibia +the Mullerian duct develops on either side into a large oviduct +(Figure 2.393 od), while the Wolffian duct acts permanently as ureter +(u). In the male Amphibia the Mullerian duct only remains as a +rudimentary organ without any functional significance, as Rathke's +canal (Figure 2.394 c); the Wolffian duct serves also as ureter, but +at the same time as spermaduct, the sperm-canals (ve) that proceed +from the testicles (t) entering the fore part of the primitive kidneys +and combining there with the urinary canals. + +In the mammals these permanent amphibian features are only seen as +brief phases of the earlier period of embryonic development (Figure +2.392). Here the primitive kidneys, which act as excretory organs of +urine throughout life in the amnion-less Vertebrates, are replaced in +the mammals by the permanent kidneys. The real primitive kidneys +disappear for the most part at an early stage of development, and only +small relics of them remain. In the male mammal the epididymis +develops from the uppermost part of the primitive kidney; in the +female a useless rudimentary organ, the epovarium, is formed from the +same part. The atrophied relic of the former is known as the +paradidymis, that of the latter as the parovarium. + +(FIGURE 2.402. Urogenital system of a human embryo of three inches in +length, double natural size. h testicles, wg spermaducts, gh +conducting ligament, p processus vaginalis, b bladder, au umbilical +arteries, m mesorchium, d intestine, u ureter, n kidney, nn accessory +kidney. (From Kollman.)) + +The Mullerian ducts undergo very important changes in the female +mammal. The oviducts proper are developed only from their upper part; +the lower part dilates into a spindle-shaped tube with thick muscular +wall, in which the impregnated ovum develops into the embryo. This is +the womb (uterus). At first the two wombs (Figure 2.399 u) are +completely separate, and open into the cloaca on either side of the +bladder (vu), as is still the case in the lowest living mammals, the +Monotremes. But in the Marsupials a communication is opened between +the two Mullerian ducts, and in the Placentals they combine below with +the rudimentary Wolffian ducts to form a single "genital cord." The +original independence of the two wombs and the vaginal canals formed +from their lower ends are retained in many of the lower Placentals, +but in the higher they gradually blend and form a single organ. The +conjunction proceeds from below (or behind) upwards (or forwards). In +many of the Rodents (such as the rabbit and squirrel) two separate +wombs still open into the simple and single vaginal canal; but in +others, and in the Carnivora, Cetacea, and Ungulates, the lower halves +of the wombs have already fused into a single piece, though the upper +halves (or "horns") are still separate ("two-horned" womb, uteris +bicornis). In the bats and lemurs the "horns" are very short, and the +lower common part is longer. Finally, in the apes and in man the +blending of the two halves is complete, and there is only the one +simple, pear-shaped uterine pouch, into which the oviducts open on +each side. This simple uterus is a late evolutionary product, and is +found ONLY in the ape and man. + +(FIGURES 2.403 TO 2.406. Origin of human ova in the female ovary. + +FIGURE 2.403. Vertical section of the ovary of a new-born female +infant, a ovarian epithelium, b rudimentary string of ova, c young ova +in the epithelium, d long string of ova with follicle-formation +(Pfluger's tube), e group of young follicles, f isolated young +follicle, g blood-vessels in connective tissue (stroma) of the ovary. +In the strings the young ova are distinguished by their considerable +size from the surrounding follicle-cells. (From Waldeyer.) + +FIGURE 2.404. Two young Graafian follicles, isolated. In 1 the +follicle-cells still form a simple, and in 2 a double, stratum round +the young ovum; in 2 they are beginning to form the ovolemma or the +zona pellucida (a). + +FIGURES 2.405 AND 2.406. Two older Graafian follicles, in which fluid +is beginning to accumulate inside the eccentrically thickened +epithelial mass of the follicle-cells (Figure 2.405 with little, 2.406 +with much, follicle-water). ei the young ovum, with embryonic vesicle +and spot, zp ovolemma or zona pellucida, dp discus proligerus, formed +of an accumulation of follicle-cells, which surround the ovum, ff +follicle-liquid (liquor folliculi), gathered inside the stratified +follicle-epithelium (fe), fk connective-tissue fibrous capsule of the +Graafian follicle (theca folliculi).) + +In the male mammals there is the same fusion of the Mullerian and +Wolffian ducts at their lower ends. Here again they form a single +genital cord (Figure 2.397 g), and this opens similarly into the +original urogenital sinus, which develops from the lowest section of +the bladder (v). But while in the male mammal the Wolffian ducts +develop into the permanent spermaducts, there are only rudimentary +relics left of the Mullerian ducts. The most notable of these is the +"male womb" (uterus masculinus), which originates from the lowest +fused part of the ducts, and corresponds to the female uterus. It is a +small, flask-shaped vesicle without any physiological significance, +which opens into the ureter between the two spermaducts and the +prostate folds (vesicula prostatica). + +(FIGURE 2.407. A ripe human Graafian follicle. a the mature ovum, b +the surrounding follicle-cells, c the epithelial cells of the +follicle, d the fibrous membrane of the follicle, e its outer +surface.) + +The internal sexual organs of the mammals undergo very distinctive +changes of position. At first the germinal glands of both sexes lie +deep inside the ventral cavity, at the inner edge of the primitive +kidneys (Figures 2.386 g and 2.392 k), attached to the vertebral +column by a short mesentery (mesorchium in the male, mesovarium in the +female). But this primary arrangement is retained permanently only in +the Monotremes (and the lower Vertebrates). In all other mammals (both +Marsupials and Placentals) they leave their original cradle and travel +more or less far down (or behind), following the direction of a +ligament that goes from the primitive kidneys to the inguinal region +of the ventral wall. This is the inguinal ligament of the primitive +kidneys, known in the male as the Hunterian ligament (Figure 2.400 +gh), and in the female as the "round maternal ligament" (Figure 2.401 +r). In woman the ovaries travel more or less towards the small pelvis, +or enter into it altogether. In the male the testicles pass out of the +ventral cavity, and penetrate by the inguinal canal into a sac-shaped +fold of the outer skin. When the right and left folds ("sexual +swellings") join together they form the scrotum. The various mammals +bring before us the successive stages of this displacement. In the +elephant and the whale the testicles descend very little, and remain +underneath the kidneys. In many of the rodents and carnassia they +enter the inguinal canal. In most of the higher mammals they pass +through this into the scrotum. As a rule, the inguinal canal closes +up. When it remains open the testicles may periodically pass into the +scrotum, and withdraw into the ventral cavity again in time of rut (as +in many of the marsupials, rodents, bats, etc.). + +The structure of the external sexual organs, the copulative organs +that convey the fecundating sperm from the male to the female organism +in the act of copulation, is also peculiar to the mammals. There are +no organs of this character in most of the other Vertebrates. In those +that live in water (such as the Acrania and Cyclostomes, and most of +the fishes) the ova and sperm-cells are simply ejected into the water, +where their conjunction and fertilisation are left to chance. But in +many of the fishes and amphibia, which are viviparous, there is a +direct conveyance of the male sperm into the female body; and this is +the case with all the Amniotes (reptiles, birds, and mammals). In +these the urinary and sexual organs always open originally into the +last section of the rectum, which thus forms a cloaca (Chapter 2.22). +Among the mammals this arrangement is permanent only in the +Monotremes, which take their name from it (Figure 2.399 cl). In all +the other mammals a frontal partition is developed in the cloaca (in +the human embryo about the beginning of the third month), and this +divides it into two cavities. The anterior cavity receives the +urogenital canal, and is the sole outlet of the urine and the sexual +products; the hind or anus-cavity passes the excrements only. + +Even before this partition has been formed in the Marsupials and +Placentals, we see the first trace of the external sexual organs. +First a conical protuberance rises at the anterior border of the +cloaca-outlet--the sexual prominence (phallus, Figure 2.402 A, e, B, +e). At the tip it is swollen in the shape of a club ("acorn" glans). +On its under side there is a furrow, the sexual groove (sulcus +genitalis, f), and on each side of this a fold of skin, the "sexual +pad" (torus genitalis, h l). The sexual protuberance or phallus is the +chief organ of the sexual sense (Chapter 2.25); the sexual nerves +spread on it, and these are the principal organs of the specific +sexual sensation. As erectile bodies (corpora cavernosa) are developed +in the male phallus by peculiar modifications of the blood-vessels, it +becomes capable of erecting periodically on a strong accession of +blood, becoming stiff, so as to penetrate into the female vagina and +thus effect copulation. In the male the phallus becomes the penis; in +the female it becomes the much smaller clitoris; this is only found to +be very large in certain apes (Ateles). A prepuce ("foreskin") is +developed in both sexes as a protecting fold on the anterior surface +of the phallus. + +(FIGURE 408. The human ovum after issuing from the Graafian follicle, +surrounded by the clinging cells of the discus proligerus (in two +radiating crowns). z ovolemma (zona pellucida, with radial porous +canals), p cytosoma (protoplasm of the cell-body, darker within, +lighter without), k nucleus of the ovum (embryonic vesicle). (From +Nagel, magnified 250 times.) (Cf. Figures 1.1 and 1.14.) + +The external sexual member (phallus) is found at various stages of +development within the mammal class, both in regard to size and shape, +and the differentiation and structure of its various parts; this +applies especially to the terminal part of the phallus, the glans, +both the larger glans penis of the male and the smaller glans +clitoridis of the female. The part of the cloaca from the upper wall +of which it forms belongs to the proctodaeum, the ectodermic +invagination of the rectum (Chapter 2.27); hence its epithelial +covering can develop the same horny growths as the corneous layer of +the epidermis. Thus the glans, which is quite smooth in man and the +higher apes, is covered with spines in many of the lower apes and in +the cat, and in many of the rodents with hairs (marmot) or scales +(guinea-pig) or solid horny warts (beaver). Many of the Ungulates have +a free conical projection on the glans, and in many of the Ruminants +this "phallus-tentacle" grows into a long cone, bent hook-wise at the +base (as in the goat, antelope, gazelle, etc.). The different forms of +the phallus are connected with variations in the structure and +distribution of the sensory corpuscles--i.e. the real organs of the +sexual sense, which develop in certain papillae of the corium of the +phallus, and have been evolved from ordinary tactile corpuscles of the +corium by erotic adaptation (Chapter 2.25). + +The formation of the corpora cavernosa, which cause the stiffness of +the phallus and its capability of penetrating the vagina, by certain +special structures of their spongy vascular spaces, also shows a good +deal of variety within the vertebrate stem. This stiffness is +increased in many orders of mammals (especially the carnassia and +rodents) by the ossification of a part of the fibrous body (corpus +fibrosum). This penis-bone (os priapi) is very large in the badger and +dog, and bent like a hook in the marten; it is also very large in some +of the lower apes, and protrudes far out into the glans. It is wanting +in most of the anthropoid apes; it seems to have been lost in their +case (and in man) by atrophy. + +The sexual groove on the under side of the phallus receives in the +male the mouth of the urogenital canal, and is changed into a +continuation of this, becoming a closed canal by the juncture of its +parallel edges, the male urethra. In the female this only takes place +in a few cases (some of the lemurs, rodents, and moles); as a rule, +the groove remains open, and the borders of this "vestibule of the +vagina" develop into the smaller labia (nymphae). The large labia of +the female develop from the sexual pads (tori genitales), the two +parallel folds of the skin that are found on each side of the genital +groove. They join together in the male, and form the closed scrotum. +These striking differences between the two sexes cannot yet be +detected in the human embryo of the ninth week. We begin to trace them +in the tenth week of development, and they are accentuated in +proportion as the difference of the sexes develops. + +Sometimes the normal juncture of the two sexual pads in the male fails +to take place, and the sexual groove may also remain open +(hypospadia). In these cases the external male genitals resemble the +female, and they are often wrongly regarded as cases of hermaphrodism. +Other malformations of various kinds are not infrequently found in the +human external sexual organs, and some of them have a great +morphological interest. The reverse of hypospadia, in which the penis +is split open below, is seen in epispadia, in which the urethra is +open above. In this case the urogenital canal opens above at the +dorsal root of the penis; in the former case down below. These and +similar obstructions interfere with a man's generative power, and thus +prejudicially affect his whole development. They clearly prove that +our history is not guided by a "kind Providence," but left to the play +of blind chance. + +We must carefully distinguish the rarer cases of real hermaphrodism +from the preceding. This is only found when the essential organs of +reproduction, the genital glands of both kinds, are united in one +individual. In these cases either an ovary is developed on the right +and a testicle on the left (or vice versa); or else there are +testicles and ovaries on both sides, some more and others less +developed. As hermaphrodism was probably the original arrangement in +all the Vertebrates, and the division of the sexes only followed by +later differentiation of this, these curious cases offer no +theoretical difficulty. But they are rarely found in man and the +higher mammals. On the other hand, we constantly find the original +hermaphrodism in some of the lower Vertebrates, such as the +Myxinoides, many fishes of the perch-type (serranus), and some of the +Amphibia (ringed snake, toad). In these cases the male often has a +rudimentary ovary at the fore end of the testicle; and the female +sometimes has a rudimentary, inactive testicle. In the carp also and +some other fishes this is found occasionally. We have already seen how +traces of the earlier hemaphrodism can be traced in the passages of +the Amphibia. + +Man has faithfully preserved the main features of his stem-history in +the ontogeny of his urinary and sexual organs. We can follow their +development step by step in the human embryo in the same advancing +gradation that is presented to us by the comparison of the urogenital +organs in the Acrania, Cyclostomes; Fishes, Amphibia, Reptiles, and +then (within the mammal series) in the Monotremes, Marsupials, and the +various Placentals. All the peculiarities of urogenital structure that +distinguish the mammals from the rest of the Vertebrates are found in +man; and in all special structural features he resembles the apes, +particularly the anthropoid apes. In proof of the fact that the +special features of the mammals have been inherited by man, I will, in +conclusion, point out the identical way in which the ova are formed in +the ovary. In all the mammals the mature ova are contained in special +capsules, which are known as the Graafian +follicles, after their discoverer, Roger de Graaf (1677). They were +formerly supposed to be the ova themselves; but Baer discovered the +ova within the follicles (Chapter 1.3). Each follicle (Figure 2.407) +consists of a round fibrous capsule (d), which contains fluid and is +lined with several strata of cells (c). The layer is thickened like a +knob at one point (b); this ovum-capsule encloses the ovum proper (a). +The mammal ovary is originally a very simple oval body (Figure 2.387 +g), formed only of connective tissue and blood-vessels, covered with a +layer of cells, the ovarian epithelium or the female germ epithelium. +From this germ epithelium strings of cells grow out into the +connective tissue or "stroma" of the ovary (Figure 2.403 b). Some of +the cells of these strings (or Pfluger's tubes) grow larger and become +ova (primitive ova, c); but the great majority remain small, and form +a protective and nutritive stratum of cells round each ovum--the +"follicle-epithelium" (e). + +The follicle-epithelium of the mammal has at first one stratum (Figure +2.404 1), but afterwards several (2). It is true that in all the other +Vertebrates the ova are enclosed in a membrane, or "follicle," that +consists of smaller cells. But it is only in the mammals that fluid +accumulates between the growing follicle-cells, and distends the +follicle into a large round capsule, on the inside wall of which the +ovum lies, at one side (Figures 2.405 and 2.406). There again, as in +the whole of his morphology, man proves indubitably his descent from +the mammals. + +In the lower Vertebrates the formation of ova in the germ-epithelium +of the ovary continues throughout life; but in the higher it is +restricted to the earlier stages, or even to the period of embryonic +development. In man it seems to cease in the first year; in the second +year we find no new-formed ova or chains of ova (Pfluger's tubes). +However, the number of ova in the two ovaries is very large in the +young girl; there are calculated to be 72,000 in the sexually-mature +maiden. In the production of the ova men resemble most of the +anthropoid apes. + +Generally speaking, the natural history of the human sexual organs is +one of those parts of anthropology that furnish the most convincing +proofs of the animal origin of the human race. Any man who is +acquainted with the facts and impartially weighs them will conclude +from them alone that we have been evolved from the lower Vertebrates. +The larger and the detailed structure, the action, and the +embryological development of the sexual organs are just the same in +man as in the apes. This applies equally to the male and the female, +the internal and the external organs. The differences we find in this +respect between man and the anthropoid apes are much slighter than the +differences between the various species of apes. But all the apes have +certainly a common origin, and have been evolved from a long-extinct +early-Tertiary stem-form, which we must trace to a branch of the +lemurs. If we had this unknown pithecoid stem-form before us, we +should certainly put it in the order of the true apes in the primate +system; but within this order we cannot, for the anatomic and +ontogenetic reasons we have seen, separate man from the group of the +anthropoid apes. Here again, therefore, on the ground of the +pithecometra-principle, comparative anatomy and ontogeny teach with +full confidence the descent of man from the ape. + + +CHAPTER 2.30. RESULTS OF ANTHROPOGENY. + +Now that we have traversed the wonderful region of human embryology +and are familiar with the principal parts of it, it will be well to +look back on the way we have come, and forward to the further path to +truth to which it has led us. We started from the simplest facts of +ontogeny, or the development of the individual--from observations that +we can repeat and verify by microscopic and anatomic study at any +moment. The first and most important of these facts is that every man, +like every other animal, begins his existence as a simple cell. This +round ovum has the same characteristic form and origin as the ovum of +any other mammal. From it is developed in the same manner in all the +Placentals, by repeated cleavage, a multicellular blastula. This is +converted into a gastrula, and this in turn into a blastocystis (or +embryonic vesicle). The two strata of cells that compose its wall are +the primary germinal layers, the skin-layer (ectoderm), and gut-layer +(entoderm). This two-layered embryonic form is the ontogenetic +reproduction of the extremely important phylogenetic stem-form of all +the Metazoa, which we have called the Gastraea. As the human embryo +passes through the gastrula-form like that of all the other Metazoa, +we can trace its phylogenetic origin to the Gastraea. + +As we continued to follow the embryonic development of the two-layered +structure, we saw that first a third, or middle layer (mesoderm), +appears between the two primary layers; when this divides into two, we +have the four secondary germinal layers. These have just the same +composition and genetic significance in man as in all the other +Vertebrates. From the skin-sense layer are developed the epidermis, +the central nervous system, and the chief part of the sense-organs. +The skin-fibre layer forms the corium and the motor organs--the +skeleton and the muscular system. From the gut-fibre layer are +developed the vascular system, the muscular wall of the gut, and the +sexual glands. Finally, the gut-gland layer only forms the epithelium, +or the inner cellular stratum of the mucous membrane of the alimentary +canal and glands (lungs, liver, etc.). + +The manner in which these different systems of organs arise from the +secondary germinal layers is essentially the same from the start in +man as in all the other Vertebrates. We saw, in studying the embryonic +development of each organ, that the human embryo follows the special +lines of differentiation and construction that are only found +otherwise in the Vertebrates. Within the limits of this vast stem we +have followed, step by step, the development both of the body as a +whole and of its various parts. This higher development follows in the +human embryo the form that is peculiar to the mammals. Finally, we saw +that, even within the limits of this class, the various phylogenetic +stages that we distinguish in a natural classification of the mammals +correspond to the ontogenetic stages that the human embryo passes +through in the course of its evolution. We were thus in a position to +determine precisely the position of man in this class, and so to +establish his relationship to the different orders of mammals. + +The line of argument we followed in this explanation of the +ontogenetic facts was simply a consistent application of the +biogenetic law. In this we have throughout taken strict account of the +distinction between palingenetic and cenogenetic phenomena. +Palingenesis (or "synoptic development") alone enables us to draw +conclusions from the observed embryonic form to the stem-form +preserved by heredity. Such inference becomes more or less precarious +when there has been cenogenesis, or disturbance of development, owing +to fresh adaptations. We cannot understand embryonic development +unless we appreciate this very important distinction. Here we stand at +the very limit that separates the older and the new science or +philosophy of nature. The whole of the results of recent morphological +research compel us irresistibly to recognise the biogenetic law and +its far-reaching consequences. These are, it is true, irreconcilable +with the legends and doctrines of former days, that have been +impressed on us by religious education. But without the biogenetic +law, without the distinction between palingenesis and cenogenesis, and +without the theory of evolution on which we base it, it is quite +impossible to understand the facts of organic development; without +them we cannot cast the faintest gleam of explanation over this +marvellous field of phenomena. But when we recognise the causal +correlation of ontogeny and phylogeny expressed in this law, the +wonderful facts of embryology are susceptible of a very simple +explanation; they are found to be the necessary mechanical effects of +the evolution of the stem, determined by the laws of heredity and +adaptation. The correlative action of these laws under the universal +influence of the struggle for existence, or--as we may say in a word, +with Darwin--"natural selection," is entirely adequate to explain the +whole process of embryology in the light of phylogeny. It is the chief +merit of Darwin that he explained by his theory of selection the +correlation of the laws of heredity and adaptation that Lamarck had +recognised, and pointed out the true way to reach a causal +interpretation of evolution. + +The phenomenon that it is most imperative to recognise in this +connection is the inheritance of functional variations. Jean Lamarck +was the first to appreciate its fundamental importance in 1809, and we +may therefore justly give the name of Lamarckism to the theory of +descent he based on it. Hence the radical opponents of the latter have +very properly directed their attacks chiefly against the former. One +of the most distinguished and most narrow-minded of these opponents, +Wilhelm His, affirms very positively that "characteristics acquired in +the life of the individual are not inherited." + +The inheritance of acquired characters is denied, not only by thorough +opponents of evolution, but even by scientists who admit it and have +contributed a good deal to its establishment, especially Weismann, +Galton, Ray Lankester, etc. Since 1884 the chief opponent has been +August Weismann, who has rendered the greatest service in the +development of Darwin's theory of selection. In his work on The +Continuity of the Germ-plasm, and in his recent excellent Lectures on +the Theory of Descent (1902), he has with great success advanced the +opinion that "only those characters can be transmitted to subsequent +generations that were contained in rudimentary form in the embryo." +However, this germ-plasm theory, with its attempt to explain heredity, +is merely a "provisional molecular hypothesis"; it is one of those +metaphysical speculations that attribute the evolutionary phenomena +exclusively to internal causes, and regard the influence of the +environment as insignificant. Herbert Spencer, Theodor Eimer, Lester +Ward, Hering, and Zehnder have pointed out the untenable consequences +of this position. I have given my view of it in the tenth edition of +the History of Creation (pages 192 and 203). I hold, with Lamarck and +Darwin, that the hereditary transmission of acquired characters is one +of the most important phenomena in biology, and is proved by thousands +of morphological and physiological experiences. It is an indispensable +foundation of the theory of evolution. + +Of the many and weighty arguments for the truth of this conception of +evolution I will for the moment merely point to the invaluable +evidence of dysteleology, the science of rudimentary organs. We cannot +insist too often or too strongly on the great morphological +significance of these remarkable organs, which are completely useless +from the physiological point of view. We find some of these useless +parts, inherited from our lower vertebrate ancestors, in every system +of organs in man and the higher Vertebrates. Thus we find at once on +the skin a scanty and rudimentary coat of hair, only fully developed +on the head, under the shoulders, and at a few other parts of the +body. The short hairs on the greater part of the body are quite +useless and devoid of physiological value; they are the last relic of +the thicker hairy coat of our simian ancestors. The sensory apparatus +presents a series of most remarkable rudimentary organs. We have seen +that the whole of the shell of the external ear, with its cartilages, +muscles, and skin, is in man a useless appendage, and has not the +physiological importance that was formerly ascribed to it. It is the +degenerate remainder of the pointed, freely moving, and more advanced +mammal ear, the muscles of which we still have, but cannot work them. +We found at the inner corner of our eye a small, curious, semi-lunar +fold that is of no use whatever to us, and is only interesting as the +last relic of the nictitating membrane, the third, inner eye-lid that +had a distinct physiological purpose in the ancient sharks, and still +has in many of the Amniotes. + +The motor apparatus, in both the skeleton and muscular systems, +provides a number of interesting dysteleological arguments. I need +only recall the projecting tail of the human embryo, with its +rudimentary caudal vertebrae and muscles; this is totally useless in +man, but very interesting as the degenerate relic of the long tail of +our simian ancestors. From these we have also inherited various bony +processes and muscles, which were very useful to them in climbing +trees, but are useless to us. At various points of the skin we have +cutaneous muscles which we never use--remnants of a strongly-developed +cutaneous muscle in our lower mammal ancestors. This "panniculus +carnosus" had the function of contracting and creasing the skin to +chase away the flies, as we see every day in the horse. Another relic +in us of this large cutaneous muscle is the frontal muscle, by which +we knit our forehead and raise our eye-brows; but there is another +considerable relic of it, the large cutaneous muscle in the neck +(platysma myoides), over which we have no voluntary control. + +Not only in the systems of animal organs, but also in the vegetal +apparatus, we find a number of rudimentary organs, many of which we +have already noticed. In the alimentary apparatus there are the +thymus-gland and the thyroid gland, the seat of goitre and the relic +of a ciliated groove that the Tunicates and Acrania still have in the +gill-pannier; there is also the vermiform appendix to the caecum. In +the vascular system we have a number of useless cords which represent +relics of atrophied vessels that were once active as blood-canals--the +ductus Botalli between the pulmonary artery and the aorta, the ductus +venosus Arantii between the portal vein and the vena cava, and many +others. The many rudimentary organs in the urinary and sexual +apparatus are particularly interesting. These are generally developed +in one sex and rudimentary in the other. Thus the spermaducts are +formed from the Wolffian ducts in the male, whereas in the female we +have merely rudimentary traces of them in Gaertner's canals. On the +other hand, in the female the oviducts and womb are developed from the +Mullerian ducts, while in the male only the lowest ends of them remain +as the "male womb" (vesicula prostatica). Again, the male has in his +nipples and mammary glands the rudiments of organs that are usually +active only in the female. + +A careful anatomic study of the human frame would disclose to us +numbers of other rudimentary organs, and these can only be explained +on the theory of evolution. Robert Wiedersheim has collected a large +number of them in his work on The Human Frame as a Witness to its +Past. They are some of the weightiest proofs of the truth of the +mechanical conception and the strongest disproofs of the teleological +view. If, as the latter demands, man or any other organism had been +designed and fitted for his life-purposes from the start and brought +into being by a creative act, the existence of these rudimentary +organs would be an insoluble enigma; it would be impossible to +understand why the Creator had put this useless burden on his +creatures to walk a path that is in itself by no means easy. But the +theory of evolution gives the simplest possible explanation of them. +It says: The rudimentary organs are parts of the body that have fallen +into disuse in the course of centuries; they had definite functions in +our animal ancestors, but have lost their physiological significance. +On account of fresh adaptations they have become superfluous, but are +transmitted from generation to generation by heredity, and gradually +atrophy. + +We have inherited not only these rudimentary parts, but all the organs +of our body, from the mammals--proximately from the apes. The human +body does not contain a single organ that has not been inherited from +the apes. In fact, with the aid of our biogenetic law we can trace the +origin of our various systems of organs much further, down to the +lowest stages of our ancestry. We can say, for instance, that we have +inherited the oldest organs of the body, the external skin and the +internal coat of the alimentary system, from the Gastraeads; the +nervous and muscular systems from the Platodes; the vascular system, +the body-cavity, and the blood from the Vermalia; the chorda and the +branchial gut from the Prochordonia; the articulation of the body from +the Acrania; the primitive skull and the higher sense-organs from the +Cyclostomes; the limbs and jaws from the Selachii; the five-toed foot +from the Amphibia; the palate from the Reptiles; the hairy coat, the +mammary glands, and the external sexual organs from the Pro-mammals. +When we formulated "the law of the ontogenetic connection of +systematically related forms," and determined the relative age of +organs, we saw how it was possible to draw phylogenetic conclusions +from the ontogenetic succession of systems of organs. + +With the aid of this important law and of comparative anatomy we were +also enabled to determine "man's place in nature," or, as we put it, +assign to man his position in the classification of the animal +kingdom. In recent zoological classification the animal world is +divided into twelve stems or phyla, and these are broadly sub-divided +into about sixty classes, and these classes into at least 300 orders. +In his whole organisation man is most certainly, in the first place, a +member of one of these stems, the vertebrate stem; secondly, a member +of one particular class in this stem, the Mammals; and thirdly, of one +particular order, the order of Primates. He has all the +characteristics that distinguish the Vertebrates from the other eleven +animal stems, the Mammals from the other sixty classes, and the +Primates from the 300 other orders of the animal kingdom. We may turn +and twist as we like, but we cannot get over this fact of anatomy and +classification. Of late years this fact has given rise to a good deal +of discussion, and especially of controversy as to the particular +anatomic relationship of man to the apes. The most curious opinions +have been advanced on this "ape-question," or "pithecoid-theory." It +is as well, therefore, to go into it once more and distinguish the +essential from the unessential. (Cf. Chapter 2.23.) + +We start from the undisputed fact that man is in any case--whether we +accept or reject his special blood-relationship to the apes--a true +mammal; in fact, a placental mammal. This fundamental fact can be +proved so easily at any moment from comparative anatomy that it has +been universally admitted since the separation of the Placentals from +the lower mammals (Marsupials and Monotremes). But for every +consistent subscriber to the theory of evolution it must follow at +once that man descends from a common stem-form with all the other +Placentals, the stem-ancestor of the Placentals, just as we must admit +a common mesozoic ancestor of all the mammals. This is, however, to +settle decisively the great and burning question of man's place in +nature, whether or no we go on to admit a nearer or more distant +relationship to the apes. Whether man is or is not a member of the +ape-order (or, if you prefer, the primate-order.) in the phylogenetic +sense, in any case his direct blood-relationship to the rest of the +mammals, and especially the Placentals, is established. It is possible +that the affinities of the various orders of mammals to each other are +different from what we hypothetically assume to-day. But, in any case, +the common descent of man and all the other mammals from one stem-form +is beyond question. This long-extinct Promammal was probably evolved +from Proreptiles during the Triassic period, and must certainly be +regarded as the monotreme and oviparous ancestor of ALL the mammals. + +If we hold firmly to this fundamental and most important thesis, we +shall see the "ape-question" in a very different light from that in +which it is usually regarded. Little reflection is then needed to see +that it is not nearly so important as it is said to be. The origin of +the human race from a series of mammal ancestors, and the historic +evolution of these from an earlier series of lower vertebrate +ancestors, together with all the weighty conclusions that every +thoughtful man deduces therefrom, remain untouched; so far as these +are concerned, it is immaterial whether we regard true "apes" as our +nearest ancestors or not. But as it has become the fashion to lay the +chief stress in the whole question of man's origin on the "descent +from the apes," I am compelled to return to it once more, and recall +the facts of comparative anatomy and ontogeny that give a decisive +answer to this "ape-question." + +The shortest way to attain our purpose is that followed by Huxley in +1863 in his able work, which I have already often quoted, Man's Place +in Nature--the way of comparative anatomy and ontogeny. We have to +compare impartially all man's organs with the same organs in the +higher apes, and then to examine if the differences between the two +are greater than the corresponding differences between the higher and +the lower apes. The indubitable and incontestable result of this +comparative-anatomical study, conducted with the greatest care and +impartiality, was the pithecometra-principle, which we have called the +Huxleian law in honour of its formulator--namely, that the differences +in organisation between man and the most advanced apes we know are +much slighter than the corresponding differences in organisation +between the higher and lower apes. We may even give a more precise +formula to this law, by excluding the Platyrrhines or American apes as +distant relatives, and restricting the comparison to the narrower +family-circle of the Catarrhines, the apes of the Old World. Within +the limits of this small group of mammals we found the structural +differences between the lower and higher catarrhine apes--for +instance, the baboon and the gorilla--to be much greater than the +differences between the anthropoid apes and man. If we now turn to +ontogeny, and find, according to our "law of the ontogenetic +connection of systematically related forms," that the embryos of the +anthropoid apes and man retain their resemblance for a longer time +than the embryos of the highest and the lowest apes, we are forced, +whether we like it or no, to recognise our descent from the order of +apes. We can assuredly construct an approximate picture in the +imagination of the form of our early Tertiary ancestors from the +foregoing facts of comparative anatomy; however we may frame this in +detail, it will be the picture of a true ape, and a distinct +catarrhine ape. This has been shown so well by Huxley (1863) that the +recent attacks of Klaatsch, Virchow, and other anthropologists, have +completely failed (cf. Chapter 2.23). All the structural characters +that distinguish the Catarrhines from the Platyrrhines are found in +man. Hence in the genealogy of the mammals we must derive man +immediately from the catarrhine group, and locate the origin of the +human race in the Old World. Only the early root-form from which both +descended was common to them. + +It is, therefore, established beyond question for all impartial +scientific inquiry that the human race comes directly from the apes of +the Old World; but, at the same time, I repeat that this is not so +important in connection with the main question of the origin of man as +is commonly supposed. Even if we entirely ignore it, all that we have +learned from the zoological facts of comparative anatomy and ontogeny +as to the placental character of man remains untouched. These prove +beyond all doubt the common descent of man and all the rest of the +mammals. Further, the main question is not in the least affected if it +is said: "It is true that man is a mammal; but he has diverged at the +very root of the class from all the other mammals, and has no closer +relationship to any living group of mammals." The affinity is more or +less close in any case, if we examine the relation of the mammal class +to the sixty other classes of the animal world. Quite certainly the +whole of the mammals, including man, have had a common origin; and it +is equally certain that their common stem-forms were gradually evolved +from a long series of lower Vertebrates. + +The resistance to the theory of a descent from the apes is clearly due +in most men to feeling rather than to reason. They shrink from the +notion of such an origin just because they see in the ape organism a +caricature of man, a distorted and unattractive image of themselves, +because it hurts man's aesthetic complacency and self-ennoblement. It +is more flattering to think we have descended from some lofty and +god-like being; and so, from the earliest times, human vanity has been +pleased to believe in our origin from gods or demi-gods. The Church, +with that sophistic reversal of ideas of which it is a master, has +succeeded in representing this ridiculous piece of vanity as +"Christian humility"; and the very men who reject with horror the +notion of an animal origin, and count themselves "children of God," +love to prate of their "humble sense of servitude." In most of the +sermons that have poured out from pulpit and altar against the +doctrine of evolution human vanity and conceit have been a conspicuous +element; and, although we have inherited this very characteristic +weakness from the apes, we must admit that we have developed it to a +higher degree, which is entirely repudiated by sound and normal +intelligence. We are greatly amused at all the childish follies that +the ridiculous pride of ancestry has maintained from the Middle Ages +to our own time; yet there is a large amount of this empty feeling in +most men. Just as most people much prefer to trace their family back +to some degenerate baron or some famous prince rather than to an +unknown peasant, so most men would rather have as parent of the race a +sinful and fallen Adam than an advancing, and vigorous ape. It is a +matter of taste, and to that extent we cannot quarrel over these +genealogical tendencies. Personally, the notion of ascent is more +congenial to me than that of descent. It seems to me a finer thing to +be the advanced offspring of a simian ancestor, that has developed +progressively from the lower mammals in the struggle for life, than +the degenerate descendant of a god-like being, made from a clod, and +fallen for his sins, and an Eve created from one of his ribs. Speaking +of the rib, I may add to what I have said about the development of the +skeleton, that the number of ribs is just the same in man and woman. +In both of them the ribs are formed from the middle germinal layer, +and are, from the phylogenetic point of view, lower or ventral +vertebral arches. + +But it is said: "That is all very well, as far as the human body is +concerned; on the facts quoted it is impossible to doubt that it has +really and gradually been evolved from the long ancestral series of +the Vertebrates. But it is quite another thing as regards man's mind, +or soul; this cannot possibly have been developed from the +vertebrate-soul."* (* The English reader will recognise here the +curious position of Dr. Wallace and of the late Dr. +Mivart.--Translator.) Let us see if we cannot meet this grave +stricture from the well-known facts of comparative anatomy, +physiology, and embryology. It will be best to begin with a +comparative study of the souls of various groups of Vertebrates. Here +we find such an enormous variety of vertebrate souls that, at first +sight, it seems quite impossible to trace them all to a common +"Primitive Vertebrate." Think of the tiny Amphioxus, with no real +brain but a simple medullary tube, and its whole psychic life at the +very lowest stage among the Vertebrates. The following group of the +Cyclostomes are still very limited, though they have a brain. When we +pass on to the fishes, we find their intelligence remaining at a very +low level. We do not see any material advance in mental development +until we go on to the Amphibia and Reptiles. There is still greater +advance when we come to the Mammals, though even here the minds of the +Monotremes and of the stupid Marsupials remain at a low stage. But +when we rise from these to the Placentals we find within this one vast +group such a number of important stages of differentiation and +progress that the psychic differences between the least intelligent +(such as the sloths and armadillos) and the most intelligent +Placentals (such as the dogs and apes) are much greater than the +psychic differences between the lowest Placentals and the Marsupials +or Monotremes. Most certainly the differences are far greater than the +differences in mental power between the dog, the ape, and man. Yet all +these animals are genetically-related members of a single natural +class. + +We see this to a still more astonishing extent in the comparative +psychology of another class of animals, that is especially interesting +for many reasons--the insect class. It is well known that we find in +many insects a degree of intelligence that is found in man alone among +the Vertebrates. Everybody knows of the famous communities and states +of bees and ants, and of the very remarkable social arrangements in +them, such as we find among the more advanced races of men, but among +no other group of animals. I need only mention the social organisation +and government of the monarchic bees and the republican ants, and +their division into different conditions--queen, drone-nobles, +workers, educators, soldiers, etc. One of the most remarkable +phenomena in this very interesting province is the cattle-keeping of +the ants, which rear plant-lice as milch-cows and regularly extract +their honeyed juice. Still more remarkable is the slave-holding of the +large red ants, which steal the young of the small black ants and +bring them up as slaves. It has long been known that these political +and social arrangements of the ants are due to the deliberate +cooperation of the countless citizens, and that they understand each +other. A number of recent observers, especially Fritz Muller, Sir J. +Lubbock (Lord Avebury), and August Forel, have put the astonishing +degree of intelligence of these tiny Articulates beyond question. + +Now, compare with these the mental life of many of the lower, +especially the parasitic insects, as Darwin did. There is, for +instance, the cochineal insect (Coccus), which, in its adult state, +has a motionless, shield-shaped body, attached to the leaves of +plants. Its feet are atrophied. Its snout is sunk in the tissue of the +plants of which it absorbs the sap. The whole psychic life of these +inert female parasites consists in the pleasure they experience from +sucking the sap of the plant and in sexual intercourse with the males. +It is the same with the maggot-like females of the fan-fly +(Strepsitera), which spend their lives parasitically and immovably, +without wings or feet, in the abdomen of wasps. There is no question +here of higher psychic action. If we compare these sluggish parasites +with the intelligent and active ants, we must admit that the psychic +differences between them are much greater than the psychic differences +between the lowest and highest mammals, between the Monotremes, +Marsupials, and armadillos on the one hand, and the dog, ape, or man +on the other. Yet all these insects belong to the same class of +Articulates, just as all the mammals belong to one and the same class. +And just as every consistent evolutionist must admit a common +stem-form for all these insects, so he must also for all the mammals. + +If we now turn from the comparative study of psychic life in different +animals to the question of the organs of this function, we receive the +answer that in all the higher animals they are always bound up with +certain groups of cells, the ganglionic cells or neurona that compose +the nervous system. All scientists without exception are agreed that +the central nervous system is the organ of psychic life in the animal, +and it is possible to prove this experimentally at any moment. When we +partially or wholly destroy the central nervous system, we extinguish +in the same proportion, partially or wholly, the "soul" or psychic +activity of the animal. We have, therefore, to examine the features of +the psychic organ in man. The reader already knows the incontestable +answer to this question. Man's psychic organ is, in structure and +origin, just the same organ as in all the other Vertebrates. It +originates in the shape of a simple medullary tube from the outer +membrane of the embryo--the skin-sense layer. The simple cerebral +vesicle that is formed by the expansion of the head-part of this +medullary tube divides by transverse constrictions into five, and +these pass through more or less the same stages of construction in the +human embryo as in the rest of the mammals. As these are undoubtedly +of a common origin, their brain and spinal cord must also have a +common origin. + +Physiology teaches us further, on the ground of observation and +experiment, that the relation of the "soul" to its organ, the brain +and spinal cord, is just the same in man as in the other mammals. The +one cannot act at all without the other; it is just as much bound up +with it as muscular movement is with the muscles. It can only develop +in connection with it. If we are evolutionists at all, and grant the +causal connection of ontogenesis and phylogenesis, we are forced to +admit this thesis: The human soul or psyche, as a function of the +medullary tube, has developed along with it; and just as brain and +spinal cord now develop from the simple medullary tube in every human +individual, so the human mind or the psychic life of the whole human +race has been gradually evolved from the lower vertebrate soul. Just +as to-day the intricate structure of the brain proceeds step by step +from the same rudiment in every human individual--the same five +cerebral vesicles--as in all the other Craniotes; so the human soul +has been gradually developed in the course of millions of years from a +long series of craniote-souls. Finally, just as to-day in every human +embryo the various parts of the brain differentiate after the special +type of the ape-brain, so the human psyche has proceeded historically +from the ape-soul. + +It is true that this Monistic conception is rejected with horror by +most men, and the Dualistic idea, which denies the inseparable +connection of brain and mind, and regards body and soul as two totally +different things, is still popular. But how can we reconcile this view +with the known facts of evolution? It meets with difficulties equally +great and insuperable in embryology and in phylogeny. If we suppose +with the majority of men that the soul is an independent entity, which +has nothing to do with the body originally, but merely inhabits it for +a time, and gives expression to its experiences through the brain just +as the pianist does through his instrument, we must assign a point in +human embryology at which the soul enters into the brain; and at death +again we must assign a moment at which it abandons the body. As, +further, each human individual has inherited certain personal features +from each parent, we must suppose that in the act of conception pieces +were detached from their souls and transferred to the embryo. A piece +of the paternal soul goes with-the spermatozoon, and a piece of the +mother's soul remains in the ovum. At the moment of conception, when +portions of the two nuclei of the copulating cells join together to +form the nucleus of the stem-cell, the accompanying fragments of the +immaterial souls must also be supposed to coalesce. + +On this Dualistic view the phenomena of psychic development are +totally incomprehensible. Everybody knows that the new-born child has +no consciousness, no knowledge of itself and the surrounding world. +Every parent who has impartially followed the mental development of +his children will find it impossible to deny that it is a case of +biological evolutionary processes. Just as all other functions of the +body develop in connection with their organs, so the soul does in +connection with the brain. This gradual unfolding of the soul of the +child is, in fact, so wonderful and glorious a phenomenon that every +mother or father who has eyes to observe is never tired of +contemplating it. It is only our manuals of psychology that know +nothing of this development; we are almost tempted to think sometimes +that their authors can never have had children themselves. The human +soul, as described in most of our psychological works, is merely the +soul of a learned philosopher, who has read a good many books, but +knows nothing of evolution, and never even reflects that his own soul +has had a development. + +When these Dualistic philosophers are consistent they must assign a +moment in the phylogeny of the human soul at which it was first +"introduced" into man's vertebrate body. Hence, at the time when the +human body was evolved from the anthropoid body of the ape (probably +in the Tertiary period), a specific human psychic element--or, as +people love to say, "a spark of divinity"--must have been suddenly +infused or breathed into the anthropoid brain, and been associated +with the ape-soul already present in it. I need not insist on the +enormous theoretical difficulties of this idea. I will only point out +that this "spark of divinity," which is supposed to distinguish the +soul of man from that of the other animals, must be itself capable of +development, and has, as a matter of fact, progressively developed in +the course of human history. As a rule, reason is taken to be this +"spark of divinity," and is supposed to be an exclusive possession of +humanity. But comparative psychology shows us that it is quite +impossible to set up this barrier between man and the brute. Either we +take the word "reason" in the wider sense, and then it is found in the +higher mammals (ape, dog, elephant, horse) just as well as in most +men; or else in the narrower sense, and then it is lacking in most men +just as much as in the majority of animals. On the whole, we may still +say of man's reason what Goethe's Mephistopheles said:-- + + Life somewhat better might content him + But for the gleam of heavenly light that Thou hast given him. + He calls it reason; thence his power's increased + To be still beastlier than any beast. + +If, then, we must reject these popular and, in some respects, +agreeable Dualistic theories as untenable, because inconsistent with +the genetic facts, there remains only the opposite or Monistic +conception, according to which the human soul is, like any other +animal soul, a function of the central nervous system, and develops in +inseparable connection therewith. We see this ontogenetically in every +child. The biogenetic law compels us to affirm it phylogenetically. +Just as in every human embryo the skin-sense layer gives rise to the +medullary tube, from the anterior end of which the five cerebral +vesicles of the Craniotes are developed, and from these the mammal +brain (first with the characters of the lower, then with those of the +higher mammals); and as the whole of this ontogenetic process is only +a brief, hereditary reproduction of the same process in the +phylogenesis of the Vertebrates; so the wonderful spiritual life of +the human race through many thousands of years has been evolved step +by step from the lowly psychic life of the lower Vertebrates, and the +development of every child-soul is only a brief repetition of that +long and complex phylogenetic process. From all these facts sound +reason must conclude that the still prevalent belief in the +immortality of the soul is an untenable superstition. I have shown its +inconsistency with modern science in the eleventh chapter of The +Riddle of the Universe. + +Here it may also be well to point out the great importance of +anthropogeny, in the light of the biogenetic law, for the purposes of +philosophy. The speculative philosophers who take cognizance of these +ontogenetic facts, and explain them (in accordance with the law) +phylogenetically, will advance the great questions of philosophy far +more than the most distinguished thinkers of all ages have yet +succeeded in doing. Most certainly every clear and consistent thinker +must derive from the facts of comparative anatomy and ontogeny we have +adduced a number of suggestive ideas that cannot fail to have an +influence on the progress of philosophy. Nor can it be doubted that +the candid statement and impartial appreciation of these facts will +lead to the decisive triumph of the philosophic tendency that we call +"Monistic" or "Mechanical," as opposed to the "Dualistic" or +"Teleological," on which most of the ancient, medieval, and modern +systems of philosophy are based. The Monistic or Mechanical philosophy +affirms that all the phenomena of human life and of the rest of nature +are ruled by fixed and unalterable laws; that there is everywhere a +necessary causal connection of phenomena; and that, therefore, the +whole knowable universe is a harmonious unity, a monon. It says, +further, that all phenomena are due solely to mechanical or efficient +causes, not to final causes. It does not admit free-will in the +ordinary sense of the word. In the light of the Monistic philosophy +the phenomena that we are wont to regard as the freest and most +independent, the expressions of the human will, are subject just as +much to rigid laws as any other natural phenomenon. As a matter of +fact, impartial and thorough examination of our "free" volitions shows +that they are never really free, but always determined by antecedent +factors that can be traced to either heredity or adaptation. We +cannot, therefore, admit the conventional distinction between nature +and spirit. There is spirit everywhere in nature, and we know of no +spirit outside of nature. Hence, also, the common antithesis of +natural science and mental or moral science is untenable. Every +science, as such, is both natural and mental. That is a firm principle +of Monism, which, on its religious side, we may also denominate +Pantheism. Man is not above, but in, nature. + +It is true that the opponents of evolution love to misrepresent the +Monistic philosophy based on it as "Materialism," and confuse the +philosophic tendency of this name with a wholly unconnected and +despicable moral materialism. Strictly speaking, it would be just as +proper to call our system Spiritualism as Materialism. The real +Materialistic philosophy affirms that the phenomena of life are, like +all other phenomena, effects or products of matter. The opposite +extreme, the Spiritualistic philosophy, says, on the contrary, that +matter is a product of energy, and that all material forms are +produced by free and independent forces. Thus, according to one-sided +Materialism, the matter is antecedent to the living force; according +to the equally one-sided view of the Spiritist, it is the reverse. +Both views are Dualistic, and, in my opinion, both are false. For us +the antithesis disappears in the Monistic philosophy, which knows +neither matter without force nor force without matter. It is only +necessary to reflect for some time over the question from the strictly +scientific point of view to see that it is impossible to form a clear +idea of either hypothesis. As Goethe said, "Matter can never exist or +act without spirit, nor spirit without matter." + +The human "spirit" or "soul" is merely a force or form of energy, +inseparably bound up with the material sub-stratum of the body. The +thinking force of the mind is just as much connected with the +structural elements of the brain as the motor force of the muscles +with their structural elements. Our mental powers are functions of the +brain as much as any other force is a function of a material body. We +know of no matter that is devoid of force, and no forces that are not +bound up with matter. When the forces enter into the phenomenon as +movements we call them living or active forces; when they are in a +state of rest or equilibrium we call them latent or potential. This +applies equally to inorganic and organic bodies. The magnet that +attracts iron filings, the powder that explodes, the steam that drives +the locomotive, are living inorganics; they act by living force as +much as the sensitive Mimosa does when it contracts its leaves at +touch, or the venerable Amphioxus that buries itself in the sand of +the sea, or man when he thinks. Only in the latter cases the +combinations of the different forces that appear as "movement" in the +phenomenon are much more intricate and difficult to analyse than in +the former. + +Our study has led us to the conclusion that in the whole evolution of +man, in his embryology and in his phylogeny, there are no living +forces at work other than those of the rest of organic and inorganic +nature. All the forces that are operative in it could be reduced in +the ultimate analysis to growth, the fundamental evolutionary function +that brings about the forms of both the organic and the inorganic. But +growth itself depends on the attraction and repulsion of homogeneous +and heterogeneous particles. Seventy-five years ago Carl Ernst von +Baer summed up the general result of his classic studies of animal +development in the sentence: "The evolution of the individual is the +history of the growth of individuality in every respect." And if we go +deeper to the root of this law of growth, we find that in the long run +it can always be reduced to that attraction and repulsion of animated +atoms which Empedocles called the "love and hatred" of the elements. + +Thus the evolution of man is directed by the same "eternal, iron laws" +as the development of any other body. These laws always lead us back +to the same simple principles, the elementary principles of physics +and chemistry. The various phenomena of nature only differ in the +degree of complexity in which the different forces work together. Each +single process of adaptation and heredity in the stem-history of our +ancestors is in itself a very complex physiological phenomenon. Far +more intricate are the processes of human embryology; in these are +condensed and comprised thousands of the phylogenetic processes. + +In my General Morphology, which appeared in 1866, I made the first +attempt to apply the theory of evolution, as reformed by Darwin, to +the whole province of biology, and especially to provide with its +assistance a mechanical foundation for the science of organic forms. +The intimate relations that exist between all parts of organic +science, especially the direct causal nexus between the two sections +of evolution--ontogeny and phylogeny--were explained in that work for +the first time by transformism, and were interpreted philosophically +in the light of the theory of descent. The anthropological part of the +General Morphology (Book 7) contains the first attempt to determine +the series of man's ancestors (volume 2 page 428). However imperfect +this attempt was, it provided a starting-point for further +investigation. In the thirty-seven years that have since elapsed the +biological horizon has been enormously widened; our empirical +acquisitions in paleontology, comparative anatomy, and ontogeny have +grown to an astonishing extent, thanks to the united efforts of a +number of able workers and the employment of better methods. Many +important biological questions that then appeared to be obscure +enigmas seem to be entirely settled. Darwinism arose like the dawn of +a new day of clear Monistic science after the dark night of mystic +dogmatism, and we can say now, proudly and gladly, that there is +daylight in our field of inquiry. + +Philosophers and others, who are equally ignorant of the empirical +sources of our evidence and the phylogenetic methods of utilising it, +have even lately claimed that in the matter of constructing our +genealogical tree nothing more has been done than the discovery of a +"gallery of ancestors," such as we find in the mansions of the +nobility. This would be quite true if the genealogy given in the +second part of this work were merely the juxtaposition of a series of +animal forms, of which we gathered the genetic connection from their +external physiognomic resemblances. As we have sufficiently proved +already, it is for us a question of a totally different thing--of the +morphological and historical proof of the phylogenetic connection of +these ancestors on the basis of their identity in internal structure +and embryonic development; and I think I have sufficiently shown in +the first part of this work how far this is calculated to reveal to us +their inner nature and its historical development. I see the essence +of its significance precisely in the proof of historical connection. I +am one of those scientists who believe in a real "natural history," +and who think as much of an historical knowledge of the past as of an +exact investigation of the present. The incalculable value of the +historical consciousness cannot be sufficiently emphasised at a time +when historical research is ignored and neglected, and when an "exact" +school, as dogmatic as it is narrow, would substitute for it physical +experiments and mathematical formulae. Historical knowledge cannot be +replaced by any other branch of science. + +It is clear that the prejudices that stand in the way of a general +recognition of this "natural anthropogeny" are still very great; +otherwise the long struggle of philosophic systems would have ended in +favour of Monism. But we may confidently expect that a more general +acquaintance with the genetic facts will gradually destroy these +prejudices, and lead to the triumph of the natural conception of +"man's place in nature." When we hear it said, in face of this +expectation, that this would lead to retrogression in the intellectual +and moral development of mankind, I cannot refrain from saying that, +in my opinion, it will be just the reverse; that it will promote to an +enormous extent the advance of the human mind. All progress in our +knowledge of truth means an advance in the higher cultivation of the +human intelligence; and all progress in its application to practical +life implies a corresponding improvement of morality. The worst +enemies of the human race--ignorance and superstition--can only be +vanquished by truth and reason. In any case, I hope and desire to have +convinced the reader of these chapters that the true scientific +comprehension of the human frame can only be attained in the way that +we recognise to be the sole sound and effective one in organic science +generally--namely, the way of Evolution. + + + +INDEX. + +Abiogenesis. + +Accipenser. + +Abortive ova. + +Achromatin. + +Achromin. + +Acoela. + +Acoustic nerve, the. + +Acquired characters, inheritance of. + +Acrania, the. + +Acroganglion, the. + +Adam's apple, the. + +Adapida. + +Adaptation. + +After-birth, the. + +Agassiz, L. + +Age of life. + +Alimentary canal, evolution of the. +structure of the. + +Allantoic circulation, the. + +Allantois, development of the. + +Allmann. + +Amblystoma. + +Amitotic cleavage. + +Ammoconida. + +Ammolynthus. + +Amnion, the. +formation of the. + +Amniotic fluid, the. + +Amoeba, the. + +Amphibia, the. + +Amphichoerus. + +Amphigastrula. + +Amphioxus, the. +circulation of the. +coelomation of the. +embryology of the. +structure of the. + +Amphirhina. + +Anamnia, the. + +Anatomy, comparative. + +Animalculists. + +Animal layer, the. + +Annelids, the. + +Annelid theory, the. + +Anomodontia. + +Ant, intelligence of the. + +Anthropithecus. + +Anthropogeny. + +Anthropoid apes, the. + +Anthropology. + +Anthropozoic period. + +Antimera. + +Anura. + +Anus, the. + +Anus, formation of the. + +Aorta, the. +development of the. + +Ape and man. + +Ape-man, the. + +Apes, the. + +Aphanocapsa. + +Aphanostomum. + +Appendicaria. + +Appendix vermiformis, the. + +Aquatic life, early prevalence of. + +Ararat, Mount. + +Archenteron. + +Archeolithic age. + +Archicaryon. + +Archicrania. + +Archigastrula. + +Archiprimas. + +Arctopitheca. + +Area, the germinative. + +Aristotle. + +Arm, structure of the. + +Arrow-worm, the. + +Arterial arches, the. +cone, the. + +Arteries, evolution of the. + +Articulates, the. +skeleton of the. + +Articulation. + +Aryo-Romanic languages, the. + +Ascidia, the. +embryology of the. + +Ascula. + +Asexual reproduction. + +Atlas, the. + +Atrium, the. +(heart), the. + +Auditory nerve, the. + +Auricles of the heart. + +Autolemures. + +Axolotl, the. + +Bacteria. + +Baer, K.E. von. + +Balanoglossus. + +Balfour, F. + +Batrachia. + +Bdellostoma Stouti. + +Bee, generation of the. + +Beyschlag, W., on evolution. + +Bilateral symmetry. +origin of. + +Bimana. + +Biogenetic law, the. + +Biogeny. + +Bionomy. + +Bird, evolution of the. +ovum of the. + +Bischoff, W. + +Bladder, evolution of the. + +Blastaea, the. + +Blastocoel, the. + +Blastocrene, the. + +Blastocystis, the. + +Blastoderm, the. + +Blastodermic vesicle, the. + +Blastoporus, the. + +Blastosphere, the. + +Blastula, the. +the mammal. + +Blood, importance of the. +recent experiments in mixture of. +structure of the. + +Blood-cells, the. + +Blood-vessels, the. +development of the. +of the vertebrate. +origin of the. + +Boniface VIII, Bull of. + +Bonnet. + +Borneo nosed-ape, the. + +Boveri, Theodor. + +Brachytarsi. + +Brain and mind. +evolution of the. +in the fish. +in the lower animals. +structure of the. + +Branchial arches, evolution of the. +cavity, the. +system, the. + +Branchiotomes. + +Breasts, the. + +Bulbilla. + +Calamichthys. + +Calcolynthus. + +Capillaries, the. + +Caracoideum, the. + +Carboniferous strata. + +Carcharodon. + +Cardiac cavity, the. + +Cardiocoel, the. + +Catallacta. + +Caryobasis. + +Caryokinesis. + +Caryolymph. + +Caryolyses. + +Caryon. + +Caryoplasm. + +Catarrhinae, the. + +Catastrophic theory, the. + +Caudate cells. + +Cell, life of the. +nature of the. +size of the. + +Cell theory, the. + +Cenogenesis. + +Cenogenetic structures. + +Cenozoic period, the. + +Central body, the. + +Central nervous system, the. + +Centrolecithal ova. + +Centrosoma, the. + +Ceratodus, the. + +Cerebellum, the. + +Cerebral vesicles, evolution of the. + +Cerebrum, the. + +Cestracion Japonicus. + +Chaetognatha. + +Chick, importance of the, in embryology. + +Child, mind of the. + +Chimpanzee, the. + +Chiromys. + +Chiroptera. + +Chirotherium. + +Chondylarthra. + +Chorda, the. +evolution of the. + +Chordaea, the. + +Chordalemma, the. + +Chordaria. + +Chordula, the. + +Choriata, the. + +Chorion, the. +development of the. +frondosum. +laeve. + +Choroid coat, the. + +Chorology. + +Chromacea. + +Chromatin. + +Chroococcacea. + +Chroococcus, the. + +Church, opposition of, to science in Middle Ages. + +Chyle. + +Chyle-vessels. + +Cicatricula, the. + +Ciliated cells. + +Cinghalese gynecomast. + +Circulation in the lancelet. + +Circulatory system, evolution of the. +structure of the. + +Classification. +evolutionary value of. + +Clitoris, the. + +Cloaca, the. + +Cnidaria. + +Coccyx, the. + +Cochineal insect, the. + +Cochlea, the. + +Coecilia. + +Coecum [Caecum], the. + +Coelenterata. + +Coelenteria. + +Coeloma, the. + +Coelomaea, the. + +Coelomaria. + +Coelomation. + +Coelom-theory, the. + +Coelomula, the. + +Colon, the. + +Comparative anatomy. + +Conception, nature of. + +Conjunctiva, the. + +Conocyema. + +Convoluta. + +Copelata, the. + +Copulative organs, evolution of the. + +Corium, the. + +Cornea, the. + +Corpora cavernosa, the. + +Corpora quadrigemina. + +Corpora striata. + +Corpus callosum, the. + +Corpus vitreum, the. + +Corpuscles of the blood. + +Craniology. + +Craniota, the. + +Cranium, the. + +Creation. + +Cretaceous strata. + +Crossopterygii. + +Crustacea, the. + +Cryptocoela. + +Cryptorchism. + +Crystalline lens, the. +development of the. + +Cutaneous glands. + +Cuttlefish, embryology of the. + +Cuvier, G. + +Cyanophycea. + +Cyclostoma, the. +ova of the. + +Cyemaria. + +Cynopitheca. + +Cynthia. + +Cytoblastus, the. + +Cytodes. + +Cytoplasm. + +Cytosoma. + +Cytula, the. + +Dalton. + +Darwin, C. + +Darwin, E. + +Darwinism. + +Decidua, the. + +Deciduata. + +Deduction, nature of. + +Degeneration theory, the. + +Dentition of the ape and man. + +Depula. + +Descent of Man. + +Design in organisms. + +Deutoplasm. + +Devonian strata. + +Diaphragm, the. +evolution of the. + +Dicyema. + +Dicyemida. + +Didelphia. + +Digonopora. + +Dinosauria. + +Dipneumones. + +Dipneusta. +ova of the. + +Dipnoa. + +Directive bodies. + +Discoblastic ova. + +Discoplacenta. + +Dissatyrus. + +Dissection, medieval decrees against. + +Dohrn, Anton. + +Dollinger. + +Dorsal furrow, the. +shield, the. +zone, the. + +Dromatherium. + +Dualism. + +Dubois, Eugen. + +Ductus Botalli, the. + +Ductus venosus Arantii. + +Duodenum, the. + +Duration of embryonic development. +of man's history. + +Dysteleology. +proofs of. + +Ear, evolution of the. +structure of the. +uselessness of the external. + +Ear-bones, the. + +Earth, age of the. + +Echidna hystrix. + +Ectoblast. + +Ectoderm, the. + +Edentata. + +Efficient causes. + +Egg of the bird. +or the chick, priority of the. + +Elasmobranchs, the. + +Embryo, human, development of the. + +Embryology. +evolutionary value of. + +Embryonic development, duration of. +disk, the. +spot, the. + +Encephalon, the. + +Endoblast. + +Endothelia. + +Enterocoela. + +Enteropneusta. + +Entoderm, the. + +Eocene strata. + +Eopitheca. + +Epiblast. + +Epidermis, the. + +Epididymis, the. + +Epigastrula. + +Epigenesis. + +Epiglottis, the. + +Epiphysis, the. + +Episoma. + +Episomites. + +Epispadia. + +Epithelia. + +Epitheria. + +Epovarium, the. + +Equilibrium, sense of. + +Esthonychida. + +Eustachian tube, the. + +Eutheria. + +Eve. + +Evolution theory, the. +inductive nature of. + +Eye, evolution of the. +structure of the. + +Eyelid, the third. + +Eyelids, evolution of the. + +Fabricius ab Aquapendente + +Face, embryonic development of the. + +Fat glands in the skin. + +Feathers, evolution of. + +Fertilisation. +place of. + +Fin, evolution of the. + +Final causes. + +Flagellate cells. + +Floating bladder, the. +evolution of the. + +Foetal circulation. + +Food-yelk, the. + +Foot, evolution of the. +of the ape and man. + +Fore brain, the. + +Fore kidneys, the. + +Fossiliferous strata, list of. + +Fossils. +scarcity of. + +Free will. + +Friedenthal, experiments of. + +Frog, the. +ova of the. + +Frontonia. + +Function and structure. + +Furcation of ova. + +Gaertner's duct. + +Ganglia, commencement of. + +Ganglionic cell, the. + +Ganoids. + +Gastraea, the. +formation of the. + +Gastraea theory, the. + +Gastraeads. + +Gastremaria. + +Gastrocystis, the. + +Gastrophysema. + +Gastrotricha. + +Gastrula, the. + +Gastrulation. + +Gegenbaur, Carl. +on evolution. +on the skull. + +Gemmation. + +General Morphology. + +Genesis. + +Genital pore, the. + +Geological evolution, length of. +periods. + +Geology, methods of. +rise of. + +Germ-plasm, theory of. + +Germinal disk. +layers, the. +scheme of the. +spot, the. +vesicle, the. + +Germinative area, the. + +Giant gorilla, the. + +Gibbon, the. + +Gill-clefts and arches. +formation of the. + +Gill-crate, the. + +Gills, disappearance of the. + +Gloeocapsa. + +Gnathostoma. + +Goethe as an evolutionist. + +Goitre. + +Gonads, the. +formation of the. + +Gonidia. + +Gonochorism, beginning of. + +Gonoducts. + +Gonotomes. + +Goodsir. + +Gorilla, the. + +Graafian follicles, the. + +Gregarinae. + +Gullet-ganglion, the. + +Gut, evolution of the. + +Gyrini. + +Gynecomastism. + +Hag-fish, the. + +Hair, evolution of the. +on the human embryo and infant. + +Hair, restriction of, by sexual selection. + +Haliphysema. + +Halisauria. + +Haller, Albrecht. + +Halosphaera viridis. + +Hand, evolution of the. +of the ape and man. + +Hapalidae. + +Harderian gland, the. + +Hare-lip. + +Harrison, Granville. + +Hartmann. + +Harvey. + +Hatschek. + +Hatteria. + +Head-cavity, the. + +Head-plates, the. + +Heart, development of the. +of the ascidia. +position of the. + +Helmholtz. + +Helminthes. + +Hepatic gut, the. + +Heredity, nature of. + +Hermaphrodism. + +Hertwig. + +Hesperopitheca. + +His, W. + +Histogeny. + +History of Creation. + +Holoblastic ova. + +Homoeosaurus. + +Homology of the germinal layers. + +Hoof, evolution of the. + +Hunterian ligament, the. + +Huxleian law, the. + +Huxley, T.H. + +Hydra, the. + +Hydrostatic apparatus in the fish. + +Hylobates. + +Hylodes Martinicensis. + +Hyoid bone, the. + +Hypermastism. + +Hyperthelism. + +Hypoblast. + +Hypobranchial groove, the. + +Hypodermis, the. + +Hypopsodina. + +Hyposoma, the. + +Hyposomites. + +Hypospadia. + +Ichthydina. + +Ichthyophis glutinosa. + +Ictopsida. + +Ileum, the. + +Immortality, Aristotle on. + +Immortality of the soul. + +Impregnation-rise, the. + +Indecidua. + +Indo-Germanic languages. + +Induction and deduction. + +Inheritance of acquired characters. + +Insects, intelligence of. + +Interamniotic cavity, the. + +Intestines, the. + +Invagination. + +Iris, the. + +Jacchus. + +Java, ape-man of. + +Jaws, evolution of the. + +Jurassic strata. + +Kant, dualism of. + +Kelvin, Lord, on the origin of life. + +Kidneys, the. +formation of the. + +Klaatsch. + +Kolliker. + +Kowalevsky. + +Labia, the. + +Labyrinth, the. + +Lachrymal glands. + +Lamarck, J. +theories of. + +Lamprey, the. +ova of the. + +Lancelet, the. +description of the. + +Languages, evolution of. + +Lanugo of the embryo. + +Larynx, the. +evolution of the. + +Latebra, the. + +Lateral plates, the. + +Laurentian strata. + +Lecithoma, the. + +Leg, evolution of the. +structure of the. + +Lemuravida. + +Lemurogona. + +Lemurs, the. + +Lepidosiren. + +Leucocytes. + +Life, age of. + +Limbs, evolution of the. + +Limiting furrow, the. + +Linin. + +Liver, the. + +Long-nosed ape, the. + +Love, importance of in nature. + +Lungs, the. +evolution of the. + +Lyell, Sir C. + +Lymphatic vessels, the. + +Lymph-cells, the. + +Macrogonidion. + +Macrospores. + +Magosphaera planula. + +Male womb, the. + +Mallochorion, the. + +Mallotheria. + +Malpighian capsules. + +Mammal, characters of the. +gastrulation of the. + +Mammals, unity of the. + +Mammary glands, the. + +Man and the ape, relation of. +origin of. + +Man's Place in Nature. + +Mantle, the. + +Mantle-folds, the. + +Marsupials, the. +ova of the. + +Materialism. + +Mathematical method, the. + +Mechanical causes. +embryology. + +Meckel's cartilage. + +Medulla capitis, the. +oblongata, the. +spinalis, the. + +Medullary groove, the. +tube, the. +formation of the. + +Mehnert, E., on the biogenetic law. + +Meroblastic ova. + +Merocytes. + +Mesentery, the. + +Mesocardium, the. + +Mesoderm, the. + +Mesogastria. + +Mesonephridia, the. + +Mesonephros, the. + +Mesorchium, the. + +Mesovarium, the. + +Mesozoic period, the. + +Metogaster, the. + +Metagastrula, the. + +Metamerism. + +Metanephridia, the. + +Metanephros, the. + +Metaplasm. + +Metastoma. + +Metatheria. + +Metazoa. + +Metovum, the. + +Microgonidian. + +Microspores. + +Middle ear, the. + +Migration, effect of. + +Milk, secretion of the. + +Mind, evolution of. +in the lower animals. + +Miocene strata. + +Mitosis. + +Monera. + +Monism. + +Monodelphia. + +Monogonopora. + +Monopneumones. + +Monotremes. +ova of the. + +Monoxenia Darwinii. + +Morea, the. + +Morphology. + +Morula, the. + +Motor-germinative layer, the. + +Mouth, development of the. +structure of the. + +Mucous layer, the. + +Mullerian duct, the. + +Muscle-layer, the. + +Muscles, evolution of the. +of the ear, rudimentary. + +Myotomes. + +Myxinoides, the. + +Nails, evolution of the. + +Nasal pits. + +Natural philosophy. +selection. + +Navel, the. + +Necrolemurs. + +Nectocystis, the. + +Nemertina. + +Nephroduct, evolution of the. + +Nephrotomes. + +Nerve-cell, the. + +Nerves, animals without. + +Nervous system, evolution of the. + +Neurenteric canal, the. + +Nictitating membrane, the. + +Nose, the, in man and the ape. +development of the. +structure of the. + +Notochorda, the. + +Nuclein. + +Nucleolinus. + +Nucleolus, the. + +Nucleus of the cell. + +Oesophagus, the. + +Oken. + +Oken's bodies. + +Oligocene strata. + +Olynthus. + +On the generation of animals. + +Ontogeny. +defective evidence of. + +Opaque area, the. + +Opossum, the. +ova of the. + +Optic nerve, the. + +Optic thalami. +vesicles. + +Orang, the. + +Ornithodelphia. + +Ornithorhyncus. + +Ornithostoma. + +Ossicles of the ear. + +Otoliths. + +Ova, number of. +of the lancelet. + +Ovaries, evolution of the. + +Oviduct, origin of the. + +Ovolemma, the. + +Ovulists. + +Ovum, discovery of the. +nature of the. +size of the. + +Pachylemurs, the. + +Pacinian corpuscles. + +Paleontology. +evolutionary evidence of. +incompleteness of. +rise of. + +Paleozoic age, the. + +Palingenesis. + +Palingenetic structures. + +Palaehatteria. + +Panniculus carnosus, the. + +Paradidymis, the. + +Parietal zone, the. + +Parthenogenesis. + +Pastrana, Miss Julia. + +Pedimana. + +Pellucid area, the. + +Pelvic cavity, the. + +Pemmatodiscus gastrulaceus. + +Penis-bone, the. + +Penis, varieties of the. + +Peramelida. + +Periblastic ova. + +Peribranchial cavity, the. + +Pericardial cavity, the. + +Perichorda, the. +formation of the. + +Perigastrula. + +Permian strata. + +Petromyzontes, the. + +Phagocytes. + +Pharyngeal ganglion, the. + +Pharynx, the. + +Philology, comparison with. + +Philosophie Zoologique. + +Philosophy and evolution. + +Phycochromacea. + +Phylogeny. + +Physemaria. + +Physiology, backwardness of. + +Phytomonera. + +Pineal eye, the. + +Pinna, the. + +Pithecanthropus. + +Pithecometra-principle, the. + +Placenta, the. + +Placentals, the. +characters of the. +gastrulation of the. + +Planocytes. + +Plant-louse, parthenogenesis of the. + +Planula, the. + +Plasma-products. + +Plasson. + +Plastids. + +Plastidules. + +Platodaria. + +Platodes, the. + +Platyrrhinae. + +Pleuracanthida. + +Pleural ducts. + +Pliocene strata. + +Polar cells. + +Polyspermism. + +Preformation theory, the. + +Primary period, the. + +Primates, the. + +Primatoid. + +Primitive groove, the. +gut, the. +kidneys, the. +mouth, the. +segments. +streak, the. +vertebrae. + +Primordial period, the. + +Prochordata. + +Prochordonia, the. + +Prochoriata. + +Prochorion, the. + +Proctodaeum, the. + +Procytella primordialis. + +Prodidelphia. + +Progaster, the. + +Progonidia. + +Promammalia. + +Pronephridia, the. + +Pronucleus femininus. +masculinus. + +Properistoma. + +Prorenal canals of the lancelet. +duct, the. +evolution of the. + +Proselachii. + +Prosimiae, the. + +Prospermaria. + +Prospondylus. + +Prostoma. + +Protamniotes. + +Protamoeba. + +Proterosaurus, the. + +Protists. + +Protonephros. + +Protophyta. + +Protoplasm. + +Protopterus. + +Prototheria. + +Protovertebrae. + +Protozoa. + +Provertebral cavity, the. +plates, the. + +Pseudocoela. + +Pseudopodia. + +Pseudova. + +Psychic life, evolution of the. + +Psychology. + +Pterosauria. + +Pylorus, the. + +Quadratum, the. + +Quadrumana. + +Quaternary period. + +Rabbit, ova of the. + +Radiates, the. + +Rathke's canals. + +Rectum, the. + +Regner de Graaf. + +Renal system, evolution of the. + +Reproduction, nature of. + +Reptiles. + +Respiratory organs, evolution of the. +pore, the. + +Retina, the. + +Rhabdocoela. + +Rhodocytes. + +Rhopalura. + +Rhyncocephala. + +Ribs, the. +number of the. + +Rudimentary ear-muscles. +organs. +list of. +toes. + +Sacculus, the. + +Sagitta. +coelomation of. + +Salamander, the. +ova of the. + +Sandal-shape of embryo. + +Satyrus. + +Sauromammalia. + +Sauropsida. + +Scatulation theory, the. + +Schizomycetes. + +Schleiden, M. + +Schwann, T. + +Sclerotic coat, the. + +Sclerotomes. + +Scrotum, the. + +Scyllium, nose of the. + +Sea-squirt, the. + +Secondary period, the. + +Segmentation. + +Segmentation-cells. + +Segmentation-sphere, the. + +Selachii. +skull of the. + +Selection, theory of. + +Selenka. + +Semnopitheci. + +Sense-organs, evolution of the. +number of the. +origin of the. + +Sensory nerves. + +Serocoelom, the. + +Serous layer, the. + +Sex-organs, early vertebrate form of the. +evolution of the. + +Sexual reproduction, simplest forms of. +selection. + +Shark, the. +nose of the. +ova of the. +placenta of the. +skull of the. + +Shoulder-blade, the. + +Sickle-groove, the. + +Sieve-membrane, the. + +Silurian strata. + +Simiae, the. + +Siphonophorae, embryology of the. + +Skeleton, structure of the. + +Skeleton-plate, the. + +Skin, the. +evolution of. +function of the. + +Skin-layer, the. + +Skull, evolution of the. +structure of the. +vertebral theory of the. + +Smell, the sense of. + +Soul, evolution of the. +nature of the. +phylogeny of the. +seat of the. + +Sound, sensations of. + +Sozobranchia. + +Space, sense of. + +Species, nature of the. + +Speech, evolution of. + +Spermaducts. + +Spermaries, evolution of the. + +Spermatozoon, the. +discovery of the. + +Spinal cord, development of the. +structure of the. + +Spirema, the. + +Spiritualism. + +Spleen, the. + +Spondyli. + +Sponges, classification of the. +ova of the. + +Spontaneous generation. + +Stegocephala. + +Stem-cell, the. + +Stem-zone, the. + +Stomach, evolution of the. +structure of the human. + +Strata, thickness of. + +Struggle for life, the. + +Subcutis, the. + +Sweat glands. + +Tactile corpuscles. + +Tadpole, the. + +Tail, evolution of the. +rudimentary, in man. + +Tailed men. + +Taste, the sense of. + +Teeth, evolution of the. +--of the ape and man. + +Teleostei. + +Telolecithal ova. + +Temperature, sense of. + +Terrestrial life, beginning of. + +Tertiary period, the. + +Theoria generationis, the. + +Theories, value of. + +Theromorpha. + +Third eyelid, the. + +Thyroid gland, the. + +Time-variations in ontogeny. + +Tissues, primary and secondary. + +Toad, the. + +Tocosauria. + +Toes, number of the. + +Tori genitales, the. + +Touch, the sense of. + +Tracheata. + +Tread, the. + +Tree-frog, the. + +Triassic strata. + +Triton taeniatus. + +Troglodytes. + +Tunicates, the. + +Turbellaria. + +Turbinated bones, the. + +Tympanic cavity, the. + +Umbilical, cord, the. +vesicle, the. + +Unicellular ancestor of all animals. +--animals. + +Urachus, the. + +Urinary system, evolution of the. + +Urogenital ducts. + +Uterus masculinus, the. + +Utriculus, the. + +Vasa deferentia. + +Vascular layer, the. +system, evolution of the. +structure of the. + +Vegetative layer, the. + +Veins, evolution of the. + +Ventral pedicle, the. + +Ventricles of the heart. + +Vermalia. + +Vermiform appendage, the. + +Vertebrae. + +Vertebraea. + +Vertebral arch, the. +column, the. +evolution of the. +structure of the. + +Vertebrates, character of the. +descent of the. + +Vertebration. + +Vesico-umbilical ligament, the. + +Vesicula prostatica, the. + +Villi of the chorion. + +Virchow, R. +on the ape-man. +on the evolution of man. + +Virgin-birth. + +Vitalism. + +Vitelline duct, the. + +Volvocina. + +Wallace, A.R. + +Water, organic importance of. + +Water vessels. + +Weismann's theories. + +Wolff, C.F. + +Wolffian bodies. + +Wolffian duct, the. + +Womb, evolution of the. + +Yelk, the. + +Yelk-sac, the. + +Zona pellucida, the. + +Zonoplacenta. + +Zoomonera. + +Zoophytes. + + + + + + + + + + + + +End of Project Gutenberg's The Evolution of Man, V.2, by Ernst Haeckel + +*** END OF THE PROJECT GUTENBERG EBOOK THE EVOLUTION OF MAN, V.2 *** + +This file should be named 6710.txt or 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