m nn c THE EVOLUTION OF MAN: A POPULAR EXPOSITION OF THE PRINCIPAL POINTS OF HUMAN ONTOGENY AND PHYLOGENY. FEOM THE GERMAN OF ERNST HAECKEL, PROFESSOR IN THE UNIVERSITY OF JENA, AUTHOR OF "THE HISTORY OF CREATION," ETC. IN TWO VOLUMES. VOL. II. NEW YOEK : D. APPLETON AND COMPANY, 549 & 551 BROADWAY. 1879. / 37 ft •• .*. aj'J i XXXIII. Systematic Survey of the ontogeny of the human XXXIV. Systematic Survey of the constitution of the human SlvGlGTOH • • • ••• • . . ••• . . , Zi { o XXXV. Systematic Survey of the phylogeny of the human skeleton ... ... ... ... 309 XXXVI. Systematic Survey of the constitution of the human intestinal system ... ... ... .... 330 XXXVII. Systematic Survey of the phylogeny of the human intestinal system ... ... ... ... 346 XXXVIII. Systematic Survey of the sequence, according to age, of the human tissue-groups (phylogenetic • sequence of the tissues) ... ... ... 366 XXXIX. Systematic Survey of the sequence, according to age, of the human organ-systems (phylogenetic sequence of the organs) ... ... ... 367 XL. Systematic Survey of the phylogeny of the human vascular system ... ... ... ... 384 XLI. Systematic Survey of the phylogeny of the human no tii t' ••« * • * • * • ••• *•• tjCjij XLII. Systematic Survey of the homologies of Worms, Articulated Animals (Arthrojwda), Soft-bodied Animals (Mollusca), and Vertebrates ... ... 387 XLIII. Systematic Survey of the phylogeny of the human urinary and sexual organs ... ... ... 428 XLIV. Systematic Survey of the homologies of the sexual organs in the two sexes of Mammals ... ... 431 THE EVOLUTION OF MAN. CHAPTER XV. THE DURATION OF HUMAN TRIBAL HISTORY. Comparison of Ontogenetic and Phylogenetic Periods of Time. — Duration of Germ-history in Man and in Different Animals. — Extreme Brevity of the Latter in Comparison with the Immeasurable Long Periods of Tribal History. — Relation of this Eapid Ontogenetic Modification to the Slow Phylogenetic Metamorphosis. — Estimate of the Past Duration of the Organic World, founded on the Relative Thickness of Sedimentary Rock-strata, or Neptunian Formations. — The Five Main Divisions in the Latter : I. Primordial, or Archilithic Epoch. II. Primary, or Palaeolithic Epoch. III. Secondary, or Mesolithic Epoch. IV. Tertiary, or Csenolithic Epoch. V. Quaternary, or Anthropolithic Epoch. — The Relative Duration of the Five Epochs. — The Results of Comparative Philology as Explaining the Phylogeny of Species. — The Inter-relations of the Main Stems and Branches of the Indo-Germanic Languages are Analogous to the Inter-relatious of the Main Stems and Branches of the Vertebrate Tribe. — The Parent Forms in both Cases are Extinct. — The Most Important Stages among the Human Ancestral Forms. — Monera originated by Spontaneous Generation. — Necessity of Sponta- neous Generation. "In vain as yet has it been attempted to draw an exact line of demarcation between historic and prehistoric times ; the origin of man and the period of his first appearance pass back into indefinable time ; the so-called archaic age cannot be sharply distinguished from the present age. This is the fate of all geological, as of all historical periods. The periods which we dis- tinguish are, therefore, more or less arbitrarily defined, and, like the divisions 2 THE EVOLUTION OF MAN. in systematic natural history, can only serve to bring the subject of our study better before us and to render it more manageable ; but not to mark real distinctions between different things." — Beknhard Cotta. (1866). Our comparative study of the Anatomy and Ontogeny of the Arnphioxus and the Ascidian has afforded us aid, the value of which can hardly be over-estimated, towards acquiring a knowledge of human Ontogeny. For in the first place we have in this way filled up, as regards Anatomy, the wide chasm which in all previous systems of the animal kingdom existed between Vertebrates and Inverte- brates ; and in the second place, in the germ-history of the Arnphioxus we have recognized primordial phases of de- velopment, which have long disappeared from the Ontogeny of Man, and which have been lost in accordance with the law of abridged heredity. Of special importance among these phases of development is the Archigastrula, the ori- ginal, genuine Gastrula-form which the Arnphioxus has retained up to the present time, and which re-appears in the same form in low invertebrate animals of the most diverse classes. The germ-history of the Arnphioxus and the Ascidian has, therefore, so far perfected our direct knowledge of human genealogy, that, notwithstanding the incompleteness of our empiric knowledge, there is no essential gap of any great moment in the pedigree. We may, therefore, at once proceed to our task, and, aided by the ontogenetic and comparative -anatomical materials at our command, may reconstruct the main outlines of human Phylogeny. The immense importance of the direct application of the funda- mental biogenetic law of the causal connection between Ontogeny and Phylogeny now becomes evident. But, before TIME REQUIRED FOR THE DEVELOPMENT OF MAN. 3 beginning this task, it will be well to note a few other general facts which may enable us better to understand the phenomena we are about to study. Firstly, it may not be out of place to insert a few remarks as to the duration of time during which Man was developing from the animal kingdom. The first thought that occurs to the mind when we consider the facts in question, is of the immense difference between the duration of the germ-history of Man on the one hand, and of his tribal history on the other. The brief period in which the Ontogeny of the human individual takes place, bears no proportion to the infinitely long period required for the Phylogeny of the human tribe. The human individual requires nine months for its perfect development from the fertilized egg- cell to the moment at which it is born and quits the mother's body. The human embryo, therefore, passes through the whole course of its development in the brief space of 40 weeks (usually in exactly 280 days). Each man is really older by this period than is usually assumed. When, for example, a child is said to be 9J years old, he is in reality 10 years old. For individual existence does not begin at the moment of birth, but at the moment of fertilization. In many other Mammals the duration of the embryonic development is the same as in Man, e.g., the Ox. In the Horse and the Ass it is somewhat longer, viz., from 43 to 45 weeks; in the Camel it is 13 months. In the largest Mammals the embryo requires a much longer time for its complete formation within the maternal body; in the Rhinoceros, for instance, 1J year, in the Elephant 90 weeks. In the latter case, therefore, gestation lasts more than twice as long as in Man — for 4 THE EVOLUTION OF MAN. nearly a year and three quarters. In the smaller Mammals, the duration of embryonic development is, on the contrary, much shorter. The smallest Mammals, the Harvest Mice, develop fully in 3 weeks ; Rabbits and Hares in 4 weeks ; Rats and Marmots in 5 weeks ; the Dog in 9, the Pig in 17, the Sheep in 21, and the Stag in 36 weeks. Development is yet more rapid in Birds. The Chick, under normal con- ditions of incubation, requires only 3 weeks, or just 21 days for its full development. The Duck, on the other hand, takes 25, the Turkey 27, the Peacock 31, the Swan 42, and the New Holland Cassowary 65 days. The smallest of all Birds, the Humming-bird, quits the egg after the twelfth day. It is, therefore, evident that in Mammals and in Birds the duration of development within the egg-membranes stands in a definite relation to the size of body attained by each vertebrate species. But the latter is not the sole determin- ing cause of the former. There are many other circum- stances which influence the duration of individual develop- ment within the membranes of the egg.126 In all cases, however, the duration of the Ontogeny appears infinitely brief when compared with the enormous, the infinitely long period during which the Phylogeny, or gradual development of the ancestral series, took place. This period is not to be measured by years and centuries, but by thousands and millions of years. Many millions of years must indeed have elapsed while the most perfect vertebrate organism, Man, gradually developed from the primeval one-celled ancestral organism. The opponents of the development theory, who regard this gradual develop- ment of Man from lower animal forms, and his original descent from a one-celled primitive animal as incredible, DURATION OF HUMAN GERM-HISTORY. 5 do not reflect that the very same marvel actually recurs before our eyes in the short space of nine months during the embryonic development of each human individual. The same series of multifariously diverse forms, through which our brute ancestors passed in the course of many millions of years, has been traversed by every Man during the first 40 weeks of his individual existence within the maternal body. All changes in organic forms, all metamorphoses of animal and plant forms, appear to us all the more remark- able and all the more wonderful in proportion as they occur more rapidly. When, therefore, our opponents pronounce that the past development of the human race from lower animal forms is incredible, they must regard the embryonic develop- ment of the human individual from the simple egg-cell as far more wonderful in comparison. This latter process — the ontogenetic modification — which takes place before our eyes, must appear more wonderful than the phylogenetic modifi- cation, in proportion as the duration of the tribal history exceeds that of the germ-history. For the human embryo must pass through the whole process of individual develop- ment, from the simple cell up to the many-celled perfect Man, with all his organs, in the brief space of 40 weeks. On the other hand, we may assign many millions of years for the accomplishment of the analogous process of phyloge- 9 netic development — the development of Man's ancestors from the simplest one-celled form. As regards these phylogenetic periods, it is impossible to fix approximately their length in hundreds or in thousands of years, or to establish any absolute measure of their duration. But the researches of geologists have long since 6 THE EVOLUTION OF MAN. enabled us to estimate and compare the relative durations of the various periods of the earth's organic history. The most direct standard for determining the relative duration of geological periods is afforded by the thickness of the so- called Neptunian strata or sedimentary rock, i.e., all those strata which have been deposited, as mud, at the bottom of the ocean, or under fresh water. These stratified sedi- mentary rocks — limestone, clay, marl, sandstone, slate, etc. — which constitute the great mass of mountain-chains, and which are often several thousand feet in thickness, afford us data for estimating the relative lengths of the various periods of the earth's history. For the sake of completeness, I must say a few words as to the development of the earth as a whole, briefly indicating a few of the more prominent facts relating to this matter. At the very outset we are confronted with the weighty fact, that life originated on our planet at a certain definite period. This is a proposition that is no longer gainsaid by any competent geologist. We now know for certain that organic life upon our planet actually began at a certain time, and that it did not exist there from eternity, as some have supposed. The indisputable proofs of this are furnished, on the one hand, by physico-astronomical cos- mogeny ; on the other, by the Ontogeny of organisms. Species and tribes, like individuals, do not enjoy a perpetual life.127 They also had a beginning. The time which has elapsed since the origin of life upon the earth up to the present time (and with this period of time alone we are here concerned) we call the " history of the organic earth," as distinguished from the " history of the inorganic earth " which embraces the period before the origin of organic life. THE FIRST DEVELOPMENT OF ORGANIC LIFE. 7 With regard to the latter, we first obtained clear ideas from the natural philosophical researches and computations of the great critical philosopher, Immanuel Kant, and on this point I must refer the reader to Kant's " Allgemeine Natur- geschichte und Theorie des Himmels " and to the numerous Cosmogenies which treat the subject in a popular style. We cannot here dwell upon questions of this kind. The organic history of the earth could begin only when water in fluid drops existed upon its surface. For the very existence of all organisms, without any exception, depends on water in the fluid state, their bodies containing a con- siderable amount of the same. Our own body, in its fully developed state, contains in its tissues 70 per cent, of water and only 30 per cent, of solid matter. The amount of water is still greater in the body of the child, and is greatest of all in the embryo. In early stages of development the human embryo contains more than 90 per cent, of water, and not 10 per cent, of solid matter. In low marine animals, especially in the Medusa?, the body contains even more than 99 per cent, of water, and not even one per cent, of solid matter. No organism can exist and perform its vital functions without water. Without water there is no life. Water in the fluid state, which is, therefore, in- dispensable for the existence of life, could not, however, appear upon the earth until after the temperature of the surface of the fiery globe had sunk to a certain point. Before this it existed only in the form of steam. As soon, however, as the first drop of water in a fluid state was precipitated by cooling from the envelope of steam, it began its geological action, and from that time to this it has effected continual changes in the modification of the hard 24 8 THE EVOLUTION OF MAN. crust of the earth. The result of this unceasing work of the water, which in the form of rain and hail, of snow and ice, of rushing torrent and surging wave crumbles and dis- solves the rocks, is the formation of ooze. As Huxley says. in his excellent "Lectures on the Causes of the Phenomena of Organic Nature," the most important fact in the past history of our earth is ooze, and the question as to the history of the past ages of the world resolves itself into a question as to the formation of ooze. AH the stratified rocks of our moun- tainous formations were originally deposited as ooze at the bottom of the waters, and only afterwards hardened into solid stone. As has already been said, it is possible, by bringing together and comparing the various rock-strata from many places on the surface of the earth, to obtain an approximate conception of the relative ages of these various strata. Geologists have long agreed that there is an entirely definite historical sequence of the various formations. The various groups of strata which lie one over another correspond to successive periods in the earth's organic history, during which they were deposited in the shape of mud at the bottom of the sea. Gradually this mud was hardened into solid rock. The latter, by alternate upheaval and depres- sion of the surface of the earth, was lifted above the water, and assumed the form of mountains. Four or five main periods in the earth's organic history, answering to the larger and smaller groups of these sedimentary rock-strata, are usually distinguished. These main periods are sub- divided into numerous subordinate or lesser periods. From twelve to fifteen of the latter are usually assumed. (Cf. Tables XII. and XIII, pp. 11, 12.) The relative thick- GEOLOGICAL TIME. 9 ness of the various groups of strata affords the means of approximately estimating the relative length of these various divisions of time. Of course we cannot say, " In a hun- dred years on the average a stratum of a certain thick- ness (say two inches) is deposited, and therefore a rock- stratum of a thousand feet in thickness is 600,000 years old." For different rock-formations of equal thick- ness may have occupied periods of very various length in their deposition and consolidation. From the thickness of the formation we may, however, approximately judge of the relative length of the period during which it was formed. Of the four or five main periods of the earth's organic history, our acquaintance with which is indispensable for our Phylogeny of the human race, the first and oldest is known as the Primordial, Archizoic, or Archilithic Epoch. If we estimate the total thickness of all the sedimentary strata as averaging about 130,000 feet, then 70,000 feet belong to this first epoch — more than one half. From this and other circumstances we may conclude that the corresponding Primordial or Archilithic Epoch must alone have been con- siderably longer than the whole long period between the close of the Archilithic and the present time. Probably the Primordial Epoch was much longer than might appear from the ratio of 7 : 6, which we have given. This Epoch is divided into three sub-periods, known as the Laurentian, Cambrian, and Silurian, corresponding to the three principal groups of sedimentary rock-strata which constitute the Archilithic rocks. The enormous length of time required for the forma- tion at the bottom of the primordial sea of these gigantic strata, of over 70,000 feet in thickness, must, at all events, 10 THE EVOLUTION OF MAN. have been many millions of years. During that time there came into existence by spontaneous generation the oldest and simplest organisms — those in which life began upon our planet — viz., the Monera. From these, one-celled plants and animals first developed — the Amcebre and many kinds of Protista. During this same Archilithic Epoch, also, all the invertebrate ancestors of the human race developed from these one-celled organisms. We draw this conclusion from the fact that towards the close of the Silurian period a few remains of fossil Fishes are already to be found, viz., Selachians and Ganoids. These are, however, much more highly organized and of later origin than the lowest Vertebrates (the Amphioxus), or. than the various skull-less Vertebrates allied to Amphioxus, which must have lived during this time. The latter must necessarily have been preceded by all the invertebrate ancestors of man. Hence we may characterize this entire epoch as the " age of man's invertebrate ancestors ; " or, with special reference to the oldest representatives of the Vertebrate tribe, as the " age of Skull-less Animals." During the whole Archilithic Epoch the inhabitants of our planet consisted exclusively of aquatic forms ; at least, no remains of terrestrial animals or plants dating from this period have as yet been found. A few remains of land-dwelling organisms which are some- times referred to the Silurian Period, are Devonian. The Primordial Epoch was followed by the Palaeolithic, Palaeozoic, or Primary Epoch, which is also separable into three sub-periods : the Devonian, the Carboniferous, and the Permian. During the Devonian Period the Old Ked Sand- stone, or Devonian system was formed ; during the Car- boniferous, those great beds of coal were deposited which ( II ) TABLE XII. Systematic Survey of the Palaeontological Periods, or the Greater Divisions in the History of the Organic Earth. I. First Epoch : The Archilithic, or Primordial Epoch. (Age of Skull-less Animals and Seaweed Forests.) 1. The Older Archilithic Epoch or Laurentian Period. 2. The Middle Archilithic Epoch „ Cambrian Period. 3. The Later Archilithic Epoch „ Silurian Period. II. Second Epoch : The Palaeolithic, or Primary Epoch. (Age of Fishes and Fern Forests.) 4. The Older Palaeolithic Epoch or Devonian Period. 5. The Middle Palaeolithic Epoch „ Coal Period. 6. The Later Palaeolithic Epoch „ Permian Period. III. Third Epoch : The Mesolithic, or Secondary Epoch. (Age of Reptiles and Pine Forests, Qoniferce.) 7. The Older Mesolithic Epoch or Triassic Period. 8. The Middle Mesolithic Epoch „ Jurassic Period. 9. The Later Mesolithic Epoch „ Chalk Period. IV. Fourth Epoch : The Csenolithic, or Tertiary Epoch. (Age of Mammals and Leaf Forests.) 10. The Older Caenolithic Epoch or Eocene Period. 11. The Middle Caenolithic Epoch „ Miocene Period. 12. The Later Caenolithic Epoch „ Pliocene Period. V. Fifth Epoch : The Anthropolithic, or Quaternary Epoch. (Age of Man and Cultivated Forests.) 13. The Older Anthropolithic Epoch or Ice Age, Glacial Period. 14. The Middle Anthropolithic Epoch „ Post Glacial Period. 15. The Later Anthropolithic Epoch „ Period of Culture. (The Period of Culture is the Historic Period, or Period of Tradition.) ( 12 ) TABLE XIII. Systematic Survey of the Paheontological Formations, or the Fossiliferous Strata of the Earth's Crust. Roclc-Groups. Synonyms of Formations. V. Quaternary Group, or Anthropolithic (Anthropozoic) groups of strata IV. Tertiary Group, or Caenolithic (Caenozoic) groups of strata III. Secondary Group, or Mesolithic (Mesozoic) groups of strata IL Primary Group, or Palaeolithic (Palaeozoic) groups of strata) XIV. Eecent (Alluvium) XIII. Pleistocene (Diluvium) { XII. Pliocene (New tertiary) XL Miocene (Middle tertiary) X. Eocene (Old tertiary) IX. Cretaceous. VIII. Jura VII. Trias VI. Permian (New red sand- stone) r. Carboniferous (Coal) IV. Devonian \ (Old red sand- stone) 36. Present 35. Recent 34. Post glacial 33. Glacial 32. Arvernian 31. Sub-Appenine 30. Falunian 29. Limburgian 28. Gypsum 27. Nummulitic 26. London clay 25. White chalk 24. Green sand 23. Neocomian 22. Wealden 21. Portlandian 20. Oxfordian 19. Bath 18. Lias 17. Keuper 16. Muschelkalk 15. Bunter sand f 14. Mountain limestone (Zechstein) 13. Red sandstone 12. Carboniferous sandstone 11. Carboniferous limestone 10. Pilton 9. Ilfracombe 8. Linton I. Primordial Group, or Archil ithic (Archizoic) groups of strata III. Silurian II. Cambrian I. Laurentian 7. Ludlow 6. Wenlock 5. Llandeilo 4. Potsdam 3. Longmynd 2. Labrador 1. Ottawa Upper alluvial Lower alluvial Upper diluvial Lower diluvial Upper Lower Upper Lower Upper Middle Lower pliocene pliocene miocene miocene eocene eocene eocene Upper cretaceous Middle cretaceous Lower cretaceous The Kentish Weald Upper oolite Middle oolite Lower oolite Lias formation Upper trias Middle trias Lower trias Upper Permian Lower Permian Upper carbonifer- ous Lower carbonifer- ous Upper Devonian Middle Devonian Lower Devonian Upper Silurian Middle Silurian Lower Silurian Upper Cambrian Lower Cambrian Upper Laurentian Lower Laurentian GEOLOGICAL PERIODS. 1 3 supply us with our principal fuel ; in the Permian, the New Red Sandstone, the Magnesian Limestone (Zechstein), and the Cupriferous Slate were formed. The approxi- mate thickness of this entire group of strata is esti- mated at 42,000 feet at most ; some geologists make it somewhat more, others considerably less. In any case, this Palaeolithic Epoch, taken as a whole, is consider- ably shorter than the Archilithic, but yet is considerably longer than all the following Epochs taken together. The strata deposited during this Primary Epoch supply fossil animal remains in great abundance ; besides numerous species of Invertebrates we find also very many Verte- brates— Fishes preponderating. As early as the Devonian, and even during the Carboniferous and the Permian Periods, there existed so great a number of Fishes, espe- cially Primitive Fishes (Sharks) and Ganoids, that we may designate the entire Palaeolithic Period as the Age of Fishes. The Palaeozoic Ganoids especially are represented by a large number of forms. But even during this period some Fishes began to accustom themselves to living upon the land, and thus gave rise to the Amphibian class. Even in the carboniferous system we find fossil remains of Amphibia — the earliest terrestrial and air-breathing animals. In the Permian Period the variety of these Amphibia becomes greater. To- wards its close the first Amnion-animals, the tribal ancestors of the true higher Vertebrate classes, seem first to appear. These are a few lizard-like animals, of which the Protero- saurus from the Cupriferous Slate at Eisenach is the best known. The appearance of the most ancient Amnion Animals (Amniota), to which the common parent-form of 14 THE EVOLUTION OF MAN. Reptiles, Birds, and Mammals must have belonged, seems in fact to be referred by these oldest reptilian remains back to the close of the Palaeolithic Epoch. During this Epoch the ancestors of the human race must accordingly have been represented, first by true Fishes, then by Mud-Fishes (Blpneusta) and Amphibia, and finally by the oldest Amnion Animals, the Protamnia. After the Palaeolithic Epoch comes a third main division of the earth's organic history, known as the Mesolithic, or Secondary Epoch. This is again distinguished into three subdivisions — the Triassic, the Jurassic, and the Cretaceous Periods. The approximate thickness of the strata-groups, formed during these three Periods from the beginning of the Triassic down to the end of the Cretaceous Period, amounts in all to about 15,000 feet, not one half the thick- ness of the Palaeolithic deposits. During this Epoch a very great and varied development took place in all divisions of the animal kingdom. In the vertebrate tribe especially a number of new and interesting forms developed. Among Fishes the Osseous Fishes (Teleostei) now first appear. But the Reptiles surpass all others both in numbers and in diversity of species — the most remarkable and the most familiar forms being the gigantic extinct Dragons (Dino- saurians), the Sea-Dragons (Halisaurians), and the Flying Lizards (Pterosaurians). In reference to this predominance of the reptilian class this time is known as the age of reptiles. But the class of Birds also developed during this period, undoubtedly originated from a branch of the lizard-like Reptiles. This is shown by the similar embryology of Birds and of Reptiles, by their Comparative Anatomy, and also by the fact that we know of fossil birds with toothed jaws FAUNA OF THE GEOLOGICAL PERIODS. 1 5 and with lizard's tail, belonging to this period (Odon- tornis Archccopteryx). Finally, it was during this period that there appeared upon the scene that most perfect and, for us, most important vertebrate class, the mammalian class. The oldest fossil remains of these have been found in the most recent Triassic strata, viz., molar teeth of a small insectivorous Pouched Animal (Marsupial). Numer- ous remains occur somewhat later in the Jura system, and • a few in the chalk. All the remains of Mammals from this Mesolithic Epoch with which we are acquainted belong to the low Pouched Animal division ; and among these were undoubtedly the ancestors of Man. On the other hand, not a single undisputed relic has yet been discovered throughout all this period of one of the higher Mammals (Placentalia). This last division, of which Man is a member, did not develop till later, in the immediately subsequent Tertiary Epoch. The fourth main division of the history of the organic earth, the Tertiary, Csenozoic, or Csenolithic Epoch, was of much shorter duration than the preceding. For the strata deposited during this period are in all only about 3000 feet in thickness. This Epoch, also, is divided into three sub- divisions, known as the Eocene, Miocene, and Pliocene Periods. During these periods the most diverse develop- ment of the higher classes of plants and animals took place and the fauna and flora of our globe now approached nearer and nearer to their present character. The most highly developed class of animals, that of Mammals, now attained pre-eminence. This Tertiary Epoch may, therefore, be called the age of Mammals. The most perfect section of this class, the Placental Animals, among which is Man, 1 6 THE EVOLUTION OF MAN. now first appeared. The first appearance of Man — or to speak more correctly — the development of man from the most nearly allied ape-form, dates probably either from the Miocene or the Pliocene Period, — from the middle or the latest section of the Tertiary Epoch. Perhaps, as is assumed by others, Man strictly so-called, i.e., Man gifted with language, first developed from the speechless man-like Apes, in the subsequent Anthropolithic Age. At all events, the perfect development and distribution % of the various races of Man dates from the fifth and last main division of the organic history of the earth, and hence this Epoch has been called the Anthropolithic, or Anthro- pozoic, and also the Quaternary Epoch. It is true that, in the present imperfect state of our palseontological and prehistoric knowledge, we cannot solve the problem as to whether the development of Man from the nearest allied Ape-forms took place in the beginning of this Anthropolithic Epoch, or as early as the middle or towards the close of the preceding Tertiary Epoch. This much, however, is certain, that the true development of human culture dates only from the Anthropolithic Epoch, and that this latter con- stitutes only an insignificantly small section of the entire enormous period of time occupied in the development of the organic earth. When we reflect upon this, it appears absurd to speak of the brief span of man's period of cul- ture as " the world's history." This so-called History of the World does not amount approximately to even one- half per cent, of the length of those enormous periods which have passed away from the beginning of the earth's organic history down to the present time. For this World's THE "AGE OF MAN. \J History, or more correctly, History of People, is itself only the latter half of the Anthropolithic Epoch, while even the first half of this Epoch must be reckoned as a prehistoric period. Hence this last main period, reaching from the close of the Crenolithic Epoch to the present time, can only be called the " age of man," inasmuch as the diffusion and differentiation of the different species and races of man, which have so powerfully influenced all the rest of the organic population of the globe, took place during its course. Since the awakening of the human consciousness, human vanity and human arrogance have delighted in regarding Man as the real main-purpose and end of all earthly life, and as the centre of terrestrial Nature, for whose use and service all the activities of the rest of creation were from the first defined or predestined by a "wise providence." How utterly baseless these presumptuous anthropocentric conceptions are, nothing could evince more strikingly than a comparison of the duration of the Anthropozoic or Quater- nary Epoch with that of the preceding Epochs. For even though the Anthropolithic Epoch may embrace several hun- dreds of thousands of years, how small is this time when compared with the millions of years that have elapsed since the beginning of the world's organic history down to the first appearance of the human race ! If the entire duration of the organic history of the earth, from the generation of the first Monera down to the present day, is divided into a hundred equal parts, and if then, corresponding with the relative average thickness of the intervening strata-systems, the respective percentages are 1 8 THE EVOLUTION OF MAN. assigned to the relative durations of the five main divisions or Epochs, the latter will be found to be about as follows : — I. Archilithic, or archizoic (primordial) Epoch . . 53.6 II. Palaeolithic, or palaeozoic (primary) Epoch . . 32.1 III. Mesolithic, or mesozoic (secondary) Epoch . .11.5 IV. Caenolithic, or cenozoic (tertiary) Epoch. . . . 2.3 V. Anthropolithic, or anthropozoic (quaternary) Epoch . 0.5 Total ... 100.0 The relative durations of the five main epochs of the earth's organic history, are yet more clearly seen in the opposite Table (XIV.), in which the relative thicknesses of the strata systems deposited within these Epochs is repre- sented on a scale corresponding to their actual depths. This table shows that the period of the so-called History of the World forms but an inconsiderable span in comparison with the immeasurable duration of those earlier epochs during which Man did not exist upon this planet. Even the great Csenozoic Epoch, the so-called Tertiary Epoch, during which the Placental Animals, the higher Mammals, developed, includes but little more than two per cent, of the whole enormous duration of the organic history of the world.128 And now before we turn to our proper phylogenetic task ; before, guided by our knowledge of ontogenetic facts and by the fundamental law of Biogeny, we attempt to trace step by step the history of the palaeontological evo- lution of our animal ancestors, let us turn aside for a short time into another and apparently very different and very remote department of science, a general review of which will make the solution of the difficult problems which now rise before us very much easier. The science is that ( 19 ) TABLE XIV. Systematic Survey of the Neptunian fossiliferous strata of the earth with reference to their relative sectional thickness (130,000 feet circa). IV. Caenolithic Strata, circa 3000 feet. Pliocene, Miocene, Eocene. III. Mesolithic Strata. IX. Chalk System. Deposits of the Secondary Epoch, circa VIII. Jurassic System. 15,000 feet. VII. Triassic System II. Palgeolithic Strata. VI. Permian System. Deposits of the Primary Epoch, circa 42,000 feet. V. Coal System. IV. Devonian System. I. Archilithic Strata. III. Silurian System, circa 22,000 feet. Deposits of the Primordial Epoch, circa II. Cambrian System, circa 18,000 feet. 70,000 feet. I. Laurentian System, circa 30,000 feet. 20 THE EVOLUTION OF MAN. of Comparative Philology. Ever since Darwin, by the theory of Natural Selection, infused new life into Biology, and raised the fundamental question of development in every branch of science, attention has frequently and from very different quarters been called to the remarkable parallelism, which exists between the evolution of the various human languages and the evolution of organic species. The comparison is quite justifiable and very instructive. Indeed it is hardly possible to find an analogy better adapted to throw a clear light on many obscure and difficult facts in the evolution of species, which is governed and directed by the same natural laws which guide the course of the evolution of language. All philologists who have made any progress in their science, now unanimously agree that all human languages have developed slowly and by degrees from the simplest rudiments. On the other hand, the strange proposition which till thirty years ago was defended by eminent au- thorities, that language is a divine gift, is now universally rejected, or at best defended only by theologians and by people who have no conception of natural evolution. Such brilliant results have been attained in Comparative Philology that only one who is wilfully blind can fail to recognize the natural evolution of language. The latter is necessarily evident to the student of nature. For speech is a physio- logical function of the human organism, developing simul- taneously with its special organs, the larynx and the tongue, and simultaneously with the functions of the brain. It is, therefore, quite natural that in the history of the evolution of languages, and in their whole system, we should find the came correlations as in the history of, the evolution of organic species and in their whole system. The various METHOD OF COMPARATIVE PHILOLOGY. 21 larger and smaller groups of speech -forms, which are distin- guished in Comparative Philology as primitive languages, fundamental languages, parent languages, derived languages, dialects, patois, etc., correspond perfectly in their mode of development with the various larger and smaller groups of organisms classed in systems of Zoology and Botany as tribes, classes, orders, families, genera, species, and varie- ties of the animal and vegetable kingdoms. The relations between these various systematic groups, or categories, are in both cases identical ; moreover, evolution follows the same course in one case as in the other. This instructive comparison was first elaborated by one of the most eminent of German philologists, one who, unfortunately, died pre- maturely— August Schleicher, not only a philologist but also a learned botanist. In his more important works, the Com- parative Anatomy and evolutionary history of languages is treated by the same phylogenetic method which we employ in the Comparative Anatomy and evolutionary history of animal forms. He has especially applied this method to the Indo-Germanic family of languages ; and in his little treatise on " The Darwinian Theory and the Science of* Language " (" Die Darwin'sche Theorie und die Sprach- wissenschaft "), he illustrated it by means of a synoptical pedigree of the Indo-Germanic family of languages.129 If the formation of the various branch languages which have developed from the common root of the primitive Indo-Germanic tongue is studied with the aid of this pedi- gree, a very clear idea of their Phylogeny will be acquired. At the same time it becomes evident how entirely analogous is the evolution of the greater and lesser groups of the Vertebrates, which have sprung from the one common root- 22 THE EVOLUTION OF MAN. form of the primitive Vertebrates. The primitive Indo- Germanic root-tongue first separated into two chief stems : the Slavo- Germanic and the Ario-Romanic main-trunks. The Slavo-Germanic then branched into a primitive German and a primitive Slavo-Lettic tongue. Similarly, the Ario- Romanic split up into a primitive Arian and a primitive Grseco-Romanic language (p. 23). If we continue our examination of this pedigree of the four primitive Indo- Germanic languages, we find that the primitive Germanic tongue divided into three chief branches — a Scandinavian, a Gothic, and a Teutonic branch. From the Teutonic branch proceeded, on the one hand, High German, and, on the other hand, Low German, to which latter belong the various Friesian, Saxon, and Low German dialects. Similarly, the Slavo-Lettic tongue developed first into a Baltic and a Slavonic language. From the Baltic spring the Lettic, Lithuanian, and Old Prussian dialects. The Slavic, on the other hand, give rise, in the South-east, to the Russian and the South Slavic dialects, and, in the West, to the Polish and Czech dialects. Turning now to the other main stem of the Indo- Germanic languages and its branches — the primitive Ario- Romanic — it is found to develop with the same luxuriance. The primitive Grseco-Romanic language gave rise, on the one hand, to the Thracian language (Albanian Greek), and on the other, to the Italo-Keltic. From the latter in turn sprung two divergent branches — in the South, the Italian branch (Romanic and Latin), and in the North, the Keltic, from which arose all the different British (Old British, Old Scottish, and Irish'* and Gallic tongues. The numerous Iranian and Indian dialects branched out in the same way from jbhe primitive Arian language. ( 23 ) TABLE XV. Pedigree of the Iudo-Germanic Languages. Lithuanians Ancient Prussians Letts Anglo-Saxons v Baltic Races Low Germans Netherlander High Germans Ancient Saxons Sorbians Polish Czechs J West Sclaves Russians South Sclaves South-east Sclaves Saxons Friesians Low Germans Scandinavians Goths Germans Primitive Germans Sclaves Sclavo-Letts Romans Latins Italians Ancient British Ancient Scotch Irish Gaels Gauls Brittanese Kelts Sclav o Germans Italo-Kelts Albanese Greeks Frimitive Thracians Indians Iranians Arians Graeco-Romans I v Ario-Romans Indo-Germans 24 THE EVOLUTION OF MAN. A close study of this pedigree of the Indo-Germanic languages is, in many respects, of great interest. Com- parative Philology, to which we are indebted for our know- ledge of this subject, thus shows itself to be a true science — a natural science. It, indeed, long ago anticipated in its own province the phylogenetic method with the aid of which we now attain the highest results in Zoology and and in Botany. And here I cannot refrain from remarking how much to the advantage of our general culture it would be if the study of languages (which is undoubtedly one of the most powerful means of culture) were comparatively prosecuted ; and if our cut and dried Philology were re- placed by a living, many-sided, comparative study of lan- guages. The latter stands in the same relation to the former as the living history of organic evolution does to the lifeless classification of species. How much deeper would the interest taken in the study of language by the students in our schools be, and how much more vivid would be the results if even the first elements of Comparative Philology were taught instead of the distasteful composi- tion of Latin exercises in Ciceronian style ! I have entered with this detail into the " Comparative Anatomy '' and the history of the evolution of languages, because it is unsurpassed as a means of illustrating the Phylogeny of organic species. We find that in structure and in development these primitive languages, parent languages, derived languages, and dialects, correspond exactly like the classes, orders, genera, and species of the animal kingdom. The " natural system ': is in both cases phylogenetic. Just as Comparative Anatomy, Ontogeny, and Palaeontology afford certain proof that all Vertebrates, MAN DESCENDED FROM EXTINCT FORMS. 2$ whether extinct or extant, are descended from a common ancestral form, so does the comparative study of the dead and living Indo-Germanic language absolutely convince us that all these languages have sprung from a common origin. This monophyletic view is unanimously adopted by all linguists of importance who have studied the question, and who are capable of passing a critical judgment upon it.130 The point, however, to which I would specially call your attention in this comparison between the various branches, on the one hand, of the Indo-Germanic language, and, on the other, of the vertebrate tribe, is that the direct descendants must never be confounded with the collateral lines, nor the extinct with the extant forms. This mistake is often made, and results in the formation of erroneous notions of which our opponents often take advantage in order to oppose the whole theory of descent. When, for instance, it is said that human beings are descended from Apes, the latter from Semi-apes, and the Semi-apes from Pouched Animals (Marsupialia), very many people think only of the familiar living species of these different mammalian orders, such as are to be found stuffed in our museums. Now, our opponents attribute this erroneous view to us, and, with more craft than judgment, declare the thing impossible ; or else they ask us as a physiological experiment to transform a Kangaroo into a Semi-ape, this into a Gorilla, and the Gorilla into a Man. Their demand is as childish as the conception on which it is founded is erroneous; for all these extant forms have varied more or less from their common parent-form, and none of them are capable of pro- ducing the same divergent posterity which were really produced thousands of years ago by that parent-form.131 26 THE EVOLUTION OF MAN. There is no doubt that Man is descended from an extinct mammalian form, which, if we could see it, we should certainly class with the Apes. It is equally certain -that this primitive Ape in turn descended from an unknown Semi-ape, and the latter from an extinct Pouched Animal. But then it is beyond a doubt that it is only in respect of essential internal structure, and on account of their similarity in the distinctive anatomical characters of the order, that all these extinct ancestral forms can be spoken of as members of the yet extant mammalian orders. In external form, in generic and specific characters, they must have been more or less — perhaps even greatly — different from all living repre- sentatives of the orders to which they belonged. For it must be accepted as a quite universal and natural fact in phylogenetic evolution that the parent-forms themselves, with their specific characters, became extinct at a more or less distant period. Those extant forms which come nearest to them, yet differ from them more or less, perhaps even very essentially. Hence in our phylogenetic researches and in our comparative view of the still living divergent descendants all we can undertake to do is to determine how far the latter depart from the parent-form. We may quite confidently assume that no single older parent-form has reproduced itself without modification down to our time. We find this same state of things on comparing various extinct and living languages, which have sprung from one common primitive tongue. If, from this point of view, we examine the genealogical tree of the Indo-Germanic languages, we may conclude, on a priori grounds, that all the earlier primitive languages, fundamental languages, and ancestral languages, from which the living dialects are COMPARATIVE METHODS OF PHILOLOGY AND ZOOLOGY. 2J descended in the first or second degree, have been extinct for a longer or shorter period. And this is the case. The Ario-Romanic and the Sclavo-Germanic tongues have long been altogether extinct, as are also the primitive Arian and Gmeco-Romanic, the Sclavo-Lettic, and primitive Germanic languages. Some even of the languages descended from these have also long been dead, and all those of the Indo- Germanic branch which are yet extant, are akin only in so far as they are divergent descendants of common parent- forms. Some have diverged from this ancestral form more, others less. This easily demonstrable fact very well illustrates analogous facts in the descent of vertebrate species. Phy- logenetic " Comparative Philology," as a powerful ally, sup- ports phylogenetic " Comparative Zoology." The former can, however, adduce far more direct evidence than the latter, because the palseontological materials of Philology, the ancient monuments of extinct tongues, have been far better preserved than the palreontological materials of Comparative Zoology, the fossil bones of vertebrates. The more these analogous conditions are considered, the more convincing is their force. We shall presently find that we can trace back the genealogical line of Man, not only to the lower Mammals, but even to the Amphibia, to the shark-like Primitive Fishes, and even far below these, to the skull-less Vertebrates allied to the Amphioxus. It must be remembered this does not mean that the living Amphioxus, Shark, or Amphibian accu- rately represent the outward appearance of the parent-forms of which we speak. Still less does it mean that the Amphioxus, or the Shark of our day, or any extant species 28 THE EVOLUTION OF MAN. of Amphibian is an actual parent-form of the higher Ver- tebrates and of Man. On the contrary, this important assertion must be clearly understood to mean, that the living forms, which have been mentioned, are side branches, which are much more nearly allied, and similar to the extinct common parent-forms, than any other known animal forms. In their internal characteristic structure they remain so similar to the unknown parent-forms, that we should class them both in one order, if we had the latter before us in a living state. But the direct descendants of the primitive forms have never remained unmodified. Hence it is quite impossible that among the living species of animals we should find the actual ancestors of the human race in their characteristic specific forms. The essential and charac- teristic features, which more or less closely connect living forms with the extinct common parent-forms, are to be found in the internal structure of the body, not in the external specific form. The latter has been much modified by adaptation. The former has been more or less retained by heredit}^. Comparative Anatomy and Ontogeny indisputably prove that Man is a true Vertebrate, so that the special genealo- gical line of Man must of course be connected with that of all those Vertebrates which are descended from the same common root. Moreover, on many definite grounds, sup- plied by Comparative Anatomy and Ontogeny, we must assume only one common origin for all Vertebrates — a monophyletic descent. Indeed, if the theory of descent is correct, all Vertebrates, Man included, can only have descended from a single common parent-form — from a single primitive Vertebrate species. The genealogical line of the Vertebrates, therefore, is also that of Man. AMCEBOID ANCESTORS. 29 Our task of ascertaining a pedigree of Man thus widens into the more considerable task of constructing the pedigree of all the Vertebrates. This is connected, as we learned from the Comparative Anatomy and Ontogeny of the Amphioxus and of the Ascidian, with the pedigree of the Invertebrate animals, and directly with that of the Worms, while no con- nection can be shown with the genealogy of the indepen- dent tribes of the Articulated Animals (Arthropoda), Soft- bodied Animals (Mollnsca), and Star-animals (Echinoderma). As the Ascidian belongs to the Mantled Animals (Tunicata), and as this class can only be referred to the great Worm tribe, we must, aided by Comparative Anatomy and Onto- geny, further trace our pedigree down through various stages to the lowest forms of Worms. This necessarily brings us to the Gastnea, that most important animal form in which we recognize the simplest conceivable prototype of an animal with two germ-layers. The Gastrsea itself must have ori- ginated from among those lowest of all simple animal forms, which are now included by the name of Primitive Animals (Protozoa). Among these we have already considered that primitive form which possesses most interest for us — the one-celled Amoeba, the peculiar significance of which depends on its resemblance to the human egg-cell. Here we have reached the lowest of those impregnable points, at which the value of our fundamental law of Biogeny is directly found, and at which, from the embryonic evolutionary stage, we can directly infer the extinct parent-form. The amoeboid nature of the young egg-cell, and the one-celled condition in which each Man begins his existence as a simple parent- cell or cytula-cell, justify us in affirming that the oldest ancestors of the human race (as of the whole animal kingdom) were simple amoeboid cells. 30 THE EVOLUTION OF MAN". Here arises another question : " Whence, in the begin- ning of the organic history of the earth, at the commence- ment of the Laurentian period, came the earliest Amoebae ? " To this there is but one reply. Like all one-celled organ- isms, the Amoebae have originally developed only from the simplest organisms know to us, the Monera. These Monera, which we have already described, are also the simplest con- ceivable organisms. Their body has no definite form, and is but a particle of primitive slime (plasson) — a little mass of living albumen, performing all the essential functions of life, and everywhere met with as the material basis of life. This brings us to the last, or perhaps the first question in the history of evolution — the question as to the origin of the Monera. And this is the momentous question as to the prime origin of life — the question of spontaneous generation (generatio spontanea or cequivoca). We have neither time, nor indeed have we any occasion, to discuss at length the weighty question of spontaneous generation. On this subject I must refer you to my " History of Creation," and, especially, to the second book of the Generelle Morplwlogie, and to the discussion on Monera and spontaneous generation in my " Studien iiber Moneren und andere Protista." 132 I have there stated my own views on this important subject in very great detail. Here I will only say a few words on the ob- scure question as to the first origin of life, and will answer it so far as it concerns our radical conception of the history of organic evolution. In the definite, limited sense in which I maintain spontaneous generation {gene- ratio spontanea) and assume it as a necessary hypo- thesis in explanation of the first beginning of life upon SPONTANEOUS GENERATION. 3 1 the earth, it merely implies the origin of Monera from inorganic carbon compounds. When animated bodies first appeared on our planet, previously without life, there must, in the first place, have been formed, by a process purely chemical, from purely inorganic carbon combinations, that very complex nitrogenized carbon compound which we call plasson, or " primitive slime," and which is the oldest material substance in which all vital activities are embodied. -In the lowest depths of the sea such homogeneous amorphous protoplasm probably still lives, in its simplest character, under the name of Bathybius.137 Each individual living particle of this structureless mass is called a Moneron. The oldest Monera originated in the sea by spontaneous generation, just as crystals form in the matrix. This assumption is required by the demand of the human understanding for causality. For when, on the one hand, we reflect that the whole inorganic history of the earth proceeds in accordance with mechanical laws and without any intervention by creative power, and when, on the other hand, we consider that the entire organic history of the world is also de- termined by similar mechanical laws ; when we see that no supernatural interference by a creative power is needed for the production of the various organisms, then it is certainly quite inconsistent to assume such supernatural creative interference for the first production of life upon our globe. At all events we, as investigators of nature, are bound at least to attempt a natural explanation. At present, the much agitated question of spontaneous generation appears very intricate, because a large number of very different, and in part quite absurd, conceptions are included under the term "spontaneous generation," and 25 32 THE EVOLUTION OF MAN. because some have supposed that the problem could be solved by means of the crudest experiments. The doctrine of spontaneous generation cannot be experimentally refuted. For each experiment with a negative result merely proves that under the conditions (always very artificial) supplied by us, no organism has been produced from inorganic combina- tions. Neither can the theory of spontaneous generation be experimentally proved unless great difficulties are over- come ; and even if in our own time Monera were produced daily by spontaneous generation — as is very possible — yet the absolute empiric proof of this fact would be extremely difficult — indeed, in most cases impossible. He, however, who does not assume a spontaneous generation of Monera, in the sense here indicated, to explain the first origin of life upon our earth, has no other resource but to believe in a supernatural miracle ; and this, in fact, is the questionable standpoint still taken by many so-called " exact naturalists," who thus renounce their own reason. Sir William Thomson has indeed tried to avoid the necessary hypothesis of spontaneous generation by assuming that the organic inhabitants of our earth originally de- scended from germs which proceeded from the inhabitants of other planets, and which, with fragments of the latter, with meteorites, accidentally fell on to the earth. This hypothesis has met with much applause, and was even supported by Helmholtz. Friederich Zoellner, an acute physicist, has, however, refuted it in his excellent natural- philosophical work " Ueber die Natur der Cometen," a critical book containing most valuable contributions to the history and theory of knowledge.127 Zoellner has plainly shown that the hypothesis is unscientific in two respects — MONERA ALONE PRODUCED BY SPONTANEOUS GENERATION. 33 firstly, in point of logic, and secondly, in its scientific tenor (p. xxvi). At the same time he rightly shows that the hypothesis of spontaneous generation, in the sense which we have defined, is the " condition necessary to the conceiv- ability of nature in accordance with the laws of causality." In conclusion, I repeat, with emphasis, that it is only in the case of Monera — of structureless organisms without organs — that we can assume the hypothesis of spontaneous generation. Every differentiated organism, every organism composed of organs, can only have originated from an undifferentiated lower organism by differentiation of its parts, and consequently by Phylogeny. Hence, even in the production of the simplest cell we must not assume the pro- cess of spontaneous generation. For even the simplest cell consists of at least two distinct constituent parts ; the inner and firmer kernel (nucleus), and the outer and softer cell-substance or protoplasm. These two distinct parts can only have come into being by differentiation of the homogeneous plasson of a moneron and of a cytod. It is for this very reason that the natural history of Monera is of the highest interest ; for it alone can remove the principal difficulties which beset the question of spontaneous genera- tion. The extant Monera do afford us organless and struc- tureless organisms, such as must have originated by spon- taneous generation at the first beginning of organic life upon the earth.133 CHAPTER XVI. THE ANCESTRY OF MAN. I. From the Monera to the Gastr^ea Relation of the General Inductive Law of the Theory of Descent to the Special Deductive Laws of the Hypotheses of Descent. — Incompleteness of the Three Great Records of Creation : Palaeontology, Ontogeny, and Comparative Anatomy. — Unequal Certainty of the Various Special Hypotheses of Descent. — The Ancestral Line of Men in Twenty-two Stages : Eight Invertebrate and Fourteen Vertebrate Ancestors. — Distri- bution of these Twenty-two Parent-forms in the Five Main Divisions of the Organic History of the Earth. — First Ancestral Stage : Monera. — The Structureless and Homogeneous Plasson of the Monera. — Differen- tiation of the Plasson into Nucleus, and the Protoplasm of the Cells. — Cytods and Cells as Two Different Plastid-forms. — Vital Phenomena of Monera. — Organisms without Organs. — Second Ancestral Stage : Amoebae. — One-celled Primitive Animals of the Simplest and most Un- differentiated Nature. — The Amoeboid Egg-cells. — The Egg is Older than the Hen. — Third Ancestral Stage : Syn-Amoeba, Ontogenetically repro- duced in the Morula. — A Community of Homogeneous Amoeboid Cells. — Fourth Ancestral Stage : Planaea, Ontogenetically reproduced in the Blastula or Planula. — Fifth Ancestral Stage : Gastrsea, Ontogenetically reproduced in the Gastrula and the Two-layered Germ-disc. — Origin of the Gastrasa by Inversion (invaginatio) of the Planaea. — Haliphysema and Gastrophysema. — Extant Gastraeads. " Now, very probably, if the course of evolution proves to be so very simple, it will be thought that the whole matter is self-evident, and that research is hardly required to establish it. But the story of Columbus and the egg is daily repeated ; and it is necessary to perform the experiment INDUCTIVE AND DEDUCTIVE METHODS. 35 for one's self. How slowly progress is made in the knowledge even of self- evident matters, especially when respectable authorities disagree, I myself have experienced sufficiently." — Karl Ernst Baer (1828). Guided by the fundamental law of Biogeny and by the sure records of creation, we now turn to the interesting task of examining the animal parent-forms of the human race in their proper sequence. To ensure accuracy, we must first become acquainted with the various mental operations which we shall apply in this natural-philosophical research. These operations are partly of an inductive, partly of a deductive nature : partly conclusions from numerous particular experiences to a general law ; partly conclusions from this general law back to particular ex- periences. Tribal history as a whole is an inductive science ; for the whole theory of descent, as an indispensable and most essential part of the whole theory of evolution, is entirely founded on inductions. From all the biological incidents in plant life, in animal life, and in human life, we have derived the certain inductive conception that the whole of the or- ganic inhabitants of our globe originated in accordance with one single law of evolution. To this law of evolution, La- marck, Darwin, and their successors gave definite form in the theory of descent. All the interesting phenomena ex- hibited by Ontogeny, Paleontology, Comparative Anatomy, Dysteleology, Chorology, the (Ekology of organisms, all the important general laws, which we infer from multitudinous phenomena of these different sciences, and which are most intimately connected together, are the broad inductive data from which is drawn the most extensive inductive law of Biology. Because the innate connection between all 3 6 THE EVOLUTION OF MAN. these infinitely various groups of phenomena in these dif- ferent departments becomes explicable and comprehensible solely through the theory of descent, therefore this theory of evolution must be regarded as an extensive inductive law. If we now really apply this inductive law, and with its help seek to discover the descent of individual organic species, we must necessarily form phylogenetic hypotheses, which are of an essentially deductive nature, and which are inferences from the general theory of descent back to indi- vidual particular cases. These special deductive conclusions are, however, in accordance with the inexorable laws of Logic, as justifiable, as necessary, and as indispensable in our department of knowledge as the general inductive conclusions of which the whole theory of evolution is formed. The doctrine of the animal parent-forms of man- kind is al o a special deductive law of this kind, which is the logical conclusion from the general inductive law of the theory of descent.134 As is now very generally acknowledged, both by the adherents of and the opponents of the theory of descent, the choice, in the matter of the origin of the human race, lies between two radically different assumptions : We must either accustom ourselves to the idea that all the various species of animals and plants, Man also included, ori- ginated independently of each other by the supernatural process of a divine " creation," which as such is entirely removed from the sphere of scientific observation — or we are compelled to accept the theory of descent in its entirety, and trace the human race, equally with the various animal and plant species, from an entirely simple primaeval parent- form. Between these two assumptions there is no third FAITH OR SCIENCE. 37 course. Either a blind belief in creation, or a scientific theory of evolution. By assuming the latter, and this is the only possible natural-scientific conception of the universe, we are enabled, with the help of Comparative Anatomy and Ontogeny, to recognize the human ancestral line with a certain approximate degree of certainty, just as is more or less the case with respect to all other organisms. Our previous study of the Comparative Anatomy and Ontogeny of Man, and of other Vertebrates, has made it quite clear that we must first seek the pedigree of mankind in that of the vertebrate tribe. There can be no doubt that (if the theory of descent is correct) Man has developed as a true Vertebrate, and that he originated from one and the same common parent-form with all other Vertebrates. This special deduction must be regarded as quite certain, correct- ness of the inductive law of the theory of descent being of course first granted. No single adherent of the latter can raise a doubt about this important deductive conclusion. We can, moreover, name a series of different forms of the vertebrate tribe, which may be safely regarded as the repre- sentatives of different successive phylogenetic stages of evolution, or as different members of the human ancestral line. We can also prove with equal certainty that the vertebrate tribe as a whole originated from a group of low invertebrate animal forms ; and among these we can again with more or less certainty recognize a series of members of the ancestral chain. We must, however, at once expressly say that the cer- tainty of the different hypotheses of descent, which are founded entirely on special deductive inferences, is very unequal. Several of these conclusions are already fully 38 THE EVOLUTION OF MAN. established ; others, on the contrary, are most doubtful ; in yet others, it depends upon the subjective proportion of the knowledge of the naturalist and on his capability of draw- ing conclusions, what degree of probability he will accord to them. It is, at all events, necessary thoroughly to dis- tinguish between the absolute certainty of the general (inductive) theory of descent, and the relative certainty of the special (deductive) hypothesis of descent. We can never in any case prove the whole ancestral line of an- cestors of an organism with the same certainty with which we regard the theory of descent as the only scientific expla- nation of the organic forms. On the contrary, the special proof of all separate parent-forms must always remain more or less incomplete and hypothetical. That is quite natural. For all the records of creation upon which Ave rely are in a great measure incomplete, and will always remain incomplete; just as in the case of Comparative Philology. Above all, Palaeontology, the most ancient of all records of creation, is in the highest degree incomplete. We know that all the petrifications with which we are acquainted form but an insignificantly small fragment of the whole number of animal forms and plant forms which have ever existed. For each extinct species obtained by us in a petrified condition, there are at least a hundred, probably thousands of extinct species which have left no trace of their existence. This extreme and most deplorable defectiveness of the palaeon- tological record of creation, upon which it is impossible to insist too strongly, is very easily accounted for. The very conditions under which organic remains become petrified necessitate it. It is also partly explicable as the result of INCOMPLETENESS OF THE BIOLOGICAL RECORDS. 39 an imperfect knowledge in this department. It must be remembered, that far the greater proportion of the rock strata which constitute the mountain masses of the surface of the earth is not yet unfolded to us. Of the count- less petrifications which are hidden in the huge moun- tain chains of Asia and Africa, we know but a few small samples. Part of Europe and of North America has alone been more minutely explored. The whole of the petri- factions accurately known and in our collections do not amount to a hundredth part of those which really exist in the crust of the earth. In this respect we may, therefore, expect a rich harvest of discoveries in the future. But, in spite of this, the palseontological record of creation (for reasons which I have amply explained in Chapter XV. of my "Natural History of Creation") will always remain extremely incomplete. Not less incomplete is the second, most important record of creation, that of Ontogeny. For the Phylogeny of the individual it is the most important of all. Yet, it also has its great defects, and often leaves us in the lurch. In this matter, we must distinguish quite clearly between palin- genetic and kenogenetic phenomena, between the original, inherited evolution and the later, vitiated evolution. We must never forget that the laws of abridged and vitiated heredity frequently disguise the original course of evolution beyond recognition. The reproduction of the Phylogeny in the Ontogeny is but rarely tolerably complete. The earliest and most important stages of germ-history are usually the most abridged and compressed. The youthful evolutionary forms have in turn often adapted themselves to new conditions, and have thus been modified. The 40 THE EVOLUTION OF MAN. struggle for existence has excited an equally strong modify- ing influence upon the various independent and yet un- developed young forms, as upon the developed and mature forms. Therefore, in the Ontogeny of the higher animal forms, the Phylogeny has been very greatly limited by Keno- genesis ; as a rule, only a blurred and much vitiated picture of the original course of evolution of their ancestors now lies before us in the Ontogeny. Only with great precaution and judgment dare we infer the tribal history directly from the germ-history. Moreover, the germ-history itself is known to us only in the case of very few species. Lastly, the highly important record of creation afforded by Comparative Anatomy is unfortunately very incomplete, and for the simple reason, that the number of extant animal species forms but a very small fragment of the whole number of different animal forms that have existed from the beginning of the organic history of the world to the present time. The total sum of the latter may safely be estimated at several millions. The number of those animals the organization of which has at present been investigated by Comparative Anatomy is very small in pro- portion. The more extended investigations of the future will, here also, open up unexpected treasures. In view of this evident and natural incompleteness of the most important records of creation, we must of course take good care, in the tribal history of Man, not to lay too great weight on single known animal forms, nor with equal certainty to consider all the stages of evolution which come under our consideration, as parent-forms. On the contrary, in hypothetically arranging our ancestral line, we must take good care to remember that the single hypothetical UNEQUAL VALUE OF THE "ANCESTRAL STAGES." 41 parent-forms are of very diverse values in relation to the certainty of our knowledge. From the few remarks which, while .speaking of the Ontogeny, we made as to the corre- sponding phylogenetic forms, it will have been understood that some germ-forms may with certainty be regarded as reproductions of corresponding parent-forms. We recog- nized the human egg-cell and the parent-cell which results from the impregnation of the latter as the first and most important form of this kind. From the weighty fact that the egg of the human being, like the egg of all other animals, is a simple cell, it may be quite certainly inferred that a one-celled parent-form once existed, from which all the many-celled animals, Man in- cluded, developed. A second very significant germ-form, which evidently reproduces a primaeval parent-form, is the germ-vesicle (Blastida), a simple hollow sphere, the wall of which con- sists of a single cell-stratum. A third extremely import- ant form in germ-history, which may be quite safely and directly referred back to the tribal history, is the true Gas- trula. This most interesting larval form already exhibits the animal body composed of two germ-layers, and fur- nished with the fundamental primitive organ, the intestinal canal. Now, as the same two-layered germ-condition, with the primitive rudiment of the intestinal canal, is common to all the other animal tribes (with the single exception of the Primitive Animals, Protozoa), we may certainly from this infer a common parent-form of similar construction to the Gastrula, the Gastnea. Equally important in their bearing on the Phylogeny of Man, are the very important ontoge- netical form conditions which correspond to certain Worms, 42 THE EVOLUTION OF MAN. Skull-less Animals (Acrania), Fishes, etc., etc. On the other hand, between these quite certain and most valuable phylo- genetic points, great gaps in our knowledge unfortunately exist, with which we shall again and again meet, and which are satisfactorily explained by reasons which have already been named, especially by the incompleteness of Palaeon- tology, of Comparative Anatomy, and Ontogeny. In the first attempts to construct the human ancestral line, which I made in my Generelle Morphologie, and in the " Natural History of Creation," I arranged first ten, and, later, twenty-two different animal forms, which, with more or less certainty, may be regarded as the animal an- cestors of the human race, and which must be looked upon as in a sense the most important stages of evolution in the long evolutionary series from the one-celled organisms up to Man.135 Of these twenty to twenty-two animal forms, about eight fall within the older division of the Inverte- brates, while twelve to fourteen belong to the more recent Vertebrate division. How these twenty-two most important parent-forms in the human ancestral line are distributed through the five main periods of the organic history of the earth, is shown in the following Table (XVI.). At least half of these twenty-two stages of evolution (that is, the eleven oldest ancestral forms) are found within the Archilithic Epoch, within that first main period of the organic history of the earth, which includes the larger half of the latter, and during which probably only aquatic organisms existed. The eleven remaining parent-forms fall within the four remaining main Epochs : three within the Palaeolithic Epoch, three within the Mesolithic Epoch, and four within the Csenolithic Epoch. In the last, the Anthropolithic Age, Man already existed. THE TWENTY-TWO ANCESTRAL STAGES. 43 If we would now undertake the difficult attempt to dis- cover the phylogenetic course of evolution of these twenty- two human ancestral stages from the very commencement of life, and if we venture to lift the dark veil which covers the oldest secrets of the organic history of the earth, we must undoubtedly seek the first beginning of life among those wonderful living beings which, under the name of Monera, we have already frequently pointed out as the simplest known organisms. They are, at the same time, the simplest conceivable organisms; for their entire body, in its fully developed and freely moving condition, consists merely of a small piece of structureless primitive slime or plasson, of a small fragment of that extraordinarily important nitro- genous carbon compound, which is now universally esteemed the most important material substratum of all the active phenomena of life. The experiences of the last ten years particularly have convinced us with more and more cer- tainty that wherever a natural body exhibits the active phenomena of life, nutrition, propagation, spontaneous movement, and sensation, a nitrogenous carbon compound, belonging to the chemical group of albuminous bodies, is always active, and represents the material substance through which these vital activities are effected. Whether, in a monistic sense, we conceive the function as the direct effect of the formed material substance, or, in a dualistic sense, we regard " Matter and Force " as distinct, it is at least certain that, hitherto, no living organism has been observed in which the exercise of vital activities wTas not inseparably connected with a plasson-body. In the Monera, the simplest con- ceivable organisms, the whole body consists merely of plasson, corresponding to the " primitive slime " of earlier natural philosophy. ( 44 ) TABLE XVI. Systematic Survey of the most Important Stages in the Animal Ancestral Line of Man. MN = Boundary between the Invertebrate and the Vertebrate Ancestors. Epochs of the Organic History of the Earth. Geological Periods of the Organic History of the Earth. Animal Ancestral Stages of Man. Nearest Living Relatives of the A ncestral Stages. I. Archilithic or Primordial Epoch 1. Laurentian Period v. 2. Cambrian Period 3. Silurian Period II. Palaeolithic or Primary Epoch III. Mesolithic or Secondary Epoch IV. Caenolithic or Tertiary Epoch 1. Monera (Monera) 2. Oldest Amoebae 3. Amoeboid Societies (S'/namcebid) 4. Ciliated planulae (Plana'ad'v) 5. Primitive Intes- tinal animals (Gastneadv) 6. Primitive Worms (Archelminthes) 1. Soft- worms (Scolecida) 8. Chorda animals (Chordoma) M ■■ 9 Bathybius Protamceba Simple Amoeba? (Automoeba) Morula larvae Blastula larvas Gastrula larvae Gliding Worms \ ( Turbellaria) ? Between the glid- ing worms and the Sea-squirts J Sea-squirts (AscidUe) t (Appendicular i <) N Skull-less animals (Acrania) 10. Round-mouths (Cyclostomi) 11. Primitive Fishes (Selachii) Lancelets (Amphioxi) Lampreys (Petromyzonta) Sharks (Squalacei) 4. Devonian Period 5. Coal Period 6. Permian Period 12. Salamander Fishes (Dipneusta) 13. Gilled Amphibia (Sozobranchia) 14. Tailed Amphibia (Sozura) { Mud fish (Protoptera) Siren (Proteus) and Axolotl (Siredon) Water-newt ( Triton) 1. Triassic Period 8. Jurassic Period 9. Chalk Period 15. Primitive Am- niota (Protamnia) 16. Primitive Mam- mals (Promammalia) 17. Pouched Animal3 (Marsupialia) i ? Between Tailed Amphibians and Beaked animals Beaked animals (Monotrcma) Pouched Pvats (Didelphyes) 10. Eocene Period 11. Miocene Period 12. Pliocene Period 18. Semi- A pes (Prosimim) 19. Tailed Narrow- nosed Apes 20. Men-like Apes or Tail-less Narrow- nosed Apes. 21. Speechless Men or Ape-like Men Lori (Stenops) Maki (Lemur) Nose Apes Holy Apes Gorilla, Chimpan- zee, Orang, Gibbon Cretins or Micro- cephali Quaternary Epoch j 13. Di ( 14. A] Diluvial Period Alluvial Period 22. Men capable of speech Australians and Papuans MONERON AND MORULA. 45 The soft slimelike plasson-substance of the body of the Moneron is commonly called " jprotoplasma" and identified with the cell-substance of ordinary animal and plant cells. As, however, Eduard van Beneden, in his excellent work upon the Gregarinse, first clearly pointed out, we must, strictly speaking, distinguish thoroughly between the plasson of cytods and the protoplasm of cells. This dis- tinction is of special importance in its bearing on the history of evolution. As was before incidentally mentioned, we must assume two different stages of evolution in those ele- mentary organisms, which, as formative particles or plastids, represent organic individuality of the first order. The older and lower stage is that of the cytods, in which the whole body consists of but one kind of albuminous substance, of the simplest plasson or formative material. The more recent and higher stage is that of cells, in which a separation or differentiation of the original plasson into two different kinds of albuminous substances, into the inner cell-kernel {nucleus), and the outer cell-substance (protoplasma), has already taken place. The Monera are the simplest permanent cytods. Their entire body consists merely of soft, structureless plasson. However thoroughly we examine them with the help of the most delicate chemical reagents and the strongest optical instruments, we yet find that all the parts are completely homogeneous. These Monera are, therefore, in the strictest sense of the word, " organisms without organs ; " or even, in a strictly philosophical sense, they might not even be called " organisms," since they possess no organs, since they are not composed of various particles. They can only be called organisms, in so far as they are capable of exercising the 46 THE EVOLUTION OF MAN. organic phenomena of life, of nutrition, reproduction, sensa- tion, and movement. If we tried to construct, a priori, the simplest conceivable organism, we should always be com- pelled to fall back upon such a Monera. Although in all real Monera the body consists merely of such a small living piece of plasson, yet, among the Monera, which have been observed in the sea and in fresh water, we have been able to distinguish several dif- ferent genera and species, varying in the mode in which their tiny bodies move and reproduce. In the ways in which movement is accomplished very noticeable differences exist. Fig. 163. — A Moneron (Protamceba) in the act of reproduction : A, the whole Moneron, which moves, like the ordinary Amoeba, by means of variable processes ; B, a contraction round its circumference parts it into two halves ; C, the two halves separate, and each now forms an independent individual (much enlarged). In some Monera, especially in the Protamceba (Fig. 163), the formless body, during its movements, invariably de- velops only a few, short, and blunt processes, which project like fingers, slowly altering their form and size, but never branching. In other Monera, on the other hand {e.g., Protomyxa, Myxastrum), very numerous, long, fine, and generally thread-like processes arise from the surface of the movable body, and these branch irregularly, inter- THE BATHYBIUS. 47 twining their free moving ends, so as to form a net. Huge masses of such slime-nets crawl upon the deepest bottom of the sea (Bathybius, Fig. 164). Within these soft slime- like plasson-nets slow currents continually pass. Such a Moneron may be fed with finely pulverized colouring matter (for instance, carmine or indigo powder), if this powder is scattered in the drop of water under the micro- scope, in which the Moneron is contained. The grains of colouring matter at first adhere to the surface of the slimy body, and then gradually penetrate, and are driven about in irregular directions. The separate smallest par- ticles, or molecules, of the Moneron-body, called " plas- tidules," 136 displace each other, change their relative positions, and thus effect a change in the position of the absorbed particles of colouring matter. This change of position, at the same time, proves positively that a hidden delicate structure does not exist. It might be argued that the Monera are not really structureless, but that their organization is so minute that, in consequence of the in- adequate power of our magnif}dng glasses, it is invisible. This objection is, however, invalid, for by the experiment of feeding, we can, at any moment, prove the entrance of foreign, formed, small bodies into the different parts of the body of the Moneron, and that these are irregularly driven about in all directions. At the same time we see that the changeable network of threads, formed by the branching of the protoplasmic threads and the coalescence of the con- fluent branches, alter their configuration every moment; just as has long been known to occur in the thread-nets of the protoplasm in the interior of the plant-cells. The Monera are, therefore, really homogeneous and structureless; 48 THE EVOLUTION OF MAN. each part of the body is every other part. Each part can absorb and digest nourishment ; each part is excitable and sensitive ; each part can move itself independently ; and lastly, each part is capable of reproduction and regenera- tion. The reproduction of Monera always occurs asexually. In the Protamceba (Fig. 163), each individual, after it has grown to a certain size, simply separates into two pieces. Round the circumference of the body a contraction arises, as in cell-division. The connection between the two halves continually becomes more slender (B), and finally parts in the middle. Thus, in the simplest possible way, two new individuals proceed by self-division from one quite simple individual (0). Other Monera, after they have grown to a certain size, gather themselves together into a spherical form. The globular protoplasmic body exudes a jelly-like protecting envelope, and a breaking-up of the whole plasson-ball takes place within this covering ; it breaks either into four pieces (Vampyrella), or into a large number of smaller globules (Protomonas, Protomyxa ; cf. Plate I. in the " Natural History of Creation "). After a time, these globules begin to move, split the integument by their movement, and emerge; after which they float about by means of a long, thin, thread-shaped process. Each again passes by simple growth into the mature form. Thus, it is possible to distinguish different genera and species of Monera, on one hand, by the form of the different processes of the body, and, on the other hand, by the different kind and manner of reproduction. In the appendix to my monograph of the Monera I enumerated eight genera and sixteen species ("Biol. Studien," vol. i. p. 182). The THE MONERON AND BATHYRIUS. 49 most remarkable of all Monera is the Bathybius, which was discovered by Huxley in 1868 (Fig. 164). This wonderful Moneron lives in the deepest parts of the sea, especially in Fig. 164. — Bathybius Hceckelii (Huxley). A small piece of the formless and continually changing plasson-net of this Moneron from the Atlantic Ocean. the Atlantic Ocean, and in places covers the whole floor of the sea in such masses, that the fine mud on the latter consists, in great measure, of living slime. The protoplasm in these formless nets does not seem differentiated at all; each little piece is capable of forming an individual. The active amoeboid movements of these formless pieces of plasson, which were first observed by the English zoologists Carpenter and Wy ville Thomson, have recently been again observed by the German Arctic voyager, Emil Bessels, in the Bathybius of the coast of Greenland.137 The origin and importance of these huge masses of living, formless plasson-bodies in the lowest depths of the 50 THE EVOLUTION OF MAN. sea, raises many different inquiries and thoughts. Spon- taneous generation, especially, is naturally suggested by the Bathybius. We have already found that, for the origin of first Monera upon our globe, the assumption of spontaneous generation is a necessary hypothesis. We shall be all the more inclined to confirm it now that, in the Monera, we have recognized those simplest organisms, the origin of which by spontaneous generation, in the present condition of our science, no longer involves very great difficulties. For the Monera actually stand on the very boundary between organic and inorganic natural bodies.138 Next to the simple cytod-bodies of the Monera, as the second ancestral stage in the human pedigree (as in that of all other animals), comes the simple cell, that most undifferen- tiated cell-form, which, at the present time, still leads an independent solitary life, as the Amoeba. For the first and oldest process of organic differentiation, which affected the homogeneous and structureless plasson-body of the Monera, caused the separation of the latter into two different sub- stances ; an inner firmer substance, the kernel, or nucleus, and an outer, softer substance, the cell-substance, or protoplasma. By this extremely important separative pro- cess, by the differentiation of the plasson into nucleus and protoplasm, the organized cell originated from the structure- less cytod, the nucleolated from the non-nucleolated plastid. That the cells which first appeared upon the earth origin- ated in this manner, by the differentiation of the Monera, is a conception which in the present condition of histological knowledge seems quite allowable ; for we can even yet directly observe this oldest histological process of differ- entiation in Ontogeny. It will be remembered that in the THE MONERULA. 51 egg-cell of animals, either before or after fertilization, the original kernel disappeared. We explained this phenomenon as a reversion or atavism, and assumed that the egg-cell, in accordance with the law of latent heredity, first falls back into the kernel-less, cytod stage (Fig. 165). It is only after fertilization is accomplished that a new cell-kernel arises in this cytod, which thus becomes the parent-cell (Cytula, Fig. 166). The transitory kernel-less cytod-con- dition, intermediate between the egg-cell and the parent- cell, is an interesting germ-form, because, in accordance with the fundamental law of Biogeny, it reproduces the original, oldest parent-form of the Moneron; we therefore call it the Monerula. (Cf. vol. i. pp. 178-183.) Fig. 165. — Monerula of Mammal (Rabbit). The fertilized egg-cell after the loss of the nucleus is a simple ball of protoplasm {d). The outer covering of the latter is formed by the modified zona pellucida (z) together with a mucous layer (h) secreted on to the outside of the latter. In this a few sperm -cells are still visible (s). Fig. 166.— Parent-cell (Cytula) of a Mammal (Rabbit): 7r, parent, kernel; n, nucleolus of the latter; p, protoplasm of the parent-cell; z, modined zona pellucida ; s, sperm-cells ; h, outer albuminous covering. 52 THE EVOLUTION OF MAN. We have already explained the one-celled germ-form, which we see in the original egg-cell and the parent-cell which is originated by the fertilization of the egg-cell, as the reproduction of a one-celled parent-form, to which we ascribed the organization of an Amoeba (cf. Chap. VI.). For the Amoeba, as it yet lives widely distributed in the fresh and salt waters of the globe, must be regarded as the most undifferentiated and most original of the various one-celled Primitive Animals. As the immature primitive egg-cells (which as "primitive eggs" or Protova are found in the ovary of animals) are indistinguishable from ordinary Amoebse, we are justified in pointing to the Amoeba as the one-celled phylogenetic form, which, in accordance with the fundamental law of Biogeny, is at the present time yet reproduced in the ontogenetic primitive condition of the "Amoeboid egg-cell." As evidence of the striking cor- respondence of the two cells, it was incidentally men- tioned that in the case of some Sponges the real eggs of these animals were formerly described as parasitic Amoebae. Large one-celled Amoeba-like organisms were seen creeping about in the interior of the Sponge, and were mistaken for parasites. It was only afterwards that it was discovered that these "parasitic Amoebae" (Fig. 168) are really the eggs of the Sponge, from which the young Sponges develop. These egg-cells of the Sponge are, however, so like the true common Amoebae (Fig. 167) in size and structure, in the nature of their nuclei and in the characteristic form of movement of their continually changing false-feet (pseudo- podia), that, unless their source is known, it is impossible to distinguish them. This phylogenetic explanation of the egg-cell and its AMCE1LE. 53 reference to the primaeval ancestral form of the Amoeba, directly enables us to give a definite answer to the old hu- morous riddle : Which was first, the egg or the hen ? We can Fig. 167. — A crawling Amoeba (much enlarged). The whole organism has the form-value of a simple naked cell and moves about by means of change- able processes, which are extended from the protoplasmic body and again drawn in. In the inside is the bright-coloured, roundish cell-kernel or nucleus. Fig. 168. — Egg-cell of a Chalk-Sponge (Olynthus). The egg-cell creeps about in the body of the Sponge by extending variable processes, like those of the ordinary Amoeba. now very simply answer this Sphinx-question, with which our opponents try to shake or even to refute the Theory of Evolution. The egg existed much earlier than the hen. Of course it did not exist in the form of a bird's egg, but as an undifferentiated amoeboid cell of the simplest form. The egg existed independently during thousands of years as a simplest one-celled organism, as the Amoeba. It was only after the descendants of these one-celled Primitive Animals had developed into many-celled animal forms, and after these had sexually differentiated, that the egg, in the present physiological sense of the word, originated from the amoe- 54 THE EVOLUTION OF MAN. boid cell. Even then, the egg was first a Gastrsea-egg, then a Worm-egg, then an Acrania-egg, then a Fish-egg, an Am- phibian-egg, a Reptile-egg, and lastly, a Bird-egg. The egg of the Bird, as it now is, is a most complex historical pro- duct, the result of countless processes of heredity, which have occurred in the course of many millions of years.139 The fact that this primitive egg-form, as it first appears in the ovary of the most dissimilar animals, is always of one form, an undifferentiated cell, of the simplest amoeboid character, has already been pointed out as an especially important phenomenon. In this earliest young condition, immediately after the individual egg-cell has originated in consequence of a separation of the cells of the maternal ovary, no essential difference is recognizable in the egg-cells of the most dissimilar animals. (Cf. Fig. 10, vol. i. p. 134.) It is not till later, when the primitive egg-cells, or the primitive eggs (protova), have absorbed different kinds of nutritive yelk, and have surrounded themselves with variously formed coverings, and in other ways differentiated — it is not till they have in this way changed into after-eggs (metova), that those of different classes of animals can usually be distinguished. These peculiarities of the developed after- egg, the mature egg, are naturally to be considered as only secondarily acquired, by adaptation to the different con- ditions of existence both of the egg itself and of the animal which forms the egg. The two first and oldest ancestral forms of the human race, which we have now considered, the Moneron and the Amoeba, are, considered from a morphological point of view, simple organisms and individuals of the first order, Plastids. All subsequent stages in the ancestral chain are, on the PRIMOBDIAL EGG-CLEAVAGE. 55 other hand, compound organisms or individuals of higher order — social aggregations of a number of cells. The earliest of these, which, under the name of Synamcebse, we must rank as the third stage of our pedigree, are quite simple societies of all homogeneous undifferentiated cells ; amoeboid communities. To be certain as to their nature and origin, we need only trace the ontogenetic product of the parent-cell step by step. After the cytula (Fig. 166) has originated, by the re-formation of a cell-kernel, from Fig. 169. — Original or primordial egg-cleavage. The parent-cell, or cytula, which resulted from the fertilization of the egg-cell, first breaks up, by a continuous and regular process of division, into two cells (A), then into four (B), then into eight (C). and, lastly, into very numerous cleavage- cells (D). the Morula (Fig. 165), the parent-cell breaks up, by repeated division, into numerous cells. We have already minutely examined this important process of egg-cleavage, and have found that all the various modes of the latter are modifications of a single mode, that of original or primordial cleavage. (Cf. Chap. VIII., p. 188.) In the Ver- tebrate line this palingenetic form of egg-cleavage has been accurately re- 26 Fig. 170. — Mulbeny- germ, or morula. 56 THE EVOLUTION OF MAN. tained to the present time only by the Amphioxus, while all other Vertebrates have assumed a modified kenogenetic form of cleavage. (Of. Table III, vol. i. p. 241.) The latter certainly originated at a later period than the former, and the egg-cleavage of the Amphioxus is, therefore, extremely interesting (vol. i. p. 442). In this the parent-cell first parts into two similar cells, the two first cleavage-cells (Fig. 169, A). From these, by continuous division, arise 4, 8, 16, 32, 64 cells, etc., etc. (Fig. 169). The final result of this primordial cleavage was, we found, the formation of a globular mass of cells, which was entirely composed of homo- geneous, undifferentiated cells of the simplest character (Figs. 170, and 171, E). On account of the resemblance which this globular mass of cells bears to a mulberry or blackberry, we called it the " mulberry-germ," or morula. This "morula" evidently at the present clay shows us the many- celled animal body in the same entirely simple primitive condition in which, in the earlier Laurentian primitive epoch, it first originated from the one-celled amoeboid primitive animal form. The morula reproduces, in accordance with the fundamental law of Biogeny, the ancestral form of the Synamceba. For the first cell-com- munities, which then formed, and which laid the first foundation of the higher many-celled animal body, must have consisted entirely of homogeneous and quite simple amoeboid cells. The earliest Amoeba lived isolated hermit lives, and the amoeboid cells, which originated from the division of these one-celled organisms, must also have long lived isolated and self-dependent lives. Gradually, however, by the side of these one-celled Primitive Animals, small amoeboid communities arose, owing to the fact that the GERMINATION OF A CORAL ■■•-.-- . ssm. - -.r% ... i \ v..-,.--. *: € o ■' M°§i P i F ISM ' ""^^rPaiM ^'"'^^i KSSta$l Fy*. ' ■- ? • -■:■-■ : -: &M£ftk. . . i J Figs. 182, 183.— Ascula of a Sponge (Olynthus). Fig. 182, from the out- side ; Fig. 183, in longitudinal section : g, primitive intestine ; o, primitive mouth ; i, intestinal layer ; e, skin-layer. REPRODUCTION IN THE GASTRJ5ADS. 69 takes place directly through the stomach-cavity. A true palingenetic gastrula (Fig. 174) develops, just as in the Monoxenia (Fig. 171), from the fertilized egg. This swims about for a time in the sea, then attaches itself, and in this state resembles a simple young-form, which occurs in the course of the evolution of many other Plant- Animals, and which is called the ascula (Figs. 182, 183). In consequence of the absorption of foreign bodies by the exoderm, it becomes the Halijphysema. When we consider that there is no other important difference between the free-swimming gastrula and this attached, simplest Plant-animal, we are fairly justified in stating that in the simplest form of Gastrsea sexual repro- duction must have taken place in the same way. In the Gastrseads, just as in Plant-animals, both kinds of sexual cells — egg-cells and sperm-cells — must have formed in the same person; the oldest Gastrceacls must, therefore, have been hermaphrodite. For Comparative Anatomy shows that hermaphroditism, that is, the union of both kinds of sexual cells in one individual, is the oldest and original con- dition of sexual differentiation ; the separation of the sexes (Gonochorismus) did not originate till a later period. ( 70 ) TABLE XVII. Systematic Survey of the five earliest evolutionary stages of the Human An- cestral Line, compared with the five earliest stages of Individual and of Systematic Evolution. Form-Value Of the five earliest stages of the animal body. First Stage. A quite simple cytod (a non-nucleated plas- tid). Second Stage. A simple cell (a nucleated plastid). Third Stage. A quite simple ag- gregation of simple, similar cells. Fourth Stage. A simple hollow globe, filled with liquid, the wall of which consists of a single stratum of homogeneous cells. Fifth Stage. A hollow body, with a single axis, the wall of which consists of different cell-strata ; with an opening at one pole of the axis. Phylogeny. The five earliest stages in the evolu- tion of the tribe. 1. Monera. The oldest animal Monera (originating by spontaneous gene- ration). 2. Amoeba. Oldest animal Amceba. Synamoeba. The oldest aggrega- tion of animal Amoeba?. 4. Planaea. An animal hollow globe, the wall of which consists of a single stratum of ciliated cells. (blastcea.') 5. Gastraea. Parent-form of in- testinal animals, or Mttazoa. Simple pri- mitive intestine with primitive mouth. The body-wall is formed by the exoderm and the entoderm. Ontogeny. The five earliest stages in the evolu- tion of the germ. Monerula. A non - nucleated animal-egg (after fer- tilization and alter loss of the germ- vesicle). 2. Cytula. A nucleated, ferti- lized animal-egg (" first cleavage globule "). 3. Morula. " Mulberry-germ." A globular mass of cleavacre-cells. 4. Blastula. A hollow globe, the wall of which consists of a single stratum of homogeneous cells (the Planula of lower animals). (blastosphcera.) Gastrula. Intestinal larva. A simple intestinal cavity with a mouth- opening. The body- wall is formed by the two primary germ- layers. The System. The five earliest stages in the animal system. 1. Monera. Protamceba, Bathy- bius, and other extant Monera. 2. Amoeba. Extant Amceba. 3. Labyrmthula. A mass of similar, one - celled primitive animals. Magosphaera. A hollow globe, the wall of which consists of a single stratum of homogeneous ciliated cells. 5. Haliphysema. A quite simple plant- animal. An unarticu- lated uniaxial person, the body-wall of which consists of the exoderm and the entoderm. CHAPTER XVII THE ANCESTEAL SEBIES OF MAN. II. — From the Primitive Worm to the Skulled Animal. The Four Higher Animal Tribes are descended from the Worm Tribe. — The Descendants of the Gastraea ; in one direction the Parent Form of Plant- Animals (Sponges and Sea-Nettles), in the other the Parent Form of Worms. — Eadiate form of the former, Bilateral form of the latter. — The Two Main Divisions of the Worms, Accelomi and Coelomati : the former without, the latter with, a Body Cavity and Blood-vessel System. — Sixth Ancestral Stage : Archelminthes, most nearly allied to Turbellaria. — Descent of the Coelomati from the Accelomi. — Mantled Animals (Tunicata) and Chorda- Animals (Chordoma). — Seventh Stage: Soft- Worms (Scolecida). — A Side Branch of the latter: the Acorn- Worm (Balanoglossus). — Differentiation of the Intestinal Tube into Gill-intes- tine and Stomach-intestine. — Eighth Stage : Chorda-Animals (Chor- doma).— Ascidian Larva exhibits the Outline of a Chorda-Animal. — Construction of the Notochord. — Mantled Animals and Vertebrates as Diverging Branches of Chorda- Animals. — Separation of Vertebrates from the other Higher Animal Tribes (Articulated Animals, Star-Animals, Soft-bodied Animals). — Significance of the Metameric Formation. — Skull-less Animals (Acrania) and Skulled Animals (Craniota). — Ninth Ancestral Stage : Skull-less Animals. — Amphioxus and Primitive Verte- brate.— Development of Skulled Animals (Construction or the Head, Skull, and Brain). — Tenth Ancestral Stage : Skulled Animals, allied to the Cydostomi (Myxinoidce and Petromyzonidce) . " Not like the gods am I ! Full well I know $ But like the worm which in the dust must go, And, finding in the dust his life and weal, Is crushed and buried by the traveller's heel. — 72 THE EVOLUTION OF MAN. Why dost thou grin at me, thou hollow skull ? As though of old thy brain, like mine, was vexed, Had looked to find bright day, but in the twilight dull, In search for truth, was sad and sore perplexed ! " Goethe. Both in prose and in poetry man is very often compared to a worm. "A miserable worm/' "a poor worm," are common and almost compassionate phrases. If we cannot detect any deep phylogenetic reference in this zoological metaphor, we might at least safely assert that it contains an unconscious comparison with a low condition of animal development which is interesting in its bearing on the pedigree of the human race. For there is no doubt that the vertebrate tribe, in common with those of the other higher classes of animals, have developed phylogenetically from that multiform group of lower invertebrate animals which are now called Worms. However closely we limit the zoological significance of the word " Worm," it yet remains indubitable that a large number of extinct Worms must be reckoned among the direct ancestors of the human race. The group of Worms (Vermes) is much more limited in the Zoology of the present day, than was the same class in the older Zoology, which followed the system of Linnseus. It, however, yet includes a great number of very diverse lower animals, which, phylogenetically, we may regard as the few last living twigs of an immense spreading tree, the trunk and main branches of which have for the most part long since died off. On the one side, among the widely divergent classes of Worms, are found the parent- forms of the four higher tribes of animals, the Molluscs, Star-animals, Articulates, and Vertebrates ; on the other side, DEVELOPMENT OF WORMS AND PLANT-ANIMALS. 73 several comprehensive groups and also single isolated genera of Worms are to be regarded as root-suckers which have sprouted directly from the rest of the primaeval family-tree of the Worms. Some of these suckers have evidently changed but little from the long-extinct parent-form, the Primitive Worm (Prothelmis), which is immediately con- nected with the Gastrsea. Comparative Anatomy and Ontogeny clearly and sig- nificantly prove that the Gastrsea must be regarded as the direct ancestor of this Primitive Worm. Even now, a gastrula develops from the egg of all Worms after its cleavage. The lowest and most imperfect Worms retain throughout life an organization so simple that they are but little raised above the lowest Plant-animals, which are also immediate descendants of the Gastrsea, and which also yet develop directly from the gastrula. If the genealogical relation of these two lower animal tribes, the Worms and the Plant-animals, is closely examined, it becomes evident that the most probable hypothesis of their descent is, that the two originated, as independent branches, directly from the Gastraea. On the one side, the common parent-form of the Worms developed from the Gastraea ; as, on the other side, did the common parent-form of the Plant-animals. (Cf. Tables XVIII. and XIX.) The tribe of Plant-animals (Zoophytes, or Ccelenterata) now comprehends, on the one side, the main class of Sponges (Spongice) ; on the other, the main class of the Sea- nettles (Acalepkoe) ; to the former belong the Gastraeads and Poriferae, to the latter the Hydroid-polyps, the Medusae, Ctenophorae, and Corals. From the Comparative Anatomy and the Ontogeny of these we may infer, with great pro- 74 THE EVOLUTION OF MAN. bability, that all these Plant-animals descend from a common and very simple parent-form, the structure of which resembled that of the ascula in essential points (Figs. 182, 183, p. 68). The uniaxial outline of the ascula and the gastrula is usually retained by the Sponges, while in most Sea-nettles (Acalephce) transverse axes have been differentiated in the course of further evolution, thus giving rise to a characteristic radiate structure with a pyramidal general outline. In distinction from this predominant radiate outline of Plant-animals, a marked bilateral general outline is de- veloped from the first in the second offshoot from the gastrula, in the Worms. As the radiate form is marked by adaptation to an adherent mode of life, so is the bilateral form by adaptation to certain definite acts of free loco- motion. The constant direction and carriage of the body which would be maintained in this mode of free locomotion, conditioned the two-sided, or bilateral outline of the symmetrical Worms. Even the parent-form of the latter, the Primitive Worm (Prothelmis) must have acquired this character, and thus have become distinguished from the uniaxial parent-form of. the Plant-animals. In this simple mechanical impetus, in the defined free locomotion of the Worms, on the one hand, and in the stationary mode of life of the earliest Plant-animals on the other, we must look for the efficient cause which produced in the one the bi- lateral or two-sided, in the other the radiate outline of the body. The former, the bilateral outline, has been inherited by the human race from the Worms. Except through the Gastrsea, the common parent-form of Plant-animals and Worms, the human race is, therefore, THE WORMS AS ANCESTORS OF MAN. 75 not related to the Plant-animals. It will be our next task to consider more closely the pedigree of Man in so far as it coincides with that of the Worms. Let us examine how far the Comparative Anatomy and Ontogeny of Worms justify us in looking among the latter for primgeval ancestors of Vertebrates, and therefore of Man. For this end we must first consider the zoological system of Worms. In accord- ance with the most recent investigations of the Comparative Anatomy and Ontogeny of Worms, we divide (without reference to the many and various peculiarities of the numerous separate classes, which in this place do not interest us) the whole mass of forms within this tribe into two large main groups. The first main group, wThich we call Bloodless Worms (Accelonii), comprehends the earlier division of the lower Worms, which have no true body-cavity, no system of blood-vessels, no heart, no blood, — in short, none of the parts connected with this organ- system. The second main group, on the contrary, called Blood-worms (Coelomati), are distinguished from the former by the possession of a true body-cavity, and also by the presence of a blood -like fluid, which fills this cavity ; most of them also develop special blood-vessels, which again cause further correlated advances in structure. The relation of these two main groups of Worms is very evi- dently phylogenetic. The Accelomi, which are very nearly allied to the Gastryea and the Plant-animals, are to be regarded as an earlier and lower group, from which the more recent and higher division of the Ccelomati developed, perhaps towards the end of the Laurentian Period. We will first carefully examine the lower group of Worms, the Acoelomi, among which we must look for the y6 THE EVOLUTION OF MAN. sixth ancestral stage of the human race, the stage imme- diately following the gastrula. The name " Accelomi " signifies " Worms without a body-cavity, or cceloma," and therefore without blood, or vascular system. The extant Accelomi are generally included in a single class, which, on account of their flattened bodies, are called Flat-worms (Plathelminthes). To this class belong the Gliding- worms (Turbellaria), which live independently in the water ; also the parasitic intestinal Sucking-worms (Trernatoda), and the Tape-worms (Cestoda), which have become yet more degraded by parasitism. The phylogenetic relations of the three forms of Flat- worms are very evident ; the Sucking- worms originated from the free Gliding-worms by adaptation to a parasitic mode of life ; and, by a yet more completely parasitic life, the Tape-worms originated from the Sucking- worms. These are striking examples of the gradually increasing degeneration of the most important organs. In addition to these well-known extant Flat- worms, great numbers of other Accelomi must have lived during the Archilithic Epoch, which in general form were very much like those of the present day, but were, in some respects, yet more simply organized, and were, in their lowest stages of development, immediately connected with the Gastraaads. The whole of these lowest Accelomi, among which the common parent-form of the whole Worm tribe (the Prothelmis) must have been, may be classed as " Primi- tive Worms " (Archelminthes). The two classes of the Accelomi, the Primitive Worms and the Flat-worms, represent in their external form the simplest bilateral condition of the animal body. The body is a simple oval, usually somewhat flattened, with- BLOODLESS WORMS. JJ out any appendage (Figs. 184, 185). The dorsal side of the leaf-like body differs from the ventral side, on which the Worm creeps. Accordingly, even in these most simple Worms there are the three definite axes which mark the bilateral type-form, and which re-occur in the human body and in that of all higher animals : (1) a longitudinal axis (main axis), which passes from front to rear ; (2) a lateral axis, passing from right to left ; and (3) a ' sagittal axis, passing from the dorsal to the ventral surface. (Of. vol. i. p. 257.) This so-called symmetrical or " bilateral" arrangement of the outline of the body is simply the mechanical result of adaptation to a creeping form of loco- motion, during which one end of the body is always directed forwards. The geometric outline of the gastrula, as of the ascula, has but one axis with unequal poles (Monaxonia dijAopola). The typical outline of Worms, as of Vertebrates, is, on the contrary, bilateral, with tranverse axes (Stau- vaxonia dipleura).lu The whole outer surface of the Gliding- worms (Turbel- laria) is covered, as in the gastrula, with a thick, fine ciliated coat ; that is, with a fur-like covering of extremely fine and close microscopic hairs, which are direct processes of the uppermost cells of the epidermis, and maintain an uninterrupted whirling or vibratory motion (Fig. 184,/). The constant vibrations of these cilia cause a continued current of water over the surface of the body. Fresh water is constantly conveyed to the surface of the skin by this current, thus permitting respiration in its simplest form (skin- respiration). A similar ciliated covering, just as is seen in the extant Gliding-worms of our fresh-water seas, pre- sumably covered our extinct ancestors of the Primitive 78 THE EVOLUTION OF MAN. Worm group, the Archelminthes. They inherited this ciliated dress directly from the Gastraea. If we now make various vertical sections (longitudinal and transverse) through the simple body of the Gliding- worms (and that of the Archelminthes which are certainly very closely allied to the former), we soon discover that their internal structure is considerably higher than that of the Gastraaads. We first observe that the two primary germ- layers (inherited from the Gastraea) have differentiated into several cell-strata. The skin-layer and the intestinal layer have each split into two strata. The four secondary germ- layers, which are thus produced, are the same that we found resulted from the first differentiation of the two primary germ-layers in the embryo of the Vertebrate also. (Cf. the transverse sections through the larval Amphioxus and Earth-worm, Figs. 50 and 51, p. 236, and Plate IV. Fig. 2; Plate V. Fig. 10.) The highly important histological differentiation of these four secondary germ-layers led directly to further organolo- gical processes of differentiation, by which the organism of the Primitive Worms was soon considerably raised above that of the Gastrseads. In the latter there was really, in a morphological sense, but a single organ, the primitive intes- tine, with its mouth-opening. The whole body was nothing but an intestinal canal ; the intestinal wall was at the same time the wall of the body. Of the two cell-layers, forming this intestinal wall, the inner accomplished the functions of nutrition, the outer those of motion and covering. As some of the cells of the primary germ-layers developed into egg-cells, and others into sperm-cells, these layers also performed the function of reproduction. In the GLIDING-WORMS. 79 Primitive Worms, however, simultaneously with the forma- tion of the secondary germ-layers, these various functions also began to be distributed to various organs, which detached themselves from the original main organ, the primitive in- testine. Special organs originated for reproduction (sexual glands), for secretion (kidneys), for motion (muscles), and for sensation (nerves and sense-organs). In order to obtain an approximate picture of the sim- plest form in which all these various organs first appeared in the Primitive Worms, it is only necessary to examine the most imperfect forms of Gliding-worms (Turbellaria), as they exist at the present time in salt and fresh water. They are mostly very small and insignificant Worms of the simplest form, many being scarcely a millimetre or a few millimetres in length. In the simplest species of Gliding- worms the greater part of the oval body is occupied by the intestinal canal. This is a very regularly shaped pouch with an opening, re- presenting both mouth and anus (Fig. 184, m). At the anterior section of the intestinal tube, which is separated as a throat (pharynx, sd), the fibrous layer is very thick, a thick muscular layer. Immediately outside the intestinal- fibrous layer lies the skin-fibrous layer, which in most worms appears as a large skin-muscle sac. Above the throat in Gliding-worms a nerve system of the simplest form is already visible in front, a pair of small nerve- knots, or ganglia, which from their position are called the " upper throat ganglia," or " brain "' (Fig. 185, g). Delicate nerve-threads (n) pass from this to the muscles and to the ciliated skin-sensory layer. A pair of quite simple eyes (ctu) and nose-pits (no) are to be found in a few Gliding- worms. The Flat-worms are also universally provided with 27 8o THE EVOLUTION OF MAN. a pair of simple kidney-canals ( " excretory organs " ), in the form of two long, thin, glandular tubes, which traverse the right and left sides of the intestine and open at the hinder end of the body (Fig. 184, mi). We found that the Fig. 184. — A simple Gliding-worm (Rhabdocoeluyn) : in, mouth; sd, thi'oat- epithelium ; sm, throat-muscles ; d, stomach-intestine ; nc, kidney ducts ; nm, opening of the kidneys ; au, eye ; 11a, nose-pit. Fig. 185. — The same Gliding-worm, showing the remaining organs : g, brain; au, eye; na, nose-pit; n, nerves; h, testes; £ , male opening; $> female opening ; e, ovary ; /, ciliated outer-skin. STRUCTURE OF THE GLIDING-WORMS. 8 1 two primitive kidney canals in the vertebrate embryo also appeared at a very early period, shortly after the first differentiation of the middle germ-layer (mesoderma). The appearance of these at so early a period shows that the kidneys are very important primordial organs. It also shows their universal existence in all Flat- worms ; for even the Tape- worms, which, in consequence of the adoption of a parasitic mode of life, have lost the intestine, yet have the two secreting primitive kidneys, or " excretory ducts." The latter seem, therefore, to be older and of greater physiologi- cal importance than the blood-vessel system, which is wholly wanting in the Flat- worms. The sexual organs appear in many of the Gliding- worms in a very complex form ; while in others their form is very simple. Most of them are hermaphrodites ; that is, each individual worm has both male and female sexual organs. In the simplest forms we find a testis in the anterior part (Fig. 185, h), a single or double ovary behind (a). One of these simplest existing Accelomi, such as we find among the lowest Rhab- doccela, may give us an approximate idea of the structure of the Primitive Worm, which forms the sixth staofe in the human pedigree. These ancestors of the human race, which, on account of their general organization, must be placed among the Bloodless Worms (Acoelomi), must have been represented during the Archilithic Epoch by a large number of various Worm forms. The lowest of these must have been directly connected with the Gastraeads (the fifth ancestral stage); the most highly developed must, on the other hand, have been directly connected with the Ccelomati (the seventh stage). As, however, our present knowledge of the Comparative 82 THE EVOLUTION OF MAN. Anatomy and Ontogeny of the Aceelomi is very fragmen- tary, and much too imperfect to enable us to point with certainty to the series of the various stages, we will not attempt a detailed arrangement of them. We will turn instead to the seventh stage in the human pedigree, which belonged to the multiform group of the Blood-bearing Worms (Codomati). The great organic advance in structure by which the Blood-bearing worms, or Coelomati, developed from the older Bloodless Worms, or Aceelomi, consisted in the for- mation of a body-cavity (coeloma), and of a nutritive juice filling the latter, the first blood. All the lower animals with which we have yet occupied ourselves in our Phy- logeny, all the Primitive Animals and Plant-animals, are, like the Aceelomi, bloodless and without a body-cavity. In the formation of a special vascular system, the earliest Cuelomati made a very great advance. Much of the com- plexity in the organic structure in the four higher tribes of animals is based on the differentiation of the vascular system, which they have inherited from the Blood-bearing Worms. The first development of a true body-cavity (coeloma) is referable to the separation of the two fibrous layers ; to the formation of a spacious cavity between the outer skin- fibrous layer and the inner intestinal-fibrous layer. In the fissure-like gaps, which formed between the two germ-layers, a juice collected, which penetrated through the intestinal wall. This juice was the first blood, and the gaps between the two germ-layers formed the first rudiment of the body- cavity. The union of these gaps formed the simple ccelom, the large cavity, containing blood or lymph, which plays so BLOOD-BEARING WORMS. S$ important a part in all the higher animals as the receptacle of the very extensive intestines. The formation of this ccelom, and of the blood-vessels developed in connection with it, exercised a very great influence on the further evolution of the animal organization. The most important result was, that it allowed the conveyance of rich nutritive juices to those parts of the body lying near the circumference, and developing at a considerable distance from the intes- tinal canal. The intimate correlation, or reciprocity of the parts, necessarily occasioned, in direct connection with the progressive development of the blood-vessel system, many other important a'dvances in the structure of the body of the Blood-bearing Worms. Just as among the Accelomi, so also among the Ccelomati, the pedigree of our race must have passed through a large number of diverse ancestral stages. But anion v extant Ccelomati (which form but a very small fraction of this once multiform group), there are but very few Worms which can with certainty be regarded as nearly allied to the long- extinct ancestors of Man. In this respect, but a single class of Ccelomati is really of prominent importance ; these are the Mantled Animals (Tunicata), to which belong the Ascidia already known to us. Our careful examination of the structure and germ-history of the Ascidian and the Amphioxus have shown the extreme importance of these very interesting animal forms. (Cf. Chapters XIII. and XIV.) That examination fully justifies us in asserting that among the ancestors of the Vertebrates (and therefore of Man) there was an unknown extinct ccelomate species, to which the nearest allied form among extant anil lals is the Appendicular! a (Fig. 187), of which we have already 84 THE EVOLUTION OF MAN. spoken, and the tailed Ascidian larva. We will for the present call this kind of Worm, which was primarily dis- tinguished by the possession of a notochord, the Chorda- animal (Chordoniwm). The Ascidians on the one hand, and the Vertebrates on the other, developed, as two diverging branches, from these Chorda-animals. The common parent- form of the Chorda-animals themselves was a ccelomate form, which finally must have descended from the Acoelomi, and from the Archelminthes. Many connecting intermediate forms must, of course, have existed between these two groups of Worms, between the Primitive Worms and the Chorda-animals. Unfortunately, however, zoological knowledge is at present especially im- perfect with regard to these important intermediate forms of the multiform Worm tribe. For very evident reasons, none of these Worms could leave fossil remains. For, like the great majority of other Worms, they had no hard parts in their bodies. Most even of the known fossil Worms are worthless, for they tell us little or nothing of the most im- portant structural features of the soft body. Fortunately, however, we can in great measure satisfactorily fill the con- siderable palaBontological gap in this part of our pedigree, with the help of the Comparative Anatomy and Ontogeny of Worms. If, on the one hand, we examine the structure and mode of development of the lower Worms from the Gliding- Worms (Turbellaria), and, on the other hand, the Anatomy and Ontogeny of the Ascidians, it is not difficult, step by step, to re-construct in imagination the connecting inter- mediate forms, and to insert a series of extinct ancestral forms between the Acoelomi and the Chordoma. This series of forms under the name of Soft-worms (Scolecida) DEVELOPMENT OF CHORDA-ANIMALS. 85 we will consider as the seventh stage in the human pedigree. An examination of the Comparative Anatomy of the various Scolecid forms, which we might perhaps distinguish here, would lead us much too far into the difficult details of the Comparative Anatomy and Ontogeny of the Worms. For our purpose it seems more important to call attention to those phylogenetic advances, by means of which the organization of the earliest Blood-bearing Worms was in the end elevated to that of the Chorda-animals. The Com- parative Anatomy and Ontogeny of the Gliding-worms and of the Ascidians justify us in giving special weight to the significant differentiation of the intestinal canal into two distinct divisions ; into an anterior division (the gill-intes- tine), which accomplishes respiration, and a posterior divi- sion (the stomach-intestine), which accomplishes digestion. As in Gastraeads and Primitive Worms, so also in the Ascidian larva, the intestinal canal is at first a simple pouch-like body, provided merely with a mouth-opening. A second opening, the anus, does not develop till a later period. Gill- openings afterwards appear in the anterior section of the intestinal canal, by which the whole anterior intestine is transformed into a gill-body. This remarkable arrange- ment is, as we found, quite peculiar to Vertebrates, and, except in the Ascidians, occurs nowhere else. Among extant Worms there is, however, a single isolated and very remark- able Worm form, which in this respect may be regarded as distantlv allied to the Ascidia and to Vertebrates, and perhaps as an off-shoot from the Soft-worms (Scolecida). This is the so-called "Acorn- worm" (Balanoglossus, Fig. 186), which lives in the sand of the sea-shore. The in- 86 THE EVOLUTION OF MAN. teresting points connecting this with Ascidians and the Skull-less Animals (Acrania) were first accurately observed and explained by Gegenbaur. Al- though this singular Balanoglossus is in many other respects peculiar in its organization, so that Gegen- baur rightly ranked it as the re- presentative of a special class (Enter oprceusta), yet the structure of the anterior section of the in- testinal tube is exactly similar to that of Ascidians and Skull-less Animals (k), a gill body, the walls of which are pierced on either side by gill-openings and are supported by gill-arches. Now, although the Acorn- worm in other points of its structure may differ very con- siderably from those extinct Soft- worms (Scolecidce), which we must regard as direct ancestors of our race, and as intermediate links between the Primitive Worms Fig. 186. — A young Acorn-worm (Bal- anoglossus). (After Alexander Agassiz.) r, acorn-like proboscis; h, collar; Tc, gill openings and gill-arches of the anterior in- testine, in a long row one behind another on each side ; d, digestive posterior intes- tine, filling the greater part of the body- cavity ; v, intestinal vessel, lying between two parallel folds of skin ; ■*^ *- 3« 8 « H < o ^ oj =« o> m r- >- S- £ -3 O CJ O fe « O o BO & © ... g 2 « ° a g.£o«) ■5 .2 "2 m 'S oj * u I B^o >■ *= .3 - ^.°£ -I 3 '-3 oj 94 THE EVOLUTION OF MAN. worm tribe ; but the parent-forms of the three former belong to worm-groups quite distinct from that of the Chordoma. It is only far down at the common root of the group of Coelomati, that we assume a common source for these various tribal forms. (Cf. Tables XVIII. and XIX.) It is especially necessary to remember that there is no direct blood-relationship between Vertebrates and Articu- lated Animals. The Articulated Animals (Arthropoda), to which the most comprehensive of all classes of animals, that of Insects, and also the Spiders, Centipedes, as well as the Crabs, or Crustaceans, belong, are descendants of articulated Worms, the nearest allies of which are the extant Ringed Worms {Annelida). The tribe of Star-animals (Echinoderma), which includes the Star-fishes, Sea-lilies, Sea-urchins, and Sea-cucumbers, must also have descended from similar articu- lated Worms.118 The parent-form of the Soft-bodied Animals (MoUusca), which include the Cuttles, Snails, Mussels, and Lamp-shells, must also be sought among the Worms. But the Coelomati, from which these three higher animal tribes originated, differed entirely in character from the Chorda- animals. Unlike the latter, they never developed a noto- chord. In them, the anterior section of the intestinal tube was never modified into a gill-body with gill-openings; nor were the upper throat-ganglia developed into a medullary tube. In a word, in Articulated Animals, Star-animals, and Soft-bodied Animals, as well as in their ancestors among the Blood-bearing Worms, the typical structural peculiari- ties which are exclusively characteristic of the vertebrate tribe and of their immediate invertebrate progenitors, were never present. Thus the great majority of all animals are DEVELOPMENT OF VERTEBRATES FROM INVERTEBRATES. 95 in no way the subject of our further investigations, which are only concerned with the Vertebrates. The development of the Vertebrates from the Inverte- . brates most nearly related to them, the Chorda-Animals, occurred millions of years ago, during the Archilithic Epoch. (See Table XII., p. 11.) This is unmistakably shown by the fact that the most recent sedimentary rock-strata which were deposited during that immense period of time, the higher layers of the Upper Silurian formation, contain remains of fossil Fishes (Primitive Fishes, Selachii). As these Fishes, although they belong to the lowest stage of the Skulled Animals (Craniota), yet possess a compara- tively high organization, and as they must necessarily have been preceded by a long progressive series of lower Skull- less Vertebrates, we must attribute the origin of the oldest Skull-less Animals (Acrania) from the Chorda-animals to a much earlier part of the Archilithic Epoch. Therefore, not only all the invertebrate ancestors of our race, but also the earliest form of our vertebrate progenitors must have developed in that primordial time, which includes the Laurentian, Cambrian, and Silurian Periods. (Cf. Tables XIIL, XIV, and XVI., pp. 12, 19, 44.) Unfortunately, Palaeontology can give us absolutely no information with regard either to the structure of our oldest vertebrate ancestors, or to the time of their appearance; for their bodies were as soft and as destitute of hard parts capable of fossilization, as were the bodies of all our preceding invertebrate ancestors. It is, therefore, not surprising, but quite natural, that we find no fossil remains of the former in the Archilithic formations. The Fishes in which the soft cartilaginous skeleton was partly g6 THE EVOLUTION OF MAN. modified into hard bone, are the earliest Vertebrates capable of leaving petrified records of their existence and structure. Fortunately, this want is more than counterbalanced by the much more important testimony of Comparative Anatomy and Ontogeny, which henceforth form our safest guides within the Vertebrate pedigree. Thanks to the classic researches of Cuvier, Johannes Muller, Huxley, and especially of Gegenbaur, we are in possession of such extensive and instructive records of creation in this most important branch of tribal history, that we can prove at least the more significant features in the development of our Vertebrate ancestors, with the most gratifying certainty. The characteristic peculiarities by which Vertebrates in general are distinguished from all Invertebrates, engaged our attention some time ago, when we examined the structure of the ideal Primitive Vertebrate (Figs. 52-56, p. 256). The most prominent characters were as follows: (1) the formation of the notochord between the medullary and intestinal tubes; (2) the differentiation of the intestinal tube into an anterior gill-intestine and a posterior stomach-intestine ; (3) the inner articulation, or formation of metamera. The Verte- brates share the first two qualities with the larval Ascidians and with the Chorda-animals ; the third quality is entirely peculiar to them. Accordingly, *the most important struc- tural advance, by which the earliest vertebrate forms origin- ated from the most nearly allied Chorda- Animals, consisted in an internal metameric structure. This showed itself first most distinctly in the articulation of the muscular system, which broke up on the right and left into a series of consecutive muscular plates. At a later period the articulation declared itself prominently in the skeleton, and CLASSIFICATION OF VERTEBRATES. 97 nervous and blood-vessel systems. As we have already seen, this process of articulation, or metameric formation, must essentially be regarded as terminal germination. Each distinct trunk-segment, or metameron, represents an individual. Thus the Vertebrates with their internal segmentation stand in a similar relation to their inarticulate Invertebrate ancestors, the Chorda Animals, as do the out- wardly segmented Ringed Worms (Annelida) and Articu- lated Animals (Arthropoda) to the simple inarticulate Worms from which they originated. The tribal history of Vertebrates is rendered much more intelligible by the natural classification of the tribe which I proposed first in my Generelle Movphologie (18G6), and afterwards improved in many ways in " The Natural History of Creation" (Chap. XX., p. 192, etc.). In accordance with that, existing Vertebrates must be divided into at least eight classes, as follows : — SYSTEMATIC SURVEY OF THE EIGHT CLASSES OF VERTEBRATES. A. Skull-less (Acrania) 1. Tube-hearted 1. Leptocardia 'a. Single-nostrilled (Monorhina) 2. Round-mouths 2. Cyclostoma I. (3. Fishps 3. Pisces n Qirniio^ 1 i Amnion-less ,•„,,! • #!. <&• Double-no-trilled ) Anamnia Craniota 4. Mud-fishes 4. Dipneusta ,5. Amphibians 5. Amphibia Amphirhina j n (6. Reptiles 6. Reptilia With Amnion < 7. Birds 1. Aves Amniota \ 8. Mammals 8. Mammalia The whole Vertebrate tribe may primarily be divided into the two main sections of the Skull-less and the Skulled Vertebrates. Of the earlier and lower section, that of the Skull-less (A crania), the Amphioxus is alone extant. To the more recent and higher section, the Skulled (Cra- niota), belong all other existing Vertebrates up to Man. The 98 THE EVOLUTION OF MAN. Craniota branched off from the Acrania, as these did from the Chorda Animals. Our exhaustive study of the Compara- tive Anatomy and Ontogeny of the Ascidian and the Amphioxus have already afforded proof of this relation. (Cf. Chapters XIII. and XIV., and Plates X. and XI. with the explanations.) I will only repeat, as the most important fact, that the Amphioxus develops from the egg in exactly the same way as the Ascidian. In both, the original Bell- gastrula (Figs. 4 and 10) originates in an exactly similar manner, by primordial cleavage from the simple parent-cell (Figs. 1 and 7). From this originates that remarkable larva, which develops a medullary tube on the dorsal side of the intestinal tube, and between the two a notochord. At a later period, both in the Ascidian and in the Amphioxus, the intestinal tube differentiates into an anterior gill-intestine and a posterior stomach-intestine. In accordance with the fundamental principle of Biogeny, from these very important facts we may deduce the following statement of great phylo- genetic importance : the Amphioxus, the lowest Vertebrate form, and the Ascidian, the most nearly allied Invertebrate form, have both descended from one single extinct Worm form, which must have possessed the essential structure of the Chorda Animals. The Amphioxus, as has already been often shown, is of extreme importance ; not only because it thus fills the great gap between the Invertebrates and the Vertebrates, but also because it represents, at the present time, the typical Vertebrate in its simplest form; and because it directly affords the best standpoint from which tc examine the gradual historic evolution of the whole tribe. If the structure and germ-history of the Amphioxus were un- THE AMPHIOXUS AS THE ANCESTOR OF MAN. 99 known to us, the whole subject of the development of the Vertebrate tribe, and thus of our own race, would be enveloped in an impenetrable veil. The accurate anatomical and ontogenetic knowledge of the Amphioxus, attained during the last few years, has alone pierced that heavy veil, formerly supposed to be impenetrable. If the Amphioxus is compared with the developed Man or any other of the higher Vertebrates, a great number of striking dissimilarities will be seen. The Amphioxus has no specialized head, no brain, no skull, no jaws, no limbs ; it is without a central- ized heart, a developed liver and kidneys, a jointed vertebral column ; every organ appears in a much simpler and more primitive form than in the higher Vertebrates and in Man. (Cf. Table X., vol. i. p. 466.) And yet, in spite of all these various deviations from the structure of other Vertebrates, the Amphioxus is a genuine, unmistakable Vertebrate ; and if, instead of the developed Man, the human embryo at an early period of its Ontogeny is compared with the Amphioxus, we shall find perfect parallelism between the two in all essential points. (Cf. Table IX., vol. i. p. 465.) This highly important parallelism justifies the conclusion that all the Skulled Animals (Craniota) have descended from a common primaeval parent-form, the structure of which was essentially that of the Amphioxus. This parent- form, the earliest Primitive Vertebrate, possessed the peculiar characters of the Vertebrates, and yet was without all those important peculiarities that distinguish the Skulled Animals from the Skull-less. Although the Amphioxus ap- pears peculiarly organized in many respects, and although it may not be regarded as an unmodified descendant of the Primitive Vertebrate, yet it must have inherited from the 100 THE EVOLUTION OF MAN. latter the distinguishing characteristic features already mentioned. We cannot therefore say that the Amphioxus is the progenitor of the Vertebrates ; but we may certainly say that the Amphioxus of all known animals is nearest allied to this progenitor ; both belong to the same limited family group, to the lowest Vertebrate class, that of the Skull-less Animals (Acrania). In the human pedigree, this group forms the ninth stage of the ancestral chain, the first among Vertebrate ancestors. From this Skull-less group was developed the Amphioxus on the one side, and on the other the parent-form of the Skulled Animals (Craniota). The comprehensive group of the Skulled Animals includes all known Vertebrates, with the single excep- tion of the Amphioxus. All these Skulled Animals possess a distinct head, inwardly specialized from the trunk, and this contains a skull, enclosing a brain. This head also carries three of the higher sense-organs, which are partially wanting in the Skull-less Animals (nose, ears, and eyes). At first, the brain appears in a very simple form, as an anterior bladder-like extension of the medullary tube (Plate XI. Fig. 16, mx). This, however, is soon distributed by several tranverse grooves — first into three, and afterwards into a series of five consecutive brain-bladders. In the formation of the head, skull, and brain, together with the higher sense-organs, lies the most essential advance made by the skulled parent-form beyond its skull-less ancestors. Other organs, however, also soon rose to a higher grade of development; a compact centralized heart appeared, a more perfect liver and kidneys; and in other directions also important advance was made. The Skull-less Animals may be primarily subdivided SKULL-LESS ANIMALS. 101 into two differing main sections, that of the Single-nostrils (Monorhina), and that of the Double-nostrils (Amphirhina). Of the former there are but very few extant forms, which are called Round-mouths (Cyclostoma). These are, however, of great interest, because in their whole structure they are intermediate between the Skull-less Animals and the Double- nostrils (Ampkirhina). Their organization is much higher than that of the Skull-less Animals, much lower than that of the Double-nostrils ; they thus form a very welcome phylogenetic link between those two divisions. We may therefore represent them as a special, tenth stage in the human ancestral series. The few existing species of the class of Round-mouths are distributed into two different orders, which are distinguished as the Hags and the Lampreys. The Hags (Myxinoides) have long, cylindrical, worm-like bodies. Linnaeus classed them among Worms, but later zoologists have placed them, sometimes among the Fishes, sometimes Amphibians, and again with Molluscs. The Hags live in the sea and are usually parasitic on Fishes, into the skin of which they penetrate by means of their round sucking mouths and their toothed tongues. They are occasionally found in the body-cavity of Fishes — for example, of the Cod and Stur- geon— having penetrated to the interior in their passage through the skin. The second order, that of the Lampreys (Petromyzontes), includes those well-known " Nine eyes," common at the seaside; the little river Lamprey (Petro- myzon fluviatilis) and the large sea Lamprey (Petromyzon marinus, Fig. 190). The animals included in the two groups of the Myxi- noides and the Petromyzontes, are called Round-mouths 102 THE EVOLUTION OF MAN. (Cyclostoma), from the fact that their mouth forms a circular or semi-circular opening. The upper and under jaws, which appear in all the higher Vertebrates, are completely wanting in the Round-mouths, as in the Amphioxus. All other Vertebrates are therefore distinguishable from them as "Jaw-mouthed" (Gnathostomi). The Round-mouths may also be called " Single-nostrils " (Monorhina), because they have but a single nasal tube, while the Gnathostomi are all furnished with a pair of nasal cavities, a right and a left nose-cavity (" Double-nostrilled," Amphirhirui). But in addition to these peculiarities, the Jaw-mouths are also distinguished by many other remarkable structural arrange- ments, and are further removed from the Fishes than the latter are from Man. They must, therefore, evidently be regarded as the last remnant of a very old and very low class of Vertebrates, which are far below the structural stage of a genuine Fish. To mention here briefly only the most important, the Round-mouths are entirely with- out any trace of limbs. Their slimy skin is quite naked and smooth, without scales. They are wholly destitute of a bony skeleton. The inner skeleton axis is a very simple inarticulate notochord, like that of the Amphioxus. In the Lampreys alone a rudimentary articu- lation is indicated by the fact that upper arches appear in the vertebral tube proceeding from the notochord sheath. At the anterior end of the chorda a skull is developed in its very simplest form. From the notochord sheath pro- ceeds a small soft-membraneous skull capsule, which becomes partly cartilaginous : this capsule encloses the brain. The important apparatus of the gill-arches, the tongue-bone, etc., which is inherited by all Vertebrates LAMPREYS. from Fishes to Man, is wholly wanting in the Round-mouths. They have, indeed, a superficial, cartilaginous gill-skeleton, but this is of quite different morphological significance. On the other hand, in them we meet, for the first time, with a brain, that important mental or«;an, which has been transmitted from the Single-nostrils up to Man. It is true that in the Round-mouths the brain appears merely as a very small and comparatively insignificant swelling of the spinal chord ; at first a simple bladder (Plate XL Fig. 16, mx), which afterwards separates into five consecu- tive brain-bladders, as in the brains of all Double -breathers. These five simple primitive brain-bladders, which reappear in a similar form in the embryos of all higher Vertebrates, from Fishes up to Man, and which undergo a very complex modification, remain in the Round-mouths, in a very low and undifferen- tiated stage of development. The histological elementary structure of the nervous system is also much more imperfect than in other Verte- brates. While in the latter the orsxan ot hearing always has three semi-circular canals, in the Lampreys it has but tivo, and in the Hags but one. In most other points also, the organization of the Round-mouths is Fig. 190. — The largo Sea-lamprey (Petromyzon mari- nus), much reduced in size. A series of seven gill-open- ings are visible below the eye. 28 I03 104 THE EVOLUTION OF MAN. much simpler and more imperfect, as, for instance, in the structure of the heart, the circulatory system, and the kidneys. In them, as in the Amphioxus, the anterior portion of the intestinal canal does, indeed, form respiratory gills ; but these respiratory organs are developed in a very peculiar way : in the form of six or seven little pouches, or sacs, which lie on both sides of the anterior intestine and communicate with the throat (pharynx) by inner openings, and by outer ones with the external skin. This is a very peculiar formation of the respiratory organs, quite cha- racteristic of this class of animals. They have therefore been called the " Pouch-gills ': (Marsiipobranchii). The absence of one very important organ found in the Fishes, the swimming-bladder, from which the lungs of the higher Vertebrates have developed, should be especially noticed. In their germ-history, as in their whole anatomical struc- ture, the Round-mouths present many peculiarities. They are even peculiar in the unequal cleavage of the egg, which most nearly approaches that of the Amphibians (Fig. 31, vol. i. p. 203). This results in the formation of a Hoocl- gastrula, like that of Amphibians (Plate II. Fig. 11). From this develops a very simple organized larval form, which is closely allied to the Amphioxus, and which, for that reason, we examined and compared with the latter (vol. i. p. 428, and Plate VIII. Fig. 1G). The gradual germ-evolution of these larvse of the Round -mouths explains very clearly and unmistakably the gradual evolution of the Skulled from the Skull-less class of Vertebrates. At a later period, from this simple Lamprey larva is developed a blind and tooth- less larval form, which is so very different from the mature Lamprey that, until twenty years ago, it was generally ROUND-MOUTHS. 105 described as a peculiar form of fish under the name of Ammocoetes. By a further metamorphosis this blind and toothless Ammocoetes is transformed into the Lamprey with eyes and teeth (Petromyzon).141 Summing up all these peculiarities in the structure and embryology of the Round-mouths, we may assert that the oldest Skulled Animals, or Craniota, diverged in two lines ; one of these lines has continued up to the present time but little modified; it is represented by the Cyclostoma, or Monorhina, forming a collateral line which has made but little progress, but has remained at a very low stage of development. The other line, the direct line in the pedigree of the Vertebrates, advanced in a straight line to the Fishes, and by new adaptations attained many important improve- ments. In order rightly to appreciate the phylogenetic signi- ficance of interesting remnants of primseval groups of animals, such as the Round-mouths, it is necessary to study minutely their various peculiar characters philosophically and with the aid of Comparative Anatomy. A careful distinction must be drawn between the hereditary cha- racters which have been accurately transmitted to the present day by heredity from common, primaeval ancestors, now extinct, on the one hand ; and, on the other, those special adaptive peculiarities which the existing remnant of that primaeval group have, in the course of time, gained secondarily by adaptation. To the latter class belong, for example, in the Round-mouths, the peculiar formation of the single nostril and the round sucking mouth ; as well as special structural arrangements of the epidermis and the pouch-shaped gills. But, on the other hand, to the io6 THE EVOLUTION OF MAN. former class of characteristics, which alone have any phylo- genetic significance, belong the primitive formation of the vertebral column and the brain, the absence of the swim- ming-bladder, of jaws, limbs, etc. In the animal system, the Round-mouths are usually classed among Fishes ; but that this is quite incorrect is apparent from the simple fact that, in all important and prominent structural peculiarities, they are further removed from the Fishes than the Fishes are from the Mammals and from Man. CHAPTER XVIII. THE PEDIGREE OF MAN. III. From the Primitive Fish to the Amniotic Animal. Comparative Anatomy of the Vertebrates. — The Characteristic Qualities of the Double-nostrilled and Jaw-monthed : the Double-Nostrils, the Gill- arch Apparatus, with the Jaw-arches, the Swimming-bladder, the Two Pairs of Limbs. — Relationship of the Three Groups of Fishes : the Pri- mitive Fishes (Selachii), the Ganoids (Ganoides), the Osseous Fishes (Telcostei). — Dawn of Terrestial Life on the Earth. — Modification of the Swimming-bladder into the Lungs. — Intermediate Position of the Dipneusta between the Primitive Fishes and Amphibia. — The Three Extant Dipneusta (Protopterus, Lepidosiren, Ceratodus). — Modification of the Many-toed Fin of the Fish into the Five-toed Foot. — Causes and Effects of the latter. — Descent of all Higher Vertebrates from a Five-toed Amphibian. — Intermediate Position of the Amphibians between the Lower and Higher Vertebrates. — Modification or Metamorphosis of Frogs. — Different Stages in Amphibian Metamorphosis. — The Gilled Batrachians (Proteus and Axolotl). — The Tailed Batrachians (Salaman- ders and Mud-fish). — Frog Batrachians (Frogs and Toads). — Chief Group of the Amnion Animals, or Amniota (Reptiles, Birds, and Mammals). — Descent of all the Amniota from a Common Lizard-like Parent-form (Protamnion) . — First Formation of the Allantois and of the Amnion. — Branching of the Amnion Animals in Two Lines : on the one side, Reptiles (and Birds), on the other side, Mammals. " The imagination is an indispensable faculty ; for it is that which, by forming new combinations, occasions important discoveries. The naturalist needs both the discriminating power of abstract reason, and the generalizing power of the imagination, and that the two should be harmoniously inter. 108 THE EVOLUTION OF MAN. related. If the proper balance of these faculties is destroyed, the naturalist is hurried into chimerical fancies by his imagination ; while the same gift leads the gifted naturalist of sufficient strength of reason to the most important discoveries." — Johannes Muller (1834). The further we proceed in human tribal history, the nar- rower does that part of the animal kingdom become within which we must look for extinct ancestors of the human race. At the same time, the evidence as to the history of the evolution of our race given by what we have called the records of creation, the evidence of Ontogeny, of Compara- tive Anatomy, and of Palaeontology, grows constantly more extensive, complete, and trustworthy. It is therefore natural that Phylogeny should assume a more definite form the nearer we approach the higher and the highest stages of the animal kino-dom. Comparative Anatomy especially has done far more for our knowledge of these higher stages of evolution in the animal kingdom than for the lower. This important science, which aims at a true philosophy of organic forms, has made greater progress in the Vertebrate tribe than in any section of the Invertebrate. Cuvier, Meckel, and Johannes Miiller had already laid a deep and extensive foundation ; and now the Comparative Anatomy of Vertebrates has recently been powerfully advanced by the admirable inves- tigations of Owen and Huxley, and, especially, has been perfected to such a degree by the unsurpassed labours of Gegenbaur, that it now forms one of the strongest supports of the Theory of Descent. Relying on the evidence thus furnished, we can now, with a great degree of certainty, recognize the most important outlines of the series of stages and the ramifications of the Vertebrate pedigree. PRIMITIVE FISHES. IO9 That part of the animal kingdom with which we are now concerned has become so narrow, even before we have left the Archilithic Epoch, that but a single one of the seven tribes of the animal kingdom forms the object of our study. Even within this tribe we have passed the lowest steps, and have risen above the Skull-less (A crania) and Double-nos trilled Vertebrates (Monorhina), to the class of Fishes. The latter are the first of the great main division of Vertebrates distinguished by mouths with jaws and by double nostrils (Amphirhina, or Gnathostoma). From Fishes we start again, as from that class of Vertebrates which are indubitably shown by Comparative Anatomy and Ontogeny to be the ancestral class of all higher Vertebrates, all Am- phirhina. Of course no existing Fish can be regarded as the direct parent-form of the higher Vertebrates. But it is equally certain that from a common extinct Fish -like parent-form Ave may trace all those Vertebrates from Fishes up to Man, which are included under the name of Am- phirhina. If this primaeval parent-form were extant, we should undoubtedly describe it as a genuine Fish and class it among Fishes. Fortunately, the Comparative Anatomy and Classification of the Fishes has been so far advanced (thanks to the labours of Johannes Muller and Gegenbaur) that we can very clearly distinguish these most important and interesting genealogical relations. In order correctly to understand the human pedigree within the Vertebrate tribe, it is very important to bear in mind the distinguishing characteristics, separating Fishes and all the other Double-nostrils (Amphirhina) from Single-nostrilled .and Skull-less Animals (Monorhina and Acrania). These very distinguishing characteristic marks no THE EVOLUTION OF MAN. Fishes have in common with all other Double-nostrils up to Man, and it is on this parallelism that we found our claim of relationship to Fishes. (Cf. Table X., vol. i. p. 466.) The following characters of the Double-nostrils must be especially indicated as the systematic anatomical features of the highest importance : (1) the double structure of the nose; (2) the internal gill-arch apparatus, together with the jaw-arches ; (3) the swimming-bladder, or lungs ; and (4) the two pairs of limbs. As to the nasal structure, on which is based the distinc- tion of the Single-nostrils (3Ionorhina) from the Double- nostrils (Amphirkina), it is certainly significant that even in Fishes the earliest rudiment of the nose consists of two en- tirely distinct lateral grooves or pits in the outer surface of the head, just as is the case in the embryo of Man and of all higher Vertebrates. On the other hand, in Single-nostrils and Skull-less Vertebrates the first rudiment of the nose is, from the first, a single pit in the centre of the forehead region. No less important is the higher development of the skeleton of the gill-arch and of the jaw apparatus connected with it, as it occurs in all Double-nostrils from Fishes to Man. It is true that the primitive modification of the anterior intestine into the gill-intestine, which occurs even in Ascidians, is developed in all Vertebrates from one simple rudiment; and in this respect the gill-openings, which in all Vertebrates and also in Ascidians pierce the wall of the gill-intestine, are quite characteristic. But the external framework of the gills, which in all Skull-less and Single- nostrilled Animals (Acraniota and Monorhina) supports the gill-body, is displaced in all Double-nostrils (Amphi- rhina) by an internal gill-skeleton which replaces the former. DOUBLE-NOSTRILS AND SINGLE-NOSTRILS. Ill This internal gill-support consists of a consecutive series of cartilaginous arches, which are situated between the gill- openings within the wall of the throat (pharynx), and extend round the throat. The foremost of these pairs of gill-arches changes into the jaw-arch (maxillary arch), which gives rise to the upper and lower jaws. A third essential character by which all Double-nostrils are well distinguished from all those lower Vertebrates which we have already considered, is the formation of a blind sac which protrudes from the anterior portion of the intestinal canal, and which in the Fishes becomes the air- filled swimming-bladder (Plate V. Fig. 13, la). As this organ, in proportion as it contains a greater or less quantity of air, or in proportion as this air is more or less compressed, imparts a higher or lower specific gravity to the Fish, it acts as a hydrostatic apparatus. By this means the Fish can rise or sink in the water. This swimming-bladder is the organ from which the lung of higher Vertebrates has developed. The fourth and last main character of Double- nostrils is the presence of two pairs of extremities or members in the primitive arrangement of the embryo ; a pair of fore limbs, which in Fishes are called pectoral fins (Fig. 191, v), and a pair of hind limbs, which in Fishes are called ventral fins (Fig. 191, h). The Comparative Anatomy of these fins is of supreme interest, because they contain the rudiments of all those parts of the skeleton which, in all the higher Vertebrates up to Man, form the skeleton or support of the extremities of the fore and hind limbs. In Skull-less and Single-nostrilled Animals there is, on the contrary, no trace of these extremities. In addition to these four most important main characters of the Amphi- 112 THE EVOLUTION OF MAN. rhina, we might further mention the presence of a sym- pathetic nerve-system, a spleen, a ventral salivary gland ; organs which are not represented in the lower Vertebrates already considered. All these important parts have trans- mitted themselves from Fishes up to Man, and from this circumstance alone it is evident how wide a chasm sepa- rates the Fishes from the Skull-less and Single-nostrilled Animals (Acraniota and Monorhina). Fishes and Man possess all these characters in common (Table X.). Turning now to consider the Fish class in greater detail, we may divide it primarily into three main groups, or sub- classes, the genealogies of which are evident. The first and most ancient group is that of the Primitive Fishes (Selachii), the best-known extant representatives of which are the members of the much-varied orders of Sharks and Rays (Figs. 101, 192). These are followed by a series of further developed Fish forms, by the sub-class of Mucous Fishes (Ganoides). The greater number of these have long been extinct, and only very few living representatives are known ; these are the Sturgeon and Huso of European seas, the Polypterus of African, and the Lepidosteus and Amia of American rivers. The earlier abundance of forms belong- ing to this interesting group is, however, proved by the abundance of their fossil remains. From these Mucous Fishes originated the third sub-class, that of the Osseous Fishes (Teleostei), to which belong most extant Fishes, espe- cially nearly all our river fish. Comparative Anatomy and Ontogeny very clearly show that the Ganoids sprang from the Selachii, just as the Teleostei sprang from the Ganoids. But, on the other hand, a second side-line, or rather the main ascending line of the Vertebrate tribe, EMBRYOS OF SHARKS 113 Fig. 191. Fig. 192. ii4 THE EVOLUTION OF MAN. Fig. 191. — Embryo of a Shark (Scymnus lichia), seen from ventral side : v, pectoral fins (in front of these five pairs of gill-openings) ; h, ventral fins ; a, anal opening ; s, tail fin ; k, external gill-tufts ; d, yelk-sac (the greater part of this has been removed) ; g, eye ; n, nose ; m, mouth fissure. Fig. 192. — Developed Man-shark (Carcharias melanopterus), seen from the left side : r1 first, r2 second dorsal fin; s, tail fin; a, anal fin; v, pectoral fins ; 7i, ventral fins. developed in another direction from the Primitive Fishes; this line leads upward through the Dipneusta group to the important class of Amphibia. This significant relationship between the three groups of Fishes has been placed beyond all doubt by the re- searches of Gegenbaur on the subject. The lucid discussion on the " systematic position of the Selachii ': which that author inserted in the introduction to his classic study of the "head skeleton of the Selachii/' must be regarded as definitely proving this important" relation.148 In Primitive Fishes {Selachii), however, the scales (skin appendages) and the teeth (jaw appendages) are identical in formation and structure, while in the other two groups of Fishes (Mucous and Osseous Fishes) these organs have already become distinct and differentiated. Moreover, in Primitive Fishes, the cartilaginous skeleton (the vertebral column and the skull, as well as the members) is of the simplest and most primitive nature, of which the bony skeletons of Mucous and Osseous Fishes must be regarded as a modification. It is true that in certain respects (in the structure of the heart and of the intestinal canal) Mucous Fishes fully coincide with Primitive Fishes, and differ from Osseous Fishes. But a comparative review of all the anatomical relations plainly shows that the Mucous Fishes constitute a connecting group between Primitive and MUD-FISHES. 1 1 5 Osseous Fishes. The Primitive Fishes (Selackii) form the most ancient and original group of Fishes. From these, in one direction, all other Fishes have developed ; the Mucous Fishes first, which, at a much later period (in the Jurassic, or the Chalk Period), gave rise to the Osseous Fishes. In another direction, the Primitive Fishes gave rise to the parent-forms of the higher Vertebrates, directly to the Dipneusta, and thus to Amphibians. Regarding the Selachii as forming the eleventh stage in our pedigree, these would be followed by the Dipneusta group as the twelfth stage, and by the Amphibian group as the thirteenth stage. The advance effected in the development of the Mud- fishes (Dipneusta) from the Primitive Fishes is of great mo- ment, and is connected with a very noticeable change, which took place in the beginning of the Palaeozoic, or Primary Period in organic life as a whole. For the very numerous fossil remains of plants and animals which are now known to belong to the first three epochs of the history of the earth — to the Laurentian, the Cambrian, and the Silurian Periods, are exclusively those of aquatic plants and animals. From this palaeontological fact, taken in connection with certain weighty geological and biological considerations, we may infer, with tolerable certainty, that at that time no land animals yet existed. During the whole of the enormous Archizoic Period — during many millions of years — the living population of our globe were all water-dwellers : a very remarkable fact, when it is remembered that this period embraces the larger half of the entire organic history of the earth. The lower animal tribes are even now exclusively, or with very few exceptions, aquatic. But during the Archizoic, or Primordial Epoch, the higher animal tribes u6 THE EVOLUTION OF MAN. continued exclusively adapted to aquatic habits of life. It was not till later that they adopted a land life. The earliest fossils of terrestrial animals occur in the Devonian strata, which were deposited in the beginning of the second great division of the earth's history (the Palaeozoic Epoch). They increase greatly in number in the deposits of the Coal and Permian Periods. Even in these early formations many terrestrial and air-breathing species, both of the Arthro- pod and of the Vertebrate tribe, occur ; while their aquatic ancestors of the Silurian Period breathed nothing but water. This physiologically significant modification of the mode of respiration is the most influential change that affected the animal organism in the transition from water to dry land. In the first place it caused the development of an air-breathing organ, the lung, the water-breathing gills having previously acted as respiratory organs. Simul- taneously, however, it effected a remarkable change in the circulation of the blood and in the organs connected with this ; for these are always most closely correlated with the respiratory organs. In addition to these, other organs also, either in consequence of more remote correlation with the respiratory organs, or in consequence of new adaptations, were more or less modified. Within the Vertebrate tribe it was undoubtedly a branch of the Primitive Fishes (SelacJiii) which, during the De- vonian Period, made the first successful effort to accustom itself to terrestrial life and to breathe atmospheric air. In this the swimming-bladder was especially of service, for it succeeded in adapting itself to respiration of air, and so became a lung. The immediate consequence of this was the modification of the heart and nose. While true Fishes EVOLUTION OF MUD-FISHES. 11/ have only two blind nose-pits on the surface of the head, these now became connected with the mouth-cavity by an open passage. A canal formed on each side, leading directly from the nose-pit into the mouth-cavity, and thus even while the mouth-opening was closed the necessary atmo- spheric air could be introduced into the lungs. While, moreover, in all true Fishes the heart consists simply of two compartments, an auricle, which receives the venous blood from the veins of the body, and a ventricle, which forces this blood through an arterial expansion into the gills, the auricle, owing to the formation of an incomplete partition wall, is now divided into a right and a left half. The right auricle alone now received the venous blood of the body, while the left auricle received the pulmonic venous blood passing from the lungs and the gills to the heart. The simple blood-circulation of the true Fishes thus became the so-called double circulation of the higher Ver- tebrates ; and this development resulted, in accordance with the laws of correlation, in further progress in the structure of other organs. The vertebrate class, which thus first adapted itself to the habit of breathing air, and which originated from a branch of the Selachii, are called Mud-fishes (Dipneusta), or Double-breathers, because, like the lowest Amphibia, they retain the earlier mode of breathing through the gills, in addition to the newly acquired lung-respiration. This class must have been represented by numerous and diverse genera during the Palaeolithic Epoch (during the Devonian, Carboniferous, and Permian Periods). As, however, the skeleton is soft and cartilaginous, like that of the Selachii, they naturally left no fossil remains. The hard teeth of nS THE EVOLUTION OF MAN. single genera (Ceratodus) could alone endure; these occur, for instance, in the Trias. At the present time there are only three extant genera of this whole class : Protopterus annectens, in the rivers of tropical Africa (White Nile, Niger, Quillimane, etc.) ; Lepidosiren paradoxa, in tropical South America (in the tributaries of the Amazon) ; and Ceratodus Fosteri, in the swamps of Southern Australia (Plate XII.).149 This wide distribution of the three isolated descendants of the class is alone sufficient to prove that they are the last remnants of a group which was formerly very widely developed. The whole structure of their bodies shows that the group to which they belong forms the transition between Fishes and Amphibia. The direct tran- sitional structure between the two classes is so clearly expressed in the whole organization of these curious animals, that zoologists yet dispute whether the Dipneusta are Fishes or Amphibia. Some well-known zoologists still class them among Amphibia, while they are usually placed among Fishes. In fact, the characters of both the classes are so united in the Dipneusta that the answer to the question as to their nature depends entirely upon the mean- ing attached to the terms " Fish " and " Amphibian." In their mode of life they are true Amphibia. During the tropical winter, in the rainy season, they swim in the water like Fishes and inhale water through the gills. During the dry season they burrow in the mud as it dries up, and during that period breathe air through lungs, like Am- phibians and higher Vertebrates. In this two-fold respira- tion they do, it is true, coincide with the lower Amphibia, and stand far above Fishes. Yet, in most other characters they more nearly resemble the latter, and stand below the EXTANT MUD-FISHES. II9 former. Their external appearance is entirely like that of Fishes. The head of the Dipneusta is not distinct from the trunk. The skin is covered with large fish-scales. The skeleton is soft, cartilaginous; its development has been arrested at a very low stage, just as in the lower Primitive Fishes. The notochord is retained entire. The two pairs of limbs are very simple fins of primitive structure, like those of the lowest Primitive Fishes. The structure of the brain, of the intestinal tube, and the sexual organs, is also as in Primitive Fishes. The Dipneusta, or Mud-fishes, have, there- fore, by heredity, accurately retained many features of a lower organization derived from our primeval Fish ancestors, while their adoption of the habit of breathing air through lungs introduced a great advance in the vertebrate organi- zation. Moreover, the three extant Mud-fishes differ a good deal from one another in important points of structure. The Australian Mud-fish (Ceratodus), which was first described at Sidney in 1870 by Gerard KrefFt, and which attains a length of six feet, appears in an especial degree to represent a primaeval and very conservative animal form (Plate XII.). This is especially true of the structure of its simple lung, and of its fins, which contain a pinnate skeleton. In the African Mud-fish (Protopterus), on the contrary, and in the American form (Lepiclosiren) the double lung is present, as in all higher Vertebrates ; nor is the fin-skeleton pinnate. In addition to the internal gills, Protopterus has also ex- ternal gills, which are wanting in Lepidosiren. Those unknown Dipneusta, which were among our direct ancestors, and which formed the connecting link between the Selachii ( 120 ) TABLE XX. Systematic Survey of the Phylogenetic Classification of Vertebrates. I. JSituIWcss (Acrania), or &ubc4jcartcu (Leptocardia) . Vertebrates without a specialized head, skull, brain, or centralized heart. 1. SkltlMfSS Acrania I. Tube-hearted Leptocardia 1. Lancelets 1. Amphioxida II. Animals fotth. skulls (Craniota) and with centralis fjrarts (Pachycardia). Vertebrates with specialized head, with skull and brain, and with a centralized heart. Main-classes of the Skulled Animals. Classes of the Skulled Animals. Sub-classes of the Skulled Animals. Systematic Name of the Sub-classes. 2. &tttfl[lc= ^ostrillcu Monorhina 3. ^on= amnionate Anamnia 4. Emmons Animals Anmiota IT. Round mouths Cyclostoma III. Fishes Pisces I IV. Mud-fishes \ Dipneusta V. Batrachians Avyihibia J \ VI. Reptiles Reptilia 2. 3. VII. Birds Axes VIII. Mammals Mammalia' Hags, or Mucous Fish Lampreys Primitive Fish Ganoid Fish Osseous Fish Single-lunged Double-lunged Mailed Batra- chians Naked Batra- chians Lizards Snakes Crocodiles Tortoises Sea-dragons Dragons Flying Reptiles Beaked Animals Long-tailed Fan-tailed Bush-tailed Cloacal Animals Pouched Ani- mals Placental Ani- mals 2. Hyperotreta (Myxinoida) 3. Hyperoartia (Petromyzontia) 4. Selachii 5. Ganoides 6. Teleostei 7.Monopneumones 8. Dipneumones 9. Phractamphibia 10. Lissamphibia 11. Lacertilia 12. Ophidia 13. Crocodilia 14. Chelonia 15. Halisauria 16. Dinosauria 17. Pterosauria 18. Anomodonta 19. Saururas 20. Carinataa 21. Ratita? 22. Monotrema 23. Marsupialia 24. Placentalia ( 121 ) TABLE XXI. Pedigree of Vertebrates. (Cf. Plate XV.) 8. fflammals Mammalia Osseous fish Teleostei Ganoid fish Ganoides Single-lunged Monopnenmones Double-lunged Bipneumones 7. Birds Aves I 6. Reptiles Iteptilia Y Mud-fish Protopteri SfotmonsUnimals Amniota I 5. 35atrachtan« Amphibia 4. fHiM'sh Dipneusta Primitive fishes Selachii 3. Fishes Pisces Douiilr^nostrillclJ Amphirhina Lampreys Hags Petromxjzontes Myxinoides 2. Round-mouths Cylostoma \_ Sea-squirts Ascidice iSmglfsnostrtllrtJ Monorhina Jokullcti^nimals Craniota 1. Tube-hearted Leptocardia Sea-barrels Thaliacea iHantlctJ Animals Tunicata BfaxlhUzs Acrania Vertebrates Vertebrata Cljortia^ntmals Chordoma Worms Vermes 122 THE EVOLUTION OF MAN. and the Amphibians, were doubtless in many respects different from their three direct descendants of the present time, but in the most essential characters they must have coincided with the latter. Unfortunately, the germ-history of the three surviving Mud -fishes is as yet entirely un- known; probably at some future time it will afford us further important information as to the tribal history of the lower Vertebrates and so of our ancestors. Very important information of this kind has been supplied by the next Vertebrate class, that of the Batra- chians (Amphibia), which is directly connected with the Dipneusta, from which it originated. To this class belong the Axolotl, Salamanders (Plate XIII.), Toads, and Frogs. Formerly, after the example of Linnaeus, all Reptiles (Lizards, Snakes, Crocodiles, and Tortoises) were also classed among Amphibia. But these animals are of a far higher organiza- tion, and in the most important characters of their ana- tomical structure are more nearly allied to Birds than to Amphibians. The true Amphibia, on the other hand, are more nearly allied to the Double-breathers and to Primitive Fishes : they are also much older than Reptiles. Even as early as the Carboniferous Period numerous very highly developed Amphibia (some of large size) were extant, whereas the earliest Reptiles first appear only towards the close of the Permian Period. In all probability the Amphibia were developed from Double-breathers at an even earlier period — during the Devonian Period. The extinct Amphibia, of which fossil remains have been preserved from that most ancient Primaeval Epoch — and these are especially numerous in the Trias — were distinguished by a large bony coat of mail overlying the skin (like that of the Crocodile), while most BATRACHIANS. 123 of the yet extant Amphibians have a smooth and slippery skin. The latter, also, are on an average smoother than the former, and must be regarded as their stunted posterity. Among the Amphibia of the present time we are, therefore, unable to find any forms that are directly referable to the pedigree of the human race, or that are to be re- garded as ancestors of the three higher Vertebrate classes ; yet, in important points of their internal anatomical struc- ture, and especially in their germ-development, they cor- respond so closely with us, that we are justified in affirming that between the Double-breathers (Bipneusta) on the one hand, and the three higher Vertebrate classes (grouped together as Amniota) on the other, there existed a series of extinct intermediate forms which, if we had them before us, we should class among Amphibia. The whole organization of the extant Amphibia represents a transitional group of this kind. In the important matters of respiration and circulation of the blood, they are still closely allied to the Double-breathers, although in other respects they rise above the latter This is especially true with respect to the ad- vanced structure of their limbs or extremities. The latter here for the first time appear as feet with five digits. The thorough researches of Gegenbaur have shown that the fins of Fishes, concerning which very erroneous views were pre- viously held, are feet with numerous digits ; that is to say, the several cartilaginous or osseous rays, many of which occur in every Fish-fin, correspond to the fingers or digits on the limbs of higher Vertebrates. The several joints of each ray correspond to the several joints of each digit. In the Double- breathers the fin yet retains the same structure as in Fishes, and it was only gradually that the five-toed form of foot, 124 THE EVOLUTION OF MAN. which occurs for the first time in Amphibians, was developed from this multi-digitate form. This reduction in the number of the digits from ten to five occurred in those Dipneusta which must be regarded as the parent-forms of the Amphibia, probably as early as the latter half of the Devonian Period — or, at latest, in the immediately subsequent Carboniferous Period. Several fossil Amphibia with five digits have already been found in the strata of the latter period. Fossil foot- prints of the same animals are very numerous in the Trias (Chevotheriuri). The great significance of the five digits depends on the fact that this number has been transmitted from the Amphibia to all higher Vertebrates. It would be impossible to discover any reason why in the lowest Amphibia, as well as in Reptiles and in higher Vertebrates up to Man, there should always originally be five digits on each of the anterior and posterior limbs, if we denied that heredity from a common five-fingered parent-form is the efficient cause of this phenomenon : heredity can alone account for it. In many Amphibia, certainly, as well as in many higher Vertebrates, we find less than five digits. But in all these cases it can be shown that separate digits have retrograded, and have finally been completely lost. The causes which effected the development of the five- fingered foot of the higher Vertebrates in this Amphibian parent-form from the many-fingered foot, must certainly be found in the adaptation to the totally altered functions which the limbs had to discharge during the transition from an exclusively aquatic life to one which was partially terrestrial. While the many-fingered fins of the Fish had previously served almost exclusively to propel the body FINS AND FINGERS. 1 25 through the water, they had now also to afford support to the animal while creeping upon land. This effected a modification both of the skeleton and of the muscles of the limbs. The number of fin rays was gradually lessened, and was finally reduced to five. These five remaining rays now, however, developed more vigorously. The soft cartilaginous rays became hard bones. The rest of the skeleton also became considerably more firm. The move- ments of the body became not only more vigorous, but also more varied. The separate portions of the skeleton system, and consequently those of the muscular system also, became more and more differentiated. Owing to the intimate correlation of the muscular to the nervous system, the latter also naturally made marked progress in point of function and structure. We therefore find that the brain is very much more developed in the higher Amphibia than in Fishes, in Mud-fis*hes, and in the lower Amphibia. The organs which are most modified in consequence of an amphibious mode of life are, as we have already seen in the Double-breathers (Dipneusta), those of respiration and of the circulation of the blood. The first advance in organization necessitated by the transition from aquatic to terrestrial habits of life was, of course, the formation of an air-breathing organ, a lung. This developed directly from the swimming-bladder which these animals had inherited from the Fishes. At first the function of this organ would be quite subordinate to the more ancient organ, used for the respiration of water, the gills. Hence we find that the lowest Amphibia, the Gilled Amphibia, like the Dipneusta, spend the greater part of their lives in the water, and that accordingly they breathe water through gills. It is only 126 THE EVOLUTION OF MAN. for brief intervals that they rise to the surface of the water or creep out of the water on to the land ; and at these times they breathe air through lungs. Some, however, of the Tailed Amphibians, the Axolotl and the Salamander, live exclusively in the water only when young, and afterwards usually remain on land. In the adult state they breathe only air through lungs. This is also the case with the most highly developed Amphibians, the Frog-amphibia (Frogs and Toads) ; some of the latter have even entirely lost the gilled larval form.153 The same is true of a few small snake-like Amphibia, the Ciecilise, which, like earth-worms, live in the ground. The high degree of interest attached to the natural history of the Amphibian class is especially due to the fact that they hold a position exactly intermediate between the higher and the lower Vertebrates. While the lower Am- phibia are in their whole organization directly allied to the Dipneusta and the Fishes, living mostly in the water and respiring water through gills, the higher Amphibia are no less directly related to the Amnion Animals, for, like the latter, they live mostly on land, and breathe air through lungs. But when young the higher forms resemble the lower, and only attain their own higher degree of development after undergoing complete modification. The* individual germ-history of most higher Amphibians still accurately reproduces the tribal history of the whole class ; and the various stages of modification which were necessitated in certain low Vertebrates by the transition from aquatic to terrestrial habits during the Devonian or Carboniferous Period, are still to be seen every spring in each Frog as it develops from the egg in our ditches and pools. IMPORTANCE OF AMPHIBIA. 127 Like the Tailed Salamanders (Fig. 193), each common Frog emerges from the egg in a larval form, totally different from that of the full-grown Frog (Fig. 194). The short Fig. 193. — Larva of Spotted Land-Newt (Salamandra rnaculata), from the ventral side. In the centre a yelk-sac yet protrudes from the intestine. The external gills are prettily branched and tree-like. The two pairs of limbs are yet very small. Fig. 194. — Larva of the Common Grass-Frog (Rana temporaria), a so- called tadpole : m, mouth ; n, a pair of suction cups used in clinging to stones ; d, skin-fold, which gives rise to the gill-roof ; behind are the gill-openings, from which the gill branches protrude ; s, tail-muscles ; /, skin-fold of the tail, forming a float. trunk is produced into a long tail, which in form and struc- 29 128 THE EVOLUTION OF MAN. ture resembles the tail of a Fish (s).. At first it has no limbs. Respiration is accomplished solely by gills, which are at first external (Jc) and afterwards internal. Corre- spondingly, the heart is also of the same form as in the Fishes, and consists of only two compartments — an auricle, which receives the venous blood of the body, and a ven- tricle, which drives it through the arterial bulb into the gills. Numbers of these fish-like Frog larvae, or " tadpoles," as they are called, swim about every spring in all ponds and pools, using their muscular tails for propulsion, just as is done by Fishes and larval Ascidians. The remarkable transformation of the fish-like form into that of the Frog: does not take place till after the tadpole has grown to a certain size. From the throat grows a closed sac which develops into a pair of large sacs; these are the kings. The simple chamber of the heart is divided into two auricles, owing to the formation of a partition wall, and simul- taneously considerable changes of structure occur in the main arterial trunks. Previously all the blood passed from the heart-chamber through the aorta arches into the gills ; but only part of it now passes to the gills, while another part passes through the newly formed lung arteries into the lungs. From the lungs arterial blood returns into the left auricle of the heart, while the venous blood of the bodv collects in the right auricle. As both of the auricles open into the simple ventricle, the latter contains mixed blood. The fish-like form has now passed into the Dipneusta form. During the further course of modification the gills, with the gill- vessels, are entirely lost, and respiration is now per- formed by the lungs alone. Yet later, the long tail is also GILLED BATRACHIANS. 1 29 rejected, and the Frog now leaps about on the land on legs which have sprouted in the mean time.150 This remarkable metamorphosis of the higher Amphibia is very instructive in its bearing on Man's ancestral history, and is especially interesting owing to the fact that the various groups of extant Amphibia have remained stationary at various stages of their tribal history, which, in accord- ance with the fundamental law of Biogeny, are reproduced in this germ-history. First, there is a very low order of Amphibia, the Gilled Batrachians (Sozobranchia), which, like Fishes, retain their gills throughout life. To this order belong, among others, the well-known blind " Olm " of the Adelsberg Cave (Proteus anguineus), the Mud-eel of South Carolina (Siren lacertinct), and the Axolotl of Mexico (Sire- don pisciformis ; Plate XIII. Fig. 1). All these Gilled Batrachians are fish-like animals with long tails, and in point of respiratory organs and of circulation of the blood they remain throughout life stationary at the Dipneusta stage. They possess both gills and lungs, and can either respire water through the gills or air through the lungs, as occasion requires. In another order, the Salamanders, the gills are lost during metamorphosis, and in the adult state air only is breathed through lungs. This order bears the name of Tailed Batrachians (Sozura) because they retain the tail throughout life. To this order belong the common Water- Newts (Triton) which swarm in all ponds during the summer, and the black, yellow-speckled Land-Salamanders (Salamandra) found in damp woods (Plate XIII. Fig. 2). The latter are among the most remarkable of our indigenous animals, sundry anatomical characters proving them to be very ancient and highly conservative Vertebrates.151 A 130 THE EVOLUTION OF MAN. few Tailed Batrachians retain the gill-opening in the side of the neck, though the gills themselves are lost (Meno- poma). If the larvre of the Salamanders (Fig. 193) and Tritons are compelled to remain in water, and not allowed to get on land, they may, under favourable conditions, be made to retain their gills. In this fish-like condition they become sexually mature, and will throughout life remain compulsorily in the lower stage of development of the Gilled Batrachians. The opposite experiment was made some years ago in the case of the Mexican Gilled Batra- chian, the fish-like Axolotl (Siredon pisciformis ; Plate XIII. Fig. 1). This animal had previously been regarded as a permanent Gilled Batrachian, remaining throughout life in this fish-like condition. But of the hundreds of these animals kept in the Jardin des Plantes at Paris, a few individuals, for some unknown reason, crept to land, lost their gills, and changed into a form closely allied to that of the Salamander (Ambly stoma, Fig. 2). In this state they became sexually mature.152 This phenomenon, which at first excited a lively interest, has since been repeatedly observed with care. Zoologists regarded the fact as some- thing peculiarly wonderful, though each spring every common Frog and Salamander passes through the same modification. In these animals we can in the same wav follow each step in the significant metamorphosis of the aquatic and gill-respiring animal into the terrestrial and lung-respiring animal. That which thus takes place in the individual during germ-evolution, took place in the same way in the whole class during the course of its tribal history. The metamorphosis which takes place in the third order haeckel's evolution of man. PLATE XI T. w:m i^.OT %l 7Jw?.' ^.lO'1 ti$ a#H A.I. K? &<; #1 j^i; ml lj;W.' i, ,\J_ ■ .11 IP A, -i; it ,;4« Sp He. I K-V^ i'fi; a |N^ Grafocfus Ibrsteri HA.ECKELS EVOLUTION OF MAN. PLATE XIII. Fy.2 '. lull Fig. i. Siredon pisciformis. Fig. 2. Salamandra maculata. TAILED BATRACHIANS AND FROG BATRACHIANS. 131 of Amphibia, the Frog Batrachians (Batrachia, or Anura), is yet more complete than in the Salamanders. To these belong all the various kinds of Toads, Water-frogs, Tree- frogs, etc. In the course of transformation these lose not only the gills, but also the tail, which drops off in some cases earlier, in others later. In this respect the various species differ somewhat from one another. In most Frog Batrachians the larv?e drop the tail very early, and the tail-less frog-like form subsequently grows considerably larger. Other species, on the contrary, as, for instance, the Pseudes paradoxus of Brazil, as also an European Toad (Pelo- batesfuscus) remain for a very long time in the fish form, and retain a lengthy tail till they have almost attained their full size; hence, after their metamorphosis is com- pleted, they appear much smaller than before. The opposite extreme is seen in some Frogs but recently brought under notice, which have lost the whole of their historic meta- morphoses, and in which no tailed and gillecl larva emerges from the egg, but the perfect Frog, without tail or gills. These Frogs inhabit isolated oceanic islands, the climate of which is very dry, and which are often for a con- siderable length of time without fresh water. As fresh water is indispensable for gill-respiring tadpoles, these Frogs have adapted themselves to this local deficiency and have entirely relinquished their original metamorphosis, e.g., Hylodes martinicensis.153 The ontogenetic loss of gills and tail in Frogs and Toads can of course only be phylogenetically explained as owing to the fact that these animals have descended from long- tailed salamander-like Amphibians. This is also proved beyond doubt by the Comparative Anatomy of the two 132 THE EVOLUTION OF MAN. groups. This remarkable transformation is, in other respects also, of general interest, as throwing a flood of light upon the Phylogeny of the Tail-less Apes and of Man. Man's ances- tors were also long-tailed gill-breathing animals, resembling Gilled Batrachians, as is irrefutably demonstrated by the tail and the gill arches in the human embryo. During the Palaeozoic Epoch, and probably in the Car- boniferous Period, there is no doubt that the Amphibian class embraced a series of forms which must be regarded as direct ancestors of Mammals, and so of Man. On grounds derived from Comparative Anatomy and Ontogeny, we must not, however, look for these Amphibian ancestors of ours — as might perhaps be supposed — among the Tail-less Frog Batrachians, but only among the lower Tailed Amphibians. We can with certainty point to at least two extinct Batra- chian forms as direct ancestors of Man, as the thirteenth and fourteenth stages in our pedigree. The thirteenth ancestral form must have been closely allied to the Double- breathers (Dipneusta), must, like these, have possessed per- manent gills, but must have been already characterized by having five digits on each foot ; and were they still living we should place them in the group of Gilled Batrachians, with the Proteus and the Axolotl (Plate XIII. Fig. 1). The fourteenth ancestral form, on the other hand, must indeed have retained the long tail, but must have lost the gills, and hence the nearest allied forms among extant Tailed Batra- chians would be the Water-Newts and Salamanders (Plate XIII. Fig. 2). Indeed, in the year 1725 the fossil skeleton of one of these extinct Salamanders (closely allied to the present giant Salamander of Japan) was described by the Swiss naturalist, Scheuchzer, as the skeleton of PRIMITIVE AMNION ANIMALS. 1 33 a fossil Man dating from the Deluge ! (" Homo diluvii testis."154) As the vertebrate form occurring in our pedigree imme- diately after these Batrachian ancestors — and, therefore, as the fifteenth stage — let us now examine a lizard-like animal, of which no fossil remains have been obtained, and which is not even proximately represented in any extant animal form, but the former existence of which we may infer with the utmost certainty from certain comparative anatomical and ontogenetical facts. This important animal form we will call the Protamnion, or Primitive Amniotic animal. All Vertebrates higher than the Amphibia — that is, the three classes of Reptiles, Birds, and Mammals — are so essentially distinct in their whole structure from all the lower Verte- brates which we have as yet considered, and, on the other hand, have so much in common, that we may class them together in one group as Amnion Animals (Amniota). It is only in these three classes of animals that we find that remarkable envelope of the embryo known as the amnion. (Cf. vol. i. p. 386.) The latter must probably be regarded as a kenogenetic adaptation, as caused by the sinking of the embryo into the yelk-sac.155 All known Amnion Animals, all Reptiles, Birds, and Mammals (Man included), coincide in so many important points of organization and development that we are fully justified in asserting their common descent from a single parent form. If the testimony of Comparative Anatomy and Ontogeny is entirely unquestionable in any point, it is certainly so here. For all the special peculiarities and characters, which appear accompanying and following the formation of the amnion, and which we found in the 134 THE EVOLUTION OF MAN. development of the human embryo; all the many peculiari- ties in the development of the organs which we shall presently notice in detail ; and, finally, the chief special arrangements of the internal structure of the body in all fully developed Amnion Animals; all these so clearly demon- strate the common origin of all Amnion Animals from a single extinct parent-form, that it is impossible to conceive their origin as polyphyletic, and that they originated from several independent parent-forms. This unknown common parent-form is the Primitive Amnion Animal {Protam- nion). In external appearance the Protamnion was most probably an intermediate form between the Salamanders and the Lizards. It was probably during the Permian Period that the Protamnion originated ; perhaps at the beginning, perhaps at the close of that period. This we know from the fact that the Amphibia did not attain their full development till the Carboniferous Period, and that toward the close of the Permian the first fossil Reptiles make their appearance — or, at least, fossils (Protevosauras, Mhopalodori) which must in all probability be referred to lizard-like Reptiles. Among the great and pregnant modifications of the vertebrate organization determined during this period by the develop- ment of the first Amnion Animals from salamander-like Amphibians, the three following are especially important : the total loss of water-breathing gills and modification of the gill-arches into other organs ; the formation of the allantois, or primitive urinary sac ; and, finally, the develop- ment of the amnion. The total loss of the respiratory gills must be regarded as one of the most prominent characters of all Amnion EVOLUTION OF AMNION ANIMALS. 1 35 Animals. All these, even such as live in the water, e.g., whales, respire only air through lungs, never water through gills. While all Amphibians, with very few exceptions, in the young state retain their gills for a longer or shorter period, and breathe through gills for some time (if not alwa}^s), from this point gill-respiration entirely ceases. Even the Protamnion must have entirely ceased to breathe water. The gill-arches, however, remain, and develop into very different organs (partly rudimentary) ; into the various parts of the tongue-bone, into certain portions of the jaw apparatus, the organ of hearing, etc. But no trace of gill-leaves, of real respiratory organs on the gill-arches, are ever found in the embryo of Amnion Animals. With this total loss of the gills is probably connected the formation of another organ, which we have already described as occurring in human Ontogeny ; this is the allantois, or primitive urinary sac. (See vol. i. p. 379.) In all probability the urinary bladder of the Dipneusta is to be regarded as the first beginning of the allantois. Even in the American Mud-fish (Lepidosiren) we find an urinary bladder, which grows from the lower wall of the posterior extremity of the intestine, and serves as a receptacle for the renal secretions. This organ has been inherited by the Amphibia, as may be seen in any Frog. But it is only in the three higher Vertebrate classes that the allantois attains a special development ; in these it protrudes at an early period from the body of the embryo, forming a large sac filled with liquid, and traversed by a considerable number of large blood-vessels. This sac also discharges a portion of the nutritive functions. In the higher Mammals and in Man the allantois afterwards forms the placenta. 136 THE EVOLUTION OF MAN. The formation of the amnion and the allantois, together with the total loss of the gills and the exclusive adoption of lung-respiration, are the most important characters by which all Amnion Animals are distinguished from the lower Vertebrates which we have been considering. In addition to these there are a few subordinate characters which are constantly inherited by Amnion Animals, and are altogether wanting in animals without an amnion. One striking em- bryonic character of the Amnion Animals is the great curva- ture of the head and neck of the embryo. In the Anamnia the embryo is from the first either nearly straight, or else the whole body is bent in a sickle-shaped curve corre- sponding to the curvature of the yelk sac, to which the embryo is attached by its ventral surface; but there are no marked angles in the longitudinal axis (Plate VI. Fig. F). In all Amnion Animals, on the contrary, the body is very noticeably bent at an early age, so that the back of the embryo is much arched outwards, the head pressed almost at right angles against the breast, and the tail inclined on to the abdomen. The tail extremity, as it bends inwards, approaches so near to the frontal side of the head, that the two often nearly touch (Plates VI. and VII.). This striking triple curvature of the embryonic body, which has already been considered when we studied the Ontogeny of Man, and in which we distinguished the skull-curve, neck-curve, and tail-curve (vol. i. p. 371), is a characteristic peculiarity common to the embryos of all Reptiles, Birds, and Mammals. But in the formation of many internal organs also, an advance is observable in all the Amnion Animals which ranks them above the highest of the non-amnionate forms. Above all, a partition wall forms MAN AS AN AMNION ANIMAL. 1 37 within the simple ventricle of the heart, dividing it into a right and a left ventricle. In connection with the complete metamorphosis of the gill-arches, a further development of the organ of hearing takes place. A considerable advance is also noticeable in the development of the brain, the skele- ton, the muscular system, and other parts. Finally, the reconstruction of the kidneys must be regarded as a most important modification. In all the lower Vertebrates as yet considered, we have found the primitive kidneys, which appear very early in the embryos of all higher Vertebrates up to Man, acting as a secretory or urinary apparatus. In Amnion Animals, however, these early primitive kidneys lose their function at an early period of embryonic life, and it is assumed by the permanent " secondary kidneys," which grow out of the terminal portion of the primitive kidney ducts. Looking back at the whole of these characters of Amnion Animals, it is impossible to doubt that all animals of this group, all Reptiles, Birds, and Mammals, had a common origin, and constitute a single main division of kindred forms. To this division belongs our own race. In his whole organization and germ-history Man is a true Amnion Animal, and, in common with all other Amniota, has descended from the Protamnion. Although this whole group originated at the end, or perhaps even in the middle, of the Palseozoic Epoch, it did not attain its full de- velopment and its full perfection till the Mesozoic Epoch. The two classes of Birds and Mammals then first appeared. Nor did the Reptilian class develop in its full variety until the Mesozoic Epoch, which is, therefore, called the "Age of Reptiles." The unknown and extinct Protamnion, the I38 THE EVOLUTION OF MAN. parent-form of the entire group, must have been very nearly allied to the Reptiles in its whole organization, even though it cannot be regarded as a true Reptile in the present meaning of the term. Of all known Reptiles, certain Lizards are most nearly allied to the Protamnion ; and in the outward form of its body we may imagine the latter as an intermediate form between the Salamander and the Lizard.156 The Comparative Anatomy and Ontogeny of the Am- nionate group clearly explains its genealogy. The group which directly descended from Protamnion gave rise to two divergent branches. The first of these, which will in future receive our whole attention, forms the Mammalian group. The other branch, which assumed an entirely different course of progressive development, and which is connected with the mammalian branch only as the root, is the compre- hensive group constituted by Reptiles and Birds. The two latter forms may be classed together as Monocondylia, or Sauropsides. The common parent-form of these is an extinct lizard-like Reptile. From this, the Serpents, Croco- diles, Tortoises, Dragons, etc. — in short, all the various forms of the Reptilian group — developed in different directions. The remarkable group formed by the Birds also developed directly from an offshoot of the Reptilian group, as is now definitely proved. Down to a late time the embryos of Reptiles and of Birds are yet identical, and even later they are in some respects surprisingly similar. (See Plate VI. Fig. T and G.) In their entire organization the resemblance between the two is so great that no anatomist now denies that the Birds originated from Reptiles. The Mammalian line is connected at its roots with the Reptilian line, but GENEALOGY OF AMNION ANIMALS. 1 39 afterwards diverged entirely from the latter, and developed in an entirely peculiar direction. The highest result of the development of the Mammalian line is Man, the so-called " Crown of Creation." CHAPTER XIX THE PEDIGREE OF MAN. IV. From the Primitive Mammal to tiie Ape. The Mammalian Character of Man. — Common Descent of all Mammals from a Single Parent-form (Promammalian). — Bifurcation of the Am nion Animals into Two Main Lines : on the one side, Eeptiles and Birds, on the other, Mammals. — Date of the Origin of Mammals : the Trias Period. — The Three Main Groups or Sub-classes of Mammals : their Genealogical Relations. — Sixteenth Ancestral Stage : Cloacal Animals (Monotremata, or Ornithodelphia) . — The Extinct Primitive Mammals (Promammalia) and the Extant Beaked Animals (Ornitlw stoma). — Seventeenth Ancestral Stage : Pouched Animals (Marsupialia, or Didel- phia). — Extinct and Extant Pouched Animals. — Their Intermediate Position between Monotremes and Placental Animals. — Origin and Structure of Placental Animals (Placentalia, or Monodelphia) . — Forma- tion of the Placenta. — The Deciduous Embryonic Membrane (Decidua). — Group of the Indecidua and of the Deciduata. — The Formation of the Decidua (vera, serotina, rejlexa) in Man and in Apes. — Eighteenth Stage: Semi-apes (Prosimice). — Nineteenth Stage : Tailed Apes (Meno- cerca). — Twentieth Stage : Man-like Apes (Anthropoides). — Speechless and Speaking Men (Mali. Homines). " A century of anatomical research brings us back to the conclusion of Linnoeus, the great lawgiver of systematic zoology, that man is a member of the same order as the apes and lemurs. Perhaps no order of mammals presents us with so extraordinary a series of gradations as this, leading us insensibly from the crown and summit of the animal creation down to creatures from which there is but a step, as it seems, to the lowest, smallest, and least intelligent of the placental mammalia. It is as if nature herself "man's place in nature." 141 had foreseen the arrogance of man, and with Roman severity had provided that his intellect, by its very triumphs, should call into prominence the slaves, admonishing the conqueror that he is but dust." — Thomas Huxley (1863). Among those zoological facts which afford us points of support in researches into the pedigree of the human race, the position of Man in the Mammalian class is one of the most important and fundamental. Much as zoologists have long disagreed in their opinions as to Man's particular place in this class, and especially in their ideas of his relation to the most nearly related group, that of the Apes, yet no naturalist has ever doubted that Man is a genuine Mammal in the whole structure and development of his body. Every anatomical museum, every manual of Comparative Anatomy, affords proof that the structure of the human body shares all those peculiarities which are common to all Mammals, and by which the latter are definitely distinguished from all other animals. Now, if we examine this established anatomical fact phylogenetically, and in the light of the Theory of Descent, we arrive immediately at the conclusion that Man is of a common stock with all the other Mammals, and springs from a root common to them. The various characteristics in which all Mammals coincide, and in which they differ from all other animals, are, moreover, of such a kind, that a polyphyletic hypothesis appears in a special degree inad- missible in their case. It is inconceivable that all existing and extinct Mammals have sprung from several different and originally separate root-forms. We are compelled, if we in any way acknowledge the Theory of Evolution, to assume the monophyletic hypothesis, that all Mammals, 142 THE EVOLUTION OF MAN. including Man, must be traced from a single common mam- malian parent-form. This long extinct primaeval root-form and its immediate descendants — which differ from each other hardly more than do several species of one genus — we will call Primitive Mammals (Promammalia). As we have already seen, this root-form developed from the ancient parent-form of the Primitive Amnion Animals in a direction wholly different from that followed by the Reptile group, which afterwards gave rise to the more highly developed class of Birds. The differences which distinguish Mammals on the one side, from Reptiles and Birds on the other, are so important and characteristic, that we may quite safely as- sume a bifurcation of this kind in the vertebrate family tree. Reptiles and Birds — which we classed together as Monocon- dylia, or Sauropsida — coincide entirely, for instance, in the characteristic structure of the skull and brain, which is strikingly dissimilar from that of the same parts in Mam- mals. In Reptiles and Birds, the skull is connected with the first cervical vertebra (the atlas) by a single joint-process (condyle) of the occipital bone; in Mammals, on the con- trary (as in Amphibians), the condyle is double. In the former, the under jaw is composed of many parts, and is connected with the skull by a peculiar bone of the jaw (the square bone) so as to be movable ; in the latter, on the contrary, the lower jaw consists of but two bone-pieces, which are directly attached to the temporal bone. Again, the skin of the Sauropsida (Reptiles and Birds) is covered with scales or feathers, that of the Mammals with hair. The red blood-cells of the former are nucleated, those of the latter non-nucleated. The eggs of the former are very large, are provided with a large nutritive yelk, and undergo REPTILES AND MAMMALS. 1 43 discoidal cleavage resulting in a Disc-gastrula ; the eggs of the latter are very small, and their unequal cleavage results in the formation of a Hood-gas trula. Finally, two charac- ters entirely peculiar to Mammals, and by which these are distinguished both from Birds and Reptiles and from all other animals, are the presence of a complete diaphragm, and of the milk-glands (mammce), by means of which the new-born young are nourished by the milk of the mother. It is only in Mammals that the diaphragm forms a transverse partition-wall across the body-cavity (cceloma), completely separating the chest from the ventral cavity. (Cf. Plate V. Fig. 16 0.) It is only among Mammals that the mother nourishes the young with her milk ; and the whole class are well named from this. These important facts in Comparative Anatomy and Ontogeny clearly show that the tribe of Amnion Animals (Amniota) bifurcated from the very first into two main diverging lines ; on the one side, the Reptilian line, from which the Birds afterwards developed ; on the other side, the Mammalian line. The same facts also prove as indu- bitably that Man originated from the latter line. For Man, in common with Mammals, shares all the characteristics we have mentioned, and is distinguished by them from all other animals. And, finally, these facts indicate as certainly those advances in vertebrate structure by which one branch of the Primitive Amnion Animals developed into the parent- form of Mammals. The most prominent of these advances were (1) the characteristic modification of the skull and brain ; (2) the formation of a covering of hair ; (3) the com- plete development of the diaphragm ; and (4) the formation of the milk-glands and the adaptation to the suckling of 144 THE EVOLUTION OF MAN. the young. Intimately connected with these, other im- portant structural modifications gradually occurred, The period at which these important advances, which laid the first foundation of the Mammalian class, took place, may most probably be placed in the first part of the Mesolithic, or Secondary Epoch, in the Triassic Period. For the oldest known fossil remains of Mammals occur in sedimentary rock-strata of the most recent deposits of the Triassic Period, in the upper Keuper. It is possible, indeed, that the parent-forms of Mammals may have appeared earlier (perhaps even at the close of the Palaeo- lithic Epoch, in the Permian Period). But no fossil remains of Mammals belonging to that period are as yet known. Throughout the Mesolithic Epoch, throughout the Triassic, Jurassic, and Calcareous Periods, fossil remains of Mammals are very scarce, and indicate a very limited development of the whole class. During this Mesolithic Epoch, Reptiles play the chief part, and Mammals are of quite secondary importance. It is, however, especially significant and interesting, that all mammalian fossil remains of the Mesozoic Epoch belong to the older and inferior division of Pouched Animals (Marswpialia), a few probably even to the yet older division of the Cloacal Animals (Mono- trema). Among them, no traces of the third and most highly developed division of the Mammals, the Placental Animals, have as yet been found. The last, to which Man belongs, are much more recent, and their fossil remains do not occur till much later — in the succeeding Camolithic Epoch ; in the Tertiary Period. This palaBontological fact is very significant, because it harmonizes perfectly with that order of the development of Mammals which is un- THE THREE MAMMALIAN GROUPS. 1 45 mistakably indicated by Comparative Anatomy and Onto- geny. These show that the whole Mammalian class is divisible into three main groups, or sub-classes, corresponding to three successive stages of phylogenetic evolution. These three stages, which consequently represent three important ancestral stages in the human pedigree, were first dis- tinguished in the year 1816 by the celebrated French zoologist, Blainville, who named them, according to the different structure of the female organs of reproduction, Ornithoclelphia, Bidelphia, and Monodelphia (&A<£u£, which, being interpreted, is uterus). It is not, however, only in the varied structure of the sexual organs that these three classes differ from one another, but in many other respects also, so that we can safely maintain the important phylogenetic statement: The Monodelphia, or Placental Animals, have descended from the Didelphia, or Pouched Animals; and the latter, again, have descended from the Cloacal Animals, or Ornithoclelphia. Accordingly we have now to consider, as the sixteenth ancestral stage in the human pedigree, the oldest and lowest main group of Mammals ; the sub-class of the Cloacal Animals (Monotremata, or Ornithoclelphia). They are so named in consequence of the cloaca, which they have in common with the other lower Vertebrates. This so-called cloaca is the common excretory channel for the excrement, the urine, and the sexual products (Fig. 827). For, in these Cloacal Animals, the urinary duct and the sexual canals yet open into the posterior parts of the intestine, while in all other Mammals they are wholly separated from the rectum and anus, and open by a special orifice (porus 146 THE EVOLUTION OF MAN. urogenitatis). The urinary bladder in the Monotremes also opens into the cloaca, and is separate from the two urinary ducts (Fig. 327, vo) ; in all other Mammals the latter open directly into the urinary bladder. The structure of the milk-glands, by means of which all Mammals suckle their new-born young for a time, is also quite peculiar in the Cloacal Animals. In them the milk-gland has no nipple which the young animal can suck ; there is only a peculiar sieve-like place in the skin, perforated with holes through which the milk passes out, and from which the young animal has to lick it. For this reason they have also been called Nipple-less Mammals (Amasta). Again, the brain of the Cloacal Animals has remained at a much lower stage of development than that of any other Mammal. The fore-brain, or cerebrum, is so small that it does not overhang the hind- brain, or cerebellum. In the skeleton (Fig. 196), the structure of the shoulder girdle, as well as of other parts, is remarkable, differing entirely from the same part in other Mammals, and resembling rather those of the lower Vertebrates, especially Reptiles and Amphibians. Like the latter, the Cloacal Animals have a well-developed coracoid bone (coracoideum), a strong bone uniting the shoulder-blade with the breast-bone. In all other Mammals the coracoid bone (as in Man) has degene- rated, has coalesced with the shoulder-blade, and appears only as an insignificant process of the latter. These and many other less striking peculiarities prove beyond doubt that the Cloacal Animals occupy the lowest rank among Mammals, and represent a direct intermediate form between the Protamnia and other Mammals. All these marked Am- phibian characters must have been present in the parent EXTANT CLOACAL ANIMALS. 147 form of the whole vertebrate class, in the Primitive Mammal, by which they must have been inherited from the Primitive Amnion Animals. During the Triassic and Jurassic Periods, the sub-class of the Cloacal Animals seems to have been represented by many Primitive Mammals of very varied form. At present it is represented only by two isolated members, which are grouped together as the Beaked Animal family (Orni- thostoma). Both of these are confined to Australia and the neighbouring island of Van Diemen's Land, or Tasmania ; both are becoming less numerous year by year, and will soon be classed, with all their blood relations, among the extinct animals of our globe. One of these forms passes its life swimming about in rivers, and builds subterranean dwellings on the banks: this is the well-known Duck- billed Platypus {Omithorhynchas 'paradoxus) : it is web- footed, has a thick, soft skin, and broad, flat jaws, which very much resemble a duck's bill (Figs. 195, 196). The other form, the Porcupine Ant-eater {Echidna hystvix), much resembles the Ant-eaters, in its mode of life, in the cha- racteristic form of its slender snout, and in the great length of its tongue ; it is covered with prickles, and can roll itself up into a ball like a hedgehog. Neither of these extant Beaked Animals possesses true bony teeth, and, in this point, they resemble the Toothless Mammals {Edentata). The absence of teeth, together with other peculiarities of the Ornithostomata, is probably the result of comparatively recent adaptation. Those extinct Cloacal Animals which embraced the parent-forms of the whole Mammalian class, the Promammalia, must certainly have been provided with a developed set of teeth, inherited from Fishes.157 Some 148 THE EVOLUTION OF MAN. Fig. 195.— The Duck-billed Platy- pus (Ornitliorhynchus paradoxus). Fig. 196.— Skeleton of Platypus. POUCHED ANIMALS. 1 49 small single molars, found in the uppermost strata of the Keuper formation in England and Wtirtemberg, and which are the oldest known vertebrate remains, probably belong to these primseval Promammalia. These teeth, by their form, indicate species that lived on insects ; the species has been called Microlestes antiquus. Teeth belonging to another closely allied Primitive Mammal (Bromatherium silvestre) have recently been discovered in the North American Trias. On the one hand, the still extant Beaked Animals, and, on the other, the parent-forms of the Pouched Animals (Mar- supialia, or Diclelphia), must be regarded as representing two distinct and divergent lines of descent from the Pro- mammalia. This second Mammalian sub-class is very interesting as a perfect link between the two other sub- classes. While the Pouched Animals, on the one side, retain many of the characters of the Cloacal Animals, they also, on the other, possess many placental characters. A few characters are quite peculiar to Pouched Animals alone ; such, for instance, is the structure of the male and female sexual organs, and the form of the lower jaw The dis- tinctive feature of the latter in these Pouched Animals is a peculiar hook-shaped bony process, passing inward hori- zontally from the angle of the lower jaw. As neither Cloacal Animals nor Placental Animals have this process, this structure is alone sufficient to distinguish the Pouched Animals (J\lavswpialia). Nearly all the known mammalian fossils from the Jurassic and Cretaceous formation are lower jaws. Our whole knowledge of numerous mesolithic mam- malia, the former existence of which would otherwise never have been known, is solely derived from their fossilized I50 THE EVOLUTION OF MAN. lower jaws, no fragment of the rest of their bodies having been reserved. According to the logic usually applied to palaeontology by the " exact " opponents of the theory of evolution, the inference drawn from this fact would be that these Mammals had no bones except lower jaws. The remarkable circumstance is, after all, very easily accounted for. The lower jaw of Mammals being a solid and excep- tionally hard bone, but very loosely attached to the skull, it is easily detached from the carcase as the latter is carried down by some river, and, falling to the bottom, is retained in the mud. The rest of the carcase is carried on further, and is gradually destroyed. As all the mammalian lower jaws found, in England, in the Jurassic strata of Stonesfield and Purbeck, exhibit this peculiar process characteristic of the Pouched Animals (Marsupialia), we may infer, from this pal?eontological fact, that they belonged to Marsupials. No Placental Animals appear to have existed during the Mesolithic Epoch. At least no fossil remains, undoubtedly belonging to these and dating from that epoch, are known. The extant Pouched Animals, the most generally known of which are the gramnivorous Kangaroos and the carni- vorous Pouched Eats, display very considerable difference in their organization, in the form of their bodies and in size, and in many respects correspond to the several orders of Placental Animals. The great majority of them live in Australia, in New Holland, and in a few of the Australian and South Asiatic islands; some few species occur in America. On the other hand, there is no longer a single indigenous Pouched Animal on the continents of Asia, of Africa, or of Europe. The case was very different during the Mesolithic, and also during the earlier Qenolithic Epochs. EXTANT POUCHED ANIMALS. I 5 I The Neptunian deposits of these epochs in all quarters of the globe, and even in Europe, contain abundant marsupial remains in great variety, some of them being of very large size. From this we may infer that the extant Pouched Animals are but the last remnant of a group which was once much more widely developed, and which was dis- tributed over the whole surface of the globe. During the Tertiary Period, these succumbed in the struggle for life with the stronger Placental Animals, and the survivors were gradually driven back by the latter into their present restricted area. From the Comparative Anatomy of the extant Pouched Animals, very important conclusions may be drawn as to their phylogenetic intermediate position between Cloacal Animals and Placental Animals. The incomplete develop- ment of the brain, especially of the fore-brain {cerebrum), the possession of marsupial bones (ossa marsupialia), the simple structure of the allantois (which does not as yet develop a placenta), with many other characters, have been inherited by the Pouched Animals from Cloacal Animals. On the other hand, they have lost the independent coracoid. bone (os coracoideiim) attached to the shoulder girdle. A more important step consists in the fact that a cloaca is no longer formed ; the cavity of the rectum, together with the anal opening, is separated by a partition wall from the urinary and sexual opening (sinus urogenitalis). Moreover, all Pouched Animals develop special nipples on the milk-glands, which are sucked by the young after birth. These nipples project into the cavity of a pouch, or marsupium, in the ventral side of the mother. This pouch is supported by a couple of marsupial bones. In it the young, which are 30 152 THE EVOLUTION OF MAN. born in a very imperfect condition, are carried by the mother for a long time ; until, in fact, they are completely developed (Fig. 197). In the large Giant Kangaroo, which Fig. 197.— The Crab-eating Pouched Eat (Philander cancrivorus). female with two young in its pouch. (After Brehm.) A THE POUCHED ANIMALS AS ANCESTORS OF MAN. 1 53 attains the height of a man, the embryo develops in the uterus but for a month ; it is then born in a very incomplete condition, and attains all its further development in the mother's pouch, where, for about nine months, it remains attached to the milk-glands. All these and other characters (especially the peculiar structure of the internal and external sexual organs of the male and female) clearly show that the whole sub-class of the Pouched Animals (Marsupialia) are a single group, which originated from the promammalian branch. From a branch of these Pouched Animals (perhaps from several branches) the parent-forms of the higher Mammals, the Placental Animals, afterwards sprang. Hence we must reckon a whole series of Pouched Animals among the an- cestors of the human race ; and these constitute the seven- teenth stage in the human pedigree.158 The remaining stages of our ancestral line, from the eighteenth to the twenty-second, all belong to the group of Placental Animals (Placentalia). This very highly de- veloped group of Mammals, the third and last, came into the world at a considerably later period. No single known fossil, belonging to any portion of the Secondary or Meso- lithic Epoch, can be referred with certainty to a Placental Animal, while we have plenty of placental fossils dating from every part of the Tertiary or Csenolithic Epoch. From this palaeontological fact we may provisionally infer that the third and last main division of Mammals did not develop from the Pouched Animals until the beginning of the C^enolithic Epoch, or, at the earliest, till the close of the Mesolithic Epoch (during the Chalk Period). In our survey of geological formations and periods (pp. 12, 19) we found 154 THE EVOLUTION OF MAX. how comparatively short this whole Tertiary or Csenolithic Epoch was. Judging from the relative thicknesses of the various strata-formations we were able to say that this whole period, during which Placental Animals first appeared, and assumed their respective forms, amounted at most to about three per cent, of the entire duration of the organic history of the earth. (Cf. p. 18.) All Placental Animals are distinguished from the two lower Mammalian groups already considered, from the Cloacal Animals and Pouched Animals, by many prominent peculiarities. All these characters are present in Man ; a most significant fact. For on the most accurate comparative anatomical and ontogenetical researches, we may base the irrefutable proposition that Man is in every respect a true Placental Animal ; in him are present all those peculiarities in the structure and in the development of the body which distinguish Placental Animals from the lower Mammalian groups, and at the same time from all other animals. Among these characteristic peculiarities the higher develop- ment of the brain, the organ of the mind, is especially prominent. The fore-brain, or large brain (cerebrum) is much more highly developed in these than in lower animals. The body (corpus callosum), which, like a bridge, connects the two hemispheres of the fore-brain, attains its full development only in Placental Animals; in the Pouched Animals and Cloacal Animals it exists merely as an insigni- ficant rudiment. It is true that in their brain structure the lowest of the Placental Animals yet resemble Pouched Animals very nearly; but within the Placental group we can trace a continuous series of progressive stages in the development of the brain, ascending quite gradually from PLACENTAL ANIMALS. 155 the lowest stage to the very highly developed mind-organ of the Monkey and of Man. (Cf. Chapter XX.) The human mind is but a more highly developed ape-mind. The milk-glands of Placental Animals, as of Marsu- pials, are provided with developed nipples; but the pouch in which the immature young of the latter are carried about and suckled is never present in the former. Nor are the marsupial bones (ossa Tnarsupialia) present in Pla- cental Animals ; these bones, which are embedded in the abdominal wall, and rest on the anterior edge of the pelvis, are common to Pouched Animals and Cloacal Animals, ori- ginating from a partial ossification of the tendons of the inner oblique muscle of the abdomen. It is only in a few beasts of prey that insignificant rudiments of these bones are found. The hook-shaped process of the lower jaw, which characterizes Pouched Animals, is also entirely wanting in Placental Animals. The character, however, which especially distinguishes Placental Animals, and which has justly given its name to the entire sub-class, is the development of the placenta, or vascular cake. We have already spoken of this organ, in describing the development of the allantois in the human embryo (vol. i. p. 382). The urinary sac or allantois, that peculiar bladder which grows out of the posterior portion of the intestinal canal, is, we found, formed at an early stage in the human embryo just as in the germs of all other Amnion Animals. (Cf. Figs. 132-135, vol. i. p. 377-380.) The thin wall of this sac consists of the same two layers, or skins, as the wall of the intestine itself ; internally of the intestinal-glan- dular layer, and externally of the intestinal-fibrous layer. The cavity of the allantois is filled with fluid ; this primi- 156 THE EVOLUTION OF MAN. tive urine must be chiefly the product of the primitive kidneys. The intestinal fibrous layer of the allantois is traversed by large blood-vessels which accomplish the nutri- ment and, especially, the respiration of the embryo ; these are the navel- vessels, or umbilical vessels (vol. i. p. 400). In all Reptiles and Birds the allantois becomes an immense sac, which encloses the embryo with the amnion, and which does not coalesce with the outer covering of the eore' (chorion). InCloacal Animals (Monotremata) and Pouched Animals (Marsupialia) the allantois is also of this nature. It is only in Placental Animals that the allantois develops into that very peculiar and remarkable formation, called the placenta, or "vascular cake." The nature of the placenta is this : the branches of the blood-vessels which traverse the wall of the allantois, penetrate into the hollow tufts of the chorion, which are inserted into corresponding depressions in the mucous membrane of the maternal uterus. As this mucous membrane is also abundantly supplied with blood- vessels, which conduct the mother's blood into the uterus, and as the partition between these maternal blood-vessels and the embryonic vessels in the chorion-tufts soon becomes extremely thin, a direct exchange of substance is soon de- veloped between the two sets of blood-vessels, which is ol" the utmost importance for the nutrition of the young Mammal. The maternal blood-vessels do not, however, pass directly (anastomosis) into the blood-vessels of the embryonic chorion-tufts, so that the two kinds of blood simply mix, but the partition between the two . sets of vessels becomes so thin, that it permits the passage of the most important food-materials, freed from unnecessary matter (transudation, or diosmosis). The larger the embryo THE PLACENTA. I 57 grows in Placental Animals, and the longer it remains in the maternal uterus, the more necessary does it become that special structural arrangements should meet the increased consumption of food. In this point there is a very striking difference between the lower and the higher Mammals. In Cloacal Animals and Pouched Animals, in which the embryo remains for a comparatively brief time in the uterus, and is born in a very immature condition, the circulation as it exists in the yelk-sac and in the allantois suffices for nutrition, as in birds and reptiles. In Placental Animals, on the contrary, in which gestation is very protracted, and the embryo remains much longer in the uterus, there attaining its full development within its investing membranes, a new ap- paratus is required to convey a direct supply of richer nutritive matter ; and this is admirably effected by the development of the placenta. In order rightly to understand and appreciate the for- mation of this placenta and its important modifications in different Placental Animals, we must once more glance at the external coverings of the mammalian egg. The outermost of these was originally, and during the cleavage of the egg and the first formation of the axial portion of the germ, formed by the so-called zona pellucida, and by the thick albuminous covering deposited externally on the latter (Fig. 19, Fig. 21, z, h, vol. i. p. 178). We called these two outer coverings, which afterwards amalgamate, the procliorion. This prochorion very soon disappears (in man perhaps in the second week of develop- ment), and is replaced by the permanent outer egg-mem- brane, the chorion. The latter, however, is simply the serous membrane, which, as we have already seen, is the 153 THE EVOLUTION OF MAN. product of the outer germ-layer of the germ-membrane vesicle. (See vol. i. p. 401, and Fig. 139, 4, 5, sh, p. 385.) This is at first a very smooth, thin membrane, surrounding the entire egg, as a closed spherical vesicle, and consisting of a single layer of exoderm cells. The chorion, however, be- comes very soon studded with a number of little protuber- ances or tufts (Fig. 139, 5, chz). These fit themselves into indentations in the mucous membrane of the uterus, and thus secure the egg to the wall of the latter. The tufts are, however, not solid, but hollow, like the fingers of a glove. Like the whole chorion, these hollow tufts consist of a thin layer of cells belonging to the horn-plate. They very soon attain an extraordinary development, growing and branching rapidly. In the spaces between them, new Fig. 198. — Egg-coverings of the human embryo (diagram- matic) : on, the thick fleshy wall of the uterus ; plu, placenta, the inner stratum (plu') of which has extended processes between the chorion-tufts (chz) (chf, tufted, chl, smooth cho- rion) ; a, amnion ; ah, amnion cavity ; as, amnion sheath of the navel. cord (passing down into the navel of the embryo, which is not represented here) ; dg, yelk-duct ; ds, yelk -sac ; dv, dr, decidua (dv, true, dr, false). The uterus. cavity (uh) opens below into the vagina, above, on the right hand side, into an oviduct (t). (After Kolliker.) tufts arise in all directions from the serous membrane, and thus before long (in the human embryo in the third week) DEVELOPMENT OF THE PLACENTA. 1 59 the whole outer surface of the egg is covered with a dense forest of tufts (Fig. 134). These hollow tufts are now penetrated from within by the branching blood-vessels, which originate from the in- testinal fibrous layer of the allantois, and which contain the blood of the embryo, introduced through the navel vessels (Fig. 198, ckz). On the other hand, dense networks of blood-vessels develop in the mucous membrane, which lines the inner surface of the uterus, particularly in the neighbourhood of the depressions into which the chorion- tufts penetrate (jplu). These vascular networks receive the blood of the mother introduced through the uterus vessels. The whole mass of these two sets of vessels, which are here most intimately connected, together with the connecting and enveloping tissues, is called the placenta, or " vascular cake." Properly speaking, the placenta consists of two quite different, though closely connected, parts ; internally, of the embryonic placenta {placenta foetalis, Fig. 198, chz), and externally of the maternal placenta (placenta uterina, Fig. 198, plu). The latter is formed byr the uterine mucous membrane and its blood vessels : the former by the secondary chorion and the navel vessels of the embryo. The mode in which these two " vascular cakes " com- bine to form the placenta, as well as the structure, form, and size of the latter, differs much in different Placental Animals, and affords valuable data for natural classification, and hence also for the tribal history of the whole sub-class. The latter is primarily divisible into two main divisions, based on these differences : the lower Placental Animals, which are called Indecidua, and the higher Placental Animals, or Deciduata. l6o THE EVOLUTION OF MAN. To the Indecidua, or lower Placental Animals, belong two very comprehensive and important vertebrate groups : (1) the Hoofed Animals (Ungulata) — the Tapirs, Horses, Swine, Ruminants, and others ; (2) the Whale-like animals (Cetomorpha) — the Sea-cows, Porpoises, Dolphins, Whales, and others. In all these Indecidua the chorion tufts are distributed, singly or in bunches, over the entire surface of the chorion, or over the greater part of it. They are but very loosely attached to the mucous membrane of the uterus, so that the entire outer egg-membrane with its tufts might easily and without using force be drawn out of the depressions in the uterine mucous membrane, just as the hand is with- drawn from a glove. The two " vascular cakes " do not really coalesce at any point of their contact. Hence, at birth the "embryonic cake" (placenta fcetalis) is alone removed ; the " maternal cake " (placenta uterina) is not displaced. The entire mucous membrane of the gravid uterus is but little altered, and, at parturition, suffers no direct loss of substance. The structure of the placenta in the second and higher division of Placental Animals, the Deciduata, is very dif- ferent. To this comprehensive and very highly developed mammalian group belong all Beasts of Prey and all Insect- eaters, Gnawers (Rodentia), Elephants, Bats, Semi-apes, and, lastly, Apes and Man. In all these Deciduata the whole surface of the chorion is also at first thickly covered with tufts. These, however, afterwards disappear from part of the surface, while they develop all the more vigorously in the remainder. The smooth chorion (chorion lqeve,Fig. 198, chl) thus becomes distinct from the tufted chorion (chorion frondosum, Fig. 198, chf). On the former there are only INDECIDUA AND DECIDUA. l6l minute and scattered tufts, or none at all ; while the latter is thickly overgrown with highly developed and large tufts. In the Deciduata the tufted chorion alone forms the placenta. Yet more characteristic of the Deciduata is the very peculiar and intimate connection which is developed in these between the tufted chorion and the contiguous portion of the uterine mucous membrane, and which must be regarded as a true coalescence. The vascular tufts of the chorion push their branches into the sanguineous tissue of this mucous membrane in such a way, and the two sets of vessels are in such close contact and are so interlaced, that the embryonic placenta is no longer distinguishable from the maternal placenta ; the two form one whole — a compact and apparently simple placenta. Owing to this intimate coalescence, a portion of the uterine mucous mem- brane of the mother comes away, at birth, with the firmly adherent egg-membrane. The portion of the mother's body which is thus removed in parturition is called, on account of its separable nature, the deciduous membrane (clecidua). All Placental Animals which possess this deciduous mem- brane are classed together as Deciduata. The removal of this membrane at parturition, of course, causes a greater or less loss of blood by the mother, which does not occur in the- Indecidua. In the Deciduata, moreover, the lost portion of the uterine mucous membrane must be replaced, after parturition, by a renewal of the tissue. The structure of the placenta and deciduous membrane is, however, by no means identical throughout the compre- hensive group of Deciduata. On the contrary, there are many important differences in this respect, which are in 1 62 THE EVOLUTION OF MAN. some degree connected with other important structural characters (e.g., the structure of the brain, of the teeth, ot the feet), and which may justly, therefore, be turned to account in the phylogenetic classification of Placentals. In the first place, two great groups of Deciduata may be distinguished according to the form of the placenta : in the one group it is ring-shaped or girdle-shaped; in the other it is discoid or cake-shaped. In Deciduata with girdle-shaped placenta (Zonoplacentalia) the poles of the oval egg take no part in the formation of the placenta. The " vascular cake ': resembles a broad ring-like girdle, embracing the central zone of the egg. It is so in Beasts of Prey (Camassia), both in the terrestrial forms (Carnivora) and in the marine forms (Pinnipedia). A similar girdle-shaped placenta is found in the False-hoofed Animals (Chelophora) : the elephants, and Klip Das (Hyrax) with its allies, which were formerly classed as Hoofed Animals. All these Zonoplacen- talia belong to one or more side-branches of the Deciduata, which are not nearly allied to Man. The second and most highly developed group is formed by the Deciduata with discoidal placenta (Disco placentalia). The formation of the placenta is here most localized and its structure most fully developed. The placenta forms a thick, spongy cake, usually in the form of a circular or oval disc, and attached only to one side of the uterine wall. The greater part of the embryonic egg-membrane is, therefore, smooth, without developed tufts. To the Disco- placentalia belong the Semi-apes and Insect-eaters, the Diggers (Effodienta) and the Sloths, Rodents and Bats, Apes and Man. Comparative Anatomy enables us to infer that of these various orders the Semi-apes are the parent- SEMI-APES. 163 group from which all other Discoplacentals, and perhaps even all Deciduous Animals, have developed as divergent branches. (Of. Tables XXIII. and XXIV.) The Semi-apes (Prosimiw) are now represented only by very few forms. These, however, are very interesting, and must be regarded as the last remnants of a group once rich in forms. This group is certainly very ancient, and was probably very prominent during the Eocene Epoch, Their present degraded descendants are scattered widely over the southern portion of the Old World. Most of the species inhabit Madagascar ; a few the Sunda Islands ; a few others the continents of Asia and Africa. No living or fossil Semi- apes have, as yet, been found in Europe, America, or Aus- tralia.159 The widely scattered posterity of the Semi-apes is considerably diversified. Some forms seem nearly allied to the Marsupials, especially to the Pouched-rats. Others (Macrotarsi) are very near akin to the Insect-eaters, and yet others (Cheiromys) to the Gnawers (Rodentia). One genus (Galeopiihccus) forms a direct transition to the Bats. Finally, some of the Semi-apes (Bvachytarsi) approach very near to true Apes. Among the latter are some tail-less forms {e.g., the Lori, Stenops, Fig. 199). From these highly in- teresting and important relations of the Semi-apes to the various Discoplacental orders, we may fairly infer that of the extant representatives of this group, they are the nearest to the common primitive parent-form. Among the direct common ancestors of Apes and Men, there must have been some Deciduata which we should class among the Semi-apes, were we to see them alive. We may therefore consider this order as a special stage, following the Pouched Animals, as the eighteenth stage in the human pedigree. 164 THE EVOLUTION OF MAN. Probably our ancestors among the Semi-apes closely re- sembled the extant Brachytarsi or Lemurs {Lemur, Lichan- Fig. 199. — The Slender Lori of Ceylon (Stenops gracilis). otus, Stenojis), and, like these, led a quiet life, climbing on trees. The extant Semi-apes are mostly nocturnal animals of gentle and melancholy disposition, subsisting on fruits. APES. 165 The Semi-apes are immediately followed by the true Apes (Simice), as the nineteenth stage in the human pedi- gree. It has long been beyond doubt that of all animals the Apes are in all respects the most nearly allied to Man. Just as, on the one side, the lowest Apes approach very near to the Semi-apes, so, on the other side, do the highest Apes most closely resemble Man. By carefully studying the Com- parative Anatomy of Apes and Man, it is possible to trace a gradual, uninterrupted advance in the Ape-organization up to the purely human structure; and on impartially testing this "Ape-question," which has lately been agitated with such passionate interest, we shall infallibly have to acknowledge the important fact, which was first explicitly laid down by Huxley, that "whatever system of organs be studied, the comparison of their modifications in the ape series leads to one and the same result — that the structural differences which separate Man from the Gorilla and Chimpanzee are not so great as those which separate the Gorilla from the lower Apes." In phylogenetic language this pregnant law established in so masterly a manner by Huxley, is equiva- lent to the popular phrase : Man is descended from the Ape. In order to become convinced of the truth of this law, let us now once more consider the placenta and deciduous membrane, on the varied structure of which we justly laid special stress. Men and Apes, in the structure of their disc- shaped placenta and in their decidua, do, indeed, coincide on the whole with all other Discoplacental Animals. But in the more delicate structure of these parts Man is dis- tinguished by peculiarities which he shares only with Apes, and which are absent in other Deciduata. Thus in Man and in the Apes three distinct parts are recognized in the 1 65 THE EVOLUTION OF MAN. deciduous membrane ; these parts may be called the outer, the inner, and the placental deciduous membrane. The outer or true membrane (d. externa or vera, Fig. 198, dv, Fig. 200, g), is that portion of the uterine mucous membrane which coats the internal surface of the uterus wherever the Fig. 200. — Human embryo, twelve weeks old, with its coverings; natural size. The navel cord passes from the navel to the placenta : b, amnion ; c, chorion ; d, placenta ; d\ remains of tufts on the smooth chorion ; /, de- cidua refiexa (inner) ; g, decidua vera (outer). (After Bernhard Schultze.) latter is not attached to the placenta. The placental or spongy deciduous membrane (d. placentalis or serotina, Fig. 198, pin, Fig. 200, d) is simply the maternal placenta THE DECIDUOUS MEMBRANE IN MAN AND APES. 167 itself, or the maternal part of the " vascular cake ' (pla- centa uterina), i.e., that part of the uterine mucous mem- brane which coalesces intimately with the chorion-tufts of Fig. 201. — Mature human embryo (at the end of pregnancy), in its natural position, taken out of the uterus. On the inner surface of the latter (on the left) is tbe placenta, which is attached to the navel of the child by the navel cord. (After Bernbavd Schultze.) the embryonic placenta (placenta fcetalis). Lastly, the inner or false deciduous membrane (d. interna or reflexa, Fig. 198, dr, Fig. 200, /) is that portion of the uterine mucous membrane which, as a peculiar thin envelope, covers all the rest of the egg-surface, lying immediately over the tuftless smooth chorion (chorion Iceve). The origin of these three distinct deciduous membranes, concerning which erroneous notions have been entertained (still retained in the nomen- clature), is plain enough ; the external or true deciduous l68 THE EVOLUTION OF MAN. membrane is a peculiar modification, afterwards lost, of the superficial layer of the original mucous membrane of the uterus. The placental membrane is that portion of the preceding which is completely modified by the intrusion of the chorion-tufts and is employed in forming the placenta. Lastly, the inner deciduous membrane is formed by a ring-shaped fold of the mucous membrane (at the point of union of the d. vera and the d. serotina) which rises, grows round the egg, and closes in the same way as the amnion.160 The peculiar anatomical characters which mark the human egg-membrane re-occur, in the same form, only in Apes. All other Discoplacental Animals present greater or less differ- ences in these points, the conditions being generally more simple. This is the case, for instance, in the structure of the placenta itself, in the coalescence of the chorion tufts with the decidua serotina. The matured human placenta is a circular (rarely oval) disc of a soft, spongy character, 6 to 8 inches in diameter, about 1 inch thick, and weighing from 1 to 1J lb. Its convex, external surface (that which coalesces with the uterus) is very uneven, and tufted. Its internal, concave surface (that which is turned towards the cavity of the egg) is quite smooth, and clothed by the amnion (Fig. 198, a). From near the centre of the placenta springs the navel cord (funiculus umbilicalis), the development of which we have already observed (vol. i. p. 383). It also is coated by the amnion as with a sheath, which at the navel end passes directly into the abdominal skin (Fig. 200, 201). The mature navel cord is a cylindrical cord, coiled spirally around its axis, and usually about 20 inches long and J inch thick. It consists of gelatinous connective tissues (" Whar- THE HUMAN PLACENTA. 1 69 ton's jelly "), in which are contained the remnants of the yelk-vessels and of the great navel vessels ; the two navel arteries which convey the blood of the embryo to the pla- centa, and the great navel vein which brings back the blood from the latter to the heart. The numerous fine branches of these embryonic navel vessels pass into the branched chorion tufts of the foetal placenta, and with these, finally, grow, in a very peculiar way, into large blood-filled cavities, which spread themselves in the uterine placenta and con- tain blood from the mother. The anatomical relations, very complex and difficult to comprehend, which are developed between the embryonic and the maternal placenta, exist in this form only in Man and in the higher Apes, while in all other Deciduous Animals their form is more or less different. The navel cord, also, is proportionately longer in Man and in Apes than in other Mammals. As in these important characters, so also in every other morphological respect, Man appears as a member of the order of Apes, and cannot be separated from the latter. The great originator of systematic description of nature, Karl Linnaeus, with prophetic penetration, united Men, Apes, Semi-apes, and Bats in a single natural division, under the name of Primates, that is, the first, the lords of the animal kingdom. Later naturalists dissolved this order of Primates. The Gottingen anatomist, Blumenbach, first placed Man in a special order, which he called that of Two-handed Animals (Bimana) : in a second order, he united Apes and Semi- apes under the name of Four-handed Animals (Quacl- rumana), while a third order included the distantly related Bats (Chiroptera). The separation of the Bimana and Quadrumana was retained by Cuvier and most succeeding 170 THE EVOLUTION OF MAN. zoologists. It seems very important, but is really wholly unjustifiable. This was first shown in the year 1863 by Huxley. Supported by very accurate Comparative Anato- mical researches, he proved that Apes are as "two-handed" as Men, or, conversely, that Men are as " four-handed " as Apes. Huxley showed, with convincing clearness, that the ideas previously held of the hand and the foot were false, and were incorrectly founded on physiological instead of on morphological distinctions. The circumstance that in the hand, the thumb may be opposed to the other four fingers, thus permitting the act of grasping, appeared especially to distinguish the hand from the foot, in which the correspond- ing great toe cannot be thus opposed to the four remaining toes. Apes, on the contrary, can grasp in this way with the hind-foot as well as with the fore-foot, and were therefore regarded as four-handed. Many tribes, however, among the lower races of men, especially many negro tribes, use the foot in the same way as the hand. In consequence of early habit and continued practice, they are able to grasp as well with the foot as with the hand (for example, in climbing, they grasp the branches of trees). Even new-born children of our own race have a very strong grasping power in the great toe, with which they can hold a spoon as fast as with the hand. The physiological distinction between hand and foot can, therefore, neither be strictly carried out, nor scientifically established. Morphological characters must be used for this purpose. A sharp morphological distinction of this kind — that is, one founded on anatomical structure — between hand and foot, between the anterior and the posterior limbs, is actually possible. There are essential and permanent differences MAN AND APE. 171 both in the structure of the bony skeleton and in that of the muscles which are attached to the hand and the foot ; and these are exactly the same in Man and in the Ape. There is, for instance, an essential difference in the arrange- ment and number of the wrist-bones of the hand {carpus) and the ankle-bones of the foot (tarsus). The muscle-masses present equally constant differences. The posterior ex- tremity, the foot, has always three muscles (a short flexor muscle, a short extensor muscle, and a long muscle attached to the muscles of the tibia) which are never present in the anterior extremity, the hand. The disposition of the muscles is also very different in the two sets of limbs. These characteristic differences between the anterior and the posterior extremities occur in Man just as in Apes. There can, therefore, be no doubt, that the foot of the Ape deserves the name as truly as that of the Man ; and that all true Apes are as genuinely two-handed animals (Bimana) as Man. Thus the usual distinction of the Apes as Quadrumana is wholly unjustifiable. It might now be asked whether, quite apart from these, there are not other marks by which Man is more widely separated from the Apes than are the different species of Apes from each other. Huxley has given a final negative to this question so convincingly, that the opposition now raised against him in many quarters must be regarded as completely unfounded and ineffective. Based on an accurate study of the Comparative Anatomy of all parts of the body, Huxley brought forward very significant proof that, in every anatomical respect, the differences between the highest and the lowest Apes are greater than the corresponding differences between the highest Apes and Man. He there- 172 THE EVOLUTION OF MAN. fore restored Linnseus's order of Primates (excluding the Bats), and divided it into three different sub-orders, the first of which is formed by the Semi-apes (Lemurida), the second by the true Apes (Simiadce), and the third by Men (Antliropidce)™1 Yet, if we proceed logically and without prejudice, in accordance with the principles of scientific reasoning, we find, on the basis of Huxley's own law, this division in- adequate, and must go considerably further. As I first showed in 18G6, in treating this question in my Gencvelle Morphologie, we are fully justified in taking at least one important step further, in assigning to Man his natural place in one of the divisions of the Ape-order. All the characters distinctive of this one division of the Apes are present in Man, while they are absent in other Apes. We are, therefore, not justified in forming a distinct order for Man apart from the true Apes. The order of the true Apes (Simice), the Semi-apes being excluded, has long been divided into two natural main groups, which, among other points, are distinguished by their geographical distribution. Those of one division (Hesperopitheci, or Western Apes) live in the New WTorld, in America. The other division, to which Man belongs, is that of the Heopitheci, or Eastern Apes ; these live in the Old World, in Asia, Africa, and, formerly, in Europe. All the Apes of the Old World, all Heopitheci, share, in common with Man, all those characteristics to which special promin- ence is justly given, in distinguishing these two groups of Apes, in zoological classification ; among these characteristics the structure of the teeth is most prominent. The objec- tion is at once evident that the teeth are, in a physiological THE TEETH. 173 sense, much too subordinate a part of the body to justify so great a weight being attached to their structure in so im- portant a question. There are, however, good reasons for this prominent consideration of the structure of the teeth ; and it is with perfect correctness and propriety that sys- tematic zoologists have, for more than a century, given special weight to this character in systematically dis- tinguishing and arranging the mammalian orders. The number, form, and disposition of the teeth are transmitted much more accurately within the respective orders of the mammals than are most other zoological characteristics. The structure of the human teeth is well known. In matu- rity there are 32 teeth in our jaws, and of these 32 teeth, 8 are front-teeth, 4 canine-teeth, and 20 molar-teeth. The eight front-teeth or incisors (denies incisivi), which are situated in the centre of the jaws, exhibit characteristic differences in the upper and lower jaw. In the upper the inner incisors are larger than the outer; in the lower jaw, on the contrary, the inner incisors are smaller than the outer. Next to these, on each side, both in the upper and lower jaw, is a corner- tooth, which is larger than the in- cisors, the so-called eye-tooth, or canine (dens caninus). Sometimes this tooth becomes very prominent in Men, as in most Apes and many other Mammals, and forms a sort of tusk. Finally, next to this, on each side, and in each jaw, are situated five back-teeth, or molar-teeth (dentes molar es), of which the two foremost (the bicuspid teeth) are small, have but a single fang, and are subject to the change of teeth, while the three hinder molars are much larger, have two fangs, and do not appear till after the temporary teeth have been shed (so-called "grinders"). The Apes of the 174 THE EVOLUTION OF MAN. Old World have exactly this human structure of the teeth, — all Apes which have as yet been found, either living or as fossils, in Africa, Asia, and Europe. All Apes of the New World, on the contrary, all American Apes, have an extra tooth on both sides of each jaw ; this is a biscupid tooth. Thus they have six back-teeth on both sides of each jaw, — in all, thirty-six teeth. This characteristic difference be- tween the Eastern and Western Apes has been so constantly transmitted within the two groups, that it is of the greatest value to us. A small family of South American Apes does, indeed, appear to form an exception in this respect. The pretty little Silk Apes, or Marmosets (ITapalida), namely, to which the Brush-monkey (Midas) and the tufted Marmoset (Jacchus) belong, have but five back-teeth in each half of the jaw, instead of six, and, accordingly, seem to approach nearer the Eastern Apes. But on closer observation it is found that, like all the Western Apes, they have the three biscupids, and that the hindmost grinder has been lost. Thus this apparent exception confirms the value of the distinction. Among the other marks by which the two main groups of the Apes are distinguished, the structure of the nose is specially important and prominent. In all Old World Apes the structure of the nose is the same as in Man ; namely, a comparatively narrow partition of the two halves, so that the nostrils are directed downwards. In a few Eastern Apes, the nose projects as prominently and is as characteristically formed as in Man. We have already called attention, in this respect, to the remarkable Nose-ape (Semno- pithecus nasicus), which has a well-curved and long nose (Fig. 202). Most of the Eastern Apes have, it is true, a THE NOSE. 175 somewhat flatter nose, as, for instance, has the white-nosed Sea-cat (Cevcopithecus petaurista, Fig. 203) ; yet in all the partition of the nose is narrow and thin. On the contrary, all American Apes have a different nasal structure. In them, the partition is peculiarly broadened and thickened below, and the wings of the nose are not developed, in con- sequence of which the nostrils are not below, but are turned outwards. This characteristic difference in the structure of the nose has also been so accurately trans- Fig. 202. — Head of Nose-ape (Semnopithecusnasicus). Fig. 203. — The white-nosed Sea-cat (Cevcopithecus petaurista). mitted in both groups, that, on account of it, the Apes of the New World have been called Flat-nosed (Platyrhince), and those of the Old World Narrow-nosed (Catarhince).' The former are, on the average, inferior in organization, 31 iy6 THE EVOLUTION OF MAN. The division of the order of Apes into two sub-orders, the Platyrhince and the Catarhince, is, on account of the constant hereditary characters, now generally accepted by zoologists, and receives much support from the geographical distribution of the two groups between the New and Old Worlds. From this follows the direct inference, very im- portant in its bearing on the Phylogeny of Apes, that, from the primaeval common parent-form of the Ape-order, two diverging lines branched out at a very early period, one of which spread over the New World, the other over the Old. It is certain that all the Flat-nosed Apes, on the one hand, are descendants of a common parent-form, and, on the other hand, all the Narrow-nosed Apes from another An inference concerning our own pedigree may be drawn from this. Man has exactly the same characters, the same peculiar formation of the teeth and nose, as all the Catarhinse, and is as thoroughly distinguished by these charateristics from the Platyrhinse. We are therefore com- pelled, in classifying the Primates, to assign to Man a place in the Narrow-nosed group. The bearing of this on our tribal history is, that Man is immediately related in blood to the apes of the Old World, and may be traced from a parent-form common to all other Catarhinse also. Man is a genuine Narrow-nosed Ape in his whole structure and in origin, and has descended from some unknown, extinct Catarhine form in the Old World. On the other hand, the Apes of the New World, the Flat-nosed group, constitute a diverging branch of our family tree, and stand in no near genealogical relation to the human race. We have now reduced the circle of our nearest allies to the small group, containing comparatively few forms, man's helation to apes. 177 which is represented by the sub-order of the Narrow-nosed, or Eastern Apes. Finally, the question which now re- mains to be answered is — what position in this sub-order must be assigned to Man, and whether other inferences as to the structure of our immediate ancestors may be drawn from this position. The comprehensive and acute researches into the Comparative Anatomy of Man and the various Catarhinse, which Huxley has recorded in his work on the " Evidence as to Man's Place in Nature," are of the greatest value in furnishing the answer to these important questions. The inevitable conclusion is, that the difference between Man and the highest Narrow-nosed Apes (the Gorilla, Chim- panzee, Orang) is slighter in every respect than the corre- sponding differences between the highest and the lowest Catarhines (the Sea-cat, Macaque, Baboon). Even within the small group of the Tail-less man-like Apes (Anthro- poides) the several genera do not differ less from each other than they do from Men. This is seen by a glance at the skeletons represented here, as arranged by Huxley (Figs. 204-208). If the skull, or the vertebral column, together with the rib-system, or the anterior or posterior members, are compared ; or if the comparison is extended to the muscular system, the circulatory system, the brain, etc., a candid and unprejudiced examination always results in the same conclusion, that Man does not differ more from the higher Catarhines than the extreme forms of the latter (for example, .the Gorilla and Baboon) differ from each other. We can, therefore, complete the important propo- sition already quoted from Huxley : We may take what- ever system of organs we will, — the comparison of their modifications within the ranks of the Catarhinse leads us 178 THE EVOLUTION OF MAN. "* bo . N t— 1 6 X ~ t=4 © t— ( _,_, c3 i—i Ph n fcb o M ^ kJ — be cu • 0> o 0 i— i X! e3 S f* n r^ 4J c ; ci CO be t0 GO > O 2* g3 ci O ^^ • i— i 00 n A/ ^ I— < o . ^2 Up ss of iflammalg Cloacal Animals (Monotrema, or Ornithodelphid) 1. Primitive Mammals 2. Beaked Anim.ils Promammalia Ornithostoma II. Second Sub-class of iHantmals Pouched Animals (Marsupialia, or Lidelphid) 3. Herbivorous Poucbed Animals Botanophaga 4. Carnivorous Pouched Animals Zoophaga III. Third Sub-class of iHammals : Placental Mammals (Placentalia, or Mono- delphia) III. (a) Placental / Mammals with- out Decidua, with Tufted Placenta Indecidua Villiplacentalia III. (6) Placental Mammals with Decidua, with ■ Girdle Placenta Deciduata Zonoplacen talia 5. Hoofed Animals f Single-hoofed Ungulata } Double-hoofed Whale-like Animals Cetomorpha Pseudo-hoofed Animals Chelophora Beasts of Prey Camassia \ Sea-cows 1 Whales !Rock Conies Elephants /Land Beasts of ) prey ~\ Marine Beasts \ prey Perissodactyla Artiodactyla Sirenia Cetacea Lamnuvgia Proboscidoa Carnivora of Pinnipedia III. (c) Placental Mammals with Decidua, with \ Discoid Placenta Deciduata Discoplacen talia 10. 11. 12. 13. 14. ( Fingered animals Semi-apes J Long-footed Prosimiai . Flying Lemur \ Lemurs Gnawing Ani- j KSpE* „ . . ) Porcupine species Bodentia ( Hare ^..J Toothless Edentata Insect-eaters Jnsectivora f Digging animals \ Sloths With Ccecum Without Ccecum Flying Animals J Flying Foxes Ckiroptera ( Bats Apes ( Flat-nosed Platyrhinai Simim \ Narrow-nosed Apes Catarhinas Leptoiactyla Macrotarsi Ptenopleura Brachytarsi Sciuromorpha Myomorpha Hystrichomorpha Lagomorpha Effodientia Bradypoda Menotyphla Lipotyphla Pterocynes Nycterides ( 138 ) TABLE XXIY. Pedigree of Mammals. Elephants Proboscidea Rock Conies Lamnungia False-hoofed Chelophora Flat -nosed Apes Platyrhince ©naming Animals Eodentia fHan Homines Bats Man-like Apes Ny derides Anthropoides Flying Foxes Narrow-nosed Apes Pterocynes Catarhince JFIm'rtg Animals Chiroptera Sea heasts of prey Pinnipedia Whales Cetacea 3prs Fingered animals Simiae Leptodactyla Land beasts of prey Camivora Flying Lemnrs Ptenopleura 33casts of ^rcg Carnassia Sea-Cows Sirenia amfjalcsfamilg Cetomorpha pjoofrti Animals TJngulata ©Hitijout Occitiua Indeciduata Lemurs Brachytarsi Toothless Edentata Insect-eaters Long-footed Insectivora Macrotarsi Semi-apes Prosimice Dwfouous Animate Deciduata -v- placental Animals Placentalia Herbivorous Pouched Animals Marsupialia botanophaga Carnivorous Pouched Animals Marsupialia zoophaga, Beaked Animals Or -nit ho stoma ^)ouri)cti Animals Marsupialia I Primitive Mammals Promamnialia Cloacal Animate Monotrema hasckel's evolution of man. PLATE XV PEDIGREE OF MAN. fMANl ~mw^. , \ Gorilla Wa yff ^d Prang | ChimpanzeTj^yjiyl^r?;^ Gibbon \ Hoofed AnininiB (Unguiata) \ Ape-Men] ;. Apes W j Bats 1 Slothaf W Semi-Apes (Lenmroidea) | Beasts of Pf ey | 'H [J^SI^MW^ Primitive Mammals (Promammalia) Beaked Animals. J [Osseous Fishes (Teleostei) H^m \ Ganoids Petromyzoi [insects [Crustaceans ffl'J g 2 > » ® Star-Animals (Echinoderma) Soft Animals (Molluscs) « Sea-Nettles I (Acalephae) ei -4-> ♦-1 _ o fl»4J « = s a, o Jj to 1: 1 l-H G 1*3 *► o • r-4 —J J ( i89 ) TABLE XXV. Pedigree of Apes. fflan Homo &pe4ifti fHan Alalus Gorilla Chimpanzee Gorilla Engeco African |Han4ike &pes Orang-Outang Satyrus Gibbon Hylobates Astatic fHan4tkc ^pcs fHan4ikc Sycs Anthropoides Silk Apes Hapalida — v~ Clutch- tails Labidocerca J Tall Apes Semnopithecus Nose Apes Nasalis Sea Cat Cercopithecus Baboons Cynocephalus I Flap-tails Apliyocerca Ipcs of |kfo EmorlU Flat-nosed Platyrhinae Catko <3pcs Menocerca &pes of ©IK raorlH Narrow-nosed Catarhinae ^pCS Simiae Prosimiae CHAPTER XX. THE HISTORY OF THE EVOLUTION OF THE EPIDERMIS AND THE NERVOUS SYSTEM. Animal and Vegetative Organ-systems. — Original Relations of these to the Two Primary Germ-layers. — Sensory Apparatus. — Constituents of Sensory Apparatus : originally only the Exoderm, or Skin-layer ; after- wards, the Skin-covering specialized from the Nerve-system. — Double Function of the Skin (as a Covering and as Organ of Touch). — Outer Skin (Epidermis) and Leather-skin (Corium). — Appendages of the Epi- dermis : Skin-glands (Sweat-glands, Tear-glands, Sebaceous Glands, Milk-glands); Nails and Hair. — The Embryonic Wool-covering. — Hair of the Head and of the Beard. — Influence of Sexual Selection. — Arrange- ment of the Nerve-system. — Motor and Sensory Nerves. — Central Marrow : Brain and Dorsal Marrow. — Constitution of the Human Brain : Large Brain (Cerebrum) and Small Brain (Cerebellum). — Comparative Anatomy of the Central Marrow. — Germ-history of the Medullary- tube. — Separation of the Medullary-tube into Brain and Dorsal Marrow. — Modification of the Simple Brain-bladder into Five Consecutive Brain- bladders : Fore-brain (Large Brain, or Cerebrum), Twixt-brain (" Centre of Sight "), Mid-brain (" Four Bulbs "), Hind-brain (Small Brain, or Cere- helium), After-brain (Neck Medulla). — Various Formation of the Five Brain-bladders in the various Vertebrate Classes. — Development of the Conductive Marrow, or " Peripheric Nervous System." "Hardly any part of the bodily frame, then, could be found better calculated to illustrate the truth that the structural differences between Man and the highest Ape are of less value than those between the highest THE DEVELOPMENT OF THE ORGANS. I9I and the lower Apes, than the hand or the foot, and yet, perhaps, there is one organ which enforces the same conclusion in a still more striking manner — and that is the brain."— Man's Place in Nature, p. 94 (1863). "As if to demonstrate, by a striking example, the impossibility of erecting any cerebral barrier between Man and the Apes, Nature has provided us, in the latter animals, with an almost complete series of gra- dations, from brains little higher than that of a Rodent to brains little lower than that of Man." — Ibid. p. 96. Our investigations, up to the present, have shown us how the whole human body has developed from an entirely simple beginning, from a single simple cell. The whole human race, as well as the individual man, owes its origin to a simple cell. The one-celled parent-form of the former is, even yet, reproduced in the one-celled germ-form of the latter. In conclusion, we must glance at the evolutionary history of the separate parts which constitute the human body. In this matter, I must, of course, restrict myself to the most general and important outlines ; for a detailed study of the evolutionary history of the separate organs and tissues would occupy too much space, and would demand a greater extent of anatomical knowledge than the generality of my readers are likely to possess. In considering the develop- ment of the organs, and of their functions, we will retain the method previously employed, except that we will consider the germ-history and the tribal history of the various parts of the body in common. In the history of the evolution of the human body as a whole we have found that Phylogeny everywhere serves to throw light on the obscure course of Ontogeny, and that the clew afforded by phylogenetic con- tinuity alone enables us to find our way through the labyrinth of ontogenetic facts. We shall experience exactly the same fact in the history of the development of the separate 192 THE EVOLUTION OF MAN. organs ; but I shall be compelled to explain the ontogenetic and the phylogenetic origin of the organs simultaneously ; for the further we penetrate into the details of organic development, and the more minutely we study the origin of the separate parts, the more clearly do we see how inseparably the evolution of the germ is connected with that of the tribe. The Ontogeny of the organs is intelligible and explicable only through their Phylogeny ; just as the germ-history of the entire body (the "person") is rendered intelligible only by the history of the tribe. Each germ- form is determined by a corresponding ancestral form. This is as true of the parts as of the whole. In endeavouring, with the help of this fundamental law of Biogeny, to obtain a general view of the main features in the development of the separate organs of man, we must, in the first place, consider the animal, and then the vegetative organ-systems of the body. The first main group of organs, the animal organ-systems, is formed by the sensory apparatus, together with the motor apparatus. To the former belong the skin-covering, the nervous system, and the organs of the senses. The motor apparatus consists of the passive organs of movement (the skeleton) and the active organs (the muscles). The second main group of organs, the vegetative organ-system, is formed by the nutritive and the repro- ductive apparatus. To the nutritive apparatus belongs especially the intestinal canal with all its appendages, together with the vascular and renal systems. The repro- ductive apparatus includes the various sexual organs (the germ-glands, germ-ducts, organs of copulation, etc.). In earlier chapters (IX. and X.) it has been stated that the animal organ-systems (the instruments of sensation and ANIMAL AND VEGETATIVE ORGAN-SYSTEMS. 1 93 of movement) proceed especially from the outer primary germ-layer, from the skin-layer. The vegetative organ- systems, on the other hand (the instruments of nutrition and reproduction), proceed principally from the inner primary germ-layer, from the intestinal layer. This radical contrast between the animal and the vegetative spheres of the body is, it is true, by no means absolute either in man or in the higher animals ; on the contrary, many separate parts of the animal apparatus (e.g., the intestinal nerve, or sympathetic) originated from cells which have proceeded from the ento- derm ; and, on the other hand, a large part of the vegetative apparatus (e.g., the mouth-cavity, and probably the greater part of the urinary and sexual organs) is formed of cells which are originally derived from the exoderm. Moreover, in the bodies of all the more highly developed animals, the most heterogeneous parts are so intermixed and blended that it is often extremely difficult to assign its true source to each one of the constituent parts. But, on the whole, we may assume as a certain and important fact, that in Man, and in all high animals, the greater part of the animal organs must be referred to the skin-layer, or exoderm ; the greater part of the vegetative organs to the intestinal layer, or entoderm. For this reason, Baer called the former the animal germ-layer, the latter, the vegetative germ-layer (Cf. vol. i. pp. 53 and 1 96). Of course, in making this important assumption, we pre-suppose the correctness of Baer's view, according to which the skin-fibrous layer (the ".flesh stratum " of Baer) must have originated (phylogenetically) from the exoderm, and, on the other hand, the intestinal- fibrous layer (Baer's "vascular layer") from the entoderm. This influential view, which is yet much disputed, is, we ( *94 ) TABLE XXVI. Systematic Survey of the Organ- Systems of the Human Body. (N.B. — The origin of the separate organs from the four secondary germ, layers is indicated by the Roman numerals (I.-IV.) : I. Skin-sensory layer ; II. Skin-fibrous layer; III. Intestinal-fibrous layer; IV. Intestinal-gland- ular layer.) 65 H 02 02 ■ o o A. Sensory Apparatus Sensorium (\. Skin-covering (Derma) 2. Central nerve- system ( Outer skin I Leather skin ( Brain { Spinal marrow < B. Motive Apparatus Locomotorium „ t, . . . I Brain nerves 3. Peripheric nerve- lSinalne]ves system } Intestinal nerves 4. Sense-organs (Organa sensuum) " ' Organ of touch (skin) Organ of taste (tongue) Organ of smell (nose) Organ of sight (eye) .Organ of hearing (ear) Skin muscles Skeleton muscles (b. Muscle - system (active motive - organs) . Skeleton-system 1 Vertebral column (passive motive < Skull organs) ( Limb skeleton Epidermis, I. Corium, II. Encephalon •» T Medulla spinalis / Nervi cerebrales, I. + II Nervi spinales, II. Sympatheticus,ll. + Ill. Org. tactus Org. gustus Org. olfactus Org. visus Org. auditus J [ I. + II. Musculi cutane M. skeleti Vertebrarium Cranium I Sk. extreniitatum / II. r w H 02 02 I < o < — a > C. Nutritive Apparatus Nutritorium (1. Intestinal system f Digestive organ (fiaster) \ Respiratory organ 0. digestiva 8. Vascular system (Organa circu- - lationis) Body cavity Lymph vessels Blood vessels , Heart 9 Renal system i ^a^dllct8 \ifirgana urinaria) | UtinJy bladd(?r D. Reproductive Apparatus Propagator ium 10 Sexual organs (Organa sexual ia) ' Sexual glands (I. Ovary) (II. Testes) .Sexual ducts (I. Oviduct) < (II. Seed duct) Copulatory organs (I. Sheath) (.(II. Penis) ia)iii.+iv. Y Cceloma, II. + III. Vasa lympha- tics V. sanguifera Cor, III. Ureteres } L CO + n- Urocystis, III. + IV. Gonades (I. Ovaria) III. + IV.(?) (II. Testts) I. + II. (r) Gonophori (I. Oviduc tus) (II. Sperma ductus) Copulativa \ (I. Vagina) \ I (II. Penis) \ !•(?) + II. + JI. THE SENSORY APPARATUS. 1 95 think, securely founded on the Gastrula — that most impor- tant of all the germ-forms of the animal kingdom — which we find recurs in similar form in the germ-history of the most different classes of animals. This significant germ- form points unmistakably to a parent-form (the Gastrsea) common to all animals, the Protozoa alone excepted ; in this long extinct parent-form the entire body of the animal consisted throughout life of the two primary germ-layers, as is yet the case, for a short time, in the Gastrula. In the Gastrsea the simple skin-layer did actually represent all the animal organs and functions, and the simple intestinal layer, on the other hand, all the vegetative organs and functions ; potentially, this is even yet the case in the Gastrula. In studying the development of the first important part of the animal sphere, the sensory apparatus, or sen- sorium, we shall now find how well adapted this Gastrsea Theory is to explain, not only in a morphological but in a physiological sense, the most important facts in the history of evolution. This sensory apparatus consists of two very distinct parts, having, apparently, nothing in common : in the first place, the external skin-covering [Derma), together with its appendages, the hair, nails, sweat-glands, etc. ; and, secondly, the nervous system, situated internally. The latter includes the central nervous system (brain and spinal chord), the peripheric brain-nerves and medullary nerves, and finally, the organs of sense. In the fully developed vertebrate body these two main constituents of the sensorium are entirely separate ; the skin lying entirely externally on the body, while the central nervous system is within, and quite separate from the former. The two are connected merely by a portion of the peripheric nerve- I96 THE EVOLUTION OF MAN. system and of the sense-organs. And yet, as we already know from the germ-history of man, the latter is developed from the former. Those organs of our body which discharge the highest and most perfect functions of animal life — those of sensation, volition, thought — in a word, the organs of the psyche, of mental life — arise from the external skin- covering. This remarkable fact, considered in itself alone, seems so wonderful, inexplicable, and paradoxical, that the truth of the fact was simply long denied. The most trustworthy embryo- logical observations were met with the erroneous statement that the central nerve-system develops, not from the outer germ-layer, but from a special cell-layer lying underneath this. The ontogenetic fact would not, however, yield ; and, now that Phylogeny has thrown light on the subject, the fact seems perfectly natural and necessary. When we reflect on the historic evolution of mind and sense activities, we must necessarily conceive the cells, which accomplish these, as originally situated on the outer surface of the animal -body. Such externally placed elementary organs could alone directly receive and deal with impressions from the outer world. Afterwards, under the influence of natural selection, the complex cell-masses which had become especially " sensitive " gradually withdrew into the shelter of the interior of the body, and there laid the first foundations of a central nervous organ. As differentiation advanced, the distance and distinstion between the external skin- covering and the central nervous system detached from this, became continually greater, and finally the two were per- manently connected merely by the conductive peripheric nerves. DERMIC ORIGIN OF THE SENSORY ORGANS. 1 97 This view is fully confirmed by the results of Comparative Anatomy. Comparative Anatomy shows that many lower animals possess no nervous system, although, in common with higher animals, they exercise the functions of sensation, volition, and thought. In the Primitive Animals (Protozoa), which do not even form germ-layers, of course the nervous system, like the skin-covering, is wanting. Even in the second main division of the animal kingdom — in the Metazoa or Intestinal Animals — there is at first no nervous system. The functions of these are performed by the simple cell- layer of the exoderm, which the lower Intestinal Animals have inherited directly from the Gastrsea (Fig. 209, e). This is the case in the lowest Plant Animals (Zoophyta), the Gas- tra3ads, Sponges, and the lowest Hydroid Polyps, which are but little higher than the Gastrreads. Just as all the vege- tative functions of these are performed by th-3 simple intes- tinal layer, so all the animal functions are discharged by the equally simple skin-layer. The simple cell stratum of the exoderm is, in these, skin-covering, motive apparatus, and nervous system simultaneously. Most probably the nervous system was also wanting in a large proportion of those Primitive Worms (Archelminthes) which were developed directly from the Gastrseads. Even those Primitive Worms in which the two primary germ-layers had already split into the four secondary germ-layers (Plate V. Fig. 10), seem not to have possessed a nervous system distinct from the skin. The skin-sensory layer must, even in these long-extinct Worms, have been at once skin-covering and nerve-system. But already in the Flat Worms (Platel- minthes), and especially in the Gliding Worms (Turbellaria) which of all existing forms approach nearest to the Primitive 198 THE EVOLUTION OF MAN. Worms, we find an independent nerve-system, distinct and separate from the outer skin-covering. This is the " upper Fig. 209. — Gastrula of Gastrophysema (Gastrsead-class). Fig. 210. — Transverse section through an embryonic Earth-worm : hs, skin-sensory layer; hm, skin-fibrous layer; (7/, intestinal-fibrous layer; dd, intestinal-glandular layer ; a, intestinal cavity ; c, body-cavity, or Coeloma ; n, nerve-ganglia; u, primitive kidneys. Fig. 211. — A Gliding Worm (Bhabdoccelum). From the brain or upper throat ganglion (a) nerves (n) radiate towards the skin (/), the eyes (aw), the organ of smell {na), and the mouth (m) : h, testes; e, ovaries. THE SKIN. I99 throat ganglion/' situated above the throat (Fig. 211, g ; Plate V. Fig. 11, m). The complex central nervous system of all higher animals has developed from this simple rudiment. In the higher Worms, e.g., the Earth-worms, according to Kowalcvsky, the earliest rudiment of the central nervous system (Fig 210, n) is a local thickening of the skin- sensory layer (/is), which afterwards becomes entirely detached from the horn-plate. Even the medullary tube of Vertebrates has the same origin. From the germ-history of Man, we already know that this medullary tube, the commencement of the central nervous system, originally develops from the outer skin-covering. Let us now turn aside from these very interesting features in evolution, and examine the development of the later human skin- covering, with its hairs, sweat-glands, etc. Physiologically, this outer covering (derma, or tegumentum) plays a double part. The skin, in the first place, forms the general protective covering (integumentwm commune) which covers the whole surface of the body, and protects all other parts. As such, it, at the same time, effects a certain ex- change of matter between the body and the surrounding atmospheric air (perspiration or skin-breathing). In the second place, the skin is the oldest and most primitive sense-organ, the organ of touch, which effects the sensation of the surrounding temperature and of the pressure or re- sistance of bodies with which it comes in contact. The human skin, like that of all higher animals, consists essentially of two distinct parts ; of the outer-skin, and of the underlying leather-skin. The outer-skin (epidermis) consists only of simple cells, and contains no blood-vessels (Fig. 212, ab). It develops from the first of the secondary 32 200 THE EVOLUTION OF MAN. germ-layers from the skin-sensory layer, and, directly, from the horn-plate of the latter. The leather-skin (corium), on the contrary, consists principally of connective or fibrous Fig. 212. — Human skin in perpendicular section (after Ecker), much en- larged : a, horny stratum of outer-skin ^epidermis) ; b, mucous stratum of outer- skin ; c, papillae of the leather-skin (corium) ; d, blood-vessels of the latter ; e, /, excretory ducts of the sweat-glands (g) ; h, fat- globules of the leather-skin ; i, nerve, passing above into a touch-body. tissue, contains numerous blood-vessels and nerves, and has a different origin. It develops from the outer stratum of the second secondary germ-layer, from the skin-fibrous layer. The leather-skin is much thicker than the outer-skin. In its deeper part, the " subcutis" lie many masses of fat-cells (Fig. 212, h). Its upper part, the true "cutis," or papillary layer, forms, over nearly the whole surface of the body, a number of microscopic cone-shaped warts, or papillae, which fit into the overlying epidermis (c). These touch-warts, or sensory papillae, contain the most delicate of all the sensory organs of the skin, the " corpuscula tactus." Other papilloe STRUCTURE OF THE SKIN. 201 contain merely the terminal loops of the nutritive blood- vessels of the skin (cd). All these different parts of the leather-skin originate, bv differentiation, from the cells, orio*i- nally homogeneous, of the leather-plate, the outer lamella of the skin-fibrous layer (Fig. 112, hpr, vol. i. p. 352; Plates IV. and V., I; Figs. 65-G9, hf, p. 277).166 Analogously, all the constituent parts and appendages of the outer-skin (epidermis) originate, by differentiation, from the homogeneous cells of the horn- plate (Fig. 213). At a Fig. 213. — Cells of the outer-skin (epidermis) of a human embryo of two months. (After Koelliker.) very early period, the simple cell-layer of this horn-plate splits into two dis- tinct strata. The inner, softer stratum (Fig. 212, b) is called the mucous layer; the outer, harder stratum (a), the horn-layer of the outer- skin. The surface of this horn-layer is continually worn out and thrown off; new cell-strata, produced by the growth of the underlying mucous layer, take its place. Originally the outer-skin forms an entirely simple cover over the surface of the body. Afterwards, however, sundry appendages develop from this both internally and ex- ternally. The internal appendages are the skin-glands ; the sweat-glands, the sebaceous glands, etc. The external appendages are hair, nails, etc. The glands of the skin-covering are at first merely solid plug-shaped growths of the outer-skin (epidermis), which penetrate into the underlying leather-skin (corium) (Fig. 214 x). A canal afterwards forms inside these solid 202 THE EVOLUTION OF MAN. plugs (2, 3), either owing to the softening and breaking up of the central cells, or as the result of a fluid internally secreted. Some of these skin-glands remain unbranched, as, for instance, the sweat-glands (e,f,g). These glands, which secrete the sweat, are of great length, their ends forming a coil ; they never branch, however ; and the same is to be said of the glands which secrete the fatty wax of the ears. Fig. 214. — Rudiments of tear-glands from a human embryo of four months. (After Koelliker.) 1. Earliest rudiment the shape of a simple, solid plug. 2 and 3. Fur- ther developed rudiments, which branch and become hollow : a, a. solid offshoot ; e, cell-covering of the hollow offshoot ; /, rudiment of the fibrons covering, which afterwards forms the leather-skin round the glands. Most other skin-glands give out shoots and branches, as, for in- stance, the tear-glands, situated on the upper eyelid, which secrete the tears (Fig. 214), and also the sebaceous glands, which produce the fatty sebaceous matter, and generally open, into the hair-follicles. The sweat and sebaceous glands occur only in Mammals. The tear- glands, on the contrary, are found in all the three classes of Amnion Animals, in Reptiles, Birds, and Mammals. They are not represented in the lower Vertebrates. Very remarkable skin-glands, found in all Mammals, and in them exclusively, are the milk-glands (glandulce mammales, Figs. 215, 216). They supply milk for the nourishment of the new-born Mammal. Notwithstanding SKIN-GLANDS. 203 their extraordinary size, these important organs are merely large sebaceous skin-glands (Plate V. Fig. 10, md). The milk is produced by liquefaction of the fatty milk-cells within the branched milk-gland pouch (Fig. 215, c), just as the sebaceous matter of the skin, and the fatty matter of the hair are produced by the breaking up of fatty sebaceous cells within the sebaceous skin-glands. The excretory passages of the milk-glands enlarge into sac-like milk-ducts (6), which again become narrower (a), and open, through from sixteen to twenty-four minute apertures, into the nipple of the breast. The first rudiment of this large and complex gland is a very simple conical plug in the Fig. 215. — The breast of the female in section : c, grape-like glandular lobules ; b, enlarged milk-ducts ; a, narrow excretory ducts, opening through the breast-nipple. (After H. Meyer.) Fig. 216. — Milk-glands of a new-born child : a, original central gland; b, smaller, and c, larger branches of the latter. (After Langer.) 204 THE EVOLUTION OF MAN. outer-skin (epidermis), which extends into the leather-skin (coriwm), and there branches. In the new-born child it consists merely of from twelve to eighteen radiating lobules (Fig. 216). These gradually branch, the excretory passages become hollow, and a large quantity of fatty matter collects between the lobules. Thus is developed the prominent breast of the female (mamma), on the summit of which rises the nipple (mammilla), adapted for being sucked.167 The nipple does not appear until after the milk -gland is already formed; this ontogenetic phenomenon is very interesting, because the more ancient Mammals (the parent- forms of the entire class) had no nipples. In them, the milk simply emerged through a plane, sieve-like perforated spot in the abdominal skin, as is even now the case in the lowest extant Mammals, the Beaked Animals (Monotremata ; p. 146). On account of this character these animals may be called Amasta (without nipple). In many of the lower mammals there are numerous milk-glands, situated at various points of the ventral side. In the human female there is usually only a pair of milk-glands, placed on the point of the breast, as in Apes, Bats, Elephants, and some other Mammals. Occasion- ally, however, even in the human female two pairs of breast glands (or even more) appear, lying one behind the other ; this must be regarded as a reversion to an older parent- form. Sometimes these glands are well developed even in the male, and are capable of being sucked, though as a rule they exist in the male sex only as rudimentary organs with- out function. Just as the skin glands originate as local growths of the outer skin in an inward direction, so the appendages of the skin, called hair and nails, originate as local growths EXTERNAL APPENDAGES OF THE SKIN. 205 of the outer skin in an outward direction. The nails (un- gues), which are important protective formations over the hind surface of the most sensitive parts of our limbs — the tips of the fingers and toes — are horny products of the epidermis, common to us with the Apes. In their place, the lower Mammals generally possess claws, and the Hoofed Animals (Ungulata) hoofs. The parent-form of Mammals undoubtedly had claws, such as appear in a rudimentary state in the Salamander. The hoofs of the Hoofed Animals and the nails of Apes and of Man originated from the claws of more ancient Mammals. In the human embryo the first rudiment of the nails first appears (between the horn-layer and the mucous layer of this outer skin) in the fourth month. Their edges do not, however, project until the end of the sixth month. The most interesting and important appendages of the outer skin are the hairs, which, on account of their peculiar structure and mode of origin, must be regarded as very characteristic of the whole Mammalian class. Hairs, it is true, appear widely distributed in many lower animals, e.g., in Insects and Worms. But these hairs, like those of plants, are thread-like processes of the outer surface, and differ from Mammalian hairs in their characteristically finer structure and in their mode of development. Hence Oken rightly called Mammals " hairy animals." The hairs of Man, as of all other Mammals, consist simply of epidermic cells peculiarly differentiated and arranged. In their first state, they appear in the embryo as solid plug-shaped pro- cesses of the epidermis which penetrate into the underlying leather-skin (corium), as do the sebaceous and the sweat glands. As in the latter, the simple plug consists originally 206 THE EVOLUTION OF MAN. of the ordinary epidermic cells. Within this a firmer central cellular mass of conical shape soon forms. This increases considerably in length, detaches itself from the surrounding cellular mass, the "root-sheath," and finally makes its way to the outside, appearing above the outer surface as a hair-stem. The deepest part, buried in the skin, the hair follicle, is the root of the hair, and is sur- rounded by the root-sheath. In the human embryo the first hairs make their appearance at the end of the fifth or in the beo-inning of the sixth month. During the last three or four months before birth the human embryo is usually covered by a thick coating of deli- cate woolly hairs. This embryonic wool-covering (lanugo) is often lost during the last weeks of embryonic life, and, at any rate, soon after birth, when it is replaced by the thinner permanent hair-covering. These later permanent hairs grow out of hair follicles which are developed from the root-sheaths of the deciduous woolly hair. In the human embryo, the embryonic woolly hair usually covers the entire body, with the exception of the palms of the hands and the soles of the feet. These parts remain bare, just as in all Apes and most other Mammals. Not un- frequently the woolly coat of the embryo differs considerably in colour from the later permanent hairy covering. Thus for instance, it sometimes happens in our own Indo-Ger- manic race that fair-haired parents are shocked to find their children, at their first appearance, covered by a dark brown, or even black, woolly covering. It is only after this has been shed, that the permanent fair hair, which the child inherits from its parents, makes its appearance. Occasionally the dark hair is retained for several weeks, THE HAIR AS A RUDIMENTARY ORGAN. 207 or even months, after birth. This remarkable woolly covering can only be explained as an inheritance from our primordial long-haired ancestors, the Apes. It is equally worthy of note that many of the higher Apes resemble Man in the thin coat of hair which covers certain parts of their body. In most Apes, especially in the higher Catarhines, the face is nearly or even quite bare, or is covered with hairs as thin and as short as those of Man. In these Apes also, just as in Man, the hair on the back of the head is usually distinguished by its length, and the males often have much beard and whisker. (Cf. Fig. 202, p. 175). In both cases this masculine adornment has been acquired in consequence of sexual selection. In some Apes the breast and the inner sides of the joints are very thinly covered with hair — far less abundantly than is the back and the outer sides of the joints. On the other hand, we not unfrequently see the shoulders, the back, and the outer sides of the limbs thickly covered with hair in men of Indo-Germanic or Semitic race. It is a well-known fact that in some families abundant hair on the body is hereditary, as is the relative vigour and character of the hair-growth of the beard and head. These great differences in the total and partial hairiness of the body, which appear very striking not only when we compare different races of man, but even when we compare many families belonging to the same race, are very simply explained by the fact that the entire hairy covering of Man is a rudimentary organ, an unused inheritance, which has been transmitted from the more hirsute Apes. In this matter, Man resembles the Elephant, Rhinoceros, Hippopotamus, Whale, and other Mammals of various orders which have also entirely or 208 THE EVOLUTION OF MAN. •partially lost their original coat of hair in consequence of adaptation.168 The form of Adaptation which has degraded the growth of hair on most parts of the human body, while preserving it, or even greatly developing it, on certain parts, was, in all probability, sexual selection. As Darwin has very clearly shown in his work on " The Descent of Man," sexual selec- tion has had especially great influence in this respect. In consequence of the male Anthropoid Apes, in selecting a partner, preferring those females which were least hairy, and in consequence of the females preferring those suitors which were distinguished by peculiarly fine beard or head- hair, the general hirsuteness of the body was gradually degraded, while the beard and the hair of the head were advanced to a higher degree of perfection. Climatic con- ditions, and other circumstances unknown to us, may, however, also have promoted the loss of the hairy coat. In proof of the assertion that the hairy covering of Man is directly inherited from the Anthropoid Apes, we find, according to Darwin, a curious evidence in the direc- tion, otherwise inexplicable, in which the rudimentary hairs lie on our arms. Both on the upper and on the lower arm the hairs are directed towards the elbow, where they meet at an obtuse angle. Except in Man, this striking arrangement occurs only in the Anthropoid Apes, the Gorilla, Chimpanzee, Orang, and several species of Gibbons. In other Gibbons the hairs of both the lower and the upper arm are directed towards the hand, as in other Mammals. This remarkable peculiarity of Anthropoids and of Man can only be explained on the assumption that our common ape- like ancestors were accustomed, as they are even now, THE NERVOUS SYSTEM. 209 during rain, to bring their hands together over their heads, or over a branch overhanging their heads. The reverse direction of the hairs, when the arms were in this position caused the rain to run off. Thus, even yet, the direction of the hairs on our lower arm testifies to this advantageous habit of our Ape-ancestors. If the skin and its appendages are minutely examined, Comparative Anatomy and Ontogeny supply many similar important "records of creation," showing that they are directly inherited from the skin-covering of the Ape. We obtained our skin and hair by inheritance, immediately from Anthropoid Apes, these from the lower Apes, which, in turn, inherited the same parts from lower Mammals. This is also true of the other great organ-system which is developed from the skin-sensory layer — of the nervous system and the sensory organs. This very highly developed organ system, which performs the highest vital functions — those of the mind — we have inherited immediately from the Apes, and mediately from Mammals of a lower order. The human nervous system, like that of all other Mammals, is, in its developed condition, a very complex apparatus, the anatomical arrangement and the physiological activity of which may, in general terms, be compared to a telegraph system. The central marrow (medulla), or cen- tral nervous system, represents the principal station, the in- numerable "ganglion cells" (Fig. 7, vol. i. p. 129) of which are connected with each other and with numerous very delicate conducting lines by their branched processes. The latter are the peripheric " nerve fibres," distributed over the whole surface of the body; these, together with their terminal apparatus, the sense-organs, etc., constitute the " conductive 2IO THE EVOLUTION OF MAN. t, V marrow," the peripheric nerve-system. Some, as sensory nerve-fibres, convey the sensations of the skin and of other sense-organs to the central medulla ; others, as motor nerve- fibres, transmit the impulses from the central marrow to the muscles. Fig. 217. — Human embryo of three months, in natural size, seen from the dorsal side ; the brain and dorsal marrow exposed (after Koelliker) : h, hemispheres of the cerebrum (fore-brain); m, "four-bulbs" (mid-brain) ; c, small brain (hind -brain, or cerebellum). Below the latter is the three-cornered "neck-medulla" (after-brain) . Fig. 218. — Central marrow of a human embrvo of four months, in natural size, seen from the dorsal side (after Koelliker) : h, large hemi- spheres; v, " four-bulbs ; " c, small brain ; mo, neck-medulla. Below this the dorsal medulla marrow). The central nervous system, or central marrow (medulla centralis), is the actual organ of mental activities, in the stricter sense. Whatever view is taken of the intimate connection between this organ and its functions, it is, at least, certain that those of its special activities which we call sensation, volition, and thought, are in man, as in all the higher animals, inseparably connected with the normal development of this material organ. Hence we must neces- sarily take a deep interest in the history of the development THE CENTRAL MARROW. 211 of this organ. As it alone can give us the most important information as to the nature of our " mind," it commands our most earnest attention. For if the central marrow develops in the human embryo exactly as in the embryos of all other Mammals, then the development of the human mental onran from the same central oro;an of other Mammals and, more remotely, from that of lower Vertebrates, cannot be questioned. It is, therefore, impossible to dispute the enormous significance of these phenomena of development. In order to appreciate these rightly, a few words must first be said as to the general form and anatomical construc- tion of the developed central marrow in Man. Like the central nervous system of all other Skulled Animals (Cra- niota), it consists of two distinct parts : firstly, of the brain or the medulla of the head (encephalon, or medulla ca- pitis), and, secondly, of the spinal marrow (medulla spi- nalis). The former is enclosed in the bony skull, or " brain case," the latter in the bony vertebral canal, which is com- posed of a consecutive series of vertebrae, shaped like signet rings. (Cf. Plate V. Fig. 16, m.) From the brain proceed twelve pairs of head nerves, from the spinal marrow thirty- one pairs of medullary or spinal nerves for the remainder of the body. The spinal marrow, when examined merely anatomically, appears as a cylindrical cord with a spindle- shaped swelling in the region of the neck (at the last of the neck- vertebrae) and another in the lumbar region (at the first lumbar vertebra, Figs. 217, 218). At the swelling at the throat the large nerves of the upper limbs pass off from the spinal marrow, and those of the lower limbs from the swelling in the lumbar region. The upper end of the spinal marrow passes through the neck-marrow (medulla oblon- 212 THE EVOLUTION OF MAN. goto) into the brain. The spinal marrow appears indeed to be a dense mass of nervous substance ; but along its axis passes a very narrow canal, which is continued in front into the larger cavities of the brain, and which, like those cavities, is filled with a clear fluid. The brain forms a considerable mass of nervous sub- stance, of very complex, minute structure, which occupies Fig. 219. — Human brain, seen from the lower side. (After H. Meyer.) Above (in front) is the large brain (cerebrum), with extensively branched furrows ; below (be- hind) is the small brain (cere- bellum), with narrow parallel furrows. The Roman numbers indicate the roots of the twelve pairs of brain nerves in order from froLt to back. the greater part of the skull-cavity ; it is roughly distin- guishable into two main parts — the large and small brain (cerebrum and cerebellum). The former is situated in front and over the latter, and its surface exhibits the well- known characteristic convolutions and furrows (Figs. 219, 220). On its upper surface it is divided by a deep longi- tudinal slit into two lateral halves, the so-called "great hemispheres," which are connected by means of a bridge, or " cross-piece " (covpus callosum). A deep transverse fissure separates the large brain (cerebrum) from the small brain THE BRAIN. 213 (cerebellum). The latter is situated more posteriorly and inferiorly, and shows on its outer surface equally numerous furrows, which are, however, much finer and more regular, ro Fig. 220. — Human brain, seen from the left side. (After H. Meyer.) The furrows of the large brain are indicated by large, thick lines, those of the small brain by finer lines. Below the latter the neck-marrow is visible. fl-fs, frontal convolutions ; Ce. a Ce. p, central convolutions ; R, fissure of Eolan- dus; 8, Sylvian fissure; T, temporal or parallel fissure; Pa, parietal lobe; An, the annectant convolutions ; PO, parietooccipital fissure ; Su, supra-marginal convolution ; IP, intra-parietal fissure ; t, temporo-sphenoidal convolution. and between them are curved ridges (Fig. 219, lower part). The small brain is also divided into two lateral halves by a longitudinal furrow; these are the "small hemispheres," which are connected at the top by a worm-like cross-piece, the " brain- worm " (vermis), and at the bottom by a bridge (pons varolii; Fig. 219, VI.). Comparative Anatomy and Ontogeny show, however, that in Man, as in all other Skulled Animals, the brain originally consists not of two but of five distinct parts lying one behind another. These originally appear in the embryo of all 214 THE EVOLUTION OF MAN. Skulled Animals (Craniota), from the Cyclostomi and Fishes up to Man, in exactly the same form, as five bladders placed one behind the other. Alike in their first rudiments, they, however, differ in their further development. In Man and all higher Mammals the first of these five bladders, the fore-brain, develops so excessively that, when mature, it forms, both in size and weight, by far the greater part of the whole brain. To it belong, not only the great hemispheres, but also the bridge (corpus callosum), which connects these two, the olfactory lobes, from which proceed the nerves of smell, and most of the processes lying on the roof and floor of the great lateral cavities of the two hemispheres ; such, for instance, as the large streaked bodies (corpora striata). On the other hand, the "centres of sight," which lie be- tween the streaked bodies, belong to the second main part, which develops from the twixt-brain ; and to the same part belong the third brain ventricle (which is single) and the processes known as the "funnel" (infiindibulum), the gray mass, and the " cone " (conarium). Behind these, and between the large brain and the small brain, we find a little mass, composed of two pairs of bosses, and called the " four bulbs," on account of two superficial furrows which cross each other at right angles, thus quartering the whole mass (Figs. 217, m, 218, v). Though these " four bulbs " are very insignificant in Man and the higher Mammalia, they constitute a distinct part of the brain, the third, or mid- brain, which is, on the contrary, especially well developed in the lower Vertebrates. The next or fourth part of the brain is the hind-brain, or small brain (cerebellum), in the strict sense of the term, with its single middle process, the " worm " (vermis), and its two lateral parts, the " small PARTS OF THE BRAIN. 21 5 hemispheres " (Figs. 217, c, 218, c). Behind this comes, finally, the fifth and last part, the " neck-marrow " (medulla oblon- gata, Fig. 218, mo), which includes the single fourth brain ventricle and the adjoining processes (pyramids, olives, and restiform bodies). The neck medulla passes directly down into the spinal marrow. The narrow central canal of the spinal marrow extends into the wider " fourth ventricle " of the neck medulla, which is rhomboidal in shape, and the fioor of which forms the " rhomboid groove." From this proceeds a narrow duct, called the " aqueduct of Sylvius," which leads through the "four-bulbs " into the third ven- tricle, situated between the two " centres of sight ; " and this cavity in turn is connected with the pair of lateral cavities which lie right and left in the large hemispheres. All the cavities of the central marrow are, therefore, directly connected together. Individually all these parts of the brain which we have enumerated have an infinitely complex, minute structure, which we cannot now study, and which hardly bears on our subject. This wonderful brain-struc- ture, as it occurs only in Man and the higher Vertebrates, is of the highest importance, simply because, in all Skulled Animals (Craniota), it develops from the same simple rudi- ments, from the five brain-bladders already enumerated. (Cf. Plates VI. and VII.) Before we direct our attention to the individual develop- ment of the complex brain from this series of simple bladders, Ave will, in order to understand the matter more clearly, glance for a moment at those lower animals which have no such brain. Even in the skull-less Vertebrates, in the Amphioxus, there is no real brain. In this case the whole central marrow is merely a simple cylindrical cord 2l6 THE EVOLUTION OF MAN. traversing the body longitudinally, and terminating in front almost as simply as at the other end : it is a simple medul- lary tube (Plate XI. Fig. 15, m). We found, however, that the rudiment of the same simple medullary tube occurs in the ascidian larva (Plate X. Fig. 5, m) and in the same cha- racteristic position, above the notochord. Moreover, when closely examined a small bladder-like swelling may be seen at the fore end of the medullary tube in these two closely allied animals ; this is the first indication of a separation of the medullary tube into brain (mx) and spinal marrow (m2). When, however, we consider the undeniable relationship of the Ascidia to the rest of the Worms, it is evident that the simple central marrow of the former exactly answers to the simple nerve -ganglion which, in the lower Worms lies above the throat (pharynx), and which has, therefore, long been called the " upper throat ganglion " (ganglion pharyngeum super ins). In the Gliding Worms (Turbellaria) the whole nerve system consists merely of this simple ganglion, which is situated on the dorsal side of the body, and from which nerve- threads radiate to the different parts of the body (Fig. Z\l,gri) This upper throat ganglion of the lower Worms is evidently the rudiment from which the more complex central marrow of the higher animals has developed. An elongation of the upper throat ganglion along the dorsal side gave rise to the medullary tube, which is characteristic of Vertebrates and the young forms of Ascidia alone. On the other hand, in all other animals, the central nerve system has de- veloped in a very different manner from the upper throat ganglion; in Articulated Animals (Arthropoda) especially, the latter has developed into a throat (pharyngeal) ring, with a ventral marrow ; this is the case, also, in the articu- THE NEKVOUS SYSTEM IN THE LOWER ANIMALS. 217 lated Ringed Worms (Annelida) and the Star-animals (Echi- noderma), which originated from Arthropods. The Soft- bodied Animals (Mollusca) also have a throat ring, which is quite unrepresented in Vertebrates. Only in Vertebrates the central marrow developed along the dorsal side, while in all other animals which have been named it developed along the ventral side of the body.169 Descending below the Worms we find very many animals which are entirely without a nerve-system, and in which the functions of that system are performed simply by the outer skin-covering — by the cells of the skin-layer, or exoderm. This is the case in many low Plant Animals (ZoopJiyta), for instance, in all Sponges, and in the common fresh-water Polyp, the Hydra. It was also undoubtedly the case in all extinct Gastraeads. In all Primitive Animals (Protozoa) the nerve-system is, of course, unrepresented, for these have not as yet attained to the development of germ- layers. In considering the individual development of the nerve- system in the human embryo, we must first of all start from the important fact already mentioned, that the first rudi- ment of the system is the simple medullary tube, which detaches itself from the outer germ-layer along the middle line of the lyre-shaped primitive germ. We found (Figs. 85-87, vol. i. p. 298) that the rectilineal primitive groove, or dorsal furrow, first arises in the centre of the lyre-shaped germ- disc. On each side of this rise the two parallel dorsal or medullary swellings. The free margins of these bend to- wards each other, coalesce, and form the closed medullary tube (Figs. 88-93, vol. i. pp. 300-309). At first this tube lies directly under the horn-plate ; it is, however, afterwards situate 218 THE EVOLUTION OF MAN. j&MflifN. »?/»•• Figs. 221-223. — Lyre-shaped (or sole-shaped) germ-shield of a Chick, in three consecutive stages of evolution, seen from, the dorsal surface : about twenty times enlarged. Fig. 221, with six pairs of primitive vertebrae. The brain a simple bladder (Jib). The medullary furrow is wide open from the point x, very wide at z. mp, Marrow (or medullary) plates ; sp, side- plates ; y, boundary between the throat cavity (sh) and the head-intestine (vd). Fig. 222, with ten pairs of primitive vertebra?. The brain consists of three bladders : v, fore-brain ; m, mid-brain ; Ji, hind-brain, c, Heart ; dv, yelk-veins. The medullary furrow is wide open behind (z). mp, Marrow- plates. Fig. 223, with sixteen pairs of primitive vertebras. The brain consists of five bladders : v, fore-brain ; z, twixt-brain ; m, mid-brain ; h, hind-brain ; n, after-brain, a, Eye-vesicles ; g, ear- vesicles ; c, heart ; dv, yelk- veins ; mp, marrow-plate, uiv, primitive vertebras. DEVELOPMENT OF THE BEAIN. 219 quite internally, the upper edges of the primitive vertebral plates, which penetrate, from right and left, in between the horn-plate and the medullary tube, uniting above the latter, and thus completely embedding it in a closed canal. As Gegenbaur most aptly remarks, " This gradual embedding in the interior of the body must be regarded as an incident acquired in connection with progressive differentiation, and with the consequent higher capacity, by which the most important organ of the system is secured in its interior." To every thoughtful and unprejudiced man it must appear an extremely important and pregnant fact, that our mental organ, like that of all other Skulled Animals (Cra- niota), commences in the same way and in exactly the same simple form in which this organ remains for life in the lowest Vertebrate, the Amphioxus (vol. i. p. 420, Fig. 151; Plate XL Fig. 15, m). In the Cyclostomi, that is, in the stage above the Acrania, the anterior extremity of the cylindrical medullary tube begins to extend, at an early period, in the form of a pear-shaped bladder, which is the first distinct rudiment of a brain (Plate XI. Fig. 16, m^. For the central medulla of Vertebrates thus first distinctly differentiates into its two main sections, the brain (mx) and the spinal marrow (m2). The first faint indication of this important differentiation is discoverable in the Amphioxus, perhaps even in the Ascidian larva (Plate X. Fig. 5). The simple bladder-like form of the brain, which is retained for a considerable time in the Cyclostomi, also appears at first in all higher Vertebrates (Fig. 221, hb). In the latter, however, it soon disappears, in consequence of the separation of the simple brain-bladder, by transverse contractions of its circumference, into several consecutive 220 THE EVOLUTION OF MAN. parts. Two of these contractions first appear, and con- sequently the brain forms three consecutive bladders (Fig. Figs. 224-226.— Central mar- row of human embryo in the seventh week, two cm. long. (After Koelliker.) Fig. 226, view of the whole embryo from the dorsal side ; the brain and dorsal marrow laid bare. Fig. 225, the brain and upper part of the dorsal marrow from the left side. Fig. 224, the brain from above : v, fore-brain ; z, twixt-brain ; m, mid-brain ; h, hind-brain ; n, after-brain. 222, v, m, h). The first and third of these three primitive bladders then again separate by transverse contractions, each into two parts, and thus five consecutive bladder-like divisions are formed (Fig. 223 : cf. also Plate V. Figs. 13-16 ; Plates VI. and VII., second cross-line). These five fundamental brain-bladders, which re-occur in the same form in the embryos of all the Skulled Animals (Craniota), were first clearly recognized by Baer, who understood their true importance and distinguished them, according to their rela- tive positions, by very appropriate names, which are still in general use : I., fore-brain (v) ; II., twixt-brain (z); III., mid- brain (m) ; IV., hind-brain (h) , and V., after-brain (n). In all Skulled Animals, from the Cyclostomi to Man, the same parts, although in very various forms, develop from these five original brain-bladders. The first bladder, the fore-brain (protopsyche, v), forms by far the largest part of the so-called "great brain" {cerebrum) ; it forms the two great hemispheres, the olfactory lobes, the streaked bodies (corpora striata), and the cross-piece {corpus callosum), together with the " arch r' (fornix). From the second THE BRAIN IN SKULLED ANIMALS. 221 bladder, the twixt-brain (deutopsyche, z,) proceed primarily the " centres of sight " and the other parts which surround the so-called "third brain-ventricle/' also the "funnel" (infundibidum), the " cone " (conarium), etc. The third bladder, the mid-brain (mesopsycJie, m), furnishes the small group of the " four bulbs/' together with the " aqueduct of Sylvius." From the fourth bladder, the hind-brain (meta- psyche, 7i), the greater part of the so-called " little brain " (cerebellum) develops; the central "worm" (vermis), and the two lateral "small hemispheres.'' The fifth bladder, finally, the after-brain (epipsyche, n), forms the neck- marrow, or the "elongated marrow" (medulla oblongata), together with the rhomboid groove, the pyramids, olives, etc. The very highest importance must certainly be ascribed to the fact, seen in Comparative Anatomy and Ontogeny, that the brain is originally formed in exactly the same way in the embryos of all Skulled Animals (Craniota), from the lowest Cyclostomi and Fishes, to Apes and Man. In all, the first rudiment of the brain is a simple bladder-like expansion at the anterior extremity of the medullary tube. In all, the five bladders are formed from this simple bladder- like expansion, and in all, these five primitive brain- bladders develop into the permanent brain, with its many complex anatomical arrangements, which afterwards appear in such extremely diverse forms in the various vertebrate classes. On comparing the mature brain of a Fish, an Amphibian, a Reptile, a Bird, and a Mammal, it is hardly conceivable that the several parts of these forms, so ex- tremely different, both internally and externally, may be traced back to one common condition. And yet, all these various brains of Craniota have originated from exactly the 222 THE EVOLUTION OF MAN. same rudimentary form. We need only compare the em- bryos of these various classes of animals at corresponding stages of development, in order to assure ourselves of this fundamental fact. (Cf. Plates VI. and VII., second cross- line.) \CL V 771 = ■nv Fig. 227. — Brains of three embryonic Skulled Animals in vertical longi- tudinal sections: A, of a Shark (Heptanchus) ; B, of a Snake (Coluber); C, of a Goat (Copra) ; a, fore-brain ; b, twixt-brain ; c, mid-brain ; d, hind-brain ; e, after-brain; s, primitive fissure of the brain. (After Gegenbaur.) Fig. 228. — Brain of a Shark (Sajllium) from the dorsal side : g, fore- brain ; h, olfactory bulbs of the fore-brain, which send the large olfactory nerves to the large nose capsules (o) ; d, twixt-brain ; b, mid-brain (behind it, the insignificant rudiment of the hind-brain) ; a, after-brain. (After Busch.) Fig. 229. — Brain and dorsal marrow of a Frog : A, from the dorsal side ; B, from the ventral side ; a, olfactory bulbs, in front of the fore-brain (b) ; i, funnel at the base of the twixt-brain ; c, mid-brain ; d, hind-brain ; s, rhomboid groove in the after-brain ; m, dorsal marrow (very short in the frog) ; m', root-processes of the spinal nerves ; t, fibre at the end of the dorsal marrow. (After Gegenbaur.) COMPARATIVE VIEW OF BRAIN DEVELOPMENT. 223 A thorough comparison of the corresponding stages of development in the brain in the various Skulled Animals (Craniota) is very instructive. If it is applied to the whole series of skulled classes, the following extremely interest- ing facts soon become evident : in the Cyclostomi (Myxi- noides and Petromyzontcs), which, as we have seen, are the lowest and earliest Skulled Animals, the whole brain remains for life at a very low and primitive stage of development, through which the embryos of the other Skulled Animals pass very rapidly; the five original sections of the brain are visible throughout life in an almost unmodified form. But even in Fishes, an essential and important transformation of the five bladders takes place ; it is evidently from the brain of the Primitive Fishes (Selachii ; Fig. 228), that, on the one side, the brain of the other Fishes, and on the other, the brain of the Amphibians and also of the higher Vertebrates, must be traced. In Fishes and Amphibians (Fig. 229), the central part, the mid-brain, and also the fifth, section, the after-brain, are especially developed, while the first, second, and fourth sections remain far behind. In the higher Vertebrates, the exact reverse is the case, for in these the first and fourth sections, the fore and hind brains, develop pre-eminently ; on the other hand, the mid-brain remains very small, and the after-brain is also much smaller. The greater part of the " four-bulbs " is covered by the large brain (cerebrum) and the after-brain by the small brain (cerebellum). Even among the higher Vertebrates themselves, numerous grada- tions occur in the structure of the brain. From the Am- phibians upward, the brain, and with it the mental life, develops in two different directions, of which the one is 33 224 THE EVOLUTION OF MAN. carried out in Reptiles and Birds, the other in Mammals. The latter are especially distinguished by the very charac- teristic development of the first section, the fore-brain. In Fig. 230. — Brain of Rabbit : A, from the dorsal side ; B, from the ventral side ; lo, olfactory lobules ; I., fore-brain ; 7i, hypophysis at the base of the twixt-brain ; III., mid-brain ; IV., hind-brain; V., after-brain; 2, optic nerve ; 3, motor nerve of the eye ; 5-8, fifth to eighth nerves of the brain. In A, the upper surface of the right large hemisphere (I.) is removed, so that the streaked bodies (corpora striata) can be seen in its side chamber (yentriculus lateralis). (After Gegenbaur.) Mammals alone (Fig. 230) does this "great brain" develop to such an extent, that it eventually covers all the other parts of the brain from above. There are also remarkable differences in the relative positions of the brain-bladders. In the lower Skulled Animals the Hve brain-bladders are at first situated one behind the other in the same plane. If the brain is re- garded from the side, a straight line may be drawn through all the five bladders. But in the three higher vertebrate classes, in the Amnion Animals (Amniota), a noticeable curving of the rudimentary brain takes place, simultaneously BRAIN CURVATURE. 225 with the head and neck curving of the whole body, owing to the fact that the whole upper dorsal surface of the brain grows much faster than the lower ventral surface. The result is that the brain is so curved that its parts are after- wards situated thus : the fore-brain lies quite in front and below, the twixt-brain somewhat higher and over it, while the mid-brain lies highest of all and projects furthest for- ward; the hind-brain is situated lower, the after-brain yet further back and below. This disposition occurs only in the three classes of the Amniota, in Reptiles, Birds, and Mammals. (Cf. Plates I, VI., and VII.) Though, in general features of growth, the brains of Mammals correspond with those of Birds and Reptiles, yet striking differences very soon appear between the two. In Birds and Reptiles (Plate VI. Figs. H and G), the mid- brain (m) and the central part of the hind-brain develop considerably. In Mammals, on the other hand, these parts remain small, and instead, the fore-brain begins to grow so rapidly that it covers the other bladders from in front and above. As it constantly grows further back, it even- tually covers the whole of the rest of the brain above, and also encloses the central part from the sides. This process is of the greatest importance, because this fore-brain is the organ of the higher mental activities, — because in it are accomplished those functions of the nerve-cells, the sum of which is generally designated as the mind, or the "spirit" in the narrower sense. The highest activities of the animal body, the wonderful manifestations of consciousness, the complex phenomena of the activities of thought, have their seat in the fore-brain. It is possible to remove the great hemispheres of a Mammal, piece by piece, without killing 226 THE EVOLUTION OF MAN. the animal, thus proving that the higher mental activities, consciousness and thought, conscious volition and sensation, may be destroyed one by one, and finally entirely anni- hilated. If the animal thus treated is artificially fed, it may be kept alive for a long time ; for the nourishment of the entire body, digestion, respiration, the circulation of the blood, secretion, in short, the vegetative functions, are in no way destroyed by this destruction of the most important mental organs. Conscious sensation and voluntary motion, the capacity for thought and the combination of the various higher mental activities, have alone been lost. This fore-brain, the source of all these most wonderful nervous activities, reaches that high degree of perfection only in the higher Placental Animals {Placentalia) ; a fact which explains very clearly why the higher Mammals so far excel the lower in intellectual capacity. While the "mind" of the lower Placental Animals does not exceed that of Birds and Reptiles, we find among the higher Placentalia an uninter- rupted gradation up to Apes and Man. Accordingly, their anterior brains show surprising differences in the degree of perfection. In the lower Mammals, the surface of the great hemispheres (the most important part) is entirely smooth and even. The fore-brain, too, remains so small that it does not even cover the mid-brain above (Fig. 230). One stage higher, and this latter is indeed entirely covered by the excessive growth of the fore-brain ; but the hind-brain remains free and uncovered. At last, in Apes and in Man, the fore-brain covers the hind-brain also. A similar gradual advance may also be traced in the development of the peculiar furrows and protuberances which are so charac- teristically prominent on the surface of the large brain CONVOLUTIONS OF BRAIN. 227 {cerebrum) of higher Mammals (Figs. 219, 220). If the brains of the various mammalian groups are compared with reference to these convolutions and furrows, it appears that their gradual development is entirely proportionate with the development of the higher intellectual activities. Much attention has recently been devoted to this particular branch of the Anatomy of the brain, and very striking individual differences have been found even within the human race. In all human individuals distinguished by peculiar ability and great intellect, these swellings and furrows on the surface of the great hemispheres exhibit a much greater development than in common average men; while in the latter, again, they are more developed than in Cretins and others of unusually feeble intellect. There are also similar gradations in the internal structure of the fore- brain in Mammals. The great cross-piece (corpus callosum), especially, the bridge between the two great hemispheres, is developed only in Placental Animals. Other arrange- ments, for example, in the structure of the lateral cavities, which seem primarily to be peculiar to Men as such, re- appear only in the higher species of Apes. It was long- believed that Man had some entirely peculiar organs in the great brain (cerebrum), which are wanting in all other animals. But close comparison has shown that this is not the case, but that rather the characteristic qualities of the human brain exist in a rudimentary state even in the lower Apes, and are developed to a greater or less degree in the higher Apes. Huxley, in his important and much -quoted book, " Evidence as to Man's Place in Nature " (1863), has shown, most convincingly, that within the Ape-series the differences in the formation of the brain are greater between the 228 THE EVOLUTION OF MAN. higher and lower Apes than between the higher Apes and Man. This statement is, indeed, equally true of all the other parts of the body. But the fact that it is true of the central marrow is especially important. This does nob become fully evident unless these morphological facts are considered in connection with the corresponding physio- logical phenomena; until we consider that every mental activity requires for its complete and normal exercise the complete and normal condition of the corresponding brain- structure. The extremely complex and perfect active phenomena within the nerve-cells, summed up in the word "mental life," can no more exist without their organs in the vertebrates, including man, than can the circulation of the blood without a heart or blood. As, however, the central marrow of Man has developed from the same medullary tube as in all other Vertebrates, so also must the mental life of Man have had the same origin. All this is of course true of the conductive marrow, or the so-called "peripheric nervous system." This consists of the sensitive nervous fibres which convey the impressions of sensation from the skin and the organs of the senses in a centripetal direction to the central marrow ; as well as of the motor nervous fibres, which, reversely, convey the movements of volition from the central marrow, in a cen- trifugal direction to the muscles. By far the greater part of these peripheric conductive nerves originates from the skin-fibrous layer, by peculiar local differentiation of the rows of cells into the respective organs. The membranous coverings and blood-vessels of the central marrow are identical in origin with the greater part of the conductive marrow; these membranous coverings ORIGIN OF THE FUNCTIONS OF THE BRAIN. 229 are the inner membrane (pia mater), the central membrane (rneninx arachnoides), and the outer membrane (dura 'mater). All these parts are developed from the skin-fibrous layer. TABLE XXVII. Systematic Survey of the most important Periods in the Phylogeny ,of the Human Skin-coverings. I. First Period : Skin of Gastrceads. The entire skin-covering (including the nervous system, not yet differ- entiated from it) consists of one simple layer of ciliated cells (exoderm, or primary skin-layer); as it is at the present day in the gastrula of the Amphioxus. II. Second Period : Skin, of Primitive Worms. The simple exoderm of the Gastrasad has thickened and split into two distinct layers, or secondary germ-layers : the skin-sensory layer (rudiment of the horn-plate and nerve-system) and the skin-fibrous layer (rudiment of the leather skin (corium), the muscle-plate and the skeleton-plate. The skin is potentially both covering and mind. III. Third Period : Skin of Chordoma. The skin-sensory layer has differentiated into the horn-plate (epidermis) , and the central marrow (upper throat ganglia) separated from it ; the latter elongates into a medullary tube. The skin-fibrous layer has differentiated into the leather plate (corium) and, below this, the skin-muscular pouch (as in all Worms). IV. Fourth Period : Skin of Acrania. The horn-plate yet forms a simple epidermis. The leather-plate is fully differentiated from the muscle and skeleton plates. 23O THE EVOLUTION OF MAN. V. Fifth Period : Shin of Cyclostoma. The outer-skin remains a simple, soft mucous layer of cells, but forms one-celled glands (cup-cells). The leather-skin {corium) differentiates into cutis and sub-culis. VI. Sixth Period : Skin of Primitive Fishes. The outer skin is still simple. The leather skin forms placoid scales or small bony tablets, as in the Selachii. VII. Seventh Period : Shin of Amphibia. The outer skin differentiates into an outer horn-layer, and an inner mucous layer. The ends of the toes are covered with horny sheaths (first rudiments of claws or nails). VIII. Eighth Period : Shin of Mammals. The outer skin forms the appendages characteristic of Mammals only ; hair, and sebaceous, sweat, and milk glands. TABLE XXVIII. Systematic Survey of the most important Periods in the Phylogeny of the Human Nervous System. I. First Period : Medulla of Gastr&ads. The nerve system is not yet distinct from the skin, and, together with the latter, is represented by the simple cell-stratum of the exoderm, or primary skin-layer; as it is at the present day in the gastrula of the Amphioxus. II. Second Period : Medulla of Primitive Worms. The central nerve system is yet, at first, a part of the skin-sensory layer, and afterwards consists of a throat medulla, a simple nerve-ganglion lying above the throat ; as it is now in the lower "Worms : the upper throat- ganglion. SURVEY OF HUMAN NERVOUS SYSTEM. 23 1 III. Third Period : Medulla of Chordoma. The central nerve system consists of a simple medullary tube, an elongation of the upper throat ganglion, which is separated from the intes- tine by a notochord (chorda dorsalis). IV. Fourth Period : Medulla of Acrania. The simple medullary tube differentiates into two parts : a head, and a dorsal part. The head medulla resembles a small, pear-shaped, simple swelling (the primitive brain, or first rudiment of the brain) on the anterior extremity of the long cylindrical spinal marrow. V. Fifth Period : Medulla of Cyclostoma. The simple, bladder-like rudiment of the brain divides into five con- secutive brain-bladders of simple structure. VI. Sixth Period : Medulla of Primitive Fishes. The five brain-bladders differentiate into a form similar to that now permanently retained by the Selachii. VII. Seventh Period : Medulla of Amphibia. The differentiation of the five brain-bladders progresses to that structure which is now characteristic of the brain in Amphibia. VIII. Eighth Period : Medulla of Mammals. The brain attains the characteristic peculiarities distinctive of Mammals. The following may be distinguished as subordinate stages of development ; 1, the brain of Monotremes ; 2, the brain of Marsupials ; 3, the brain of Semi-apes ; 4, the brain of Apes ; 5, the brain of Man-like Apes ; 6, the brain of Ape-men j and 7, the brain of Man. ( 2l2 ) TABLE XXIX. Systematic Survey of the Evolution of the Skin-covering and Nerve System. XXIX. A. Survey of the Evolution of the Skin-covering. Skin (Derma, or Integumentum) Outer-skin (Epidermis) Product of the Skin- sensory-layer Horn-layer of the outer / Hair skin Nails (Stratum corneum) | Sweat glands Mucous layer of the \ Tear glands outer skin (Stratum mucosum) Sebaceous glands vMilk glands Leather-skin (Corium) f Fibrous layer of leather skin the I Connective tissue Fatty tissue Muscular tissue J r , , (Cutl/?u ! .t, 4 Blood-vessels Froduct of the Skin- ) Fatty layer of the leather } t>..„:11 f tr fibrous-layer skin (Subcutis) Papillas of taste and nerves of the leather skin XXIX. B. Survey of the Evolution of the Central Marrow. Central Marrow, or Central Nerve System (Psyche, or Medulla \ Centralis). Product of the Skin-sensory layer Fore-brain (Protopsyclie) II. Twixt-brain (Deut'opsyche) III. Mid-brain (Mesopsyche) IV. Hind-brain (Metapsyche) 'Great hemispheres Olfactory lobules Lateral chambers Streaked bodies Arch I. Cross piece f Centre of sight | Third chamber of the •< brain | Pineal body L Funnel I Four bulbs Aqueduct of Sylvius (_ Brain stalks After-brain (Epipsyche) ( Small hemispheres -j Brain worm L Brain bridge ( Pvramids | Olives ■{ Eestiform bodies j Fourth chamber of the (^ brain \VI. Dorsal Marrow Xotopsyche Eem ifph aerce cerebri J.obi olfactorii Ycntriculi lateralcs Cor i or a striata Fornix Corpus callosum Thalami optici Yentriculus tertius Con avium Infundibulum Corpus bigeminum Aqueeductus Sylvii Ptdunculi cerebri JTemisphara' cerebelli Vermis cerebelli Pons Varolii Corpora pyramidalia Corpora oliraria Corpora restiformia Yentriculus quartus Medulla spinalis Medullary coverings (Meninges) 'Enveloping mem- branes, with the nutritive blood- - vessels of the brain and spinal cord 1. Soft medullary skin Pia mater 2. Central medullary skin Arachnoidea 3. Hard medullary 'skin Dura matev (Products of the skin-fibrous layer) CHAPTER XXI. DEVELOPMENT OF THE SENSE-ORGANS. Origin of the most highly Purposive Sense-organs by no Preconceived Purpose, but simply by "Natural Selection. — The Six Sense-organs and the Seven Sense-functions. — All the Sense-organs originally Developed from the Outer Skin-covering (from the Skin-sensory Layer). — Organs of the Pressure Sense, the Heat Sense, the Sexual Sense, and the Taste Sense. — Structure of the Organ of Scent. — The Blind Nose-pits of Fishes. — The Nasal Furrows change into Nasal Canals. — Separation of the Cavities of the Nose and Mouth by the Palate Eoof. — Structure of the Eye. — The Primary Eye Vesicles (Stalked Protuberances from the Twixt-brain). — Inversion of this Eye Vesicle by the Crystalline Lens, separated from the Horn-plate. — Inversion of the Vitreous Body. — The Vascular Capsule and the Fibrous Capsule of the Eyeball. — Eye- lids.— Structure of the Ear. — The Apparatus for Perception of Sound : Labyrinth and Auditory Nerve. — Origin of the Labyrinth from the Primitive Ear Vesicles (by Separation from the Horn-plate). — Conduct- ing Apparatus of Sound : Drum Cavity, Ear Bonelets, and Drum Mem- brane.— Origin of these from the First Gill-opening and the Parts immediately round it (the First and Second Gill-arch). — Rudimentary Outer Ear. — Rudimentary Muscles of the Ear-shell. " Systematic Physiology is based especially upon the history of develop- ment, and unless this is more complete, can never make rapid progress ; for the history of development furnishes the philosopher with the materials necessary for the secure construction of a system of organic life. Hence anatomical and physiological researches should be prosecuted more from tho 234 • THE EVOLUTION OF MAN. point of view of development than is now the case ; that is, we should study each organ, each tissue, and even each function simply with the view of determining whence they have arisen." — Emil Huschke (1832). The sense-organs are undeniably among the most important and most interesting parts of the human body ; through their activity alone we recognize the objects in the world around us. " Nihil est in intellectu, quod non prius fuerit in sensu." They are the true springs of our mental life. In no other part of the animal body can we point to such extremely delicate and complex anatomical contrivances, co-operating for a definite physiological aim ; and in no other part of the body do these wonderful and very apt contrivances seem, at first, to indicate a premeditated creative design so conclu- sively. Hence it is that, in accordance with the received teleological view, it has been customary to admire the so- called " wisdom of the Creator ' and the " purposive con- trivances of His Creation " especially in this matter. But on more mature consideration it will be observed that the Creator, according to this conception, does after all but play the part of an ingenious mechanic or of a skilful watch- maker ; just, indeed, as all these cherished teleological conceptions of the Creator and His Creation are based on childish anthropomorphism. We admit that at first sight this teleological explana- tion seems to afford the simplest and fittest interpretation of these very apt contrivances. If the structure and func- tions of the very highly developed sense-organs are alone regarded, it seems as though their origin is hardly explic- able except on the assumption of a supernatural creative act. But it is exactly on this point that the history of ORGANS OF SENSE. 235 evolution proves most clearly that this received conception is radically false. The history of evolution convinces us that the highly purposive and admirably constituted sense organs, like all other organs, have developed without premeditated aim ; that they originated by the same mechanical process of Natural Selection, by the same constant interaction of Adaptation and Heredity, by which all the other pur- posive contrivances of the animal organization have been slowly and gradually evolved during the " Struggle for Existence." Like most other Vertebrates, Man possesses six distinct organs of sense, which accomplish seven distinct sensations. The external skin-covering accomplishes the sensation of pressure (resistance) and of temperature (warmth and cold). This is the earliest, the lowest, and the least differentiated organ of sense; it is distributed over the entire surface of the body. The other sensorial activities are localized. The sexual sense is limited to the skin-covering of the external sexual organs, just as the sense of taste is limited to the mucous membrane of the mouth-cavity (tongue and palate), and the sense of smell to the mucous membrane of the nose-cavity. Special mechanical contrivances of great com- plexity exist for the two highest and most differentiated organs of sense, the eye for the sense of sight, and the ear for that of hearing. Comparative Anatomy and Physiology show that in the low animals specialized sense-organs are entirety wanting, and that all sensations are transmitted through the outer surface of the skin-covering. The undifferentiated skin-layer, or exo- derm, of the Gastnea is the simple cell-layer from which the 236 THE EVOLUTION OF MAN. differentiated sense-organs of all Intestinal Animals (Metazoa), and, therefore, of all Vertebrates, originally developed. Start- ing from the consideration that necessarily only the most superficial parts of the body, those immediately exposed to the outer world, could have accomplished sensations, we should be justified in conjecturing a priori that the organs of sense also owe their origin to the same part. This is, indeed, the fact. The most important part of all sense-organs develops from the outermost germ-layer, from the skin- sensory layer ; in part, directly from the horn-plate, and, in part, from the brain, the foremost section of the medullary tube, after this has separated from the horn-plate. On comparing the individual development of the various organs of sense, we see that at first they make their appearance in the simplest conceivable form : only very gradually does that wonderful perfect structure develop by which the higher sense-organs eventually become the most remarkable and the most complex mechanisms of the entire organiza- tion. All organs of sense are, however, originally merely portions of the external skin-covering, in which sensorial nerves are distributed. Even these nerves were originally homogeneous and undifferentiated in character. Gradually, by division of labour, the various functions or " specific energies" of the different sensorial nerves developed. Simul- taneously the simple terminal expansions of these sense nerves in the skin-covering developed into extremely com- plex organs. The important bearings of these historic facts upon the just appreciation of mental life will readily be perceived. The whole philosophy of the future will assume another NATURAL SYSTEM OF PSYCHOLOGY. 237 form as soon as Psychology has gained an accurate know- ledge of these genetic facts, and has made them the basis of its speculations. If the psychological teachings, published by the best- known speculative philosophers, and still generally received, are impartially studied, the simplicity with which the authors bring forward their airy metaphysical speculations, regardless of all the significant ontogenetic facts by which their doctrines are clearly refuted, cannot fail to cause great sur- prise. And yet the history of evolution, in conjunction with the rapidly advancing Comparative Anatomy and Physiology of the sense-organs, affords the only safe founda- tion for the natural theory of the mind. With reference to the terminal expansions of the sensory nerves, the human organs of sense may be distri- buted into three groups, corresponding to three different stages of development. The first group includes those sense-organs, the nerves of which distribute themselves simply in the free surface of the skin-covering (organs of the sense of pressure, of heat, and of the sexual sense). In the second group, the nerves distribute themselves in the mucous membrane of cavities, which are originally grooves or inversions of the skin-covering (organs of taste and of smell). Finally, the third group is constituted by those very highly developed sense-organs, the nerves of which distribute themselves over an internal vesicle detached from the skin-covering (organs of sight and hearing). This remarkable genetic relation is represented in the following table : — 238 THE EVOLUTION OF MAN. Three Groups. Sense-organs. Sense-nerves. Sense-functions. A. Sense-organs in which the ter- minal expansions of the nerves are<\ distributed in the outer skin-cover- ing. B. Sense-organs in which the ter- minal expansions of the nerves are distributed over inverted grooves of the outer skin- covering. I. Skin-covering (outer skin, or epidermis, and leather - skin, or corium) II. External sexual parts (penks and cli- toris) III. Mucous mem- brane of the mouth - cavity (tongue and palate) IV. Mucous mem- brane of the nose-cavity C. Sense-organs in f which the ter- minal expansions of the nerves are distribut e 1 o ver-< vesicles sepa- rated from the external skin- covering. V. Eye VI. Ear I. Skin nerves i Sense of pressure (nervi cutanei) i Sense of warmth II. Sexual nerves (nervi pudendi) 3. Sexual sense III. Taste nerve 4. Sense of taste (nervus glosso- pharyngeus) IV. Olfactory nerve (n. olfactorius) 5. Sense of smell V. Sight nerve 6. Sense of sight (n. opticus) VI. Ear-nerve 7. Sense of hear- (n. acousticus) ing Of the developmental history of the lower organs of sense I have but little to say. The development of the skin- covering, which is the organ of the sense of pressure (sense of touch) and of warmth, we have already traced (p. 209). I need only add that in the leather skin (corium) of Man, as of all higher Vertebrates, innumerable microscopic sense- organs develop, the direct relations of which to the sensa- tions of pressure or resistance, of warmth and of cold, are not yet ascertained. These organs, in or upon which the sensitive skin-nerves terminate, are the so-called " touch bodies " and the " Pacinian bodies," named after their dis- MUCOUS MEMBRANE OF THE TONGUE AND NOSE. 239 coverer, Pacini. Similar bodies are also found in the organs of the sexual sense, in the penis of the male and in the clitoris of the female ; these are processes of the integument, and the development of which we shall consider presently, in connection with that of the other organs of generation. The development of the organ of taste, the tongue and the palate, we will also consider presently, in connection with that of the intestinal canal, to which these parts belong. To one point, however, I vill now call particular attention, viz., the mucous membrane of the tongue and palate, in which the taste-nerve terminates, is also in its origin a portion of the external skin-covering. For, as we found, the entire mouth-cavity originates, not as a part of the actual intes- tinal canal, but as a groove-like inversion of the external skin (vol. i. p. 838). Its mucous membrane, therefore, is formed, not from the intestinal layer, but from the skin- layer, and the taste-cells on the upper surface of the tongue and palate arise, not from the intestinal-glandular layer, but from the skin-sensory layer. This is equally true of the mucous membrane of the organ of smell, the nose. The history of the development of this sense-organ is, however, of far higher interest. Although the human nose, externally viewed, seems simple and single, yet in Man, as in all higher Vertebrates, it consists of two perfectly distinct halves, of a right and a left nasal cavity. These two cavities are entirely separated by a vertical partition, so that the passage into the right nasal cavity lies only through the right nostril, and into the left cavity only through the left nostril. Posteriorly the two nasal cavities open separately through the two posterior nasal apertures into the head of the pharynx, so that the 240 THE EVOLUTION OF MAN. pharynx may be entered without touching the cavity of the mouth. This is the passage by which air is usually inhaled; the mouth being shut, it enters the pharynx, and thence passes through the windpipe into the lungs. Both nasal cavities are separated from the mouth-cavity by the hori- zontal bony palate roof, to the back of which the soft palate and the uvula is attached, like a hanging curtain. In the upper and hinder portion of both nasal cavities the olfactory nerve extends over the mucous membrane, which lines these parts. This is the first pair of brain nerves, which issue from the skull-cavity through the sieve bone. Its branches extend partly over the partition wall, and partly over the inner side-walls of the nasal cavities, to which are attached the "shells," or spongy bones of the nose — complex bony structures. These " shells " are much further developed in many of the higher Mammals than in Man. In all Mammals there are three of these * shells " in each of the two nasal cavities. The sensation of smell is produced by a current of air, containing odoriferous matters, passing over the mucous membrane of the cavities, and there coining in contact with nerve-ends. The peculiar characters which distinguish the olfactory organ of Mammals from that of lower Vertebrates, are represented in Man. In all specific points the human nose exactly resembles that of the Catarhine Apes, some of which indeed possess an entirely human external nose (see face of the Nose-ape, Fig. 202, p. 175). The first rudiment of the olfactory organ in the human embryo does not, however, show any signs of the fine form of the future catarhine nose. Indeed, it first appears in the same form which persists for life in Fishes; in the form of two simple pits, THE NOSE. 241 or grooves in the skin of the upper surface of the head. In all Fishes two of these mere blind nose-pits are found in the upper surface of the head ; sometimes they are situated at the back, near the eyes, sometimes near the snout, or, again, near the mouth-opening (Fig. 191, n, p. 113). They are lined by mucous membrane in folds, over which the end branches of the olfactory nerves spread. In this its original condition the double nose of all Amphirhina (p. 101) is entirely unconnected with the pri- mitive mouth-cavity. The connection, however, begins to Fig. 231.— Head of a Shark {ScyJ- lium), from the ventral side : w, mouth opening ; o, nose grooves, or pits ; r, nasal furrow ; n, nose-flap in its natural position ; n', nose-flap turned up. (The dots are openings of mucous ducts.) (After Gegenbaur.) appear even in some Primitive Fishes (SelachO) ; a super- ficial skin-furrow extends on each side from the nose-groove down to the adjacent corner of the mouth. This furrow, the nasal channel, or furrow (Fig. 231, r), is of great sig- nificance. In many Sharks (e.g., Scyllium) a special process of the frontal skin, the nasal flap, or " inner nasal process," overlaps the nasal furrow (n, n'). Opposite to this the outer edge of the furrow rises and forms the "outer nasal process." In Dipneusta and Amphibia these two nasal processes meet over the furrow and coalesce, thus forming a canal, the " nasal canal." There is now a passage from the external nasal groove through this canal directly into the mouth- 242 THE EVOLUTION OF MAN. cavity, which latter was developed independently of the groove. In the Dipneusta and the lower Amphibia the internal opening of the nasal canal lies well forward (behind the lips) ; in the higher Amphibia it lies further back. In the three highest vertebrate classes, the Amniota, the primary mouth-cavity is separated by the formation of the horizontal palate roof into two perfectly distinct cavities, the superior (or secondary) nasal cavity, and the inferior (or secondary) mouth-cavity. The nasal cavity is also separated by the vertical partition into two distinct halves, into a right and a left nasal cavity. Comparative Anatomy thus still shows us simultaneously, in the ascending series of the double-nostrilled Vertebrates, from Fishes up to Man, all the various stages of develop- ment of the nose which the very highly developed olfactory organ of the higher Mammals has passed through succes- sively in the different periods of its tribal history. The first rudiment of the organ of smell in the embryo of Man and in that of all the higher Mammals, makes its appearance in the same entirely simple form which is retained throughout life by the nose of Fishes. At a very early stage, and while no trace of the characteristic facial structure of Man is yet visible, a pair of small grooves appear on the front of the head, and before the primitive mouth-cavity ; these were first discovered by Baer, and by him properly enough named " olfactory grooves ': (" Riechgruben," Figs. 232, n, 233, n). These primitive nasal grooves are quite separate from the primitive mouth-cavity, or mouth indentation, which, as we found, likewise makes its appearance as a groove-like indentation of the external skin-covering, in front of the blind anterior extremity of the intestinal canal. DEVELOPMENT OF THE NOSE. 243 This pair of nasal grooves, as well as the single mouth groove (Fig. 235, m), is lined by the horn-plate. The Fig. 232. Fig. 233. m a n Fig. 235. Fig. 236. Figs. 232, 233.— Head of an embryonic Chick, on the third day of incubation : 232, from the front ; 233, from the right side, n, Nose-rudi- ment (olfactory grooves) ; I, eye-rudiment (sight-grooves) ; g, ear-rudiment (auditory grooves) ; v, fore-brain ; gl, eye-slits ; o, upper jaw process ; u, lower jaw process of the first gill arch. Fig. 234. — Head of an embryonic Chick, on the fourth day of incubation, from below : n, nose-groove ; o, upper jaw process of the first gill arch ; u, lower jaw process of the same ; k", second gill-arch ; sp, choroidal fissure of the eye ; s, throat (pharynx). Figs. 235, 236. — Two heads of embryonic Chicks : 235, at the end of the fourth day ; 236, at the end of the fifth day of incubation. The letters as in Fig. 234. Additional letters are in, inner, and an, outer nasal process ; nf, nasal furrow ; st, frontal process ; tn, mouth-cavity. (After Koelliker.) All these figures are proportionately enlarged. 244 THE EVOLUTION OF MAN. original separation of the nasal groove from the mouth groove is, however, soon interrupted, for the frontal process (Fig. 235, st, Rathke's " Nasenfortsatz der Stirnwand ") is immediately formed above the mouth groove. Right and left the edges of this process project in the form of two lateral processes : these are the inner nasal processes, or nasal flaps (Fig. 235, in). On each side, opposite to these rises a parallel ridge between the eye and the nasal groove. These ridges are the outer nasal processes (Rathke's "Nasen- dacher," Fig. 235, an). Between the inner and outer nasal process a channel-like depression thus extends on each side from the nose groove toward the mouth groove (m), and this channel is, of course, the same nasal furrow or channel which we found in the Shark (Fig. 231, ?"). As the two parallel edges of the inner and the outer nasal processes bend towards each other and coalesce above the nasal channel, the latter becomes a small tube — the primitive " nasal canal." In this stage of its Ontogeny, therefore, the nose of Man and of all other Amnion Animals consists of two small narrow tubes — the " nasal canals " — leading from the outer surface of the frontal skin into the simple pri- mitive mouth-cavity. This transient condition resembles the permanent condition of the nose in Dipneusta and Amphibia. (Cf. Plate I., Frontispiece, with explanation.) Specially significant in the modification of the open nasal channel into the closed nasal canal, is a plug-shaped forma- tion, which extends from below up to the lower extremities of both the nasal processes on each side, and unites with them. This is the upper jaw process (Figs. 232, o, 236, o, Plate I., o). Below the mouth groove lie the gill arches, which are separated from one another by the gill openings UPPER JAW PROCESS. 245 (Plates I., VI., and VII., h). The first of these gill arches, at present the most interesting to us, which we may call the jaw arch, develops the jaw-skeleton of the mouth (Plate I., u). A small process first grows out from the base of the front gill-arch : this is the upper jaw process. The first gill-arch itself develops a cartilage on its inner side, called after its discoverer, "Meckel's cartilage," on the outer surface of which the lower jaw forms (Figs. 232, u, 236, u). The upper jaw process forms the principal part of the entire framework of the upper jaw, viz., the palate bone and the wing bone. On its outer side the upper jaw bone, in the narrower sense, afterwards arises, while the middle portion of the upper jaw skeleton, the twixt jaw (intermaxillary bone) develops from the anterior portion of the frontal process. (See development of the face in Plate I.) In the further characteristic development of the face in the three higher vertebrate classes, the two upper jaw pro- cesses are of the highest importance. From them proceeds the palate roof, the important horizontal partition which grows into the simple primitive mouth-cavity, separating it into two quite distinct cavities. The upper cavity, into which the two nasal cavities open, now develops into the nasal cavity — a respiratory air passage and an olfactory organ. The lower cavity, on the other hand, forms, by itself, the permanent secondary mouth-cavity (Fig. 237, m) — the digestive food passage and the organ of taste. Both the upper smell-cavity and the lower taste-cavity open at the back into the throat (pharynx). The palate roof, separating these two cavities, is formed by the coalescence of two lateral portions — of the horizontal plates of the two upper jaw processes (palate-plates; Fig. 237, p). When these do not perfectly 246 THE EVOLUTION OF MAN. adhere in the middle line, the result is a permanent longi- tudinal cleft, through which there is an open passage from the mouth-cavity directly into the nasal cavity. The so- Fig. 237. — Diagrammatic transverse section through the mouth and nose cavity. While the palate-plates (p) separate the original mouth-cavity into the lower secondaiy mouth-cavity (m) and the upper nasal cavity, the latter is parted by the ver- tical partition wall of the nose (e) into two distinct halves (n, n). (After Gegenbaur.) called " wolf's jaws " are thus caused. The " hare-lip " and "split lip" is a slighter degree of this arrested develop- ment.116 Simultaneously with the horizontal partition of the palate roof, a vertical wall by which the single nasal cavity is divided into two, a right and a left cavity, develops (Fig. 237, n, n). This vertical partition of the nose (e) is formed by the middle part of the frontal process : above this gives rise by ossification to the vertical lamella of the sieve bone (cubiform plate), and below the great vertical bony partition wall — the " plough-share '; (vomer), and in front to the twixt-jaw (os intermaxillare). Goethe was the first to show that in Man, just as in all the other Skulled Animals, the twixt-jaw appears as an independent bone between the two halves of the upper jaw. The vertical partition wall of the nose finally coalesces with the horizontal palate roof. The two nasal cavities are now as entirely separate from one another as from the secondary mouth- cavity. These three cavities open, however, at the back into the pharnyx, or jaw-cavity. THE HUMAN NOSE. 247 The double-nostrilled nose has now attained the structure characteristic of Man in common with all other Mammals. Its further development is very easily intelligible : it is limited to the formation of internal and external processes of the walls of both nasal cavities. Within the cavities develop the " nose shells," spongy bony structures, over which the olfactory mucous membrane spreads. The first brain nerve, the olfactory nerve, with its delicate branches, passes Figs. 238, 239. — Upper part of the body of a human embryo (16 mm. in length) during the sixth week : Fig. 238, from the left side ; Fig. 239, from the front. The origin of the nose in two lateral halves, originally separate, is still plainly visible. The nose and upper lip are disproportionately great in comparison with the rest of the face, especially with the lower lip. (After Eollman.) Fig. 240. — Face of a human embryo of eight weeks. (After Ecker.) Cf. Frontispiece, Plate I. Fig. Mi— Mm. 34 248 THE EVOLUTION OF MAN. from the large brain through the roof of both nasal cavities into the cavities, and extends over the olfactory mucous membrane. At the same time, by inversion of the nasal mucous membrane, the minor cavities of the nose, which are afterwards filled with air, and which communicate directly with the two nasal cavities, arise (frontal cavities, cavities of the sphenoid bone, jaw cavities, etc.). In this special stage of development they occur only in Mammals.171 The external nose is not developed until long after all these essential internal parts of the olfactory organ have been formed. The first trace in the human embryo appears at the end of the second month (Figs. 238-240). Any human embryo during the first month shows that originally there is no trace of the external nose. It afterwards grows out from the anterior nasal portion of the primitive skull. The form of nose which is characteristic of Man does not appear till a period far later. Much stress is usually laid on the shape of the external nose as a noble organ, occurring exclusively in Man ; but there are Apes which have very human noses, as, for instance, the Nosed Ape already mentioned. On the other hand, the external nose, the fine shape of which is so extremely important to the beauty of the facial structure, possesses in certain inferior races of Man a shape anything but beautiful. In most Apes the external structure of the nose remains undeveloped. Especially remarkable is the important fact already cited that it is only in the Apes of the Old World, in the Cata- rhines, that the nasal partition wall (septum) remains as small as it is in Man ; in Apes of the New World it widens considerably at the base, so that the nostrils open outwards (Platyrhini, p. 175). ( 249 ) TABLE XXX. Systematic Survey of the Chief Phylogenetic Stages of the Human Nose. First Stage: Nose of the earlier Primitive Fishes. The nose is formed by a pair of simple skin-grooves (nose-pits) in the outer surface of the head (like those which are now permanently retained by the lower Selachians). Second Stage : Nose of the more recent Primitive Fislies. Each of the two blind nasal grooves becomes connected by a furrow (nasal-furrow) with one end of the month (as is yet permanently the case in the higher Selachians). Third Stage : Nose of the Dipneusta. The two nasal furrows change, in consequence of the coalescence of their edges, into closed canals (primary nose-canals), which open at their front ends, within the soft edges of the lip, into the primary month-cavity; as is yet permanently the case in the Dipneusta and the earlier lower Amphibia (Sozobranchia). Fourth Stage: Nose of Amphibia. The inner openings of the nasal canals penetrate further back into the primary mouth-cavity, so that they are surrounded by hard bony portions of the jaw (as is yet permanently the case in the higher Amphibia). Fifth Stage : Nose of the Protamnia. The primitive mouth-cavity, into which both nasal canals open, separates, in consequence of the formation of a horizontal partition (the palate-roof), into an upper nasal cavity and a lower (secondary) mouth-cavity. The formation of the spongy bones of the nose commences (as in the earlier Amnion Animals). Sixth Stage : Nose of the earlier Mammals. The simple nose-cavity separates, in consequence of the development of a vertical partition wall (the "plough," vomer), into two distinct nose-cavities, each of which is occupied by one of the nasal canals (as is yet the case in all Mammals). The spongy nose-bones differentiate. Seventh Stage : Nose of the more recent Mammals. Within both nose-cavities the development of the spongy bones proceeds further, and an external nose begins to form. Eighth Stage : Nose of the Catarhine Apes. The internal and the external nose attain the full development ex- clusively characteristic of Catarhine Apes and of Man. . . 250 THE EVOLUTION OF MAN. The history of the development of the eye is equally remarkable and instructive. For although the eye, owing to its exquisite optical arrangement and wonderful struc- ture, is one of the most complex and most nicely adapted organs, yet it develops, without a preconceived design, from a very simple rudiment in the outer skin-covering. /; Fig. 241. — The human eye in transverse section: a, protective membrane (sclerotica) ; b, horn membrane (cornea) ; c, outer membrane (conjunctiva) ; d, circular veins of iris ; e, vascular membrane (choroidea) ; /, ciliary muscle; g, corona ciliaris; A, rainbow membrane (iris); i, optic nerve (n. opticus) ; k, anterior limit of the retina ; I, crystalline lens (lens crystal- Una); m, inner cover of the horn membrane (water membrane, membrana Descemeti); n, pigment membrane (pigmentosa); o, retina; p, " petit s -canal;" q, yellow spot of the retina. (After Helmholtz.) When fully developed, the human eye is a globular capsule (the eyeball, bulbus, Fig. 241). This lies in the THE EYE. 251 bony orbit of the skull, surrounded by protective fat and by motor muscles. The greater part of this eyeball is occupied by a semi-fluid, clear gelatinous substance, the vitreous body (corpus vitreum). The crystalline lens (Fig. 241, I) is embedded in the anterior surface of the vitreous body. It is a lentil-shaped, bi-convex, transparent body — the most important of the light-refracting media of the eye. Among these media is, in addition to the lens and vitreous body, the aqueous humour (humor aqueus, at m, in Fig. 241), in front of the lens. These three pellucid, light-refracting media — the vitreous body, the crystalline lens, and the aqueous humour — by which the rays of light, incident on the eye, are refracted and concentrated, are enclosed in a firm globular capsule consisting of several different membranes, comparable with the concentric layers of an onion. The outer and thickest of these forms the white protective membrane of the eye (sclerotica, a). It consists of firm, compact white connective tissue. In front of the lens a circular, very convex, transparent plate, re- sembling a watch glass, is inserted in the white protective membrane; this is the horny membrane (cornea, b). On its outer surface the horny membrane is covered by a very thin coating of outer skin (epidermis) ; this coating is called the connecting membrane (conjunctiva); it extends from the horny membrane over the inner surface of both eyelids — the upper and lower folds of skin which on closing the eyes are drawn together over them. At the inner corner of our eye there is, as a sort of rudimentary organ, the remnant of a third (inner) eyelid, which, as the " nic- titating membrane," is highly developed in the lower Vertebrates (vol. i. p. 110). Below the upper eyelid are lodged 252 THE EVOLUTION OF MAN. the tear-glands, the secretion o£ which keeps the surface of the eye smooth and clean. Directly under the protective membrane is a delicate dark-red, highly vascular membrane, the vascular mem- brane (choroidea, e), and within this the retina (o), which is a dilatation of the optic nerve (i). This latter is the second brain nerve. It extends from the " centre of sight " (the second brain-bladder) to the eye, penetrates the outer coats of this, and then extends, as the retina, between the vascular membrane (choroidea) and the vitreous body (corpus vitreum). Between the retina and the vascular membrane lies another very delicate membrane, which is commonly, but wrongly, considered as part of the latter. This is the black pigment membrane (■pigmentosa, lamina pigmenti, n), or the " black carpet ' (tapetum nigrum). It consists of a single layer of beautiful hexagonal cells accurately joined together and filled with black pigment granules. This pigment membrane lines, not only the inner surface of the actual vascular membrane, but also the pos- terior surface of its" anterior muscular prolongation, which, as a circular ring-like membrane, covers the edge of the lens, and prevents the penetration of lateral rays. This is the well-known "rainbow membrane" (iris, h), which is differently coloured in different persons (blue, gray, brown, etc.). This " rainbow membrane " is the limit towards the front of the vascular membrane. The round hole in the iris is the pupil, through which the rays of light pass into the interior of the eye. Where the iris proceeds from the edge of the actual vascular membrane, the latter is much thickened and forms a beautiful ciliated crown (corona ciliaris, g), which surrounds the edge of the lens with about seventy large, and many smaller rays. DEVELOPMENT OF THE EYE. 253 In the embryo of Man, as in that of all other Amphi- rhina, two pear-shaped vesicles grow out laterally, at a very early period, from the foremost part of the first brain bladder (Fig. 223, a, p. 218). These bladder-like protuberances are the primary eye- vesicles. At first they are directed outward and forward, but they soon make their way further downward, so that after the specialization of the live brain- bladders, they lie at the base of the twixt-brain. The internal spaces within the two pear-shaped vesicles, which soon attain a considerable size, communicate through their hollow stalks with the cavity of the twixt-brain. Their outer covering is formed by the outer skin-covering (horn- plate and leather-plate). Where, on each side, the latter comes directly in contact with the most curved portion of the primary eye-vesicles, a thickening (I) arises, and at the same time a groove-like indentation (0) in the horn-plate (Fig. 242, 1). This groove, which we will call the lens groove, changes into a closed sac, the thick- walled lens vesicle (2, 1), owing to the fact that the edges of the groove coalesce above Fig. 242.— Eye of an embryonic Chick in longitudinal section (1, of a germ after sixty-five hours of incubation; 2, of a somewhat older germ; 3, of a germ four days old) : h, horn-plate ; o, lens groove ; I, lens (in 1^ it still forms part of the epidermis, while in 2 and 3 it has separated) ; x, thickening of the horn-plate at the point from which the lens separated ; gl, vitreous body; r, retina; u, pigment membrane. (After Remak.) 254 THE EVOLUTION OF MAN. it. Exactly as the medullary tube originally separates from the outer germ-layer does this lens-sac separate from the horn-plate, in which it originated. The space within this sac is afterwards entirely filled by the cells of its thick wall, and the solid crystalline lens is thus formed. The latter is, therefore, purely a formation of the epidermis. Together with the lens the small fragment of the leather-plate (corium) lying below the lens separates from the outer skin-covering. This small piece of the leather-skin very soon forms a highly vascular sac round the lens {capsula vasculosa lentis). Its anterior portion at first covers the pupillary orifice, and is then known as the pupillary membrane (membrana jmpillaris). Its back portion of the same membrane is called the " membrana carpsVblo-rpwpillaris" This "vascular lens capsule, which merely serves to nourish the growing lens," afterwards entirely disappears. The later, permanent lens capsule contains no vessels, and is a structureless secretion of the lens cells. As the lens thus separates from the horn-plate and grows inward, it must necessarily indent the adjoining primary eye-vesicles from without (Fig. 242, 1-3). This process may be compared to the inversion of the germ-mem- brane vesicle (blastida), which in the Amphioxus and in many low animals gives rise to the gastrula (vol. i. p. 192). In both instances the inversion of one side of the closed vesicle proceeds until finally the inner, inverted portion touches the outer, uninverted portion of the wall of the vesicle, so that the cavity disappears. Just as in the gastrula the former part changes into the intestinal layer {entodermd), and the latter into the skin-layer (exodevma), so in the inverted primary eye-vesicle the retina develops from the former DEVELOPMENT OF THE EYE. 255 (inner) part (Fig. 242, r), and the black pigment membrane (w) from the latter (the outer, uninverted part). The hollow stalk of the primary eye-vesicle changes into the optic nerve. The lens (I) which enacts so important a part in this inverting process of the primary eye-vesicle, lies at first directly upon its inverted part, that is, on the retina (r). Very soon, however, the two separate, a new body, the vitreous body (corpus vitreum, gl), coming in between them. While the lens-sac is detaching itself, and the primary eye- vesicle is being inverted from without, another inversion simultaneously proceeds from beneath — from the superficial portion of the skin-fibrous layer, i.e., from the leather-plate of the head. At the back of the lens and below it, a ledge-like process of the leather-plate arises (Fig. 243, g), which inverts the primary eye-vesicle (now shaped like a cup) from below, and presses in between the lens (I) and the retina (r) Thus the primary eye-vesicle assumes the form of a hood. The opening of this hood, answering to the face, is covered by the lens ; but the opening, through which the neck would pass, answers to the indentation through which the leather-skin passes in between the lens and the retina (the inner wall of the hood). The space within this secondary eye-vesicle is almost filled by the vitreous body, which answers to the head wrapped in this hood. The hood itself is, properly speaking, double : the inner hood itself is the retina, and the outer one, directly surrounding the former, is the pigment membrane. The comparison with a hood renders this process of inversion, which is sometimes hard to explain, more clearly understood. The rudiment of the vitreous body (corpus vitreum) is at first very incon- 256 THE EVOLUTION OF MAN. siderable (Fig. 243, g), and the retina disproportionally thick. As the former expands, the latter becomes much thinner, till at last the retina appears only as a very delicate Fig. 243. — Horizontal transverse section through the eye of a human embryo of four weeks ; 100 times enlarged (after Koelliker) : t, lens (the dark wall of which is equal to the diameter of the central cavity) ; g, vitreous body (connected with the leather-plate by a stalk, g') ; v, vas- cular loop (penetrating through the stalk (g') into the Vitreous body be- hind the lens) ; r, retina (inner, thicker, inverted lamella of the primary eye-vesicle) ; a. pigment membrane (outer, thinner, uninverted lamella of the same) ; h, intermediate space between the retina and the pigment membrane (remnant of the cavity of the primary eye- vesicle). coat of the thick, almost globular vitreous body, which fills the greater part of the secondary eye-vesicle. The outer layer of the vitreous body changes into a highly vascular capsule, the vessels of which afterwards disappear. The slit-like passage through which the rudiment of the vitreous body grows from below in between the lens and the retina, of course causes a break in the retina and the pigment-membrane. This break, which appears on the inner surface of the vascular membrane as a colourless streak, has been inaptly called the choroidal cleft, though the true vascular membrane is not cleft at all at this point (Fig. 234, sp} 235, sp, p. 243). A thin process of the vitreous body passes inward on the under surface of the optic nerve, which it inverts in the same way as the primary eye-vesicle was inverted. The hollow cylindrical optic nerve (the stalk of DEVELOPMENT OF THE EYE. 257 the primary eye-vesicle) is thus transformed into a channel, opening downward. The inverted lower surface attaches itself to the uninverted upper surface of the hollow stalk, so that the hollow space within the stalk, forming the com- munication between the cavity of the twixt-brain and of the primary eye-vesicle, now disappears. The two edges of the channel now grow downward toward each other, enclose the band-like process of the leather-plate, and coalesce beneath it. Thus this process now lies within the axis of the solid secondary optic nerve. It develops into a cord of connective tissue carrying the central blood-vessel of the retina (vasa centralia retinas). An entirely fibrous covering, the fibrous capsule of the eye, now finally forms round the outside of the secondary eye-vesicle and its stalk (the secondary optic nerve). It originates from the head-plate, from that part of the skin- fibrous layer which immediately encloses the eye-vesicle. This fibrous covering takes the form of a completely-closed globular sac, which surrounds the whole ball of the eye, and on the outer side of this, grows in between the lens and the horn-plate. The globular wall of the capsule soon separates, by fission of the surface, into two distinct membranes. The inner membrane becomes the choroidea, or vascular layer; in front it forms the ciliated crown (corona ciliavis) and the iris. The outer membrane, on the other hand, becomes the white enveloping, or protective membrane (sclerotica), and, in front, forms the transparent horny membrane (cornea). The rudiments of all the essential parts of the eye are now formed, and its further development is only io details, in the complex differentiation and combination of the several parts. ( 258 ) TABLE XXXI. Systematic Survey of the Development of the Human Eye. I. Systematic Survey of those parts of the Human Eye which develop from the first of the Secondary Germ-layers, the Skin-sensory Layer. / Products of the h Marrow-plate V 1. Stem of the primary 1. Optic nerve Nervus opticus eye-vesicle 2. Inner (inverted) part of 2. Retina Retina the primary eye- vesicle 3. Outer (uuinverted) part 3. Screen, or pig- Pigmentosa (lamina of the primary eye- ment-coat pigmenti) vesicle 4. Vesicle separated from 4. Crystalline lens Lens crystallina the horny plate Products of the ' 5- 0uter epidermic skin 5. Connective mem- Conjunctiva brane Horn -plate 6. Inverted portions of the 6. Tear-glands epidermic skin Glandulce lacrymales II. Systematic Survey of those parts of the Human Eye which develop from the second of the Secondary Germ-layers, the Skin-fibrous Layer. C. Products of the < Leather-plate (1, 8. Ledge-like process of 7. Vitreous body Corpus vitreum the corium on the 8. Vascular mem- Capsula vasculosa brane of the corporis vitrei vitreous body 9. Central vessels of Vasa centralia the retina lower side of the pri- mary eye- vesicle 9. Continuation of the corium process 10. Pupillary membrane, 10. Vascular mem-' with its capsule brane of the lens 11. Folds of the leather 11. Eyelids ^ skin (corium) retinae Capsula vasculosa lentis crystallince Palpebral D. Products of the Skull-plate • 12, 13. Vascular mem- 12. Vascular mem- Ch.oroid.ea brane of the eye- brane ball (capsula vas- 13. E,ainbovv mem- Iris culosa bulbi)) brane 14, 15. Fibrous membrane 14. Protective mem- Sclerotica of the eyeball (cap- brane k sula fibrosa bulbi) 15. Horny membrane Cornea THE NICTITATING MEMBRANE. 259 The most important fact in this remarkable process of eye-development is the circumstance that the optic nerve, the retina, and the pigment-membrane originate from a part of the brain, from a protuberance of the twixt-brain, while the crystalline lens, the most important refracting medium, develops from the outer skin (epidermis). From the outer skin — the horny lamina — originates also the delicate connecting membrane (conjunctiva) which after- wards envelopes the outer surface of the eyeball. The tear- glands proceed, as branched processes, from the conjunctiva (Fig. 214, p. 202). All the other parts of the eye originate from the skin-fibrous layer; the vitreous body and the vascular lens-capsule from the leather-plate, the choroid coat with the iris, and the protective membrane (sclerotica) with the horny membrane (cornea) from the head-plates. The outer protective organs for the eye, the eyelids, are merely simple folds of skin, which, in the human embryo, appear in the third month. In the fourth month the upper eyelid adheres to the lower, and the eye then remains covered by them till birth. (Plate VII. Fig. M ill., R ill., etc.) The two eyelids usually again separate shortly before birth, but sometimes not till after. Our skulled ancestors had, in addition to these, a third eyelid, the nictitating membrane, which was drawn over the eye from the inner corner. Many Primitive Fishes (Selachii) and Amnion Animals yet retain this. In Apes and in Man it has atrophied, and only a small remnant of it exists in the inner corner of the eye as the " crescent-shaped fold," as a useless " rudi- mentary organ." (Cf. vol. i. p. 109.) Apes and, Man have also lost the " Harder gland," opening below the nictitating membrane, which appears in other Mammals, and in Birds, Reptiles, and Amphibians. 260 THE EVOLUTION OF MAN. The ear of Vertebrates develops in many important points similarly to the eye and nose, but yet in other respects very differently.172 The organ of hearing of the developed human being resembles that of other Mammals in all essential particulars, and is especially similar to that of Apes. As in the latter, it consists of two principal parts, an apparatus for the conveyance of sound (external and middle ear) and an apparatus for producing the sensation of sound (internal ear). The outer ear opens in the ear-shell (concha Fig. 244. — Auditory organ of man (left ear, seen from the front ^ natural size) : a, ear-shell : o, external ear-canal ; c, drum, or tympanic membrane ; d, cavity of drum ; e, ear-trumpet; /, g, h, the three ear bonelets (/, hammer ; g, anvil ; h, stirrup) ; i, ear-pouch (utriculus) ; k, the three semi-circular canals ; I, ear-sac (sacculus) ; m. snail (cochlea) ; n, auditory nerve. auris), situated at the side of the head (Fig. 224, a). From this the outer ear-canal, which is usually about an inch long, leads to the inside of the head (6). The inner end of this THE EAR 26l tube is closed by the well-known tympanic membrane or drum (tympanum) ; a thin membrane of oval form (c), placed in a vertical position, but slightly inclined. This membrane separates the outer ear-canal from the so-called cavity of the drum (cavum tympani). This is a small cavity enclosed in the petrous part of the temporal bone, which is filled with air and connected by a special tube with the mouth-cavity. This tube is somewhat longer, but much narrower than the outer ear-canal ; it leads inward and forward in an oblique direction from the inside wall of the tympanum and opens behind the inner nostrils (or Choana) into the upper part of the cavity of the throat (pharynx). This canal is called the Eustachian tube (tuba Eustachii). It equalizes the pressure of the air in the tympanic cavity, and the outer atmospheric air which enters by the ear canal. Both the Eustachian tube and the tympanic cavity are lined by a thin, mucous membrane, which is a direct continuation of the mucous membrane of the throat. Within the tympanic cavity are the three bonelets of the ear, which, from their characteristic shape, are called the hammer, the anvil, and the stirrup (Fig. 244 /, g, h). The hammer (/) lies furthest outward, just within the tympanic membrane ; the anvil (g) is wedged in between the two others, above the hammer, and further in than the hammer ; and, lastly, the stirrup (h) lies next to the anvil toward the inside, and touches with its base the outer wall of the internal ear, or the auditory sac. All these parts of the middle and external ear belong to the sound-conducting apparatus. Their principal office is to convey the waves ol sound from without through the thick side-wall of the head, to the internal ear. In Fishes these parts are entirely unre- 262 THE EVOLUTION OF MAN. presented. In them, the sound-waves are conveyed directly through the wall of the head itself to the internal ear. The inner apparatus, that which produces the sensation of sound, receiving the sound-waves thus conveyed to it, consists in Man, as in all other Vertebrates (with the single exception of the Ainphioxus), of a closed auditory sac filled with fluid, and of an auditory nerve, the ends of which are distributed over the wall of this sac. The vibrations of the waves of sound are conveyed by that medium to these nerve-ends. In the auditory fluid (endolymph), which fills the labyrinth, and opposite the places at which the auditory nerves enter, are some small stones, composed of a mass of microscopic calcareous crystals (otoliths). The organs of hearing of most Invertebrates have essentially the same construction. In them, also, it usually consists of a closed sac filled with fluid, containing otoliths, and having the auditory nerve distributed over its wall. But while in Invertebrates the auditory vesicle is usually of a very simple spherical or oval form, in all Amphirhina, on the contrary, that is, in all Vertebrates above the Fishes up to Man, it is distinguished by a very characteristic and singular form known as the auditory labyrinth. This thin membra- nous labyrinth is enclosed in a bony envelope of the same form, the osseous labyrinth (Fig. 245), which lies within the petrous bone of the skull. The labyrinth in all Amphirhina is divided into two sacs. The larger sac is called the auditory pouch (iitriculus), and has three curved appendages, called the semi-circular canals (c, d, e) ; the smaller sac is called the auditory sac (saccidus), and is connected with a peculiar appendage, which in Man and the higher Mammals is distinguished by a spiral form, like the shell of a snail, and DEVELOPMENT OF THE EAR. 263 hence is called the " snail " {cochlea, b). On the thin wall of this delicate membranous labyrinth, the auditory nerve, which passes from the after-brain to the labyrinth, is dis- tributed in a very complex manner. It divides into two main branches, the nerve of the cochlea, and the nerve of vestibule, for the remaining part of the labyrinth. The former seems specially to determine the quality of the sound heard, the latter its quantity. The nerve of the cochlea Fig. 245. — The bony labyrinth of the human ear (left side) : a, vestibule ; b, cochlea ; c, upper semi- circular canal; d, posterior semi- circular canal; e, outer semi-circular canal ; f, fenestra ovalis ; g, fenestra rotunda. (From Meyer.) *$> tells us the pitch and quality of sounds, the nerve of the vestibule their strength. The first rudiment of this extremely complex organ of hearing is very simple in the human embryo, as in those of all other Skulled Animals (Craniota) ; it is a groove-like depression of the outer skin (epidermis). At the back of the head, near the after-brain, at the upper end of the second gill-opening, a little wart-like thickening of the horn-plate arises on each side (Figs. 246, A,fl; 248, g). This deepens into a small groove, and separates from the outer-skin, just as does the lens of the eye. (Cf. p. 253.) A small vesicle filled with fluid, the primitive ear-vesicle, is thus formed on each side, immediately below the horn-plate of the back part of the head ; this is also called the " primary laby- rinth " (Plates VI. and VII.). As this separates from its original site, the horn-plate, and grows inward and down- ward in the skull, it changes from a globular to a pear- shaped form (Figs. 246, B, Iv ; 249, 0). The outer part has 264 THE EVOLUTION OF MAN, elongated into a thin stalk, which at first opens outward in a narrow canal. (Cf. Fig. 137,/, vol. i. p. 382.) This is called the appendage of the labyrinth recessus labyrinthi, Fig. 246, Ir). Fig. 246. — Development of the ear-labyrinth of a Chick, in five con- secutive stages (A-E) (cross-sections through the rudimentary skull) t /?, ear-groove ; Iv, ear- vesicle ; It, labyrinth appendage ; c, rudiment of the cochlea ; csp, hind semi-circular canal ; cse, outer semi-circular canal ; jr, jugular vein. (After Eeissner.) Figs. 247, 248. — Head of an embryonic Chick, on the third day of incuba- tion : 247 in front, 248 from the right ; n} rudimentary nose (olfactory groove) ; 7, rudimentary eye (ocular groove) ; g, rudimentary ear (auditory groove) ; v, fore-brain ; gl, eye-slit ; o, process of the upper jaw ; u, process of the lower jaw of the first gill-arch. (After Koelliker.) Fig. 249. — Primitive brain of human embryo of four weeks, in vertical section, and the left half observed from within : v, z, m, h, n, the five grooves of the skull cavity in which the five brain bladders are situated (fore, twixt, mid, hincl, and after brains) ; o, primary, pear-shaped auditory vesicle (showing through) ; a, eye (showing through) ; no, optic nerve ; p, canal of the hypophysis ; t, central skull-pieces. (From Koelliker.) DEVELOPMENT OF THE EAR. 265 In lower Vertebrates, this develops into a peculiar cavity filled with calcareous crystals, which in some Primitive Fishes (Selachii) remains permanently open, and opens above on the skull (ductus endolymphaticus). In Mam- mals, on the contrary, the appendage of the labyrinth atrophies. In these, it is of interest only as a rudimentary organ, which has no longer any physiological significance. Its useless remnant traverses the osseous wall of the petrous bone in the form of a narrow canal, and is called the aque- duct of the vestibule (aquceductus vestibuli). Only the inner and lower part (extended like a bladder) of the detached ear-vesicle develops into the differentiated and extremely complex structure which is afterwards known as the " secondary labyrinth." This vesicle separates at a very early stage into an upper, larger section, and a lower, smaller section. The former gives rise to the ear-pouch (utriculws) with the three semi-circular canals; from the latter proceeds the ear-sac (sacculus) with the "snail" (cochlea, Fig. 246, c). The three semi-circular canals originate as simple pocket-like processes from the ear- pouch (Fig. 246, E, cse and csp). In the centre of each of these processes, the two walls coalesce, and separate them- selves from the utricle, while their extremities still commu- nicate with its cavity. In all Double-nostrils (Amphirhina) there are three semi-circular canals, as in Man, while of the Cyclostomi the Lampreys have but two, and the Myxinoides but one (p. 103). The highly-developed structure of the '•'snail" (cochlea), which is one of the most delicate and admirable products of adaptation in the mammalian body, originally develops very simply as a bottle-like process from the ear-sac (sacculus). As Hasse has shown, the 266 THE EVOLUTION OF MAN. various stages in its ontogenetic development still exist permanently side by side in the ranks of the lower Verte- brates.173 Even in Monotremes the snail-like spiral curving of the cochlea is not present ; it is exclusively characteristic of the other Mammals and Man. The auditory nerve (nervws acusticus), or the eighth brain-nerve, — one of the main branches of which distributes itself over the " snail * (cochlea), the other over the other parts of the labyrinth, — is, as Gegenbaur has shown, the sensory dorsal branch of a spinal brain-nerve, the motor ventral branch of which is the motor nerve of the facial muscles (nervus facialis). Phylogenetically it has, there- fore, originated from an ordinary skin-nerve, and is, conse- quently, of wholly different origin from the optic and olfactory nerves, which represent the two direct processes of the brain. In this respect the organ of hearing differs essentially from the organs of sight and of smell. The auditory nerve originates from the cells of the head-plate ; therefore, from the skin-fibrous layer. From this also develop all the membranous, cartilaginous, and bony cover- ings of the ear-labyrinth. The development of the apparatus for the conveyance of sound, situated in the middle and external ear of Mammals, is entirely distinct from that of the apparatus of auditory sensation. It must be regarded, phylogenetically as well as ontogenetically, as an independent, secondary formation, which only afterwards connects itself with the primary internal ear. Its development is, however, not less in- teresting, and is equally clearly explained by Comparative Anatomy. In all Fishes, and in the yet lower Vertebrates, there is no special apparatus for the conveyance of sound, ( 267 ) TABLE XXXII. Systematic Survey of the Chief Stages in the Development of the Human Ear. I. First Stage. The auditory nerve is an ordinary sensitive skin-nerve, which, during the differentiation of the horn-plate, appears at a certain point on the skin of the head. II. Second Stage. The differentiated place of the horn-plate, at which the auditory nerve appeared, forms a small special auditory groove in the skin, which has an outer orifice in the appendage called the " labyrinth." III. Third Stage. The auditory groove has detached itself from the horn -lamina, and forms a small closed auditory vesicle filled with fluid. The " labyrinth-appendage" becomes rudimentary {AquwdAictus vestibuli) . IV. Fourth Stage. The auditory vesicle differentiates into two connected parts, the ear- pouch (utriculus) and the ear-sac (sacculus). Each of the two vesicles receives a special main branch of the auditory nerve. V. Fifth Stage. Three semi-circular canals grow from the ear-pouch (as in all Amphi- rhina). VI. Sixth Stage. The "snail" (cochlea) grows from the ear-sac in Fishes and Amphibia; it is very insignificant, and is only developed as an independent part in the Amniota. VII. Seventh Stage. The first gill-opening (the blow-hole of Selachians) changes into the tympanic cavity and the Eustachian tube ; the former is externally closed by the tympanic membrane (Amphibia). VIII. Eighth Stage. The small bones of the ear (ossicula auditus) (the hammer (malleus) and anvil (incus) from the first gill-arch, the stirrup (stapes) from the second) develop from parts of the first and second gill-arches. IX. Ninth Stage. The external ear is developed, together with the bony ear-canal. The shell of the ear is pointed and movable (as in most lower Mammals). X. Tenth Stage. The ear-shell, with its muscles, becomes disused and a rudimentary organ. It is no longer pointed, but, on the contrary, has a curved rim with a small ear-flap (as in Anthropoid Apes and Men) . ( 268 ) TABLE XXXIII. Systematic Survey of the Development of the Human Ear. I. Survey of the parts of the Internal Ear. (Apparatus perceptive of sound.) Products of the < Horn-plate 1. Stalk of the primary 1. Aqueduct of the Aquceductus vestibuli ear- vesicle vestibule (Due- s. Recessus labyrinthi tus endolym- phaticus) 2, 3. Upper part of the 2. Ear-pouch Utriculus primary ear-vesicle 3. Three semi-circu- Canales semi-circu- lar, or curved canals 4, 5. Lower part of the 4. Ear-sac primary ear-vesicle 5. "The snail" lares Sacculus Cochlea B. Products of the Head-plate 1 6. Auditory nerve 6. Auditory nerve Nervus acusticus 7. Bony covering of the 7. Osseous labyrinth Labyrinthus osseus membranous laby- rinth 8. Bony covering of the 8. " The stony bone " Os petrosum \ whole internal ear II. Survey of the parts of the Intermediate and External Ear. (Apparatus for the conveyance of sound.) Products of the first Gill-opening / D. Products of the first two Gill-arches E. Product of the Head-plate Product of the Skin-covering f 9. 10. 11, ,12. 13. 14. /l5. I 16. Inner part of the first gill-opening Central part of the first gill-opening Closed part of the first gill-opening Upper part of the second gill-arch Upper part of the first gill-arch Central part of the first gill-arch Tympanic circle (Anmilus tympanicus) Circular membranous fold at the closed part of the first gill- opening 9. Eustachian tube Tuba Eustachii 10. Tympanic cavity Cavum tympani (Interior of the drum) 11. Tympanic mem- Membrana tympani brane (Head of the drum) 12. Stirrup (First Stapes bonelet of the ear) 13. Anvil (Second Incus bonelet of the ear) 14. Hammer (Third Malleus bonelet of the ear) 15. Bony outer audi- Meatus auditorius tory passage osseus 16. Ear-shell Concha auris 17. Rudimentary ear- Musculi concha? muscles DEVELOPMENT OF THE EAR. 269 no external and middle ear ; in these animals there is only a labyrinth, an internal ear, situated within the skull. The tympanic membrane, its cavity, and all the connected parts are unrepresented. The middle ear first develops in the Amphibian class, in which a tympanic membrane, a tym- panic cavity, and an Eustachian tube are first found. All these essential parts of the middle ear develop from the first gill-opening, with its surrounding parts, which in the Pri- mitive Fishes (Selachii) remains through life as an open blow-hole, situated between the first and second gill-arches. In the embryos of higher Vertebrates it closes in the centre, the point of concrescence forming the tympanic membrane. The remaining outer part of the first gill-opening is the rudiment of the outer ear-canal. From the inner part originates the tympanic cavity, and further inward, the Eustachian tube. In connection with these, the three bone- lets of the ear develop from the first two gill-arches ; the hammer and anvil from the first, and the stirrup from the upper end of the second gill-arch.174 Finally, as regards the external ear, the ear-shell (concha auris), and the outer ear-canal, leading from the shell to the tympanic membrane — these parts develop in the simplest way from the skin-covering which borders the outer orifice of the first gill-opening. At this point the ear-shell rises in the form of a circular fold of skin, in which cartilage and muscles afterwards form (Fig. 238, p. 247). This organ is also limited to Mammals. Among them, it is originally wanting only in the lowest division, in the Beaked Animals, (Monotrema). In the others, on the contrary, it appears in very different stages of development and partly also of atrophy. The ear-shell has atrophied in most aquatic 270 THE EVOLUTION OF MAN. Mammals. Most of these have even lost it entirely ; this is so, for example, in the Sea-cows and Whales, and most Seals. On the other hand, in the great majority of Pouched Animals (Marsupialia) and Placental Animals (Placentalia), the ear-shell is well developed, receives and concentrates the waves of sound, and is provided with a highly-developed muscular apparatus, by means of which it can be turned freely to all sides, and at the same time can be changed in form. Every one must have noticed how strongly and freely our domestic Mammals, Horses, Cows, Dogs, Kabbits, etc., can " prick " their ears, erect them and turn them in different directions. Most Apes yet retain the power of doing this, and our ancient Ape progenitors could also do it. The more Fig. 250. Rudimentary ear-muscles on the human skull : a, upward muscle (m. attollens) ; b, forward muscle (m. attrahens) ; c, backward muscle (m. retrahens) ; d, larger muscle of the helix (m. helicis major) ; e, smaller muscle of the helix (w. helicis minor) ; /, muscle of the tragus (m. tragicus) ; g, muscle of the antitragus (w. antitragicus) . (After H. Meyer.) recent Ape ancestors, common to Men and to the Anthropoid Apes (Gorilla, Chimpanzee, etc.), discontinued the habit of moving their ears, and hence the motor muscles gradually THE EAR IN MAN AND APES. 27 1 became rudimentary and useless. We still, however, possess them (Fig. 250). A few individual men can even move their ears forward or backward a little by the use of the forward muscle (b) and the backward muscle (c) ; and by long practice these motions can be gradually increased. On the other hand, no man is able to erect the ear-shell by the upward muscle (a), or to change its form by the little inner muscles of the ear (cZ, e, f, g). These muscles, which were very useful to our ancestors, have become entirely un- important to us. This is equally true of Anthropoid Apes. We also share only with the higher Anthropoid Apes — the Gorilla, Chimpanzee, and Orang — the characteristic form of our human ear-shell, especially the rolled edge, the helix, and the ear-flap. The lower Apes, like all other Mammals, have pointed ears without the helix, and without ear-flaps. Darwin has, however, shown that in some men a short, pointed process, not occurring in most individuals, is per- ceptible at the upper part of the folded rim of the ear. In some few individuals, this process is very well developed. It can only be explained as the remnant of the original point of the ear which, in consequence of the folding of the edge of the ear, has been bent forward and inward. (Cf. the similarly folded ear in the embryo of the Pig and Cow, Plate VII. Fig. H III. and G III.) On carefully comparing the ear-shells of Man and of the various Apes in this particular, we find that they form a connected series of retrograde steps. In the common catarhine ancestors of the Anthropoids and of Man, this retrogression began with the folding down of the ear-shell. In consequence of this, the ear-ed^e was formed on which that significant corner appears, the last trace of the free prominent point of the ear 35 272 THE EVOLUTION OF MAN. in our older Ape ancestors. Thus it is possible even here, with the help of Comparative Anatomy, to trace this human organ from the similar but more highly-developed organ of the lower Mammals, with certainty. At the same time, Com- parative Physiology shows us that this organ is of more or less high physiological value to the latter, while in Anthropoids and Man it is a useless rudimentary organ. Men with their ears cut off can hear as well as they did before. The conveyance of sound is not affected by the loss of the ear-shell. This explains the great diversity in the form and size of the ear-shell in different persons ; it shares this high degree of variability with other rudimentary organs.175 CHAPTER XXII. DEVELOPMENT OF THE ORGANS OF MOTION. The Motive Apparatus of "Vertebrates. — These are constituted by the Passive and Active Organs of Motion (Skeleton and Muscles). — The Significance of the Internal Skeleton of Vertebrates. — Structure of the Vertebral Column. — Formation and Number of the Vertebrae. — The Ribs and Breast-bone. — Germ-history of the Vertebral Column. — The Noto- chord. — The Primitive Vertebral Plates. — The Formation of the Meta- mera. — Cartilaginous and Bony Vertebrae. — Intervertebral Discs. — Head-skeleton (Skull and Gill-arches). — Vertebral Theory of the Skull (Goethe and Oken, Huxley and Gegenbaur). — Primitive Skull, or Primordial Cranium. — Its Formation from Nine or Ten Coalescent Metamera. — The Gill-arches (Ribs of the Head). — Bones of the Two Pairs of Limbs. — Development of the Five-toed Foot, adapted for Walking, from the Many-toed Fin of the Fish. — The Primitive Fin of the Selachians (Archipterygium of Gegenbaur). — Transition of the Pinnate into the Semi-pinnate Fin. — Atrophy of the Rays or Toes of the Fins. — Many-fingered and Five-fingered Vertebrates. — Com- parison of the Anterior Limbs (Pectoral Fins) and the Posterior Limbs (Ventral Fins). — Shoulder Girdle and Pelvis Girdle. — Germ-history of the Limbs. — Deselopment of the Muscles. " In forming his estimate of my entire theory, the reader may begin with the details and examine the fundamental facts on which I base my con- clusions. But it is equally necessary to connect the detached facts, and estimate their bearing on the whole. He who in the world of organisms sees only disconnected existences, in which some Organic similarities appear as 274 THE EVOLUTION OF MAN. accidental coincidences, will remain a stranger to the results of this investigation ; not merely because he does not comprehend the conclu- sions, but principally because the significance of the facts on which they are grounded, escapes him. A fact in itself is no more a scientific result, than a mere collection of facts is a science. That which makes a science of these facts, is their combination by that organizing mental faculty which determines the relations of the facts to each other." — Karl Gegenbauu (1872). Among those features of the organization which are specially characteristic of the vertebrate tribe as such, the peculiar arrangement of the motive apparatus, or " locomotorium," undoubtedly occupies a principal place. As in all the higher animals, the active organs of motion, the muscles, form the most important part of this apparatus : these are the fleshy bands which, by means of their peculiar contrac- tibility, of their power of contracting and shortening, move the various parts of the body, and thus change the position of the entire body. The arrangement of these muscles is, however, entirely peculiar in Vertebrates, and differs from the arrangement common to all Invertebrates. In most lower animals, especially in Worms, we find that the muscles form a simple, thin flesh-layer immediately below the outer skin-covering. This " skin-muscle pouch " is most intimately connected with the skin itself, and the same feature occurs in the tribe of the Soft-bodied Animals (Mollusca). In the great group of the Articulated Animals (Arthropoda), in the Crab, Spider, Centipede, and Insect classes, we also find a similar feature, but with the difference that in these the skin-covering forms a hard coat of mail ; an inflexible skin-skeleton, formed of chitine, and often of carbonated chalk. This outer chitinous coat of mail is jointed in a great varie'ty of ways both on the trunk and THE SKELETON. 275 on the limbs of Articulated Animals, and the muscular system, the contractile fleshy bands of which are attached to the inside of the chitinous tubes, is correspondingly jointed in an extremely varied manner. The case is exactly reversed in Vertebrates. In these alone an internal hard skeleton develops ; an inner cartilaginous or bony frame to which the fleshy muscles are externally attached, and in which they find a firm support. This bony frame forms a combined lever-apparatus, a passive apparatus of motion. The hard parts of this, the arms of the lever, or the bones, are moored against each other by the active movable muscular bands, as by hawsers. This admirable locomotive apparatus, and especially its firm central axis, the vertebral column, is quite peculiar to Vertebrates, on account of which the- whole group has long been called that of Vertebrates. This internal skeleton, notwithstanding the similarity of its first rudiment, has, however, developed so variously and characteristically in the different vertebrate classes, and in the higher classes forms so complex an apparatus, that Comparative Anatomy finds one of its richest mines in this feature. This was recognized as long ago as the beginning of the century by the older Natural Science, which at once seized these very welcome materials with peculiar pleasure. That science also, which is now called in the higher and more philosophical sense, " Comparative Anatomy," has reaped its richest harvest from this field. The Comparative Anatomy of the present day has studied the skeleton of Vertebrates more thoroughly, and revealed the laws of its formation more successfully, than has been the case with any other system of organs of the animal body. Here the well-known and oft-quoted passage, in which Goethe 276 THE EVOLUTION OF MAN. summed up the general result of his investigations in Mor- phology is especially appropriate : " All forms have a resemblance ; none is the same as another, And their chorns complete points to a mystical law." * Now that, by the Theory of Descent, we have discovered this " mystical law," have solved this " sacred enigma," now that we can explain the similarity of forms by Heredity, and their dissimilarity by Adaptation, we can find no weapon in the whole rich arsenal of Comparative Anatomy which defends the truth of the Theory of Descent more powerfully than the comparison of the internal skeletons of the various Vertebrates. We may, therefore, expect & priori that such comparison is of special importance in our History of the Evolution of Man. The inner vertebrate skeleton is one of those organs as to the Phylogeny of which Comparative Anatomy affords us conclusions far more important and deeper than those to be gained from its Ontogen}^.176 More than any other system of organs, the internal skeleton of Vertebrates, when studied comparatively, clearly and immediately impresses the observer with the necessity of the phylogenetic connection between these allied and yet very varied forms. A thoughtful comparison of the bony frame of Man with that of other Mammals, and of these asrain ivith that of lower Vertebrates, is alone sufficient to afford conviction of the true tribal relationship of all Vertebrates. All the separate parts of which this bony frame is composed appear in other Mammals, in a great * " Alle Gestalten sind ahnlich, doch keine gleichet der andern; Und so deutet der Chor auf ein geheimes Gesetz." IMPORTANCE OF THE SKELETON. 277 variety of forms indeed, but yet in the same characteristic arrangement and relative position ; and if the comparison of the anatomical conditions of the skeleton is carried out below Mammals, we can prove that a direct and uninter- rupted connection exists throughout between these various forms which are apparently so utterly unlike, and can finally be traced from a most simple, common, fundamental form. These facts alone must fully convince every ad- herent of the Theory of Development that all Vertebrates, including Man, must be traced from a single common parent-form, from a Primitive Vertebrate; for the mor- phological features of the inner skeleton, and of the mus- cular system which stands in the closest correlative rela- tions to it, are of such a kind that it is quite impossible to conceive a polyphyletic origin, a descent from several different root-forms. It is impossible, on mature reflection, to accept the theory that the vertebral column with its various appendages, or the skeleton of the limbs with their variously differentiated parts, could have originated on several occasions during the course of the earth's history, and that, consequently, the various Vertebrates must be referred in various lines of descent from Invertebrates. Indeed, it is exactly in this point that Comparative Anatomy and Ontogeny irresistibly drive us to the monophyletic conclusion, that the human race is a very recent offshoot of the same great single trunk, from branches of which all other Vertebrates have also sprung. In order to obtain a view of the outlines of the develop- ment of the human skeleton, we must first take a general survey of its arrangement in the developed Man. (Cf. Table XXXIV. and Fig. 251, the human skeleton from the ( 27S ) TABLE XXXIY. Systematic Survey of the Arrangement of the Human Skeleton. A. Central Skeleton, or Axial Skeleton. Spine. A. a. Vertebral Bodies and Upper Arches. A.b. Lower Vertebral Arches. 1 . Skull I 1 a. Pre-vertebral skull (Cranium) \ 2 b. Vertebral skull ( 1. Products of the gill- Producta arcuum 1 arches branchialium f 1 Neck vertebrae 2. Vertebral | 12 Chest „ column -^ 5 Hip „ (Colc.mna | 5 Vertebras of the sacrum vertebralis) [_ 4 „ „ „ tail (coccyx) ( 2. Ribs and breast- Costa el sUrnum I bone B. Bones connecting the Extremities. B 1. (2- (3. 4. a. Pones connecting the Anterior Limbs: Pones of the Shoulder. Shoulder-blade Scapula, Primitive key-bone Pi-ocoracoidesf) Raven 1 one Cmacoides j) Collar-bone, or key -bone Clavicula 1. 2. 3. B.b. Pones connecting the Lower Limbs: Pones of the I'tlvis. Intestinal bone Os ilium Pubic bone Os pubis Hip-bone Os i&chi C. Jointed Skeleton of the Limhs. C. a. Skeleton of the Fore Limbs. C. b. Skeleton of the Ilind Limbs. I. First Division : Ui'per Arm. I. First Division: Thigh. 1. Upper arm bone Humerus 1. Thigh-bone Femur II. Second Division' : Lower Arm. II. Second Division : Leg. 2. Spoke-bone 3. Ell-bone Padius Ulna 2. Shin-bone 3. Calf-bone Tibia Fibula III. Third Division : Hand. III. Third Dn ision : Foot. III. A. Wrist Campus III. Ankle Tarsus Original parts. t a. Radical \ b. Intermedium | c. Ulnar ' [d. Central Modified parts. = Scaphoideum =. Lunatum = Triquetrum = Intermedium f] Original parts. /a. Tibial i b. Intermedium J c. Fibular \d. Cent nil Modified parts. J = Astragalus =■ Calcaneus = Naviculars / e. Carpal I. )/• ., H. )g. „ ii r. U. „ iv. + v. = Trapezium = Trapezoides — Capitatum = Hamaturn / e. Tarsal I. /• » II. \g. „ hi. U. „ iv. + v. = Cuneiform I. II. = „ III. = Cuboides III. P. Palm of the Hand Metacarpus (5) III. B. Sole of the Foot Metatarsus (5) III. C. Five Fingers (14 bones Jjigiti Phalanges') III. C. Five Toes (14 bones Digiti Phalanges) HUMAN SKELETON. 279 « 1 .';,' Fig. 251. Fig. 252. 28o THE EVOLUTION OF MAN. A tc3 4 Fig. 253.— Human vertebral column (in an up- right position ; from the right side). (After H. Meyer.) right side (without arms); Fig. 252, the entire skeleton from the front.) In Man, as in all other Mammals, the skeleton is primarily distinguishable into the axial skeleton, or spine, and the skeleton of the appendages, or the bony frame of the limbs. The spine consists of the vertebral column and of the skull ; the latter being the pecu- liarly modified anterior part of the former. The ribs are the appendages of the vertebral column; the tongue-bone (os lingiice), the lower jaw, and the other products of the gill-arches, are those of the skull. The skeletons of the two pairs of limbs, or ex- tremities, are composed of two different parts : of the bony frame of the actual, pro- minent extremities, and of the inner girdle skeleton, by which the limbs are attached to the vertebral column. The girdle skele- ton of the arms (or fore limbs) is the shoulder girdle ; the girdle skeleton of the legs (or the hind limbs) is the pelvic girdle. The bony vertebral column in human beings (columna vertebralis, or vertebra- rium, Fig. 253) is composed of thirty- three or thirty-four circular pieces of bone, which lie one behind the other (one above the other in the usual upright position of man). These bones (vertebrce) are sepa- rated from each other by elastic cushions, DEVELOPMENT OF THE SKELETON. 28l the intervertebral discs (ligamenta intervertebralia), and at the same time, are connected by joints, so that the entire vertebral column forms a firm and solid axis, which is, however, flexible and elastic, capable of moving freely in all directions. In the various regions of the trunk, the vertebras differ in form and connection, so that the following are distinguished in the human vertebral column, beginning from above : seven neck- vertebras, twelve breast- vertebras, five lumbar-vertebras, ^.ve cross-vertebras, and four to five tail-vertebras. The uppermost, those directly in Fig. 254. — Third neck-vertebra of man. Fig. 255. — Sixth breast- vertebra of man. Fig. 256. — Second lumbar-vertebra of man. contact with the skull, are the neck-vertebras (Fig. 254), and are distinguished by a hole found in each of the two lateral processes. There are seven neck-vertebras in Man, as in nearly all other Mammals, whether the neck is long, as in the Camel and the Giraffe, or short, as in the Mole and the Hedgehog. The fact that the number of these neck-vertebras is always seven, — and there are but few exceptions (explicable by adaptation), — is a strong argu- ment for the common descent of all Mammals ; it can only be accounted for as a strict transmission from a common 282 THE EVOLUTION OF MAN. parent-form, from some Promammal which had seven neck- vertebrae. If each animal species had been a distinct crea- tion, it would have been far more to the purpose to have furnished the long-necked Mammalia with a larger, and the short-necked with a smaller number of neck- vertebrae. The neck- vertebrae are immediately followed by those of the breast or thorax, which, in Man and most other Mammals, number twelve or thirteen (usually twelve). Attached to the sides of each breast- vertebra (Fig. 255) is a pair of ribs — long- curved processes of bone lying in and supporting the wall of the thorax. The twelve pairs of ribs, with the connecting intercostal muscles and the breast-bone (sternum) constitute the breast -body (thorax, Fig. 252, p. 279). In this elastic and yet firm thorax lie the double lung, and between the two halves of this, the heart. The chest-vertebrae are followed by a short but massive section of the vertebral column, formed by five large vertebrae. These are the lumbar-vertebrae (Fig. 256), which bear no ribs and have no perforations in their lateral processes. Next comes the cross-bone (sacrum), which is inserted between the two halves of the pelvic girdle. This cross-bone consists of five fixed and amalgamated cross-vertebrae. Last comes a small rudimentary tail- vertebral column, the rump-bone (coccyx). This bone consists of a varying number (usually four, more rarely three or five) of small aborted vertebra? ; it is a useless rudimentary organ, retaining no physiological sig- nificance either in Man or in the Tail-less Apes or Anthro- poids. (Cf. Figs. 204-208.) Morphologically it is, however, very interesting, as affording incontrovertible evidence of the descent of Man and of Anthropoids from Long- tailed Apes. For this assumption affords the only possible THE VERTEBRAE. 283 explanation of this rudimentary tail. In the human embryo, indeed, during the earlier stages of germ-history, the tail projects considerably. (Cf. Plate VII. Fig. M II., and Figs. 123, s, 124, s, vol. i. p. 370.) It afterwards becomes adherent, and is no longer externally visible. Yet traces of the aborted tail-vertebrae, as well as of the rudimentary muscles, which formerly moved them, persist throughout life. According to the earlier anatomists the tail in the female human being has one vertebra more than that of the male (four in the latter, five in the former).177 Number of Vertebrae in various Catarhini. Neck Verte- brae. Tail- less l 'Man (Fig. 208) Orang (Fig. 205) * Gibbon (Fig. 204) Gorilla (Fig. 207) ^Chimpanzee (Fig. 20G) Tailed 'Mandril (Mormon choras) Drill (Mormon leucophceus) ... Rhesus (Inuus rhesus) Sphinx (Papio sphinx) L Simpai (Semnopithecus melus) 7 7 7 7 7 Chest or tho- racic Verte- brae. 7 7 7 7 7 12 12 13 13 14 Lum- bar Verte- bra. 13 12 12 13 12 5 5 5 4 4 Cross or sacral Verte- brae. Tail Verte- brae. 6 7 7 6 7 5 4 4 4 4 3 3 2 3 3 4 5 3 5 5 5 8 18 24 31 Total. 33 33 32 33 34 34 37 46 53 60 The number of vertebrae in the human vertebral column is usually thirty-three in all ; but it is an interesting fact that this number frequently varies, one or another vertebra failing, or a new, supernumerary vertebra inserting itself. Not unfrequently, also, a rib, capable of free motion, forms on the last neck-vertebra or on the first lumbar-vertebra, so that thus there are thirteen breast, and six neck, or four lumbar vertebrae. In this way contiguous vertebrae in the different sections of the vertebral column may replace each 284 THE EVOLUTION OF MAN. other. On the other hand, the above comparison of the number of vertebras in different tail-less and tailed Catarhines shows considerable fluctuations in these numbers even in this one family.178 To understand the history of the development of the human vertebral column, we must now study the form and combination of the vertebras in somewhat greater detail. The main outline of each vertebra is that of a signet ring (Figs. 254-256). The thicker part, which faces the ventral side, is called the body of the vertebra, and it forms a short disc of bone ; the thinner forms a semi-circular arch — the vertebral arch, which is turned toward the dorsal side of the body. The arches of all the consecutive vertebras are so con- nected by thin ligaments (Jiigamenta intercruralia) that the space enclosed by them all in common forms a long canal. In this spinal, vertebral canal lies, as we have seen, the hind portion of the central nervous system, the spinal marrow. The front part of this, the brain, is enclosed in the skull- cavity, and hence the skull itself is merely the anterior section of the vertebral column, modified in a peculiar way. The base or ventral side of the bladder-shaped brain-capsule was originally formed by a number of coalescent vertebral bodies, the amalgamated upper vertebral arches of which formed the arched or ventral side of the skull. While the firm, massive vertebral bodies constitute the true central axis of the skeleton, the dorsal arches serve to enclose and protect the central marrow. Analogous arches also develop on the ventral side as a protection for the thoracic and abdominal viscera. These inferior or ventral vertebral arches, proceeding from the ventral side of the vertebral bodies, form a canal in many low Vertebrates in, NATURE OF THE VERTEBRAE. 285 which are enclosed the large blood-vessels on the under surface of the vertebral column — the aorta and the tail vein. In higher Vertebrates most of these inferior vertebral arches are lost or become merely rudimentary. But in the breast section of the vertebral column they develop into strong, independent bony arches, the ribs (costce). The ribs are, in fact, merely large vertebral arches which have become independent, and have broken their original connection with the vertebral bodies. The gill arches, of which we have spoken so often, are of similar origin ; they are actual head-ribs in the strictest sense — processes which have actually originated from the lower arches of the skull- vertebrae, and which correspond with the ribs. Even the mode of connection of the right and left halves of the arches on the ventral side is the same in both instances. The chest is closed in front by the intervention, between the upper ribs, of the breast-bone (sternum) — a single bone originating from two corresponding side-halves. The gill- body is also closed in front by the intervention of a single piece of bone — the copula lingualis. In now turning from this anatomical examination of the constitution of the vertebral column to the question of its development, I may, as regards the first and most important features in the evolution, refer the reader to the explanation already given of the germ-history of the vertebral column (Chap. XII, vol. i. pp. 369-378). In the first place, it is ne- cessary to recollect the important fact that in Man, as in all other Vertebrates, a simple, unarticulated cartilaginous rod at first occupies the place of the articulated vertebral column. This firm but flexible and elastic cartilaginous rod is the well-known notochord (chorda dorsalis). In the lowest Ver- 286 THE EVOLUTION OF MAN. tebrate, the Amphioxus, this persists throughout life in this very simple form, and permanently constitutes the whole internal skeleton (Fig. 151, i,vol i. p. 420 ; Plate XI. Fig. 15). But even in the Mantle Animals (Tunicata), the nearest invertebrate allies of Vertebrata, we find this same noto- chord; transitorily in the transient larval tail of Ascidia (Plate X. Fig. 5, ch) ; permanently in the Appendicularia (Fig. 162). The Mantle Animals, as well as the Acrania, have undoubtedly inherited the notochord from a common worm-like parent-form, and these primaeval worm ancestors are the Chorda Animals (Chovdonia, p. 91). Long before any trace of a skull, limbs, etc., appears in the human embryo or in that of any of the higher Vertebrates — in that early stage when the whole body is represented only ,b by the lyre-shaped germ-disc — in the cen- tral line of this latter, directly under the primitive groove or medullary furrow, ap- pears the simple chorda dorsalis. (Cf. Figs. 84-87, vol. i. pp. 297, 298, surface view; Figs. 66-70, 89-93, transverse section ; also Plates IV., V, ch) As a cylindrical chord it traverses the longitudinal axis of the body, and is equally pointed at both ends. The cells which compose the chord (Fig. 257, b) come, in common with all the other cells of tion of notochord the skeletoilj from t]ie skin-fibrous layer. (chorda dor sah s) of . an embryo sheep : They most resemble certain cartilage cells ; a, sheath ; b, cells. a Special " chordal tissue " is often said to (After Koelliker.) ^.^ _ ^ ^ ^^ ^ ^ regarded ag more than a special form of cartilaginous tissue. At an early period the notochord envelopes itself in a structureless sheath (a) as clear as glass, which is secreted by its cells. kt- Fig. 257.— Por- DEVELOPMENT OF THE VERTEBRAL COLUMN. 287 This perfectly simple, inarticulate, primary axial skeleton is soon replaced by an articulated, secondary axial skeleton, called the " vertebral column." On each side of the notochord the primitive vertebral bands or primitive vertebral plates (vol. i. p. 306, Fig. 92, uiv) differentiate from the inner portion of the skin-fibrous layer. The inner part of these primitive vertebral bands, which immediately sur- rounds the notochord, is the skeleton-plate, or skeleton stratum (i.e., the cell-layer forming the skeleton), which furnishes the tissue for the rudiments of the permanent vertebral column and of the skull. In the anterior half of the body the primitive vertebral plate remains a simple, continuous, unbroken layer of tissue, and soon expands into a thin- walled vesicle, which surrounds the brain ; this is the primordial skull. In the posterior half, on the contrary, the primitive vertebral plate breaks up into a number of homologous cube-shaped pieces, lying one behind the other, these are the several primitive vertebrae. The number of these is at first very small, but soon increases, as the germ grows in the posterior direction (Figs. 258-260, uw). The first and earliest primitive vertebras are the foremost neck-vertebras ; the posterior neck-vertebras then originate ; then the anterior breast-vertebrae, etc. The lowest of the tail-vertebrse arise last. This successive ontogenetic growth of the vertebral column in a direction from front to rear may be explained phylogenetically by regarding the many- membered vertebrate body as a secondary product, which has originated from an originally inarticulate parent-form by progressive metameric development, or articulation. Just as the many-membered Worms (Earth-worm, Leech) and the closely allied Arthropods (Crabs, Insects) originally 288 THE EVOLUTION OF MAN. a*<» ■■■■$ — '«VnP "■•Rite ■ v» . ■I' / X2> Figs. 258-260. — Lyre-shaped germ-shield of a Chick, in three consecutive stages of development; seen from the dorsal side; enlarged about twenty times. Fig. 258, with six pairs of primitive vertebras. The brain is a sim- ple bladder (lib). The spinal furrow from x remains wide open ; behind, at z, it is much enlarged, mp, Marrow-plates ; sp, side-plates ; y, limit be- tween the pharynx cavity (s7i) and the head-intestine (vd). Fig. 259, with ten pairs of primitive vertebrae. The brain has separated into three bladders : v, fore-brain ; m, mid-brain ; h, hind-brain ; c, heart ; dv, yelk- veins. The spinal furrow is still wide open (z). nip, Marrow-plates. Fig. 260, with sixteen pairs of primitive vertebras. The brain has separated PRIMITIVE VERTEBRAE. 289 into five bladders: v, fore-brain; z, twixt-brain; ra, mid-brain; h, hind- brain ; n, after-brain ; a, eye-vesicles ; g, ear-vesicles ; c, heart ; dv, yelk- veins ; rap, marrow-plate ; uw, primitive vertebra. developed from an inarticulate worm-form by terminal budding, so the many-membered vertebrate body has originated from an inarticulate parent-form. The nearest extant allies of. this parent-form are the Appendicularia (Fig. 162) and the Ascidian (Plate XL Fig. 14). As has been repeatedly pointed out, this primitive vertebral, or metameric structure has a very important bearing on the higher morphological and physiological de- velopment of Vertebrates. (Cf. vol. i. p. 346.) For the articu- lation is by no means confined to the vertebral column, but equally affects the muscular, nervous, vascular, and other systems. As is shown by the Amphioxus, the metameric structure appeared much earlier in the muscular than in the skeleton system. Each so-called primitive vertebra is in fact far more than the mere rudiment of a future verte- bra. In each primitive vertebra exists the rudiment of a segment of the dorsal muscles, of a pair of spinal nerve- roots, etc. Only the inner portion — that which lies directly next to the notochord and the medullary tube — is employed, as the skeleton-plate, in the formation of actual vertebrae. We have already seen how these true vertebras develop from the skeleton-plate of the primitive vertebras or metamera. The right and left lateral halves of each primitive vertebra, originally separate, unite. The ventral edges, meeting below the medullary tube, surround the chord and thus form the rudiments of the vertebral bodies ; the dorsal edges, meeting above the medullary tube, form the first rudiments of the vertebral arches. (Cf. Figs. 95-98, and Plate IV. Figs. 3-8.) 290 THE EVOLUTION OF MAN. li In all Skulled Animals (Cvaniota), most of the soft, undifferentiated cells which originally constitute the skeleton-plate, afterwards change into cartilage cells, which secrete a firm, elastic "intercellular sub- stance," and thus produce cartilaginous tissue. Like most other parts of the skeleton, the rudimentary vertebrse soon pass into a cartilaginous condition, and, in the higher Vertebrates, the cartila- ginous tissue is afterwards replaced by the rigid bony tissue with its peculiar radiate bone-cells (Fig. 5, vol. i. p. 12G). The original axis of the vertebral column, human embryo of eight the notochord, is more or less compressed weeks, in lateral Ion- , .-, , .-. ,. i • i by the cartilaginous tissue which grows vigorously round it. In lower Verte- brates (i.e., in Primitive Fishes) a more or less considerable portion of the noto- chord remains within the vertebral bodies. In Mammals, on the contrary, it disappears almost entirely. In the human embryo, even at the end of the second month, the notochord is seen only as a thin thread which passes through the axis of the thick cartilaginous ver- tebral column (Fig. 261, ch). In the cartilaginous vertebral bodies themselves, which afterwards ossify, the thin remnant of the notochord (Fig. 262, ch) soon disappears entirely. A remnant remains, however, throughout life in the elastic " intervertebral discs " which develop, from the skeleton plate, between each pair of vertebral bodies (Fig. 261, li). In a new-born child, a large, pear-shaped cavity, filled with a gelatinous cell-mass, is visible in each intervertebral disc Jt/iig.&'iiv*?.1 Fig. 261.— Three breast-vertebrte of a gitudinal section : v, cartilaginous vertebral bodies ; li, interverte- bral discs ; ch, noto- chord. (After Koel- liker.) EVOLUTION OF THE NOTOCHORD. 29I (Fig. 2G3, a). This " gelatinous nucleus " of the elastic ver- tebral disc becomes less sharply defined, but persists throughout life in all Mammals, while in Birds and Rep- tiles, even the last remnant of the notochord vanishes. Ftg. 262. — A breast-vertebra of the same embryo in lateral cross-section: cv, cartilaginous vertebral bodies ; ch, notochord ; pr, square process ; a, vertebral arch (upper) ; c, upper end of rib (lower arch). (After Koelliker.) Fig. 263. — Intervertebral disc of new-born child in cross-section : a, remnant of the notochord. (After Koelliker.) When the cartilaginous vertebras afterwards ossify, the first deposit of bone-substance (the first "bone-nucleus") in the vertebral bodies is formed immediately round the rem- nant of the notochord, and soon completely displaces the latter. A special bone kernel or nucleus is then formed in each half of the cartilaginous vertebral arch. It is not till after birth that the ossification progresses so far that the three bone -nuclei approach each other. The two bony halves of the arch unite during the first year, but it is not till much later, till between the eighth and the twelfth year, that they unite with the bony vertebral body. The bony skull (cranium), which must be regarded as 292 THE EVOLUTION OF MAN. the foremost, peculiarly modified section of the vertebral column, develops in an exactly similar manner. Just as, in the spinal column, the vertebral canal envelopes and pro- tects the dorsal marrow, so the skull forms a bony covering round the brain ; and, as the brain is merely the anterior, peculiarly differentiated portion of the dorsal marrow, we might conclude on a priori grounds, that the bony envelope of the brain is a peculiar modification of that of the dorsal marrow. It is true, that if the developed human skull (Fig. 264) is considered by itself, it is impossible to under- stand how it can be merely the modified anterior portion of the vertebral column. It is a complex, capacious bony structure, consisting of no less than twenty bones, differing widely in form and size. Seven of these skull-bones constitute the spacious case which encloses the brain, and in which we distinguish the strong, massive floor of the skull (basis cranii) below, and the boldly arched roof of the skull (fornix cranii) above. The other thirteen bones form the "facial skull," which especially provides the bony envelopes of the higher sense-organs, and at the same time as the jaw- skeleton, encircles the entrance to the intestinal canal. The lower jaw (usually regarded as the twenty-first skull-bone) is jointed to the skull-floor, and behind this, embedded in the roots of the tongue, we find the tongue- bone, which, like the lower jaw, has originated from the gill-arches, together with a portion of the lower arch, which originally developed as " skull-ribs " from the ventral side of the skull-floor. Fig. 264.— Human skull, from the right side. VERTEBRAL THEORY OF THE SKULL. 293 Although, therefore, the developed skull of the higher Vertebrates, in its peculiar form, its very considerable size, and its complex structure, seems to have nothing in common with ordinary vertebrae, yet the old comparative anatomists at the close of the eighteenth century correctly believed that the skull is originally merely a series of modified vertebra?. In 1790, Goethe "picked up out of the sand of the Jews' burying-ground among the downs near Venice, a dismembered skull of a sheep ; he at once per- ceived that the face bones (like the three vertebrae of the back of the skull) are also derivable from vertebrae." And, in 1806, Oken (without knowing of Goethe's discovery), at Ilsenstein, on the way to the Brocken, " found a beautifully bleached skull of a hind ; the thought flashed through him, It is a vertebral column ! " 179 For the last seventy years, this celebrated " Vertebral Theory of the Skull ' has interested the most prominent zoologists ; the most important representatives of Compara- tive Anatomy have exercised their ingenuity in attempting to solve this philosophical skull-problem ; and the question has engaged attention in yet wider circles. It was not till 1872 that the solution was found, after seven years of labour, by the comparative anatomist, who, both in the wealth of his real empirical knowledge and in the pro- fundity of his philosophic speculations, surpasses all other students of this science. Karl Gegenbaur, in his classic " Researches in the Comparative Anatomy of Vertebrates " (third part), showed that the skull skeleton of the Selachii is the only record which affords definite proof of the verte- bral theory of the skull. Earlier comparative anatomists erred in starting from the developed mammalian skull, and 294 THE EVOLUTION OF MAN. in comparing the several component bones with the separate parts of vertebrae ; they supposed that in this way they could prove that the developed mammalian skull consists of from three to six original vertebrae. The hindmost of these skull-vertebrae was, according to them, the occipital bone. A second and a third vertebra were represented by the sphenoid bone, with the parietal bones, and by the frontal bone, etc. The elements of anterior skull vertebrae were even supposed to exist in the face bones. In opposi- tion to this view, Huxley first called attention to the fact that in the embryo this bony skull originally develops from a simple cartilaginous vesicle, and that in this simple cartilaginous " primitive skull " not the slightest trace of a constitution of vertebrate parts is visible. This is equally true of the skulls of the lowest and most ancient Skulled Animals (Cvaniota), the Cyclostomi and the Selachii. In these the skull retains throughout life the form of a simple cartilaginous capsule — of an inarticulate " primitive or primordial skull." If the older skull-theoiy, as it was accepted from Goethe and Oken by most comparative anatomists, were correct, then in these lowest Skulled Animals especially, and in the embryos of the higher Skulled Animals, the constitution of the " primitive skull ': by a series of " skull-vertebrae " would be very clearly evident. This simple and obvious consideration, first duly em- phasized by Huxley, indeed overturns the famous " Verte- brate Theory of the Skull," as held by the older comparative anatomists. Yet the entirely correct fundamental idea holds good, i.e., the hypothesis that the skull develops from the anterior portion of the spinal column by differentiation and peculiar modification, just as the brain develops from HUXLEY'S SKULL THEORY. 295 the anterior portion of the dorsal marrow. But the true mode of empirically establishing this philosophic hypothesis was yet to be discovered ; and this discovery we owe to Gegenbaur.180 He was the first to employ the phylogenetic method, which, in this as in all morphological questions, leads most surely and quickly to the result. He showed that the Primitive Fishes (Selachii, Figs. 191, 192, p. 113), as the parent-forms of all Amphirhina, yet retain per- manently in their skull-structure that form of primordial skull, from which the modified skull of the higher Verte- brates, and therefore that of Man, has developed phylo- genetically. He also pointed out that the gill-arches of the Selachii show that their primordial skull was originally formed of a considerable number — at least nine or ten — primitive vertebrae, and that the brain-nerves, which branch from the base of the brain, entirely confirm this. These brain-nerves — with the exception of the first and the second pairs (the olfactory and the optic nerves) — are merely modi- fied spinal nerves, and, in their peripheric distribution, essentially resemble the latter. The Comparative Anatomy of these brain-nerves is one of the strongest arguments for the newer vertebral theory of the skull. It would lead us too far aside if we were to enter into the particulars of this ingenious theory of Gegenbaur, and I must content myself with referring to the great work already quoted ; in it the theory is fully demonstrated by empirical and philosophical arguments. The same author has given a brief abstract in his " Outlines of Comparative Anatomy " (1874), the study of which it is impossible to recommend too highly. In this work Gegenbaur indi- cates as original "skull-ribs," or " lower arches of skull- 36 296 THE EVOLUTION OF MAN. vertebrae/' in the selachian skull (Fig. 265), the following pairs of arches : I. and II. are two lip cartilages, of which the anterior (a) consists only of an upper, and the inferior (be) of an upper and a lower piece ; III., the jaw-arch, which also consists of two pieces on each side, — viz., the primitive upper jaw (os palato-qvbadratum, o) and the Fig. 265. — Head skeleton of a Primitive Fish: n, nose- groove; cth, region of the sieve-bone ; orb, eye-cavity : la, wall of ear-labyrinth ; occ, occipital region of the primitive sknll ; cv, vertebral column ; a, front ; be, hind lip- cartilage ; o, primitive npper jaw (palato quadratum) ; u, primitive lower jaw; II., tongue- arch; III.-VI1I., first to sixth gill-arches. (After Gegen- baur.) primitive lower jaw (u); IV., the tongue arch (II.), and V. to X., six true gill arches, in the stricter sense of that term (III.-VIIL). The anatomical features of these nine or ten skull-ribs, or " lower vertebral arches," and of the brain nerves distributed over them, show that the apparently simple, . cartilaginous "'primordial skull' of the Primitive Fishes originally develops from an equal number (nine at the least) of primitive vertebrae. The base of the skull is formed by the vertebral bodies ; the roof of the skull by the upper vertebral arches. The coalescence and amalgamation of these into a single capsule is, however, so ancient, that EVOLUTION OF THE SKULL. 297 their primordial separate condition now appears effaced by the action of the "law of abridged heredity/' and is no longer demonstrable in the Ontogeny. In the human primitive skull (Fig. 266), and in that of all higher Vertebrates, which has been modified, phyloge- netically, from the primitive skull of the Selachii, five con- secutive divisions are visible at a certain early period of development; these one might be tempted to refer to five Fig. 266. — Primitive skull of human embryo of four weeks ; vertical section, the left half seen from the inside : v, z, ra, h, n, the five grooves in the skull cavity, in which lie the live brain -bladders (fore-brain, twixt-brain, mid-brain, hind- brain, after-brain) ; 0, pear-shaped pri- mary ear-vesicle ; a, eye ; no, optic nerve ; p, canal of the hypophysist ; t, central part of the cranial basis. (After Koelliker.) original primitive vertebrae ; they are, however, merely the result of adaptation to the five primitive brain-bladders, and, like the latter, they rather correspond to a larger number of metamera. The fact that the primitive verte- brate skull is a much modified and profoundly transformed organ, and by no means a primitive structure, is also evi- dent in the circumstance that its rudiment, originally a soft membrane, commonly assumes the cartilaginous state only at its base and on the sides, while it remains membranous at the skull-roof. Here the bones of the later bony skull develop in the soft membranous rudiment as an external bony roof, without a previous intermediate cartilaginous state, as in the base of the skull. Thus a great part of the skull-bones originally developed as roof-bones from the 298 THE EVOLUTION OF MAN. leather-skin (corium), and only secondarily, come into closer relations with the skull. How, in Man, this most simple and primordial rudiment of the primitive skull develops, onto- genetically, from the head-plates, and how, in the mean- time, the anterior extremity of the notochord is enclosed in the base of the skull, has already been explained. (Cf. vol. i. p. 378 ; Figs. 145 and 146, p. 393.) The main features in the history of the development of the gill-arches, which must now be regarded as skull-ribs, has been told. Of the four original rudimentary gill-arches of Mammals (Plates I. and VII., Figs. 232-236, p. 243), the first lies between the primitive mouth-opening and the first gill- opening. From the base of this gill-arch the "upper jaw process " develops, and this unites, in the manner already described, with the internal and the external nasal processes on each side, and forms the chief parts of the upper jaw skele- ton palate-bones, wing-bones, etc. (Cf. p. 245 and 268.) The rest of the first gill-arch, now distinguished as the " lower-jaw process," forms out of its base two ear bonelets — the hammer (malleus) and the anvil (incus) ; the rest of its mass becomes a long strip of cartilage, called, after its discoverer, " Meckel's cartilage." On the external surface of this cartilage origin- ates, as a surface-bone (formed of cellular matter from the leather-plate), the permanent bony lower jaw. From the base of the second gill-arch in Mammalia originate the third ear bonelet, the stirrup (stapes), and from the subse- quent parts, in order, the stirrup-muscle, the styloid process of the temporal bone, the styloid band, and the small horn of the tongue-bone. Finally, the third gill-arch becomes cartilagin- ous only at its anterior portion, and here, by the union of its two halves, is formed the body of the tongue-bone (co'p-uia EVOLUTION OF THE SKULL. 299 hyoidea) and its great horn on each side. The fourth gill- arch appears in the mammalian embryo only as a transient, rudimentary embryonic organ, and does not develop into special parts. Of the posterior gill-arches (the fifth and sixth pairs), which are permanent in the Primitive Fishes, no trace is visible in the embryo of higher Vertebrates. The latter have long been lost. The four gill-openings in the human embryo are also only interesting as transient rudi- mentary organs, which soon disappear entirely by concre- scence. The first gill-opening (between the first and second gill-arches) alone is of permanent importance ; from it develops the drum, or tympanic cavity of the ear, and the Eustachian tube. (Cf. p. 269, and Plate I., with explan- ation.) Not only did Gegenbaur, in his model " Researches into the Comparative Anatomy of Vertebrates," first correctly explain the skull and its relation to the vertebral column, but he also first performed the no less weighty and interest- ing task of showing the phylogenetic derivation of the skeleton of the limbs in all Vertebrates from one primordial form. Few parts of the body in the different Vertebrates are subjected, by adaptation to various circumstances, to such an infinite variety of modifications as the limbs, in point of size, form, and special fitness for certain purposes, and yet we are now able to refer them all to one common here- ditary form. Vertebrates are distinguishable as regards the structure of their limbs into three large main groups. The lowest and most ancient Vertebrates, the skull-less and jaw- less classes, like all their invertebrate ancestors, had no paired limbs ; this condition is yet represented in the Am- phioxus and in the Cyclostomi (Figs. 189, 190). The second 300 THE EVOLUTION OF MAN. main group consists of the two classes of true Fishes, and of the Dipneusta ; in these, two pairs of lateral limbs, in the shape of many-fingered swimming-fins — one pair of pectoral fins (the fore legs) and one pair of abdominal fins (hind legs) — are originally always present (Figs. 191, 192, Plate XII.). Finally, the third main group embraces the four higher vertebrate classes: Amphibia, Reptiles, Birds, and Mammals; in these the same two pairs of legs exist originally, but in the form of five-fingered feet. The digits or fingers are often fewer than five ; sometimes, also, the feet are quite aborted. But the original parent-form of the entire group had anteriorly and posteriorly five digits (Pentaclactylism, p. 12-3). As regards the Phylogeny of the limbs, from their Comparative Anatomy it appears, therefore, that the extre- mities originated in the Fishes, in the Primitive Fishes (Selachii), and were transmitted from these to all higher Vertebrates (all the Amjphirliina), first in the form of many-fingered fins, and afterwards as five-fingered feet (Fio-s. 267-272). • The anterior extremity — the pectoral fin (or the fore leg) — is originally shaped iDrecisely like the posterior extremity — the ventral fin (or the hind leg). In the one, as in the other, the true limb, externally promi- nent, is distinguishable from the internal, concealed girdle, by which the limb is attached to the spinal column — the shoulder-girdle above, the pelvic girdle below. The genuine primitive form of the paired limbs, as it existed in the most ancient of the Primitive Fishes during the Silurian Period, occurs to this day in perfect preserva- tion in the ancient Ceratodus, and very curious Mud-fish of Australia (p. 119, Plate XII.). In this, both the pectoral and THE LIMBS. 3OI the ventral fin is a flat, oval paddle, in which we find a feathered or biserial cartilaginous skeleton (Fig. 267). This skeleton consists firstly of a strong, articulated fin-rod or " stem " (Fig. 267, A B), which extends from the base to the tip of the fin, and secondly, of a double row of thin, feathered rays (rr), which are attached to both sides of the central rod, like the pinnee of a pinnate leaf. This primi- tive fin, first recognized by Gegenbaur, and by him called the Arclivpterygiwm, is attached to the spinal column by means of a simple girdle in the shape of a cartilaginous arch.181 In some Sharks and Rays, especially when very young, this same primitive fin also occurs in a more or less modified form. But in most Primitive Fishes the fin is already essentially modified, in that the rays on one side of the stem are partly or altogether lost, and are retained only on the other side (Fig. 268). Hence arises the half-feathered, or uniserial fish-fin, inherited by the other fishes from the Selachii (Fig. 269). Gegenbaur first showed how the five-fingered leg of Amphibia is developed from this uniserial fin (Fig. 270) and is inherited by three classes of Amniota. In those Dip- neusta which were the ancestors of the Amphibia, the fin rays on the other side of the stem also were gradually degraded in development, and were in a great measure lost (the light- coloured cartilages in Fig. 269). Only the four lowest rays (shaded in Fig. 269) were retained ; and these are the four outer digits of the foot (second to fifth digits). The first, or great digit (toe), on the contrary, originated from the lower part of the fin-rod. From the middle and upper parts of this fin-rod developed the long main stem of the limbs 302 THE EVOLUTION OF MAN. Fig. 267. Fig. 269. Fig. 270. Fig. 272. EVOLUTION OF THE LIMBS. 303 Fig. 267. — Bones of pectoral fins of Oeratodus (Archipterygium, or bilateral pinnate skeleton) : A B, series of cartilaginous pieces forming the ventral stem of the fin ; rr, rays of the fin. (After Gunther.) Fig. 268. — Bones of pectoral fin of an earlier Primitive Fish ( Acanthias) . Most of the rays of the medial edge of the fin (B) have disappeared ; only a few (B') remain. B B, rays of the lateral edge of fin ; mt, Metap- terygium ; ms, Mezopterygium ; p, Propterygium. (After Gegenbaur.) Fig. 269. — Bones of pectoral fin of a more recent Primitive Fish, or Selachian. The rays of the medial edge of the fin have entirely dis- appeared. The shaded part on the right is that portion which develops into the five-fingered hand of higher Vertebrates (b, the three basal pieces of the fin ; mt, Metapterygium ; rudiment of the humerus ; ms, Mezoptery- gium ; p, Propterygium) . (After Gegenbaur.) Fig. 270. — Bones of the fore-limb of an Amphibian : h, upper arm (humerus) ; r, u, lower arm (r, radius ; u, ulna) ; r, c, i, c, u, root-bones of the hand, first row (r, radial ; i, intermediate ; c, central ; u, ulnary) ; 1, 2, 3, 4, 5, root-bones of the hand, second row. (After Gegenbaur.) Fig. 271. — Bones of hand of Gorilla. (After Huxley.) Fig. 272. — Bones of human hand, seen from the back. (After H. Meyer.) which is so prominent in the higher Vertebrata as the upper arm (or leg) (Fig. 270, v and u) and the lower arm (or leg, h). The many -fingered fish-fins thus gave rise, by a process of gradual reversion and differentiation, to the five-fingered amphibian foot, which occurs first in the Sozobranchia, and which, from them, has been transmitted on the one hand to Reptiles, and to Mammals, up to Man, on the other (Fig. 272) Simultaneously with the reduction of the number of the fin- rays to four, a further differentiation affected the fin-stem or rod ; it became transversely divided into the upper and lower arms (or legs), and a modification took place in the girdle, which in the higher Mammals originally consists, both anteriorly and posteriorly, of three bones. The simple arch of the original shoulder-girdle separates, on each side into an upper (dorsal) piece — the shoulder-blade {scapula) 304 THE EVOLUTION OF MAN. and a lower (ventral) piece ; the anterior portion of the latter constitutes the pro-key (or collar) bone (procoracoi- deum) and its posterior part the raven-bone (coracoideum). The simple arch of the pelvic girdle breaks up, correspond- ingly, into an upper (dorsal) piece — the intestinal bone (os ilium), and a lower (ventral) piece ; the anterior portion of the latter becomes the pubic bone (os pubis) and the posterior portion the hip-bone (os ischii). Table XXXIV., p. 278, shows the correspondence of these three parts of the pelvic girdle with those of the shoulder-girdle. The latter, however, in the key-bone or collar-bone (clavicula), possesses a fourth, wanting in the former. (Cf. Gegenbaur.182) As in the girdle, so in the trunk of the limbs there is originally an absolute agreement between the anterior and posterior limbs. The first section of the trunk is supported by a single strong bone — in the anterior limbs, the upper arm (humerus) ; in the posterior, the upper leg (femur). The second section, on the other hand, contains two bones — on the anterior extremity the spoke-bone (radius, Fig. 270, r), and the ell-bone (ulna, Fig. 270, u) ; in the posterior the two corresponding bones, the shin-bone (tibia) and calf-bone (fibula). (Cf. skeletons in Fig. 196 and Figs. 204-208). Moreover, the subsequent small and numerous bones of the wrist (carpus) and of the ankle (tarsus) cor- respond ; so do the five bones of the middle of the hand (metacarpus) and of the middle of the foot (metatarsus). Finally, the same is true of the five digits attached to these parts, which in their characteristic structure of a series of bone-pieces correspond in the anterior and posterior limbs. Charles Martins, of Montpellier, an excellent morphologist has shown that, in detail, the anterior and posterior limbs correspond.183 HOMOLOGY OF THE LIMBS. 305 As Comparative Anatomy thus shows that the skeleton of the limbs in Man is composed of the same bones, and in the same manner as the skeleton in the four higher verte- brate classes, Ave may justly infer their common descent from a single parent-form. This parent-form was the most ancient Amphibian possessing five digits both on the fore and on the hind limbs. The outermost part of the limbs has, indeed, been very much modified by adaptation to various conditions of life. The diversities in this point within the mammalian class are enormous. The slender limbs of the swift Deer and the strong, springy legs of the Kangaroo, the climbing feet of the Sloth and the digging paws of the Mole, the fins of the Whale and the wings of the Bat, are all instances. It will, of course, be admitted by all that these organs of locomotion are as diverse as possible in point of size, form, and special function. And yet the internal bony skeleton is substantially the same in them all. In all these different forms of limbs the same characteristic bones are always represented in essentially the same strongly inherited combination ; and here we have a weighty confirm- ation of the theory of descent, such as is hardly afforded by the Comparative Anatomy of any other organ. (Cf. Plate IV. p. 34, vol. ii. of " History of Creation.") True, in the limbs of the different Mammals, the skeleton is subject to various arrests of development and reversions, in addition to those due to special adaptation (Fig. 273). Thus, in the fore foot (or hand) of the Dog the first digit, or thumb, is aborted (Fig. 273 II.). In the Pig (III.) and the Tapir (V.) this digit has entirely disappeared. So, too, in the Rumi- nants (e.g., the Ox, Fig. IV.) the second and fifth digits are also aborted, and only the third and fourth are well deve- o6 THE EVOLUTION OF MAN. loped. Finally, in the Horse, only one digit, the third, is perfectly developed (Fig. VI., 3). And yet all these diverse fore-feet, as also the hand of the Ape (Fig. 271) and the human hand (Fig. 272), have originated from the same common live-lingered parent-form. This is proved, not only by the rudiments of the aborted digits, but also by the homologous disposition of the wrist-bones (Fig. 273, a-p). (Vide supra, p. 124.) The same story is also told by the germ-history of the limbs, which is originally identical, not only in all Mammals, but in all Vertebrates. However different the limbs of the various Skulled Animals (Cvaniota) afterwards appear in their fully developed state, they nevertheless all originate from the same simple rudiment. (Cf. Plates VI. and VII., 4- 3 Fig. 273. — Skeleton of hand or fore -foot of six Mammals. I. Man; II. Dog; III. Pig; IV. Ox; V. Tapir; VI. Horse, r, Kadius; u, ulna; c, scaphoid ; b, semi-lunar ; c, triquetrum (cuneiform) ; d, trapezium ; e, trapezoid ; /, capitatum (unciform process) ; g, hamatum (unciform bone) ; p, pisiform; 1, thumb; 2, digit; 3, middle finger; 4, ring finger; 5, little finger. (After Gegenbaur.) ORIGIN OF THE LIMBS. 307 vol. i. p. 362 ; /, fore-leg, b, hind-leg.) In all, the first rudi- ment of each limb in the embryo is a simple wart, or small knob, which grows from the side of the body between the dorsal and ventral sides (Figs. 119 and 120, vol. i. pp. 357, 359 ; 136 and 137, pp. 381, 382). The cells composing these knobs belono- to the skin-fibrous layer. The outer surface is coated b}7 the horn-plate, which is rather thicker at the apex of the protuberance (Plate IV. Fig. 5, x). The two anterior protuberances appear at a rather earlier period than the two posterior. By differentiation of the cells, these simple rudiments develop immediately, in Fishes and in the Dipneusta, into fins. In the higher vertebrate classes, on the contrary, each of the four protuberances, in the course of its development, assumes the form of a stalked plate, the inner portion of which being narrower and thicker, the outer broader and thinner. The inner portion, or the handle of the plate, then divides into two sections : the upper and lower legs (or arms). Four notches then appear in the free edge of the plate, and these gradually become deeper ; these are the divisions between the five digits (Plate VIII. Fig. 1). The latter soon become more prominent. At first, however, all the five digits, both on the fore and on the hind limbs, are joined by a thin connecting web-like membrane; this recalls the original adaptation of the foot as a swimming-fin. The further development of the limbs from this most simple rudiment takes place in the same way in all Vertebrates ; that is, by the modification of certain groups of the cells of the skin-fibrous layer into cartilage, of other groups into muscles, yet others into blood-vessels, nerves, etc. Probably the differentiation of all these various tissues occurs actuallv in the limbs. Like the vertebral column and the skull, the 308 THE EVOLUTION OF MAN. bony parts of the limbs are also formed at first from soft undifferentiated cell-groups of the skin-fibrous layer. These afterwards change into cartilage, and from these the per- manent bones originate by a tertiary process.184 The development of the muscles, or the active organs of locomotion, is, as yet, of much less interest than that of the skeleton, or the passive instruments of motion. The Com- parative Anatomy of these is, indeed, of much higher im- portance than their Embryology. But as very little attention has, as yet, been paid to the Comparative Anatomy and Ontogeny of the muscular system, we have only very general ideas of its Phylogeny also. The muscular system as a whole has developed in the most intimate reciprocal correlation with the bone system.185 ( 309 ) TABLE XXXV. Systematic Survey of the most Important Periods in the Phylogeny of the Human Skeleton. I. First Period : Skeleton of the Chordoma (Fig. 187, p. 90). The entire skeleton is formed by the notochord. II. Second Period : Skeleton of the Acrania (Fig. 189, p. 91). A notochord-niembrane, the dorsal continuation of which forms a cover- ing round the medullary tube, is formed round the notochord. III. Third Period : Skeleton of the Cyclostomi (Fig. 190, p. 103). A cartilaginous primordial skull develops round the anterior extremity of the notochord, from the notochord-membrane. An outer cartilaginous gill-skeleton forms round the gills. IV. Fourth Period : Skeleton of the older Selachii (Fig. 268, p. 302). A primitive vertebral column, with upper and lower arches (the gill- arches and ribs) forms round the notochord. The remnant of the outer gill- skeleton remains with the inner. Two pairs of limbs, with pinnate (biserial) skeletons, appear. V. Fifth Period: Skeleton of the more recent Selachii (Fig. 269, p. 302). The anterior gill-arches change into lip-cartilage and jaw-arches. The external gill-skeleton is lost. The skeleton of the two pairs of fins becomes uniserial (semi -pinnate). VI. Sixth Period : Skeleton of the Dipneusta (Fig. 2, Plate XII.). The skull becomes partially ossified ; as does the shoulder-girdle. VII. Seventh Period : Skeleton of the Amphibia (Fig. 270, p. 302). The gill-arches are modified into parts of the tongue-bone, and of the jaw- apparatus. On the semi -pinnate skeletons of the fins the rays diminish in number to four, thus giving rise to the five-toed foot. The vertebral column ossifies. 310 THE EVOLUTION OF MAN. VIII. Eighth Period: Skeleton of the Monotremata (Fig. 196, p. 148). The vertebral column, skull, jaws, and limbs, acquire the definite characteristics of Mammals. IX. Ninth Period : Skeleton of the Marsupialia (Fig. 197, p. 152). The coracoid bone of the shoulder. girdle becomes atrophied, and the remnant of it amalgamates with the shoulder-blade. X. Tenth Period : Skeleton of the Semi-apes (Fig. 199, p. 164). The pouch-bones, which distinguish Monotremes and Marsupials, disappear. XI. Eleventh Period : Skeleton of the Anthropoid Apes (Figs. 204-208, p. 179). The skeleton acquires the peculiar development shared by Man ex- clusively with the Anthropoid Apes. CHAPTER XXIII. DEVELOPMENT OF THE INTESTINAL SYSTEM. The Primitive Intestine of the Gastrula. — Its Homology, or Morphological Identity in all Animals (excepting the Protozoa). — Survey of the Structure of the Developed Intestinal Canal in Man. — The Mouth- cavity. — The Throat (pharynx). — The Gullet (oesophagus). — The Wind- pipe (trachea) and Lungs. — The Larynx. — The Stomach. — The Small Intestine. — The Liver and Gall-bladder. — The Ventral Salivary Gland (pancreas). — The Large Intestine. — The Rectum. — The First Rudiment of the Simple Intestinal Tube. — The Gastrula of the Amphioxus and of Mammals. — Separation of the Germ from the Intestinal Germ Vesicle (Gastrocystis). — The Primitive Intestine (Protogaster) and the After Intestine (Metagaster). — Secondary Formation of the Mouth and Anus from the Outer Skin. — Development of the Intestinal Epithelium from the Intestinal-glandular Layer, and of all other parts of the Intestine from the Intestinal-fibrous Layer. — Simple Intestinal Pouch of the Lower Wox*ms. — Differentiation of the Primitive Intestinal Tube into a Respiratory and a Digestive Intestine. — Gill-intestine and Stomach- Intestine of the Amphioxus and Ascidian. — Origin and Significance of the Gill-openings. — Their Disappearance. — The Gill-arches and the Jaw- skeleton. — Formation of the Teeth. — Development of the Lungs from the Swim-bladder of Fish. — Differentiation of the Stomach. — Development of the Liver and Pancreas. — Differentiation of the Small and Large Intestines. — Formation of the Cloaca. "Cautious people require us to confine ourselves to gathering materials, and to leave it to posterity to raise a scientific structure from those materials ; because only in that way can we escape the ignominy of having the theories we believed in overthrown by the advance of knowledge. The unreasonableness of this demand is apparent enough from the fact that 312 THE EVOLUTION OF MAN. Comparative Anatomy, like every other science, is endless ; and therefore the endlessness of the accumulation of materials would never allow men, if they complied with this demand, to reap any harvest from this field. But, further than this, history teaches us clearly, that no age in which scientific inquiry has been active, has been able so to deny itself, as, setting the goal of its researches in the future, to refrain from drawing conclusions for itself from its larger or smaller treasury of observations,, and from trying to fill the gaps with hypotheses. It would, indeed, be a hopeless proceeding, if, in order to avoid losing any part of our possessions, we should refuse to acquire any possessions whatever." — Karl Ernst Baer (1819). Among the vegetative organs of the human body, to the development of which we now turn our attention, the intes- tinal canal is the most important. For the intestinal tube is the oldest of all the organs of the animal body, and carries us back to the earliest time of organological differ- entiation, to the first period of the Laurentian Epoch. As we have already seen, the result of the first division of labour in the homogeneous cells of the earliest many-celled animal body must have been the formation of a nutritive intestinal canal. The first duty and the first need of every organism is self-support. This task is accomplished by the two functions of nutrition and of the covering of the body. When, therefore, in the primaeval collection of homogeneous cells (Synamoebium), of the phylogenetic existence of which we yet have evidence in the ontogenetic developmental form of the mulberry-germ (Morula), the several members of the community began to divide the work of life, they were first obliged to engage in two separate tasks. One half modified into nutritive cells, enclosing a digestive cavity, the intestinal canal ; the other half, on the contrary, developed into covering cells, forming the outer cover- ing of this intestinal canal, and, at the same time, of the whole body. Thus arose the first two germ-layers : the PRIMITIVE INTESTINAL CANAL. 313 inner, nutritive, or vegetative layer, and the outer, covering, or animal layer. If we try to construct for ourselves an animal body of the simplest conceivable form, possessing such a primitive intestinal canal, and the two primary germ-layers forming its wall, the result is necessarily the very remarkable germ-form of the gastrula, which we have shown to exist in wonderful uniformity throughout the whole animal Fig. 274. —Gastrula of a Chalk-sponge (Olynthus) : A, from outside ; Bj in longitudinal section through the axis ; g, primitive intestine ; 0, primi- tive mouth ; i, intestinal layer, or entoderm ; e, skin-layer, or exoderm. series: in the Sponges, Sea-nettles (Acalephce), Worms, Soft-bodied Animals (Mollusca), Articulated Animals {Arthvo- poda), and Vertebrates (Figs. 174-179, p. 65). In all these various animal tribes the gastrula reappears in the same entirely simple form (Fig. 274). Its whole body is really merely the intestinal canal ; the simple cavity of the body, the digestive intestinal cavity, is the primitive intestine 314 THE EVOLUTION OF MAN. (protogaster, g) ; its simple opening, the primitive mouth (jorotostoma, 6), is at once mouth and anus ; and the two cell-strata which compose its wall, are the two primary germ-layers : the inner, the nutritive, or vegetative germ- layer, is the intestinal layer (entoderma, i) ; and the outer, covering layer, which, by means of its cilia, is also the agent of motion, is the animal layer, or skin-layer (exoderma, e). This highly important fact, that the gastrula appears as an early larval condition in the individual development of the most varied animals, and that this gastrula always exhibits the same structure, and that the very differently developed intestinal canals of the most varied animals, arises, onto- genetically, from the same extremely simple gastrula- intestine, this very important fact justifies, in accordance with the fundamental law of Biogeny, two conclusions, which involve important results, and of which one is general and one special. The general conclusion is an inductive one, and may be stated thus : The very variously formed intestinal canal of all the different Intestinal Animals has developed, phylogenetically, from one common and extremely simple primitive intestine, from the intestinal cavity of the Gastrsea, that primaeval common parent-form which is at the present reproduced, in accordance with the fundamental law of Biogeny, in the gastrula. The second, the special conclusion, which is connected with the former, is deductive, and may be stated thus : The intestinal canal in Man as a whole is homologous with the intestinal canal in all other animals ; it has the same original significance, and has developed from the same rudimentary form.186 Before proceeding to trace the history of the develop- ment of the human intestinal canal in detail, it will be THE HUMAN INTESTINAL CANAL. 3 I 5 necessary briefly to get a correct idea of the more general conditions of the formation of the intestinal canal in the developed Man. Not until this is known can the development of the several parts be correctly understood. (Cf. Plates IV. and V., vol. i. p. 321.) The intestinal canal in the developed Man is, in all essential points, exactly similar in form to those of all other higher Mammals, and, especially, to that of the Catarhines, the Narrow-nosed Apes of the Old World. The entrance to the intestinal canal is the mouth-opening (Plate V. Fig. 16, o). Food and drink pass first through this into the mouth-cavity, in the lower part of which is the tongue. The human mouth-cavity is hedged with thirty-two teeth, attached in two rows to the two jaws, the upper and lower. It has already been stated that the series of teeth is formed in Man exactly as in the Catarhine Apes, but differs from the corresponding part in all other animals (p. 173). Above the mouth-cavity is the double nose-cavity ; the two parts of this are separated by the par- tition-wall of the palate. But, as we have seen, the nasal cavity is not originally separated at all from the mouth- cavity, a common nasal and mouth cavity being primarily formed in the embryo, and this separates at a later period into two separate stories by the hard palate-roof : the upper is the nasal cavity, the lower is the mouth-cavity. The nasal cavity is connected with certain air-filled bony cavities: the jaw-cavities in the upper jaw, the frontal cavities in the frontal bone, and the sphenoid cavities in the sphenoid bone. Numerous glands of various kinds open into the mouth-cavity, particularly many small mucous glands and three pairs of large salivary glands. The human mouth-cavity is half closed at the back by 3l6 THE EVOLUTION OF MAN. the vertical curtain which Ave call the soft palate, and in the centre of the lower part of Avhich is situated the uvula. A glance with the mouth open into a mirror is sufficient to show the form. The uvula is of importance, because it occurs only in Men and in Apes. On both sides of the soft palate are the tonsils (tonsillce). Through the gate-like arched opening situated beneath the soft palate, we pass into the throat-cavity (pharynx; Plate V. Fig. 16, sh), which lies behind the mouth-cavity. This is only partly visible in the open mouth when reflected in the mirror. Into the throat-cavity a narrow passage opens on each side (the Eustachian tube of the ear), which leads directly into the tympanic cavity of the ear (Fig. 244, e, p. 260). The throat-cavity is continued into a long narrow tube, the gullet {oesophagus, sr). Through this the masticated and swrallowed food passes down into the stomach. The wind-pipe (trachea, Ir) also opens into the upper part of the throat, and leads thence to the lungs. The opening of this is protected by the epiglottis, over which the food passes. The respiratory organs, the two lungs (Plate IV. Fig. 8, hi), are situated, in Man, as in all Mammals, in the right and left sides of the breast-cavity (thorax), and midway between them is the heart (Fig. 8, hr, hi). At the upper end of the wind-pipe (trachea), below the epiglottis just spoken of, is a peculiarly differ- entiated section, the larynx, which is protected by a carti- laginous frame. The larynx is the most important organ of the human voice and speech, and also develops from a part of the intestinal canal. In front of the larynx lies the thyroid gland (thyreoidea), which occasionally enlarges to the so-called " goitre." THE HUMAN INTESTINAL CANAL. 317 The gullet {oesophagus) passes downward through the thorax, along the vertebral column, behind the lungs and the heart, and enters the ventral cavity, after penetrating the diaphragm. The latter (Fig. 16, z) is a membranous, muscular, transverse partition, which in all Mammals (and only in these) completely separates the chest-cavity (thorax, c) from the ventral cavity (c„). As has been said, this division does not originally exist; at first a common chest and ventral cavity, the cceloma, or the pleuro- peritoneal cavity, is formed in the embryo. It is only afterwards that the diaphragm forms a muscular, horizontal partition between the chest and the ventral cavities. This partition then completely separates the two cavities, and is penetrated only by separate organs, passing through the FrG. 275. — Human stomach and gall-intestine in longitudinal section : a, cardia (limit of the oesophagus) ; b, fundus (blind sac of the left side) ; c, pylorus fold; d, pyloras valve; e, pylorus-cavity ; / a h, gall-intestine ; i, mouth of the gall-duct and of the pancreas duct. (After H. Meyer.) chest-cavity into the ventral cavity. One of the most important of these organs is the gullet (oesophagus). After this has passed through the diaphragm into the ventral cavity it enlarges into the stomach in which digestion 318 THE EVOLUTION OF MAN. especially takes place. The stomach of an adult man (Fig. 275, Plate V. Fig. 16, ong) is an oblong sac, placed somewhat obliquely, the left side of which widens into a blind-sac, the base of the stomach or fundus (b), while the right side narrows, and passes at the right end, called the pylorus (e), into the small intestine. Between these two parts of the intestine is a valve, the pyloric valve (d), which only opens when the food-pulp (chyme) passes from the stomach into the small intestine. The stomach itself is the most important digestive organ, and serves especially to dissolve the food. The muscular wall of the stomach is comparatively thick, and, on the outside, has strong muscle- layers, which effect the digestive movements of the stomach ; — on the inside, it has a great number of small glands, the gastric glands, which secrete the gastric juice. Next to the stomach follows the longest part of the whole intestinal canal, the central, or small intestine (chylog aster). Its principal function is to effect the absorp- tion of the fluid mass of digested food, or the food-pulp (chyme), and it is again divided into several sections, the first of which, the one immediately following the stomach, is called the gall-intestine, or " twelve-finger intestine " (duodenum, Fig. 275, fg h). The gall-intestine forms a short loop curved like a horse-shoe. The largest glands of the intestinal canal open into it : the liver, the most important digestive gland, which furnishes the bile, or gall, and a very large salivary gland, the ventral salivary gland, or pancreas, which secretes the digestive saliva. Both of these glands pour the juices they secrete, the bile and pancreatic juice, into the duodenum (i) near each other. In adults the liver is a very large gland, well supplied with blood, lying on the THE HUMAN INTESTINAL CANAL. 319 right side immediately below the diaphragm, and separated by the latter from the lungs (Plate V. Fig. 16, lb). The pancreas lies somewhat further back and more to the left (Fig. 16, p). The small intestine is so long that it has to lie in many folds in order to find room in the limited space of the ventral cavity ; these coils are the bowels. They are divided into an upper intestine, called the empty intestine {jejunum), and a lower, the crooked intestine {ilium). In this latter part lies that part of the small intestine at which, in the embryo, the yelk-sac opens into the intestinal tube. This long, thin intestine then passes into the large intestine, from which it is separated by a peculiar valve. Directly behind this " Bauhinian valve " the first part of the large intestines forms a broad pouch- like expansion, the blind intestine {ccecum), the atrophied extremity of which is a well-known rudimentary organ, the vermiform process {processus vermiformis). The large intestine {colon) consists of three parts ; an ascending part on the right, a transverse central part, and a descending- part on the left. The latter finally curves like an S, called the " sigmoid flexure," into the last part of the intestinal canal, above the rectum, which opens at the back by the anus (Plate V. Fig. 16, a). Both the large intestine and the small intestine are furnished with numerous glands, most of them very small, and which secrete mucous and other juices. Along the greater part of its length the intestinal canal is attached to the inner dorsal surface of the ventral cavity, or to the lower surface of the vertebral column. It is fastened by means of the thin, membranous plate, called the mesentery, which develops directly under the notochord 37 320 THE EVOLUTION OF MAN. from the intestinal-fibrous layer, at the point where this curves into the outer lamina of the side-layer, into the skin-fibrous layer (Plate IV. Fig. 5, g). The curving-point was distinguished as the middle-plate (Fig. 99, mp). The mesentery is, at first, very short (Plate V. Fig. 14, gr) ; but it soon lengthens considerably at the central part of the intes- tinal canal, and takes the form of a thin, transparent, membranous plate, which has to be the more extended the further the folds of the intestine diverge from the place where they are first attached to the vertebral column. The blood-vessels, lymphatic vessels, and nerves which enter the intestinal canal traverse this mesentery. Although, therefore, the intestinal canal, in the adult human being forms an extremely complex organ, and though it shows in its details so many intricate and delicate structural arrangements, — into which we cannot enter here, — this entire structure has developed, historically, from that simplest form of primitive intestine which was possessed by our gastrsead ancestors, and which the extant gastrula now exhibits. We have already shown (in Chapter VIII.) that the peculiar Hood-gastrula (Amphi- gastrula) of Mammals (Fig. 277) may be referred back to the original Bell-gastrula (Archigastrula) form, which, among Vertebrates, is now accurately retained solely by the Amphioxus (Fig. 276 ; Plate X. Fig. 10). Like the latter, the gastrula of Man and of all Mam- mals must be regarded as the ontogenetic reproduction of that phylogenetic evolution-form which we call the Gastreea, and in which the whole body of the animal is intestine. The peculiar form and mode in which the complex DEVELOPMENT OF THE INTESTINAL CANAL. 321 human intestinal canal develops from the simple gastrula and which is similar to that in other Mammals, can there- fore be only correctly understood when it is considered in the light of Phylogeny. We must, accordingly, distinguish Fig. 276. — Archigastrula of Amphioxus (in longitudinal section) : d, primitive intestine ; 0, primitive mouth ; i, intestinal layer ; e, skin-layer. Fig. 277. — Amphigastrula of Mammal (in longitudinal section). The primitive intestine (d) and primitive mouth (o) are filled up by the cells of the intestinal layer (i) ; e, skin-layer. between the original primary intestine ("the primitive intestine, or protogaster ") of the Skull-less Animals (Acrania), and the differentiated or secondary intestine ( " after intestine, or mctagaster " ) of the Skulled Animals (Craniota). The intestine of the Amphioxus (the repre- sentative of the Acrania) forms no yelk-sac, and develops, palingenetically, from the entire primitive intestine of the gastrula. The intestine of the" Skulled Animals, on the other hand, has a modified, kenogenetic form of evolution, and differentiates at a very early period into two different parts : into the permanent secondary intestine, which alone 322 THE EVOLUTION OF MAN. gives rise to the various parts of the differentiated intestinal system, and the transient yelk-sac, which serves only as a storehouse of materials for the building of the embryo. The yelk-sac attains its greatest development in Primitive Fishes (Selachii), Bony Fishes (Teleostei), Reptiles, and Birds. In Mammals, and especially in Placental Animals, it is atrophied. The peculiar intestinal development of the Cyclostomi, Ganoids, and Amphibia must be regarded as an intermediate form, between the palingenetic intestinal development of the Skull-less animals, and the kenoge- netic intestinal development of the Amnion Animals (Am- niota).181 We have already seen in what a peculiar way the development of the intestine takes place ontogenetically in the human embryo and in that of other Mammals. Imme- diately from the gastrula of these originates a globular intestinal germ-vesicle (gastrocystis), filled with fluid (Figs. 72, 73, vol. i. p. 289). In the wall of this is formed the lyre-shaped germ-shield, on the lower side of which, along the middle line, appears a shallow groove, the first rudi- ment of the future, secondary intestinal tube. This intestinal groove grows constantly deeper, and its edges curve toward each other, to grow together at last and form a tube (Fig. 100, vol. i. p. 333). The wall of this secondary intestinal tube consists of two membranes of the inner, intestinal-glandular layer, and of the outer, intestinal- fibrous layer. The tube is completely closed at the ends, having only an opening in the centre of the lower wall, by which it is connected with the intestinal germ-vesicle (Plate V. Fig. 14). The latter, in the course of development, becomes continually smaller, as the intestinal canal continues DEVELOPMENT OF THE INTESTINAL CANAL. 323 to grow larger and more perfect. While, at first, the intes- tinal tube appears only as a little appendage on one side of the great intestinal germ-vesicle (Fig. 278), the remnant of the latter afterwards forms only a very inconsiderable appen- dage of the great intestinal canal. This appendage is the yelk-sac, or navel-vesicle. It entirely loses its importance, and at length disappears, while the intestinal canal is finally closed at the original central opening, where it forms the so-called intestinal navel (Fig. 94, vol. i. p. 312). It has also been said that this simple cylindrical intestinal tube, in Man as in all Vertebrates, is at first entirely closed at both ends (Plate V. Fig. 14), and that the two permanent openings of the intestinal canal — at the anterior extremity, the mouth, at the posterior, the anus — form only second- arily, and from the outer skin. At the fore end, a shallow mouth-furrow originates in the outer skin, and this grows toward the blind, anterior end of the head intestinal cavity, into which it finally breaks. In the same way a shal- low furrow for the anus is formed behind in the skin, and this soon grows deeper, and grows toward the blind posterior end of the pelvic intestinal cavity, with which it finally unites. At both extremities there is, at first, a thin partition between the outer skin-furrow and the blind end of the intestine, and this disappears when the opening is made.188 Directly in front of the anus the allantois grows out of the posterior intestine ; this is the important embryonic appendage which develops, in Placental Animals, and only in these (thus in Man too) into the placenta (Figs. 278, 279, 1 ; Plate V. Fig. 14, al). In this more developed form — repre- sented in the diagram (Fig. 94, 4, vol. i. p. 312) — the intestinal 324 THE EVOLUTION OF MAN, canal of Man, like that of all other Mammals, now forms a slightly-curved, cylindrical tube, which has an opening at both ends, and from the lower wall of which depend two sacs; the anterior navel-bladder, or yelk-sac, and the pos- terior allantois, or primitive urinary sac. Microscopic observation shows that the thin wall of this simple intestinal tube and of its two bladder-like append- ages is composed of two distinct cell-strata. The inner, which coats the entire cavity, consists of larger, darker cells, Fig. 278. — Human embryo of the third week, with the amnion and allantois. The great globular yelk-sac is below, the bladder-like allantois on the right ; there are as yet no limbs. The germ, with its appendages, is enclosed in the tufted membrane (chorion). Fig. 279. — Human embryo, with amnion and allantois, in the fourth week. (After Krause.) The amnion (w) lies pretty close to the body. The greater part of the yelk-sac (d) has been torn away. Behind this the allan- tois appears as a small pear-shaped bladder. Arms (/) and legs (b) are already commenced : v, fore-brain ; z, twixt-brain ; m, mid-brain ; h, hind- brain j n} after-brain ; a, eye ; Jc, three gill-arches ; c, heart ; s, tail. RUDIMENT OF THE INTESTINAL CANAL. 325 and is the intestinal-glandular layer. The outer stratum consists of lighter, smaller cells, and is the intestinal fibrous- layer. The cavities of the mouth and the anus are the only exceptions to this, because they originate from the outer skin. The inner cell-coating of the entire mouth-cavity is therefore furnished, not by the intestinal glandular-layer, but by the skin-sensory layer, and its muscular lower layer, not by the intestinal-fibrous layer, but by the skin-fibrous layer. This is equally true of the wall of the anal cavity (Plate V. Fig. 15). If the question be asked, what relation these component germ-layers of the primitive intestinal wall bear to the infinitely varied tissues and organs which we afterwards find in the developed intestine, the answer is extremely simple. The relations of these two layers to the formation and differentiation of the tissues of the intestinal canal with all its parts, may be condensed into a single sentence : The intestinal epithelium, that is, the inner, soft cell-stratum which coats the cavities of the intestinal canal and of all its appendages, and which directly accomplishes the nutritive process, develops solely from the intestinal-glandular layer ; on the contrary, all other tissues and organs belong- ing to the intestinal canal and its appendages, proceed from the intestinal-fibrous layer. From this latter, therefore, originates the entire outer covering of the intestinal tube and its appendages ; the fibrous connective tissue and the smooth muscles which compose its fleshy skin ; the carti- lages which support these, for example, the cartilage of the larynx and of the trachea ; the numerous blood and lymph vessels which absorb nutrition from the wall of the intestine; in short, everything belonging to the intestine, with the 326 THE EVOLUTION OF MAN. exception of the intestinal epithelium. From the intestinal- fibrous layer originates also the entire mesentery with all the adjacent parts, the heart, the large blood-vessels of the body, etc. (Plate V. Fig. 16). Let us now turn aside for a moment from this original rudimentary intestine of Mammals, in order to institute a comparison between it and the intestinal canal of those lower Vertebrates and Worms, which we have learned to recognize as the ancestors of Man. In the simplest Gliding- worm, or Turbellaria (Rhabdoccelum, Fig. 280), we find a very simple intestinal form. As in the gastrula, the intes- tine in these Worms is a simple pouch with a single open- ing, which latter acts both as mouth and anus (m). The intestinal pouch has, however, differentiated into two sec- tions, an anterior throat-intestine (sd) and a posterior stomach-intestine (d). This differentiation becomes more important in the Ascidia (Fig. 281) and in the Amphioxus (Fig. 282), which connects the Worms with the Vertebrates. In these two animal forms the intestine is essentially identical; the anterior portion forms the respiratory gill- intestine, the posterior forms the digestive stomach-intes- tine. In both it develops, palingenetically, directly from the primitive intestine of the gastrula (Plate XI. Figs. 4, 10). But the original mouth- opening of the gastrula, or the primitive mouth, afterwards closes, and in its place is formed the later anus. In the same way, the mouth- opening of the Amphioxus and of the Ascidian is a new formation, as is the mouth-opening of Man, and generally, of all Skulled Animals (Graniota). The secondary forma- tion of the mouth of the Lancelet is connected, as may be conjectured with some probability, with the formation ol EARLY FORMS OF THE INTESTINAL CANAL. 327 the gill-openings, which appear directly behind it on the intestine. The front portion of the intestine has thus ivnv Fig. 280. — A simple Gliding Worm (Rhabdoccehim) m, mouth; sd, throat- epithelium ; sm, throat muscle-mass ; d, stomach-intestine; nc, renal ducts; /, ciliated outer-skin ; », openings of the latter ; em, eye ; na, nose-pit. Fig. 281. — Structure of an Ascidian (seen from the left side, as in Plate XL Fig. 14). The dorsal side is turned toward the right, the ventral side to the left ; the mouth-opening (0) is above ; at the opposite, tail end, the ascidian has become adherent. The gill-intestine (br), perforated by many openings, extends into the stomach-intestine. The terminal intestine opens through the anus (a) into the gill-cavity (cl), from which the excre- ment is passed out with the respirated water through the gill-pore, or cloacal opening (a1) ; m, mantle. (After Gegenbaur.) 328 THE EVOLUTION OF MAN. t nv- d im become a respiratory organ. I have already pointed out how characteristic this adaptation is of Vertebrates and Mantle Animals (Tunicata, p. 87). The phylogenetic origin of the gill-openings in- dicates the beginning of a new epoch in the tribal history of Vertebrates. The most important process we meet with in the further ontogenetic development of the intestinal canal in the human embryo, is the origin of the gill-openings. At the head of the human embryo, the wall of the throat very early unites with the outer wall of the body, and four openings then form on the right and left sides of the neck, behind the mouth, and these lead directly from without into the throat-cavity. These openings are the gill-openings, and the par- titions separating them are the gill-arches (Figs. 116-118, vol. i. p. 356; Plates I. and V., Fig. 15, ks). These embryonic for- mations are very interesting ; for they show Fig. 282. — Lancelet (Amphioxus lanceolatus) , double the natural size, seen from the left side (the longi- tudinal axis is perpendicular, the mouth end above, the tail end below (as in Plate XL Fig. 15) : or, mouth- opening, surrounded by bristles ; b, anal opening ; c, gill-pore (porus -branchialis) ; d, gill-body ; e, stomach; /, liver; g, small intestine; h, gill-cavity; i, notochord, below which is the aorta; it, aortal arch; J, main stem of the gill-artery ; m, swellings on the branches of the latter ; n, hollow vein ; o, intestinal vein. that all the higher Vertebrates when in a very young state, ORIGIN OF THE GILL-OPENINGS. 329 reproduce, in accordance with the fundamental principle of Biogeny, the same process which was originally of the greatest importance to the development of the whole verte- brate tribe. This process was the differentiation of the intestinal canal into two sections : an anterior, respiratory part, the gill-intestine, which serves only for breathing, and a posterior, digestive part, the stomach-intestine, which serves only for digestion. As we meet with this very characteristic differentiation of the intestinal tube into two, physiologically, very distinct main sections, not only in the Amphioxus, but also in the Ascidian and the Appen- dicularia, we can safely conclude that it also existed in our common ancestors, the Chorda Animals (Chordoma), especially as even the Acorn Worm (Balanoglossus) has it (Fig. 186, p. 86). All other Invertebrate Animals are entirely without this peculiar arrangement. The number of the gill-openings is still very large in the Amphioxus, as in Ascidians and in the Acorn Worm. In the Skulled Animals it is, on the contrary, very much lessened. Fishes mostly have from four to six pairs of gill- openings. In the embryos of Man and the higher Verte- brates also, only three or four pairs are developed, and these appear at a very early period. The gill-openings are perma- nent in Fishes, and afford a passage to the water which has been breathed in through the mouth (Figs. 191, 192, p. 113 ; Plate V. Fig. 13, ks). On the other hand, the Amphibians lose them partially, and all the higher Vertebrates entirely. In the latter, only a single vestige of the gill-openings remains, the remnant of the first gill-opening. This changes into a part of the organ of hearing; from it originates the outer ear-canal, the tympanic cavity, and the Eustachian tube. °^0 JJ>V TABLE XXXVI. Systematic Survey of the Development of the Human Intestinal System. N.B. — The parts marked thus f are processes from the intestinal tube. Mouth-cavity (Cavum oris) First main section of the Intestinal System : the Respiratory ( Intestine (Gill Intestine). Pneogaster. ( Tractus respiratorius.) II. Second main section of the Intestinal System : Digestive Intestine (Stomach Intes- tine). Peptogastek. (Tractus digestivus.) 4. Nose-cavity (Cavum nasi) Throat-cavity (Cavum pharyn- gis) , Mouth-opening Lips Jaws Teeth Tongue Tongue bone f Salivary glands Soft palate \ Uvula / Nose canal ) f Jaw cavities "j f Frontal cavities (f Ethmoid cavity Isthmus of the throat Tonsils Pharynx { -j- Eustachian tube -j- Tympanic cavity f Brain-appendage V f Thyroid gland Lung-cavity ( T (Cavum pulmonis) \ T f Larynx Windpipe Lungs Anterior Intestine J (Prosogaster) Gullet Stomach-opening Stomach Stomach exit 6. Central Intestine (Mesogastcr) 1. Posterior Intestine (Epigaster) Urinary Intestine (Urogaster) r Gall-intestine f Liver f Pancreas : Empty intestine (f Yelk-sac, or navel- bladder) V Crooked intestine Large intestine f Blind intestine f Vermiform process of the ccecum Piectum Anal opening (f Primitive urinary sac rinary tube Urinary bladder to vfU Rima oris \ Labia 43 Maxilla £l! Dentes T3 f= Lingua Os hyoides 1 H Glandulaz salivates ^ Velum palatinum 'T£ Uvula Meatus narium Sinus maxillares S5.S Sinus frontales %* Sinus ethmoidales / Isthmus faucium » Tonsillar C Pharynx 0 Tuba Eustachii as Cavum tympani S.S Hypophysis Thyreoidea ° .a Larynx Trachea Pulmones H (Esophagus p Cardia c.2 Stomachus Pylorus Duodenum the inte! us, whic Hepar Pancreas c — Jejunum ( Vesicula umbilica- lis) ^ Ileum £ ° Colon "5 '> Coicum Processus vermi- formis 1 s Rectum a; (- Anus 1-1 £ Allantois) CH 8 Urethra A Urocystis THE MOUTH-SKELETON. 33 I We have already considered this remarkable formation, and will only call attention once more to the interesting fact that the human middle and external ear is the last remnant of the gill-opening of a Fish. The gill-arches, also, which separate the gill-openings, develop into very various parts. In Fishes they remain permanently as gill-arches, carrying the respiratory gill-tufts ; so also in the lowest Amphibia ; but in the higher Amphibia they undergo various modifica- tions in the course of development, and in all the three higher vertebrate classes, thus also in Man, the tongue-bone (ps hyoides) and the bonelets of the ear originate from the gill-arches. (Cf. Plates VI and VII.) From the first gill-arch, from the centre of the inner surface of which the muscular tongue grows, proceeds the rudimentary jaw-skeleton ; the upper and lower jaws which enclose the cavity of the mouth and carry the teeth. The Acrania and Monorhina are entirely destitute of these important parts. They first appear in the genuine Fishes, and have been transmitted by these to the higher Vertebrates. The original formation of the human mouth- skeleton, of the upper and lower jaws, can thus be traced back to the earliest Fishes, from which we have inherited them. The teeth originate from the outer skin-covering which covers the jaws ; for, as the formation of the whole mouth-cavity takes place from the outer germ-layer, the teeth must, of course, also have developed originally from the skin- layer. This can be actually proved by close microscopic examination of the most delicate structural features of the teeth. The scales of Fishes, especially of Sharks, are, in this respect, exactly similar to their teeth (Fig. 283). Thus the human teeth, in their earliest origin, are modified fish- 332 THE EVOLUTION OF MAN. scales.188 On similar grounds we must regard the salivary glands, which open into the mouth-cavity, as really outer- skin (epidermic) glands, which have not developed, like the other intestinal glands, from the intestinal-glandular layer of the intestinal canal, but from the outer skin, from the horn-plate of the outer germ-layer. It is evident that, as the mouth develops in this way, the salivary glands must be placed genetically in the same series with the sweat, sebaceous, and milk glands of the epidermis. The human intestinal canal is therefore quite as simple in its original formation as the primitive intestine of the gastrula. It also resembles that of the lowest Worms. It then differentiates into two sec- tions, an anterior gill-intestine, and Jj a posterior stomach-intestine, like the intestinal canal of the Lancelet and the Ascidian. By the develop- ment of the jaws and gill-arches it is modified into a true Fish- intestine. Afterwards, however, the gill-intestine, which is a memorial of the Fish-ancestors, as such, is entirely lost. The parts that remain Fig. 283. — Scales of a J r Shark (Centrcphoras caiceus). take a wholly different form ; but On each rhomboid bone-tablet, notwithstanding that the anterior lying in the leather-skin, rises . . , , a small, three-cornered tooth, section of our intestinal canal thus (After Gegenbaur.) surrenders entirely its original form of gill-intestine, it yet retains its physiological func- tion as a respiratory intestine; for the extremely in- THE BREATHING APPARATUS. 333 teresting and remarkable discovery is now made that even the permanent respiratory organ of the higher Vertebrates, the air-breathing lungs, has also developed from this anterior section of the intestinal canal. Our lungs, together with the wind-pipe {trachea) and the larynx, develop from the ventral wall of the anterior intestine. This entire great breathing-apparatus, which occupies the greater part of the chest (thorax) in the developed Man, is at first merely a very small and simple vesicle or sac, which grows out from the intestinal canal immediately behind the gills, and soon separates into two lateral halves (Figs., 284, c, 285, c ; Plate V. Figs. 13, 15, 16, hi). This vesicle occurs in all Vertebrates except in the two lowest classes, the Acrania and Cyclostomi. In the lower Vertebrates, however, it develops, not into lungs, but into an air-filled bladder of considerable size, occupying a great part of the body-cavity (cceloma), and which is of quite a different significance from the lungs. It serves, not for breathing, but as an hydrostatic apparatus: for vertical swimming movements it is the swimming-bladder of Fish ; but the lungs of Man and of all other air-breathing Vertebrates develop from the same simple bladder-like appendage of the anterior intestine, which, in Fishes, becomes the swimming-bladder. Originally this sac also has no respiratory function, but serves only as an hydrostatic apparatus, augmenting or diminishing the specific gravity of the body. Fishes, in which the swimming-bladder is fully developed, are able to compress it, and thus to condense the air contained in it. The air sometimes also escapes from the intestinal canal through an air-passage which connects the swimming- bladder with the throat (pharynx), and is expelled .through 334 THE EVOLUTION OF MAN. the mouth ; in this way the circumference of the swim- ming-bladder is diminished, and the fish becomes heavier and sinks. When the animal is again about to ascend, the swimming-bladder is distended by remitting the com- Fig. 284. — Intestine of an embryonic Dog (which is representee in Fig. 137, vol. i. p. 382; after Bischoff), from the ventral side : a, gill-arches (four pairs) ; b, rudimentary throat and larynx ; c, lungs ; d, stomach ; /, liver ; g, walls of the opened yelk-sac, into which the central intestine opens by a wide aperture ; h, rectum. Fig. 285. — The same intestine, seen from the right side: a, lungs ; b, stomach ; c, liver ; d, yelk-sac ; e, rectum. pressing force. This hydrostatic apparatus begins to be transformed into a respiratory organ in the Mud-fishes (Dipneusta), the blood-vessels in the wall of the swim- ming-bladder no longer merely separating air, but also inhaling fresh air, which has come in through the air- passage. This process is fully developed in all Amphibia. The original swimming-bladder here generally becomes a EVOLUTION OF THE LUNGS. 335 lung, and its air-passage a wind-pipe. The amphibian lung has been transmitted to the three higher vertebrate classes, and even in the lowest Amphibia the lung on either side is as yet a very simple, transparent, thin-walled sac — as, for instance, in our common Water-Newts, or Tritons, and very like the swimming-bladder of Fishes. The Amphibia have, it is true, two lungs, a right and a left; but in many Fishes also (in the ancient Ganoids) the swim- ming-bladder is double, the organ being divided into a right and a left half. On the other hand, the lung of the Ceratodus is single (p. 119). The earliest rudiment of the lung in the human embryo and in the embryo of all higher Vertebrates is also a simple, single vesicle, which does not separate till afterwards into a pair of halves — the right and the left lung. At a later period, the two vesicles grow con- siderably, occupy the greater part of the chest cavity, and lie one on each side of the heart ; even in Frogs we find that the simple sac, in the course of its development, is transformed into a spongy body of a peculiar, froth-like texture. This lung-tissue develops as a tree-like, branched gland, bearing berry -like appendages. The process by which the lung-sac was attached to the anterior intestine, which was originally very short, lengthens, by simple growth, into a long thin tube ; this tube is the wind-pipe {trachea) ; it opens above into the throat (pharynx), and below divides into two branches which pass into the two lungs. In the wall of the wind-pipe ring-shaped cartilages develop, which keep the whole distended ; at the upper end of this wind-pipe, below its entrance into the throat, the larynx, the organ of voice and speech, develops. The larynx occurs even in Amphibia in very various stages of development, and with the aid of 336 THE EVOLUTION OF MAN. Comparative Anatomy we can trace the progressive develop- ment of this important organ from its very simple rudiment in the lower Amphibia up to the complex and vocal appara- tus represented by the larynx of Birds and Mammals. Though these organs of voice, speech, and air-respiration develop so differently in the various higher Mammals, they yet all arise from the same simple original rudiment — from a vesicle which grows out of the wall of the anterior intestine. We have thus satisfied ourselves of the interest- ing fact that both the respiratory apparatus of Vertebrates develop from the fore part of the intestinal canal ; first, the primary and more primitive water-respiring apparatus, the gill-body, which is altogether lost in the three higher vertebrate classes ; and, afterwards, the secondary and more recent air-breathing apparatus, which acts in Fishes only as a swimming-bladder, but as a lung from the Dipneusta upwards. We must say a few words about an interesting rudi- mentary organ of the respiratory intestine, the thyroid gland (thyreoidea), the large gland situated in front of the larynx, and below the so-called " Adam's apple," and which, especially in the male sex, is often very prominent ; it is produced in the embryo by the separation of the lower wall of the throat {pharynx). This thyroid gland is of no use whatever to man; it is only aesthetically interesting, because in certain mountainous districts it has a tendency to enlarge, and in that case it forms the " goitre " which hangs from the neck in front. Its dysteleological interest is, however, far higher ; for as Wilhelm Muller of Jena has shown, this useless and unsightly organ is the last remnant of the " hypobranchial groove," which we have THE STOMACH. 337 already considered, and which, in the Ascidia and in the Amphioxus, traverses the middle of the gill-body, and is of great importance in conducting the food into the stomach (vol. i. p. 420; Plate XI. Figs. 14-16, y)}m The second main section of the intestinal canal, the stomach or digestive intestine, undergoes modifications no less important than those affecting the first main section. On tracing the further development of this digestive section of the intestinal tube, we again find a very complex and composite organ eventually produced from a very simple rudiment. For the sake of rendering the matter more intelligible, we may distinguish the digestive intestine into three parts : the fore intestine (with the gullet and stomach) ; the middle intestine, the gall-intestine (with the liver and pancreas) ; the empty intestine (jejunum), and crooked intestine (ileus) ; and the hind intestine (large intestine and rectum). Here we again find protuberances or appendages of the originally simple intestinal tube which change into very various structures. We have already discussed two of these appendages — the yelk-sac, which protrudes from the middle of the intestinal tube (Fig. 286, c), and the allantois, which grows out of the last portion of the pelvic intestine as a large sac-like protuberance (u). The protuberances from the middle of the intestine are the two great glands which open into the duodenum, the liver (h) and the ventral salivary gland. Immediately behind the bladder-like rudiment of the lungs (Fig. 286, I) comes that portion of the intestinal tube which forms the most important part of the digestive apparatus, viz., the stomach (Figs. 284, d, 285, b). This sac- l 338 THE EVOLUTION OF MAN. shaped organ, in which the food is especially dissolved and digested, is not so complex in structure in the lower Verte- brates as in the higher. Thus, for instance, in many Fishes, it appears as a very simple spindle-shaped expansion at the r putium penis) {Prozputium clitoridis) 10. Sexual folds 10. Testes-sac 10. Labiapudendi majores {Plicce genitales) {Scrotum) 11. Fissure between the two sexual 11. Seam of the testis-sac 11. Female Vulva folds {Raphe scroti) 12. Sexual edges (edges of the 12. Edges of the sexual 12. Labia pudendi minora sexual furrow) furrow coalesce 13. Urogenital canal 13. Ureters 13. Antechamber of the vagina {Sinus urogenital is) ( Urethra) {lestibulum vagina,) 14. Glandular appendages of the 14. Cowper's glands 14. Bartholi's glands urogenital canal CHAPTER XXVI. RESULTS OF ANTHROPOGENY. Review of the Germ -hi story as given. — Its Explanation by the Fundamental Law of Biogeny. — Its Causal Relation to the History of the Tribe. — Rudimentary Organs of Man. — Dysteleology, or the Doctrine of Pur- poselessness. — Inheritances from Apes. — Man's Place in the Natural System of the Animal Kingdom. —Man as a Vertebrate and a Mammal. — Special Tribal Relation of Men and Apes. — Evidences regarding the Ape Question. — The Catarhina and the Platyrhina. — The Divine Origin of Man. — Adam and Eve. — History of the Evolution of the Mind. — Important Mental Differences within a Single Class of Animals. — The Mammalian Mind and the Insect Mind. — Mind in the Ant and in the Scale-louse (Coccus). — Mind in Man and in Ape. — The Organ of Mental Activity : the Central Nervous System. — The Ontogeny and Phy- logeny of the Mind. — The Monistic and Dualistic Theories of the Mind. — Heredity of the Mind. — Bearing of the Fundamental Law of Biogeny on Psychology. — Influence of Anthropogeny on the Victory of the Monistic Philosophy and the Defeat of the Dualistic. — Nature and Spirit. — Natural Science and Spiritual Science. — Conception of the World reformed by Anthropogeny. " The Theory of Descent is a general inductive law which results with absolute necessity from the comparative synthesis of all the phenomena of organic nature, and especially from the threefold parallel of phylogenetic, ontogenetic, and systematic evolution. The doctrine that man has de- veloped from lower Vertebrates, and immediately from genuine Apes, is a special deductive conclusion, which results with absolute necessity from the general inductive law of the Theory of Descent. This view of ' man's SUMMARY. 433 place in nature,' cannot, we believe, be made too prominent. If the Theory of Descent is correct as a whole, then the theory that man has developed from lower Vertebrates is simply an unavoidable deductive conclusion from that general inductive law. Hence, all farther discoveries which may in future enrich our knowledge of the phyletic development of man, can only be confirmative of special points of that deduction, which rests on the broadest inductive basis." — Generelle Morphologie (1866). As we have now traversed the wonderful territory of the history of human development, and learned its most important parts, it seems appropriate that, at the close of our travels, we should look back on the road behind us, and, on the other hand, glance forward along the further path of knowledge into which our road will lead in future. We started from the simplest facts of the history of man's individual development; ontogenetic facts which can, at any moment, be shown and established by microscopic or anatomic research. The first and most important of these ontogenetic facts is, that every man, like every other animal, is at the commencement of his individual existence, a simple cell. This egg-cell exhibits precisely the same structure and mode of origin as that of any other Mammal. From this cell proceeds, by repeated division, a many-celled body, the mulberry-germ (morula) ; this changes into a cup-germ (gastrula), and this, again, into an intestinal germ-vesicle (gastrocystis). The two distinct cell-strata which compose its wall are the two primary germ- layers ; the skin-layer (exodevma) and the intestinal layer (entoclevma). This double-layered germ-form is the onto- genetic reproduction of that extremely important phylo- genetic parent-form of all Intestinal Animals, to which we have given the name Gastrsea. As the human germ, like that of other Intestinal Animals, 434 THE EVOLUTION OF MAN. passes through this gastrula-form, we are enabled to trace its phylogenetic origin back to the Gastrrea. By tracing the germ-history of the two-layered germ still farther, we found that, by fission, four secondary layers are produced from the two original germ-layers. These have exactly the same constitution and genetic significance in Man as in all other Vertebrates. From the skin-sensory layer develops the outer skin (epidermis) and the central nervous system, and, probably, the kidney system. The skin-fibrous layer forms the leather-skin (corium) and the organs of motion (the skeleton and muscle systems). From the intestinal-fibrous layer originates the vascular system and the fleshy wall of the intestine. The intestinal-glandular layer, finally, forms only the epithelium, or the inner cellular layer of the intestinal-mucous membrane and of the intestinal glands. The manner in which these various organic systems develop from the four secondary germ-layers, is, from the very first, exactly the same in Man as in all other Verte- brates. The germ-history of each separate organ afforded proof that the human embryo takes exactly the same special direction in its differentiation and formation, which, except in Man, occurs only in the other Vertebrates. Within this great animal tribe we then traced, step by step, and stage after stage, the farther development which takes place in the entire body as well as in all its several parts. This higher development takes place in the human embryo in the form peculiar to Mammals. Finally, we saw, that even ivithin this class the various stages of phylogenetic development, which determine the natural classification of Mammals, correspond throughout to the various stages of ontogenetic formation through which the human embryo passes in the THE FUNDAMENTAL LAW OF BIOGENY. 435 further course of its development. We were thus enabled to determine the place of Man more definitely in the system of this class, and accordingly to establish the nature of his relation to the various mammalian orders. The course of reasoning which we adopted in explaining these ontogenetic facts, was simply the logical carrying out of the fundamental law of Biogeny. In so doing we have constantly tried to carry out the significant distinction between palingenetic and kenogenetic phenomena. Palin- genesis, or "the history of inheritance," alone enabled us to draw direct conclusions from observed germ-forms as to the tribal forms transmitted by heredity. On the other hand, these conclusions were more or less endangered, wherever Kenogenesis, or "vitiated evolution," was introduced by new adaptations. The whole understanding of the history of in- dividual evolution depends on the recognition of this most important relation. We stand here on the border-line which sharply divides the new from the old method of scientific investigation, the new from the old conception of the world. All the results of recent morphological research drive us with irresistible force to the recognition of this fundamental principle of Biogeny, and of its far-reaching consequences. These are, it is true, irreconcilable with the customary mythological ideas of the world, and with the powerful prejudices engrafted into us in early youth by theosophic instruction ; but, without this fundamental law of Biogeny, without the distinction between Palingenesis and Keno- genesis, and without the Theory of Descent, upon which these are based, we are entirely unable to understand the facts of organic development ; without these, we cannot afford the faintest explanation of any part of this great and 436 THE EVOLUTION OF MAN. wonderful world of phenomena. But, if we recognize the causal relation between the development of the germ and that of the tribe, if we recognize the true causal connection of Ontogeny and Phylogeny, which is expressed in that law, then the wonderful phenomena of Ontogeny explain them- selves most simply; then the facts of germ-development appear but the necessary mechanical effects of the develop- ment of the tribe, conditioned by the laws of Heredity and Adaptation. The inter-operation of these laws among the everywhere-active influences of the struggle for existence, — or, as we may simply say with Darwin, Natural Selection, — is amply sufficient to explain to us the entire process of germ-history by the history of the tribe. Darwin's chief merit lies in the fact, that by the discovery of the inter- action of the phenomena of Heredity and Adaptation, he prepared the way for a correct, logical understanding of the history of Evolution. Among the numerous and important evidences that we have found for the truth of this view of our development history, I will only call attention here once more to the peculiarly valuable records of creation afforded by Dystele- ology, or the doctrine of purposelessness, the science dealing with rudimentary organs. It is impossible to emphasize too often and too strongly the high morphological import- ance of those remarkable parts of the body, which are, physiologically, completely worthless and useless. In every system of organs we find, in Man and in all higher Verte- brates, some of these worthless primaeval heirlooms, which have been inherited from our lower vertebrate ancestors. Thus, first, we find on the outer surface of the body a scanty rudimentary covering of hair, which is thicker only on the RUDIMENTARY ORGANS. 437 head, in the armpits, and on some other parts of the body. The short hairs on the greater part of the surface of our bodies are entirely useless, are without any physiological significance ; they are the last scanty remains of the much more fully developed hairy covering of our Ape ancestors (p. 208). The sense-organs exhibit a series of the most remarkable rudimen- tary parts. As we have seen, the whole external shell of the ear, with its cartilages, muscles, and membranes, is, in Man, a useless appendage, destitute of the physiological importance that was formerly, erroneously, attributed to it. It is the atrophied remnant of the pointed, freely-moving, and much more highly developed mammalian ear, the muscles of which we retain, although we can no longer use them (p. 271). Again, we found, at the inner corner of the human eye, the remarkable little crescent-shaped fold, which is of no use to us, and is of interest only as being the last vestige of the nictitating membrane ; of that third inner eyelid which is still of great physiological importance in Sharks and many Amnion Animals (p. 259). Numerous and interesting dysteleological proofs are also afforded by the apparatus of motion, both by the bony and the muscular systems. I will only cite the free, projecting tail of the human embryo, and the rudimentary caudal vertebras developed in the latter, together with the pertinent muscles ; this whole organ is entirely useless to Man, but is of great interest as the atrophied remnant of the long tail of our earlier Ape ancestors, which was composed of numerous vertebras and muscles (p. 283). From these same ancestors we have also inherited various bone-processes and muscles, which were of great use to them in their climbing life among the trees, but with us have fallen out of use. At various points under the 433 THE EVOLUTION OF MAN. skin we also have entirely unused skin-muscles ; vestiges of the largely developed skin-muscles of our lower mam- malian ancestors. It was the function of this " panniculus carnosus " to contract and wrinkle the skin, as we may see- any day done by horses to drive away flies. We still possess an active remnant of this great skin-muscle in the muscle of the forehead, by means of which we wrinkle the forehead and draw up the eyebrows ; but we are no longer able to move at will another considerable remnant of it, the great skin-muscle of the neck (jplaty&ma myoides). As in these animal organ-systems of our body, so also in the vegetative apparatus, we meet with many rudimentary organs, most of which we have incidentally noticed. I will only cite the remarkable thyroid gland (thyreoidea), the rudiment of the crop and the remnant of the ciliated groove (hypobranchial groove) present in Chordoma, Ascidia, and Arcrania, on the lower part of the gill-body (pp. 336, 353) ; also the vermiform process of the blind-intestine (ccecum) (p. 344). In the vascular system we find many useless ducts, the vestiges of disused vessels which were formerly active blood-channels; such, for instance, are the "ductus Botalli" between the lung-artery and the aorta, and the "ductus venosus Arantii," between the vena povtce and vena cava, and many others. The numerous rudi- mentary organs of the urinary and sexual systems (p. 415) are especially interesting. Most of these are developed in one sex and rudimentary in the other. Thus, in the male, the seed-ducts form from the Wolffian ducts, of which the only traces remaining in the female are the Gartnerian canals. On the other hand, from the Mullerian ducts in the female are developed the oviducts and the uterus; while in HISTORICAL IMPORTANCE OF THE RUDIMENTARY ORGANS. 439 the male, only the lower extremities of these ducts remain, forming the useless male uterus (vesicula prostatica). In the nipples and mammary glands, the male possesses other rudimentary organs which, as a rule, are functional only in the female (p. 204). A closer anatomical examination of the human body would bring to our notice a number of other rudimentary organs, all of which can be explained only by the Theory of Descent. They are among the most important evidences for the truth of the mechanical theory of nature, and among the most overwhelming proofs against the prevailing teleological ideas of creation. If, in accordance with this latter view, Man and every other organism had been designed for his life- purpose from the beginning, and had been called into existence by an act of creation, the existence of these rudimentary organs would be an incomprehensible enigma ; it would be impossible to understand why the Creator should have laid this useless burden on his creatures in their life-journey, so arduous at the best. On the other hand, b}r means of the Theory of Descent we can explain their existence in the most simple way, and say : The rudimentary organs arc parts of the body, which, in the course of centuries, have gradually fallen out of use ; organs which performed definite functions in our animal ancestors, but which, in us, have lost their physiological importance. They have become useless in consequence of our adaptation to new circumstances, but yet are transmitted from generation to generation by heredity, and have only slowly atrophied. Like these rudimentary organs, so also all the other organs of our body have been transmitted to us from Mammals, and, immediately, from our Ape ancestors. The 42 440 THE EVOLUTION OF MAN. human body includes no single organ which is not in- herited from Apes ; but, by means of our fundamental law of Biogeny, we can trace the origin of our several systems of organs yet further down to various lower ancestral grades. Thus, for instance, we can say that we have inherited the earliest organs of our body, the outer-skin (epidermis) and the intestinal canal, from the Gastraeads, the nervous and muscular systems from the lower Worms (Archelminthes), the vascular system, body -cavity (cceloma), and blood from Soft Worms (Scolecida), the notochord and the gill-intestine from Chorda Animals, the differentiated organs of sense from the Cyclostoma, the limbs and the Mullerian ducts from Primitive Fishes (Selachii), and the ex- ternal reproductive organs from Primitive Mammals (Pro- mammalia). When we stated the " law of the ontogenetic connection of systematically allied forms," and determined the relative age of the organs, we saw how we could draw such phylogenetic conclusions as these from the ontogenetic succession of the organ-systems (vol. i. p. 390 ; ii. 357). By the help of this important law and of Comparative Anatomy, we were also enabled to determine definitely "man's place in nature," or, as we may say, to assign to man his position in the system of the animal kingdom. It is now usual, in the more recent zoological systems, to distribute the whole animal kingdom into the seven tribes, or phyla, which are again sub-divided, in round numbers, into about forty classes ; and these classes into about two hundred orders. According to his whole organization, Man is undoubtedly, primarily, a member of but a single tribe, that of Vertebrates ; secondly, he is a member of but a single class, that of Mammals ; and, thirdly, a member "man's place in nature." 441 of but a single order, that of the Apes. All the character- istic peculiarities, distinguishing Vertebrates from the other six tribes, distinguishing Mammals from the other forty classes, and distinguishing Apes from the remaining two hundred orders of the animal kingdom, are also present in Man. Turn and twist as we may, we cannot escape this anatomical and systematic fact. Quite recently this very fact has led to the liveliest discussion, and has occasioned, especially, many disputes about the specific anatomical relationship of Man to Apes. The most astounding views on this "ape question," or "pithecoid theory," have been uttered. It will therefore be well to examine it closely once more at this point, and to separate the essential from the non-essential in it. We will start from the undisputed fact, that Man, at all events, — whether his special blood-relationship to Apes is acknowledged or denied, — is a genuine Mammal, is a Pla- cental Mammal. This fundamental truth can be so easily proved at any moment by investigations in Comparative Anatomy, that it has been unanimously acknowledged since the separation of the Placental from the lower Mammals (Pouched Animals, or Marswpialia, and Beaked Animals, or Ornithostoma). But, from this, every logical adherent of the doctrine of development at once draws the conclusion, that man is descended from one and the same common parent-form, together with all other Placental Animals, from the progenitor of the Placentalia, just as, further, we must necessarily suppose a common mammalian ancestral form of all the various Mammals (Placentalia), Pouched Animals, and Cloacal Animals (Monotremata) ; but by this the great, all-agitating main question of man's place in nature is 442 THE EVOLUTION OF MAN. conclusively settled, whether we ascribe to Man a nearer or a more remote relationship to Apes. No matter whether Man is, in a phylogenetic sense, a member of the Ape order (or, if it is preferred, of the Primate order) or not, — in any case, his direct blood-relationship to all other Mammals, and especially to the Placental Mammals, is established. It may be that the inter-relations of the various Mammals are quite different from those now hypothetically assumed ; but, in any case, the common descent of Man and all other Mammals from a common parent-form is indis- putable. This primaeval, long since extinct parent-form, which probably developed during the Triassic Period, was the monotreme ancestral form of all Mammals. If this fundamental and extremely significant principle is borne in mind, the " ape question ': will appear to us in a wholly different light from that in which it is usually presented. A little reflection will bring conviction that this question has not the importance that has of late been attributed to it ; for the origin of the human race from a series of various mammalian ancestors, and the historical development of the latter from an earlier series of lower vertebrate ancestors, remains indubitably established, no matter whether the genuine " Apes " are regarded as the nearest animal ancestors of the human race or not. But, it having become habitual to lay the principal weight of the entire question of the origin of man on this very " descent from Apes," I find myself compelled to return once more to it here, and to recall those facts in Com- parative Anatomy and Ontogeny, which conclusively settle this "ape question." The shortest way to the goal is the one taken by " HUXLEY'S LAW.' 443 Huxley in his celebrated work, which we have so often quoted, on the " Evidences as to Man's Place in Nature," — the way afforded by Comparative Anatomy and Ontogeny. We have to compare objectively all the several organs of Man with the same organs in the higher Apes, and then to •ascertain whether the differences between the former and the latter are greater than the corresponding differences between the higher and lower Apes. The indubitable and indisputable result of this comparative anatomical investi- gation which was conducted with the greatest candour and accuracy, was the important law, which, in honour of its discoverer, we have named Huxley's Law ; namely, that the physical differences between the organization of Man and that of the most highly developed Apes known to us, are much smaller than the corresponding differences between the higher and lower Apes. We might even define this law yet more exactly by excluding entirely the Platyrhina or American Apes as being more remote relatives, and limiting our comparison to the narrower circle of relatives, the Catarhina, or Apes of the Old World. Even within this small group of Mammals, we found the differences of struc- ture between the higher and lower Narrow-nosed Apes, for example between the Gorilla and the Baboon, much greater than the differences between these Man-like Apes and Man. When, in addition, we now turn to Ontogeny, and when we find there, according to our " law of the ontogenetic con- nection of systematically related forms, that the embryos of Man and of the Man-like Apes, are identical for a longer period than the embryos of the highest and of the lowest Apes, we are certainly obliged to bring ourselves, whether with a good or a bad grace, to acknowledge our origin from 444 THE EVOLUTION OF MAN. the Ape order. From the facts exhibited by Comparative Anatomy, we can undoubtedly form in imagination an approximate image of the structure of our ancestors during the older Tertiary Period ; we may fill out the details as we will, yet this image will be a genuine Ape, and a true Catarhine. For Man has all the physical characters dis- tinguishing the Catyrhina from the Platyrhina. Accord- ingly, in the mammalian pedigree, we must derive the human race directly from the Catarhine group, and refer the origin of Man to the Old World. For the entire group of the Catarhine Apes has, as yet, been confined to the Old World, just as the group of the Platyrhine Apes has been limited to the New. Only the earliest root-form, that from which both groups sprang, was common to them ; probably it originated from the Semi-apes of the Old World. Therefore, although it is thus indubitably established as the result of our objective scientific inquiry, that the human race is directly descended from the Apes of the Old World, yet we will once more state emphatically that this signifi- cant fact is not of as great importance to the main question of the origin of Man, as is generally supposed. For, even if we entirely ignore the fact or thrust it aside, this will not affect all that the zoological facts of Comparative Anatomy and the history of development have taught us concerning the placental character of Man. These clearly prove the common descent of Man and the other Mammals. It is evident also, that the main question cannot be in the least evaded or set aside by the statement : " Man is, indeed, a Mammal; but he branched off from the others quite at the root of the class, and has no nearer relationship with any other extant Mammal." At all events, the relationship EVOLUTION AND SENTIMENT. 445 is evidently more or less close if we comparatively examine the relation of the Mammalian class to the remaining forty classes of the animal kingdom. All Mammals, including Man, are, at least, of common origin, and it is equally certain that their common parent-forms gradually developed from a long series of lower Vertebrates. Feeling, evidently, rather than understanding, induces most people to combat the theory of their "descent from Apes." It is simply because the organism of the Ape appears a caricature of Man, a distorted likeness of ourselves in a not very attractive form, because the customary aesthetic ideas and self-glorification of Man are touched by this in so sensitive a point, that most men shrink from recognizing their descent from Apes. It seems much pleasanter to be descended from a more highly developed, divine being, and hence, as is well known, human vanity has, from the earliest times, flattered itself by assuming the original descent of the race from gods or demi-gods. The church, with that sophistical distortion of ideas of which she is so great an adept, has managed to extol this ridiculous pride as Christian humility ; and those people who reject with haughty horror every suggestion of descent from lower animals, and consider themselves children of God, those very people are exceedingly fond of boasting about their childlike humility of spirit. In most of the sermons delivered against the progress of the doctrine of evolution, human vanity and conceit play throughout a prominent part ; and, although we have inherited this characteristic weakness from Apes, yet we must confess to having developed it to a degree of perfection which completely overthrows the unprejudiced judgment of the 446 THE EVOLUTION OF MAN. " sound understanding of man." We ridicule the childish follies occasioned by the pride of ancestry among the nobility, from the splendid Middle Ages down to our own time, and yet no small portion of this groundless pride of nobility lurks in a great majority of men. Just as most people prefer to trace their pedigree from a decayed baron or, if possible, from a celebrated prince, rather than from an unknown, humble peasant, so they prefer seeing the pro- genitor of the human race in an Adam degraded by the Fall, rather than in an Ape capable of higher development and progress. It is a matter of taste, and such genealogical preferences do not, therefore, admit of discussion. Still I must confess that, personally, I am as proud of my paternal grandfather, who was simply a Silesian peasant, as of my maternal grandfather, who raised himself from the position of a Rhenish lawyer to the highest posts in the council of state. And it is also much more to my individual taste to be the more highly developed descendant of a primaeval Ape ancestor, who, in the struggle for existence, had de- veloped progressively from lower Mammals, as they from still lower Vertebrates, than the degraded descendant of an Adam, god-like, but debased by the Fall, who was formed from a clod of earth, and of an Eve, created from a rib of Adam. As regards this celebrated " rib," I must here ex- pressly add as a supplement to the history of the develop- ment of the skeleton, that the number of ribs is the same in man and in woman. In the latter as well as in the former, the ribs originate from the skin-fibrous layer, and are to be re- garded phylogenetically as lower or ventral vertebra? (p. 285). Now I certainly hear some one say : " That may all be right and correct as far as the human body is concerned, and, EVOLUTION OF THE MIND. 447 from the facts presented, it is certainly no longer to be doubted that this has actually developed gradually, step by step, from the long ancestral series of Vertebrates ; but it is quite otherwise with the ' spirit of man,' with the human mind, which cannot possibly have developed in a similar way from the mind of lower Vertebrates." Let us see if the known facts of Comparative Anatomy, Physiology, and Evolution can meet this grave objection. We shall best gain firm ground from which to start in this matter by comparatively examining the minds of the different Verte- brates. Side by side within the various classes, orders, genera, and species of Vertebrates, we find so great a variety of vertebral intellects, that, at first sight, one can scarcely deem it possible that they can all be derived from the mind of a common " Primitive Vertebrate." First, there is the little Lancelet, which has no brain at all, but only a simple medullary tube, the entire mental capacity remaining at the very lowest grade occurring among Vertebrates. The Cyclostomi, also, standing just above, exhibit a hardly higher mental life, though they have a brain. Passing on to Fishes, we find their intelligence, as is well known, also at a very low point. Not until from these we ascend to the Amphibia, is any essential progress in mental development observable. This is much greater in Mammals, although, even here, in the Beaked Animals (Ornithostoma), and the next higher class, the stupid Pouched Animals (Marciipials), the entire mental activity is still of a very low order; but if we pass on from these to Placental Animals, within this multiform group we find such numerous and important steps in differentiation and improvement, that the mental differences between the most stupid Placental Animals (for 44$ THE EVOLUTION OF MAN. instance, Sloths and Armadillos) and the most intelligent animals of the same group (for instance. Dogs and Apes), seem much more considerable than the intellectual dif- ferences between those lowest Placentals and the Pouched Animals, or even the lower Vertebrates. Those differences are, at any rate, much more considerable than the dif- ferences in the intellectual life of dogs, apes, and men. And yet all these animals are allied members of a single class.198 This fact is shown to a yet more surprising degree in the Comparative Psychology of another class of animals, which is specially interesting for many reasons, that of Insects. It is well known that many Insects exhibit a mental capacity approximately as highly developed as is possessed by Man only of the vertebrate group. It is needless to speak of the celebrated organized communities and states of Bees and Ants ; every one knows that very remarkable social arrangements occur among these, such as occur in an equal degree of development only in the higher races of men, and nowhere else in the animal kingdom. I will only allude to the civil organization and government among Monarchical bees and Republican ants, to their division into various orders : the queen, the drone nobility, the workers, the nurses, soldiers, and so on. Among the most remarkable phenomena in this extremely interesting field of life, is certainly the cattle-keeping of certain Ants, which tend plant-lice for the sake of their milk and regularly collect their honey-juice. Even more remarkable is the slave-holding of the large red Ants, which steal the young of the small black species and rear them to slave-labour. It has long been known that all these civil and social arrangements of the Ants were originated by the systematic COMPARATIVE MENTAL CAPACITIES. 449 co-operation of numerous citizens, understanding each other. Numerous observations have placed the astoundingly high intellectual development of these little Articulated Animals beyond all doubt. With this let us compare, as Darwin has done, the intellectual capacity of many lower, and, especially, of many parasitic, Insects. There, for example, are the Scale Insects (Coccus) which, when mature, consist of an entirely immovable shield-shaped body attached to the leaves of plants. Their feet are atrophied. Their mouths are embedded into the tissue of the plant, the juices of which they suck. The whole mental activity of this motionless female parasite consists in the enjoyment it derives from sucking these juices and from sexual inter- course with the unattached male. The same is true of the maggot-like female of the Twisted- wings (Streps ipt era), which spends its whole life, wingless and footless, as a motionless parasite in the body of the wasp. There can be no suspicion of any higher mental activity there. If these brutish parasites are compared with the mentally active and sensible ants, it will certainly be admitted, that the psychical differences between the two are much greater than those between the highest and lowest Mammals, between Beaked Animals (Omithostoma), Pouched Animals (Marswpialia), and Armadillos on the one hand, and Dogs, Apes, and Men on the other. And yet all those insects belong, without question, to the single class of Arthropoda, just as all these Mammals undoubtedly belong to the single class of Vertebrates ; and just as every logical adherent of the doctrine of evolution must assume a common parent- form for all those Insects, so also he must necessarily assert a common descent for all these Mammals. 450 THE EVOLUTION OF MAN. Turning now from observing the comparative mental capacity of the various animals to the question as to the organs of these functions, we receive the answer, that in all higher animals they are invariably connected with certain groups of cells, those cells which compose the central nervous system. All naturalists, without exception, agree that the central nervous system is the organ of the mental life of animals, and this assertion is at any time capable of experimental proof. If the central nervous system is wholly or partially destroyed, the " mind," or the psychical activity of the animal, is wholly or partially annihilated at the same time. We must, therefore, next inquire what is the character of the mental organ in man. The undeniable answer to this question has already been given. Man's mental organ is, in its whole structure and origin, the same as that of all other Vertebrates. It originates as a simple medullary tube from the outer skin of the embryo, from the skin-sensory layer, or the first of the secondary germ- layers. In the course of its gradual development it passes through the same stages of progression in the human embryo as in that of all other Vertebrates, and as these latter have undoubtedly a common origin, so must also the brain and spinal cord be of the same origin in all. Physiological observation and experiment teaches, more- over, that the relation of the " mind " to its organ, the brain and spinal marrow, is exactly the same in Man as in all other Mammals. The former can in no case act without the latter; the one is connected with the other, as is muscular movement with muscle. Therefore, the mind can develop only in connection with its organ. Adherents of the Theory of Descent, who concede the causal connection DEVELOPMENT OF THE HUMAN MIND, 45 I between Ontogeny and Phylogeny, are now compelled to recognize the following propositions : The mind, or " psyche," of man has developed together with, and as the function of the medullary tube, and just as even now the brain and spinal marrow develop in each human individual from the simple medullary tube, so the human " mind," or the mental capacity of the entire human race, has developed gradually., step by step, from the mind of lower Vertebrates. Just as even now in every individual of the human race the wonderful and complex structure of the brain develops step by step from exactly the same rudiment, from the same five simple brain-bladders, as in all other Skulled Animals (Craniota), so the human mind has gradually developed in the course of millions of years from the mind of lower Skulled Animals ; and as now the brain of every human embryo differentiates according to the special type of the Ape-brain, so also the human psyche has historically differentiated from the Ape-mind. This monistic idea will, of course, be indignantly re- jected by most people, who accept the contrary dualistic view, which denies the inseparable connection of the brain and the mind, and regards " body and mind ': as entirely separate and distinct ; but how shall we reconcile this commonly accepted view with the facts taught by the history of evolution ? The dualistic view is, at least, as irreconcilably opposed to Ontogeny as to Phylogeny. If we agree with the majority of men, that the mind is a self- existent, independent being, which has originally nothing to do with the body, but only dwells in it for a time, and which gives expression to its emotions through the brain, as the piano-player through his instrument, then we must 452 THE EVOLUTION OF MAN. suppose a period in the human germ-history, at which the mind enters the body, enters the brain ; and we must also suppose a moment at death, at which it leaves the body; and further, as every man inherits certain individual mental qualities from each parent, we must suppose that portions of the mind of each were transferred to the germ at the time of its procreation. A little piece of the father's mind accompanied the sperm-cell, a little piece of the mother's mind remained with the egg-cell. This dualistic view entirely fails to explain the phenomena of evolution. We all know that the new-born child has no consciousness, no knowledge of itself and of the objective world. Who- ever has children of his own, and follows their mental development candidly, cannot possibly deny that processes of biological evolution are at work there. Just as all other functions of the body develop in connection with their organs, so does the mind develop in connection with the brain. And this gradual development of the child's mind is such a wonderful and beautiful phenomenon, that every mother and every father with eyes to see takes unwearied delight in observing it. The text-books of Psychology alone are ignorant of any such development, and we are almost forced to the conclusion that their authors them- selves never had any children. The human mind, as it is represented in the great majority of psychological works, is only the one-sided mind of a learned philosopher, who, indeed, knows many books, but nothing of the process of evolution, and does not suspect that even his own mind has developed. These same dualistic philosophers must, of course, if they are consistent, also assume that there was a moment REASON. 453 in the Phylogeny of the human mind at which this mind first entered the vertebrate body of man. Accordingly, at the time when the human body developed from the body of the Anthropoid Ape (thus, probably, in the latter part of the Tertiary Period), a specific human mind-element — or, as it is usually expressed, a " divine spark " — must have suddenly entered or been breathed into the brain of the Anthropoid Ape, and there have associated itself with the already existing Ape-mind. I need not point out the theoretic difficulties involved in this conception. I will only remark that even this " divine spark," by which the mind of Man is said to be distinguished from that of all other animals, must itself be a thing capable of evolution, and has actually developed progressively in the course of human history. This "divine spark" is usually understood to be "reason," and is ascribed to man as a mental function distinguishing him from all "irrational animals." Comparative Psycho- logy, however, teaches that this frontier-post between man and beast is altogether untenable.198 We must either take the idea of reason in its broader sense, in which case it belongs to the higher Mammals (the Ape, Dog, Elephant, Horse), as much as to the majority of men ; or we must conceive it in its narrower sense, and then it is lacking in the majority of men, as well as in most animals. On the whole, that which Goethe's Mephistopheles said of his time, is true of Man's reason to-day : " He might have kept himself more right Hadst Thou ne'er shewn to him a glimpse of heaven's light. He calls it Reason, but Thou seest Its use but makes him beastlier than the beast." If, therefore, we must abandon this generally preferred, 454 THE EVOLUTION OF MAN. and, in many respects, very pleasant dualistic theory of the mind, as being wholly untenable, because irreconcilable with genetic facts, then the opposite monistic view alone remains to us. according to which the human mind, like that of any other animal, is a function of the central nervous system, with which it has developed in inseparable con- nection. Ontogenetically, we see this in every child ; phylogenetically, we must assert it in accordance with the fundamental law of Biogeny. In every human embryo the medullary tube develops from the skin-sensory layer, and from the anterior part of that tube the five brain- bladders of Skulled Animals (Craniota), and from these the mammalian brain (at first with the characteristics of the lower, then with those of the higher Mammals). Just as this entire ontogenetic process is but a short repro- duction, occasioned by Heredity, of the same process in the Phylogeny of Vertebrates, so also the wonderful mental activity of the human race has gradually developed, step by step, in the course of many thousands of years, from the less perfect mental activity of the lower Vertebrates. And the evolution of the mind in each child is only a brief reproduction of that long phylogenetic process. The extraordinary and important bearing of Anthro- pogeny on Philosophy, in the light of the fundamental prin- ciple of Biogeny, now becomes apparent. The speculative philosophers who will take possession of the facts of On- togeny and explain them phylogenetically (according to that law), will introduce a greater advance in the history of Philosophy than has been made by the greatest thinkers of all previous centuries. Undoubtedly every clear and logical thinker must draw from the facts of Comparative Anatomy PHILOSOPHICAL ASPECT OF EVOLUTION. 455 and Ontogeny which have been brought forward, a mass of suggestive thoughts and reflections which cannot fail of their effect on the further development of the philo- sophical study of the universe. Neither can it be doubted that these facts, if properly weighed, and judged without pre- judice, will lead to the decisive victory of that philosophical tendency, which we distinguish, briefly, as monistic or mechanical, in distinction from the dualistic or teleological, on which most philosophical systems of ancient, mediseval, and modern times are based. This mechanical, or monistic philosophy, asserts that everywhere the phenomena of human life, as well as those of external nature, are under the control of fixed and unalterable laws, that there is everywhere a necessary causal connection between pheno- mena, and that, accordingly, the whole knowable universe forms one undivided whole, a " monon." It further asserts, that all phenomena are produced by mechanical causes {causae ejjficientes), not by pre-arranged, purposive causes (causce finales). Hence there is no such thing as "free- will " in the usual sense. On the contrary, in the light of this monistic conception of nature, even those phenomena which we have been accustomed to regard as most free and independent, the expressions of the human will, appear as subject to fixed laws as any other natural phenomenon Indeed, each unprejudiced and searching test applied to the action of our " free-will '! shows that the latter is never really free, but is always determined by previous causal conditions, which are eventually referable either to Heredity or to Adaptation. Accordingly, we cannot assent to the popular distinction between nature and spirit. Spirit exists everywhere in nature, and we know of no spirit out- 456 THE EVOLUTION OF MAN. side of nature. Hence, also, the usual distinction between natural science and mental science is entirely untenable. Every real science is at the same time both a natural and a mental science. Man is not above nature, but in nature. The opponents of the doctrine of evolution are very fond of branding the monistic philosophy grounded upon it as "materialism," by confusing philosophical materialism with the wholly different and censurable moral materialism. Strictly, however, our " monism " might, as accurately or as inaccurately, be called spiritualism as materialism. The real materialistic philosophy asserts, that the vital pheno- mena of motion, like all other phenomena of motion, are effects or products of matter. The other, opposite extreme, spiritualistic philosophy, asserts, on the contrary, that matter is the product of motive force, and that all material forms are produced by free forces entirely independent of the matter itself. Thus, according to the materialistic conception of the universe, matter, or substance, precedes motion, or active force. According to the spiritualistic con- ception of the universe, on the contrary, active force or motion precedes matter. Both views are dualistic, and we hold them both to be equally false. A contrast to both views is presented in the monistic philosophy, which can as little believe in force without matter, as in matter without force. It is only necessary to reflect on this for a time, from a strictly scientific standpoint, to find that on close examination it is impossible clearly to represent the one without the other. As Goethe says, "Matter can never exist and act without spirit; neither can spirit without matter." 12 The " spirit " and " mind " of man are but forces which REASON A FUNCTION OF MIND. 457 are inseparably connected with the material substance of our bodies. Just as the motive force of our flesh is involved in the muscular form-element, so is the thinking force of our spirit involved in the form-element of the brain. Our spiritual forces are as much functions of this part of the body, as every force is a function of a material body. We know of no matter which does not possess force, and, con- versely, of no forces that are not connected with matter. When the forces manifest themselves in the phenomena of motion, they are called active forces; if, on the other hand, the forces are in a state of rest, or of equilibrium, they are called latent forces.199 This is as true of inorganic natural substances as of organic. The magnet attracting iron- filings, powder exploding, steam driving the locomotive, are active inorganic substances; they work by active force just as does the sensitive mimosa, when it folds its leaves at a touch, — as does the Amphioxus, when it buries itself in the sand, — as does man, when he thinks. Only in these latter cases the combination of the different forces, appearing as phenomena of motion, are much more complex and much less easily recognized than in the former cases. Anthropogeny has led us to the conclusion that even in the entire history of the evolution of man, in the history of the germ, as well as in that of the tribe, no other active forces have been at work, than in the rest of organic and inorganic nature. All the forces at work there can be reduced at last to growth — to that fundamental function of evolution by which the forms of inorganic, as well as of organic bodies, originate. Growth, again, itself rests on the attraction and repulsion of like and unlike particles.194 It has given rise to Man and to Ape, to Palm and Alga, to 458 THE EVOLUTION OF MAN. crystal and water. Hence the evolution of man has taken place according to the same " eternal, immutable laws," as has the evolution of any other natural body. It is true that the prejudices that stand in the way of the general recognition of this " Natural Anthropogeny ': are even yet intensely powerful ; otherwise the ancient strife between the various philosophical systems would already have been decided in favour of " Monism." But it can be foreseen with certainty that a more general acquaintance with genetic facts, will gradually destroy those prejudices and bring about the victory of the natural idea of "Man's Place in Nature." The fear is often expressed in opposition to this view that it will cause a retrogression in the intellectual and moral development of man ; but, on the contrary, I cannot withhold my convic- tion, that the very reverse will be true, that by it the pro- gressive development of the human spirit will be advanced in an unusual degree. At all events, I hope and trust that I have, in these chapters, afforded convincing proof that the only way to attain a true scientific knowledge of the human organism, is by employing the method which we must acknowledge to be alone correct and successful in the study of organic nature, — by following the course of the History of Evolution.200 NOTES. REMARKS AND REFERENCES TO LITERATURE. 1 (vol. i. p. 2). Anthropogeny ( Greek ) = History of the .Evolu- tion of Man; from Anthropos (avOpwTros) = man, and genea (yevea) = Evolution history. There is no especial Greek word for "the history of evolution;" in its place is used either yeved (= de- scent), or yoviia (= generation). If goneia is preferred to genea, the word must be written Anthropogony. The word " Ani]iro])Oc\ony" used first by Josephus, means, however, only " the generation of man." Genesis (yeVeo-is) means " origination, or evolution ; " therefore Anthropogenesis = " the evolution of man." 2 (i. 3). Embryo (Greek) = germ (efx/3pvov). Really to ivros t^s yao-rpos fipvov (Eust.), i.e. the unborn germ in the mother's body (Latin foetus, or, better, fetus). In accordance with this original sense, the term embryo should only be applied to those young organisms which are still enclosed in the egg-coverings. (Cf. " Generelle Morphologie," vol. ii. p. 20.) Inaccurately, how- ever, various free-moving young forms of low animals (larvae) are often spoken of as embryos. Embryonic life ends at birth. 3 (i. 5). Embryology (Greek) = Germ-science, from embryon (e/x/3pvov) = germ, and logos (Aoyos) = science. Even now the whole history of the evolution of the individual is erroneously called "embryology." For corresponding with, the term "embryo" (see note 2), by " embryology," or " embryogony," should only be understood " the history of the evolution of the 460 NOTES. individual within the egg- coverings." As soon as the organism has left there, it is no longer a real " embryo." The later changes of this form the subject of the science of Metamorphoses, or Met a morpli 0 logy. 4 (i. 5). Ontogeny (Greek) = "germ-history," or "the history of the evolution of the individual;" from 6vra = indi- viduals, and genea (yei/ea) = history of evolution. (Cf. note 1.) Ontogeny, as the "history of the evolution of the individual," embraces both Embryology and Metamorphology (note 3). — " Generelle Morphologie," vol. ii. p. 30. 5 (i. 5). Phylogeny (Greek) = tribal history, or " the pa- laeontological history of evolution ; " from phylon (<£uAoi/) = tribe, and genea (yei/ea) = history of evolution. The phylon includes all organisms connected by blood, which are descended from a common typical parent-form. Phylogeny includes Palaeontology and Genealogy. — " Generelle Morphologie," vol. ii. p. 305. 6 (i. 6). Biogeny (Greek) = the history of the evolution of organisms or of living natural bodies in the widest sense. (Genea tu biu.) /3ios = life. 7 (i. 6). The fundamental law of Biogeny. Cf. my "General History of the Evolution of Organisms" (" Generelle Morphologie," 18G6, vol. ii.), p. 300 (Essays on the causal connection of biogenetic and phyletic evolution) ; also the " Monograph of Chalk Sponges " (" Monographic der Kalkschwamme," 1872, vol. i. 471); also my " Natural History of Creation." 8 (i. 10). Palingenesis (Greek) = original evolution, from palingenesia (7raA.il/ye1/eo-ta) = new-birth, renewal of the former course of evolution. Therefore, Palingeny = inherited history (from TraAtv = reproduced, and yei/ea = history of evolution). 9 (i. 10). Kenogenesis (Greek) = modified evolution, from kenos (/cevo's) = strange, meaningless; and genea (yei/ea) = history of evolution. The modifications introduced into Palingenesis by Kenogenesis are vitiations, strange, meaningless additions to the original, true course of evolution. Kenogeny = vitiated history. 10 (i. 12). Latin definition of the fundamental law of NOTES. 461 Biogeny : " Ontogenesis summarium vel recapitulatio est phy- logeneseos, tanto integrius, quanto hereditate palingenesis con- servatur, tanto minus integrum, quanto adaptatione kenogenesis introducitur." Cf. my "Aims and Methods of Recent History of Evolution " (" Ziele und Wege der Heutigen Entwickelungs- geschichte," p. 77. Jena, 1876). • 11 (i. 17). Mechanical and purposive causes. Mechanical natural philosophy assumes that throughout nature, in organic as well as in inorganic processes, only non-purposive, mechanical, necessarily-working causes exist {causae, efficientes, mechanism, causality). On the other hand, vitalistic natural philosophy asserts that the latter are at work only in inorganic processes, which in certain other, purposive, special causes are at work, conscious or purposive causes, working for a definite end (causce finales, Vitalism, Teleology). (Cf. " Generelle Morphologie," vol. i. p. 94.) 12 (i. 17). Monism and Dualism. Unitary philosophy, or Monism, is neither extremely materialistic nor extremely spirit- ualistic, but resembles rather a union and combination of these opposed principles, in that it conceives all nature as one whole and nowhere recognizes any but mechanical causes. Binary philosophy, on the other hand, or Dualism, regards nature and spirit, matter and force, inorganic and organic nature as distinct and independent existences. (Cf. vol. ii. p. 456.) 13 (i. 20). Morphology and Physiology. Morphology (as the science of forms) and Physiology (as the science of the functions of organisms) are indeed connected, but co-ordinate sciences, independent of each other. The two together constitute Biology, or the " science of organisms." Each has its peculiar methods and aids. (Cf. " Generelle Morphologie," vol. i. pp. 17-21.) 14 (i. 24). Morphogeny and Physiogeny. Biogeny, or the " history of the evolution of organisms," up to the present time has been almost exclusively Morphogeny. Just as this first opens the way to a true knowledge of organic forms, so will Physiogeny afterwards make a true recognition of functions 462 NOTES. possible, by discovering their historic evolution. Its future promises to be most fruitful. Cf. " Aims and Methods of the Recent History of Evolution" (" Ziele und Wege der Heutigen Entwickelungsgeschichte," pp. 92-98. Jena, 1876). 15 (i. 27). Aristotle. Five books on the generation and evolution of animals (7repi £gjwv yeyeo-eos). 16 (i. 28). Parthenogenesis. On " virginal generation," or the "immaculate conception" of Invertebrates, especially of Articulated Animals (Crustacea, Inseda, etc.), see Siebold, "Remarks on Parthenogenesis among Arthropoda " (" Beitrage zur Parthenogenesis der Arthropoden." Leipzig, 1871). Georo- Seidlitz, " Parthenogenesis and its Relation to other Forms of Generation in the Animal Kingdom " (" Die Parthenogenesis und ihr Verhaltniss zu den tibrigen Zeugungs-Arten im Thierreich." Leipzig, 1872). 17 (i. 34). The Preformation-theory. This theory is, in Germany, usually called " Evolutions-theorie," in distinction from the " Epigenesis-theorie." As, however, in England, France, and Italy, the latter is, on the contrary, usually called the theory of evolution, evolution and epigenesis being used as synonymous terms, it appears better to call the former " the theory of pre- formation." Recently Kolliker has called his "theory of hetero- genous generation" "Evolutionism" (note 47). Cf. preface, p. XXX. 18 (i. 37). Alfred Kirchhoff, "Caspar Friedrich Wolff, his Life and Teaching in the Science of Organic Evolution." — " Jenaische Zeitschrift fur Naturwissenschaft," 1868, vol. iv. p. 193. 19 (i. 43). Part of the writings left by Wolff have not yet been published. His most important works are the dissertation for the degree of doctor, Theoria generationis (1759), and his model treatise " de formatione intestinorum " (on the formation of the intestinal canal). — "Nov. Comment. Acad. Sc. Petropol.," xii. 1768; xiii. 1769. Translated into German by Meckel. Halle, 1812. 20 (i. 51). Christian Pander, " Historia metamorphoseos, quam NOTES. 463 ovum incubatum prioribus quinque diebus subit." Vicebergi, 1817. (Dissertatio inauguralis.) " Contributions toward the history of the evolution of the chick within the egg." (" Beitrage zur Entwickelungsgeschichte des Hiihnchens im Eie." Wiirz- burg, 1817.) 21 (i. 52). Karl Ernst Baer, "On the Evolution of Animals. Observations and Reflections " (" Ueber Entwickelung-ssfeschichte der Thiere. Beobachtung und Reflexion." 2 vols. Konigsberg, 1827-1837). In addition to this chief work, cf. " Story of the Life and Writings of Dr. Karl Ernst Baer, told by himself " (" Nachrichten iiber Leben und Schriften des Dr. Karl Ernst Baer, mitgetheilt von ihm selbst." Petersburg, 1865). 22 (i. GO). Albert Kolliker. His "History of the Evolution of Man and the Higher Animals " (" Entwickelungsgeschichte des Menschen und der hoheren Thiere "). The 2nd (corrected) edition, 1876, contains (pp. 28-40) a catalogue of ontogenetic literature. On the newer contributions to this, cf. the "Jahresberichte iiber die Leistungen und Eortschritte der Medicin " (Berlin), by Virchow and Hirsch (the " History of Evolution," by Waldeyer) ; also the " Jahresberichte iiber die Fortschritte der Anatomie und Physiologie," by Hofmann and Schwalbe (Leipzig) ; the "History of Evolution," by R. Hertwig and Nitsche. Most of Kowalev- sky's researches are contained in the " Memoires de l'Academie imperial e de St. Petersburg " (from the year 1866). Others are published in Max Schultze's "Archiv fur mikroskopische Anatomie," and in other periodicals. 23 (i. 60). Theodor Schwann, " Microscopic Researches into the Identity in Structure and Growth of Plants and Animals " (" Mikroskopische Untersuchungen iiber die Uebereinstimmung in der Structur und Wachsthum der Thiere und Pflanzen." Berlin, 1839). 24 (i. 69). Ernst Haeckel, the Gastraaa Theory, phylogenetic classification of the animal kingdom and homology of the germ- layers. — " Jenaische Zeitschrift fiir Naturwissenschaft," vol. viii. 1874, pp. 1-56. 25 (i. 75). Ernst Haeckel, "The History of Creation." London, 1876. 43 4&4 NOTES. 26 (i. 81). Fritz Schultze, "Kant and Darwin." A con- tribution to the history of the science of evolution. Jena, 1875. 27 (i. 81). Immanuel Kant, " Critique of Teleological Rea- son " ("Kritik der teleologischen Urtheilskraft "). 1790. § 74 and § 79. Cf. also my " History of Creation," vol. i. p. 103. 28 (i. 83). Jean Lamarck, " Philosophie Zoologique, ou Exposition des Considerations relatives a l'histoire naturelle des animaux," etc. 2 Tomes. Paris, 1809. Nouvelle edition, revue et precedee d'une introduction biographique par Charles Martins. Paris, 1873. 29 (i. 88). Wolfgang Goethe on Morphology (zur Morpho- logic). The formation and re-formation of organic bodies. On Goethe's morphological studies, cf. Oscar Schmidt (" Goethe's Verhaltniss zu den organischen Naturwissenschaften." Jena, 1853). Rudolph Yirchow, "Goethe as a Naturalist" (Berlin, 1861). Helmholtz, "On Goethe's Natural Scientific Works" (Brunswick, 1865). 30 (i. 96). Charles Darwin. His chief work is " On the Origin of Species by means of Natural Selection " (1859). 31 (i. 99). Darwin and Wallace. The general outlines of the theory of selection were discovered independently by Darwin and Wallace. It does not, however, follow that the services of the latter in furthering the science of evolution are at all comparable with those of the former. As many opponents of Darwin, especially the English Jesuit Mivart, have recently endeavoured to exalt Wallace at the expense of Darwin, and to depreciate the latter, I take this opportunity of expressly assert- ing that Darwin's services are very far the greater. 32 (i. 101). Thomas Huxley. In addition to the works mentioned in the text, the following popular works are especially to be recommended : " On Our Knowledge of the Causes of Phenomena in Organic Nature," and the " Elementary Phy- siology" (1871). 33 (i. 101). Gustav Jaeger, " Zoological Letters" ("Zoologische Briefe." Vienna, 1876), and the " Text-book of General Zoology" (" Lehrbuch der Allgemeinen Zoologie." Stuttgart, 1875). NOTES. 465 34 (i. 101). Friedrich Rolle, "Man, his Descent and Morality represented in the light of the Darwinian Theory, and on the basis of Recent Geological Discoveries " (" Der Mensch, seine Abstammung und Gesittung im Lichte der Dar-vvin'schen Lehre," etc.). Frankfort, 1866. 35 (i. 102). Ernst Haeckel, " Generelle Morphologie der Organismen." General outlines of the science of organic forms, mechanically shown in accordance with the theory of descent as reformed by Charles Darwin. Vol. i , " General Anatomy ; " vol. ii., " General History of Evolution." Berlin, 1866. 36 (i. 103). Charles Darwin, "The Descent of Man, and Selection in Relation to Sex." 2 vols. London, 1871. 37 (i. 108). Karl Gegenbaur, "Outlines of Comparative Anatomy" (" Grundziige der vergleichenden Anatomic" Leipzig. 2nd ed., 1870). "Elements of Comparative Anatomy " (" Grunclriss der vergleichenden Anatomie." 3rd (improved) edition, 1874). 38 (i. 114). Migration-theory. Moritz Wagner, " The Dar- winian Theory and the Law of Migration of Organisms " ("Die Darwin'sche Theorie und das Migrations-gesetz der Organ- ismen." Leipzig, 1868). August Weismann, " On the Influence of Isolation in the Formation of Species " (" Ueber den Einfluss der Isolirung auf die Artenbidung." Leipzig, 1871). 39 (i. 116). Cams Sterne, "Evolution and Dissolution" ("Wer- den und Vergehen"). A popular history of the evolution of nature as a whole. Berlin, 1876. Agassiz a "founder" of natural science. " Gegenwart." Berlin, 1876. 40 (i. 117). Ernst Haeckel, "The Chalk-sponges" ("Die Kalkschwamme ; Calcispongien oder Grantien." Berlin, 1872). A monograph and an attempted solution of the problem of the origin of species. Vol. i., "Biology of Chalk-sponges ; " vol. ii., "Classification of Chalk-sponges;" vol. iii., "Atlas of Chalk- sponges " (with 60 plates). 41 (i. 124). On the Individuality of Cells and recent reforms in the cell-theory, cf. my " Individnalitatslehre," or " Tectologie " ("Generelle Morphologie," vol. i. pp. 239-274). Rudolf Virchow, " Cellular Pathologic." 4th edition. Berlin, 1871. 466 NOTES. 42 (i. 130). "The Plastid-theory and the Cell-theory."— " Jenaische Zeitschrift fur Naturwissenschaft," 1870, vol. v. p. 492. 43 (i. 138). Gegenbaur, " On the Structure and Evolution of Vertebrate Eggs with Partial Yelk-cleavage." — " Archiv f. Anat. u. Phys." 1861, p. 491. 44 (i. 153). Ernst Haeckel, "On Division of Labour in Nature and Human Life," in the collection of Lectures by Virchow- Holtzendorf, 1869. Sect. 78 2nd edition. 45 (i. 160). Monogony (Generatio neutralis). On the various forms of asexual reproduction (Schizogony, Sporogony, etc.), cf. " Generelle Morphologie," vol. ii. pp. 36-58. 46 (i. 160). Amphigony {Generatio sexualis). On the various forms of sexual reproduction (Hermaphroditism, Gonochorism, etc.), see " Grenerelle Morphologie," vol. ii. pp. 58-69. 47 (i. 168). Fitful evolution and gradual evolution. The theory of fitful evolution has recently been developed especially by Kolliker, who, under the title of heterogeneous generation, opposes it to gradual evolution as maintained by us (" Zeitschr. f. Wissens. Zool.," vol. xiv. 1864, p. 181, and " Alcyonaria," 1872, pp. 384-415). This theory is distinguished by assuming entirely unknown causes for the "fitful evolution of species," a so-called "great law of evolution" (an empty word indeed!). On the contrary, we see, with Darwin, in the facts of Heredity and Adaptation sufficient known (partly inner, partly external) physiological causes, which explain the gradual evolution of species under the influence of the struggle for existence. 48 (i. 170). Immaculate Conception never occurs in the vertebrate tribe. On the other hand, parthenogenesis frequently occurs among Articulated Animals (Arthropoda) (note 16). 49 (i. 171). Fertilization of Flowers by insects. Charles Darwin on " The various contrivances by which British and Foreign Orchids are fertilized by Insects." Hermann Miiller on " The Fertilization of Flowers by Insects, and the correlative adaptations of both " (" Die Befruchtung der Blumen durch Insecten und die gegenseitigen Anpassungen Beider "). A con- NOTES. 467 tribution to our knowledge of causal connection in organic nature. Leipzig, 1873. 50 (i. 178). The Process of Fertilization has been very variously viewed, and was formerly often regarded as an entirely mysterious process, or even as a supernatural miracle. It now appears no more " wonderful or supernatural " than the process of digestion, of muscular movement, or of any other physiological function. For the earlier views, cf. Leuckart, Article " Zeugung " (generation) in R. Wagner's " Dictionary of Physiology," 1850. 51 (i. 179). Monerula. The simple, very transient, kernel- less condition, which we briefly call the " monerula," and, in accordance with the fundamental law of Biogeny, regard as a palingenetic reproduction of the phylogenetic Moneron parent- form, appears to vary to some extent in different organisms, especially in the matter of duration. In those cases in which it no longer occurs, and in which the kernel of the fertilized egg persists wholly or partially, we may regard this phenomenon as a later, kenogenetic curtailment of Ontogeny. 52 (i. 181). The Plasson of the monerula appears, mor- phologically, a homogeneous and structureless substance, like that of the Moneron. This is not contradicted by the fact that we ascribe a very complex molecular structure to the plastidules, or " plasson-molecules," of the monerula ; this latter will naturally be more complex in proportion as the organism which it ontogenetically constitutes is higher, and as the ancestral series of that organism is longer, in proportion as the preceding processes of Heredity and Adaptation are more numerous. 53 (i. 182). The Fundamental Significance of the Parent-cell, or cytula, as the foundation-stone of the young organism in the course of development, can only be rightly appreciated, if the part taken in its constitution by the two generating cells is rightly appreciated, the part taken by the male sperm-cell and by the female egg-cell. 54 (i. 183). The One-celled Germ-organism, like the act of fertilization from which it results, has been very variously 468 NOTES. viewed. Cf. on this subject, in addition to the four important works, here quoted, by Auerbach, Biitschli, Hertwig, and Stras- burger, the most recent annals of the progress of the history of evolution (Waldeyer in Yirchow-Hirsch's " Jab.resberichten," Berlin; Hertwig in Hofmann-Schwalbe's " Jahresberichten," Leipzig). 55 (i. 185). Protozoa and Metazoa. Cf. vol. i. p. 248 ; ii. 92. The Protozoa and Metazoa are genetically and anatomically so very distinct, that the former, as Protista, may even be excluded entirely from the animal kingdom, and may be regarded as a neutral intermediate kingdom between the plant and animal kingdoms. — " Generelle Morphologie," vol. i. pp. 191-230. Ac- cording to this view the Metazoa alone are really animals. 56 (i. 186). The Unity of the Zoogenetic Conception, result- ing from the Gastrjea- theory, has as yet not been destroyed by the numerous attacks directed against that theory : for none of these attacks have succeeded in substituting anything positive ; by pure negation no advance can be made in this dark and difficult subject. 57 (i. 187). The Egg-cleavage and Gastrulation of Man, as represented diagrammatically in Figs. 12-17 of Plate II., is most probably in no essential way different from that of the Rabbit, which has as yet been most closely examined in this point. 58 (i. 188). Ernst Haeckel, " Arabian Corals " (" Arabische Korallen"). "A Journey to the Coral Banks of the Red Sea, and a Glimpse into the Life of Coral Animals. A popular lecture, with scientific explanations." With 5 coloured plates, and 20 woodcuts. Berlin, 1876. 59 (i. 189). The Number of the Segmentella, or cleavage- cells, increases, in the original, pure forms of palingenetic egg- cleavage, in regular geometric progression. But the point to which this proceeds varies in the various archiblastic animals, so that the Morula, as the final result of the cleavage-pi*ocess, consists sometimes of 32, sometimes of 64, sometimes of 128 cells, and so on. 60 (i. 189). The Mulberry- germ, or Morula. The seg- NOTES. 469 mentella, or cleavage-cells, which constitute the Morula at the close of palingenetic egg-cleavage, generally appear entirely similar, with morphological difference in size, form, or con- stitution. This does not, however, hinder the fact that these cells have separated, even during cleavage, into animal and vegetable cells, have differentiated physiologically, as is indicated in Figs. 2 and 3, Plate II., as probable. 61 (i. 189). The Bladder-germ of Archiblastic Animals (blastula, or blastosphcera?), which is now commonly known as the germ- vesicle, or, more accurately, as the " germ-membrane vesicle," must not be confused with the essentially different " germ- vesicle " of amphiblastic mammals, which is better called the "intestinal-germ vesicle ': (gastrocystis). The gastrocystis and the blastula are still often united under the name of " germ- vesicle, or vesicula blastodermica." Cf. vol. i. p. 290. 62 (i. 192). The Definition of the Gastrula was first established by me in 1872, in my "Monograph of Chalk-sponges " (vol. i. pp. 333, 345, 466). There I already gave due weight to the " extremely great significance of the gastrula in reference to the general Phylogeny of the animal kingdom" (p. 333). " The fact that these larval forms re-occur in the most different animals, cannot, I think, be sufficiently estimated, and bears plain witness to the former common descent of all from the Gastrsea " (p. 345). 63 (i. 194). The Uniaxial Outline of the Gastrula is, on account of the two different poles of the axis, more accurately described as a diplopolic uniaxial form (a sternometric outline : conoid-form, or cone). Cf. my " Promorphology " (" Generelle Morphologie," vol. i. p. 426). 64 (i. 194). Primitive Intestine and Primitive Mouth. My distinction of the primitive intestine and primitive mouth ('pwtogaster and protostoma) from the later, permanent intestine and mouth (metagaster and metastoma) has been variously attacked ; it is, however, as much justified as the distinction of the primitive kidney from the permanent kidney, of the primitive vertebras from the permanent vertebra?. The primitive intestine 470 NOTES. forms but a part of the permanent intestine, and the primitive mouth (at least in the higher animals) does not become the permanent mouth. 65 (i. 196). Primitive germ-layers (blastophylla) . As the two primary germ-layers (entoderma and exoderma) originally form the sole mitogenetic rudiment of the whole body, and as the mesoderma, the nutritive yelk, and all other accessory parts of the germ have developed only secondarily from the former, 1 consider it very important to distinguish between the primary and secondary germ-layers. The latter, to distinguish them from the former, might be called "after germ-layers" {blas- telasma). 66 (i. 201). Unequal Cleavage and Hood-gastrula (Seg- mentatio inmqualis et Amphigastruld). Next to Amphibia the most accessible examples for observation of unequal cleavage and the Amphigastrula are afforded by the indigenous Soft- bodied Animals (Mollusca) and Worms (Snails and Mussels, Earth Worms and Leeches). 67 (i. 202). The Colour of Amphibian-eggs is occasioned by the accumulation of dark colouring-matter at the animal pole of the egg. In consequence of this the animal-cells of the exoderm appear darker than the vegetative cells of the entoderm. In most animals the reverse is the case; the protoplasm of the entoderm cells being usually darker and more coarsely granulated (vol. i. p. 197). 68 (i. 207). Hood-gastrula of Amphibia. Cf. Robert Remak, " On the Evolution of Batrachia " (" Ueber die Entwickelung der Batrachier," p. 126 ; Plate XII. Figs. 3-7). Strieker's " Manual of Tissues" ("Handbuch der Gewebelehre," vol. ii. p. 1195- 1202; Figs. 399-402). Goette, "History of the Evolution of Bombinator" (" Entwickelungsgeschichte der Unke," p. 145; Plate II. Figs. 32-35). 69 (i. 214). Hood-gastrula of Mammals. Eduard van Beneden, " La maturation del'ceuf, la fecondation et les premieres phases du developpement embryonnaire des Mammiferes, d'apres des recherches faites chez le lapin." Brussels, 1875. No figures NOTES. 471 are given with these " Communication preliminaire ; ': Van Beneden's description is, however, so clear, so thorough and care- ful, that they afford an entirely satisfactory insight into unequal eo-cr. cleavage and the formation of the Hood-gastrula in Mammals. All other observers, who have studied the germination of Mam- malian eggs (among the most recent Kolliker, Rauber, and Hewson may be especially mentioned), have overlooked or failed to recognize the important features discovered by Yan Beneden. 70 (i. 218). The Disc-gastrula (Bisco-gastrula) of Osseous Fishes (Teleostei). Yan Bambeke, " Recherches sur l'embry- ologie des poissons osseux." Brussels, 1875. The transparent Fish-eggs, in which I observed discoid cleavage (Segmentatio discoidalis) and the formation of the Disc-gastrula by invagination, are accurately described in my article on " The Gastrula and Egg-cleavage of Animals " (" Jen. Zeitschrift fur Naturwis- senschaft," 1875, vol. ix. p. 432-444; Plates IY, Y). On the Disc-gastrula of Selachii, cf. Balfour, " The Development of Elasmo branch Fishes." — " Journ. of Anat. and Pbysiol.," vol. x. p. 517; Plates XX., XXIII. 71 (i. 221). Yelk-cells of Birds. The cell-like constituent parts, which occur in great number and variety in the nutritive yelk of Birds and Reptiles, as in most Fishes, are nothing less than true cells, as His and others have asserted. This does not mean that in this matter a distinct limit everywhere exists between the nutritive and the formative yelks, as in our oceanic Fish-eggs (Figs. 42, 43, note 70). On the contrary, originally (phylogenetically) the nutritive yelk originated from part of the entoderm. 72 (i. 223). Egg-cells of Birds. Notwithstanding the large nutritive yelk, the " after-egg " (metovurn*) of Birds and Reptiles is, in form- value, a single cell. The very small, active protoplasm of the " tread " does, however, indeed fall far short, in volume, of the huge mass of the yellow yelk -ball. The bird's eggs are absolutely the largest cells of the animal body. Cf. note 43, and Eduard van Beneden, " Recherches sur la composition et la 472 NOTES. signification de l'ceuf." Brussels, 1870. Hubert Ludwig, "On Egg-structure in the Animal Kingdom " ("Ueber die Eibildung in Thierreiche." Wiirzburg, 1874). 73 (i. 226). Discoidal cleavage (Segmentatio discoidalis) of Bird's eggs. Cf. Kolliker, " History of the Evolution of Man and the Higher Animals " (" Entwickelungsgeschichte des Men- schen und der hoheren Thiere." 2nd edition, 1876, pp. 69-81 ; Figs. 16-22). 74 (i. 227). Disc-gastrula (Disco-gastrida) of Birds. Cf. Rauber, " On the Place of the Chick in the System of Evolu- tion " (" Ueber die Stellung des Hiihnchens im Entwickelungs- plan"). Leipzig, 1876. Foster and Balfour, " The Elements of Embryology." London, 1874. 75 (i. 231). Bladder-gastrula (J? eri gastrula) of Articulated Animals (Arthropoda) . Cf. Bobretzky, " Russian Essay on the Germ-history of Astacus and Pala?mon." Kiew, 1873. Also my own article on the gastrula and egg-cleavage. — "Jen. Zeitschrift fur Naturwissenschaft." Vol. ix. pp. 444-452, Plate VI. 76 (i. 234). The Four-layer Theory, which was first clearly stated by Baer in 1837 (" Entwickelungsgeschichte der Thiere," vol. ii. pp. 46, 68), and which we have here carried out logically, yet appears the only form of the germ-layer theory, which, on comparative observation of all higher animals, supplies a universal law of germination for all and at the same time meets the inconsistent reputations of many observers. 77 (i. 239). Caspar Friedrich Wolff first indicated the Four- layer Theory (note 76). Cf. the remarkable sentence, quoted at vol. i. p. 45, from his pregnant work on the formation of the intestinal canal (note 19). 78 (i. 240). The Four Main Types of Gastrulation, which are diagrammatically distinguished in Plates II. and III., and in Tables III. and IV. (vol. i. pp. 241, 242), are of course connected by intermediate forms. These are transitions both between the primordial and the unequal forms, and between the primordial and the superficial forms ; similarly, the unequal form of egg-cleavage is connected by twixt- forms with the discoidal forms, which latter notes. 473 is again, perhaps, connected in the same way with the superficial form. 79 (i. 241). The G-astrulation of the various classes of animals has been far too little studied to enable us thoroughly to summarize the distribution of the various forms within the separate classes. Yet it is already evident that primordial egg- cleavage and the formation of the Archigastrula occur in the lowest classes of each tribe. 80 (i. 243). The Rhythm of egg-cleavage is by no means as regular as might appear from the four first examples in the five tables. There are, on the contrary, many variations, and not infrequently an entirely irregular and very variable sequence of numbers occurs (especially in discoided cleavage). 81 (i. 246). Definition of the Type. Cf, Gcgenbaur, " Elements of Comparative Anatomy," 1874, p. 59. 82 (i. 246). Types and Phyla. According to the prevailing " Type-theory," the types of the animal kingdom are parallel, and entirely independent ; according to my " Gastrasa-theory," on the contrary, they are divergent tribes, connected at the roots ; according to the view of Claus and other opponents, the latter is no essential distinction. 83 (i. 248). The one-celled condition of Infusoria entirely forbids their morphological comparison with Metazoa. Cf. my article " On the Morphology of Infusoria " (" Jen. Zeitschrift fur Naturwissenschaft " 1873, vol. vii. p. 516-568). 84 (i. 257). The axes of the Vertebrate outline. Cf. my " Promorphology " (Stereometry of Organisms). — " Grenerelle Morphologie," vol. i. pp. 374-574. " Singly double-outlines " (Dipleura), p. 519. "Bilateral-symmetrical" forms in the fourth signification of the word. 85 (i. 255). The Primitive Vertebrate Type, as it is repre- sented in Figs. 52-56, is a hypothetic diagram, which is principally founded on the outline of the Amphioxus, but in which the Comparative Anatomy of Ascidia and Appendicularia on the one side, of Cyclostomi and Selachii on the other, is regarded. This diagram is by no means meant to be an " exact figure," but 474 NOTES. a provisional stage in the hypothetic reconstruction of the unknown, long extinct parent-form of Vertebrates, an "Archi- type." 86 (i. 258). Only very uncertain assumptions can be made as to the sense-organs of the hypothetic parent-form, for these organs, more than any others, have been subject to adaptations, and in Ascidia, as in the Amphioxus, have probably been much atrophied. The earliest Vertebrates probably inherited a pair of eyes of very simple character and a pair of simple ear- vesicles from Worms. 87 (i. 267). The primitive kidneys were perhaps already metameric in the hypothetic parent-form of Vertebrates, so that in addition to the two longitudinal main canals (primitive kidney ducts) numerous transverse tubes (segmental canals) were connected with these main canals, a pair in each metameron of the middle part of the body. Perhaps these already opened through ciliated funnels into the body-cavity (coeloma), as is now the case in Annelids, and, according to Balfour, in the embryos of Selachii. Cf. Balfour, " Development of Elasmo- branch Fishes." — u Quarterly Journal of Microscopical Science." New Series, vol. xiv. p. 323 ; " Journal of Anat. and Physiol." vol. x. 88 (i. 273). The germination of Primitive Vertebrates. Cf. with Table VI., Table VII. (vol. i. p. 327), Table XI. (p. 467); also the diagrammatic figures in Plates IV. and V. with explana- tion (p. 321). 89 (i. 276). The Germ-forms of the earliest Vertebrates, as they are represented in diagrammatic cross sections in Figs. 62-69, can only, of course, be approximately guessed, and with the aid of Comparative Anatomy and Ontogeny. These hypothetic diagrams, therefore, by no means claim to be ac- cepted dogmatically, any more than do those in Figs. 52-56. (Cf. note 85.) 90 (i. 280). Main incidents in Vertebrate germination. Of the main palingenetic incidents here enumerated, perhaps the sixth, ninth, and tenth originally occurred in a very dif- notes. 475 ferent form. The other seven now appear to be pretty well established. 91 (i. 285). The flat germ-disc of Birds, which even now, in the opinion of most embryologists, represents the first starting- point in the formation of the embryo, and to which all other germ-forms have been referred, is, on the contrary, a late and mnch modified germ-form, which has arisen in consequence of the extension of the gastrula over the greatly enlarging nutritive yelk. 92 (i. 289). Site of Fertilization. In Man, as in other Mammals, fertilization of the eggs probably usually takes place in the oviduct : here, the eggs which, at the rupture of the Graafian follicles, have emerged from the female ovary and passed into the outer opening of the oviduct, meet with the active sperm-cells of the male seed, which, during copulation, penetrated into the uterus, and from there passed into the inner opening of the oviduct. Rarely, fertilization occurs even on the ovary, or not till within the uterus. (Cf. Chapter XXY.) 93 (i. 293). The origin of the mesoderm in Mammals, as in other animals, is, at present, among the most obscure and con- tested points of Ontogeny. Remak, Balfour, and others derive it from the entoderm, Kolliker and others from the exoderm. Waldeyer, His, and others assert that both primary germ-layers take part in the formation of the mesoderm. The last assump- tion is, I believe, correct. (Cf. notes 76, 77.} 94 (i. 297). The Germ-shield (Notaspis). The ordinary view, that the germ-shield (= Remak's " Doppelschild ") is the earliest rudiment of the actual embryo, results in many erroneous conclusions. It is, therefore, necessary to point out especially that the germ-shield represents the first well-defined central dorsal part of the embryo. 95 (i. 317). Body Wall and Intestinal Wall. The morpho- logical distinction between the body wall and the intestinal wall, certainly primordial, is probably referable to the simple primary germ-layers of the Gastrasa. If the skin-fibrous layer is derived from the exoderm, and the intestinal-fibrous layer from the 4/6 NOTES. entoderm, this most simply explains the progressive development of this distinction, which may be traced through the series of Worms, and up to Vertebrates. 96 (i. 320). Palingenetic and Kenogenetic germination. In the germ-history of Vertebrates no clear conception of the embryological process has yet been attained, because all authors have started from the higher Vertebrates (usually from the Chick) and have assumed that the form of evolution occurring in this case is original and typical. It is only since the germ- history of the Amphioxus has taught us the palingenetic, really original form of germination of Vertebrate organisms, that we have been enabled, by Comparative Ontogeny (and especially by the principles of the Gastrsea theory), rightly to understand and to explain phylogenetically the kenogenetic forms of germination of higher Vertebrates. 97 (i. 321). The Diagrams in Plates IV. and V. are as simple and abstract as possible, in order to render the desired general explanation as easy as possible. 98 (i. 346). Primitive Vertebras and Metamera. For the right conception of " primitive vertebral " structure it is espe- cially necessary to point out that the primitive vertebra? are much more than their name indicates. They must, in fact, be conceived as individual, consecutive sections of the trunk, which have arisen one after the other, as true "metamera," or consecutive pieces (" Generelle Morphologie," vol. i. p. 312). Each primitive vertebra of a Vertebrate, like each trunk-segment or metameron of an Annelid or Arthropod, contains all the essential, morphological constituent parts, characteristic of the corresponding animal-tribe. 99 (i. 349). Origin of the Primitive Vertebras. My concep- tion of these as individual, morphological "consecutive pieces," which, like the metamera of Cestods and Annelids, have arisen by terminal budding from a single unarticulated piece, has been much attacked. I therefore emphatically remark that I only understand this process in the widest sense. In both cases there is certainly a reproduction of individual, like parts, which have originated (in time and space) consecutively. notes. 477 100 (i. 361). The agreement among the germ-forms of various Mammals is instructive especially because it shows us how, by diversity in the mode of evolution, the most diverse structures can originate from one and the same form. As we actually see this in germ-forms, we may hypothetically assume the same to have occurred among tribe-forms. Moreover, this agreement is never absolute identity, but always only the very greatest similarity. Even the germs of the various individuals of a species are never actually identical. 101 (i. 366). The law of the ontogenetic connection of systematically allied animal-forms has many apparent exceptions. These are, however, fully explained by the adaptation of the germ to kenogenetic conditions of existence. Where the pal in- genetic form of evolution of the germ has been accurately transmitted by heredity, that law is always in force. Cf. Fritz Muller, "Fur Darwin " (note 111). 102 (i. 367). Earliest human germs. Cf. Kolliker, " History of the Evolution of Man " (" Entwickelungsgeschichte des Men- schen." 2nd edition, 1876, pp. 303-319). Also Ecker, "Icones physiologies." Leipzig, 1859. Plates XXY.-XXXI. The earliest human germs which have yet been certainly recognized, were from twelve to fourteen days old, and were observed by Prof. Allen Thomson, of Glasgow. No opportunity has ever occurred for the observation of earlier germs. 103 (i. 369). Human germs of three weeks (twenty to twenty- one days) exhibit in their whole structure thatphylogenetic stage of evolution which, among extant Vertebrates, is represented by the Cyclostomi (Lampreys and Hags, vol. ii. p. 103), and which must be referable to extinct Monorhine ancestors of similar structure. 104 (i. 370). Human germs of four weeks (twenty-five to thirty days), on the whole, exhibit in their whole structure that phylogenetic stage of evolution, which is exhibited in Sharks and Rays, among extant Vertebrates, and which is referable to similar extinct Primitive Fish ancestors (Proselachii). Of course this comparison is affected by various kenogenetic modifications 478 NOTES. (both heterotopic and heterochronic), just as in the former. (Cf. note 108.) 105 (i. 374). The nose of Nosed-apes is much more different from that of other Apes than from that of Man. Moreover, even the extreme variety and variability in the external form of the human nose shows how small is the morphological value of this organ, so important to the physiognomy. 106 (i. 383). The bladder-like form of the human Allantois. Cf. W. Krause, " On the Allantois in Man " (" Ueber die Allan- tois des Menschen."-— "Archiv fiir Anat. u. Physiol.," 1875, p. 215, Plate VI.)- 107 (i. 400). The navel-cord (funiculus umiilicalis), like the placenta, is an organ shared by Man exclusively with Pla- cental Animals. Cf . Chap. XIX. pp. 155-168, and Figs. 200, 201. On the more minute structure of this organ, and on the special features of the embryonic blood-circulation, cf. Kolliker, " His- tory of the Evolution of Man." 2nd edition, 1876, pp. 319-363. 108 (i. 401). The Kenogeny of Man. In pointing out the phylogenetic significance of the separate incidents and periods of human germ-history, and in explaining them by reference to cor- responding processes and stages in the tribal history of our animal ancestors, we must always bear in mind that in Man, as in all higher animals, the original palingenetic cause of germination has undergone much kenogenetic modification in consequence of many adaptations to the very various conditions of embryonic life, that it has thus been much violated and contracted. The higher the organism develops, the more are especially these earliest stages of evolution abbreviated. 109 (i. 404). The sections of human germ-history, of which only four larger and ten smaller are mentioned here in reference to their phylogenetic significance, allow of much more division if their comparative Ontogeny is minutely examined. This phylogenetic significance may also be very well explained with fitting reference to kenogenetic displacements in place and time (vol. i. p. 13). 110 (i. 405). Figures of human embryos in all stages of notes. 479 germ-history were given in very beautififl detail by M. P. Erdl thirty years ago : " The Evolution of Man, and of the Chick in the Egg" ("Die Entwickelung des Menschen, mid des Hiihnchens imEi." Leipzig, 1845). 111 (i. 409). Fritz Miiller, "Fiir Darwin." Leipzig, 1864. A very excellent little book, in which the modification of the fundamental law of Biogeny (with reference to the Phylogeny of Crustacea) are explained for the first time. 112 (i. 413). The Method of Phylogeny is of the same morphological value as the well-known method of Geology, and may, therefore, claim exactly the same scientific acceptation. Cf. the excellent discourse by Eduard Strasburger, " On the Importance of Phylogenetic Methods in the Study of Living Beings." — "Jenaische Zeitschrift fiir Naturwissenschaft," 1874, vol. viii. p. 56. 113 (i. 415). Johannes Miiller, " On the Structure and Vital Phenomena of Amr)liioxus lanceolatus." — Transactions of the Berlin Academy, 1844. 114 (i. 415). Kecent works on the Amphioxus. W. Rolph and E. Bay Lankester especially have recently added to our knowledge of the organology of the Amphioxus, Wilhelm Miiller and P. Langerhans to that of its histology. The literature of this subject is fully represented by W. Rolph, in his "Researches into the Structure of the Amphioxus " (" Untersuchungen iiber den Bau des Amphioxus." — " Morpholog. Jahrb.," vol. ii. p. 87, Plates V. and VII.), and in P. Langerhans, " On the Anatomy of the Amphioxus" ("Zur Anatomie des Amphioxus." — " Archiv. fiir Mikr. Anat.," vol. xii. p. 290, Plates XII.-XV). 115 (i. 416). Acrania and Craniota. The separation of Vertebrates into Skull-less Animals {Acrania) and Skulled Animals {Craniota), which I first indicated in 1866 in my " Generelle Morphologie," appears to me absolutely essential for the phylogenetic explanation of the Vertebrate-tribe. 116 (i. 428). Max Schultze, "History of the Evolution of Petromyzon" ("Entwickelungsgeschichte von Petromyzon" Haar- lem, 1856). The Ontogeny of the Hags, which promises very important results, is yet, unfortunately, entirely unknown. 480 NOTES. 117 (i. 430). Savigny, " Memoires sur les Animaux sans Vertebres." Vol. ii., Ascidies, 1816. Giard, " Recherches sur les Synascidies." — "Archives de Zoologie Experimental, " vol. i., 1872. 118 (i. 435). Syn-ascidia and Echinoderms. The Corm-theory of Echinoderms, which I explained in 1866 (" Grenerelle Mor- phologic," vol. ii. p. Ixiii), and which has been much attacked as " paradoxical," is as yet the sole theory attempting the genetic explanation of this remarkable group of animals. 119 (i. 442). Kowalevsky, "History of the Evolution of the Amphioxus and of Simple Ascidians" ("Memoires de l'Acad. de S. Petersbourg." 7 Serie. Tom. x. and xi. 1867-8). 120 (i. 450). The metameric structure of the Amphioxus which is indicated in its nerve aud muscle systems, undoubtedly shows that the notochord exists in Vertebrates previous to their metameric structure, and consequently that it is inherited from unarticulated Chorda Animals. 121 (i. 454). The Metamorphosis of the Amphioxus, through which the larva passes into the adult form, is not yet fully known in all its details. This does not, however, affect the extraordinarily important bearing of the thoroughly known, earliest incidents in its germination on the palingenesis of Verte- brates. 122 (i. 455). Fertilization of Ascidia (Phallusia mammillata), Eduard Strasburger, " On Cell-structure and Cell-division, with Studies of Fertilization." 2nd edition. Jena, 1876, p. 306, Plate VIII. 123 (i. 462). Kupffcr. The tribal relation of Ascidia to Vertebrates ("Archiv fur Mikros. Anat.," 1870, vol. vi. pp. 115-170). Oscar Hertwig, " Researches into the Structure and Evolution of the Cellulose Mantles of Tunicata " (" Untersu- chunsren liber den Bau und die Entwickelun^ des Cellulose- Mantels der Tunicaten"). Richard Hertwig, "Contribution to Knowledge of Ascidian Structure '; (" Beitrage zur Kenntniss des Baues der Ascidien." — " Jenaische Zeitschrift fur Naturwis- senschaft," 1873, vol. vii.). NOTES. 481 124 (i. 464). The Phylogenetic Importance of the Amphi- oxus cannot be too highly insisted on. Without knowledge of its Anatomy and Ontogeny, the origin of Vertebrates would be entirely dubious, and their descent from Worms would appear incredible. 125 (i. 467). The Ontogenetic Cell-pedigree, as it is repre- sented, with reference to the Amphioxus, in Table XI., probably holds good, in its most important features, for all Vertebrates, and, therefore, also for Man. For, more than any other form, the Amphioxus by strict Heredity has accurately retained its Palingenesis. This histogenetic cell-pedigree is apparently well established as regards most and the chief features ; on the other hand, it yet appears doubtful with regard to the origin of the primitive kidneys, the testes, and ovaries. 126 (ii. 4). Milne-Edwards, "Lecons sur la Physiologic Comparee," vol. ix. 127 (ii. 6). Eternity of Organic Life. xVccording to the monistic view, organic life is a further form of evolution of the inorganic word-processes, and had a beginning in time on our planet. In opposition to this, A. Fechner, among others, in his "Thoughts on the Creation and Evolution of Organisms," has stated certain opposed " kosmorganic phantasys " which appear entirely irreconcilable with the ontogenetic facts given here. 128 (ii. 18). Bernhard Cotta (" Geologie der Gegenwart," 1866; 4th edition, 1874) and Karl Zittel("Aus der Urzeit;" Munchen, 1875, 2nd edition) have made some excellent remarks on the duration and the whole course of the organic history of the world. 129 (ii. 21). August Schleicher, "The Darwinian Theory and Philology" ("Die Darwin'sche Theorie und die Sprach- wissenschaft." Weimar, 1863. 2nd edition, 1873). 130 (ii. 25). At first sight, most polyphyletic hypotheses appear more simple and easy than do monophyletic, but the former always present more difficulties the more they are considered. 131 (ii. 25). Those physiologists who desire an experi- 4§2 NOTES. mental proof of the theory of descent, merely thereby prove their extraordinary ignorance of the morphological scientific facts re- lating to this matter. 132 (ii. 30). Spontaneous generation. — " Generelle Mor- phologic," vol. i. pp. 167-190. "Monera and Spontaneous Gene- ration."— "Jenaische Zeitschrift fur Naturwissenschaft," 1871, vol. vi. pp. 37-42. 133 (ii. 33). The Absence of Organs in Monera. In saying that Monera are "organisms without organs," we understand the definition of organs in a morphological sense. In a physiological sense, on the other hand, we may call the variable plasson- processes of the body of the Moneron the " fseudopodia " organs. 134 (ii. 36). Induction and Deduction in Anthropogeny. "Generelle Morphologie," vol. i. pp. 79-88; vol. ii. p. 427. " History of Creation," vol. ii. p. 357. 135 (ii. 42). Animal Ancestors of Man. The number of species (or, more accurately, form-stages, which are distinguished as " species ") must, in the human ancestral line (in the course of many millions of years !), have amounted to many thousands ; the number of genera to many hundreds. 136 (ii. 47). Following Elsberg, we give the name of "plas- tidules" to the "molecules of plasson," to the smallest like parts of that albuminous substance which, according to the " plastid- theory," is the material substratum of all the active phenomena of life. Cf . my work on " The Perigenesis of Plastidules " (" Perigenesis der Plastidule ocler Wellenzeugung der Lebens- theilchen." Berlin, 1876). This is an attempt to explain mechanically the elementary processes of evolution. 137 (ii. 49). Bathybius and the free protoplasm of ocean depths. Cf. my " Studies on Monera and other Protista." Leipzig, 1870, p. 86. The most recent observations on living Bathybius are those of Dr. Emil Bessel, who found this form on the coast of Greenland (in Smith's Sound), at a depth of about 550 ft. He noticed very active amoeboid movements in them, as well as the assumption of foreign particles (carmine, etc.). " It consists of nearly pure protoplasm, tinged most intensely by NOTES. 483 a solution of carmine in ammonia. It contains fine gray granules of considerable refracting power, and besides the latter a great number of oleaginous drops, soluble in ether. It manifests very marked amoeboid motions, and takes up particles of carmine, etc." — Packard, " Life Histories of Animals, including Man." New York, 1876. 138 (ii. 50). The Philosophical Importance of Monera in explaining the most obscure biological questions cannot be sufficiently emphasized. Monograph of Monera. — "Jenaische Zeitschrift fiir Naturwissenschaft," vol. iv., 1868, p. 64. 139 (ii. 54). The Nature and Significance of the Egg-cell can only be philosophically understood by means of phylogenetic examination. 140 (ii. 58). Synamceba. Cienkowski, " On the Structure and Evolution of Labyrinthula " ("Uber den Bau und die Entwic- kelung der Labyrinthuleen." — Arch, fiir Mikrosk. Anat., 1870, vol. iii. p. 274). Hertwig, " Microgromia Socialis." — Ibid. 141 (ii. 61). Catallacta, a new Protista-group (MagospJicera planula). See " Jenaische Zeitschrift fiir Naturwissenschaft," vol. vi., 1871, p. 1. 142 (ii. 66). Haliphysema and Gastrophysema. Extant Gastreeads. See " Jenaische Zeitschrift fiir Naturwissenschaft," vol. xi., 1876, p. 1, Plates I.-VI. 143 (ii. 70). The five first stages in the evolution of the animal body, which are compared in Table XVII., and which are common to Man and all higher Animals, are established beyond all doubt as existing in the Ontogeny of most extant animals. As Comparative Anatomy shows that corresponding form-stages yet exist in the system of the lower animals, we may assume, in accordance with the fundamental law of Biogeny, that similar forms existed phylogenetically as most important ancestral forms. 144 (ii. 77). On the distinction of the axes, and on the geometric outline of the animal body, see " Promorphologie " (" Generelle Morphologie," vol. i. pp. 374-574). 145 (ii. 87). The hermaphrodite structure of our ancestral 4$4 NOTES. series was perhaps transmitted from the Chorda Animals even as far as the lower stages of Vertebrate ancestors. Cf . Chapter XXV. 146 (ii. 89). I am inclined to regard the Appendicularia as living Chorda Animals of the present day ; they are the only Invertebrates permanently possessing a notochord, and thus, as by many other peculiarities, distinguished from genuine Tuni- cates. 147 (ii. 105). Metamorphosis of Lampreys. That the blind Ammocoetes change into Petromyzon was known two hundred years ago (1666) to the fisherman Leonhard Baldner of Stras- burg ; but this observation remained unrecognized, and the modification was first discovered by August Muller in 1854 (" Archiv fur Anat.," 1856, p. 325). Cf. Siebold, "The Fresh- water Fishes of Central Europe" ("Die Siisswasserftsche von Mittel-Europa," 1863). 148 (ii. 114). Selachii as Primitive Fishes. The old disputes as to the systematic position and kindred of Selachii were first definitely settled by Gegenbaur, in the introduction to his classical work on " The Head-skeleton of Selachii." 149 (ii. 118). Gerard Krefft, "Description of a Gigantic Am- phibian ; " and Albert Giinther, " Ceratodus, and its Systematic Position." — "Archiv fur Naturgeschichte," 37, 1871, vol. i. p. 321 ; also "Phil. Trans.," 1871, Part II. p. 511, etc. 150 (ii. 129). The duration of metamorphosis of Amphibia varies much in the different forms of Frogs and Toads, the whole forming a complete phylogenetic series from the original, quite complete form, to the later, much shortened and vitiated heredity of modification. 151 (ii. 129). "All the histological features of the Land Salamander (Salamandra metadata) force the impression that it belongs to an entirely different epoch of terrestrial life than that of the Water Salamander (Triton), externally so similar."' — Robert Remak (" Entwickelung der Wirbelthiere," p. 117). 152 (ii. 130). Siredon and Amblystoma. Yery various views have lately been expressed as to the phylogenetic significance to be attributed to the much-discussed modification of the Mexican NOTES. 485 Axolotl into an Amblystoma. Cf. on this subject especially August Weismann, in " Zeitsch. fiir wissensch. Zoologie," vol. xxv., Sup., pp. 297-334 153 (ii. 131). The Leaf- frog of Martinique (Hylodes mar- tinicensis) loses its gills on the seventh day, its tail and yelk-sac on the eighth day of egg-life. On the ninth or tenth day after fertilization the complete frog emerges from the egg. — Bavay, "Sur l'Hylodes Martinicensis et ses Metamorphoses." "Journal, de Zool. par Grrevais," vol. ii. 1873, p. 13. 154 (ii. 133). " Homo diluvii testis" = Andrias Scheuchzeri. " Sad bone of an ancient evil-doer ; Soften, stone, the heart of the new children of evil " (Diaconus Miller). Quenstedt. "Formerly and Now " (" Sonst und Jetzt," 1856, p. 239). 155 (ii. 133). The Amnion-structure of the three higher "Vertebrate-classes, wanting in all lower Vertebrates, has no connection with the similar, but independently acquired Amnion- structure (analogous, but not homologous) of higher Articu- lated Animals [Arthropoda) . 156 (ii. 138). The former existence of a Protamnion, the common parent-form of all Amniota, is undoubtedly shown by the Comparative Anatomy and Ontogeny of Reptiles, Birds, and Mammals. No fossil remains of such a Protamnion have, how- ever, yet been discovered. They must be sought in the Permian or Carboniferous formation. 157 (ii. 147). The former organisation of the Promammalia may be hypothetically reconstructed from the Comparative Anatomy of the Salamander, Lizards, and Beaked. Animals ( Ornitlwrliynclius) . 158 (ii. 153). The Didelphic ancestors of Man may have been externally very different from all known Pouched Animals (Alar- siqrialia), but possessed all the essential internal characters of Marsupialia. 159 (ii. 163). The phylogenetic of the Semi-apes, as the primaeval placental parent-group, is not influenced by our ignor- ance of any fossil Prosimise, for it is never safe to estimate palaeontological facts as negative, but only as positive. 486 NOTES. 160 (ii. 168). On the structure of the Decidua very various theories have been given. Cf. Kdlliker, "History of the Evolution of Man " (" Entwickelungsgeschichte des Menschen." 2nd edition, 187] , pp. 319-376). Ercolani (Giambattista), " Sul pro- cesso formativo della placenta." Bologna, 1870. " Le glandole otricolari del'utero." Bologna, 1868, 1873. Haxley, "Lectures on the Elements of Comparative Anatomy," 1864, pp. 101-112. 161 (ii. 172). Huxley, " Anatomy of Vertebrates," 1873, p. 382. Previously Huxley had separated the " Primates " into seven families of nearly equal systematic value. (See " Man's Place," etc., p. 119.) 162 (ii. 179). Darwin. Sexual selection in Apes and Man. — " Descent of Man," vol. ii. pp. 210-355. 163 (ii. 180). Man-like Holy Apes. Of all Apes, some Holy Apes (Semnojrithecus) most resemble Man, in the form of their nose and the character of their hair (both that on the head and that on the beard). — Darwin, "Descent of Man," vol. i. p. 335; vol. ii. p. 172. 164 (ii. 182). Friedrich Miiller (" Allgemeine Ethnographic" Vienna, 1873, p. 29), on the supposed age of man. Families of languages (pp. 5, 15, etc.). 165 (ii. 183). The plate (XV.) representing the migrations, given in the "History of Creation," merely claims the value of a first attempt, is an hypothetic sketch, as I there expressly said, and as, in consequence of repeated attacks, I must here insist. 166 (ii. 201). The Leather-plate. The phylogenetic distinction of a special leather-plate, the outermost lamella separating from the skin-fibrous layer, is justified by Comparative Anatomy. 167 (ii. 201). Milk-glands. Huss, "Contributions to the History of the Evolution of the Milk-glands" ("Beitrage zur Entwickelungsgeschichte der Milchdriisen ") ; and Gegenbaur, " On the Milk-gland Papillae " (" Jenaischc Zeitschrift fin- Naturwissenschaft," 1873, vol. vii. pp. 176, 204). 168 (ii. 208). On the hairy covering of Man and Apes, see Darwin, "Descent of Man," vol. i. pp. 20, 167, 180; vol. ii. pp. 280, 298, 335, etc. NOTES. 487 169 (ii. 217). Dorsal side and ventral sides are homologous in Vertebrates, Articulated Animals (Arthropod a), Soft-bodied Animals (Mollusca), and Worms, so that the dorsal marrow and the ventral marrow are not comparable. Cf. Gegenbaur, "Morph. Jahrbuch," vol. i. pp. 5, 6. 170 (ii. 22c8). The unknown ontogenetic origin of the sym- pathetic nerve-system must probably, for phylogenetic reasons, be sought chiefly in the intestinal layer, not in the skin-layer. 171 (ii. 248). On the cavities connected with the nose, see Gegenbaur, " Elements of Comparative Anatomy," p. 580. 172 (ii. 260). The analogies in the germination of the higher sense organs were rightly, grasped even by the earlier natural philosophers. The first more accurate sketches of the very obscure germ-history of the sense-organs, especially of the eye and ear, were given (1830) by Emil Huschke, of Jena (Isis, Meckel's Archiv, etc.). 173 (ii. 265). Hasse, " Anatomical Studies " (" Anatomische Studien "), chiefly of the organ of hearing. Leipzig, 1873. 174 (ii. 269). Johannes Rathke, "On the Gill-apparatus and the Tongue-bone " (" Ueber den Kiemen-apparat und des Zungenbein," 1832). Gegenbaur, " On the Head-skeleton of Selachii," 1872. (See note 124.) 175 (ii. 272). On the Rudimentary Ear-shell of Man, cf. Darwin, "Descent of Man," vol. i. pp. 17-19. 176 (ii. 276). Scarcely anywhere does Comparative Anatomy prove its high morphological value as with reference to the skeleton of Vertebrates : in this matter it accomplishes much more than Ontogeny. There is all the more reason to insist on this here, as Goette, in his gigantic history of the evolution of Bombinator, has recently denied all scientific value to Com- parative Anatomy, and asserted that Morphology is explained solely by Ontogeny. Cf. my " Aims and Methods of the Recent History of Evolution " (" Ziele und Wege der heutigen Ent- wickelungsgeschichte," 1875, p. 52, etc.). 177 (ii. 283). The Human Tail, like all other rudimentary organs, is very variable in point of size and development. In 41 488 NOTES. rare cases it remains permanently, projecting freely : usually it disappears at an early period, as in Anthropoid Apes. 178 (ii. 284). On the Number of Vertebras in different Mam- mals, cf. Cuvier, "Lecons d'Auatomie Comparee." 2nd edition, tome i., 1835, p. 177. 179 (ii. 203). On the earlier Skull-theory of Goethe and Oken, cf. Yirchow, " Goethe as a Naturalist " (" Goethe als Natur- forscher," 1861, p. 103). 180 (ii 295.). Karl Gegenbaur, "The Head-skeleton of Selachii" ("Das Kopfskelet der Selachier"). As the foundation of a study of the head-skeleton of Vertebrates (1872). 181 (ii. 301). Karl Gegenbaur, " On the Archipterygium." — "Jenaische Zeitschrift fur Naturwissenschaft," vol. vii. 1873, p. 131. 182 (ii. 304). Gegenbaur, " Researches iuto the Comparative Anatomy of Vertebrates" (" Untersuchungen zur Vergleichen- den Anatomie der Wirbelthiere "). Part I. Carpus and Tarsus (1864). Part II. The shoulder girdle of Vertebrates. Pectoral tins of Fishes (1866). 183 (ii. 305). Charles Martins, " Nouvelle comparaison des membres pelviens et thoraciques chez 1'homme et chez les mammiferes." — " Memoires de l'Acad. de Montpellier," vol. iii. 1857. 184 (ii. 308). Ossification. Not all bones of the human body are first formed of cartilage. Cf. Gegenbaur, " On Primary and Secondary Bone-formation, with special reference to the Pri- mordial Skull Theory." — " Jenaisch. Zeitschrift fur Natur- wissenschaft," 1867, vol. iii.. p. 54. 185 (ii. 308). Johannes Miiller, "Comparative Anatomy of Myxinoides." — " Transactions of the Berlin Academy," 1834-1842. 186 (ii. 314). The Homology of the Primitive Intestine and the two primary germ-layers is the postulate for morphological comparison of the various Metazoa-tribes. 187 (ii. 322). In the Evolution of the Intestine, Amphibia and Ganoids have, by heredity, retained the original Craniota-form more accurately than have Selachii and Osseous Fishes (Tdeoslei). NOTES. 489 The palingenetic germination of Selachii has been much altered by kenogenetic adaptations. 188 (ii. 323). On the Homology of Scales and Teeth, cf. Gegenbaur, " Comparative Anatomy ': (" Grundriss der vergl. Anatomie," 1874, pp. 426, 582) ; also Oscar Hertwig, "Jenaische Zeitschrift fur Naturwissenschaft," 1874, vol. viii. On the important distinction of homology (morphological resemblance) and Analogy (physiological resemblance), see Gegenbaur, as above, p. 63; also my " Generelle Morphologie," vol. i. p. 313. 189 (ii. 337). Wilhelm Miiller, "On the Hypobranchial Groove in Tunicates, and its Presence in the Amphioxus and Cyclostomi." — "Jenaische Zeitschrift fur Naturwissenschaft," 1873, vol. viii. p. 327. 190 (ii. 358). The Nerve- muscular Cells of the Hydra throw the earliest light on the simultaneous, phylogenetic differentiation of nerve and muscle tissue. Cf. "Klemenberg, Hydra." Leipzig, 1872. 191 (ii. 383). The germ-history of the human heart accurately reproduces in all essential points its tribal history. This palin- genetic reproduction is, however, much contracted in particular points and vitiated by kenogenetic modifications of the original course of evolution, displacements partly in time, partly in place, which are the result of embryonic adaptations. 192 (ii. 383). On the Special Germ-history of the Human vascular system, cf. Kolliker, "History of the Evolution of Man" ("Entwickelungsgeschichte des Menschen." 2nd edition, 1876) ; also Rathke's excellent work on Ontogeny. 193 (ii. 387). The Homologies of the Primitive Organs, as they are here provisionally described in accordance with the Casteea- theory (note 24), can only be established by further co- operation between Comparative Anatomy and Ontogeny. Cf. Gegenbaur on Comparative Anatomy (" Grundriss der verglei- chenden Anatomie "). 194 (ii. 390). The Mechanism of Reproduction. As the functions of reproduction and of heredity, connected with re- production, are referable to growth, so the former as well as the 490 NOTES. latter are finally explicable as the results of the attraction and rejection of homogeneous and heterogeneous particles. 195 (ii. 397). Eduard van Beneden, " De la Distinction origi- nelle du Testicule et de l'Ovaire." Brussels, 1874. 196 (ii. 399). On the Original Hermaphrodite Structure of Vertebrates, cf. Waldeyer, "Ovary and Egg" ("Eierstock und Ei," 1872, p. 152) ; also Gegenbaur (" Grnndriss der vergleichen- den Anatomie," 1874, p. 615). On the origin of the eggs from the ovary-epithelium, cf. Pniiger, " On the Ovaries of Mammals and Man" ("Die Eierstocke der Siiugetkiere und des Menscheu/' 1863). 197 (ii. 423). On the special germ-history of the urinary and sexual organs, cf. Kolliker, "History of the Evolution of Man." On the homologies of these organs, see Gegenbaur (" Grundriss der vergleichenden Anatomie," 1874, pp. 610-628). 198 (ii. 41S). Wilkelm Wundt, "Lectures on the Human and Animal Mind" (" Vorlesungen iiber die Menschen- und Thier- seele." 1863). W. Wundt, "Outlines of Physiological Psy- chology" ("Grundziige der Physiologishen Psychologie," 1874). 199 (ii. 457). On Active (actual) and Latent (preteritial) forces, cf. Hermann Helmholtz, " Interoperation of Natural Forces " (" Wechselwirkung der Naturkraf te," Part II., 1871). 200 (ii. 457). " Anthrojoology as Part of Zoology." — " Generelle Morphologie," vol. ii. p. 432. "History of Creation," vol. i. 7; vol. ii. 347. INDEX. Acaleph.e, ii. 73, 92 Accelomi, ii. 75, 92 Acorn-worms, ii. 86 Acrania, i. 116 ; ii. 97 Adam's apple, ii. 336 Adaptation, i. 158 After-birth, i. 400 Agassiz, thoughts on creation, i. 116 Air-tube (trachea), ii. 330, 333 Alali, ii. 182 Allantois, i. 380; ii. 135, 411 Alluvial period, ii. 12 Amasta, ii. 146, 204 Amnion, i. 314, 386 animals, ii. 120, 133 sheaths of, i. 387 water, i. 314 Amniota, ii. 120, 133 Amoeba, i. 142 ; ii. 152 false feet of, i. 142 Amoeboid egg-cells, i. 144; ii. 53 ■ ■ movements, i. 142; ii.c3 states, ii. 56 Amphibia, ii. 120, 122 AmpJiigastrula, i. 200, 241 Amphigonia, i. 160 Amphioxus, i. 413; ii. 98 blastula of, i. 443 body-form of, i. 417 ■ — cells of, their pedigree, i. 467 chorda of, i. 417 distribution of, i. 415 gastrula of, i. 414 germ-layers of, i. 447 Amphioxus, medullary tube of, i. 41S place of, in natural system, i. 416 sexual organs of, i. 425 side canals of, i. 423 significance of, i. 254, 427 Amphirhina, ii. 97, 101 Analosrv, ii. 412 Anamnia, ii. 97, 120 Ancestral series of man, ii. 44, 184 Animalculists, i. 37 Animal germ-layer, i. 194, 327 • organs, ii. 192, 194 Anorgana, i. 156; ii. 30 Anthropocentric conception, ii. 457 Anthropoids, ii. 177, 189 Anthropolithic epoch, ii. 11, 16 Anthropozoie periods, ii. 12, 17 Antimera, i. 257 Anus, i. 339 ; ii. 323, 345 Anus-groove, i. 339 Anvil (Incus of ear), ii. 261, 268 Ape-men, ii. 44, 181 Apes, ii. 165, 1S9 eastern, ii. 172, 189 flat-nosed, ii. 172, 189 ■ narrow-nosed, ii. 172, 189 question as to descent of, ii. 165, 441 tailed, ii. 172, 189 western, ii. 172, 189 Aorta, i. 265; ii. 378 roots of, ii. 375 stem of, ii. 375 Aortal arches, ii. 375, 378 Appendicularia, i. 459 ; ii. 90 492 INDEX. Archelminthes, ii. 76 Archigastrula, i. 198, 241 Archilithic epoch, ii. 9, 19 Archipterygium, ii. 301 Archizoic periods, ii. 9, 11 Area germinativa, i. 292 opaca, i. 296 pellucida, i. 296 Aristotle, i. 27 ; ii. 368 epigenesis, i. 29 heart formation, ii. 368 his history of evolution, i. 27 Arm, lower, ii. 278, 304 ■ upper, ii. 278, 304 Arterice omphalo-mesentericce, i. 395 umbilicales, i. 400 vertebrates, i. 395 vitellines, i. 395 Arteries, i. 393 Artery-arches, ii. 377 stalk, ii. 380 Arthropoda, ii. 92, 94 Articulated animals, ii. 92, 94 Articulation in man, i. 346 Ascidia, i. 429 ; ii. 90 ■ blastula of, i. 455 chorda of, i. 456 communities of, i. 455 gastrula of, i. 455 gill-sac of, i. 431 heart of, i. 433 homologies of, i. 465, 466 intestine of, i. 432 mantle of, i. 430, 461 ■ medullary tube of, i. 458 sexual organs of, i. 434 tail of, i. 456 Ascula, ii. 68 Atrium, ii. 374, 381 Auditory nerve, ii. 262 organ, ii. 260 passage, ii. 269 vesicles, ii. 262 Auricular processes of heart, ii. 381 Axes of the body, i. 255 ; ii. 77 Axial cord, i. 301 rod (notochord), i. 302 skeleton, ii. 280, 299 Axis-plate, i. 299 Axolotl, ii. 126 B Baer, Karl Ernst, i. 50 his germ-layer theory, i. 51 his law, i. 58 life of, i. 52 on the bladder-like outline, ii. 62 on the human egg, i. 55; ii. 424 on the notochord, i. 55 on type theory, i. 54 Balanogloss^ls, ii. 85 Bathybius, ii. 49 Batrachia, ii. 131 Bats, ii. 169, 187 Beaked animals, ii, 147, 187 Bell-gastrula, i. 198 Bilateral outline, i. 257 ; ii. 74 Bhnana, ii. 169 Biogeny, i. 24; ii. 434 fundamental law of, i. 6, 24; ii. 434 Birds, ii. 120, 138 gastrula of, i. 223 Bischoff, Wilhelm, i. 59 Bladder-gastrula, i. 229, 241 Blastata, ii. 61 Blast ocoeloma, i. 189 Blastoderma, i. 189 Blastodiscus, i. 227 Blastogeny, i. 24 Blastophylla, i. 195 Blastophyly, i. 24 Blastosph&ra, i. 191 Blastula, i. 191, 242 Blind-intestine, ii. 330,-343 Blood-cells (corpuscles), i. 159; ii. 366 relationship, i. 112 vessels, ii. 370 Bloodless worms, ii. 75 Bonnet, i. 40 Brain, i. 212, 232 bladders of, i. 343 ; ii. 214 parts of, ii. 212 skull of, ii. 292 Breast-body, ii. 282 bone, ii. 282 cavity, i. 261 vertebra^, ii. 2S2 INDEX 493 Budding (gemmation), ii. 391 Bulbus arteriosus, ii. 374 Bulbus oculi, ii. 250 c CiENOLTTHIC EPOCH, ii. 11, 15 Csenozoic period, ii. 15, 19 Calf-bone, ii. 278, 304 Cambrian period, ii. 9, 19 Canalis auricularis, ii. 381 Carboniferous period, ii. 10, 19 Cardinal veins, i. 391 Carpus, ii. 278 Catarhince, ii. 176, 189 Catastrophes, theory of, i. 76 Causae efficientes, i. 16, 80 ; ii. 455 finales, i. 16, 80 ; ii. 455 Cavum tympani, ii. 261, 270 Cell-division, i. 124 kernel {nucleus), i. 125 state, i. 124 substance, i. 125 Cells, i. 125 female, i. 171 ; ii. 392 male, i. 171 ; ii. 392 theory of, i. 60, 121 Centi-al heart, ii. 120 medulla, ii. 210, 232 nerve-system, ii. 210 skeleton, ii. 280, 299 " Centre of sight," ii. 252 Ceratodus Fosteri, ii. 119 Cerebellum, ii. 212, 232 Cerebrum, ii. 212, 232 Cetacea, ii. 187 Cetomorpha, ii. 160, 187 Chalk period, ii. 14, 19 Chalk-sponges, i. 117 Chick, importance of, i. 31 Chimpanzee, ii. 178, 180 Chorda animals, ii. 84, 87 dorsalis, i. 255, 301 sheath, ii. 286 tissue of, ii. 286 vertebralis, i. 255, 301 Chordoma, i. 84, 87 Chorioidea, ii. 252, 258 Chorion, i. 387 ; ii. 158 frondosum, ii. 160 Iceve, ii. 160 Chorion, smooth, ii. 160 tufted, ii. 160 Chorology, i. 113 Chyle vessels, ii. 374 Cicatricula, i. 138 Circulation in Amphioxus, i. 423 Ascidia, i. 433 Fishes, ii. 375 — germ-area, i. 397 Mammals, ii. 378 Clavicula, ii. 278, 304 Cleavage cells, i. 185 ■ forms of, i. 242 of egg, i. 185, 241 partial, of bird's egg, i. 224 rhythm, i. 243 superficial, i. 200, 241 unequal, i. 200, 241 Clitoris, ii. 423, 431 Cloaca, ii. 145, 418 Cloacal animals, ii. 145, 187 Coalescence, i. 164 Coal period, ii. 11, 19 Coccyx, ii. 282 Cochlea, ii. 263, 268 Ccelenterata, ii. 73 Cceloma, i. 260 ; ii. 75 Coelomati, ii. 75, 92 Columna vertebralis, i. 349 ; ii. 285 Comparative Anatomy, i. 107, 245 Concrescence, i. 164 Conjunctiva, ii. 259 Connective membrane of eye, ii. 251 tissue, ii. 363 Connectivum, ii. 361, 366 Convolutions of brain, ii. 226 Copulation organs, ii. 421 Copulativa, ii. 421 Coracoideum, ii. 278, 304 Corium, ii. 200, 232 Cormogeny, i. 24 Corniophyly, i. 24 Cornea, ii. 251, 258 Costcs, ii. 278, 282 Covering tissue, ii. 361 Craniota, ii. 100, 120 Cranium, ii. 291 Creation, i. 74, 79 ; ii. 183 Crooked intestine, ii. 319, 330 Cross-vertebrae, ii. 282 Crystalline lens, ii. 253, 258 Culture period, ii. 11 494 INDEX. Curves of embryo, i. 369 Cutis, ii. 200, 232 Cuvier, theory of catastrophes, i. 76 theory of types, i. 57 Cyclostoma, ii. 101, 120 Cytods, i. 103 Cytula, i. 176 Cytococcus, i. 176 D Dalton, i. 51 Darwin, Charles, i. 96 descent of man, i. 103 selection, theory of, i. 95 sexual selection, i. 103 Erasmus, i. 96 Darwinism, i. 95 Decidua, ii. 161, 165 animals, ii. 161 Decidua-less animals, ii. 161 Deciduata, ii. 161, 187 Deduction, i. 101 ; ii. 37 Degree of development, i. 58 Derma, ii. 232 Descent, theory of, i. 81 of man, i. 101 Devonian period, ii. 10, 19 Didelphia, ii. 149, 187 Differentiation, i. 152, 159 Digestive intestine, ii. 330 Digits, ii. 278 Diluvial period, ii. 12, 15 Dipneusta, ii. 115, 120 Discogastrula, i. 219, 211 Discoidal cleavage, i. 225, 242 Discoplacentalia, ii. 162, 187 Discus blastodennicus, i. 139, 226 Doellinger, i. 50 Dorsal furrow, i. 302 marrow, ii. 221 swellings, i. 303 Double -breathers, ii. 117, 120 nostrils, ii. 97, 101 " Double-shield," i. 297 Dualism, i. 17 ; ii. 456 Dualistic philosophy, i. 17 Ductus Gartneri, ii. 416, 431 Millleri, ii. 415, 431 Bathkei, ii. 415, 431 Wolffii, ii. 415, 431 Dysteleology, i. 109 E Ear, bonelets of, ii. 268 labyrinth of, ii. 262, 26S muscles of, ii. 271 nerve of, ii. 266 pouch {utricultis) , ii. 262 sac (sacculus), ii. 262 shell of, ii. 269 snail of, ii. 263, 268 trumpet, ii. 260 vesicles, ii. 265 wax, glands of, ii. 262 Echidna, ii. 147 Echinoderma, i. 435; ii. 92 Egg-cell, i. 132 of Chick, i. 139 Bird, i. 139 Mammal, i. 137 Man, i. 137 ; ii. 425 sponge, i. 144 cleavage, i. 185, 242 holoblastic, i. 215 human, i. 137 ; ii. 425 membranes, i. 375 ; ii. 158 meroblastic, i. 216 Elbow, ii. 278, 304 Elementary organism, i. 124 Embryo of Vertebrates, i. 360 Embryology, i. 3 Empty intestine, ii. 319 Encephdlon, ii. 232 Endocoelarium, ii. 366, 400 Enteropnensta, ii. 86 Entoderma, i. 206, 236 Eocene period, ii. 11, 15 Epidermis, ii. 200, 232 Epididymis, ii. 417, 428 Epigenesis, i. 39, 41 E pi genesis, theory of, i. 39, 41 Epithelial tissue, ii. 361 Epithelium, ii. 361, 366 Epochs, duration of, ii. 3 Evolution of forms, i. 19 of functions, i. 19 history of, i. 1, 2t theory of, i. 34 * Excretory organs, i. 267 ; ii. Exocwlarium, ii. 369, 400 Exoderma, i. 195, 236 403 INDEX. 495 Extremities, ii. Ill, 306 Eye, ii. 250, 258 connective membrane of, ii. 251 lids, ii. 259 netted membrane of, ii. 252, 258 protective membrane of, ii. 251 pupil of, ii. 250 rainbow membrane of, ii. 252 vascular membrane of, ii. 252 vesicles, i. 357 ; ii. 253 ¥ Fabricius ab Aquapendente, i. 31 Face, development of, ii. 245, 316 skull of, ii. 278, 291 Fallopian canals, ii. 431 hydatids, ii. 431 Fatty layer of corium, ii. 232 Female breast, ii. 202 cells, i. 171, 392 copulatory organs, ii. 423 excretory ducts, ii. 415, 431 germ-glands, ii. 398 germ-layer, ii. 308 milk-glands, ii. 202 phallus (Clitoris), ii. 423 sexual organs, ii. 423 sexual plate, ii. 401 uterus, ii. 417 Femur, ii. 278, 304 Fertilization,]', 169, 176 Fibula, ii. 278, 304 Fin, central rod of, ii. 302 Fin, rays of, ii. 302 skeleton of, ii. 302 Final causes, i. 16 Fingers, ii. 278 Fishes, ii. 109, 120 fins of, ii. Ill gastrula of, i. 219 scales of, ii. 331 Five-digited foot, ii. 123 Flat-worms, ii. 76 Flesh, i. 259 Flesh-layer, i. 236 Foot, ii. 170 Force and matter, ii. 456 Forces, active, ii. 457 latent, ii. 457 Formative functions, i. 15 yelk, i. 216 Forms, science of, i. 20 Frog-Batrachia, ii. 131 Frogs, ii. 131 egg- cleavage of, i. 203 gastrula of, i. 207 iarva of, ii. 127 metamorphosis of, ii. 126 Frontal process, ii. 244 Functions of evolution, ii. 155 science of, i. 19 Funiculus genitalis, ii. 418 umbilicalis, i. 383 ; ii. 168 G Gall-bladder, ii. 341 ducts, ii. 341 intestine, ii. 317, 330 Ganoid Fishes, ii. 112, 120 Gartnerian duct, ii. 416, 431 Gastrma, i. 232 ; ii. 66 theory of, i. 247 ; ii. 195 Gastraeads, ii. 62, 70 Gastrocystis, i. 291 Gastrodiscus, i. 292 Gastrula, i. 192; ii. 65 Bell-, i. 198 Bladder-, i. 200 Disc-, i. 200 Hood-, i. 200 Gegenbaur, i. 108; ii. 96 on Comparative Ana- tomy, ii. 96 Gegenbaur on head-skeleton, ii. 293 on theory of descent, i. 108 skull theory, ii. 293 theory of limbs, ii. 299 Generatio spontanea, ii. 30 " Generelle Morphologie," i. 102 Geological hypotheses, i. 410 Germ, i. 3 Germ-area, i. 292 dark, i. 297 light, i. 297 cavity, i. 189 disc, i. 139, 226 496 INDEX. Germ-epithelium, ii. 401 glands, ii. 398 history, i. 6, 24 layer, middle, i. 13 membrane, i. 189 membrane vesicle, i. plate, ii. 401 ■ point, i. 135 shield, i. 297 spot, i. 135 vesicle, i. 179, 291 1S9 Gibbon, ii. 178, 181 Glacial period, ii. 11 Glands of intestine, ii. 330 skin, i. 201 Glans phalli, ii. 422 Glomeruli renales, ii. 407 Gnathostomi, ii. 109 Goethe, Wolfgang, i. 88 his skull theory, ii. 293 morphology, i. 88 on metamorphosis, i. 90 on reason, ii. 453 on specification, i. 90 Goette, Alexander, i. 65 Gonades, ii. 398 Gonochorismus, ii. 69, 395 Gonophori, ii. 402 Gorilla, i. 178, 180 Graafian follicles, ii. 424 Gubernaculum Hunteri, ii. 431 Hair, ii. 205, 232 Hair-animals, ii. 205 Hairy covering, ii. 206 Haliphysema, ii. 66 Haller, Albrecht, i. 38 Hand, ii. 169 skeleton of, ii. 302 Hare-lip, ii. 246 Harvey, i. 31 Head-cap, i. 386 marrow, ii. 210 plate, i. 335 ribs, ii. 298 sheath, i. 387 Heart, auricle of, ii. 381 auricular processes of, ii. 381 Heart cavity, i. 394 development of, ii. 385 human, ii. 379, 382 mesentery, i. 394 ventricle of, ii. 381 Heopitheci, ii. 172 Heredity, i. 161 vitiated, i. 408 Hermaphrodites, ii. 395 Hermaphrodite gland, ii. 401 Vertebrates, ii. 403 Hermaphroditismus, ii. 69, 395 Hesperopitheci, ii. 172 Heterochronism, i. 13 Heterotopism, i. 13 Hind-brain, ii. 221, 232 intestine, ii. 343 limbs, ii. Ill Hip-bone, ii. 278 His, Wilhelm, i. 64 Histogeny, i. 24 Histology, i. 24 Histophyly, i. 24 Hollow-worms, ii. 76 Holoblastic eggs, i. 215 Hologastrula, i. 241 Homology of primitive intestine, i. 247 ; ii. 321 of the animal tribes, ii. 387 ■ germ-layers, i. 247 sexes, ii. 431 Hood-gastrula, i. 200, 241 Hoofed animals, ii. 160, 188 Horn-plate, i. 307 Horn-stratum of Epidermis, ii. 200 Humerus, ii. 278, 304 Huxley, i. 101 ; ii. 294 germ -layer theory, i. 67 his Evidences, i. 101 Man and Ape, i. 101 primates, law of, ii. 177 skull theory, ii. 291 Hylooates, ii. 181, 1S9 Hypospadia, ii. 423 I. Immaculate conception, i. 170 Indecidua ii. 159, 187 Individuality, i. 123 INDEX. 497 Individuality of cells, i. 123 of metamera, i. 348 Indo- Germanic pedigree, ii. 23 Induction, i. 104 ; ii. 35 Inorganic history of earth, ii. 5 Insects, mental capacity of, ii. 448 Integumentum, ii. 199 Intestinal germ-disc, i. 291 vesicle, i. 291 head-cavity, i. 335 Intestine, after, ii. 321 blind, ii. 330 crooked, ii. 319, 330 digestive, ii. 330 empty, ii. 330 middle, ii. 330 stomach, ii. 330 Invertebrates, i. 414 Iris, ii. 252, 258 Jaeger, Gustav, i. 101 Jaw arches, ii. 102 lower, ii. 102 upper, ii. 102 Jurassic period, ii. 14, 19 K Kant, Immanuel, i. 79 Kidneys, ii. 403. 412 Kidney system, ii. 403 primitive, ii. 306, 410 Kenogenesis, i. 12 Kenogenetic cleavage, i. 231 Kernel of cell (nucleus), i. 127 Kleinenberg, Nicolaus, ii. 358 Kolliker, Albert, i. 59, 62 Kowalevsky, August, i. 59, 441 L Labyrinth of ear, ii. 260 Labyrinthulae, ii. 58 Lamarck, Jean, i. 82 his life, i. 82 Man and Ape, i. 85 " Philosophic Zoologique," i. 83 Lamina dermalis i. 273, 327 gastralis, i. 273, 327 inodermalis, i. 327 inogastralis, i. 327 myxogastralis, i. 327 neuro dermalis, i. 327 Lampreys, ii. 101, 121 Lancelot, i. 253, 413 Laukester, Ray, i. 60 Lanugo, ii. 206 Larynx, ii. 330 Latebra (of bird's egg), i. 138 Laurentian period, ii. 9, 19 Layers (Lamina}), i. 273, 327 Leather-plate, i. 327 skin, ii. 200, 232 Leeuwenhoek, i. 37 Leg, lower, ii. 278 upper, ii. 278 Leibnitz, i. 39 Lemuria, ii. 183 Lemurs, ii. 164 Lens, ii. 251, 254 Lepidosiren paradoxal, ii. 119 Leptocardia, ii. 120 Limbs, ii. Ill, 306 fore, ii. 302 hind, ii. 306 skeleton of, ii. 305 theory of, ii. 305 Linnaeus, Karl, i. 73 Lip-cartilage, ii. 245 fissure, ii. 246 Liver, ii. 330, 341 Lizards, ii. 120, 129 Locomotorium, ii. 194, 274 Lori, ii. 163 Lyell, Charles, i. 77 Lymph-cells, ii. 366 vessels, ii. 373 M Macula, germinativa, i. 133 Magosphcera planv.la, ii. 60 Male breast, ii. 204 cells, i. 171 ; ii. 392 copulatory organs, ii. 423 excretory ducts, ii. 414, 431 germ-glands, ii. 398 germ-lay er, ii. 398 498 INDEX. Male milk-glands, ii. 204 phallus {Penis), ii. 423 sexual organs, ii. 431 sexual plate, ii. 401 — i — uterus, ii. 419 Malpighi, i. 31 Malthas, i. 98 Mamilla, ii. 204 Mamma, li. 201 Mammalia, ii. 141, 187 Mammals, egg-cleavage of, i. 210 gastrula of, i. 213 mental capacities of, ii. 448 Man-apes, ii. 178 Mantle animals, ii. 83 Marsupobrancliii, ii. 104 Marsupialia, ii. 149, 187 Martins, Charles, ii. 304 Materialism, ii. 456 Maternal placenta, ii. 1G0 Matter, ii. 457 Mechanism in nature, i. 80 Meckel's cartilage, ii. 298 Medulla, ii. 211,232 capitis, ii. 211 centralis, ii. 211 oblongata, ii. 211 ————— spinalis, ii. 211 Medullary furrow, i. 302 membranes, ii. 22S plate, i. 327 swellings, i. 303 tube, i. 305 Meninges, ii. 228, 232 Meroblastic eggs, 216 Merogastrula, i. 211 Mesentery, ii. 320 Mesoderma, i. 236, 278 Mesolithic epoch, ii. 14, 19 Mesozoic periods, ii. 12, 14 Metacarpus, ii. 278, 304 Metagaster, ii. 321 Metagastrula, i. 199 Metamera, i. 346 Metameric structure, i. 317 Metanephra, ii. 412 Metatarsus, ii. 278, 304 Metazoa (intestinal animals), i. 248 ; ii.92 Microlestes, ii. 149 Mid-brain, ii. 221, 232 Middle germ-layer, i. 278 ■ intestine, ii. 330 layers, i. 278 part of foot, ii. 278, 304 hand, ii. 278, 304 Migration, theory of, i. 114 Milk, ii. 202 glands, ii. 143, 202 Mind, ii. 226, 447 activity of, ii. 210 cells of, i. 129 development of, ii. 450 — ■ heredity of, ii. 452 Mollusca, ii. 92, 94 Monads, i. 39 Monera, i. 180 ; ii. 43 Monerula, i. 179 JNlonistic philosophy, i. 16; ii. 456 Monocondyles, ii. 138 Monodelphia, ii. 151, 187 Monogeny, i. 160 Monophyletic origin, ii. 277 Monorhina, ii. 101, 120 Monotrema, ii. 145, 187 Monstrous evolution, i. 1G8 Morphogeny, i. 21, 21 Morphology, i. 21 Morphophyly, i. 24 Morula, i. 189 Motor apparatus, ii. 194, 274 germinative layer, i. 330 Mouth, i. 338; ii. 315,330 cavity, ii. 315, 330 groove, i. 338 Mud-fishes, ii. 115, 120 Mulberry -germ, i. 189 Miiller, Fritz, i. 59, 408 Hermann, i. 170 Johannes, i. 59 ; ii. 96, 414 Mullerian duct, ii. 414, 431 Muscles, i. 259 ; ii. 364 j\Iuscle-plate, i. 353 ■ system, ii. 308 Myxinoides, ii. 101, 120 N Nails, ii. 204, 232 Natural history of creation, i. 103 philosophy, i. 82 Navel, i. 315, 335 INDEX 499 Navel arteries, i. 400 cord, i. 384 ; ii. 168 mesentery arteries, i. 395 — veins, i. 399 veins, i. 399 vesicle, i. 337, 377 Neck curvature, i. 369 marrow, ii. 211, 222 — vertebras, ii. 281 Nerve-cells, i. 126 system, ii. 211, 232 Neuro-muscular cells, ii. 232, 236 Nictitating membrane, ii. 259 Nipples of milk-gtands, ii. 202 Nipple-less animals, ii. 146, 204 Nose, i. 374; ii. 247 of Ape, ii. 175 cavities, ii. 245 flaps, ii. 243 furrow, ii. 244 grooves, ii. 244 processes, ii. 242 roofs, ii. 242 Notaspis, i. 297 Nucleolus, i. 133 Nucleus, i. 125 Nutrition, i. 158 Nutritive yelk, i. 216 O CEkology, i. 114 Oken, Lorenz, i. 49 Olfactory grooves, ii. 240 nerve, ii. 239 organ, ii. 239 Ontogenesis (evolution of the germ), i. 12 Ontogenetic fission of the layers, i. 238 unity, i. 366 Ontogeny, i. 5 Oophora, ii. 398, 429 Optic nerve, ii. 250 Orang-outang, ii. 178, 181 Orchides, ii. 399, 429 Organic history of the earth, ii. 7 Organisms without organs, ii. 45 Organogeny, i. 24 Organology, ii. 192 Organ op hyly, i. 24 Organ-systems, age of, ii. 357, 367 • ■ human, ii. 191 Original cleavage, i. 198, 241 Ornithodelphia, ii. 145, 187 Omithorhynchus, ii. 147 Ornithostoma, ii. 117, 187 Os ilium, ii. 278, 304 Os ischii, ii. 278, 304 Os pubis, ii. 278, 304 Outer skin, ii. 200, 232 Ovary, ii. 398, 430 plate, ii. 401 Oviduct, ii. 403, 429 Ovococcus, i. 183 Ovoplasma, i. 183 Ovula holoblasta, i. 215, 241 meroblasta, i. 216, 241 Ovulists, i. 37 Ovidum, i. 171, 183 Pachycardia, i. 120 Palate, ii. 330 roof, ii. 330 soft, ii. 330 Palaeolithic epoch, ii. 10, 19 Palaeontology, i. 106 Palaeozic periods, ii. 12, 13 Palingenesis, i. 10 Palingenetic cleavage, i. 211 Paliugeny, i. 11 Pancreas, ii. 330, 343 Pander, Christian, i. 51 Paradise, ii. 183 Parallelism in evolution, ii. 70 Parent-cell, i. 176 kernel, i. 176 Parovarium, ii. 417, 431 Parthenogenesis, i. 40, 170 Partial cleavage, i. 216, 240 of bird's egg, I 22 1 Tistrana, Julia, i. 374 Pedigree, i. 112 ■ of animals, ii. 93 Apes, ii. 189 cells, i. 467 Indo-Germauic lan- guages, ii. 23 Mammals, ii. 188 man, ii. 23 5oo INDEX. Pedigree of Vertebrates, ii. 121 Pelvic girdle, ii. 278, 304 intestinal cavity, i. 335 Penis, ii. 423, 431 . Pentadactylia, ii. 123, 302 Perigastrula, i. 230, 241 Peripheric nerve- system, ii. 228 Permian period, ii. 10 Petromyzontes, ii. 101, 120 Phallus, ii. 422, 431 Phallusia, i. 454 Pharynx, ii. 316, 330 Philology, ii 20 comparative, ii. 21 Philosophy, i. 17 ; ii. 456 Phylogenesis (evolution of the tribe), i. 12 Phylogenetic fission of the layers, i. 238 ■ hypotheses, i. 413 Phylogeny, i. 5, 72 Physiogeny, i. 21, 24 Physiology, i. 20 comparative, i. 20 Physiophyly, i. 24 Pig, i. 362 Pigment-membrane, ii. 252 Pithecanthropi, ii. 181 Pithecoid theory, ii. 441 Placenta, i. 383 ; ii. 155, 168 disc-shaped, ii. 162, 187 embryonic, ii. 1^60 fcetalis, ii. 160 girdle-shaped, ii. 162, 187 maternal, ii. 160 uterina, ii. 160 Placental animals, ii. 153, 187 Placentalia, ii. 153, 187 Pianola, ii. 61 Planseads, ii. 61 Plant-animals, Planula, ii. 59 Plasson, i. 130 ; ii. 43 Plastids, i. 130 ; ii. 45 Plastid ancestors, ii. 184 Plastid theory, i. 130 Plastidules, ii. 47 Plates {lamellae), i. 303 Plathebninthes, ii. 76 Platyrhinw, ii. 175, 189 Pleuro-peritoneal cavity, i. 260 Pliocene period, ii. 11, 15 Polydactylia, ii. 123 Poms genitalis, ii. 402 Post-glacial period, ii. 11 Pouch-bones, ii. 151 Pouched animals, ii. 149, 187 Prsedelineation theory, i. 37 Prae formation, i. 34 theory, i. 34 Praiputium, ii. 423, 431 Pressure, sense of, ii. 238 Primary axial skeleton, ii. 285 age, ii. 10, 11 germ-layers, i. 196 Primates, ii. 169 Primitive amnion animals, ii. 133 animal ancestors, ii. 184 animals, or Protozoa, i. 248 clavicula, ii. 278 egg (Protovum), i. 131 fins, ii. 303 Fishes, ii. 112, 120 furrow, i. 226 germ-layers, i. 195 groove, i. 335 intestine, i. 444 ; ii. 313 kidney, i. 306 ; ii. 410 ducts, ii. 406 Mammal, ii. 142, 187 Man, ii. 182 mouth, i. 444 ; ii. 313 skull, ii. 296 slime, ii. 43 streak, i. 299 urine-bladder, ii. 411 urine-sac, i. 379; ii. 411 vertebra?, i. 346 vertebral cords, i. 305 vertebral plates, i. 316 - Vertebrate (ideal), i. 256 (real), ii. 98 Worm, ii. 74, 80 Primordial cleavage, i. 198, 211 kidneys, i. 307 ; ii. 410 skull, ii. 297 — times, ii. 9, 11 Prochorion, ii. 157 Procoracoideum, ii. 278, 304 Promammalia, ii. 142, 1S7 Prosimia, ii. 163, 187 Protamosba, ii. 46 Protamnion, ii. 133 Prothelmis, ii. 76 INDEX. 50I Protogaster, i. 444 ; ii. 314 Protomyxa, ii. 46 Protonephra, ii. 410 Protoplasnia, i. 131 Prolopterus annectens, ii. 119 Protozoa, ii. 248 Protureter, ii. 406 Pseudopodia of Ainoebse, i. 142 Psyche, ii. 225, 416 Psychology, ii. 448 Pubic bone, ii. 278 Panctum germinativum, i. 135 Pupil, ii. 252, 258 membrane, ii. 254 R Radius, ii. 278, 304 Ratlike, Heinrich, i. 59 Rathke's duct, ii. 415, 431 Reason, ii. 453 Reichert, Bogulaus, i. 61 Remak, Robert, i. 62 Renes, ii. 412 Reproduction, i. 159 Reproductive organs, ii. 392, 413 Reptiles, ii. 120, 138 Respiratory intestine, i. 262 ; ii. 300 organs, i. 262 j ii. 333 Retina, ii. 252, 258 Ribs, ii. 285 Rolle, Friedrich, i. 101 Round-mouths, ii. 101, 120 Rudimentary organs, i. 109 Rump bone, ii. 282 vertebrae, ii. 282 Rusconi, anus of, i. 206 Rusconi's nutritive cavity, i. 207 s Salamander, ii. 127 Salivary glands, Sauropsida, ii. 138 Scapula, ii. 278, 303 Schleiden, M. J., i. 60, 123 Schwann, Theodor, i. 60 Sclerotica, i. 251, 258 Scolecida, ii. 86 Scrotum, ii. 423, 431 Sea-nettles, ii. 73, 92 Secondary age, ii. 11, 14 axial skeleton, ii. 291 germ-layers, i. 235, 273 kidneys, ii. 412 sexual character, ii. 396 strata of earth, ii. 12 Seed (male), i. 36 animalcules, i. 173 cells, i. 173 duct, ii. 403, 429 Segmental canals, ii. 406 Segmentation, i. 186 Segmentella, i. 186 Selachii, ii. 112, 121 Selection, theory of, i. 95 Semi-apes, ii. 164, 187 Semicircular canals of ear, ii. 263, 268 Semper, Karl, i. 91, 426 Sense of pressure, ii. 238 of warmth, ii. 238 Sense-organs, ii. 238 Sensorium, ii. 194 Sensory apparatus, ii. 194 functions, ii. 238 - layer, i. 236, 229 nerves, ii. 238 Sexes, separation of, ii. 69, 396 Sexual cejls, origin of, cord, ii. 418 ducts, ii. 402, 429 folds, ii. 422, 431 furrow, ii. 422 431 glands, ii. 398 nerves, ii. 238 organs, i. 266 plates, ii. 399 selection, i. 103 ; ii. 394 sense, ii. 238 Sheath (Vagina), ii. 417, 431 of amnion, i. 387 Shin-bone, ii. 278, 304 Shoulder-blade, ii. 278, 304 girdle, ii. 278, 301 Side-layers, i. 303 plates, i. 303 sheath, i. 303 Silurian period, ii. 9, 19 Simian, ii. 165 502 INDEX. Single-nostrils, ii. 101, 120 Sinus urogenitalis, ii. 419, 421 Siredon, ii. 129 Skeleton, ii. 278 Skeleton-forming cell-layer, ii. 287 Skeleton, muscles of, ii. 194 ■ plate, ii. 287 Skin, ii. 195, 229 covering, ii. 195, 232 fibrous layer, i. 236 glands, ii. 201, 232 layer, i. 236 muscle layer, i. 236 muscles, ii. 194 navel, i. 317 nerves, ii. 238 sensory layer, i. 236 stratum, i. 236 Skull, ii. 292 floor, ii. 292 roof, ii. 292 vertebra?, ii. 294 theory of, ii. 295 Skulled Animals, ii. 100 Skull less Animals, i. 416 ; ii. 99 Small brain, ii. 213, 232 Soft-bodied Animals, ii. 92, 94 Soft worms (Scolecida), ii. 86 Sozobranchia, ii. 129 Sozura, ii. 129 Species (idea of), i. 73, 115 Sperma, i. 171 Spermaductus, ii. 403, 429 Spermalists, i. 37 Spermatazoa, i. 172 Sperm-cells, i. 172 Spermococcus, i. 183 Spermoplasma, i. 183 Spermulum, i. 183 Spine, ii. 280 Spiritualism, ii. 456 Spoke-bone (Radius), ii. 278, 304 Sponges, ii. 73, 92 Spontaneous generation, ii. 30 Star-animals, i. 435 Stenops, ii. 164 Sternum, ii. 278 Stomach, ii. 330 intestine, ii. 330 Struggle for existence, i. 95 Subcutis, ii. 232 Sucking worms, ii. 76 Superficial cleavage, i. 229 Sweat-glands, ii. 202 Swimming-bladder, ii. Ill, 335 Sylvian aqueduct, ii. 221 Synamcebium, ii. 56 System of animals, ii. 92 germ-layers, i. 273, 327 mammals, ii. 1S8 organs, ii. 191 tissues, ii. 366 vertebrates, ii. 126 T Tadpoles, ii. 128 Tail, human, i. 372 ; ii. 283 Tail-cap, i. 387 curvature, i. 369 sheath, i. 387 vertebrae, ii. 283 Tailed apes, ii. 180 Batrachia, ii. 129 Tarsus, ii. 278 Taste, nerve o!", ii. 238 — sense of, ii. 238 Teeth, ii. 173, 331 Tegumentum, ii. 199 Teleology, i. 16, 109 Teleostei, ii. 115, 120 Terminal budding, i. 349 Tertiary age, ii. 11, 15 Testes, ii. 399, 429 change of place of, ii. 419 sac, ii. 423, 431 Testiculi, ii. 399, 429 Theoria Generationis, i. 41 Thorax, ii. 282 Thyroid gland, ii. 336 Tibia, ii. 278, 301 Tissues, ii. 362, 366 age of, ii. 361, 366 connective, ii. 363 covering, ii. 361, 366 vascular, ii. 361, 366 Total cleavage, i. 217, 242 Tongue, ii. 331 arch, ii. 296 bone, ii. 298 Tortoise, ii. 120 Touch bodies, ii. 220 INDEX. 503 Touch, organ of, ii. 190, 238 Transition forms, i. 117 Tread, i. 138 Triassic period, ii. 14, 19 Tribal history, i. 7, 24 Trophic germ-layer, i. 239 Tub mF allodia-, ii. 431 Tube-hearts, ii. 120 Tunicata, ii. 83, 92 Turbellaria, ii. 79 Twixt-brain, ii. 220, 232 jaw, ii. 246 Tympanic cavity, ii. 261, 268 membrane, ii. 261, 268 Tympanum, ii. 261, 268 Types in animal kingdom, i. 56, 216 theory of, i. 56, 246 U Ulna, ii. 278, 304 Ungulata, ii. 160, 188 Unitary conception of the world, i. 17 ; ii. 456 Urachus, ii. 413 Ureter, ii. 413 Urethra, ii. 423, 431 Urinary bladder, ii. 413 ducts, ii. 406 organs, ii. 403 sac, i. 379 sexual cavity, ii. 419 - sexual duct, ii. 403 system, ii. 403 Uterus, ii. 417, 431 bicomis, ii. 418 masculinus, ii. 419 Uvula, ii. 430 Vagina, ii. 417, 431 Vampyrella, ii. 48 Van Beneden, Eduard, i. 60, 209 ; ii. 398 Vasa deferentia, ii. 403, 429 umbilicalia i. 399 Vascular system, ii. 38 i Vegetative germ-layer, i. 196, 327 organs, ii. 193, 194 Veins, i. 393 Ventral cavity, i. 316 ■ plates, i. 316 vessel, i. 423 wall, i. 316 Ventricle of heart, ii. 374 Vermal appendage of coecuni, ii. 3 14 Vertebra?, ii. 280 number of, ii. 283 Vertebral arches, ii. 284 bodies, ii. 284 canal, ii. 284 column, ii. 280 Vertebrarium, ii. 278 Vertebrates, ii. 92 Vertebrates, ancestors of, ii. 185 mental capacities of, ii. 446 pedigree of, ii. 93 system of, ii. 97 Vesicula blastodermica , i. 290 germinativa, i. 133 prostatica, ii. 419 umbilicalis, i. 377 Vestibulum vagina?, ii. 431 Virginal generation, i. 170 Vitellus, i. 135 w Wagner, Mokitz, i. 114 Wallace, Alfred, i. 98, 99 Water, amount of in body, ii. 7 Whale-like Animals, ii. 160, 189 Whales, ii. 160 Wolff, Caspar Friedrich, i. 40 his life, i. 41 his Natural Philosophy, i. 47 on formation of intestine, i. 44 on germ -layers, i. 45 Theoria Generationis, i. 41 Wolffian bodies, ii. 411 duct, ii. 414, 431 Wolff's primitive kidneys, ii. 411, 431 Woolly hair of embryo, ii. 206 Worms, i. 246 ; ii. 74 ancestors of, ii. 73 tribe of, ii. 73 Wrist, ii. 278 504 INDEX. Yelk, i. 135 arteries, i. 395 cavity, i. 138 duct, i. 338; ii. 1G8 formative, i. 216 membrane, i. 138 nutritive, i. 216 sac, i. 337 Yelk veins, i. 395 vessels, i. 395 Z Zona pellucida, i. 135 Zonaplacentalia, ii. 162, 187 Zoojphytdf i. 246 ; ii. 73 V THE END. /37