XXX. RESULTS OF ANTHROPOGENY
Now that we have traversed the wonderful region of human embryology and are familiar with the principal parts of it, it will be well to look back on the way we have come, and forward to the further path to truth to which it has led us. We started from the simplest facts of ontogeny, or the development of the individual—from observations that we can repeat and verify by microscopic and anatomic study at any moment. The first and most important of these facts is that every man, like every other animal, begins his existence as a simple cell. This round ovum has the same characteristic form and origin as the ovum of any other mammal. From it is developed in the same manner in all the Placentals, by repeated cleavage, a multicellular blastula. This is converted into a gastrula, and this in turn into a blastocystis (or embryonic vesicle). The two strata of cells that compose its wall are the primary germinal layers, the skin-layer (ectoderm), and gut-layer (entoderm). This two-layered embryonic form is the ontogenetic reproduction of the extremely important phylogenetic stem-form of all the Metazoa, which we have called the Gastræa. As the human embryo passes through the gastrula-form like that of all the other Metazoa, we can trace its phylogenetic origin to the Gastræa.
As we continued to follow the embryonic development of the two-layered structure, we saw that first a third, or middle layer (mesoderm), appears between the two primary layers; when this divides into two, we have the four secondary germinal layers. These have just the same composition and genetic significance in man as in all the other Vertebrates. From the skin-sense layer are developed the epidermis, the central nervous system, and the chief part of the sense-organs. The skin-fibre layer forms the corium and the motor organs—the skeleton and the muscular system. From the gut-fibre layer are developed the vascular system, the muscular wall of the gut, and the sexual glands. Finally, the gut-gland layer only forms the epithelium, or the inner cellular stratum of the mucous membrane of the alimentary canal and glands (lungs, liver, etc.).
The manner in which these different systems of organs arise from the secondary germinal layers is essentially the same from the start in man as in all the other Vertebrates. We saw, in studying the embryonic development of each organ, that the human embryo follows the special lines of differentiation and construction that are only found otherwise in the Vertebrates. Within the limits of this vast stem we have followed, step by step, the development both of the body as a whole and of its various parts. This higher development follows in the human embryo the form that is peculiar to the mammals. Finally, we saw that, even within the limits of this class, the various phylogenetic stages that we distinguish in a natural classification of the mammals correspond to the ontogenetic stages that the human embryo passes through in the course of its evolution. We were thus in a position to determine precisely the position of man in this class, and so to establish his relationship to the different orders of mammals.
The line of argument we followed in this explanation of the ontogenetic facts was simply a consistent application of the biogenetic law. In this we have throughout taken strict account of the distinction between palingenetic and cenogenetic phenomena. Palingenesis (or “synoptic development”) alone enables us to draw conclusions from the observed embryonic form to the stem-form preserved by heredity. Such inference becomes more or less precarious when there has been cenogenesis, or disturbance of development, owing to fresh adaptations. We cannot understand embryonic development unless we appreciate this very important distinction. Here we stand at the very limit that separates the older and the new science or philosophy of nature. The whole of the results of recent morphological research compel us irresistibly to recognise the biogenetic law and its far-reaching consequences. These are, it is true, irreconcilable with the legends and doctrines of former days, that have been impressed on us by religious education. But without the biogenetic law, without the distinction between palingenesis and cenogenesis, and without the theory of evolution on which we base it, it is quite impossible to understand the facts of organic development; without them we cannot cast the faintest gleam of explanation over this marvellous field of phenomena. But when we recognise the causal correlation of ontogeny and phylogeny expressed in this law, the wonderful facts of embryology are susceptible of a very simple explanation; they are found to be the necessary mechanical effects of the evolution of the stem, determined by the laws of heredity and adaptation. The correlative action of these laws under the universal influence of the struggle for existence, or—as we may say in a word, with Darwin—“natural selection,” is entirely adequate to explain the whole process of embryology in the light of phylogeny. It is the chief merit of Darwin that he explained by his theory of selection the correlation of the laws of heredity and adaptation that Lamarck had recognised, and pointed out the true way to reach a causal interpretation of evolution.
The phenomenon that it is most imperative to recognise in this connection is the inheritance of functional variations. Jean Lamarck was the first to appreciate its fundamental importance in 1809, and we may therefore justly give the name of Lamarckism to the theory of descent he based on it. Hence the radical opponents of the latter have very properly directed their attacks chiefly against the former. One of the most distinguished and most narrow-minded of these opponents, Wilhelm His, affirms very positively that “characteristics acquired in the life of the individual are not inherited.”
The inheritance of acquired characters is denied, not only by thorough opponents of evolution, but even by scientists who admit it and have contributed a good deal to its establishment, especially Weismann, Galton, Ray Lankester, etc. Since 1884 the chief opponent has been August Weismann, who has rendered the greatest service in the development of Darwin’s theory of selection. In his work on The Continuity of the Germ-plasm, and in his recent excellent Lectures on the Theory of Descent (1902), he has with great success advanced the opinion that “only those characters can be transmitted to subsequent generations that were contained in rudimentary form in the embryo.” However, this germ-plasm theory, with its attempt to explain heredity, is merely a “provisional molecular hypothesis”; it is one of those metaphysical speculations that attribute the evolutionary phenomena exclusively to internal causes, and regard the influence of the environment as insignificant. Herbert Spencer, Theodor Eimer, Lester Ward, Hering, and Zehnder have pointed out the untenable consequences of this position. I have given my view of it in the tenth edition of the History of Creation (pp. 192, 203). I hold, with Lamarck and Darwin, that the hereditary transmission of acquired characters is one of the most important phenomena in biology, and is proved by thousands of morphological and physiological experiences. It is an indispensable foundation of the theory of evolution.
Of the many and weighty arguments for the truth of this conception of evolution I will for the moment merely point to the invaluable evidence of dysteleology, the science of rudimentary organs. We cannot insist too often or too strongly on the great morphological significance of these remarkable organs, which are completely useless from the physiological point of view. We find some of these useless parts, inherited from our lower vertebrate ancestors, in every system of organs in man and the higher Vertebrates. Thus we find at once on the skin a scanty and rudimentary coat of hair, only fully developed on the head, under the shoulders, and at a few other parts of the body. The short hairs on the greater part of the body are quite useless and devoid of physiological value; they are the last relic of the thicker hairy coat of our simian ancestors. The sensory apparatus presents a series of most remarkable rudimentary organs. We have seen that the whole of the shell of the external ear, with its cartilages, muscles, and skin, is in man a useless appendage, and has not the physiological importance that was formerly ascribed to it. It is the degenerate remainder of the pointed, freely moving, and more advanced mammal ear, the muscles of which we still have, but cannot work them. We found at the inner corner of our eye a small, curious, semi-lunar fold that is of no use whatever to us, and is only interesting as the last relic of the nictitating membrane, the third, inner eye-lid that had a distinct physiological purpose in the ancient sharks, and still has in many of the Amniotes.
The motor apparatus, in both the skeleton and muscular systems, provides a number of interesting dysteleological arguments. I need only recall the projecting tail of the human embryo, with its rudimentary caudal vertebræ and muscles; this is totally useless in man, but very interesting as the degenerate relic of the long tail of our simian ancestors. From these we have also inherited various bony processes and muscles, which were very useful to them in climbing trees, but are useless to us. At various points of the skin we have cutaneous muscles which we never use—remnants of a strongly-developed cutaneous muscle in our lower mammal ancestors. This “panniculus carnosus” had the function of contracting and creasing the skin to chase away the flies, as we see every day in the horse. Another relic in us of this large cutaneous muscle is the frontal muscle, by which we knit our forehead and raise our eye-brows; but there is another considerable relic of it, the large cutaneous muscle in the neck (platysma myoides), over which we have no voluntary control.
Not only in the systems of animal organs, but also in the vegetal apparatus, we find a number of rudimentary organs, many of which we have already noticed. In the alimentary apparatus there are the thymus-gland and the thyroid gland, the seat of goitre and the relic of a ciliated groove that the Tunicates and Acrania still have in the gill-pannier; there is also the vermiform appendix to the cæcum. In the vascular system we have a number of useless cords which represent relics of atrophied vessels that were once active as blood-canals—the ductus Botalli between the pulmonary artery and the aorta, the ductus venosus Arantii between the portal vein and the vena cava, and many others. The many rudimentary organs in the urinary and sexual apparatus are particularly interesting. These are generally developed in one sex and rudimentary in the other. Thus the spermaducts are formed from the Wolffian ducts in the male, whereas in the female we have merely rudimentary traces of them in Gaertner’s canals. On the other hand, in the female the oviducts and womb are developed from the Mullerian ducts, while in the male only the lowest ends of them remain as the “male womb” (vesicula prostatica). Again, the male has in his nipples and mammary glands the rudiments of organs that are usually active only in the female.
A careful anatomic study of the human frame would disclose to us numbers of other rudimentary organs, and these can only be explained on the theory of evolution. Robert Wiedersheim has collected a large number of them in his work on The Human Frame as a Witness to its Past. They are some of the weightiest proofs of the truth of the mechanical conception and the strongest disproofs of the teleological view. If, as the latter demands, man or any other organism had been designed and fitted for his life-purposes from the start and brought into being by a creative act, the existence of these rudimentary organs would be an insoluble enigma; it would be impossible to understand why the Creator had put this useless burden on his creatures to walk a path that is in itself by no means easy. But the theory of evolution gives the simplest possible explanation of them. It says: The rudimentary organs are parts of the body that have fallen into disuse in the course of centuries; they had definite functions in our animal ancestors, but have lost their physiological significance. On account of fresh adaptations they have become superfluous, but are transmitted from generation to generation by heredity, and gradually atrophy.
We have inherited not only these rudimentary parts, but all the organs of our body, from the mammals—proximately from the apes. The human body does not contain a single organ that has not been inherited from the apes. In fact, with the aid of our biogenetic law we can trace the origin of our various systems of organs much further, down to the lowest stages of our ancestry. We can say, for instance, that we have inherited the oldest organs of the body, the external skin and the internal coat of the alimentary system, from the Gastræads; the nervous and muscular systems from the Platodes; the vascular system, the body-cavity, and the blood from the Vermalia; the chorda and the branchial gut from the Prochordonia; the articulation of the body from the Acrania; the primitive skull and the higher sense-organs from the Cyclostomes; the limbs and jaws from the Selachii; the five-toed foot from the Amphibia; the palate from the Reptiles; the hairy coat, the mammary glands, and the external sexual organs from the Pro-mammals. When we formulated “the law of the ontogenetic connection of systematically related forms,” and determined the relative age of organs, we saw how it was possible to draw phylogenetic conclusions from the ontogenetic succession of systems of organs.
With the aid of this important law and of comparative anatomy we were also enabled to determine “man’s place in nature,” or, as we put it, assign to man his position in the classification of the animal kingdom. In recent zoological classification the animal world is divided into twelve stems or phyla, and these are broadly sub-divided into about sixty classes, and these classes into at least 300 orders. In his whole organisation man is most certainly, in the first place, a member of one of these stems, the vertebrate stem; secondly, a member of one particular class in this stem, the Mammals; and thirdly, of one particular order, the order of Primates. He has all the characteristics that distinguish the Vertebrates from the other eleven animal stems, the Mammals from the other sixty classes, and the Primates from the 300 other orders of the animal kingdom. We may turn and twist as we like, but we cannot get over this fact of anatomy and classification. Of late years this fact has given rise to a good deal of discussion, and especially of controversy as to the particular anatomic relationship of man to the apes. The most curious opinions have been advanced on this “ape-question,” or “pithecoid-theory.” It is as well, therefore, to go into it once more and distinguish the essential from the unessential. (Cf. pp. 261–5.)
We start from the undisputed fact that man is in any case—whether we accept or reject his special blood-relationship to the apes—a true mammal; in fact, a placental mammal. This fundamental fact can be proved so easily at any moment from comparative anatomy that it has been universally admitted since the separation of the Placentals from the lower mammals (Marsupials and Monotremes). But for every consistent subscriber to the theory of evolution it must follow at once that man descends from a common stem-form with all the other Placentals, the stem-ancestor of the Placentals, just as we must admit a common mesozoic ancestor of all the mammals. This is, however, to settle decisively the great and burning question of man’s place in nature, whether or no we go on to admit a nearer or more distant relationship to the apes. Whether man is or is not a member of the ape-order (or, if you prefer, the primate-order.) in the phylogenetic sense, in any case his direct blood-relationship to the rest of the mammals, and especially the Placentals, is established. It is possible that the affinities of the various orders of mammals to each other are different from what we hypothetically assume to-day. But, in any case, the common descent of man and all the other mammals from one stem-form is beyond question. This long-extinct Promammal was probably evolved from Proreptiles during the Triassic period, and must certainly be regarded as the monotreme and oviparous ancestor of all the mammals.
If we hold firmly to this fundamental and most important thesis, we shall see the “ape-question” in a very different light from that in which it is usually regarded. Little reflection is then needed to see that it is not nearly so important as it is said to be. The origin of the human race from a series of mammal ancestors, and the historic evolution of these from an earlier series of lower vertebrate ancestors, together with all the weighty conclusions that every thoughtful man deduces therefrom, remain untouched; so far as these are concerned, it is immaterial whether we regard true “apes” as our nearest ancestors or not. But as it has become the fashion to lay the chief stress in the whole question of man’s origin on the “descent from the apes,” I am compelled to return to it once more, and recall the facts of comparative anatomy and ontogeny that give a decisive answer to this “ape-question.”
The shortest way to attain our purpose is that followed by Huxley in 1863 in his able work, which I have already often quoted, Man’s Place in Nature—the way of comparative anatomy and ontogeny. We have to compare impartially all man’s organs with the same organs in the higher apes, and then to examine if the differences between the two are greater than the corresponding differences between the higher and the lower apes. The indubitable and incontestable result of this comparative-anatomical study, conducted with the greatest care and impartiality, was the pithecometra-principle, which we have called the Huxleian law in honour of its formulator—namely, that the differences in organisation between man and the most advanced apes we know are much slighter than the corresponding differences in organisation between the higher and lower apes. We may even give a more precise formula to this law, by excluding the Platyrrhines or American apes as distant relatives, and restricting the comparison to the narrower family-circle of the Catarrhines, the apes of the Old World. Within the limits of this small group of mammals we found the structural differences between the lower and higher catarrhine apes—for instance, the baboon and the gorilla—to be much greater than the differences between the anthropoid apes and man. If we now turn to ontogeny, and find, according to our “law of the ontogenetic connection of systematically related forms,” that the embryos of the anthropoid apes and man retain their resemblance for a longer time than the embryos of the highest and the lowest apes, we are forced, whether we like it or no, to recognise our descent from the order of apes. We can assuredly construct an approximate picture in the imagination of the form of our early Tertiary ancestors from the foregoing facts of comparative anatomy; however we may frame this in detail, it will be the picture of a true ape, and a distinct catarrhine ape. This has been shown so well by Huxley (1863) that the recent attacks of Klaatsch, Virchow, and other anthropologists, have completely failed (cf. pp.263–264). All the structural characters that distinguish the Catarrhines from the Platyrrhines are found in man. Hence in the genealogy of the mammals we must derive man immediately from the catarrhine group, and locate the origin of the human race in the Old World. Only the early root-form from which both descended was common to them.
It is, therefore, established beyond question for all impartial scientific inquiry that the human race comes directly from the apes of the Old World; but, at the same time, I repeat that this is not so important in connection with the main question of the origin of man as is commonly supposed. Even if we entirely ignore it, all that we have learned from the zoological facts of comparative anatomy and ontogeny as to the placental character of man remains untouched. These prove beyond all doubt the common descent of man and all the rest of the mammals. Further, the main question is not in the least affected if it is said: “It is true that man is a mammal; but he has diverged at the very root of the class from all the other mammals, and has no closer relationship to any living group of mammals.” The affinity is more or less close in any case, if we examine the relation of the mammal class to the sixty other classes of the animal world. Quite certainly the whole of the mammals, including man, have had a common origin; and it is equally certain that their common stem-forms were gradually evolved from a long series of lower Vertebrates.
The resistance to the theory of a descent from the apes is clearly due in most men to feeling rather than to reason. They shrink from the notion of such an origin just because they see in the ape organism a caricature of man, a distorted and unattractive image of themselves, because it hurts man’s æsthetic complacency and self-ennoblement. It is more flattering to think we have descended from some lofty and god-like being; and so, from the earliest times, human vanity has been pleased to believe in our origin from gods or demi-gods. The Church, with that sophistic reversal of ideas of which it is a master, has succeeded in representing this ridiculous piece of vanity as “Christian humility”; and the very men who reject with horror the notion of an animal origin, and count themselves “children of God,” love to prate of their “humble sense of servitude.” In most of the sermons that have poured out from pulpit and altar against the doctrine of evolution human vanity and conceit have been a conspicuous element; and, although we have inherited this very characteristic weakness from the apes, we must admit that we have developed it to a higher degree, which is entirely repudiated by sound and normal intelligence. We are greatly amused at all the childish follies that the ridiculous pride of ancestry has maintained from the Middle Ages to our own time; yet there is a large amount of this empty feeling in most men. Just as most people much prefer to trace their family back to some degenerate baron or some famous prince rather than to an unknown peasant, so most men would rather have as parent of the race a sinful and fallen Adam than an advancing, and vigorous ape. It is a matter of taste, and to that extent we cannot quarrel over these genealogical tendencies. Personally, the notion of ascent is more congenial to me than that of descent. It seems to me a finer thing to be the advanced offspring of a simian ancestor, that has developed progressively from the lower mammals in the struggle for life, than the degenerate descendant of a god-like being, made from a clod, and fallen for his sins, and an Eve created from one of his ribs. Speaking of the rib, I may add to what I have said about the development of the skeleton, that the number of ribs is just the same in man and woman. In both of them the ribs are formed from the middle germinal layer, and are, from the phylogenetic point of view, lower or ventral vertebral arches.
But it is said: “That is all very well, as far as the human body is concerned; on the facts quoted it is impossible to doubt that it has really and gradually been evolved from the long ancestral series of the Vertebrates. But it is quite another thing as regards man’s mind, or soul; this cannot possibly have been developed from the vertebrate-soul.”[35] Let us see if we cannot meet this grave stricture from the well-known facts of comparative anatomy, physiology, and embryology. It will be best to begin with a comparative study of the souls of various groups of Vertebrates. Here we find such an enormous variety of vertebrate souls that, at first sight, it seems quite impossible to trace them all to a common “Primitive Vertebrate.” Think of the tiny Amphioxus, with no real brain but a simple medullary tube, and its whole psychic life at the very lowest stage among the Vertebrates. The following group of the Cyclostomes are still very limited, though they have a brain. When we pass on to the fishes, we find their intelligence remaining at a very low level. We do not see any material advance in mental development until we go on to the Amphibia and Reptiles. There is still greater advance when we come to the Mammals, though even here the minds of the Monotremes and of the stupid Marsupials remain at a low stage. But when we rise from these to the Placentals we find within this one vast group such a number of important stages of differentiation and progress that the psychic differences between the least intelligent (such as the sloths and armadillos) and the most intelligent Placentals (such as the dogs and apes) are much greater than the psychic differences between the lowest Placentals and the Marsupials or Monotremes. Most certainly the differences are far greater than the differences in mental power between the dog, the ape, and man. Yet all these animals are genetically-related members of a single natural class.
[35] The English reader will recognise here the curious position of Dr. Wallace and of the late Dr. Mivart.—Translator.
We see this to a still more astonishing extent in the comparative psychology of another class of animals, that is especially interesting for many reasons—the insect class. It is well known that we find in many insects a degree of intelligence that is found in man alone among the Vertebrates. Everybody knows of the famous communities and states of bees and ants, and of the very remarkable social arrangements in them, such as we find among the more advanced races of men, but among no other group of animals. I need only mention the social organisation and government of the monarchic bees and the republican ants, and their division into different conditions—queen, drone-nobles, workers, educators, soldiers, etc. One of the most remarkable phenomena in this very interesting province is the cattle-keeping of the ants, which rear plant-lice as milch-cows and regularly extract their honeyed juice. Still more remarkable is the slave-holding of the large red ants, which steal the young of the small black ants and bring them up as slaves. It has long been known that these political and social arrangements of the ants are due to the deliberate cooperation of the countless citizens, and that they understand each other. A number of recent observers, especially Fritz Müller, Sir J. Lubbock (Lord Avebury), and August Forel, have put the astonishing degree of intelligence of these tiny Articulates beyond question.
Now, compare with these the mental life of many of the lower, especially the parasitic insects, as Darwin did. There is, for instance, the cochineal insect (Coccus), which, in its adult state, has a motionless, shield-shaped body, attached to the leaves of plants. Its feet are atrophied. Its snout is sunk in the tissue of the plants of which it absorbs the sap. The whole psychic life of these inert female parasites consists in the pleasure they experience from sucking the sap of the plant and in sexual intercourse with the males. It is the same with the maggot-like females of the fan-fly (Strepsitera), which spend their lives parasitically and immovably, without wings or feet, in the abdomen of wasps. There is no question here of higher psychic action. If we compare these sluggish parasites with the intelligent and active ants, we must admit that the psychic differences between them are much greater than the psychic differences between the lowest and highest mammals, between the Monotremes, Marsupials, and armadillos on the one hand, and the dog, ape, or man on the other. Yet all these insects belong to the same class of Articulates, just as all the mammals belong to one and the same class. And just as every consistent evolutionist must admit a common stem-form for all these insects, so he must also for all the mammals.
If we now turn from the comparative study of psychic life in different animals to the question of the organs of this function, we receive the answer that in all the higher animals they are always bound up with certain groups of cells, the ganglionic cells or neurona that compose the nervous system. All scientists without exception are agreed that the central nervous system is the organ of psychic life in the animal, and it is possible to prove this experimentally at any moment. When we partially or wholly destroy the central nervous system, we extinguish in the same proportion, partially or wholly, the “soul” or psychic activity of the animal. We have, therefore, to examine the features of the psychic organ in man. The reader already knows the incontestable answer to this question. Man’s psychic organ is, in structure and origin, just the same organ as in all the other Vertebrates. It originates in the shape of a simple medullary tube from the outer membrane of the embryo—the skin-sense layer. The simple cerebral vesicle that is formed by the expansion of the head-part of this medullary tube divides by transverse constrictions into five, and these pass through more or less the same stages of construction in the human embryo as in the rest of the mammals. As these are undoubtedly of a common origin, their brain and spinal cord must also have a common origin.
Physiology teaches us further, on the ground of observation and experiment, that the relation of the “soul” to its organ, the brain and spinal cord, is just the same in man as in the other mammals. The one cannot act at all without the other; it is just as much bound up with it as muscular movement is with the muscles. It can only develop in connection with it. If we are evolutionists at all, and grant the causal connection of ontogenesis and phylogenesis, we are forced to admit this thesis: The human soul or psyche, as a function of the medullary tube, has developed along with it; and just as brain and spinal cord now develop from the simple medullary tube in every human individual, so the human mind or the psychic life of the whole human race has been gradually evolved from the lower vertebrate soul. Just as to-day the intricate structure of the brain proceeds step by step from the same rudiment in every human individual—the same five cerebral vesicles—as in all the other Craniotes; so the human soul has been gradually developed in the course of millions of years from a long series of craniote-souls. Finally, just as to-day in every human embryo the various parts of the brain differentiate after the special type of the ape-brain, so the human psyche has proceeded historically from the ape-soul.
It is true that this Monistic conception is rejected with horror by most men, and the Dualistic idea, which denies the inseparable connection of brain and mind, and regards body and soul as two totally different things, is still popular. But how can we reconcile this view with the known facts of evolution? It meets with difficulties equally great and insuperable in embryology and in phylogeny. If we suppose with the majority of men that the soul is an independent entity, which has nothing to do with the body originally, but merely inhabits it for a time, and gives expression to its experiences through the brain just as the pianist does through his instrument, we must assign a point in human embryology at which the soul enters into the brain; and at death again we must assign a moment at which it abandons the body. As, further, each human individual has inherited certain personal features from each parent, we must suppose that in the act of conception pieces were detached from their souls and transferred to the embryo. A piece of the paternal soul goes with-the spermatozoon, and a piece of the mother’s soul remains in the ovum. At the moment of conception, when portions of the two nuclei of the copulating cells join together to form the nucleus of the stem-cell, the accompanying fragments of the immaterial souls must also be supposed to coalesce.
On this Dualistic view the phenomena of psychic development are totally incomprehensible. Everybody knows that the new-born child has no consciousness, no knowledge of itself and the surrounding world. Every parent who has impartially followed the mental development of his children will find it impossible to deny that it is a case of biological evolutionary processes. Just as all other functions of the body develop in connection with their organs, so the soul does in connection with the brain. This gradual unfolding of the soul of the child is, in fact, so wonderful and glorious a phenomenon that every mother or father who has eyes to observe is never tired of contemplating it. It is only our manuals of psychology that know nothing of this development; we are almost tempted to think sometimes that their authors can never have had children themselves. The human soul, as described in most of our psychological works, is merely the soul of a learned philosopher, who has read a good many books, but knows nothing of evolution, and never even reflects that his own soul has had a development.
When these Dualistic philosophers are consistent they must assign a moment in the phylogeny of the human soul at which it was first “introduced” into man’s vertebrate body. Hence, at the time when the human body was evolved from the anthropoid body of the ape (probably in the Tertiary period), a specific human psychic element—or, as people love to say, “a spark of divinity”—must have been suddenly infused or breathed into the anthropoid brain, and been associated with the ape-soul already present in it. I need not insist on the enormous theoretical difficulties of this idea. I will only point out that this “spark of divinity,” which is supposed to distinguish the soul of man from that of the other animals, must be itself capable of development, and has, as a matter of fact, progressively developed in the course of human history. As a rule, reason is taken to be this “spark of divinity,” and is supposed to be an exclusive possession of humanity. But comparative psychology shows us that it is quite impossible to set up this barrier between man and the brute. Either we take the word “reason” in the wider sense, and then it is found in the higher mammals (ape, dog, elephant, horse) just as well as in most men; or else in the narrower sense, and then it is lacking in most men just as much as in the majority of animals. On the whole, we may still say of man’s reason what Goethe’s Mephistopheles said:—
Life somewhat better might content him But for the gleam of heavenly light that Thou hast given him. He calls it reason; thence his power’s increased To be still beastlier than any beast.
If, then, we must reject these popular and, in some respects, agreeable Dualistic theories as untenable, because inconsistent with the genetic facts, there remains only the opposite or Monistic conception, according to which the human soul is, like any other animal soul, a function of the central nervous system, and develops in inseparable connection therewith. We see this ontogenetically in every child. The biogenetic law compels us to affirm it phylogenetically. Just as in every human embryo the skin-sense layer gives rise to the medullary tube, from the anterior end of which the five cerebral vesicles of the Craniotes are developed, and from these the mammal brain (first with the characters of the lower, then with those of the higher mammals); and as the whole of this ontogenetic process is only a brief, hereditary reproduction of the same process in the phylogenesis of the Vertebrates; so the wonderful spiritual life of the human race through many thousands of years has been evolved step by step from the lowly psychic life of the lower Vertebrates, and the development of every child-soul is only a brief repetition of that long and complex phylogenetic process. From all these facts sound reason must conclude that the still prevalent belief in the immortality of the soul is an untenable superstition. I have shown its inconsistency with modern science in the eleventh chapter of The Riddle of the Universe.
Here it may also be well to point out the great importance of anthropogeny, in the light of the biogenetic law, for the purposes of philosophy. The speculative philosophers who take cognizance of these ontogenetic facts, and explain them (in accordance with the law) phylogenetically, will advance the great questions of philosophy far more than the most distinguished thinkers of all ages have yet succeeded in doing. Most certainly every clear and consistent thinker must derive from the facts of comparative anatomy and ontogeny we have adduced a number of suggestive ideas that cannot fail to have an influence on the progress of philosophy. Nor can it be doubted that the candid statement and impartial appreciation of these facts will lead to the decisive triumph of the philosophic tendency that we call “Monistic” or “Mechanical,” as opposed to the “Dualistic” or “Teleological,” on which most of the ancient, medieval, and modern systems of philosophy are based. The Monistic or Mechanical philosophy affirms that all the phenomena of human life and of the rest of nature are ruled by fixed and unalterable laws; that there is everywhere a necessary causal connection of phenomena; and that, therefore, the whole knowable universe is a harmonious unity, a monon. It says, further, that all phenomena are due solely to mechanical or efficient causes, not to final causes. It does not admit free-will in the ordinary sense of the word. In the light of the Monistic philosophy the phenomena that we are wont to regard as the freest and most independent, the expressions of the human will, are subject just as much to rigid laws as any other natural phenomenon. As a matter of fact, impartial and thorough examination of our “free” volitions shows that they are never really free, but always determined by antecedent factors that can be traced to either heredity or adaptation. We cannot, therefore, admit the conventional distinction between nature and spirit. There is spirit everywhere in nature, and we know of no spirit outside of nature. Hence, also, the common antithesis of natural science and mental or moral science is untenable. Every science, as such, is both natural and mental. That is a firm principle of Monism, which, on its religious side, we may also denominate Pantheism. Man is not above, but in, nature.
It is true that the opponents of evolution love to misrepresent the Monistic philosophy based on it as “Materialism,” and confuse the philosophic tendency of this name with a wholly unconnected and despicable moral materialism. Strictly speaking, it would be just as proper to call our system Spiritualism as Materialism. The real Materialistic philosophy affirms that the phenomena of life are, like all other phenomena, effects or products of matter. The opposite extreme, the Spiritualistic philosophy, says, on the contrary, that matter is a product of energy, and that all material forms are produced by free and independent forces. Thus, according to one-sided Materialism, the matter is antecedent to the living force; according to the equally one-sided view of the Spiritist, it is the reverse. Both views are Dualistic, and, in my opinion, both are false. For us the antithesis disappears in the Monistic philosophy, which knows neither matter without force nor force without matter. It is only necessary to reflect for some time over the question from the strictly scientific point of view to see that it is impossible to form a clear idea of either hypothesis. As Goethe said, “Matter can never exist or act without spirit, nor spirit without matter.”
The human “spirit” or “soul” is merely a force or form of energy, inseparably bound up with the material sub-stratum of the body. The thinking force of the mind is just as much connected with the structural elements of the brain as the motor force of the muscles with their structural elements. Our mental powers are functions of the brain as much as any other force is a function of a material body. We know of no matter that is devoid of force, and no forces that are not bound up with matter. When the forces enter into the phenomenon as movements we call them living or active forces; when they are in a state of rest or equilibrium we call them latent or potential. This applies equally to inorganic and organic bodies. The magnet that attracts iron filings, the powder that explodes, the steam that drives the locomotive, are living inorganics; they act by living force as much as the sensitive Mimosa does when it contracts its leaves at touch, or the venerable Amphioxus that buries itself in the sand of the sea, or man when he thinks. Only in the latter cases the combinations of the different forces that appear as “movement” in the phenomenon are much more intricate and difficult to analyse than in the former.
Our study has led us to the conclusion that in the whole evolution of man, in his embryology and in his phylogeny, there are no living forces at work other than those of the rest of organic and inorganic nature. All the forces that are operative in it could be reduced in the ultimate analysis to growth, the fundamental evolutionary function that brings about the forms of both the organic and the inorganic. But growth itself depends on the attraction and repulsion of homogeneous and heterogeneous particles. Seventy-five years ago Carl Ernst von Baer summed up the general result of his classic studies of animal development in the sentence: “The evolution of the individual is the history of the growth of individuality in every respect.” And if we go deeper to the root of this law of growth, we find that in the long run it can always be reduced to that attraction and repulsion of animated atoms which Empedocles called the “love and hatred” of the elements.
Thus the evolution of man is directed by the same “eternal, iron laws” as the development of any other body. These laws always lead us back to the same simple principles, the elementary principles of physics and chemistry. The various phenomena of nature only differ in the degree of complexity in which the different forces work together. Each single process of adaptation and heredity in the stem-history of our ancestors is in itself a very complex physiological phenomenon. Far more intricate are the processes of human embryology; in these are condensed and comprised thousands of the phylogenetic processes.
In my General Morphology, which appeared in 1866, I made the first attempt to apply the theory of evolution, as reformed by Darwin, to the whole province of biology, and especially to provide with its assistance a mechanical foundation for the science of organic forms. The intimate relations that exist between all parts of organic science, especially the direct causal nexus between the two sections of evolution—ontogeny and phylogeny—were explained in that work for the first time by transformism, and were interpreted philosophically in the light of the theory of descent. The anthropological part of the General Morphology (Book vii) contains the first attempt to determine the series of man’s ancestors (vol. ii, p. 428). However imperfect this attempt was, it provided a starting-point for further investigation. In the thirty-seven years that have since elapsed the biological horizon has been enormously widened; our empirical acquisitions in paleontology, comparative anatomy, and ontogeny have grown to an astonishing extent, thanks to the united efforts of a number of able workers and the employment of better methods. Many important biological questions that then appeared to be obscure enigmas seem to be entirely settled. Darwinism arose like the dawn of a new day of clear Monistic science after the dark night of mystic dogmatism, and we can say now, proudly and gladly, that there is daylight in our field of inquiry.
Philosophers and others, who are equally ignorant of the empirical sources of our evidence and the phylogenetic methods of utilising it, have even lately claimed that in the matter of constructing our genealogical tree nothing more has been done than the discovery of a “gallery of ancestors,” such as we find in the mansions of the nobility. This would be quite true if the genealogy given in the second part of this work were merely the juxtaposition of a series of animal forms, of which we gathered the genetic connection from their external physiognomic resemblances. As we have sufficiently proved already, it is for us a question of a totally different thing—of the morphological and historical proof of the phylogenetic connection of these ancestors on the basis of their identity in internal structure and embryonic development; and I think I have sufficiently shown in the first part of this work how far this is calculated to reveal to us their inner nature and its historical development. I see the essence of its significance precisely in the proof of historical connection. I am one of those scientists who believe in a real “natural history,” and who think as much of an historical knowledge of the past as of an exact investigation of the present. The incalculable value of the historical consciousness cannot be sufficiently emphasised at a time when historical research is ignored and neglected, and when an “exact” school, as dogmatic as it is narrow, would substitute for it physical experiments and mathematical formulæ. Historical knowledge cannot be replaced by any other branch of science.
It is clear that the prejudices that stand in the way of a general recognition of this “natural anthropogeny” are still very great; otherwise the long struggle of philosophic systems would have ended in favour of Monism. But we may confidently expect that a more general acquaintance with the genetic facts will gradually destroy these prejudices, and lead to the triumph of the natural conception of “man’s place in nature.” When we hear it said, in face of this expectation, that this would lead to retrogression in the intellectual and moral development of mankind, I cannot refrain from saying that, in my opinion, it will be just the reverse; that it will promote to an enormous extent the advance of the human mind. All progress in our knowledge of truth means an advance in the higher cultivation of the human intelligence; and all progress in its application to practical life implies a corresponding improvement of morality. The worst enemies of the human race—ignorance and superstition—can only be vanquished by truth and reason. In any case, I hope and desire to have convinced the reader of these chapters that the true scientific comprehension of the human frame can only be attained in the way that we recognise to be the sole sound and effective one in organic science generally—namely, the way of Evolution.
INDEX
A
Abiogenesis, 26
Accipenser, 234
Abortive ova, 55
Achromatin, 42
Achromin, 42
Acœla, 221
Acoustic nerve, the, 289, 290
Acquired characters, inheritance of, 349
Acrania, the, 182
Acroganglion, the, 268, 275
Adam’s apple, the, 184
Adapida, 257
Adaptation, 3, 5, 27
After-birth, the, 167
Agassiz, L., 34
Age of life, 200
Alimentary canal, evolution of the, 13, 14, 133, 308–17 — — structure of the, 169, 308–10
Allantoic circulation, the, 171
Allantois, development of the, 166
Allmann, 20
Amblystoma, 243
Amitotic cleavage, 40
Ammoconida, 217
Ammolynthus, 217
Amnion, the, 115 — formation of the, 134, 244
Amniotic fluid, the, 134
Amœba, the, 47–9, 210
Amphibia, the, 239
Amphichœrus, 221
Amphigastrula, 80
Amphioxus, the, 105, 181–95 — circulation of the, 184 — cœlomation of the, 95 — embryology of the, 191–95 — structure of the, 183–88
Amphirhina, 230
Anamnia, the, 115
Anatomy, comparative, 208
Animalculists, 12
Animal layer, the, 16
Annelids, the, 142, 219
Annelid theory, the, 142
Anomodontia, 246
Ant, intelligence of the, 353
Anthropithecus, 174, 262
Anthropogeny, 1
Anthropoid apes, the, 166, 173, 262
Anthropology, 1, 35
Anthropozoic period, 203
Antimera, 107
Anura, 243
Anus, the, 317
Anus, formation of the, 139
Aorta, the, 327 — development of the, 170
Ape and man, 157, 164, 261, 307, 351
Ape-man, the, 263
Apes, the, 257–60
Aphanocapsa, 210
Aphanostomum, 221
Appendicaria, 197
Appendix vermiformis, the, 32
Aquatic life, early prevalence of, 235
Ararat, Mount, 24
Archenteron, 64, 74
Archeolithic age, 203
Archicaryon, 55
Archicrania, 230
Archigastrula, 65, 193
Archiprimas, 263
Arctopitheca, 261
Area, the germinative, 121
Aristotle, 9
Arm, structure of the, 306
Arrow-worm, the, 191
Arterial arches, the, 325–26 — cone, the, 324
Arteries, evolution of the, 170, 323–24
Articulates, the, 142, 219 — skeleton of the, 294
Articulation, 141–42
Aryo-Romanic languages, the, 203
Ascidia, the, 181, 188–90 — embryology of the, 196–98
Ascula, 217
Asexual reproduction, 51
Atlas, the, 247
Atrium, the, 183, 185 — (heart), the, 326
Auditory nerve, the, 289, 290
Auricles of the heart, 325
Autolemures, 257
Axolotl, the, 243
B
Bacteria, 38, 210
Baer, K. E. von, 15–17
Balanoglossus, 226
Balfour, F., 21
Batrachia, 241
Bdellostoma Stouti, 78
Bee, generation of the, 9
Beyschlag, W., on evolution, 50
Bilateral symmetry, 66 — — origin of, 221
Bimana, 258
Biogenetic law, the, 2, 21, 23, 179, 349
Biogeny, 2
Bionomy, 33
Bird, evolution of the, 245 — ovum of the, 44–6, 80–1
Bischoff, W., 17
Bladder, evolution of the, 244, 339
Blastæa, the, 206, 213
Blastocœl, the, 62, 74
Blastocrene, the, 99
Blastocystis, the, 62, 119, 120
Blastoderm, the, 62
Blastodermic vesicle, the, 119
Blastoporus, the, 64
Blastosphere, the, 62, 119
Blastula, the, 62, 74 — the mammal, 119
Blood, importance of the, 318 — recent experiments in mixture of, 172 — structure of the, 319
Blood-cells, the, 319
Blood-vessels, the, 318–25 — development of the, 168 — of the vertebrate, 110 — origin of the, 320–21
Boniface VIII, Bull of, 10
Bonnet, 13
Borneo nosed-ape, the, 164
Boveri, Theodor, 185
Brachytarsi, 257
Brain and mind, 278, 354–56 — evolution of the, 8, 275–80 — in the fish, 276 — in the lower animals, 275 — structure of the, 273–74
Branchial arches, evolution of the, 303 — cavity, the, 183, 189 — system, the, 110
Branchiotomes, 149
Breasts, the, 113
Bulbilla, 184
C
Calamichthys, 234
Calcolynthus, 217
Capillaries, the, 323
Caracoideum, the, 249
Carboniferous strata, 202
Carcharodon, 234
Cardiac cavity, the, 170
Cardiocœl, the, 328
Caryobasis, 38, 54
Caryokinesis, 42
Caryolymph, 38, 54
Caryolyses, 42
Caryon, 37
Caryoplasm, 37
Catallacta, 213
Catarrhinæ, the, 173, 261
Catastrophic theory, the, 24
Caudate cells, 53
Cell, life of the, 41–3 — nature of the, 36–7 — size of the, 38
Cell theory, the, 18, 36
Cenogenesis, 4
Cenogenetic structures, 4
Cenozoic period, the, 203
Central body, the, 38, 42
Central nervous system, the, 273
Centrolecithal ova, 68
Centrosoma, the, 38, 42
Ceratodus, the, 76, 237
Cerebellum, the, 274
Cerebral vesicles, evolution of the, 276
Cerebrum, the, 273
Cestracion Japonicus, 75, 79
Chætognatha, 94
Chick, importance of the, in embryology, 11, 16
Child, mind of the, 8, 355
Chimpanzee, the, 174, 262
Chiromys, 257
Chiroptera, 258
Chirotherium, 239
Chondylarthra, 257
Chorda, the, 17, 95, 107, 183 — evolution of the, 296
Chordæa, the, 97
Chordalemma, the, 296
Chordaria, 97
Chordula, the, 3, 96, 191
Choriata, the, 166
Chorion, the, 119 — development of the, 165–6 — frondosum, 255 — læve, 255
Choroid coat, the, 286
Chorology, 33
Chromacea, 209
Chromatin, 42
Chroococcacea, 210
Chroococcus, the, 210
Church, opposition of, to science in Middle Ages, 10
Chyle, 318
Chyle-vessels, 324
Cicatricula, the, 45, 81
Ciliated cells, 53, 193
Cinghalese gynecomast, 114
Circulation in the lancelet, 184
Circulatory system, evolution of the, 321–25 — — structure of the, 318
Classification, 103 — evolutionary value of, 33
Clitoris, the, 345
Cloaca, the, 249, 317
Cnidaria, 217
Coccyx, the, 295
Cochineal insect, the, 354
Cochlea, the, 289
Cœcilia, 241
Cœcum, the, 310, 317
Cœlenterata, 20, 91, 93, 104
Cœlenteria, 221
Cœloma, the, 21, 64, 91
Cœlomæa, the, 98
Cœlomaria, 21, 91, 104, 221
Cœlomation, 93–4
Cœlom-theory, the, 21, 93
Cœlomula, the, 98
Colon, the, 310, 317
Comparative anatomy, 31
Conception, nature of, 51
Conjunctiva, the, 286
Conocyema, 215
Convoluta, 221
Copelata, the, 197
Copulative organs, evolution of the, 344–45
Corium, the, 108, 268
Cornea, the, 286
Corpora cavernosa, the, 345, 346
Corpora quadrigemina, 274
Corpora striata, 274
Corpus callosum, the, 274
Corpus vitreum, the, 285
Corpuscles of the blood, 319
Craniology, 303
Craniota, the, 182, 229
Cranium, the, 299
Creation, 23–4
Cretaceous strata, 202
Crossopterygii, 234
Crustacea, the, 142, 219
Cryptocœla, 221
Cryptorchism, 114
Crystalline lens, the, 285 — — development of the, 287
Cutaneous glands, 268
Cuttlefish, embryology of the, 9
Cuvier, G., 17, 24
Cyanophycea, 209
Cyclostoma, the, 188, 230–32 — ova of the, 75
Cyemaria, 214
Cynopitheca, 262
Cynthia, 191, 196
Cytoblastus, the, 37
Cytodes, 40
Cytoplasm, 37, 38
Cytosoma, 37
Cytula, the, 54
D
Dalton, 15
Darwin, C., 2, 5, 23, 28–9
Darwin, E., 28
Darwinism, 5, 28
Decidua, the, 167
Deciduata, 255
Deduction, nature of, 208
Degeneration theory, the, 219
Dentition of the ape and man, 259
Depula, 62
Descent of Man, 30
Design in organisms, 33
Deutoplasm, 44
Devonian strata, 202
Diaphragm, the, 309 — evolution of the, 328
Dicyema, 215
Dicyemida, 215
Didelphia, 248
Digonopora, 223
Dinosauria, 202
Dipneumones, 238
Dipneusta, 235–38 — ova of the, 75
Dipnoa, 236
Directive bodies, 54
Discoblastic ova, 68
Discoplacenta, 255
Dissatyrus, 174
Dissection, medieval decrees against, 10
Dohrn, Anton, 219
Döllinger, 15
Dorsal furrow, the, 125 — shield, the, 123 — zone, the, 129
Dromatherium, 248
Dualism, 6
Dubois, Eugen, 263
Ductus Botalli, the, 350
Ductus venosus Arantii, 350
Duodenum, the, 309, 317
Duration of embryonic development, 199 — of man’s history, 199
Dysteleology, 32 — proofs of, 349
E
Ear, evolution of the, 288–92 — structure of the, 288 — uselessness of the external, 32
Ear-bones, the, 289
Earth, age of the, 200–201
Echidna hystrix, 249
Ectoblast, 20, 64
Ectoderm, the, 20, 64
Edentata, 250
Efficient causes, 6
Egg of the bird, 44–6, 81 — or the chick, priority of the, 211
Elasmobranchs, the, 79
Embryo, human, development of the, 158
Embryology, 2 — evolutionary value of, 34
Embryonic development, duration of, 199 — disk, the, 121–22 — spot, the, 125
Encephalon, the, 273
Endoblast, 20, 64
Endothelia, 321
Enterocœla, 93, 223
Enteropneusta, 226
Entoderm, the, 20, 64
Eocene strata, 203
Eopitheca, 259
Epiblast, 20, 64
Epidermis, the, 108, 268
Epididymis, the, 342
Epigastrula, 80
Epigenesis, 11, 13
Epiglottis, the, 309
Epiphysis, the, 108
Episoma, 129
Episomites, 130, 194
Epispadia, 346
Epithelia, 37
Epitheria, 243, 253
Epovarium, the, 342
Equilibrium, sense of, 291
Esthonychida, 257
Eustachian tube, the, 289
Eutheria, 253
Eve, 12
Evolution theory, the, 11, 208 — inductive nature of, 30
Eye, evolution of the, 285–88 — structure of the, 285
Eyelid, the third, 32
Eyelids, evolution of the, 288
F
Fabricius ab Aquapendente, 10
Face, embryonic development of the, 284
Fat glands in the skin, 269
Feathers, evolution of, 270
Fertilisation, 51 — place of, 119
Fin, evolution of the, 239, 304
Final causes, 6
Flagellate cells, 193
Floating bladder, the, 233, 241 — — evolution of the, 314
Fœtal circulation, 170–71
Food-yelk, the, 67,
Foot, evolution of the, 241, 304–6 — of the ape and man, 258–59
Fore brain, the, 278
Fore kidneys, the, 336, 337
Fossiliferous strata, list of, 201
Fossils, 180 — scarcity of, 208
Free will, 356
Friedenthal, experiments of, 172
Frog, the, 241–42 — ova of the, 71–2
Frontonia, 224
Function and structure, 7
Furcation of ova, 72
G
Gaertner’s duct, 341, 350
Ganglia, commencement of, 268
Ganglionic cell, the, 39
Ganoids, 233, 234
Gastræa, the, 3, 20, 206 — formation of the, 213
Gastræa theory, the, 20, 64, 69
Gastræads, 69, 214
Gastremaria, 214
Gastrocystis, the, 62, 119, 120
Gastrophysema, 215
Gastrotricha, 224
Gastrula, the, 3, 20, 62
Gastrulation, 62
Gegenbaur, Carl, 220 — on evolution, 32 — on the skull, 300–1
Gemmation, 331
General Morphology, 8, 29
Genesis, 23
Genital pore, the, 335
Geological evolution, length of, 200 — periods, 201
Geology, methods of, 180 — rise of, 24
Germ-plasm, theory of, 349
Germinal disk, 46, 81 — layers, the, 14, 16 — — scheme of the, 92 — spot, the, 44 — vesicle, the, 43, 54
Germinative area, the, 121
Giant gorilla, the, 176
Gibbon, the, 173, 262
Gill-clefts and arches, 110 — formation of the, 151–52, 303
Gill-crate, the, 183, 189
Gills, disappearance of the, 244
Glœocapsa, 210
Gnathostoma, 230, 232
Goethe as an evolutionist, 27, 299
Goitre, 110
Gonads, the, 111 — formation of the, 149–50
Gonidia, 334
Gonochorism, beginning of, 322
Gonoducts, 335
Gonotomes, 146, 149
Goodsir, 189
Gorilla, the, 174, 176, 262
Graafian follicles, the, 17, 119, 347
Gregarinæ, 211
Gullet-ganglion, the, 190
Gut, evolution of the, 310–17
Gyrini, 242
Gynecomastism, 114
H
Hag-fish, the, 188
Hair, evolution of the, 270 — on the human embryo and infant, 271
Hair, restriction of, by sexual selection, 271
Haliphysema, 215
Halisauria, 202
Haller, Albrecht, 12
Halosphæra viridis, 213
Hand, evolution of the, 250, 304–6 — of the ape and man, 258
Hapalidæ, 261
Harderian gland, the, 288
Hare-lip, 284
Harrison, Granville, 161
Hartmann, 262
Harvey, 10
Hatschek, 192
Hatteria, 243, 246
Head-cavity, the, 138
Head-plates, the, 149
Heart, development of the, 7, 10, 111, 151, 170, 322, 324–27 — of the ascidia, 190 — position of the, 327
Helmholtz, 207
Helminthes, 223
Hepatic gut, the, 109, 316
Heredity, nature of, 3, 5, 27, 56–7, 349
Hermaphrodism, 9, 23, 114, 218, 322, 346
Hertwig, 21
Hesperopitheca, 259
His, W., 19
Histogeny, 18, 19
History of Creation, 6, 30
Holoblastic ova, 67, 71, 77
Homœosaurus, 244, 246
Homology of the germinal layers, 20
Hoof, evolution of the, 270
Hunterian ligament, the, 344
Huxleian law, the, 171, 257, 262
Huxley, T. H., 7, 20, 29
Hydra, the, 69, 217
Hydrostatic apparatus in the fish, 315
Hylobates, 173, 262
Hylodes Martinicensis, 241
Hyoid bone, the, 299
Hypermastism, 113
Hyperthelism, 113
Hypoblast, 20, 64
Hypobranchial groove, the, 110, 184, 226, 316
Hypodermis, the, 268
Hypopsodina, 257
Hyposoma, the, 129
Hyposomites, 130, 194
Hypospadia, 346
I
Ichthydina, 224
Ichthyophis glutinosa, 80
Ictopsida, 257
Ileum, the, 310
Immortality, Aristotle on, 10
Immortality of the soul, 58
Impregnation-rise, the, 55
Indecidua, 255
Indo-Germanic languages, 203
Induction and deduction, 31, 208
Inheritance of acquired characters, 349
Insects, intelligence of, 353
Interamniotic cavity, the, 165
Intestines, the, 309, 316–17
Invagination, 62
Iris, the, 286
J
Jacchus, 261
Java, ape-man of, 263, 264
Jaws, evolution of the, 301
Jurassic strata, 202
K
Kant, dualism of, 25
Kelvin, Lord, on the origin of life, 207
Kidneys, the, 111 — formation of the, 150–51, 336–42
Klaatsch, 262
Kölliker, 21
Kowalevsky, 191
L
Labia, the, 346
Labyrinth, the, 290
Lachrymal glands, 269
Lamarck, J., 23, 25–7 — theories of, 26, 349
Lamprey, the, 230 — ova of the, 75
Lancelet, the, 60, 181–95 — description of the, 105
Languages, evolution of, 203
Lanugo of the embryo, 271
Larynx, the, 309 — evolution of the, 314
Latebra, the, 45
Lateral plates, the, 129
Laurentian strata, 201
Lecithoma, the, 117
Leg, evolution of the, 304 — structure of the, 306
Lemuravida, 257
Lemurogona, 257
Lemurs, the, 257
Lepidosiren, 257
Leucocytes, 319
Life, age of, 200
Limbs, evolution of the, 152, 239, 304
Limiting furrow, the, 133
Linin, 42
Liver, the, 309, 317
Long-nosed ape, the, 164
Love, importance of in nature, 332
Lungs, the, 110 — evolution of the, 241, 314–15
Lyell, Sir C., 24
Lymphatic vessels, the, 318
Lymph-cells, the, 319
M
Macrogonidion, 331
Macrospores, 331
Magosphæra planula, 213
Male womb, the, 344, 350
Mallochorion, the, 166
Mallotheria, 257
Malpighian capsules, 339, 341
Mammal, characters of the, 112 — gastrulation of the, 84
Mammals, unity of the, 247–48
Mammary glands, the, 113, 269
Man and the ape, relation of, 262, 351 — origin of, 29
Man’s Place in Nature, 7, 29, 351
Mantle, the, 189
Mantle-folds, the, 185
Marsupials, the, 250–52 — ova of the, 85
Materialism, 356
Mathematical method, the, 30
Mechanical causes, 6 — embryology, 8, 19, 22
Meckel’s cartilage, 304
Medulla capitis, the, 273 — oblongata, the, 274 — spinalis, the, 273
Medullary groove, the, 125 — tube, the, 107, 128 — — formation of the, 131, 133, 227, 267, 276
Mehnert, E., on the biogenetic law, 5
Meroblastic ova, 67, 71, 78
Merocytes, 68, 321
Mesentery, the, 98, 109, 310, 316
Mesocardium, the, 327
Mesoderm, the, 20, 64, 90, 93
Mesogastria, 215
Mesonephridia, the, 338
Mesonephros, the, 336
Mesorchium, the, 344
Mesovarium, the, 344
Mesozoic period, the, 202
Metogaster, the, 64
Metagastrula, the, 67
Metamerism, 142
Metanephridia, the, 341
Metanephros, the, 336
Metaplasm, 39
Metastoma, 64, 222
Metatheria, 248
Metazoa, 20, 62
Metovum, the, 81
Microgonidian, 331
Microspores, 331
Middle ear, the, 291
Migration, effect of, 33
Milk, secretion of the, 269
Mind, evolution of, 353–54 — in the lower animals, 353
Miocene strata, 203
Mitosis, 40, 41
Monera, 40, 206, 209
Monism, 6, 356
Monodelphia, 248
Monogonopora, 223
Monopneumones, 238
Monotremes, 118, 249 — ova of the, 84
Monoxenia Darwinii, 60
Morea, the, 212
Morphology, 2, 27
Morula, the, 62, 212
Motor-germinative layer, the, 19
Mouth, development of the, 124, 139 — structure of the, 308
Mucous layer, the, 16
Müllerian duct, the, 341
Muscle-layer, the, 16
Muscles, evolution of the, 307 — of the ear, rudimentary, 292
Myotomes, 108, 146
Myxinoides, the, 188, 230
N
Nails, evolution of the, 270
Nasal pits, 284
Natural philosophy, 25 — selection, 26, 28, 349
Navel, the, 117, 134
Necrolemurs, 257
Nectocystis, the, 314
Nemertina, 224–26
Nephroduct, evolution of the, 338–39
Nephrotomes, 149, 338
Nerve-cell, the, 39
Nerves, animals without, 267
Nervous system, evolution of the, 7, 267
Neurenteric canal, the, 127
Nictitating membrane, the, 32, 286, 288
Nose, the, in man and the ape, 164 — development of the, 282–85 — structure of the, 283
Notochorda, the, 107
Nuclein, 37
Nucleolinus, 44
Nucleolus, the, 38, 44, 54
Nucleus of the cell, 37
O
Œsophagus, the, 309, 316
Oken, 5, 27, 300
Oken’s bodies, 339
Oligocene strata, 203
Olynthus, 217
On the generation of animals, 9
Ontogeny, 2, 23 — defective evidence of, 208
Opaque area, the, 122
Opossum, the, 252 — ova of the, 85
Optic nerve, the, 287
Optic thalami, 274 — vesicles, 286
Orang, the, 174, 262
Ornithodelphia, 248
Ornithorhyncus, 85, 249
Ornithostoma, 249
Ossicles of the ear, 289
Otoliths, 289
Ova, number of, 347 — of the lancelet, 192
Ovaries, evolution of the, 333–34
Oviduct, origin of the, 335, 342
Ovolemma, the, 44
Ovulists, 12
Ovum, discovery of the, 16 — nature of the, 40, — size of the, 44
P
Pachylemurs, the, 257
Pacinian corpuscles, 282
Paleontology, 2 — evolutionary evidence of, 31 — incompleteness of, 208 — rise of, 24
Paleozoic age, the, 202
Palingenesis, 4
Palingenetic structures, 4
Palæhatteria, 244, 246
Panniculus carnosus, the, 350
Paradidymis, the, 342
Parietal zone, the, 129
Parthenogenesis, 9, 13
Pastrana, Miss Julia, 164
Pedimana, 252
Pellucid area, the, 122
Pelvic cavity, the, 138
Pemmatodiscus gastrulaceus, 215
Penis-bone, the, 346
Penis, varieties of the, 345
Peramelida, 254
Periblastic ova, 68
Peribranchial cavity, the, 185, 190
Pericardial cavity, the, 328
Perichorda, the, 108, 183 — formation of the, 136
Perigastrula, 89
Permian strata, 202
Petromyzontes, the, 188, 230
Phagocytes, 49, 320
Pharyngeal ganglion, the, 275
Pharynx, the, 309
Philology, comparison with, 203
Philosophie Zoologique, 25
Philosophy and evolution, 6
Phycochromacea, 209
Phylogeny, 2, 23
Physemaria, 214
Physiology, backwardness of, 7
Phytomonera, 209
Pineal eye, the, 108
Pinna, the, 291
Pithecanthropus, 263, 264
Pithecometra-principle, the, 171
Placenta, the, 166, 253–54
Placentals, the, 166 — characters of the, 253 — gastrulation of the, 86
Planocytes, 49, 320
Plant-louse, parthenogenesis of the, 13
Planula, the, 89
Plasma-products, 38, 39
Plasson, 40, 59
Plastids, 36, 40, 209
Plastidules, 59
Platodaria, 221
Platodes, the, 221
Platyrrhinæ, 261
Pleuracanthida, 234
Pleural ducts, 328
Pliocene strata, 203
Polar cells, 54
Polyspermism, 58
Preformation theory, the, 11
Primary period, the, 202
Primates, the, 157, 257–60
Primatoid, 263
Primitive groove, the, 69, 82, 124, 125 — gut, the, 20, 63, 214 — kidneys, the, 111, 337 — mouth, the, 20, 63 — segments, 143 — streak, the, 100, 122 — vertebræ, 144, 195, 206, 229
Primordial period, the, 201
Prochordata, 192
Prochordonia, the, 192, 218
Prochoriata, 253
Prochorion, the, 44, 119
Proctodæum, the, 345
Procytella primordialis, 210
Prodidelphia, 256
Progaster, the, 20, 63
Progonidia, 333
Promammalia, 247
Pronephridia, the, 151
Pronucleus femininus, 54 — masculinus, 54
Properistoma, 69
Prorenal canals of the lancelet, 186 — duct, the, 132, 139, 186 — — evolution of the, 338
Proselachii, 234
Prosimiæ, the, 257
Prospermaria, 333
Prospondylus, 105, 229
Prostoma, 20, 63, 222
Protamniotes, 243–44
Protamœba, 210
Proterosaurus, the, 202, 244
Protists, 36, 38
Protonephros, 111, 336
Protophyta, 210
Protoplasm, 37, 209
Protopterus, 238
Prototheria, 248
Protovertebræ, 142, 144
Protozoa, 20, 210
Provertebral cavity, the, 148 — plates, the, 136, 144
Pseudocœla, 93, 221
Pseudopodia, 48
Pseudova, 13
Psychic life, evolution of the, 8
Psychology, 8
Pterosauria, 202
Pylorus, the, 309
Q
Quadratum, the, 247
Quadrumana, 258
Quaternary period, 203
R
Rabbit, ova of the, 86–7
Radiates, the, 103
Rathke’s canals, 341
Rectum, the, 317
Regner de Graaf, 119
Renal system, evolution of the, 335–42
Reproduction, nature of, 330–31
Reptiles, 245–47
Respiratory organs, evolution of the, 314–15 — pore, the, 183, 189
Retina, the, 286
Rhabdocœla, 222
Rhodocytes, 319
Rhopalura, 215
Rhyncocephala, 243
Ribs, the, 295 — number of the, 353
Rudimentary ear-muscles, 292 — organs, 32 — — list of, 349–50 — toes, 306
S
Sacculus, the, 289
Sagitta, 65, 66, 191 — cœlomation of, 93
Salamander, the, 241 — ova of the, 74
Sandal-shape of embryo, 128–29
Satyrus, 174, 262
Sauromammalia, 246
Sauropsida, 245
Scatulation theory, the, 12
Schizomycetes, 210
Schleiden, M., 18, 36
Schwann, T., 18, 36
Sclerotic coat, the, 286
Sclerotomes, 108, 143, 148
Scrotum, the, 344
Scyllium, nose of the, 283
Sea-squirt, the, 181, 188–90
Secondary period, the, 202
Segmentation, 60, 141–42
Segmentation-cells, 54
Segmentation-sphere, the, 17
Selachii, 223 — skull of the, 301
Selection, theory of, 28
Selenka, 166, 168
Semnopitheci, 262
Sense-organs, evolution of the, 151, 280 — number of the, 281 — origin of the, 281
Sensory nerves, 279
Serocœlom, the, 165
Serous layer, the, 16
Sex-organs, early vertebrate form of the, 111 — evolution of the, 333
Sexual reproduction, simplest forms of, 331 — selection, 30, 271–72
Shark, the, 233 — nose of the, 283 — ova of the, 75 — placenta of the, 9 — skull of the, 301
Shoulder-blade, the, 306
Sickle-groove, the, 82, 121
Sieve-membrane, the, 167
Silurian strata, 202
Simiæ, the, 257–60
Siphonophoræ, embryology of the, 21
Skeleton, structure of the, 294
Skeleton-plate, the, 148
Skin, the, 151 — evolution of, 266–69 — function of the, 269
Skin-layer, the, 16
Skull, evolution of the, 149, 299–303 — structure of the, 299 — vertebral theory of the, 300
Smell, the sense of, 282
Soul, evolution of the, 353–56 — nature of the, 58, 356 — phylogeny of the, 8 — seat of the, 278
Sound, sensations of, 289–90
Sozobranchia, 242
Space, sense of, 291
Species, nature of the, 23, 34
Speech, evolution of, 264
Spermaducts, 335, 342
Spermaries, evolution of the, 333–34
Spermatozoon, the, 52–3 — discovery of the, 12, 53
Spinal cord, development of the, 8 — structure of the, 273
Spirema, the, 42
Spiritualism, 356
Spleen, the, 318
Spondyli, 142
Sponges, classification of the, 34 — ova of the, 49
Spontaneous generation, 26, 206
Stegocephala, 239
Stem-cell, the, 54
Stem-zone, the, 129
Stomach, evolution of the, 311–14, 316 — structure of the human, 309
Strata, thickness of, 200–201
Struggle for life, the, 28
Subcutis, the, 268
Sweat glands, 269
T
Tactile corpuscles, 268, 282
Tadpole, the, 242
Tail, evolution of the, 242–43 — rudimentary, in man, 159, 295, 350
Tailed men, 160–61
Taste, the sense of, 282
Teeth, evolution of the, 314 — of the ape and man, 259
Teleostei, 234
Telolecithal ova, 67, 68
Temperature, sense of, 282
Terrestrial life, beginning of, 235
Tertiary period, the, 203
Theoria generationis, the, 13
Theories, value of, 181
Theromorpha, 246
Third eyelid, the, 286, 288
Thyroid gland, the, 110, 184, 315
Time-variations in ontogeny, 5
Tissues, primary and secondary, 37
Toad, the, 241
Tocosauria, 246
Toes, number of the, 240
Tori genitales, the, 346
Touch, the sense of, 282
Tracheata, 142, 219
Tread, the, 45, 81
Tree-frog, the, 241
Triassic strata, 202
Triton tæniatus, 74
Troglodytes, 174
Tunicates, the, 189
Turbellaria, 222
Turbinated bones, the, 283
Tympanic cavity, the, 288
U
Umbilical, cord, the, 117 — vesicle, the, 138
Unicellular ancestor of all animals, 47 — animals, 38, 47
Urachus, the, 317, 341
Urinary system, evolution of the, 335–42
Urogenital ducts, 335
Uterus masculinus, the, 344, 350
Utriculus, the, 289
V
Vasa deferentia, 335
Vascular layer, the, 16, 168 — system, evolution of the, 321–25 — — structure of the, 318
Vegetative layer, the, 16
Veins, evolution of the, 323–24
Ventral pedicle, the, 166
Ventricles of the heart, 325
Vermalia, 220, 223
Vermiform appendage, the, 32, 310, 317
Vertebræ, 142, 294
Vertebræa, 105
Vertebral arch, the, 148, 295 — column, the, 144 — — evolution of the, 296 — — structure of the, 294
Vertebrates, character of the, 104–10 — descent of the, 219–20
Vertebration, 142
Vesico-umbilical ligament, the, 341
Vesicula prostatica, the, 344, 350
Villi of the chorion, 165
Virchow, R., 35 — on the ape-man, 303 — on the evolution of man, 264
Virgin-birth, 9, 13
Vitalism, 6
Vitelline duct, the, 138
Volvocina, 213
W
Wallace, A. R., 29
Water, organic importance of, 200
Water vessels, 336
Weismann’s theories, 349
Wolff, C. F., 13
Wolffian bodies, 339
Wolffian duct, the, 341
Womb, evolution of the, 342–43
Y
Yelk, the, 43, 45, 67
Yelk-sac, the, 117, 134
Z
Zona pellucida, the, 44
Zonoplacenta, 255
Zoomonera, 209
Zoophytes, 20, 64, 104