The Evolution of Man Volume Ii Part 18

FIGURE 2.404. Two young Graafian follicles, isolated. In 1 the follicle-cells still form a simple, and in 2 a double, stratum round the young ovum; in 2 they are beginning to form the ovolemma or the zona pellucida (a).

FIGURES 2.405 AND 2.406. Two older Graafian follicles, in which fluid is beginning to acc.u.mulate inside the eccentrically thickened epithelial ma.s.s of the follicle-cells (Figure 2.405 with little, 2.406 with much, follicle-water). ei the young ovum, with embryonic vesicle and spot, zp ovolemma or zona pellucida, dp discus proligerus, formed of an acc.u.mulation of follicle-cells, which surround the ovum, ff follicle-liquid (liquor folliculi), gathered inside the stratified follicle-epithelium (fe), fk connective-tissue fibrous capsule of the Graafian follicle (theca folliculi).)

In the male mammals there is the same fusion of the Mullerian and Wolffian ducts at their lower ends. Here again they form a single genital cord (Figure 2.397 g), and this opens similarly into the original urogenital sinus, which develops from the lowest section of the bladder (v). But while in the male mammal the Wolffian ducts develop into the permanent spermaducts, there are only rudimentary relics left of the Mullerian ducts. The most notable of these is the "male womb" (uterus masculinus), which originates from the lowest fused part of the ducts, and corresponds to the female uterus. It is a small, flask-shaped vesicle without any physiological significance, which opens into the ureter between the two spermaducts and the prostate folds (vesicula prostatica).

(FIGURE 2.407. A ripe human Graafian follicle. a the mature ovum, b the surrounding follicle-cells, c the epithelial cells of the follicle, d the fibrous membrane of the follicle, e its outer surface.)

The internal s.e.xual organs of the mammals undergo very distinctive changes of position. At first the germinal glands of both s.e.xes lie deep inside the ventral cavity, at the inner edge of the primitive kidneys (Figures 2.386 g and 2.392 k), attached to the vertebral column by a short mesentery (mesorchium in the male, mesovarium in the female). But this primary arrangement is retained permanently only in the Monotremes (and the lower Vertebrates). In all other mammals (both Marsupials and Placentals) they leave their original cradle and travel more or less far down (or behind), following the direction of a ligament that goes from the primitive kidneys to the inguinal region of the ventral wall. This is the inguinal ligament of the primitive kidneys, known in the male as the Hunterian ligament (Figure 2.400 gh), and in the female as the "round maternal ligament" (Figure 2.401 r). In woman the ovaries travel more or less towards the small pelvis, or enter into it altogether. In the male the t.e.s.t.i.c.l.es pa.s.s out of the ventral cavity, and penetrate by the inguinal ca.n.a.l into a sac-shaped fold of the outer skin. When the right and left folds ("s.e.xual swellings") join together they form the s.c.r.o.t.u.m. The various mammals bring before us the successive stages of this displacement. In the elephant and the whale the t.e.s.t.i.c.l.es descend very little, and remain underneath the kidneys. In many of the rodents and carna.s.sia they enter the inguinal ca.n.a.l. In most of the higher mammals they pa.s.s through this into the s.c.r.o.t.u.m. As a rule, the inguinal ca.n.a.l closes up. When it remains open the t.e.s.t.i.c.l.es may periodically pa.s.s into the s.c.r.o.t.u.m, and withdraw into the ventral cavity again in time of rut (as in many of the marsupials, rodents, bats, etc.).

The structure of the external s.e.xual organs, the copulative organs that convey the fecundating sperm from the male to the female organism in the act of copulation, is also peculiar to the mammals. There are no organs of this character in most of the other Vertebrates. In those that live in water (such as the Acrania and Cyclostomes, and most of the fishes) the ova and sperm-cells are simply ejected into the water, where their conjunction and fertilisation are left to chance. But in many of the fishes and amphibia, which are viviparous, there is a direct conveyance of the male sperm into the female body; and this is the case with all the Amniotes (reptiles, birds, and mammals). In these the urinary and s.e.xual organs always open originally into the last section of the r.e.c.t.u.m, which thus forms a cloaca (Chapter 2.22).

Among the mammals this arrangement is permanent only in the Monotremes, which take their name from it (Figure 2.399 cl). In all the other mammals a frontal part.i.tion is developed in the cloaca (in the human embryo about the beginning of the third month), and this divides it into two cavities. The anterior cavity receives the urogenital ca.n.a.l, and is the sole outlet of the urine and the s.e.xual products; the hind or a.n.u.s-cavity pa.s.ses the excrements only.

Even before this part.i.tion has been formed in the Marsupials and Placentals, we see the first trace of the external s.e.xual organs.

First a conical protuberance rises at the anterior border of the cloaca-outlet--the s.e.xual prominence (phallus, Figure 2.402 A, e, B, e). At the tip it is swollen in the shape of a club ("acorn" glans).

On its under side there is a furrow, the s.e.xual groove (sulcus genitalis, f), and on each side of this a fold of skin, the "s.e.xual pad" (torus genitalis, h l). The s.e.xual protuberance or phallus is the chief organ of the s.e.xual sense (Chapter 2.25); the s.e.xual nerves spread on it, and these are the princ.i.p.al organs of the specific s.e.xual sensation. As erectile bodies (corpora cavernosa) are developed in the male phallus by peculiar modifications of the blood-vessels, it becomes capable of erecting periodically on a strong accession of blood, becoming stiff, so as to penetrate into the female v.a.g.i.n.a and thus effect copulation. In the male the phallus becomes the p.e.n.i.s; in the female it becomes the much smaller c.l.i.toris; this is only found to be very large in certain apes (Ateles). A prepuce ("foreskin") is developed in both s.e.xes as a protecting fold on the anterior surface of the phallus.

(FIGURE 408. The human ovum after issuing from the Graafian follicle, surrounded by the clinging cells of the discus proligerus (in two radiating crowns). z ovolemma (zona pellucida, with radial porous ca.n.a.ls), p cytosoma (protoplasm of the cell-body, darker within, lighter without), k nucleus of the ovum (embryonic vesicle). (From Nagel, magnified 250 times.) (Cf. Figures 1.1 and 1.14.)

The external s.e.xual member (phallus) is found at various stages of development within the mammal cla.s.s, both in regard to size and shape, and the differentiation and structure of its various parts; this applies especially to the terminal part of the phallus, the glans, both the larger glans p.e.n.i.s of the male and the smaller glans c.l.i.toridis of the female. The part of the cloaca from the upper wall of which it forms belongs to the proctodaeum, the ectodermic inv.a.g.i.n.ation of the r.e.c.t.u.m (Chapter 2.27); hence its epithelial covering can develop the same h.o.r.n.y growths as the corneous layer of the epidermis. Thus the glans, which is quite smooth in man and the higher apes, is covered with spines in many of the lower apes and in the cat, and in many of the rodents with hairs (marmot) or scales (guinea-pig) or solid h.o.r.n.y warts (beaver). Many of the Ungulates have a free conical projection on the glans, and in many of the Ruminants this "phallus-tentacle" grows into a long cone, bent hook-wise at the base (as in the goat, antelope, gazelle, etc.). The different forms of the phallus are connected with variations in the structure and distribution of the sensory corpuscles--i.e. the real organs of the s.e.xual sense, which develop in certain papillae of the corium of the phallus, and have been evolved from ordinary tactile corpuscles of the corium by erotic adaptation (Chapter 2.25).

The formation of the corpora cavernosa, which cause the stiffness of the phallus and its capability of penetrating the v.a.g.i.n.a, by certain special structures of their spongy vascular s.p.a.ces, also shows a good deal of variety within the vertebrate stem. This stiffness is increased in many orders of mammals (especially the carna.s.sia and rodents) by the ossification of a part of the fibrous body (corpus fibrosum). This p.e.n.i.s-bone (os priapi) is very large in the badger and dog, and bent like a hook in the marten; it is also very large in some of the lower apes, and protrudes far out into the glans. It is wanting in most of the anthropoid apes; it seems to have been lost in their case (and in man) by atrophy.

The s.e.xual groove on the under side of the phallus receives in the male the mouth of the urogenital ca.n.a.l, and is changed into a continuation of this, becoming a closed ca.n.a.l by the juncture of its parallel edges, the male urethra. In the female this only takes place in a few cases (some of the lemurs, rodents, and moles); as a rule, the groove remains open, and the borders of this "vestibule of the v.a.g.i.n.a" develop into the smaller l.a.b.i.a (nymphae). The large l.a.b.i.a of the female develop from the s.e.xual pads (tori genitales), the two parallel folds of the skin that are found on each side of the genital groove. They join together in the male, and form the closed s.c.r.o.t.u.m.

These striking differences between the two s.e.xes cannot yet be detected in the human embryo of the ninth week. We begin to trace them in the tenth week of development, and they are accentuated in proportion as the difference of the s.e.xes develops.

Sometimes the normal juncture of the two s.e.xual pads in the male fails to take place, and the s.e.xual groove may also remain open (hypospadia). In these cases the external male genitals resemble the female, and they are often wrongly regarded as cases of hermaphrodism.

Other malformations of various kinds are not infrequently found in the human external s.e.xual organs, and some of them have a great morphological interest. The reverse of hypospadia, in which the p.e.n.i.s is split open below, is seen in epispadia, in which the urethra is open above. In this case the urogenital ca.n.a.l opens above at the dorsal root of the p.e.n.i.s; in the former case down below. These and similar obstructions interfere with a man's generative power, and thus prejudicially affect his whole development. They clearly prove that our history is not guided by a "kind Providence," but left to the play of blind chance.

We must carefully distinguish the rarer cases of real hermaphrodism from the preceding. This is only found when the essential organs of reproduction, the genital glands of both kinds, are united in one individual. In these cases either an ovary is developed on the right and a t.e.s.t.i.c.l.e on the left (or vice versa); or else there are t.e.s.t.i.c.l.es and ovaries on both sides, some more and others less developed. As hermaphrodism was probably the original arrangement in all the Vertebrates, and the division of the s.e.xes only followed by later differentiation of this, these curious cases offer no theoretical difficulty. But they are rarely found in man and the higher mammals. On the other hand, we constantly find the original hermaphrodism in some of the lower Vertebrates, such as the Myxinoides, many fishes of the perch-type (serra.n.u.s), and some of the Amphibia (ringed snake, toad). In these cases the male often has a rudimentary ovary at the fore end of the t.e.s.t.i.c.l.e; and the female sometimes has a rudimentary, inactive t.e.s.t.i.c.l.e. In the carp also and some other fishes this is found occasionally. We have already seen how traces of the earlier hemaphrodism can be traced in the pa.s.sages of the Amphibia.

Man has faithfully preserved the main features of his stem-history in the ontogeny of his urinary and s.e.xual organs. We can follow their development step by step in the human embryo in the same advancing gradation that is presented to us by the comparison of the urogenital organs in the Acrania, Cyclostomes; Fishes, Amphibia, Reptiles, and then (within the mammal series) in the Monotremes, Marsupials, and the various Placentals. All the peculiarities of urogenital structure that distinguish the mammals from the rest of the Vertebrates are found in man; and in all special structural features he resembles the apes, particularly the anthropoid apes. In proof of the fact that the special features of the mammals have been inherited by man, I will, in conclusion, point out the identical way in which the ova are formed in the ovary. In all the mammals the mature ova are contained in special capsules, which are known as the Graafian follicles, after their discoverer, Roger de Graaf (1677). They were formerly supposed to be the ova themselves; but Baer discovered the ova within the follicles (Chapter 1.3). Each follicle (Figure 2.407) consists of a round fibrous capsule (d), which contains fluid and is lined with several strata of cells (c). The layer is thickened like a k.n.o.b at one point (b); this ovum-capsule encloses the ovum proper (a).

The mammal ovary is originally a very simple oval body (Figure 2.387 g), formed only of connective tissue and blood-vessels, covered with a layer of cells, the ovarian epithelium or the female germ epithelium.

From this germ epithelium strings of cells grow out into the connective tissue or "stroma" of the ovary (Figure 2.403 b). Some of the cells of these strings (or Pfluger's tubes) grow larger and become ova (primitive ova, c); but the great majority remain small, and form a protective and nutritive stratum of cells round each ovum--the "follicle-epithelium" (e).

The follicle-epithelium of the mammal has at first one stratum (Figure 2.404 1), but afterwards several (2). It is true that in all the other Vertebrates the ova are enclosed in a membrane, or "follicle," that consists of smaller cells. But it is only in the mammals that fluid acc.u.mulates between the growing follicle-cells, and distends the follicle into a large round capsule, on the inside wall of which the ovum lies, at one side (Figures 2.405 and 2.406). There again, as in the whole of his morphology, man proves indubitably his descent from the mammals.

In the lower Vertebrates the formation of ova in the germ-epithelium of the ovary continues throughout life; but in the higher it is restricted to the earlier stages, or even to the period of embryonic development. In man it seems to cease in the first year; in the second year we find no new-formed ova or chains of ova (Pfluger's tubes).

However, the number of ova in the two ovaries is very large in the young girl; there are calculated to be 72,000 in the s.e.xually-mature maiden. In the production of the ova men resemble most of the anthropoid apes.

Generally speaking, the natural history of the human s.e.xual organs is one of those parts of anthropology that furnish the most convincing proofs of the animal origin of the human race. Any man who is acquainted with the facts and impartially weighs them will conclude from them alone that we have been evolved from the lower Vertebrates.

The larger and the detailed structure, the action, and the embryological development of the s.e.xual organs are just the same in man as in the apes. This applies equally to the male and the female, the internal and the external organs. The differences we find in this respect between man and the anthropoid apes are much slighter than the differences between the various species of apes. But all the apes have certainly a common origin, and have been evolved from a long-extinct early-Tertiary stem-form, which we must trace to a branch of the lemurs. If we had this unknown pithecoid stem-form before us, we should certainly put it in the order of the true apes in the primate system; but within this order we cannot, for the anatomic and ontogenetic reasons we have seen, separate man from the group of the anthropoid apes. Here again, therefore, on the ground of the pithecometra-principle, comparative anatomy and ontogeny teach with full confidence the descent of man from the ape.

CHAPTER 2.30. RESULTS OF ANTHROPOGENY.

Now that we have traversed the wonderful region of human embryology and are familiar with the princ.i.p.al parts of it, it will be well to look back on the way we have come, and forward to the further path to truth to which it has led us. We started from the simplest facts of ontogeny, or the development of the individual--from observations that we can repeat and verify by microscopic and anatomic study at any moment. The first and most important of these facts is that every man, like every other animal, begins his existence as a simple cell. This round ovum has the same characteristic form and origin as the ovum of any other mammal. From it is developed in the same manner in all the Placentals, by repeated cleavage, a multicellular blastula. This is converted into a gastrula, and this in turn into a blastocystis (or embryonic vesicle). The two strata of cells that compose its wall are the primary germinal layers, the skin-layer (ectoderm), and gut-layer (entoderm). This two-layered embryonic form is the ontogenetic reproduction of the extremely important phylogenetic stem-form of all the Metazoa, which we have called the Gastraea. As the human embryo pa.s.ses through the gastrula-form like that of all the other Metazoa, we can trace its phylogenetic origin to the Gastraea.

As we continued to follow the embryonic development of the two-layered structure, we saw that first a third, or middle layer (mesoderm), appears between the two primary layers; when this divides into two, we have the four secondary germinal layers. These have just the same composition and genetic significance in man as in all the other Vertebrates. From the skin-sense layer are developed the epidermis, the central nervous system, and the chief part of the sense-organs.

The skin-fibre layer forms the corium and the motor organs--the skeleton and the muscular system. From the gut-fibre layer are developed the vascular system, the muscular wall of the gut, and the s.e.xual glands. Finally, the gut-gland layer only forms the epithelium, or the inner cellular stratum of the mucous membrane of the alimentary ca.n.a.l 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 cla.s.s, the various phylogenetic stages that we distinguish in a natural cla.s.sification of the mammals correspond to the ontogenetic stages that the human embryo pa.s.ses through in the course of its evolution. We were thus in a position to determine precisely the position of man in this cla.s.s, and so to establish his relations.h.i.+p to the different orders of mammals.

The line of argument we followed in this explanation of the ontogenetic facts was simply a consistent application of the biogenetic law. In this we have throughout taken strict account of the distinction between palingenetic and cenogenetic phenomena.

Palingenesis (or "synoptic development") alone enables us to draw conclusions from the observed embryonic form to the stem-form preserved by heredity. Such inference becomes more or less precarious when there has been cenogenesis, or disturbance of development, owing to fresh adaptations. We cannot understand embryonic development unless we appreciate this very important distinction. Here we stand at the very limit that separates the older and the new science or philosophy of nature. The whole of the results of recent morphological research compel us irresistibly to recognise the biogenetic law and its far-reaching consequences. These are, it is true, irreconcilable with the legends and doctrines of former days, that have been impressed on us by religious education. But without the biogenetic law, without the distinction between palingenesis and cenogenesis, and without the theory of evolution on which we base it, it is quite impossible to understand the facts of organic development; without them we cannot cast the faintest gleam of explanation over this marvellous field of phenomena. But when we recognise the causal correlation of ontogeny and phylogeny expressed in this law, the wonderful facts of embryology are susceptible of a very simple explanation; they are found to be the necessary mechanical effects of the evolution of the stem, determined by the laws of heredity and adaptation. The correlative action of these laws under the universal influence of the struggle for existence, or--as we may say in a word, with Darwin--"natural selection," is entirely adequate to explain the whole process of embryology in the light of phylogeny. It is the chief merit of Darwin that he explained by his theory of selection the correlation of the laws of heredity and adaptation that Lamarck had recognised, and pointed out the true way to reach a causal interpretation of evolution.

The phenomenon that it is most imperative to recognise in this connection is the inheritance of functional variations. Jean Lamarck was the first to appreciate its fundamental importance in 1809, and we may therefore justly give the name of Lamarckism to the theory of descent he based on it. Hence the radical opponents of the latter have very properly directed their attacks chiefly against the former. One of the most distinguished and most narrow-minded of these opponents, Wilhelm His, affirms very positively that "characteristics acquired in the life of the individual are not inherited."

The inheritance of acquired characters is denied, not only by thorough opponents of evolution, but even by scientists who admit it and have contributed a good deal to its establishment, especially Weismann, Galton, Ray Lankester, etc. Since 1884 the chief opponent has been August Weismann, who has rendered the greatest service in the development of Darwin's theory of selection. In his work on The Continuity of the Germ-plasm, and in his recent excellent Lectures on the Theory of Descent (1902), he has with great success advanced the opinion that "only those characters can be transmitted to subsequent generations that were contained in rudimentary form in the embryo."

However, this germ-plasm theory, with its attempt to explain heredity, is merely a "provisional molecular hypothesis"; it is one of those metaphysical speculations that attribute the evolutionary phenomena exclusively to internal causes, and regard the influence of the environment as insignificant. Herbert Spencer, Theodor Eimer, Lester Ward, Hering, and Zehnder have pointed out the untenable consequences of this position. I have given my view of it in the tenth edition of the History of Creation (pages 192 and 203). I hold, with Lamarck and Darwin, that the hereditary transmission of acquired characters is one of the most important phenomena in biology, and is proved by thousands of morphological and physiological experiences. It is an indispensable foundation of the theory of evolution.

Of the many and weighty arguments for the truth of this conception of evolution I will for the moment merely point to the invaluable evidence of dysteleology, the science of rudimentary organs. We cannot insist too often or too strongly on the great morphological significance of these remarkable organs, which are completely useless from the physiological point of view. We find some of these useless parts, inherited from our lower vertebrate ancestors, in every system of organs in man and the higher Vertebrates. Thus we find at once on the skin a scanty and rudimentary coat of hair, only fully developed on the head, under the shoulders, and at a few other parts of the body. The short hairs on the greater part of the body are quite useless and devoid of physiological value; they are the last relic of the thicker hairy coat of our simian ancestors. The sensory apparatus presents a series of most remarkable rudimentary organs. We have seen that the whole of the sh.e.l.l 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 nict.i.tating membrane, the third, inner eye-lid that had a distinct physiological purpose in the ancient sharks, and still has in many of the Amniotes.

The motor apparatus, in both the skeleton and muscular systems, provides a number of interesting dysteleological arguments. I need only recall the projecting tail of the human embryo, with its rudimentary caudal vertebrae and muscles; this is totally useless in man, but very interesting as the degenerate relic of the long tail of our simian ancestors. From these we have also inherited various bony processes and muscles, which were very useful to them in climbing trees, but are useless to us. At various points of the skin we have cutaneous muscles which we never use--remnants of a strongly-developed cutaneous muscle in our lower mammal ancestors. This "panniculus carnosus" had the function of contracting and creasing the skin to chase away the flies, as we see every day in the horse. Another relic in us of this large cutaneous muscle is the frontal muscle, by which we knit our forehead and raise our eye-brows; but there is another considerable relic of it, the large cutaneous muscle in the neck (platysma myoides), over which we have no voluntary control.

Not only in the systems of animal organs, but also in the vegetal apparatus, we find a number of rudimentary organs, many of which we have already noticed. In the alimentary apparatus there are the thymus-gland and the thyroid gland, the seat of goitre and the relic of a ciliated groove that the Tunicates and Acrania still have in the gill-pannier; there is also the vermiform appendix to the caec.u.m. In the vascular system we have a number of useless cords which represent relics of atrophied vessels that were once active as blood-ca.n.a.ls--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 s.e.xual apparatus are particularly interesting. These are generally developed in one s.e.x 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 ca.n.a.ls. On the other hand, in the female the oviducts and womb are developed from the Mullerian ducts, while in the male only the lowest ends of them remain as the "male womb" (vesicula prostatica). Again, the male has in his nipples and mammary glands the rudiments of organs that are usually active only in the female.

A careful anatomic study of the human frame would disclose to us numbers of other rudimentary organs, and these can only be explained on the theory of evolution. Robert Wiedersheim has collected a large number of them in his work on The Human Frame as a Witness to its Past. They are some of the weightiest proofs of the truth of the mechanical conception and the strongest disproofs of the teleological view. If, as the latter demands, man or any other organism had been designed and fitted for his life-purposes from the start and brought into being by a creative act, the existence of these rudimentary organs would be an insoluble enigma; it would be impossible to understand why the Creator had put this useless burden on his creatures to walk a path that is in itself by no means easy. But the theory of evolution gives the simplest possible explanation of them.

It says: The rudimentary organs are parts of the body that have fallen into disuse in the course of centuries; they had definite functions in our animal ancestors, but have lost their physiological significance.

On account of fresh adaptations they have become superfluous, but are transmitted from generation to generation by heredity, and gradually atrophy.

We have inherited not only these rudimentary parts, but all the organs of our body, from the mammals--proximately from the apes. The human body does not contain a single organ that has not been inherited from the apes. In fact, with the aid of our biogenetic law we can trace the origin of our various systems of organs much further, down to the lowest stages of our ancestry. We can say, for instance, that we have inherited the oldest organs of the body, the external skin and the internal coat of the alimentary system, from the Gastraeads; the nervous and muscular systems from the Platodes; the vascular system, the body-cavity, and the blood from the Vermalia; the chorda and the branchial gut from the Prochordonia; the articulation of the body from the Acrania; the primitive skull and the higher sense-organs from the Cyclostomes; the limbs and jaws from the Selachii; the five-toed foot from the Amphibia; the palate from the Reptiles; the hairy coat, the mammary glands, and the external s.e.xual 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, a.s.sign to man his position in the cla.s.sification of the animal kingdom. In recent zoological cla.s.sification the animal world is divided into twelve stems or phyla, and these are broadly sub-divided into about sixty cla.s.ses, and these cla.s.ses 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 cla.s.s 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 cla.s.ses, 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 cla.s.sification. Of late years this fact has given rise to a good deal of discussion, and especially of controversy as to the particular anatomic relations.h.i.+p of man to the apes. The most curious opinions have been advanced on this "ape-question," or "pithecoid-theory." It is as well, therefore, to go into it once more and distinguish the essential from the unessential. (Cf. Chapter 2.23.)

We start from the undisputed fact that man is in any case--whether we accept or reject his special blood-relations.h.i.+p 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 relations.h.i.+p 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-relations.h.i.+p 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 a.s.sume 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 Tria.s.sic 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 fas.h.i.+on 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 a.s.suredly construct an approximate picture in the imagination of the form of our early Tertiary ancestors from the foregoing facts of comparative anatomy; however we may frame this in detail, it will be the picture of a true ape, and a distinct catarrhine ape. This has been shown so well by Huxley (1863) that the recent attacks of Klaatsch, Virchow, and other anthropologists, have completely failed (cf. Chapter 2.23). All the structural characters that distinguish the Catarrhines from the Platyrrhines are found in man. Hence in the genealogy of the mammals we must derive man immediately from the catarrhine group, and locate the origin of the human race in the Old World. Only the early root-form from which both descended was common to them.

It is, therefore, established beyond question for all impartial scientific inquiry that the human race comes directly from the apes of the Old World; but, at the same time, I repeat that this is not so important in connection with the main question of the origin of man as is commonly supposed. Even if we entirely ignore it, all that we have learned from the zoological facts of comparative anatomy and ontogeny as to the placental character of man remains untouched. These prove beyond all doubt the common descent of man and all the rest of the mammals. Further, the main question is not in the least affected if it is said: "It is true that man is a mammal; but he has diverged at the very root of the cla.s.s from all the other mammals, and has no closer relations.h.i.+p to any living group of mammals." The affinity is more or less close in any case, if we examine the relation of the mammal cla.s.s to the sixty other cla.s.ses of the animal world. Quite certainly the whole of the mammals, including man, have had a common origin; and it is equally certain that their common stem-forms were gradually evolved from a long series of lower Vertebrates.

The resistance to the theory of a descent from the apes is clearly due in most men to feeling rather than to reason. They shrink from the notion of such an origin just because they see in the ape organism a caricature of man, a distorted and unattractive image of themselves, because it hurts man's aesthetic complacency and self-enn.o.blement. It is more flattering to think we have descended from some lofty and G.o.d-like being; and so, from the earliest times, human vanity has been pleased to believe in our origin from G.o.ds or demi-G.o.ds. 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 G.o.d,"

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 G.o.d-like being, made from a clod, and fallen for his sins, and an Eve created from one of his ribs. Speaking of the rib, I may add to what I have said about the development of the skeleton, that the number of ribs is just the same in man and woman.

In both of them the ribs are formed from the middle germinal layer, and are, from the phylogenetic point of view, lower or ventral vertebral arches.

But it is said: "That is all very well, as far as the human body is concerned; on the facts quoted it is impossible to doubt that it has really and gradually been evolved from the long ancestral series of the Vertebrates. But it is quite another thing as regards man's mind, or soul; this cannot possibly have been developed from the vertebrate-soul."* (* The English reader will recognise here the curious position of Dr. Wallace and of the late Dr.

Mivart.--Translator.) Let us see if we cannot meet this grave stricture from the well-known facts of comparative anatomy, physiology, and embryology. It will be best to begin with a comparative study of the souls of various groups of Vertebrates. Here we find such an enormous variety of vertebrate souls that, at first sight, it seems quite impossible to trace them all to a common "Primitive Vertebrate." Think of the tiny Amphioxus, with no real brain but a simple medullary tube, and its whole psychic life at the very lowest stage among the Vertebrates. The following group of the Cyclostomes are still very limited, though they have a brain. When we pa.s.s 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 cla.s.s.

We see this to a still more astonis.h.i.+ng extent in the comparative psychology of another cla.s.s of animals, that is especially interesting for many reasons--the insect cla.s.s. 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-n.o.bles, workers, educators, soldiers, etc. One of the most remarkable phenomena in this very interesting province is the cattle-keeping of the ants, which rear plant-lice as milch-cows and regularly extract their honeyed juice. Still more remarkable is the slave-holding of the large red ants, which steal the young of the small black ants and bring them up as slaves. It has long been known that these political and social arrangements of the ants are due to the deliberate cooperation of the countless citizens, and that they understand each other. A number of recent observers, especially Fritz Muller, Sir J.

Lubbock (Lord Avebury), and August Forel, have put the astonis.h.i.+ng 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, s.h.i.+eld-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 s.e.xual 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 cla.s.s of Articulates, just as all the mammals belong to one and the same cla.s.s.

And just as every consistent evolutionist must admit a common stem-form for all these insects, so he must also for all the mammals.