The Physical KinshipMan an AnimalMan a VertebrateMan a MammalMan a PrimateRecapitulationThe Meaning of HomologyThe Earth an EvolutionThe Factors of Organic EvolutionThe Evidences of Organic EvolutionThe Genealogy of AnimalsConclusion‘Like the Roman emperors, who, intoxicated by their power, at length regarded themselves as demigods, so the ruler of the earth believes that the animals subjected to his will have nothing in common with his own nature. Man is not content to be the king of animals. He insists on having it that an impassable gulf separates him from his subjects. The affinity of the ape disturbs and humbles him. And, turning his back upon the earth, he flies, with his threatened majesty, into the cloudy sphere of a special “human kingdom.” But Anatomy, like those slaves who followed the conqueror’s car crying, “Thou art a man,” disturbs him in his self-admiration, and reminds him of those plain and tangible realities which unite him with the animal world.’— Broca.The Universal KinshipThe Physical KinshipI. Man an Animal.It was in the zoology class at college. We had made all the long journey from amoeba to coral, from coral to worm, from worm to mollusk, from mollusk to fish, from fish to reptile, and from reptile to mammal—and there, in the closing pages of faithful old Packard, we found it. ‘A mammal of the order of primates,’ the book said, with that unconcern characteristic of the deliverances of science. I was almost saddened. It was the first intimation I had ever received of that trite but neglected truth thatman is an animal.But the intimation was so weak, and I was at that time so unconscious, that it was not till years later that I began, through reflection, actually to realise the truth here first caught sight of. During these years I knew that man was not a mineral nor a plant—that, indeed, he belonged to the animal kingdom. But, like most men still, I continued to think of him as being altogether different from other animals. I thought of manand the animals,notof man and theotheranimals. Man was somehowsui generis. He had had, I believed, a unique and miraculous origin; for I had not yet learned of organic evolution. The pre-Darwinian belief that I had come down from the skies, and that non-human creatures of all kinds had been brought into existence as adjuncts of the distinguished species to which I belonged, occupied prominent place in my thinking. Non-human races, so I had been taught, had in themselves no reason for existence. They were accessories. A chasm, too wide for any bridge ever to span, yawned between the human and all other species. Man was celestial, a blue-blood barely escaping divinity. All other beings were little higher than clods. So faithfully and mechanically did I reflect the bias in which I had grown up.But manisananimal. It was away out there on the prairies, among the green corn rows, one beautiful June morning—a long time ago it seems to me now—that this revelation really came to me. And I repeat it here, as it has grown to seem to me, for the sake of a world which is so wise in many things, but so darkened and wayward regarding this one thing. However averse to accepting it we may be on account of favourite traditions, man is an animal in the most literal and materialistic meaning of the word. Man has not a spark of so-called ‘divinity’ about him. In important respects he is the most highly evolved of animals; but in origin, disposition, and form he is no more ‘divine’ than the dog who laps his sores, the terrapin who waddles over the earth in a carapace, or the unfastidious worm who dines on the dust of his feet. Man is not the pedestalled individual pictured by his imagination—a being glittering with prerogatives, and towering apart from and above all other beings. He is a pain-shunning, pleasure-seeking, death-dreading organism, differing in particulars, but not in kind, from the pain-shunning, pleasure-seeking, death-dreading organisms below and around him. Man is neither a rock, a vegetable, nor a deity. He belongs to the same class of existences, and has been brought into existence by the same evolutional processes, as the horse, the toad that hops in his garden, the firefly that lights its twilight torch, and the bivalve that reluctantly feeds him.Man’s body is composed fundamentally of the same materials as the bodies of all other animals. The bodies of all animals are composed of clay. They are formed of the same elements as those that murmur in the waters, gallop in the winds, and constitute the substance of the insensate rocks and soils. More than two-thirds of the weight of the human body is made up of oxygen alone, a gas which forms one-fifth of the weight of the air, more than eight-ninths of that of the sea, and forty-seven per cent, of the superficial solids of the earth.Man’s body is composed of cells. So are the bodies of all other animals. And the cells in the body of a human being are not essentially different in composition or structure from the cells in the body of the sponge. All cells are composed primarily of protoplasm, a compound of carbon, hydrogen, nitrogen, and oxygen. Like all other animals, man is incapable of producing a particle of the essential substance of which his body is made. No animal can produce protoplasm. This is a power of the plant, and the plant only. All that any animal can do is to burn the compounds formed in the sun-lit laboratories of the vegetable world. The human skeleton, like the skeletons of nearly all other animals, is composed chiefly of lime—lime being, in the sea, where life spent so many of its earlier centuries, the most available material for parts whose purpose it is to furnish shape and durability to the organism. Man grows from an egg. So do all creatures of clay. Every animal commences at the same place—in a single, lowly, almost homogeneous cell. A dog, a frog, a philosopher, and a worm cannot for a long time after their embryonic commencement be distinguished from each other. Like the oyster, the ox, the insect, and the fish, like all that live, move, and breathe, man is mortal. He increases in size and complexity through an allotted period of time; then, like all his kindred, wilts back into the indistinguishable flux from which he came. Man inhales oxygen and exhales carbon dioxide. So does every animal that breathes, whether it breathe by lungs, gills, skin, or ectosarc, and whether it breathe the sunless ooze of the sea floor or the ethereal blue of the sky. Animals inhale oxygen because they eat carbon and hydrogen. The energy of all animals is produced mainly by the union of oxygen with the elements of carbon and hydrogen in the tissues of animal bodies, the plentiful and ardent oxygen being the most available supporter of the combustion of these two elements.Man is, then, an animal, more highly evolved than the most of his fellow-beings, but positively of the same clay, and of the same fundamental make-up, with the same eagerness to exceed and the same destiny, as his less pompous kindred who float and frolic and pass away in the seas and atmospheres, and creep over the land-patches of a common clod.II. Man a Vertebrate.Man is avertebrateanimal.[1]He has (anatomically at least) a backbone. He belongs to that substantial class of organisms possessing an articulating internal skeleton—the family of the fishes, amphibians, reptiles, birds, and mammals. Most animals have some sort of skeleton, some sort of calcareous contrivance, whose business it is to give form and protection to the softer parts of the organism. Some animals, as the starfishes, have plates of lime scattered throughout the surface parts of the body; others, as the corals and sponges secrete plant-like frames, upon and among the branches of which the organisms reside; and still others, as the clams, crustaceans, and insects, have skeletons consisting of a shell or sheath on the outside of, and more or less surrounding, the softer substances of the body. The limbs of insects are tiny tubes on the inside of which are the miniature muscles with which they perform their marvels of locomotion. The skeleton of vertebrates, consisting of levers, beams, columns, and arches, all skilfully joined together and sunk deep within the muscular tissue, forms a conspicuous contrast to the rudimentary frames of other animals. The vertebrate skeleton consists of a hollow axis, divided into segments and extending along the dorsal region of the body, from the ventral side of which articulate, by means of awkwardly-constructed girdles, an anterior and a posterior pair of limbs. This dorsal axis ends in front in a peculiar bulbous arrangement called the head, which contains, among other valuables, the brain and buccal cavern. The thoracic segments of the backbone send off pairs of flat bones, which, arching ventrally, form the chest for the protection of the heart and other vitals. The limbs (except in fishes) consist each of a single long bone, succeeded by two long bones, followed by two transverse rows of short, irregular wrist or ankle bones, ending normally in five branching series of bones called digits. This is essentially the skeleton of all fishes, amphibians, reptiles, birds, and mammals. In short, it is the universal vertebrate type of frame. There are minor modifications to suit the various kinds of environment, adaptations to the necessities of aquatic, terrestrial, and aerial locomotion and life, some parts being specialised, others atrophied, and still others omitted, but there is never anywhere, from fishes to philosophers, any fundamental departure from the established vertebrate type of skeleton.[2]The pectoral fins of fishes correspond to the fore-limbs of frogs and reptiles, the wings of birds, and the arms of men. The pelvic fins of fishes are homologous with the hind-limbs of frogs, reptiles, and quadrupeds, and the legs of birds, apes, and men. The foot of the dog and crocodile, the hand of the orang, and the flipper of the dolphin and seal, all have the same general structure as the hand of man; and the wings of the bat and bird, the forelimbs of the lizard and elephant, and the comical shovels of the mole and ornithorhynchus, notwithstanding the great differences in their external appearance and use, contain essentially the same bones and the same arrangement of the bones as do the arms of men and women. The human body has two primary cavities in it. So have the bodies of all vertebrates: a neural cavity containing the brain and spinal cord, and a visceral cavity containing the heart, liver, lungs, and alimentary canal. Invertebrates have only one body cavity—the one corresponding to the visceral cavity of vertebrates—and the main nerve trunk, instead of extending along the back, as among vertebrates, is in invertebrates located ventrally. Vertebrates are the only animals on the earth that have a highly developed circulatory system, a system entirely shut off from the other systems, and containing a heart, arteries, veins, and capillaries. In all invertebrates the digestive and circulatory systems remain to a greater or less extent connected, the blood and food mingling more or less in the general cavity of the body. Worms and insects have pulsating tubes instead of heart and arteries. Crustaceans have hearts with one chamber, and mollusks have two or three chambered hearts, but the blood, instead of returning to the heart after its journey through the arteries, passes into the body cavity. In man and other vertebrates the circulating current is confined strictly to the bloodvessels, no particle of it ever escaping into the general body cavity. The heart of vertebrates is distinguished from that of invertebrates by being located ventrally. The heart of invertebrates is in the back. The blood of vertebrates differs from that of invertebrates in containing both red and white corpuscles. Invertebrates have white corpuscles only. Worms have yellow, red, or bright green blood. The blood of crustaceans is bluish, that of mollusks is white, and that of insects dusky or brown. The blood of all vertebrates, excepting amphioxus, is red. All backboned beings, whether they dwell in seas or cities, and whether they build nests or empires, have two eyes, two ears, nose and mouth, all located in the head, and always occupying the same relative position to each other. Invertebrates may have their brains in their abdomen, as do the mites; hear with their legs or antennae, as many insects do; see with their tunics, like the scallops; and breathe with their skin, as do the worms. The crayfish hears with its ‘feelers,’ the cricket and katydid with their fore-legs, the grasshopper with its abdomen, the clam with its ‘foot,’ and mysis and other low crustaceans have their auditory organs on their tails.Man is, then, like the fishes, frogs, reptiles, birds, and quadrupeds, a vertebrate animal. Excepting in his infancy, when he is a quadruped going on all fours, he uses his posterior limbs only for locomotion, and his anterior for prehension and the like. His spinal axis is erect instead of horizontal, and his tail is atrophied. But he possesses all of the unmistakable qualities of the vertebrate type of structure—a two-chambered body cavity, a highly developed and dorsally located nerve trunk, vertebrate vitals, a closed circulatory system, a ventral heart, red blood, a head containing sense organs and brain, and a well-ordered internal skeleton, consisting of a vertebral column with skull and ribs and two pairs of limbs, the limbs consisting each of one long bone, two long bones, two transverse rows of irregular bones, and five branches at the end.1.See ‘Classes of Animals,’ at the end of the chapter.2.Snakes are limbless, and hind-limbs are lacking in whales and other degenerates; but rudimentary limbs are found in the embryonic stages of these animals. Frogs, it may be said also, have no ribs.Classes of AnimalsIII. Man a Mammal.Man is amammal. He belongs to the most brilliant and influential of the five classes of vertebrates—the class to which belong so many of his associates and victims, the class to which belong the horse, the dog, the deer, the ox, the sheep, the swine, the squirrel, the camel, the unattenuated elephant, and the timid-hearted hare. To this class belong also the lion, the tiger, the kangaroo, the beaver, the bear, the bat, the monkey, the mole, the wolf, the ornithorhynchus, and the whale—in short,all animals that have hair. Fishes and reptiles have scales; birds have feathers; all mammals are covered to a greater or less extent with hair. The aquatic habits of whales render hair of no use to them. Hence, while the unborn of these animals still cling to the structural traditions of their ancestors and are covered with hair, the adults are almost hairless. The sartorial habits of human beings and the selective influences of the sexes have had a similar effect on the hairy covering of the human body. Hair exists all over the human body surface, excepting on the soles of the hands and feet, but in a greatly dwarfed condition. It is only on the scalp and on the faces of males, where it is scientifically assisted for purposes of display, that it grows luxuriantly. It is by no means certain that even the hair on the masculine scalp will last forever. For if the hermetical derby and other deadly devices worn by men continue their devastations as they have in the past, we may expect to have, in the course of generations, men with foreheads reaching regularly to the occiput. Most animals lay eggs. Man does not. Like the dog, the horse, the squirrel, and the bat, man is viviparous, the eggs hatching within the parental body. Human young are born helpless, and are sustained during the period of their infancy by the secretions of the milk glands. So are all the sons and daughters of mammals. Whether they come into the world among the waters or among the desert sands, in the hollow of a tree, in a hole in the earth, or in a palace, the children of mammals are frail and pitiful, and they survive to grow and multiply only because they are the object of the loving and incessant sacrifices of a mother.Mammals are distinguished from all other animals by the possession of two kinds of skin glands—the sweat glands and the oil glands—and by the development of certain of these glands in the female into organs for the nourishing of the young. Among reptiles and birds the lower jaw is suspended from the skull by a bone called the quadrate bone. Among men and other mammals the lower jaw is joined directly to the skull, the quadrate bone becoming, in the vicissitudes of evolution, the hammer (malleus) of the mammalian ear. Man has a four-chambered heart—two reservoirs which receive, and two pumps which propel, the scarlet waters of the body. Fishes have two-chambered hearts; frogs and most reptiles have three-chambered hearts; all mammals and birds have four-chambered hearts. The red corpuscles in the blood of fishes, frogs, reptiles, and birds, are discs, double-convex, nucleated, and in shape oval or triangular. In man and in all other mammals (except the archaic camel) the red corpuscles are double-concave, non-nucleated, and circular. ‘Man has a diaphragm dividing the body cavity into chest and abdomen, and a shining white bridge of interlacing fibres, calledcorpus callosum, uniting his cerebral hemispheres. And man is a mammal because, like other mammals, he has, in addition to the qualities already mentioned, these valuable and distinct characteristics.IV. Man a Primate.Man is aprimate. There are four divisions in the order of primates—lemurs, monkeys, apes, and men. But the most interesting and important of these, according to man, is man. Man is a primate because, like other primates, he has arms and hands instead of fore-legs. And these are important characteristics. It was a splendid moment when the tendencies of evolution, pondering the possibilities of structural improvement, decided to rear the vertebrate upon its hind-limbs, and convert its anterior appendages into instruments of manipulation. So long as living creatures were able simply to move through the airs and waters of the earth and over the surface of the solids, they were powerless to modify the universe about them very much. But the moment beings were developed with parts of their bodies fitted to take hold of and move and fashion and compel the universe around them, that moment the life process was endowed with the power of miracles. With the invention of hands and arms commenced seriously that long campaign against the tendencies of inanimate nature which finds its most marvellous achievements in the sustained and triumphant operations of human industry. None of the primates excepting man use their hind-limbs as a sole means of changing their place in the universe, but in all of them the fore-limbs are regularly used as organs of manipulation. Man is a primate because his fingers and toes, like those of other primates (except the tiny marmosets of Brazil), end in nails. Man has neither claws to burrow into the earth, talons with which to hold and rend his victims, nor hoofs to put thunder into his movements. The human stomach, like that of all the other primates, is a bagpipe. The stomach of the carnivora is usually a simple sack, while rodents have, as a rule, two stomachs, and ruminants four. Man is a primate because his milk glands are located on the breast and are two in number. The mammary glands vary in number in the different orders of mammals, from two in the horse and whale to twenty-two in some insectivora. Most ruminating animals have four, swine ten, and carnivora generally six or eight. These glands may be located in the region of the groin, as in the horse and whale; between the forelimbs, as in the elephant and bat; or arranged in pairs extending from the fore to the hind limbs, as in the carnivora and swine. In man and all other primates (except lemurs) the mammary glands are pectoral and two in number. All primates, including man, have also a disc-shaped placenta. The placenta is the organ of nutrition in mammalian embryos. It is found in all young-bearing animals above the marsupials, and consists of a mass of glands between the embryo and the parental body. In some animals it entirely surrounds and encloses the embryo; in others it assumes the form of a girdle; and in still others it is bell-shaped. The primates are the only animals in which this peculiar organ is in the shape of a simple disc.[1]The nearest relatives by blood man has in this world are the exceedingly man-like apes—the tailless anthropoids—the gorillas and chimpanzees of Africa, and the orangs and gibbons of southern and insular Asia. The fact that man is an actual relative and descendant of the ape is one of the most disagreeable of the many distasteful truths which the human mind in its evolution has come upon. To a vanity puffed, as is that of human beings, to the splitting, the consanguinity of gorilla and gentleman seems horrible. Man prefers to have arrived on the earth by way of a ladder let down by his imagination from the celestial concave. Within his own memory man has been guilty of many foolish and disgraceful things. But this attempt by him to repudiate his ancestors by surreptitiously fabricating for himself an origin different from, and more glorious than the rest is one of the most absurd and scandalous in the whole list. It is a shallow logic—the logic of those who, without worth of their own, try to shine with a false and stolen lustre. No more masterly rebuke was ever administered to those in the habit of sneering at the truth in this matter than the caustic reply of Huxley to the taunt of the fat-witted Bishop—that he would rather be the descendant of a respectable ape than the descendant of one who not only closed his eyes to the facts around him, but used his official position to persuade others to do likewise. Man’s reluctance to take his anatomical place beside his simian kinspeople has been exceeded only by his selfish and high-handed determination to exclude all other terrestrial beings from his heaven.Man is a talkative and religious ape. He is an ape, but with a much greater amount of enterprise and with a greater likelihood of being found in every variety of climate. Like the anthropoid, man has a bald face and an obsolete tail. But he is distinguished from his arboreal relative by his arrogant bearing, his skilled larynx, and especially by the satisfaction he experiences in the contemplation of the image which appears when he looks in a mirror.The man-like apes are from three to six feet tall, and are all of them very strong, the gorilla, who sometimes weighs over three hundred pounds, being about the bravest and most formidable unarmed animal on the planet. They are erect or semi-erect, have loud voices, plantigrade feet, and irritable dispositions—in all of these particulars being strikingly like men. The gorilla, chimpanzee, and gibbon are highlanders, preferring the uplands and mountains. The orang is a lowlander, living phlegmatically among the sylvan swamps of Sumatra and Borneo. The gorilla and chimpanzee are terrestrial, seldom going among the trees except to get food or to sleep. The orang and gibbon are arboreal, seldom coming to the ground except to drink or bathe. They all walk on their hind-limbs, generally in a stooping posture, with their knuckles or fingers touching the ground. But they sometimes walk with their arms hanging down by their sides, and sometimes with their hands clasped back of their heads to give them balance. None of them ever place their palms on the ground when they walk—that is, none of them walk on four feet. The anthropoid races, in the shape of their heads and faces and in the general form and structure of their bodies, and even in their habits of life, resemble in a remarkable manner the lowest races of human beings. This resemblance is recognised by the negro races, who call the gorilla and chimpanzee ‘hairy men,’ and believe them to be descendants of outcast members of their own species.There are differences in structure between man and the apes, just as there are differences in structure between the Caucasian and the Caffre, or even between individual Caucasians or individual Caffres. There are differences in structure and topography, often very noticeable differences, even among members of the same family. But in all of its essential characters, and extending often to astonishing particulars, the structure of man is identical with that of the anthropoid.[2]In external appearances the man-like races differ from men in having a luxuriant covering of natural hair. But anthropoids differ very much among themselves in this particular. The orang, usually covered with long hair, is sometimes almost hairless. There are, too, races of human beings whose bodies are covered with a considerable growth of hair. The Todas (Australians) and Ainus (aborigines of Japan) are noted for the hairiness of their bodies, certain individuals among them being covered with a real fur, especially on the lower limbs.[3]Individuals also often appear in every race with a remarkable development of the hair. Adrian and his son Fedor, exhibited years ago over Europe as ‘dog-men,’ are examples. The father was completely covered with a thick growth of fine dirty-yellow hair two or three inches long. Long tufts grew out of his nostrils and ears, giving him a striking resemblance to a Skye terrier. Fedor, and also his sister, were covered with hair like the father, but another son was like ordinary men. The man-like races have also longer arms in proportion to the height of the body than man generally has. But this is also true of human infants and negroes. The gibbon has relatively much longer arms than the other anthropoids. It differs from the chimpanzee in this respect more than the chimpanzee differs from man. When standing upright and reaching down with the middle finger, the gibbon can touch its foot, while the chimpanzee can reach only to the knee. Man ordinarily reaches part way down the thigh, but negroes have been known to have arms reaching to the knee-pan.[4]The skeleton of the African races contains many characters recognised by osteologists as ‘pithecoid,’ or ape-like. It is massive, the flat bones are thick, and the pelvis narrow. In the manlike apes the large toe is opposable to the other four, and is used by them much as the thumb is used. But this difference between the two races of beings is just what might be expected from the differences in their modes of life. Man has little need of this opposability on account of his exclusively terrestrial life, while to the ape it is indispensable on account of his arboreal environment and life. ‘But there are,’ says Haeckel, ‘wild tribes of men who can oppose the large toe to the other four just as if it were a thumb, and even new-born infants of the most highly-developed races of men can grasp as easily with their hind-hands as with their forehands. Chinese boatmen row with their feet, and Bengal workmen weave with them. The negro, in whom the big toe is freely movable, seizes hold of the branches of trees with it when climbing, just like the four-handed apes’.[5]Many men have lost their arms by accident and have learned to use their feet as hands with wonderful skill. Not many years ago there died in Europe an armless violinist who had during his lifetime played to cultured audiences in most of the capitals of the world. Some of the most accomplished of penmen hold their pen between their toes. The man-like apes live to about the same age as man, and all of them, like man, have beards. The anthropoid beard, too, like the human, appears at the age of sexual maturity. The human beard often differs in colour from the hair of the scalp, and whenever it does it has been observed to be invariably lighter—never darker—than the hair on the scalp. This is true among all races of men. The same rule and the same uniformity exists among anthropoids. The races of mankind are divided into two primary groups depending upon the shape of the head and the character of the hair: the short-headed races (Brachycephali), such as the Malays, Mongols, and Aryans, with round or oval faces, straight hair, and vertical profiles; and the long-headed races (Dolichocephali), with woolly hair and prognathous faces, such as the Papuans and Africa races. The skin of the short-headed races is orange or white, while the skin and hair of the long-headed races are glossy black.It is, at least, interesting that the orang and gibbon, who live in Asia and its islands, where the brachycephalic races of men supposedly arose, are themselves brachycephalic; and that the gorilla and chimpanzee, who live in Africa, where the dolichocephalic races chiefly live, are dolichocephalic. The gorilla and chimpanzee also have, like the men and women of Africa, black skin and hair; while the hair of the orang is a reddish-brown, and its skin sometimes yellowish-white. The dentition of the anthropoids and men is in all essentials identical. They all have two sets of teeth: a set of milk-teeth, twenty in number, and thirty-two permanent teeth, the permanents consisting of two incisors, one canine, two premolars, and three molars, in each half-jaw. Man has ordinarily twelve pairs of ribs and thirty-two vertebrae. So has the orang. The other anthropoids have thirteen pairs of ribs. But the number of ribs in both human and anthropoid beings is not uniform, man occasionally having thirteen pairs, and the gorilla fourteen. Man has also the same number of caudal vertebrae in his rudimentary tail as the anthropoid has. The hands and feet of anthropoids, bone for bone and muscle for muscle, correspond with those of men, no greater structural differences existing than among different species of men. The human foot has three muscles not found in the human hand—a short flexor muscle, a short extensor muscle, and a long muscle extending from the fibula to the foot. All of these muscles are found in the anthropoid foot just as in the foot of man. There are also the same differences between the arrangement of the bones of the anthropoid wrist and ankle as between the wrist and ankle bones of man. Whatever set of anatomical particulars may be selected, whether it be hands, arms, feet, muscles, skull, viscera, ribs, or dentition, it is found that the anthropoid races and men are in all essentials the same. The differences are such as have arisen as a result of different modes of life, and such as exist between different tribes of either group of animals.‘The structural differences which separate man from the gorilla and chimpanzee,’ says Huxley, in summing up the conclusion of his brilliant inquiry into ‘Man’s Place in Nature,’ ‘are not so great as those which separate the gorilla from the lower apes.’‘The body of man and that of the anthropoid are not only peculiarly similar,’ says Haeckel, ‘but they are practically one and the same in every important respect. The same two hundred bones, in the same order and structure, make up our inner skeleton; the same three hundred muscles effect our movements; the same hair clothes our skin; the same four-chambered heart is the central pulsometer in our circulation; the same thirty-two teeth are set in the same order in our jaws; the same salivary, hepatic, and gastric glands compass our digestion; the same reproductive organs insure the maintenance of our race’.[6]‘Not being able,’ says Owen in his paper on ‘The Characters of Mammalia,’ ‘to appreciate or conceive of the distinction between the psychical phenomena of a chimpanzee and of a Boschisman or of an Aztec with arrested brain-growth, as being of a nature so essential as to preclude a comparison between them, or as being other than a difference in degree, I cannot shut my eyes to the significance of that all-pervading similitude of structure—every tooth, every bone, strictly homologous—which makes the determination of the difference betweenHomoandPithecusthe anatomist’s difficulty.’‘If before the appearance of man on the earth,’ says Ward in his ‘Dynamic Sociology,’ ‘an imaginary painter had visited it, and drawn a portrait embodying the thorax of the gibbon, the hands and feet of the gorilla, the form and skull of the chimpanzee, the brain development of the orang, and the countenance ofSemnopithecus, giving to the whole the average stature of all of these apes, the result would have been a being not far removed from our conception of the primitive man, and not widely different from the actual condition of certain low tribes of savages. The brain development would perhaps be too low for the average of any existing tribe, and would correspond better with that of certain microcephalous idiots and cretins, of which the human race furnishes many examples.’And it is not true, as is commonly supposed, that, after all other resemblances between the human and anthropoid structures have been made out, there still exists somewhere some undistinguishable difference in the organic structure of their brains. All differences in structure from time to time suspected or asserted to exist between the brain of man and that of the man-like apes have been one after another completely swept away. And it is now known to all neurologists that the human and anthropoid brains differ structurally in no particulars whatever, both of them containing the same lobes, the same ventricles and cornua, and the same convolutional outline. Even the posterior lobe, the posterior cornu, and the hippocampus minor, so long triumphantly asserted to be characteristic features of the human brain, have been pitilessly identified in all anthropoids by the profound and terrible Huxley. There is not an important fold or fissure in the brain of man that is not found in the brain of the anthropoid. ‘The surface of the brain of a monkey,’ says Huxley, ‘exhibits a sort of skeleton map of man’s, and in the man-like apes the details become more and more filled in, until it is only in minor characters that the chimpanzee’s or the orang’s brain can be structurally distinguished from man’s’.[7]The great difference physically between man and the anthropoids, aside from man’s talented larynx and erect posture, lies in man’s abnormal cranial capacity. The normal human cranium never contains less than 55 cubic inches of space, while the largest gorilla cranium contains only 34½ cubic inches. This is a difference of 20½ cubic inches. And 20½ cubic inches of thinking matter is an alarming amount to be lacking in a single individual. But this cranial gap between gorilla and man is deprived of some of its significance by the fact that human crania sometimes measure 114 cubic inches, making a difference between the smallest and largest human brains of 59 cubic inches. The difference between the gorilla and the savage in cranial capacity is, therefore,only about one-third as great as the cranial chasm between the savage and the sage.1.The bat and a few other animals have a disc-like placenta, but it develops into the disc shape by a different route from what it does in the primates.2.Hartmann:Anthropoid Apes; New York, 1901.3.Quatrefages:The Human Species; New York, 1898.4.Tyler:Anthropology; New York; 1899.5.Haeckel:History of Creation, 2 vols.; New York, 1896.6.Haeckel:The Riddle of the Universe; New York, 1901.7.Huxley:Man’s Place in Nature; New York, 1883.V. Recapitulation.The anatomical gulf between men and apes does not exist. There are, in fact, no gulfs anywhere, only gradations. All chasms are completely covered by unmistakable affinities, in spite of the fact that the remains of so many millions of deceased races lie hidden beneath seas or everlastingly locked in the limy bosoms of the continents. There are closer kinships and remoter kinships, but there are kinships everywhere. The more intimate kinships are indicated by more definite and detailed similarities, and the more general relationships by more fundamental resemblances. All creatures are bound to all other creatures by the ties of a varying but undeniable consanguinity.Man stands unquestionably in the primate order of animals, because he has certain qualities of structure which all primates have, and which all other animals have not: hands and arms and nails, a bagpipe stomach, great subordination of the cerebellum, a disc-like placenta, teeth differentiated into incisors, canines, and molars, and pectoral milk glands.Man is more closely akin to the anthropoid apes than to the other primates on account of his immense brain, his ape-like face, his vertical spine, and in being a true two-handed biped. The manlike apes and men have the same number and kinds of teeth, the same limb bones and muscles, like ribs and vertebrae, an atrophied tail, the same brain structure, and a suspicious similarity in looks and disposition. Men and anthropoids live about the same number of years, both being toothless and wrinkled in old age. The beard, too, in both classes of animals appears at the same period of life and obeys the same law of variation in colour. Even the hairs on different parts of the bodies of men and anthropoids, as on the arms, incline at a like angle to the body surface. The hair on the upper arm and that on the forearm, in both anthropoids and men, point in opposite directions—toward the elbow. This peculiarity is found nowhere in the animal kingdom excepting in a few American monkeys.Man’s mammalian affinities are shown in his diaphragm, his hair, his four-chambered heart, hiscorpus callosum, his non-nucleated blood-corpuscles, and his awkward incubation.The fishes, frogs, reptiles, birds, and non-human mammals are human in having two body cavities, segmented internal skeletons, two pairs of limbs, skulls and spinal columns, red blood, brains, and dorsal cords; and in possessing two eyes, two ears, nostrils, and mouth opening out of the head. And finally all animals, including man, are related to all other animal forms by the great underlying facts of their origin, structure, composition, and destiny. All creatures, whether they live in the sea, in the heavens, or in subterranean glooms; whether they swim, fly, crawl, or walk; whether their world is a planet or a water-drop; and whether they realise it or not, commence existence in the same way, are composed of the same substances, are nourished by the same matters, follow fundamentally the same occupations, all do under the circumstances the best they can, and all arrive ultimately at the same pitiful end.VI. The Meaning of Homology.The similarities and homologies of structure existing between man and other animals, and between other animals and still others, are not accidental and causeless. They are not resemblances scattered arbitrarily among the multitudinous forms of life by the capricious levities of chance. That all animals commence existence as an egg and are all made up of cells composed of the same protoplasmic substance, and all inhale oxygen and exhale carbon dioxide, and are all seeking pleasure and seeking to avoid pain, are more than ordinary facts. They are filled with inferences. That vertebrate animals, differing in externals as widely as herring and Englishmen, are all built according to the same fundamental plan, with marrow-filled backbones and exactly two pairs of limbs branching in the same way, is an astonishing coincidence. That the wing of the bird, the foreleg of the dog, the flipper of the whale, and the fore-limb of the toad and crocodile, have essentially the same bones as the human arm has is a fact which may be without significance to blind men, but to no one else. The metamorphosis of the frog from a fish, of the insect from a worm, and of a poet from a senseless cell, are transformations simply marvellous in meaning. And it is not easy, since Darwin, to understand how such lessons could remain long unintelligible, even to stones and simpletons. Not many generations have passed, however, since these revelations, now so distinct and wonderful, fell on the listless minds of men as ineffectually as the glories of the flower fall on the sightless sockets of the blind.It is hardly two generations since the highest intelligences on the earth conceived that not only the different varieties of men—the black, the white, and the orange—but all the orders and genera of the animal world, and not only animals, but plants, had all been somehow simultaneously and arbitrarily brought into existence in some indistinct antiquity, and that they had from the beginning all existed with practically the same features and in approximately the same conditions as those with which and in which they are found to-day. The universe was conceived to be a fixed and stupid something, born as we see it, incapable of growth, and indulging in nothing but repetitions. There were no necessary coherencies and consanguinities, no cosmical tendencies operating eternally and universally. All was whimsical and arbitrary. It was not known that anything had grown or evolved. All things were believed to have been given beginning and assigned to their respective places in the universe by a potential and all-clever creator. The serpent was limbless because it had officiously allowed Eve to include in her dietary that which had been expressly forbidden. The quadruped walked with its face towards the earth as a structural reminder of its subjection to the biped, who was supposed to be especially skilled in keeping his eyes rolled heavenward. The flowers flung out their colours, not for the benefit of the bugs and bees, and the stars paraded, not because they were moved to do so by their own eternal urgings, but because man had eyes capable of being affected by them. Man was an erect and featherless vertebrate because his hypothetical maker was erect and featherless. (I wonder whether, if a clam should conceive a creator, it would have the magnanimity to make him an insect or a vertebrate, or anything other than a great big clam.)VII. The Earth an Evolution.The world now knows—at least, the scientific part of it knows—that these things are not true, that they are but the solemn fancies of honest but simple-minded ancients who did the best they could in that twilight age to explain to their inquiring instincts the wilderness of phenomena in which they found themselves. The universe is a process. It is not petrified, but flowing. It is going somewhere. Everything is changing and evolving, and will always continue to do so. The forms of life, of continents and oceans, and of streams and systems, which we perceive as we open our senses upon the world to-day, are not the forms that have always existed, and they are not the forms of the eternal future. There was a time, away in the inconceivable, when there was no life upon the earth, no solids, and no seas. The world was an incandescent lump, lifeless and alone, in the cold solitudes of the spaces. There was a time—there must have been a time—when life appeared for the first time upon the earth, simple cellules without bones or blood, and without a suspicion of their immense and quarrelsome posterity. There was a time when North America was an island, and the Alleghany Mountains were the only mountains of the continent. The time was—in the coal-forming age—when the Mississippi Valley, from the Colorado Islands to the Alleghanies, was a vast marsh or sea, choked with forests of equisetum and fern, and swarming with gigantic reptiles now extinct. There was a time when palms grew in Dakota, and magnolias waved in the semi-tropical climate of Greenland and Spitzbergen. There was a time when there were no Rocky Mountains in existence, no Andes, no Alps, no Pyrenees, and no Himalayas. And that time, compared with the vast stretches of geological duration, was not so very long ago, for these mountains are all young mountains. The time was when Jurassic saurians—those repulsive ruffians of that rude old time—represented the highest intelligence and civilisation of the known universe. There were no men and women in the world, not even savages, when our ape-like forefathers wandered and wondered through the awesome silences of primeval wilds; there were no railroads, steamboats, telegraphs, telephones, typewriters, harvesters, electric lights, nor sewing machines; no billionaires nor bicycles, no socialists nor steam-heat, no ‘watered stock’ nor ‘government by injunction,’ no women’s clubs, captains of industry, labour unions, nor ‘yellow perils’—there was none of these things on the earth a hundred years ago. All things have evolved to be what they are—the continents, oceans, and atmospheres, and the plants and populations that live in and upon them.There will come a time, too, looking forward into the future, when what we see now will be seen no more. As we go backward into the past, the earth in all of its aspects rapidly changes; the continents dwindle, the mountains melt, and existing races and species disappear one after another. The farther we penetrate into the past, the stranger and the more different from the present does everything become, until finally we come to a world of molten rocks and vapourised seas without a creeping thing upon it. As it has been in the past so will it be in time to come. The present is not everlasting. The minds that perceive upon this planet a thousand centuries in the future will perceive a very different world from that which the minds of this day perceive—different arts, animals, events, ideals, geographies, sciences, and civilisations. The earth seems fixed and changeless because we are so fleeting. We see it but a moment, and are gone. The tossing forest in the wrath of the storm is motionless when looked at by a flash of lightning. The same tendencies that have worked past changes are at work to-day as tirelessly as in the past. By invisible chisels the mountains are being sculptured, ocean floors are lifting, and continents are sinking into the seas. Species, systems, and civilisations are changing, some crumbling and passing away, others rising out of the ruins of the departed. Mighty astronomical tendencies are secretly but relentlessly at work, and immense vicissitudes are in store for this clod of our nativity. The earth is doomed to be frozen to death. In a few million years, according to astronomers, the sun will have shrunken to a fraction of his present size, and will have become correspondingly reduced in heat-giving powers. It is estimated that in twelve or fifteen million years the sun, upon whose mighty dispensations all life and activity on the earth are absolutely dependent, will become so enfeebled that no form of life on the earth will be possible. The partially-cooled earth itself is giving up its internal warmth, and will continue to give it up until it is the same temperature as the surrounding abysms, which is the frightful negative of something like 270 centigrade degrees. These are not very cheerful facts for those who inhabit the earth to contemplate. But they that seek the things that cheer must seek another sphere. No power can stay the emaciation of suns or the thievery of enveloping immensities. Old age is inevitable. It is far off, but it is as certain as human decay, and as mournful. In that dreadful but inevitable time no living being will be left in this world; there will be no cities nor states nor vanities nor creeping things, no flowers, no twilights, no love, only a frozen sphere. The oceans that now rave against the rocky flanks of the continents will be locked in eternal immobility; the atmospheres, which to-day drive their fleecy flocks over the azure meads of heaven and float sweet sounds and feathered forms, will be, in that terrible time, turned to stone; the radiant woods and fields, the home of the myriads and the green play-places of the shadows, will, like all that live, move, and breathe, have rotted into the everlasting lumber of the elements. There will be no Europe then, no pompous philosophies, no hellish rich, and no gods. All will have suffered indescribable refrigeration. The earth will be a fluidless, lifeless, sunless cinder, unimaginably dead and desolate, a decrepit and pitiful old ruin falling endlessly among heartless immensities, the universal tomb of the activities.The universe is an evolution. Change is as extensive as time and space. The present has come out of that which has been, and will enter into and determine that which is to be. Everything has a biography. Everything has evolved—everything—from the murmur on the lips of the speechless babe to the soul of the poet, and from the molecule to Jehovah.VIII. The Factors of Organic Evolution.The animal kingdom represents one of the two grand branches of the organic universe. It has been evolved—evolved in a manner as simple and straightforward as it is revolting. It has all been brought about bypartialityorselection. Generations of beings have come into existence. The individual members of each generation have differed from each other—differed in size, strength, speed, colour, shape, sagacity, luck, and likelihood of life. No two beings, not even those born from the same womb, are in all respects identical. Hardships have come. They have come from the inanimate universe in the form of floods, fires, frosts, accidents, diseases, droughts, storms, and the like; from other species, who were competitors or enemies; and from unbrotherly members of the same species. Some have survived, but the great majority have perished. Only a fraction, and generally an appallingly small fraction, of each generation of a species have lived to maturity. The lobster lays 10,000 eggs in a season, yet the mortality is such that the number of lobsters do not increase from one year to another. The elephant is the slowest breeder of all animals, yet, if they should all live, the offspring of a single pair in 750 years would, according to Darwin, number nearly 19,000,000. It has been shown that at the normal rate of increase of English sparrows, if none were to die save of old age, it would take but twenty years for a single pair to give one sparrow to every square inch in the State of Indiana.[1]A single cyclops (one of the humbler crustaceans) may have 5,000,000 descendants in a season. One aphis will produce 100 young, and these young will reproduce in like manner for ten generations in a season, when, if they should all live, there would be a quintillion of young. A female white ant, when adult, does nothing but lie in a cell and lay eggs. She lays 80,000 eggs a day regularly for several months. An oyster lays 2,000,000 eggs in a season, and if all these eggs came to maturity a few dozen oysters might supply the markets of the world. The tapeworm is said to produce the incredible number of 1,000,000,000 ova, and some of the humbler plants three times this number of spores. If each egg of the codfish should produce an adult, a single pair in twenty-five years would produce a mass of fish larger than the earth. Lower forms of life are even more prolific than the higher. Maupas said that certain microscopic infusorians which he studied multiplied so rapidly that, if they should continue to multiply for thirty-eight days, and all of them should live, any one of them would produce a mass of protoplasm as big as the sun.Those of each generation that have died have been inferior, or unfitted to the environment in which they found themselves. Those that have survived have been superior, superior in something—bigness, cunning, courage, virtue, vitality, strength, speed, littleness, or ferocity—something that has related them advantageously to surrounding conditions. The surviving remnant of each generation have become the progenitors of the next generation, and have transmitted, or tended to transmit, to their offspring the qualities of their superiority. This winnowing has gone on in each generation of living beings during many millions of years—almost ever since life commenced to be on the earth. Some have continued themselves, and others have died childless. The environment of each species has been an immense sieve, and only the superior have gone through it. Different environments have emphasised different qualities of structure and disposition, and have thus given rise to permanent varieties in survival. These varieties, through the accumulated effects of many generations of selection, have diverged into species; species, after a still longer series of selections, have evolved into genera; genera have evolved into families; families into orders; and so on. In this simple, terrible manner have all the branches of organic beings (thanks to the horrors of a million ages) been brought into existence.Variation, therefore, which furnishes variety in offspring;Heredity, which tends to perpetuate peculiarities by causing offspring to resemble more or less the characters of their parents; andEnvironment, which determines the character of the selections, are the three factors, and the only three factors, in organic evolution.1.Jordan:Footnotes of Evolution; New York, 1898.IX. The Evidences of Organic Evolution.That the forms of life to-day found on the earth have come into existence by the evolution of the more complex forms from the simpler, and of these simpler forms from still simpler, through the ever-operating law of Selection, is a necessary conclusion from the following facts:1. The existence in the animal world of all grades of structures, from the humblest possible protozoan, whose body consists of a single simple speck, to the most powerful and complex of mammals. There are estimated to be something like a million species of animals living on the earth to-day. There may be several times this number. These species are linked together by millions of varieties, and are so related to each other that they may be all gathered together into various genera; these genera may be grouped into families, the families into orders, and the orders into seven or eight great primary phyla. By taking existing species and adding to them the extinct species of the rocks, and placing them all according to their structural affinities, it is possible to arrange them in the form of a tree with the various phyla, orders, families, genera, and species, branching and rebranching from the main trunk. The existence of structures, so graduated as to render such an arrangement possible, is in itself suggestive of a common relationship and origin.2. Evolution is suggested by the similarities and homologies of structure found throughout the animal kingdom. Some of these similarities and homologies have already been mentioned. They are everywhere—remoter and more fundamental, some of them, others closer and more detailed. To the untrained mind, which sees surfaces only, and not even surfaces well, the animal world is an interminable miscellany of forms. But to the biologist, who looks deeper and with immense acumen over the whole field of animal life, there are only seven or eight different types of structure in the entire animal world. These seven or eight types correspond with the primary classes, or phyla, into which animals are divided, viz., protozoa, sponges, celenterates, echinoderms, worms, mollusks, arthropods, and vertebrates. However widely the members of each of these great groups may differ among themselves in colour, size, habits of life, and the like, the members of each group all resemble each other fundamentally. Moles differ from monkeys, bats from men, and birds from crocodiles and toads. They differ enormously. But they are all vertebrates with red blood, double body cavities, backbones, two pairs of limbs, and five fingers on each limb. When they are looked at superficially, there is not much similarity between a water-strider and a butterfly or between a stag-beetle and a gnat. But they are all, in reality, built according to the same plan. Like all other insects, they have six legs, a sheath-like skeleton, and bodies characteristically divided into head, thorax, and abdomen. It is the same with all other great classes of beings. All worms resemble each other; and so do all mollusks, although they may differ in particulars as widely as nautiluses and clams. Echinoderms have a radiate structure, celenterates and sponges are vase-like in shape, and protozoa are one-celled. The differences in structure among the members of a group consist in different modifications of a fundamental type. Among the vertebrates the fore-limb may be an arm, a leg, a wing, a shovel, a flipper, or a fin. But in all cases it is the same organ—that is, the same implement modified to serve different ends. Take the mouth-parts of insects. In the grasshopper and cricket these parts are fitted for grinding; in the moths and butterflies they are fashioned into long tubes for sucking the sweets of flowers; in the mosquito they form an elaborate apparatus for drilling and drinking; and in the mayfly the mouth-parts, though present, are not used at all. In all of these animals these parts are essentially the same, although differing so much in their forms and purposes that the unscientific can scarcely be made to believe they are fundamentally alike. There is no fact more familiar to the biologist or more frequently met with in the fields of animal morphology than the fact that the same general type may be hammered into dozens, or hundreds, or even thousands, of different patterns by the incessant industry of its surroundings, and that the same organic part may be moulded into various implements serving totally different ends by the environmental vicissitudes of time and space. On the hypothesis that the members of each group of animals possessing common characteristics, whether the group be large or small, have sprung from a common ancestry, and that the differences in structure have arisen as a result of differences in environment, the similarities and homologies of structure existing among animals are perfectly intelligible. But on any other supposition they are inexplicable.3. Evolution is suggested by the remarkable series of phenomena presented by embryology. There are at least four facts in the developmental history of every creature which can hardly be accounted for on any other supposition than that of organic evolution.First, the fact that every animal, above the lowest, individually passes through an evolution between the beginning of its existence and its maturity. Terrestrial beings are not born, like Minerva, full-grown. They grow. They evolve. They commence close down to the very atoms. And from this lowly genesis they rise, through a series of marvellous changes, to that high state of perfection and greatness from which they descend to dissolution.If we knew by actual observation as little concerning the evolution of individuals as we do of the evolution of species—if we had always been used to seeing animals, including ourselves, in full bloom—had never watched the tadpole, the pupa, and the babe pass through their wonderful metamorphoses on their way to maturity, it would probably be just as hard for many minds to believe that animals evolve individually to be what they are as it is for them to believe that species have grown to be what they are. In the case of individuals, however, the evolution takes place right before our eyes largely, while the evolution of species goes on so slowly and stretches back so far into the past that it can only be inferred.Second, the fact that animals, no matter how much they may differ from each other at maturity, all begin existence at the same place. Every animal commences its organic existence as an egg—as a one-celled animal—as an organism identical in structure with the simplest protozoan. The ova of whales ‘are no larger than fern seeds.’ The eggs of the coral, the crab, the ape, and the man are so precisely alike that the highest powers of the microscope cannot distinguish between them.Third, the fact that the members of the same great group of animals in their individual development pass through similar stages of evolution. The ‘worm’ stage in the development of most insects and the ‘fish’ stage of frogs are well known.There are no more remarkable instances of individual evolution in the whole range of animal life. The fish, the reptile, the bird, the dog, and the human being—all vertebrates, in short—cannot for some time after their embryonic commencement be distinguished from each other. ‘The feet of lizards and mammals, the wings and feet of birds, and the hands and feet of men,’ says the illustrious Von Baer, as quoted by Darwin, ‘all arise from the same fundamental form’.[1a]‘It is quite in the later stages of development,’ says Huxley, ‘that the human being presents marked differences from the ape, while the latter departs as much from the dog in its development as the man does’.[2]Not only frogs, but reptiles, birds, and mammals, including man, all have gills at a certain stage in their embryonic development. Nearly all the lower invertebrate animals are hermaphroditic—that is, in the body of each animal is found the two kinds of sex organs which in the higher animals exist in distinct animals. And frogs, birds, and other higher animals, which as adults are unisexual, have, as an inheritance from these primitive forms, hermaphroditic embryos.[3a]Fourth, the fact that the structural stages through which animals in embryo pass correspond in a wonderful manner with the permanent structures of those lower forms which extend serially back to the beginnings of life. It is the proudest boast of the embryologist that he is able to know the route through which any species has come to be what it is by a simple study of the individual evolution of its members. Each animal repeats in its individual evolution the evolution of its species. This recapitulation is not always complete—is, in fact, frequently vague, sometimes circuitous, and often broken or abbreviated. Processes requiring originally centuries or thousands of years to accomplish are here telescoped into a few months, or even days. It is not strange that the process is imperfect. But so firmly is the belief in the correspondence of ontogeny and phylogeny fixed in the minds of modern biologists that, in determining the classification and affinities of any particular animal, more reliance is placed on the facts of embryology than on those of adult structure.The first thing that an animal becomes after it is an egg—unless it is a one-celled animal, in which case it remains always an egg—is two cells; these two cells become four; these four become eight; and so on, until the embryo becomes a many-celled ball, consisting of a single layer of cells surrounding a fluid interior. A dimple forms in the cell layer on one side of this ball, and, by deepening to a hollow, changes the ball into a double-walled sac. This is the gastrula—the permanent structure of the sponges and celenterates, and an (almost) invariable stage in the larval development of all animals above the sponges and celenterates. The gastrula becomes a worm (or an insect or a fish through the worm) by elongation and enlargement, and by the development of the endoderm, which is the inner layer of the cell wall, into organs of nutrition and reproduction, and by the development of the ectoderm, which is the outer cell layer, into organs of motion and sensation.The embryonic development of a human being is not different in kind from the embryonic development of any other animal. Every human being at the beginning of his organic existence is a protozoan, about1⁄125inch in diameter; at another stage of development he is a tiny sac-shaped mass of cells without blood or nerves, the gastrula; at another stage he is a worm, with a pulsating tube instead of a heart, and without head, neck, spinal column, or limbs; at another stage he has, as a backbone, a rod of cartilage extending along the back, and a faint nerve cord, as in amphioxus, the lowest of the vertebrates; at another stage he is a fish with a two-chambered heart, mesonephric kidneys, and gill-slits with gill arteries leading to them, just as in fishes; at another stage he is a reptile with a three-chambered heart, and voiding his excreta through a cloaca like other reptiles; and finally, when he enters upon post-natal sins and actualities, he is a sprawling, squalling, unreasoning quadruped. The human larva from the fifth to the seventh month of development is covered with a thick growth of hair and has a true caudal appendage, like the monkey. At this stage the embryo has in all thirty-eight vertebrae, nine of which are caudal, and the great toe extends at right angles to the other toes, and is not longer than the other toes, but shorter, as in the ape.These facts are unmistakable. There is a reason for everything, and there is a reason for these transformations through which each generation of living beings journeys. The individual passes through them because the species to which he belongs has passed through them. They represent ancestral wanderings. As if to emphasise the kinship of all of life’s forms and to render incontrovertible the fact of universal evolution, Nature compels every individual to commence existence at the same place, and to recapitulate in his individual evolution the phylogenetic journeyings of his species.4. That existing forms of life have been evolved from other forms, and that these ancestral forms have been different from those derived from them, is shown by the occasional appearance of antecedent and abandoned types of structure among the offspring of existing species. Occasionally a human child is born strangely unlike its parents, but bearing an unmistakable resemblance in looks and disposition to his great-grandfather or some other remote ancestor. This isatavism, that tendency to revert to ancestral types which is prevalent among all animals. We may think of it figuratively as a flash of indecision when Nature hesitates for a moment whether to adopt a new form of structure or cling to the old and tried. Horses and mules are sometimes born with three toes on each foot, and zebra-like stripes on their legs and shoulders; and domestic pigeons, such as are naturally black, red, or mottled, occasionally produce offspring with blue plumage and two black wing-bars, like the wild rock-dove, from which all domestic breeds have sprung. In man the cheekbone and the frontal bone of the forehead consist normally each of a single bone. But in children and human embryos these bones are always double, as is normally the case in adults among some of the anthropoids and other mammals. Gills appear regularly in the embryos of reptiles, birds, and mammals, and human young are sometimes born with gill-slits on the neck. There are times when, owing to inaccurate or incomplete embryological development, these fish-like characteristics are so perfect at birth as to allow liquids, on being swallowed, to pass out through them and trickle down on the outside of the neck. Many muscles are occasionally developed in man which are normal in the apes and other mammals. As many as seven different muscular variations have been found in a single human being, every one of which were muscles found normally in the structure of the apes.[1b]5. Closely akin to atavism, which is the occasional persistence of ancestral types of character, is the regular occurrence of vestigial organs or structures, organs which in ancestral forms have definite functions, but which in existing species, owing to changed conditions, are rudimentary and useless. On the back of each ankle of the horse are two splints, the atrophied remains of the second and fourth toes. Similar vestiges of two obsolete toes are also found just back of the wrists and ankles on all the two-toed ungulates, such as the cow and sheep. In the body of the whale where hind-limbs would naturally be, there are found the anatomical ruins of these organs in the form of a few diminutive bones. The same thing is true in the sirenians. In the Greenland whale there are remnants of both femur and tibia in the region of the atrophied hind-limbs. The snakes are limbless, but the pythons and boas have internal remnants of hind-limbs and sometimes even clawed structures representing toes. The so-called ‘glass-snake’ or ‘joint-snake’ (which is really a limbless lizard) has four complete internal limbs. Young turtles, parrots, and whalebone whales have teeth, but the adults of these animals are toothless. Cows, sheep, deer, and other ruminants, never have as adults any upper incisors, but these teeth are found in the foetal stages of these animals just under the gums. The female frog has rudimentary male reproductive organs, and the male has corresponding vestiges of female organs. Similar remnants of the reproductive structures exist in many other animals. They represent stages in the transition from the hermaphroditism of primitive animals to the unisexuality of the higher forms, the separation of the sex organs into those of male and female having come about through the decay of one set of structures in each individual.For reasons which it is not necessary to mention here, biologists believe that insects all originated from a common parental form, with two pairs of wings and six legs. Insects all retain their original allowance of legs, but in many species one or the other pair of wings has become more or less degenerated. In the whole order of flies the back pair of wings is represented by a couple of insignificant knobs. In the Strepsiptera, a sub-order of beetles, the front-wings are similarly reduced, being mere twisted filaments. Many parasites, such as fleas and ticks, whose mode of life renders organs of aerial locomotion unnecessary, are entirely wingless. The insects of small isolated islands are also largely without wings, the proportion of wingless species being much larger than among insects inhabiting continents. This is due to their greater liability on small land masses of being carried out to sea and drowned, owing to the feebleness and uncertainty of insect flight. On the island of Madeira, out of the 550 species found there, 220 species no longer have the power of flight.Air-breathing animals—amphibians, reptiles, birds, and mammals—have normally a pair of lungs—a right one and a left one. But in snakes and snake-like lizards, where the body is very slender and elongated, only one lung, sometimes the right one, and sometimes the left, is fully developed. The right ovary is likewise aborted in all birds, the left one yielding all the eggs. The swifts and frigate birds live almost their whole lives long on the wing, and the legs of these birds have grown so short and weak and rudimentary, as a result of their constant life in the air, that they can scarcely walk. The chimney swift is said never to alight anywhere except on the sooty inner walls of the chimney where its nest is. Its food consists of insects which it gathers in the air, and the few dead twigs used in making its nest are nipped from the tree while the bird continues its flight. The ostriches, cassowaries, and many other birds, have, on the other hand, developed their legs at the expense of their wings. The ostrich is said to be able to outrun the horse, but it has no power of flight, although it has wings and wing muscles, and even the skin-folds covering the wings corresponding to those of birds that fly. But its whole flying apparatus is in ruins. The rudimentary hind-toe of birds is a vestigial organ, and so are the claws which appear on the thumb and first finger of all young birds. So also are the rudiments of eyes in cave crickets, fishes, and other inhabitants of total darkness. The flounder and other so-called flat fishes swim straight up, as ordinary fishes do, when young. But as they grow they incline more and more to one side, and finally swim entirely on their side, the eye on the lower side migrating around, and joining the other on the upper side of the head.
Man an AnimalMan a VertebrateMan a MammalMan a PrimateRecapitulationThe Meaning of HomologyThe Earth an EvolutionThe Factors of Organic EvolutionThe Evidences of Organic EvolutionThe Genealogy of AnimalsConclusion
‘Like the Roman emperors, who, intoxicated by their power, at length regarded themselves as demigods, so the ruler of the earth believes that the animals subjected to his will have nothing in common with his own nature. Man is not content to be the king of animals. He insists on having it that an impassable gulf separates him from his subjects. The affinity of the ape disturbs and humbles him. And, turning his back upon the earth, he flies, with his threatened majesty, into the cloudy sphere of a special “human kingdom.” But Anatomy, like those slaves who followed the conqueror’s car crying, “Thou art a man,” disturbs him in his self-admiration, and reminds him of those plain and tangible realities which unite him with the animal world.’— Broca.
‘Like the Roman emperors, who, intoxicated by their power, at length regarded themselves as demigods, so the ruler of the earth believes that the animals subjected to his will have nothing in common with his own nature. Man is not content to be the king of animals. He insists on having it that an impassable gulf separates him from his subjects. The affinity of the ape disturbs and humbles him. And, turning his back upon the earth, he flies, with his threatened majesty, into the cloudy sphere of a special “human kingdom.” But Anatomy, like those slaves who followed the conqueror’s car crying, “Thou art a man,” disturbs him in his self-admiration, and reminds him of those plain and tangible realities which unite him with the animal world.’
— Broca.
It was in the zoology class at college. We had made all the long journey from amoeba to coral, from coral to worm, from worm to mollusk, from mollusk to fish, from fish to reptile, and from reptile to mammal—and there, in the closing pages of faithful old Packard, we found it. ‘A mammal of the order of primates,’ the book said, with that unconcern characteristic of the deliverances of science. I was almost saddened. It was the first intimation I had ever received of that trite but neglected truth thatman is an animal.
But the intimation was so weak, and I was at that time so unconscious, that it was not till years later that I began, through reflection, actually to realise the truth here first caught sight of. During these years I knew that man was not a mineral nor a plant—that, indeed, he belonged to the animal kingdom. But, like most men still, I continued to think of him as being altogether different from other animals. I thought of manand the animals,notof man and theotheranimals. Man was somehowsui generis. He had had, I believed, a unique and miraculous origin; for I had not yet learned of organic evolution. The pre-Darwinian belief that I had come down from the skies, and that non-human creatures of all kinds had been brought into existence as adjuncts of the distinguished species to which I belonged, occupied prominent place in my thinking. Non-human races, so I had been taught, had in themselves no reason for existence. They were accessories. A chasm, too wide for any bridge ever to span, yawned between the human and all other species. Man was celestial, a blue-blood barely escaping divinity. All other beings were little higher than clods. So faithfully and mechanically did I reflect the bias in which I had grown up.
But manisananimal. It was away out there on the prairies, among the green corn rows, one beautiful June morning—a long time ago it seems to me now—that this revelation really came to me. And I repeat it here, as it has grown to seem to me, for the sake of a world which is so wise in many things, but so darkened and wayward regarding this one thing. However averse to accepting it we may be on account of favourite traditions, man is an animal in the most literal and materialistic meaning of the word. Man has not a spark of so-called ‘divinity’ about him. In important respects he is the most highly evolved of animals; but in origin, disposition, and form he is no more ‘divine’ than the dog who laps his sores, the terrapin who waddles over the earth in a carapace, or the unfastidious worm who dines on the dust of his feet. Man is not the pedestalled individual pictured by his imagination—a being glittering with prerogatives, and towering apart from and above all other beings. He is a pain-shunning, pleasure-seeking, death-dreading organism, differing in particulars, but not in kind, from the pain-shunning, pleasure-seeking, death-dreading organisms below and around him. Man is neither a rock, a vegetable, nor a deity. He belongs to the same class of existences, and has been brought into existence by the same evolutional processes, as the horse, the toad that hops in his garden, the firefly that lights its twilight torch, and the bivalve that reluctantly feeds him.
Man’s body is composed fundamentally of the same materials as the bodies of all other animals. The bodies of all animals are composed of clay. They are formed of the same elements as those that murmur in the waters, gallop in the winds, and constitute the substance of the insensate rocks and soils. More than two-thirds of the weight of the human body is made up of oxygen alone, a gas which forms one-fifth of the weight of the air, more than eight-ninths of that of the sea, and forty-seven per cent, of the superficial solids of the earth.
Man’s body is composed of cells. So are the bodies of all other animals. And the cells in the body of a human being are not essentially different in composition or structure from the cells in the body of the sponge. All cells are composed primarily of protoplasm, a compound of carbon, hydrogen, nitrogen, and oxygen. Like all other animals, man is incapable of producing a particle of the essential substance of which his body is made. No animal can produce protoplasm. This is a power of the plant, and the plant only. All that any animal can do is to burn the compounds formed in the sun-lit laboratories of the vegetable world. The human skeleton, like the skeletons of nearly all other animals, is composed chiefly of lime—lime being, in the sea, where life spent so many of its earlier centuries, the most available material for parts whose purpose it is to furnish shape and durability to the organism. Man grows from an egg. So do all creatures of clay. Every animal commences at the same place—in a single, lowly, almost homogeneous cell. A dog, a frog, a philosopher, and a worm cannot for a long time after their embryonic commencement be distinguished from each other. Like the oyster, the ox, the insect, and the fish, like all that live, move, and breathe, man is mortal. He increases in size and complexity through an allotted period of time; then, like all his kindred, wilts back into the indistinguishable flux from which he came. Man inhales oxygen and exhales carbon dioxide. So does every animal that breathes, whether it breathe by lungs, gills, skin, or ectosarc, and whether it breathe the sunless ooze of the sea floor or the ethereal blue of the sky. Animals inhale oxygen because they eat carbon and hydrogen. The energy of all animals is produced mainly by the union of oxygen with the elements of carbon and hydrogen in the tissues of animal bodies, the plentiful and ardent oxygen being the most available supporter of the combustion of these two elements.
Man is, then, an animal, more highly evolved than the most of his fellow-beings, but positively of the same clay, and of the same fundamental make-up, with the same eagerness to exceed and the same destiny, as his less pompous kindred who float and frolic and pass away in the seas and atmospheres, and creep over the land-patches of a common clod.
Man is avertebrateanimal.[1]He has (anatomically at least) a backbone. He belongs to that substantial class of organisms possessing an articulating internal skeleton—the family of the fishes, amphibians, reptiles, birds, and mammals. Most animals have some sort of skeleton, some sort of calcareous contrivance, whose business it is to give form and protection to the softer parts of the organism. Some animals, as the starfishes, have plates of lime scattered throughout the surface parts of the body; others, as the corals and sponges secrete plant-like frames, upon and among the branches of which the organisms reside; and still others, as the clams, crustaceans, and insects, have skeletons consisting of a shell or sheath on the outside of, and more or less surrounding, the softer substances of the body. The limbs of insects are tiny tubes on the inside of which are the miniature muscles with which they perform their marvels of locomotion. The skeleton of vertebrates, consisting of levers, beams, columns, and arches, all skilfully joined together and sunk deep within the muscular tissue, forms a conspicuous contrast to the rudimentary frames of other animals. The vertebrate skeleton consists of a hollow axis, divided into segments and extending along the dorsal region of the body, from the ventral side of which articulate, by means of awkwardly-constructed girdles, an anterior and a posterior pair of limbs. This dorsal axis ends in front in a peculiar bulbous arrangement called the head, which contains, among other valuables, the brain and buccal cavern. The thoracic segments of the backbone send off pairs of flat bones, which, arching ventrally, form the chest for the protection of the heart and other vitals. The limbs (except in fishes) consist each of a single long bone, succeeded by two long bones, followed by two transverse rows of short, irregular wrist or ankle bones, ending normally in five branching series of bones called digits. This is essentially the skeleton of all fishes, amphibians, reptiles, birds, and mammals. In short, it is the universal vertebrate type of frame. There are minor modifications to suit the various kinds of environment, adaptations to the necessities of aquatic, terrestrial, and aerial locomotion and life, some parts being specialised, others atrophied, and still others omitted, but there is never anywhere, from fishes to philosophers, any fundamental departure from the established vertebrate type of skeleton.[2]The pectoral fins of fishes correspond to the fore-limbs of frogs and reptiles, the wings of birds, and the arms of men. The pelvic fins of fishes are homologous with the hind-limbs of frogs, reptiles, and quadrupeds, and the legs of birds, apes, and men. The foot of the dog and crocodile, the hand of the orang, and the flipper of the dolphin and seal, all have the same general structure as the hand of man; and the wings of the bat and bird, the forelimbs of the lizard and elephant, and the comical shovels of the mole and ornithorhynchus, notwithstanding the great differences in their external appearance and use, contain essentially the same bones and the same arrangement of the bones as do the arms of men and women. The human body has two primary cavities in it. So have the bodies of all vertebrates: a neural cavity containing the brain and spinal cord, and a visceral cavity containing the heart, liver, lungs, and alimentary canal. Invertebrates have only one body cavity—the one corresponding to the visceral cavity of vertebrates—and the main nerve trunk, instead of extending along the back, as among vertebrates, is in invertebrates located ventrally. Vertebrates are the only animals on the earth that have a highly developed circulatory system, a system entirely shut off from the other systems, and containing a heart, arteries, veins, and capillaries. In all invertebrates the digestive and circulatory systems remain to a greater or less extent connected, the blood and food mingling more or less in the general cavity of the body. Worms and insects have pulsating tubes instead of heart and arteries. Crustaceans have hearts with one chamber, and mollusks have two or three chambered hearts, but the blood, instead of returning to the heart after its journey through the arteries, passes into the body cavity. In man and other vertebrates the circulating current is confined strictly to the bloodvessels, no particle of it ever escaping into the general body cavity. The heart of vertebrates is distinguished from that of invertebrates by being located ventrally. The heart of invertebrates is in the back. The blood of vertebrates differs from that of invertebrates in containing both red and white corpuscles. Invertebrates have white corpuscles only. Worms have yellow, red, or bright green blood. The blood of crustaceans is bluish, that of mollusks is white, and that of insects dusky or brown. The blood of all vertebrates, excepting amphioxus, is red. All backboned beings, whether they dwell in seas or cities, and whether they build nests or empires, have two eyes, two ears, nose and mouth, all located in the head, and always occupying the same relative position to each other. Invertebrates may have their brains in their abdomen, as do the mites; hear with their legs or antennae, as many insects do; see with their tunics, like the scallops; and breathe with their skin, as do the worms. The crayfish hears with its ‘feelers,’ the cricket and katydid with their fore-legs, the grasshopper with its abdomen, the clam with its ‘foot,’ and mysis and other low crustaceans have their auditory organs on their tails.
Man is, then, like the fishes, frogs, reptiles, birds, and quadrupeds, a vertebrate animal. Excepting in his infancy, when he is a quadruped going on all fours, he uses his posterior limbs only for locomotion, and his anterior for prehension and the like. His spinal axis is erect instead of horizontal, and his tail is atrophied. But he possesses all of the unmistakable qualities of the vertebrate type of structure—a two-chambered body cavity, a highly developed and dorsally located nerve trunk, vertebrate vitals, a closed circulatory system, a ventral heart, red blood, a head containing sense organs and brain, and a well-ordered internal skeleton, consisting of a vertebral column with skull and ribs and two pairs of limbs, the limbs consisting each of one long bone, two long bones, two transverse rows of irregular bones, and five branches at the end.
1.See ‘Classes of Animals,’ at the end of the chapter.2.Snakes are limbless, and hind-limbs are lacking in whales and other degenerates; but rudimentary limbs are found in the embryonic stages of these animals. Frogs, it may be said also, have no ribs.
Man is amammal. He belongs to the most brilliant and influential of the five classes of vertebrates—the class to which belong so many of his associates and victims, the class to which belong the horse, the dog, the deer, the ox, the sheep, the swine, the squirrel, the camel, the unattenuated elephant, and the timid-hearted hare. To this class belong also the lion, the tiger, the kangaroo, the beaver, the bear, the bat, the monkey, the mole, the wolf, the ornithorhynchus, and the whale—in short,all animals that have hair. Fishes and reptiles have scales; birds have feathers; all mammals are covered to a greater or less extent with hair. The aquatic habits of whales render hair of no use to them. Hence, while the unborn of these animals still cling to the structural traditions of their ancestors and are covered with hair, the adults are almost hairless. The sartorial habits of human beings and the selective influences of the sexes have had a similar effect on the hairy covering of the human body. Hair exists all over the human body surface, excepting on the soles of the hands and feet, but in a greatly dwarfed condition. It is only on the scalp and on the faces of males, where it is scientifically assisted for purposes of display, that it grows luxuriantly. It is by no means certain that even the hair on the masculine scalp will last forever. For if the hermetical derby and other deadly devices worn by men continue their devastations as they have in the past, we may expect to have, in the course of generations, men with foreheads reaching regularly to the occiput. Most animals lay eggs. Man does not. Like the dog, the horse, the squirrel, and the bat, man is viviparous, the eggs hatching within the parental body. Human young are born helpless, and are sustained during the period of their infancy by the secretions of the milk glands. So are all the sons and daughters of mammals. Whether they come into the world among the waters or among the desert sands, in the hollow of a tree, in a hole in the earth, or in a palace, the children of mammals are frail and pitiful, and they survive to grow and multiply only because they are the object of the loving and incessant sacrifices of a mother.
Mammals are distinguished from all other animals by the possession of two kinds of skin glands—the sweat glands and the oil glands—and by the development of certain of these glands in the female into organs for the nourishing of the young. Among reptiles and birds the lower jaw is suspended from the skull by a bone called the quadrate bone. Among men and other mammals the lower jaw is joined directly to the skull, the quadrate bone becoming, in the vicissitudes of evolution, the hammer (malleus) of the mammalian ear. Man has a four-chambered heart—two reservoirs which receive, and two pumps which propel, the scarlet waters of the body. Fishes have two-chambered hearts; frogs and most reptiles have three-chambered hearts; all mammals and birds have four-chambered hearts. The red corpuscles in the blood of fishes, frogs, reptiles, and birds, are discs, double-convex, nucleated, and in shape oval or triangular. In man and in all other mammals (except the archaic camel) the red corpuscles are double-concave, non-nucleated, and circular. ‘Man has a diaphragm dividing the body cavity into chest and abdomen, and a shining white bridge of interlacing fibres, calledcorpus callosum, uniting his cerebral hemispheres. And man is a mammal because, like other mammals, he has, in addition to the qualities already mentioned, these valuable and distinct characteristics.
Man is aprimate. There are four divisions in the order of primates—lemurs, monkeys, apes, and men. But the most interesting and important of these, according to man, is man. Man is a primate because, like other primates, he has arms and hands instead of fore-legs. And these are important characteristics. It was a splendid moment when the tendencies of evolution, pondering the possibilities of structural improvement, decided to rear the vertebrate upon its hind-limbs, and convert its anterior appendages into instruments of manipulation. So long as living creatures were able simply to move through the airs and waters of the earth and over the surface of the solids, they were powerless to modify the universe about them very much. But the moment beings were developed with parts of their bodies fitted to take hold of and move and fashion and compel the universe around them, that moment the life process was endowed with the power of miracles. With the invention of hands and arms commenced seriously that long campaign against the tendencies of inanimate nature which finds its most marvellous achievements in the sustained and triumphant operations of human industry. None of the primates excepting man use their hind-limbs as a sole means of changing their place in the universe, but in all of them the fore-limbs are regularly used as organs of manipulation. Man is a primate because his fingers and toes, like those of other primates (except the tiny marmosets of Brazil), end in nails. Man has neither claws to burrow into the earth, talons with which to hold and rend his victims, nor hoofs to put thunder into his movements. The human stomach, like that of all the other primates, is a bagpipe. The stomach of the carnivora is usually a simple sack, while rodents have, as a rule, two stomachs, and ruminants four. Man is a primate because his milk glands are located on the breast and are two in number. The mammary glands vary in number in the different orders of mammals, from two in the horse and whale to twenty-two in some insectivora. Most ruminating animals have four, swine ten, and carnivora generally six or eight. These glands may be located in the region of the groin, as in the horse and whale; between the forelimbs, as in the elephant and bat; or arranged in pairs extending from the fore to the hind limbs, as in the carnivora and swine. In man and all other primates (except lemurs) the mammary glands are pectoral and two in number. All primates, including man, have also a disc-shaped placenta. The placenta is the organ of nutrition in mammalian embryos. It is found in all young-bearing animals above the marsupials, and consists of a mass of glands between the embryo and the parental body. In some animals it entirely surrounds and encloses the embryo; in others it assumes the form of a girdle; and in still others it is bell-shaped. The primates are the only animals in which this peculiar organ is in the shape of a simple disc.[1]
The nearest relatives by blood man has in this world are the exceedingly man-like apes—the tailless anthropoids—the gorillas and chimpanzees of Africa, and the orangs and gibbons of southern and insular Asia. The fact that man is an actual relative and descendant of the ape is one of the most disagreeable of the many distasteful truths which the human mind in its evolution has come upon. To a vanity puffed, as is that of human beings, to the splitting, the consanguinity of gorilla and gentleman seems horrible. Man prefers to have arrived on the earth by way of a ladder let down by his imagination from the celestial concave. Within his own memory man has been guilty of many foolish and disgraceful things. But this attempt by him to repudiate his ancestors by surreptitiously fabricating for himself an origin different from, and more glorious than the rest is one of the most absurd and scandalous in the whole list. It is a shallow logic—the logic of those who, without worth of their own, try to shine with a false and stolen lustre. No more masterly rebuke was ever administered to those in the habit of sneering at the truth in this matter than the caustic reply of Huxley to the taunt of the fat-witted Bishop—that he would rather be the descendant of a respectable ape than the descendant of one who not only closed his eyes to the facts around him, but used his official position to persuade others to do likewise. Man’s reluctance to take his anatomical place beside his simian kinspeople has been exceeded only by his selfish and high-handed determination to exclude all other terrestrial beings from his heaven.
Man is a talkative and religious ape. He is an ape, but with a much greater amount of enterprise and with a greater likelihood of being found in every variety of climate. Like the anthropoid, man has a bald face and an obsolete tail. But he is distinguished from his arboreal relative by his arrogant bearing, his skilled larynx, and especially by the satisfaction he experiences in the contemplation of the image which appears when he looks in a mirror.
The man-like apes are from three to six feet tall, and are all of them very strong, the gorilla, who sometimes weighs over three hundred pounds, being about the bravest and most formidable unarmed animal on the planet. They are erect or semi-erect, have loud voices, plantigrade feet, and irritable dispositions—in all of these particulars being strikingly like men. The gorilla, chimpanzee, and gibbon are highlanders, preferring the uplands and mountains. The orang is a lowlander, living phlegmatically among the sylvan swamps of Sumatra and Borneo. The gorilla and chimpanzee are terrestrial, seldom going among the trees except to get food or to sleep. The orang and gibbon are arboreal, seldom coming to the ground except to drink or bathe. They all walk on their hind-limbs, generally in a stooping posture, with their knuckles or fingers touching the ground. But they sometimes walk with their arms hanging down by their sides, and sometimes with their hands clasped back of their heads to give them balance. None of them ever place their palms on the ground when they walk—that is, none of them walk on four feet. The anthropoid races, in the shape of their heads and faces and in the general form and structure of their bodies, and even in their habits of life, resemble in a remarkable manner the lowest races of human beings. This resemblance is recognised by the negro races, who call the gorilla and chimpanzee ‘hairy men,’ and believe them to be descendants of outcast members of their own species.
There are differences in structure between man and the apes, just as there are differences in structure between the Caucasian and the Caffre, or even between individual Caucasians or individual Caffres. There are differences in structure and topography, often very noticeable differences, even among members of the same family. But in all of its essential characters, and extending often to astonishing particulars, the structure of man is identical with that of the anthropoid.[2]
In external appearances the man-like races differ from men in having a luxuriant covering of natural hair. But anthropoids differ very much among themselves in this particular. The orang, usually covered with long hair, is sometimes almost hairless. There are, too, races of human beings whose bodies are covered with a considerable growth of hair. The Todas (Australians) and Ainus (aborigines of Japan) are noted for the hairiness of their bodies, certain individuals among them being covered with a real fur, especially on the lower limbs.[3]
Individuals also often appear in every race with a remarkable development of the hair. Adrian and his son Fedor, exhibited years ago over Europe as ‘dog-men,’ are examples. The father was completely covered with a thick growth of fine dirty-yellow hair two or three inches long. Long tufts grew out of his nostrils and ears, giving him a striking resemblance to a Skye terrier. Fedor, and also his sister, were covered with hair like the father, but another son was like ordinary men. The man-like races have also longer arms in proportion to the height of the body than man generally has. But this is also true of human infants and negroes. The gibbon has relatively much longer arms than the other anthropoids. It differs from the chimpanzee in this respect more than the chimpanzee differs from man. When standing upright and reaching down with the middle finger, the gibbon can touch its foot, while the chimpanzee can reach only to the knee. Man ordinarily reaches part way down the thigh, but negroes have been known to have arms reaching to the knee-pan.[4]
The skeleton of the African races contains many characters recognised by osteologists as ‘pithecoid,’ or ape-like. It is massive, the flat bones are thick, and the pelvis narrow. In the manlike apes the large toe is opposable to the other four, and is used by them much as the thumb is used. But this difference between the two races of beings is just what might be expected from the differences in their modes of life. Man has little need of this opposability on account of his exclusively terrestrial life, while to the ape it is indispensable on account of his arboreal environment and life. ‘But there are,’ says Haeckel, ‘wild tribes of men who can oppose the large toe to the other four just as if it were a thumb, and even new-born infants of the most highly-developed races of men can grasp as easily with their hind-hands as with their forehands. Chinese boatmen row with their feet, and Bengal workmen weave with them. The negro, in whom the big toe is freely movable, seizes hold of the branches of trees with it when climbing, just like the four-handed apes’.[5]
Many men have lost their arms by accident and have learned to use their feet as hands with wonderful skill. Not many years ago there died in Europe an armless violinist who had during his lifetime played to cultured audiences in most of the capitals of the world. Some of the most accomplished of penmen hold their pen between their toes. The man-like apes live to about the same age as man, and all of them, like man, have beards. The anthropoid beard, too, like the human, appears at the age of sexual maturity. The human beard often differs in colour from the hair of the scalp, and whenever it does it has been observed to be invariably lighter—never darker—than the hair on the scalp. This is true among all races of men. The same rule and the same uniformity exists among anthropoids. The races of mankind are divided into two primary groups depending upon the shape of the head and the character of the hair: the short-headed races (Brachycephali), such as the Malays, Mongols, and Aryans, with round or oval faces, straight hair, and vertical profiles; and the long-headed races (Dolichocephali), with woolly hair and prognathous faces, such as the Papuans and Africa races. The skin of the short-headed races is orange or white, while the skin and hair of the long-headed races are glossy black.
It is, at least, interesting that the orang and gibbon, who live in Asia and its islands, where the brachycephalic races of men supposedly arose, are themselves brachycephalic; and that the gorilla and chimpanzee, who live in Africa, where the dolichocephalic races chiefly live, are dolichocephalic. The gorilla and chimpanzee also have, like the men and women of Africa, black skin and hair; while the hair of the orang is a reddish-brown, and its skin sometimes yellowish-white. The dentition of the anthropoids and men is in all essentials identical. They all have two sets of teeth: a set of milk-teeth, twenty in number, and thirty-two permanent teeth, the permanents consisting of two incisors, one canine, two premolars, and three molars, in each half-jaw. Man has ordinarily twelve pairs of ribs and thirty-two vertebrae. So has the orang. The other anthropoids have thirteen pairs of ribs. But the number of ribs in both human and anthropoid beings is not uniform, man occasionally having thirteen pairs, and the gorilla fourteen. Man has also the same number of caudal vertebrae in his rudimentary tail as the anthropoid has. The hands and feet of anthropoids, bone for bone and muscle for muscle, correspond with those of men, no greater structural differences existing than among different species of men. The human foot has three muscles not found in the human hand—a short flexor muscle, a short extensor muscle, and a long muscle extending from the fibula to the foot. All of these muscles are found in the anthropoid foot just as in the foot of man. There are also the same differences between the arrangement of the bones of the anthropoid wrist and ankle as between the wrist and ankle bones of man. Whatever set of anatomical particulars may be selected, whether it be hands, arms, feet, muscles, skull, viscera, ribs, or dentition, it is found that the anthropoid races and men are in all essentials the same. The differences are such as have arisen as a result of different modes of life, and such as exist between different tribes of either group of animals.
‘The structural differences which separate man from the gorilla and chimpanzee,’ says Huxley, in summing up the conclusion of his brilliant inquiry into ‘Man’s Place in Nature,’ ‘are not so great as those which separate the gorilla from the lower apes.’
‘The body of man and that of the anthropoid are not only peculiarly similar,’ says Haeckel, ‘but they are practically one and the same in every important respect. The same two hundred bones, in the same order and structure, make up our inner skeleton; the same three hundred muscles effect our movements; the same hair clothes our skin; the same four-chambered heart is the central pulsometer in our circulation; the same thirty-two teeth are set in the same order in our jaws; the same salivary, hepatic, and gastric glands compass our digestion; the same reproductive organs insure the maintenance of our race’.[6]
‘Not being able,’ says Owen in his paper on ‘The Characters of Mammalia,’ ‘to appreciate or conceive of the distinction between the psychical phenomena of a chimpanzee and of a Boschisman or of an Aztec with arrested brain-growth, as being of a nature so essential as to preclude a comparison between them, or as being other than a difference in degree, I cannot shut my eyes to the significance of that all-pervading similitude of structure—every tooth, every bone, strictly homologous—which makes the determination of the difference betweenHomoandPithecusthe anatomist’s difficulty.’
‘If before the appearance of man on the earth,’ says Ward in his ‘Dynamic Sociology,’ ‘an imaginary painter had visited it, and drawn a portrait embodying the thorax of the gibbon, the hands and feet of the gorilla, the form and skull of the chimpanzee, the brain development of the orang, and the countenance ofSemnopithecus, giving to the whole the average stature of all of these apes, the result would have been a being not far removed from our conception of the primitive man, and not widely different from the actual condition of certain low tribes of savages. The brain development would perhaps be too low for the average of any existing tribe, and would correspond better with that of certain microcephalous idiots and cretins, of which the human race furnishes many examples.’
And it is not true, as is commonly supposed, that, after all other resemblances between the human and anthropoid structures have been made out, there still exists somewhere some undistinguishable difference in the organic structure of their brains. All differences in structure from time to time suspected or asserted to exist between the brain of man and that of the man-like apes have been one after another completely swept away. And it is now known to all neurologists that the human and anthropoid brains differ structurally in no particulars whatever, both of them containing the same lobes, the same ventricles and cornua, and the same convolutional outline. Even the posterior lobe, the posterior cornu, and the hippocampus minor, so long triumphantly asserted to be characteristic features of the human brain, have been pitilessly identified in all anthropoids by the profound and terrible Huxley. There is not an important fold or fissure in the brain of man that is not found in the brain of the anthropoid. ‘The surface of the brain of a monkey,’ says Huxley, ‘exhibits a sort of skeleton map of man’s, and in the man-like apes the details become more and more filled in, until it is only in minor characters that the chimpanzee’s or the orang’s brain can be structurally distinguished from man’s’.[7]
The great difference physically between man and the anthropoids, aside from man’s talented larynx and erect posture, lies in man’s abnormal cranial capacity. The normal human cranium never contains less than 55 cubic inches of space, while the largest gorilla cranium contains only 34½ cubic inches. This is a difference of 20½ cubic inches. And 20½ cubic inches of thinking matter is an alarming amount to be lacking in a single individual. But this cranial gap between gorilla and man is deprived of some of its significance by the fact that human crania sometimes measure 114 cubic inches, making a difference between the smallest and largest human brains of 59 cubic inches. The difference between the gorilla and the savage in cranial capacity is, therefore,only about one-third as great as the cranial chasm between the savage and the sage.
1.The bat and a few other animals have a disc-like placenta, but it develops into the disc shape by a different route from what it does in the primates.2.Hartmann:Anthropoid Apes; New York, 1901.3.Quatrefages:The Human Species; New York, 1898.4.Tyler:Anthropology; New York; 1899.5.Haeckel:History of Creation, 2 vols.; New York, 1896.6.Haeckel:The Riddle of the Universe; New York, 1901.7.Huxley:Man’s Place in Nature; New York, 1883.
The anatomical gulf between men and apes does not exist. There are, in fact, no gulfs anywhere, only gradations. All chasms are completely covered by unmistakable affinities, in spite of the fact that the remains of so many millions of deceased races lie hidden beneath seas or everlastingly locked in the limy bosoms of the continents. There are closer kinships and remoter kinships, but there are kinships everywhere. The more intimate kinships are indicated by more definite and detailed similarities, and the more general relationships by more fundamental resemblances. All creatures are bound to all other creatures by the ties of a varying but undeniable consanguinity.
Man stands unquestionably in the primate order of animals, because he has certain qualities of structure which all primates have, and which all other animals have not: hands and arms and nails, a bagpipe stomach, great subordination of the cerebellum, a disc-like placenta, teeth differentiated into incisors, canines, and molars, and pectoral milk glands.
Man is more closely akin to the anthropoid apes than to the other primates on account of his immense brain, his ape-like face, his vertical spine, and in being a true two-handed biped. The manlike apes and men have the same number and kinds of teeth, the same limb bones and muscles, like ribs and vertebrae, an atrophied tail, the same brain structure, and a suspicious similarity in looks and disposition. Men and anthropoids live about the same number of years, both being toothless and wrinkled in old age. The beard, too, in both classes of animals appears at the same period of life and obeys the same law of variation in colour. Even the hairs on different parts of the bodies of men and anthropoids, as on the arms, incline at a like angle to the body surface. The hair on the upper arm and that on the forearm, in both anthropoids and men, point in opposite directions—toward the elbow. This peculiarity is found nowhere in the animal kingdom excepting in a few American monkeys.
Man’s mammalian affinities are shown in his diaphragm, his hair, his four-chambered heart, hiscorpus callosum, his non-nucleated blood-corpuscles, and his awkward incubation.
The fishes, frogs, reptiles, birds, and non-human mammals are human in having two body cavities, segmented internal skeletons, two pairs of limbs, skulls and spinal columns, red blood, brains, and dorsal cords; and in possessing two eyes, two ears, nostrils, and mouth opening out of the head. And finally all animals, including man, are related to all other animal forms by the great underlying facts of their origin, structure, composition, and destiny. All creatures, whether they live in the sea, in the heavens, or in subterranean glooms; whether they swim, fly, crawl, or walk; whether their world is a planet or a water-drop; and whether they realise it or not, commence existence in the same way, are composed of the same substances, are nourished by the same matters, follow fundamentally the same occupations, all do under the circumstances the best they can, and all arrive ultimately at the same pitiful end.
The similarities and homologies of structure existing between man and other animals, and between other animals and still others, are not accidental and causeless. They are not resemblances scattered arbitrarily among the multitudinous forms of life by the capricious levities of chance. That all animals commence existence as an egg and are all made up of cells composed of the same protoplasmic substance, and all inhale oxygen and exhale carbon dioxide, and are all seeking pleasure and seeking to avoid pain, are more than ordinary facts. They are filled with inferences. That vertebrate animals, differing in externals as widely as herring and Englishmen, are all built according to the same fundamental plan, with marrow-filled backbones and exactly two pairs of limbs branching in the same way, is an astonishing coincidence. That the wing of the bird, the foreleg of the dog, the flipper of the whale, and the fore-limb of the toad and crocodile, have essentially the same bones as the human arm has is a fact which may be without significance to blind men, but to no one else. The metamorphosis of the frog from a fish, of the insect from a worm, and of a poet from a senseless cell, are transformations simply marvellous in meaning. And it is not easy, since Darwin, to understand how such lessons could remain long unintelligible, even to stones and simpletons. Not many generations have passed, however, since these revelations, now so distinct and wonderful, fell on the listless minds of men as ineffectually as the glories of the flower fall on the sightless sockets of the blind.
It is hardly two generations since the highest intelligences on the earth conceived that not only the different varieties of men—the black, the white, and the orange—but all the orders and genera of the animal world, and not only animals, but plants, had all been somehow simultaneously and arbitrarily brought into existence in some indistinct antiquity, and that they had from the beginning all existed with practically the same features and in approximately the same conditions as those with which and in which they are found to-day. The universe was conceived to be a fixed and stupid something, born as we see it, incapable of growth, and indulging in nothing but repetitions. There were no necessary coherencies and consanguinities, no cosmical tendencies operating eternally and universally. All was whimsical and arbitrary. It was not known that anything had grown or evolved. All things were believed to have been given beginning and assigned to their respective places in the universe by a potential and all-clever creator. The serpent was limbless because it had officiously allowed Eve to include in her dietary that which had been expressly forbidden. The quadruped walked with its face towards the earth as a structural reminder of its subjection to the biped, who was supposed to be especially skilled in keeping his eyes rolled heavenward. The flowers flung out their colours, not for the benefit of the bugs and bees, and the stars paraded, not because they were moved to do so by their own eternal urgings, but because man had eyes capable of being affected by them. Man was an erect and featherless vertebrate because his hypothetical maker was erect and featherless. (I wonder whether, if a clam should conceive a creator, it would have the magnanimity to make him an insect or a vertebrate, or anything other than a great big clam.)
The world now knows—at least, the scientific part of it knows—that these things are not true, that they are but the solemn fancies of honest but simple-minded ancients who did the best they could in that twilight age to explain to their inquiring instincts the wilderness of phenomena in which they found themselves. The universe is a process. It is not petrified, but flowing. It is going somewhere. Everything is changing and evolving, and will always continue to do so. The forms of life, of continents and oceans, and of streams and systems, which we perceive as we open our senses upon the world to-day, are not the forms that have always existed, and they are not the forms of the eternal future. There was a time, away in the inconceivable, when there was no life upon the earth, no solids, and no seas. The world was an incandescent lump, lifeless and alone, in the cold solitudes of the spaces. There was a time—there must have been a time—when life appeared for the first time upon the earth, simple cellules without bones or blood, and without a suspicion of their immense and quarrelsome posterity. There was a time when North America was an island, and the Alleghany Mountains were the only mountains of the continent. The time was—in the coal-forming age—when the Mississippi Valley, from the Colorado Islands to the Alleghanies, was a vast marsh or sea, choked with forests of equisetum and fern, and swarming with gigantic reptiles now extinct. There was a time when palms grew in Dakota, and magnolias waved in the semi-tropical climate of Greenland and Spitzbergen. There was a time when there were no Rocky Mountains in existence, no Andes, no Alps, no Pyrenees, and no Himalayas. And that time, compared with the vast stretches of geological duration, was not so very long ago, for these mountains are all young mountains. The time was when Jurassic saurians—those repulsive ruffians of that rude old time—represented the highest intelligence and civilisation of the known universe. There were no men and women in the world, not even savages, when our ape-like forefathers wandered and wondered through the awesome silences of primeval wilds; there were no railroads, steamboats, telegraphs, telephones, typewriters, harvesters, electric lights, nor sewing machines; no billionaires nor bicycles, no socialists nor steam-heat, no ‘watered stock’ nor ‘government by injunction,’ no women’s clubs, captains of industry, labour unions, nor ‘yellow perils’—there was none of these things on the earth a hundred years ago. All things have evolved to be what they are—the continents, oceans, and atmospheres, and the plants and populations that live in and upon them.
There will come a time, too, looking forward into the future, when what we see now will be seen no more. As we go backward into the past, the earth in all of its aspects rapidly changes; the continents dwindle, the mountains melt, and existing races and species disappear one after another. The farther we penetrate into the past, the stranger and the more different from the present does everything become, until finally we come to a world of molten rocks and vapourised seas without a creeping thing upon it. As it has been in the past so will it be in time to come. The present is not everlasting. The minds that perceive upon this planet a thousand centuries in the future will perceive a very different world from that which the minds of this day perceive—different arts, animals, events, ideals, geographies, sciences, and civilisations. The earth seems fixed and changeless because we are so fleeting. We see it but a moment, and are gone. The tossing forest in the wrath of the storm is motionless when looked at by a flash of lightning. The same tendencies that have worked past changes are at work to-day as tirelessly as in the past. By invisible chisels the mountains are being sculptured, ocean floors are lifting, and continents are sinking into the seas. Species, systems, and civilisations are changing, some crumbling and passing away, others rising out of the ruins of the departed. Mighty astronomical tendencies are secretly but relentlessly at work, and immense vicissitudes are in store for this clod of our nativity. The earth is doomed to be frozen to death. In a few million years, according to astronomers, the sun will have shrunken to a fraction of his present size, and will have become correspondingly reduced in heat-giving powers. It is estimated that in twelve or fifteen million years the sun, upon whose mighty dispensations all life and activity on the earth are absolutely dependent, will become so enfeebled that no form of life on the earth will be possible. The partially-cooled earth itself is giving up its internal warmth, and will continue to give it up until it is the same temperature as the surrounding abysms, which is the frightful negative of something like 270 centigrade degrees. These are not very cheerful facts for those who inhabit the earth to contemplate. But they that seek the things that cheer must seek another sphere. No power can stay the emaciation of suns or the thievery of enveloping immensities. Old age is inevitable. It is far off, but it is as certain as human decay, and as mournful. In that dreadful but inevitable time no living being will be left in this world; there will be no cities nor states nor vanities nor creeping things, no flowers, no twilights, no love, only a frozen sphere. The oceans that now rave against the rocky flanks of the continents will be locked in eternal immobility; the atmospheres, which to-day drive their fleecy flocks over the azure meads of heaven and float sweet sounds and feathered forms, will be, in that terrible time, turned to stone; the radiant woods and fields, the home of the myriads and the green play-places of the shadows, will, like all that live, move, and breathe, have rotted into the everlasting lumber of the elements. There will be no Europe then, no pompous philosophies, no hellish rich, and no gods. All will have suffered indescribable refrigeration. The earth will be a fluidless, lifeless, sunless cinder, unimaginably dead and desolate, a decrepit and pitiful old ruin falling endlessly among heartless immensities, the universal tomb of the activities.
The universe is an evolution. Change is as extensive as time and space. The present has come out of that which has been, and will enter into and determine that which is to be. Everything has a biography. Everything has evolved—everything—from the murmur on the lips of the speechless babe to the soul of the poet, and from the molecule to Jehovah.
The animal kingdom represents one of the two grand branches of the organic universe. It has been evolved—evolved in a manner as simple and straightforward as it is revolting. It has all been brought about bypartialityorselection. Generations of beings have come into existence. The individual members of each generation have differed from each other—differed in size, strength, speed, colour, shape, sagacity, luck, and likelihood of life. No two beings, not even those born from the same womb, are in all respects identical. Hardships have come. They have come from the inanimate universe in the form of floods, fires, frosts, accidents, diseases, droughts, storms, and the like; from other species, who were competitors or enemies; and from unbrotherly members of the same species. Some have survived, but the great majority have perished. Only a fraction, and generally an appallingly small fraction, of each generation of a species have lived to maturity. The lobster lays 10,000 eggs in a season, yet the mortality is such that the number of lobsters do not increase from one year to another. The elephant is the slowest breeder of all animals, yet, if they should all live, the offspring of a single pair in 750 years would, according to Darwin, number nearly 19,000,000. It has been shown that at the normal rate of increase of English sparrows, if none were to die save of old age, it would take but twenty years for a single pair to give one sparrow to every square inch in the State of Indiana.[1]A single cyclops (one of the humbler crustaceans) may have 5,000,000 descendants in a season. One aphis will produce 100 young, and these young will reproduce in like manner for ten generations in a season, when, if they should all live, there would be a quintillion of young. A female white ant, when adult, does nothing but lie in a cell and lay eggs. She lays 80,000 eggs a day regularly for several months. An oyster lays 2,000,000 eggs in a season, and if all these eggs came to maturity a few dozen oysters might supply the markets of the world. The tapeworm is said to produce the incredible number of 1,000,000,000 ova, and some of the humbler plants three times this number of spores. If each egg of the codfish should produce an adult, a single pair in twenty-five years would produce a mass of fish larger than the earth. Lower forms of life are even more prolific than the higher. Maupas said that certain microscopic infusorians which he studied multiplied so rapidly that, if they should continue to multiply for thirty-eight days, and all of them should live, any one of them would produce a mass of protoplasm as big as the sun.
Those of each generation that have died have been inferior, or unfitted to the environment in which they found themselves. Those that have survived have been superior, superior in something—bigness, cunning, courage, virtue, vitality, strength, speed, littleness, or ferocity—something that has related them advantageously to surrounding conditions. The surviving remnant of each generation have become the progenitors of the next generation, and have transmitted, or tended to transmit, to their offspring the qualities of their superiority. This winnowing has gone on in each generation of living beings during many millions of years—almost ever since life commenced to be on the earth. Some have continued themselves, and others have died childless. The environment of each species has been an immense sieve, and only the superior have gone through it. Different environments have emphasised different qualities of structure and disposition, and have thus given rise to permanent varieties in survival. These varieties, through the accumulated effects of many generations of selection, have diverged into species; species, after a still longer series of selections, have evolved into genera; genera have evolved into families; families into orders; and so on. In this simple, terrible manner have all the branches of organic beings (thanks to the horrors of a million ages) been brought into existence.
Variation, therefore, which furnishes variety in offspring;Heredity, which tends to perpetuate peculiarities by causing offspring to resemble more or less the characters of their parents; andEnvironment, which determines the character of the selections, are the three factors, and the only three factors, in organic evolution.
1.Jordan:Footnotes of Evolution; New York, 1898.
That the forms of life to-day found on the earth have come into existence by the evolution of the more complex forms from the simpler, and of these simpler forms from still simpler, through the ever-operating law of Selection, is a necessary conclusion from the following facts:
1. The existence in the animal world of all grades of structures, from the humblest possible protozoan, whose body consists of a single simple speck, to the most powerful and complex of mammals. There are estimated to be something like a million species of animals living on the earth to-day. There may be several times this number. These species are linked together by millions of varieties, and are so related to each other that they may be all gathered together into various genera; these genera may be grouped into families, the families into orders, and the orders into seven or eight great primary phyla. By taking existing species and adding to them the extinct species of the rocks, and placing them all according to their structural affinities, it is possible to arrange them in the form of a tree with the various phyla, orders, families, genera, and species, branching and rebranching from the main trunk. The existence of structures, so graduated as to render such an arrangement possible, is in itself suggestive of a common relationship and origin.
2. Evolution is suggested by the similarities and homologies of structure found throughout the animal kingdom. Some of these similarities and homologies have already been mentioned. They are everywhere—remoter and more fundamental, some of them, others closer and more detailed. To the untrained mind, which sees surfaces only, and not even surfaces well, the animal world is an interminable miscellany of forms. But to the biologist, who looks deeper and with immense acumen over the whole field of animal life, there are only seven or eight different types of structure in the entire animal world. These seven or eight types correspond with the primary classes, or phyla, into which animals are divided, viz., protozoa, sponges, celenterates, echinoderms, worms, mollusks, arthropods, and vertebrates. However widely the members of each of these great groups may differ among themselves in colour, size, habits of life, and the like, the members of each group all resemble each other fundamentally. Moles differ from monkeys, bats from men, and birds from crocodiles and toads. They differ enormously. But they are all vertebrates with red blood, double body cavities, backbones, two pairs of limbs, and five fingers on each limb. When they are looked at superficially, there is not much similarity between a water-strider and a butterfly or between a stag-beetle and a gnat. But they are all, in reality, built according to the same plan. Like all other insects, they have six legs, a sheath-like skeleton, and bodies characteristically divided into head, thorax, and abdomen. It is the same with all other great classes of beings. All worms resemble each other; and so do all mollusks, although they may differ in particulars as widely as nautiluses and clams. Echinoderms have a radiate structure, celenterates and sponges are vase-like in shape, and protozoa are one-celled. The differences in structure among the members of a group consist in different modifications of a fundamental type. Among the vertebrates the fore-limb may be an arm, a leg, a wing, a shovel, a flipper, or a fin. But in all cases it is the same organ—that is, the same implement modified to serve different ends. Take the mouth-parts of insects. In the grasshopper and cricket these parts are fitted for grinding; in the moths and butterflies they are fashioned into long tubes for sucking the sweets of flowers; in the mosquito they form an elaborate apparatus for drilling and drinking; and in the mayfly the mouth-parts, though present, are not used at all. In all of these animals these parts are essentially the same, although differing so much in their forms and purposes that the unscientific can scarcely be made to believe they are fundamentally alike. There is no fact more familiar to the biologist or more frequently met with in the fields of animal morphology than the fact that the same general type may be hammered into dozens, or hundreds, or even thousands, of different patterns by the incessant industry of its surroundings, and that the same organic part may be moulded into various implements serving totally different ends by the environmental vicissitudes of time and space. On the hypothesis that the members of each group of animals possessing common characteristics, whether the group be large or small, have sprung from a common ancestry, and that the differences in structure have arisen as a result of differences in environment, the similarities and homologies of structure existing among animals are perfectly intelligible. But on any other supposition they are inexplicable.
3. Evolution is suggested by the remarkable series of phenomena presented by embryology. There are at least four facts in the developmental history of every creature which can hardly be accounted for on any other supposition than that of organic evolution.
First, the fact that every animal, above the lowest, individually passes through an evolution between the beginning of its existence and its maturity. Terrestrial beings are not born, like Minerva, full-grown. They grow. They evolve. They commence close down to the very atoms. And from this lowly genesis they rise, through a series of marvellous changes, to that high state of perfection and greatness from which they descend to dissolution.
If we knew by actual observation as little concerning the evolution of individuals as we do of the evolution of species—if we had always been used to seeing animals, including ourselves, in full bloom—had never watched the tadpole, the pupa, and the babe pass through their wonderful metamorphoses on their way to maturity, it would probably be just as hard for many minds to believe that animals evolve individually to be what they are as it is for them to believe that species have grown to be what they are. In the case of individuals, however, the evolution takes place right before our eyes largely, while the evolution of species goes on so slowly and stretches back so far into the past that it can only be inferred.
Second, the fact that animals, no matter how much they may differ from each other at maturity, all begin existence at the same place. Every animal commences its organic existence as an egg—as a one-celled animal—as an organism identical in structure with the simplest protozoan. The ova of whales ‘are no larger than fern seeds.’ The eggs of the coral, the crab, the ape, and the man are so precisely alike that the highest powers of the microscope cannot distinguish between them.
Third, the fact that the members of the same great group of animals in their individual development pass through similar stages of evolution. The ‘worm’ stage in the development of most insects and the ‘fish’ stage of frogs are well known.
There are no more remarkable instances of individual evolution in the whole range of animal life. The fish, the reptile, the bird, the dog, and the human being—all vertebrates, in short—cannot for some time after their embryonic commencement be distinguished from each other. ‘The feet of lizards and mammals, the wings and feet of birds, and the hands and feet of men,’ says the illustrious Von Baer, as quoted by Darwin, ‘all arise from the same fundamental form’.[1a]
‘It is quite in the later stages of development,’ says Huxley, ‘that the human being presents marked differences from the ape, while the latter departs as much from the dog in its development as the man does’.[2]
Not only frogs, but reptiles, birds, and mammals, including man, all have gills at a certain stage in their embryonic development. Nearly all the lower invertebrate animals are hermaphroditic—that is, in the body of each animal is found the two kinds of sex organs which in the higher animals exist in distinct animals. And frogs, birds, and other higher animals, which as adults are unisexual, have, as an inheritance from these primitive forms, hermaphroditic embryos.[3a]
Fourth, the fact that the structural stages through which animals in embryo pass correspond in a wonderful manner with the permanent structures of those lower forms which extend serially back to the beginnings of life. It is the proudest boast of the embryologist that he is able to know the route through which any species has come to be what it is by a simple study of the individual evolution of its members. Each animal repeats in its individual evolution the evolution of its species. This recapitulation is not always complete—is, in fact, frequently vague, sometimes circuitous, and often broken or abbreviated. Processes requiring originally centuries or thousands of years to accomplish are here telescoped into a few months, or even days. It is not strange that the process is imperfect. But so firmly is the belief in the correspondence of ontogeny and phylogeny fixed in the minds of modern biologists that, in determining the classification and affinities of any particular animal, more reliance is placed on the facts of embryology than on those of adult structure.
The first thing that an animal becomes after it is an egg—unless it is a one-celled animal, in which case it remains always an egg—is two cells; these two cells become four; these four become eight; and so on, until the embryo becomes a many-celled ball, consisting of a single layer of cells surrounding a fluid interior. A dimple forms in the cell layer on one side of this ball, and, by deepening to a hollow, changes the ball into a double-walled sac. This is the gastrula—the permanent structure of the sponges and celenterates, and an (almost) invariable stage in the larval development of all animals above the sponges and celenterates. The gastrula becomes a worm (or an insect or a fish through the worm) by elongation and enlargement, and by the development of the endoderm, which is the inner layer of the cell wall, into organs of nutrition and reproduction, and by the development of the ectoderm, which is the outer cell layer, into organs of motion and sensation.
The embryonic development of a human being is not different in kind from the embryonic development of any other animal. Every human being at the beginning of his organic existence is a protozoan, about1⁄125inch in diameter; at another stage of development he is a tiny sac-shaped mass of cells without blood or nerves, the gastrula; at another stage he is a worm, with a pulsating tube instead of a heart, and without head, neck, spinal column, or limbs; at another stage he has, as a backbone, a rod of cartilage extending along the back, and a faint nerve cord, as in amphioxus, the lowest of the vertebrates; at another stage he is a fish with a two-chambered heart, mesonephric kidneys, and gill-slits with gill arteries leading to them, just as in fishes; at another stage he is a reptile with a three-chambered heart, and voiding his excreta through a cloaca like other reptiles; and finally, when he enters upon post-natal sins and actualities, he is a sprawling, squalling, unreasoning quadruped. The human larva from the fifth to the seventh month of development is covered with a thick growth of hair and has a true caudal appendage, like the monkey. At this stage the embryo has in all thirty-eight vertebrae, nine of which are caudal, and the great toe extends at right angles to the other toes, and is not longer than the other toes, but shorter, as in the ape.
These facts are unmistakable. There is a reason for everything, and there is a reason for these transformations through which each generation of living beings journeys. The individual passes through them because the species to which he belongs has passed through them. They represent ancestral wanderings. As if to emphasise the kinship of all of life’s forms and to render incontrovertible the fact of universal evolution, Nature compels every individual to commence existence at the same place, and to recapitulate in his individual evolution the phylogenetic journeyings of his species.
4. That existing forms of life have been evolved from other forms, and that these ancestral forms have been different from those derived from them, is shown by the occasional appearance of antecedent and abandoned types of structure among the offspring of existing species. Occasionally a human child is born strangely unlike its parents, but bearing an unmistakable resemblance in looks and disposition to his great-grandfather or some other remote ancestor. This isatavism, that tendency to revert to ancestral types which is prevalent among all animals. We may think of it figuratively as a flash of indecision when Nature hesitates for a moment whether to adopt a new form of structure or cling to the old and tried. Horses and mules are sometimes born with three toes on each foot, and zebra-like stripes on their legs and shoulders; and domestic pigeons, such as are naturally black, red, or mottled, occasionally produce offspring with blue plumage and two black wing-bars, like the wild rock-dove, from which all domestic breeds have sprung. In man the cheekbone and the frontal bone of the forehead consist normally each of a single bone. But in children and human embryos these bones are always double, as is normally the case in adults among some of the anthropoids and other mammals. Gills appear regularly in the embryos of reptiles, birds, and mammals, and human young are sometimes born with gill-slits on the neck. There are times when, owing to inaccurate or incomplete embryological development, these fish-like characteristics are so perfect at birth as to allow liquids, on being swallowed, to pass out through them and trickle down on the outside of the neck. Many muscles are occasionally developed in man which are normal in the apes and other mammals. As many as seven different muscular variations have been found in a single human being, every one of which were muscles found normally in the structure of the apes.[1b]
5. Closely akin to atavism, which is the occasional persistence of ancestral types of character, is the regular occurrence of vestigial organs or structures, organs which in ancestral forms have definite functions, but which in existing species, owing to changed conditions, are rudimentary and useless. On the back of each ankle of the horse are two splints, the atrophied remains of the second and fourth toes. Similar vestiges of two obsolete toes are also found just back of the wrists and ankles on all the two-toed ungulates, such as the cow and sheep. In the body of the whale where hind-limbs would naturally be, there are found the anatomical ruins of these organs in the form of a few diminutive bones. The same thing is true in the sirenians. In the Greenland whale there are remnants of both femur and tibia in the region of the atrophied hind-limbs. The snakes are limbless, but the pythons and boas have internal remnants of hind-limbs and sometimes even clawed structures representing toes. The so-called ‘glass-snake’ or ‘joint-snake’ (which is really a limbless lizard) has four complete internal limbs. Young turtles, parrots, and whalebone whales have teeth, but the adults of these animals are toothless. Cows, sheep, deer, and other ruminants, never have as adults any upper incisors, but these teeth are found in the foetal stages of these animals just under the gums. The female frog has rudimentary male reproductive organs, and the male has corresponding vestiges of female organs. Similar remnants of the reproductive structures exist in many other animals. They represent stages in the transition from the hermaphroditism of primitive animals to the unisexuality of the higher forms, the separation of the sex organs into those of male and female having come about through the decay of one set of structures in each individual.
For reasons which it is not necessary to mention here, biologists believe that insects all originated from a common parental form, with two pairs of wings and six legs. Insects all retain their original allowance of legs, but in many species one or the other pair of wings has become more or less degenerated. In the whole order of flies the back pair of wings is represented by a couple of insignificant knobs. In the Strepsiptera, a sub-order of beetles, the front-wings are similarly reduced, being mere twisted filaments. Many parasites, such as fleas and ticks, whose mode of life renders organs of aerial locomotion unnecessary, are entirely wingless. The insects of small isolated islands are also largely without wings, the proportion of wingless species being much larger than among insects inhabiting continents. This is due to their greater liability on small land masses of being carried out to sea and drowned, owing to the feebleness and uncertainty of insect flight. On the island of Madeira, out of the 550 species found there, 220 species no longer have the power of flight.
Air-breathing animals—amphibians, reptiles, birds, and mammals—have normally a pair of lungs—a right one and a left one. But in snakes and snake-like lizards, where the body is very slender and elongated, only one lung, sometimes the right one, and sometimes the left, is fully developed. The right ovary is likewise aborted in all birds, the left one yielding all the eggs. The swifts and frigate birds live almost their whole lives long on the wing, and the legs of these birds have grown so short and weak and rudimentary, as a result of their constant life in the air, that they can scarcely walk. The chimney swift is said never to alight anywhere except on the sooty inner walls of the chimney where its nest is. Its food consists of insects which it gathers in the air, and the few dead twigs used in making its nest are nipped from the tree while the bird continues its flight. The ostriches, cassowaries, and many other birds, have, on the other hand, developed their legs at the expense of their wings. The ostrich is said to be able to outrun the horse, but it has no power of flight, although it has wings and wing muscles, and even the skin-folds covering the wings corresponding to those of birds that fly. But its whole flying apparatus is in ruins. The rudimentary hind-toe of birds is a vestigial organ, and so are the claws which appear on the thumb and first finger of all young birds. So also are the rudiments of eyes in cave crickets, fishes, and other inhabitants of total darkness. The flounder and other so-called flat fishes swim straight up, as ordinary fishes do, when young. But as they grow they incline more and more to one side, and finally swim entirely on their side, the eye on the lower side migrating around, and joining the other on the upper side of the head.