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 that man 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 man and the animals, not of man and the other animals. Man was somehow sui 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 man is an animal. 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 a vertebrate animal.[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.