Part 3

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 by _partiality_ or _selection_. 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; and _Environment_, 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’.[1]

‘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.[3]

_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, about 1/125 inch 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.