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, 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.
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 is atavism, 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.
About the first thing a human infant does on coming into the world is to prove its arboreal origin by grasping and spitefully clinging to everything that stimulates its palms. A little peeperless babe an hour old can perform feats of strength with its hands and arms that many men and women cannot equal. It can support the entire weight of its body for several seconds hanging by its hands. Dr. Robinson, an English physician, found as a result of sixty experiments on as many infants, more than half of whom were less than an hour old, that with two exceptions every babe was able to hang to the finger or to a small stick, and sustain the whole weight of the body for at least ten seconds. Twelve of those just born held on for nearly a minute. At the age of two or three weeks, when this power is greatest, several succeeded in sustaining themselves for over a minute and a half, two for over two minutes, and one for two minutes and thirty-five seconds. The young ape for some weeks after birth clings tenaciously to its mother’s neck and hair, and the instinct of the child to cling to objects is probably a survival of the instinct of the young ape. I believe it is Wallace who relates somewhere an incident which illustrates the instinct of the young simian to cling to something. Wallace had captured a young ape, and was carrying it to camp, when the little fellow happened to get its hands on the naturalist’s whiskers, which it mistook, evidently, for the hirsute property of its mother, and, driven by the powerful instinct of self-preservation, it hung on to them so desperately it could scarcely be pulled loose. Many mammals are provided with a well-developed muscular apparatus for the manipulation of their ears. But in man there does not exist the same necessity for auricular detection of enemies, and while these muscles still exist, and are capable of being used to a slight extent by occasional individuals, they are generally so emaciated as to be useless.
Another vestigial organ in the body of man, and one of significance from the standpoint of morphology, is the tail. The tail is an exceedingly unpopular part of the human anatomy, most men and women being unwilling to admit that they have such an appendage. But many a person who has hitherto dozed in ignorance on this matter has learned with considerable dismay, when he has for the first time looked upon the undraped lineaments of the human skeleton, that man actually has a tail. It consists of three or four (sometimes five) small vertebrae, more or less fused, at the posterior end of the spinal column. That this is really a rudimentary tail is proved beyond a doubt by the fact that in the embryo it is highly developed, being longer than the limbs, and is provided with a regular muscular apparatus for wagging it. These caudal muscles are generally represented in grown-up people by bands of fibrous tissue, but cases are known where the actual muscles have persisted through life.[4]
The nictitating membrane, which in birds and many reptiles consists of a half-transparent curtain acting as a lid to sweep the eye, is in the human eye dwindled to a small membranous remnant, draped at the inner corner. The growth of hair over the human body surface may be regarded, in view of the sartorial habits of man, as a vestigial inheritance from hairy ancestors. One of the most notorious of the vestigial organs of man is the vermiform appendix, a small slender sac opening from the large intestine near where the large intestine is joined by the small intestine. In some animals this organ is large and performs an important part in the process of digestion. But in man it is a mere rudiment, not only of no possible aid in digestion, but the source of frequent disease, and even of death.
There are in all, according to Darwin, about eighty vestigial organs in the human body. But these organs occur everywhere throughout the animal kingdom. There is not an order of animals, nor of plants either, without them. They are necessary facts growing out of evolution. Organic structures are the result of adjustment to surrounding conditions. The continual changes in environment to which all organisms are exposed necessitate corresponding changes in structure. And the vestiges found in the bodies of all animals represent parts which in the previous existence were useful and necessary to a complete adjustment of the organism, but which, owing to a change of emphasis in surroundings, have become useless, and consequently shrunken. They are the obsolete or obsolescent parts of animal structure—parts which have been outgrown and superseded—the ‘silent letters’ of morphology. They sustain the same relation to the individual organism as dead or dwindling species sustain to a fauna. They furnish indisputable proof of the kinship and unity of the animal world.
6. It is only on the supposition that the life of the earth has evolved step by step with the evolution of the land masses, and that the forms of life from which existing forms were evolved were dispersed over the earth at a time when physiographic conditions were very different from what they are now, that it is possible to account for the peculiar manner in which animals are distributed over the earth. The cassowary is a flightless bird of the ostrich order inhabiting Australia and the islands to the north of it. This bird is found nowhere else in the world, and each area has its own particular species. The same things are also true of the kangaroo. It is found over a similar region, with a different species occupying each land mass. Now, on the hypothesis of special creation there is no thinkable reason why these animals should be divided, as they are, into distinct species, and restricted to this particular region. But on the hypothesis of evolution it is perfectly plain. All of these regions at one time were united with one another, and were subsequently submerged in part, forming islands. Each group of animals, being isolated from every other group and subjected to somewhat different conditions, developed a style of departure from the original type of structure different from that of every other group in response to the peculiar conditions operating upon it. This has led, in the course of centuries of selection, to the formation of distinct species such as exist to-day.
Lombock Strait, a narrow neck of water between Bali and Lombock Island, and Macassar Strait, separating Celebes from Borneo, are parts of a continuous passage of water which in remote times separated two continents—an Indo-Malayan continent to which belonged Borneo, Sumatra, Java, and the Malay Peninsula; and an Austro-Malayan continent, now represented by Australia, Celebes, the Moluccas, New Guinea, Solomon’s Islands, etc. Wallace first announced this ancient boundary, and it has been called ‘Wallace’s line.’ He was led to infer its existence by the fact which he observed as he travelled about from island to island, that, while the faunas of these two regions are as wholes very different from each other, the faunas of the various land patches in each area have a wonderful similarity. Australia is a veritable museum of old and obsolete forms of both plants and animals. Its fauna and flora are made up prevailingly of forms such as have on the other continents long been superseded by more specialised species. No true mammals, excepting men and a few rats, lived in Australia when Englishmen first went there. The most powerful animals were the comparatively helpless marsupials. The explanation of these remarkable facts is probably this: The Australian continent, which formerly included New Guinea and other islands to the north, has not been connected with the other land masses for a very long period of time. The development upon the other continents of the more powerful mammals, especially of the ungulates and the carnivora, resulted in the extermination of the more helpless forms from most of the earth’s surface. But Australia, protected by its isolation, has retained to this day its old-fashioned forms of life, neither land animals nor plants having been able to navigate the intervening straits. This supposition is strengthened by the fact that fossil remains of marsupials are to-day found scattered all over the world, while, with the exception of the American opossums, living marsupials are found only in Australia and its islands. There is to-day not a single survivor of these once-numerous races in either Europe, Asia, or Africa. Similar facts of distribution are furnished by the lemurs—those small, monkey-like animals with fox faces, which are sometimes called ‘half-apes,’ since they are supposed to be the link connecting the true apes with lower forms. Fossil lemurs are found in both America and Europe, but lemurs are now extinct in both continents. Those of America were probably exterminated by the carnivora, who are known to be very fond of monkey meat of all kinds. The European lemurs seem to have migrated southward into eastern Africa at a time when Madagascar formed a part of the mainland. ‘There they have been isolated, and have developed in a fashion comparable to that which has occurred in the case of the Australian marsupials. Of fifty living species, thirty are confined to Madagascar, and the lemurs are there exceedingly numerous in individuals. Outside of Madagascar they only maintain a precarious footing in forests or on islands, and are usually few in number’.[3b]
If the earth were peopled by migrations from Ararat, it would require a good deal of intellectual legerdemain to show why the sloths are confined to South America and the monotremes to Australia and its islands. The reindeer of northern Europe and Asia, and the elk and caribou of Arctic America, are so much alike they must have descended from a common ancestry, and been developed into distinct species since the separation of North America and Eurasia. The same thing is probably also true of the puma and jaguar, who inhabit the middle latitudes of the New World, and the lion, tiger, and leopard, occupying like latitudes of the Old World. They all belong to the cat family, and represent divergences from a common feline type of structure. The camel does not exist normally outside of northern Africa and central and western Asia. And when the camel-like llama of South America first became known to zoologists, it was a problem how this creature could have become separated so far from the apparent origin of the camel family. But since then fossil camels have been found all over both North and South America. And it has even been suspected that perhaps America was the original home of the camel, and that, like the horse, the camel migrated to the eastern hemisphere at a time when the eastern and western land masses were connected. The foxes, hares, and other mammals of the upper Alps, also many Alpine plants, are like those of the Arctic regions. The most probable explanation of these resemblances is that these Alpine species climbed up into these inhospitable altitudes, and were left stranded here on this island of cold, when their relatives, on the return of warmth at the close of the glacial period, retreated back to the ice-bound fastnesses around the pole. It is for a similar reason, probably, that the flora of the upper White Mountains resembles that of Labrador.
7. One of the strongest pieces of evidence bearing on evolution that is furnished by any department of knowledge is that furnished by geology. It is the evidence of the rocks. Geology is, among other things, a history of the earth. This history has been written by the earth itself on laminae of stone. It is from these records that we learn incontestably the order in which the forms of life have made their appearance on the earth.
Three-fourths of the surface of the earth is sea. Over the surface of the remaining fourth, excepting in mountainous places, is a layer of soil, varying from a few feet to a few hundred feet in depth. Beneath this coverlet of soil, extending as far as man has penetrated into the earth, is rock. Excepting in regions overflowed by lava poured out from beneath, or along the backbones of continents where the surface rocks have been upheaved into folds and carried away by denudation, the rocks immediately beneath the soil, to a thickness often of thousands of feet, are in the form of layers, or sheets, arranged one above another. These rocks are called sedimentary rocks, as distinguished from the unlaminated rocks of the interior. They have been formed at the bottom of the sea, and have, hence, all been formed since the condensation of the oceans. They have been formed out of the detritus of continents brought down by the rivers and the accumulated remains of animal and vegetal forms which have slowly settled down through the waters. They are the successive cemeteries of the dead past. Such rocks are now forming over the floors of all oceans—forming just as they have formed throughout the long eons of geological history. Along the axes of ancient mountains and in deep-cut canyons the rock layers are exposed to a thickness of thousands of feet, in some cases thirty or forty thousand feet. Here they lie, piled up, one on top of another, the great, broad pages upon which are written the long, dark story of our planet. It is the mightiest and most everlasting of all annals—the autobiography of a world. It is possible, by studying these rock records, to know not only the kind of life that lived in each age, but a good deal regarding the conditions in which that life lived and passed away. Just as the naturalist is able, from a single bone of an unknown animal, to reconstruct the entire animal and to infer something of its surroundings and habits of life, and as the archeologist, by going back to the graves of deceased races and digging up the dust upon which these races wrought, is able to tell much of their history and characteristics, so the geologist, by studying the bones of those more distant civilisations, the civilisations sandwiched among the fossiliferous rocks, is able to know, not only just the kind of life that lived in each age, but, by comparing the species of successive strata, can construct with astonishing fulness the genealogical outline of the entire life process. The succession of life forms as they appear in the rocks, with a sketch of their probable genealogy, is traced elsewhere in this chapter. It is only necessary to say here that the order in which the forms of life appear in the sedimentary strata is that of a gradually increasing complexity. The invertebrates appear first; then the fishes, the lowest of the vertebrates; after these come the amphibians; following these the reptiles; and finally the birds and mammals.
8. There is another reason for a belief in evolution furnished by geology, but of a somewhat different kind from that just stated. It consists in the fact that there are found in the rocks series or grades of structures, which fit with amazing accuracy on to the structures of existing species. Now, this is precisely what, according to the evolutional hypothesis, is to be expected. For, if evolution is true, existing species represent the tops of things. They are the existing and visible parts of processes which extend indefinitely back into the past, and whose deceased stages may reasonably be expected to be found fossil in the earth. Considering the youth and inexperience of paleontology and the torn and incoherent character of the record, it is surprising that anatomists have been able to accomplish what they have accomplished. In many cases—notably, those of man, the snail, the crocodile, and the horse—antecedent forms of structure have been found in almost unbroken gradations leading back to types differing immensely from their existing representatives. Bones and fossils of men have been found buried beneath the alluvium of rivers, under old lava-beds, and in caves, crusted over by the deposits of percolating waters. Many such fossils are found in quaternary rocks, along with the bones of animals still living and some extinct. Some of these remains indicate unmistakable affinities with the ape. The most celebrated of these discoveries is the fossil of an erect ape-man (Pithecanthropus erectus), found by a Dutch Governor on the island of Java in 1894. This fossil, in the shape and size of the head and in its general structure, strikes about as near as could be the middle between man and ape. That it is the fossil of an ambiguous form is indicated by the fact that, when it was examined by a company of twelve specialists at Berlin soon after its discovery, three of them declared it to be the remains of an individual belonging to a low variety of man; three others thought it was a large anthropoid; while the other six held that it was neither man nor anthropoid, but a genuine connecting link between them. It is discussed at length by Haeckel in ‘The Last Link,’ a paper read before the International Congress of Zoology, at Cambridge, in 1898. ‘It is,’ says the veteran biologist, ‘the much-sought “missing link” supposed to be wanting in the chain of primates which stretches unbroken from the lowest catarhine to the most highly developed man.’ Associated with this fossil ape-man were the fossils of the elephant, hyena, and hippopotamus, none of which any longer exist in that part of the world, also the fossil remains of two orders of animals now extinct. The genealogy of the crocodile has been traced by Huxley, through all intermediate stages, back to the giant reptiles of the early Tertiary.[5]And the pedigree of the horse has been even more completely worked out by the indefatigable Marsh. In the museum of Yale University may be seen the fossil history of this splendid ungulate, from the time it was a clumsy little quadruped only 14 inches high, and with four or five toes on each foot, down to existing horses. The earliest known ancestor of the horse, the eohippus, lived at the beginning of the Eocene epoch. It had five toes, almost equal, on each front foot (four toes behind), and was about the size of a fox. The orohippus, which lived a little later, had four toes on each front-foot, and three behind. The mesohippus, found in the Miocene, had three toes and one rudimentary toe on each front-foot, and three toes behind. It was about the size of a sheep. The miohippus, which is found later, had three toes on each of its four feet, with the middle toe on each foot larger than the other two. The pliohippus, living in the Pliocene epoch, had one principal toe on each foot, and two secondary toes, the two secondary toes not reaching to the ground. It was about the size of a donkey. Existing horses have one toe on each foot—the digit corresponding to the big middle finger—and the ruins of two others in the form of splints on the back of each ankle. In the embryo of the horse these splints are segmented, each of them, into three phalanges. Fossil remains representing all stages in the development of the horse have been found in the regions about the upper waters of the Missouri River.
It is an important fact that the types of structure forming any series grow more and more generalised as the distance from the present increases, and that different lines of development, when traced back into the past, often converge in types which combine the main characters of various existing groups. The horses, rhinoceroses, and tapirs, great as are the differences among them now, can be traced back step by step through fossil forms, their differences gradually becoming less marked, until ‘the lines ultimately blend together, if not in one common ancestor, at all events into forms so closely alike in all essentials that no reasonable doubt can be held as to their common origin.’ ‘The four chief orders of the higher mammals—the primates, ungulates, carnivora, and rodents—seem to be separated by profound gulfs, when we confine our attention to the representatives of to-day. But these gulfs are completely closed, and the sharp distinctions of the four orders are entirely lost, when we go back and compare their extinct predecessors of the Cenozoic period, who lived at least three million years ago. There we find the great sub-class of the placentals, which to-day comprises more than two thousand five hundred species, represented by only a small number of insignificant pro-placentals, in which the characters of the four divergent orders are so intermingled and toned down that we cannot in reason do other than consider them as the precursors of those features. The oldest primates, the oldest ungulates, the oldest carnivora, and the oldest rodents, all have the same skeletal structure and the same typical dentition (forty-four teeth) as these pro-placentals; all are characterised by the small and imperfect structure of the brain, especially of the cortex, its chief part, and all have short legs and five-toed, flat-soled (plantigrade) feet. In many cases among these oldest placentals it was at first very difficult to say whether they should be classed with the primates, ungulates, carnivora, or rodents, so very closely and confusedly do these four groups, which diverge so widely afterwards, approach each other at that time. Their common origin from a single ancestral group follows incontestably’.[6]
9. Man is the most powerful and influential of animals. He rules the world—rules it with a sovereignty more despotic and extensive than that hitherto exercised by any other animal. Many races of beings are, and have been for centuries, completely dominated by him. These races, during their long subjection, have been changed and transformed by man in a wonderful manner through his control of their power to breed. All domestic animals have come from wild animals; they have been derived by a process of selective evolution conducted by man himself. By continually choosing as the progenitors of each generation those with qualities best suited to his whims and purposes, man has evolved races as different from each other in appearance and structure, and as different from the original species, as many groups which, in the wild state, constitute distinct species; indeed, man has in some cases created entirely new species, both of plants and animals—species that breed true and are what biologists call ‘good’—by his own selections.
There are something over 150 different varieties of the domestic pigeon. Some of these varieties—as many as a dozen, Mr. Darwin thinks—differ from each other sufficiently to be reckoned, if they are considered solely with reference to their structures, as entirely distinct species. The carrier, for instance, the giant of the pigeons, measures 17 inches from bill-tip to the end of its tail, and has a beak 1 3⁄4 inches long. Around each eye is a large dahlia-like wattle, and another large wattle is on the beak, giving the beak the appearance of having been thrust through the kernel of a walnut. The tumbler is small, squatty, and almost beakless. It has the preposterous habit of rising high in the air and then tumbling heels over head. The roller, one of the many varieties of the tumbler, descends to the ground in a series of back somersaults, executed so rapidly that it looks like a falling ball. The runt is large, weighing sometimes as much as the carrier. The fantail has thirty or forty feathers in its tail, while all other varieties have only twelve or fourteen, the normal number for birds. The trumpeter, so named on account of its peculiar coo, has an umbrella-like hood of feathers covering its head and face, and its feet are so heavily feathered that they look like little wings. In the correct specimens of this variety the feathers have to be clipped from the face before the birds can see to feed themselves. The pouter has the absurd habit of inflating its gullet to a prodigious size, and the Jacobin wears a gigantic ruff. The homing pigeon has such a strong attachment for its cote that it will travel hundreds of miles, sometimes as many as 1,400 miles, in order to reach the home from which it has been separated. But it is not simply in their colour, size, habits, and plumage, that pigeons vary. There are corresponding differences in their structures, in the number of their ribs and vertebrae, in the shape and size of the skull, in the bones of the face, in the development of the breast-bone, and in the length of the neck, legs, and bill. Pigeons also differ in the shape and size of their eggs, and in their dispositions and voice. ‘There is,’ says Huxley in summing up his discussion of the great variety in these birds, ‘hardly a particular of either internal economy or external shape which has not by selective breeding been perpetuated and become the foundation of a new race’.[7]
All of the 150 different varieties of domestic pigeons have been evolved by human selection during the past three or four thousand years from the blue rock-doves which to-day inhabit the seacoast countries of Europe.
What is true of pigeons is also true largely of most of the other races associated with man—of cats, cattle, horses, sheep, swine, goats, fowls, and the like. All varieties of the domestic chicken—the clumsy Cochin with its feather-duster legs, the tall and stately Spanish, the great-crested Minorca, the Dorking with its matchless; comb and wattle, the almost combless Polish, the blue Andalusian, the gigantic Brahma, the tiny Bantam, the Wyandottes in all colours (black, white, buff, silver, and golden), the magnificent Plymouth Rocks, and the exceedingly pugnacious Game-cock—these and dozens of other varieties, all flightless, have come from the jungle-bird whose morning clarion still greets Aurora from the wilds of distant India. The dog is a civilised wolf, and the wild-boar is the progenitor of the oleaginous swine. The Merino and South Down breeds of sheep have come from the same stock in the last century and a half. In 1790 a lamb was born on the farm of Seth Wright in Massachusetts. It had a long body and short, bowed legs. It was noticed that this lamb could not follow the others over the fences. The owner thought it would be a good thing if all his sheep were like it. So he selected it to breed from. Some of its offspring were like it, and some were like the ordinary sheep. By continual selection of those with long bodies and short legs the ancon breed of sheep was finally produced. In 1770 in a herd of Paraguay cattle a hornless male calf appeared, and from this individual in a similar way came the stock of Muleys. The occasional appearance of horned calves and lambs among the offspring of hornless breeds of cattle and sheep are examples of atavism indicating the presence of a vestigial tendency to breed true to their horned ancestors. The Hereford cattle originated as a distinct variety about 1769 through the careful selections of a certain Englishman by the name of Tompkins. All domesticated quadrupeds, except the elephant, have come from wild species with erect ears, the ears acting as funnels to harvest the sound-waves. But there are few of them in which there is not one or more varieties with drooping ears—cats in China, horses in parts of Russia, sheep in Italy, cattle in India, and pigs, dogs, and rabbits in all long-civilised lands. We are so accustomed to seeing dogs and pigs with pendent ears that we are surprised to know there are varieties with erect ears. The goldfish is a carp, and in its native haunts in the waters of China it has the colour of the carp. The golden hue seen in the occupants of our aquaria has been given to this fish by the Chinese through the continual selection of certain kinds. The goldfish, almost as much as the pigeon, has been the sport of fanciers, and the strangest varieties have resulted. Some have outlandishly long fins, while others have no dorsal fin at all. Some are streaked and splotched with gold and scarlet; others are pure albinos. One of the most monstrous varieties has a three-lobed tail-fin, and its eyeballs, without sockets, are on the outside of its head. All of our common barnyard fowls—turkeys, ducks, geese, and chickens—are flightless, but the varieties from which the domesticated forms have come all have functional wings, two of these varieties crossing continents in their annual migrations.
Not only animals, but plants also, many of them, have been greatly changed by man in his efforts to adapt them to his uses as food, ornamentation, and the like. On the seaside cliffs of Chili and Peru may still be found growing the wild-potato—the small, tough, bitter ancestor of the mammoth Burbank, Peerless, Early Rose, and the nearly two hundred other varieties of this matchless tuber found in the gardens of civilised man. The cabbage, kale, cauliflower, and kohlrabi are all modifications of the same wild species (Brassica oleracea), the cauliflower being the developed flower, kohlrabi the stalk, and kale and cabbage the leaves. The peach and the almond, Darwin thinks, have also come from a common ancestral drupe, the peach being the developed fruit, and the almond the seed. There are nearly 900 different varieties of apples, varying in the most wonderful manner in size, colour, flavour, texture, and shape, but all of them probably derived from the little, sour, inedible Asiatic crab. The many times ‘double’ roses of our gardens have come from the five-petalled wild-rose of the prairies. The cultivated varieties of viburnum and hydrangea have showy corymbs of infertile flowers only, but the wild forms from which the domestic varieties have been derived have only a single marginal row of showy infertile flowers surrounding a mass of inconspicuous fertile flowers. It has been due to their efforts to please men that bananas, pineapples, and oranges have got into the habit of neglecting to produce seeds. There are certain species of grapes that are seedless, also seedless sugar-cane, and a seedless apple has just been announced by horticulturists. The development of domesticated plants is only in its infancy, and it is probably impossible even for the most agile imagination to dream of the miracles the horticulturist is destined to work in the ages to come. There is every reason to believe that seedless varieties of all our common fruits will ultimately be produced, and that in size, flavour, nutrient constituents, and appearance, they will be developed into forms utterly different from existing varieties. Just within the last few years the U.S. Department of Agriculture has developed a cotton-plant immune to the bacterial diseases of the soil, which had completely driven the cotton-raising industry out of large districts of the South. The cultivation of many of the cereals has gone on so long, and has proceeded so far, that their origin is lost in antiquity.
Whether or not it is possible for new varieties and species to be evolved is a question, therefore, which does not need to depend for reply wholly upon theory. It is known to have taken place; and the process by which the different varieties of domestic animals and plants have been evolved—domestic selection—is not different in principle from the process of natural selection, the chief operation by which life in general, both plant and animal, is assumed to have been evolved.
10. There are other reasons for a belief in organic evolution, but the last one I shall mention is the fact that the theory of organic evolution harmonises with the known tendencies of the universe as a whole. The organic kingdoms of the earth—animals and plants—are as truly parts of the terrestrial globe as the inorganic kingdom is; and as such they share in, and are actuated by, the same great tendency or instinct as that which actuates the whole. Nine-tenths of the substance of all animals and plants is oxygen, hydrogen, carbon, and nitrogen—the very elements which make up the entire ocean and air, and enter largely into the composition of the continents. The human body, which has essentially the same chemical composition as the bodies of animals in general, is made up of four solids, five gases, and seven metals—in all, sixteen elements of the something like seventy which constitute the entire planet. ‘In the past, man appeared to be a creature foreign to the earth, and placed upon it as a transitory inhabitant by some incomprehensible power. The more perfect insight of the present day sees man as a being whose development has taken place in accordance with the same laws as those that have governed the development of the earth and its entire organisation—a being not put upon the earth accidentally by an arbitrary act, but produced in harmony with the earth’s nature, and belonging to it as do the flowers and the fruits to the tree which bears them.’ Animals are not outside of, nor distinct from, the universe, as one might suspect who has listened much to the recital of tradition so long accepted as science. They are more or less detached portions of the planet earth which move over its surfaces and through its fluids and multiply, but which in their phenomena obey the same laws of chemistry and physics as those in accordance with which the rest of the universe acts. Animals are moulds through which digressing matters from the soil, sea, and sky pass on rounds of eternal itineracy.
Now, the earth as a planet is in process of evolution. Not many things are more certain than this. The earth has come out of fire. It has grown to be what it is. Its mountains, valleys, plains, seas, shores, islands, lakes, rivers, and continents—these were not always here. They have been evolved. Not only the earth, but the entire family of spheres of which the earth is a member—the solar system—are all evolving. Mr. Spencer never did anything more profound than when he demonstrated in his ‘Law and Cause of Progress’ the universal migration of things from a condition of homogeneity toward a condition of greater and greater heterogeneity. The whole universe, or as much of it as can be examined by terrestrial instruments, has probably evolved out of the same primordial matters. The organic part of the earth has evolved, therefore, and is destined to continue to evolve, because it is a part of a whole whose habit or ambition it is to evolve.
The evidence is overwhelming. The theory of organic evolution is sustained by a mass of facts not less authoritative and convincing than that which supports the Copernican theory of the worlds. Evolution is, in fact, a doctrine so apparent that it only needs to be honestly and intelligently looked into to be accepted unreservedly. It is, indeed, more than a doctrine. It is a known fact. It is a necessary effect of the conditions known to exist among the animals and plants of the earth. If beings vary among themselves generation after generation, if only the fittest of each generation survive and if the survivors tend to transmit to their offspring the qualities of their superiority (and the animals and plants of the earth are known to do continually all of these things), then it follows with mathematical certainty that evolution is going on, and that it will continue to go on as long as these conditions continue. It is inevitable. It could not be otherwise. We would know that evolution were going on among organisms where these conditions existed, even though we had never observed it.
The boldest and most enthusiastic opponents of evolution have always been those with the least information about it. But the evidence is accumulating so rapidly, and is being drawn up in such unanswerable array, that, if it is not already the case, it will not be many years before it will be an intellectual reproach for anyone to discredit, or to be known to have discredited, this splendid and inspiring revelation.
[1a.] [1b.] Darwin: Descent of Man, 2nd edit.; London, 1874.
[2.] Huxley: Man’s Place in Nature; New York, 1883.
[3a.] [3b.] Thompson: Outlines of Zoology, 3rd edit.; Edinburgh, 1899.
[4.] Drummond: Ascent of Man; New York, 1894.
[5.] See table of geological ages, at the end of the chapter.
[6.] Haeckel: The Riddle of the Universe; New York, 1901.
[7.] Huxley: On the Origin of Species, lecture iv.
