6 The Age of Mammals

6.1 A New Age of Life

The third great division of the geological record, the Cainozoic opens with a world already physically very like the world we live in to-day. Probably the day was at first still perceptibly shorter, but the scenery had become very modern in it character. Climate was, of course, undergoing, age by age, its incessant and irregular variations; lands that are temperate to-day have passed, since the Cainozoic age began, through phases of great warmth, intense cold, and extreme dryness; but the landscape, if it altered, altered to nothing that cannot still be paralleled to-day in some part of the world or other. In the place of the cycads, sequoias, and strange conifers of the Mesozoic, the plant names that now appear in the lists of fossils include birch, beech, holly, tulip trees, ivy, sweet gum, bread-fruit trees. Flowers had developed concurrently with bees and butterflies. Palms were now very important. Such plants had already been in evidence in the later levels of the (American Cretaceous,) Mesozoic, but now they dominated the scene altogether. Grass was becoming a great fact in the world. Certain grasses, too, had appeared in the later Mesozoic, but only with the Cainozoic period came grass plains and turf spreading wide over a world that was once barren stone.

The period opened with a long phase of considerable warmth; then the world cooled. And in the opening of this third part of the record, this Cainozoic period, a gigantic crumpling of the earth’s crust and an upheaval of mountain ranges was in progress. The Alps, the Andes, the Himalayas, are all Cainozoic mountain ranges; the background of an early Cainozoic scene to be typical should display an active, volcano or so. It must have been an age of great earthquakes.

Geologists make certain main divisions of the Cainozoic period, and it will be convenient to name them here and to indicate their climate. First comes the Eocene (dawn of recent life), an age of exceptional warmth in the world’s history, subdivided into an older and newer Eocene; then the Oligocene, (but little of recent life), in which the climate was still equable. The Miocene (with living species still in a minority) was the great age of mountain building, and the general temperature was falling. In the Pliocene (more living than extinct species), climate was very much as its present phase; but with the Pleistocene (a great majority of living species) there set in a long period of extreme conditions-it was the Great Ice Age. Glaciers spread from the poles towards the equator, until England to the Thames was covered in ice. Thereafter to our own time came a period of partial recovery. We may be moving now towards a warmer phase. Half a million years hence this may be a much sunnier and pleasanter world to live in than it is to-day.

6.2 Tradition Comes Into the World

In the forests and. following the grass over the Eocene plains there appeared for the first time a variety and abundance of mammals. Before we proceed to any description of these mammals, it may be well to note in general terms what a mammal is.

From the appearance of the, vertebrated animals in the Lower Palaeozoic Age, when the fish first swarmed out into the sea, there has been a steady progressive development of vertebrated creatures. A fish is a vertebrated animal that breathes by gills and can live only in water. An amphibian may be described as a fish that has added to its gill-breathing the power of breathing air with its swimming-bladder in adult life, and that has also developed limbs with five toes to them in place of the fins of a fish. A tadpole is for a time, a fish, it becomes a land creature as it develops. A reptile is a further stage in this detachment from water; it is an amphibian that is no longer amphibious; it passes through its tadpole stage its fish stage that is in an egg. From the beginning it must breathe in air; it can never breathe under water as a tadpole can do.

Fig. 39 – Some Oligocene Mammals

Now a modern mammal is really a sort of reptile that has developed a peculiarly effective protective covering, hair; and that also retains its eggs in the body until they hatch so that it brings forth living young (viviparous), and even after birth it cares for them and feeds them by its mammae for a longer or shorter period. Some reptiles, some vipers for example, are viviparous, but none stand by their young as the real mammals do. Both the birds and the, mammals, which escaped whatever destructive forces made an end to the Mesozoic reptiles, and which survived to dominate the Cainozoic world, have these two things in common; first, a, far more effective protection against changes of temperature than any other variation of the reptile type ever produced, and, secondly, a peculiar care for their eggs, the bird by incubation and the mammal by retention, and a disposition to look after the young for a, certain period after hatching or birth. There is by comparison the greatest carelessness about offspring in the reptile.

Hair was evidently the earliest distinction of the mammals from the rest of the reptiles. It is doubtful if the particular Theriodont reptiles who were developing hair in the early Mesozoic were viviparous. Two mammals survive, to this day which not only do not suckle their young,[5] but which lay eggs, the Ornithorhynchus and the Echidna, and in the Eocene there were a number of allied forms. They are the survivors of what was probably a much larger number and variety of small egg-laying hairy creatures, hairy reptiles, hoppers, climbers, and runners, which included the Mesozoic ancestors of all existing mammals up to and including man.

Fig. 41 – Miocene Mammals

Now we may put the essential facts about mammalian reproduction in another way. The mammal is a family animal. And the family habit involved the possibility of a new sort of continuity of experience in the world. Compare the completely closed-in life of an individual lizard with the life of even a quite lowly mammal of almost any kind. The former has no mental continuity with anything beyond itself; it is a little self-contained globe, of experience that, serves its purpose and ends; but the latter «picks up» from its mother, and «bands on» to its offspring. All the mammals, except for the two genera we have named, had already before the lower Eocene age arrived at this stage of pre-adult dependence and imitation.

They were all more or less imitative in youth and capable of a certain modicum of education; they all, as a part of their development, received a certain amount of care and example and even direction from their mother. This is as true of the hyaena and rhinoceros as it is of the dog or man; the difference of educability is enormous, but the fact of protection and educability in the young stage is undeniable. So far as the vertebrated animals go, these new mammals, with their viviparous, young-protecting disposition, and these new birds, with their incubating, young-protecting disposition, introduce at the opening of the Cainozoic period a fresh thing into the expanding story of life, namely, social association, the addition to hard and inflexible instinct of tradition, and the nervous organisation necessary to receive tradition.

All the innovations that come into the history of life begin very humbly. The supply of blood-vessels in the swimming-bladder of the mudfish in the lower Palaeozoic torrent-river, that enabled it to pull through a season of drought, would have seemed at that time to that bodiless visitant to our planet we have already imagined, a very unimportant side fact in that ancient world of great sharks and plated fishes, sea scorpions, and coral reefs and seaweed; but it opened the narrow way by which the land vertebrates arose to predominance. The mudfish would have seemed then a poor refugee from the too crowded and aggressive life of the sea. But once lungs were launched into the world, every line of descent that had lungs went on improving them. So, too, in the upper Palaeozoic, the fact that some of the Amphibia were losing their «amphibiousness» by a retardation of hatching of their eggs, would have appeared a mere response to the distressful dangers that threatened the young tadpole. Yet that prepared the conquest of the dry land for the triumphant multitude of the Mesozoic reptiles. It opened a new direction towards a free and vigorous land-life along which all the reptilian animals moved. And this viviparous, young- tending training that the ancestral mammalia underwent during that age of inferiority and hardship for them, set going in the world a new continuity of perception, of which even man to-day only begins to appreciate the significance.

6.3 An Age of Brain Growth

A number of types of mammal already appear in the Eocene. Some are differentiating in one direction, and some in another, some are perfecting themselves as herbivorous quadrupeds, some leap and climb among the trees, some turn back to the water to swim, but all types are unconsciously exploiting and developing the brain which is the instrument of this new power of acquisition and educability. In the Eocene rocks are found small early predecessors of the horse (Eohippus), tiny camels, pigs, early tapirs, early hedgehogs, monkeys and lemurs, opossums and carnivores. Now, all these were more or less ancestral to living forms, and all have brains relatively much smaller than their living representatives. There is, for instance, an early, rhinoceros-like beast, Titanotherium, with a brain not one-tenth the size of that of the existing rhinoceros. The latter is by no means a perfect type of the attentive and submissive student, but even so it is ten times more observant and teachable than its predecessor. This sort of thing is true of all the orders and families that survive until to-day. All the Cainozoic mammals were doing this one thing in common under the urgency of a common necessity; they were all growing brain. It was a parallel advance. In the same order or family to-day, the brain is usually from six to ten times what it was in the Eocene ancestor.

The Eocene period displayed a series of herbivorous brutes of which no representative survives to-day. Such were the Uintatheres and the Titanotheres. They were ousted by more specialized graminivorous forms as grass spread over the world. In pursuit of such beasts came great swarms of primitive dogs, some as big as bears, and the first cats, one in particular (Smi-lodon), a small fierce-looking creature with big knife-like canines, the first sabre-toothed tiger, which was to develop into greater things. American deposits in the Miocene display a great variety of camels, giraffe camels with long necks, gazelle camels, llamas, and true camels. North America, throughout most of the Cainozoic period, appears to have been in open and easy continuation with Asia, and when at last the glaciers of the Great Ice Age, and then the Bering Strait, came to separate the two great continental regions, the last camels were left in the old world and the llamas in the new.

In the Eocene the first ancestors of the elephants appear in northern Africa as snouted creatures; the elephant’s trunk dawned on the world in the Miocene.

One group of creatures is of peculiar interest in a history that is mainly to be the story of mankind. We find fossils in the Eocene of monkeys and lemurs, but of one particular creature we have as yet not a single bone. It must have been a creature half ape, half monkey; it clambered about the trees and ran, and probably ran well, on its bind-legs upon the ground. It was small-brained by our present standards, but it had clever hands with which it handled fruits and beat nuts upon the rocks and caught up sticks and stones to smite its fellows. Spite of the lack of material evidence, the facts of biological science almost compel us to believe that such a creature existed, the common ancestor of the anthropoid apes and the two species of men we will describe in the next chapter.

6.4 The World Grows Hard Again

Through millions of simian generations the spinning world circled about the sun; slowly its orbit, which may have been nearly circular during the equable days of the early Eocene, was drawn by the attraction of the circling outer planets into a more elliptical form. Its axis of rotation, which had always heeled over to the plane of its orbit, as the mast of a yacht under sail heels over to the level of the water, heeled over by imperceptible degrees a little more and a little more. And each year its summer point shifted a little further from perihelion round its path. These were small changes to happen to a one-inch ball, circling at a distance of 330 yards from a flaming sun nine feet across, in the course of a few million years. They were changes an immortal astronomer in Neptune, watching the earth from age to age, would have found almost imperceptible. But from the point of view of the surviving mammalian life of the Miocene, they mattered profoundly. Age by age the winters grew on the whole colder and harder and a few hours longer relatively to the summers in a thousand years; age by age the summers grew briefer. On an average the winter snow lay a little later in the spring in each century, and the glaciers in the northern mountains gained an inch this year, receded half an inch next, came on again a few inches…

The Record of the Rocks tells of the increasing chill. The Pliocene was a temperate time, and many of the warmth-loving plants and animals had gone. Then, rather less deliberately, some feet or some inches every year, the ice came on.

An arctic fauna, musk ox, woolly mammoth, woolly rhinoceros, lemming, ushers in the Pleistocene. Over North America, and Europe and Asia alike, the ice advanced. For thousands of years it advanced, and then for thousands of years it receded, to advance again. Europe down to the Baltic shores, Britain down to the Thames, North America down to New England, and more centrally as far south as Ohio, lay for ages under the glaciers. Enormous volumes of water were withdrawn from the ocean and locked up in those stupendous ice caps so as to cause a world-wide change in the relative levels of land and sea. Vast areas were exposed that are now again sea bottom.

The world to-day is still coming slowly out of the last of four great waves of cold. It is not growing warmer steadily. There have been fluctuations. Remains of bog oaks, for example, which grew two or three thousand years ago, are found in Scotland at latitudes in which not even a stunted oak will grow at the present time. And it is amidst this crescendo and diminuendo of frost and snow that we first recognize forms that are like the forms of men. The Age of Mammals culminated in ice and hardship and man.