The Outline of History - H. G. Wells (most recommended books txt) 📗

animals not closely related to the Mesozoic reptiles and certainly not descended from any of their ruling types. A new kind of life is in possession of the world.

 

This apparently abrupt ending up of the reptiles is, beyond all question, the most striking revolution in the whole history of the earth before the coming of mankind. It is probably connected with the close of a vast period of equable warm conditions and the onset of a new austerer age, in which the winters were bitterer and the summers brief but hot. The Mesozoic life, animal and vegetable alike, was adapted to warm conditions and capable of little resistance to cold. The new life, on the other hand, was before all things capable of resisting great changes of temperature.

 

Whatever it was that led to the extinction of the Mesozoic reptile, it was probably some very far-reaching change indeed, for the life of the seas did at the same time undergo a similar catastrophic alteration. The crescendo and ending of the Reptiles on land was paralleled by the crescendo and ending of the Ammonites, a division of creatures like squids with coiled shells which swarmed in those ancient seas. All though the rocky record of this Mesozoic period there is a vast multitude and variety of these coiled, shells; there are hundreds of species, and towards the end of the Mesozoic period they increased in diversity and produced exaggerated types. When the record resumes these, too, have gone. So far as the reptiles are con c erned, people may perhaps be inclined to argue that they were exterminated because the Mammals that replaced them, competed with them, and were more fitted to survive; but nothing of the sort can be true of the Ammonites, because to this day their place has not been taken. Simply they are gone. Unknown conditions made it possible for them to live in the Mesozoic seas, and then some unknown change made life impossible for them. No genus of Ammonite survives to-day of all that vast variety, but there still exists one insolated genus very closely related to the Ammonites, the Pearly Nautilus. It is found, it is to be noted, in the warm waters of the Indian and Pacific oceans.

 

And as for the Mammals competing with and ousting the less fit reptiles, a struggle of which people talk at times, there is not a scrap of evidence of any such direct competition. To judge by the Record of the Rocks as we know it to-day, there is much more reason for believing that first the reptiles in some inexplicable way perished, and then that later on, after a very hard, time for all life upon the earth, the mammals, as conditions became more genial again, developed and spread to fill the vacant world.

 

5.5 The First Appearance of Fur and Feathers

 

Were there mammals in the Mesozoic period?

 

This is a question not yet to be answered precisely. Patiently and steadily the geologist's gather fresh evidence and reason out completer conclusions. At any time some new deposit may reveal fossils that will illuminate this question. Certainly either mammals, or the ancestors of the mammals, must have lived throughout the Mesozoic period. In the very opening chapter of the Mesozoic volume of the Record there were those Theriomorphous Reptiles to which we have already alluded, and in the later Mesozoic a number of small jawbones are found, entirely mammalian in character. But there is not a scrap, not a bone, to suggest that there lived any Mesozoic Mammal which could look a dinosaur in the face. The Mesozoic mammals or mammal-like reptiles"for we do not know clearly which they were"seem to have been All obscure little beasts of the size of mice and rats, more like a downtrodden order of reptiles than a distinct class; probably they still laid eggs and were developing only slowly their distinctive covering of hair. They lived away from big waters, and perhaps in the desolate uplands, as marmots do now; probably they lived there beyond the pursuit of the carnivorous dinosaurs. Some perhaps went on all fours, some chiefly went on their hind-legs and clambered with their fore limbs. They became fossils only so occasionally that chance has not yet revealed a single complete skeleton in the whole vast record of the Mesozoic rocks by which to check these guesses.

 

These little Theriomorphs, these ancestral mammals, developed hair. Hairs, like feathers, are long and elaborately specialized scales. Hair is perhaps the clue to, the salvation of the early mammals. Leading lives upon the margin of existence, away from the marshes and the warmth, they developed an outer covering only second in its warmth-holding (or heat-resisting) powers to the down and. feathers of the Arctic seabirds. And so they held out through the age of hardship between the Mesozoic and Cainozoic ages, to which most of the true reptiles succumbed.

 

[Fig 0035 Hesperornis]

 

[Fig 0035]

 

All the main characteristics of this flora and sea and land fauna that came to an end with the end of the Mesozoic age were such as were adapted to an equable climate and to shallow and swampy regions. But in the case of their Cainozoic successors, both hair and feathers gave a power of resistance to variable temperatures such as no reptile possessed, and with it gave a range far greater than any animal had hitherto attained.

 

The range of life of the Lower Palaeozoic Period was confined to warm water.

 

The range of life of the Upper Palaeozoic Period was confined to warm water or to warm swamps and wet ground.

 

The range of life of the Mesozoic Period as we know it was confined to water and fairly low-lying valley regions under equable conditions.

 

Meanwhile in each of these periods there were types involuntarily extending the range of life beyond the limits prevailing in that period; and when ages of extreme, conditions prevailed, it was these marginal types which survived to inherit the depopulated world.

 

That perhaps is the most general statement we can make about the story of the geological record; it is a story of widening range. Classes, genera, and species of animals appear and disappear, but the range widens. It widens always. Life has never had so great a range as it has to-day. Life to-day, in the form of man, goes higher in the air than it has ever done before; man's geographical range is from pole to pole, he goes under the water in submarines, he sounds the cold, lifeless darkness of the deepest seas, he burrows into virgin levels of the rocks, and in thought and knowledge he pierces to the centre of the earth and reaches out to the uttermost star. Yet in all the relics of the Mesozoic time we find no certain memorials of his ancestry. His ancestors, like the ancestors of all the kindred mammals, must have been creatures so rare, so obscure, and so remote that they have left scarcely a trace amidst the abundant vestiges of the monsters that wallowed rejoicing in the steamy air and lush vegetation of the Mesozoic lagoons, or crawled or hopped or fluttered over the great river plains of that time.

 

6.0 The Age of Mammals

 

6.1 A New Age of Life

 

6.2 Tradition Comes Into the World

 

6.3 An Age of Brain Growth

 

6.4 The World Grows Hard Again

 

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 0039 Some Oligocene Mammals]

 

[Fig 0039]

 

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