The Outline of History: Being a Plain History of Life and Mankind

by H. G. Wells

A third great group of causes influencing climate are to be found in the forces within the world itself. Throughout the long history of the earth there has been a continuous wearing down of the hills and mountains by frost and rain and a carrying out of their material to become sedimentary rocks under the seas. There has been a continuous process of wearing down the land and filling up the seas, by which the seas, as they became shallower, must have spread more and more over the land. The reverse process, a process of crumpling and upheaval, has also been in progress, but less regularly. The forces of upheaval have been spasmodic; the forces of wearing down continuous. For long ages there has been comparatively little volcanic upheaval, and then have come periods in which vast mountain chains have been thrust up and the whole outline of land and sea changed. Such a time was the opening stage of the Cainozoic period, in which the Alps, the Himalayas, and the Andes were all thrust up from the sea-level to far beyond their present elevations, and the main outlines of the existing geography of the world were drawn.

Now, a time of high mountains and deep seas would mean a larger dry land surface for the world, and a more restricted sea surface, and a time of low lands would mean a time of wider and shallower seas. High mountains precipitate moisture from the atmosphere and hold it out of circulation as snow and glaciers, while smaller oceans mean a lesser area for surface evaporation. Other things being equal, lowland stages of the world’s history would be ages of more general atmospheric moisture than periods of relatively greater height of the mountains and greater depth of the seas. But even small increases in the amount of moisture in the air have a powerful influence upon the transmission of radiant heat through that air. The sun’s heat will pass much more freely through dry air than through moist air, and so a greater amount of heat would reach the land surfaces of the globe under the conditions of extremes of elevation and depth, than during the periods of relative lowness and shallowness. Dry phases in the history of the earth mean, therefore, hot days. But they also mean cold nights, because for the same reason that the heat comes abundantly to the earth, it will be abundantly radiated away. Moist phases mean, on the other hand, cooler days and warmer nights. The same principle applies to the seasons, and so a phase of great elevations and depressions of the surface would also be another contributory factor on the side of extreme climatic conditions.

And a stage of greater elevation and depression would intensify its extreme conditions by the gradual accumulation of ice caps upon the polar regions and upon the more elevated mountain masses. This accumulation would be at the expense of the sea, whose surface would thus be further shrunken in comparison with the land.

Here, then, is another set of varying influences that will play in with and help or check the influence of the astronomical variations stated in § 1 and § 2. There are other more localized forces at work into which we cannot go in any detail here, but which will be familiar to the student of the elements of physical geography; the influence of great ocean currents in carrying warmth from equatorial to more temperate latitudes; the interference of mountain chains with the moisture borne by prevalent winds and the like. As in the slow processes of nature these currents are deflected or the mountain chains worn down or displaced by fresh upheavals, the climate over great areas will be changed and all the conditions of life changed with it. Under the incessant slow variations of these astronomical, telluric, and geographical influences life has no rest. As its conditions change it must change or perish.

§ 4

And while we are enumerating the forces that change climate and the conditions of terrestrial life, we may perhaps look ahead a little and add a fourth set of influences, at first unimportant in the history of the world so far as the land surface is concerned, but becoming more important after the age of Reptiles, to which we shall proceed in our next chapter. These are the effects produced upon climate by life itself. Particularly great is the influence of vegetation, and especially that of forests. Every tree is continually transpiring water vapour into the air; the amount of water evaporated in summer by a lake surface is far less than the amount evaporated by the same area of beech forest. As in the later Mesozoic and the Cainozoic Age, great forests spread over the world, their action in keeping the air moist and mitigating and stabilizing climate by keeping the summer cool and the winter mild must have become more and more important. Moreover, forests accumulate and protect soil and so prepare the possibility of agricultural life.

Water-weeds again may accumulate to choke and deflect rivers, flood and convert great areas into marshes, and so lead to the destruction of forests or the replacement of grass-lands by boggy wildernesses.

Finally, with the appearance of human communities, came what is perhaps the most powerful of all living influences upon climate. By fire and plough and axe man alters his world. By destroying forests and by irrigation man has already affected the climate of great regions of the world’s surface. The destruction of forests makes the seasons more extreme; this has happened, for instance, in the northeastern states of the United States of America. Moreover, the soil is no longer protected from the scour of rain, and is washed away, leaving only barren rock beneath. This has happened in Spain and Dalmatia and, some thousands of years earlier, in South Arabia. By irrigation, on the other hand, man restores the desert to life and mitigates climate. This process is going on in Northwest India and Australia. In the future, by making such operations worldwide and systematic, man may be able to control climate to an extent at which as yet we can only guess.

VI
THE AGE OF REPTILES

§ 1. The Age of Lowland Life. § 2. Flying Dragons. § 3. The First Birds. § 4. An Age of Hardship and Death. § 5. The First Appearance of Fur and Feathers.