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

he Sun a Steadfast Star. § 3. Changes from Wi

owly and painfully in an investigation that is still incomplete and still in progress. The Carboniferous Rocks, the "coal-measures," give us a vision of the first great expansion of life over the wet lowlands. Then come the tor

ometimes checked life. Every species of living thing is always adapting itself more and more closely to its conditions. A

of cooling down. The centre of the earth is certainly very hot to this day, but we feel nothing of that internal heat at the surface; the internal heat, except for volcanoes and hot springs, has not been perceptible at the surface since first the rocks grew solid. Even in the Azo

ost nearly the centre. When the orbit becomes most elliptical, then the position of the sun becomes most remote from the middle or, to use the astronomer's phrase, most eccentric. When the orbit is most nearly circular, then it must be manifest that all the year round the earth must be getting much the same amount of heat from the sun; when the orbit is most distorted, then there will be a season in each year when the earth is nearest the sun (this phase is called Perihelion) and getting a great deal of heat comparatively, and a season when it will be at its farthest from the sun (Aphelion) and getting very little warmth. A planet at aphelion is travelling its slowest, and its fastest at perihelion; so that the hot part of its

no change in the seasons at all. The sun would always be overhead at the equator, and the day and night would each be exactly twelve hours long throughout the year everywhere. It is this inclination which causes the difference in the seasons and the unequal length of the day in summer and winter. There is, according to Laplace, a possible variation of nearly three degrees (from 22° 6' to 24° 50') in this inclin

ears. Any one who watches a spinning top as it "sleeps," will see its axis making a slow circular movement, exactly after the fashion of this circling movement of the e

ot at aphelion and perihelion, but at the half-way points between them. When the summer of one hemisphere happens at perihelion and the winter at aphelion, it will be clear that the summer of the other hemisphere will happen at aphelion and its winter at perihelion. One hemisphere will have a short hot summer and a very cold wint

lar, the equator is at its least inclination from the plane of the earth's orbit, and the spring and autumn are at perihelion and aphelion, then all these causes will be conspiring to make climate warm and uniform; there will be least difference of summer and winter. When, on the other hand, the orbit is in its most eccentric stage of deformation, when also the equator is most tilted up and when further the summer and winter are at aphelion and perih

e of the World Change Slowly but Continuously. It does not change in regular periods. It fl

ee systems of influence come together with a common tendency towar

n the separate rhythms of these three systems kept them out of agreement and the atmosphere was temperate, ages of world-wide warmth, and other

s when there was a great weeding out and disappearance of species, genera, and classes, and the learning of stern lessons by all that survived. Such a propitious conjunction it

treme conditions. Half a million years ahead it may be a winterless world with trees and vegetation even in the polar circles. At present we have no certainty in such a forecast,

in the past there have been periods of greater and lesser intensity. About that we know nothing; human experience has been too short; and so far we have been able to find no evidence on this matter in the geological record. On the whole, sc

d 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 t

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 lowne

ulation of ice caps upon the polar regions and upon the more elevated mountain masses. This accumulation wo

s 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 chai

ext 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 fores

and convert great areas into marshes, and so lead to the destruction

ts 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 ear