Page:The Outline of History Vol 1.djvu/58

 towards warmth or severity, or as they contradict and cancel each other.

We can trace in the Record of the Rocks an irregular series of changes due to the interplay of these influences; there have been great ages when the separate rhythms of these three systems kept them out of agreement and the atmosphere was temperate, ages of world-wide warmth, and other ages when they seemed to concentrate bitterly to their utmost extremity, to freeze out and inflict the utmost stresses and hardship upon life.

And in accordance we find from the record in the rocks that there have been long periods of expansion and multiplication when life flowed and abounded and varied, and harsh ages 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 must have been that gave the age of luxuriant low-grade growth of the coal-measures; such an adverse series of circumstances that chilled the closing æons of the Palæozoic time.

It is probable that the warm spells have been long relatively to the cold ages. Our world to-day seems to be emerging with fluctuations from a prolonged phase of adversity and extreme 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, but later on, as knowledge increases, it may be possible to reckon with more precision, so that our race will make its plans thousands of years ahead to meet the coming changes.

§ 2

Another entirely different cause of changes in the general climate of the earth may be due to variations in the heat of the sun. We do not yet understand what causes the heat of the sun or what sustains that undying fire. It is possible that 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, scientific men