Here are three wavering systems of change all going on independently of each other; the precession of the equinoxes, the change in the obliquity of the equator to the orbit, and the changes in the eccentricity of the orbit. Each system tends by itself to produce periods of equability and periods of greater climatic contrast. And all these systems of change interplay with each other. When it happens that at the same time the orbit is most nearly circular, 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 perihelion, then climates will be at their extremest and winter at its bitterest. There will be great accumulations of ice and snow in winter; the heat of the brief hot summer will be partly reflected back into space by the white snow, and it will be unequal to the task of melting all the winter’s ice before the earth spins away once more towards its chilly aphelion. The earth will accumulate cold so long as this conspiracy of extreme conditions continues. [Illustration: DIAGRAM TO ILLUSTRATE ONE SET OF CAUSES, THE ASTRONOMICAL VARIATIONS, WHICH MAKE THE CLIMATE OF THE WORLD CHANGE SLOWLY BUT CONTINUOUSLY. It does not change in regular periods. It fluctuates through vast ages. As the world’s climate changes, life must change too or perish.] So our earth’s climate changes and wavers perpetually as these three systems of influence come together with a common tendency 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.