Among the climatic sequences, glacial fluctuations are perhaps of the most vital import from the standpoint of organic evolution; from the standpoint of human history the same is true of climatic pulsations. Glacial epochs have repeatedly wiped out thousands upon thousands of species and played a part in the origin of entirely new types of plants and animals. This is best seen when the life of the Pennsylvanian is contrasted with that of the Permian. An historic pulsation may wipe out an entire civilization and permit a new one to grow up with a radically different character. Hence it is not strange that the causes of such climatic phenomena have been discussed with extraordinary vigor. In few realms of science has there been a more imposing or more interesting array of theories. In this book we shall consider the more important of these theories. A new solar or cyclonic hypothesis and the hypothesis of changes in the form and altitude of the land will receive the most attention, but the other chief hypotheses are outlined in the next chapter, and are frequently referred to throughout the volume. Between glacial fluctuations and historical pulsations in duration, but probably less severe than either, come orbital precessions. These stand in a group by themselves and are more akin to seasonal alternations than to any other type of climatic sequence. They must have occurred with absolute regularity ever since the earth began to revolve around the sun in its present elliptical orbit. Since the orbit is elliptical and since the sun is in one of the two foci of the ellipse, the earth's distance from the sun varies. At present the earth is nearest the sun in the northern winter. Hence the rigor of winter in the northern hemisphere is mitigated, while that of the southern hemisphere is increased. In about ten thousand years this condition will be reversed, and in another ten thousand the present conditions will return once more. Such climatic precessions, as we may here call them, must have occurred unnumbered times in the past, but they do not appear to have been large enough to leave in the fossils of the rocks any traces that can be distinguished from those of other climatic sequences.