A curious reflection is prompted in connection with this division of the earth's story into periods of relative prosperity and quiescence, separated by periods of disturbance. There was--on the most modest estimate--a stretch of some fifteen million years between the Cambrian and the Permian upheavals. On the same chronological scale the interval between the Permian and Cretaceous revolutions was only about seven million years, and the Tertiary Era will comprise only about three million years. One wonders if the Fourth (Quaternary) Era in which we live will be similarly shortened. Further, whereas the earth returned after each of the earlier upheavals to what seems to have been its primitive condition of equable and warm climate, it has now entirely departed from that condition, and exhibits very different zones of climate and a succession of seasons in the year. One wonders what the climate of the earth will become long before the expiration of those ten million years which are usually assigned as the minimum period during which the globe will remain habitable. It is premature to glance at the future, when we are still some millions of years from the present, but it will be useful to look more closely at the facts which inspire this reflection. From what we have seen, and shall further see, it is clear that, in spite of all the recent controversy about climate among our geologists, there has undeniably been a progressive refrigeration of the globe. Every geologist, indeed, admits "oscillations of climate," as Professor Chamberlin puts it. But amidst all these oscillations we trace a steady lowering of the temperature. Unless we put a strained and somewhat arbitrary interpretation on the facts of the geological record, earlier ages knew nothing of our division of the year into pronounced seasons and of the globe into very different climatic zones. It might plausibly be suggested that we are still living in the last days of the Ice-Age, and that the earth may be slowly returning to a warmer condition. Shackleton, it might be observed, found that there has been a considerable shrinkage of the south polar ice within the period of exploration. But we shall find that a difference of climate, as compared with earlier ages, was already evident in the middle of the Tertiary Era, and it is far more noticeable to-day.