In discussions of climate, as of most subjects, a peculiar psychological phenomenon is observable. Everyone sees the necessity of explaining conditions different from those that now exist, but few realize that present conditions may be abnormal, and that they need explanation just as much as do others. Because of this tendency glaciation has been discussed with the greatest fullness, while there has been much neglect not only of the periods when the climate of the earth resembled that of the present, but also of the vastly longer periods when it was even milder than now. How important the periods of mild climate have been in geological times may be judged from the relative length of glacial compared with inter-glacial epochs, and still more from the far greater relative length of the mild parts of periods and eras when compared with the severe parts. Recent estimates by R. T. Chamberlin[58] indicate that according to the consensus of opinion among geologists the average inter-glacial epoch during the Pleistocene was about five times as long as the average glacial epoch, while the whole of a given glacial epoch averaged five times as long as the period when the ice was at a maximum. Climatic periods far milder, longer, and more monotonous than any inter-glacial epoch appear repeatedly during the course of geological history. Our task in this chapter is to explain them. Knowlton[59] has done geology a great service by collecting the evidence as to the mild type of climate which has again and again prevailed in the past. He lays special stress on botanical evidence since that pertains to the variable atmosphere of the lands, and hence furnishes a better guide than does the evidence of animals that lived in the relatively unchanging water of the oceans. The nature of the evidence has already been indicated in various parts of this book. It includes palms, tree ferns, and a host of other plants which once grew in regions which are now much too cold to support them. With this must be placed the abundant reef-building corals and other warmth-loving marine creatures in latitudes now much too cold for them. Of a piece with this are the conditions of inter-glacial epochs in Europe, for example, when elephants and hippopotamuses, as well as many species of plants from low latitudes, were abundant. These conditions indicate not only that the climate was warmer than now, but that the contrast from season to season was much less. Indeed, Knowlton goes so far as to say that "relative uniformity, mildness, and comparative equability of climate, accompanied by high humidity, have prevailed over the greater part of the earth, extending to, or into, polar circles, during the greater part of geologic time--since, at least, the Middle Paleozoic. This is the regular, the ordinary, the normal condition." ... "By many it is thought that one of the strongest arguments against a gradually cooling globe and a humid, non-zonally disposed climate in the ages before the