It is further remarkable that while there is geological evidence of climates colder than the present in the temperate regions, there is equally good proof of warmer climates even within the arctic circle than those of the cold temperate regions at present. It is difficult to account for these vicissitudes of climate, and much controversy exists on the subject; but it seems certain that in the earlier Tertiary and Cretaceous periods, for example, the supplies of heat and light were so diffused over the earth as to permit the growth of a temperate vegetation in Greenland, and even in Spitzbergen. Geologists, however unwillingly, have been obliged to admit this as one of those great possibilities, altogether unexpected beforehand, which have been developed in the history of our planet. Various modes of explaining this succession of cold and warm periods have been adopted, all more or less hypothetical. Lyell has argued that it may be explained by a different distribution of land and water and of the ocean currents. Croll accounts for it by the varying eccentricity of the earth's orbit, in connection with the precession of the equinoxes. Evans by a shifting of the axis of rotation of the earth. Drayson, Bell, Warring, and others, by a change in the inclination of the earth's axis. Others by the secular diminution of the internal heat of the earth, and of that of the sun. Others by the supposed recurrence of periods in which the sun gives more or less heat, or in which the earth is passing through colder or warmer regions of space. As the subject is of interest with reference to possible correspondences of these great summers and winters of the earth with the stages of the creative work, it may be well to notice shortly the relative merits of these theories. (1.) The hypothesis of Croll is one of the most ingenious and elaborate of the whole; but it has two great defects. One is that the causes alleged are so uncertain and so complicated that it is difficult to estimate their real value. Another is that it proves too much, namely, a regular succession of cold and warm periods throughout geological time, of which we have no good evidence, and which is on many grounds improbable. (2.) That the earth's axis of rotation has continued unchanged throughout the whole of the geological ages seems proved by the fact that the principal lines of crumpling and upheaval from the Laurentian period downward are arranged in great circles of the earth tangent to the polar circle; and that the lines of deposit of sediment in the Palaeozoic age are coincident with the present direction of the arctic currents.