A much more elaborate theory was advanced by Dr. Croll, and is still entertained by many. The path of the earth round the sun is not circular, but elliptical, and there are times when the gravitational pull of the other planets increases the eccentricity of the orbit. It was assumed that there are periods of great length, separated from each other by still longer periods, when this eccentricity of the orbit is greatly exaggerated. The effect would be to prolong the winter and shorten the summer of each hemisphere in turn. The total amount of heat received would not alter, but there would be a long winter with less heat per hour, and a short summer with more heat. The short summer would not suffice to melt the enormous winter accumulations of ice and snow, and an ice-age would result. To this theory, again, it is objected that we do not find the regular succession of ice-ages in the story of the earth which the theory demands, and that there is no evidence of an alternation of the ice between the northern and southern hemispheres. More recent writers have appealed to the sun itself, and supposed that some prolonged veiling of its photosphere greatly reduced the amount of heat emitted by it. More recently still it has been suggested that an accumulation of cosmic or meteoric dust in our atmosphere, or between us and the sun, had, for a prolonged period, the effect of a colossal "fire-screen." Neither of these suppositions would explain the localisation of the ice. In any case we need not have recourse to purely speculative accidents in the world beyond until it is clear that there were no changes in the earth itself which afford some explanation. This is by no means clear. Some writers appeal to changes in the ocean currents. It is certain that a change in the course of the cold and warm currents of the ocean to-day might cause very extensive changes of climate, but there seems to be some confusion of ideas in suggesting that this might have had an equal, or even greater, influence in former times. Our ocean currents differ so much in temperature because the earth is now divided into very pronounced zones of climate. These zones did not exist before the Pliocene period, and it is not at all clear that any redistribution of currents in earlier times could have had such remarkable consequences. The same difficulty applies to wind-currents. On the other hand, we have already, in discussing the Permian glaciation, discovered two agencies which are very effective in lowering the temperature of the earth. One is the rise of the land; the other is the thinning of the atmosphere. These are closely related agencies, and we found them acting in conjunction to bring about the Permian Ice-Age. Do we find them at work in the Pleistocene?