Nothing is clearer than that great fluctuations of climate have occurred in the past. Many theories have been put forth in explanation. It has been suggested that it was caused by loss of heat from the earth itself. That the earth was once a ball of incandescent matter, like the sun, and has since cooled down, is of course admitted. More than that, this process still continues; and the time must come when the earth, having yielded up its internal heat, will cease to be an inhabitable globe. But the climate of the surface of the earth is not dependent upon the heat of the interior. This now depends “according to the proportion of heat received either directly or indirectly from the sun; and so it must have been during all the ages of which any records have come down to us.”12 Some have supposed that the sun, traveling as it does through space, carrying the earth and the other planets with him, might, in the course of ages, pass through portions of space either warmer or colder than that in which it now moves. When we come to a warm region of space, a genial climate would prevail over the earth; but, when we struck a cold belt, eternal Winter would mantle a large part of the globe with snow and ice. This, of course, is simply guess-work. No less than seven distinct causes have been urged; most of them either purely conjectural, like the last, or manifestly incompetent to produce the great results which we have seen must be accounted for. But, amongst these, two causes have been advanced—the one astronomical, the other geographical; and, to the one or the other, the majority of scholars have given their consent. It will be no harm to see what can be said in favor of both theories. So, we will ask the reader’s attention, as it is our earnest desire to make as plain as possible a question that has so much to do with our present inquiry. In the course of our investigations, we can not fail to catch glimpses of wonderful changes in far away times; and can not help seeing what labor is involved in the solution of all questions relating to the same.13 Earth’s Orbit. The earth revolves around the sun in an orbit called an ellipse. This is not a fixed form, but slowly varies from year to year. It is now gradually becoming circular. It will, however, not become an exact circle. Astronomers assure us that, after a long lapse of time, it will commence to elongate as an ellipse again. Thus, it will continually change from an ellipse to an approximate circle, and back again. In scientific language, the eccentricity of, the earth’s orbit is said to increase and decrease.