To account for these extraordinary changes of climate has generally been regarded as the most difficult and perplexing problem which has fallen to the lot of the geologist. Some have attempted to explain them by assuming a displacement of the earth’s axis of rotation in consequence of the uprising of large mountain masses on some part of the earth’s surface. But it has been shown by Professor Airy,[3] Sir William Thomson,[4] and others, that the earth’s equatorial protuberance is such that no geological change on its surface could ever possibly alter the position of the axis of rotation to an extent which could at all sensibly affect climate. Others, again, have tried to explain the change of climate by supposing, with Poisson, that the earth during its past geological history may have passed through hotter and colder parts of space. This is not a very satisfactory hypothesis. There is no doubt a difference in the quantity of force in the form of heat passing through different parts of space; but space itself is not a substance which can possibly be either cold or hot. If, therefore, we were to adopt this hypothesis, we must assume that the earth during the hot periods must have been in the vicinity of some other great source of heat and light besides the sun. But the proximity of a mass of such magnitude as would be sufficient to affect to any great extent the earth’s climate would, by its gravity, seriously disarrange the mechanism of our solar system. Consequently, if our solar system had ever, during any former period of its history, really come into the vicinity of such a mass, the orbits of the planets ought at the present day to afford some evidence of it. But again, in order to account for a cold period, such as the glacial epoch, we have to assume that the earth must have come into the vicinity of a cold body.[5] But recent discoveries in regard to inter-glacial periods are wholly irreconcilable with this theory. A change in the obliquity of the ecliptic has frequently been, and still is, appealed to as an explanation of geological climate. This theory appears, however, to be beset by a twofold objection: (1), it can be shown from celestial mechanics, that the variations in the obliquity of the ecliptic must always have been so small that they could not materially affect the climatic condition of the globe; and (2), even admitting that the obliquity could change to an indefinite extent, it can be shown[6] that no increase or decrease, however great, could possibly account for either the glacial epoch or a warm temperate condition of climate in polar regions. The theory that the sun is a variable star, and that the glacial epochs of the geologists may correspond to periods of decrease in the sun’s heat, has lately been advanced. This theory is also open to two objections: (1), a general diminution of heat[7] never could produce a glacial epoch; and (2), even if it could, it would not explain inter-glacial periods.