It has all along been an admitted opinion among geologists that the climatic condition of our globe has not, during past ages, been uniformly the same as at present. For a long time it was supposed that during the Cambrian, Silurian, and other early geological periods, the climate of our globe was much hotter than now, and that ever since it has been gradually becoming cooler. And this high temperature of Palæozoic ages was generally referred to the influence of the earth’s internal heat. It has, however, been proved by Sir William Thomson[2] that the general climate of our globe could not have been sensibly affected by internal heat at any time more than ten thousand years after the commencement of the solidification of the surface. This physicist has proved that the present influence of internal heat on the temperature amounts to about only 1/75th of a degree. Not only is the theory of internal heat now generally abandoned, but it is admitted that we have no good geological evidence that climate was much hotter during Palæozoic ages than now; and much less, that it has been becoming _uniformly_ colder.
The great discovery of the glacial epoch, and more lately that of a mild and temperate condition of climate extending during the Miocene and other periods to North Greenland, have introduced a complete revolution of ideas in reference to geological climate. Those discoveries showed that our globe has not only undergone changes of climate, but changes of the most extraordinary character. They showed that at one time not only an arctic condition of climate prevailed in our island, but that the greater part of the temperate region down to comparatively low latitudes was buried under ice, while at other periods Greenland and the Arctic regions, probably up to the North Pole, were not only free from ice, but were covered with a rich and luxuriant vegetation.
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.