"In the regular course of events the southern hemisphere would, in its turn, be subject to a severe glacial period, with the northern hemisphere rendered warmer; and then the southern temperate forms would invade the equatorial lowlands. The northern forms which had before been left on the mountains would now descend and mingle with the southern forms. These latter, when the warmth returned, would return to their former homes, leaving some few species on the mountains, and carrying southward with them some of the northern temperate forms, which had descended from their mountain fastnesses. Thus we should have some few species identically the same in the northern and southern temperate zones, and on the mountains of the intermediate tropical regions." We are sorry to spoil so ingenious a theory as this to account for plant-migration from the temperate zones north to the corresponding zones south. But in spite of all the great names which will frown down upon us in the attempt, we are obliged to demolish this altitudiness structure, even at the risk of its tumbling about our own ears. But first let us lay down a few undeniable propositions, on the strength of which this ingenious and purely speculative theory of Mr. Darwin must rest:-- 1. It is universally conceded by the scientific world that these glacial epochs, however many of them there may have been in the past and however few there may be in the future, depend, for their occurrence, upon the maxima of eccentricity in the earth's orbit about the sun. 2. The actual amount of heat which the earth annually receives from the sun is in no way affected by the eccentricity of its orbit. It is a constant quantity, and only unequally distributed on the earth's surface, being neither increased nor diminished, as our winters occur in aphelion or perihelion. 3. The actual amount of ice-cap accumulated about the two poles of the earth, is also a constant quantity. And to measure the severity of any glacial epoch, we have only to determine the exact amount of ice (not altogether an impossible problem) about the two poles at any given time, and then determine the effect of its entire transference from one pole to the other. 4. It is not probable that the present ice-cap of the south pole extends continuously and permanently much farther north than 80° or 81°. Mt. Erebus, in Victoria Land, lies in about this latitude, and it was only a few years since that the coast line of that island or continent was traversed, by English exploring vessels, from Mt. Erebus to a point some ten or twelve degrees further north. [18]