But let us see into what an inextricable labyrinth of difficulty we are led by this theory of plant-migration from the equatorial to the sub-arctic zone, and _vice-versa,_ and even beyond the equator to the sub-antarctic zone, and still _vice versa_. Before proceeding to consider the probable duration of the several geographical epochs, called glacial periods, on which their theory of plant-migration depends, or considering the evidence touching these glacial periods, we will state their position in regard to these possible migrations as briefly and concisely as we know how. Mr. Darwin's solution of this problem is the generally accepted one of the evolutionists, as well as most of the present scientific world. As the truth, or rather the falsity, of his pet theory of evolution depended on the satisfactory solution of this vexed problem, it became necessary for him to give his best and entire mental energies to the gigantic task which was, by universal consent, assigned him. The reader shall see how admirably the thermal equator is crossed by Mr. Darwin, with his vast swarms of flies, mosquitoes, insectivorous and other plants, forest trees, anthropoid apes, and general menagerie of wild animals, such as would gladden the heart of the "great American showman" beyond the most extravagant comparison. The question, bear in mind, which he was specially called upon to solve, was how the temperate forms north--those, for instance, of the warm and cold temperate zones--managed to cross the thermal equator, and invade the corresponding zones in the southern hemisphere; just as though there was any more necessity of determining this question than the opposite one, of how the southern forms came to invade the northern hemisphere. We will give his solution of this problem in his own language, that we may not be charged with misrepresentation. He says, in speaking of the glacial periods: "As the cold became more and more intense, we know that arctic forms invaded the temperate regions; and, from the facts just given, there can hardly be a doubt that some of the more vigorous, dominant, and widest-spread temperate forms invaded the equatorial lowlands. The inhabitants (flora and fauna) of these hot lowlands would at the same time have migrated to the tropical and sub-tropical regions of the south; for the southern hemisphere was at this period warmer. On the decline of the glacial period, as both hemispheres gradually recovered their former temperatures, the northern forms living on the lowlands under the equator would have been driven to their former homes or have been destroyed, being replaced by the equatorial forms returning from the south. Some, however, of the northern temperate forms would almost certainly have ascended any adjoining highland, where, if sufficiently lofty, they would have long survived, like the arctic forms on the mountains of Europe.