The high lands of tropical Africa, above the altitude of three thousand feet, and situated in places of great precipitation, were probably covered with snow and ice during the glacial age. Travellers have reported that islands composed partly of granite bowlders are found in the lakes at the head-waters of the Nile. But the glaciers that invaded the tropical latitudes were of short duration compared with the ice-sheets that burdened the lands of the temperate zones. Besides, such tropical ice as flowed to the low lands was so near a melting condition that it made small impression on the rocks; but on steep mountain slopes, where the movement of the ice was comparatively rapid, it possessed considerable eroding power. The climate of the tropical zone on both continents during the perfection of an ice period was so cold that such animals as could not endure a low temperature retreated into the warmest regions of the equatorial latitudes, while many species who failed to reach such places perished. And especially was this the case with the pre-glacial fauna of the western continent. Mr. W. B. M. Davidson, in his treatise on Florida phosphates, says: “The great mammal hordes of the glacial epoch were driven into Florida in their flight southward for life and warmth, and there perished because of the deadly cold which ever moved southward. The Florida waters grew so icy cold, fishes, reptiles, and mammoth animals died, and added their frames and teeth to the valley of bones now found in that southern region.”
Such species of the tropical fauna of the ocean as survived the ice age could have existed only in torrid seas with small connection with the cold oceans during the frigid epochs. For, with the diminished oceans of a cold period, it seems that the conditions were favorable for the maintenance of such seas in the region of the East India Islands.
Such parts of Southern Europe and Northern Africa as bordered on the Mediterranean Sea probably possessed a milder climate during the ice age than regions in the same latitudes on the Atlantic coast, for the reason that the North Atlantic was proportionally a greater receptacle for icebergs which were launched into it from the numerous glaciers of North-eastern America, Greenland, Iceland, and North-western Europe than the great inland sea obtained from its less frigid shores. And it may have happened that during such times the tropical waters of the Indian Ocean had some connection with the Mediterranean through the Red Sea and Suez, and so during portions of the year the waters of the tropical Indian Ocean were forced by the periodical winds into the inland sea. It is the opinion of several writers that man, along with other species of animal life, existed previous to the glacial period; for, since the seas and lands of the globe were chilled, the conditions seem to have been less favorable for the spontaneous generation of animate bodies than during the previous warm ages. Therefore, it appears that the generative ages should be ascribed to the long genial eras prior to the glacial epochs. For it is probable that the lower parts of the ocean, which now possess a low temperature even in the tropical latitudes, were, during the warm eras, wholly composed of warm water, because the surface waters of the antarctic seas of that age, which supply the great under-currents of the ocean, would possess a high temperature; and it is probable that the temperature of a large portion of the seas of the torrid zone was for a long time maintained at blood heat. For it should be considered that the waters which moved from the torrid seas, after making their journey through the warm regions of the high latitudes, would on their return to the tropics retain a large portion of the heat they acquired in the torrid zone before making their journey to the mild polar regions.
And, when we reflect how the heat of the sun’s rays was conserved by the ocean waters, and that their circulation during such times was almost wholly performed by the winds, as the difference of temperature between the polar latitudes and the equator was small, it appears that during the eras previous to the glacial age the oceans must have obtained a higher temperature than possessed by the warmest seas of to-day.
According to the discoveries of Professor Wright and others, ancient stone implements have been found beneath the glacial drift, as well as the bones of animals whose descendants are now living, which goes to prove that man, with other species of fauna which now inhabit the earth, existed anterior to the glacial epoch.
And on consideration it seems unreasonable to suppose that any of the superior species of animals could have been brought into existence since the waters and lands of the earth were chilled by the cold of a glacial age. And it appears that many species of animals which are known to have survived the cold periods were indebted for such survivals to the slow process through which a frigid period is brought about, thus affording time for evolutionary inurement to the slow increase of cold which at length perfects a glacial epoch.
The inurement to cold acquired by animals during the glacial age is still an attribute possessed by many species of fauna to-day. For, when a warm climate took possession of the tropical zone, it was deserted by a large portion of the animals that found refuge there during the glacial age.
Thus, while the seas and shores of the cooler latitudes swarm with animate bodies, the torrid latitudes seem comparatively lonely to the voyagers on the tropical oceans.