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. CHAPTER IV. THE GLACIERS OF THE TEMPERATE ZONES. Having asserted that during the culmination of a frigid period the ice-sheets spread over a portion of the lands of the tropical zone, I will give my views, with those of several writers, on the spread of ice-sheets within the now temperate latitudes; and meanwhile I will repeat a portion of my former essays on the subject. Professor Hitchcock, in his lectures on the early history of North America, says that “the history opens with igneous agency in the ascendant, aqueous and organic forces become conspicuous later on, and ice has put on the finishing touches to the terrestrial contours.” But there appear to be various opinions held by geologists respecting the changes brought about on the earth’s surface during the glacial period. Some think that glaciers have never been an important geological agent, while others assert that during the glacial epoch heavy ice-sheets covered the elevated portions of Western North America as far south as the thirty-sixth parallel of latitude, and Eastern North America was overspread with ice-sheets, which attained a depth of five or six thousand feet, and were able to move their débris over wide lands of little declivity toward the sea, their immense deposits forming the lands of Cape Cod, and also the islands of Nantucket and Martha’s Vineyard.