The changes thus witnessed on a comparatively small scale, on familiar areas, help us in some degree to estimate the vast physical revolutions that have taken place throughout the northern hemisphere within that recent geological period which succeeded the formation of the pleiocene strata. One of the most remarkable phenomena now recognised as affecting the conditions of life in recent geological epochs is the prolonged existence, throughout the whole northern hemisphere, of a temperature resembling that of the Arctic regions at the present time. After a period more nearly assimilating in climatic character to the tropics, though otherwise under varying conditions, the temperature of the whole northern hemisphere gradually diminished towards the end of the tertiary epoch, until the highlands of Scotland and Wales—then at a much higher elevation,—resembled Greenland at the present time, and an Arctic temperature extended southward to the Pyrenees and the Alps. Glaciers formed under the influence of perpetual frost and snow descended into the valleys and plains over the greater portion of Central Europe and Northern Asia, and an Arctic winter reigned throughout. This condition of things, pertaining to what is known as the glacial period, was unquestionably of long duration. But after some partial variations of temperature, and a consequent advance and retrocession of the glacial influences along what was then the border lines of a north temperate zone, the first period of extreme cold drew to a close. Between the Alps and the mountain ranges of Scotland and Wales, the winter resembled that which even now prevails on the North American continent, in latitudes in which the moose, the wapiti, and the grizzly bear, freely range over the same areas where during a brief summer of intense heat enormous herds of buffalo annually migrate from the south. A similar alternation of seasons within the European glacial period can alone account for the presence, alongside of an Arctic fauna, of animals such as the hippopotamus and the hyæna, known only throughout the historical period as natives of the tropics. The range of temperature of Canadian seasons admits of the Arctic skua-gull, the snow-goose, the Lapland bunting, and the like Arctic visitors, meeting the king-bird, the humming-bird, and other wanderers from the gulf of Mexico.