melted away in the existing northern or southern summer; fresh ice accumulates on top of the old mass with each winter; prevailing winds, blowing over this ice, chill regions lying much farther toward the tropics; icebergs detach themselves and float off, thus lowering the temperature of the sea in the middle zones; arctic or antarctic currents spread round the coasts and absorb the solar heat in enormous quantities. We have only to remember the trenchant difference in England between a parching cold east wind and a mild sou'wester to realize what an immense part these polar ice caps and frozen highlands play in the production of our existing winter. Alps, Pyrenees, Himalayas, Rocky Mountains, further assist in the same direction.
On the other hand, currents in the sea may cut either way; the Gulf Stream makes England warm, while the arctic current makes Labrador, much farther south, practically uninhabitable.
Ever since the Glacial epoch, therefore, it has been quite easy for man in the temperate and frigid zones to recognize the year as a natural reality. The annual cycles of heat and cold are far too marked to be overlooked by anybody. Organically, they made themselves felt at once by extraordinary changes induced in the fauna and flora. Before the steady advance of the annual cold wave, vegetation had perforce to alter its ways. The large-leaved evergreens went out altogether in frigid and high temperate regions; deciduous trees, or needle-leaved types like the pines and firs, took the place of the luxuriant Miocene foliage in Europe and North America. Every autumn the larger number of trees and shrubs learned to shed their leaves all togeher; every spring they came out anew in fresh green and in masses of blossom. Similarly with animals. Birds learned to migrate, or to accommodate themselves to the winter; insects learned to hibernate in the egg or the cocoon; pigs fattened themselves on mast against the frozen time; moles slept over winter; squirrels hoarded nuts for a store to bridge over heavy frosts; frogs retired to the warmer mud in the depths of ponds; adders coiled themselves in holes and dozed away the cold season. Innumerable adaptations sprang up at once, those species or individuals which failed to meet the new conditions perishing in the struggle. In proportion as we recede from the tropics, the more marked do the annual cycles of life thus induced become, many species practically ceasing to exist as such for several months of the year, and being only potentially represented by eggs, germs, or seeds, and sometimes by dormant pregnant females.
At the same time, while the cause of the seasons as a whole is the obliquity of the earth's axis, with the resulting inclination of either pole toward the sun alternately, we must not forget that the
