240 POPULAR SCIENCE MONTHLY. 



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 enor- 

 mous quantities. We have only to remember the trenchant differ- 

 ence 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 them- 

 selves 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 ever- 

 greens went out altogether in frigid and high temperate regions; de- 

 ciduous trees, or needle-leaved types like the pines and firs, took the 

 place of the luxuriant Miocene foliage in Europe and North Amer- 

 ica. 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 win- 

 ter; insects learned to hibernate in the egg or the cocoon; pigs fat- 

 tened 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 indi- 

 viduals 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 



