Page:Popular Science Monthly Volume 40.djvu/833

Rh to that of such geographical conditions as the distribution of land and sea, and of mountains and lowlands.

The idea that glaciation was dependent on extreme cold has been rejected by other students. J. de Charpentier recognized the conditions as inconsistent. Lecoq, of Clermont, "affirmed a correlation between a great solar heat, provoking a powerful evaporation, and the formation of glaciers." Tyndall has shown that the ice of the Alps "derives its origin from the heat of the sun," and that if that were diminished their source of supply would be cut off. The thoughts of some other writers, as Le Blanc, Forbes, and Charles Martins, have been turned to showing that the depression of temperature, if there was any, need not have been great.

Another group of writers, whose views have been summarized by M. Millot, of Nancy, hold that warmer climates than now prevail were more favorable to glaciation, and gave character to the Glacial period; and that the present conditions of limited glaciation are the result of the sun's cooling, whereby the supply of evaporated moisture has fallen off. They claim that their theory furnishes the simplest explanation of the presence of warmth-loving plants and animals along with evidences of ice-action. The hot and the glaciated region were so close to one another that the mixture easily took place.

Prof. G. F. Becker, of the United States Geological Survey, has also expressed the opinion (Popular Science Monthly, February, 1884) that the Glacial period was one of higher mean temperature at the sea-level than the present; that while the formation of glaciers may have been affected by all contemporaneous changes, including secular revolutions, it is not necessary to have recourse to such causes; the question is chiefly one of differences between the temperatures at the sea-level and those at the level where the glacier was formed.

M. Blytt, studying the distribution of the Scandinavian fauna, has found it subject to considerable local variations at short distance, which have relation to differences in conditions of exposure and the character of the soil. He concludes that no great changes, but only small variations in the extremes of temperature and rainfall, are required to explain these departures. Such variations may be produced, for his country, by fluctuations in the direction, force, and temperature of ocean currents and winds that need in no case be great; but he believes that these variations are coincident with periodical changes of climate corresponding with secular incidents.

The considerable effects of exposure on local climates are