to relax the mental and moral fiber, induces indolence, self-indulgences and various excesses which lower the physical tone.[1435] The social control of public opinion in the new environment is weak, while temptation, due to both climatic and social causes, is peculiarly strong.  The presence of an inferior, more or less servile native population, relaxes both conscience and physical energy just when both need a tonic.  The result is general enervation, deterioration both as economic and political agents.

[Sidenote:  Historical significance of deterioration]

This is the effect of climate which has had the most far-reaching and persistent historical consequences.  Our study of the historical movements of peoples in the northern hemisphere revealed a steady influx from colder into tropical and sub-tropical lands, followed always by enervation and loss of national efficiency, due partly to the debilitating heat of the new habitat, partly to its easier conditions of living, whether the intruders came as conquerors and appropriated the fat of the land, or as immigrant colonists who dropped into slack methods of agriculture, because rain and sun and soil made their reluctant labor scarcely necessary.  Everywhere in the Tropics the enervating effects of heat, moisture, and abundance make not only the natives averse to steady work, but start the energetic European immigrant down the same easy descent to Avernus.  Passing over the deterioration of the Aryans in India, the Persians in Mesopotamia, and the Vandals in Africa, we find that modern instances show the transformation to be very rapid.  The French who since 1715 have occupied the islands of Reunion and Mauritius have lost much of their thrift and energy, though their new homes lie just within the southern tropic, and are blessed with an oceanic climate.  Yet the volunteer troops sent by Reunion to aid in the recent subjugation of the Hovas in Madagascar proved to be utterly useless.[1436] The Spaniards who come to-day to Mexico have great energy, born of their former hard conditions of life in Spain.  But their children are reared in a country whose mean annual temperature, even on the plateau, exceeds that of Spain by 10 deg.C. (or 18 deg.F.), a difference equal to that between Mobile and New York, or Madrid and Christiania.  Hence they are less energetic and vigorous, while the third generation are typical Mexicans in their easy-going way of life.[1437] The Germans who recently have colonized southern Brazil in great numbers show a similar deterioration under similar increase of mean annual temperature, combined with somewhat greater humidity, which intensifies the debilitating effects of the heat.  An investigation made in 1900 by the International Harvester Company of America revealed the fact that the German farmer in the State of Santa Catharina rarely cultivated over one acre of grain.[1438] Much of the iron in the blood and conscience of the New England missionary stock which went to Hawaii two generations ago has been dissolved out by the warm rain and balmy air of the islands.