The low temperatures of the Canadian Northwest for the same reason have not repelled settlers even from the Southern States.  Negroes, however, meet a climatic barrier in America at the isotherm of 5 deg.  Centigrade (41 deg.  F.).  They are found in New England and Nova Scotia, generally with a large admixture of white blood; but there and farther north where the climate is moist as well as cold, they show a fatal tendency to pulmonary diseases.

[Sidenote:  The small amount of tropical emigration]

The acclimatization of tropical people in temperate regions will never be a question of widespread importance.  The negroes were involuntary immigrants to America, under conditions that can never recur.  Their concentration in the “black belt,” where they find the heat and moisture in which they thrive, and their climatically conditioned exclusion from the more northern states are matters of local significance.  Economic and social retardation have kept the hot belt relatively underpopulated.  The density map shows much the largest part of it with a population less than 25 to the square mile.  Only the small portion contained in India, southernmost China, and Java shows a density over 125 to the square mile (or 50 to the square kilometer).  This density has to rise to 500 or more to the square mile before emigration begins.  The would-be exiles then have a wide choice of new homes in other tropical lands, where they find congenial climate and phases of economic development into which they will fit.  East Indian coolies are found in Cape Colony, Natal, Zanzibar, Trinidad, and British Guiana, where they constitute 38 per cent. of the population.

[Sidenote:  Effects of tropical climate.]

The redundant population of crowded western and southern Europe also seek these sparsely inhabited Tropics, but they come heavily handicapped by the necessity of acclimatization.  They leave their homes from Trondhjem and Stockholm in the north to the Mediterranean in the south, where the mean annual temperatures vary from 5 deg. to 17 deg.  C, (41 deg. to 63 deg.  F.), to seek the Torrid Zone which averages 25 deg.  C. or 77 deg.  F. over most of its territory.  The effects of a tropical climate are due to intense heat, to its long duration without the respite conferred by a bracing winter season, and its combination with the high degree of humidity prevailing over most of the Torrid Zone.  These are conditions advantageous to plant life, but hardly favorable to human development.  They produce certain derangements in the physiological functions of heart, liver, kidneys and organs of reproduction.  Bodily temperature rises, while susceptibility to disease and rate of mortality show an increase ominous for white colonization.  The general effect is intense enervation; this starts a craving for stimulants and induces habits of alcoholism which are accountable for many bodily ills usually attributed to direct climatic influences.  Transfer to the Tropics tends