[Sidenote:  Direct and indirect effects of climate.]

For a clear understanding of climatic influences, it is necessary to adhere to the chief characteristics of the atmosphere, such as heat and cold, moisture and aridity, and to consider the effect of zonal location, winds and relief in the production and distribution of these; also to distinguish between direct and indirect results of climate, temporary and permanent, physiological and psychological ones, because the confusion of these various effects breeds far-fetched conclusions.  The direct modification of man by climate is partly an a priori assumption, because the incontestable evidences of such modification are not very numerous, however strong the probability may be.  The effect of climate upon plant and animal life is obvious, and immediately raises the assumption that man has been similarly influenced.  But there is this difference:  in contrast to the helpless dependence upon environment of stationary plants and animals, whose range of movement is strictly determined by conditions of food and temperature, the great mobility of man, combined with his inventiveness, enables him to flee or seek almost any climatic condition, and to emancipate himself from the full tyranny of climatic control by substituting an indirect economic effect for a direct physical effect.

The direct results of climate are various, though some are open to the charge of imperfect proof.  Even the relation of nigrescence to tropical heat, which seems to be established by the geographical distribution of negroid races in the Old World, fails to find support from the facts of pigmentation among the American Indians from Alaska to Tierra del Fuego.  Nevertheless climate undoubtedly modifies many physiological processes in individuals and peoples,[1411] affects their immunity from certain classes of diseases and their susceptibility to others, influences their temperament, their energy, their capacity for sustained or for merely intermittent effort, and therefore helps determine their efficiency as economic and political agents.

While producing these direct effects, climate also influences man indirectly by controlling the wide range of his life conditions dependent upon the plant and animal life about him.  It dictates what crops he may raise, and has it in its power to affect radically the size of his harvest.  It decides which flocks and herds are best suited to his environment, and therefore directs his pastoral activities, whether he keeps reindeer, camels, llamas, horses or horned cattle.  By interdicting both agriculture and stock-raising, as in Greenland whose ice cap leaves little surface free even for reindeer moss, it condemns the inhabitants forever to the uncertain subsistence of the hunter.  Where it encourages the growth of large forests which harbor abundant game and yield abundant fruits, as in the hot, moist equatorial belt and on rainy mountain slopes, it prolongs the hunter stage of development, retards the advance to agriculture.  Climate thus helps to influence the rate and the limit of cultural development.  It determines in part the local supply of raw material with which man has to work, and hence the majority of his secondary activities, except where these are expended on mineral resources.  It decides the character of his food, clothing, and dwelling, and ultimately of his civilization.