Besides its obvious bearing on the long-vexed and still unsettled question of the unity of the human species, and on the closely related one of acclimatization, the present inquiry is of great medical importance. Tropical pathology, whether of native or foreign races, cannot be fairly studied until we thoroughly know its physiology; nor can we recognize and properly estimate disturbed action of organs till we understand their healthy functions. Otherwise natural phenomena may be mistaken for symptoms of sickness. Many so-called tropical diseases are merely exaggerations of the ordinary effects of climate, physiological merged into pathological phenomena; a knowledge of the one is the first step to an accurate acquaintance with and philosophical method of treating or preventing the other. No inconsiderable part of our present knowledge of the vital phenomena induced in the human economy in passing from cold to warm regions, o the reverse, is derived from experiments carried out in artificially made on seldom encountered climates. By hot-air chambers we illustrate the effect of augmented temperature on the respiration, pulse, &c., and by the rarefied atmosphere of mountain-tops show how diminished density acts. Neither of these, however, are fair examples of natural climates. Thus the former, dry and warm, is unlike the tropics, with its triple combination of increased heat, rarefied air, and excessive moisture; as the latter, dry and chilly, is dissimilar from the usual surface-climates of extra-tropical latitudes which conjoin cold, condensation, and moisture. The dry and warm, o dry and cold, climates which occur in nature are usually local and limited. Nor do such abrupt and temporary exposures to heat and cold have any parallel in ordinary life, or are they likely to induce results similar to the comparatively slow transition involved in an ordinary change of climate: and though the rarefied air of heated chambers will decrease, while that of great altitudes will accelerate respiration, the former will do so less, and the latter more, than they otherwise would, from the skin, and especially the more slowly acting liver and kidneys, being unable at once to increase their action so as to aid the lungs in eliminating carbon. The functional changes so induced cannot therefore be taken as a fair criterion of what occurs in nature; and as mere approximates to truth, such observations, though interesting, are evidently wanting in practical importance.
TECHNIQUES IN TROPICAL PATHOLOGY