Physiology, in its progress, has passed through the same stages as physics. Living beings were once considered to be beyond the power of external influences, the various physiological functions being carried forward by a feigned immaterial principle, called the vital agent. But when it was discovered that the heart is constructed upon the recognized rules of hydraulics; the eye upon the most refined principles of optics; that the ear was furnished with the means of dealing with the three characteristics of sound--its tympanum for intensity, its cochlea for pitch, and its semicircular canals for quality; and that the air, brought into the great air passages, calling into play atmospheric pressure, was conveyed upon physical principles into the ultimate cells of the lungs, and thence to the blood; when these and very many other like facts were brought into prominence by modern research, it became necessary to admit that animated beings do not constitute the exception once supposed, and that organic operations are the result of physical agencies. 'If thus, in the recesses of the individual economy, these natural agents bear sway, must they not operate in the social economy too? 'Has the great, shadeless desert nothing to do with the habits of the nomade tribes who pitch their tents upon it--the fertile plain no connection with flocks and pastoral life--the mountain fastnesses with the courage that has so often defended them--the sea with habits of adventure? Indeed, do not all our expectations of the stability of social institutions rest upon our belief in the stability of surrounding physical conditions? From the time of Bodin, who nearly three hundred years ago published his work 'De Republica,' these principles have been well recognized: that the laws of nature cannot be subordinated to the will of man, and that government must be adapted to climate. It was these things which led to the conclusion that force is best resorted to for northern nations, reason for the middle, and superstition for the southern.'