THE sensory organization with which an animal is equipped by nature permits it, within the framework set, to pursue its individual life and to maintain its species. If, with von Uexküll, we consider the life plans of animals as biologically primitive phenomena, behind which we cannot go, then the sensory organizations of the various species of animals seem equally successful and equally meaningful. But, of course, this conception does not forbid us to compare one with another the performances of the sense organs of various animals. Our first comparative point of view is that of differential sensitivity to external stimulation.
The question about differential sensitivity is twofold. It is directed either to the "minimum perceptibile," i.e. the weakest stimulus which can still produce a reaction (absolute threshold) or to the difference between two stimuli which can just be distinguished by a sense organ (difference threshold). In human sensory psychology these investigations, which date from the earliest years of experimental psychology, have found their general expression in the formulation of the Weber-Fechner Law. As is known, this law states that--apart from extremely high and low values of stimuli--the intensities of sensation do not increase in arithmetical proportion to the intensities of the stimuli, but in a much slower ratio, the logarithmic. The law has been variously interpreted. The physiological interpretation makes the logarithmic decrease of the stimulus effects