And this naturally suggests the question whether those sense-organs in the invertebrates which contain otoliths may not be regarded with more probability as organs for the appreciation of changes of motion than as auditory organs. This for some years has been my own belief. I have always felt a difficulty in understanding how the otoliths are set a-dance by auditory vibrations. But their inertia would materially aid in the appreciation of changes of motion. In some forms the otoliths are held in suspension in a gelatinous material. In others--the molluscs, for example--the otolith (which is generally single) is retained in a free position by ciliary action. In aquatic creatures an organ for the appreciation of changes of motion might be of more service than an auditory organ. And if one be permitted to speculate, one may surmise that the sense of hearing may be a refinement of the sense through which changes of motion are appreciated. First would come a sense of movements of the organism in the medium through the stimulation of the sense-hairs by the relative motion of the otolith; then these sense-hairs, with increased delicacy, might appreciate shocks in the medium; and, eventually, those more delicate shocks which we know as auditory waves. In this way we might account for the fact that in the vertebrates the same organ, through different parts of its structure, appreciates both change of motion and auditory vibrations. And thus the organs in the invertebrata which are generally regarded as auditory, and for which has been suggested the function of reacting to changes of motion, may, in truth, subserve both purposes--may be organs in which the differentiation I have hinted at is taking place. * * * * *