vided with a complicated acoustic apparatus, in the same way that the eye is provided with a lens, which renders into terms of noise and music those air-waves which to the rest of the skin are imperceptible.
But a sound conveys no more information in itself than does the field of colors presented by the eye; only when we can tell from what it comes, and what consequences have been connected with it in our past experience, does it have any practical meaning to us. And, again, this explanation can only be furnished by our sense of touch, or by our sense of sight, which, as we have seen, is entirely dependent upon our sense of touch.
The senses of smell and taste should properly be resolved into one sense, for they are probably only late modifications of the same property of the mucous membrane lining the mouth and nose. This membrane is only an invading growth of the skin surrounding the mouth, so, morphologically, this sense is the same as the two just examined. The sense of smell is undoubtedly present in some insects, as, for instance, in the burying beetles, and may perhaps be found lower.
In man this double sense is undoubtedly retrogressive, and probably reaches its highest development in some of the lower mammalia. With us it is at best only a source of transitory pleasure, and seems in no way to contribute to our higher mental development.
But the senses of sight and hearing are very different in this respect. If Darwin is right, they have played a most important part in the evolution of the past as the instruments of sexual selection. And, in the future development of our race, it seems as if their perfection would be reached only with the perfection of the human mind. For if the impulse to development is given from without by the environment, these organs must be continually improved so as to perceive the nicer and nicer distinctions in the environment which will be the means of elevating the mind. If the impulse to development is given from within the mind, these sense-organs must be developed more highly in order to provide the enlarging mind with the continually nicer perception which it will require.
Man's mind develops, not his body. With the exception of these two sense-organs, his body has been nearly stationary for thousands of years, but these two organs respond to comparatively little change. The ear of the savage differs from the ear of the civilized man more than the two men differ in any other respect.
Touch, smell, and taste seem as complete as they need be for any conceivable human being, but that the eye is yet incomplete is very strikingly shown by the so-called actinic rays of the solar spectrum. In this spectrum there are rays beyond the violet which have an action on certain chemical substances much like the action of the blue and violet rays. But to the human eye these rays are absolutely invisible. The perception of this unknown color seems but a short step in the development of the eye.