"Light cannot penetrate everywhere, and as a consequence, animals which live habitually in places which it cannot reach, do not have an opportunity of using eyes, even though they have got them; but animals which form part of a system of organization which comprises eyes as an invariable rule among its organs, must have had eyes originally. Since then we find among these animals some which have lost their eyes, and which have only concealed traces of these organs, it is evident that the impoverishment, and even disappearance of the organs in question, must be the effect of long-continued disuse. "A proof of this is to be found in the fact that the organ of hearing is never in like case with that of sight; we always find it in animals of whose system of organization hearing is a component part; and for the following reason, namely, that sound, which is the effect of vibration upon the ear, can penetrate everywhere, and pass even through massive intermediate bodies. Any animal, therefore, with an organic system of which the ear is an essential part, can always find a use for its ears, no matter where it inhabits. We never, therefore, come upon rudimentary ears among the vertebrata, and when, going down the scale of life lower than the vertebrata, we come to a point at which the ear is no longer to be found; we never come upon ears again in any lower class. "Not so with the organ of sight: we see this organ disappear, reappear, and disappear again with the possibility or impossibility of using eyes on the part of the creature itself.[374] "The great development of mantle in the acephalous molluscs has rendered eyes, and even a head, entirely useless to them. These organs, though belonging to the type of the organism, and by rights included in it, have had to disappear and become annihilated owing to continued default of use. . . . . . . "Many insects which, by the analogy of their order and even genus, should have wings, have nevertheless lost them more or less completely through disuse. A number of coleoptera, orthoptera, hymenoptera, and hemiptera give us examples, the habits of these animals never leading them to use their wings."[375] * * * * *