Not less apparent cogency for the argument for modifications by use have those cases where the modification has been apparently due to disuse. It is well known that living creatures found in the total darkness of great limestone caverns, like those at Kentucky, are blind, through imperfections of one kind or another in the organs of sight. But the rudimentary structures which remain tell us that these creatures had ancestors which were once fully equipped in this respect, and which had wandered into the caverns from the sunlit outer world. Thus the case of a crab has been noted, in which the stalks on which a crab’s eyes are set were preserved, while the eyes had disappeared: it is, as Darwin observes, as if the stand of a telescope had been retained while the telescope itself had gone. Sometimes the eyes of cave-fishes are covered with a horny layer, sometimes the whole structure is atrophied and withered. But never is an animal found under these conditions which has retained its power of sight. The conclusion seems obvious. In individuals, a muscle or other organ is known to strengthen and develop by use and to atrophy by disuse. As use and disuse appear to be invariably accompanied by precisely the same effects in the species as in the individual, and as there seems no way of accounting for this by any known physiological law without assuming that modifications acquired by the individual are transmitted to its progeny, the case for the inheritability of such modifications appears, at first sight, irresistible.[54]