In other words, the adaptation of blind animals to the cave is only apparent; they were adapted to cave life before they entered the cave. Many animals are obviously burdened with a germinal abnormality giving rise to imperfection and smallness of the eye--the hereditary factor involved may have to do with the development of the blood-vessels and lymphatics of the eye. Such mutants can survive more easily in the cave, where they do not have to meet the competition of seeing forms, than in the open. In man also an hereditary form of blindness is known, the so-called hereditary glaucoma. It has nothing to do with light, but the disease seems to be due to an hereditary anomaly of the circulation in the eye. Kammerer[278] has recently reported that by keeping the blind European cave salamander _Proteus anguinus_ under certain conditions of illumination he succeeded in producing two specimens with larger eyes. According to him the eyes of _Proteus_ may develop to a certain point and then retrogress again. He states that by keeping young salamanders alternately for a week or two in sunlight and in a dark room where they were exposed to red incandescent light, two males formed somewhat larger eyes. The first year no alteration was visible. In the second year a slight increase in the size of the eyes was noticeable under the skin. In the third year the eye protruded slightly and this increased somewhat in the fourth year. [278] Kammerer, P., _Arch. f. Entwcklngsmech._, 1912, xxxiii., 349. There is thus far only one case on record in animal biology in which the light influences the formation of organs. The writer found that the regeneration of the polyps of the hydroid _Eudendrium_ does not take place if the animals are kept in the dark, while the polyps will regenerate if exposed to the light;[279] and the time of exposure may be rather short according to Goldfarb.[280] It is possible that _Proteus_ resembles in this respect _Eudendrium_; it should be stated, however, that of many different forms tried by the writer over a number of years, _Eudendrium_ was the only one which gave evidence of such an influence of light. Of course it is not impossible that the light might influence reflexly the development of blood-vessels in the eyes of certain animals, _e. g._, _Proteus_, and thus allow the eyes of _Proteus_ to grow a little larger. [279] Loeb, J., _Arch. d. f. ges. Physiol._, 1896, lxiii., 273. [280] Goldfarb, A. J., _Jour. Exper. Zoöl._, 1906, iii., 129; 1910, viii., 133. We therefore come to the conclusion that it is not the cave that made animals blind but that animals with a hereditary tendency towards a degeneration of the eyes can survive in a cave while they can only exceptionally survive in the open. The cause of the degeneration is a disturbance in the circulation and nutrition of the eye, which is as a rule independent of the presence or absence of light.