is comprehensible why utterly useless tropisms or structures should occur in organisms. [275] Loeb, J., _Der Heliotropismus der Tiere and seine Übereinstimmung mit dem Heliotropismus der Pflanzen_. Würzburg, 1890 (appeared in 1889). 2. A famous case for the apparent adaptation of animals to environment has been the blind cave animals. It is known that in caves blind salamanders, blind fishes, and blind insects are common, while such forms are comparatively rare in the open. This fact has suggested the idea that the darkness of the cave was the cause of the degeneration of the eyes. A closer investigation leads, however, to a different explanation. Eigenmann has shown that of the species of salamanders living habitually in North American caves, two have apparently quite normal eyes. They are _Spelerpes maculicauda_ and _Spelerpes stejnegeri_. Two others living in caves have quite degenerate eyes, _Typhlotriton spelæus_ and _Typhlomolge rathbuni_. If disuse is the direct cause of blindness we must inquire why _Spelerpes_ is not blind. Another difficulty arises from the fact that a blind fish _Typhlogobius_ is found in the open (on the coast of southern California) in shallow water, where it lives under rocks in holes occupied by shrimps. The question must again be raised: How can it happen that in spite of exposure to light _Typhlogobius_ is blind? The most important fact is perhaps the one found by Eigenmann in the fishes of the family of Amblyopsidæ. Six species of this group live permanently in caves, are not found in the open, and have abnormal eyes, while one lives permanently in the open, is never found in caves, and one comes from subterranean springs. The one form which is found only in the open, _Chologaster cornutus_, has a simplified retina as well as a comparatively small eye, in other words, its eye is not normal. This indicates the possibility that the other representatives which are found only in caves also might have abnormal eyes even if they had never lived in caves. Through these facts the old idea becomes questionable, namely, that the cave animals had originally been animals with normal eyes which owing to disuse had undergone a gradual hereditary degeneration. Recent experiments made on the embryos of the fish _Fundulus_ have yielded the result that it is possible to produce blindness in fish by various means other than lack of light.[276] Thus the writer found that by crossing the egg of _Fundulus_ with the sperm of a widely different species, namely, _Menidia_, blind embryos were produced very frequently; that is to say such embryos had the degenerate eyes characteristic of blind cave fishes. Very often no other external trace of an eye, except a gathering of pigment, could be found, while a close histological examination would possibly have resulted in the demonstration of rudiments of a lens and other tissues of the eye. [276] Loeb, J., _Biol. Bull._, 1915, xxix., 50.