Note 54 — Chapter IV
Reading edition
The subject of degenerated and lost organs is very fully treated by M. Edmond Perrier in his Traité de Zoologie, pp. 325 sqq. It may be noted that animals which are fixed usually lack eyes, even in light. In the depths of the sea, where total darkness reigns except for the phosphorescence emitted by certain animals, it is found that some creatures have completely lost their organs of sight, while others have them extraordinarily developed. Those which have lost them are the walkers (Crustaceæ); those which show an exceptional development are the swimmers. This goes to show that the needs of the animal, rather than the external conditions, are the determining cause.
Cave fishes are all extremely sensitive to light, which affects them disagreeably, even when the optic nerve is wholly destroyed. See Armand Viré, La Faune Actuelle des Cavernes, Revue des Idées, March 15, 1905, and La Faune Souterraine de France, 1900.