The pigment cells can not be made to show themselves, even by a prolonged stay in the light. The old, if kept in the light, will not become darker, and a young one reared in the light until ten months old not only showed no increase in the pigmentation but lost the pigment it had at birth, taking on the exact pigmentless coloration of the adult. Pigment cells are late in appearing in Amblyopsis. When the young are two months old pigment is abundant. This pigmented condition is evidently a hereditarily transmitted condition. It disappears with age. Primarily this disappearance was probably individual. But, as in the flounder, the depigmentation has also become hereditarily transmitted, for even those individuals reared in the light lose the color. Numerous facts and experiments show that while pigment may be, and is, developed in total darkness, the amount of color in an individual animal depends, other things equal, directly on the amount of light to which it is habitually exposed.
Fig. 1.—Chologaster Agassizii from Cedar Sinks Cave, Kentucky.
The lower and upper surfaces of the flounder, the one protected and the other exposed to the light, give the most striking example, and the argument is clinched here by the fact, noted by Cunningham, that a flounder whose lower side is for long periods exposed to the light takes on color. Loeb has shown that in the yolk sacs of Fundulus embryos more pigment cells are developed if the embryos are kept in the light than when they are kept in the dark. However, in the body, and especially in the eye, the pigmentation was not affected by the absence of light.
The general absence of color in cave animals is conceded. Packard states, “As regards change of color, we do not recall an exception to the general rule that all cave animals are either colorless or nearly white, or, as in the case of Arachnida and insects, much paler than their out-of-door relatives.” Chilton has made the same observation on the underground animals of New Zealand. Similar observations have been recorded by Lönnberg, Carpenter, Schmeil, and Viré. Hamann enumerates a number of species living both in caves and above ground. In such cases the underground individuals are paler than the others. This confirms similar observations by Packard.
Fig. 2.—Chologaster papilliferus from Illinois.
Poulton has mentioned that Proteus becomes darker when exposed to the light. This has been verified by others. Typhlotriton larvæ living at the entrance of a cave are dark, while the adult living farther in the cave are much lighter, but with many chromatophores containing a small amount of color. Epigæan fishes found in caves are always lighter in color than their confrères outside.
Fig. 3.—Typhlichthys subterraneus.