COLOR OF THE AMBLYOPSIDvE. 79 



red light, and in absolute darkness." This was to be expected, for even in the 

 young of cave animals pigment is, as a rule, well developed. 



(J) Pouchet (Arch, de Physiol, et d'Anat., 1876, and Rev. Scient, vol. xm, 1897) 

 has demonstrated that change in color cells, such as are mentioned under (b) and (d), 

 is brought about by the reflex control of the eye. The section of the great sympa- 

 thetic nerve puts an end to the changes of color under the influence of light. 



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 and McMann, 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 Fundiilus 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 Lonnberg, Carpenter, Schmeil, and Vire. 



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 of Packard. 



Poulton has mentioned that Proteus becomes darker when exposed to the light. 

 This has been verified by others. In Typhlotriton, larvae living at the entrance of 

 a cave are dark, while the adult living farther in are much lighter, but with many 

 chromatophores containing a small amount of color. Epigean fishes found in 

 caves are always lighter in color than their confreres outside. 



We have thus numerous examples of colored epigean animals bleaching in 

 caves, and also bleached cave animals turning dark when exposed to the light. We 

 have also animals in which the side habitually turned to the dark is colorless, while 

 the side habitually turned to the light is colored. Finally we have cave animals 

 that are permanently bleached. 



Natural selection can not have affected the coloration of the cave forms, for it 

 can be of no consequence whether a cave species is white or black. It could only 

 affect the coloration indirectly in one of two ways : first, as a matter of economy, but 

 since the individual is in part bleached by the direct effect of the darkness, there is 

 no reason why natural selection should come into play at all in reducing the pig- 

 ment as a matter of economy; second, Romanes has supposed that the color 

 disappeared through the selection of correlated structures, a supposition he found 

 scarcely conceivable when the variety of animals showing the bleached condition 

 was considered. 



Panmixia can not account for the discharge of the color, since it returns in some 

 species when they are exposed to the light and disappears to a certain extent in 

 others when kept in the dark. Panmixia, Romanes thinks, may have helped to 

 discharge the color. In many instances the coloration is a protective adaptation, 

 and therefore maintained by selection. Panmixia might in such instances lower 

 the general average to what has been termed the "birth mean." Proteus is perhaps 



