THE EYES OF RHINEURA FLORIDANA 535 



tain that any fossil members of an existing genus all of whose living 

 species have degenerate eyes, must have had eyes that were to a greater 

 or less extent degenerate. The time suggested by this find of Baur 

 during which the eyes of Rhineura have been degenerating is sur- 

 prisingly long, extending as it does through about 5 to 10 percent of 

 the formation of sedimentary rocks. This is in distinct contrast to the 

 Amblyopsidse, the family of blind fishes, in which the eyes have 

 reached their present condition largely since the glacial epoch, during 

 which the caves of the Ohio valley were not habitable. The eyes of 

 the latter were, however, very probably degenerate to a certain degree 

 before they entered the caves. 



Rhineura is a burrowing animal, and blind animals which burrow 

 in the ground are not found in naturally made caves. The latter are 

 largely populated by species that tend to hide in crevices or natural 

 cavities under rocks. It would seem from this that the cave fauna was 

 incipient before the existence of caves, and that the latter were colo- 

 nized as soon as they were large enough to admit their present inhabi- 

 tants.^ 



GENERAL ACCOUNT OF THE EYE. 



The eye of Rhineura Jloridana is not visible externally, nor is 

 there any indication where it formerly came to the surface. The side 



' I have in divers other places emphasized the fact of the voluntary coloniza- 

 tion of caves by animals predisposed to shun the light or creep under rocks or into 

 crevices. In other words, the predisposition to become cave animals existed 

 before the present caves were formed. There is no doubt whatever that the 

 colonizers have been profoundly modified since they entered the caves. Ernest 

 Krause (Prometheus, ix, 652) enters the following protest : 



" Well, whoever believes it may also attain blessedness in the belief that caves 

 were created to serve as the playground and elysian fields, as it were, of the blind 

 of all classes of animals. We have repeatedly heard these siren songs out of the 

 mystic darkness of the enemies of light and of the development theory ; they are 

 in harmony with the growing effort to upset Lamarckism, Darwinism and even 

 Weissmannism, and hence, before we admit the convincing force of Eigenmann's 

 conclusions, we must look a little more closely at their foundations." 



Mr. Krause continues for another column to show how very wrong it is not 

 to agree with him that the degeneration of the eyes of cave creatures is due to the 

 absence of light. It is, however, necessary to get our animals into the caves 

 before their eyes may degenerate as the result of the absence of light, and they 

 must be able to maintain themselves in the dark after we have got them there. A 

 sudden and accidental colonization is, therefore, out of the question. If species 

 depending on their eyes for food are excluded as candidates for cave existence, 

 nocturnal animals, negatively heliotropic or positively stereotropic ones, must 

 have supplied the present cave fauna. An examination of the inhabitants of any 

 cave will readily demonstrate that its fauna was derived from the latter classes 

 and that, in spite of the absence of light, many of them have not yet undergone 

 any appreciable degeneration as far as their eyes are concerned. 



