THE BLINDNESS OF THE CAVE FAUNA. 51 



mann in the fishes of the family of Amblyopsidae. Six species 

 of this group live permanently in caves and are not found in the 

 open, while one lives permanently in the open and is never 

 found in caves and one comes from subterranean springs. The 

 one form which is only found in the open, Chologaster cornutiis, 

 has a simplified retina aside from having a comparatively small 

 eye, in other words, its eye is not normal. This indicates the 

 possibility that the other representatives which are only found 

 in caves also might have abnormal eyes even if they never had 

 lived in caves. 



Through these facts the old idea becomes doubtful, namely, 

 that the cave animals had originally been animals with normal 

 eyes in which the disuse had led to a gradual hereditary de- 

 generation of the eyes. Instead we must consider the possi- 

 bility that in the blind cave animals as well as in the blind animals-, 

 which live in the open the tendency towards blindness developed 

 independently of presence or absence of light. From this point 

 of view the tendency tow r ard degeneration of the eye appears 

 as a hereditary mutation comparable to the inherited glaucoma 

 which is known in the human. Glaucoma is a form of blindness 

 caused by the atrophy of the optic nerve in consequence of an 

 increased intraocular pressure and this high pressure seems to be 

 caused by a certain disturbance in the circulation in the eye.. 

 The fact that these patients are born with normal eyes and do- 

 not become blind until later in life shows that lack of light has 

 no share in the development of this hereditary disease or muta- 

 tion. The question then remains, how can we account under 

 such an assumption for the fact that blind animals are so preva- 

 lent in caves and so rare in the open? We shall return to this, 

 question later on. 



II. 



We will now review briefly the literature dealing with observa- 

 tions and experiments on the influence of light on the formation 

 of eyes. The fact that the eyes of mammals are formed in 

 complete darkness (in the uterus) may serve at the outset as a 

 warning against overestimating the effect of light on the forma- 

 tion of eyes. F. Payne 1 has raised sixty-nine generations of a fly 



1 F. Payne, BIOL. BULL., XXI., p. 297, 1911. 



