CAUSES OF DEGENERATION. 241 



cess. There must have been ever widening bounds within which the variation of 

 the eye would not subject the possessor to elimination. 



Chologaster is in a stage of panmixia as far as the eye is concerned. It is true 

 the eye is still functional, but that the fish can do without its use is evident by its 

 general habit and by the fact that it sometimes lives in caves. 



The present conditions have apparently existed for many generations, as long 

 as the present habits have existed, and yet the eye still maintains a higher degree 

 of structure than reversed selection, if operative, would lead us to expect, and a 

 lower degree than the birth mean of fishes depending on their eyes the condi- 

 tion that the state of panmixia alone would lead us to expect. There is a staying 

 quality about the eye with the degeneration, and this can only be explained by the 

 degree of use to which the eye is subjected. 



The results in Chologaster are due to panmixia and the limited degree of use 

 to which the eye is put. Chologaster agassizii shows the rapid diminution of the 

 eye with total disuse. 



The difference in the conditions between Chologaster and Amblyopsis, Typh- 

 lichthys and Troglichthys is that in the former the eyes are still in use, except 

 when living in caves ; in the latter they have not been in a position to be used for 

 hundreds of generations. The transition between conditions of possible use and 

 absolute disuse may have been rapid with each individual after permanently enter- 

 ing a cave. Panmixia, as regards the minute eye, continued. Reversed selection 

 was inoperative, for economy can not have affected the eye for reasons already 

 stated. Simply the loss of the force of heredity, unless this was caused by disuse 

 or the process of germinal selection, can not have brought about the conditions, 

 because some parts have been affected more than others. 



Considering the parts most affected and the parts least affected, the degree of 

 use is the only cause capable of explaining the conditions. Those parts most 

 active during use are the ones reduced most, viz., the muscles, the retina, optic 

 nerve, and dioptric appliances, the lens and vitreous parts. Those organs occupy- 

 ing a more passive position, the scleral cartilages, have been much less affected 

 and the bony orbit least. The lens is one of the latest organs affected, and not 

 at all during use, possibly because during use it would continually be in use. It 

 disappears most rapidly after the beginning of absolute disuse both ontogenetically 

 and phylogenetically. All indications point to use and disuse as the effective agent 

 in molding the eye. The process does not, however, give results with mathe- 

 matical precision. In Typhlichthys snbtcrraneus the pigmented layer is affected 

 differently from that of Amblyopsis. The variable development of the eye muscles 

 in different species would offer another objection if we did not know of the variable 

 condition of these structures in different individuals. Chilton has objected to the 

 application of the Lamarckian factor to explain degeneration on account of the 

 variable effects of degeneration in various invertebrates. But such differences in 

 the reaction are still less explicable by any of the other theories. 



