CAUSES OF DEGENERATION IN BLIND FISHES. 405 



economy, can not have affected the eye for reasons already stated. The 

 mere loss of the force of heredity, unless this was caused by disuse, or 

 the process of germinal selection, can not have brought about the condi- 

 tions, 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 occupying a more passive position, 

 e. g., the scleral cartilages, have been much less affected. The lens is 

 one of the latest organs affected, not at all during use, possibly because 

 during use it would continuously be in use. It disappears most rapidly 



Amblyopris before all the yolk 

 is absorbed. 



after the beginning of absolute disuse both ontogenetically and phylo- 

 genetically. All indications point to use and disuse as the effective 

 agents in molding the eye. The process, however, does not give results 

 with mathematical precision. In Typhlichthys subterraneus the pig- 

 mented 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 applica- 

 tion 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 explainable by any of the other 

 theories. 



