THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 191 



that in their highest development the parts between the inner reticular and the pig- 

 mented layers are below the lowest point reached by the corresponding parts in the 

 epigean species mentioned. The same is true of the pigmented epithelium. 



The simplification of the structure of the retina from its maximum to its minimum 

 in ontogeny is of greater extent than its simplification from the lowest- differentiated 

 retina found in epigean species to the maximum found in Amblyopsis. 



From the foregoing we may conclude that there is no constant ratio between 

 the extent and degree of ontogenic and phylogenic degeneration, and that the 

 observed rate of ontogenic degeneration is not necessarily proportionate to the rate 

 of phylogenic degeneration inferred from the degree of degeneration of the eye at its 

 optimum. 



XIV. THE FUTURE OF THE EYE. 



There can be no doubt that the phylogenic fate of the eye, exclusive of connective 

 tissue, sheaths, sclera, etc., is total disappearance. (There is no indication that the 

 scleral cartilages will meet a like fate.) The most degenerate ontogenic eye indicates 

 as much. There are no relatives of Amblyopsis that have reached this condition, but 

 Troglichthys has an eye distinctly more degenerate than that of Amblyopsis. It may 

 offer a clue as to whether any of the ontogenically degenerate eyes, such as are found 

 in old specimens of Amblyopsis, are prophetic of the condition through which the eye 

 will pass in its route to the vanishing point. The most highly developed eye found 

 in any specimens of Troglichthys (PI. XII, Figs. 11, 12)is comparable in a general way 

 with the eyes of the old of Amblyopsis. The pigmented epithelium is larger than the 

 requirements of the eye in both cases, and the scleral cartilages are disproportionately 

 developed in both cases. The ganglionic cells extending through the centre of the 

 eye of the younger Amblyopsis are absent in both cases.* When we attempt a closer 

 comparison our efforts fail. 



We may conclude that if Troglichthys indicates one of the steps through which 

 the eye of Amblyopsis will pass in its annihilation, the degenerative phases seen in 

 the oldest specimens of Amblyopsis indicate only in a general way the phylogenic 

 path through which the eye will pass in the future. 



* Only three cells have been found in this region in all the eyes of Troglichthys examined. 



