190 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



In order to compare the ratio between the ontogenic and phylogenic rates of 

 degeneration, it is necessary to use some stage in the development of the eye as the 

 point where phylogenic degeneration has been reached. For such a point we shall 

 use the optimum reached by various parts of the eye during their development. It is 

 certain that the phylogenic stage is below this optimum, that some of the degenera- 

 tion in individual eyes is due to phylogeny, but since we do not know how much of 

 the descent from the optimum is due to heredity and how much to the peculiarities of 

 the environment and the resulting functionless lif e of the parts during the lif e of the 

 individual, it will be best to take the optimum as above indicated. 



All phylogenic time is taken as a unit, although some parts of the eye have been 

 degenerating longer than others. 



The ontogenic degeneration leads from the optimum to the vanishing point for 

 most parts of the eye. 



i. The Lens. — Ontogenically the lens degenerates very rapidly, reaching its van- 

 ishing point from its optimum during the period in which the fish grows not more than 

 5 millimetres in length. The rate of its phylogenic degeneration must have been 

 proportionately rapid, for at its optimum in Amblyopsis it is minute and its cells are 

 undifferentiated. In the epigean relatives of Amblyopsis the lens is one of the parts 

 least affected, so that it must have degenerated very rapidly in its later phylogenic 

 history, after the fish had entered the caves. 



j 2. The Vitreous Body — At its best the vitreous body is so inappreciable in 

 amount that I have not been able to consider its ontogenic degeneration. Its phy- 

 logeny has approached the vanishing point toward which most parts of the eye are 

 heading. 



3. The Retina. — The retina may be considered in its extent and in the degree of 

 the histogenic differentiation of its parts. In the matter of its extent or size there is 

 little change from its optimum until its disintegration in old age. Its ontogenic 

 changes are slight. Its optimum is comparable with that of the lens and indicates a 

 rapid and great reduction from the lowest retina of epigean relatives. The ontogenic 

 and phylogenic rates of degeneration in the extent of the retina differ greatly, the 

 former having come practically to a standstill. 



In its histogenic differentiation the retina is not comparable with the lens, for it 

 rises above the embryonic phases. In fact in its histogenic differentiation the retina 

 rises far above the requirements of the case, and the most highly developed eye of 

 Amblyopsis approaches the lowest of its epigean relatives. Over any given area it 

 is doubtful whether the ganglionic and inner reticular layers are more degenerate or 

 as degenerate as the same parts in the eyes of Chologaster comutus. It is certain 



