200 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



have ceased to be so in the adult. . . . But another explanation is possible, which is 

 that organs which are becoming functionless, and disappearing at all stages, may in 

 some cases disappear unevenly, that is to say, they may remain at one stage after 

 they have totally disappeared at another" (p. 50). 



The question seems to me not quite so simple as imagined by Sedgwick. 



Degenerate organs may or may not be better developed in the young than they 

 are in the adult. 1. They are better developed in the young if they are still func- 

 tional in the young while they have become functionless in the adult. 2. They may 

 be better developed in the young, if they were of use to the young, after they ceased to 

 be of use to the adult. 3. They may be well developed in the young after complete 

 disappearance in the adult if the material is used for other purposes in later life. 4. 

 They are better developed in the young if their presence is essential to provide the 

 necessary stimulus to bring about or to inhibit cell movements or cell differentiation 

 in the development of other organs. 5. They are supposed to be no better devel- 

 oped in the young than in the adult if they have never been of use to the young 

 after they had lost their use in the adult. 



The material entering into the formation of the eyes is not used for the building 

 up of other organs, and it is uncertain whether the eyes positively or negatively influ- 

 ence the development of other organs so that a discussion of numbers 3 and 4 of the 

 above possibilities is not profitable. Inasmuch as both young and adult live perma- 

 nently in total darkness, and the eye of the young cannot be functional under the 

 present mode of existence, the first possibility is also eliminated from the discussion. 



In Amblyopsis, which carries its young in its gill-cavity, we are undoubtedly 

 dealing with an animal in which the eyes are useless in the young as well as in the 

 adult and in which they became totally useless in the young at the same time that they 

 became totally useless in the adult, that is, at the time the species took up permanent 

 quarters in the caves. Do the eyes in this case repeat the phylogenic history of the 

 eye or have the eyes in the embryo degenerated in proportion to their degeneration 

 in the adult? In this form the question is whether a perfect or better eye is pro- 

 duced to be finally metamorphosed into the condition found in the adult, or whether 

 development of the eye is direct. 



We have seen in the preceding pages that the foundations of the eye are nor- 

 mally laid, but that the superstructure instead of continuing the plan with new 

 material completes it out of the material provided for the foundations, and that in 

 fact not even all of this (lens) material enters into the structure of the adult eye. 

 The development of the foundations of the eye are phylogenic, the stages beyond 

 the foundations are direct. 



