172 BLIND VERTEBRATES AND THEIR EYES. 



toes of bird's foot (and probably of horse's foot), the reduced fingers of a bird's hand. * * * 

 2. Organs which have (presumably) recently become reduced or enlarged in the adult are also 

 reduced or enlarged in the embryo. * * * 3. Organs which have been recently acquired may 

 appear at the very earliest possible stage. * * * The latter arrangement ["ancestral organs 

 have disappeared without leaving a trace"] seems to be the rule, the former the exception. 



I think it can be shown that the retention of ancestral organs by the larvae [embryos?] after 

 they have been lost by the adult is due to the absorption of a larval or immature free stage into em- 

 bryonic life. A larval character thus absorbed into the embryonic life, its disappearance is no 

 longer a matter of importance to the organism, because, the embryo being protected from the struggle 

 for existence, the pressure of rudimentary functionless organs is unimportant to it. Characters 

 which disappear during free life disappear also in the embryo, but characters which, though lost 

 by the adult, are retained in the larva may ultimately be absorbed into the embryonic phase and 

 leave their traces in embryonic development. 



To put the matter in another and more general way. The only functionless ancestral structures 

 which are preserved in development are those which at some time or another have been of use to 

 the organism during its development after they 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 case disappear unevenly, that is to say, they may remain at one stage after 

 they have totally disappeared at another. 



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

 generate organs may or may not be better developed in the young than in the adult. 



(1) They are better developed in the young if they are still functional in the 

 young after 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 pro- 

 vide the necessary stimulus to bring about or to inhibit cell movements or cell dif- 

 ferentiation in the development of other organs. 



(5) They are supposed to be no better developed in the young than in the 

 adult, if they ceased to be of use to the young when they 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 can not 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 tiseless 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 phylogenetic 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 produced 

 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 



