168 BLIND VERTEBRATES AND THELR EYES. 



(phyletic) of development found in the eyes of the different members of the Ambly- 

 opsidae are all referable to the difference in time in which they have been subjected 

 to their present environment. The only environmental condition surrounding the 

 developing eggs of Amblyopsis to which the peculiarities of development might 

 be attributed is the total absence of light. 



Temperature, oxygen pressure, chemical composition, etc., of the surrounding 

 medium may be entirely excluded from the possible agents affecting the eye, inas- 

 much as normal eyes are developed by other fishes in the same water and under 

 all possible fluctuations of the above conditions within the limits of the possibility 

 of fish life. But the same objection holds in attributing the lack of development to 

 the absence of light. Chologasler agassizii, a member of the Amblyopsidae, which 

 always lives in caves in exactly the same conditions under which Amblyopsis lives, 

 has nevertheless normally developed, though small, eyes. 



While guarding against the possibility of attributing too much weight to the 

 results obtained in other families of animals, it still may be mentioned that many 

 fishes living perpetually in total darkness develop normal eyes. This is also true of 

 the young of all viviparous animals which develop in more or less complete darkness. 



If, then, so closely related fishes as Chologasler and Amblyopsis are subjected 

 to the same environment which is minus a certain element and both develop their 

 normal parental structure, one developing a normal eye, the other a very abnormal 

 degenerate one, it is scarcely warrantable to say that the abnormal structure in one 

 of them is due to the absence of the one element (light) from the environment. 

 Moreover, if the development is controlled by the absence of light, there is no reason 

 why development should be normal, even to the extent of forming a normal start and 

 should then be arrested or retarded. The fact that the presence or absence of light 

 is not the controlling factor in the retarded development of the eye of Amblyopsis 

 does not vitiate the supposition that a certain amount of change may not be pro- 

 duced on the eyes of an individual by rearing it in the light. Such change would, 

 however, stand on a par with the ontogenetic degeneration of the eye with age in 

 the absence of light ; that is, it would be a functional adaptation due to use. 



Experiments have been in progress to test the effect of light. So far only nega- 

 tive results have been obtained. One young has been reared till it was 6 months 

 old. It was obtained from the caves at a time when it was ready to swim about 

 freely; that is, when the eye was already fully formed. There was no difference in 

 the gross anatomy of the eye of this individual as compared with that of others. The 

 minute anatomy, as the result of an accident, was not available for study. The 

 others examined in earlier stages have not been reared beyond a length of a few 

 millimeters, and the effect of the light, if any, was not appreciable. From the 

 observations on the development of the eyes — which show that some processes are 

 arrested very early — it would seem that the only rational way to determine the 

 effect of light on the total development is to colonize the adults in an outdoor pool 

 where the young can be reared, from the fertilization on, in normally lighted waters. 



The lack of development of the eye not being chargeable to any factor in the 

 environment, is there any factor within the fish that inhibits its development, or 

 whose absence fails to furnish the stimulus necessary to the development? If so, 

 this factor must be present or absent at the time the retardation begins or some time 

 before. 



