194 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



vertebrates in general, I have insisted that cross-country conclusions must be guarded 

 against, I then saw no objection, and now see none, to considering the different mem- 

 bers of the Amblyopsidse as homogeneous material within the bounds of which we 

 may expect similar causes to effect similar results. The different stages (phyletic) of 

 development found in the eyes of the different members of the Amblyopsidse are all 

 referable to the difference in time during which they have been subjected to their 

 present environment. 



The only environmental condition surrounding the developing eggs of Ambly- 

 opsis 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. Chologaster agassizii, a member of the Amblyopsidse, 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 Chologaster 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. More- 

 over, 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 ontogenic 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 six months 



