398 FISHES 



do they have great difficulty in keeping under and soon come 

 to the surface again. 



The development of the eyes in this fish has been described 

 in the Chapter on breeding, where it was pointed out that the 

 peculiarities arise by a secondary change after the eyes are devel- 

 oped in the normal fashion, although the special structure is 

 attained before the young are born. It must be admitted that 

 we cannot at present show exactly how the conditions of vision 

 following from the habit of surface swimming could in the indi- 

 vidual tend to cause the changes in the eye which have taken 

 place, but it is difficult to avoid the conclusion that these 

 changes have been the direct result of the position of the eye 

 to which they so exactly correspond. Other explanations should 

 be able to produce some evidence of their probablity, and there 

 is no evidence of the occurrence of such modifications of the 

 eye except in the fish which habitually swims with its eye half 

 out of water. If the modifications only occurred under the 

 conditions to which they are adapted, the conditions must 

 be logically considered as the cause of the modifications, not 

 by selection but by direct influence. Dialommus, a marine 

 blenny from the Panama region, has its eye horizontally divided 

 in a similar manner, and seems to have habits similar to those of 

 Anableps. 



In several species of fish the eyes are absent, or so rudi- 

 mentary that vision is impossible. As in other animals this de- 

 generation of the organs of sight occurs in fishes which live in 

 darkness, and the case has attracted much interest among biolo- 

 gists in connection with the question of the evolution of adapta- 

 tions : by one school of evolutionists the facts are considered 

 to afford support to the belief in the direct action of conditions 

 on the organism, while those who deny the possibility of the 

 inheritance of acquired characters maintain that the loss of the 

 eyes can be explained by variations or mutations which were 

 independent of the external conditions. The most celebrated 

 blind fish is Amblyopsis spelcea which lives in the stream flowing 

 through the Mammoth Cave of Kentucky. This fish is not only 

 eyeless but colourless, both conditions being either directly or 

 indirectly the result of the absence of light. Amblyopsis is 

 closely allied to the Cyprinodontidae, and was formerly placed 

 in that family. Several other species are, however, now known 



