ADAPTATIONS 401 



effects of the absence of light, than with the view that it arose 

 from spontaneous variations ; certainly the gradual degenera- 

 tion in the embryo is in no way similar to the sudden variations 

 which are called mutations, which develop directly. 



Whether by means of the positive selection of individuals 

 with regressive variations of eyes and pigment, or as some 

 maintain by the mere cessation of selection, or by the direct 

 effect of the absence of light, subterranean conditions of life 

 have produced similar results in fishes of different families and 

 also in invertebrates. A blind and colourless crayfish lives in 

 the underground waters of Kentucky and neighbouring states 

 with the blind Amblyopsis. In caves of Pennsylvania there is a 

 blind species of cat-fish called Amiurus or Gronias nigrilabris, 

 closely related to species of Amiurus which are common in the 

 rivers on the surface of the earth in that region. In caves of 

 Cuba species of an entirely different family have become 

 adapted to the absence of light : the Cuban cave-fishes belong 

 to the genera Stygicola and Lucifuga of the family Zoarcidae, 

 and are evidently derived from marine ancestors, since they are 

 the only fresh-water species of the family. Many of the Zoar- 

 cidae live near the shore, and it is easy to understand how 

 specimens might be carried by storms, or might make their 

 way of their own accord, into the waters of caves, and survive 

 there. These fishes, like Brotula, their nearest marine relative, 

 have long dorsal and ventral fins confluent with the tail-fin. 

 It is curious that one of the Cuban cave-fishes, namely 

 Lucifuga, like the Amblyopsidae, is viviparous, but this is not 

 the result of subterranean conditions, since in both cases 

 members of the same family living under ordinary conditions 

 are also viviparous. 



It is certain that the conditions of life in the deep abysses 

 of the ocean are not similar, at any rate with regard to the 

 absence of light, to those of subterranean caves, for abyssal 

 fishes are not generally blind. In fact, there are very few deep- 

 sea fishes which are totally blind, for example Ipnops, TypJdo- 

 nus, Aphyonus, and Tauredophidium ; of these all except the 

 first belong to the family Zoarcidae. 



The habit of seeking concealment under stones or in 

 crevices is characteristic of this family generally, and the loss of 

 the eyes in the three abyssal members mentioned must be at- 



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