16 BLIND VERTEBRATES AND THEIR EYES. 



isolated elements of a discontinuous environment that can not, in the nature of the 

 case, at any time have formed parts of a continuous environment. 



Amblyopsis is found on both sides of the Ohio River. The caves of the two 

 sides have certainly never formed part of the same complex. It is possible, though 

 scarcely probable, that the caves south of the Ohio, inhabited by Typhlichthys at 

 one time, formed a continuous environment. It seems evident that Amblyopsis 

 could not have migrated from the caves south of the Ohio to those north of the 

 Ohio. The different colonies probably had similar but independent histories. 

 The cave salamander, Spelerpes maculicauda, is widely distributed in the Missis- 

 sippi Valley. It enters caves wherever they are found within its area of distribu- 

 tion. It is becoming adapted to a cave existence in widely isolated places. What 

 is at present taking place with Spelerpes may have taken place with Amblyopsis, 

 except that Spelerpes found its caves ready made, while Amblyopsis was present 

 during their making. 



The ancestry of the Amblyopsidae we may assume to have had a tendency to 

 seek dark places, wherever found, and incipient blind forms would thus arise 

 over their entire distribution. Certainly the fearless, conspicuous blind fishes, 

 as at present developed, would have no chance of surviving in the open water. 

 Their wide dispersal after their present characters had been assumed would be 

 out of the question entirely, except through subterranean waters. The same 

 would not be true of the incipient cave forms when they had reached the stage 

 at present found in Chologaster. This genus has the habit of hiding under- 

 neath objects in the darker sides of an aquarium. These dark-seeking crea- 

 tures would be especially well fitted to become distributed in caves throughout 

 their habitat. S. Carman's able argument for the single origin and dispersal 

 of the blind fishes through epigean waters was based on the supposition that 

 the cis-Mississippi and trans-Mississippi forms were identical. The differences 

 between these species are such as to warrant the inference, not only that they 

 have been independently segregated, but that they are descended from different 

 genera. The external differences between these species are insignificant, but 

 this is to be expected in an environment where all the elements that make for 

 external color markings are lacking. The similarity between Typhlichthys and 

 Amblyopsis is so great that the former has been considered to be the young of the 

 latter. For reasons that will be fully set forth there is every probability that the 

 Cuban blind fishes developed with the caves which they inhabit. 



In conclusion it may be said : 



(i) That the cave fauna is in large part the result of the formation of the caves 

 themselves, that environment and habitat developed pari passu. 



(2) That to this original fauna have been added and are being added species 

 (such as Spelerpes maculicauda) which, because they are negatively heliotropic or 

 positively stereotropic, are gradually becoming adapted to the deeper and deeper 

 recesses of caves. 



(3) That to the fauna of the larger caves may also have been added animals 

 which had become adjusted to cave existence in crevices, under banks or rocks, etc., 

 that is, in small caves. 



(4) That accident has played little or no part in developing the cave fauna. 



