198 BLIND VERTEBRATES AND THEIR EYES. 



gradually, as the reef was raised, became adapted to fresh-water conditions. But 

 if this rift with its well contain descendants of its original marine inhabitants, there 

 is no reason why the same should not be true of the wells and caves and rifts of the 

 more elevated coral reefs of Cuba. In other words, there is no reason why the 

 blind fishes should not have developed over the entire area and risen with the entire 

 area over which they are now known to be distributed. Stygicola is found from 

 Cahas at least as far east as Jovellanos ; Lucifuga only west of Guira and at least 

 as far as Cahas. There is, furthermore, no special reason why the blind fishes 

 which have been reported from the natural wells at Merida in Mexico and from 

 Jamaica should not be identical or related to the Cuban species, why they should not 

 have been independently derived in different places from one or more species 

 widely distributed in cracks and crevices of coral reefs. 



PHYSICAL ENVIRONMENT OF STYGICOLA AND LUCIFUGA AND THEIR 



REACTIONS TO IT. 



LIGHT. 



Whatever conditions may have been in the past, at the present light is entirely 

 absent from some of the places inhabited by the blind fishes while others are as well 

 lighted as any stream. In the only cave I entered which light does not penetrate, 

 the pools of water, in every respect similar to those in other caves, contained no blind 

 fishes. On the other hand, in Ashton Cave, parts of which are as well lighted as 

 any stream, blind fishes live side by side with eyed fishes. In a few of the best- 

 lighted caves no blind fishes were found, but in Modesta, where an n-foot opening 

 in the ceiling lights a space 35 by 45 feet so that pebbles and fishes can be seen with 

 perfect distinctness at a depth of water of 10 feet and more, blind fishes are abundant. 

 The same is true of similar caves, well or partially lighted. 



Blind fishes were abundant in Tranquilidad, a dark cave into which light pene- 

 trates through a narrow shaft over 20 feet deep and then only illuminates the margin. 

 They were also abundant in the open well at the Carboneria, about 6 feet in 

 diameter and with a total depth of about 10 feet. 



It is to be emphasized that blind fishes are abundant in well-lighted caves only 

 when these are connected with underground channels that extend into the dark. 

 Such caves contain many more fishes than caves that are totally dark. The reason 

 for this lies entirely in the much greater abundance of the food supply in caves 

 open to the surface; the lighting of the cave is incidental. 



The reaction of the blind fishes to light can be as well studied in the " M " Cave 

 as in any aquarium ideally constructed for the experiment. The pool of water in this 

 cave varies from 5 to about 20 feet across, and from a few inches in depth to many 

 feet — certainly over 10 feet and possibly 50. The pool is probably between 150 

 and 300 feet long. A direct shaft of light reaches the pool near one end so that 

 the water is well illuminated within this shaft. The right end, near which the shaft 

 of light reaches, is shaded by rocks and is so dark that a lamp is of distinct assistance 

 in exploring its 2 to 3 feet of depth. The other end of the cave is in total and per- 

 petual darkness. Fishes are abundant in this cave. I have seen very few within 

 the shaft of light and most of those were driven there by my movements. In the 

 shade of the rocks to the right, on the contrary, they are abundant, and in the larger 

 dark parts of the cave to the left they are also abundant though relatively less so 

 than on the right. Here we have a very distinct reaction to the light — all the fishes 



