14 BLIND VERTEBRATES AND THEIR EYES. 



animal. Certain parasitic insects are in the habit of boring through the hard mud 

 walls of the nests of mud wasps to deposit their eggs. It seems difficult to explain 

 the origin of so complex a habit and of the organ sufficient to pierce the hard wall. 

 A mutation to account for it seems inconceivable. It is, however, quite possible 

 that the hard wall is a partial adaptation against these very enemies, and that the 

 habit of building heavier and heavier walls, and the development of more and 

 more efficient organs for piercing them were developed as armor plate and armor- 

 piercing shells are interrelated developments. 



From the lulls about Horse Cave, Kentucky, one sees valleys about 250 feet 

 deep stretching out in four directions. Of the river that is responsible for them 

 nothing is to be seen. It is 185 feet beneath the bottom of the valley at the town 

 of Horse Cave. The hills are capped with over 70 feet of sandstone. The river 

 has had a continuous existence from the time it formed the valley in the sand- 

 stone capping, through its later history when it continued the process of valley 

 formation in the limestone underlying it, and later still when it hollowed out its 

 underground channel in the limestone. There is nowhere any indication that 

 there has been a cataclysm in the history of the river. It lies south of the glacial 

 area. What is true of the river may be true of the inhabitants still within it. There 

 is no reason to think that the ancestors of the blind fishes may not have lived in 

 the stream when it flowed over the sandstone capping the hills. 1 Some fishes of 

 any stream stay in the light, others in the shade, others under rocks. The ances- 

 tors of the blind fishes probably lived in the shade under rocks and became ad- 

 justed to the dark or dusk, existing there long before the caves were formed. When 

 placed in open pools Amblyopsis still has that habit. What more natural than 

 that this fish should descend farther and farther with the river after it began its 

 subterranean course — not suddenly, but gradually ? At first only part of its water 

 found its way underground ; but when all its water could flow beneath the surface 

 under normal conditions, a part flowed above ground after every freshet, just as 

 the water of Lost River of Indiana does at present. It could not sink beneath 

 the ground at all until Greene River, into which it empties, had cut a considerable 

 distance beneath the surface of the limestone, and thus gave the water in the lime- 

 stone rifts a chance to flow out and be replaced with fresh water from the river 

 above. As the stream sank beneath the surface, naturally those fishes depending 

 on light for food and courtship left it, and only those either negatively heliotropic 

 or positively stereotropic remained. 



The blind aquatic fauna looked at from this standpoint is not a new acquisi- 

 tion of the present cave stream, but a relict of the fauna of the river when it still 

 flowed above ground. The cave and its fauna have developed hand in hand. 

 The presence of the cave fishes and other aquatic cave dwellers do not so much 

 need explanation (they were present long ago) as does the absence of all of the 

 other forms that must have been present when the stream flowed in its epigean 

 valley. The prime requisite for a candidate for underground existence is a nega- 

 tive reaction to light, or positive stereotropism, or both. 



It must also be evident that a fish depending on its sight to procure its food 

 can never become a cave form. Sun-fishes, which are annually carried into the 

 present fully developed caves, belong to this class of fishes. They are always 



1 Shaler, 1875, considers that during the glacial epoch the conditions in the caves of Kentucky were such 

 that the present fauna could not have existed there. 



