MUSEUM OF COMPARATIVE ZOOLOGY. 233 



localities, from knowing that exact parallels in the development of 

 animals in nature, if they exist, are excessively rare. If our caution 

 prevents ready acceptance of two apparently exact evolutionary parallels 

 as really coincident, we become much more sceptical when the number 

 of parallels or coinciding lines is increased. There is no doubt that the 

 representatives of Typhlichthys subterraneus in the various caves were 

 derived from a single common ancestral species. The doubts concern 

 only the probability of the existence of three or more lines of develop- 

 ment, in as many different locations, starting from the same species and 

 leading to such practical identity of result. Suc|i . identical results 

 would demand substantially similar modifying elements, — darkness, 

 temperature, food, enemies, etc., ^i— and the same length of time subjected 

 to their influence. The likelihood of the existence of so many like ele- 

 ments in distant regions is inversely to the number demanded, though 

 one cannot say it is impossible. To accept the conclusion favoring inde- 

 pendent developments of the same species would involve acceptance of 

 the idea that the caves in each of the districts had been occupied for 

 about the same period of time. This, of course, would not furnish us 

 with any clue to the time of formation of the caves. 



As an alternative, the opinion is here advanced that these blind fishes 

 originated in a particular locality, and have been, and are being, dis- 

 tributed among the caves throughout the valley. We are in the habit 

 of looking upon great rivers like the Ohio or Mississippi as impassable 

 obstacles to passage from 'cave to cave, rather than as thoroughfares. 

 In this we have certainly assumed too much. Various instances are on 

 record of the discovery of blind fishes that have strayed into the 

 open streams from their caverns. If there were means of determin- 

 ing the frequency of the occurrence of such instances, it would un- 

 doubtedly much exceed what we are now inclined to credit. Persons 

 acquainted with the streams of the Mississippi basin will agree that 

 their undermined banks provide series of recesses or caverns, extend- 

 ing from the rills at the sources of the tributaries to the Gulf. The 

 currents do not prove insurmountable to multitudes of fishes, no better 

 provided with locomotive organs than the blind fishes, passing up the 

 streams every season. Swept from the caves by the torrents in the 

 flooded mouths, the blind species would find itself protected at once 

 from light or enemies by the turbid waters. The temperature of the 

 water at such times is low, and, should the light penetrate so as to prove 

 detrimental, retreats exist on every hand in the excavations of the banks 

 or the mud of the bottom. What migrations these fishes mav make in 



