4 BLIND VERTEBRATES AND THEIR EYES. 



those of the south polar sea, these form a discontinuous unit of environment, a unit 

 whose parts do not form a portion of a continuous area and whose inhabitants 

 can not migrate from one part to the other. 



If we assume the conditions in the equatorial Atlantic to be the same as those of 

 the equatorial Pacific, we are again dealing with a discontinuous unit — discontinuous 

 because the inhabitants of one part can not migrate to the other. If we examine 

 these two units more closely, it becomes evident that the Arctic and Antarctic 

 oceans have always formed a discontinuous unit. Arctic conditions have never 

 prevailed between the two. On the other hand, the equatorial Atlantic and the 

 equatorial Pacific were formerly connected in Colombia and formed one continu- 

 ous environment. The land area and the fresh waters near the equator from 

 Para to the Andes form a continuous unit of environment, and the Galapagos 

 Islands to the west of it form a discontinuous unit, each separate island forming a 

 continuous unit of a smaller order. It is evident that there are degrees of discon- 

 tinuity, depending in part on the length of time the discontinuity has existed, and 

 in part on the space separating the nearest parts of the unit. 



Caves are discontinuous units of environment whose elements have always 

 been separate. It is possible that in some areas a large complex of different under- 

 ground channels exists. An east to west fault has lowered the southern part of 

 Texas, or has raised the northern part, many feet. The dividing line is an abrupt 

 escarpment across the State. This fault has favored the formation of underground 

 watercourses, and inasmuch as river valleys do not cut down to the underground 

 channels, it is possible that they form a network of channels or a continuous unit 

 which permits the ready migration of its inhabitants from one part to another. 



The lower area on the southern slope of Cuba, between Canas on the west and 

 an undetermined point east of Union, is drained by underground rivers. No 

 valleys cut down to these rivers, and since this part of Cuba has sunk in recent 

 times, the land being only a few feet above sea-level, it is possible that we again 

 have a complex of underground channels permitting the migration of its inhabit- 

 ants. However, it is also possible that the streams run in separate courses. The 

 absence of Lucifuga from the eastern caves favors this hypothesis. At best we 

 have here several degrees of continuity. 



The large streams cut. the cave region of Kentucky, Indiana, and Missouri into 

 sections, their beds lying deeper than the caves. These caves are, therefore, part 

 of a discontinuous environment. These facts must be constantly borne in mind 

 in considering the origin and dispersal of cave faunas. 



It is quite out of the question in this connection to give even a partial list of 

 North American caves, or an account of the North American cave regions. The 

 region to which Mammoth Cave belongs reaches from near Bloomington, Indiana, 

 through Kentucky into Tennessee and embraces many thousand square miles of 

 territory. Only the larger streams whose rapidly deepening channels have made 

 the caves possible flow on the surface. "One may travel on horseback all day, 

 through certain parts of Kentucky, without crossing a single running stream ; all 

 the rain water that falls being carried down through the sink holes into caverns 

 below where are the gathering beds that feed the few large open streams of the 

 region, of which Green River is an example. It is reported that there are 4,000 

 sink holes and 500 known caverns in Edmondson County (Kentucky) alone." ' 



1 For an account of the principal caves of North America see Hovey, Celebrated American Caverns, Cin- 

 cinnati, 18S2 and 1896; and Packard, The Cave Fauna of North America, Memoirs of the Nat. Acad. Sci. 

 vol. 4, 188S. 



