92 SCIENCE SKETCHES. 



intervening space may have been easily bridged over for 

 the passage of fishes by a slight geological change affect- 

 ing the level of the water-shed or even by temporary 

 floods ; and a communication of this kind, if existing for 

 a limited period only, would afford the ready means of an 

 exchange of a number of species previously peculiar to 

 one or the other of these river or lake systems. Some 

 fishes provided with gill-openings so narrow that the 

 water moistening the gills cannot readily evaporate, and 

 endowed, besides, with an extraordinary degree of vitality, 

 like many Siluroids (Clarias, Callichthys), Eels, etc., are 

 enabled to wander for some distance over land, and may 

 thus reach a water-course leading them thousands of 

 miles from their original home. Finally, fishes or their 

 ova may be accidentally carried by water-spouts, by 

 aquatic birds or insects, to considerable distances." 



A somewhat detailed statement of the known 

 facts, arranged in the form of twenty-eight propo- 

 sitions, was given by me in iS/S. 1 To these some 

 further data were added in a paper by Professor 

 Gilbert and myself on the fishes of Arkansas and 

 Texas, 2 published during the past year. These 

 few memoirs, four or five in number, and dealing 

 chiefly with other things, give about all that has 

 been done in the way of generalization on this 

 subject; and in none of these is the question of 

 causes or methods in distribution dealt with in 

 detail or to any important extent. 



1 On the Distribution of the Fishes of the Alleghany Region, of 

 South Carolina, Georgia, and Tennessee, with Descriptions of new 

 or little-known Species. Bull. U. S. Nat. Mus., xii. 1878, pp. 91- 



95- 



2 List of Fishes collected in Arkansas, Indian Territory, and 

 Texas, in September, 1884, with Notes and Descriptions. Proc. 

 U. S. Nat. Mus., 1886, pp. 1-25. 



