Qg2 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 1878.1. To these some 
further data were added in a paper by Professor 
Gilbert and myself on the fishes of Arkansas and 
Texas,? 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- 
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. 
