Dispersion of Fresh-water Fishes 289 



existed, at any time, but especially since the last submergence, 

 by which the transfer of species might occur? " Some evidence 

 of such transfer exists in the wide distribution of certain species, 

 especially those which seek the highest streamlets in the moun- 

 tains; but except to call attention to the cavernous character 

 of the Subcarboniferous and Devonian limestones, Prof. Cope 

 has made little attempt to account for it. 



Prof. Cope finally concludes with this important generali- 

 zation : 



" It would appear, from the previous considerations, that 

 the distribution of fresh-water fishes is governed by laws similar 

 to those controlling terrestrial vertebrates and other animals, 

 in spite of the seemingly confined nature of their habitat." 



Views of Gunther. Dr. Giinther * has well summarized some 

 of the known facts in regard to the manner of dispersion of 

 fishes : 



"The ways in which the dispersal of fresh- water fishes has 

 been affected were various. They are probably all still in opera- 

 tion, but most work so slowly and imperceptibly as to escape 

 direct observation ; perhaps they will be more conspicuous after 

 science and scientific inquiry shall have reached a somewhat 

 greater age. From the great number of fresh-water forms 

 which we see at this present day acclimatized in, gradually 

 acclimatizing themselves in, or periodically or sporadically mi- 

 grating into, the sea, we must conclude that under certain cir- 

 cumstances salt water may cease to be a barrier at some period 

 of the existence of fresh- water species, and that many of them 

 have passed from one river through salt water into another. 

 Secondly, the headwaters of some of the grandest rivers, the 

 mouths of which are at opposite ends of the continents which 

 they drain, are sometimes distant from each other a few miles 

 only. The 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 num- 

 ber of species previously peculiar to one or the other of these 

 river or lake systems. Some fishes provided with gill-openings 



* Introduction to the Study of Fishes, 1880, p. 211. 



