DISPERSION OF FRESH-WATER FISHES. 91 



seek the highest streamlets in the mountains ; but 

 except to call attention to the cavernous character 

 of the Subcarboniferous and Devonian limestones, 

 Professor Cope has made little attempt to account 

 for it. 



Professor Cope finally concludes with this im- 

 portant generalization : 



"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." 



Dr. Giinther 1 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 effected were various. They are probably all 

 still in operation, but most work so slowly and imper- 

 ceptibly 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 accli- 

 matizing themselves in, or periodically or sporadically mi- 

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

 circumstances 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 head-waters 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 



1 Guide to the Study of Fishes, 1880, p. 211. 



