218 Dr. Mac Culloch on the changing 



seems to inhabit indifferently the sea shores and the mouths of the 

 neighbouring rivers. 



The Cyclopterus Liparis has also been observed to ascend from 

 the sea into fresh waters; and the same fact is familiar with respect 

 to the sturgeon, the common lamprey, and the lesser lamprey, or 

 Petromyzon Fluviatilis. 



Lastly, the Delphinus Leucas, or white whale, is known to ascend 

 the fresh-water rivers of Northern America; but as this animal 

 breathes air, it does not, in that point at least, coincide with the 

 true fishes, which respire water. The appearance of this species 

 of whale seems to have been the chief evidence by which Hearne 

 and Mackenzie attempted to prove that they had reached the sea 

 in their respective expeditions. It is known to ascend the Hudson 

 to a distance of 100 miles and more, above the salt water, and is 

 taken by an established fishery high up in some of the fresh rivers 

 of Hudson's-bay. . 



Here then is a large body of evidence, derived not only 

 from the occasional, but from the permanent, residence of 

 many sea-fish in fresh waters, and, on the contrary, of some fresh- 

 water fish in salt lakes, to prove the existence, or possibility, of 

 these convertible habits, at least in those species. But it will be 

 convenient to subdivide the considerations which arise out of this 

 subject, as they affect those functions in fish which, as far as this 

 question is concerned, must be considered as of a vital or essential 

 nature; either as they regard the life and health of the individual, 

 or the continuation of the species. 



The first of these, is the act of respiration. The first doubt na- 

 turally arising on this subject is, whether salt-waiter fish can with 

 impunity breathe fresh water, and the contrary. From the great 

 number of the sea-fish which, either systematically or occasionally, 

 visit fresh water without inconvenience, it is fair to conclude that 

 the latter in no way disagrees with the function of respiration in 

 them. A much stronger confirmation of this is afforded by the 

 facility with which the plaice, mullet, and flounder have been per- 

 manently naturalized to fresh water; and by the fact that so many 



