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 deserve 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 adistance 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-water 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 haye been per- 
manently naturalized to fresh water; and by the fact that so many 
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