the Residence of certain Fishes. 219 
others which are described in the preceding catalogue, seem by 
nature to inhabit both indifferently. It remains indeed to be proved 
that any fresh-water species now known as limited to rivers and 
lakes, can be permanently confined to the sea; but this is a point 
which can obviously never be determined. 
A species of argument might be derived, on this subject, from 
the probable state of the earth at former distant periods, and from 
that which has probably been the original condition of many inland 
lakes besides the Caspian. It is probable that many such lakes 
were portions of the salt ocean, and that they have been rendered 
fresh since their separation from it, by the effects of the rivers flow- 
ing into them. In this case, the fish which these contain were 
once sea-fish; and thus perhaps we may account for the double 
existénce of the pike and of those Cyprini above described, in 
the salt waters of the Caspian and in the fresh lakes of other inland 
districts. But I will not here lay much stress on this reasoning. 
It is evident at least, from the preceding remarks, that a change 
of the medium of respiration is not injurious or poisonous to all 
those fish which even incidentally pass into fresh waters from the 
Sea, as this effect, if any, ought to be immediate, or at least speedy. 
If so many species can bear that change in the medium of respira- 
tion, it is not unlikely that the whole might, as the general struc- 
ture of the respiratory organ is the same in all; and it is not 
therefore likely that this function will be the cause of any great 
obstruction in attempts to change permanently the residence of 
fishes from one variety of water to another. 
The next important function to be considered is that of nutrition, 
or the probability that food may be found or provided for those 
sea-fish which any projects of naturalizing them in fresh waters, 
may confine to inland lakes. We are so little acquainted with the 
food of many fishes, that it is not possible to throw much light on 
this subject ; but it is probable that the most important and insur- 
mountable obstacle will be found here. Of many species, it seems 
to be ascertained that they feed on marine vegetables. Others, 
like the mullet, are known to plough the sand in search of lum- 
brici; probably also, of the spawn of other fishes. Some species 
