214 Dr. Mac Culloch on the changing 
The Gadus Barbatus, or whiting, and the Tricirratus, or rockling, 
occur in abundance in those Highland sea-lochs where the water is 
at times perfectly fresh, from the magnitude of the riversin rainy sea~ 
sons; not quitting their haunts even when it is deeply tinged by the 
colour derived from peat. From their permanence in those situa- 
ations, and from being taken of all sizes, they probably spawn 
there; and, if so, they offer, like the common river flounder and the 
pike, perfect examples of the permanent convertibility of the habits 
of these species. 
The Cottus Quadricornis, a native of the Baltic, also ascends 
rivers, as does the GasterosteusPungitius, or stickleback, in our own 
country. The Pleuronectes Platessa, or plaice, as has already been 
observed, is naturalized to fresh water in East Friesland: and the 
P. Flessus, or common flounder, is. well known to be permanent 
in the Thames and other rivers, far within the fresh water, although 
equally a salt-water fish. The P. Roseus has also been taken in the 
Thames. Iam further informed that a sole was kept in a fresh- 
water pond in a garden, by a person whose name I need not quote, 
for a great many years ; and if the plaice and flounder can be so 
naturalized, it is not unlikely that this would prove true of the 
whole genus. 
Although the mackerel is rare in Scotland, it is sometimes taken 
in the lochs of the western highlands, where the water, from the 
entrance of rivers, is nearly or absolutely fresh ; a proof at least, 
as in many other fishes, that whatever aversion they may have to 
residing permanently in fresh water, whether from the want of food 
or for other reasons unknown to us, they experience no difficulty 
in respiring in it. 
The Mugil Cephalus, or mullet, which is a sea-fish, not only 
ascends rivers, but has been introduced and detained in ponds; 
offering another example, like the plaice, of the possibility of per- 
manently naturalizing a sea-fish to fresh water. This fish does not 
necessarily spawn in rivers; since, in England, it performs this 
operation on the sandy and muddy shores of the sea. Yet, in 
Asia minor, it appears that it always spawns in the rivers, ascending 
the Sturmus, the Meander, and others for this purpose, and pro- 
