350 JOURNAL OF SCIENCE. 
must be remembered, are constantly floating through the water, 
ready to fasten on any vulnerable fish. 
The Tweed, then, may be taken as the type of salmon rivers 
in Britain, which have not their source in large lakes, as the Nith, 
Solway, Severn, &c. But rivers such as the Tay, flowing out of 
a loch, where one would expect nature had laid up an ample 
compensation for the modern loss of water in the river lower 
down, caused by excessive drainage, have also been found to be 
infected. In such cases I suspect the lake water cannot be suff- 
cient in quantity or quality to effect the above object. At the 
same time it must be borne in mind that there is no evidence as 
yet of the disease in British waters draining a less cultivated 
area—as Loch Lomond and its tributaries, the river Awe, and 
the rivers of the North of Scotland. On the other hand, North 
America and New Zealand furnish examples of fungus on salmon 
or trout living in waters where no appreciable alteration in che- 
mical constitution can as yet have resulted from agricultural 
operations. 
The above ideas, stated generally, explain what I call the 
predisposing or primary causes of the outbreak of the salmon 
disease. But before discussing these further under my last divi- 
sion, the remedy, I may observe that I cannot entirely agree with 
the opinion expressed by Dr. Cooke and other scientific men, 
viz.—that river pollutions, arising from town and factory drainage, 
have nothing to do with it. I am well aware that the fearful 
condition of the Tyne, below Newcastle-on-Tyne, has not as yet 
resulted in salmon forsaking that river, up which they still force 
their way on the top of a flood. It is likewise true that a score 
or two of this noble fish still run the gauntlet of the abominations 
of the Clyde at Glasgow, under favourable autumnal spates, and 
push up to their ancestral haunts below the Falls of Clyde. But 
I contend that not only is it but a question of time when the 
salmon will desert such rivers; I also believe that all excess of 
nitrogen contained in sewage will have the same bad effects on 
the fish as the manures I have already mentioned employed in 
agriculture. By excess of nitrogen, I mean the ratio of nitrogen 
to oxygen being too high for the health of the fish. 
3. The proposed remedy.—Here it becomes necessary to refer 
to the natural history of the salmon, so far as now known, as 
being inseparable from a consideration of the three causes of the 
disease. The recruiting and feeding ground of the salmon is in 
salt, nct fresh water. I speak, of course, of Salma salar. Asa 
rule, with few exceptions, it does not feed during its ascent of 
fresh water rivers. One proof of this is in the fact that no food 
has as yet been found in the stomachs of salmon which had been 
any short time ina river. The salmon leaves the sea regularly 
and cleaves its way up a river in search of a nest or a place 
where it may, so to speak, form a seed-bed for its ova, and that 
as near the river’s source as possible. This, its principal instinct, 
it follows to the neglect of all other considerations, food included. 
But when spawning is accomplished, then it seeks for nourish- 
