115 



named fish does show that predisposition may tell in certain cases ; but the professor 

 salmon does not get any advantage with years. The truth, however, is, that the ROLLESTON. 

 analogy of such fungi as the fermentation and putrefaction — producing fungi, __ 



does not help us much. They kill and destroy not merely by mechanical 

 increase, but by the poisonous effect of the result of that growth, operating 

 on fermentescible and putrescible substances "in the infested organism, and 

 expressed in terms of alcohol, butyric acid, lactic acid, nitrogenized, and many 

 other products ; and this result is an achievement of which Saprolegnia and 

 its allies cannot be accused, however luxuriant their growth may be. 



It is easier also to arrest fermentative and putrefactive action, than the mor e 

 general nutritive action which is limited to vegetative increase ; and hopes 

 therefore, which might be founded on such success as we have attained in the, 

 way of arresting infection by bacteria, as well as other fermentative work, are 

 fallacious. 



Even if this were not so, the quantitative disproportion which must exist, 

 firstly, between any available quantity of disinfectant and the cubical bulk 

 of a salmon river, and secondly, between the concentration of any disinfec- 

 tant solution strong enough to kill a fungus, and the resistance opposable 

 by the gills of the fish affected by it, are obviously incompatible with hopes 

 of help from the employment of disinfectants. 



It has often been observed that closely allied species, or even varieties, so 

 called, of one and the same species, may vary from each other as to their 

 amenability to disease ; and some hopes might on this general ground have 

 been entertained as to benefiting our salmon-consuming population by the 

 introduction of a new variety of salmon. But the Saprolegnia appears not to 

 be specialized, as are some other parasites, to one host only. Several fresh- 

 water fish have been already mentioned as being fatally infested by it, the eel 

 alone of the freshwater fishes observed by Dr. W. S. Church in a pond at 

 Ightham, in Kent,* enjoying an immunity from its attacks. A similarly 

 indiscriminating parasitism appears to characterize the Hemileia vastatrix 

 which does not spp.re any variety of the coffee plant which comes in its way ; 

 any more than (with perhaps the single exception of Isabella) the animal 

 Phylloxera spares any variety of vine. 



The first of two practical points which I should, with my present knowledge, 

 or rather with my present ignorance, insist upon, would be the expediency 

 of removing, if possible, and as early as possible, all infected fish out of the 

 river. But what are these possibilities? This you know much better than 

 I do, and you can, I daresay, suggest or order plans with dredge or trawl, 

 which would effectually catch up and scoop out the dying or dead fish 

 .enveloped in their fleece of Saprolegnia, and would so far stay the plague. 



I wish to suggest as my second practical point that by the employment of 

 some such contrivances as the "hempen tangles" used on board H.M.S. 

 " Challenger " for sweeping other organisms from off the deep-sea bottom, 

 we might strain or filter a very considerable quantity of salmon fungus out of 

 our rivers. But to the " swabs " or " tangles " for the salmon fungus I should 

 add coal tar, thinned with coal oil, just as the Americans use it smeared 

 over their coal tar " pans " for the destruction of locusts. The admirable 

 " First Annual Report of the United States Entomological Commission," 1878, 

 gives at p. 281-401, a detailed account of the efficiency of coal tar as a locusti- 

 cide, with many illustrations of the various mechanical contrivances for 

 expressing as large a square area as possible, shall be, when thus prepared, 

 put in the way of these insects. Figures of the " hempen swabs " or " tangles " 

 will be found in Sir Wyville Thompson's " Depths of the Sea." And I cannot 

 think that it is entirely absurd to suppose that we may not, with a due square 

 area of coal tar, thus spread out at once to entangle and to poison, being at once 

 sticky and toxic, the amount of fungus, and the risks run by salmon may be 

 considerably lessened in many of our rivers at no very great expense. 

 Instances of success following the use of similar agencies against similar 

 enemies are not altogether wanting. 



I have, &c. 

 (Signed) George Rolleston, M.D., F.R.S. 



Anatomical Department, University Museum, Oxford, May 20, 1880. 



* See Mr. A. B. Stirling, Proceedings, Edin. Royal Society, 1879-1830, p. 273. 



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