113 



4thly. The possible predisposing causes towards the salmon becoming weak MR - H. LEE. 



and sickly are, of course, numerous. I believe that overstocking is one of the 



chief of them. 



I have, &c. 



Margate, July 20, 1880. (Signed) Henry Lee. 



IV. — Letter from Professor Rollestox, M.D. PROFESSOR 



ROLLESTOIS. 



Gentlemen, — 



I have little to add about the diseased condition of the various 

 specimens of diseased salmon lately sent to me besides and beyond what is 

 contained in the letters already published in " Land and Water"* by myself, by 

 Mr. William Hatchett Jackson, F.L.S., and Mr. A. Heneage Cocks, after 

 examination of specimens as they come to this museum. 



There are, however, one or two more or less general remarks which I will 

 take this opportunity of making, as they arise more or less directly out of the 

 observations recorded in those letters, or out of the reading which those letters 

 led me into. 



The problem we have before us is the possibility of abating the violence 

 and intensity of a fungoid disease which we have some reason for thinking 

 is never wholly absent from our fresh waters, but which we have the best 

 reason for knowing is just now very unusually destructive. 



The first remark which anybody with experience of epidemics affecting our 

 own species will make is, that similar alternations between intensity and obso- 

 lescence similarly characterise these latter epidemics. Cholera is probably 

 never wholly absent in any one year from the Ganges Valley, but it is only 

 occasionally that we have it intensified into the proportions it assumed in 

 the year when it struck Lord Hastings' army. The salmon disease epidemic 

 of 1880 may be paralleled by the cholera epidemic of 1817. 



The increase of the disease in either case must be held, under all the circum- 

 stances, to have been due either to some quantitative increase in the amount of 

 the poisonous agents or to some qualitative intensification of their malignity. 

 For there is no special reason to assign any very large share in the causation 

 of the increase in question to any increase in the predisposition of the passive 

 subjects of the attack. 



Firstly, as regards the alternative which might seek to explain the increase 

 is the activity for mischief of the salmon-fungus (Saproleynia ferax) by the 

 hypothesis of some qualitative increase in its modus operandi, I have to say 

 that there does not appear to me to be any reason for adopting this view. It 

 is true enough that the malignity of certain epidemics affecting ourselves may 

 be illustrated and made more intelligible, if indeed not explained, by the cases 

 in which variation or sporting as of a peach into a nectarine has been observed 

 to intensify the peculiar properties of a well-marked and otherwise presumably 

 stable species. And secondly, in the way of experiment, it has been discovered 

 that by successive transplantation into successive hosts the virulence of septic 

 agents may be greatly increased. But I do not think that these parallelisms 

 apply here, however suggestive they may be as to the phenomena of other 

 epidemics. For the working of the salmon-fungus appears to me to be mainly 

 of a mechanical kind ; it smothers rather than poisons ; it chokes rather than 

 intoxicates ; it interferes with the functions of the gills in the way that a wet 

 blanket properly applied might interfere with the functions of our lungs rather 

 than in the way in which other fungoid diseases are reasonably supposed to act, 

 namely, by throwing a quantity of poisonous material into the circulation 

 to work on what it finds there, and by subsequently, in some cases, entering 

 it themselves. And its operation on the external integument is similarly 

 mechanical ; the mycelium of the fungus lives at the expense of the fish's 

 integument ; it may spread from head to tail in as short a period as three 

 days in a thick feltings ; and, as nothing can come of nothing, this growth of 



* See " Land and Water," Jan. 21, 1880, p. 74; Jan. 31, 1880 ; April 10, Prof. Rolleston ; April 

 10, 1880, Mr. W. H. Jackson ; April o, 1880, Mr. A. Heneage Cocks. 



P712. H 



