376 
Proceedings of the Royal Society 
probably a slower way, is when the fungus grows over and closes up 
the opercular openings of the gills, which seems to be the way those 
specimens have been suffocated, being shrouded while alive in fatal 
fungus, they have died in their beauty, with their silvery skins 
unbroken. 
There is one fact connected with the Ightham epidemic, namely, 
that the large pike, perch, and eels were not affected by the fungus 
disease. I am unable to account for this immunity on physiological 
principles, and refer it to the hypothesis of the “struggle for 
existence and survival of the fittest.” It would be difficult to find 
anywhere a purer collection of water than the Ightham ponds. The 
main stream, upper pond, and stews, being virgin spring water, 
uncontaminated with any pollution, so that I am convinced that S. 
ferax can and does exist where no source of pollution is present, and 
exercises its destructive influence upon the fish as is evidenced by 
the numerous deaths in the epidemic of 1874 in the Ightham upper 
pond. Up to the present time, it has generally been held that 
fungus epidemic, or, as it has been called, salmon disease, was con 
fined to and had its origin in rivers frequented by the migratory 
Salmonidce. At an early stage of the inquiry, Sir Robert Christison 
referred to this, and urged that if possible it should be ascertained 
whether the disease had ever been observed in the head waters of 
any salmon river above any impassable obstruction, either natural or 
artificial. The epidemic at Ightham moat and ponds answers the 
question Sir Robert desired to be cleared up, and proves that S. 
ferax is not confined to rivers frequented by salmon. 
In a former paper, I stated that the so-called salmon disease did 
not depend upon a pre-existing functional disorder in the fish. I 
am still of this opinion, and point to the fish from Ightham as a 
further proof that this is the case. I also stated my belief that S. 
ferax existed at all times and probably in all waters, and that the 
presence of fish and S. ferax in the same water under certain climatic 
or other at present unknown influence, seems all that is necessary to 
originate fungus epidemic. 
The epidemic at Ightham in my opinion does away with the 
theories of overcrowding, including overstocking. Overcrowding of 
salmon in a pool in a river is not analagous to overcrowding of 
people in a room or in a prison cell, where only a certain amount of 
