182 ON THE SALMON DISEASE. 



observed on decaying bodies in salt water. Thus it becomes, to 

 say the least, a highly probable conclusion that the origin of the 

 disease is to be found in the Saproiegftice which infest dead organic 

 bodies in our fresh waters. Neither drought, pollution, nor over- 

 stocking will produce the disease, so long as Saprolegjiia is absent ; 

 though doubtless these conditions will favour its development or 

 diffusion wherever the fungus already exists. 



The results, then, of observations and experiments recently 

 made appear to justify the following conclusions : — 



I. — That the Saprolegnia attacks the healthy, living salmon 

 exactly in the same way that it attacks the dead insect ; and that 

 it is the sole cause of the disease, whatever other circumstances 

 may — in a secondary degree— assist its operations. 



2. — That death may result, without any other organ than the 

 skin being attacked ; and that, under these circumstances, it is the 

 consequence partly of the exhaustion of nervous energy through 

 the incessant irritation of the felted mycelium, with its charge of 

 fine sand, — partly of the drain of nutriment directly and indirectly 

 caused by the fungus. 



3.— That the penetration of the hyphce of the Saprolegnia into 

 the skin renders it at least possible that the disease may break out 

 in a fresh-run salmon without re-infection. 



4. — That Saprolegnia^ the cause of the disease, may flourish in 

 any fresh water, in the absence of salmon, as a saprophyte upon 

 dead insects and other animals. 



5. — That the chances of infection for a healthy fish entering a 

 river are enormously increased by the existence of diseased fish in 

 that river ; since the bulk of Saprolegnia on a few diseased fish 

 greatly exceeds what would exist there without them. 



6. — That, as in the case of the potato disease, the careful 

 extirpation of every diseased fish is the treatment theoretically 

 indicated ; though it may not be worth while in practice to adopt 

 that treatment. 



