and in a few days give rise to an abundant crop of the 

 fungus. 



It is not only possible, but probable, that there are yet 

 other forms totally different from the ordinary Saprolegnia, 

 under which the mould may continue its existence. The 

 recent investigations of Brefeld on the torula condition of 

 many fungi (Ustilago, Tilletid), known hitherto only as 

 parasites on plants, are very significant in connection with 

 this question. 



Permit me now to sum up, in a few propositions, the 

 present state of our knowledge respecting the salmon 

 disease : 



1. The sole cause of the disease is the fungus Saprolegnia 

 ferax, which burrows into and destroys the skin of the 

 fish. 



2. This fungus habitually lives on dead organic matter, 

 and exists only in fresh water. 



3. The Saprolegnia is propagated by zoospores, oospores, 

 and possibly also in other ways. The zoospores and the 

 oospores give rise either to the saprophyte Saprolegnia, 

 which lives at the expense of dead organic matter, or to 

 the parasite Saprolegnia, which lives at the expense of 

 living freshwater fishes. 



4. The zoospores of the Saprolegnia, grown on fish, attack 

 dead flies, and the zoospores of the Saprolegnia, grown on 

 flies, attack living fish. 



5. It follows, therefore, that the existence of the cause 

 of salmon disease, or, to speak more generally, of the 

 integumentary mycosis of freshwater fishes, is independent 

 of the existence of fishes ; and consequently that the extir- 

 pation of all the diseased fish in a river does not involve 

 the extirpation of the cause of the disease in that river. 



6. There is reason to believe that the Saprolegnia exists 

 VOL. vi. c. C 



