70 The Fungus of Salmon Disease. 



under varying conditions of temperature or aeration, consider- 

 abh- to change its accepted life history. It is just possible 

 that this may be a reason that the disease did not appear long 

 ago, or, having appeared, may be a cause of its still continuing 

 its yearly ravages. It is not unusual for kindred organisms 

 gradually to become suited to conditions that, if suddenly sup- 

 plied, would have been quite unfavourable ; and the fungus 

 of salmon disease may have become far more suited to pro- 

 duce the injury it at present causes than it formerly was, 

 owing to a gradual series of adapting reproduction in genera- 

 tions becoming slowly better able to grow upon and assimilate 

 a food to which they were previously unaccustomed or un- 

 suited. 



Our fungus has, I believe, been called Saprolegnia ferax since 

 the salmon disease became prevalent, but it had been known 

 and studied before then. Its present destructive function is, 

 then, only a lately recognised one, and one to which it may 

 have been partly trained by some favourable, but formerly 

 unusual, condition that is now present. It would be as wrong 

 to think that fish destruction is its special mission as that it 

 had lately sprung to life solely to bring about the wholesale 

 injury at present so common. It appears to me far more just 

 to think that some unstudied influence has latterly existed to 

 disturb the nice balance of favourable life conditions essential to 

 the fish, when considered with the dangerous function of the 

 parasite, and that such an unfavourable condition can alone 

 cause the fungus to thrive with a special function in rivers 

 where it was formerly harmless, and is so still in the same 

 waters for long periods of each year. 



It would be very difficult to prove, and as unsafe to infer, 

 that the fungus of salmon disease does not exist in all open 

 running waters where fish live. Such water is its natural 

 element, and its ordinary function appears to be to purify the 

 water by attacking and reducing any dead animal matter it 

 can find ; but other organisms, better suited under ordinary 

 conditions, are generally more prevalent for this work, especially 



