256 Modern Fishculture in Fresh and Salt Water. 



cause and not a mere accompaniment of the salmon 

 disease." I have treated of this in the preceding chap- 

 ter. Prof. Huxley thus sums up the present state of 

 our knowledge respecting the salmon disease : 



"i. The sole cause of the disease is the fungus 

 Saprolegnia ferax, which burrows in and destroys the 

 skin of the fish. 



"2. This fungus habitually lives on dead organic 

 matter, and only lives in fresh water." 



3 and 4 give the mode of propagation of the fungus. 



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

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

 the integumentary mycosis of fresh-water fishes, is in- 

 dependent of the existence of fishes; and consequently 

 that the extirpation of all the diseased fish in a river 

 does not involve the extirpation of the cause of the dis- 

 ease in that river. 



"6. There is reason to believe that the Saprolegnia 

 exists in its saprophytic form in most fresh waters, 

 and that it attacks the fish of most rivers occasionally. 

 In other words, the mycosis of fresh-water fishes is a 

 widespread sporadic disease. 



"7. That which it is now desirable to ascertain is the 

 nature of the influences under which the sporadic dis- 

 ease suddenly assumes an epidemic character. On 

 this point we have very little light at present, for al- 

 though there is some reason for thinking that deficient 

 oxygenation, whether produced by overcrowding or 

 otherwise, may favor the development of the disease, 

 and though it is possible that some kinds of pollution 

 may favor it, yet the disease sometimes becomes epi- 

 demic under conditions in which these two predispos- 

 ing causes are excluded; and it does not always appear 

 when they are present. 



