106 



DH. M. C. question. Unless excessive it can hardly be cause of appreciable injury in 

 COOKE. running streams. These remarks must not be construed in favour of per- 

 — mitting dead fish, suffering from this disease, to remain in proximity to the 



living. An exception which is necessary to be borne in mind. There is no 

 evidence that the fish fungus will grow upon dead quadrupeds, and all expe- 

 rience is at variance with the supposition. Hence dead dos^s and other quad- 

 rupeds do not serve as a source of the disease. Another alleged cause is the 

 presence of bacteria in diseased spots, upon which the fungus is supposed 

 afterwards to locate itself. I fear that this view cannot be maintained, as the 

 presence of bacteria is the consequence, and not the cause, of morbid tissues. 

 A third cause is that alleged by Dr. Erasmus Wilson, who is no mean authority 

 on cutaneous diseases. He says that he is "more and more convinced that 

 " the fungus is a morbid growth of the mucus, produced by the skin of a 

 " diseased animal, and not a vegetable parasite." The evident meaning of 

 this rather vague statement is, that the so-called fungus is not a true fungus, 

 or vegetable parasite, but a morbid development of the mucus. I wish that I 

 could agree with this, but I cannot, because this growth, whatever it may be, 

 produces within itself certain germs, which may be transplanted, and will 

 grow, and produce other growths just like that from which the germs were 

 taken. Even as an acorn produces an oak like the tree which produced the 

 acorn, so these germs produce a fungus like the parent from which they are 

 taken. There is no morbid growth of mucus which could do this ; but the 

 power of reproduction exhibited must belong to some distinct organism, capable 

 of reproducing its species. " Morbid growth of mucus " would simply mean 

 an unusual and diseased development of the skin of the fish, which could 

 consequently perform no vital functions of its own. 



I should be most willing to believe with those who assert that the salmon 

 disease is a contagious disease which spreads from fish to fish, producing 

 blotches or eruptions, upon which a parasite afterwards establishes itself. I 

 would accept this as a reasonable and sufficient cause, if I could find any 

 evidence in its support. It is certainly not an impossible cause, and none of 

 the evidence really contradicts it ; but I fear that is all which can be said in its 

 favour. If there'were any proof of the existence of a few fish which had 

 given evidence of the presence of this disease, but which had not suffered 

 from the fungus, it would suffice. A single fish with the skin and flesh 

 diseased in the identical manner as in the ordinary disease, but from which 

 the fungus was wholly absent, would suffice to prove that the fungus is not 

 the cause of the disease. In the absence of this single evidence I am afraid 

 I must confess that, as far as we at present know, the fungus (Saprolegnia) 

 appears to be the active agent in the salmon disease. 



Let us endeavour to ascertain how far history gives a collateral support to 

 this view. In the year 1821 Gruithuis described this fungus under the name 

 of Conferva ferax* and in 1823 it is again alluded to under the name of 

 Hydronema by Carus ;f and even at this early period it is recognised as a fish- 

 parasite, although only until lately proved to be a fungus. All the early 

 writers considered it a water-weed, Conferva, or Alga, which seems to have 

 been most common on the Water Salamander. In 1831 Nees von Esenbeck 

 referred to it, and it was probably mixed up with another species of the same 

 family, the Achlya prolifera,X which grows upon dead insects in water. 

 Passing over some intermediate writers we find in 1841 Valentin writes of it 

 as occurring " on the ova of fishes, it constitutes a very powerul preventive," 

 he says, " to their development, and its progress is so rapid that a single egg 

 " infected with it will in a very short time infest many hundreds and thus 

 " destroy them." Afterwards he adds, " in fishes also, as the Cyprinus nasus, 

 " when kept in narrow vessels, and the water not quite sweet, he observed the 

 " same fungus on all parts which might be abraded, as, for instance, the head 

 " and tail."§ Strangely coincident this, which was written 40 years ago, with 

 the facts of to-day. 



* Nova Acta Natura Curiosorum, 1821. 

 f Ibid, 1823, p. 491 



X Nova Acta Curiosorum Naturae, xv. (1831) p. 375. 



§ Valentin's Eepertorium, 1841, p. 58. Cooper's Microscopical Journal, 1841, 

 p. 155. 



