332 SPENCER WALPOLE AND T. H. HUXLEY. 



Again, it is known with respect to many of the common 

 moulds, such as Penicillium and Mucor, which are habitu- 

 ally saprophytes (that is to say, live on decaying organic 

 matter, as Saprolegnia does), that they flourish in certain 

 artificial solutions containing salts of ammonia. It is quite 

 possible, though whether the fact is so will have to be ex- 

 perimentally determined, that Saprolegnia is capable of 

 living under the same conditions. Fungi are also extremely 

 sensitive to slight differences in the acidity or alkalinity of 

 water, so that even apparently insignificant changes in this 

 respect may come into play as secondary conditions of 

 salmon disease. Hence, although there is not the slightest 

 ground for regarding " pollutions,"' whether they arise from 

 agricultural or from manufacturing industries, as primary 

 causes of salmon disease, they may have a most important 

 secondary influence ; they may, in fact, determine whether, 

 in any river, the disease shall be sporadic or epidemic. 



But of all the conditions which determine the increase of 

 Saprolegnia, and therefore multiply the chances of infection 

 of healthy fish, the presence of already diseased fish is 

 obviously one of the most important. A large fully diseased 

 salmon may have as much as two square feet of its skin 

 thickly covered with Saprolegnia, and its crop of spores 

 may be taken as equivalent to that of several hundred flies. 

 It may be safely assumed that forty such salmon might 

 furnish one spore to the gallon for all the water of the 

 Thames which flows over Teddington Weir (380,000,000 

 gallons) in the course of a day. 



In 1878, 350 dead salmon were taken out of a very small 

 river, the Esk,' in three days. If the zoospores which these 

 gave off had been evenly diffused through the water of the 

 Esk, the difficulty is to understand how any fish entering it 

 could escape infection. 



In fact the objection easily arises that these arguments 

 prove too much ; and that, if the Saprolegnia is the cause 

 of the disease and its spores thus widely diffused in an in- 

 fected river, not a fish which ascends that river should 

 escape the disease. 



But such an objection loses its force if it is remembered 

 that, though the Saprolegnia is the cause of the disease and 

 though a single spore is undoubtedly sufficient to kill a 

 salmon, yet, in order to produce that effect, the spore must, 

 in the first place, reach and adhere to the epidermis of the 

 salmon; in the second place, it must germinate ; and, in the 



' Stirling, 'Proceedings of Royal Society of Edinburgh.' vol. ix, p. 



726. 



