BULLETIN OF THE UNITED STATES FISH COMMISSION. 447 



presence of already diseased fish is obviously one of tlie most important. 

 A large fully diseased salmon may have as much as two square feet of 

 its skin thickly covered with Sajyrolegnia, and its crop of spores may be 

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

 assumed that 40 such salmon might furnish one spore to the gallon for 

 all the water of the Thames which flows over Teddington Weir (3S0,- 

 000,000 gallons) in the course of a day. 



In 1878, 3j0 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 difiiculty is to under- 

 stand 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 are thus widely diffused in an infected river, not a fish which as- 

 cends 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 tlie epider- 

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

 third place, the delicate hypha which it sends out must bore its way 

 through the ejiidermis into the derma. 



Each of these conditions of successful infection may be modified in 

 endless ways of which we know nothiug — by the state of the epidermis 

 of the fish; by the motility and the general vital energy of the spore; 

 by the composition of the water, and especially bj' that of its gaseous 

 and acid or alkaline contents. 



To take only one of these conditions. If the spores germinate within 

 the zoosporangia, or are not locomotive after they leave it, their chances 

 of diffusion, and hence of reaching a healthy fish, will be vastly less 

 than if they are locomotive, for even a short time. And again, their 

 chances will be far less if they germinate after the first locomotive state, 

 which lasts only a few minutes, than if they enter into the second loco- 

 motive state, which may endure for four and twenty hours or more. So, 

 if the salmon Saprolegnia produces oosporangia in tbe late summer, and 

 these lie dormant at the bottom until the following sju-ing, the chances 

 of infection of fresh-run fish will be greater than they will be if the con- 

 tinuance of the existence of the Saprolegnia, through the whiter, de- 

 pends upon the accident of a sufficient sui)ply of dead organic sub- 

 stances. 



Moreover, any one who has practised the cultivation of Saprolegnia is 

 familiar with the difficulties which arise from the swarms of Infusoria 

 and Bacteria which devour, or otherwise destroy, the fungus, notwith- 

 standing all his efforts to preserve it. 



The struggle for existence rages among fungi as elsewhere; and the 



* Stirling, "Proceedings Royal Society of Edinburgh, " vol. is, p. 728. 



