SAPROLEGNIA IN RELATION TO THE SALMON DISEASE. 333 



third [)lace, the delicate hypha which it sends out must bore 

 its way through the epidermis into the derma. 



Each of these conditions of successful infection may be 

 modified in endless ways of which we know nothing — 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 by that of its gaseous and acid 

 or alkaline contents. 



To take only one of these conditions. If the spores ger- 

 minate within the zoosporangia, or ave not Idcomotive after 

 they leave it, their chances of diffusion, and hence of receiv- 

 ing a healthy fish, will be vastly less than if they are loco- 

 motive, 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 locomotive state, which may endure for four- 

 and-twenty hours or more. So, if the salmon Saprolegnia 

 produces oosporangia in the late summer, and these lie 

 dormant at the bottom until the following spring, the 

 chances of infection of fresh-run fish will be greater than 

 they will be if the continuance of the existence of the 

 Saprolegnia through the winter depends upon the accident 

 of a sufficient supply of dead organic substances. 



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, notwithstanding all his 

 efforts to preserve it. 



The struggle for existence rages among fungi as elsewhere; 

 and the question whether a salmon which enters water in 

 which Saprolegnia is present shall be infected or not depends 

 upon the mutual adjustment of a vast variety of conflicting 

 agencies. Until we have learned something more than we 

 at present know of these agencies, and of the history of 

 the salmon Saprolegnia itself, there can be no thoroughly 

 safe foundation for any view which may be put forward as 

 to the best mode of dealing with the disease. 



Nevertheless, since it is evident that every diseased salmon 

 which remains in a river must immensely increase the 

 chances of the infection of the healthy fish in that river, 

 the policy of extirpating every diseased fish appears, theo- 

 retically, to be fully justified. But whether, in endeavouring 

 to carry such a policy into effect in any given river, the cost 

 would not exceed the loss from the disease, is a point 

 which must be left for the consideration of Boards of 

 Conservators; 



