64 The Fungus of Salmon Disease. 



even when isolated cases of the disease occur, they may be 

 pretty evenly distributed in currents ; also, that they settle on 

 the banks or on weeds, and in quiet places, from which, however, 

 they are always liable to be displaced by floods and carried out 

 to sea, or buried in muddy deposits. It is however, probable 

 that there is in all open rivers a continuous reproduction going 

 on, as the fungus will grow on almost any animal matter (and 

 I am assured that it will grow freely on vegetable matter also, 

 but I have had no actual experience on this point) ; and, con- 

 sidering the immense number of spores liberated, even from the 

 growth on a small fly, it is difficult to imagine their absence 

 from any open water. In the dry condition the distribution is, 

 I fear, also very probable. The point does not appear to have 

 been studied, yet it is the most likely way of accounting for a 

 general distribution of the fungus, and may, perhaps, have 

 caused the disease to spread to neighbouring rivers, where, 

 possibly, it had become extinct from starvation or other causes. 

 Let me here describe an experiment of my own that touches 

 this question. 



At Christmas time last year I intended leaving home for 

 some little while, and believing my cultures of the fungus might 

 be destroyed during my absence, I thought of collecting some 

 zoospores on paper, by filtration. Accordingly, I poured off the 

 water from a cultivation of the fungus on a fly, through filter 

 paper, feeling assured that the water was charged with zoospores, 

 which would be retained by the paper, while the water, of course, 

 readily passed through it. Afterwards the filter-paper was tied 

 up and hung on the wall of a dry room. My cultivations were 

 then all thrown away, and for a full month I had no time to 

 test the vitality of my dried zoospores ; when I did so the result 

 was rather surprising, as full development occurred on a suitable 

 object in the usual time, and is continuing to-day from the 

 same source.* 



* I have not been able to confirm these observations on the resisting vitality 

 of the zoospores of Sap. ferax to the conditions of dryness, although I tried to 

 do so. The conditions of culture at the time of these second experiments were, how- 

 ever, unfavourable, and I do not wish to withdraw anything from the details of an ob- 



