BULLETIN OF THE UNITED STATES FISH COMMISSION. 445 



Although all the evidence leads to the conclusion that the Saprolegnia 

 is the immediate and primary cause of the salmon disease, and that, in 

 the absence of the fungus, the disease never makes its appearance, how- 

 ever polluted the water may be, or however closely the fish may be 

 crowded, yet in this as in other epidemics caused by parasitic organ- 

 isms, the prevalence and the mortality of the malady, at any given 

 time and in any given place, must be determined by a multitude of 

 secondary conditions independent of the immediate cause of the dis- 

 ease. 



In the case of the potato disease, it is well known that dry weather 

 is extremely unfavourable to the growth and diffusion of the Peronos- 

 pora. In such a season a plant may be affected here or there, but cases 

 of disease are so rare that they escape notice. But if even a few days 

 of rain with a thoroughly damp atmosphere supervene, the fungus 

 spreads from plant to plant with extraordinary rapidity, and field after 

 field is devastated as if struck by a sudden blight. So with the epi- 

 demic disorders of mankind. In a hsrge town, isolated cases of small- 

 pox, measles, diphtheria and the like constantly occur, and every case 

 is the source of a vast quantity of infectious material. Nevertheless, it 

 is only under certain conditions that this infectious material takes ef- 

 fect and gives rise to an epidemic. 



At a moderate estimate, the SaproUgnke on a single dead fly may 

 carry a thousand zoosporangia. If each sporangium contains twenty 

 zoospores, and runs through the whole course of its development in twelve 

 hours, tlie result will be the production of 40,000 zoospores in the course 

 of a day, which is a number more than sufficient to furnish one zoospore 

 to the cubic inch of twenty cubic feet of water. Even if we have this 

 rate of production, it is easy to see that the Saprolegnuc on a single fly 

 may yield a sufficient abundance of zoospores to render any small and 

 shallow stream, such as salmon often ascend for spawning purposes, 

 dangerous for several days. For a single one of these spores, if it ad- 

 heres to the surface of thfe skin of a salmon and germinates, is sufficient 

 to establish the disease. Other things being alike, of course the greater 

 the quantity of Saprolegnia in a stream the greater the chances of in- 

 fection for the fish which enter it. 



In looking for the causes of an epidemic of salmon disease we have 

 therefore to inquire, in the first place, into the conditions which favour 

 the growth of the Saprolegyiia. It is known that the Saprolegniw subsist 

 not only on dead insects and on dead Crustacea and mollusks, but on 

 some other dead animal matters and on decaying plants. The particular 

 form which infests the salmon, as we have seen, flourishes as well upon 

 dead flies ; it can also be grown upon pieces of bladder, but whether it 

 can be transferred to decaying vegetable substances has yet to be de- 

 termined. 



Hence it follows that, within certain limits (active putrefaction ap- 

 pearing to be unfavourable to Sajyrolegnia), an increase of the quantity 



