330 SPENCER WALPOLE AND T. H. HUXLEY. 



period of three or four months, that is to say, sufficient to 

 preserve them till the next return of the salmon. 



On all these points persons conversant with the use of the 

 microscope, who are resident in the neighbourhood of salmon 

 streams, might obtain information of great value, hardly to 

 be procured in any other way. 



Although all the evidence leads to the conclusion that the 

 Saproleffniais the immediate and primary cause of the salmon 

 disease, and that, in the absence of the fungus, the disease 

 never makes its appearance, however polluted the water may 

 be, or however closely the fish may be crowded; yet, in this, 

 as in other epidemics caused by parasitic organisms, the pre- 

 valence 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 disease. 



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

 weather is extremely unfavorable to the growth and diffusion 

 of the Paronospora. In such a season a plant n>ay tie 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 Avith extraordinary rapidity, and field after field is 

 devastated as if struck by a sudden blight. So with the 

 epidemic disorders of mankind. In a large town isolated 

 cases of smallpox, measles, diphtheria, and the like, con- 

 stantly 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 effect and gives 

 rise to an epidemic. 



At a moderate estimate the Saprolegnia 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, the 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 zoo- 

 spore to the cubic inch of twenty cubic feet of water. Even 

 if we halve this rate of production it is easy to see that the 

 SaprolegnicB 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 adheres 

 to the surface of the 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 infection for the fish which enter it. 



