320 SPENCER WALPOLE AND T, H. HUXLEY. 



may present notable variations. Thus, the zoospores may 

 germinate without passing into an active condition ; or they 

 may germinate immediately after they assume the first 

 quiescent state. Again, in one and the same form, anthe- 

 ridia are sometimes developed and sometimes absent. In 

 some forms, indeed, antheridia never make their appear- 

 ance and consequently fertilisation does not occur. Never- 

 theless, the unfertilised oospores germinate and produce 

 new SaprolegnicB, apparently just as well as if they were 

 fertilised. 



The commonest species of Saprolegnia has received the 

 name of S.ferax, and both Pringsheim and De Bary agree 

 that several so-called species, namely, 'S'. monoica, S. thureti, 

 and S. torulosa are merely more or less permanent varieties 

 of S. ferax ; that they are all, in fact, members of the 

 S. ferax group. 



It has been seen that the fungus which grows on diseased 

 salmon is unquestionably a species of the genus Saprolegnia ; 

 and it is commonly identified with S. ferax. But this 

 identification has rested upon very slender grounds. It is 

 practically almost impossible to determine the species of a 

 Saprolegnia until the characters of its oosporangia and of its 

 antheridia (if it have any) have been accurately made out. 

 At present not only are we without any sufficient account 

 of these organs in the salmon Saprolegnia, but it is certain 

 that they are, at most seasons, extremely rare. Mr. Stirling^ 

 speaks of having observed only four in the course of all his 

 investigations \ and not a single specimen has presented 

 itself in the considerable number of diseased salmon from 

 the Conwiiy, the Tweed, and the North Esk, which have 

 come under our observation during the last four months. 



When our inquiries commenced there was, strictly speak- 

 ing, no proof that the salmon Saprolegnia could live on 

 anything but a salmon. It was therefore quite possible that, 

 since thei"e are many species of Saprolegnia, that of the 

 salmon might be peculiar to it, just as, in the analogous 



' Mr. Stirling's valuable contributions to our knowledpje of the salmon 

 disease are contained in the ' Proceedinss of the Royal Society of Edin- 

 burgh for 187S and 1879. Mr. W. G. Smith, in a paper on tlie salniou 

 disease in the ' Gardener's Chronicle,' May 4, 1878, not only aflarms that 

 " the resting spores are common enough," but figures them. However, 

 Mr. Smith's figures of the zoosporangia are so unlike anything ordinarily 

 observed in the salmon Saprolegnia, and his statement that " the fungus 

 has invariably vanished with tlie death of the fish," is so strangely con- 

 trary to common experience, that it is difficult to know how much weight 

 ought to be attached to his observations. 



