114 



TLcjp7y<rr!®T* ^ ne f un g us entails a corresponding loss of substance in the way of erosion 



"° ' and alteration of the fish. These lesions of course put the functions of the 



skin into abeyance, and call out reactions in the fish affected in the way of 

 rubbing the sores, &c, but they are not due to anything of the nature of a 

 qualitatively toxic agent. 



The problem we have to deal with, then, reduces itself to an inquiry as to 

 the causes and as to the arrest of the quantitative increase of a particular 

 fungus which infests and affects salmon much as many Perorosporece and 

 Mucorini, and notably the Hernileia vastatrix, which produces the " coffee- 

 leaf-disease," and the Pellicularia kolerog a, which produces the "coffee rot," 

 infest and affect plants. Instances of increase in the numbers of individuals of 

 species which in another year may be comparatively rare are very numerous. 

 Everybody has observed how in one year some species of insect, say such as the 

 Humming Bird Moth, may be as abundantly as pleasantly present, whilst in 

 another it may never be seen except by a skilled entomologist familiar with 

 its special haunts and habits. Of course the less pleasant presence inter- 

 mittently of insect plagues is another exemplification of the same rising and 

 falling off in the numbers of the same species, and other exemplifications are 

 not far to seek in other sub-kingdoms of animal and plant life. Upon what 

 do these more or less chronometrically measurable increases depend ? As I 

 imagine in most cases, and know in some, upon a diminution in the numbers 

 of the natural enemies of the organisms, animal or vegetable, thus inter- 

 mittently increasing. Insects multiply into plagues when birds have been 

 reduced in number by frosts, and rabbits desolate a district which has lost the 

 protection of domestic and wild carnivora, cats, ferrets, and mustelines gene- 

 rally. The natural enemies of the fungi are, I suspect, those of their own 

 household ; other forms, in this particular case, and still lower forms of fungoid 

 life. But how to command these lower forms, or how to favour their mul- 

 tiplication, I have no very definite idea. Still I very strongly incline to 

 thinking that it is in this direction that we ought to turn our eyes and our 

 microscopes, if we are to obtain proper notions as to the causation, not to 

 say as to the prevention, of the salmon disease. It does not seem to me 

 that any greater abundance of fungus food, or of other favouring conditions 

 of the surrounding medium, is likely to be the cause of the increase in the 

 disease-producing organism. For the salmon is the food of the fungus, and as 

 far as we know there is no other host concerned with this lodgment or 

 sustentation. Indeed, the Suprolegnia which multiplies both sexually and 

 asexually, goes through both modes of self-multiplication whilst growing on 

 or over-growing the " king of fish," as well as other less dignified and less 

 migratory occupants of fresh water. In the language of botanists, it is not 

 " heterokisch ;" it does not, like the wheat rust, infest the cereal at one stage of 

 its existence, and a Rhamnns or Berberis at another ; nor like Roestelia, infest 

 a juniper (sabina) at one time, and the pear at another; nor finally, like the 

 fluke of the sheep rot, infest the sheep in the sexual, and the snail, or, as I 

 believe, a slug in its non-sexual stage. The concatination of mischief which 

 we have to deal with here is a shorter and, I fear, a stronger chain ; at any 

 rate it has much fewer links, and we therefore have much fewer chances of 

 striking one out and setting our clients free. 



I have net gathered from my observation and reading any hint that inorganic 

 agencies, such as climatic changes, chemical pollutions, or any other forces 

 unconnected with living operations, except perhaps the lowering of the water, 

 have very much to do with even favouring, not to say with producing, the 

 increase of the malady. 



Not do I think that predisposition counts for very much in the matter 

 either. No doubt an animal in a depressed state, such as that of a " spent 

 fish," is more liable to contract many diseases, though this cannot be said 

 to have been made out with reference to some of the diseases dependent on 

 eontagia viva, which affect our own species. No doubt organs and tissues in 

 a depressed state of vitality will succumb to the septic action of lower fungi 

 than our Saprolegnia more readily than will organs in full force, but then we 

 have not here to deal with septic, but with mechanical work ; and as a matter 

 of fact, perfectly healthy salmon, as well as perfectly healthy roach, dace, and 

 gudgeon, young perch, and young pike, may be killed with the all-embracing 

 but cruel kindness of Saprolegnia. The fact that age protects the two last- 



