l6 PATHOLOGICAL MYCOLOGY. 



are constantly found in relation to the tissues of their hosts, which 

 are in each case typical, and in which they find conditions' adapted 

 to their requirements, and favourable to the development of their 

 products. 



Selective Affinities. 



13. The inquirer into the relations between micro-organisms and 

 disease is met at the very outset by a more difficult question. What 

 peculiarity in the micro-organism itself, or in its host, determines its 

 development or non-development in the tissues of the infected animal ? 

 And how is it that micro-organisms appear to exercise a certain 

 selective power, growing readily in one animal, but remaining inactive 

 in another? 



Why should the anthrax bacillus attack man and not the pig, an 

 ox or a mouse, but not a dog? Why also should the tubercle 

 bacillus have a special affinity for certain animals, and not for others. 

 How do the house-mouse and field-mouse differ, that in one, " mouse 

 septicaemia " is readily induced, whilst in the other, this is seldom or 

 never the case ? 



Klein found, as a result of his experiments on swine plague, that 

 he could produce this disease by inoculation in rabbits and mice, 

 but that it is impossible to obtain similar results in man or birds, in 

 the guinea-pig or carnivorous animals. 



He further found that infusions of the flesh of any of these groups 

 of animals served equally well as a nutrient material for artificial 

 cultivation of either anthrax bacillus or the bacillus of swine plague. 

 There can therefore be nothing in the dead flesh itself, which, by 

 chemical or other reactions, aids or hinders the growth of the specific 

 micro-organisms. As to the exact nature of the aiding or retarding 

 factors in the living tissues, there are as yet no exact experiments. 

 Anything as yet advanced is mere theory. Klein throws aside the 

 general vital theory, and advances the statement that "the most 

 feasible theory see^ps to me to be this, that this inhibitory power 

 is due to the presence of a chemical substance produced by the 

 living tissue.'' He therefore specialises the vital theory, but retains 

 it as a means of explaining the chemical. This is as yet incapable of 

 exact proof, but there is a Certain amount of evidence in its favour. 



