224 EVOLUTION AND DISEASE. 



Let us take yeast for example. Torula spores exist in 

 countless myriads, and it is necessary merely to expose a 

 solution of sugar to the air in a warm place and in the 

 course of a few hours it is filled with toruls ; according 

 to the amount of sugar present in the solution depends 

 the degree of fermentation established by them, and the 

 amount of alcohol resulting from their activity. The 

 time occupied in producing it depends largely upon 

 the favouring influence of temperature. 



Thus it is clear that in this case the determining condi- 

 tions are the presence of a solution of sugar exposed to air, 

 and a suitable temperature. The spores of the yeast 

 being always around us ready to develop as soon as en- 

 - vironing conditions are favourable. So with pathogenic 

 organisms, it by no means follows because they gain 

 entrance into an organism they necessarily flourish. 

 Pasteur demonstrated in the case of silkworms that the 

 micro-organisms found in cases of flacherie are only to be 

 found among the pounded leaves in their alimentary 

 canal when the worms digested badly ; when the digestive 

 functions of silkworms are active the germs of the 

 micro-organisms are either digested or hindered in their 

 development. 



Similar conditions may be studied in man. It is now 

 cle^r that the disease known as erysipelas is due to 

 pathogenic bacteria which gain entrance into the body 

 \/through abrasions of the skin. This is more likely to 

 happen when individuals with wounds are surrounded 

 by insanitary conditions, are badly fed, and crowded 

 together. It by no means follows that every wound ex- 

 posed to the poison of erysipelas necessarily becomes 



