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 torulz ; 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 
clear 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 
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