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easy life of civilization, brought up a large family, and 

 destroyed or consumed no more than was necessary for 

 the support of the community; while the ferment thrust 

 into the recesses of the vat and forced to tear oxygen 

 from the material around him, figuratively speaking, cuts 

 down a tree in order to cook a dinner, and destroys a 

 forest in order to obtain a little breathing space. As the 

 savage may acquire strength and ferocity by his mode of 

 life, so it may be inferred that the germ actually acquires 

 virulence by exercise in its anaerobic mode of life. The 

 question is, does this virulence become fixed by heredity, 

 in any case, in such a way as to amount to the establishment 

 of a new species, with peculiar attributes which will enable 

 it, not only to tear oxygen as a saprophyte from dead organic 

 matter, but as a parasite from living tissues. For it must 

 be borne in mind that, so far as reliable experiment has yet 

 gone, the process absolutely stops at this point. The trans- 

 formation of the harmless saprophyte into the deadly animal 

 disease has not yet been conclusively shown. 



12. — Modern scientific ideas and discoveries do not so much 

 displace old ideas as spring from them. There is usually a 

 certain basis of experience for the old ideas, and experience 

 is really a basis of fact, which must be true. Hence it is 

 natural that inquirers into the variability of germs and their 

 pathogenic relations should turn to oxygen as being likely 

 to play an important part in this connection. The influence 

 of oxygen as a purifier of water attracted the attention of Dr. 

 Angus Smith long before Pasteur's attenuation experiments. 

 As he himself has explained, this was a natural consequence 

 of the ideas of the older chemists as to the influence of 

 oxygen as the active agent of decay. Again, the value of 

 ventilation and fresh air in cases of consumption was insisted 

 upon by medical men long before Koch's discovery of the 

 Bacillus tuberculosis. From the beginning of his inquiries 

 Pasteur has been strongly disposed to regard oxygen as an 



