198 SCIENCE AND THE HUMAN MIND 



recent years, it is well to trace the whole story at 

 this point. 



It has been found that, in some cases at all events, 

 it is the presence of a definite substance in the microbic 

 cells, or its production by their activity, that causes 

 the changes associated with their life. Thus in 1897 

 Biichner discovered the first-known of these active 

 substances or enzymes in yeast cells, and showed that, 

 when extracted from them, it could cause the same 

 fermentation that living yeast cells produce. Ap- 

 parently the enzyme itself remains unchanged at the 

 end of the operation ; its mere presence is sufficient 

 to start and maintain the chemical action. 



At the end of the eighteenth century, Jenner had 

 introduced the process of vaccination, by which in- 

 oculation with the virus of small-pox, after its atten- 

 uation into cow-pox by transmission through a calf, 

 causes partial or complete immunity to the severer 

 form of the disease. This principle of attenuation 

 was extended to other diseases by Pasteur, and, in 

 the case of rabies or hydrophobia, it was shown that 

 inoculation was generally effectual even after infection. 

 The mortality from this horrible and previously in- 

 curable disease was thus reduced to about one per cent, 

 of the cases treated ; while in England the rigid en- 

 forcement of a muzzling order for dogs for a short 

 period, and subsequent quarantine precautions against 

 the reintroduction of the specific poison from the 

 Continent, has freed the country from the shadow of 

 this dread scourge. 



The life-history of these pathogenic organisms is 

 often very complicated, and some of them pass certain 

 stages of their career in different hosts. Only by 



