IN THE LAST HALF-CENTURY. 129 



But within that time, they have already 

 rendered highly important services by the 

 exploration of the phenomena of parasi- 

 tism. Not only have the history of the 

 animal parasites, such as the tapeworms 

 and the trichina, which infest men and 

 animals, with deadly results, been cleared 

 up by means of experimental investiga- 

 tions, and efficient modes of prevention 

 deduced from the data so obtained ; but 

 the terrible agency of the parasitic fungi 

 and of the infinitesimally minute microbes, 

 which work far greater havoc among 

 plants and animals, has been brought to 

 light. The 'particulate' or 'germ' the- 

 ory of disease, as it is called, long since 

 suggested, has obtained a firm foundation, 

 in so far as it has been proved to be true 

 in respect of sundry epidemic disorders. 

 Moreover, it has theoretically justified pro- 

 phylactic measures, such as vaccination, 

 which f ormerly rested on a merely empiri- 

 cal basis ; and it has been extended to 



