242 CRITIQUES AND ADDRESSES. [x. 



particles, which have grown and multiplied where they 

 have been planted ? The observations of M. Chauveau, 

 extended and confirmed by Dr. Sanderson himself, appear 

 to leave no doubt upon this head. Experiments, similar 

 in principle to those of Helmholtz on fermentation and 

 putrefaction, have proved that the active element in the 

 vaccine lymph is non-diffusible, and consists of minute 

 particles not exceeding 2 -oTTo f an i ncn i n diameter, 

 which are made visible in the lymph by the microscope. 

 Similar experiments have proved that two of the most 

 destructive of epizootic diseases, sheep-pox and glanders, 

 are also dependent for their existence and their propa- 

 gation upon extremely small living solid particles, to 

 which the title of microzymes is applied. An animal 

 suffering under either of these terrible diseases is a 

 source of infection and contagion to others, for precisely 

 the same reason, as a tub of fermenting beer is capable 

 of propagating its fermentation by " infection," or 

 " contagion," to fresh wort. In both cases it is the 

 solid living particles which are efficient ; the liquid 

 in which they float, and at the expense of which they 

 live, being altogether passive. 



Now arises the question, are these microzymes the 

 results of Homogenesis, or of Xenogenesis ; are they 

 capable, like the Tor nice of yeast, of arising only by 

 the development of pre-existing germs ; or may they 

 be, like the constituents of a nut-gall, the results of a 

 modification and individualization of the tissues of the 

 body in which they are found, resulting from the opera- 

 tion of certain conditions ? Are they parasites in the 

 zoological sense, or are they merely what Virchow has 

 called " heterologous growths " ? It is obvious that this 

 question has the most profound importance, whether 

 we look at it from a practical or from a theoretical 

 point of view. A parasite may be stamped out by 



