VIII BIOGENESIS AND ABIOGENESIS 261 



element in the vaccine lympli is non-diffusible, 

 and consists of minute particles not exceeding 

 Y oooo ^^ of an inch in diameter, which are made 

 visible in the lymj)h 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 propagation upon extremely small living 

 soHd particles, to which the title of microzymes is 

 appHed. 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 pro- 

 pagating 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 microzjTnes 

 the results of Homogenesis^ or of Xcnogenesis ? are 

 they capable, like the Tm^ulce of yeast, of arising 

 only by the development of pre-existing germs? 

 or may they be, hke the constituents of a nut-gall, 

 the results of a modification and individuahsation 

 of the tissues of the body in which they are 

 found, resulting from the operation 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. 



