370 LAY SERMONS, ADDRESSES, AND REVIEWS. [xv. 



lymph by tlie 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 solid particles, to which the title of mi- 

 crozymes 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 Torulce 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 modifica- 

 tion and individualisation of the tissues of the body in 

 which they are found, resulting from the operation of 

 certain conditions? Are they parasites in the zoolo- 

 gical 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 theo- 

 retical point of view. A parasite may be stamped out 

 by destroying its germs, but a pathological product 

 can only be annihilated by removing the conditions 

 which give rise to it. 



It appears to me that this great problem mil have 

 to be solved for each zymotic disease separately, for 

 analogy cuts two ways. I have dwelt upon the anal- 

 ogy of pathological modification, which is in favour of 

 the xenogenetic origin of microzymes ; but I must now 



