336 THE CANADIAN NATURALIST. [Sept. 



Now arises the questioD, are tliese microzymes the results of 

 Homogenesis or oi Xenogenesis ; are they capable, like the Torulce 

 of yeast, of arising only by the developement 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 operation of certain 

 conditions 1 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 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 will have to be solved 

 for each zymotic disease separately, for analogy cuts two ways. I 

 have dwelt upon the analogy of pathological modification, which is 

 in favour of the xeuogenetic origin of microzymes ; but I must 

 now speak of the equally strong analogies in favour of the origin 

 of such pestiferous particles by the ordinary process of the genera- 

 tion of like from like. 



It is, at present, a well-established fact that certain diseases, 

 both of plants and of animals, which have all the characters of 

 contagious and infecti#us epidemics, are caused by minute 

 organisms. The smut of wheat is a well-known instance of such a 

 disease, and it cannot be doubted that the grape-disease and the 

 potato-disease fall under the same category. Among animals, 

 insects are wonderfully liable to the ravages of contagious and 

 infectious diseases caused by microscopic Fungi. 



In autumn, it is not uncommon to see flies, motionless, upon a 

 window-pane, with a sort of magic circle, in white, drawn round 

 them. On microscopic examination, the magic circle is found to 

 consist of innumerable spores, which have been thrown ofi" in all 

 directions by a minute fungus called Empiisa muscce, the spore- 

 forming filaments of which stand out like a pile of velvet from the 

 body of the fly. These spore-forming filaments are connected with 

 others, which fill the interior of the fly's body like so much fine 

 wool, having eaten away and destroyed the creature's viscera. 

 This is the full-grown condition of the Empusa. If traced back 

 to its earlier stages, in flies which are still active, and to all appear- 

 ance healthy, it is found to exist in the form of minute corpuscles 

 which float in the blood of the fly. These multiply and lengthen 



