x.] BIOGENESIS AND ABIOGENESIS. 243 



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 patho- 

 logical modification, which is in favour of the xeno- 

 genetic 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 generation 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 infectious 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 examina- 

 tion, the magic circle is found to consist of innumerable 

 spores, which have been thrown off in all directions by a 

 minute fungus called Empysa 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 



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