xv.] SPONTANEOUS GENERATION. 371 



speak of the equally strong analogies in favour of the 

 origin of such pestiferous particles by the ordinary pro- 

 cess 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 can- 

 not 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 con- 

 tagious and infectious diseases caused by microscopic 

 Fungi. 



In autumn, it is not uncommon to see flies, motion- 

 less upon a window-pane, with a sort of magic circle, 

 in white, drawn around them. On microscojnc exam- 

 ination, the magic circle is found to consist of innu- 

 merable spores, which have been thrown off in all 

 directions by a minute fungus called Em/pusa muscw, 

 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 Mnpusa. If 

 traced back to its earlier stages, in flies which are still 

 active, and to all appearance healthy, it is found to 

 exist in the form of minute corpuscles which float in 

 the blood of the fly. These multiply and lengthen 

 into filaments, at the ex|)ense of the fly's substance ; 

 and, when they have at last killed the patient, they 

 grow out of its body and give off spores. Healthy 

 flies shut up with diseased ones catch this mortal dis- 

 ease and perish like the others. A most competent 

 observer, M. Cohn, who studied the development of 



