2.60 ANNUAL OF SCIENTIFIC DISCOVERY. 



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 bo 

 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 favor of the xenogenetic origin of microzymes; but I must 

 now speak .of the equally strong analogies in favor of the origin 

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

 eration 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 micro- 

 scopic 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 round them. On microscopic examination, the magic 

 circle is found to consist of innumerable spores, which have been 

 thrown off in all directions by a minute fungus called Empusa 

 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 condi- 

 tion of the Empusa. 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 expense 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 disease and perish like the others. A most competent ob- 

 server, M. Conn, who studied the development of the Empusa in 

 the fly very carefully, was utterly unable to discover in what 

 manner the smallest germs of the Empusa got into the fly. The 

 spores could not be made to give rise to such germs by cultiva- 

 tion, nor were such germs discoverable in the air, or in the food of 

 the fly. It looked exceedingly like a case of abiogenesis, or, at 

 any rate, of xenogenesis ; and it is only quite recently that the 

 real course of events has been made out. Jt has been ascertained 

 that when one of the spores falls upon the body of a fly, it begins 

 to germinate and sends out a process which bores its way through 

 the fly's skin ; this, having reached the interior cavities of its 

 body, gives off the minute floating corpuscles which are the 

 earliest stage of the Empusa. The disease is ' contagious,' be- 

 cause a healthy fly coming in contact with a diseased one, from. 



