244 CRITIQUES, AND ADDRESSES. [x. 



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 observer, M. Cohn, who studied the develop- 

 ment of the Empusa 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 cultivation ; nor 

 were such germs discoverable in the air, or in the food 

 of the fly. It looked exceedingly like a case of Abio- 

 genesis, or, at any rate, of Xenogenesis ; and it is only 

 quite recently that the . real course of events has been 

 made out. It 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," because a 

 healthy fly coming in contact with a diseased one, 

 from which the spore-bearing filaments protrude, is 

 pretty sure to carry off a spore or two. It is " infec- 

 tious" because the spores become scattered about all 

 sorts of matter in the neighbourhood of the slain 

 flies. 



The silkworm has long been known to be subject to a 

 very fatal and infectious disease called the Musc:irdin& 

 Audouin transmitted it by inoculation. This disease is 

 entirely due to the development of a fungus, Botrytis 

 Bassiana, in the body of the caterpillar ; and its con- 

 tagiousness and infectiousness are accounted for in the 

 same way as those of the fly-disease. But, of late years, 

 a still more serious epizootic has appeared among the 



