372 LAY SERMONS, ADDRESSES, AND REVIEWS. [xr. 



the JEmpusa in the fly very carefully, was utterly un- 

 alble to discover in what manner the smallest germs of 

 the JEmpusa got into the fly. The spores could not 

 he 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 inte- 

 rior cavities of its body, gives off the minute floating 

 corpuscles which are the earliest stage of the JEmpusa. 

 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 

 pff a spore or two. It is "infectious" 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 Muscar- 

 dine. Audouin transmitted it by inoculation. This 

 disease is entirely due to the development of a fungus, 

 JBotrytis Bassiana, in the body of the caterpillar ; and 

 its contagiousness 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 silkworms ; and I may mention a few facts 

 which will give you some conception of the gravity of 

 the injury which it has inflicted on France alone. 



The production of silk has been for centuries an im- 

 portant branch of industry in Southern France, and in 

 the year 1853 it had attained such a magnitude that 

 the annual produce of the French sericulture was esti- 



