IxXXvi REPORT 1 870. 



It is, at present, a -svell-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 in- 

 fectious 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 

 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 musca, the spore-forming filaments of wliich 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, 

 ha\-ing 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 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 observer, M. 

 Cohn, 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 cultivation ; 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. It has been ascertained, that when one of the 

 spores falls upon the body of a fly, it begius 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 oft' 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 

 " 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 conta- 

 gious and infectious disease called the Miiscardine. Audouin transmitted it 

 by inoculation. This disease is entirely due to the development of a fungus, 

 Boirytis 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 sUkwoiTQS ; and I may mention a few facts which wiR 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 important 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 estimated 

 to amount to a tenth of that of the whole world, and represented a money 

 value of 117,000,000 of francs, or nearly five millions sterling. What may 

 be the sum which would represent the money-value of all the industries con- 

 nected with the working up of the raw silk thus produced is more than I can 

 pretend to estimate. Suffice it to say, that the city of Lyons is built upon 



