SPONTANEOUS CONGESTION AND INFECTIOUS DISEASES. 761 
logical product can only be 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 
favour of the xenogenetic origin of microzymes ; but I must now speak of 
equally strong analogies in favour of the origin of such pestiferous particles 
by the ordinary process 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 ail 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 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 fnngus called Empusa muscce, the spore-forming filaments of which 
stand out like a pile of velvet from the body of a 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 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 ex- 
pense 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 
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 begins to ger- 
minate, 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 “infectious,” because the spores 
become scattered about all sorts of manner in the neighbourhood of the slain 
flies. 
The silkworm has long been known to be subject to a very fatal contagious 
and infectious disease called the muscardine. Audouin transmitted it by 
inoculation. This disease is entirely due to the development of the fungus, 
botrytis bassiana , in the body of the caterpillar ; and its contagiousness and 
infectiousness are accounted for in the same way as those of the fly-discase. 
But of late years a still more serious epizootic has appeared among the silk- 
worms ; and I may venture a few facts which will give you some conoeption 
of the gravity of the injury which it has inflicted on Trance alone. The pro- 
duction of silk has been for centuries ati important branch of industry in 
Southern France, and in the year 1853 it had attained such a magnitude, 
that the annual produce of the Trench sericulture was estimated to amount 
to a tenth of that of the whole world, and represented a money value of 
