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398 | Address of Professor Husley. 
rule of yeast, of arising only by the development of preéxist- 
ing germs; or may they be, like the constituents of a nutgall, 
the results of a modification and individualization of the tissues 
of the body in which they are found, resulting from the opera- 
tion of certain conditions? Are they parasites in the zoologi- 
cal sense, or are they merely what Virchow has called “ hetero- 
logous growth?” 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 1 rms, but a pathological 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. 
T 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 generation 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 or- 
ganisms. 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 ani- 
mals, insects are wonderfully liable to the ravages of contagious 
and infectious diseases caused by microscopic Fung?. 
In autumn, it is not uncommon to see flies, motionless upon & 
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 musce, the 
spore-forming filaments of which stand out like a pile of velvet 
from the body of the fly. These spore-forming filaments ar 
connected with others wigs fill the interior of the fty’s body 
still active, and to all appearance healthy, it is found to exist in 
pay they grow out of its body and give off spores. Healthy 
diseased ones catch this mortal disease and 
carefully, was utterly una ple to discover in what manner the 
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