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DUST AND DISEASE. 287

that the true ferments are organized beings, which find in
the reputedferments their necessary food.

Side by side with these researches and discoveries, and
fortified by them and others, has run the germ-theory of
epidemic disease. The notion was expressed by Kircher
and favored by Linnaeus, that epidemic diseases are due
to germs which float in the atmosphere, enter the- body, and
produce disturbance by the development within the body
of parasitic life. While it was still struggling against
great odds, this theory found an expounder and a defender
in the President of this institution. At a time when most
of his medical brethren considered it a wild dream, Sir
Henry Holland contended that some form of the germ-
theory was probably true. The strength of this theory
consists in the perfect parallelism of the phenomena of con-
tagious disease with those of life. As a planted acorn
gives birth to an oak competent to produce a whole crop
of acorns, each gifted with the power of reproducing its
parent-tree; and- as thus from a single seedling a whole
forest may spring; so, it is contended, these epidemic dis-
eases literally plant their seeds, grow, and shake abroad
new germs, which, meeting in the human body their proper
food and temperature, finally take possession of whole
pOpulations. There is nothing to my knowledge in pure
chemistry which resembles the power of self-multiplication
possessed by the matter which produces epidemic disease.
If you sow wheat you do not get barley ; if you sow small-
pox you do not get scarlet fever, but small-pox indefinitely
multiplied, and nothing else. The matter of each con-
tagious disease reproduces itself as rigidly as if it were (as
Miss Nightingale puts it) dog or cat.

Parasitic Diseases of Sills-worms. Pasteur’s Researches.

It is admitted on all hands thatsome diseases are the
product of parasitic growth. Both in man and lower crea-

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