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INTROD UCTOR Y. 3

Of these four groups, bacteria command by far the greatest
interest. They are the chief agents in the production of those
intense decompositions designated as fermentation, putrefaction,
etc. By this means they not only prevent the accumulation of
dead animal and vegetable matter upon the earth's surface, but,
at the same time, produce from the complicated organic sub-
stances of the latter the simple compounds, carbonic acid (COj)
and ammonia (N'H3), which are absolutely necessary to the con-
tinuauL'e of a chlorophyl-bearing vegetation.
Without these processes all higher vegetation, and conse-
(juently all animal life, would in the course of time become ex-
tinct. Unfortunately, however, the activity of the bacteria is
by no means restricted to the performance of this important role
in the household of nature. They are also capable of exertinL^
a most deleterious influence on the living animal body, and hare
been recogoized as the exciting cause of the majority of all dis-
eases to which mankind is subject.
Mould-fungi (Moulds), although very widely distributed in
nature, do not by far play so important a part as the fission-
fungi. They do not produce as intense decompositions of organic
substances, nor, with the exception of certain cutaneous diseases,
are they capable of bringing about such disturbances in the
human body as result from the invasion of fission-fungi. Many
diseases of plants and of lower animals (insects) have, however,
been traced to the agency of mould-fungi.
Bud-fungi (yeast-fungi) exert even less disturbing influence
on human life than moulds; with the single exception of
thrush, no disease is at present known to be duo to their action.
Our knowledge of the physiological and hygienic significance
of the " animal-fungi" is, as yet, very inaccurate.
The lower Mycetozoa (Monadina) occur as parasites on algae
and fungi, as well as on higher plants. They have also been
found in enormous quantities in the human intestines, in dis-
eased as well as in healthy conditions, without it being possible
to assign to them an indubitably pathogenic action.