a somewhat anomalous fact that a special branch of science,
interesting such a large number of people, should be developed
around a small group of low plants. The importance of bacteriology
is not due to any importance bacteria have as plants or as members
of the vegetable kingdom, but solely to their powers of producing
profound changes in Nature. There is no one family of plants that
begins to compare with them in importance. It is the object of
this work to point out briefly how much both of good and ill we
owe to the life and growth of these microscopic organisms. As we
have learned more and more of them during the last fifty years, it
has become more and more evident that this one little class of
microscopic plants fills a place in Nature's processes which in
some respects balances that filled by the whole of the green
plants. Minute as they are, their importance can hardly be
overrated, for upon their activities is founded the continued life
of the animal and vegetable kingdom. For good and for ill they are
agents of neverceasing and almost unlimited powers.

HISTORICAL.

The study of bacteria practically began with the use of the
microscope. It was toward the close of the seventeenth century
that the Dutch microscopist, Leeuwenhoek, working with his simple
lenses, first saw the organisms which we now know under this name,
with sufficient clearness to describe them. Beyond mentioning
their existence, however, his observations told little or nothing.
Nor can much more be said of the studies which followed during the
next one hundred and fifty years. During this long period many a
microscope was turned to the observation of these minute
organisms, but the majority of observers were contented with