the ninth decade of the century these methods were simplified by
Robert Koch. This simplification of method for the first time
placed this line of investigation within the reach of scientists
who did not have the genius of Pasteur. It was now possible to get
pure cultures easily, and to obtain with such pure cultures
results which were uniform and simple. It was now possible to take
steps which had the stamp of accuracy upon them, and which further
experiment did not disprove. From the time when these methods were
thus made manageable the study of bacteria increased with a
rapidity which has been fairly startling, and the information
which has accumulated is almost formidable. The very rapidity with
which the investigations have progressed has brought considerable
confusion, from the fact that the new discoveries have not had
time to be properly assimilated into knowledge. Today many facts
are known whose significance is still uncertain, and a clear
logical discussion of the facts of modern bacteriology is not
possible. But sufficient knowledge has been accumulated and
digested to show us at least the direction along which
bacteriological advance is tending, and it is to the pointing out
of these directions that the following pages will be devoted.

WHAT ARE BACTERIA?

The most interesting facts connected with the subject of
bacteriology concern the powers and influence in Nature possessed
by the bacteria. The morphological side of the subject is
interesting enough to the scientist, but to him alone. Still, it
is impossible to attempt to study the powers of bacteria without
knowing something of the organisms themselves. To understand how
they come to play an important part in Nature's processes, we must