other characters, such as the shape of the colony produced in
solid gelatine, the power to produce disease, or to oxidize
nitrites, etc. Thus at present the different species are
distinguished rather by their physiological than their
morphological characters. This is an unsatisfactory basis of
classification, and has produced much confusion in the attempts to
classify bacteria. The problem of determining the species of
bacteria is to-day a very difficult one, and with our best methods
is still unsatisfactorily solved. A few species of marked
character are well known, and their powers of action so well
understood that they can be readily recognised; but of the great
host of bacteria studied, the large majority have been so slightly
experimented upon that their characters are not known, and it is
impossible, therefore, to distinguish many of them apart. We find
that each bacteriologist working in any special line commonly
keeps a list of the bacteria which he finds, with such data in
regard to them as he has collected. Such a list is of value to
him, but commonly of little value to other bacteriologists from
the insufficiency of the data. Thus it happens that a large part
of the different species of bacteria described in literature to-
day have been found and studied by one investigator alone. By him
they have been described and perhaps named. Quite likely the same
species may have been found by two or three other bacteriologists,
but owing to the difficulty of comparing results and the
incompleteness of the descriptions the identity of the species is
not discovered, and they are probably described again under
different names. The same process may be repeated over and over
again, until the same species of bacterium will come to be known
by several different names, as it has been studied by different
observers.