While it has generally been recognised that bacteria are plants, any further classification has proved a matter of great difficulty, and bacteriologists find it extremely difficult to devise means of distinguishing species.  Their extreme simplicity makes it no easy matter to find points by which any species can be recognised.  But in spite of their similarity, there is no doubt that many different species exist.  Bacteria which appear to be almost identical, under the microscope prove to have entirely different properties, and must therefore be regarded as distinct species.  But how to distinguish them has been a puzzle.  Microscopists have come to look upon the differences in shape, multiplication, and formation of spores as furnishing data sufficient to enable them to divide the bacteria into genera.  The genus Bacillus, for instance, is the name given to all rod-shaped bacteria which form endogenous spores, etc.  But to distinguish smaller subdivisions it has been found necessary to fall back upon 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.

Variation of bacteria.