This matter is made even more confusing by the fact that any species of bacterium may show more or less variation.  At one time in the history of bacteriology, a period lasting for many years, it was the prevalent opinion that there was no constancy among bacteria, but that the same species might assume almost any of the various forms and shapes, and possess various properties.  Bacteria were regarded by some as stages in the life history of higher plants.  This question as to whether bacteria remain constant in character for any considerable length of time has ever been a prominent one with bacteriologists, and even to-day we hardly know what the final answer will be.  It has been demonstrated beyond peradventure that some species may change their physiological characters.  Disease bacteria, for instance, under certain conditions lose their powers of developing disease.  Species which sour milk, or others which turn gelatine green, may lose their characters.  Now, since it is upon just such physiological characters as these that we must depend in order to separate different species of bacteria from each other, it will be seen that great confusion and uncertainty will result in our attempts to define species.  Further, it has been proved that there is sometimes more or less of a metamorphosis in the life history of certain species of bacteria.  The same species may form a short rod, or a long thread, or break up into spherical spores, and thus either a short rod, or a thread, or a spherical form may belong to the same species.  Other species may be motile at one time and stationary at another, while at a third period it is a simple mass of spherical spores.  A spherical form, when it lengthens before dividing, appears as a short rod, and a short rod form after dividing may be so short as to appear like a spherical organism.

With all these reasons for confusion, it is not to be wondered at that no satisfactory classification of bacteria has been reached, or that different bacteriologists do not agree as to what constitutes a species, or whether two forms are identical or not.  But with all the confusion there is slowly being obtained something like system.  In spite of the fact that species may vary and show different properties under different conditions, the fundamental constancy of species is everywhere recognised to-day as a fact.  The members of the same species may show different properties under different conditions, but it is believed that under identical conditions the properties will be constant.  It is no more possible to convert one species into another than it is among the higher orders of plants.  It is believed that bacteria do form a group of plants by themselves, and are not to be regarded as stages in the history of higher plants.  It is believed that, together with a considerable amount of variability and an occasional somewhat long life history with successive stages, there is also an essential constancy.  A systematic classification has been made which is becoming more or less satisfactory.  We are constantly learning more and more of the characters, so that they can be recognised in different places by different observers.  It is the conviction of all who work with bacteria that, in spite of the difficulties, it is only a matter of time when we shall have a classification and description of bacteria so complete as to characterize the different species accurately.