Even with our present incomplete knowledge of what characterizes a species, it is necessary to use some names.  Bacteria are commonly given a generic name based upon their microscopic appearance.  There are only a few of these names.  Micrococcus, Streptococcus, Staphylococcus, Sarcina, Bacterium, Bacillus, Spirillum, are all the names in common use applying to the ordinary bacteria.  There are a few others less commonly used.  To this generic name a specific name is commonly added, based upon some physiological character.  For example, Bacillus typhosus is the name given to the bacillus which causes typhoid fever.  Such names are of great use when the species is a common and well-known one, but of doubtful value for less-known species It frequently happens that a bacteriologist makes a study of the bacteria found in a certain locality, and obtains thus a long list of species hitherto unknown.  In these cases it is common simply to number these species rather than name them.  This method is frequently advisable, since the bacteriologist can seldom hunt up all bacteriological literature with sufficient accuracy to determine whether some other bacteriologist may not have found the same species in an entirely different locality.  One bacteriologist, for example, finds some seventy different species of bacteria in different cheeses.  He studies them enough for his own purposes, but not sufficiently to determine whether some other person may not have found the same species perhaps in milk or water.  He therefore simply numbers them—­a method which conveys no suggestion as to whether they may be new species or not.  This method avoids the giving of separate names to the same species found by different observers, and it is hoped that gradually accumulating knowledge will in time group together the forms which are really identical, but which have been described by different observers.

Where bacteria are found.

There are no other plants or animals so universally found in Nature as the bacteria.  It is this universal presence, together with their great powers of multiplication, which renders them of so much importance in Nature.  They exist almost everywhere on the surface of the earth.  They are in the soil, especially at its surface.  They do not extend to very great depths of soil, however, few existing below four feet of soil.  At the surface they are very abundant, especially if the soil is moist and full of organic material.  The number may range from a few hundred to one hundred millions per gramme. [Footnote:  One gramme is fifteen grains.] The soil bacteria vary also in species, some two-score different species having been described as common in soil.  They are in all bodies of water, both at the surface and below it.  They are found at considerable depths in the ocean.  All bodies of fresh water contain them, and all sediments in such bodies of water are filled with bacteria.  They are in streams of