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.

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 running water in even greater quantity than in standing water. This is simply because running streams are being constantly supplied with water which has been washing the surface of the country and thus carrying off all surface accumulations. Lakes or reservoirs, however, by standing quiet allow the bacteria to settle to the bottom, and the water thus gets somewhat purified. They are in the air, especially in regions of habitation. Their numbers are greatest near the surface of the ground, and decrease in the upper strata of air. Anything which tends to raise dust increases the number of bacteria in the air greatly, and the dust and emanations from the clothes of people crowded in a close room fill the air with bacteria in very great numbers. They are found in excessive abundance in every bit of decaying matter wherever it may be. Manure heaps, dead bodies of animals, decaying trees, filth and slime and muck everywhere are filled with them, for it is in such places that they find their best nourishment. The bodies of animals contain them in the mouth, stomach, and intestine in great numbers, and this is, of course, equally true of man. On the surface of the body they cling in great quantity; attached to the clothes, under the finger nails, among the hairs, in every possible crevice or hiding place in the skin, and in all secretions. They do not, however, occur in the tissues of a healthy individual, either in the blood, muscle, gland, or any other organ. Secretions, such as milk, urine, etc., always contain them, however, since the bacteria do exist in the ducts of the glands which conduct the secretions to the exterior, and thus, while the bacteria are never in the healthy gland itself, they always succeed in contaminating the secretion as it passes to the exterior. Not only higher animals, but the lower animals also have their bodies more or less covered with bacteria. Flies have them on their feet, bees among their hairs, etc.

In short, wherever