The last fifteen years have, however, seen a wonderful change. The difficulties had been mostly those of methods of work, and with the ninth decade of the century these methods were simplified by Robert Koch. This simplification of method for the first time placed this line of investigation within the reach of scientists who did not have the genius of Pasteur. It was now possible to get pure cultures easily, and to obtain with such pure cultures results which were uniform and simple. It was now possible to take steps which had the stamp of accuracy upon them, and which further experiment did not disprove. From the time when these methods were thus made manageable the study of bacteria increased with a rapidity which has been fairly startling, and the information which has accumulated is almost formidable. The very rapidity with which the investigations have progressed has brought considerable confusion, from the fact that the new discoveries have not had time to be properly assimilated into knowledge. Today many facts are known whose significance is still uncertain, and a clear logical discussion of the facts of modern bacteriology is not possible. But sufficient knowledge has been accumulated and digested to show us at least the direction along which bacteriological advance is tending, and it is to the pointing out of these directions that the following pages will be devoted. WHAT ARE BACTERIA? The most interesting facts connected with the subject of bacteriology concern the powers and influence in Nature possessed by the bacteria. The morphological side of the subject is interesting enough to the scientist, but to him alone. Still, it is impossible to attempt to study the powers of bacteria without knowing something of the organisms themselves. To understand how they come to play an important part in Nature's processes, we must know first how they look and where they are found. A short consideration of certain morphological facts will therefore be necessary at the start. FORM OF BACTERIA. In shape bacteria are the simplest conceivable structures. Although there are hundreds of different species, they have only three general forms, which have been aptly compared to billiard balls, lead pencils, and corkscrews. Spheres, rods, and spirals represent all shapes. The spheres may be large or small, and may group themselves in various ways; the rods may be long or short, thick or slender; the spirals may be loosely or tightly coiled, and may have only one or two or may have many coils, and they may be flexible or stiff; but still rods, spheres, and spirals comprise all types.