By mechanical improvements in microscopes, and subtle methods for colouring bacteria, enabling us to study their form with precision, by ingenious devices for supplying them with suitable food materials, or, in other words, by the creation of bacterial nurseries, providing the means for watching their growth and observing their distinctive habits and character, this important branch of the vegetable kingdom has been raised from obscurity to one of the principal places in our catalogue of sciences, and Bacteriology has won for itself an individual footing in the scientific curriculum of our great educational institutions, and is represented in literature by such famous serials devoted to the publication of bacterial and allied researches as the _Annales de l'Institut Pasteur_, the _Centralblatt für Bakteriologie_, the _Zeitschrift für Hygiene_, the _Annali d'Igiene Sperimentale_, and other well-known journals which constitute an essential but ever-increasing burden upon the library shelves as well as pocket of the investigator. Museums of bacteria have been established where not only specimens of particular varieties of a permanent character for comparison and reference can be obtained, but living cultivations of hundreds of different micro-organisms are maintained; and only those who have had the charge of bacteria can realise the enormous amount of skilled labour involved in the catering for such a multitude, in which individual likes and dislikes in regard to diet and treatment must, if success is to be secured, be as carefully considered as is necessary in the case of the most delicate and highly pampered patient. Bacteria, by means of these depôts, can, in fact, be bought or exchanged by collectors with as much facility as postage stamps, with the all-important difference that this collecting of bacteria is not a mere mania or speculation, but serves a most useful purpose. To the busy investigator who cannot afford either the time or space in which to maintain a large bacterial family, it is of immense convenience to be able to obtain at a moment's notice a trustworthy culture, say, of typhoid or tuberculosis, or specimens of obscurer origin from air or water for purposes of investigation. These bacterial cultures are all guaranteed pure, free from contamination or admixture with other and alien micro-organisms, and are strictly what they are represented to be. Although such a declaration is attached to many commodities at the present day with ludicrous incongruity, in the case of micro-organisms such a breach of faith is unknown, and the antecedents of a microbe may be said to be regarded as of as much moment and to be as jealously preserved as is the pedigree of the most ambitious candidate for honours at a cattle or dog show!