1872. Bollinger, spores in anthrax. 1872-75. Cohn, definite classification. 1873. Obermeier, recurrent fever, _Spirochæta obermeieri_. 1873. Schlösing and Münz, nitrification due to organisms. 1875. Lösch, amebic dysentery, _Amœba coli_. 1875-76. Tyndall, germs in the air. 1876. Robert Koch, anthrax, _Bacillus anthracis_. _The first instance of a bacterium shown to be the cause of disease in an animal._ 1877. Bollinger, actinomycosis, _Actinomyces bovis_ (_Streptothrix bovis_). 1877. Weigert, used anilin dyes for staining. 1877. Woronin, cabbage disease, _Plasmodiophora brassicæ_. _The first instance of a protozoan shown to be the cause of a disease in a plant._ 1878. Koch, wound infections, bacterial in origin. 1881. Koch, gelatin plate cultures, Abbé, improvements in the microscope. CHAPTER I. POSITION--RELATIONSHIPS. Bacteria are considered to belong to the plant kingdom not because of any one character they possess, but because they most nearly resemble organisms which are generally recognized as plants. While it is not difficult to distinguish between the higher plants and higher animals, it becomes almost, if not quite, impossible to separate the lowest, forms of life. It is only by the method of resemblances above mentioned that a decision is finally reached. It has even been proposed to make a third class of organisms neither plants nor animals but midway between in which the bacteria are included, but such a classification has not as yet been adopted. In many respects the bacteria are most nearly related to the lowest _algæ_, since both are unicellular organisms, both reproduce by transverse division and the forms of the cell are strikingly similar. The bacteria differ in one important respect, that is, they do not contain _chlorophyl_, the green coloring matter which enables all plants possessing it to absorb and break up carbon dioxide in the light, and hence belong among the fungi. Bacteria average much smaller than even the smallest algæ. Bacteria are closely connected with the _fission yeasts_ and the _yeasts_ and _torulæ_. All are unicellular and without chlorophyl. The bacteria, as has been stated, reproduce by division but the others characteristically by budding or gemmation, though the fission yeasts also by division. There is a certain resemblance to the _molds_ in their absence of chlorophyl. But the molds grow as branching threads and also have special fruiting organs for producing spores as a means of reproduction, neither of which characteristics is found among the _true_ bacteria. The higher thread bacteria do show true branching and rudimentary fruiting bodies (Streptothrix) and appear to be a link connecting the true bacteria and the molds. [Illustration: FIG. 8.--A thread of blue-green algæ.] [Illustration: FIG. 9.--A thread of small blue-green algæ.] [Illustration: FIG. 10.--A thread of bacteria. Compare with Figs. 8 and 9.]