A simple experiment to show this may be performed by taking two test tubes and partly filling them with an infusion of almost any organic substance (dried leaves or hay, or a bit of meat will answer). The fluid should now be boiled so as to kill any germs that may be in it; and while hot, one of the vessels should be securely stopped up with a plug of cotton wool, and the other left open. The cotton prevents access of all solid particles, but allows the air to enter. If proper care has been taken, the infusion in the closed vessel will remain unchanged indefinitely; but the other will soon become turbid, and a disagreeable odor will be given off. Microscopic examination shows the first to be free from germs of any kind, while the second is swarming with various forms of bacteria. [Illustration: FIG. 8.--Bacteria.] These little organisms have of late years attracted the attention of very many scientists, from the fact that to them is due many, if not all, contagious diseases. The germs of many such diseases have been isolated, and experiments prove beyond doubt that these are alone the causes of the diseases in question. If a drop of water containing bacteria is examined, we find them to be excessively small, many of them barely visible with the strongest lenses. The larger ones (Fig. 8) recall quite strongly the smaller species of oscillaria, and exhibit similar movements. Others are so small as to appear as mere lines and dots, even with the strongest lenses. Among the common forms are small, nearly globular cells; oblong, rod-shaped or thread-shaped filaments, either straight or curved, or even spirally twisted. Frequently they show a quick movement which is probably in all cases due to cilia, which are, however, too small to be seen in most cases. [Illustration: FIG. 9.--_Euglena_. _A_, individual in the active condition. _E_, the red "eye-spot." _c_, flagellum. _n_, nucleus. _B_, resting stage. _C_, individual dividing, × 300.] Reproduction is for the most part by simple transverse division, as in oscillaria; but occasionally spores are produced also. CLASS III.--GREEN MONADS (_Volvocineæ_). This group of the protophytes is unquestionably closely related to certain low animals (_Monads_ or _Flagellata_), with which they are sometimes united. They are characterized by being actively motile, and are either strictly unicellular, or the cells are united by a gelatinous envelope into a colony of definite form. Of the first group, _Euglena_ (Fig. 9), may be selected as a type.