that they have some internal structure, we must recognise that as yet microscopists have not been able to make it out.  In short, the bacteria after two centuries of study appear to us about as they did at first.  They must still be described as minute spheres, rods, or spirals, with no further discernible structure, sometimes motile and sometimes stationary, sometimes producing spores and sometimes not, and multiplying universally by binary fission.  With all the development of the modern microscope we can hardly say more than this.  Our advance in knowledge of bacteria is connected almost wholly with their methods of growth and the effects they produce in Nature.

Animals or plants?

There has been in the past not a little question as to whether bacteria should be rightly classed with plants or with animals.  They certainly have characters which ally them with both.  Their very common power of active independent motion and their common habit of living upon complex bodies for foods are animal characters, and have lent force to the suggestion that they are true animals.  But their general form, their method of growth and formation of threads, and their method of spore formation are quite plantlike.  Their general form is very similar to a group of low green plants known as Oscillaria.  Fig. 17 shows a group of these Oscillariae, and the similarity of this to some of the thread-like bacteria is decided.  The Oscillariae are, however, true plants, and are of a green colour.  Bacteria are therefore to-day looked upon as a low type of plant which has no chlorophyll, [Footnote:  Chlorophyll is the green colouring matter of plants.] but is related to Oscillariae.  The absence of the chlorophyll has forced them to adopt new relations to food, and compels them to feed upon complex foods instead of the simple ones, which form the food of green plants.  We may have no hesitation, then, in calling them plants.  It is interesting to notice that with this idea their place in the organic world is reduced to a small one systematically.  They do not form a class by themselves, but are simply a subclass, or even a family, and a family closely related to several other common plants.  But the absence of chlorophyll and the resulting peculiar life has brought about a curious anomaly.  Whereas their closest allies are known only to botanists, and are of no interest outside of their systematic relations, the bacteria are familiar to every one, and are demanding the life attention of hundreds of investigators.  It is their absence of chlorophyll and their consequent dependence upon complex foods which has produced this anomaly.

Classification of bacteria.