Thus, these multitudinous forms of living creatures, both animals and plants, are reducible to certain categories in harmony with their modes of life, and the relations existing between them and all surrounding influences. We may see that, without compliance with certain of such laws, their existence would be impossible, and we see that there is a general correspondence between their shape and structure on the one hand, and their environment (that is, the totality of all surrounding agencies and influences) on the other. Are we to consider that such influences are the _causes_ of their form and structure? Obviously the biological facts before us, as yet, are insufficient to enable us to give a satisfactory answer to this question. It will for the present be enough to bear in mind that by some writers the environment _is_ deemed the one and sufficient cause of all the characters of living creatures. But as yet we have not even seen what _is_ the environment. Evidently physical influences--the earth, sea, or air, light, heat, and motion--do not exhaust it. One important factor would be omitted if we neglected to note the share taken in the environment of each living creature by a multitude of other living creatures which are in various ways related to it. This question must occupy us later. But by the forms of living creatures is not meant merely their external form. Some general notion then should here at starting be obtained of their internal form--that is, of their essential structure. The minutest and probably the simplest forms of living creatures (whether plant or animal) are such as are presented by _Bacteria_,[84] the yeast-plant and _Protoccus_. Bacteria are those minute creatures the mode of origin of which in sealed infusions has been so much of late disputed, but the activity of which in promoting the decomposition of dead substances is undisputed. A _bacterium_ is a particle of protoplasmic matter, either spheroidal or oblong, or like a short rod, or shaped like a corkscrew, and bacteria may also be in the form of a short chain of spheroids, or of oblong particles, or of rods united in a zigzag manner. Their breadth may vary from the 1/30000 to 1/10000 of an inch. They may also assume quite another appearance, by surrounding themselves with a gelatinous envelope, which condition is called their _zooglæa_ state of existence. They may be readily obtained by making some hay tea, and keeping it for a day or two, when they will be found to abound in the scum which forms on the surface, and to be in active motion. In the corkscrew form, _Spirillum volitans_, each end of the body is produced into a minute hair-like process or _cilium_, and it is by the lashings of these cilia that the minute organism moves about.