Animals and Vegetables.--The first classification of living creatures separates them into two great kingdoms, animals and vegetables. Although it is very easy to define the general characteristics of each of these classes, it is impossible to fix upon any single peculiarity which will be applicable in every case. Most vegetable organisms remain stationary, while some possess organs of locomotion, and swim about in the water in a manner much resembling the movements of certain animals. Most vegetables obtain their nutriment from the earth and the air, while animals subsist on living matter. A few plants seem to take organic matter for food, some even catching and killing small insects. It is found impossible to draw the precise line between animals and vegetables, for the reason just mentioned. The two kingdoms blend so intimately that in some cases it is impossible to tell whether a certain microscopic speck of life is an animal or a vegetable. But since these doubtful creatures are usually so minute that several millions of them can exist in a single drop of water, it is usually of no practical importance whether they are animal or vegetable, or sometimes one and sometimes the other, as they have been supposed to be by some biologists. All living creatures are _organized_ beings. Most possess a structure and an organism more or less complicated; but some of the lowest forms are merely little masses of a transparent, homogeneous jelly, known as protoplasm. Some of the smallest of these are so minute that one hundred millions of them could occupy the space of a cube one-thousandth of an inch on each side; yet each one runs its course of life as regularly as man himself, performing its proper functions even more perfectly, perhaps.