The number of all the various kinds of living creatures is so enormous that it would be impossible to study them profitably, were they not classified in an orderly manner. Therefore the whole mass has been divided, in the first place, into two supreme groups, fancifully termed kingdoms—the "animal kingdom" and the "vegetal kingdom." Each of these is subdivided into an orderly series of subordinate groups, successively contained one within the other, and named sub-kingdoms, classes, orders, families, genera and species. The lowest group but one is the "genus," which contains one or more different kinds termed "species," as e.g., the species "wood anemone" and the species "blue titmouse." The lowest group of all—a species—may be said to consist of individuals which differ from each other only by trifling characters, such as characters due to difference of sex, while their peculiar organization is faithfully reproduced by generation as a whole, though small individual differences exist in all cases.
The vegetal, or vegetable, kingdom, consists of the great mass of flowering plants, many of which, however, have such inconspicuous flowers that they are mistakenly regarded as flowerless, as is often the case with the grasses, the pines, and the yews. Another mass, or sub-kingdom, of plants consists of the really flowerless plants, such as the ferns, horsetails (Fig. 1), lycopods, and mosses. Sea and fresh-water weeds (algæ), and mushrooms, or "moulds," of all kinds (fungi), amongst which are the now famous "bacteria," constitute a third and lowest set of plants.
The animal kingdom consists, first, of a sub-kingdom of animals which possess a spinal column, or backbone, and which are known as vertebrate animals. Such are all beasts, birds, reptiles, and fishes. There are also a variety of remotely allied marine organisms known as tunicates, sea-squirts, or ascidians (Fig. 2). There is, further, an immense group of arthropods, consisting of all insects, crab-like creatures, hundred-legs and their allies, with spiders, scorpions, tics and mites. We have also the sub-kingdom of shell-fish or molluscs, including cuttle-fishes, snails, whelks, limpets, the oyster, and a multitude of allied forms. A multitudinous sub-kingdom of worms also exists, as well as another of star-fishes and their congeners. There is yet another of zoophytes, or polyps, and another of sponges, and, finally, we have a sub-kingdom of minute creatures, or animalculæ, of very varied forms, which may make up the sub-kingdom of Protozoa, consisting of animals which are mostly unicellular.
Multitudinous and varied as are the creatures which compose this immense organic world, they nevertheless exhibit a very remarkable uniformity of composition in their essential structure. Every living creature from a man to a mushroom, or even to the smallest animalcule or unicellular plant is always partly fluid, but never entirely so. Every living creature also consists in part (and that part is the most active living part) of a soft, viscid, transparent, colorless substance, termed protoplasm, which can be resolved into the four elements, oxygen, hydrogen, nitrogen and carbon. Besides these four elements, living organisms commonly contain sulphur, phosphorus, chlorine, potassium, sodium, calcium, magnesium and iron.
In the fact that living creatures always consist of the four elements, oxygen, hydrogen, nitrogen and carbon, we have a fundamental character whereby the organic and inorganic (or non-living) worlds are to be distinguished, for as we have seen, inorganic bodies, instead of being thus uniformly constituted, may consist of the most diverse elements and sometimes of but two or even of only one.
Again, many minerals, such as crystals, are bounded by plain surfaces, and, with very few exceptions (spathic and hematite iron and dolomite are such exceptions) none are bounded by curved lines and surfaces, while living organisms are bounded by such lines and surfaces.
Yet, again, if a crystal be cut through, its internal structure will be seen to be similar throughout. But if the body of any living creature be divided, it will, at the very least, be seen to consist of a variety of minute distinct particles, called "granules," variously distributed throughout its interior.
All organisms consist either—as do the simplest, mostly microscopic, plants and animals—of a single minute mass of protoplasm, or of a few, or of many, or of an enormous aggregation of such before-mentioned particles, each of which is one of those bodies named a "cell" (Fig. 3). Cells may, or may not, be enclosed in an investing coat or "cell-wall." Every cell generally contains within it a denser, normally spheroidal, body known as the nucleus.