There are two kingdoms, an Animal and a Vegetable, to one of which everything that lives may be assigned with more or less certainty. The contrast between the higher or multicellular animals and plants is too great and constant both in an anatomical and physiological sense to leave room for doubt. An animal possesses the power of locomotion; it has a compact form, a special digestive organ for the reception of solid food, and it is unable to utilise Carbon dioxide as a source of carbon for the production of carbohydrates; it has organs of special sense with a nervous system, specialised contractile cells or muscular tissue, complex excretory products, and in the majority special organs of excretion, lymph or blood, with circulatory organs, etc. Its tissue cells, with few exceptions, are not isolated or all but isolated by closed and firm cell-envelopes. Supposing that it is fixed, its other characteristics remain; if it takes on a branched or plant-like mode of growth, examination shows that it consists of a connected multitude of typical animals, each one perfect in itself. A few instances are known in which chlorophyl bodies are present; and it appears probable that they enable the organism to utilise Carbon dioxide for the preparation of starch under the influence of light.
Whether, however, the chlorophyl bodies are in all these instances intrinsic parts of the organism is a matter of dispute (pp. 242-5). A typical multicellular plant, on the other hand, is either branched, and it then consists of a root with a stem, bearing a number of repeated organs, the leaves, which are subject to modification, or it is compact and its cells very similar one to another. It is fixed; it has chlorophyl bodies, which under the influence of light enable it to utilise Carbon dioxide as a source of carbon; it is able to build up protoplasm and therefore derive tissue elements from the Carbon dioxide of the air, ammonia, nitrates and mineral constituents of the soil; it is devoid of digestive organs, of special sense organs, nervous system, excretory organs, special contractile tissue. Its tissue cells become isolated completely or all but completely by closed and firm cell-envelopes. It may require for food partly elaborated material or fairly complex organic compounds in solution, and may then be devoid of chlorophyl bodies, but its other characters remain unchanged. If it has a motile initial stage the course of development proves its plantlike nature.
In those rare instances, i. e. in Insectivorous plants where solid food is digested and the products of digestion utilised, the process of digestion is carried on externally to the organism, and absorption takes place by the outer surface.
What is true of multicellular animals and plants is true, within limitations, of unicellular. There are of course in a unicellular animal no specialised systems of organs such as the digestive, for example, but power of locomotion remains, and the natural irritability, automatism and contractility of the protoplasm are very strongly developed. Solid organic food is ingulfed within the protoplasm and is broken down, giving rise to fat, albumen, glycogen or other starchy bodies as in higher animals; it also leaves generally a faecal residue, and there is reason to think that complex and sometimes crystalline excretory products are formed. But the organism may in some cases utilise organic food in solution, in other words it is saprophytic, e.g. some Flagellata, and probably the My-cetozoa; in other cases, e. g. the Flagellate Euglena, owing to the presence of chlorophyl bodies, nutrition becomes holophytic or completely plantlike. In these instances recourse can be had only to considerations of structure, life-history, comparison with other forms, or the behaviour of the doubtful organism under altered conditions of life. Good examples of these considerations may be drawn from Flagellata and My cetozoa.
The position however of some few forms, e. g. the Volvocina, remains a matter of doubt, and they are claimed by botanists and zoologists alike. Their nutrition is holophytic, and their structure is paralleled in undoubted vegetable organisms1
However complex in structure a multicellular animal or plant may be, it can be traced without exception to an origin from a single cell. Many animals, the whole group known as Protozoa, and many plants never attain a higher degree of morphological complexity than a single cell. But in some Protozoa, at any rate, that cell possesses highly developed vital energies and a corresponding specialisation of parts. In its simplest aspect a cell may be defined as a mass of protoplasm (cytoplasm) containing one or more nuclei. It has been shown that non-nucleated masses of protoplasm, derived from nucleated, are in the Protozoa capable of growth in size, but they have no power of reproduction. On the other hand, there are a few Protozoa (certain Proteomyxa) which appear to consist really of non-nucleated protoplasm, capable not only of growth but reproduction. They may be distinguished as 'cytods' from the cell to which a nucleus is essential. It is possible however that the elements of the nucleus are in these cases disseminate.
Protoplasm, or 'the physical basis of life,' is a substance of complex chemical composition containing Nitrogen, Carbon, Oxygen and Hydrogen, with Sulphur, Phosphorus, Sodium and Potassium. From the physical point of view it is viscid, of variable refrangibility, more or less doubly refractile, colourless, hyaline in its purest condition. It appears sometimes to be structureless, but as a rule it is more or less vesicular, consisting of a denser substance (mitome) enclosing droplets of a more fluid character (enchylema, para-mitome), and it is endowed with certain physiological properties, the sum of which constitute life. It is contractile, irritable, possessed of automatism, able to convert other protoplasm or less complex compounds, sometimes organic only, sometimes only inorganic, into its own substance. And this nutrition not only maintains the status quo, but if over sufficient for that purpose leads first to the storage of superfluous material in the shape of fat, albumen and starchy bodies; and secondly, causes a positive increase of bulk, with which is connected the power of reproduction in its most primitive form - a division of the mass into two similar parts.