The term organism, which is commonly used as having the same meaning as living being, owes its derivation to the complexity of structure common among the higher forms of life, which are made up of several distinct organs. This organic construction does not hold good as a distinguishing mark between living beings and inanimate matter, because we are acquainted with a vast number of living organisms, both plants and animals, which are not made up of organs, but are composed of a minute piece of a soft, jelly-like material, which is simply granular throughout, and devoid of structural differentiation during the life of the creature.
We may classify the general characters of living beings as follows: -
1. Structural and physical properties.
3. Activities during life (vital phenomena).
The minute structure of living beings as shown by the microscope no doubt helps to distinguish the textures of organisms from inorganic structures. Although organic textures are found to differ very widely in their characters, they are all related in one respect, namely, that at the earliest period of their existence they consist of a minute mass of a substance called Protoplasm, known as a cell. In plants a cellular structure remains obvious in all parts of the adult, no matter how much the texture may be modified by adaptation to the requirements of any given duty or function. If we examine with the microscope the leaves, bark, wood, or pith of a plant, in all of them a cellular structure can be recognized. In the less developed members of the animal kingdom, and during the initial stages in the existence of the highest animals, the textures are composed exclusively of aggregations of living cell elements. We shall shortly see that in the more fully developed condition of the higher animals, the cells become variously modified in form and function, and the protoplasm manufactures various structures adapted to the performance of the diverse functions of the different parts. In all organic textures which can be said to be living, cells are dispersed in greater or less number, and regulate their nutrition and repair.