The kingdom of nature is divided into organic and inorganic bodies. Organic bodies have life; inorganic bodies are without life. Organic bodies are composed of several reciprocal parts, each of which is necessary to, and dependent upon, all the other parts. Organic bodies, therefore, include plants and animals, and inorganic bodies include earths, metals, and minerals. Organic bodies spring from some parent or immediate producing agent; they are supported by means of nourishment, and die without it; they increase in size by the addition of new particles of matter to all parts of their substances. Inorganic bodies are formed by some chemical law or union, and grow only by addition to their surfaces.
Organic and inorganic bodies are continually wasting away or wearing out. Waste takes place in all objects, animate and inanimate. The minutest change in position in any plant, animal, or rock cannot be effected without some loss of substance. It has always been beyond the power of man to make anything that would not wear out. But there is this important distinction between organic and inorganic bodies. Only organic bodies can repair their waste, and add to their substance; they alone have life, or vital force. When anything wears out in a stone or a steam-engine, there is no power in the stone or the engine to replace the lost matter; and when a plant or an animal dies, the power of repairing waste is gone from it forever. Hence it is with animate bodies, or bodies endowed with life, that we have to do in considering the subject of food.
Life is that form of energy in creation that results in development from within the object. The energy may be purely physical, as in plants; or it may involve mental and moral considerations, as in animals.
There are some essential distinctions between the various forms of organic life. Animals grow proportionally in all directions, and, at a certain time of life, attain their average size. Plants grow upwards and downwards from a collet only, and continue to grow through a term of existence. Animals feed upon organic matter, consume oxygen from the air, and throw off carbonic acid; plants feed upon inorganic matter, consume carbonic acid, and restore oxygen to the air.
Living plants or vegetables are, with few exceptions, fixed to the spot of earth from which they spring, and receive their nourishment from external sources. It is furnished them by the soil, air, light, and heat by which they are surrounded; and they are every moment receiving all that is necessary for their sustenance. If one of these essential conditions be withdrawn, death follows. Living animals have the power of locomotion, and, being obliged to wander, they are not always directly in contact with their sources of nourishment. They have, therefore, a storehouse in which they lay up at intervals a supply of material. The possession of this stomach, or storehouse, characterizes all animal beings.
The changes that occur in animal life are more rapid and variable than those in vegetable life. Not being, like vegetables, always in connection with their food, animals need some monitor to warn them when to seek it. This is provided them in the appetite, or the sensations of hunger and thirst. There is also a pleasure in the regulated indulgence of these sensations, which never fails to insure attention to their demands.
The vegetable kingdom is the original source of all organic matter. All our food is derived directly from the vegetable world, or indirectly through animals which have been nourished on vegetable products. The ox and sheep, which are consumed in the form of beef and mutton, have not fed on flesh, but on grass, hay, oats, and other grains. It is only under exposure to the sun's rays that plants will grow. Hence to its influence we must refer the production of food in the first instance, and therefore the sustenance of all life.
Life and growth in human beings are dependent upon two conditions, - motion and warmth.