GROWTH, increase in size or volume. It may be divided into inorganic and organic growth. As an example of the former is the increase in size of minerals. Inorganic growth is not always process and hence inorganic growth is really a misnomer. Living beings or organisms grow by adding to the substances (protoplasm, etc.) forming their bodies similar matters as food, which are digested, assimilated and thus taken into the body of the plant or animal by interstitial deposit. Organic growth is thus fundamentally a physico-chemical proc ess,. a sort of synthesis of structuralized ex perience occurring in the growth mechanism. The result of this absorption of food is that the body increases in size, that is, grows. All growth is attended by movement; and growth movements are, as Verworn states, common to all living bodies, but they take place so slowly that they can scarcely be followed with the eye. Growth goes on more freely and the size of the body increases most rapidly in those organisms in which the body presents a large raying sur face, in distinction from the microscopic bodies of the one-celled plants or animals. The sim plest phenomenon of growth is seen in cells, which during growth rapidly mukiply by self division, which causes the increase in volume in the embryo.
The physical agents or factors in the growth of plants and animals are abundance of food, together with the influences exerted by heat, light and other forms of energy. During growth the simple molecule of living proteid continually attracts elements to itself from the food (Hatschek).
Food and Chemical As digestion and assimilation are chemical processes, they require certain materials to work with. These are called food. The elements which constitute food and which occur in protoplasm and flesh are carbon, oxygen, nitrogen, lime, phosphorus. potassium, sodium, chlorine, magnesium, sul phur, silicon and iron. All or any of these enter the body in various combinations, each playing a definite part in growth. Phosphorus is especially abundant in the tissues of embryos; potassium appears to be of great importance in imbibition, while iron is essential in the early processes of cell-division. Besides these inor ganic substances, organic food, as flesh or vegetables, are essential to the growth of ani mals. Water is also essential and embryos de velop most rapidly in moist places or in water.
Light.— Without light there would be no growth, indeed no life. Light may retard or hasten growth, under different circumstances. Young growing plants and embryos of animals need to be protected from too direct sunlight.
Temperature.— Organisms need sufficient heat in order to grow. The requisite amount for normal, maximum growth is called the optimum temperature, for at .such a degree of warmth growth takes place faster than at any other. If the temperature be lowered, the rate of growth gradually diminishes; if the tem perature be raised too much above the opti mum, the rate of growth diminishes more rapidly. Excess of cold dwarfs both plants and animals.
Space and Movement— If too much crowded, plants become slender and weak; snails become dwarfed if reared in too small vessels; mankind when confined to too narrow quarters in large cities tend to become under sized, from not having sufficient space and good air to live small trout live in small brooks and large ones in larger streams. All organisms need room to move or at least to grow.
Heredity.— Besides the factors already mentioned heredity has its share as an agent. Growth, development and reproduction are now in the plant and animal world proceeding as it were in grooves, or along more or less definite paths, in accordance with long established laws or relations, and the mechanism of growth is subject to heredity. Heredity is only thus an other term for past experience of the race.
Growth and Longevity.— The elephant and whale attain a colossal size because they grow throughout life and live long. The large size of man as compared with many other mammals is due to the fact that he grows for a longer period; while many mammals get their growth in one, two or three years, man does not stop growing until he is 30.
It is to he observed that individual growth is not only dependent upon a suitable amount of food, but on proper environment and favor able conditions of life, and all these agencies also are the primary factors of organic life. It is the changes in the conditions of life, coupled with heredity and selection, that have caused the evolution of the world of plants and animals. Thus we see that the fundamental causes of the evolution of species are the same as those which determine the growth of any individual organ ism; we by no means understand all the phe nomena of simple growth; there are unex plained laws and causes, as there are in general evolution; both in this respect are of a piece and are similar in their nature and results. The origin of species is as natural a process as the growth of the individual, and both in many respects are alike inexplicable by the science of the present day.