In plant life, also, the same principle holds. As long as a grain of corn, wheat, etc., is kept in a dry place, the life principle stored up within the seed is unable to manifest itself in growth. When, on the other hand, it is appropriately stimulated by water, heat, and light, the seed awakens to life, or germinates. In other words, the seed reacts upon the external stimulations of water, heat, and light, and manifests the activity known as growth, or development. Thus all physical growth, whether of the plant or the animal, is conditioned on the energizing of the inherent life principle, in response to appropriate stimulation of the environment.
[Illustration: A. Simple amoeba. B. An amoeba developing as a result of assimilating food. C. An amoeba about to divide, or propagate.]
=B. Development in Human Life.=--In addition to its physical nature, human life has within it a spiritual law, or principle, which enables the individual to respond to suitable stimulations and by that means develop into an intelligent and moral being. When, for instance, waves of light from an external object stimulate the nervous system through the eye, man is able, through his intelligent nature, to react mentally upon these stimulations and, by interpreting them, build up within his experience conscious images of light, colour, and form. In like manner, when the nerves in the hand are stimulated by an external object, the mind is able to react upon the impressions and, by interpreting them, obtain images of touch, temperature, and weight. In the sphere of action, also, the child who is stimulated by the sight of his elder pounding with a hammer, sweeping with a broom, etc., reacts imitatively upon such stimulations, and thus acquires skill in action. So also when stimulated by means of his human surroundings, as, for example, through the kindly acts of his mother, father, etc., he reacts morally toward these stimulations and thus develops such social qualities as sympathy, love, and kindness. Nor are the conditions of development different in more complex intellectual problems. If a child is given nine blocks on which are printed the nine digits, and is asked to arrange them in the form of a square so that each of the horizontal and the vertical columns will add up to fifteen, there is equally an inner growth through stimulation and response. In such a case, since the answer is unknown to the child, the problem serves as a stimulation to his mind. Furthermore, it is only by reacting upon this problem with his present knowledge of the value of the various digits when combined in threes, as 1, 6, 8; 5, 7, 3; 9, 2, 4; 1, 5, 9; etc., that the necessary growth of knowledge relative to the solution of the problem will take place within the mind.