One of the greatest contributions ever made to scientific knowledge is Herbert Spencer's profound and luminous exposition of Life as the continuous adjustment of inner relations to outer relations. The extreme simplicity of the subject in its earliest illustrations is such that the student at first hardly suspects the wealth of knowledge toward which it is pointing the way. The most fundamental characteristic of living things is their response to external stimuli. If you come upon a dog lying by the roadside and are in doubt whether he is alive or dead, you poke him with a stick; if you get no response you presently conclude that it is a dead dog. So if the tree fails to put forth leaves in response to the rising vernal temperature, it is an indication of death. Pour water on a drooping plant, and it shows its life by rearing its head. The growth of a plant is in its ultimate analysis a group of motions put forth in adjustment to a group of physical and chemical conditions in the soil and atmosphere. A fine illustration is the spiral distribution of leaves about the stem, at different angular intervals in different kinds of plants, but always so arranged as to ensure the most complete exposure of the chlorophyll to the sunbeams. Every feature of the plant is explicable on similar principles. It is the result of a continuous adjustment of relations within the plant to relations existing outside of it. It is important that we should form a clear conception of this, and a contrasted instance will help us. Take one of those storm-glasses in which the approach of atmospheric disturbance sets up a feathery crystallization that changes in shape and distribution as the state of the air outside changes. Here is something that simulates vegetable life, but there is a profound difference. In every one of these changes the liquid in the storm-glass is passive; it is changed and waits until it is changed again. But in the case of a tree, when the increased supply of solar radiance in spring causes those internal motions which result in the putting forth of leaves, it is quite another affair. Here the external change sets up an internal change which leads to a second internal change that anticipates a second external change. It is this active response that is the mark of life.