PHYSICAL ENVIRONMENT. By the terms en vironment, surroundings, conditions of life, or medium (milieu) is meant the nature or natural condition of any region or area of the earth's surface. It is the milieu ambiant—the monde ambient of Geoffrey St. Hilaire. Life consists of constant reactions and readjustments to the influence of, or changes in. the environment. and this molding of life-forms to the medium or surroundings is what we call adaptation. Were there no diversity in, no changes of, the environ ment,there would be no differentiation or speciali zation, no species, no varieties or variation. Life is dependent on motion, change, and change in environment has been the initial cause of evolu tion. This fact has been vaguely hinted at by Ilip pocrates. by Aristotle, and others down to the time of Buffon, Erasmus Darwin, Lamarck, and Geoffroy. The environment may be analyzed into molar or mechanical, physical. chemical, geologi cal, geographical conditions, food coalitions, and biological surroundings. Examples of the molar or mechanical environment are gravity, atmospheric pressure, confined space, currents of water. the action of wind. Under the head
of the physical environment may be comprised electricity, light, darkness, temperature. sound. To the ehemieal nature of food-materials. odors, tasks, etc., animals respond, the physical and chemical stimuli setting the evolutionary ball in motion. or starling the machinery of life in the seed, the egg. and in the primary modes of reproduction, such as fission, budding, and eell division, the gases in the air, the degree of saltness or freshness of the water. The in fluence of food, its nature and amount, over feeding and fasting, cannot be overestimated. The geological changes—those of the physical geography or topography of a region, changes in altitude, moisture and dryness—directly af fect plant and animal life, as do changes of the seasons. Some of these changes tend to build up, others to drag down the organism. See EVOLU