[14] This enumeration of the elements that enter into the structure of plants and animals is taken from Professor F.J. Allen's paper already referred to. [15] _Early History of Mankind_, 2nd ed. p. 227. CHAPTER XI THE PHYSICAL CONDITIONS ESSENTIAL FOR ORGANIC LIFE The physical conditions on the surface of our earth which appear to be necessary for the development and maintenance of living organisms may be dealt with under the following headings:-- 1. Regularity of heat-supply, resulting in a limited range of temperature. 2. A sufficient amount of solar light and heat. 3. Water in great abundance, and universally distributed. 4. An atmosphere of sufficient density, and consisting of the gases which are essential for vegetable and animal life. These are Oxygen, Carbonic-acid gas, Aqueous vapour, Nitrogen, and Ammonia. These must all be present in suitable proportions. 5. Alternations of day and night. SMALL RANGE OF TEMPERATURE REQUIRED FOR GROWTH AND DEVELOPMENT Vital phenomena for the most part occur between the temperatures of freezing water and 104° Fahr., and this is supposed to be due mainly to the properties of nitrogen and its compounds, which between these temperatures only can maintain those peculiarities which are essential to life--extreme sensitiveness and lability; facility of change as regards chemical combination and energy; and other properties which alone render nutrition, growth, and continual repair possible. A very small increase or decrease of temperature beyond these limits, if continued for any considerable time, would certainly destroy most existing forms of life, and would not improbably render any further development of life impossible except in some of its lowest forms. As one example of the direct effects of increased temperature, we may adduce the coagulation of albumen. This substance is one of the proteids, and plays an important part in the vital phenomena of both plants and animals, and its fluidity and power of easy combination and change of form are lost by any degree of coagulation which takes place at about 160° Fahr.