The first essential for organic life is a certain very limited range of temperature. We are so accustomed to consider the change of temperature from winter to summer, from day to night, and that which occurs when we pass from the tropics to the Polar regions as being very great, that we do not realise what a small proportion such changes bear to the whole range of temperature that exists in the known universe. The absolute zero of temperature is calculated to be minus 461° F., while the heat of the sun has been determined to be over 10,000° F., and many of the stars are known to be much hotter than the sun. The actual range of temperature is therefore enormous; but any development of organic life is possible only within the very narrow limits of the freezing and boiling points of water, since within those temperatures only is the existence of liquid water possible. But a much less range than this is really required, because albumen, one of the commonest forms of protoplasm, is coagulated or solidified at a temperature of about 160° F. Now, if, as is generally believed, the earth has been once a liquid or even a gaseous mass and has since cooled to its present temperature on the surface, and the sun is undergoing a similar process of cooling, we are able to understand that the very limited range of temperature within which life development is possible implies an equally limited period of time as compared with that occupied by the whole process of solar and planetary development. [Sidenote: We Live by the Heat of the Sun] It must be understood, however, that the present temperature of the earth’s surface is due entirely to sun-heat, and that if that were withdrawn or greatly diminished the whole surface of the globe would be permanently far below the freezing point and all the oceans be frozen for a considerable depth; so that all organic life would become extinct. Under such conditions no renewed development of life would be possible; and it is therefore quite certain that the sun has actually maintained the uniform moderate temperature required, and must continue to maintain it for whatever future period man is destined to continue his existence upon the earth. But it is not only a certain amount of heat that is required, but also a sufficient quantity of light; and this implies a further restriction of conditions, because light is due to vibrations of a limited range of wave-length, and without these particular rays plants cannot take the carbon from the carbonic acid in the atmosphere, and by its means build up the wonderful series of carbon compounds, including protoplasm, which are essential for the life of animals. What is commonly termed dark heat, therefore, would not be sufficient for the development of any but the lowest forms of life, even though it produced the necessary temperature during a sufficient period of time.