Of course, all this is rather suggestion than proof. It is possible that in a world of perpetual day or in one of perpetual night, life _might_ have been developed. But on the other hand, considering the great variety of physical conditions which are seen to be necessary for the development and preservation of life in its endless varieties, any prejudicial influences, however slight, might turn the scale, and prevent that harmonious and continuous evolution which we know _must_ have occurred. So far I have only considered the question of day and night as regards the presence or absence of light. But it is probably far more important in its heat aspect; and here its period becomes of great, perhaps vital, importance. With its present duration of twelve hours day and twelve night on the average, there is not time, even between the tropics, for the earth to become so excessively heated as to be inimical to life; while a considerable portion of the heat, stored up in the soil, the water, and the atmosphere, is given out at night, and thus prevents a too sudden and injurious contrast of heat and cold. If the day and night were each very much longer--say 50 or 100 hours--it is quite certain that, during a day of that duration, the heat would become so great as to be inimical, perhaps prohibitive, to most forms of life; while the absence of all sun-heat for an equally long period would result in a temperature far below the freezing point of water. It is doubtful whether any high forms of animal life could have arisen under such great and continual contrasts of temperature. We will now proceed to point out the special features which, in our earth, have combined to bring about and to maintain the various and complex conditions we have seen to be essential for life as it exists around us. CHAPTER XII THE EARTH IN ITS RELATION TO THE DEVELOPMENT AND MAINTENANCE OF LIFE