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
The first circumstance to be considered in relation to the habitability of a planet is its distance from the sun. We know that the heating power of the sun upon our earth is ample for the development of life in an almost infinite variety of forms; and we have a large amount of evidence to show that, were it not for the equalising power of air and water, distributed as they are with us, the heat received from the sun would be sometimes too great and sometimes too little. In some parts of Africa, Australia, and India, the sandy soil becomes so hot that an egg can be cooked by placing it just below the surface. On the other hand, at an elevation of about 12,000 feet in lat. 40° it freezes every night, and throughout the day in all places sheltered from the sun. Now, both these temperatures are adverse to life, and if either of them persisted over a considerable portion of the earth, the development of life would have been impossible. But the heat derived from the sun is inversely as the square of the distance, so that at half the distance we should have four times as much heat, and at twice the distance only one-fourth of the heat. Even at two-thirds of the distance we should receive more than twice as much heat; and, considering the facts as to the extreme sensitiveness of protoplasm and the coagulation of albumen, it seems certain that we are situated in what has been termed the temperate zone of the solar system, and that we could not be removed far from our present position without endangering a considerable portion of the life now existing upon the earth, and in all probability rendering the actual development of life, through all its phases and gradations, impossible.
The Obliquity of the Ecliptic