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 The effect of the obliquity of the earth's equator to its path round the sun, upon which depend our varying seasons and the inequality of day and night throughout all the temperate zones, is very generally known. But it is not usually considered that this obliquity is of any great importance as regards the suitability of the earth for the development and maintenance of life; and it seems to have been passed over as an accident hardly worth notice, because almost any other obliquity or none at all would have been equally advantageous. But if we consider what the direction of the earth's axis might possibly have been, we shall find that it is really a matter of great importance from our present point of view.