Since, now, the temperature of the earth-crust would much more quickly sink from 100° to 55° than it had fallen from 1000° to 100°, we may imagine that only a few thousands of years may have intervened between the formation of the first crust of the earth and the cooling down to a temperature such as would sustain life. Since that time the temperature has probably never been so low that the larger portion of the earth’s surface would not have been able to support organisms, although there have been several glacial ages in which the arctic districts inaccessible to life must have extended much farther than at present. The ocean will also have been free of ice over much the greatest portion of its surface at all times, and may therefore have been inhabited by organisms in all ages. The interior of the earth cools continually, though slowly, because heat passes from the inner, warmer portions to the other, cooler portions through the crust of the earth. The earth is able to serve as the abode of living beings because its outer portions are cooled to a suitable temperature (below 55°) by radiation, and because the cooling does not proceed so far that the open sea would continually be frozen over, and that the temperature on the Continent would always remain below freezing-point. We owe this favorable intermediate stage to the fact that the radiation from the sun balances the loss of heat by radiation into space, and that it is capable of maintaining the greater portion of the surface of the earth at a temperature above the freezing-point of water. The temperature conditioning life on a planet is therefore maintained only because, on the one side, light and heat are received by radiation from the sun in sufficient quantities, while on the other side an equivalent radiation of heat takes place into space. If the heat gain and the heat loss were not to balance each other, the term of suitable conditions would not last long. The temperature of the earth-crust could sink in a few hundreds or thousands of years from 1000° to 100°, because when the earth was at this high temperature its radiation into space predominated over the radiation received from the sun. On the other hand, about a hundred million years have passed, according to Joly, since the age when the ocean originated. The temperature of the earth, therefore, required this long space of time in order to cool down from 365° (at which temperature water vapor can first be condensed to liquid water) to its present temperature. The cooling afterwards proceeded at a slower rate, because the difference between the radiations inward and outward was lessened with the diminishing temperature of the earth. Various methods have been applied in estimating these periods. Joly based his estimate on the percentage of salt in the sea and in the rivers. If we calculate how much salt there is in the sea, and how much salt the rivers can supply to it in the course of a year, we arrive at