The earth, which started life as a hot mass of gas, has gradually cooled, until it has now about touched bottom, and has almost no heat beyond that which it receives from the sun. This just about balances the amount it radiates away into space, so that it would stay at its present temperature for ever if external conditions did not change, and any changes in its condition will be forced on it by changes occurring outside. These external changes may be of many kinds. The sun’s loss of weight causes the earth to recede from it at the rate of about a yard a century, so that after a million million years, the earth will be 10 per cent. further away from the source of its light and life than now. Consequently even if the sun then radiated as much light and heat as now, the earth would receive 20 per cent. less of this radiation, and its mean temperature would be some 15 degrees Centigrade or so lower than at present. But after a million million years the sun will not radiate as much light and heat as now; it will have lost some 6 per cent. of its present weight through radiation, and, judging from other stars, this loss will probably reduce its energy-generating capacity by about 20 per cent. This will reduce the earth’s temperature by about another 15 degrees, so that after a million million years the inevitable course of events will have reduced the earth’s temperature by about 30 degrees Centigrade. It would be rash to attempt to predict how such a fall of temperature may affect terrestrial life, and human life in particular. Given sufficient time, life has such an enormous capacity for adapting itself to its environment that it seems possible that, even with a temperature 30 degrees Centigrade lower than now, life may still exist on earth a million million years hence. If so, I am glad that my life has not fallen in this far distant future. Mountains and seas, which provide some of the keenest pleasures of our present life, will exist only as traditions handed down from a remote and almost incredible past. The denudation of a million million years will have reduced the mountains almost to plains, while seas and rivers will be frozen packs of solid ice. We may well imagine that man will have infinitely more knowledge than now, but he will no longer know the thrill of pleasure of the pioneer who opens up new realms of knowledge. Disease, and perhaps death, will have been conquered, and life will doubtless be safer and incomparably better-ordered than now. It will seem incredible that a time could have existed when men risked, and lost, their lives in traversing unexplored country, in climbing hitherto unclimbed peaks, in fighting wild beasts for the fun of it. Life will be more of a routine and less of an adventure than now; it will also be more purposeless when the human race knows that within a measurable space of time it must face extinction and the eternal destruction of all its hopes, endeavours, and achievements.