Any one, however, who has lived with discerning thought through the opening years of the twentieth century, must be aware that something has happened to chasten and subdue these wildly enthusiastic hopes of the mid-Victorian age. Others beside the "gloomy dean" of St. Paul's, whether through well-considered thought or through the psychological shock of the Great War, have come to look upon this rash, unmitigated enthusiasm about the earth's future as a fool's paradise. At any rate, no treatment of the idea of progress would be complete which did not dwell upon the limitations to that idea, now definitely obvious to thoughtful men. As early as 1879, in Saporta's "Le Monde des Plantes," we run upon one serious setback to unqualified expectations of progress. Men began to take into account the fact that this earth is not a permanent affair. "We recognize from this point of view as from others," wrote Saporta, "that the world was once young; then adolescent; that it has even passed the age of maturity; man has come late, when a beginning of physical decadence had struck the globe, his domain." [12] Here is a fact to give enthusiasm over earthly progress serious pause. This earth, once uninhabitable, will be uninhabitable again. If not by wholesale catastrophe, then by the slow wearing down of the sun's heat, already passed its climacteric, this planet, the transient theatre of the human drama, will be no longer the scene of man's activity, but as cold as the moon, or as hot as colliding stars in heaven, will be able to sustain human life no more. "The grandest material works of the human race," wrote Faye in 1884, "will have to be effaced by degrees under the action of a few physical forces which will survive man for a time. Nothing will remain, not even the ruins." [13] Every suggested clew to a possible escape from the grimness of the planet's dissolution has been followed up with careful search. The discovery of radioactivity seemed to promise endlessly extended life to our sun, but Sir E. Rutherford, before the Royal Astronomical Society, has roundly denied that the discovery materially lengthens our estimate of the sun's tenure of life and has said that if the sun were made of uranium it would not because of that last five years the longer as a giver of heat.[14] Whether we will or not, we have no choice except to face the tremendous fact, calmly set down by von Hartmann in 1904: "The only question is whether . . . the world-process will work itself out slowly in prodigious lapse of time, according to purely physical laws; or whether it will find its end by means of some metaphysical resource when it has reached its culminating point. Only in the last case would its end coincide with the fulfilment of a purpose or object; in the first case, a long period of purposeless existence would follow after the culmination of life." [15]