THE PROBABLE AGE OF THE WORLD. 675 



tion of the prodigious length of time that it must have taken to exca- 

 vate the gorge ? We should certainly feel startled when on making 

 the necessary calculations we found that the stream had performed 

 this enormous amount of work in something less than a million years. 

 The absolute settlement of the question must ever be above our 

 powers. For a few centuries only we have the comparative daylight 

 of historical times ; thence backward lies the rapidly-gathering twi- 

 light of tradition ; beyond that, geological periods the duration of 

 which can be only vaguely guessed at, and beyond all these, far back 

 in past eternity, the epoch when Time began. The old belief, which 

 limited the existence of the earth to less than seven thousand years, 

 gave way once for all, almost within living memory. All men are 

 now agreed that the six days of creation were periods of indefinite 

 extent. They are not solar days for evening happened and morning 

 happened three times over before the sun was created. Not being 

 days measured by the sun, we know not how many thousands of years 

 they may have endured. The reaction was sudden and complete. 

 Geology jumped to the conclusion that the past history of the world 

 was without any limits that human imagination could conceive. But 

 in quite recent years, as we have tried to show, the calm light of sci- 

 ence has proved that the practical eternity of matter is not more 

 tenable than the arbitrary limitation by which thought was formerly 

 confined. 



"I dare say," says Prof. Tait, "that many of you are acquainted with the 

 speculations of Lyell and others, especially of Darwin, who tell us that even 

 for a comparatively hrief portion of recent geological history three hundred 

 millions of years will not suffice ! We say so much the worse for geology as 

 at present understood hy its chief authorities, for ... . physical considera- 

 tions render it impossible that more than ten or fifteen millions of years can be 

 granted." 



Sir William Thomson is not so sweeping in his assertion : but 

 then the nature of the problem before him did not require any such 

 opinion at his hands. His argument aimed at disproving Playfair's 

 assertion that neither the heavenly bodies nor the earth offered any 

 evidence of a beginning, or any advance toward an end. If, there- 

 fore, Sir William Thomson was able to show that there was good 

 evidence both of a beginning and an end, he was not concerned to 

 speculate how long past time had existed, or # when the end would 

 come. His summing up is this: 



"We must admit some limit. . . . Dynamical theory of the sun's heat ren- 

 ders it almost impossible that the earth's surface has been illuminated by the 

 sun many times ten million years. And when finally we consider underground 

 temperature we find ourselves driven to the conclusion that the existing state of 

 things on the earth, life on the earth, and all geological history showing conti- 

 nuity of life, must be limited within some such period of past time as one hun- 

 dred million years." 



