in THE PAST AND THE FUTURE 291 



the sources of the earth's heat were complete, and that there 

 was no ground for assuming any such completeness. The 

 dogmatism of the mathematician prevailed until the dis- 

 covery of radio-activity, revealing entirely new sources of 

 heat, proved the justice of Huxley's caution, and placed 

 the whole question of the terrestrial past and future in a 

 new light. We have now every reason to think that the 

 durability of the earth as a habitable planet is immensely 

 greater than Lord Kelvin supposed, that it is to be 

 measured in hundreds rather than in units of millions, and 

 that we are in no sense witnessing the latter stages of 

 evolution on a dying planet. It may be said that, never- 

 theless, ultimate decay is certain, but it may be replied that 

 the supposed certainty once more arises from drawing 

 mathematical deductions from facts supposed to be known 

 in their completeness, and the lesson of radio-activity is 

 precisely that we may be very far from so knowing them. 

 As to the Dissipation of Energy, this is still more clearly an 

 incomplete account of the world-process as a whole. For 

 it can proceed only by assuming an infinite quantum of 

 original energy at high potential, of which it pretends to 

 give no account whatever. Its validity is merely in the 

 account that it gives of mechanical process as such, and 

 the more certain it is the more it proves that mechanical 

 processes cannot exhaust reality. It proves that there 

 must within the sphere of reality be, or at least have 

 been, an unknown compensatory process building up what 

 mechanism dissipates. 



Neither can we, in face of modern inventions and of our 

 whole account of the growth of mind, set any limit to the 

 possibility of the control of external nature. It may seem 

 grotesque to suggest that the time may come when man 

 will control the movements of the earth or at need accom- 

 plish migration to another planet. But a few generations 

 ago it would have seemed equally grotesque to fancy a 

 means of communication across the ocean without so much 

 as a visible connecting mechanism. What can fairly be 

 said against an optimistic view of the future of human 

 control is that it is not impossible, but unverified. What 

 reason have we for adopting it? Why should we think 



