124 EVOLUTION 



heat will be combatted for a time it is idle to conjecture. 

 The humanity of that time will be more different from 

 us than we are from our lemur ancestor of the Eocene 

 period. But the heart of the system dies when the sun 

 ceases to give sufficient light and heat, and no one can 

 doubt that all life will perish on the planets. Whether 

 Jupiter and Saturn will by that time have developed far 

 enough to support life we may doubt; certainly the 

 story of life cannot run far on those as yet immature 

 planets. In the case of Mars we cannot even regard the 

 canals as a sure proof of the presence of intelligence, 

 but in spite of Dr. Wallace's able but inspired 

 speculations it seems that the conditions of life are 

 realised on Mars, and on general principles it Is reason- 

 able to assume that the evolution of some type or other 

 of life has run there to even greater heights than on 

 earth. The question of Venus is more difficult, but it 

 certainly cannot be said that we must exclude life of 

 any type from it. These, however, are as yet idle 

 speculations. Whatever populations there be on the 

 planets of our solar system are surely doomed when the 

 sun goes down for ever. 



When will these things be ? Let us say, quite can- 

 didly, that we have not the slightest idea, but it is not 

 likely to be less than many million years. On the older 

 theory, that the heat of the sun was created almost 

 entirely by its condensation, the physicist had some 

 ground on which to approach the problem. Assuming 

 that our sun had passed its prime of life, it was calcu- 

 lated that It would last for between ten and fifteen 

 million years yet. But It is not admitted by all astrono- 

 mers that our sun is In Its decline, and the period 

 allotted would have to be enlarged. Now, however, the 

 discovery of radium enlarges it still further, and makes 

 it quite indefinite. We do not know what radium there 



