252 THE LESSON OF EVOLUTION 



would not be suitable for a planet which was either 

 larger or smaller than the earth. This is evident 

 from the fact that the ratio of the surface to the 

 volume varies with the size of the planet. Indeed, 

 from physical considerations alone, we may feel sure 

 that at the present time living protoplasm could not 

 exist in any part of the solar system, except on the 

 surface of the earth. 



It is quite possible that in other parts of the 

 Universe other substances than protoplasm may be 

 alive. But we must remember that many of the 

 elementary substances found on the earth are also 

 found in the stars, and it is doubtful if there is any 

 substance in the stars which is not found on the 

 earth. If hydrogen exists in the sun and in the stars 

 it must have the same properties and the same 

 chemical affinities there as here, and so with the 

 other elements. 



Any substance which exhibits the phenomena of 

 life must necessarily have a mobile constitution, 

 for life exists by the constant rearrangement of mole- 

 cules. Consequently it must contain water, for 

 there is no other thing on the earth which could 

 be substituted for w r ater. And there are no elemen- 

 tary substances in the sun, or in the stars, which 

 could make another substance which might replace 

 water. Practically we know the possible combina- 

 tions which might be formed in other worlds, if we 

 assume, as we must, that the physical and chemical 

 laws are the same throughout the universe. 



Now water exists as a liquid only between cer- 

 tain narrow limits of temperature, so that no living 



