FORMATION OF PLANETARY SYSTEMS 175 



side of his conclusions.* Finally we may stress one 

 point. Mars has every appearance of being a planet 

 long past its prime; and it is in any case improbable that 

 two planets differing so much as Mars and the Earth 

 would be in the zenith of biological development con- 

 temporaneously. 



Formation of Planetary Systems. If the planets of the 

 solar system should fail us, there remain some thousands 

 of millions of stars which we have been accustomed to 

 regard as suns ruling attendant systems of planets. It 

 has seemed a presumption, bordering almost on impiety, 

 to deny to them life of the same order of creation as 

 ourselves. It would indeed be rash to assume that 

 nowhere else in the universe has Nature repeated the 

 strange experiment which she has performed on the 

 earth. But there are considerations which must hold us 

 back from populating the universe too liberally. 



On examining the stars with a telescope we are sur- 

 prised to find how many of those which appear single 

 points to the eye are actually two stars close together. 

 When the telescope fails to separate them the spectro- 

 scope often reveals two stars in orbital revolution round 

 each other. At least one star in three is double — a pair 

 of self-luminous globes both comparable in dimensions 

 with the sun. The single supreme sun is accordingly 

 not the only product of evolution; not much less fre- 

 quently the development has taken another turn and 

 resulted in two suns closely associated. We may prob- 

 ably rule out the possibility of planets in double stars. 



♦Mars is not seen under favourable conditions except from low lati- 

 tudes and high altitudes. Astronomers who have not these advantages 

 are reluctant to form a decided opinion on the many controversial points 

 that have arisen. 



