168 MAN'S PLACE IN THE UNIVERSE 



all parts. We observe nebulae and clusters in different 

 stages of advance. Some stars are still highly diffuse; 

 others are concentrated like the sun with density greater 

 than water; others, still more advanced, have shrunk to 

 unimaginable density. But no doubt can be entertained 

 that the genesis of the stars is a single process of evolu- 

 tion which has passed and is passing over a primordial 

 distribution. Formerly it was freely speculated that the 

 birth of a star was an individual event like the birth of 

 an animal. From time to time two long extinct stars 

 would collide and be turned into vapour by the energy of 

 the collision; condensation would follow and life as a 

 luminous body would begin all over again. We can 

 scarcely affirm that this will never occur and that the 

 sun is not destined to have a second or third innings; 

 but it is clear from the various relations traced among 

 the stars that the present stage of existence of the 

 sidereal universe is the first innings. Groups of stars are 

 found which move across the sky with common proper 

 motion; these must have had a single origin and cannot 

 have been formed by casual collisions. Another aban- 

 doned speculation is that lucid stars may be the excep- 

 tion, and that there may exist thousands of dead stars 

 for every one that is seen shining. There are ways of 

 estimating the total mass in interstellar space by its 

 gravitational effect on the average speed of the stars; 

 it is found that the lucid stars account for something 

 approaching the total mass admissible and the amount 

 left over for dark stars is very limited. 



Biologists and geologists carry back the history of the 

 earth some thousand million years. Physical evidence 

 based on the rate of transmutation of radioactive sub- 

 stances seems to leave no escape from the conclusion 

 that the older (Archaean) rocks in the earth's crust were 



