20 EVOLUTION 



approach closer, generate an ever-increasing heat. The 

 smaller the mass, the more quickly will the work of 

 concentration proceed. Each planetary mass, having 

 cast off its own fringe of arms or rings or tidal bulges, 

 will round into a ball of incandescent matter with a 

 temperature of possibly 10,000C. at its surface. Eight 

 or nine small suns will course round the parent mass, 

 still a hazy half-concentrated nebula. The smallest 

 bodies the moons will first run through their period 

 of brilliancy, sink to a dull red, and at last have their 

 molten interior hidden under a solid crust. One by one, 

 according to size, the planets will run the same course : 

 first Mercury (2,946 miles in diameter), then Mars* 

 (4,172 miles), then Venus (7,894 miles), and Terra (7,926 

 miles). Jupiter and Saturn seem not yet sufficiently 

 cooled at the surface to support oceans ; dense belts of 

 clouds envelop their gigantic frames. The sun, 324,000 

 times heavier than the earth (or 2,000 trillion tons in 

 weight), is still in the throes of condensation. Whatever 

 share radium may have in its outpouring of heat, the 

 simple condensation of its mass would sustain its enor- 

 mous temperature (probably 7,000C. at the surface, and 

 higher within) for millions of years. 



To the future evolution of the sun and its planets we 

 will return later. At present we must follow the direct 

 thread of our story, and trace the development of that 

 one of the arms or masses detached from it that became 

 our planet. Before we do so, however, two further 

 points must be briefly touched in this chapter. 



In the first place it v/ill be asked " when these things 

 were." Can modern astronomers give us some idea of 



* Assuming that the planets were not formed at long inter- 

 vals. I must point out that the "planetesimal" theory denies 

 this incandescent stage, and assumes a less violent fall of its 

 particles into the planetary and solar masses. 



