PHYSICAL EVOLUTION 5 



not known on the earth, but revealed by the spec- 

 troscope helium, asterium, and perhaps some 

 others. When the star cools these substances 

 re-appear in the same order in which they 

 disappeared. 



Now, as a matter of fact, the spectroscope shows 

 us that stars in all these stages actually exist at the 

 present day in the heavens. In some the tempera- 

 ture is increasing, in others it is decreasing; and, 

 although small stars must run through their 

 development quicker than large ones, this is quite 

 insufficient to account for all the present differences. 

 From which it follows that some of the stars are 

 much older than others. The sun was amongst the 

 earliest of formed stars. When it was born, the sky 

 must have presented an almost uniform blackness. 

 There was no Milky- Way ; no Orion nor Southern 

 Cross; no Pleiades nor Dog-star. All these, and 

 many others, have been added since : not all 

 together, but one after the other, through the long 

 ages during which the sun was undergoing develop- 

 ment. Judging by the relative ages of the stars, it 

 seems probable that the process of concentration 

 of the original cosmic dust commenced near the 

 Solar System and spread outwards to the Milky- 

 Way. But, however this may be, the process is not 

 yet over. Many nebulae have not yet condensed 

 into stars. Swarms of meteorites still traverse 

 space ; and even in the neighbourhood of the Solar 

 System they are so abundant, that the Earth alone 

 is estimated to collect more than twenty millions 

 each day. 



