122 EVOLUTION 



are slowly escaping the earth's gravitation and passing 

 into space. We may regard the slight escape without 

 concern. The evolutionist will turn rather to the 

 astronomer, and ask him what lesson he learns for the 

 future from the other contents of the universe. And 

 from the astronomer we learn that, beyond any question, 

 life will come to an end on our planet by the extinction 

 of the sun. 



Our chapter on the birth of our solar system has, in 

 fact, fully prepared us for its death. Our earth was 

 once a small sun, glittering brilliantly in space (if there 

 were any to see it). As surely as it was extinguished, 

 and from the same causes, the parent sun will grow 

 dark. When we examine the sun through a (protected) 

 telescope, we find its surface covered with a network of 

 dark streaks, terminating here and there in the black 

 patches we call "sun-spots." Spectroscopic investiga- 

 tion shows that these spots are oceans of cooler vapour 

 lying on the brilliant bed of incandescent metal. It 

 rains liquid metal on the sun. From the appalling 

 ocean of the photosphere the vapours of molten metals 

 rise high up with the atmosphere, cool and fall again 

 upon the surface, and run together into lakes and 

 oceans. Black as the " spot " seems, it is really 

 brighter than white-hot iron, and is merely darkened by 

 the contrast of the dazzling photosphere. But the 

 process will no more go on for ever than it did in the 

 case of the earth or Jupiter, or any of the dead or dying 

 suns in the heavens. No matter what the sun be 

 composed of it cannot give out heat indefinitely. The 

 cold of space will gain on it in time. The cooler vapours 

 will gather thicker over its photosphere, and men will 

 look up to a red and failing luminary above their heads. 



We saw that the stars are now classified according to 

 their spectra, and illustrate the whole process of the 



