4 THE LESSON OF EVOLUTION 



and history. But these would be speculations, 

 which could not, at present, be verified; and so we 

 must content ourselves with the chondroi as the 

 earliest form of matter known to us. 



Through the action of gravitation much of the 

 cosmic dust is supposed to have aggregated into 

 meteorites, whose irregular movements were, in 

 certain places, reduced to order; and so arose a 

 number of meteoritic streams, or swarms, moving 

 through space. Still under the force of gravitation , 

 each of these swarms got more and more dense, until, 

 at last, collisions took place between the meteorites ; 

 light and heat were given out, and the swarm became 

 a nebula. The heat produced by the collisions 

 would, at first, be slight, but would gradually 

 increase, until the whole of the solid material was 

 resolved into vapour, and a star was formed. Con- 

 centration, however, would still go on, and the 

 temperature of the star would rise, until, in time, 

 the loss by radiation more than counterbalanced the 

 gain by concentration, when the star would begin to 

 cool. At last light would no longer be given off, 

 and the star would end by becoming a cold dark 

 body moving in space. Of course, some stars would 

 attain a higher maximum temperature than others ; 

 and either a single or a double star might be the 

 result of the condensation ; but all would follow a 

 somewhat similar development. As the tempera- 

 ture of the star increases the metals gradually 

 disappear. Then the silicon vanishes ; then carbon, 

 nitrogen and oxygen, until in the very hottest we 

 find only proto-hydrogen that is a form of hydrogen 



