THE EVOLUTION OF THE WORLD. 247 



beginning of the century. In particular, a closer 

 acquaintance with comets, meteorites, star-clusters, 

 and nebulae has helped us to realise the great signi- 

 ficance of the smaller bodies which are found in 

 millions in the space between the stars. 



We now know that the paths of the millions of 

 heavenly bodies are changeable, and to some extent 

 irregular, whereas the planetary system was formerly 

 thought to be constant, and the rotating spheres were 

 described as pursuing their orbits in eternal regularity. 

 Astrophysics owes much of its triumph to the immense 

 progress of other branches of physics, of optics, and 

 electricity, and especially of the theory of ether. And 

 here, again, our supreme law of substance is found to 

 be one of the most valuable achievements of modern 

 science. We now know that it rules unconditionally 

 in the most distant reaches of space, just as it does in 

 our planetary system, in the most minute particle of 

 the earth as well as in the smallest cell of our human 

 frame. We are, moreover, justified in concluding, if 

 we are not logically compelled to conclude, that the 

 persistence of matter and force has held good through- 

 out all time as it does to-day. Through all eternity 

 the infinite universe has been, and is, subject to the 

 law of substance. 



From this great progress of astronomy and physics, 

 which mutually elucidate and supplement each other, 

 we draw a series of most important conclusions with 

 regard to the constitution and evolution of the cosmos, 

 and the persistence and transformation of substance. 

 Let us put them briefly in the following theses : — 



I. — The extent of the universe is infinite and 

 unbounded ; it is empty in no part, but everywhere 

 filled with substance. 



