EVOLUTION IN GEOLOGY 25 



until at last a central mass was left, and this became 

 the sun. Satellites were thrown off from several of 

 the planets just in the same way as the planets them- 

 selves arose from the original nebula, and Saturn's 

 rings are pointed to as showing this process even now 

 in course of operation. 



Such a description as this may appear fanciful at 

 first sight, but it was worked out quantitatively as 

 well as qualitatively by its author, and was shown to 

 explain in detail a multitude of phenomena. Spencer 

 points out that when we have, worked out by one of 

 the first of mathematicians, a definite theory of plane- 

 tary evolution based on established mechanical laws* 

 and one which accounts in a satisfactory way for all 

 the known phenomena* the conclusion that the solar 

 system really did arise by a process of evolution is, to 

 say the least, difficult to avoid. 



The establishment and propagation of the idea that 

 the present condition of the earth's surface arose 

 through a course of gradual evolution, by the agency 

 of such processes only as are known to be in operation 

 at the present day* is the great contribution of Sir 

 Charles Lyell to the science of geology. We may 

 briefly trace the evolution of the idea itself, beginning 

 with the speculations of Werner, who, from observa- 

 tions of the geological formations of a limited tract of 

 country, came to the conclusion that the successive 

 strata were precipitated one by one from an universal 

 ocean. Here we see the first germ of the idea of 

 evolution embodied in the notion that the stratified 



