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 
