THE PAIwETIOLOGICAL SCIENCES. 531 



only a type of similar studies with respect to all the 

 elements, which, in the history of the earth's inha- 

 bitants, have been constantly undergoing a series of 

 connected changes. 



But, wide as is the view which such consi- 

 derations give us of the class of sciences to which 

 geology belongs, they extend still further. "The 

 science of the changes which have taken place in 

 the organic and inorganic kingdoms of nature," 

 (such is the description which has been given of 

 Geology 3 ,) may, by following another set of con- 

 nexions, be extended beyond "the modifications of 

 the surface of our own planet." For we cannot 

 doubt that some resemblance, of a closer or looser 

 kind, has obtained between the changes and causes 

 of change, on other bodies of the universe, and on 

 our own. The appearances of something of the 

 kind of volcanic action on the surface of the moon, 

 are not to be mistaken. And the inquiries concern- 

 ing the origin of our planet and of our solar system, 

 inquiries to which Geology irresistibly impels her 

 students, direct us to ask what information the 

 rest of the universe can supply, bearing upon this 

 subject. It has been thought by some, that we can 

 trace systems, more or less like our solar system, 

 in the process of formation ; the nebulous matter, 

 which is at first expansive and attenuated, con- 

 densing gradually into suns and planets. Whether 

 this Nebular Hypothesis be tenable or no, I shall 



3 Lyell, Principles of Geology, p. 1 . 



M M '-' 



