138 THE ADVANCE OF SCIENCE 



be forgotten that what we call ' catastro- 

 phes,' are, in relation to the earth, changes, 

 the equivalents of which would be well 

 represented by the development of a few 

 pimples, or the scratch of a pin, on a 

 man's head. Vast regions of the earth's 

 surface remain geologically unknown ; 

 but the area already fairly explored is 

 many times greater than it was in 1837 ; 

 and, in many parts of Europe and the 

 United States, the structure of the super- 

 ficial crust of the earth has been investi- 

 gated with great minuteness. 



The parallel between Biology and Ge- 

 ology, which I have drawn, is further 

 illustrated by the modern growth of that 

 branch of the science known as Petrology, 

 which answers to Histology, and has made 

 the microscope as essential an instrument 

 to the geological as to the biological in- 

 vestigator. 



The evidence of the importance of 

 causes now in operation has been wonder- 



