INTRODUCTORY 



Evolution is a term that has been 

 used in a great variety of ways. We 

 speak of the evolution of the stars, 

 meaning thereby the process by which 

 stars have grown from gaseous masses 

 to incandescent bodies, such as our 

 sun, and finally to the cold inert con- 

 ditions of stellar death. We speak of 

 the evolution of the earth, in that we 

 picture the growth of that body as a 

 part of the solar system whose central 

 element, the sun, yields the energy by 

 which the earth is moulded. Under 

 the varying heat of this luminary our 

 atmosphere is made to move as wind, 

 water is evaporated and condensed, 

 continents are eroded and dissected, 

 materials are disintegrated, trans- 

 ported, and deposited — in short, the 

 surface of the earth is put under con- 



