WHAT EVOLUTION IS 7 



ond heading, a group of doctrines 

 that have to do with the way in which 

 descent with modification has been 

 accompHshed. Granting that plants 

 and animals have arisen by the modi- 

 fication of earlier forms, what have 

 been the driving forces in nature that 

 have induced this modification? This 

 is a newer and much less certain field 

 of work than that which deals with 

 the simple fact of change or trans- 

 mutation in organisms. It includes 

 a consideration of Lamarckism, of 

 Darwinism or the theory of natural 

 selection and such subordinate theo- 

 ries as sexual selection, of orthogene- 

 sis, of the mutation theory, and of a 

 host of other views which from time 

 to time have been advanced as ex- 

 planations of descent with modifica- 

 tion. In the following pages, after 

 some brief historical comment, the 

 subject matter will be dealt with 



