vi PREFACE 



of the evolutionary process. In the third lec- 

 ture tlie j^hysical basis of heredity and the com- 

 position of the germ plasm stream are examined 

 in the light of new observations; while in the 

 fourth lecture the thesis is developed that 

 chance variation combined v/ith a property of 

 living things to manifold themselves is the 

 key note of modern evolutionary thought. 



T. H. Morgan. 

 July, 1016. 



