PAGE 



CONTENTS 



I. Introductory. Structure of the Germ Plasm. Genetic 

 Variations Resulting from New Combinations of 

 Genes i 



II. (i) Genetic Variations Resulting from Irregulari- 

 ties in the Structure and Operation of the Genetic 

 System. (2) Gene Mutations 27 



III. Are the Gene Mutations Observed in Experimental 

 Genetics the Basic Material of Progressive Evo- 

 lution ? 58 



IV. Genetic Variation in Unicellular Organisms, with 

 Special Relation to the Inheritance of Environmen- 

 tal Effects and Reactions 95 



V. Genetic Variation in Unicellular Organisms, con- 

 tinued. What Is the Method of Operation of the 

 Genetic System in Bringing about Genetic Varia- 

 tions under the Long-continued Action of Environ- 

 mental Conditions ? 119 



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