xii Contents 



CHAPTER V 



PAGE 



The Theory of Natural Selection (continued} . . .129 

 Objections to the Theory of Natural Selection .... 129 



Sterility between Species 147 



Weismann's Germinal Selection 154 



CHAPTER VI 



Darwin's Theory of Sexual Selection 167 



Sexual Selection 167 



General Criticism of the Theory of Sexual Selection . . .213 



CHAPTER VII 



The Inheritance of Acquired Characters .... 222 



Lamarck's Theory . 222 



Darwin's Hypothesis of Pangenesis 233 



The Neo-Lamarckian School 240 



CHAPTER VIII 



Continuous and Discontinuous Variation and Heredity . 261 



Continuous Variation 261 



Heredity and Continuous Variation 270 



Discontinuous Variation 272 



Mendel's Law 278 



The Mutation Theory of De Vries 287 



Conclusions 297 



CHAPTER IX 



Evolution as the Result of External and Internal Factors 300 



The Effect of External Influences . . ... 300 



Responsive Changes in the Organism that adapt it to the New 



Environment ......... 319 



Nageli's Perfecting Principle 325 



