TABLE OF CONTENTS 15 



CHAPTER V 



PAGE 



Natural Selection since Darwin (continued) . 72 



The appearance of variations: their numerical 

 importance; Delboeuf's law. — Frequency of vari- 

 ation. — Nature of variations ; usefulness and de- 

 gree of development of certain characters. — The 

 neck of the giraffe and the femur of the whale. — 

 Abnormal development of characters. Butterflies 

 that mimic leaves. — Parallel adaptation. — Over- 

 perfected organs. — Analogy between natural se- 

 lection and artificial selection. — Secondary objec- 

 tions. — The true role of selection. 



CHAPTER VI 

 Sexual Selection 95 



The origin of secondary sexual characters. — 

 Darwin's hypothesis; objections formulated against ^ 



it. — The numerical proportion of males and fe- 

 males. — The aesthetic sense of the female. — Orna- 

 mental characters of animals with external fertil- 

 isation. — New theories : means of identification, 

 desire to frighten enemies, surplus energy in the 

 male, internal secretions of genital organs. 



CHAPTER VII 



Theories of Heredity — Spencer's Physiological 



Units 108 



Relations between the theories of evolution and 

 the theories of heredity. — The various hypotheses 

 as to the structure of the protoplasm. — Micromer- 

 ists and organicists. — Uniform particles and rep- 

 resentative particles. — Physiological units typical 



