CONTENTS. xi 



CHAPTER VI. 



THE EVIDENCE FROM HYBRIDS. 



Importance of the subject — It furnishes a means of analyzing 

 or isolating tlie influence of each sexual element — Hybrids 

 very variable — Hybrids from domesticated races more 

 variable than those from wild races — The descendants of 

 hybrids more variable tlian the hybrids themselves— The 

 offspring of a male hybrid and the female of a pure species 

 are mucli more variable than those of a female hybrid and 

 the male of a pure species — These facts inexplicable on 

 any view, except the one here presented — Reciprocal 

 crosses — They differ in fertility and in structure — The 

 difference is exactly what our theory requires — Difficulty 

 in explaining transmission of characters without fusion — 

 Reversion caused by crossing — Two kinds of reversion — 

 Summary 118 



CHAPTER VII. 



THE EVIDENCE FROM VARIATION. 



Causes of variation — Changed conditions of life induce varia- 

 bility — No particular kind of change is necessary — Varia- 

 bility is almost exclusively confined to organisms produced 

 from fertilized ova — Bud variation very rare — History of 

 the Italian orange — The frequency of variation in organ- 

 isms produced from sexual union, as compared with its 

 infrequency in those produced asexually, receives a direct 

 explanation by our theory of heredity — Bud variation 

 more frequent in cultivated than in wild plants — Our 

 theory would lead us to expect this — Changed conditions 

 do not act directly, but they cause subsequent generations 

 to vary — Tendency to vary is hereditary — These facts 

 perfectly explicable by our theory — Specific characters 

 more variable than generic — Species of large genera more 

 variable than those of small genera — A part developed in 

 an unusual way highly variable — Law of equable variation 

 — Secondary sexual characters variable — Natural selection 

 cannot act to produce permanent modification unless 

 many individuals vary together — Our theory is the only 

 explanation of the simultaneous variation of many in- 

 dividuals — This theory also simplifies the evolution of 

 complex structures — Saltatory evolution — This is ex- 

 plained by our theory of heredity — Correlated variation 

 of homologous parts — Parts confined to males more vari- 

 able than parts confined to females — Males more variable 

 than females — Summary of last two chapters 140 



CHAPTER VIII. 



'THE EVIDENCE FROM SECONDARY SEXUAL CHARACTERS 166 



