CONTENTS. x i 



CHAPTER VI. 



THE EVIDENCE FROM HYBRIDS. 



Importance of the subject It furnishes a means of analyzing 

 or isolating the 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 than the hybrids themselves The 

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

 are much 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 

 theoty 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 



