CHAPTER VII. 



THE EVIDENCE FROM VARIATION. 



Causes of variation Changed conditions of life induce varia- 

 bility No particular kind of change is necessary Variabil- 

 ity is almost exclusively confined to organisms produced 

 from fertilized ova Bud variation very rare History of 

 the Italian orange The frequenc} r of variation in organ- 

 isms produced from sexual uuion, as compared with its in fre- 

 quency in those produced asexually, receives a direct expla- 

 nation 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 ex- 

 plicable by our theoiy 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 sex- 

 ual 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 individuals This theory 

 also simplifies the evolution of complex structures Salta- 

 tory evolution This is explained by our theory of heredity 

 Correlated variation of homologous parts Parts confined 

 to males more variable than parts confined to females 

 Males more variable than females Summary of last two 

 chapters. 



TJie Causes of Variation. 



CERTAIN authors have held that variability is a neces- 

 sary accompaniment of reproduction; that it is deter- 

 mined by something within rather than without the or- 



