xii Contents. 



PAGE 



12. Cereals io6 



Pedigree and Elite, no; Nilsson's Results, 114. 



13. The Limits to the Amount of Change that Can be Ef- 



fected by Selection 118 



Linear Variation, 118; Regression, 120; Instability 

 of Improved Races, 120; Adaptation, 121. 



14. The Behavior of Improved Races After the Cessation 



of Selection 122 



Progeny of the Original Seed, 126; Change of Seed, 

 126; Intermediate Generations, 128. 



IV. Controversial Questions 130 



15. Acquired Characters and Variations Caused by Nutri- 



tion 130 



16. On the Inheritance of Acquired Characters 135 



Papavcr somnifcrum polycephalum, 138; Selection 

 and Nutrition, 142. 



17. On Partial Variability and Selection by Vegetative 



Methods of Propagation 143 



Alpine Plants, 145; Sugar Cane, 148. 



18. Variation and Adaptation 149 



19. Variability in Man and Social Questions 154 



20. Some Subjects for Future Investigation 159 



Correlative Variation, 160; External Conditions, 161; 

 Regression, 161 ; Retrogressive Selection, 163. 



V. The Origin of Species by Mutation 165 



21. Species, Subspecies and Varieties 165 



Varieties, 169; Elementary Species, 171. 



22. Species in Nature 172 



22). Species in Cultivation 176 



Age of Races, 176; Cereals, 178; Apples, 179; Age 

 of Garden Varieties, 183. 



24. Species and Specific Characters 185 



25. Mutations in Cultivation 187 



Historical Accounts of the Origin of Species, 188; 

 Sterile Forms, 195 ; Constancy from Seed, 196. 



26. The Hypothesis of Indiscriminate Mutability 198 



27. The Hypothesis of Periodic Mutability 205 



The Migration Theory, 206. 



28. The Phenomenon of Mutation Within the Limits of 



the Mutation Periods 207 



Delboeuf's Law, 208. 

 VI. Conclusion 211 



