X CONTENTS 



CHAPTER PAGE 



III. Experiments of De Vries .114 



IV. American Experiences 129 



V. Economic Significance of Mutations . . . . . . 135 



VI. Biological Significance of Mutations 136 



References . 139 



PART II CAUSES OF VARIATION 



INTRODUCTION . .141 



VII. THE MECHANISM OF DEVELOPMENT AND DIFFERENTIATION . 142 



I. Protoplasm the Physical Basis of Life . . . u ' . 142 



II. The Cell the Unit of Structure . . . . .143 



III. Mechanism of Cell Division (Mitosis) 145 



IV. Cell Division with and without Differentiation . . 149 

 V. Physiological Units . 152 



References , -154 



VIII. INTERNAL CAUSES OF VARIATION 155 



/. INTERNAL INFLUENCES AFFECTING PRIMARILY THE IN- 

 DIVIDUAL 155 



I. Cell Division 155 



II. Bisexual Reproduction a Fundamental Cause of Variation . 160 



III. Maturation and the Reduction of the Chromosomes a Cause 



of Variation 163 



IV. Bud Variation 181 



V. Influence of the Condition of the Germ upon Development . 182 



VI. Xenia, or Fertilization of the Endosperm . . . .183 

 VII. Telegony 185 



VIII. Intra-Uterine Influences . . 189 



IX. Reversion and Atavism .192 



X. Individual Characters dependent upon Sex . . . .194 

 II. INTERNAL INFLUENCES AFFECTING THE RACE AS A 



WHOLE . 196 



XI. Relative Fertility, or Genetic Selection 196 



XII. Physiological Selection .201 



XIII. Selective Death Rate ; Longevity 201 



XIV. Bathmic Influences 202 



XV. Physiological Units 208 



XVI. Germinal Selection 213 



References * 217 



IX. EXTERNAL INFLUENCES AS CAUSES OF VARIATION . . . 220 

 I. General Effect of Locality upon Plant and Animal Develop- 

 ment 221 



II. Influence of Food upon Variability ..... 225 



III. Effect of Moisture upon Development 230 



IV. Effect of Contact upon Protoplasmic Activity .... 233 

 V. Effect of Gravity upon Living Matter ; Geotropism . . 236 



VI. Effect of Light upon Living Matter 239 



