Contents ix 



CHAPTER XII 



PAGE 



Inbreeding , i86 



Experiments with Mice and Rats i88 



Experiments with the Pomace Fly ....... 190 



Behavior of Germ-cells in Inbreeding 195 



CHAPTER XIII 



Influence of Selection , . . 198 



Fluctuating or Individual Differences ....... 199 



Selection of Fluctuating Differences ....... 202 



Elimination within the Species ........ 204 



Variation and Parthenogenesis .... ... 206 



The Results of Selection and Hybridization of Wild Elementary Species 207 

 Selection under Domestication of Mutations, Saltations, Sports, and 



Discontinuous Varieties in General 208 



CHAPTER XIV 



The Theory of Evolution 213 



The Analogy with Artificial Selection . . . . . . .214 



The Influence of the Environment 215 



De Vries's Results with Oenothera 217 



Variation and Mutation in Helix . .221 



Evolution by Means of Definite Variation ...... 224 



Adaptation ............ 229 



The Selection Theory and the Theory of the Survival of Mutations . 232 



EXPERIMENTAL STUDY OF GROWTH 

 CHAPTER XV 



Introductory 239 



Normal Growth 239 



Senescence 246 



Length of Life in Different Species 248 



Absorption of Parts by Larvae . . . . , . . • 25 1 



CHAPTER XVI 



External Factors that influence Growth 253 



Influence of Food 253 



Stimulants affecting Growth ......... 256 



Effects of Salt on Growth 257 



Effects of Heat on Growth 260 



