xvi TABLE OF CONTENTS 



PAGE 



Chapter XIX. The Factor Hypothesis as Applied to Plants. 



John M. and Merle C. Coulter 253 



I. Presence and Absence Hypothesis 253 



II. Blends 255 



III. The Factor Hypothesis 257 



Chapter XX. The Factor Hypothesis as Applied to Animals . 269 



Illustrations of the Factor Hypothesis 269 



The Factorial Analysis of Color in Mice 269 



Different Kinds of Albinos 270 



Factorial Analysis of Coat Color in Swine 271 



Coat Colors in Guinea Pigs 272 



Chapter XXI. Review op Mendelism 273 



Chapter XXII. Sex-linked Heredity 277 



Chapter XXIII. Linkage, Crossing-over, and the Architecture 



of the Germ Plasm 285 



Linkage 285 



Crossing-over 288 



Chromosome Maps Indicating the Arrangement of Mendelian Fac- 

 tors, or Genes, in the Chromosomes 291 



Linkage in Other Organisms 296 



Chapter XXIV. Cross-breeding and Inbreeding 299 



The Role of Hybridization in Evolution 299 



Some Animal Hybrids 300 



Secondary Effects of Cross-breeding and Inbreeding . . . .302 



A. Cross-breeding 302 



B. Inbreeding 304 



Chapter XXV. Change Factors. Introduction 308 



Changing Views as to the Origin of New Hereditary Characters .' 308 



Lamarck 308 



Darwin 308 



Weismann 309 



Mutations 312 



Chapter XXVI. The Mutation Theory 313 



New Species (Mutants) of Oenothera. Hugo Dc Vries . . . . 315 



Summary of De Vries's Mutation Theory. Thomas Hunt Morgan . 322 



Later Investigations of Mutations 327 



The Neo-Mutationist Position. R. Ruggles Gates 328 



Chromosome Mutations 329 



Mutation. H. J. Midler 333 



