CONTENTS xi 



CHAPTER PAGE 



11. No known mechanism for impressing germplasm with 



somatic characters ..... 84 



12. Evidence for the inheritance of acquired characters 



inconclusive 86 



a. Mutilations ....... 87 



6. Environmental effects ..... 88 



c. The effects of use or disuse . . . .91 



d. Disease transmission ..... 92 



13. The germplasm theory sufficient to account for the 



facts of heredity ...... 94 



14. The opposition to Weismann 95 



15. Conclusion 96 



VI. THE PURE LINE. 



1. The unit character method of attack ... 97 



2. Galton's law of regression ..... 98 



3. The idea of the pure line 102 



4. Johanssen's nineteen beans 103 



5. Cases similar to Johanssen's pure lines . . . 107 



6. Tower's potato-beetles 108 



7. Jennings' work on Paramecium .... 110 



8. Phenotypical and genotypical distinctions . .113 



9. The distinction between a population and a pure line 115 

 10. Pure lines and natural selection .... 118 



VII. SEGREGATION AND DOMINANCE. 



1. Methods of studying heredity 120 



2. The melting-pot of cross-breeding .... 120 



a. Blending inheritance . . . . .121 



6. Alternative inheritance ..... 121 



c. Particulate inheritance 121 



3. Johann Gregor Mendel ...... 123 



4. Mendel's experiments on garden peas . . .124 



5. Some further instances of Mendel's law . . . 128 



6. The principle of segregation . . . . .130 



7. Homozygotes and heterozygotes .... 131 



8. The identification of a heterozygote .... 132 



9. The presence and absence hypothesis . . . 132 

 10. Dihybrids 133 



