CONTENTS xi 



CHAPTER * AGH 



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. Johannsen's nineteen beans . . . . . 103 



5. Cases similar to Johannsen'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 



