xii CONTENTS 



CHAPTER 



3. Lamarck's evening primrose 42 



4. Plant mutations found in nature 47 



5. Some mutations among animals 48 



6. Kinds of mutation 51 



7. The origin of mutations 53 



8. When mutations occur 55 



9. Possible causes of mutation 57 



IV. THE INHERITANCE OF ACQUIRED CHARACTERS ... 62 



1. Summary of preceding chapters 62 



2. The bearing of this chapter upon genetics . . 63 



3. The importance of the question 63 



4. A historical sketch of opinion 64 



5. Confusion in definitions 66 



6. Weismann's conception of acquired characters . 67' 



7. The distinction between germinal and somatic 



characters 67 



8. What variations reappear? 69 



9. How may germplasm acquire new characters? . 69 



10. Weismann's reasons for doubting the inheritance 



of acquired characters 71 



a. No known mechanism for impressing germ- 



plasm with somatic characters ... 71 



b. Evidence for the inheritance of acquired 



characters inconclusive 74 



a. Mutilation 75 



b. Environmental effects 76 



c. The effects of use or disuse .... 78 



d. Transmission of disease 80 



e. Immunity and the effect of drugs . . 82 



f. Prenatal influences 83 



c. The germplasm theory sufficient to account 



for the facts of heredity 84 



11. The comparative independence of germ and soma 85 



12. Acquired characters in the protozoa .... 86 



13. The opposition to Weismann 88 



14. Various results upon the offspring of parental 



acquisitions 90 



15. Conclusion 92 



V. MENDELISM 93 



1. Methods of studying heredity 93 



2. The melting pot of cross-breeding 93 



3. Johann Gregor Mendel 95 



4. Mendel's experiments on garden peas .... 97 



5. Some further instances of Mendel's law . . . 101 



6. The cardinal principle of segregation .... 103 



