CONTENTS 



CHAPTER PAGE 



I. INTRODUCTION 1 



1. The idea of species 1 



2. The triangle of life 3 



3. A definition of heredity 6 *-" 



4. The maintenance of life 8 



5. Somatoplasm and germplasm 12 \) 



II. VARIATION 17 



1. The most invariable thing in nature 17 



2. The universality of variation 18 



3. The kinds of variation with respect to their: 



a. Nature 19 



b. Duplication 20 



c. Utility 20 



d. Direction in evolution 21 



e. Source . 21 



f. Normality 22 



g. Degree of continuity 22 



h. Character 22 



i. Relation to an average standard .... 23 



j. Heritability 23 < 



4. Methods of studying variation 



5. Biometry 23 



6. Fluctuating variation 24 



7. The interpretation of variation curves .... 28 



a. Relative variability 28 



b. Bimodal curves 29 



c. Skew curves 31 



8. Graduated and integral variations 33 ^ 



9. The causes of variation 34 / 



a. Darwin's attitude 34 * 



b. Lamarck's attitude 34 ^ 



c. Weismann's attitude 35 tx 



d. Bateson's attitude 37 



III. HERITABLE DIFFERENCES 



1. The mutation idea 38 ^ 



2. A summary of the mutation theory 41 



xi 



