I. INTRODUCTORY. THE PROBLEM OF SPECIES AND VARIETY 1 II. MERISTIC PHENOMENA 31 III. SEGMENTATION, ORGANIC AND MECHANICAL 60 IV. THE CLASSIFICATION OF VARIATION AND THE NATURE OF SUBSTANTIVE VARIATION 83 NOTE TO CHAPTER IV 94 V. THE MUTATION THEORY 97 NOTE TO CHAPTER V 116 VI. VARIATION AND LOCALITY 118 VII. LOCAL DIFFERENTIATION--_continued_. OVERLAPPING FORMS 146 VIII. LOCALLY DIFFERENTIATED FORMS--_continued_. CLIMATIC VARIETIES 164 IX. THE EFFECTS OF CHANGED CONDITIONS 187 X. THE EFFECTS OF CHANGED CONDITIONS--_continued_. THE CAUSES OF GENETIC VARIATION 212 XI. THE STERILITY OF HYBRIDS. CONCLUDING REMARKS 233 APPENDIX TO CHAPTER X 250 INDEX 251 PROBLEMS OF GENETICS CHAPTER I INTRODUCTORY The purpose of these lectures is to discuss some of the familiar phenomena of biology in the light of modern discoveries. In the last decade of the nineteenth century many of us perceived that if any serious advance was to be made with the group of problems generally spoken of as the Theory of Evolution, methods of investigation must be devised and applied of a kind more direct and more penetrating than those which after the general acceptance of the Darwinian views had been deemed adequate. Such methods obviously were to be found in a critical and exhaustive study of the facts of variation and heredity, upon which all conceptions of evolution are based. To construct a true synthetic theory of Evolution it was necessary that variation and heredity instead of being merely postulated as axioms should be minutely examined as phenomena. Such a study Darwin himself had indeed tentatively begun, but work of a more thorough and comprehensive quality was required. In the conventional view which the orthodoxy of the day prescribed, the terms variation and heredity stood for processes so vague and indefinite that no analytical investigation of them could be contemplated. So soon, however, as systematic inquiry into the natural facts was begun it was at once found that the accepted ideas of variation were unfounded. Variation was seen very frequently to be a definite and specific phenomenon, affecting different forms of life in different ways, but in all its diversity showing manifold and often obvious indications of regularity. This observation was not in its essence novel. Several examples of definite variation had been well known to Darwin and