viii CONTENTS 



VI. Tlic Chromosoiiies and MendeVs Two 



Laws 73 



THE CELLULAR BASIS OF HEREDITY AND 



DEVELOPMENT 73 



THE MECHANISM OF MENDEl's TWO 



LAWS 82 



VII. Tlie Linkage Groups and the 



Chromosomes 87 



THE FOUR GREAT LINKAGE GROUPS OF 



DROSOPHILA MELANOGASTER 89 



VIII. Sejc-Linkcd Inh eritance 101 



IX. Crossing -over 111 



X. Natural Selection and Evolution 119 



THE THEORY OF NATURxVL SELECTION 119 



THE MEASUREMENT OF VARIATION 121 



SELECTION AND VARIATION 129 



PURE LINES 130 



GENETIC VARIABILITY 133 



CONCLUSIONS 135 



