Blood heritage--Kind determined by origin--Ancestry a network-- Ancestry in royalty--Offspring derived from one parent only--Dual ancestry an aid in studying heredity--Reversion--Telegony-- Prenatal influences apart from heredity--Parent body and germ not identical--A hereditary character defined--Hereditary mingling a mosaic rather than a blend--Determiners of characters, not characters themselves, transmitted--Our knowledge of heredity derived along three lines--The method of experimental breeding-- The statistical method--Galton's law of regression--Correlations between parents and offspring--The biometrical method, statistical, not physiological--Mental as well as physical qualities inheritable. II THE BEARERS OF THE HERITAGE 20 The cell the unit of structure--Unicellular organisms--Importance of cell-theory--Heredity in unicellular forms--Reproduction and heredity in colonial protozoa--Conjugation--Specialization of sex-cells--The fertilized ovum--Advancement seen in the Volvox colony--Natural death--Specialization in higher organisms--Sexual phenomena in higher forms--Cell-division--Chromosomes constant in number and appearance--Significance of the chromosomes--Cleavage of the egg--Chief processes operative in building the body--The origin of the new germ-cells--Significance of the early setting apart of the germ-cells--Individuality of chromosomes--Pairs of chromosomes--Reduction of the number of chromosomes by one-half-- Maturation of the sperm-cell--Maturation of the egg-cell--Parallel between the two processes--Fertilization--Significance of the behavior of the chromosomes--A single set of chromosomes sufficient for the production of an organism--The duality of the body and the singleness of the germ--The cytoplasm in inheritance--Chromosomes possibly responsible for the distinctiveness of given characters--Sex and heredity--Many theories of sex determination--The sex-chromosome--Sex-linked characters in man--In lower forms. III MENDELISM 67