I propose now to pass in review some characters in man known to be inherited, choosing preferably those that come nearest to the field of pathology, or belonging to it. I shall begin with comparatively simple cases, about which there can be little doubt, and pass to more and more difficult situations. I am taking the risk of reaching an anticlimax, but nevertheless such a procedure will, I hope, serve our purpose this evening if I can point out where the evidence is satisfactory and where it is deficient. My first illustration of inheritance in man may be said to be a physiological one, mainly because we do not know at present any structural or chemical basis for the reaction. [Illustration: FIG. 6. Inheritance of color blindness of man which is sex-linked (i.e., the factor for color blindness is carried in the X-chromosome). This X-chromosome is stippled in the figure while the X-chromosomes for normal eyes are represented by black X’s. The color-blind eye is also stippled, and the normal eye (which distinguishes between red and green) is here represented by an eye half black and half cross-lined. The 1st generation offspring (F₁) are normal eyed. In the 2d generation offspring, half the sons are color blind.] Color-blindness in man is clearly a case of sex-linked inheritance. It conforms to the general scheme of inheritance in other animals; in Drosophila, for example, we have about sixty mutant characters which show this form of inheritance. A color-blind man married to a normal woman has only normal daughters and sons; all of the daughters, however, transmit color-blindness to half of their sons, Fig. 6. Color-blind women are rare, because they can never arise unless a color-blind man marries a woman who is color-blind, or else marries a normal woman who had a color-blind father, or had a mother heterozygous for color-blindness, Fig. 7. [Illustration: FIG. 7. Reciprocal of the cross shown in Fig. 6. Here a normal-eyed male marries a color-blind female, giving all color-blind sons and normal daughters. When two individuals like these marry, the expectation is for half of the daughters and half of the sons to be color blind, and half of the daughters and half of the sons to be normal eyed.] The pedigrees of color-blind families—and they are many—leave little doubt as to the mode of inheritance of this character.