First of all, we find useful the principle of the unit-character. According to this principle, characters are, for the most part, inherited independently of each other, and each trait is inherited as a unit or may be broken up into characters that are so inherited. Next, it must be recognized that characters, as such, are not inherited. Strictly, my son has not my nose, because I still have it; what was transmitted was something that determined the shape of his nose, and that is called in brief a "determiner." So the second principle is that unit-characters are inherited through determiners in the germ cells. And finally, it is recognized that there really is no inheritance from parent to child, but that parent and child resemble each other because they are derived from the same germ plasm, they are chips from the same old block; and the son is the half-brother to his father, by another mother. These three principles are the three corner stones of heredity as we know it today, the principles of the independent unit-characters each derived from a determiner in the germ plasm. How far are the known facts of heredity in man in accord with these principles? No doubt all human traits are inherited in accordance with these principles; but knowledge proceeds slowly in this field. As a first illustration I may take the case of human eye color. The iris is made up of a trestle-work of fibers, in which are suspended particles that give the blue color. In addition, in many eyes much brown pigment is formed which may be small in amount and gathered around the pupil or so extensive as to suffuse the entire iris and make it all brown. It is seen, then, that the brown iris is formed by something additional to the blue. And brown iris may be spoken of as a _positive_ character, depending on a determiner for brown pigment; and blue as a _negative_ character, depending on the absence of the determiner for brown. Now when both parents have brown eyes and come from an ancestry with brown eyes, it is probable that all of their germ cells contain the determiner for brown iris pigmentation. So when these germ cells, both carrying the determiner, unite, all of the progeny will receive the determiner from both sides of the house; consequently the determiners are double in their bodies and the resulting iris pigmentation may be said to be _duplex_. When a character is duplex in an individual, that means that when the germ cells ripen in the body of that individual each contains a determiner. So that individual is capable, so far as he is concerned, of transmitting his trait in undiminished intensity.