A frequent source of doubt and misunderstanding is the universal occurrence of such distinct types among brothers and sisters. The query at once arises, "Have not these children the same heredity?" Brothers and sisters have the same ancestors, but not the same heredity. Recent biological discoveries teach us that the individual develops from a bundle of units derived from the two parents, but the units supplied by a parent never represent the totality of the parents' composition, nor do all the units that are passed on come to manifest themselves as parts of the character. The parent passes on sample units from her or his own inheritance, so that no two combinations are ever exactly alike. It is a commonplace observation that Johnny may have his maternal grandmother's chin, his paternal grandmother's eyes, his father's walk, his Uncle George's lips, his Aunt Mary's sharp tongue, his grandfather's alertness, and his mother's good judgment. Of course, he has _not_ his grandmother's eyes or his uncle's lips: these relatives still retain their respective facial organs, and his father still has his quick temper. What Johnny has inherited is a something, perhaps in the nature of a ferment, which _determines_ the color of his eyes, a certain something that makes his lips develop into that particular shape, a certain something that causes his brain to respond to annoyance in the same manner as that of his Aunt Mary's. And the various ancestors and relatives have received from their parents similar determining factors that have manifested themselves in similar peculiarities. We do not inherit from our relatives, or even from our parents: we are built up of the same elements as those of which our relatives are built, but each one of us has received his individual combination of factors. Hence, no two brothers or sisters are exactly alike, although they have the same parents and the same ancestors. While it is universally recognized that no two individuals are exactly alike, we are not at all clear in our minds as to whether the important differences arise from differences in experience or _nurture_, or from essential differences in _nature_. We know that children of the same parents are essentially different from birth, and that no matter how similar the treatment they receive afterward they will always remain different, or even become more different as they become older. It is becoming more clear every day, as a result of scientific study, that every individual is absolutely unique, excepting only "true" twins.