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.