Heredity is "descent," or "breed." Heredity, as the word is here used,
means those qualities which are handed down from one generation to the
next. It means those qualities which a new generation inherits from the
generation from whom it descends.
It means all that "is bred in the bone." If a man inherits a Grecian
nose, a violent temper, well-knit muscles, a love of excitement, or a
good ear for music, from his father or mother, that quality or feature
is part of his heredity. It is "bred in him."
Every quality a child possesses at the moment of birth, _every_ quality
of body or of mind, is inherited from his parents and their ancestors.
And the whole of those qualities--which _are_ the child--are what we
call "heredity."
No child brings into the world one single quality of body or mind that
has not been handed down to it by its ancestors.
And yet no two children are exactly alike, and no child is exactly like
any one of its forbears.
This difference of children from each other and from the parent stock is
called "variation."
Hundreds of books and papers have been written about "variation," and
to read some of them one might suppose variation to be a very difficult
subject. But it is quite simple, and will not give us any trouble at
all. Let us see.