The cause of variation can be easily understood.
Variation is due to the fact that every child has two parents. If these
two parents were exactly alike, and if their ancestors had been all
exactly alike, their children would be exactly like each other and like
their parents.
But the father and mother are of different families, of different
natures, and perhaps of different races. And the ancestors of the father
and mother--millions in number--were all different from each other in
nature and in descent.
Now, since a child inherits some qualities from its father and some from
its mother, it follows that if the father and mother are different from
each other, the child must differ from both, and yet resemble both.
For he will inherit from the father qualities which the mother has
not inherited from her ancestors, and he will inherit from the mother
qualities which the father did not inherit from his ancestors. So the
child will resemble both parents, without being an exact copy of either.
It "varies" from both parents by inheriting from each.
The child of a black and a white parent is what we call a half-caste: he
is neither a negro nor a white man. The pup of a bulldog and a terrier
is neither a bull-dog nor a terrier; he is a bull-terrier terrier.
But heredity goes farther than that, and variation is more complex than
that.
We must not think of a man as inheriting from his father and mother
only. He inherits from the parents of both his parents; and from
thousands of ancestors before those. He inherits from men and women
who died thousands of years before he was born. He inherits from the
cave-man, from the tree-man, from the ape-man, from the ape, and from
the beast before the ape.
The child in the womb begins as a cell, and develops through the stages
of evolution, becoming an embryo worm, fish, quadruped, ape, and,
finally, a human baby.
The child is born with the bodily and mental qualities inherited from
many generations of beasts and many generations of men.
Any one of the many ancient qualities of mind or body may crop up again
in a modern child. Children have been born with tails: children have
been born with six nipples, like a dog, instead of with two, like a
human being.
And now I will explain, simply and briefly, what we mean by the word
"Atavism."