The "variation" in brothers and sisters is like the variation in the mixing of beads in our bottles.
It is as though we made several tartan plaids of the same four colours, but in different patterns.
It is like dealing hands of cards from a shuffled pack. There are four suits, but one hand may be rich in clubs, another in diamonds.
And who in a game of whist would blame his partner for holding no trumps in his hand? The partner could only play the trumps dealt out to him.
In no way can a child control the pre-natal shuffling or dealing of the ancestral pack.
Now, as to atavism, or breeding back. In the ancestral bottles called men and women there are millions of different kinds of beads. And it sometimes happens that a particular kind of bead (or quality) which has lain dormant for a long time—perhaps for a thousand years—will crop up in a new mixing that goes to make a "child-bottle," and so that child may be less like its own parents than like some ancestor who has been dead and forgotten for centuries.
In the case of the man with the seven ape muscles, mentioned by Darwin, the breeding back must have reached millions of years.
This "lying doggo," or inactive, of some hereditary trait, may be likened to the action of a kaleidoscope. We do not see all the fragments of coloured glass at every turn. But they are all there.
We do not see the same pattern twice; yet the patterns are made almost of the same colours and the same pieces.
And now I think we have got a clear idea of the meanings of the words "heredity," "variation," and "atavism," and the most timid reader will not be afraid of them any more.