As man can produce, and certainly has produced, a great result by his methodical and unconscious means of selection, what may not natural selection effect? Man can act only on external and visible characters: Nature, if I may be allowed to personify the natural preservation or survival of the fittest, cares nothing for appearances, except in so far as they are useful to any being.

She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life.

Man selects only for his own good; Nature only for that of the being which she tends.

Every selected character is fully exercised by her, as is implied by the fact of their selection.