Charles Robert Darwin (18091882). Origin of Species. The Harvard
Classics. 190914. VI. Difficulties of the Theory Summary: the
Law of Unity of Type and of the Conditions of Existence Embraced
by the Theory of Natural Selection WE have in this chapter
discussed some of the difficulties and objections which may be
urged against the theory. Many of them are serious; but I think
that in the discussion light has been thrown on several facts,
which on the belief of independent acts of creation are utterly
obscure. We have seen that species at any one period are not
indefinitely variable, and are not linked together by a
multitude of intermediate gradations, partly because the process
of natural selection is always very slow, and at any one time
acts only on a few forms; and partly because the very process of
natural selection implies the continual supplanting and
extinction of preceding and intermediate gradations. Closely
allied species, now living on a continuous area, must often have
been formed when the area wa