The origin of subspecies and varieties as found in nature was not
proved, but only generally recognized as evident. A broader knowledge
has brought about the same state of opinion for greater groups of
relationships. Systematic affinities find their one possible explanation
by the aid of this principle; without it, all similarity is only
apparent and accidental. Geographic and paleontologic facts, brought
together by Darwin and others on a previously unequalled scale, point
clearly in the same direction. The vast amount of evidence of all [4]
comparative sciences compels us to accept the idea. To deny it, is to
give up all opportunity of conceiving Nature in her true form.
The general features of the theory of descent are now accepted as the
basis of all biological science. Half a century of discussion and
investigation has cleared up the minor points and brought out an
abundance of facts; but they have not changed the principle. Descent
with modification is now universally accepted as the chief law of nature
in the organic world. In honor of him, who with unsurpassed genius, and
by unlimited labor has made it the basis of modern thought, this law is
called the "Darwinian theory of descent."
Darwin's second contribution to this attainment was his proof of the
possibility of a physiological explanation of the process of descent
itself.
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