structural differences, I should have no hesitation in saying that Mr.
Darwin had demonstrated the existence of a true physical cause, amply
competent to account for the origin of living species, and of man among
the rest.

But, in addition to their structural distinctions, the species of
animals and plants, or at least a great number of them, exhibit
physiological characters--what are known as distinct species,
structurally, being for the most part either altogether incompetent to
breed one with another; or if they breed, the resulting mule, or hybrid,
is unable to perpetuate its race with another hybrid of the same kind.

A true physical cause is, however, admitted to be such only on one
condition--that it shall account for all the phenomena which come within
the range of its operation. If it is inconsistent with any one
phenomenon, it must be rejected; if it fails to explain any one
phenomenon, it is so far weak, so far to be suspected; though it may
have a perfect right to claim provisional acceptance.

Now, Mr. Darwin's hypothesis is not, so far as I am aware, inconsistent
with any known biological fact; on the contrary, if admitted, the facts
of Development, of Comparative Anatomy, of Geographical Distribution,
and of Palaeontology, become connected together, and exhibit a meaning
such as they never possessed before; and I, for one, am fully convinced,
that if not precisely true, that hypothesis is as near an approximation
to the truth as, for example, the Copernican hypothesis was to the true
theory of the planetary motions.

But, for all this, our acceptance of the Darwinian hypothesis must be
provisional so long as one link in the chain of evidence is wanting; and