er question
of the tenability, or untenability of Mr. Darwin's views. But here
we enter upon difficult ground, and it behoves us to define our exact
position with the greatest care.

It cannot be doubted, I think, that Mr. Darwin has satisfactorily proved
that what he terms selection, or selective modification, must occur, and
does occur, in nature; and he has also proved to superfluity that such
selection is competent to produce forms as distinct, structurally, as
some genera even are. If the animated world presented us with none but
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
so long as all the animals and plants certainly produced by selective
breeding from a common stock are fertile, and their progeny are fertile
with one another, that link will be wanting. For, so long, selective
bre