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
breeding will not be proved to be competent to do all that is required
of it to produce natural species.

I have put this conclusion as strongly as possible before the reader,
because the last position in which I wish to find myself is that of an
advocate for Mr. Darwin's, or any other views--if by an advocate is
meant one whose business it is to smooth over real difficulties, and to
persuade where he cannot convince.

In justice to Mr. Darwin, however, it must be admitted that the
conditions of fertility and sterility are very ill understood, and that
every day's advance in knowledge leads us to regard the hiatus in his
evidence as of less and less importance, when set against the multitude
of facts which harmonize with, or receive an explanation from, his
doctrines.

I adopt Mr. Darwin's hypothesis, therefore, subject to the production of
proof that physiological species may be produced by selective breeding;
just as a physical philosopher may accept the undulatory theory of
light, subject to the proof of the existence of the hypothetical ether;
or as the chemist adopts the atomic theory, subject to the proof of the
existence of atoms; and for exactly the same reasons, namely, that it
has an immense amount of prima facie probability: that it is the only
means at present within reach of reducing the chaos of observed facts to
order; and lastly, that it is the most powerful instrument of
investigation which has been presented to naturalists since the
invention of the natural system of classification, and the commencement