idence
is wanting; and, so long as all the animals and plants certainly
produced by selective breeding from a common stock 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 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 the naturalists since
the invention of the natural system of classification, and the
commencement of the systematic study of embryology."--_Man's
Place in Nature_, p. 149.[E]

FOOTNOTES:

[Footnote E: Further details on the subject of this chapter may be
obtained in Clodd's excellent volume, _Pioneers of Evolution_, where
an account of the history of the idea of evolution from the earliest
times is given; and in Poulton's _Charles Darwin and the Theory of
Natural Selection_, where there is a particularly valuable chapter
upon Huxley's relation to Darwinism.]

CHAPTER VIII

VERTEBRATE ANATOMY

The Theory of the Vertebrate Skull--Goethe, Oken, Cuvier, and
Owen--Huxley Defends Goethe--His Own Contributions to the
Theory--The Classification of Birds--Huxley Treats them as
"Extinct Animals"--Geographical Distribution--Sclater's
Regions--Huxley's Suggestions.

We have seen that some of the most important of the contributions made
by Huxley to zooelogical knowledge were in the field of the lower
animals, especially of those marine forms for the study of which he
had so great opportunities on the _Rattlesnake_. A great bulk of his
zooelogical work, however, related to the group of back-boned animals.
These, by their natural affinities and anatomical structure, are more
closely related to man, and, as Huxley began his scienti