"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 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.