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N this year of the Darwin centenary it is worth while to raise two questions which have, in the mass of literature elicited by the occasion, received less consideration than they merit. At what date can the evidence in favor of the theory of organic evolution—as distinct from the hypothesis of natural selection—be said to have been fairly complete: in other words, how early were the facts and principles from which the truth of that theory is now ordinarily inferred sufficiently known to all competent men of science, to require the inference, even though it was not, in fact, generally made? And by what English writer was a logically cogent argument for the theory first brought together and put before the public? The interest attaching to these questions is much more than merely historical. The answer to them will afford a sort of object-lesson in the logic of scientific reasoning. Here is a doctrine now accepted by all naturalists: at what point, in the century-long accumulation, through half a dozen separate sciences, of the evidences inclining to that doctrine, ought we to say that the balance of logical probability turned decisively in its favor? The inquiry will also be found, I think, to throw a somewhat instructive light upon the psychology of belief, and to show how far, even in the minds of acute and professedly unprejudiced men of science, the emotion of conviction may lag behind the presentation of proof.

By this time, no doubt—though it has not long been so—every schoolboy knows that Darwin did not invent the theory of evolution. The Darwin centenary itself has served to remind the public of the names and works of at least some of the earlier protagonists of the doctrine: of the elder Darwin, namely, of Lamarck, of Geoffroy St. Hilaire, of the author of the "Vestiges of the Natural History of Creation," and of Herbert Spencer. It is less commonly remembered, but perhaps not universally forgotten, that among English-speaking naturalists, the theory was a commonplace topic of discussion for two or three decades before 1859, and especially after the publication and immense circulation of the successive editions of Robert Chambers's "Vestiges," of which the first appeared in 1844. Geological text-books of the period referred to the "theory of transmutation of species" as