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512 transformed, through the accumulation of variations, into new species differing from their progenitors by the final test of specificity of character. In the 1850's such a radical distinction seemed to hold; even by the sixth edition of the "Origin," dated 1872, Darwin was able to point to only four somewhat debatable instances, in plants, of the infertility of varieties when intercrossed. If this difficulty appeared to Huxley and other zoologists an insuperable objection to evolutionism before 1858, it was not, in Huxley's opinion, removed after that date. Yet he no longer found the difficulty insuperable; it was purely a negative argument, e silentio, and he had faith to believe that by further investigation it would be removed. In his Edinburgh lectures of 1862, "he warned his audience of the one missing link in the chain of evidence—the fact that selective breeding has not yet produced species sterile to one another. But it is to be accepted as a working hypothesis, like other scientific generalizations, 'subject to the production of proof that physiological species may be produced by selective breeding'" In the same year Huxley wrote Darwin:

It is certainly interesting thus to observe that, as Huxley, before his conversion, saw no potency in arguments which afterwards seemed to him conclusive, so also he was able, in his second phase, to pass over by an act of faith one of the most serious of the pre-Darwinian objections to evolutionism. This provisional disregard of the "missing link" in an argument otherwise impressively well concatenated was, under the circumstances, far from unreasonable. But it would have been equally reasonable in 1846 or in 1851.

Leaving these antecedent considerations in favor of evolutionism drawn from the general principles of scientific method, I turn to the more specific facts which—when illumined by those principles—provide the now usual and familiar arguments for the theory. All the more essential of these facts were known before 1844; and attention was duly called to their bearings by the neglected prophets of evolutionism during the fifteen years preceding the publication of the "Origin of Species." In speaking of these "facts," it is well to explain what is meant, in this connection, by the expression. The theory of evolution does not rest immediately upon an induction of individual phenomena; and the evidence for it did not increase by a slow arithmetical progression, through the accumulation of observations of individual phenomena. It is a generalization established inferentially, by