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made. There was an intellectual explosion, a flash of the mind, and from that moment his life-work was devoted to elaborating the consequences of the principle. The facts which he had been gathering and reflecting upon were explained as the effects of the cause which Malthus presented, and gathered a new significance from it. “Here, then, I had at last got a theory by which to work,” he said. The groping was at an end. His future work was outlined. The confession in that sentence can be appreciated only by one who has in his own experience passed from the mental strain and perplexity of a purely inductive effort to the solid ground afforded by even a fairly probable hypothesis. Doubtless his work was thenceforth many times more rapid than it could otherwise have been; for with so vast a number of facts to be considered the theory itself was the only pathfinder. Only after the discovery of the principle could the work of gathering up and classifying known facts and of searching for new ones, of reducing exceptions and apparently unexplainable groups of facts, go on apace. The logical process by which adaptations, variations, and the struggle for existence were brought together into the relation of cause and effect was deductive; and the principle of Natural Selection still depends for its logical support upon that power of deductive explanation which Darwin recognized in it the day he read Malthus on Population. It has penetrated every field of thought, but in the field in which it first gathered strength it is still without direct demonstration. It has been made the basis for countless deductive operations, but it leans for support on the very structures thus erected. Writing to Bentham, in 1863, concerning the proofs of natural selection and the descent of species, Darwin said, “Belief in natural selection must at present be grounded entirely on general considerations: (1) on its being a vera causa from the struggle for existence; . . . (2) from the analogy of change under domestication by man's selection; (3) and chiefly from this view connecting under an intelligible point of view a host of facts.” . To Huxley he said, in December, 1860, “I can pretty plainly see that, if my view is ever to be generally adopted, it will be by young men growing up and replacing the old workers, . . . and finding out that they can group facts and search out new lines of inves: tigation better on the notion of descent than on that of creation.”” * Life and Letters, Vol. II. p. 21o. 3 Ibid., p. 147.
From a logical point of view the work of the last thirty years in the various fields of biology has been a series of deductions and verifications of the original propositions laid down by Darwin. He saw from the beginning that belief in his theory must rest on general considerations, the chief of which was its power to facilitate deductive investigation; and there it still rests. At this late day the chief apostle of natural selection says that it is really difficult to imagine the process of natural selection in its details, and that it is, impossible to this day to demonstrate it in any one point.” It is the logical relation of the principle to the facts that makes it invaluable in modern thought. The whole logical history of Darwin's principles illustrates what Mill said of the deductive method. “To the Deductive Method thus characterized in its three constituent parts, Induction, Ratiocination, and Verification, the human mind is indebted for its most conspicuous triumphs in the investigation of nature. To it we owe all the theories by which vast and complicated phenomena are embraced under a few simple laws, which, considered as the laws of these great phenomena, could never have been detected by their direct study.” Writing of the celestial motions as an illustration, he continued, “How could we ever have ascertained the combination of forces on which the motions of the earth and planets are dependent by merely comparing the orbits or velocities of different planets or the different velocities and positions of the same planet?”
1 Weissmann, Contemporary Review, September, 1893, Vol. LXIV. p. 322.
Darwin himself did not discover the cause by the direct study of the effects; but his efforts to reach a cause inductively gave him such an insight into variations and adaptations that he could prosecute vigorously the other two steps, deduction and verification, when once the cause was given. What Mill said of celestial motions could be almost literally quoted of adaptations. It would hardly be going beyond the facts to say that the history of theories proves that usually not even preliminary hypotheses concerning causes are worked out directly from an analysis of effects; but the causes are usually caught in action during the effort to discover them inductively, or are reached in a roundabout way.
Much might be said concerning the personal qualities of the man that did so much scientific work of such uniformly high character. The moral force that overcame lifelong physical suffering, that stood through many years of silent toil face to face with the certainty of abuse for its reward, that never knew defeat and remained calm during the years of victory, has a powerful influence on the student of Darwin. The utter lack of partisanship for any idea, the rare judicial temper that made truth seem better than any theory, the penetration, the power of concentration, the firm mental grasp, the inability to leave anything unexplained, - all these high qualities have their silent evidence in the character of Darwin's scientific work. But his intellectual and moral traits have been touched upon here only in so far as was necessary in order to discuss clearly his use of the logical processes. The effectiveness of these processes