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HE theories with which Darwin dealt were so general, and the facts that had to be handled as evidence were so vast in number, that probably no man was ever exposed to greater temptation to collect his evidence promiscuously from all quarters, picking up in each field what was already known, and supplementing it by a few test observations. But he never contented himself with sketching theories and adorning them with dashes of evidence. He made himself invincible by the exhaustiveness with which he determined the quality of his evidence. The great confidence which scientific men have had in him has been due to the fact that he did not leave it to them to test the theories which he presented. He convinced the world of the truth of a doctrine which others had striven in vain for fifty years to establish. To my mind one of the chief characteristic differences between his work and that of Lamarck and others, apart from differences in the explanations offered, is its superb exhaustiveness. It is as impossible now to take the ideas of descent and of natural selection out of the world as to take a star out of the sky. The firm establishment of these ideas was due to the quality and quantity of Darwin's work, and both of these were determined by the same exhaustiveness in method. When he started out to describe the single little abnormal cirriped from the west coast of South America, he was characteristically led, as he said, for the sake of comparison, to examine the internal parts of as many genera as he could procure. This untamed determination to find out all there was to know about what he was describing was associated with a fine contempt for the kind of work that merely describes new things without showing all their connections. One of the greatest and most constant obstacles to his progress was that this intellectual quality was so rare or so little cultivated in other naturalists; so much of the scientific material with which he had to deal was so superficially or carelessly worked out that he never knew what to trust. Among the best examples of this spirit of Movement in Plants" was written, it has been shown that the conclusion of the authors that “an object which yields with the greatest ease will deflect a radicle " is wrong. The whole superstructure of reasoning which resulted in the notion that the tips of radicles are sensitive to contact was therefore built on sand. The experiments and reasoning will be discussed from the point of view of the authors at the time they were made, and afterwards attention will be called to the corrections that have since been made by others. In their work on the movements of radicles, Charles and Francis Darwin found that “an object which yields with the greatest ease will deflect a radicle.” Extremely thin tin-foil on soft sand was not at all impressed, and deflected the root at right angles. Hence, they reasoned, the cause of the deflection could not be mechanical contact. A conceivable hypothesis was that “the gentlest pressure might arrest growth and the apex grow only on one side; but this view leaves unexplained the curvature of the upper part, extending for a length of 8–10 mm. . . . We were therefore led to suspect that the apex was sensitive to contact, and that an effect was transmitted from it to the upper part.”". By the

exhaustiveness is his study of pigeons." As usual, he knew clearly what he was after, and this gave him the power of selecting judiciously the lines along which to make investigations and of using to the best advantage the materials he worked on. In his remarks on the search for the cause of the modification of species, he said, “Believing that it is always best to study Some special group, I have, after deliberation, taken up domestic pigeons.” With other ends in view, pigeons might be studied in different ways. But remembering his purpose, his work on pigeons is a model of exhaustiveness as well as of reasoning. He not only studied the variation of breeds, but sought its explanation by a minute study of individual differences. He considered the skeleton as well as the feathers, and gathered facts and specimens from all over the world. What is true of his study of pigeons is true of his work on orchids. The adaptation of flowers for cross-fertilization had interested him for many years, and he had collected a large mass of observations; but he was true to his instinct: “It seemed to me a better plan to work out one group of plants as carefully as I

1 Variation of Animals and Plants under Domestication, Chaps. V. and VI., pp. 137-235.

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could rather than publish many miscellaneous
and imperfect observations.” Orchids furnished
an extreme case; and his work on them is fas-
cinating from the nature of the subject, the end
aimed at, and the ingenuity of the reasoning
employed. He showed “how admirably these
plants are constructed so as to permit of, or to
favor, or to necessitate cross-fertilization ”; but
the way in which he did it is as admirable from
a logical point of view as the flowers them-
selves are in their peculiar adaptation. By
thus selecting judiciously the most extreme
special cases for exhaustive examination, he
threw the strongest light on all the collateral
evidence, and made it easy for him to under
stand its significance. On the shoulders of
such work his theories sat firmly, and it made
it easy for those who came after him to work
out the classes of facts which he was not able
to exhaust.
In each of the many special studies which he
carried on there are many models of method in
the pursuit of details. The following case is
especially interesting because it illustrates both
the habitual care of the authors in their experi-
ments on the movements of plants and the ex-
treme liability to error that results from a
wrong start. Since the book on the “Power of

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* Power of Movement in Plants, pp. 131–140.

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