this for his subjects before he gave them to the world. His work on the "Expression of the Emotions" began in 1838 and closed in 1872; "Insectivorous Plants," 1860-1876; "Vegetable Mould and Earthworms," 1837-1881. One of the most notable legacies that he left to the ambitious student is his example of great energy and great patience, his incarnation of the truth that time, as well as reason, is the handmaid of science.

Coupled with the habit of treating exhaustively the subjects with which he dealt, and the willingness to bide his time for publication until his views had reached their full maturity, was his extreme conscientiousness in giving full force to the objections against his, general results. The habit of pursuing all the facts to their meaning made it possible for him to say that he had been able to consider in advance all the objections that were afterwards made to any of his views. The summing up of his experiments on Adonis æstivalis, in the "Effects of Cross- and Self-Fertilization," is an example of the great length to which he went in recording facts, especially if they were in any way opposed to his conclusions. He said, "The results of my experiments on this plant are hardly worth giving, as I remarked in my notes

made at the time, 'seedlings from some unknown cause, all miserably unhealthy,' nor did they ever become healthy; yet I feel bound to give the present case, as it is opposed to the general results at which I have arrived.”1

He did not hesitate to diminish the positive results of his experiments or the effect of his views by incorporating all exceptions, unless they were clearly due to some known extraneous cause. But his inability to leave any thing unexplained was so great that he rarely left exceptional facts without at least suggestions of possible explanations. He was extremely ingenious in guessing explanations for facts that could not be brought under the same general explanation as the other facts of their class. A good instance of this art of wriggling is his attempt to explain the sloping terraces of Coquimbo.2

1 Effects of Cross- and Self-Fertilization, p. 128.

2 Geological Observations, etc., pp. 256-258.

6

IT

V.

NEGATIVE EVIDENCE.

T would naturally be expected, from Darwin's clear notions of evidence in general, and the necessity that he was under all his life of handling vast bodies of complicated evidence, that his work would furnish examples for the treatment of negative evidence. His estimate of its value is well shown by his treatment of the question whether Primula veris, P. vulgaris, and P. elatior are different forms of the same species.1 After discussing the evidence in favor of this view he said, "Negative evidence is of little value; but the following facts may be worth giving." Then follows the recital of his efforts to determine whether the cowslip varies enough to justify the belief. He transplanted cowslips from the fields into a shrubbery, and then into highly-manured land; the next year they were protected from insects, artificially fertilized, and seed grown, which

1 Different Forms of Flowers on Plants of the same Species, p. 62.

was sown in a hot-bed. The young plants were set out, some in very rich soil, some in stiff, poor clay, some in old peat, and others in pots in the greenhouse, seven hundred and sixty-five in all. Though they and their parents were subjected to all this diversity of treatment, "not one of them presented the least variation except in size." Negative evidence is indeed of little value, unless it can be shown that it covers the whole ground. In order to transform these experiments into proof it would be necessary to show that, if the three forms belong to one species, the cowslip should have varied under the conditions to which it was subjected. It is far more difficult to disprove a proposition by negative evidence than by proving the truth of its contradictory. Darwin accordingly demonstrated what several other botanists had surmised: that the oxlip is a hybrid between the cowslip and the primrose.

His efforts to determine whether Orchis morio secretes nectar also furnish a good illustration of his treatment of negative evidence.1 A nectary implies nectar, but Sprengel had thoroughly searched many flowers of O. maculata and morio, and could not find a drop. Of his own efforts in this direction, Darwin said,

1 Fertilization of Orchids, pp. 36-41.

"I have looked to all our common British species and could find no trace of nectar; I examined, for instance, eleven flowers of O. maculata, taken from different plants growing in different districts, and taken from the most favorable position on the spike, and could not find under the microscope the smallest bead of nectar." Sprengel believed that these plants exhibit an organized system of deception, "for he well knew that the visits of insects were indispensable for their fertilization"; but Darwin could not believe in so gigantic an imposture. “Notwithstanding these several facts,” he went on, "I still suspected that nectar must be secreted by our own orchids, and I determined to examine O. morio rigorously. As soon as the flowers were open I began to examine them for twenty-three consecutive days; I looked at them after hot sunshine, after rain, and at all hours; I kept the spikes in water and examined them at midnight and early the next morning." He irritated the nectaries with bristles and exposed them to irritating vapors. He examined flowers whose pollinia had been removed, and others which would probably have them soon removed. But the nectary was invariably dry. Only after he had made the negative evidence as complete as it could be made, by examining