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the nectaries of very many flowers from different places under all the possible circumstances in which nectar might be secreted, did he feel justified in saying, “We may therefore safely conclude that the nectaries of the above named orchids neither in this country nor in Germany ever contain nectar.” Even then he restricted the negative conclusion to the two countries in which the exhaustive examinations had been made. But he did not rest with this negative evidence. It was strong enough to convince him that there was no ordinary nectar, but the further evidence that he presents shows how quickly negative evidence falls into the background in the presence of even the most indirect positive evidence. He was thoroughly convinced that these orchids require the visits of insects for fertilization, that insects visit flowers for the attractions offered in the way of nectar, pollen, etc.; that nature could not deceive insects by a permanent imposture, and yet that in these orchids the ordinary attraction was absent. It was as if a crime had been committed, and he were asked to believe there was no criminal. In examining the nectaries of the several species of orchids he was “surprised at the degree to which the inner and outer membranes forming the tube or spur were separated from each other, also at the delicate nature of the

inner membrane, . . . and, lastly, at the quantity of fluid contained between the two membranes.” He found the space between the

membranes of other nectaries quite dry. He then examined other forms that do secrete nectar in the ordinary way, and found the membranes closely united, instead of separated by a space. “I was therefore led to conclude,” he said, “that insects penetrate the lax membrane of the nectaries of the above named orchids and suck the copious fluid between the two membranes. This was a bold hypothesis, for at the time no case was known of insects penetrating with their proboscides even the laxest membrane.” He afterwards learned that at the Cape of Good Hope moths and butterflies penetrate peaches and plums, and in Queensland, Australia, a moth penetrates the rind of the orange. These facts merely proved his anticipation less anomalous than it had seemed. The bees which he saw visiting Orchis morio kept their proboscides inserted in the nectaries for some time. He opened several nectaries, and found brown specks, due, as he believed, to punctures made some time before. Herman Müller has since corroborated Darwin's interpretation, saying, “My own observations have confirmed this view, as well as every detail of the rest of Darwin's account.”" The negative evidence was, by its very completeness, a stumbling-block to Darwin's beliefs. As must sooner or later be done with all instances of negative evidence, he again set about replacing it by positive evidence, which removed the necessity of the belief which the negative evidence destroyed. Until that was done the negative evidence increased rather than diminished the mystery that needed solution.

* H. Müller, Fertilization of Flowers, pp. 535–538.

VI.
CLASSIFICATION.

ERFORMED consciously or unconsciously, the act of classification is indispensable to and accompanies every scientific inference. A mind is orderly or slovenly, according as it does or does not habitually and accurately classify the facts with which it comes in contact. The success of an investigation, the worth of a conclusion, are in direct proportion to the fidelity to this principle and the exhaustiveness with which the process is carried out. In nature, constant forces at work upon varying materials necessarily produce segregation; the like are brought together, and the unlike separated. The result is a literally “natural classification.” This simple result is rarely realized. The forces at work are so numerous, and have acted so long, that, especially with reference to living things, Nature's serial classifications in space and time have been broken up and thrown into confusion to such an extent that they are seldom recognizable. The result is a superficial chaos of phenomena. The recognition of natural classifications was an excessively slow growth; they were finally worked out by the slow collection of material and successive attempts at a natural arrangement. In nature's arrangement of living things over the earth it has been very difficult to recognize law, and at first it was possible only where isolation has been long continued and the forces at work upon living things have been few and steady in their action. Even then the recognition has required extensive travel and a powerful inclination to classify and recognize the relations of distant facts to each other. It was the recognition of several such arrangements or classifications in Nature that first led Darwin to reflect on the Origin of Species. What immortalized his observations is not the simple fact that they were made, but that by their cumulative presentation they led Darwin to seek an adequate cause for these natural arrangements. After pointing out, in the narrative of his voyage, the striking relation between the fossil and the living animals of South America, he said: “This wonderful relationship in the same continent between the dead and the living will, I do not doubt, hereafter throw more light on

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