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VII.

ANALOGY.

NALOGICAL reasoning plays a very important part in all scientific work; and Darwin frequently availed himself of its help in making discoveries and establishing conclusions. He used every logical device to establish and extend his theories, and there is no lack of material from which to choose examples. But analogy, when used on a large scale, proves so treacherous, that it is useful for the most part only in giving clues to discoveries. There are but few examples of analogical reasoning on a large scale in Darwin's works. The most important, perhaps, is his work on Insectivorous Plants. It has already been told how, while resting at Hartfield after years of labor on the Origin of Species, he was struck by the number of insects caught by the leaves of the common Sundew. It soon became evident to him that “Drosera was excellently adapted for the special purpose of catching insects, so that the subject

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seemed well worthy of investigation.” As soon as he began to work on Drosera, and was led to believe that the leaves absorbed nutritious matter from the insects, he began to reason by analogy from the well understood digestive capacity of animals. One needs but to imagine an attempt to do the work without any knowledge of animal digestion to understand at once its impossibility under such conditions. By connecting his observations with the well known animal processes he proceeded on a course of rapid discovery that must otherwise have remained entirely closed to him. At almost every step he drew suggestions from and checked his results by reference to animal digestion. He made preliminary “crucial" experiments by immersing some leaves of Drosera in nitrogenous and others in nonnitrogenous fluids of the same density to determine positively whether the former affected the leaves differently from the latter.” The discovery that Drosera detects “with almost unerring certainty the presence of nitrogen" in various fluids “led me to inquire,” he said, “whether Drosera possessed the power of dissolving solid animal matter; the experiments proving that the leaves are capable of true digestion, and that the glands absorb the digested matter.”” “These are, perhaps, the most interesting of all my observations on Drosera, as no such power was before distinctly known to exist in the vegetable kingdom.” Having by analogy established the power of true digestion in plants, analogy led him to seek in plants the elements that do the work of digestion in animals. Bringing together what was known of plants, he pointed out that the juices of many plants contain an acid, and so one element of a digestive fluid was at hand; and that all plants possess the power of dissolving albuminous or proteid substances, protoplasm, chlorophyll, etc., and that “this must be effected by a solvent, probably consisting of a ferment together with an acid.”” After writing the last quoted sentence he learned that a ferment which converted albuminous substances into true peptones had been extracted from the seeds of the vetch. Sachs mentioned the discovery of the ferment, recorded the fact that peptones had themselves been actually found in the seeds of the lupine, and added “as we come to know the proteinaceous reserve materials of plants better, and if we follow their behavior in the

2 Insectivorous Plants, p. 2. * Ibid., p. 76.

* Insectivorous Plants, p. 268. * Ibid., p. 362.

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animal body also, it can scarcely be doubtful that, in spite of incomplete knowledge, the assumption is nevertheless warranted that peptonizing ferments are perhaps universally distributed in plants.” “Attention was first drawn to the occurrence of peptonizing ferments in the vegetable kingdom by the remarkable phenomena observed in the so called insectivorous plants.” By analogical reasoning a whole new field of study was opened; a new view of the powers of plants was gained, and a much closer analogy between plant and animal functions was established. But if recent studies are taken into account, the question may be raised whether this stupendous analogical structure has not been undermined. Tischutkin contends that the “digestion” of insectivorous plants is not accomplished in the same way as in animals, but is due to bacteria; that the pepsin of the leaves is not a secretion of the plant, but a by-product of the activity of the bacteria.” He proves that bacteria capable of dissolving egg albumen are always present in the secretion of the leaves; that they come principally from the air, that the plant only furnishes a medium for them to live in, that disintegration of albuminous substances begins only after enough micro-organisms are devel. oped to do the work, and that the plant simply assimilates what these lower organisms have set free. The relation between the insectivorous plants and the bacteria is one of genuine symbiosis. If the whole of Tischutkin's contention is true, the great body of facts brought out by Darwin must still be placed to the credit of analogical reasoning. The facts concerning plant and animal digestion would still remain parallel, both in the succession of the phenomena and in the results. It would be another illustration of the vast importance of analogy in scientific method, and of the fact that every analogy, the strongest as well as the faintest, will sooner or later fail. In another instance, analogical reasoning from animals to plants actually deterred him from discovering the truth to which other logical processes might have led him. He states the case so clearly himself that it will almost suffice to quote him.” “The adaptation of flowers for cross-fertilization is a subject which has interested me for the last thirtyseven years. . . . From my own observations

* Sachs, Physiology of Plants, p. 344. * Berichte der Deutschen Botanischen Gesellschaft, 1889.

1. Effects of Cross- and Self-Fertilization, pp. 6–8.

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