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here as everywhere, compelled recognition by forcing themselves in increasing numbers upon the attention of biologists. The effort to suppress them by the old theory gave place gradually to the effort to base the new idea of continuity upon them. This period of reversal in scientific activity and the accompanying rapid reinterpretation of both the old and new phenomena, while it is recognizable in the histories of many sciences, is probably most striking in the history of evolution.

Every hypothesis by its nature accords with the facts from which it sprang. But it is the weakness of all hypotheses, true and false alike, that they are at first based on only a small part of the facts, and these are nearly always the most unsafe, because they are, as has been shown, the most highly specialized. The true hypothesis has to pass successfully through the ordeal of assimilating large bodies of facts that are already known by observation apart from the hypothesis. The theory of evolution did this as thoroughly and perhaps more rapidly since Darwin's time than any other scientific theory. There is a vast number of illustrations of this, but a typical one will suffice. Anatomy had long ago established the presence of valves in human veins, and physiology assigned to them the only intelligible function—that of preventing the blood from flowing back toward the capillaries. Had they been distributed throughout the venous system, there would have been no problem; but they are present in some veins and absent in others. No law regulating their distribution could be assigned, and students of human anatomy had to learn their distribution by sheer force of memory. Here was a fine group of arbitrary facts established by empirical observation. Not only was there no law to explain their distribution—their actual arrangement was utterly irrational if it were true that they were intended to prevent the backward flow of blood. It was easy enough to understand, from the old view of creation, why there should be valves in the veins of the arms and legs; but it was stultifying to learn that the spinal, iliac, portal, and above all the inferior vena cava, the largest vein in the body carrying blood upward, are without valves. To make the facts and their functional explanation still more incongruous, there are valves in the intercostal veins, in which the blood flows horizontally; and in the thyroid and internal and external jugulars, in which the blood flows down hill. Valves and gravitation apparently had nothing to do with each other.

Dr. Clevenger[1] was the first to explain this group of facts by an application of the theory of evolution. If the theory is true,

  1. Physiology and Psychology, Clevenger, pp. 38-46. American Naturalist, January, 1884.