3 86 THE POPULAR SCIENCE MONTHLY. 



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 re- 

 interpretation 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 assimi- 

 lating 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 explana- 

 tion 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 * was the first to explain this group of facts by 

 an application of the theory of evolution. If the theory is true, 



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

 1884, 



