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be one of disappointment. Its immediate influence seems to have been strangely small in proportion to the magnitude of the discovery. It would be interesting if we could tell to what extent it influenced Harvey's own practice. It seems to have had very little on that of his followers. 'Tis curious to notice how Willis in his Tract on Fevers published in 1659 recognised, that the discovery of the circulation had established a new founda- tion for medicine, and discredited the Galenical doctrine of the constitution of the blood; and then to notice how he clears away the ancient rubbish of the four humours only to replace it with another hypothesis, and takes this for the basis of all his reasonings on fevers, for his ex- planations of their phenomena and his indications for their treatment. He shows unconsciously how much need there was in those times of Harvey's exhortation "to search out the secrets of Nature hy way of experiment " Why the immediate practical fruits of Harvey's discovery were so small, seems to me to have been for this reason : that practice at that time was far in advance of theory. Physicians had already acquired by observation