Having attained to a knowledge of the circulation of the blood, and of the conditions on which its motion depends, Harvey had a ready deductive solution for problems which had puzzled the older physiologists. Thus the true significance of the valves in the veins became at once apparent. Of no importance while the blood is flowing in its normal course toward the heart, they at once oppose any accidental reversal of its current which may arise from the pressure of adjacent muscles or the like. And in like manner the swelling of the veins on the farther side of the ligature, which so much troubled Cæsalpinus, became at once intelligible as the natural result of the damming up of the returning current. In addition to the great positive results which are contained in the treatise which Harvey modestly calls an _Exercise_ and which is, in truth, not so long as many a pamphlet about some wholly insignificant affair, its pages are characterized by such precision and simplicity of statement, such force of reasoning, and such a clear comprehension of the methods of inquiry and of the logic of physical science, that it holds a unique rank among physiological monographs. Under this aspect, I think I may fairly say that it has rarely been equalled and never surpassed. Such being the state of knowledge among his contemporaries, and such the immense progress effected by Harvey, it is not wonderful that the publication of the _Exercitatio_ produced a profound sensation. And the best indirect evidence of the originality of its author, and of the revolutionary character of his views, is to be found in the multiplicity and the virulence of the attacks to which they were at once subjected. Riolan, of Paris, had the greatest reputation of any anatomist of those days, and he followed the course which is usually adopted by the men of temporary notoriety toward those of enduring fame. According to Riolan, Harvey's theory of the circulation was not true; and besides that, it was not new; and, furthermore, he invented a mongrel doctrine of his own, composed of the old views with as much of Harvey's as it was safe to borrow, and tried therewith to fish credit for himself out of the business. In fact, in wading through these forgotten controversies, I felt myself quite at home. Substitute the name of Darwin for that of Harvey, and the truth that history repeats itself will come home to the dullest apprehension. It was said of the doctrine of the circulation of the blood that nobody over forty could be got to adopt it; and I think I remember a passage in the _Origin of Species_ to the effect that its author expects to convert only young and flexible minds.