Physiological science generally was not sufficiently advanced in Harvey’s time to admit of a truly great and enduring work being produced on a subject so abstruse, and involving so many particulars as that of Generation. On the doctrine of the circulation the dawn had long been visible; Harvey came and the sun arose. On the subject of animal reproduction, all was night and darkness two centuries ago; and though the light has still been waxing in strength since Harvey wrote, it is only in these times that we have seen it brightening into something like the day. In Harvey’s time the very means and instruments that were indispensable to the investigation were not yet known, or were used of powers inadequate to bring the prime facts within the cognizance of the senses. Harvey doubtless did as much as any man living could have accomplished when he wrote. He announced the general truth: Omne animal ex ovo; he showed the cicatricula of the egg as the point where the reproductive process begins; he corrected numerous errors into which his master Fabricius had fallen; he further pointed out the path of observation and experiment as the only one that could lead to satisfactory results in the investigation of a subject which gradually displayed itself as one of natural history; and, it may be added, by his wanderings in the labyrinth of the metaphysics of physiological science, he did enough to deter any one from attempting to tread such barren ground again. In his work on the Heart and Blood, Harvey had all the essential facts of the subject clearly before him, and he used them at once in such masterly-wise, that he left little or nothing for addition either by himself or others. Secure of his footing here, he could well dispense with “vital spirits,” “innate heat,” and other inscrutable agencies, he could leave “adequate and efficient causes,” and other metaphysical phantoms on one side--it was physics that he was dealing with, and the physician was at home. With the information we now possess, we see clearly how indifferently weaponed was the physiologist of the year 1647 for encountering such a subject as Animal Generation; a Leeuwenhoek and a De Graaf, a Spallanzani and a Haighton, a Wolff, a Purkinje, a Von Baer, a Valentin, a Rudolph Wagner, a Bischoff, and many more, had successively to appear, before the facts of the subject could be ascertained, and a Schleiden and a Schwann were further necessary as ultimate interpreters of the things observed before they could be either rightly or wholly understood. No wonder then that The Physiologist of the 17th century, meets us in the guise of one rather puzzled with the burthen he has made up his mind to bear, and, contrary to his former wont, eking out the lack of positive knowledge by reiterated disquisitions on topics where certainty is unattainable.