RENAISSANCE II7 



the old vascular theory, we find fundamental differences in the two concep- 

 tions, both anatomically and physiologically. According to the old theory 

 the heart was not a muscular organ; it dilated purely passively and allowed 

 the blood to enter in order to be provided with " vital spirit," this being the 

 primary life-function of the heart, if it were not also, as Aristotle and 

 his followers until Cesalpino held, the centre of intelligence. Again, the 

 blood moved of itself owing to the specifically living qualities which the 

 " spirifus" lent it. Harvey, on the other hand, proves that the movement of 

 the blood is due to the purely mechanical function of the heart: the heart's 

 muscular contraction propels the blood out into the blood-vessels, through 

 the arteries out into the body, thence back to the heart through the veins, 

 and so farther through the lungs. In this contrast lies, one may say, the great 

 ditference between the ancient and the modern biological conception. Even 

 Harvey's way of producing his proofs is purely modern; while Servet still 

 refers back to philosophical speculations and the interpretation of classical 

 authors, Harvey propounds a purely mathematical calculus on the volume 

 of the heart and vascular system and continues to prove his thesis by means 

 of observations and experiments on a number of both higher and lower ani- 

 mal forms. He thus fulfils in the sphere of biology the requirement which 

 his contemporary Bacon laid down as a principle of science: to explain na- 

 ture by experience based upon observations and experiment. And even Gali- 

 leo's fundamental principle governing natural research — to measure what 

 can be measured and to make measurable what cannot be measured — is 

 applied to living nature by Harvey for the first time. Galileo also thought 

 that science can only explain how the forces of nature operate; what their 

 innate essential quality is will never be known under any circumstance. In 

 his explanation of the circulation of the blood Harvey does indeed fulfil the 

 first half of this principle; on the other hand, by adhering to the ancient 

 belief in the vital spirits in the blood he remains in his theoretical concep- 

 tions entirely on ancient ground. 



On the generation of animals 

 This conservatism of Harvey's displays itself conspicuously in a work which 

 he published in his old age: Exercitationes de generatione animalium (165 1). 

 Like his work on the circulation of the blood, this book is the fruit of many 

 years' labour, but in contrast to the former it is somewhat lengthy and far 

 less perfect in form. In this Harvey gives a comparative account of the em- 

 bryonic development in higher and lower animal forms. He is able here to 

 quote as his precursor his old teacher Fabrizio, and he does so with the 

 utmost piety. But above all he proves himself to be a follower of Aristotle, 

 whose conception of the true essence of life he has made entirely his own. 

 Along Aristotelean lines he endeavours to find a formal unity in the mani- 

 fold aspects of phenomena, as displayed in the evolution of the embryo, and 



