In the second year after that of the Exercises to Riolanus Harvey's final publication, his treatise On Generation with appended essays, was given to the world, not long before his seventy-third birthday. During how many years this work had been in preparation we do not know; but it is avowedly based upon the views of Aristotle, whom Harvey styles his "_dux_"--his leader--as regards the subject of this treatise.[170] In it, to be sure, the ancient master is often weighed in the balance and found wanting by Harvey, who even questions whether Aristotle had seen for himself what he "narrates as to the generation of the chick," or "had accepted it from some expert."[171] Nevertheless, it is with the doctrines of Aristotle that Harvey incessantly compares the results of observation. Here the veteran records anew his denial of the Aristotelian primacy of the heart, and records as well his final emphatic assertion of the primacy of the blood. In regard to these matters it is interesting to note the various grades of expression which appear to mirror in this single work the various phases of Harvey's thought. In the following florid passage doubt of the primacy of the heart seems hardly even hinted at. Harvey says:-- "Certain of the parts themselves are said to be generative, such as the heart, from which Aristotle declares that the rest of the parts derive their origin; as is also clear from the history which I have given. The heart, I say--or at least its first beginning, to wit, the vesicle and leaping point--constructs the rest of the body to be its future abode; enters this when once built up, and hides in it, vivifies and governs it; fortifies it with ribs and sternum super-imposed as a bulwark; and is a kind of household shrine, as it were, the first seat of the soul, the first receptacle and perennial soul-endowed[172] hearth of the innate heat, the source and origin of all the faculties, and their sole relief in calamity."[173] Divergence from Aristotle in the matter of the heart is plainly marked, however, in the following passage of the same treatise, where Harvey says:--