Despite emphatic denials of Aristotelian doctrine which Harvey freely makes in his treatise On Generation, the Aristotelian flavor of the foregoing passages is obvious. It is not easy to make out from Harvey's writings the exact nature of his views as to spontaneous generation. He sometimes seems to assume the truth thereof; but it is by no means certain that he believed in it in the same simple sense in which it had been accepted by Aristotle.[439] Nothing, however, can be clearer than this: that, though for Harvey the "innate heat" is not ethereal spirits nor even an analogue of the ether, but is simply identical with the living blood; nevertheless, for him as emphatically as for Aristotle the "heat in animals" which "is not fire and does not take its origin from fire," derives its origin from the solar ray. To Harvey, the lifelong thinker upon the meaning of the circulation, the prodigious history of generation was but continued in the history of the blood, with which he had identified the "innate heat" and which he saw appear and live, before all other parts, in the minute first rudiment of the embryo. We have seen him pondering the cycle wherein unending life is transmitted from egg to fowl and from fowl to egg beneath the sun's life-giving rays. So now shall we see him pondering a cycle of life wherein, from generation to generation, the circulating blood takes over and exercises the generative powers of the semen from which it is derived, and transmits them in turn to the semen evolved out of itself. He says:-- "Further, since we have just seen the study of the semen to be so difficult; that is to say, the way in which the structure of the body is built up by the semen with foresight, art, and divine intelligence; why should we not equally admire the excellent nature of the blood, and make the same reflections upon it as upon the semen? Especially since the semen itself is made of blood,[440] as is proved in the case of the egg; and since the whole body is seen not only to take origin from the blood as from a generative part, but also to owe its preservation to the same."[441] To this same theme Harvey returns in his Exercise On the Innate Heat near the end of his treatise On Generation, saying:--