To this hypothesis it may be objected, that while there is no direct evidence in support of it, it is exceedingly unreasonable, inasmuch as we can scarcely believe that the semen can go the whole round of circulation, and then find its way to the ovary in such a pure unaltered state as the experiments of Spallanzani prove it must be in, that it may impregnate. A third set of theorists have maintained that an imperceptible something, which they have called _aura seminalis_, passes from the semen lodged in the vagina to the ovary, and excites those actions which are essential to the development of an ovum. Others, again, have told us that it is all done by sympathy. That neither the semen nor any volatile part of it finds its way to the ovary; but that the semen excites the parts with which it is in contact in a peculiar manner, and by the law of animal economy, termed sympathy, or consent of parts, a peculiar action commences in the ovary, by which an ovum is developed. To both these conjectures it may be objected that they have no other foundation but the supposed necessity of adopting them, to account for the effect of impregnation; and, further, they "make no provision for the formation of mules; for the peculiarities of, and likeness to, parents, and for the propagation of predisposition to disease, from parent to child; for the production of mulattoes," etc. A fifth, and to me far more satisfactory view of the subject than any other, is that advanced by our distinguished countryman, Dr. Dewees, of Philadelphia. It appears to harmonize with all known facts relating to the conception and something from analogy may also be drawn in its favor. It is this, that there is a set of absorbent vessels, leading directly from the inner surface of the _labia externa_ and the vagina, to the ovaries, the whole office of which vessels is to absorb the semen and convey it to the ovaries.* I do not know that these vessels have yet been fully discovered, but in a note on the sixteenth page of his "Essays on Various Subjects," the doctor says: "The existence of these vessels is now rendered almost certain, as Dr. Gartner, of Copenhagen, has discovered a duct leading from the ovary to the vagina."