It is not my intention, in discussing this branch of the subject, to examine the hundred ingenious theories of generation which ancient and modern physiologists have put forth. I shall not enquire whether the future human being owes its first existence, as Hippocrates and Galen asserted, and Buffon very ingeniously supports, to the union of two life-giving fluids, each a sort of extract of the body of the parent, and composed of organic particles similar to the future offspring; or whether, as Harvey and Haller teach, the embryo reposes in the ovum until vivified by the seminal fluid, or perhaps only by the _aura seminalis_; or whether, according to the theories of Leuvenhoeck and Boerhaave, the future man first exists as a spermatic animalcula, for which the ovum becomes merely the nourishing receptacle; or whether, as the ingenious Andry imagines, a vivifying worm be the more correct hypothesis; or whether, finally, as Perault will have it,[25] the embryo beings (too wonderfully organized to be supposed the production of any mere physical phenomenon) must be imagined to come directly from the hands of the Creator, who has filled the universe with these little germs, too minute, indeed, to exercise all the animal functions, but still self-existent, and awaiting only the insinuation of some subtle essence into their microscopic pores, to come forth as human beings. Still less am I inclined to follow Hippocrates and Tertullian in their enquiries, whether the soul is merely introduced into the fœtus, or preexists in the semen, and becomes, as it were, the architect of its future residence, the body;[26] or to attempt a refutation of the hypothesis of the metaphysical naturalist,[27] who asserts, (and adduces the infinite indivisibility of matter in support of the assertion,) that the actual germs of the whole human race, and of all that are yet to be born, existed in the ovaria of our first mother, Eve. I leave these and fifty other hypotheses as ingenious and as useless, to be discussed by those who seem to make it a point of honour to leave no fact unexplained by some imagined theory; and I descend at once to the _terra firma_ of positive experience and actual observation.