That organized being should give rise to other organized beings under favorable circumstances as to nourishment, warmth, etc., is no more wonderful than that fire should give rise to fire when air and fuel are present. To be sure, there are some minute steps in the processes which are not fully known to us; still, if they ever should be known, we should unquestionably see that there is a natural cause for every one of them; and that they are all consonant with certain laws of the animal economy. We should see no necessity of attempting to explain the process of generation by bringing to our aid, or rather to the darkening of the subject, any imaginary principle, as the _visus formaticus_ of Blumenbach. As to the "observations and experiments of Haller and Spallanzani," I think, with Dr. Bostock, that they weigh but little, if any, against the theory before us. I shall not be at the labor of bringing them forward and showing their futility as objections to this theory, for I am far from insisting on the correctness of it; that is, I do not insist that any part of the female secretion, during coition, unites with the male semen in the formation of the rudiments of the foetus. The second hypothesis or theory, I shall notice, as to the rudiments of the foetus, is that of Leeuwenhoek, who regarded the seminal animalculse of the male semen as the proper rudiments of the foetus, and thought that the office of the female is to afford them a suitable receptacle where they may be supported and nourished until they are able to exist by the exercise of their own functions. This is essentially the view of the subject which I intend to give more particularly presently. I know of no serious objections to this hypothesis, nothing but the "extreme improbability," as its opponents say, "that these animalculae should be the rudiments of being so totally dissimilar to them." But I wish to know if there is more difference between a foetus and a seminal animalcule than there is between a foetus and a few material particles in some other form than that of such animalcule? The third hypothesis, or that of pre-existing germs, proceeded upon a precisely opposite view of the subject to that of Leeuwenhoek, namely, that the foetus is properly the production of the female; that it exists previous to the sexual congress, with all its organs, in some parts of the uterine system; and that it receives no proper addition from the male, but that the seminal fluid acts merely by exciting the powers of the foetus, or endowing it with vitality.