Whatever may be the likeness between the first appearances of the human embryo and the forms of lower animals, they are not the effect of a stable existence, but of a transitory and shifting existence, which does not constitute a species, but is merely and essentially a movement toward the formation of the species. On the contrary, the forms presented by animals already constituted in their being belong to a stable and permanent existence, which diversifies one species from another. The difference, then, between the former and the latter is interior and substantial, and cannot be changed into exterior and accidental, as it would be if it consisted in {74} stopping or in travelling further on. The movement or tendency which takes place in the germ to become another thing until the said germ assumes a perfect organization relative to the being it must produce, is not a quality which can be discarded, since it is intimately combined with the subject itself in which it is found. The essence itself must be changed in it in order to obtain stability and consistency. But if the essence be changed, we are out of the question, since in that case we should have, not the human embryo arrested at this or that stage on its road, but a different being substituted for it; of analogous exterior appearance, perhaps, but substantially different, which would constitute an annual of inferior degree. In short, each animal is circumscribed in its own species, like every other being in nature. If to reach to the perfection required by its independent existence it needs development, every step in that journey is an inchoation of the next, and cannot exist but as such. To change its nature and to make it a permanent being, is as impossible as to change one essence into another. Again: From the opinion we are refuting it would follow that all animals, man excepted, are so many monsters, since they are nothing else but deviations, for want of ulterior development, from what nature really intends to do as a term of its action. Thus anomaly is converted into law, disorder into order, an accidental case into a constant fact.