"Human organogeny is a comparative transitory anatomy, as comparative anatomy is the fixed and permanent state of the organogeny of man; and, on the contrary, if we reverse the proposition, or method of investigation, and study animal life from the lowest to the highest, instead of considering it from the highest to the lowest, we shall see that the organisms of the series reproduce incessantly those of the embryos, and fix themselves in that state which for animals becomes the term of their development. The long series of changes of form presented by the same organism in comparative anatomy is but the reproduction of the numerous series of transformations to which this organism is subjected in the embryo in the course of its development. In the embryo the passage is rapid, in virtue of the power of the life which animates it; in the animal the life of the organism is exhausted, and it stops there, because it is not permitted to follow the course traced for the human embryo. Distinct stages on the one hand, progressive advance on the other, here is the secret of development, the fundamental difference which the human mind can perceive between comparative anatomy and organogeny. The animal series thus considered in its organisms is but a long chain of embryos which succeed each other gradually and at intervals, reaching at last man, who thus finds his physical development in comparative organogeny." Thus speaks Serres. And in another place: "The whole animal kingdom appears only like one animal in the course of formation in the different organisms. It stops here sooner, there later, and thus at the time of each interruption determines, by the state in which it then is, the distinctive and organized characters of classes, families, genera, and species." II. THIS OPINION REFUTED BY PHILOSOPHICAL REASONS. The futility of the above doctrine is manifest, in the first place, from the weakness of the foundation on which it rests. That foundation is no other than a kind of likeness which appears at first sight between the rudimental forms which, in the first steps of its development, are assumed by the human embryo, and the forms of some inferior animals. For the germ, by the very reason that it has not, as it was once believed, all the organism of the human body in microscopic proportions, but in order to acquire it must pass from potential to actual existence--by that very reason, is {73} subjected to continual metamorphoses, that is, to successive transformations, which give it different aspects, from that of a little disc to the perfect human figure. Now, it is clear that, in this gradual transition from the mere power to the act of perfect organization, a kind of analogy or likeness to some of the numberless forms of inferior organizations of the animal kingdom may, and must, be found in its intermediate and incomplete state.