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.
Finally, in that hypothesis we should have to affirm not only that the inferior and more imperfect species appeared on earth before the nobler and the more akin to the unique and perfect type, but also that on the appearance of a more perfect species the preceding one had disappeared; being inferior in the scale of perfection. For what other reason could be alleged for nature's stopping at a bird when it intends to make a man, but that the causes are not properly disposed, or that circumstances are not quite favorable to the production of that perfect animal? Then when the causes are ready, and the circumstances propitious, it is necessary that man be fashioned and that the bird disappear. Now all that is contrary to experience. For all the species, together with the type, are of the same date, and we see them born constantly in the same circumstances which are common to all, either of temperature or atmosphere or latitude, etc.
The theory, then, of the unity of type in the animal kingdom and of stages of development falls to the ground, if we only look at it from a philosophical point of view.
III.
IT IS REFUTED BY PHYSIOLOGICAL REASONS.
However, physiological arguments have more force in this matter than the philosophical; since they are more closely connected with the subject, and have in their favor the tangible evidence of fact.
We shall take our arguments from three celebrated naturalists as the representatives of an immense number, whom want of space forbids us to quote.
Flourens shows the error of that opinion by referring to the diversity of the nervous system. The nervous system is the foundation of the animal organism; it is the general instrument of vital functions, of sensation, and of motion. If then one archetypal idea presides over the formation of the different organisms, only one nervous system ought to appear in each, more or less developed or arrested. But experience teaches us the contrary. It shows nervous systems differing in different animals ordained to different functions, each perfect in its kind. "Is there a unity of type?" asks this celebrated naturalist. "To say that there is but one type is to say that there is but one form of [{75}] nervous system; because the form of the nervous system determines the type; that is, it determines the general form of the animal. Now, can we affirm that there is but one form of nervous system? Can we hold that the nervous system of the zoophyte is the same as that of the mollusk, and this latter the same as that of the articulata, or this again the same as that of the vertebrata? And if we cannot say that there is only one nervous system, can we affirm that there is only one type?"
He speaks likewise of the unity of plan. Every creature is built differently, and the difference is especially striking between members of the several grand divisions of the animal kingdom. The plan then of each is different, and so is the typical idea which prescribes its formation. No animal can then be considered as the proof or outline of another.