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?"