Volume 1 — 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.

"Is there a unity of plan? The plan is the relative location of the parts. One can conceive very well the unity of plan without the unity of number; for it is sufficient that all the parts, whatever their number may be, keep always relatively to each other the same place. But can one say that the vertebrate animal, whose nervous system is placed above the digestive canal, is fashioned after the same plan as the mollusk, whose digestive canal is placed above the nervous system? Can one say that the crustacean, whose heart is placed above the spinal marrow, is fashioned after the same pattern as the vertebrate, whose spinal marrow is placed above the heart? Is the relative location of the parts maintained? On the contrary, is it not overthrown? And if there is a change in the location of parts, how is there a unity of plan?"

Müller draws nearer to the consideration of the development of the human embryo, and forcibly illustrates the falsehood of the pretended theory. "It is not long since it was held with great seriousness that the human foetus, before reaching its perfect state, travels successively though the different degrees of development which are permanent during the whole life of animals of inferior classes. That hypothesis has not the least foundation, as Baer has shown. The human embryo never resembles a radiate, or an insect, or a mollusk, or a worm. The plan of formation of those animals is quite different from that of the vertebrate. Man then might at most resemble these last, since he himself is a vertebrate, and his organization is fashioned after the common type of this great division of the animal kingdom. But he does not even resemble at one time a fish, at another a reptile, a bird, etc. The analogy is no greater between him and a reptile or a bird, than it is between all vertebrate animals. During the first stages of their formation, all the embryos of vertebrate animals present merely the simplest and most general delineations of the type of a vertebrate; hence it is that they resemble each other so much as to render it very difficult to distinguish them. The fish, the reptile, the bird, the mammal, and man are at first the simplest expression of a type common to all; but in proportion as they grow, the general resemblance becomes fainter and fainter, and their extremities, for instance, after being alike for a certain time, assume the characters of wings, of hands, of feet, etc."

Mr. Milne-Edwards takes the same view of embryonic generation:

"I agree with Geoffrey Saint Hilaire, that often a great analogy is observed between the final state of certain parts of the bodies of some inferior animals, and the embryonic state of the same parts of other animals belonging to the same type the organism of which is further developed, and with the same philosopher, I call the cause of the state of permanent inferiority arrests of development. But I am far from thinking with some of his disciples that the embryo of man or of mammals exhibits in its different degrees of formation the species of the less perfect of animate creation. No! a {76} mollusk or an anhelid is not the embryo of a mammal, arrested in its organic development, any more than the mammal is a kind of fish perfected. Each animal carries within itself, from the very origin, the beginning of its specific individuality, and the development of its organism, in conformity to the general outline of the plan of structure proper to its species, is always a condition of its existence. There is never a complete likeness between an adult animal and the embryo of another, between one of its organs and the transitory state of the same in the course of formation; and the multiplicity of the products of creation could never be explained by a similar transmutation of species. We shall see hereafter, that in every zoological group composed of animals which seem to be derived from a common fundamental type, the different species do not exhibit at first any marked difference, but soon begin to be marked by various particularities of constructure always growing and numerous. Thus each species acquires a character of its own, which distinguishes it from all others in the way of development, and each of its organs becomes different from the analogous part of every other embryo. But the changes which the organs and the whole being undergo after they have deviated from the common genesiac form, are generally speaking the less considerable in proportion as the animal is destined to receive a less perfect organism, and consequently they retain a kind of resemblance to those transitory forms."

Reason then and experience, theory and fact, philosophy and physiology, agree in protesting against the arbitrary doctrine of the unity of type in the animal kingdom; a doctrine which has its origin in an absence of sound scientific notions and a superficial observation of the phenomena of nature. Through the former defect men failed to consider that if the end of each animal species is different, different also must be its being, and therefore a different type must preside as a rule and supreme law over the formation of the being. By the latter, some very slight and partial analogies have been mistaken for identity and universality, and mere appearances have been assumed as realities.