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It follows from this general exposition of all observations on the particular organization [of different species of fishes], that the fishes form an animal class distinct from all the others, and totally determined by its structure to live, move and carry out its essential activities in an aqueous element. This is their place in the creation. They have been there since their origin. They will remain there until the destruction of the existing order of things. It is only by vain metaphysical speculations, or by very superficial comparisons, that their class has been considered a development, perfection or ennoblement of the molluscs, or as a first sketch, or embryonic state, of the other classes of vertebrates.
Without doubt, the molluscs, like the fishes, breathe by gills; they have in common with them and with the other vertebrates a nervous and circulatory system, an intestinal canal and liver, and no one knows this better than myself, since I had been the first to make known, with some small degree of completeness, their anatomy and their zoological relations. Since the animal level of existence has only received a limited number of organs, it is necessary that at least some of these organs should be common to several classes. But where, otherwise, is there a resemblance? Is the framework of these animals, their system of locomotion, comparable in the least of their parts? And in what way are the organs common to the molluscs and fishes? Can they be shown to have the relations and connections to each other in the molluscs that they have in the fishes and other vertebrates? What transition in nature leads us from one to the other?
One could, I am aware, propose a definition which only took account of the features these groups have in common, and disregarded their differences. But this definition would always remain a pure mental abstraction, a purely nominal definition, a vain assemblage of words, which could never be represented by a common plan unless it was stripped of all the details needed to conceive of it. Such a method could then be used to connect all the organisms one wished together, since, in the final analysis, any two creatures, however remote they might be from each other, would always show some resemblances, if in nothing else but existence itself.
The single heart in the molluscs is placed in a way contrary to that of the fishes; it is located at the conjunction of the branchial veins and the arteries of the body. In several of them, the appendages are on the head. In others, the organs of generation are on the side. Often the organs of respiration are located above those of digestion, or are spread out on all or part of the dorsal surface. In short, they and the fishes have gills. This is all the similarity they have. Every time one wishes to begin from these purely verbal and metaphysical formulas, they are led to make prefigured comparisons.
For example, by one author, the shells of bivalves are taken to represent the opercula of fishes. Another takes the bone of a cuttlefish for a true bone; yet a third interprets the great plates of the sturgeon or box fish for an external skeleton.
Others seek their analogical relations in the crustaceans. The segments of the thorax are taken to represent the opercular bones of the fishes, and to be sure, one does find the gills under these segments. But if we penetrate a little further, everything is reversed. The nerve cord is on the ventral side in the crustaceans, and the heart is on the back, and this heart, like that of the molluscs, receives, rather than sends, the blood from the gills. Other authors, without explanation of the causes involved, have wished to see the rays or spiny apophyses of the vertebrae of fishes in the feet of the crustaceans. But then, one no longer has a perfection of the fishes, but rather a manifest degradation.
The parallelism of the fishes and the other vertebrates is not as completely misconceived. Here, at least, some sensible relationships in the number of organ systems and in their mutual connections takes its rise. But this is still far not only from an identity of structure, but even from the appearance of a progressive sequence.
The head of fishes, or better their skull, is divided into a number of bones that are almost like those seen in the skull of birds and lizard-like reptiles. There is also some resemblance, although much less complete, between these bones and those of the mammalian fetus, just as the circulation in the reptiles has some relation with that of the mammalian fetus. Thus the oviparous classes, and especially the reptiles, have been considered to be mammals arrested at an early period in their development. Extending the comparison to the fishes, in which respiration and circulation, as far as it concerns the (arrangement of) the vessels, is almost the same as in the frog tadpole and the other batracian reptiles, it has been concluded that the fishes prefigure these tadpoles. They are, as a consequence some kind of fetus of the second degree, a fetus of a fetus.
But even if these relationships in the number of bones are as complete as they are rare, and even if one forgets that the reptiles most close to the fishes—the frogs and salamanders—have, in all their manifestations, fewer bones of the skull and face than the fishes and even the mammals, this manner of viewing them will be no less defective. It considers, we might say, only one or two points of similarity and ignores all the rest, or else connects them to this system of arrangement only by making suppositions contrary to intuition.
It is no advance in understanding to arrive by arbitrary concessions at the conclusion that entire organ systems are reversed; or that the bones which belong to one organ are to be seen interspersed between those of another; or that bones arranged in one group of organisms in series are found one upon the other in the succeeding class: or lastly, that a whole group of bones, which shows a constant diminution and simplification in one group, like the small tympanic bones, should suddenly regain its original number of segments and increase in volume so as to exercise a totally different function, the protection of the gills. For examination reveals that there is not the proper number of bone segments, nor are the parts in the proper connection. In short, nothing yet supports these claimed analogies that have allegedly been attained by such a laborious process.
Let us suppose, as an example, that the vertebral spine (apophyse épineuse) of a fish vertebra is detached, and that one of its two halves is raised above the other. Let us even allow that in these circumstances, nature shaped these parts differently, and made this articulation so complex that it could be termed a “chain-like” articulation. Does this then give us the radial (interépineux) and ray of the dorsal fin which articulate together? No, because the radial itself is composed of three parts, and the ray, if it is a simply spiny ray, is still divided vertically into two halves. Or take the case of a soft ray, divided at the end into numerous branches and hundreds of small articulations. With regard to the six separate muscles attached to each of these rays, the evidence for a lack of an analogy (with the serial body muscles) is such that no one would dare assign them one. The same would be true, although the contrary has been claimed, if one tries to compare the opercular muscles to those connected to the small bones of the ear.
Without doubt, the bony apparatus bearing the gills of fishes has some relationship, although a rather distant one, with that bearing the branchial fringes of the tadpole or salamander. But that itself would prove that it cannot be the analog of the larynx and bronchial tubes, since the larynx and bronchials exist in these animals simultaneously with the gill apparatus. Furthermore, is there the least comparison to be made between the muscles of the gill apparatus in the two classes?
Now, if nature had created one set of muscles expressly for the reptiles, and another for the fishes, why could it not also have created separate bones for them?
Some anatomists have wanted to see an analogy between the opercular bones [pieces operculaires des ouies] of fishes and the ear bones of the mammals. But then the former could not be rudiments of the latter, but rather would be an enormous development of them. How can this be reconciled with the fact that it is precisely these same bones in the (amphibious) reptiles, such as the salamanders and frogs, which in their first stages are almost true fishes, which are most similar to those of the fishes. Yet the amphibians are, of all the vertebrates, those which show the most weak and rudimentary development of the small bones of the ear.
We ought, therefore, conclude that if there are resemblances between the structures of the fishes and those of other classes of vertebrates, it is only insofar as there are similarities in their functions. Let us conclude that if it can be said that these animals are ennobled molluscs, molluscs elevated by a degree, or the fetuses of reptiles, reptiles in an embryonic stage, it is only to be understood in an abstract and metaphysical sense. Even then it is a necessary conclusion only insofar as this abstract expression organizes our legitimate ideas. Particularly, we should conclude that there are no links in that imaginary chain of successive forms, in which one form can serve as the source of the other, since none of these could exist by itself. Nor is there that other, no less fictitious, chain of simultaneous and graded forms, which has its existence only in the imagination of some naturalists, more poets than observers of nature. Rather fish belong to the real chain of coexistent beings of creatures necessary to each other and to the whole, which, by their mutual interaction, maintain the order and harmony of the universe, a chain in which no portion can exist without all the others, and in which the coils, ceaselessly united or dispersed, embrace the globe in their contours.
Translated by P. R. Sloan
Reading and Discussion Question
1.What anatomical differences does Cuvier enumerate between the mollusks and the fishes? In what way do these points serve as a critique of Lamarck?