So far we have considered Geoffroy's theories in their application to the facts. We go on to discuss the theories themselves, and the general conception of living things which underlies them.
The principle of unity of plan and composition is the keynote of Geoffroy's work. It states that the same materials of organisation are to be found in all animals, and that these materials stand always in the same general spatial relations to one another. The "materials of organisation" are not necessarily organs in the physiological sense, and indeed the principle of the unity of plan cannot be upheld if the unity has reference to organs only. This became clear to Geoffroy, especially in his later years. In 1835 he wrote, speaking of the principle of the unity of plan, "I have, moreover, regenerated this principle, and obtained for it universality of application, by showing that it is not always the organs as a whole, but merely the materials composing each organ, that can be reduced to unity."[109] Even in the _Philosophie anatomique_ he deals rather with parts than with organs; he deals, for instance, with the elementary parts of the sternum, not with the organ "sternum" in its totality. The functions of the sternum vary, and the primary protective function of the sternum may be assumed by quite other parts, _e.g._, by the clavicles in fish, which protect the heart.[110]
True homologies can be established between materials of organisation but not always between organs, which may be composed of different "materials."
Almost as a corollary to this comes the further view that form is of little importance in determining homologies. An organ is essentially an instrument for doing a particular kind of work, and its form is determined by its function. Organs which perform the same function are usually similar in form though the elementary materials composing them may be different. This is seen in many cases of convergence. Organs, therefore, which perform the same function and are similar in external form are not necessary homologous. Conversely, the same complex of materials, say a fore limb, may take on the most varied shapes according as the function of the organ changes--but homology remains though form changes. Accordingly, form is one of the least important elements to be considered in determining a homology. "Nature," he wrote in one of his early papers, "tends to repeat the same organs in the same number and in the same relations, and varies to infinity only their form. In accordance with this principle I shall have to draw my conclusions, in the determining the bones of the fish's skull, not from a consideration of their form, but from a consideration of their connections."[111]
Again, after comparing a vertebra of the Aurochs with an abdominal segment of the crab, he says, "I have insisted upon an identity which has extended to the least important relation of all, that of form."[112]
Geoffroy's morphological units or materials of organisation were in the case of the skeleton--with which his researches principally deal--the single bones. But the interesting point is that he sought his skeleton-units in the embryo, and considered each separate centre of ossification as a separate bone. Coalescence of bones originally separate is one of the most usual events in development, and it is an occurrence which, more than any other, tends to obscure homologies.
Because of its coalescence with the maxillaries, the intermaxillary in man was not discovered until Vicq d'Azyr and Goethe found it separate in the embryo. Apparently quite independently of Goethe, Geoffroy hit upon this plan of seeking in the embryo the primary elements or materials of organisation. In an early paper on the skull of Vertebrates,[113] where he is concerned with showing that each bone of the fish's skull has its homologue in the skull of higher Vertebrates, he is faced with the difficulty that the skull of the fish has more bones than the skull of higher Vertebrates. "Having had the inspiration," he writes, "to reckon as many bones as there are distinct centres of ossification, and having made a consistent trial of this method, I have been able to appreciate the correctness of the idea: fish, in their earliest stages, are in the same conditions relatively to their development as the foetuses of mammals, and hence bear out the theory" (p. 344). So, too, in dealing with the homologies of the sternal elements (_supra_, p. 57) he treats as separate bones the "annexes" of the sternum in birds, though these are separate only in the young.
If the same materials of organisation are present in all animals, and if they are arranged always in the same positions relatively to one another, how does it come about that animal forms are so varied, what explanation can be offered of the diversities of organic structure?
Geoffroy's main answer to this question is his _Loi de balancement_. The law was enunciated by him already in 1807.[114] We take the following quotation, which represents his thought most nearly, from the _Cours de l'histoire naturelle des Mammiferes_ (1829). "According to our manner of regarding the organisation of mammals, there is only a single animal modified by the inverse reciprocal variation of all or some of its parts. Now, from the fact that there is only one single general animal, it follows that for each section of its components or for each of its organs there is available only a given quantity of formative materials.
Now suppose that the distribution of these materials has not been made in such a way as to ensure an exact equilibrium between all the parts concerned, one organ will get more than its share, another less. My law of the compensation of organs is founded on these principles" (i., _Lecon_ 16, p. 12). "The atrophy of one organ turns to the profit of another; and the reason why this cannot be otherwise is simple, it is because there is not an unlimited supply of the substance required for each special purpose."[115] The nutritive material available is limited for each species; if one part gets more than its share the other parts must get less--that is all the law means. As an example, take the minuteness of the episternals and xiphisternals in birds, as contrasted with the huge size of the entosternal. "The minuteness of the episternals and xiphisternals might be imputed to this gigantic piece diverting to its own profit the nutritive fluid, since the bigger it is the smaller these are."[116]
One has constantly to remember in dealing with Geoffroy's theories that he was not an evolutionist, but purely a morphologist. It is therefore, perhaps, to ask too much to require of him an explanation of the causes of diversity. The morphologist describes, classifies, generalises; he does not seek for causes. But we must leave this question aside in order to discuss how far Geoffroy's theory of the unity of plan and composition fits the facts. As Geoffroy himself admitted on several occasions, his theory was an _a priori_ one, a theory hit upon by hasty induction, then erected into a principle and imposed upon the facts. No more than Goethe did he extract his principle from a sufficient mass of data.
Now he found his theory to be in its pure form unworkable; he found, for example, that the skeleton of fishes could not be compared directly, bone for bone, with the skeleton of higher Vertebrates; he had to admit differences of position of whole sets of organs in the two groups, he had to admit various _metastases_, before he could bring the skeleton of fish into line. And these metastases are due to functional requirements--for example, the forward position of sternum and thoracic organs in fish is an adaptation to swimming.
So he does not so much demonstrate the unity of plan of whole organisms as the unity of plan of particular corresponding parts of them. Thus he does not prove or attempt to prove that Articulates are in all points like Vertebrates, but simply that their skeleton is built upon the same plan as that of Vertebrates. The rest of the organs, while still comparable with the organs of Vertebrates, stand in different relations to the skeleton. An Articulate therefore, on his own showing, is not, _as a whole_, built upon the same general structural plan as a Vertebrate.
Further, he does not always remain true to his principles, for he does not establish homologies of parts entirely by their connections but sometimes by their functions as well. Thus the sternum, or rather the complex of sternal elements, is defined and discovered in particular cases not by its connections only but also by its functions. The framework of the gills is homologised part by part with the framework of the lungs, not because the relations of the framework to the rest of the skeleton are the same in fish and air-breathing Vertebrates, but simply because gills are considered the equivalents of lungs--a comparison which is purely physiological.
Even with these concessions to the functional view of living things, Geoffroy was unable to make good his contention that all animals are built upon the same plan. His arguments failed to carry conviction to his contemporaries, and Cuvier in particular subjected them to destructive, and indeed final, criticism.
The paper, already referred to, in which Cuvier disposed of the transcendentalists' comparison of Cephalopods and Vertebrates is of great significance, for it states in the clearest way the radical opposition between the functional and the formal attitudes to living things.
Cuvier points out that if by unity of composition is meant identity, then the statement that all animals show the same composition is simply not true--compare a polyp with a man!--on the other hand, if by unity is meant simply resemblance or homology, the statement is true within certain limits, but it has been employed as a principle since the days of Aristotle, and the theory of unity of composition is original only in so far as it is false. He admits, however, that Geoffroy has seized upon many hidden homologies, especially by his valuable discovery of the importance of foetal structure. In all this Cuvier is undoubtedly right.
Unity of plan and composition, as Geoffroy conceived it, simply does not exist. Cuvier goes on to say that this principle of Geoffroy's, in the greatly modified form in which it can be accepted, and has been accepted from the dawn of zoology, is not the sole and unique principle of the science. On the contrary, it is merely a subordinate principle, subordinate to a higher and more fruitful principle, that, namely, of the conditions of existence, of the adaptation (_convenance_) of the parts, of the co-ordination of the parts for the role which the animal is to play in Nature. "That is the true philosophical principle," he says, "whence may be deduced the possibility of certain resemblances, the impossibility of certain others; it is the rational principle from which follows the principle of the unity of plan and composition, and in which at the same time it finds those limits, which some would like to disregard" (p. 248).
Geoffroy's position is the direct contrary. He holds that the principle of the unity of plan and composition is the true base of natural history,[117] and that this unity limits the possible transformations of the organism. Thus, speaking of the influence of the respiratory medium, he says, "All the same this influence of the external world, if it has ever become a cause which disturbed organisation, must necessarily have been confined within fairly narrow limits; animals must have opposed to it certain conditions inherent to their nature, the existence of the same materials composing them, and a manifest tendency to resemble one another, and to reproduce invariably the same primordial type."[118] Unity of plan and composition is, on this view, prior to adaptation and limits adaptation. Cuvier's view, on the contrary, is that the necessity of functional and ecological adaptation accounts for the repetition of the same types of structure. There are, of all the possible combinations of organs, only a few viable types--those whose structure is adapted to their life. Therefore it is reasonable that these few types should be repeated in innumerable exemplars. One must remember, in order to appreciate Cuvier's view, that he was not obsessed, as we are, by the idea of evolution.
Cuvier thought in terms of organs, not in terms of "materials of organisation." He held that the resemblances between the organs of one class of animals and the organs of another were due to the similarity of their functions. "Let us conclude, then, that if there are resemblances between the organs of fish and those of other classes, it is only in the measure that there is a resemblance between their functions."[119] There are only a few kinds of organs, each adapted for a particular function, and these organs are necessarily repeated from class to class.--"As the animal kingdom has received only a limited number of organs, it is inevitable that some at least of these organs should be common to several classes."[120]
Geoffroy thought in terms of "materials," of parts of indefinite function, parts which might take on any function. He insists upon the necessity of disregarding function when tracing out the unity of composition. He considers, in direct opposition to Cuvier's interpretation of structural resemblance as due to similarity of function, that unity of composition is the primary fact, and similarity of function subsidiary. In his reply in the _Mammiferes_ (1829) to Cuvier's criticisms in the _Histoire naturelle des Poissons_ (1828), he insists on the necessity of excluding function from consideration in any truly philosophical treatment of comparative anatomy (Discours prel., p.
25). Cuvier held that function determined structure, or at least that the necessity of adaptation ruled the transformations of form. Geoffroy considered that structure determined function, that changes of structure, however they might arise, caused changes of function.
"Animals," he writes, "have no habits but those that result from the structure of their organs; if the latter varies, there vary in the same manner all their springs of action, all their faculties and all their actions."[121]
Again, "a vegetarian regime is imposed upon the Quadrumana by their possession of a somewhat ample stomach, and intestines of moderate length."[122] The hand of the bat has become so modified as to constrain the bat to live in the air.[123]
The best example of Geoffroy's insistence upon the priority of structure to function, and so of his purely morphological attitude, is perhaps his interpretation, already alluded to, of the appendages of Articulates.
The segments of the Articulate are, he says, the equivalents of the bodies of the vertebrae of higher forms. Now "from the circumstance that the vertebra is external, it results that the ribs must be so too; and, as it is impossible that organs of such a size can remain passive and absolutely functionless, these great arms, hanging there continually at the disposition of the animal, are pressed into the service of progression, and become its efficient instruments."[124] The ribs become locomotory appendages.
We may compare the similar thought that the ear ossicles are simply opercular bones reduced and turned to other uses.
Geoffroy could not but recognise the correlation of structure to function, for this is a fact which imposes itself upon every observer.
He recognised also correlation between functions, as when he pointed out the connection between increased respiration and enhanced muscular activity in birds.[125] He interpreted structure at times in terms of function, the short, strong clavicle of the mole as an adaptation to digging, the keeled sternum of birds as an adaptation to flying, and so on. But we may say that his whole tendency was to disregard function, to look upon it as subsidiary. He protests against arguing from function and habits to structure, as an "abuse of final causes."[126] He was not so convinced as Cuvier was of the all-importance of functional correlation; in this view he was probably confirmed by his work on teratology. It did not surprise him that Insects, in which lungs, heart and circulation have disappeared(!), should yet have a skeleton built upon the same plan as the skeleton of Vertebrates, which possess these organs; the correlation of organ-systems is not so close as to prevent this.[127] So too, although the other organs of the insect are all inside the body of the vertebrae, they are yet comparable with the organs of Vertebrates.[128]
The existence of rudimentary organs also seemed to him an argument against too strict a correlation of parts.
The contrast between the teleological attitude, with its insistence upon the priority of function to structure, and the morphological attitude, with its conviction of the priority of structure to function, is one of the most fundamental in biology.
Cuvier and Geoffroy are the greatest representatives of these opposing views. Which of them is right? Is there nothing more in the unity and diversity of organic forms than the results of functional adaptation, or is Geoffroy right in insisting upon an element of unity which cannot be explained in terms of adaptation? If there be an irreducible element of unity, is there any truth in Geoffroy's suggestion that this unity results from a power which is exercised in the world of atoms where are elements of inalterable character?[129]
The problem as Geoffroy and Cuvier understood it was not an evolutionary one. But the problem exists unchanged for the evolutionist, and evolution-theory is essentially an attempt to solve it in the one direction or the other. Theories such as Darwin's, which assume a random variation which is not primarily a response to environmental changes, answer the problem in Geoffroy's sense. Theories such as Lamarck's, which postulate an active responsive self-adaptation of the organism, are essentially a continuation and completing of Cuvier's thought.
[86] "Memoire sur les rapports naturels des makis,"
_Magasin Encyclopedique_, vii.
[87] Discours preliminaire, pp. xv.-xxiv.
[88] _etudes progressives d'un Naturaliste_, p. 50, Paris, 1835.
[89] _Philosophie Anatomique_., i., Introduction, p. 1.
[90] "Sur une colonne vertebrale et ses cotes dans les insectes apiropodes," (_Acad. Sci._, Feb. 12, 1820).
Printed in _Isis_, pp. 527-52, 1820 (2).
[91] "Sur l'organisation des insectes," p. 458. _Isis_, pp. 452-62, 1820 (2).
[92] _Mem. Mus. d'Hist. nat._, ix., pp. 89-119, Pls.
v-vii.
[93] _Sur l'organisation des insectes_, p. 459.
[94] _Isis_, p. 549.
[95] Published in _Ann. Sci. Nat._, xix., pp. 241-59, 1830.
[96] _Cf._ Aristotle (_supra_, p. 10).
[97] For an account of the controversy reference may be made to I. Geoffroy St Hilaire, _Vie Travaux et Doctrine scientifique d'Etienne Geoffroy St Hilaire_, Paris, 1847; also Semper, _Arb. zool. zoot. Instit. Wurzburg_, iii., 1876-7, K. E. von Baer, _Lebensgeschichte Cuviers_, ed. L. Stieda, 1897, and J. Kohlbrugge, in _Zoolog.
Annalen_, v., pp. 143-95. 1913.
[98] "Recherches sur l'organisation des Gavials," _Mem.
Mus. d'Hist. nat._, xii., 1825.
[99] _Mem. Mus. d'Hist. nat._, xvii., pp. 209-29.
[100] _Mem. Acad. Sci._, xii., pp. 63-92, 1833.
[101] _Mem. Acad. Sci._, xii., pp. 43-61, 1833.
[102] Geoffroy's French style is at times incredibly bad, and more or less literal translations of his sentences are apt to read queerly!
[103] _Mem. Mus. d'Hist. nat._, xiii., p. 289, 1826.
[104] _Mem. Mus. d'Hist. nat._, xviii., p. 221, 1828. His teratological work is important, and is chiefly contained in the second volume of the _Philosophie anatomique_.