Owen's argument for limbs might strike us today as contrived and peculiar. His effort does not represent the high water mark of formalist logic even in its own terms and times, but may still win our grudging respect for ingenious-ness and pure chutzpah. The apparent problem, after all, can only be described as daunting. After using all the lateral and ventral parts of a vertebra to build the girdle, what remains for constructing the prominent complexity of humerus, radius and ulna, carpals and metacarpals, right down to the most distal phalanx of the digits? Surely these bones can only be “novel” structures unrelated to the archetypal vertebra, and the integrative program of archetypal reduction and genesis must fail. We might choose to downplay the supernumerary status of a stapes or hyoid bone; but we can scarcely disregard the need to encompass limbs within any general theory of the vertebrate skeleton — for an archetype that omits such a major structure can only provide a partial and paltry explanation indeed.
Owen's improbable solution homologizes the vertebrate limb, in all its complexity, with a simple, unbranched projection from the haemapophysis (see Fig. 4-13), called a diverging ray (note the ray on each vertebra of the archetype in Fig. 4-14).
But how could Owen justify a comparison of so many articulated bones with a hypothetical single rod? Owen used the time-honored comparative method by attempting to trace back the complexity of vertebrate limbs in a structural series of simplification, leading to the lungfish Lepidosiren and its minimal pectoral ray. Lest this series be rejected as a concatenation of heterogeneous objects, Owen presented a tripartite argument: (1) the structural series denotes a descent by simplification; (2) simplification occurs by “arrest of development,” bringing the reduced form closer to an embryonic state; (3) the embryo, following von Baer's principles, reveals the generating archetype in a way that the complexly modified adult cannot. Sensing that opponents might view the proposition as a “transcendental dream,” Owen defended his structural series (Fig. 4-15) as a voyage to the archetype: “It is no mere transcendental dream, but true knowledge and legitimate fruit of inductive research, that clear insight into the essential nature of the organ, which is acquired by tracing it step by step from the unbranched pectoral ray of the lepidosiren to the equally small and slender but bifid pectoral ray of the amphiuma, thence to the similar but trifid ray of the proteus and through the
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4-14. The key plate from Owen's 1849 monograph on the nature of limbs. The archetype, built of a series of vertebrae, shown at the top, with skeletons of a fish and a reptile below. In his boldest move, Owen attempts to derive the entire limb of later vertebrates from the single diverging ray of each vertebral element in the archetypal form. The diverging ray is the little spike projecting upward at about 10 o'clock from the junction of two vertical elements below each vertebral centrum of the archetype. (Author's collection.)
progressively superadded structures and perfections in higher reptiles and in mammals” (p. 70).
But these strained homologies then incur a second, equally serious problem in requiring a pronounced shift of position among vertebrate classes — an interpretation inconsistent with the formalist principle that topology and connection serve as the primary criteria of homology. Geoffroy had developed his concept of metastasis (see p. 300) to explain exceptions in the same troubling example, and Owen followed this continental solution. The pectoral girdle of fishes attaches to the rear of the skull. In fact, Owen regarded the bones of the girdle as the haemal portions of the fourth, or occipital, skull vertebra. (The arm and hand arise from the diverging ray of this vertebra and also become parts of the head by homology.) Owen recognizes the counterintuitive oddity of such a claim, but must follow the formalist logic: “However strange and paradoxical the proposition may sound, the scapular arch [Page 321] and its appendages, down the last phalanx of the little finger, are truly and essentially bones of the skull” (p. 112).
But all tetrapods separate the shoulder girdle and front appendages from the skull by a sequence of intercalated vertebrae. Owen argues, as did Geoffroy, that the haemal portion of the fourth skull vertebra has migrated back in the terrestrial classes. And why should this degree of transposition be
4-15. Owen justifies his bold claim that the entire limb might be derived from a diverging ray by showing a series of maximally reduced limbs in living vertebrates from Proteus with several elements but only two digits, to Amphiuma, also with two digits but fewer elements, and finally to Lepidosiren with a single element maximally similar to the hypothetical diverging ray. (Author's collection).
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regarded as improbable? After all, the pelvic fin of many living teleost fishes, the undeniable serial homolog of the pectoral, moves sufficiently far forward to lie in front of the pectoral. If this more radical movement occurs in modern species, why balk at a less profound metastasis to separate forelimb from skull? “But it may be objected that the ordinary costal or haemal arch has been detached from its centrum for the purpose of this comparison. True! And the scapular arch in mammals, birds and reptiles, is a haemal arch so dislocated, — a statement which I do not hesitate to make under a pledge to demonstrate the proper centrum and the rest of the segment or vertebra to which it belongs” (p. 50).
Having thus brought the entire skeleton, with all its complexity and adaptive variety, into homology with the archetypal vertebra, Owen could proclaim both the glory and the generality of formalist morphology. On the largely rhetorical subject of glory, Owen joined Agassiz in refuting the Paleyan link of functionalism to God's beneficence. Shall we not regard a generalized archetype, a sublime and abstract pattern for all manifest variety, as a loftier testimony to a truly omnipotent God than the mean material fitting, however exact, of some unique and particular object to an immediate environment? “The satisfaction felt by the rightly constituted mind must ever be great in recognizing the fitness of parts for their appropriate functions; but when this fitness is gained, as in the great toe of the foot of man or the ostrich, by a structure which at the same time betokens harmonious concord with a common type, the prescient operation of the One Cause of all organization becomes strikingly manifested to our limited intelligence” (p. 38).
Archetypal thinking also exalts our own status, for if God ordained the archetype, he certainly recognized all potential modifications in advance, and the concept of human existence therefore long predated our actual appearance: “The recognition of an ideal Exemplar for the vertebrated animals proves that the knowledge of such a being as man must have existed before man appeared. For the Divine mind which planned the Archetype also foreknew all its modifications” (pp. 85-86).
In fact, the entire geological history of vertebrates may be interpreted as a movement towards humanity, guided by natural forces ordained by God as secondary causes. Owen's oft-quoted last paragraph provides a genuine expression of evolutionary views in this limited sense (transformations within an archetypal framework under unknown, but natural, laws established by God to implement His plans of progress):
To what natural laws or secondary causes the orderly succession and progression of such organic phenomena may have been committed we as yet are ignorant. But if, without derogation of the Divine power, we may conceive the existence of such ministers, and personify them by the term “Nature,” we learn from the past history of our globe that she has advanced with slow and stately steps, guided by the archetypal light, amidst the wreck of worlds, from the first embodiment of the Vertebrate [Page 323] idea under its old Ichthyic vestment, until it became arrayed in the glorious garb of the Human form (p. 86).
Owen drew from his archetype all the standard implications that set the research program of formalism — the correlations that define the essence (and continuing relevance) of this pole on the dichotomy: importance of constraint and mistrust of adaptationism (accompanied by demotion to secondary status).
Constraint. This term includes two distinct meanings, bo
th in vernacular English usage (see Gould, 1989a, and Chapter 10, pp. 1025–1061) and in biological jargon — the negative concept of restriction, and the positive sense of channeling. Those who belittle the evolutionary importance of the subject do not deny the phenomenon itself, but rather limit their concept to the negative meaning.
Owen properly depicted constraint as both limitation and channeling. In the former meaning, for example, he notes that the first digit of the generalized mammalian hand or foot develops only two phalanges, while the others grow three. These numbers do not change, even when utility would dictate otherwise — as in elephants where the first and fifth toes do not differ in length, and all digits are enclosed in a large pad (1849, p. 37); or in humans, where the first toe becomes massive and weight-bearing (but cannot gain an additional phalanx), and the little toe almost vestigial (while still retaining its full complement of three phalanges).
On the positive theme of channels, Owen regards an archetype as a blueprint of myriad possibilities (made all the more intelligible by limiting their range to products of common elements in unvarying topological order). All realized examples on earth therefore include only a small subset of possible forms. Owen even felt free to speculate about the anatomy of life on other worlds, provided that the vertebral archetype can lay claim to universal status: “Our thoughts are free to soar as far as any legitimate analogy may seem to guide them rightly in the boundless ocean of unknown truth. And if censure be merited for here indulging, even for a moment, in pure speculation, it may, perhaps, be disarmed by the reflection that the discovery of the vertebrate archetype could not fail to suggest to the anatomist many possible modifications of it beyond those that we know to have been realized in this little orb of ours” (1849, p. 83).
For example, no earthly vertebrate grows more than two pairs of limbs, but the archetype bears diverging rays (the source of limbs by general homology), on each vertebra, and additional pairs therefore become possible:
We have been accustomed to regard the vertebrate animals as being characterized by the limitation of their limbs to two pairs, and it is true that no more diverging appendages are developed for station, locomotion and manipulation. But the rudiments of many more pairs are present in many species. Although they may never be developed as such in this planet, it is quite conceivable that certain of them may be so developed, [Page 324] if the Vertebrate type should be that on which any of the inhabitants of other planets of our system are organized. The conceivable modifications of the vertebrate archetype are very far from being exhausted by any of the forms that now inhabit the earth, or that are known to have existed here at any period (1849, p. 83).
Adaptation. Neither Owen nor any prominent formalist has ever denied interest or importance to the manifestly obvious phenomenon of adaptation. Formalists do not question the high frequency of adaptation, but only dispute the relative ranking of utility as a causal argument. In the view of functionalists, from creationists like Paley to evolutionists like Darwin, adaptation embodies the source and cause of morphological order and change. For formalists, adaptation becomes a secondary phenomenon, imposed upon primary and underlying laws of form to fit a particular organism to an immediate environment. Adaptation remains vital; for without such specific utility, the organic world would feature only abstract models, but no real creatures in their stunning variety. Yet, adaptation still works in a sequential and secondary fashion to place an overlay upon the archetype. Thus, while Owen continually speaks of morphology and teleology, we must not view him as a mushy pluralist, advocating equality of the two poles. His mode of blending ranks the poles, with adaptation distinctly subservient to archetype in the classical mixture of formalist thought.
Owen argued that two great laws build actual organisms from the archetypal form. The first, called irrelative (or vegetative) repetition, iterates the archetypal element into a series of similar parts. The second, adaptive or teleological force then modifies the various segments in different ways demanded by their mode of life.
Since the adaptive force imposes secondary modifications upon an initial string of identical archetypal elements, we must penetrate behind this imposed veil of specific utility and specialization if we wish to apprehend the archetype itself. Various formalist principles lead us to fruitful strategies for peering behind the adaptive mask: embryos as more archetypal than adults; early and simple forms as closer than later and more complex creatures, following “the law that the Archetype is progressively departed from as the organization is more and more modified in adaptation to higher and more varied powers and actions” (p. 49).
This secondary and derivative character of adaptation leads to a linguistic convention of structuralism, where functional fit becomes an impediment to research upon laws of form. The movement of the tetrapod forelimb away from its initial position within the last vertebra of the skull, for example, shows “the antagonizing power of adaptive modification by the removal of that arch from its proper segment” (p. 59). We focus on embryos and simple anatomies in our study of the archetype because, in these forms, “the archetype is least obscured by purposive adaptations” (p. 55).
The derivative nature of adaptation also debars this important phenomenon from any role as a primary organizing principle of morphology. Owen [Page 325] begins his treatise with an incisive argument, cutting to the heart of Paley's error in ignoring relationships among organisms and speaking only of particular designs and their individual excellences. Using Paley's own device of analogy to machines, Owen undermines functionalism from within. Manufactured structures may be individually optimized for their utility; therefore, such contrivances of human technology will not be strongly constrained by homological elements of common design. If organisms had been similarly built on mechanical principles of optimality in adaptation, they would show more structural variation, and not be morphologically clustered as varied manifestations of archetypes. The archetypes themselves, therefore, cannot represent principles of merely functional design:
To break its ocean-bounds, the islander fabricates his craft, and glides over the water by means of the oar, the sail, or the paddle wheel. To quit the dull earth man inflates the balloon, and soars aloft, and, perhaps, endeavors to steer or guide his course by the action of broad expanded sheets, like wings. With the arched shield and the spade or pick he bores the tunnel: and his modes of accelerating his speed in moving over the surface of the ground are many and various. But by whatever means or instruments man aids, or supersedes, his natural locomotive organs, such instruments are adapted expressly and immediately to the end proposed. He does not fetter himself by the trammels of any common type of locomotive instrument, and increase his pains by having to adjust the parts and compensate their proportions, so as best to perform the end required without deviating from the pattern previously laid down for all.... Nor should we anticipate, if animated in our researches by the quest of final causes in the belief that they were the sole governing principle of organization, a much greater amount of conformity in the construction of the natural instruments by means of which these different elements are traversed by different animals. The teleologist would rather expect to find the same direct and purposive adaptation of the limb to its office as in the machine (1849, pp. 9-10).
Moreover, to stress the key methodological point, immediate utility does not imply design for a current end. Complex shapes and anatomies, developed under formalist rules of structural transformation, may find utility after arising for nonadaptive reasons. The delayed fusion of mammalian skull bones may now serve as a prerequisite for parturition through a small birth canal, but birds and reptiles show a similar delay, and this “adaptive” feature did not arise “for” its current and indispensable use in mammals:
I think it will be obvious that the principle of final adaptation fails to satisfy all the conditions of the problem. That every segment in almost every bone which is present in the human hand and arm shou
ld exist in the fin of the whale, solely because it is assumed that they were required in such number and collocation for the support and movements of that undivided and inflexible paddle, squares as little with our idea of the simplest [Page 326] mode of effecting the purpose, as the reason which might be assigned for the great number of bones in the cranium of the chick, viz. to allow the safe compression of the brain case during the act of exclusion, squares with the requirements of that act. Such a final purpose is indeed readily perceived and admitted in regard of the multiplied points of ossification of the skull of the human fetus, and their relation to safe parturition, but when we find that the same ossific centers are established, and in similar order, in the skull of the embryo kangaroo, which is born when an inch in length, and in that of the callow bird that breaks the brittle egg, we feel the truth of Bacon's comparison of “final causes” to the Vestal Virgins, and perceive that they would be barren and unproductive of the fruits we are laboring to attain, and would yield us no clue to the comprehension of that law of conformity of which we are in quest (1849, pp. 39-40).
Owen and Darwin
The changing and uncertain relationship between Darwin and Owen presents an intriguing story in Victorian scientific sociology. Darwin's statement in his autobiography has been frequently quoted: “I often saw Owen, whilst living in London, and admired him greatly, but was never able to understand his character and never became intimate with him. After the publication of the Origin of Species he became my bitter enemy, not owing to any quarrel between us, but as far as I could judge out of jealousy at its success.”
The Structure of Evolutionary Theory Page 52