The Structure of Evolutionary Theory

by Stephen Jay Gould

  I shall show, in the rest of this chapter, that formalist, or structuralist, evolutionary thought, from the immediate post-Darwinian years to the codifica­tion of the Modern Synthesis, continued to emphasize the twinned concepts of saltation and channels — for these notions represent two sides of the same conviction that internal structure can set and constrain the pathways of change. The modern plea to put history and organic integrity back into evolu­tionary theory echoes the same call. If “constraint” has become a buzzword of contemporary evolutionary theory, then I must assume that the shades of Galton and Geoffroy are smiling, for the structure of their thought has with­stood the formalist's ultimate test of timelessness.

  Orthogenesis as a Theory of Channels and One-Way Streets: The Marginalization of Darwinism

  MISCONCEPTIONS AND RELATIVE FREQUENCIES

  The German zoologist Wilhelm Haacke devised the word “orthogenesis” in 1893 (see Kellogg, 1907; Bowler, 1983); but the concept implicitly moti­vated the entire formalist tradition that sandwiched Darwin (in a chronologi­cal sense) between Goethe and Geoffroy on one side, and searchers for the [Page 352] mechanical rules of embryological development on the other. The word liter­ally means “straight (line) generation,” but the term never bore a merely de­scriptive meaning, and all evolutionists understood the wider import. Ortho­genesis denotes the claim that evolution proceeds along defined and restricted pathways because internal factors limit and bias variation into specified chan­nels. In this key sense, orthogenesis must be regarded as a formalist theory, standing against the central Darwinian principle that natural selection im­parts direction by shaping isotropic variation (and doesn't only act in a nega­tive and subsidiary way to eliminate the unfit while some other process cre­ates the fit). (Evolutionists recognized, of course, that natural selection could also produce a directional anagenesis — first called “orthoselection” by Ludwig Plate — and that claims for orthogenesis must therefore demonstrate a causal basis for internal channeling beyond the power of natural selection to shape, and not only record the simple pattern of monotonic change itself).

  Later on, after the Modern Synthesis congealed, and a latter day Darwin­ian consensus needed to recruit some whipping boys from the past, ortho­genesis became a convenient foil for illustrating the bad old days of failure to grasp selection's power. Ever since, most textbook one-liners have dismissed orthogenesis as a theistic remnant operating as a mild pollutant within sci­ence, an almost mystical theory of arcane and inexorable direction. I shall present the arguments of three prominent supporters — G. H. T. Eimer, A. Hyatt, and C. O. Whitman — to explicate orthogenesis as a viable and well-wrought formalist alternative (or supplement) to Darwinism at a time when natural selection could muster no compelling defense. But in hopes of encour­aging a more sympathetic hearing, I begin with three points raised to allay our conventional misconceptions in a more general way.

  1. Some prominent non-Darwinians may justly be designated as “theistic evolutionists” — St. George Mivart and Pierre Teilhard de Chardin, for exam­ple. But orthogenesis does not fall into this category. Rather, and entirely to the contrary, all leading orthogeneticists insisted vociferously that their argu­ments for internal directionality included no teleological or theistic compo­nent. Most leading orthogeneticists held strictly mechanistic views in the mainstream of the highly deterministic late 19th century scientific consensus. They argued that internal channels arose as products of conventional, physi­cal causes, based upon properties of hereditary and developmental systems. (These properties may have been unknown, hence “mysterious” in the ver­nacular sense, but certainly not spiritual or teleological.) In stating claims for predictability of phyletic directions, and for parallelism of numerous inde­pendent lineages constrained by the same internal mechanics, most ortho­geneticists considered themselves in better tune with the physical and deter­ministic spirit of the age, whereas Darwinians fell into disharmony by committing themselves to undirected variation and unpredictable contin­gency of change. Moreover, the orthogeneticists argued, how could a charge of theistic progressionism be leveled when orthogenetic channels drove lin­eages to extinction as often as to complexity?

  2. If the concept of internally constrained channels only represented a [Page 353] vague theoretical notion invented to oppose Darwin's principle of undirected variation, then the usual charge of vacuousness might apply. But all leading orthogeneticists, as my three case studies will demonstrate, identified a pri­mary channel fully consistent with a late 19th century biological consensus. Haeckel's biogenetic law had become widely accepted as the preeminent prin­ciple for tracing phylogeny (Gould, 1977b). This law of recapitulation re­quired that new evolutionary features be added to the ends of previous onto­genies, and that early stages of development be continually speeded up (law of acceleration) to provide room for these additions; adult stages of ancestors therefore migrated “backwards” in ontogeny to become juvenile features of descendants. This principle validated a primary ontogenetic channel as the major determinant of highly constrained evolutionary variability. Features consistent with established ontogenetic trajectories — as additions (by hypermorphosis) or regressions (by secondary slowing of developmental rate) — might easily arise as new evolutionary variants. Other configurations, even if potentially useful to organisms, might never arise in such a constrained sys­tem. In short, ontogeny itself served as the primary channel of constrained variation in orthogenetic theory.

  3. The “hard version” of orthogenesis (held by some proponents, including Hyatt in my case studies) of inexorable one-way streets leading straight to extinction by degeneration of form compels little attention today (and enjoyed little support even in its own time). But the “softer” version of two-way chan­nels — furrows of constrained variation that provided biased material to natu­ral selection, but could not drive a trend to extinction all by itself and against Darwinian forces — expresses a primary and enduring theme of a formalist and structuralist biology that should still engage our close attention.

  To explicate orthogenesis, I turn once again to Vernon Kellogg (1907), au­thor of the finest book on varieties of evolutionary theories and their distinc­tions (in a time of maximum disaffection with Darwinism and general agnos­ticism about alternatives). As discussed on pages 163–169, my framework for this book owes much to Kellogg's argument that Darwinism should be de­fined by a meaningful essence of minimal commitments — and that other no­tions can therefore be classified as basically helpful (“auxiliary” in his termi­nology) or contrary (“alternative”).

  Kellogg identified three major “alternatives” to natural selection — one as functionalist as Darwinism, but offering a different explanation of adapta­tion (Lamarckism), and the other two as structuralist denials that adaptation must guide the origin of new species (orthogenesis, and heterogenesis or saltationism in de Vries's macromutational style). But Kellogg showed partic­ular sensitivity to the nuances, shadings and subtleties that arguments about relative frequency always impart to natural history. He clearly stated that the logic of all three alternatives stands squarely against natural selection if we argue for prevailing strength of effect and relative frequency: “All of these theories offer distinctly substitutional methods of species forming” (1907, p. 262).

  However, Kellogg also recognized that “milder” versions might be seen [Page 354] as auxiliary — not consonant to be sure (for the nonselectionist logic cannot be contravened) — but supplementary rather than substitutional. Thus, for example, if acquired characters are inherited only rarely and weakly, then Lamarckism might aid natural selection in developing adaptation more quickly (by secondary reinforcement from a different source) — a position ad­vocated by Darwin himself throughout the Origin (1859, pp. 134-139, for example). But if acquired characters are inherited faithfully all the time, then natural selection will be overwhelmed and Lamarckism becomes a refutation of Darwinism. Relative frequency determines the distinction. Kel
logg writes, in a statement just before the quotation of the last paragraph (1907, p. 262): “Few biologists would hold any of these theories to be exclusively alternative with natural selection; de Vries himself would restrict natural selection but lit­tle in its large and effective control or determination of the general course of descent.”

  A similar situation prevails for orthogenesis. In Hyatt's “hard” (and truly antiselectionist) version, the internal pathway dominates all lineages, liter­ally pushing the impotent force of natural selection aside, and forcing lin­eages to extinction by phyletic senility. Natural selection exists as a “true” force, but can only operate as a peripheral factor that can, at most, delay the inevitable. In milder versions, the relative frequencies equalize (and the orthogenetic pathways need not lead so clearly to inadaptive forms). In soft versions, still defendable today (though we have ceased, for good reasons, to use the term “orthogenesis”), such internal drives become auxiliaries to selec­tion in providing an initial boost of directed variation for the “incipient stages” of useful structures that posed so many problems for early Darwin­ians (see Mivart, 1871) — and then letting the ultimately more powerful force of natural selection prevail in the larger realm of evident utility. Kellogg writes of such potentially “friendly” versions for Darwinism:

  In true orthogenesis the variation, and hence the lines of modification, are predetermined. It seems obvious, however, to any believer in natural selection that sooner or later the fate of these lines of development will come into the hands of selection. And most orthogeneticists do indeed admit this. But it is precisely in the making of a start in modification that orthogenesis fills a long-felt want, and if capable of proof, should be gladly received by Darwinians as an important auxiliary theory in the explanation of modification, species-forming, and descent [my italics, and an interesting choice of words since Kellogg classifies orthogenesis as an alternative but recognizes here that a sufficiently mild version would fall into his other category of auxiliaries to Darwinism] (1907, p. 276).

  None of the three versions discussed below presents quite so mild a view (though C. O. Whitman approaches such a formulation). All three conceive orthogenesis as a competitor to Darwinism and a more powerful force than natural selection. But these versions do illustrate a spectrum from the “cen­trist” Eimer (the primary popularizer of the name and idea), who viewed natural [Page 355] selection as deprived of power but not contravened; to the “hard liner” Hyatt on one side, who interpreted the orthogenetic drive as contrary to se­lection; to the conciliatory Whitman on the other, who hoped to find appro­priate and mutually reinforcing status for all viable contenders in a pluralistic evolutionary theory.

  I present orthogenesis as a spectrum grounded in relative strength and frequency because I believe that a potential role for modern versions of such structuralist theories should be judged in the same manner. A mild formal­ism of constraint, akin to some ideas within the unfairly reviled theory of orthogenesis, may now enrich our Darwinian world (see Chapters 10 and 11) — and the potential fusion, in its richest form, would not be designated as strict selectionism with a little bit added, but would be recognized as a poten­tially integrated theory of a new kind (with a persistent Darwinian core). In this light, an understanding of the original formulations of orthogenesis and their varying relationships with Darwinism may enlighten us in our current struggle to integrate structuralist and functionalist approaches to evolution­ary causality.

  THEODOR E1MER AND THE OHNMACHT OF SELECTION

  Theodor Eimer's evolving views followed a channel every bit as directional (though inadaptive by current, and perhaps transient, standards) as the con­straining orthogenetic pathways that he ascribed to organisms. His empirical work of the 1870's, on coloration of lizards from Capri, presented a predomi­nantly functionalist argument, with a boost from internal channels to foster movement through incipient stages, and to reinforce the process along the way. In the first of his two volumes on orthogenesis — published in German in 1888 and translated into English in 1890 as “Organic evolution as the result of the inheritance of acquired characters according to the laws of organic growth” — Eimer stressed internal channels, relegated Darwin to a periphery, but still sought a genuine fusion (as the title proclaims) of formalist and func­tionalist perspectives. The second and last volume (for Eimer died soon there­after) — published in 1897 as Orthogenesis der Schmetterlinge: ein Beweis bestimmt gerichteter Entwickelung und Ohnmacht der Naturlichen Zuchtwahl bei der Artbildung — presents an anti-selectionist polemic (directed more at his sparring partner Weismann than at Darwin) and a defense of internal direction as preeminent. Its title proclaims Eimer's change of emphasis from fusion to exclusivity: Orthogenesis of butterflies: a proof of definitely directed development and the weakness of natural selection in the origin of species.

  Gustav Heinrich Theodor Eimer (1843-1898) was born near Zurich and eventually became professor of zoology at Tubingen, Germany. He imbibed the late 19th century mechanistic tradition that so permeated German science (in such movements as Entwicklungsmechanik) — an attitude strongly op­posed to speculative phylogenizing, the main thrust of the “Darwinian” (read Haeckelian) tradition in Germany (see Gould, 1977b, chapter 6). But Eimer also expressed sympathy for “our great philosopher Oken” (1890, p. 433), [Page 356] the early 19th century formalist oracle of Naturphilosophie, who had pro­ceeded him at Tubingen, and had taught Agassiz, among others. Thus, Eimer doubted Darwinian functionalism from both sides of 19th century German biology — the romanticism of early 19th century formalist morphology, and the mechanism of late 19th century experimentalism.

  All sources agree that Eimer's treatise (1890 translation) became the major English language source for the theory of orthogenesis. Contemporaries ei­ther set their own discussion in its light (see Whitman, 1919 — a posthumous publication of work done before 1910), or recommended its primary study to those unfamiliar with, or hostile to, orthogenesis (Kellogg, 1907, p. 322). Modern historians of sciences (Bowler, 1979, 1983) continue to view Eimer as “the major popularizer” of orthogenesis (Bowler, 1983, p. 141, in his book on non-Darwinian evolutionists of the late 19th century). I shall therefore treat Eimer's views first.

  Eimer's mechanist side led him to reject any vitalist or “teleological” tinge to orthogenesis. “I repudiate any special internal force of evolution. Accord­ing to my view, everything in evolution is due to perfectly natural processes, to material, physical causes” (1890, p. 64). In fact, Eimer's philosophical de­fense of orthogenesis relies largely on its putative superiority over Darwinism as an evolutionary mechanics in the determinist tradition; for a discovery of law like order and direction in the key domain that Darwin had surrendered to chance — the origin of variation — would represent a notable triumph for a physicalist worldview. Eimer's opening words (1890, p. 1) set his entire argu­ment in this context: “It seemed to me long ago of the greatest importance to undertake an investigation of the question whether the modification (varia­tion) of the species of animals is not governed by definite laws.” Eimer, of course, concluded in the affirmative (1890, p. 1): “If the principles of Dar­winism are true because they can be shown to follow from natural laws, then it was to be expected that obedience to laws would also be discovered in that province which Darwin had surrendered to chance. But if variation were shown to follow certain laws, the same demonstration would apply to the or­igins of species.”

  If the directions of variation are strongly channeled and law like, then evolutionary history may one day achieve the predictability of physical science (in its late 19th century deterministic version): “The evolution — the growth — of species one from another proceeds onwards as though following a plan drawn out beforehand” (1890, p. 29). This leaning towards predictability flows from the particular theory of channeling adopted by all leading orthogeneticists — phyletic cooptation of the ontogenetic pathway. By virtually synonymizing “evolution” and “growth” in the sta
tement cited above, Eimer expressed the common Haeckelian belief that if ontogeny and phylogeny can­not be exactly equated, both processes proceed under a common nexus of causes (“phylogeny is the mechanical cause of ontogeny,” to cite a familiar Haeckelian maxim — see Gould, 1977b).

  Since the predictable character of ontogeny cannot be denied, this compari­son establishes a prima facie case for orthogenetically channeled evolutionary [Page 357] change. Eimer (1890, p. 379) emphasizes the distinction without a difference: “We have to distinguish from one another (a) individual (personal) growth, (b) the growth of the race (the species), or phyletic growth. The latter is, how­ever, merely the sum of the modifications due to growth which the individuals of a line of descent have undergone in course of time.” Then, in a stronger statement of unity, he adds:

  The individual growth of every plant, every animal is a brief and rapid repetition, under the continued influence of similar stimulation, of the series of effects produced by external stimuli in the course of vast periods of time on the tissues of its ancestors. The character of the individual growth of every living being therefore depends essentially on phyletic growth; the individual growth includes phyletic growth in itself. Since the individual growth of every living being is thus a stage of phyletic growth, since the latter . . . presents a sum of individual growths, both are traced back to one and the same process — fundamentally they can­not be separated (pp. 381-382).

  Eimer used the ontogenetic channel as a device to elucidate a range of evolutionary phenomena beyond simple directionality of change. Why, for example, do some populations of a species “move on” to more advanced stages of phylogeny (and to formal status as a new species), while others languish in stability? Some contemporaries had argued that populations must first become isolated and then may diverge as selection dictates (while paren­tal forms remain stable in their unchanged environment). But Eimer denied allopatry as a precondition for speciation: some groups within a species sim­ply show more phyletic “activity” in varying beyond the ancestral onto­genetic trajectory. As these groups advance in form, they proceed further than their stable neighbors, eventually to a distance beyond the range of interbreedability with parental forms. Eimer thus argued that orthogenesis could explain both directional change and diversification.* Speciation marks the fractioning of a phylogenetic sequence into separate segments representing persistent and altered populations. “Varieties and species are therefore in re­ality nothing but groups of forms standing at different stages of evolution, that is, at different stages of phyletic growth, whether it be that they outstripped their fellows or their fellows them in the process of evolution, so that connection by intermediate forms was lost.... The essential cause of the separation of species is seen to be the persistence of a number of individuals of a definite lower grade of this evolution, while the rest advance farther in modification” (pp. 30-31).