The Structure of Evolutionary Theory

by Stephen Jay Gould

  All these objections can be combined into a single claim, which Weismann found so compelling that he eventually surrendered panmixia as a fully adequate [Page 207] explanation of degeneration. Panmixia is a genuine, but weak, force; it can reduce the average value of an organ to a state somewhat below its for­mer functional size. But panmixia cannot solve the central question of degen­eration: what propels a useless organ all the way down the slide and into his­tory's dumpster? Weismann admitted his failure (1896, p. 22), and later summarized this ultimately unsuccessful episode in his quest to understand degeneration:

  As my doubts regarding the Lamarckian principle grew greater and greater, I was obliged to seek for some other factor in modification, which should be sufficient to effect the degeneration of a disused part, and for a time I thought I found this in panmixia, that is, in the mingling of all together, well and less well equipped alike. This factor does cer­tainly operate, but the more I thought over it the clearer it became to me that there must be some other factor at work as well, for while panmixia might explain the deterioration of an organ, it could not explain its decrease in size, its gradual wearing away, and ultimate total disappear­ance. Yet this is the path followed, slowly indeed, but quite surely, by all organs, which have become useless (1903, vol. 2, p. 115).

  Weismann therefore needed another kind of auxiliary hypothesis to pre­serve the Allmacht of selection against resurgent Lamarckism. He had tried the mechanics of inheritance as expressed in the doctrine of panmixia; now he would expand the domain of selection itself. He would depart from Dar­win's distinctive focus on struggle among organisms, and attempt to identify a source of directional variation in an analogous competition among determi­nants of heredity within germ cells — a “germinal selection.” Weismann de­vised a truly ingenious argument: if natural selection can produce trends in the morphology of phenotypes, then an intracellular, germinal selection might yield directionality in the variation presented to conventional selec­tion upon organisms. If the determinants of a useless organ predictably lose in an intracellular struggle for existence, then a trend to complete elimina­tion — an apparent example of Lamarckian inheritance by the principle of dis­use — might still be attributed to selection. This new mechanism could not be equated with Darwinian selection upon struggling organisms, but “germinal selection” did represent a process of the same form and logic, but applied to replicating objects at a subcellular scale rather than to entire organisms.

  Weismann first proposed the theory of germinal selection as a brief note in his last rebuttal to Herbert Spencer, thus marking Britain's Victorian pun­dit as a chief source (in reaction) to the first explicit theory of hierarchical selection. (Neue Gedanken zur Vererbungsfrage, eine Antwort an Herbert Spencer, Jena, 1895). Weismann then elaborated the theory in 1896 (pre­sented to the International Congress of Zoology at Leiden on September 16, 1895; first published in The Monist in January, 1896, then as a separate pamphlet, translated into English later that year). Weismann's fullest develop­ment, with some remarkable changes by extension, appeared in his most im­portant book, Vortrage iiber Descendenztheorie (1902), translated into English [Page 208] by J. Arthur and Margaret R. Thomson as The Evolution Theory in 1903. A comparison of the original 1896 version with the fullest exposi­tion of 1902 provides a fascinating exercise in itself, and also becomes a cru­cial argument for this book — for Weismann moved from a limited hypothe­sis proposed only as an adjunct to natural selection to a fully articulated theory of hierarchy, including concepts of independence and conflict between levels.

  SOME ANTECEDENTS TO HIERARCHY IN GERMAN

  EVOLUTIONARY THOUGHT

  Germinal selection certainly finds its immediate source in Weismann's war with Lamarckism, his debate with Spencer, and his severe, longstanding dif­ficulty with the problem of degeneration. But Weismann's eventual embrace of hierarchy as an ultimate argument against Lamarckism also grew from a deeper foundation in German evolutionary thought. This lineage of argument is virtually unknown to English-speaking evolutionists, for the roots lie in the two most important untranslated documents of 19th century German evolu­tionary biology — the Generelle Morphologie (1866) of Ernst Haeckel and the Jugendwerk of a man who eventually made his considerable mark in an­other area of biology, Wilhelm Roux's (1881) Der Kampf der Theile im Organismus (The Battle of Parts in the Organism). Neither Haeckel nor Roux proposed a theory of causal hierarchy across levels of selection; both, in fact, spoke in the name of reductionism. Yet by denying, in very different ways, the exclusivity, or even the privileged status, of the organism as a causal agent in evolution, and by focusing attention on a structural hierarchy of lev­els, both Haeckel and Roux provided central ingredients to Weismann's the­ory of evolutionary hierarchy.

  Haeckel's descriptive hierarchy in levels of organization

  Generelle Morphologie der Organismen (1866), Haeckel's first book, repre­sents an eclectic mixture of militant reductionism and old-fashioned idealis­tic morphology; all united to an evolutionary theory every bit as idiosyn­cratic. (Haeckel dedicated volume two, jointly, to Darwin, Lamarck, and Goethe — and its central argument represents an odd fusion of their disparate ideas.) Haeckel's later notoriety rested almost entirely on the second volume, with its celebrated evolutionary trees (so often reproduced in modern text­books), based largely on his “biogenetic law,” ontogeny recapitulates phylogeny (Gould, 1977b). The first volume, entitled “Allgemeine Anatomie” and dedicated to Carl Gegenbaur, has largely been forgotten. This first volume consists of two major parts, each attempting to establish a formal science for morphological study and each, following Haeckel's invariable practice, stud­ded with a baroque terminology of his own construction. (Haeckel, with a sure sense of what R. K. Merton (1965) would later call the eponymous strat­egy for renown, coined new terms shamelessly, recognizing (I suspect) that a few would probably hang on to bear his legacy (an r-selection approach to [Page 209] the courting of fame). The vast majority quickly succumbed to the negative selection of incomprehensibility, but survivors include ontogeny, phylogeny, heterochrony, ecology, and Monera.)

  The second science, “promorphology,” tried to establish a physical, or crystallographic basis for organic form. Haeckel created a forest of terms illustrated in two complex plates, but never established any useful connections with physical or chemical principles. (Haeckel promoted his much vaunted mechanistic reductionism more by verbal proclamation than by deed, but the influence of a well-articulated philosophy consonant with social trends of an age must never be doubted.) The first science, “tectology,” tried a different approach to reductionism — not subsumption under physical laws, but break­down to component parts.

  As the “basic principle” of tectology, Haeckel stated that all organic ob­jects must be built from components in a structural hierarchy of six ascending levels. But, in applying this pronouncement to actual cases, Haeckel makes a fascinating intellectual move, proving that his allegiance lay as much with ho­listic traditions of an older idealistic morphology, as with the militant physi­cal reductionism that won his lip service and fit with many of his social and political goals (Gasman, 1971). For Haeckel did not argue, in the manner of most 19th century reductionists, that his first and lowest level stands as fun­damental and basic (also “closer” to physics), with subsequent levels only treated as amalgamations based on principles of joining. Instead, Haeckel proclaimed a form of equality among the six levels (while not denying the compositional theme that lower units join to build higher entities). He re­ferred to tectology as the “doctrine of organic individuality” (Lehre von der organischen Individuality), and insisted that the objects at each of the six levels be designated as “individuals” in their own right — “individuals of the first order,” “individuals of the second order,” etc. He placed “plastids” (cells and cell components) on the first rung, organs (including tissues and organ systems) on the second, antimeres or Gegenstiicke (symmetrical parts, includ­ing rays or
body halves of bilateral creatures, literally “counterparts”) on the third, metameres or Folgestucke (body segments, literally “following pieces”) on the fourth, persons (or vernacular “individuals”) on the fifth as “morphologische Individuen funfter Ordnung,” and colonies or “corms” on the sixth and last plane.

  This equalization of status prompted the interesting consequence, with reference to natural selection, of denying to organisms their privileged Darwin­ian role as exclusive evolutionary agents. Natural selection surely ascribed evolutionary change to a struggle among individuals for reproductive success. But Haeckel insisted that objects at all six levels counted as “individuals,” and that no level could claim any special status as evolutionary agent. Organ­isms represent only one waystation in the ascending hierarchy. Perched on the fifth rung, they are made of metameres and aggregate into corms — just as or­gans are made of plastids and aggregate into antimeres. In an insightful state­ment on the role of language in prejudicing thought, Haeckel wrote of his fifth level (1866, vol. 1, pp. 318-319): [Page 210]

  An unbiassed and more deeply probing conception of organic individu­ality shows that these “true” or absolute individuals are, in fact, only rel­ative . . . Although these “true” individuals are, in most higher plants and coelenterates, only the subordinate components of a higher-standing unity (the colony), nonetheless the individuality of humans and higher animals leads us to the erroneous conception that morphological indi­viduals of the fifth order are the “true” organic individuals. This concept has become so general, and has been so strongly fixed in both scientific and vernacular consciousness, that we must mark it as the major source of the numerous and varied interpretations and debates that prevail on the subject of organic individuality.

  Haeckel's concept of structural levels and the non-distinctive status of or­ganisms entered Weismann's argument in two crucial places — first, very gen­erally, when Weismann used the same style of thinking to establish a hierar­chy of (hypothetical) entities as the physical bearers of heredity within germ cells (see p. 214); and, second, quite specifically, when Weismann invoked Haeckel's six-part hierarchy (1896, p. 42) to argue that the struggle for exis­tence starts within germ cells, but then extends up through all Haeckelian cat­egories to colonies at the top.

  Roux's theory of intracorporeal struggle

  Wilhelm Roux's Der Kampf der Theile im Organismus evoked a wide range of reactions. Roux's teacher, Gustav Schwalbe, warned him against ever pub­lishing such a “philosophical” book again. Haeckel, another teacher, liked the work for its consonance with his own ideas, while Darwin himself, during the last year of his life, became greatly intrigued, writing to G. J. Romanes on April 16, 1881:

  Dr. Roux has sent me a book just published by him ... It is full of rea­soning, and this in German is very difficult for me, so that I have only skimmed through each page; here and there reading with a little more care. As far as I can imperfectly judge, it is the most important book on Evolution which has appeared for some time ... Roux argues that there is a struggle going on within every organism between the organic mole­cules, the cells and the organs ... If you read it, and are struck with it (but I may be wholly mistaken about its value), you would do a public service by analyzing and criticizing it in 'Nature' (in F. Darwin, 1887, vol. 3, p. 244).

  (Note how, contrary to the prevalent historical myth of the aged Darwin as the reclusive “sage of Down,” he actually (and actively) kept his ears alert, and his fingers right on the pulse of evolutionary debate. Romanes repre­sented just one among several younger colleagues and supporters whom Dar­win often recruited, both overtly and nonsubtly, to carry forth his interests in both the public and the professional arena.) [Page 211]

  Obviously, Roux had adopted Darwinian language for his title. Just as obviously, he hoped to apply the Darwinian apparatus at a level below its conventional locus of organisms. Roux's book surely occupies a place in the history of hierarchy theory, if only because its verbal image of struggling parts led many evolutionists, notably Weismann himself, to consider multiple levels of selection. But, curiously, as several critics soon noted (Plate, 1905; Kellogg, 1907; and even, with some ambiguity, Weismann, 1904, as well), Roux's the­ory does not really treat descent at all. Weismann's germinal selection, as we shall see, is a true theory of suborganismal selection and inheritance; but Roux's battle of the parts includes no statement about heredity, and ranks in­stead as a theory of functional adjustment in development.

  Roux argued that the construction of a harmonious and well-designed organism emerges from a struggle among parts competing for limited nutri­ment. Lung cells compete with liver cells, and bone cells battle with other bone cells for best locations in the flow of nutriment. To cite Roux's favorite example, made even more famous by D'Arcy Thompson's later analysis (1917, and see Chapter 11, pp. 1195–1196), the bony trabeculae in the head of a hu­man femur form a virtual diagram of forces imposed on the bone during loco­motion, and must therefore be optimally designed to counter stress. But no one can argue that details of the arrangement in any single bone represent an evolutionary adaptation, if only because the trabeculae of a broken, and im­properly mended, femur reform along the new stress lines of a limping walk.

  Roux argued that stresses establish lines of preferred flow for nutriment. Bone cells that happen to lie in the stream prosper and proliferate; others in less advantageous positions wither and die — leading to a functional honey­comb of struts and empty spaces. Roux used this argument to explain the functional design of tissues and organs in general, but he focused upon such complex and exquisite examples of optimal form as the barbules on bird feathers, the hairs that cover the spiracles of many insects, the arrangement of muscle fibers in the walls of blood vessels, and the bony trabeculae discussed above.

  This “battle of the parts” may account for the flexible construction of opti­mal form in each organism. Indeed, such a principle, appropriately modern­ized, remains essential for a developmental biology that cannot invoke a specially tailored gene for each villus on an intestinal surface. But Roux's pro­posal cannot operate as a theory of evolutionary change for two reasons. First, the struggling parts do not vary in heritable ways, and victory cannot lead to beneficial changes in future generations. Bone cells that prosper on the growing trabeculae cannot be designated as superior to, or even in any sense intrinsically different from, the losing cells that die for lack of nutriment in the spaces between. The winners owe their success only to the good fortune of a favorable location. Kellogg (1907, p. 207) wrote: “This competition de­pends chiefly on the hazard of position ... Not the best qualified but the best situated fibers have vanquished the others by robbing them of food and thus finally destroying them.” Second, Roux's Kampf der Theile includes no theory [Page 212] about inheritance. No matter how exquisite or optimal the outcome for any one organism, the results of the struggle cannot be imparted to offspring. (The capacity for functional adaptation might, of course, be heritable and might evolve by ordinary natural selection, but Roux never discusses this quite different issue.)

  Weismann reacted to Roux's theory in a complex and ambiguous manner. He always credited Roux as an antecedent of germinal selection (a reasonable attribution, if only because an explicit metaphor of struggling parts can direct another scientist's thinking to a truly selectionist theory, even if the original proposal operated in a different domain). Weismann, particularly in his early work, seems to credit Roux — incorrectly — as a true suborganismal selection­ist: “Functional adaptation is itself nothing else than the efflux of intrabiontic selective processes” (1896, p. 15). Roux's theory, he argues in several passages, rests upon a variational base, and is therefore Darwinian.

  But, by 1904, Weismann had recognized that Roux's suborganismal strug­gle could not operate as a theory of evolutionary change: “There is an essen­tial difference between personal and histonal selection, inasmuch as the latter can give rise to adaptive structural modifications
corresponding to the needs of the tissue at the moment, but not to permanent and cumulative changes in the individual elements of the tissue” (1904, volume 1, p. 248). “No one will be likely to suppose that the distorted position of the spongiosa of a badly healed fracture could reappear in the straight bone of a descendant” (ibid., p. 251).

  Moreover, Weismann added, even the metaphorical linkages of Roux to Darwin cannot be logically sustained. Most of Roux's examples do not in­clude competition among members of the same cell population (as in bone cells within the developing femur), but between entirely different organs: liver cell with lung, or kidney or heart. This process cannot be viewed as a struggle for existence at all, but only as a sorting out of different “species” into their appropriate places: “The struggle for existence and for descendants, in this case, is between two kinds of cell which were different from the beginning, and of which one has the advantage at one spot, another at another” (1904, volume 1, p. 248). Weismann then drew a striking analogy* between different [Page 213] tissues in an organism, and different species of birds in a broad geograph­ical region:

  The case may be compared to that of a flock of nearly allied species of bird, of which one species thrives best in the plains, another among the hills, and a third among the mountain forests, all mingled together in a vast new territory to which they had migrated, and in which all three kinds of conditions were represented. A struggle would arise among the different species, in which in every case the particular species would be victorious which was best adapted to the local conditions ... This would be the result of a struggle between the three species, not between individ­uals within each species, and it could not therefore bring about an im­provement of a single species, but only the local prevalence of one or an­other (1904, volume 1, pp. 248-249).