The Structure of Evolutionary Theory

by Stephen Jay Gould

  The complexities, interrelationships (in some cases amounting to near contradictions) and comprehensive character of ideas at the core of de Vries' mutation theory may best be illustrated by his attempt to epitomize his system as a set of seven laws (1905, pp. 558-571, 578). Note particularly the tug of war (both logical and psychological) between his understanding that the the­ory refutes Darwinian principles on one hand, and his desire, on the other hand, to retain fealty with Darwin as a personal hero.

  1. “New elementary species appear suddenly, without intermediate steps” (p. 558). De Vries's first paragraph of description boldly expresses the contra­diction between this statement and Darwinian principles. (But note how he declines to attach Darwin's name to the orthodoxy he opposes — speaking in­stead only of “current scientific belief,” or “the ordinary conception”):

  This is a striking point, and the one that is in the most immediate contradiction to current scientific belief. The ordinary conception assumes very slow changes, in fact, so slow that centuries are supposed to be required to make the differences appreciable. If this were true, all chance of ever seeing a new species arise would be hopelessly small. Fortunately, the evening primroses exhibit contrary tendencies . . . The mutants that constitute the first representatives of their race, exhibit all the attri­butes of the new type in full display at once. No series of generations, no selection, no struggle for existence are needed to reach this end (1905, p. 558).

  2. “New forms spring laterally from the main stem” (p. 560). De Vries presents a cogent defense of cladogenetic vs. anagenetic modes for conceptu­alizing evolutionary change, including the full set of implications that con­tinue to evoke debate today:

  The current conception concerning the origin of species assumes that species are slowly converted into others. The conversion is assumed to affect all the individuals in the same direction and in the same degree. The whole group changes its character, acquiring new attributes.... The birth of the new species necessarily seemed to involve the death of the old one ... The general belief is not supported by the evidence of the eve­ning primroses. There is neither a slow nor sudden change of all the indi­viduals. On the contrary, the vast majority remains unchanged; thousands are seen exactly repeating the original prototype yearly, both in the na­tive field and in my garden. There is no danger that Lamarckiana might die out from the act of mutating, nor that the mutating strains it would be exposed to ultimate destruction from this cause (pp. 560–561). [Page 437]

  3. “New elementary species attain their full constancy at once” (p. 562). Again, de Vries states his first words of explanation in a forthrightly and ex­plicitly anti-Darwinian manner, in this case confuting gradualism and the adaptationist perspective. “Constancy is not the result of selection or of im­provement. It is a quality of its own. It can neither be constrained by selection if it is absent from the beginning nor does it need any natural or artificial aid if it is present” (pp. 562–563).

  4. “Some of the new strains are evidently elementary species, while others are to be considered as varieties” (p. 564). De Vries regarded his taxonomy of relative merit and evolutionary potency of mutations (discussed on pp. 431–434) as sufficiently important to rank as one of the seven cardinal statements.

  5. “The same new species are produced in a large number of individuals” (p. 566). De Vries also recognized the importance of his distinctive, non-gene­alogical principle (see p. 433) that mutations forming new “elementary spe­cies” may arise several times (thus imparting a greater chance of success to the novel taxon).

  6. “The relation between mutability and fluctuating variability” (p. 568) — so stated as a phrase rather than a declarative sentence. De Vries recognized this causal claim for a fundamental distinction between two modes of varia­tion as the focus of his theory (see pp. 430–432). No other point received so much discussion in his texts. I do not know why he placed this fundamental statement of his reductionist program in 6th position among 7 statements.

  7. “The mutations take place in nearly all directions” (p. 570 — I shall pre­sent a more extensive discussion of this claim on pp. 446–451). De Vries em­phasized this statement as his major tactic for maintaining fealty with Darwin at macroevolutionary scales, while destroying his mentor's theory for the ori­gin of species. If the phenotypic ranges of new species form an isotropic dis­tribution about the parental type, then the manifest directionality of evolu­tion at geological scales must record the action of a higher selection process upon these species-level variations. Can a form of Darwinian argument there­fore prevail among species (to produce trends), even while the sudden origin of new species precludes selectionism in Darwin's own favored realm?

  From this conceptual foundation, de Vries reached further to promote his mutation theory as the basis for an overarching worldview. To illustrate the range of implications explicitly developed by de Vries, consider just two is­sues of widely differing import. On the first, and practical, question of bene­fits to agriculture, the mutation theory suggested that conventional selection (on fluctuating variation) could only yield limited and easily reversed im­provements. But new mutations might secure large and permanent benefits. Yet, as a practical dilemma, new mutations are rare and cannot be induced by our efforts. What benefit can emerge from scientific horticulture if this disci­pline must wait patiently for good fortune, and can then only apply the jour­neyman's procedure of preservation and propagation: “the practice of the horticulturist in producing new varieties is limited to isolation, whenever chance affords them” (1905, p. 606). As a legitimate escape from this dis­abling consequence of his theory, de Vries proposed that future knowledge of [Page 438] the causes of mutation might place evolutionary alteration under our control, and give us power so far beyond the scope of selection that we might truly be­come the masters of nature: “We may search for mutable plants in nature, or we may hope to induce species to become mutable by artificial methods. The first promises to yield results most quickly, but the scope of the second is much greater and it may yield results of far more importance. Indeed, if it should once become possible to bring plants to mutate at our will and per­haps even in arbitrarily chosen directions, there is no limit to the power we may finally hope to gain over nature” (1905, p. 688).

  On the second, and theoretical, issue of insights from evolutionary theory for human cultural and racial differences, de Vries stated, with principled consistency, that his views on the origin of species suggested no implications whatever: “Our knowledge of the origin of species in nature has no bearing on social questions” (1909a, volume 1, p. 156). De Vries regarded human ra­cial distinctions as arising entirely from selection (or drift) upon fluctuating variability. Homo sapiens resides, with the vast majority of species, in a long­standing phase of stability; not, like the evening primrose, in a rare state of mutability: “Since the beginning of the diluvial period, man has not given rise to any new races or types. He is, in fact, immutable, albeit highly variable” (1909a, volume 1, p. 156). This fluctuating variation provides a source for all racial differences, which, however “profound” in phenotypic appearance, must therefore remain as limited and changeable as any alteration fashioned in this weak Darwinian mode:

  Many mistakes may in the future be avoided if a clear distinction be drawn between mutability and variability in the ordinary sense. The variability exhibited by man is of the fluctuating kind: whereas species arise by mutation. The two phenomena are fundamentally different. The assumption that human variability bears any relation to the variation, which has or is supposed to have caused the origin of species, is to my mind absolutely unjustified. Man is a permanent type, like the vast ma­jority of species of animals and plants ... As we have seen, it is charac­teristic of these types to exhibit a certain amount of fluctuating variabil­ity. Man is no exception to this rule. Therefore all that we can apply to the treatment of social questions is our knowledge of ordinary variabil­ity. The facts of specific differ
entiation are interesting but not relevant (1909a, volume 1, pp. 154-155).

  In sum, de Vries' Mutation Theory became the most important set of con­cepts in evolutionary biology during the early 20th century. The theory at­tained this central status by (1) its radically different and non-Darwinian view of the origin of species; (2) the breadth of its concerns, ranging from variation at the smallest scale to modes of geological pattern at the largest; (3) the range of its implications, as illustrated above, thus expanding the doctrine from a scientific theory to a comprehensive worldview. [Page 439]

  Darwinism and the mutation theory

  Confusing rhetoric, and the personal factor. The Muta­tion Theory — in its logic, on its face (and clearly in the eyes of de Vries' con­temporaries) — seems so evidently contrary to the central tenets of Darwin­ism. Kellogg classified de Vries' theory as one of the three major alternatives to natural selection (with Lamarckism and orthogenesis as the other candi­dates). De Vries himself, and with relish, explained his theory in the light of Galton's polyhedron (pp. 342–351), the primary anti-Darwinian metaphor of his day:

  Little shocks make it totter; it oscillates round its position of equilibrium and finally returns to it. A slightly stronger push however can make it go so far that it comes to lie on a new side. The oscillations round a position of equilibrium are the fluctuations; the transitions from one position of equilibrium to another correspond to the mutations. The track left be­hind by the rolling polyhedron can be regarded as the line of descent of the species; each subdivision of this track, corresponding to a side of the polyhedron, representing a particular elementary species; each transi­tional movement to a new position of mutation (1909a, volume 1, p. 55).

  Yet I began this section on de Vries with a strange story about the uniquely sour note that he introduced into Darwin's biggest centenary party by tortur­ing a Darwinian quotation to gain his master's supposed approval for a mani­festly un-Darwinian view about the nature of variability (pp. 415–417). I then discussed the powerful psychological and intellectual hold that Darwin exerted upon de Vries through his status as personal hero (pp. 421–423). I claim no insight into the subtleties of psychology, but de Vries' relationship with Darwin surely ranks as the most complex, enigmatic, and contradictory personal interaction discussed in this book. Other paired opponents — Cuvier and Geoffroy, Weismann and Spencer, for example — battled in public and provide the usual stuff of controversy. But de Vries met Darwin only once, and their struggle unfolded later, and largely within de Vries' own head.

  De Vries managed (and apparently needed) to support several contradic­tory propositions, to play several roles at the same time: a loyal disciple, who would neither propagate nor tolerate any diminution of his master's fame and Tightness; a shrewd compromiser, who would bring a glorious past into har­mony with later discoveries; a novel revolutionary, who could sweep aside the old and establish a startlingly different theory as a source of personal fame. In documenting the range of de Vries' rhetorical strategies, one can only expe­rience the frustration of any careful and attentive reader in trying to locate a coherent center among the welter of contradictory claims.* Consider de Vries' several positions: [Page 440]

  1. The mutation theory is fully Darwinian. (Darwin might have flirted with a false idea about the efficacy of fluctuating variation, but he ultimately rec­ognized the primacy, if not the exclusivity, of mutational variation.)

  It is in fullest harmony with the great principle laid down by Darwin. In order to be acted upon by that complex of environmental forces, which Darwin has called natural selection, the changes must obviously first be there. The manner in which they are produced is of secondary impor­tance and has hardly any bearing on the theory of descent with modifica­tion. A critical survey of all the facts of variability of plants in nature as well as under cultivation had led me to the conviction, that Darwin was right in stating that those rare beneficial variations, which from time to time happen to arise — the now so-called mutations — are the real source of progress in the whole realm of the organic world (1909a, volume 1, p. 74).

  This comment advances the stubborn position that so frustrated A. C. Seward, and inspired his justified rejoinder at Darwin's birthday party. De Vries' argument must be designated as a remarkable example of “stonewall­ing,” a word just entering the English language in de Vries' time (according to the OED) from a combination of British cricket slang, Australian political terminology, and the memory of Confederate General Stonewall Jackson. In the light of Darwin's firm and consistent emphasis on imperceptibly gradual change, and his equally clear denial of the efficacy of “sports” (a term that de Vries acknowledged as synonymous with his mutations), how could de Vries count Darwin as a saltationist? De Vries admits that Darwin didn't really say the proper words, but he then claims that Darwin must have intended to do so: “Darwin's view, although he never definitely formulated it, was that it was these occasional single variations which brought about the continual dif­ferentiation of living forms” (1909a, volume 1, pp. 86-87). De Vries then at­tributed the truly Darwinian belief in gradualism to Wallace and other less worthy epigones: “Wallace's view is that the material for species forming se­lection is furnished by fluctuating variability; and that these infinitesimal dif­ferences are gradually heaped up in the same direction until ultimately they attain the dimensions of specific differences” (ibid., p. 87) — a good definition of Darwin's actual view!

  2. The mutation theory is Darwinian, while also incorporating a few minor glosses and corrections of Darwin's own views.

  The mutation theory is intended to be a support and corollary to the selection theory of Darwin. There can be no doubt that Darwin correctly set forth the essential steps in the evolutionary process and that changes in his views mostly relate to those minor points, for which, at this time, [Page 441] the material of facts was not adequate to a correct decision. The muta­tion theory claims to remove many of the difficulties, inherent to the Darwinian doctrine, as e.g. the general occurrence of useless characters and the impossibility of explaining the first beginning of a selection on the ground of its usefulness (1922, p. 223).

  3. The mutation theory is Darwinian. We must admit Darwin's errors on some issues, even for important points. But we can't blame him, given the limitations of knowledge in his time: “My work claims to be in full accord with the principles laid down by Darwin and to give a thorough and sharp analysis of some of the ideas of variability, inheritance, selection, and muta­tion, which were necessarily vague at his time. It is only just to state, that Darwin established so broad a basis for scientific research upon these sub­jects, that after half a century many problems of major interest remained to be taken up” (1905, p. ix).

  4. Darwin recognized both fluctuating and mutational variation, but he never formulated a judgment about their relative importance (a direct contra­diction of claim 1 on the dominance or exclusivity of mutational variation): “Darwin almost always speaks of these two types in his discussion on selec­tion but never separates them, and is always in doubt as to their relative im­portance in the origin of species” (1909a, volume 1, p. 31).

  In a variation upon this position, de Vries sometimes claimed that Darwin's agnosticism about the relative importance of these two types of variation holds no significance for evolution, and indicates no weakness in Darwin's logic, because the issue of how variation arises becomes subordinate to the role of natural selection once we feel confident that organisms will generate sufficient variation in any case: “Darwin has left the decision on this difficult and obviously subordinate point to his followers” (1909b, p. 84).

  5. Darwin recognized both fluctuating and mutational variation and re­garded both modes as important. Wallace later restricted “Darwinism” to the fluctuating mode alone (de Vries, 1905, p. 8). True Darwinians, who continue to recognize both modes, tend to be favorable towards the Mutation Theory (though they must revise their views on the relative significance of fluctuating var
iation):

  Unlike the prevailing form of the theory of selection, the doctrine of mutation lays stress on sudden or discontinuous changes, and regards only these as active in the formation of species. The Darwinian form of the theory of selection regards both these and fluctuating variations as operative in the origin of new forms, whilst Wallace favors the other extreme, according to which all formation of species goes by a slow and gradual process of change. The two schools of thought naturally adopt different attitudes towards the doctrine of mutation. It is at once rejected by Wallace's adherents, whilst those who incline to Darwin's own form of the theory are less unreservedly inimical; many of them have even greeted it with open arms (1909a, volume 2, p. 599). [Page 442]

  6. Darwin recognized both kinds of variation. Early in his career he cor­rectly emphasized the mutational mode. Unfortunately, his critics later bad­gered him into a more extreme and less generous commitment to the primacy of fluctuating variation (note the direct contradiction to claim 4 and, in turn, the contradiction of 4 and 1 — quite an intellectual odyssey: from exclusivity of one form, to pluralism, to exclusivity of the other. Note also that de Vries often stated all these positions in the same publication; thus, I am not only re­cording a consistent and legitimate ontogenetic change in opinion): “To sum up, we see that Darwin always distinguished between individual differences and single variations and that he ascribed to the latter at least a very consider­able role in the origin of species. It was only by the pressure of criticism that he finally gave up this view and gave the place of honor to the ever-present in­dividual variations” (1909a, volume 1, p. 39).