Weismann's strong and valid critique of Roux leaves us with a puzzle: why did Darwin, who understood the nature of selection so much better than anyone else (see next section), become so intrigued with Roux's book, if Kampf der Theile does not really develop a selectionist, or even a truly evolutionary, theory at all? Several resolutions may be suggested. Most mundanely, Darwin was no German scholar, and he may not, as he himself suggested to Romanes, have properly understood the theory in his cursory reading. Secondly, Darwin was not a strict selectionist, and may simply have valued Roux's insights on functional adaptation, including the Lamarckian implications for a theory of heredity by extension. But, in a third and intellectually more intriguing hypothesis, perhaps Darwin valued Kampf der Theile for two genuine benefits or consonances that Roux's book granted to natural selection — one practical, the other metaphorical.
In a practical sense, Roux explicitly provided the resolution of a paradox that had plagued natural selection — the problem of too much adaptation (“organs of extreme perfection” in Darwin's designation in Chapter 6 of the Origin). Can we really believe that organismal selection constructs each barbule on every feather — even with the immensity of geological time and the hecatomb of deaths in each generation? Roux offered Darwin a sensible exit from such an untenable implication: selection builds the capacity for an automatic functional response that can directly shape each organism in minutely adaptive ways during growth: “Through the capacity of the struggle of parts, a much higher perfection, the purposefulness of the functioning part down to the last molecule, can arise, and occur much more rapidly, than if it had to originate, by the Darwin-Wallace principle, through selection of variation in the struggle for existence among individuals” (Roux, 1881, p. 239).
But Roux offered even more, by way of metaphor, to Darwin's cardinal vision — the paradox of higher stability arising through struggle among lower elements. Functional adaptation might not rank as an evolutionary theory, [Page 214] but such a process does produce internal order within a body by struggle — just as natural selection engenders the external harmonies of adaptive design and ecological balance: “As the struggle of parts [Kampf der Theile] yields purposefulness within an organism ... so does the analogous struggle for existence [Kampf urn's Dasein] among individuals yield purposefulness with respect to external conditions of existence” (Roux, 1881, p. 238). Roux also echoed Darwin's most general and most important philosophical principle:
To many, the direction of this book may well seem very strange — for it holds that, in an animal, in which everything is so exquisitely ordered, in which all the different parts interlock with such excellence, and work together in such perfected coordination, that a struggle of parts occurs, so that in one place, where everything works together according to firm principles, a conflict among the individual parts exists. But how can an entity [ein Games] exist, whose parts are at variance? ... How shall the good and the stable arise from struggle and battle?... All good can only arise from struggle [alles Gute nur aus dem Kampfe entspringt] (1881, p. 64).
Darwin himself could not have penned a better epitome for his most radical claim.
GERMINAL SELECTION AS A HELPMATE TO PERSONAL
SELECTION
Weismann proposed the theory of germinal selection as a logical solution to the problem of degeneration in a non-Lamarckian world. But germinal selection only makes sense under Weismann's concept of inheritance — yet another theory of structural hierarchy, and explicitly linked by Weismann to Haeckel's 6-fold sequence as a further breakdown and elaboration (for germ plasm) of categories within Haeckel's lowest unit of “plastids,” or cellular constitutents (1896, p. 42).
In Weismann's admittedly hypothetical system, the fundamental sub-microscopic particles of heredity are called “biophors.” Biophors aggregate to “determinants,” the key unit for the theory of germinal selection. The logic of panselectionism requires a high degree of easy dissociability among genetic “particles” responsible for “traits” that can be individually optimized to construct well-adapted organisms — for if “particles” become too tightly linked or coordinated, then each change entails too many consequences for other traits, and constraints begin to prevail over adaptation. “Determinants” play this necessary role in Weismann's panselectionist theory of heredity. Each determinant builds an organ or a particular part of the body — in other words, an “item” of the phenotype that selection can mold independently.
Determinants, like their constituent biophors, are invisible and hypothetical. They aggregate into the first observable unit, the “id” — an earlier use of a term that Freud coopted for a much different role and purpose (just as paleontologists had coined and developed a meaning for “mutation” (Waagen, 1869) before the new science of genetics outcompeted us with a later and altogether [Page 215] different definition). Each id contains a determinant for every trait, and can therefore build a body. Weismann identified ids with the disk-like microsomes, recently observed as linearly ordered on chromosomes. The chromosomes themselves, or “idants” in Weismann's system, carry ids in rows, and stand atop the hierarchy of hereditary units.
Germinal selection rests upon the notion that determinants within germ cells may be analogized to organisms within habitats. Just as organisms struggle for limited resources (and not all can survive), determinants battle for the restricted flow of nutriment available to any cell. The winners grow and proliferate; the losers wither or disappear entirely. The strength of determinants governs the phenotypic expression of their resulting trait. Thus, if the determinants of a particular trait decrease or wither by germinal selection within cells, the trait will suffer in expression by exhaustion of its molecular base.
Weismann viewed germinal selection as an analog to interspecific struggle, rather than to competition among members of a single population — that is, determinants for one organ battle for limited nutriment with determinants for other parts of the body. But, unlike Roux's Kampfder Theile, Weismann's germinal selection does operate as a theory of altered heredity, and therefore as a potential evolutionary mechanism — for determinants weakened by germinal selection not only build a diminished body part; they also pass fewer (and debilitated) offspring determinants to germ cells of the next generation.
The ingenuity of this argument lies in its capacity to take the most serious potential challenges to the Allmacht of selection — a group of overt phenomena that seem to lie outside the possible control of organismal selection, and therefore within the domain of Lamarckism or orthogenesis — and to reinterpret them as consequences of selection acting at a lower level. For if organismal selection can produce directional trends in phenotypes during geological time, then germinal selection can forge trends in strengthening or weakening determinants (and their phenotypic expressions) across generations. The gradual and unidirectional shriveling to oblivion of an organ not subject to personal selection certainly suggests Lamarckian inheritance and evolutionary loss by disuse; but if we descend a level and peer within the germ cells, we may envision (though we cannot directly see) a constant competition and selection among determinants, with losers paying the usual price of gradual and inexorable elimination. The Allmacht of strict Darwinism may be sacrificed, as organismal selection loses its exclusivity; but selection itself remains preeminent by expansion:
Powerful determinants in the germ cell will absorb nutriment more rapidly than weaker determinants. The latter, accordingly, will grow more slowly and will produce weaker descendants than the former . . . Since every determinant battles stoutly with its neighbors for food, that is, takes to itself as much as it can, consonantly with its power of assimilation and proportionately to the nutriment supply, therefore the unimpoverished neighbors of this minus determinant will deprive it of its nutriment more rapidly than was the case with its more robust ancestors (1896, p. 24). [Page 216]
Weismann offered the same pugnacious defense for germinal selection th
at he had long championed for the Allmacht of organismal selection; the argument must hold, lest we be driven either to mysticism or to the patently false Lamarckian mechanism:
No one who is unwilling to accept germinal selection can be compelled to do so, as he might be to accept the Pythagorean propositions. It is not built up from beneath upon axioms, but is an attempt at an explanation of a fact established by observation — the disappearance of disused parts. But when once the inheritance of functional modifications has been demonstrated to be a fallacy ... he who rejects germinal selection must renounce all attempt at explanation. It is the same as in the case of personal selection. No one can demonstrate mathematically that any variation possesses selection value, but whoever rejects personal selection gives up hope of explaining adaptations, for these cannot be referred to purely internal forces of development (1903, volume 2, p. 121).
Weismann, at least for public consumption, insisted that germinal selection represented the advancing wave of triumphant Neo-Darwinism. (In the late 19th century, “Neo-Darwinism,” a term coined by Romanes, referred to the panselectionist school of Wallace and Weismann, not to the pluralism of Darwin himself. The modern meaning, associated with the evolutionary synthesis of the 1930's and onward, is not genealogically linked to this earlier definition.) Many critics responded by charging that this invisible process represented little more than an ad hoc hypothesis invented to save selection from the otherwise unexplainable phenomenon of degeneration. Kellogg (1907) provided, I believe, the most balanced perspective. He labelled germinal selection as “a new and radically un-Darwinian theory” (1907, p. 134) — recognizing that Darwin's own theory of “natural selection” specified organisms as the locus of selection. But he respected the theory, recognized its similarity with the selectionist logic of classical Darwinism, and regarded germinal selection as a credible attempt to explain, in expanded Darwinian terms, the apparently un-Darwinian property of directional variation. He wrote: “Obviously Weismann in his theory of germinal selection has preserved the actuality of the struggle and the selection, but with a 'rehabilitation' of natural selection in the real Darwinian meaning and only fair application of the phrase, the new theory has nothing to do. It is, much more, a distinct admission of the inadequacy of natural selection to do what has long been claimed for it. It is the first serious attempt at a causomechanical explanation of a theory of orthogenesis, that is, variation along determined lines” (Kellogg, 1907, p. 199).
I particularly value Kellogg's interpretation because the logic of his argument correctly represents, in my view, the relationship of modern hierarchical selection theory to classical Darwinism and to the Modern Synthesis as well. Hierarchy should be viewed as an expansion of Darwinism in its continuing reliance on selection as the primary mechanism of evolutionary change. But, [Page 217] at the same time, hierarchical selection does not merely extend Darwin's exclusively organismal version in a simple, comfortable, or inexorable manner. Rather, in fracturing Darwin's reliance on organisms as evolutionary agents, and in rendering evolutionary variation and change by interaction among levels, the theory of hierarchical selection introduces enough conceptual novelty, and disperses enough inadequate orthodoxy, to rank as a new, and in some respects radical, formulation, rather than a fully comfortable expansion.
Kellogg also grasped the weaknesses of germinal selection (which, of course, would soon become irrelevant when the codification of Mendelism disproved Weismann's conjecture about the physical mechanism of heredity). He asked (1907, pp. 200-201) (1) why measured variation so often conformed to a normal distribution if germinal selection could act so powerfully to promote directional variation; (2) why species generally displayed such geological stability if germinal selection provided such a strong mechanism for orthogenesis; and (3) why, if determinants wage such constant battle, each against all others, severe deprivation of food often produced a proportionately dwarfed organism, rather than a creature lacking phenotypic expression for particular losing determinants in such intensified struggle.
Weismann originally developed germinal selection to explain the disappearance of degenerate organs, once he recognized that panmixia could only yield a partial reduction. But he soon expanded the scope of his new theory into the domain of positive selection as well, hoping to resolve thereby most of the remaining dilemmas in classical Darwinism. The main promise of germinal selection lay in its capacity to explain a phenomenon that could scarcely be more inconvenient for Darwinism — directed variation. Darwin had emphasized the “random” or undirected character of variational raw material as a prerequisite for advocating natural selection as the cause of evolutionary change (see previous discussion of this crucial principle on pp. 144–146). For directed variation, if sufficiently powerful, would demote natural selection to a negative force for eliminating the unfit (while the fit arise automatically by differential variation), and a minor accelerator of trends originating for internal reasons (since random mortality can sustain an evolutionary direction imparted by biased variation).
But germinal selection could now explain directed variation by differential survival of struggling components within germ cells. Weismann therefore made a sweep through evolutionary problems that might be resolved by his new and selectionist theory of directed variation. In his original formulation of 1896, Weismann identified three possible roles for germinal selection in positive adaptation.
1. Fleeming Jenkin (1867) had troubled Darwin with his analogy of potential variation within a species to a rigid glass sphere. Selection could be effective along any radius, but the movement of a modal form from center to surface exhausted all possible variation, and positive selection must therefore damp itself to oblivion before achieving any substantial change. But, Weismann argued, germinal selection of successful determinants establishes a [Page 218] highway for new variation, and any positive natural selection can automatically engender, by germinal selection, a wave of increasing variation ahead of the advancing mean.
2. Spencer (1893a and b) had invested his major defense of Lamarck in the phenomenon of co-adaptation. How could natural selection, working separately on each trait, produce an intricate coordination of numerous parts, all changing in the same direction? But Weismann now argued that any positive organismal selection will strengthen the determinants of all traits involved, thereby triggering a coordinated trend in germinal selection. Co-adaptation becomes less puzzling if all positively selected traits achieve such a strong boost from germinal selection: “As soon as utility itself is supposed to exercise a determinative influence on the direction of variation, we get an insight into the entire process and into much else besides that has hitherto been regarded as a stumbling block to the theory of selection ... as, for example, the like directed variation of a large number of already existing similar parts, seen in the origin of feathers from the scales of reptiles” (1896, p. 39).
3. In his furthest extension — a remarkable claim given his previous faith in the Allmacht of purely organismal selection — Weismann now argued that any intricately precise adaptation probably requires a boost from germinal selection to reach a pinnacle of exquisite design. Writing of mimicry in Lepidoptera, he argued:
It would have been impossible for such a minute similarity in the design, and particularly in the shades of the coloration, ever to have arisen, if the process of adaptation rested entirely on personal selection ... In such cases there can be no question of accident, but the variations presented to personal selection must themselves have been produced by the principle of the survival of the fit! And this is effected, as I am inclined to believe, through such profound processes of selection in the interior of the germ plasm as I have endeavored to sketch ... under the title of germinal selection (1896, p. 32).
This list, while granting broad scope and power to the newly formulated domain of suborganismal struggle, also illustrates the strictly limited conceptual role that Weismann envisaged fo
r germinal selection when he first formulated the concept in 1895 and 1896. The action of germinal selection must always be synergistic with conventional organismal selection. In degeneration, the “type” example if you will, germinal selection “finishes off” what natural selection began but could not complete. In all other cases of evolutionary construction (maintenance of variation, co-adaptation, and “extreme perfection”), germinal selection works hand in hand with organismal selection, either to supply positive variation, or to accelerate change by supplying an impetus in the same direction from another level.
Over and over again, Weismann emphasizes the purely synergistic action of germinal with organismal selection. Thus, for example, ordinary natural selection can initiate the decline of an organ rendered useless by environmental change. In [Page 219] fishes that have migrated into dark caves, organismal selection against eyes must lead to differential survival of animals with weakened determinants for these organs. Once debilitated, these determinants continue to lose battles of germinal selection, and phenotypic expression sinks below the threshold that organismal selection could regulate: “In short, only by this means are the determinants of the useless organ brought upon the inclined plane, down which they are destined slowly but incessantly to slide towards their complete extinction” (1896, p. 25).
The case for synergism becomes even clearer when selection acts for the increasing complexity of an organ — for the two levels need not engage in a relay, but may now work continuously together in the same direction. Organismal selection favors a stronger part, thereby preserving organisms with more powerful determinants for the part, and unleashing a necessarily synergistic germinal selection: “As soon as personal selection favors the more powerful variations of the determinant ... at once the tendency must arise for them to vary still more strongly in the plus direction . . . From the relative vigor or dynamical status of the particles of the germ plasm, thus, will issue spontaneously an ascending line of variation, precisely as the facts of evolution require” (1896, p. 27).
The Structure of Evolutionary Theory Page 36