Partly as a rhetorical device to be sure, but largely from deep conviction about the essence (and attractiveness) of his system, Eimer presented his style of orthogenesis as a reasonable and happy intermediate, an Aristotelian [Page 358] golden mean, between the two extreme views of purely internal and exclusively external forces as causes of evolutionary change. At one counterproductive extreme of strict internalism, his colleague Nageli acknowledged external forces of environmental shaping and adaptation only as hindrances to the expansive and progressive character of inherently driven evolution. Nageli granted but two roles to externality: the accountant's job of crafting clearer taxonomic groups by eliminating intermediates; and the grim reaper's task of pruning a bush that would otherwise have grown even more luxuriant. All evolutionary advance and diversification arises from intrinsic properties of life, particularly from a Vervollkommungskraft, or perfecting force.
Nageli expressed this most internalist of all post-Darwinian evolutionary theories with a striking metaphor of gardeners and growing bushes — an iconography well worth remembering as a guide to this style of theorizing (note the obviously intended comparison of Darwinians with children):
Still better may we compare the vegetable kingdom to a great tree branching from its base upwards, of which the ends of the twigs represent the plant forms living at one time. This tree has an enormous power of sprouting, and it would, if it could develop without hindrance, form an inextricable bush-like confusion of innumerable branchings. Extermination in the struggle for existence, like a gardener, prunes the tree continually, takes twigs and branches away, and produces an orderly arrangement with clearly distinguishable parts. Children who see the gardener daily at his task may well suppose that he is the cause of the formation of the branches and twigs. Yet the tree, without the constant pruning of the gardener, would have been much greater, not in height, but in extent, and in the richness and complexity of its branching. In the perfecting process (progression) and adaptation lie the mechanical impulses, which lead to the abundance of forms; in competition and extermination, or in Darwinism proper, only the mechanical cause of the formation of gaps in the two organic kingdoms (quoted in Eimer, 1890, p. 19).
Incidentally, and to show the power of mechanistic thinking in scientific culture at the time, Nageli (though usually cited as a leading vitalist today) insisted as vociferously as Eimer that his orthogenetic forces, though unresolved, must be entirely natural consequences of the physiochemical construction of organisms.
At the other extreme of overextended externalism, strict Darwinians hold that organisms contribute only isotropic raw material (hence no direction from internal forces), and that natural selection produces all evolutionary change as adaptation. Eimer did not engage Darwin himself as a chief opponent — for Darwin had held more pluralistic views and was, moreover, no longer available for polemical battle. Instead, Eimer focused his anti-selectionist arguments against August Weismann, chief disciple of the exclusivistic version that adopted the label of “neo-Darwinism” (see Romanes, 1900, on the difference, non-continuity, and “non-homology” of this [Page 359] “brand” of Neo-Darwinism — Weismann and Wallace's hyperadaptationism — with the same term as adopted by the Modern Synthesis of the 1940's, and continuing today). Weismann, as discussed extensively in Chapter 3, strongly advocated the Allmacht (“all-power,” or omnipotence) of natural selection. In a rhetorical dig, Eimer countered with the Ohnmacht (“without-power,” or weakness) of Darwin's primary force.*
Eimer attempted to land squarely in the middle of this debate (1890, p. 63): “Neither Nageli's view, which ascribes the principle of utility an almost infinitesimal effect, nor Weismann's which regards adaptation as all powerful, can be unreservedly accepted. The truth lies between them.” Eimer joins inside and outside with a model of internal orthogenesis as the architect of possible pathways for change (with ontogenetic trajectories as the most important channels) and environmental forces as potentiators of the channels into actual expression as evolutionary alteration. As a simple, but instructive example, channels feature two directions. Orthogenesis builds the channel — thus constraining change to two paths in a potential infinity — but doesn't specify the direction. Environment supplies the required push, thus assuring that the prefigured change will be either adaptive or at least neutral, for selection operates as an efficient executioner of the ill designed. (Or, taking an even larger role, environment might choose the channel, if several stand open for possible entry.) [Page 360]
Morphological “degeneration” provides the best example of environmental impetus within Eimer's concept of balance between internal and external forces. The ontogenetic channel supplies the internal component, since simplification can be achieved most easily by backing down the ontogenetic pathway. But environment must provide the downward push — for change usually proceeds in the other (and upward) direction, as the biogenetic law asserts. Environment, in this case, might act in a positive sense (when simplification leads to local adaptation, as in parasitism), or in a more negative way (when unfavorable climates prevent the full passage of ontogeny, and a resulting juvenilization then becomes inherited in Lamarckian fashion). Eimer writes: “The abundance of the species which have been formed by degeneration, by retrogression, is known to every zoologist. It is self-evident that their origin is to be traced to the action of external conditions” (1890, p. 53).
Eimer found his best example of balance, or environmentally triggered orthogenesis, in a favorite case of all early evolutionists — the Mexican axolotl, or sexually mature tadpole of the genus Ambystoma. (Axolotl formerly bore the separate generic name Siredon, while the parental form often received a misplaced “1” as Amblystoma — as in the quote below.) Ontogeny set the orthogenetic channel and change could only proceed in the prefigured direction, up or down. But the actual path taken depends upon the environment — down for warm and permanent ponds, up in more terrestrial climates:
Siredon is for the problem of evolution one of the most important living animals, in that it brings so beautifully before our eyes the transition of lower sexually mature into a higher sexually mature form, and at the same time shows so clearly the causes of the transformation. We discern these causes simply in the reaction of the organism under external conditions, the increased exertion of an organ already in process of formation (the lung), and the disappearance of another (gills) in consequence of definite demands of the environment, and changes connected with these by correlation. Amblystoma appears where the axolotl has too little water to live in, where it is compelled to live on dry land. That this is the case is proved by the possibility of artificially rearing Amblystoma from the axolotl by gradually withdrawing water (1890, p. 46).
At this point, Darwinian forces might have played a major role in Eimer's system. He needed external potentiators to trigger prefigured orthogenetic pathways. Had he chosen selection as a preferred external force, his resulting amalgam would still have departed from a Darwinian rubric — for the idea of internal orthogenetic channels runs so contrary to the key Darwinian postulate of undirected variation. But selection would still have played an important role as an instigator of trends.
However, Eimer chose Lamarckism as the preferred external potentiator for his “brand” of functionalism. He opted for use and disuse with inheritance of acquired characters as the standard mode for transferring environmental information to organisms — and thus as the primary mechanism for [Page 361] adaptive change. Selection, to Eimer, therefore became a negative force — an eliminator of the unfit, once other factors had channeled a trend.
Eimer's 1890 subtitle neatly expressed his balanced view of external and internal forces — “. . . the result of the inheritance of acquired characters according to the laws of organic growth.” The laws of organic growth act as internal (structural) channels of orthogenesis; the catch phrase of the Lamarckian system then identifies the primary external pushe
r. Natural selection therefore becomes marginalized to the periphery of half a theory.
Eimer's low opinion of selection represents a common viewpoint among evolutionists of his generation, as expressed in the most familiar of all late 19th century critiques (see pp. 137–141) — the denial of “creativity” to natural selection (viewed entirely as a negative force, while new and favorable features must arise by some other “creative” process). Eimer places himself squarely in this majoritarian tradition when he writes (1890, pp. 383-384): “Natural selection can, as I have repeatedly remarked, create nothing new. It only so far contributes to the growth of the organic world that it selects the forms which are most fitted for life, and preserves them for the future action of new stimuli and of crossing . . . Thus the power of selection lies chiefly in the promotion and diversification of organic growth. It is ... only an indirect cause of the evolution of living beings.”
Eimer's original contribution lay in his characterization and defense of the creative force that could displace selection to such an insignificant periphery. Eimer understood the crucial role of undirected variability in the logic of Darwinian argument. He knew that his orthogenetic channels would derail the Darwinian system by vesting creativity for change in the process of variation itself; selection could then only speed up or hinder what internally generated directionality had previously supplied. Eimer contrasts orthogenesis against selectionism, while also reemphasizing the mechanistic, and non-vitalistic, character of channeled variation:
This conclusion is in a certain sense opposed to Darwin's, since it recognizes a perfectly definite direction in the evolution and continuous modification of organisms, which even down to the smallest detail is prescribed by the material composition (constitution) of the body. According to this conclusion, the real Darwinian principle, that of selection depending on utility, is only effective within the limits which are prescribed by the material composition of the body, that is, by the fixed directions of evolution. Accordingly there is nothing fortuitous, but everything in evolution to the smallest detail is governed by laws (1890, p. 431).
If the creative force of evolution resides in the process of directed variation itself, then the nature of internal channeling assumes crucial importance. In the absence of a documented mechanistic theory for the nature of inheritance, Eimer and all leading orthogeneticists followed the empirical tradition of inducing supposed regularities of channeled variation from common features of case studies. Eimer presented his list, with varying numbers and orderings of categories, in all his major publications (1890, pp. 28-30; 1897, pp. 18-21). [Page 362] Each principal “law” of channeled variation identifies a putative ontogenetic pathway, and therefore indicates the status of individual growth as the primary determinant of evolutionary direction. Eimer includes in his list:
1. As an overarching principle, the biogenetic law of recapitulation, which specifies that the sequence of terminally added features in phylogeny shall, through acceleration of development, become the ontogenetic channel of future changes. But what regularities specify the character of these terminal additions?
2. The invariant series of changes in color markings from longitudinal stripes, to spots, to transverse stripes, and finally (in most cases, when intensification occurs) to darker and more uniform coloring. Today, we may be surprised and puzzled (as, by the way, were many scientists at the time, for example Whitman, 1919) by Eimer's decision to present so apparently particular and contingent a sequence as not only effectively universal, but also as the major specific rule of channeling stressed in all his publications. Eimer first enunciated this principle in his early work on lizards. The doctrine then became the foundation for his most important study on coloration of butterfly wings (the focus of the final volume (1897) of his treatise on orthogenesis and evolutionary theory). Eimer's specific claims for butterflies did not gain a wide following, as many naturalists recognized the circular character of his argument. (He assumed the law of longitudinal striping ® spots ® transverse striping a priori and then used this principle to establish “phyletic” sequences of living species with no other criterion of cladistic order).
3. “The law of wave-like evolution, or law of undulation” (1890, p. 29). New characters appear at particular parts of the body — almost always the posterior end — and then pass forward during growth. A series of progressive waves may sweep over the body during a single ontogeny. This law, apparently so arbitrary and riddled with exceptions, also met with little favor, even among fellow orthogeneticists. Whitman (1919), for example, accepted the notion of spatial “waves” in ontogeny as channels of variation, but argued that progressive color variation passed from front to back in pigeons, in direct opposition to Eimer's pathway. (A “rational” basis for spatial waves could be sought in the biogenetic law, as older parts of the body should act as the source for new phyletic features added terminally. But the posterior end of an animal may be either old or young depending on the body's mode of growth.)
4. “The law of male preponderance.” In Eimer's words (1890, p. 28): “that where new characters appear, the males, and indeed the vigorous old males, acquire them first, that the females on the contrary remain always at a more juvenile lower stage, and that the males transmit these new characters to the species.” This principle could claim a rationale beyond simple sexism (though this social context should not be disregarded as a source either), for male preponderance followed from the general theme behind all Eimer's channels — the biogenetic law. If, as most biologists believed in Eimer's time, males tend to move beyond terminal female stages in a common ontogenetic channel, [Page 363] then new features will first appear in males, since novelty must be added terminally under Haeckel's law.
The specification of such channels as constraints on adaptation does not, of itself, push natural selection to a periphery of unimportance; for, as Darwin argued in another context (see pp. 251–260), existing channels must have evolved for some adaptive reason in ancestors — and if selection constructs adaptation, then Darwinian forces can reclaim their creative power, whatever limits these inherited channels may place upon current adaptation. In fact, Eimer did toy with the idea that selection might have built the primary channels — doing so in a burst of fanciful speculation, much in the mode that he himself would later castigate as the primary weakness of adaptationist thinking.
For the primary law of longitudinal stripes to spots to transverse stripes in coloration, for example, Eimer suggested that the initial state might represent an adaptation of ancestors (under the false assumption that monocots precede dicots in the geological record of angiosperms): “The fact of the original prevalence of longitudinal striping might be connected with the original predominance of the monocotyledonous plants, whose linear organs and linear shadows would have corresponded with the linear stripes of the animals” (1890, p. 57). Eimer then extended this speculation by guessing that conversion to spots “might be connected with the development of a vegetation which cast spotted shadows”; and the final transition to transverse stripes “with the shadows, for example, of the branches of woody plants — thus the marking of the wild cat escapes notice among the branches of trees” (p. 57).
From this phyletic fancy, Eimer moved to more conventional Darwinian reasons for other channels. Male preponderance “might possibly be explained by the fact that the males fight the battle of existence more than the females, and therefore must always be first to respond to new demands” (p. 58). And ornamental waves moving from posterior to anterior might also gain a selectionist basis “by the fact that the part of the body farthest from the head is most in need of mimicry, because it is least protected in other ways by the sense organs, and because it is at a special disadvantage; that it is the last part to be withdrawn from the pursuit of an enemy” (p. 58).
But Eimer could not, ultimately, grant even this much power to selection and adaptation for two major reasons. First, he decided that several of his channe
ls expressed no evident utility and, even if adaptive in final expression, could not have possessed any selective value in incipient states (1890, p. 59):
But all this does not explain the first occurrence of the new characters, nor the undeviating course of the evolution in a particular direction. For when a number of varying individuals are compared it is seen that the variations of all tend to a definite end, and that the majority of the intermediate forms show stages in the development of the characters, which are absolutely without use to them. This cannot be explained except by [Page 364] natural growth, whose operations are changed, intensified, or diminished to a certain extent by the stress of adaptation, and may also at times be entirely restrained.
The Structure of Evolutionary Theory Page 58