358 THE STRUCTURE OF EVOLUTIONARY THEORY
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