The Fruitful Facets of Galton's Polyhedron 357
change. Eimer (1890, p. 379) emphasizes the distinction without a difference: "We
have to distinguish from one another (a) individual (personal) growth, (b) the
growth of the race (the species), or phyletic growth. The latter is, however, merely
the sum of the modifications due to growth which the individuals of a line of
descent have undergone in course of time." Then, in a stronger statement of unity,
he adds:
The individual growth of every plant, every animal is a brief and rapid
repetition, under the continued influence of similar stimulation, of the series
of effects produced by external stimuli in the course of vast periods of time
on the tissues of its ancestors. The character of the individual growth of
every living being therefore depends essentially on phyletic growth; the
individual growth includes phyletic growth in itself. Since the individual
growth of every living being is thus a stage of phyletic growth, since the
latter... presents a sum of individual growths, both are traced back to one
and the same process—fundamentally they cannot be separated (pp. 381-
382).Eimer used the ontogenetic channel as a device to elucidate a range of
evolutionary phenomena beyond simple directionality of change. Why, for
example, do some populations of a species "move on" to more advanced stages of
phylogeny (and to formal status as a new species), while others languish in
stability? Some contemporaries had argued that populations must first become
isolated and then may diverge as selection dictates (while parental forms remain
stable in their unchanged environment). But Eimer denied allopatry as a
precondition for speciation: some groups within a species simply show more
phyletic "activity" in varying beyond the ancestral ontogenetic trajectory. As these
groups advance in form, they proceed further than their stable neighbors,
eventually to a distance beyond the range of interbreedability with parental forms.
Eimer thus argued that orthogenesis could explain both directional change and
diversification. * Speciation marks the fractioning of a phylogenetic sequence into
separate segments representing persistent and altered populations. "Varieties and
species are therefore in reality nothing but groups of forms standing at different
stages of evolution, that is, at different stages of phyletic growth, whether it be that
they outstripped their fellows or their fellows them in the process of evolution, so
that connection by intermediate forms was lost.... The essential cause of the
separation of species is seen to be the persistence of a number of individuals of a
definite lower grade of this evolution, while the rest advance farther in
modification" (pp. 30-31).
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
*Darwin faced the same issue when he realized that natural selection, as an agent of
anagenesis, could not fully encompass evolution without a separate explanation for
multiplication of species—a gap that Darwin attempted to fill with his "principle of
divergence" (see pp. 224-236).