682 THE STRUCTURE OF EVOLUTIONARY THEORY
any assertion about a fixed number of units in the expanded hierarchy of
Darwinian action.
Any such claim of definity could only rank as both foolish and incoherent for
at least two reasons. First, the hierarchy has not been set by structural or logical
principles, but historically evolved in a contingent manner. Thus, before the
inventions of sexual reproduction and multicellular organisms, neither species nor
organisms (as a level distinct from cells) existed, and a quadripartite hierarchy held
sway (and still does today in the dominant world of asexual unicells)—gene, cell,
clone, and clade. Second, several of the levels discussed here coagulate numerous
phenomena because they lie between two clear boundaries. As Buss (1987) points
out, for example, we might, in certain contexts, recognize several items that encase
genes but serve as parts of multicellular organisms: chromosomes, organelles,
cells, organs, etc. Before the multicellular organism evolved, and began to act with
such effectiveness as a suppressor of intraorganismic selection, we might have
construed this domain of "proto-individuality" quite differently, and with finer
resolution.
As a second argument against granting necessary or inherent status to these
six levels, I have followed nearly all students of this field in preferring a fully
nested hierarchy of increasing inclusion, to other legitimate interactors that
function only occasionally, transiently, or in special circumstances. This fully
nested hierarchy operates with Linnaean logic in requiring that lower units
amalgamate completely, and under strict genealogical constraint—so that no lower
unit can belong to more than one higher unit, while no higher unit can "forage"
outside its hereditary line to incorporate the lower units of other distinct
evolutionary branches at the same level. Just as a genus can't belong to two
families, a species of flies cannot incorporate some onychophores and a few
myriapods to construct a more versatile species-individual.
We logically require this property of nesting to correlate the nonhistorical
process of selection with a set of quintessentially historical phenomena in
evolutionary biology, including phylogenetics and the study of adaptation. Without
such a fully inclusive hierarchy, for example, we could not use one level as a
surrogate or convenient descriptor for events at other levels in the same nest—as
when we choose the gene level for keeping the general books of evolution (see pp.
632 - 637 on the error of gene selectionism).
Nature, of course, does not always obey this logical stricture, though we may
appeal to the empirical success of this formulation as an indicator that nature does
comply at a preponderant relative frequency. If life did not generally work within a
hierarchy of inclusion, the biotic world would present such a different appearance
that our conventional ordering devices would not operate usefully, and would
never have been proposed or accepted. (I am not a naive realist, and I have argued
throughout this book that we impose our social preferences upon nature in
constructing our theories. But nature does provide a strong input, and does impose
a powerful constraint upon our formulations.) No one would ever have suggested a
nested system like Linnaeus's, if common experience proclaimed that novel taxa
generally arise by distant amalgamation—if, for example, each new mammal arose