696 THE STRUCTURE OF EVOLUTIONARY THEORY
we treated the subject in evolutionary terms as a historical result of the cell's initial
capacity, retained from its phylogenetic past as an entire organism, for differential
proliferation over other cells (formerly competitors as separate organisms, not
compatriots as components of other organs). Of course, modern human cells that
escape this constraint do themselves no ultimate good, for they have no access to
the germ line, and their unrestrained growth eventually eliminates both their own
lineage and the entire surrounding organism. To this extent, the organism's general
strategies do eventually prevail, following an initially successful assault by a cell
lineage. But what a pyrrhic victory! Nonetheless, the double effectiveness of a
virulent cancerous cell lineage— crowding out in place and distant metastasis to
other locations in the body— recalls the more "benign" strategies of other
successful evolutionary plurifiers within a constrained space (genie proliferation by
tandem duplication and transposition; budding off of new demes and "capture" of
existing demes by immigration and transformation).
If selection at the level of cell lineages now plays only a minor role in most
groups of multicellular organisms, we should not view this hierarchical level as
intrinsically impotent, but rather as historically suppressed in "the evolution of
[multicellular] individuality," to cite the title of Leo Buss's seminal book on this
intriguing subject (Buss, 1987). In Buss's terminology selection upon cells must
now unfold in the "somatic environment," where suppression reigns in the service
of organismic integrity, whereas such selection once occurred in the "external
environment," where unicellular organisms could experience the full independence
and competitive range of Darwin's world. (In fact, since most organisms on earth
remain unicellular—see Gould, 1996a, on the persistence of the bacterial mode
throughout the history of life—this transition has never occurred for the vast
majority of organisms on earth.)
This cellular level therefore provides our best demonstration that the current
evolutionary hierarchy in styles of individuality arose both historically and
contingently, and not with necessity as a timeless, predictable, invariant
consequence of natural law. Levels have surely been added sequentially through
time, as Buss has emphasized. If life began with naked replicators at the genie or
subgenic level, then these earliest times for life may have featured, uniquely for
this initial interval, the property that strict Darwinians have tried so hard to impose
upon our richer world of modern life—selection at one level only. The evolution of
cells led to a tripartite hierarchy that characterized most of life's 3.5 billion year
history, and still regulates the majority of earthly organisms: genes, cells, and
clones. The evolution of sexual reproduction added species, while the complex
processes that constructed the multicellular individual then added the organism
(the body that encloses cells and cell lineages).
Suppression of cell lineage selection by the multicellular organism has greatly
restricted a once vibrant and multifarious level. I must confess to my own
parochialism in recognizing just one unit, the cell, as a surrogate for all entities that
enclose genomes and form parts of organisms. Certainly