602 THE STRUCTURE OF EVOLUTIONARY THEORY
that a proper theory of macroevolution, the central concern of this book, rests upon
such a proposition; and (2) because species seem so maximally unlike discrete
"things" to many biologists, thus making the correction of this false impression a
prerequisite for accepting the full hierarchical model of selection. But species can
claim no favored status in the hierarchical model, and I use them here only as an
example—so that the argument may then proceed to a full set of levels, each
characterized by a valid kind of individual acting in a distinctive way.
Criteria for Individuality
When we apply the term "individual" in ordinary English, we envisage a set of
properties centered upon uniqueness, discreteness, functionality, and cohesion
considered both spatially and temporally. To be a unique "thing," and not just a
part of a continuum, a named object must clearly begin and end— and must remain
its definable self throughout a continuous existence. We may, I think, best
summarize this intuition in three criteria. To be called an individual, a material
entity must have:
- a discrete and definable beginning, or birth;
- an equally discrete and definable ending, or death; and
- sufficient stability (defined as coherence of substance and constancy of
form) during its lifetime to merit continuous recognition as the same
"thing."
I realize, of course, that the third criterion amalgamates several crucial
notions into a single statement. We might specify at least four properties involved
in our ordinary concept of "sufficient stability" for individuality:
CHANGE. An individual may undergo some, even substantial, change during
its lifetime, but not so much either to become unrecognizable or to encourage
redefinition as a different thing—and, particularly, for temporal sequences of
individuals, not so much alteration that late stages come to resemble the next-
named individual in a sequence more than the early stages of the same individual.
DISCRETENESS AND COHESION. An individual must maintain clear and
coherent boundaries during its lifetime. Parts should not "ooze out" into other
individuals, while components of other individuals should not enter and become
incorporated.
CONTINUITY. An individual cannot fade in and out of existence during its
lifetime, but must maintain material continuity throughout. Members of classes, on
the other hand, are not so constrained, for classes are defined by common
properties, not by historical continuity. As Hull (1980) argues, the class of gold
atoms does not require continuity or filiation. If all gold disappeared, its position
on the periodic table would remain—and an element later reconstituted with the
right atomic particles and requisite properties would still, and legitimately, be
called gold. But if all peacocks die, the species-individual Pavo cristatus
disappears forever. Even if some human engineer retained