896 THE STRUCTURE OF EVOLUTIONARY THEORY
groups of life's tree. If we can accomplish this speciational reformulation of
macroevolution, we will understand that many classical "trends" emerge as passive
consequences of temporal variation in numbers of species within a clade often
enhanced when structural constraints channel the potential directions of such
variation, and not as selectively driven vectors in the biomechanical form of
"average" organisms within the clade. We should be asking questions about
numbers of actual species, rather than rates of flux for anatomical archetypes or
abstractions. We should be studying the dynamics of differential species success as
the causal basis of macroevolutionary pattern, not placing our hopes for
explanation upon undefinable optima for competitive triumph of organic designs.
We will finally recognize that causes for the evolution of form and the
evolution of diversity do not interact in the conceptual opposition that defined
Lamarck's original formulation of evolution (see Chapter 3), and that persists today
in common statements (see G. G. Simpson on p. 562, J. S. Huxley on p. 563, or F.
Ayala, 1982) that speciation (or cladogenesis) builds the luxury of iterated
variation, while a different, and altogether more important, process of
transformational anagenesis fashions the trends of form that culminate in such
glories as the human brain and the dance language of bees. We will then grasp that
many—though not all—phenomena in the evolution of form arise as noncausally
correlated consequences of patterns in the changing diversity of species.
If heaven exists (and the management let Darwin in), he must be greeting this
prospect with the same thought that the founders of America emblazoned on the
new nation's Great Seal: annuit coeptis (he smiles on our beginnings). For Darwin
tried (see Chapters 2 and 3) to disassemble Lamarck's sterile dichotomy between
fundamental, but probably unresolvable, causes of progressive evolution in form,
and secondary, albeit testable, causes of lateral diversification—and to reformulate
all evolution in terms of the previously trivialized tangent, while branding the
supposed main line as illusory. I am proposing an analogous reform at the level of
macroevolution—with the "diversity machine" of speciation, previously labelled as
secondary and merely luxurious, also recognized as the generator of what we
perceive as trends in form within clades. Again, the humble and testable factor,
once relegated to a playground of triviality, becomes the cause of a supposedly
higher process formerly judged orthogonal, if not oppositional. But this time, both
the atom of agency and the cause of change reside at the higher level of
macroevolution, and must therefore be accessed in the unfamiliar framework of
deep time, rather than directly observed in human time. Our intellectual resources
are not unequal to such a task, and we could not ask for a better leader than
Darwin.
To illustrate how such a speciational reformulation might proceed, let us
consider, at three levels of inquiry, the consequences of documenting
macroevolution as expansion, contraction and changing form in the distribution of
all species within clades through time. I regard this unfamiliar categorization as an
empowering substitute for the usual tactic of summarizing the history