1058 THE STRUCTURE OF EVOLUTIONARY THEORY
the usual ethos of science in placing special value on new ideas that challenge
complacent conventionalities.) In this crucial sense, the identification of certain
causes as constraints depends upon the claims and nature of a standard theory.
The historical and structural vertices of the aptive triangle become sources of
constraint when this standard theory emanates from the third, or functional, vertex—
as classical Darwinism does, and as Darwin himself so clearly specified in his own
analysis of the three vertices (without explicitly identifying such a model) in closing
Chapter 6, entitled "Difficulties on Theory," in the Origin of Species—again, see pp.
251 - 260 of this book for my analysis of this key Darwinian argument). In making this
"pure end-member" analysis of canonical causes vs. constraints, I am consciously
presenting extreme, or cardboard, versions of central theories in order to clarify a
logical (and terminological) point about the naming of expectations and exceptions.
(Just as I argued previously that no actual empirical case would fall precisely at a
vertex of total determination by one factor alone, I also acknowledge that no subtle
thinker's theory will fall right on a vertex either. Nonetheless, discussion in terms of
pure end-members may be defended as a conceptual device for clarifying the central
content and primary commitment of more complex theories.)
Figure 10-11 illustrates the changing terminology of constraint and convention
under three pure end-member theories of causation at vertices of the aptive triangle.
For the pure adaptationist, committed to natural selection as the controlling and
functionalist mechanism of evolutionary change, all causes of currently adaptive form
that cannot be attributed to direct selection for immediate utility must count as
constraints. (Fig. 10-1 la depicts this version, with the canonical cause placed at the
functional vertex, and with other vertices making contributions that then be called
constraints upon the full and free operation of current natural selection to forge
immediate utility.)
As another virtue of these simplified representations, we can also grasp how any
pure end-member theorist must treat the exceptions ("constraints") that cannot be
denied as causal contributors to currently adaptive form. In this case (Fig. 10-lla), I
have already noted Darwin's own excellent strategy (see pp. 251-260): admit the
historical inputs, but attribute their cause to natural selection in the past; then admit
the structural inputs as genuine exceptions, but relegate them to a low and
insignificant relative frequency. Thus, all "constraints" either record the operation of
the canonical mechanism in the past, or stand as genuine exceptions rendered
impotent by their rarity.
But if I were committed to a view that the direct action of physical forces (as
expressed in the spatiotemporal invariance of natural law) builds the adaptive forms
of organisms directly, and without any appeal to functionalist or distinctively
biological principles like natural selection—D'Arcy Thompson, in fact, advocated
this general view in as pure a form as any 20th century biologist dared to espouse
(see pp. 1179-1208)—then the structural