1052 THE STRUCTURE OF EVOLUTIONARY THEORY
amalgamations of three idealized end-members, in full expectation that few, if any,
real rocks will include only one end-member and plot right at one of the triangle's
vertices.
In keeping with my previous discussion, and with Seilacher's original
conception, we may call these idealized end-members "functional," "historical," and
"structural." In other words, any phenotypic feature now "working well" for an
organism may have been constructed by a process that directly crafted the feature for
its current function (the first corner), inherited from an ancestral form (the second
corner), or built by some structural mechanism or process not directly related to, or
engendered by, the functional needs of the organism.
As discussed in my previous analysis of Darwin's brilliant argument in Chapter
6 of the Origin (pp. 251- 2 60), the argument for natural selection as the dominant
cause of evolutionary change must be made in the following way under the aegis of
this model (as Darwin did, but without constructing any formal picture like Fig. 10-
10): At the functional vertex, natural selection stands alone as the only known and
effective cause in this mode. If the Lamarckian mechanism operated in nature, then
inheritance of acquired and adaptive characters would provide another functionalist
option for explaining the origin of working design. But inheritance does not so
operate, on this planet at least. (Darwin took the more generous view that Lamarckian
inheritance might exist, but at a relative frequency distinctly subsidiary to natural
selection.)
At the historical vertex, working features passively inherited from ancestors did
not originate to meet current functional needs. But so long as these
10 - 10. Standard triangular diagrams for depicting basic causes of form as functional
(immediate adaptation to current circumstances), historical (inherited by homology, whatever
the basis of ancestral origin), and structural, or arising either as physical consequence of other
features or directly from the nature of physical forces acting on biological materials. All
vertices may yield aptive traits of great utility to the organism.