1194 THE STRUCTURE OF EVOLUTIONARY THEORY
membrane was inevitably bound, by molecular forces, to become a surface of
the least possible area which the circumstances permitted; that in the present
case, the symmetry and "freedom" of the system permitted, and ipso facto
caused, this surface to be a surface of revolution; and that of the few surfaces
of revolution which, as being also surfaces minimae areae, were available, the
unduloid was manifestly the one permitted, and ipso facto caused, by the
dimensions of the organisms. We also see that the actual outline of this or that
particular unduloid is also a very subordinate matter, such as physico-
chemical variants of a minute kind would suffice to bring about; for between
the various unduloids which the various species of Vorticella represent, there
is no more real difference than that difference of ratio or degree which exists
between two circles of different diameter, or two lines of unequal length.In cases like this, strict selectionists conventionally assert that no genuine
problem exists, but only a conceptual or terminological confusion. After all, any
devotee of natural selection knows that adaptive shapes must be explained both in
terms of survival value and immediate mode of construction. Therefore, the
selectionist would continue, "I am happy to suppose that natural selection built the
adaptive unduloid by fostering the differential reproductive success of growth
variants that could attain the advantageous form along a single dimension of
selection, rather than by having to construct each property in a piecemeal fashion,
character by character."
In fact, selectionists can even cite a terminology to bolster their understanding
that any adaptation requires, for its full explication, an account of both survival value
and mode of construction (which, in truth, only reflects Aristotle's old distinction of
final and efficient causes, and might as well bear these names rather than their
currently favored neologisms). Selectionists generally refer to these two
complementary modalities as "ultimate" and "proximate" causes—often supposing
that they have, by this terminology, won some preciously new insight to clear away
the conceptual fog of centuries. However, as stated above, the distinction only
codifies a particular expression of Aristotle's argument on the multiple meanings and
aspects of causality.
Nonetheless, even if not new, this argument about the complementarity and non-
oppositional nature of ultimate and proximate causation cannot be gainsaid—and
Darwinians advance this point with complete justice. That is, when selectionists cite
the adaptive advantage of a form, they surely do not deny the need for a different
statement about the immediate mode of genetic and developmental origin in any
individual as well. However, we also need to recognize that this legitimate defense of
adaptationist language does not apply to D'Arcy Thompson's point of genuine
contention (logically genuine that is, not necessarily empirically correct in any given
instance).
D'Arcy Thompson does not merely argue that he has found the mode by which
natural selection worked to build adaptive unduloids. Rather, he advances the
radically different, and truly oppositional, argument that natural