Structural Constraints, Spandrels, and Exaptation 1195
selection need not be invoked at all, and as any kind of cause in this case. For he
holds that physical forces shaped the unduloid directly, without any selection of
favored forms from a range of variants. In other words, he believes that the efficient
cause of mechanical imposition constructs the final cause automatically, thus
obviating the need for any separate and explicitly biological or functional explanation
to fashion the adaptive shape of the unduloid. (In selectionist jargon, D'Arcy
Thompson argues that the proximate cause fashions the ultimate cause all by itself,
thus explaining two properties for the price of one mechanism. I also happen to think
that D'Arcy Thompson was probably wrong in this case, and that the traditional
Darwinian scheme, with different forms of explanation needed for ultimate and
proximate causes, probably applies to this case. But, the logic of D'Arcy Thompson's
argument remains sound.)
- At intermediary sizes, the automatically realized forms of inorganic objects
often map the "conflicting" expressions of surficial forces holding things up and
volumetric forces pulling them down. In a fascinating section added to the 2nd
edition of 1942, D'Arcy Thompson studied drops of more viscid material falling
through water. He compares the resulting forms (balancing surface tensions that
retard descent and spread out the drops, with gravitational forces that pull the dense
drops towards the bottom of the vessel) to the strikingly similar (and often quite
complex) radially symmetrical shapes of jellyfishes (Figure 11-5). D'Arcy Thompson
wrote (1942, pp. 397-398): "Not only do we recognize in a vorticoid drop a 'schema'
or analog of medusoid form, but we seem able to discover various actual phases of
the splash or drop in the all but innumerable living types of jellyfish ... It is hard
indeed to say how much or little all these analogies imply. But they indicate, at the
very least, how certain simple organic forms might be naturally assumed by one fluid
mass within another, when gravity, surface tension and fluid friction play their part." - At still larger sizes, surface tension becomes so negligible that rigid hard parts
become necessary to maintain shape, lest gravity create a world of pancakes.