1190 THE STRUCTURE OF EVOLUTIONARY THEORY
out of context in a book of course readings, have no inkling of its anchoring purpose
in a much broader theory that they would, no doubt, heartily reject.) If physical forces
shape organisms directly, then our best test resides in the preeminence of the S/V
principle, and the linear scaling of this ratio with increasing organismal size. Tiny
animals must dwell in a world dominated by forces acting upon their surfaces, while
large animals will be ruled by gravitational forces operating upon volumes. We can
therefore test the efficacy of physical forces by noting whether organisms show the
"right" conformations for direct molding by the appropriate relative strengths of these
forces at their size.
The following chapter, entitled "the rate of growth," then develops the dynamic
argument that physical forces will be exerted upon vectors of growth during an
organism's ontogeny, not merely upon a realized final form. The subsequent 15
chapters then follow a sequence, beginning with single cells, where growth plays a
minimal role and forms may be construed as simple responses to a small number of
constraining conditions and imposing forces, as in D'Arcy Thompson's most famous
comparison (Figure 11-1) of protistan cells to Plateau's surfaces of revolution—a set
of shapes exhibiting minimal areas in designs that are radially symmetrical about a
single axis.
D'Arcy Thompson then moves on to simple aggregations of cells or units, but
proceeding no "further" (up the traditional chain of complexity) than fairly uniform
tissues of a single organ, minimally differentiated metazoans like sponges, and
colonial organisms made of similar units crowded together. He presents a wide
taxonomic range of putative cases for direct mechanical construction, but with strong
emphasis upon the most plausible circumstance of geometric forms automatically
engendered by closest packing of malleable units of the same basic size and
composition (the "soap-bubble" paradigm, if you will)—including an ingenious
analysis of sponge and holothurian spicules as mineralized maps of the junctions
between units, and not as