Structural Constraints, Spandrels, and Exaptation 1199
heredity and phylogeny—for explaining the complex and intricately multivariate
Bauplane of complex metazoan animals. In other words, he accepted that the basic
designs must be admitted as "primitive terms" or "background conditions" within his
theory—as "givens" to be acknowledged (and attributed to other kinds of causes), and
as basic inputs before any further analysis could be conducted in his favored terms.
In making such an admission D'Arcy Thompson swallowed a bitter pill. He had
to accept the existence and contrary construction of "hipponess" or "eagleness" at the
outset, and then to determine what field might be left to his favored causes of direct
mapping by physical forces. The theory of transformed coordinates presents his
positive approach to this dilemma, his attempt to keep his theory maximally relevant
in the light of his enforced concession to historicism in general, and to Darwinism in
particular. He could not lay claim to the basic forms themselves, but he would still
make a play for the taxonomic variety produced by their transformations.
If the differences among related species could be expressed as simple distortions
11 - 6. An example of D'Arcy Thompson's theory of transformed coordinates. To understand
his view, we must recognize that these figures are meant to feature the transformation grid
lines themselves—as indications of physical forces that directly impose phyletic changes upon
the organisms. From D'Arcy Thompson, 1917.