D’Arcy Thompson’s Morphological Transformations: Issues of Causality and Dimensionality

Abstract

AbstractD’Arcy Thompson’s drawings showing coordinated differences between the shape of an individual of one species and the shape of an individual of another have been reproduced and discussed countless times. However, while they have been widely regarded as inspirational, their interpretation in causal terms has proved difficult, and there is as yet no consensus on this matter. Here, I approach these Thompsonian transformations from a particular angle, namely their dimensional insufficiency. I argue that this problem must be solved before the issue of causality can be considered. This approach leads to the conclusion that Thompsonian transformations—or “morphological transformations”—have not taken place in evolution, and logically can never do so, because they involve the direct conversion of theadultform of one species into that of another. In contrast, “developmental transformations” do occur in the short term, within the lifetime of a single individual. And “evolutionary transformations” occur in the long term, in a context that can be described, following Scott Gilbert, as five-dimensional. I argue that these two kinds of transformation—developmental and evolutionary—have different causal agencies. I consider the possible nature of these agencies and, related to that, the way in which Thompson’s work connects with Darwinian evolutionary theory.