D’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 the adult form 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. (shrink)

One of the most fundamental features of the body plan of arthropods is its segmental design. There is considerable variation in segment number among arthropod groups (about 20‐fold); yet, paradoxically, the vast majority of arthropod species have a fixed number of segments, thus providing no variation in this character for natural selection to act upon. However, the 1000‐species‐strong centipede order Geophilomorpha provides an exception to the general rule of intraspecific invariance in segment number. Members of this group, and especially our (...) favourite animal Strigamia maritima, may thus help us to understand the evolution of segment number in arthropods. Evolution must act by modifying the formation of segments during embryogenesis. So, how this developmental process operates, in a variable‐segment‐number species, is of considerable interest. Strigamia maritima turns out to be a tractable system both at the ecological level of investigating differences in mean segment number between populations and at the molecular level of studying the expression patterns of developmental genes. Here we report the current state of play in our work on this fascinating animal, including our recent finding of a double‐segment periodicity in the expression of two Strigamia segmentation genes, and its possible implications for our understanding of arthropod segmentation mechanisms in general. BioEssays 27:653–660, 2005. © 2005 Wiley Periodicals, Inc. (shrink)