Mapping the Two Hierarchies

Both ontogeny and phylogeny are, in certain respects, hierarchical processes. A theme running through the work of most contributors to what can now be described as evolutionary developmental biology is the relationship between these two hierarchies. This is true throughout the history of the subject (see Chapter 4) from the pioneering studies of von Baer (1828) and Haeckel (1866, 1896) to recent work, notably that of Thomson (1988) and Hall (1992). Goodwin (1994a, p.234) states: “Developmental processes are hierarchical. So are biological classification schemes. I shall follow a well-trodden route in attempting to relate them.” And a possible way in which they may be related is given by Thomson (1988, p.92): “In principle we should be able to reconstruct for any species or any higher group a sequence of levels of morphological characteristics that define all the higher groups to which the taxon belongs, and to match these up with particular points in the hierarchy of morphogenesis.” I would now like to extend Thomson's proposed relationship by superimposing on it, in the following paragraph, a series of further proposals. As we will shortly see, some of these are oversimplified or even incorrect. However, the aim for the moment is to build an extreme picture that will serve as a means to an end.