dealing with gustation and «facial» sensitivity, as well as with the
control of rythmic vegetative functions.
2. Conservation and change in evolution
In this second, more speculative section, I will explore the idea
that all extant triploblasts arose from a single, highly sophisticated
ancestral species. The proposal I will argue for is that this
unexpected monophyly derives at least in part from two linked
factors. First, a major constraint of metazoan evolution is that,
because of the complexity of the developmmental programme of
any organism, changes are not allowed (rule of conservative
changes). Second, only those developmental processes that
have become extremely stable can withstand substantial varia-
tion without collapsing.