Because of the nature of a complex developmental program, some forms of variation may not be possible. Historical constraints may cause adaptive imperfection. So possibilities for future adaptation are always limited by structures put in place by prior history of adaptation. An organism's design may be a trade-off between different adaptive needs. As in any engineering project, the adaptive design of an organism necessarily involves compromises among conflicting needs.
Examples are abundantly easy to imagine.
For example, both large size and small size have intrinsic advantages. In this section, Ridley surveys the consequences of recognizing evolutionary constraints. Consequently, second, the methods for studying adaptation must be informed by recognition of constraints. Biologists differ in the relative importance they ascribe to adaptation vs. its constraints. History suggests that this is a difficult problem.