I.14.1 Evolutionary constraints can substantially predetermine the direction of evolutionary processes
The term "constraints” has both a negative limiting meaning and also a positive meaning, the meaning of an active agent that can function as a creative factor in a certain process (Gould 2002). If a tube through which water is flowing is narrowed at a certain place, this narrowing (constraint) will both have a limiting effect on the overall flow rate of water through the tube, but will simultaneously increase the flow rate of the water at the particular point, which can be used to perform useful work, for example for creating under-pressure in a pump.
During the study of the historical course of evolution, long-term trends have been repeatedly observed in changes in some properties of a species in the individual higher taxons. In certain cases, these changes in properties (in the phenotype) of subsequent species can be explained by systematic changes in a certain environmental factor to which the new species must gradually adapt, for example an increase in the temperature. In other cases, such an explanation seems inadequate. Structuralists generally assume that the formation of a great many of these macroevolutionary trends is the direct result of ontogenetic constraints, which predetermine which ontogenetic and thus which evolutionary changes can occur within the the particular taxon (see also XXVI.7.4). Thus an evolutionary constraint can be the driving force for an active evolutionary trend.