We can generalize the point made in the last chapter about the role of environmental factors in determining human height to the role of these factors in determining the body sizes of most if not all animals. Indeed, we can generalize further, because environmental factors play a role in determining the values of many characters in most or all animals, including those that are unrelated to size. In other words, the genes do not completely determine the course of development; rather, this course is set by a mixture of genetic and environmental influences, and the interactions between them.

There are several ways of describing the effects of environmental factors on development. One way is to say that a character such as body size upon whose value there is an environmental influence has only a partial heritability. Another is to say that there is an element of plasticity in the character concerned. More complete terms for the latter are phenotypic plasticity or developmental plasticity, the second term being preferable for reasons already given in Chapter 22.