This chapter is a discussion of ‘direct adaptations’, instances where mechanical forces operate upon a living structure in such a way as to modify it and make it mechanically efficient. There immediately arises the question, rather carefully avoided by D'Arcy Thompson, of the relations of these adaptations to the problem of inheritance. That there is a relation follows not only from the requisites of the Darwinian conception of evolution, but also from direct evidence.

In some instances a particular structure may not be inherited (such as the configuration of bone trabeculae in a poorly set broken leg), but the sensitivity of the cells to physical forces surely is a factor capable of inheritance and obviously of adaptive value.

In other cases, a structure is both stimulated into existence by mechanical factors and present in the embryo before the mechanical factors could possibly have operated. The soles of the feet are already thickened in the human foetus, although clearly the abrasion of walking barefoot vastly exaggerates the embryonic beginning. We have already suggested an explanation of how the organism might inherit a reactivity to the environment that would possess adaptive value, and now comes the question of how the structure itself could be directly inherited without use.

The simplest possibility (sometimes referred to as the Baldwin effect) is that if certain gene combinations appeared that produced a structure that was identical to the one produced by mechanical factors, obviously it would be advantageous and be retained by the population.