In this chapter and the following ones D'Arcy Thompson is straggling against the notion that all form can simply be explained by heredity, and that therefore changes in form inevitably map out phylogenetic relations. Instead he repeatedly suggests that physical forces (such as those which produce the variations of shapes of snow-flakes) are of prime importance and relationships of shape may not justify any family tree or a sequence in time, but simply show mathematical kinship. Today we are inclined to combine the two and say that the genes, the units of heredity, do control shapes, but that the activities of genes are constrained by the physico-chemical properties of the chemical substances and the configuration of these substances present in the organism. This does not touch upon the question of whether or not all the shapes produced are adaptively significant. D'Arcy Thompson's strong arguments that they are not is a reaction against those who see a selective advantage to all structures. But this issue cannot be resolved without an ecological study of each example, a Gargantuan task that is unlikely ever to be achieved. All we can say at the moment is that there is no a priori reason why some structures, which have been initiated by mutation and formed within the confines of physico-chemical laws, should not utterly lack any adaptive significance, and yet remain fixed in the population. It might even be argued that a particular gene-change produced other effects that were adaptively significant and these less obvious gene-effects elicited the selection pressure which preserved the adaptively inert structure in the population.