Transformations

I'm rereading The Origin and, geekishly, really enjoying this sort of thing (from Morphology, Chapter XIV):

Most physiologists believe that the bones of the skull are homologous with — that is, correspond in number and in relative connexion with—the elemental parts of a certain number of vertebræ. The anterior and posterior limbs in all the members of the vertebrate classes are plainly homologous. So it is with the wonderfully complex jaws and legs of crustaceans. It is familiar to almost every one, that in a flower the relative position of the sepals, petals, stamens, and pistils, as well as their intimate structure, are intelligible on the view that they consist of metamorphosed leaves, arranged in a spire. In monstrous plants, we often get direct evidence of the possibility of one organ being transformed into another; and we can actually see in flowers during their early development, as well as in crustaceans and many other animals during their embryonic states, that organs, which when mature become extremely different, are at an early stage of growth exactly alike.

Darwin asks questions such as: 'Why should the brain be enclosed in a box composed of such numerous and such extraordinarily shaped pieces of bone?' And then shows how natural selection is the necessary explanation.