Although foreshadowing if often associated with plot developments in fiction, Darwin sometimes uses a kind of rhetorical foreshadowing to help guide his readers through his argument. For example, in Chapter 11, he foreshadows his discussion of development and embryology in Chapter 14:
Agassiz and several other highly competent judges insist that ancient animals resemble to a certain extent the embryos of recent animals belonging to the same classes; and that the geological succession of extinct forms is nearly parallel with the embryological development of existing forms. This view accords admirably well with our theory. In a future chapter I shall attempt to show that the adult differs from its embryo, owing to variations having supervened at a not early age, and having been inherited at a corresponding age.
Darwin gives a brief summary of his argument about embryology here. Namely, he believes that the geological record resembles the embryonic development of modern life forms because life forms have gradually become more complicated through additional adaptations. These more complicated adaptations develop in an individual life form, he believes, after the embryo has gone through multiple stages of more basic development. Therefore, an embryo's stages of development are a compressed version of the longer history of that organism's evolution from simple to more complex.
Darwin does indeed delve further into this part of his theory in a later chapter. He seems to bring it up briefly here for two main reasons. First, it is relevant to the claims he is making about the geological record. It is not time yet for him to delve into embryology, but embryology and geology are entangled in his theory. Other scientists have also posited a connection between the two. Darwin's reference to "Aggasiz and several other highly competent judges" demonstrates his awareness that many of his educated readers will be expecting him to address this connection. By mentioning here that he will get to embryology in a later chapter, Darwin gets ahead of any accusations that he is not considering the full picture.
Second, by foreshadowing his discussion of embryology, Darwin prepares readers for his more complicated claims about it later on. By the time readers arrive at Chapter 14, they have had a few chapters to sit with the idea that there is a connection between geology and embryology. This little bit of preparation helps create a through-line in the argument. It also signals to the reader that if they get lost in Chapter 14, it might be worth returning to Chapter 11 to remind themselves of important details.
