In chapter 25, Mina is showing to her friend Michael a section of an encyclopedia. The section is all about dinosaurs. Mina points to a picture of a particular dinosaur and says that it looks like a "heavy, bony thing." She also points out that the dinosaur has a "clumsy, leaden tail" and that dinosaurs were capable of "nothing but short, sudden flights." Mina is explaining that dinosaurs could not fly because they were very heavy creatures. She uses the example of the dinosaur's tail, which looks very heavy in the picture, to help her prove her point.
Mina is speaking to her friend Michael about dinosaurs so that they can better work out the nature of the strange winged man whom Michael has found in his garage. She is also using the example of dinosaurs to explain the theory of evolution. The implication is that Skellig is perhaps a more evolved form of a human being. If dinosaurs can evolve from heavy, "leaden" and flightless creatures, like the ones in the encyclopedia, into birds who can fly for hours, then it is possible that human beings might one day evolve into winged creatures like Skellig. At least this seems to be Mina's hypothesis. Indeed Mina says to Michael, "Maybe this is not how we are meant to be for ever." By this, Mina means that perhaps humans are not meant to remain in their current stage of evolution for ever, but rather evolve into creatures like Skellig. Perhaps Skellig is to humans what birds are to the "leaden" dinosaurs.