that rabbits are descended from frogs, or frogs from dog-fish, but that these three forms are remote cousins, derived from some ancient and far simpler progenitor. But since both rabbit and frog pass through phases like the adult condition of the dog-fish, it seems probable that the dog-fish has remained more like the primordial form than these two, and similarly, the frog than the rabbit.
Text Book of Biology, Part 1: Vertebrata · The Wunder Library — complete classics, free to read, with narration.