A few of these Pokémon, as a result of new evolutions, become a part of evolutionary families with branched evolutions. Those Pokémon which evolve into a Pokémon of a previous generation, instead of from it, are specifically baby Pokémon.
The amount of cross-generational Pokémon introduced in each generation has been inconsistent. Generation II and IV introduced many new evolutions, Generation V introduced none at all and other generations introduced minimal amounts.
Generation IV introduced the most evolutionary relatives to Pokémon from previous generations, with 29, and Generation VI introduced the fewest evolutionary relatives to Pokémon from previous generations, with only one, Sylveon. The only generations that didn't introduce any new cross-generational evolutions (not counting regional variants) are Generation V and Generation VII.
Roselia is the only Pokémon to have both a pre-evolution and an evolution (Budew and Roserade, respectively) introduced in the same generation.
Porygon is the only Pokémon to have an evolution introduced in a later generation that was later able to evolve once again.