[quote=@Dusksong]
[@Cerces22] Mr. Mime likes crowds, so dunno if that's going to be around the park. Maybe in the city? But, that reminds me: isn't Mr. Mime the evolved form of Mime Jr.? So that'd make it and Jigglypuff (Iggybuff) the only evolved forms on the list.
[/quote]

So the way this works...

You can only catch baby Pokemon if you were to find the Pokemon Nest for that species of Pokemon (something that will be introduced later). Mr. Mime is still considered the base form, and Mime Jr. is a step below that. 

Also, say you were to go after a Rattata. Rattata evolve at level 20. Meaning that you are allowed to run into a level 20 Raticate right now.