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So I got Pokemon Diamond and a DS. I'm a good One-third in I think And a have a Roselia. Heres the thing, Before, I had a Budew and I was getting mad because she wouldn't evolve. Then, she starts to evolve at level 27 right.)I stopped it for some strange reason.( Then I go somewhere a guess what wild pokemon pops out. A Roselia at Level 19! So now I'm pissed and confused.

1/3 of the way through the game?
You've already got a obtained pokedex of 164? (and one third) That's a lot for not knowing how Baby pokemon evolve. But uh, yeah, ZOMBEH is correct about how baby pokemon evolve, You need a maxed out happiness before they evolve.
Carrying them in your party, giving them Poffins they like, giving them massages, giving them items to hold, using them in battle, and the Soothe bell are really good for that. Likewise, keeping them in your box, feeding them poffins they don't like, and leaving them fainted are not beneficial.

Actually, Happiny also evolves by happiness. It just needs to be day and holding a Oval stone as the additional triggers.
Tyrogue, Smoochum, Elekid, Magby, Wynaut. evolve at a certain level. (20, 30, 30, 30, and 15 respectively)
Mime Jr, and Bonsley need to know a certain move. (mimic)
Mantyke needs a remoraid in the party.
and the babies that evolve with happiness (and additional requirements to the happiness) Pichu, Cleffa, Igglybuff, Azurill, Budew, Chingling, Happiny, Munchlax. (so, 50% still pretty monumental considering how many different ways baby pokemon are triggered to evolve)
Rilou and Togepi are considered basic pokemon but have baby pokemon qualities.

What does your house phone have to do with Chansey evolving?

Bulbapedia wasn't completely wrong, the entire Chansey line evolves via happiness. I assume they just expected people to understand that.

I never trust a single website for my pokemon information. Bulbapedia, Serebii, Smogon, gameFAQs, and Marriland are all websites that I use when doing much of anything in my pokemon game. (although right now I'm just filling up my dex without worrying about what to train them in, so Smogon isn't as important now as it will be later.)