Answer:
Ok, that post was extremely stupid for several reasons, so I'm improving it.
No, you cannot trade crystal to emerald sadly. That'd be really cool if they made it so you could technically get yellow Pokemon to black by continuing down the line of games, but they didn't.
You can however trade Red/Blue/Yellow to Crystal. Not 100% sure about trading with gold/silver to r/b/y, because I've only played Crystal of those three, but that's pretty likely. The only restriction is that you can only trade to 1st gen if the Pokemon is from kanto and doesn't have a 2nd gen item on it, for obvious reasons.
I also want to point out the last guy said something along the lines of, "You can trade Emerald/Ruby/Sapphire over to Fire Red/Leaf Green/Water Blue." ...Water blue doesn't exist.
But he was right that you can trade E/R/S with FR/LG, but that's pretty obvious.
You can also send E/R/S/FR/LG over to diamond/pearl/platinum, but its called migrating. You send over the Pokemon you want in you're 3rd gen over to 4th gen, and it will go to Pal Park. Pal Park is just a park where the Pokemon go, and you have to catch them like a normal wild Pokemon.
I have no idea if you can or can't send P/D/P to Black and White, because my DS broke a while back, so I can't actually play Black or White. If someone wants to edit this thing to say if you can or can't go ahead!
Also, the last guy actually posted, "By Joshua," at the end of his little post. Why the hell would you do that? That's both douchey and dumb.
...By Maddii