How to "evolve" cards?

[Jairo]

Guys, please help me.
I can't figure out hoe the "evolve cards" feature works.I simplyly CAN'T press the "evolving" button in the Enhance sub menu...

[-bookie-]

Need to be level 50 to do it. Then need at least 11 silver cards to evolve. 10 will be consumed or disappear and one will become level 51 (reset to level 41 but in "yellow" text). You then pay rubies to upgrade one level at a time again and get similar hp and attack increases each time.

[Vimara]

you choose the cards to sacrifice?

I only have bears at level 46 ... but within a few months I would like to know.