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Evolution (進化, shinka?) is a sudden change in form of a Pokémon, more akin to a metamorphosis than actual evolution, and is usually accompanied by an increase in stat values. Evolution can occur in Pokémon for several different reasons, the most common of which is gaining enough battle experience. There are many other factors that can determine if, when, and into what, different Pokémon will evolve. For example, some evolutions will only occur if the Pokémon is holding a certain item; at a certain time of day; if the Pokémon's Loyalty is high enough; if the Pokémon is a certain gender; and if it is traded to another player. The player can choose to halt evolution at any time before the animation is finished (only by level up), simply by pressing the B button, unless the evolution was purposely initiated using an evolutionary stone.
The Pokémon Trading Card Game introduced the idea of numerical stages for referring to different points in a Pokémon’s evolution. This has translated into colloquial usage among fans of the games. All Pokémon can be placed at one of four evolutionary stages (though no existing Pokémon line includes more than three out of the four stages): Basic Pokémon, Stage 1 Pokémon which evolve from Basic Pokémon, Stage 2 Pokémon which evolve from Stage 1 Pokémon, and Baby Pokémon which are acquired by breeding certain Basic or Stage 1 Pokémon and hatching their eggs. A Pokémon higher up on this evolutionary scale is called an evolved form of the previous stages; a Pokémon lower down on the scale is said to be a pre-evolved form of later Pokémon in the chain. In general, the lower the evolutionary stage that the Pokémon is, the faster it will learn moves.