Frogs in a pot

A modern popular parable states that frogs thrown into a pot of boiling water will jump out, while frogs sitting in water that is slowly brought to a boil will just sit there until they are boiled.

While there are certain flaws in this parable (such as a frog thrown into boiling water will likely die and a frog sitting in uncomfortably hot water will certainly jump out unless you've tied him up nicely) it is used to illustrate the principle of Gradualism, which states that profound change is more often the result of steady, incremental steps rather than sudden great leaps.

Steady, incremental steps... nothing but steady, incremental steps... that lead to a profound and dramatic change.