Mickey Mouse has evolved. This was noticed and brough to the world by Stephen J. Gould in his book, "The Panda's Thumb", deserving a whole chapter.
As you can see, in this case, Mickey Mouse is not portraited as an individual, but as a population, or species of consecutive Mickeys. The Mickeys started to suffer small changes and, since they were successful (the character became more dear to the public) they kept those traits, by a special kind of natural selection.

And what traits are those that make Mickey each time more a favourite of people? The traits of a child! The chubby, big eyed, big headed, rounder mouse has been becoming everytime more like the being we instinctively tend to love and protect: our kids.