A summoner's eidolon can gain the Flight evolution, which says the following:
An eidolon grows large wings, like those of a bat, bird, insect, or dragon, gaining the ability to fly. The eidolon gains a fly speed equal to its base speed.
There are many ways to increase a creature's base speed. For example, the eidolon could put on a pair of Boots of Striding and Springing, which increases its land movement:
These boots increase the wearer’s base land speed by 10 feet. In addition to this striding ability (considered an enhancement bonus), these boots allow the wearer to make great leaps. She can jump with a +5 competence bonus on Acrobatics checks.
By default, an eidolon's base land speed depends on its base form (biped, quadruped, serpentine, etc.), and so its default fly speed should equal that amount of movement.
If an eidolon has the flight evolution, and gets an enhancement bonus to its base speed (such as using the Boots of Striding and Springing), will its fly speed also increase? Or does the eidolon's fly speed derive from its unmodified land speed?