When a caterpillar emerges from the egg on a tree branch, it immediately climbs upward until it finds a leaf bud to eat. Biologists thought that this behavior displayed an innate tendency to move in the direction opposite to the pull of gravity. In a recent experiment, a strong light source was placed at the bottom of a tree, and caterpillars, after hatching, climbed downward.

Which of the following hypotheses is best supported by the statements given?