Oscillate
Oscillate (os"sil*lāt) , intransitive verb
[Latin oscillare to swing, from oscillum a swing, a little mask or puppet made to be hung from trees and swing in the wind, prob. orig., a little mouth, a dim. from os mouth. See Oral, and compare Osculate.]
1.
To move backward and forward; to vibrate like a pendulum; to swing; to sway.
2.
To vary or fluctuate between fixed limits; to act or move in a fickle or fluctuating manner; to change repeatedly, back and forth.
The amount of superior families oscillates rather than changes, that is, it fluctuates within fixed limits.