gradation (noun) - a process or change taking place through a series of stages or degrees [grey-dey-shuhn]
BREAKDOWN: GRAD- (step) + -ATION (process of)
to gradate means to change or cause to change imperceptibly; to arrange in grades or rank
gradational means taking place by degrees; gradual
“Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.” —Charles Darwin