Primitive and derived traits
In a sequence of evolutionary change primitive traits are succeeded by derived ones. But the derived trait is itself primitive in relation to any that succeed it. For example, in the evolution of vertebrate limbs the absence of paired limbs (I), is the primitive state. The sequence move through the limb of a rhipidistian fish (2) and a five-toed land vertebrate(3). The simplified limbs of the fosil horse(4) and modern horse(5), which appear more primitive than (3) are actually derived traits- bones have been lost in the course of evolution