in·ter·grade 
(ĭn′tər-grād)
(ĭn′tər-grād)intr.v. in·ter·grad·ed, in·ter·grad·ing, in·ter·grades
To merge into each other in a series of stages, forms, or types.
n. (ĭntər-grād′)
A transitional stage, form, or type.
in′ter·gra·dation (-grā-dāshən) n.