As applied to an estate, perdurable signifies lasting long or forever. Thus, a disseisor or tenant in fee upon condition has as high and great an estate as the rightful owner or tenant in fee-simple absolute, but not so perdurable. The term is chiefly used with reference to the extinguishment of rights by unity of seisin, which does not take place unless both the right and the land out of which it issues are held for equally high and perdurable estates. Co. Litt. 313a, 313 ft; Gale, Easem. 582; Sweet.